Brenner and Rector's The Kidney, 8th ed.

CHAPTER 33. Tubulointerstitial Diseases

Giuseppe Remuzzi Norberto Perico Marc E. De Broe

		Historical View Including Structure-Function Relationships, 1174
		Mechanisms of Tubulointerstitial Injury, 1175
		Glomerular-Related Events, 1175
		Tubulointerstitial Antigens and Costimulatory Signals, 1178
		Interstitial Inflammation, 1180
		Interstitial Fibrosis and Scarring, 1182
		Acute Interstitial Nephritis, 1183
		Etiology, 1183
		Pathology, 1184
		Clinical Manifestations and Diagnosis, 1185
		Prognosis and Management, 1185
		Chronic Tubulointerstitial Nephritis, 1186
		Analgesics, 1186
		Nonsteroidal Anti-inflammatory Drugs, 1188
		5-Aminosalicylic Acid, 1188
		Chinese Herbs—Aristolochic Nephropathy, 1190
		Lithium, 1192
		Lead, 1193
		Cadmium, 1194
		Balkan Endemic Nephropathy, 1195
		Hyperuricemia/Hyperuricosuria, 1196
		Chronic Hypokalemia, 1196
		Sarcoidosis, 1197

Tubulointerstitial disease is common to all chronic progressive renal diseases, irrespective of the initial trigger or site of injury. Once viewed as an inconsequential corol-lary to pathologic events enveloping glomeruli, tubulointerstitial disease is now recognized as an indispensable and prominent participant in the progression of renal disease.^[1] Many, if not most, forms of progressive, noncystic renal disease are glomerular in origin, and yet, the intensity of accompanying or evolving injury of the tubulointerstitial compartment, rather than the extent of glomerular changes, is what predicts overall decline in renal function.^[1] Studies have explored, among others, the following questions: How do processes originating in glomeruli recruit tubulointer-stitial events? Are there processes within the tubulointerstitium itself that can sustain tubulointerstitial disease? And, finally, why do these tubulointerstitial changes became the final arbiter of the functional fate of the kidney? Also worthy of consideration is the finding that seemingly diminishes tubulointerstitial disease as a determinant of progressive injury: namely, primary tubulointerstitial nephritides are, as a group, among the more indolent and slowly progressive of all nephritides.^[2]

HISTORICAL VIEW INCLUDING STRUCTURE-FUNCTION RELATIONSHIPS

In the middle of 19th century, tubulointerstitial compartment was first considered an independent entity inside the kidney. This rests on the seminal observation by Bowman in 1842^[3] that the malpighian bodies were directly connected to tubules and may participate in urine formation and the recognition, few years later by Kolliker,^[4] that the interstitium was a separate anatomic compartment. In 1860, the development of an experimental model of interstitial injury by mercury bichloride^[5] and the finding of interstitial infiltrates at autopsy led to the consideration that the tubulointerstitium was not only relevant for renal physiology but also a key player in renal diseases. Thus, based on previous observation of the presence of fibroblast-like cells in the renal interstitium,^[6] in 1870, Traube^[7] hypothesized that interstitial changes documented in Bright's disease were responsible for kidney scarring and shrinking associated with end-stage renal failure. “Cellular and fluid exudation in the interstitial tissue” was described by Councilman in 1898 when he examined the kidneys of patients dying of scarlet fever and diphtheria.^[8] In particular, he noted that the organs were sterile, thus raising the possibility of an allergic-type phenomenon. This entity was termed acute tubulointerstitial nephritis. This report and the insights from several models of tubulointerstitial injury developed across the century provided the rationale to Volhard and Fahr^[9] in 1914 for the inclusion of interstitial nephritis in their classification of kidney diseases. Hints to a possible association between drugs and interstitial nephritis in humans emerged in the 1940s with the observation that first antibiotics and then analgesics could damage the interstitial compartment.^[10] This opened the way to also consider other drugs as potential etiologic factors of interstitial nephritis.

After the early evidence in 1913 that injection of heterologous proteins into rabbits led to lymphocyte infiltration in the renal interstitium,^[11] the attention to the immunologic basis of interstitial injury was renewed only in 1971 by Steblay and Rudofsky,^[12] who described a model of tubulointerstitial nephritis (TIN) induced by antibodies reactive with tubular basement membrane (TBM) in guinea pigs. Since then, it has been established that a num-ber of species, including mice, rats, and rabbits, develop autoimmune tubulointerstitial injury when immunized with heterologous TBM.^[13] Although all of these models illustrated immune mechanisms that, in one form or another, have been reported in humans with interstitial nephritis, none of them did offer the intrinsic value of a spontaneously occurring renal lesion. In 1984, Neilson and coworkers^[14] characterized a model of spontaneous interstitial nephritis in kdkd mice. Although the failure to document the presence of antibodies to determinants in the tubulointerstitium, they found that the development of the interstitial lesion was markedly inhibited by thymectomy with T cell depletion, but could not be adoptively transferred with cells or serum from nephritic mice. These findings led to the hypothesis that susceptibility to the expression of interstitial nephritis in this inheritable model of spontaneous interstitial nephritis involved the cellular limb of the immune system and was related, at least in part, to alterations in regulatory T cell function.

In virtually all forms of progressive experimental and human chronic renal disease, a prominent inflammatory infiltrate exists within the interstitial compartment. The extent of the infiltrate and associated areas of fibrosis correlate with progressive decline in renal function. The concept of tubulointerstitial damage mediating impaired renal function is not new.^[10] Many studies have pointed out the prognostic significance of severe injury of the tubulointerstitial compartment in lupus nephritis^[15] and membranous nephropathy.^[16] About 40 years ago, Risdon and colleagues^[16] first described the association between the degree of renal impairment and the extent of tubulointerstitial damage in patients with glomerular disease. In 50 cases of persistent glomerular nephritis, a significant correlation was found between the extent of the eosinophilous deposits in glomeruli and the creatinine clearance, plasma creatinine concentration, and the ability of the kidney to concentrate the urine. Nevertheless, the correlation between these functions and the glomerular changes were less striking than those documented between the extent of tubular lesions and the alteration in renal function. These findings suggested that, in chronic glomerulonephritis, the structural damage in the tubules may have much more effect on the glomerular filtration rate (GFR) than does the structural injury in the glomeruli. A morphometric contribution to this topic was provided by Bohle and coworkers,^[17] who studied tubulointerstitial changes in a wide variety of glomerulopathies. The correlation curves showed that, as the urine osmolality decreased, the renal function (as creatinine clearance) did drop. Conversely, decreasing maximal urine osmolality was correlated best with increasing interstitial volume, lowering cross-sectional area of proximal tubular epithelium or epithelium from the thick segment of the loop of Henle. This was not a unique feature, because other key interstitial changes documented by light microscopy, immunofluorescence, or histochemistry—which include the presence of immune inflammatory cells, activated fibro-blasts, extracellular matrix (ECM) components, antibodies and complement—could also predict the long-term prognosis in chronic glomerulonephritis. ^{[17] [18]}

How the tubulointerstitial disease affects renal function detrimentally, however, remains ill defined. Several mechanisms, not mutually exclusive, have been suggested. A simple explanation is the tubular obstruction as a result of interstitial inflammation and fibrosis that would impede urine flow and increase intratubular pressure and, eventually, lower glomerular filtration.^[17] Although no direct measurements are available, tubular atrophy and cell debris within tubules would represent an hurdle to fluid drain, as shown in a rat model of unilateral ureteral ligation.^[19] A second possible mechanism implicates the reduction in the volume of peritubular capillaries^[20] and the number and cross-sectional area of the postglomerular capillaries^[17] as the interstitial inflammation and fibrosis increase. In this setting, the tubulointerstitial compartment becomes relatively avascular and, therefore, somewhat ischemic. As a result of the increase in vascular resistance in the postglomerular region, the hydrostatic pressure in the glomerular capillaries may also increase. Intraglomerular hypertension ensuing initially as compensatory adaptation to the impaired glomerular arteriolar outflow can cause progressive deterioration of renal function upon severe structural damage of the glomerular tuft. Perhaps more significant would be the contribution of alterations in the tubuloglomerular feedback, as a third explanation. The presence of edema and inflammation in the renal interstitium, by increasing the interstitial pressure, may lower the sensitivity of the feedback mechanism,^[21] possibly through local control of production of vasoactive substances such as angiotensin II, nitric oxide, and prostaglandins. When tubulointerstitial fibrosis develops, the autoregulation of renal blood flow is permanently disrupted, as is shown in chronic glomerulonephritis in rats.^[22]

Findings of atrophic tubules and of glomeruli no longer connected to their proximal tubules are associated with tubulointerstitial injury and parallel signs of renal dysfunction. The glomerular-tubule disconnection, as a fourth hypothetical mechanism, has been reported in several experimental models such as radiation nephropathy^[23] or nephropathy induced by tubular toxic substances such as lithium.^[24] Also, in a model of renal mass ablation^[25] or in the accelerated passive Heymann nephritis model (which mimics chronic proteinuric diseases in humans),^[26] the declining renal function with time paralleled tubular atrophy and disconnection. This is in harmony with human data that show a positive correlation between the volume fraction of the interstitium, the percentage of proximal tubuli without connection to a glomerulus, and renal function decline in patients with chronic pyelonephritis.^[27]

Collectively, none of these mechanisms predominate to dictate the interaction between tubulointerstitial damage and progressive renal function deterioration.

MECHANISMS OF TUBULOINTERSTITIAL INJURY

Progressive inflammation or injury to the renal interstitium can either begin as a secondary event following glomerular or vascular injury or form within the tubulointerstitial compartment. It is difficult to make useful comparisons of the pathogenesis of tubulointerstitial lesions between experimental animals and humans, because nearly all we know about this process in humans comes from work in experimental models.

Glomerular-Related Events

Glomerular diseases incite tubulointerstitial disease through multiple pathways^[1]: (1) Impaired glomerular permselecti-vity allows the escape into the urinary space of substances that are toxic to tubuli. (2) Altered glomerular hemodynamics can damage nephrons via intraglomerular hypertension. Alternatively, glomerular hypoperfusion may diminish postglomerular blood flow and provoke tubular ischemia. (3) Immunologic mechanisms in glomeruli may incur the loss of tolerance and thereby instigate tubulointerstitial injury. (4) Inflammatory mediators may seep from glomeruli into the urinary space or percolate down the mesangium into the interstitium. In addition, leukocytes may migrate out into the interstitium via the mesangial stalk and vascular pole of the glomerulus. (5) Nephron loss owing to destruction of the glomeruli and attached tubules may instigate metabolic adaptations in surviving nephrons that induce tubulointerstitial injury. This loss of containment of injury within the confines of the glomerulus and subsequent invasion of the surrounding tubuli and interstitium now involve a compartment that makes up the bulk of the kidney, encompassing as it does some 85% to 90% of kidney volume rather than the 3% to 4% occupied by glomeruli. Chronic inflammation changes and accompanying interstitial fibrosis, driven by glomerular processes, can continue to ramify widely within the tubulointerstitium; the latter, in effect, providing a conduit for the easy transmission of inflammation and injury into areas previously uninvolved by the original disease. Tubulointerstitial disease can thus bridge the areas that separate injured and noninjured nephrons. One important mechanism of glomerular-tubulointerstitial interaction is through proteinuria.

Proteinuria-Induced Tubular Cell Activation and Damage

Although historically proteinuria has been considered as simply a surrogate marker of the severity of underlying glomerular damage, clinical and experimental data reported during more than a decade of intensive investigation indicate that proteinuria is an independent risk factor and plays an important role in the pathogenesis of the progression of renal disease. ^{[28] [29]}

In 1932, Chanutin and Ferris^[30] observed that removing of three quarters of the total renal mass in the rat led to a slowly progressive deterioration in the function of the remaining nephrons , with progressive azotemia and glomerulosclerosis. The glomerular lesions of the remnant kidneys were associated with abnormal glomerular permeability and proteinuria. At that time, proteinuria was considered a marker of the extent of the glomerular damage, despite the fact that Volhard and Fahr in 1914^[9] and Mollendorrf and Stohr in 1924^[31] already found that renal damage was related to exuberant protein excretion in the urine. In 1954, Oliver and associates^[32] recognized protein droplets in the cytoplasm of tubular cells. They suggested that such findings were possibly the result of an impairment of the process of reabsorption of plasma proteins normally carried out by renal tubules and proposed that proteinuria could lead to nephron structural and functional damage.

The mechanisms by which increased urinary protein concentration leads to nephrotoxic injury are certainly to be multifactorial and involve complex interactions between numerous pathways of cellular damage. Obstruction of tubular lumen by casts and obliteration of the tubular neck by glomerular tuft adhesions may contribute to tubulointerstitial damage by proteinuria. However, accumulating evidence emphasizes direct effects of filtered macromolecules on tubular cells.^[33] Proteins that have escaped into the glomerular filtrate and reach the tubular urine are largely reabsorbed in the proximal segments at the apical pole of the tubular epithelium. This involves receptor-mediated endocytosis, followed by clustering of the ligand-receptor complex into clathrin-coated pits, giving rise to endocytic vesicles or endosomes. Upon endocytic uptake, progress to the lysosomes requires endosomal acidification to dissociate proteins from the receptors, permitting their degradation in lysosomes by the action of specific enzymes. The tandem endocytic receptors megalin and cubilin, which are abundantly expressed at the brush border of proximal tubular cells, interact to mediate the reabsorption of a large amount of proteins, including carrier proteins important for transport and cellular uptake of vitamins and lipids.^[34]

Insights into specific mechanisms linking the excess traffic of plasma proteins and tubulointerstitial injury have come from in vitro studies using polarized proximal tubular cells to assess the effect of apical exposure to proteins. Collectively, these studies show that protein overload activates proximal tubular cells into acquiring a proinflammatory phenotype. ^{[35] [36] [37] [38] [39]} Indeed, up-regulation of inflammatory and fibrogenic genes and production of related proteins have been reported upon challenge of proximal tubular cells with plasma proteins. They include cytokines and chemokines such as monocyte chemoattractant protein-1 (MCP-1), RANTES (regulated upon activation, normal T cell expressed and secreted), interleukin-8 (IL-8), ^{[36] [37] [38]} and fractalkine.^[39] Moreover, the profibrogenic cytokine transforming growth factor-β (TGF-β) and its type II receptor,^[40] as well as tissue inhibitors of metalloproteinase (TIMP)-1 and TIMP-2, and membrane surface expression of the avb5 integrin^[41] were also highly increased in vitro by plasma proteins. These events are triggered by protein kinase C-dependent generation of reactive oxygen species,^[42] nuclear translocation of nuclear factor kb (NF-κb), ^{[37] [38] [39] [43]} and activation of mitogen-activated protein kinase. ^{[39] [44]}

It is obvious that extrapolation from such in vitro data to the human in vivo situation may also be difficult, considering the somewhat conflicting data observed with different proteins in different cell systems ^{[45] [46]} as well as the reported changes in the expression of several genes of still unknown function.^[47] One of the concerns relates to the possibility that in vitro evidence of the phenotypic changes induced in the proximal tubular cells by protein overload can adequately reflect the in vivo proteinuric conditions in animals and humans, arguing that the concentrations of albumin used to challenge proximal tubular cells in culture have usually been much higher than one would expect in the disease condition in vivo.

The challenge to the overall hypothesis of proteinuria-induced tubulointerstitial changes is the ability to specifically interfere with the cascade of events leading to renal damage. An established approach has been to prevent or reduce proteinuria notably by blocking the renin-angiotensin system. ^{[48] [49] [50] [51] [52]} Angiotensin-converting enzyme (ACE) inhibitors given to animals with experimental models of proteinuric chronic nephropathies markedly reduced urinary protein excretion and, at the same time, lowered tubular injury and attenuated interstitial inflammation and fibrosis. Further evidence suggests that inhibition of renin-angiotensin system, in addition to a reduction of proteinuria, may also attenuate albumin-induced signal activation in tubular cells.^[53] Moreover, it has recently been shown that transfection with a monocyte chemotactic protein-1 antagonist^[54] or a truncated form of IkBa, thereby inhibiting NF-κb,^[55] inhibits albumin-overload induced tubulointerstitial injury. Nevertheless, despite many years of concerted efforts, it is still unclear whether in vivo protein overload and uptake of proximal tubular cells results in a proinflammatory phenotype with clinical significance toward chronic tubulointerstitial injury.

Proteinuria-Induced Tubular Cell Apoptosis

Apoptosis is emerging as one of the mechanisms underlying protein-induced tubular cell injury. Protein overload caused a dose- and duration-dependent induction of apoptosis in cultured proximal tubular cells, as disclosed by evidence of internucleosomal DNA fragmentation, morphologic changes including cell shrinkage and nuclear condensation, and plasma membrane alterations.^[45] Apoptosis in this setting was associated with activation of Fas-FADD-caspase 8 pathway. Evidence of apoptotic responses to protein load is not confined to cultured tubular epithelial cells. Persistent proteinuria was indeed associated with increased numbers of terminal deoxyuridine triphosphate (dUTP) nick-end labeling positive apoptotic cells both in the tubulointerstitial compartment and in the glomeruli in the rat model of albumin overload. In tubuli, most of the positive cells belonged to profile expressing angiotensin II type 2 (AT2) receptor. Findings of reduced phosphorylation of extracellular signal-regulated kinase (ERK) and Bcl-2 were suggested to reflect an AT2 receptor-mediated mechanism underlying tubular cell apoptosis.^[56] Proximal tubular cell apoptosis, which may contribute to glomerular-tubule disconnection and atrophy, was also found in response to proteinuria in the accelerated model of passive Heymann nephritis.^[26]

Apoptotic cells were recently detected in both proximal and distal tubular profiles in biopsy specimens of patients with primary focal segmental glomerulosclerosis. In support of the pathophysiologic significance of such observation, a strong positive correlation was found between proteinuria and the incidence of tubular cell apoptosis, which was identified as a strong predictor of outcome in these patients.^[57]

Filtered Growth Factors/Cytokines

Plasma contains many growth factors and cytokines at considerable concentrations, usually in high-molecular-weight precursor forms or bound to specific binding proteins that regulate their biologic activity. They are present in nephrotic tubular fluid.

Insulin-like growth factor-1 (IGF-1) is present in serum at 20 to 40 nM, which is more than 1000-fold its biologic activity. Almost all of the circulating IGF-1 is present in higher-molecular-weight complexes at about 50 and 150 kDa, normally preventing glomerular ultrafiltration. However, in experimental proteinuria in rats there is translocation of this growth factor into tubular fluid (primarily as the 50-kDa complex), as shown by micropuncture collection of early proximal tubular fluid.^[58] Tubular fluid from nephrotic rats activates IGF-1 receptors in cultured tubular cells that are expressed in basolateral as well as in apical membranes in some tubular segments.^[58] Hepatocyte growth factor (HGF) is largely of hepatic origin and is present in serum in a bio-inactive, monomeric form (97 kDa) and as heterodimeric HGF (80–92 kDa depending on glycosylation). Its specific signaling receptor, the p190^merprotein, is expressed in apical membranes in proximal tubuli in normal rats and at increased levels in diabetic animals. HGF is present in early proximal tubular fluid from rats with streptozotocin-induced diabetic nephropathy and is excreted with urine in diabetic animals.^[59] Circumstantial evidence suggests that HGF undergoes glomerular ultrafiltration in proteinuric states, probably as the mature, bioactive form of the molecule. TGF-β is a pluripotent cytokine (25 kDa) present in serum at considerable levels and in high concentrations in platelets. In these reservoirs, almost all of the peptide is maintained in bio-inactive complexes by binding to the latency-associated protein (LAP), which is further bound to latent TGF-β–binding protein (LTBP) forming a high-molecular-weight LAP-LTBP complex (220 kDa). TGF-β is also associated in serum with α₂-macroglobulin (900 kDa). The high-molecular-weight TGF-β complexes prevent glomerular ultrafiltration under physiologic conditions. However, in proteinuric glomerular diseases, TGF-β is present in early proximal tubular fluid and at least a portion is bioactive. The remainder is likely activated during downstream tubular flow by acidification of tubular fluid and, perhaps, by the increasing urea concentrations and by the presence of enzymes such as plasminogen activator inhibitor (PAI)-1. The concentration of TGF-β in glomerular ultrafiltrate from rats with diabetic nephropathy is approximately 30 pM, which is one to two orders of magnitude greater than required for documented biologic responses.^[59] TGF-β receptors are expressed in most tubular segments. ^{[60] [61]} IGF-1, HGF, and TGF-β are also present in urine in patients with proteinuric diseases. ^{[62] [63]} Urinary excretion of these proteins, however, does not prove glomerular ultrafiltration as IGF-1 and TGF-β are also expressed in tubular segments in some renal disease; nevertheless, the presence of these growth factors/cytokines in urine in patients with proteinuric diseases is certainly compatible with their glomerular ultrafiltration. Ultrafiltered IGF-1, HGF, and TGF-β appear to act on tubular cells through their apical signaling receptors. There are several responses by tubular cells that, collectively, can be described as activation or a moderate change in all phenotype toward a program resembling cell injury. This includes a moderate increase in collagen type I and IV production in response to IGF-1.^[58] HGF modestly increases the expression of fibronectin in tubular cells.^[61] HGF has also unique effects in proximal tubular cells. It actually completely blocks expression of collagen α₁III (Col3A1),^[61] which is consistent with an antifibrogenic role. Incubation of proximal tubular cells with pooled early proximal tubular fluid, collected by micropuncture from rats with diabetic nephropathy, increases fibronectin expression. TGF-β also increases the transcription of the genes encoding Col3A1 and collagen α₂I (Col1A2) as well as fibronectin in proximal tubular cells. Thus, ultrafiltered growth factors induce moderately increased expression of ECM proteins in tubular cells that most likely contribute to interstitial fibrosis.

Protein-Bound Lipids

Beside proteins themselves, fatty acids carried by filtered proteins have been advocated as trigger of tubulointerstitial injury. In rats with overload proteinuria, a potent chemotactic lipid was isolated from the urine, which attracted monocytes but not neutrophils.^[64] Similar results were obtained in mice transgenic for human liver-type fatty acid-binding protein, which in human proximal tubular cells was shown to bind free fatty acids in the cytoplasm and to carry them to mitochondria or peroxisomes for metabolism by beta-oxidation.^[65] These mice developed less macrophage infiltration and tendency to reduced tubulointerstitial damage than the wild-type control, possibly suggesting that intracellular accumulation of overload free fatty acids can be regulated to modulate cell activation. Attempts have also been made to differentiate the effects of individual fatty acids (palmitate, stearate, oleate, and linoleate) on cell toxicity and fibronectin production in cultured proximal tubular cells.^[66] Oleate and linoleate were identified as the most profibrogenic and tubulotoxic fatty acids. An additional pathogenic pathway has recently been linked to a form of low-density lipoprotein (LDL) modified by hypochlorous acid (HOCl), found to accumulate in tubular epithelial cells in settings of injury.^[67] Hypochlorous acid/hypochlorite is a major oxidant generated from hydrogen peroxide (H₂O₂) by myeloperoxidase during oxidative burst. In the Hk-2 proximal tubular cell line, hypochlorite-modified LDL caused a rapid increase in the expression of several genes encoding for proteins engaged in control of cell proliferation and apoptosis (Gadd153), reactive oxygen species metabolism (hemeoxygenase 1, cytochrome b5 reductase), tissue remodeling and inflammation (connective tissue growth factor [CTGF], vascular cell adhesion molecule-1 [VCAM-1], IL-1β, matrix metalloproteinase-7 [MMP-7], vascular endothelial growth factor [VEGF]). Hypochlorite-modified LDL but not naive LDL also had antiproliferative and proapoptotic effects in these cells. Comparable changes in gene expression were found in renal biopsy samples microdissected from proteinuric patients with declining renal function. The presence of hypochlorite-modified LDL in damaged tubular cells was confirmed by immunohistochemistry.^[67] These observations seem to mirror altered patterns of gene expression occurring selectively in response to oxidative LDL modifications to enhance inflammatory and fibrogenic processes in chronic proteinuric conditions.

Activation of Complement Components

Among specific components of proteinuria, serum-derived complement factors can be highly harmful, especially upon activation in the proximal tubulus.^[68] Renal tubular epithelial cells appear most susceptible to luminal attack by C5b-9 because of the relative lack of membrane-bound complement regulatory proteins such as membrane cofactor protein (CD46), decay-accelerating factor, or CD55 and CD59 on the apical surface,^[69] as opposed to other cell types such as the endothelium or circulating cells ordinarily exposed to constant challenge from complement. C3 is an essential factor of both the classical and the alternative pathways of complement activation that lead to the formation of C5b-9 membrane attack complex. In vitro, proximal tubular cells exposed to human serum were found to activate comple-ment via the alternative pathway leading to fixation of the C5b-9 membrane attack complex neoantigen on the cell surface.^[70] These events were followed by marked cytoskeleton alterations with disruption of the network of actin stress fibers, formation of blebs, and cytolysis. Increased production of superoxide anion and H₂O₂ and synthesis of proinflammatory cytokines such as IL-6 and TNF-α were also observed.^[71]

Within the kidney, complement proteins form deposits along the tubule luminal side and are internalized by proximal tubular cells in rats with protein overload proteinuria,^[72] renal mass ablation,^[73] and aminonucleoside nephrosis,^[74] a pattern that is commonly observed in kidneys of patients with nonselective proteinuria. In C6-deficient rats with 5/6 nephrectomy, a marked improvement of tubulointerstitial injury and function in respect to normocomplementemic rats was also demonstrated,^[75] which suggested that treatments to reduce C5b-9 attack in tubular cells may slow disease progression and facilitate functional recovery independent of the initial inciters of glomerular injury.

Intracellular C3 staining was evident in proximal tubules early after renal mass ablation in a stage closely preceding the appearance of inflammation. C3 colocalized with immunoglobulin G (IgG) to the same tubules in adiacent sections. Protein accumulation in proximal tubular cells was followed by local recruitment of infiltrating mononuclear cells that concentrated almost exclusively in regions containing C3-positive proximal tubuli.^[76]

The amidation of C3 by ammonia in the presence of high protein catabolism was suggested to contribute to luminal formation of C5b-9^[77] and generation of a monocyte-activating factor (amidated C3).^[78] Treatment with ACE inhibitor, while preventing proteinuria, limited at the same time both the tubular accumulation of C3 and IgG and interstitial inflammation.^[73] It should be remarked that the previously mentioned results converge to suggest that the pivotal role of complement as a mediator of progressive tubulointerstitial damage requires an environment of protein-enriched ultrafiltrate. That this is indeed the case has been recently substantiated by data showing that, in the absence of proteinuria, C5b-9 could not exert significant pathogenic potential to mediate chronic tubulointerstitial disease.^[79] In three pathophysiologically distinct models of nonproteinuric tubulointerstitial disease in Piebal-Viral Glaxo (PVG) rats, an increased deposition of C5b-9 at peritubular sites was associated with tubular and interstitial changes. In each model, the severity of the disease was equivalent regardless of whether the animals were from breeding pairs with normal complement activity or C6-deficiency. Finding that C6 deficiency does not alter the severity and progression of structural damage despite the up-regulation of C5b-9 on basolateral membranes of tubuli would also suggest that, in contrast to proteinuric state, C5b-9 does not have a significant impact on the progression of primary nonglomerular chronic disease of the kidney.^[79] Renal parenchymal tissues express a limited repertoire of receptors, including CR1, CR3, CD88, that may directly bind complement proteins present in the ultrafiltrate. Whether the stimulation of complement receptors on tubular cells may have functional consequences in progressive renal disease has not yet been established.

In addition to activating exogenous complement, proximal tubular epithelial cells can synthesize a number of complement components including C3, C4, factor B, and C5.^[80] The exposure of cultured tubular epithelial cells to total serum proteins at the apical surface up-regulated C3 mRNA expression and protein biosynthesis.^[81] The enhanced secretion of the protein was predominantly at the basolateral site, providing in vitro evidence to suggest roles for locally synthesized complement in the process of tubulointerstitial damage. Serum fractionation experiments identified the substance(s) responsible for such effects in the molecular size range of 30 to 100 kD. This fraction contains proteins that pass through the glomerular barrier in proteinuric states, including transferrin. After incubation with apical transferrin, C3 mRNA was overexpressed and both apical and basolateral C3 secretion increased.^[82] A similar degree of C3 up-regulation was obtained when iron-poor transferrin, or apotransferrin, was used, indicating that the synthesis of C3 in proximal tubular cells is up-regulated by transferrin, for which protein rather than iron moiety may account for the observed effects. These findings raise the potential role of intrarenal C3 synthesis in progressive renal disease and the relative contribution of locally synthesized versus ultrafiltered complement components in promoting inflammation and fibrosis. Recent data in C3-deficient mice have shown significant attenuation of the interstitial accumulation of cells expressing the F4/80 marker of monocytes/macrophages and dentritic cells in response to protein overload with serum albumin. The latter caused significant C3 mRNA up-regulation in the whole kidney.^[83] Finally, complement activation may directly regulate the renal immunologic response. Of great interest is the observation that the local synthesis of C3 may stimulate transmigration of T cells across tubular epithelial cells.^[84] This pathway involves direct action of tissue C3 with infiltrating T cells expressing C3 receptors and is a candidate target of lymphocyte inhibitor agents, such as mycophenolate mofetil, shown to be effective if combined with antiproteinuric therapy against primary nonimmune disease characterized by tubular deposition of complement.^{[73] [85]}

Direct Transfer of a Glomerular Injury Into the Tubulointerstitium

An additional hypothesis to explain the interstitial damage associated with chronic glomerular disease has recently been proposed.^[86] Based on histology studies of the development and progression of animal and human renal disease, two new mechanisms have been proposed, namely the misdirected filtration and the epithelial cell proliferation. According to the misdirection mechanism, there is an extension of a proteinaceous crescent into the outer aspect of the proximal tubule. As a consequence of persistent misdirected filtration, the proteinaceous filtrate, via the glomerulotubular junction, expands in the space between the tubular epithelial and the TBM and may spread within this space along the entire proximal convolution. This process is predominantly associated with degenerative glomerular disease^[87] but may also be found as the dominant process in individual nephrons in inflammatory disease.^[88] The other mechanism is the enchroachment of a growing cellular crescent upon the glomerulotubular junction. Thereby, the initial segment of the proximal tubulus gets incorporated into the crescent. This process is more characteristic of the inflammatory models. ^{[88] [89]} However, cellular proliferation is complemented frequently in inflammatory models by misdirected filtrate spreading, leading to mixed crescents. In both cases, this results in the loss of nephrons and subsequent fibrosis, which is, however, considered a reparative process important for the maintenance of renal structure rather than a determinant of further injury.^[86] Although this hypothesis does not exclude a direct effect of proteins filtered into the tubular lumen, it underlines the fact that therapeutic intervention to prevent renal disease progression should target these glomerular changes rather than processes activating proximal tubular cells.

Tubulointerstitial Antigens and Costimulatory Signals

Inflammation and injury of the renal interstitium can begin directly from the tubulointerstitial compartment. In this case, the key event is the expression of nephritogenic antigens. They are derived from renal cells and TBMs or exogenous antigens processed by tubular cells. Peculiar is also the case of molecules, including drugs, that may become nephritogenic antigens by acting as haptens or through molecular mimicry.

Antigens from Renal Cells and Tubule Basement Membrane

Injection of a crude preparation of tubular brush border extract, termed Fx1A, with Freund's adjuvant into allogeneic rats elicits an antibody-mediated response that allows the development of one of the most intensively studied experimental model, Heymann nephritis.^[90] Although the disease mimics membranous nephropathy in humans, the animals develop long-term tubulointerstitial injury. Megalin was originally identified as a major antigen in Heymann nephritis. It was purified from rat kidney brush border and named gp330 on the basis of molecular weight as estimated by its mobility during gel electrophoresis.^[91] Subsequent cloning analysis of the protein^[92] revealed a 600-kDa glycoprotein, leading to the term “Megalin.” Megalin is an approximately 4600–amino acid transmembrane protein with a large NH₂-terminal extracellular domain, a single transmembrane domain, and a short cytoplasmic tail.^[92] The protein belongs to the LDL-receptor family, sharing features with mammalian receptors including the LDL receptor, the LDL-receptor-related protein (LPR), the very low-density lipoprotein (VLDL) receptor, and the apolipoprotein E (apo E) receptor-2.^[93] Megalin was localized in the brush border and in the luminal apical endocytic pathway of renal proximal tubule.^[94] The cytoplasmic tail of megalin interacts with cytoplasmic proteins, such as disabled protein-2, that possibly modulate the endocytosis.^[95]

Immunofluorescence and immunochemical analyses by light and electron microscopy have also documented the presence of Tamm-Horsfall glycoprotein (THP)—as an additional antigenic component—in kidney cells of thick ascending limb of Henle's loop and early distal convoluted tubules.^[96] THP is the most abundant urinary protein in mammals, and its excretion occurs by proteolytic cleavage of the large ectodomain of the glycosyl phosphatidylinositol-anchored counterpart exposed at the luminal cell surface.^[96] Clinical studies indicate that renal diseases, namely chronic interstitial nephritis, medullary cystic disease, and reflux nephropathy, very often are accompanied by abnormal THP deposits in the renal interstitium, and there is an immunologic response to THP.^[97] THP is a powerful autoantigen, and immune deposits containing THP and anti-THP antibodies have been localized in the intercellular space of the thick ascending limb of Henle in animals challenged with homologous THP.^[96] In all likelihood, healthy mammals, including humans, do not produce anti-THP antibodies because the exclusive localization of THP at the luminal face of tubular cell segregates the protein from immune system. Segregation may be abolished, however, by various alteration of cells expressing THP, such as loss of their apical/basolateral polarity and the consequent release of THP from the wrong side as well as conditions altering cell integrity.^[96] Several studies have also shown that interstitial deposits of THP and THP immune complexes frequently are surrounded by neutrophils, mononuclear cells, and plasma cells. Binding of THP to neutrophils may be responsible for the acute inflammatory response, whereas that to mononuclear cells may extend the reaction to the chronic phase characterized by fibrosis. Although the proinflammatory activity of THP has been put forward as involved in TIN pathogenesis, both clinical and experimental observations, however, indicate that abnormal THP deposits in the kidney do not have a critical role in the inflammatory process.^[96]

There is also the antigen of TIN, originally identified as a nephritogenic antigen involved in anti-TBM antibody-mediated interstitial nephritis in humans.^[98] Immunofluorescent staining using sera from patients with anti-TBM nephritis or monoclonal antibodies specific to TIN antigen revealed its localization in the basement membrane of the proximal and, to a lesser extent, distal tubules and Bowman's capsule in the kidney.^[99] Isolation studies using enzymatic digestion and denaturing agents led to identifications of 54- to 58- and 40- to 50-kD glycoprotein isoforms as TIN antigen recognized by anti-TBM antibodies. ^{[98] [99]} Subsequent studies revealed the affinity of this molecule to type IV collagen and laminin,^[100] implying that TIN antigen is important for the maintenance of the integrity of the basement membrane in renal tubules. Its defect in hereditary tubulointerstitial disorders such as juvenile nephronophthisis^[99] indicates its role in renal development by facilitating cells-to-matrix interaction in concert with other basement membrane proteins. The human TIN antigen has been mapped to chromosome 6p 11.2-12, and cDNAs encoding rabbit TIN antigen and its human homolog have been cloned and sequenced. ^{[99] [101]} Polymorphism of expression is present in rats and humans, and allotypic differences in its expression among humans may occasion-ally result in anti-TBM disease after renal transplantation. Recently, a novel protein that is homologous to both TIN antigen and cathepsin B has been identified.^[102] Cloning and sequence comparisons of this novel protein, termed TIN antigen-related protein (TIN-ag-RP), revealed that it is more closely related to TIN antigen than to cathepsin B-like proteases. Based on the high degree of homology between TIN-ag-RP and TIN antigen, it appears that a distinct family of TIN antigen-like proteins may exist.

Exogenous and Endogenous Antigen Presentation by Tubular Cells

T cells recognize foreign antigens only when they are properly digested into small fragments and presented on the surface of antigen-presenting cells (APC) bound to major histocompatibility antigen (MHC) molecules.^[103] The recognition of this bimolecular complex on the surface of APC leads to activation of T cells. Recognition of class II MHC molecules by CD4⁺ T cells results in proliferation, whereas in comparison, CD8⁺ T cells cause target cell death on encountering antigen-class I complex-bearing cells.

Certainly, tubular cells in vivo have the capacity to hydrolyze and process exogenous proteins, afforded by a luminal brush border rich in enzymes, and the ability to endocytose macromolecules. Although much is unknown about antigen presentation by tubular cells, a hypothetical model has been proposed.^[104] After endocytosis of intact proteins from either luminal surface or through basolateral surface, fusion of endosomes with lysosomes allows protein digestion. However, it is unknown whether and how peptides are shuttled back and forth between acid-rich, MHC class II-devoid lysosomes and endosomes containing only a limited repertoire of hydrolases, but containing the class II molecules required for surface expression.^[105] Although class II molecules bind peptides almost exclusively from exogenously derived proteins, they can also bind endogenously derived or “self” peptides.^[106] Thus, an indeterminable number of exogenous and endogenous proteins could be candidate to generate immunogenic peptides in tubulointerstitial disease. It is difficult, however, in TIN, particularly in the clinical setting, to properly define the specific antigen presented by tubular epithelial cells. A possible explanation can be derived by the recent observation that antigenic peptides may derive by fusion of two distinct shorter sequences from the same protein, the consequence of an excision-and-splicing event. ^{[107] [108]} It has been argued that peptide splicing is catalyzed by the proteosome during protein degradation.^[108] Thus, cutting and pasting of exogenously or endogenously derived proteins by proximal tubular cells could generate new peptide variants with immunogenic properties presented on the cell surface in the context of MHC molecules. In addition to processing multiple potentially immunogenic peptides, proximal tubular cells could also be exposed to filtered low-molecular-weight proinflammatory cytokines such as interferon-γ (IFN-γ), IL-1, and tumor necrosis factor-α (TNF-α). Cytokines have been detected in urine and could augment the antigen-presenting capacity of tubules by up-regulating both class II and adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1).^[104] Actually, phenotypic characteristics of interstitial nephritis include augmented expression of MHC class II on tubular epithelial cells^[109] and enhanced tubular cell expression of adhesion molecules. Of note, expression of MHC class II is seen in less than 5% of tubular cells from normal kidneys.

Drugs as Nephritogenic Antigens

Medications may induce renal interstitial inflammation mainly through a cell-mediated hypersensitivity reaction to a drug.^[110] This is supported by the observation that T cells are the predominant cell type making up the interstitial infiltrate. Medications are thought to bind to, or mimic, renal tubular antigens or to induce an immune reaction following deposition in the interstitium. A humoral response underlies rare cases of interstitial nephritis in which a portion of a drug (i.e., methicillin) may act as a hapten, bind to the TBM, and elicit anti-TBM antibodies.^[110]

Costimulation by Tubular Epithelial Cells

Costimulation refers to signals independent of the antigen receptor that are required for full activation of lymphocyte.^[111] This represents a bidirectional communication between the APC and the lymphocyte that could result in activation or inhibition of the immune response. Costimulatory molecules exist as pairs, with a receptor on T cell and a ligand on APC. There are two classes of costimulatory receptors on T cells based on sequence homologies: the Ig family including CD28, CTLA-4, PD-1, and ICOS, with their ligands from the B7 family; and the TNF receptor family, including CD40, OX40, 4-1BB and CD30, with their respective ligands.^[111] The best-characterized costimulatory molecules are B7.1 (CD80) and B7.2 (CD86), which are predominantly expressed by professional APC (dendritic cells and monocytes). Whether tubular epithelial cells express B7.1 and B7.2 has been a matter of debate.^[112] However, tubular epithelial cells were found to express other accessory molecules, such as CD40, ICAM-1, and VCAM-1, which are all involved in T cell activation.^[113] CD40 is weakly expressed in tubules of normal mice but is increasingly expressed by tubules and interstitium of animals with progressive adriamycin nephropathy.^[114] Blockade of CD40-CD40 ligand with the monoclonal anti-CD40L antibody MR1 was found to reduce severity of renal injury in this murine model.^[114]

In addition, new costimulatory molecules have been identified that are more broadly expressed and may play a role in the regulation of peripheral T cell activation by tissue cells. One of these molecules is inducible T cell costimulator ligand (ICOS-L), which is expressed in lymphoid and nonlymphoid tissues, including kidney.^[115] Interaction between ICOS-L and ICOS, which is expressed on activated T cells, has been implicated in T cell proliferation and the regulation of cytokine production by effector T cells.^[116] Another member of the B7 family recently identified is B7-H1 (PD-L1). The receptor for B7-H1 is programmed death-1 (PD-1). Evidence has become available that, in human tubular epithelial cells in culture, the ICOS-L and B7-H1 expression was increased upon stimulation with IFN-γ.^[115] Moreover, coculture experiments showed that tubular cell-T cell interaction results in T cells with a low proliferating, IL-10–producing phenotype, which is partially regulated by ICOS-L and B7-H1. In line with these in vitro findings are immunohistochemistry data of significant B7-H1 staining in renal tubules in tissue biopsy of patients with IgA nephropathy, interstitial nephritis, or lupus nephritis,^[117] suggesting that the expression of this costimulatory molecule is up-regulated also in vivo. These interesting, albeit preliminary, observations provide novel insights on the role of tubular cells to regulate activation of immune cells infiltrating the interstitium in kidney diseases. Thus, interaction of tubular epithelial cells and T cells may alter the balance of positive and negative signals to the T cells. One of the consequences of this interaction seems to be the development of IL-10–producing T cells, a process that might aid in limiting or attenuating local T cell responses. Because, ultimately, the B7-H1/PD-1 pathway might play a role in protecting tubular epithelium from immune-mediated damage, active delivery of the B7-H1 inhibitory signal would represent, in the future, a novel therapeutic strategy in tubulointerstitial diseases.

Interstitial Inflammation

In the normal kidney, there are small number of interstitial leukocytes. These are predominantly macrophages, although T cells are also present. Infiltration of inflammatory cells is a constant feature in conditions associated with chronic renal damage. Tubulointerstitial inflammation may result from antigen-specific stimulation, but may also occur in the absence of antigenic stimulation.^[118] Phenotypic characterization of these cells infiltrating the interstitium provides evidence of a mixture of effector and regulatory cells.

Effector Cells

The interstitial infiltrate of most human chronic renal diseases consist of a number of different effector cells, including macrophages and CD4⁺ and CD8⁺ T cells. Most studies report that T cells predominate, and the majority of these are CD4⁺ cells, although there is considerable variation between these analyses.^[119] These results should be interpreted with caution, however, as macrophage enumeration in these studies was performed by immunohistochemistry with primary antibodies directed against the surface antigen CD14. As the expression of this antigen varies with the maturity and activation status of the cell, macrophage numbers have probably been underestimated. Furthermore, the predominant T cell population may also be altered by immunosuppressive therapy before biopsy or by the stage of disease at the time of biopsy. Corticosteroids can markedly deplete the number of lymphocytes seen in interstitial nephritis. Studies in animal models indicate that macrophages are the dominant infiltrating cells in the initiation and progression of injury in chronic renal disease. Specifically, tubulointerstitial macrophage accumulation in chronic nephropathies correlates with the severity of the glomerular and interstitial lesions and the degree of renal dysfunction.^[118] Resident and infiltrating macrophages play a central role in innate immunoprotection both through the clearance of infective pathogens and through the repair of tissue injury that occurs, in part, as a consequence of this response. For example, the initial response of macrophage to bacterial infections is cytotoxic and proinflammatory; then, on control of the infection, macrophages phagocytose cellular debris and apoptotic bodies and begin tissue repair. However, in many noninfective renal diseases associated with macrophage infiltrates, although the primary cause may abate, interstitial inflammation and tubulointerstitial injury worsens.^[118] Direct damage to resident cells is caused through the generation by macrophages of radical oxygen species (ROS), nitric oxide (NO), complement factors, and proinflammatory cytokines.^[118] Macrophages can also affect supporting matrix and vasculature through the expression of metalloproteinases and vasoactive peptides. Resident interstitial fibroblasts and myofibroblasts proliferate in response to macrophage-derived profibrogenic cytokines, and their number correlates with the subsequent formation of a scar.^[120] They may be derived from transdifferentiated tubular epithelial cells, a process promoted by profibrogenic cytokines, including TGF-β expressed by macrophages, and also by tubular epithelial cells as a consequence of macrophage-tubular cell interaction.^[121] Nevertheless, much remains to be learned about macrophages in tubulointerstitial injury. In this respect, enlightening is the case of the role of interstitial macrophages in mice with progressive adriamycin nephropathy.^[122] Treating mice with a monoclonal antibody ED7 directed against CD11b/CD18 integrin, which is expressed by macrophages, the renal cortical macrophages (ED1-positive cells) were reduced by almost 50%, whether ED7 was administered before or after adriamycin administration.^[123] Even so, ED7 reduced renal structural and functional injury only when treatment was started prior to adriamycin administration.^[123] Among several possible explanations for these observations is a temporal change in the predominant macrophage phenotype. If pathogenetic macrophages predominated early and protective macrophages later in the course of the disease, then only early antimacrophage treatment would be expected to protect against progression. Recent observations also support the importance of macrophage phenotype. For example, in mice with unilateral ureteric obstruction reconstituted with bone marrow of AT1 receptor gene knock-out or wild-type mice, infiltrating macrophages were shown to play a beneficial antifibrotic role.^[124] Other studies have demonstrated marked macrophage heterogeneity and context specificity, depending on the nature of the injury and location within the kidney.^[125] On this line, evidence is available that macrophages perform both injury-inducing and repair-promoting tasks in different models of inflammation. This has been shown in a reversible model of liver injury, in which the injury and the recovery phases are distinct.^[126] Macrophage depletion when liver fibrosis was advanced resulted in reduced scarring and fewer myofibroblasts. Macrophage depletion during recovery, by contrast, led to a failure of matrix degradation.^[126] These findings provide the clear evidence that functionally distinct subpopulations of macrophages exist in the same tissue. Further studies on possible temporal variations in the phenotype, activation status, and net effect on injury of macrophages should give a better understanding of the complex role of macrophages in tubulointerstitial damage of chronic renal disease.

Macrophages are only one component of the cellular infiltrate that characterizes inflammation in the renal interstitium. In experimental chronic ischemia induced by unilateral renal artery stenosis in the rat, the ischemic kidney showed marked tubulointerstitial damage, including an abundant inflammatory infiltrate and tubulitis in 42% of the tubular cross sections.^[127] The infiltrating cells were mostly macrophages, B cells, and T-helper lymphocytes, and there were important changes in the antigenic constitution of the tubules. Similarities were found between the phenotypic characteristics of the infiltrate in chronic ischemia and the infiltrate in autoimmune TIN,^[128] aminonucleoside nephrosis,^[129] and cyclosporine nephrotoxicity.^[130] Recent models of overload proteinuria have emphasized the importance of tubulointerstitial infiltration with mononuclear cells. The possibility of participation of immunocompetent cells in the pathogenesis of this model is not ruled out. Studies have shown that there is an early (<7 days) and prominent macrophage infiltration that remains unabated throughout the duration of the overload proteinuria; furthermore, after 2 weeks, T-helper and T-cytotoxic cells are also observed in the tubulointerstitial infiltrate.^[72] Whereas T-helper infiltrate tends to decrease after the 3rd week, the T-cytotoxic cells remain for at least 7 weeks. T cell depletion with intraperitoneal anti-T cell monoclonal antibody administration did not modify the macrophage infiltration, indicating that the influx of these cells was not dependent on the lymphocytes,^[72] but more likely resulted from locally expressed osteopontin, MCP-1, and VCAM and ICAM by tubular cells. Nevertheless, structural changes in the tubules are present in this experimental model, and the toxic damage could result in the expression of autoantigens that could elicit a delayed cellular response. For instance, vimentin filaments found in the tubules of rats with overload proteinuria are not expressed in normal tubules and neoantigens expressed after activation of the C5b-9 attack complex have been demonstrated in the brush border of tubular cells. ^{[72] [131]} In the reduction of renal mass, a well-defined nonimmune model of progressive renal failure, macrophage infiltration in the tubulointerstitium is already found after 10 days, increases after 4 weeks, and peaks after 12 weeks postablation.^[132] The infiltration of macrophages is facilitated by the expression of MCP-1 and stimulated by AT.^[118]Approximately one third of the infiltrating macrophages are proliferating in the remnant kidney, and the total number of these cells correlate with the functional parameters of renal failure. T lymphocytes are also conspicuously present in the tubulointerstitial infiltrate early postablation, and remain in significant numbers for the following weeks.^[132] The disease is associated with overexpression of MHC class II antigens, and the infiltrating T cells are predominantly of the T-helper phenotype. The pathogenetic importance of immunocompetent cells in this hemodynamically induced chronic renal damage has been dramatically shown by the improvement obtained with immunosuppressive treatment.^[132]

CD8⁺ T cells are regarded as the predominant effector cells in a number of forms of renal injury including anti-TBM disease, Heymann nephritis,^[123] and murine adriamycin nephrosis.^[133] Not surprisingly, experimental models of acute kidney injury are also associated with intense inflammatory infiltrate in association with tubular necrosis. Mercuric chloride nephrotoxicity is characterized by proximal tubular necrosis and intense mononuclear cell infiltration that precedes growth factor expression. The macrophage and lymphocyte infiltration may be stimulated by the generated reactive oxygen species, because recent work found that it decreases with antioxidant therapy.^[134]

In experimental models and in humans, the infiltration of immunocompetent cells requires both the attraction exerted by the local expression of chemotactic factors and the expression of adhesion molecules.^[118] Whereas the infiltration of macrophages is part of a nonspecific inflammatory reaction, the presence of lymphocyte within lesions raises questions about their role in the maintenance and amplification of the inflammatory response and whether or not their recruitment and activation are mediated by an antigen-specific immune response in nonimmune renal disease. These questions remain largely unanswered at the present time. Inasmuch as the T cells are the effectors of the cellular immune responses, their presence, in association with evidence of cytokine production, raises the possibility that they are recognizing and reacting to endogenous antigens newly expressed on tubulointerstitial cells. This does not imply a selective contribution of tubular cells, but may also involve endogenous dendritic cells—professional APCs—that can be identified in tubulointerstitial lesions.^[85]

Regulatory Cells

CD4⁺ T cells constitute a critical component of the adaptive immune system and are typified by their capacity to help both humoral and cell-mediated responses. However, there is a substantial functional diversity among CD4⁺ T cells, and it is clear that certain subpopulations hinder rather than help immune response. The most well-characterized example of an inhibitory subpopulation is the CD4⁺CD25⁺, which appears to play an active role in down-regulating pathogenic autoimmune responses.^[135] CD4⁺CD25⁺ T cells are potent immunoregulatory cells that not only suppress T cell proliferation in vitro but also have the capacity to suppress immune responses to auto- and alloantigens tumor antigens and infectious antigens in vivo.^[136] The regulatory activity of these cells in the setting of chronic renal diseases is highlighted by studies in SCID mice reconstituted with CD4⁺CD25⁺ T cells after induction of adriamycin nephrosis.^[123] Mice reconstituted with these regulatory cells had significantly reduced glomerulosclerosis, tubular injury, and interstitial expansion compared with unreconstituted mice with adriamycin nephrosis. This protective effect raises the possibility that enhancement of CD4⁺CD25⁺ regulatory activity may be a useful maneuver for slowing progression of chronic nephropathies. The recent description of the green fluorescence protein (GFP)-Foxp3 mouse suggests that Foxp3 expression identifies the T regulatory population and is expressed in ab T cells.^[137] Evidence is available that Foxp3-transduced T cells had a regulatory phenotype by functional and phenotypic assays.^[138] In the murine model of adriamycin nephropathy, the adoptive transfer of Foxp3-transduced T cells protected against renal injury. Urinary protein excretion and serum creatinine were reduced, and there was significantly less glomerulosclerosis, tubular damage, and interstitial infiltrates.^[138]

γδ T cells are an ancient lineage of T cells that play important roles in antimicrobial immunity as well as in chronic inflammatory processes.^[139] γδ T cells were found to be expanded in kidneys of rats with Heymann nephritis and to express a restricted set of Vγ6/Vδ1 TCR genes.^[140] High levels of regulatory cytokines including TGF-β, IL-4, IL-5 and low levels of IL-2 were expressed by the infiltrating γδ T cells in Heymann nephritis.^[140] It is tempting to speculate that, like in Heymann nephritis, γδ T cells is a subpopulation of inflammatory cells infiltrating the interstitium also in other models of nonimmune and immune renal diseases, where they could promote a regulatory response.

Interstitial Fibrosis and Scarring

Fibrosis is a common final pathway leading to end-stage renal disease (ESRD), irrespective of the nature of the initial renal injury. The process of tubulointerstitial fibrosis involves the loss of renal tubules and the accumulation of myofibroblasts and ECM proteins, such as collagen (types I, III, IV, V, and VII), fibronectin, and laminin.^[141] Cells infiltrat-ing the renal interstitium have long been believed to play a major role in the initiation and progression of tubulointerstitial fibrosis. This is because the degree to which a number of cell types, macrophage, lymphocytes, and fibroblasts accumulate in the renal interstitium parallels the extent of fibrogenesis. Moreover, it has been difficult to distinguish harmful cells from beneficial cells using standard histopathologic techniques. Recent advances in molecular technology, however, now enable one to analyze different cell types separately, and activated fibroblasts have been iden-tified as the principal effector mediating tubulointerstitial fibrosis.^[142]

Since the identification of fibroblast specific protein-1 (FSP-1) as a marker of tissue fibroblasts,^[143] they have been found to originate or multiply from a variety of sources, including local epithelial-mesenchymal transition (EMT), the bone marrow, resident fibroblasts, or myofibroblasts.^[144] Emerging evidence has recently shown that fibroblasts derived from EMT play a critical role in tubulointerstitial fibrosis.^[144] With that as a background, we focus now on the newly proposed mechanism of tubulointerstitial fibrosis, namely that it emphasizes the role of EMT and cellular activation.

Epithelial-Mesenchymal Transition

Epithelial line tubules and ducts in the kidney and renal fibroblasts formed prior to inflammation are generated by EMT during development and organ growth. Tubular epithelia undergoing EMT after persistent injury are commonly found in the kidney, and nearly 35% of renal fibroblasts are produced locally by EMT in response to persistent inflammation.^[145] Approximately 14% to 25% are derived from EMT events that occur in the bone marrow. Fibroblasts from this niche may circulate to peripheral tissue. All populations of fibroblasts can increase locally by proliferation; this process can account for up to 50% of new fibroblasts in progressive models of renal fibrosis.^[145] Persistent cytokine activity during renal inflammation and disruption of the underlying basement membrane by local proteases initiates the process of EMT.^[146] Rather than collapsing into the tubular lumens and dying, some epithelial cells transition into fibroblasts while translocating back into the interstitial space behind decondensating tubules through holes in the ruptured basement membrane. Wnt proteins, integrin-linked kinases, IGF-1 and -2, epidermal growth factor (EGF), fibroblast growth factor-2 (FGF-2) and TGF-β are among the archetypal modulators of EMT.^[147] For example, FGF-2 promotes the transition of tubular epithelia by inducing the release of MMP-2 and -9 that eventually damages the underlying basement membranes.^[148] FGF-2 also synergizes with TGF-β and EGF in mediating EMT by reducing cytokeratin expression and stimulating the movement of transitioning cells across damaged basement membranes. Together, TGF-β and EGF provide the strongest stimulus for completion of epithelial transitions. During EMT, FGF-2 activates its receptor FGFR-1 on the cell surface. Together, the growth factor and its receptor are imported into the nucleus, where they engage a variety of sequence-specific transcription factors. Similar processes occur through activation of the TGF-β receptor.^[149] The net result is the emergence of the EMT proteome and repression of epithelial proteins.^[146] Loss of E-cadherins and cytokeratins, rearrangement of actin stress fibers, and expression of FSP1, vimentin, interstitial collagens, and occasionally α-smooth muscle actin, mark the morphologic transition of epithelial cells into fibroblasts. Of note, an offsetting feature of EMT in mature renal tissues is signaling involving hepatocyte growth factor (HGF) and bone morphogenetic protein-7 (BMP-7) that antagonizes epithelial transitions driven by FGF-2 and TGF-β.^[150] The combined effects of HGF on EMT retard renal fibrogenesis in mice.^[151] Similarly, in several models of kidney injury, administration of BMP-7 also attenuates renal fibrogenesis while restoring the structure of tubular epithelial units.^[150] At what stage of fibrosis these mesenchymal-epithelial transition (MET) modulators are most effective is not yet known. The transcriptional modulators for BMP-7 signaling in MET are yet to be identified, but the intracellular Smad/b catenin activity may be a reasonable candidate.^[149]

An additional regulation of EMT is through Trap proteins, accessory molecules that circulate in interstitial spaces as soluble moieties to block receptor activation by free ligand.^[147] This dual-control mechanism favors a state of differen-tiation in epithelia, which preserves organ structure and function. What is becoming increasingly clear is that mature epithelial are in the dynamic, but not terminal, state of homeostasis. Morphogenic forces that normally maintain epithelial phenotypes are pitted against countervailing forces trying to weaken that stability. Chronic inflammation into the interstitium destabilizes epithelial tissues by favoring fibrogenesis.

Chronic Hypoxia

One of the most important contributors to the development of tubulointerstitial fibrosis is chronic ischemia.^[152] Production of AT and inhibition of production of NO underlie chronic vasoconstriction, which may contribute to tissue ischemia and hypoxia.^[153] In that regard, histologic studies of animal models and human kidney have documented that there is often a loss of peritubular capillaries in areas of tubulointerstitial fibrosis.^[153] Down-regulation of VEGF may be functionally implicated in the progressive attrition of peritubular capillaries and tissue hypoxia, as shown in mouse folic acid nephropathy.^[154] Moreover, given that the size of the interstitial compartment determines the diffusion distance between peritubular capillaries and tubular cells, interstitial fibrosis further impairs tubular oxygen supply. Focal reduction of capillary blood flow leading to starvation of tubuli may underlie tubular atrophy and loss. Under these conditions, the remaining tubules are subjects to functional hypermetabolism with increased oxygen consumption, which in turn creates an even more severely hypoxic environment in the renal interstitium. In vitro, such hypoxia stimulates fibroblast proliferation and ECM production by tubular epithelial cells.^[155]

ACUTE INTERSTITIAL NEPHRITIS

Acute interstitial nephritis (AIN) defines a pattern of renal injury usually associated with an abrupt deterioration in renal function characterized histopathologically by inflammation and edema of the renal interstitium. The term was first used by Councilman^[8] in 1898 when he noted the histopathologic changes in autopsy specimens of patients with diphtheria and scarlet fever. Although the term acute interstitial nephritis is more commonly used, acute TIN more accurately describes this diseases entity, because the renal tubules as well as the interstitium are involved. Of the diseases causing acute TIN, the most common, acute pyelonephritis and renal tubular necrosis and acute allograft rejection, are covered in Chapters 29 , 34 and 63 . This section describes the features common to acute tubulointerstitial injury, whatever its cause. AIN has become an important cause of acute renal failure due to drug hypersensitivity reactions as a result of the increasing use of antibiotics and other medications that may induce an allergic response in the interstitium. AIN has been reported to occur in approximately 1% of renal biopsies during the evaluation of hematuria or proteinuria. In some studies of patients with acute renal failure, approximately 5% to 15% had AIN.^[156]

Etiology

The most frequent cause of AIN can be found in one of the three general categories: drug-induced, infection-associated, and cases associated with autoimmune idiopathic lesions ( Table 33-1 ).

TABLE 33-1 -- Acute Interstitial Nephritis: Causative Factors

Drugs
Antibiotics	Cephalosporins, ciprofloxacin, ethambutol, isoniazid, macrolides, penicillins, rifampin, sulfonamides, tetracycline, vancomycin
NSAIDs	Almost all agents
Diuretics	Furosemide, thiazides, triamterene
Miscellaneous	Acyclovir, allopurinol, amlodipine, azathioprine, captopril, carbamazepine, clofibrate, cocaine, creatine, diltiazem, famotidine, indinavir, mesalazine, omeprazole, phenteramine, phenytoin, pranlukast, propylthiouracil, quinine, ranitidine
Infectious agents
Bacteria	Corynebacterium diphtheriae, Legionella, Staphylococci, Streptococci, Yersinia, Brucella, Escherichia coli, Campylobacter
Viruses	Cytomegalovirus, Epstein-Barr virus, hantaviruses, hepatitis C, herpes simplex virus, human immunodeficiency virus, mumps, polyoma virus
Other agents	Leptospira, Mycobacterium, Mycoplasma, rickettsia, syphilis, toxoplasmosis, Chlamydia
Idiopathic
Immune	Anti-tubule basement membrane disease, tubulointerstitial nephritis and uveitis (TINU) syndrome

NSAIDs, nonsteroidal anti-inflammatory drugs.

Drugs

The list of drugs implicated in causing AIN continues to expand. Drugs are more frequently recognized as etiologic factors in AIN because of the increased use of renal biopsy and the characteristic clinical presentation. Antibiotics have long been, and remain, a major cause of drug-related renal toxicity producing both acute renal failure and tubulointerstitial disease depending on the drug. Tubulointerstitial disease is more commonly seen with β-lactam antibiotics (including cephalosporins), but other antibiotics (sulfonamides, rifampin, vancomycin, ciprofloxacin) have also been involved. Clarithromycin^[157] (the newer ketolide semisynthetic erythromycin-A derivative) and telethromycin^[158] have been implicated in renal biopsy-proven AIN in case reports. Approximately a third of cases of drug-related AIN are due to antibiotics.^[159] Two complicating factors make the nephrotoxicity of antibiotics more common than expected. First, many of these drugs are given in combination, either with themselves or with other drugs. Consequently, the toxicity of one agent may be aggravated by the other one, as exemplificated by the coadministration of aminoglycosides and nonsteroidal anti-inflammatory drugs (NSAIDs). Second, many antimicrobials are removed from the body essentially, or at least predominantly, through the renal route. The serum levels of these drugs will, therefore, increase as renal function becomes impaired either as a consequence of the drug toxicity itself or because of concomitant renal damage caused by another drug or by another cause of nephrotoxic reaction. β-Lactams cause interstitial nephritis because they behave like haptens, which may bind to serum or cellular proteins to be subsequently processed and presented by MHC molecules as hapten-modified peptides. The most common form of haptenization for penicillins is the penicillolyl configuration, which arises from the opening of the strained β-lactam ring, yielding an additional carboxylic function that allows the molecule to covalently bind to the lateral and terminal aminofunctions of proteins. Serum molecules thus facilitate haptenization. This reaction occurs with the prototype benzylpenicillin and virtually all semisynthetic penicillins, but other derivatives (called minor determinants) can be formed in small quantities and stimulate variable immune responses. Because all β-lactams share the same basic structure, they are all disposed to give rise to haptenization. Variation in the side chains and the corresponding differences in the chemical nature of the haptens, however, explain why clinical consequences are variable from one class of β-lactams to another.^[160] Cross-reactivity between penicillins and cephalosporins is accordingly far from complete and is even rare.

Approximately 1% to 5% of patients exposed to NSAIDs develop diverse nephrotoxic syndromes warranting potential physician intervention.^[161] Whereas, on the surface, this relatively low prevalence is not alarming, the extensive use profile of these analgesic, anti-inflammatory, and antipyretic agents implies that an enormous number of citizens are at risk for consequential kidney dysfunction. For instance, approximately 1 in 7 patients with rheumatologic disorders is likely to receive such a prescription, and approximately 1 in 5 (50 million) U.S. citizens report they use an NSAID for other acute complaints.^[162] Thus, it is possible to estimate that some type of renal abnormalities is likely to develop among the 500,000 to 2.5 millions U.S. citizens exposed to NSAIDs on a regular or intermittent basis per annum.^[161] The problem takes on added dimensions in that 20% of NSAID-patients at risk are predisposed to the development of renal toxicity because of volume-contracted states, low cardiac output, or other conditions tending to compromise renal perfusion. Use of NSAIDs may spiral upward with the aging of the population and the attendant rise in chronic musculoskeletal disorders such as the arthritides. The combination of AIN with moderate or heavy proteinuria and otherwise nearly normal glomeruli showing extensive epithelial foot process spreading (fusion)—minimal change glomerulopathy—is characteristically seen in fewer than 0.2/1000 subjects who take NSAIDs other than aspirin.^[163] The onset of proteinuria combined with interstitial nephritis may manifest after several days or months of NSAID exposure (range 2 wk–18 mo).^[161] Fenoprofen is the drug carrying the highest risk. The cyclo-oxygenase-2 (COX-2) inhibitor rofecoxib has also been implicated in renal biopsy-proven AIN.^[164] The disease is believed to be a toxic effect of NSAIDs on the immune system resulting from blockade of cyclo-oxygenase causing arachidonic acid metabolism to be directed toward the alternative lipoxygenase pathway within the kidney, thus increasing production of proinflammatory leukotrienes or expoxy- and hydroxy-eicosanoids.^[163] However, that some cases of NSAID-related interstitial disease are in fact allergic is suggested by the presence in them of eosinophils in the infiltrate or other manifestations of IgE-mediated hypersensitivity. The risk factors for this form of NSAID-induced nephropathy have not been defined. Preexisting renal impairment does not appear to be a factor; however, the intercurrent presence of an autoimmune/connective tissue disease, such as systemic lupus erythematosus, may become apparent when a NSAID apparently induces the nephrotic syndrome and other possible causes are evaluated. Advancing age has been proposed as a risk factor, and this relationship may simply reflect the prevailing use of NSAIDs by the elderly. Essentially all NSAIDs have been reported to cause the nephrotic syndrome. Nonetheless, on a per capita use basis, the majority of reports have been associated with the use of fenoprofen therapy, suggesting that the molecular structure of a NSAID is of importance in the pathogenesis of this clinical entity.

Infections

AIN is associated with primary renal infections such as acute bacterial pyelonephritis, renal tuberculosis, and fungal nephritis. Systemic infections can cause direct injury because of pathologic processes in the kidney or can be associated with indirect injury caused by medications used in the treatment of infections. For example, human immunodeficiency virus (HIV) can be responsible for AIN caused by opportunistic infections or using drugs such as indinavir, sulfonamide antibiotics, and others. Sometimes, depressed cell-mediated immunity may have the impact of protecting the patients from developing AIN.

Idiopathic Acute Interstitial Nephritis

Immunologic diseases such as Behöet's disease, Sjögren's syndrome, sarcoidosis, systemic lupus erythematosus, or vasculitides may cause severe renal interstitial involvement.^[165] However, acute renal failure due to idiopathic AIN is quite uncommon.

Anti-TBM nephritis is occasionally manifested in association with membranous nephropathy.^[166] The characteristics of the patients with this combination include a predominance of males, onset in early childhood, microscopic hematuria, and nephrotic-range proteinuria. In addition, patients show tubular dysfunction (complete or incomplete Fanconi syndrome), circulating anti-TBM antibodies, and the progression to ESRD, the features of which can differentiate this disorder from idiopathic membranous nephropathy. Circulating anti-TBM antibodies from these patients react exclusively with the proximal TBM, not with GBM, exhibiting binding to TIN antigen.

The precise mechanisms of combined immune complex deposition in the glomeruli and antibody formation against TIN antigen is unclear. Soluble TIN antigen binding to its relevant antibody may participate in the formation of immune complexes in the glomerular lesions. However, TIN antigen is not detected within the immune complex deposits. The cases showing membranous nephropathy preceding anti-TBM nephritis suggest that anti-TBM antibodies are formed by TIN antigen modified by certain components or exposed by certain enzymes present within the massive proteinuria. The role of human leukocyte antigens (HLAs) in the evolution of these autoimmune disorders has also been suggested.^[166]

In 1975, Dobrin and associates described two young women with a new syndrome characterized by anterior uveitis, bone marrow granulomas, hypergammaglobulinemia, increased erythrocyte sedimentation rate, acute renal failure with renal histologic features of AIN due to numerous interstitial inflammatory cells consisting mainly of eosinophils.^[167] The acute renal failure due to idiopathic TIN associated with bilateral uveitis was thus termed TINU syndrome. Though Dobrin's syndrome may be unique, the majority of about 50 cases published so far had the combination of anterior uveitis and idiopathic AIN without granulomas in various organs. ^{[159] [168]} TINU syndrome among adults occurs predominantly in females (3:1). Familial occurrence has been described,^[169] and a genetic pattern probably plays some role in the pathogenesis. The anterior uveitis may precede, concur with, or follow the nephropathy. These patients generally suffer from weigh loss and anemia and have a raised erythrocyte sedimentation rate. Although associations with both chlamydia and mycoplasma infections have been suggested, the etiology remains obscure.^[170] It has been suggested that the syndrome is immune-mediated, and the T cell proliferation within the kidney in the AIN confirms this.^[168] Moreover, the possibility of a delayed-type hypersensitivity reaction has been also suggested, on the basis of the 2:1 ratio of CD4⁺ to CD8⁺ renal interstitial lymphocytes. A significant association has been reported in the frequency of HLA-DR6 and TINU syndrome.

Prolonged steroid therapy usually leads to improvement in both renal function and uveitis, although the latter may relapse. ^{[159] [168]}

Pathology

The hallmark of AIN is the infiltration of inflammatory cells within the renal interstitium, with associated edema, usually sparing the glomeruli and blood vessels.^[156]

In the acute allergic interstitial nephritis, the predominant pathology is interstitial, is cortical rather than medullary, and comprises edema and inflammatory infiltrate, which may be sparse, focal, or intense. The most numerous cells are lymphocytes, with CD4⁺ T cells rather more frequent than CD8⁺ T cells, B lymphocytes and plasma cells, natural killer cells, or macrophages.^[171] Polymorphonuclear granulocytes, usually eosinophils, are often present. The inflammatory reaction may be concentrated around, or even be seen to be invading, tubular epithelium (so-called tubulitis). In more severe cases, tubulitis is associated with epithelial cell degeneration resembling patchy tubular necrosis and some disruption of the TBM. Granulomas may occur in the interstitium, but vasculitis is uncommon. Increased ECM, followed by destructive fibrogenesis, may appear as early as the 2nd week of acute in-flammation.^[171] Immunofluorescence microscopy or immunoperoxidase staining may show, with diminishing frequency: no complement or immunoglobulin; immune complexes sometime with complement along the TBM; or linear IgG and complement on the TBM.^[171] Thus, the pathology indicates a cell-mediated delayed-type hypersensitivity reaction directed against tubular cells or nearby interstitial structures. However, dimethoxyphenyl-penicilloyl radicals may attach to TBM as hapten in β-lactam-associated nephritis, and antibody to this combination is occasionally present. In the AIN with minimal change glomerulopathy induced by NSAIDs, the interstitial inflammatory exudate resembles that of acute allergic interstitial nephritis except that cytotoxic T cells predominate and eosinophils are uncommon.^[172] There is no evidence of antibody-mediated injury in this form of interstitial nephritis.

Clinical Manifestations and Diagnosis

The clinical presentations comprise local and systemic manifestations of acute diffuse inflammation of the kidneys. Patients with AIN typically present with nonspecific symptoms of acute renal failure, including oliguria, malaise, anorexia, or nausea and vomiting with acute or subacute onset.^[173] The clinical features can range from asymptomatic elevation in creatinine or blood urea nitrogen (BUN) or abnormal urinary sediment to generalized hypersensitivity syndrome with fever, rash, eosinophilia, and oliguria renal failure. The classic triad of low-grade fever, skin rash, and arthralgias was primarily described with methicillin-induced AIN, but it was present only about a third of the time. Recent pool analysis of the three largest contemporary series yield a total of 128 patients with AIN. ^{[159] [174] [175]} These series span a period from 1968 to 2001 and consist of 72 of 128 (56.3%) males with a mean age of 46.6 years. At presentation, rash was present in 14.8%, fever in 27.3%, and eosinophilia in 23.3%. The classic triad of fever, rash, and arthralgias was present in only 6 of 60 patients for whom the information was available. Nevertheless, this finding is in stark contrast to earlier series in which allergic-type features dominated the clinical picture.^[159] Blood pressure is usually not high except with oliguric renal failure.

AIN should be considered in any patients with a rising serum creatinine but little or no evidence of glomerular or arterial disease, no prerenal factors, and no dilatation of the urinary collecting system on ultransonography. The clinical history of exposure to a β-lactam or aminoglycoside antibiotic or a NSAID, susceptibility to ascending urinary infection, or a recent or coexisting condition causing shock will indicate the diagnosis directly. Most difficulty is experienced in those patients exposed to a nephritogenic or nephrotoxic agent at about the same time as a major operation, serious infection, or other significant illness that may itself have caused tubular necrosis. The presence of other features of a hypersensitivity reaction or of significant eosinophiluria in the case of β-lactam antibiotics or of moderate or heavy albuminuria when a NSAIDs is implicated indicates a drug-related etiology. Urine eosinophils are frequently tested to provide confirmatory evidence of AIN. Early studies^[176] found that Hansel stain for eosinophils was more sensitive than Wright stain but did not conclusively demonstrate that urine eosinophils were diagnostically useful in confirming or excluding AIN. A more recent study^[177] found a positive predictive value of 38% (95% confidence interval [CI] 15%–65%) and a negative predictive value of 74% (95% CI, 57%–88%) among 51 patients for whom urine eosinophils were tested to help diagnose an acute renal disease; 15 of these were suspected to have AIN, though biopsy was not performed in all patients. Other conditions such as cystitis, prostatitis, and pyelonephritis can also be associated with eosinophiluria. Thus, the diagnostic value of urine eosinophils remains unclear. Renal ultrasonography may demonstrate kidneys that are normal to enlarged in size with increased cortical echogenicity, but no ultrasonographic findings will reliably confirm or exclude AIN versus other causes of acute renal failure. Gallium 67 scanning has been proposed^[178] as a useful test to diagnose AIN. In one small series, nine patients with AIN had positive gallium 67 scans, whereas six patients with ATN had negative scans. In other studies, other renal disorders such as minimal-change glomerulonephritis, cortical necrosis, and AIN have resulted in positive gallium 67 scans. Nonrenal disorders such as iron overload or severe liver disease also can result in positive gallium 67 scan. Likewise, patients with biopsy-proven acute tubulointerstitial disease have had negative gallium 67 scans; therefore, the predictive value of this test remains limited. Renal biopsy is the “gold standard” for diagnosis of AIN, with the typical histopathologic findings of lymphocytic infiltrates in the peritubular areas of the interstitium, usually with interstitial edema. Renal biopsy, however, is not needed in all patients, for whom a probable precipitating drug can be easily withdrawn or who improve readily after withdrawal of a potentially offending drug. Supportive management can proceed safely without renal biopsy. Patients who did not improve after withdrawal of likely precipitating medications, who have no contraindications to renal biopsy and do not refuse the procedure, and who are being considered for steroid therapy are good candidates for renal biopsy.

Prognosis and Management

Most patients with AIN in whom offending medications are withdrawn early can be expected to recover normal or near-normal renal function within a few weeks. Patients who discontinue offending medications within 2 weeks of the onset of AIN (measured by increased serum creatinine) are more likely to recover nearly baseline renal function than those who remain on the precipitating medication for 3 or more weeks. Reviewing the three modern series of AIN, only 64.1% of patients made a full recovery (serum creatinine < 132 mmol/L), whereas 23.4% gained a partial recovery (serum creatinine > 132 mmol/L) and 12.5% remained on renal replacement therapy.^[159] This relatively poor outcome may reflect the different case-mix in recent series, with fewer patients having traditional allergic-type AIN. Clearly, it would be useful to have prognostic indicators for AIN, and it has been suggested previously that the long-term outcome is worse if renal failure lasts for more than 3 weeks.^[159] However, this is not useful prospectively. Two series have shown worse prognosis with increasing age, but there appears to be no correlation with peak creatinine concentration.^[159] Attempts have also been made to gain prognostic information from the renal biopsy. Some authors have reported that patchy cellular infiltration predicts a better outcome than diffuse disease.^[159]However, more recent studies have not supported a correlation between the degree of cellular infiltration or tubulitis and outcome.^[179] The degree of interstitial fibrosis has been correlated to outcome,^[179] but such relationship was not confirmed in other studies.^[109] These conflicting observations may be due to the patchy nature of the disease and random sampling on renal biopsy.

Withdrawal of medications that are likely to cause AIN is the most significant step in early management of suspected or biopsy-proven AIN.^[173] If multiple potentially precipitating medications are being used by the patient, it is reasonable to substitute other medications for as many of these as possible and to withdraw the most likely etiologic agent among medications that cannot be substituted. The majority of patients with AIN improve spontaneously after the withdrawal of medications that resulted in renal failure. Other supportive care interventions include fluid and electrolyte management, maintenance of adequate hydration, symptomatic relief for fever and systemic symptoms, symptomatic relief for rash. Indications for dialysis in the management of acute renal failure include uncontrolled hyperkalemia, azotemia with mental status changes, and other symptomatic fluid or electrolyte derangements. The role of steroids in the treatment in AIN remains to be defined. There are those who continue to question the use of or indications for steroid therapy. It is true that there are no controlled, randomized trials supporting this recommendation.^[159] However, small case reports and studies have demonstrated rapid diuresis, clinical improvement, and return of normal renal function within 72 hours after starting steroid treatment, although some case reports indicate lack of efficacy, especially in cases of NSAID-induced AIN.^[179a] The decision to use steroids should be guided by the clinical course following withdrawal of offending medications. Convincing clinical evidence for a role for steroid therapy, however, comes from the idiopathic form of AIN, particularly that with coexistent anterior uveitis, the so-called TINU syndrome. Both the ocular and the renal changes of these patients respond dramatically to a brief course of steroid therapy. Clearly, steroids are not to be used in cases due to infectious agents, in which proper therapy is directed at eliminating infection. If steroid therapy is started, a reasonable dosage is prednisone 1 mg/kg/day orally (or equivalent intravenous dose) for 2 or 3 weeks, followed by a gradually tapering dose over 3 to 4 weeks.^[180] The merits of immunosuppressive agents, specifically cyclophosphamide or cyclosporine, are questionable. They should be considered in those cases in which the biopsy reveals immune complex deposits and those with evidence of circulating antitubular basement antibodies or complement consumption. Considerations should be also given to patients who fail to respond to a 2-week course of steroid therapy. A 4-week course of cyclophosphamide (2 mg/kg of body weight per day) while monitoring renal function and white blood cell count should suffice to determine the response. Therapy for longer than 4 to 6 weeks is not indicated. Plasmapheresis has been used in patients with circulating anti-TBM antibodies. It must be considered experimental and used only when no response to steroids and cyclophosphamide is observed.

CHRONIC TUBULOINTERSTITIAL NEPHRITIS

The pathologic features of chronic TIN include atrophy of tubular cells with flattened epithelial cells and tubule dilatation, interstitial fibrosis, and areas of mononuclear cell infiltration within the interstitial compartment and between tubules. TBMs are frequently thickened. The cellular infiltrate in chronic interstitial disease is composed of lymphocytes, macrophages, and B cells with only occasional neutrophils, plasma cells, and eosinophils.

In chronic interstitial disease, the glomeruli may remain remarkably normal by light microscopy, even when marked functional impairment is present. As chronic interstitial injury progresses, glomerular abnormalities are more evident and consist of periglomerular fibrosis, segmental sclerosis, and ultimately, global sclerosis. Small arteries and arterioles show fibrointimal thickening of variable severity, but vasculitis is not a feature of chronic interstitial disease ( Table 33-2 ).

TABLE 33-2 -- Causes of Chronic Interstitial Nephritis

Primary
Drug induced	Analgesics
	5-Aminosalicylic acid
	NSAIDs
	Chinese herbs
	Lithium
Toxin induced	Lead
	Cadmium
	Balkan endemic nephropathy
Metabolic disorders	Abnormal uric acid metabolism
	Hypokaliemia
	Hypercalcemia
	Hyperoxaluria
Immune mediated	Sarcoidosis
	Sjögren syndrome
Infection	Bacterial pyelonephritis
	Hantavirus
	Leptospirosis
Hematologic disorders	Sickle cell disease
	Light chain nephropathy
	Amyloidosis
Secondary Secondary chronic tubulointerstitial nephritis encompasses disease in which the primary pathogenic process begins in vascular, glomerular, or tubular systems followed by damage to the tubulointerstitial compartment.

NSAIDs, nonsteroidal anti-inflammatory drugs.

Analgesics

Analgesic nephropathy (AN) is a specific form of renal disease characterized by renal papillary necrosis (RPN) and chronic interstitial nephritis caused by prolonged and excessive consumption of analgesic mixtures. It is invariably caused by compound analgesic mixtures containing aspirin or antipyrine in combination with phenacetin, paracetamol, or salicylamide and caffeine or codeine in popular over-the-counter proprietary mixtures^[181] (see Table 33-2 ).

AN has been one of the more common causes of chronic renal failure (CRF), particularly in Australia and parts of Europe.^[182] Previous estimates, obtained before phenacetin was removed from over-the-counter analgesics and prior to legislation was enacted making combined analgesics available only by prescription (in Sweden, Canada, and Australia), suggested that AN was responsible for 1% to 3% of cases of ESRD in the United States as a whole, up to 10% in areas of North Carolina, and 13% to 20% in Australia and some countries in Europe (such as Belgium and Switzerland). In the 1990s, there was a clear decrease in the prevalence and incidence of AN among patients undergoing dialysis in several European countries and Australia. Most authors associated this decrease with the removal of phenacetin from analgesic mixtures.^[183]

Pathogenesis and Pathology

The etiology of AN remains a controversial issue, and particularly the question of which kind of analgesics are nephrotoxic is still a matter of debate. From 1955, experimental studies on the nephrotoxicity of analgesics were performed, mainly using rats fed with large amounts of drugs. It could be concluded that RPN was most frequently observed after the administration of aspirin in combination with phenacetin or paracetamol.^[184]

In humans, the long-standing excessive use of analgesics observed in patients with AN is concentrated on combination analgesics. Abusers prefer analgesic mixtures rather than single analgesics, taking these products rather for their mood-altering effects than for the relief of physical complaints. Hence, all these mixtures contain caffeine and/or codeine, substances that can create a psychological dependence toward these drugs. In the majority of the early AN reports, nearly all patients had taken large amounts of analgesic mixtures containing phenacetin. Part of the epidemiologic evidence of the nephrotoxicity of analgesics derived from the case-control studies performed in this field.^{[186] [187] [188] [189] [190]} All these studies included patients with renal failure ranging from newly diagnosed CRF (stages 1-3) (two studies) to ESRD (five studies) and the very specific diagnosis of RPN (one study). Controls were randomly selected from a hospitalized population (three studies), outpatient clinic (one study), or the general population (four studies). The definition of minimal abuse varied considerably from a frequency of twice a week to daily for a period of 1 month to 1 year. Overall results showed an exceptionally high relative risk of 17.2 in the study with RPN cases and comparable significantly increased risks between 2.2 and 2.9 in four studies. In the remaining studies, overall results were nonconclusive or not reported.

Most of these case-control studies also reported on the nephrotoxicity of different kind of analgesics. Except for single analgesics, however, these products were part of analgesic mixtures. Moreover, the composition of most of these mixtures changed considerably over the time period that they were available on the market. Only Pommer and co-workers^[190] were able to calculate reliable risk ratios for the different analgesics in the combinations as they were marketed, controlled for the use of other analgesics and for their “historical” compositions. They found an overall relative risk of 2.4 (95% CI 1.8–3.4), increased risks ranging from 2.6 to 4.8 for the different combinations (with or without phenactin) marketed in Germany, and no increased risk for single-ingredient analgesics.

The withdrawal of phenacetin from analgesic mixtures in Western Europe, Australia, and the United States, however, gives rise to the question of the nephrotoxic potency of different kinds of products without phenacetin that are currently on the market. Clinical observations in countries where analgesics without phenacetin are on the market for periods of more than 20 years (e.g., Australia, Belgium, Germany) learned that identical renal pathology is observed in patients abusing analgesic mixtures that never contained phenacetin.^[191] Moreover, we observed a cohort of 226 patients with AN diagnosed according to objective renal imaging criteria. In all except 7 patients, the abuse of analgesic mixtures was documented, and in 46 patients, nephrotoxicity was found in the absence of any previous phenacetin consumption. These patients abused the combinations: aspirin-paracetamol, aspirin-pyrazolones, paracetamol-pyrazolones, and two pyrazolones.^[192]

The mechanisms responsible for the renal injury are incompletely understood ( Fig. 33-1 ). Phenacetin is metabolized to acetaminophen and to reactive intermediates that can injure cells, in part by lipid peroxidation.^[193] These metabolites tend to accumulate in the medulla along the medullary osmotic gradient (created by the countercurrent system). As a result, the highest concentrations are seen at the papillary tip, the site of the initial vascular lesions.^[194] The potentiating effect of aspirin with both phenacetin and acetaminophen may be related to two factors:

	•	Acetaminophen undergoes oxidative metabolism by prostaglandin H synthase to reactive quinoneimine that is conjugated to glutathione. If acetaminophen is present alone, there is sufficient glutathione generated in the papillae to detoxify the reactive intermediate. However, if acetaminophen is ingested with aspirin, the aspirin is converted to salicylate, which becomes highly concentrated and depletes glutathione in both the cortex and the papillae of the kidney. With the cellular glutathione depleted, the reactive metabolite of acetaminophen then produces lipid peroxides and arylation of tissue proteins, ultimately resulting in necrosis of the papillae (see Fig. 33-1 ). [183] [195]
	•	Aspirin (also NSAIDs) suppress prostaglandin production by inhibiting cyclooxygenase enzymes. Renal blood flow, particularly within the renal medulla living at edge of hypoxia, is highly dependent upon systemic and local production of vasodilatory prostaglandins. The final injury is, therefore, due to both hemodynamic and cytotoxic effects of these drugs resulting in papillary necrosis and interstitial fibrosis.

FIGURE 33-1 Synergistic toxicity of analgesics in the inner medulla and centrally acting dependence-producing drugs lead to analgesic nephropathy. Acetaminophen undergoes oxidative metabolism by prostaglandin H synthase to reactive quinoneimine that is conjugated to glutathione. If acetaminophen is present alone, sufficient glutathione is generated in the papillae to detoxify the reactive intermediate. If the acetaminophen is ingested with aspirin, the aspirin is converted to salicylate, and salicylate becomes highly concentrated in both the cortex and the papillae of the kidney. Salicylate is a potent depletor of glutathione. With the cellular glutathione depleted, the reactive metabolite of acetaminophen then produces lipid peroxidases and arylation of tissue proteins, ultimately resulting in necrosis of the papillae. (Redrawn from Kincaid-Smith P, Nanra RS: Lithium-induced and analgesic-induced renal diseases. In Schrier RW, Gottschalk CW [eds]: Diseases of the Kidney, 5th ed. Boston, Little Brown, 1993, pp 1099-1129; and Duggin GG: Combination analgesic-induced kidney disease: The Australian experience. Am J Kidney Dis 28(suppl 1):S39–S47, 1996.)

The renal damage induced by analgesics is most prominent in the medulla. The earliest changes consist of prominent thickening of the vasa recta capillaries (capillary sclerosis) and patchy areas of tubular necrosis; similar vascular lesions can be found in the renal pelvis and ureter, suggesting that the primary effect is damage to the vascular endothelial cells.^[195] Later changes include areas of papillary necrosis and secondary cortical injury with focal and segmental glomerulosclerosis and interstitial infiltration and fibrosis.

Clinical Features

The renal manifestations of AN are usually nonspecific: slowly progressive CRF, with serum urea and creatinine analysis that may be normal or may reveal sterile pyuria and a mild proteinuria (<1.5 g/day).^[196] Hypertension and anemia are commonly seen with moderate to advanced disease; more prominent proteinuria that can exceed 3.5 g/day can also occur at this time, a probable reflection of secondary hemodynamically mediated glomerular injury. Most patients have no symptoms referable to the urinary tract, although flank pain or macroscopic/microscopic hematuria from a sloughed or obstructing papilla may occur or a transectional cell carcinoma may produce pain. Urinary tract infection is also somewhat more common in women with this disorder. Most patients are commonly between the ages of 30 and 70. Careful questioning often reveals a history of chronic headaches or low back pain that leads to the analgesic use. Also common are other somatic complaints (such as malaise and weakness) and ulcer-like symptoms or a history of peptic ulcer disease owing, in part, to chronic aspirin ingestion.^[196]

The decline in renal function can be expected to progress if analgesics are continued. Conversely, renal function stabilizes or mildly improves in most patients if analgesic consumption is discontinued.^[191] If, however, the renal disease is already advanced, then progression may occur in the absence of drug intake, presumably owing to second-ary hemodynamic and metabolic changes associated with nephron loss.^[197] The late course of AN also may be complicated by two additional problems: malignancy and atherosclerotic disease. It is estimated that a urinary tract malignancy will develop in as many as 8% to 10% of patients with AN, but in well under 1% of phenacetin-containing analgesic users without kidney disease.^[198] In women under the age of 50, for example, analgesic abuse is the most common cause of bladder cancer, an otherwise unusual disorder in young women.^[199] The potential magnitude of this problem has also been illustrated by histologic examination of nephrectomy specimens obtained prior to renal transplantation; the incidence of urothelial atypia in this setting approaches 50%.^[200] The tumors generally become apparent after 15 to 25 years of analgesic abuse,^[200] usually, but not always, in patients with clinically evident AN.^[201] Most patients are still taking the drug at the time of diagnosis, but clinically evident disease can first become apparent several years after cessation of analgesic intake and even after renal transplantation has been performed.^[200] It is presumed that the induction of malignancy results from the intrarenal accumulation of N-hydroxylated phenacetin metabolites that have potent alkylating action.^[201] Because of urinary concentration, the highest concentration of these metabolites will be in the renal medulla, ureters, and bladder, possibly explaining the predisposition to carcinogenesis at these sites. The pathogenetic importance of phenacetin metabolites is suggested indirectly from the observation that there appears to be no association with tumor formation with the prolonged ingestion of other analgesics that can cause papillary necrosis but do not form these metabolites, such as acetaminophen and the NSAIDs. ^{[192] [203]} The major presenting symptom of urinary tract malignancy in AN is microscopic or gross hematuria. Thus, continued monitoring is essential, and new hematuria should be evaluated with urinary cytology, and if indicated, cystoscopy with retrograde pyelography.^[200] The incidence of urothelial carcinoma after renal transplantation in patients with AN is comparable with the general incidence of up to 10% of urothelial carcinomas in ESRD patients with AN. Removal of the native kidneys prior to renal transplantation has also been suggested, but the efficacy of this regimen has not been proved.^[200]

Diagnosis and Treatment

In view of the lack of reliable criteria and the high prevalence of AN in the 1980s in Belgium (17.9% in 1984), a series of prospective, multicenter, controlled studies to define and validate diagnostic criteria for this disease were performed. ^{[204] [205]} We could provide strong evidence that specific anatomic changes, best seen by noncontrast computer tomography (CT), show much greater sensitivity and specificity than other clinical signs and symptoms in the diagnosis of ESRD due to AN.^[204] These changes are (1) decrease in renal volume, (2) bumpy renal contours, and (3) papillary calcifications. In a more recent study, these observations were validated in a representative sample of patients with analgesics abuse with ESRD and extended to patients with moderate renal failure.^[204] In clinical practice, however, it is important to remember that the predictive value of this test, like any other diagnostic test, is very much dependent on the prevalence of the disease in the population under study. This test should, therefore, be utilized in patients with a reasonable risk for AN and not as a general screening test ( Fig. 33-2 ).

FIGURE 33-2 Renal imaging criteria of analgesic nephropathy as observed in a postmortem kidney and computed tomography (CT) scans without contrast material, including a decreased renal size, bumpy contours, and papillary calcifications. RA, renal artery; RV, renal vein; SP, spine. (Adapted from De Broe ME, Elseviers MM: Analgesic nephropathy. N Engl J Med 338:446–452, 1998.)

Nonsteroidal Anti-inflammatory Drugs

NSAIDs are popular for a wide range of clinical conditions. They are available in both over-the-counter and prescription strengths. The most common renal disorder associated with NSAIDs is acute, largely reversible, insufficiency due to the inhibition of renal vasodilatory prostaglandins in the clinical setting of a stimulated renin-angiotensin system. ^{[163] [206]} Older age, hypertension, concomitant use of diuretics or aspirin, preexisting renal failure, diabetes, and plasma-volume contraction are known risk factors for renal failure after the ingestion of NSAIDs.^[163] The chronic effects of NSAIDs are less well documented. Three epidemiologic studies addressed the question of the chronic effects of NSAIDs on the kidney. Two (Perneger et al^[187] and Ibanezet al^[189]) could not find an increased risk of chronic renal disease associated with daily NSAID use. One study (Sandler et al)^[185] did find a relative risk of 2.1 (CI 1.1–4.1). This positive result was, however, attributed to an excessive high risk (16.6) in one group of the subjects studied (males > 65 yr), whereas the risk was absent (0.9, 1.3, 0.9) in the other three groups. A recent report, however, demonstrated that NSAIDs are the most frequent cause of permanent renal insufficiency after AIN.^[175] Risk factors for permanent renal insufficiency are preexisting renal damage, long intake of the causative drug, slow oligosymptomatic disease development, and histologic signs of chronicity with those of AIN. Although RPN and CRF can occur after prolonged use of NSAIDs, the actual risk of these serious complications is not known.

5-Aminosalicylic Acid

Recently, an association has been suggested between the use of 5-aminosalicylic (5-ASA) in patients with chronic inflammatory bowel disease (IBD) and the development of a particular type of chronic TIN ( Table 33-3 ).

TABLE 33-3 -- Differential Diagnosis of Some Forms of Chronic Interstitial Nephritis

	Analgesic Nephropathy	Chinese Herb Disease	Balkan Endemic Nephritis	5-Aminosalicylic Acid Nephritis
Course	>10–15 Yr	6 Mo–2 Yr	>20 Yr	>6 Mo
Kidney imaging	Shrunken	Shrunken	Shrunken	Slightly shrunken
	Irregular contours	Irregular contours	Smooth surface	Smooth
	Papillary calcifications	No calcifications	No calcification	No calcification
Histology
Cellular infiltration	++	+	+	+++
Fibrosis	++	++	++	++
Atrophy	++	++	+++	+
Capillarosclerosis	+	?/+	+	?-
Apoptosis	?	?	+	?
Urothelial malignancies	+ ( (*) )	+	+	+
Familial occurrence	-	-	+	-
Etiology	Analgesics	Aristolochic acid	?	5-Aminosalicylic acid
	+ addictive substances	+ diuretics		+?
		+ vasoconstrictive substances?

(*)	As long as phenacetin was part of the analgesic mixture.

In the last decade, these new 5-ASA products replaced sulfasalazine as the first-line therapy for mildly to moderately active IBD. However, a literature search (1998) revealed 17 published cases of renal impairment with 5-ASA therapy in patients with IBD, and in several, it was shown that the patients did not recover completely upon stopping the drug, even after a follow-up period of several years. A European prospective registration study was performed aiming to register all patients with IBD and renal impairment and to control for a possible association with 5-ASA therapy. A cohort of 1449 patients with IBD seen during 1 year in the outpatient clinics of 28 European gastroenterology departments was investigated: Preliminary results showed 30 patients (2%) with decreased renal function, and a possible association with 5-ASA therapy was found in half.^[206] A recent study estimated the incidence of clinical nephrotoxicity in patients taking 5-ASA therapy as approximately 1 in 4000 patients/yr.^[207]

Determining the cause of renal disease in those patients with IBD is not straightforward. The most frequent renal complications are oxalate stones and their consequences, such as pyelonephritis, hydronephrosis, and in the long-term, amyloidosis . Chronic IBD may also be associated with different forms of glomerulonephritis. As for many drugs, re-versible, AIN has been described with the use of 5-ASA compounds. In view of this complexity, the association of 5-ASA and chronic interstitial nephritis in patients with IBD can be difficult to interpret because renal involvement may be an extraintestinal manifestation of the underlying disease. However, the particular form of chronic TIN in patients with IBD treated with 5-ASA is characterized by an important cellular infiltration of the interstitium with macrophages, T cells, and also B cells (even granulomas) surrounding atrophic tubules, suggesting a cell-mediated immune response such as T cell response against an autoantigen modified by the drug.^[208]

Pathogenesis and Pathology

That 5-ASA causes renal disease is supported by the number of case reports appearing in the recent literature of patients with IBD using 5-ASA as their only medication, the improvement, at least partial, of the impaired renal function upon stopping of the drug, and a worsening after resuming 5-ASA use. Furthermore, the molecular structure of 5-ASA is very close to those of salicylic acid, phenacetin, and aminophenol, drugs with well-documented nephrotoxic potential. The mechanism of renal damage, possibly caused by 5-ASA itself, may be analogs to that of salicylates by inducing hypoxia of renal tissues either by uncoupling oxidative phosphorylation in renal mitochondria, by inhibiting the synthesis of renal prostaglandins, or by rendering the kidney susceptible to oxidative damage by a reducing renal glutathione concentration after inhibition of the pentose phosphate shunt.^[208]

Clinical Features

A typical case is shown in Figure 33-3 . An intriguing aspect of this type of toxic nephropathy is the documented persistence of the inflammation of the renal interstitium even after several months/years arrest of the drug. The disease is more prevalent in men, with a male-to-female ratio of 15:2. The age of reported cases ranged from 14 to 45 years. By contrast with AN, in which renal lesions are observed only after several years of analgesic abuse, interstitial nephritis associated with 5-ASA was already observed during the 1st year of treatment in 7 out of 17 reported cases, most of whom had started 5-ASA therapy with documented normal renal function. The cumulative dose of 5-ASA is not predictive for development of interstitial nephritis. In several patients, particularly in those in whom there is a delayed diagnosis of renal damage, recovery of renal function does not occur, and some needed renal replacement therapy. ^{[209] [210]}

FIGURE 33-3 Case report of nephrotoxicity of 5-aminosalicylic acid (Pentasa) in inflammatory bowel disease. A, Evolution of renal failure. B, First renal biopsy. C, Second renal biopsy.

Diagnosis and Treatment

Because this type of chronic TIN produces few if any symptoms, and if diagnosed at a late stage progresses to irreversible chronic ESRD, serum creatinine levels should be measured in any patient with IBD treated with 5-ASA at the start of the treatment, every 3 months for the remainder of the 1st year, and annually thereafter. In some patients, favorable response to steroid therapy has been reported. If serum creatinine increases in a patient with IBD treated with 5-ASA, a renal biopsy is the only way to determine a correct diagnosis.^[210]

Chinese Herbs—Aristolochic Nephropathy

In 1992, nephrologists in Belgium noted an increasing number of women presenting with renal failure, often near end stage, following their exposure to a slimming regimen containing Chinese herbs^[211] (see Table 33-3 ). An initial survey of seven nephrology centers in Brussels identified 14 women under the age of 50 who had presented with advanced renal failure due to biopsy-proven, chronic TIN over a 3-year period; 9 of these had been exposed to the same slimming regimen.^[212] As of early 2000, more than 120 cases had been identified. The epidemiology is unknown, as is the risk for the development of severe renal damage, but the recent publication of case reports from several countries in Europe and Asia indicate that the incidence of herbal medicine-induced nephrotoxicity is more common than previously believed. ^{[214] [215] [216]}

A plant nephrotoxin, aristolochic acid (AA), has been proposed a possible etiologic agent. Support for this hypothesis is provided by findings in animal models. In a first study, rabbits were given intraperitoneal injections of AA (0.1 mg AA/kg 5 days/wk for 17-21 mo).^[216] Histologic examina-tions of the kidneys and genitourinary tract revealed renal tubular atrophy, interstitial infiltration/fibrosis, and atypical and malignant uroepithelial cells.

In a more recent study, the daily subcutaneous administration of 10 mg/kg of AA to salt-depleted rats induced, after 35 days, moderate renal failure associated with tubular atrophy and interstitial fibrosis.^[217]

In vitro (opossum kidney cell line) and in vivo (rats) proximal tubular injury occurs early after AA intoxication in rats. A link exists between specific AA-DNA adduct formation and decreased megalin expression and inhibition of receptor-mediated endocytosis of low-molecular-weight proteins.^[218] Recent cytotoxicity data obtained in LLC-PK cells suggest that the nitro and methoxy groups are critical determinants of the nephrotoxicology potency of AA.^[219] The main histologic lesion in human biopsies is extensive interstitial fibrosis (located principally in the cortex) with atrophy and loss of the tubules ( Fig. 33-4 ). Cellular infiltration of the interstitium is scarce. Important tryptase-positive mast cells were observed in the fibrotic areas in renal biopsies.^[214] Thickening of the walls of the interlobular and afferent arterioles result from endothelial cell swelling. The glomeruli are relatively spared, and immune deposits are not observed. These findings suggest that the primary lesions may be centered in the vessel walls, thereby leading to ischemia and interstitial fibrosis.^[220]

FIGURE 33-4 Case of Chinese herb nephropathy. Kidney biopsy shows tubular atrophy, widening of the interstitium, cellular infiltration, important fibrosis, and glomeruli surrounded by a fibrotic ring. A, Masson staining. B, Hematoxylin-eosin staining.

At one center in Belgium, 19 native kidneys and ureters were removed in a series of 10 patients during and/or after renal transplantation: multifocal, high-grade, flat, transitional cell carcinoma (carcinoma in situ) was observed in 4 (40%), and all had multifocal moderate atypia. Tissue samples revealed AA-related DNA adducts, indicating a possible mechanism underlying the development of malignancy. In another study of 39 patients with Chinese herbal nephropathy and ESRD who underwent prophylactic removal of the native kidneys and ureters, urothelial carcinoma was discovered in 18 and mild-to-moderate urothelial dysplasia in 19. All atypical cells were found to overexpress a p53 protein, suggesting the presence of a mutation in the gene.^[221] How far transdifferentiation and apoptosis play an important role in this fast-developing type of chronic tubulointerstitial disease of the kidneys is the subject of active research in several groups in the world. In addition to AA, patients with Chinese herb nephropathy also received the appetite suppressants fenfluramine and diethylpropion, agents with vasoconstrictive properties and diuretics at low dose.

Together, these observations suggest that the relatively fast-developing chronic tubulointerstitial renal disease may have been caused by combined exposure to a potent nephrotoxic substance, that is, AA, and to renal vasoconstrictors fenfluramine/diethylpropion associated with clinical risk factors such as volume depletion. However, Debelle and associates^[222] showed, in rats given the combination of AA and high doses of fenfluramine, a comparable degree of interstitial fibrosis as the rats given AA alone. The findings of Debelle and colleagues^[217] of the necessity to induce a stimulated intrarenal renin-angiotensin system in order to obtain fibrointerstitial lesions is in line with the combined exposure concept observed with several potential nephrotoxins. Recently, Debelle and co-workers^[223] demonstrated that blockade of the renin-angiotensin system does not prevent renal interstitial fibrosis induced by AA.

Another uncertain factor is why only some patients exposed to the same herbal preparations develop renal disease. Women appear to be at greater risk than men. Other possibly important factors include toxin dose, batch-to-batch variability in toxin content, individual differences in toxin metabolism, and a genetically determined predisposition toward nephrotoxicity and/or carcinogenesis.^[213]

Clinical Features

Patients present with renal insufficiency and other features indicating a tubulointerstitial disease. The blood pressure is either normal or only mildly elevated, and the urine sediment reveals only a few red and white cells. The urine contains protein, less than 1.5 g/day, consisting of both albumin and low-molecular-weight proteins that are normally reabsorbed by the proximal tubules; hence, tubular dysfunction—also marked by glycosuria—contributes to the proteinuria.^[224] The plasma creatinine concentration at presentation has ranged from 1.4 to 12.7 mg/dL (123–1122 mmol/L). Follow-up studies have revealed relatively stable renal function in most patients with an initial plasma creatinine concentration below 2 mg/dL (176 mmol/L). However, progressive renal failure resulting in dialysis or transplantation may ensue in patients with more severe disease, even if further exposure to Chinese herbs is prevented.

Diagnosis and Treatment

No specific criteria exist for the diagnosis of this type of renal disease. The diagnosis should be suggested in any patient with unexplained relatively fast progressive renal disease who is using/abusing herbal remedies. The presence of tubular proteinuria may be a clue to the diagnosis, particularly in the early stages. The histologic appearances are not specific. On CT scan, bilateral shrunken kidneys with irregular contours and no parenchymal calcification can be observed.

No proven effective therapy exists for this disorder. An uncontrolled study suggested that corticosteroids may slow the rate of loss of renal function.^[212] A recent experimental study showed that circulating transgene-derived HGF-attenuated interstitial fibrosis in the AA-treated rats.^[225] The high incidence of cellular atypia of the genitourinary tract suggests that, as a minimum, these patients should undergo regular surveillance for abnormal urinary cytology. Whether more aggressive management strategies, such as bilateral native nephroureterectomies (particularly in those undergoing renal transplantation), are required is unclear. Findings from a recent report support the more aggressive option.^[226]

Lithium

Lithium is used extensively in the treatment of manic-depressive psychosis. Different forms of renal effects/injury have been described: most frequently, nephrogenic diabetes insipidus, but also renal tubular acidosis, chronic interstitial nephritis, nephrotic syndrome, and focal segmental glomerular sclerosis/global glomerular sclerosis.^[227] Hyperparathyroidism is observed in patients treated with lithium.

Pathogenesis and Pathology

Lithium is eliminated from the body almost entirely by the kidney, being filtered and reabsorbed in the proximal tubule, resulting in a clearance of one third of the creatinine clearance. It moves in and out of cells only slowly and accumulates in the kidney, particularly in the collecting tubule, entering these cells through sodium channels in the luminal membrane.^[228] Hence, its principal toxicity relates to distal tubular function, in which inhibition of adenylate cyclase and generation of cyclic adenosine monophosphate (cAMP) results in down-regulation of aquaporin-2, the collecting tubule water channel, and a decrease in antidiuretic hor-mone receptor density, leading to resistance to antidiuretic hormone.^[229] A low intracellular level of cAMP leads to increased cellular levels of glycogen observed in the kidney biopsy of patients taking lithium, as does the fact that lithium also directly inhibits enzymes involved in glycogen breakdown. The ensuing glycogen storage may interfere with distal tubular function and be responsible for the nephrogenic diabetes insipidus and, hence, polyuria and polydipsia in lithium-treated patients.^[230] A lithium-induced decrease in the activity of the H⁺-ATPase pump in the collecting tubule may be responsible for the impaired ability to acidify the urine.

Lithium treatment has been etiologically related to parathyroid hypertrophy and hyperfunction, the latter seeming to be due to an upward resetting of the level at which the plasma calcium concentration depresses parathyroid hormone release.^[231] The hyperparathyroidism observed in patients receiving lithium treatment is characterized by elevated parathyroid hormone levels, hypercalcemia, hypocalciuria, and normal serum phosphate levels, by contrast to primary hyperparathyroidism in which hypophosphatemia and hypercalciuria are seen. Renal biopsies from patients taking lithium show a specific histologic lesion (100% of cases) in the distal tubule and collecting duct ( Fig. 33-5 ). On light microscopy, this lesion consists of swelling and vacuolization in cells associated with considerable accumulation of periodic acid-Schiff (PAS)–positive glycogen.

FIGURE 33-5 Top, Severe lithium-associated chronic tubulointerstitial nephropathy with the additional finding of focal tubular cysts arising in a background of severe interstitial fibrosis and tubular atrophy. Periodic acid-Schiff (PAS) stain, ×40. Bottom, High-power view of tubular cysts lined by simple cuboidal epithelium (c). Adjacent tubules show tubular dilatation (d). PAS stain, ×100. (Reproduced with permission from Markowitz GS, Radhakrishnan J, Kambham N, et al: Lithium nephrotoxicity: A progressive combined glomerular and tubulointerstitial nephropathy. J Am Soc Nephrol 11:1439–1448, 2000.)

Hestbech and associates^[232] were the first to suggest that progressive chronic interstitial lesions occurred in the kidneys of patients receiving lithium. However, a controlled study showed no difference between biopsies from patients taking lithium and those from a group of patients who had affective disorders but were not doing so.^[230] Specifically, there was no difference in the incidence of glomerular sclerosis, interstitial fibrosis, tubular atrophy, cast formation, or interstitial volume, but there was a significant increase in the number of microcyst formations in the lithium-treated patients. One reason why it has been difficult to determine the nature of lithium-induced chronic renal damage has been the lack, until recently, of an animal model in which lesions similar to those noted in human biopsies can be demonstrated. However, a recent study on lithium nephrotoxicity in the rabbit showed clear-cut evidence of progressive histologic and functional impairment, with the development of significant interstitial fibrosis, tubular atrophy, glomerular sclerosis, and cystic tubular lesions. A recent publication by Markowitz and colleagues^[233]revealed a chronic TIN in 100% of 24 patients having received lithium for several years, associated with cortical and medullary tubular cysts or dilatation. There was also a surprisingly high prevalence of focal segmental glomerulosclerosis and global glomerulosclerosis, sometimes of equivalent severity to the chronic tubulointerstitial disease. Despite discontinuation of lithium treatment, 7 of 9 patients with initial serum creatinine values above 2.5 mg/dL progressed to ESRD. A recent French follow-up study of lithium-treated patients demonstrated that the duration of lithium therapy and the cumulative dose of lithium were the major determinants of nephrotoxicity and estimated a prevalence of lithium-related ESRD in 2/1000 dialysis patients (0.22% of all cases). Twelve out of 74 patients in this study reached ESRD at a mean age of 65 years with an average latency between onset of lithium therapy and ESRD of 20 years.^[234] Lepkifker and co-workers^[235] studied retrospectively 114 subjects with major depressive or schizoaffective disorders who had been taken lithium for 4 to 30 years from 1968 to 2000. Long-term lithium therapy did not influence glomerular function in the majority of patients. However, 20% of long-term lithium patients exhibited “creeping creatinine,” developing chronic renal insufficiency.^[235]

Clinical Features

Apart from acute lithium intoxication, chronic poisoning can occur in patients whose lithium dosage has been increased or in those with a decreased effective circulating volume, decreased sodium intake, diabetes mellitus, gastroenteritis, and renal failure, thereby resulting in an increase in serum lithium levels (>1.5 mEq/L of Li). Symptoms associated with poisoning include lethargy, drowsiness, coarse hand tremor, muscle weakness, nausea, vomiting, weight loss, polyuria, and polydipsia. Severe toxicity (>2.5 mEq/L of Li) is associated with increased deep tendon reflexes, seizures, syncope, renal insufficiency, and coma. Chronic lithium poisoning is frequently associated with electrocardiogram changes, including ST segment depression and inverted T waves in the lateral precordial leads. Lithium is concentrated within the thyroid and inhibits the synthesis and release of thyroxine, which can lead to hypothyroidism and hypothermia. It may also cause thyrotoxicosis and hyperthermia. Symptoms of hypercalcemia may also be present, such as exacerbating the urinary concentrating defect already present in these patients.^[236] In patients with glomerular lesions such as minimal change or focal glomerular sclerosis, proteinuria generally begins within 1.5 to 10 months after the onset of therapy, completely or partially resolving in most patients within 4 weeks after lithium is discontinued. Reinstitution of lithium has led to recurrent nephrosis in some patients.

Diagnosis and Treatment

Polyuria and polydipsia due to nephrogenic diabetes insipidus usually disappear rapidly if lithium is withdrawn. In most cases, the lithium is so clearly beneficial that the polyuria is accepted as a side effect and treatment continued. It is likely that the serum concentration of lithium is important and that the renal damage is more likely to occur if the serum concentration is consistently high or if symptoms of lithium toxicity recur. The serum lithium concentration should, therefore, be monitored carefully (at least every 3 months) and maintained at the lowest level that will provide adequate control of the manic-depressive psychosis. Much more difficult to handle is the situation in which a patient on long-term lithium therapy is found to have impaired renal function for which there is no obvious alternative cause. As stated previously, renal failure may progress even if lithium therapy is withdrawn, and in some patients, the discontinuation of lithium can lead to a devastating deterioration in their psychiatric condition.

Lead

Lead toxicity affects many organs, resulting in encephalopathy, anemia, peripheral neuropathy, gout, and renal failure. The epidemic of lead nephropathy in Queensland (Australia) provided the strongest link between lead and chronic TIN. Inglis and co-workers^[237] noted the excess mortality due to the chronic interstitial nephritis to be present in Queensland but not in other parts of Australia and correlated the incidence of granular contracted kidneys found at autopsy with the lead content of the skull in people from Queensland and Sydney, showing that this correlated closely with the incidence of renal failure. Exposure was due to lead-based paints used between 1890 and 1930, but recently, the source of lead is industrial exposure. This type of exposure is often insidious, occurring over a very long period. Two studies have shown an inverse relationship between low-level lead exposure and renal function in the general population. Although low-level lead exposure in the general population is associated with mild but significant depression of renal function, its role in the development of ESRD is still a matter of debate. However, a recent prospective study by Yu and associates^[238] showed that low-level environmental lead exposure is associated with accelerated deterioration of renal insufficiency. Even at levels below the normal ranges, both increased blood lead levels and body lead burden (ethylenediaminetetraacetic acid [EDTA] mobilization test) predict accelerated progression of CRF in patients without diabetes or occupational lead exposure.^[238] Muntner and colleagues^[239] examined the association between blood lead and renal function among a representative sample of the civilian U.S. population with and without hypertension, age 20 years or older. In this cross-sectional study, they observed hypertension and exposure to lead (blood lead levels) even at low levels are associated with chronic kidney disease.

Etiology, Pathogenesis, and Pathology

The pathogenesis of the renal disease seen in the context of lead exposure may be related to proximal tubule reabsorption of filtered lead with subsequent accumulation in proximal tubule cells. Aminoaciduria, glycosuria, and phosphaturia representing the Fanconi syndrome are observed after lead exposure and believed to be related to an effect of lead on mitochondrial respiration and phosphorylation. Because lead is also capable of reducing 1,25-dihydroxyvitamin D synthesis, prolonged hyperphosphaturia and hypophosphatemia caused by lead poisoning in children may result in bone demineralization and rickets. Chronic lead poisoning can affect glomerular function: After an initial period of hyperfiltration, the GFR is reduced and nephrosclerosis and CRF may ensue. Protracted lead exposure also interferes with distal tubular secretion of urate, leading to hyperuricemia and gout. Renal biopsies in patients with subclinical lead nephropathy and a mild-to-moderate decrease in GFR primarily show focal tubular atrophy and interstitial fibrosis with minimal cellular infiltration. Electron microscopy shows mitochondrial swelling, loss of cristae, loss of basal infoldings, and a lysosomal-like structure containing dense bodies in the proximal tubules.

Clinical Features

Renal failure becomes apparent years after the exposure and is associated with gout in most, if not all, cases. Hypertension is a very common feature of lead nephropathy, and an association between hypertension without renal failure and low-level lead exposure has gained increasing recognition over the past 2 decades. Although hyperuricemia is common in renal failure, gout is unusual and its presence should raise the possiblity of lead nephropathy.

Many studies of occupational lead poisoning, however, have not taken into account the coexposure to other toxins such as cadmium.^[240] In addition, the relationship between early markers of renal tubular dysfunction, such as the urinary excretion of low-molecular-weight proteins or N-acetyl β-d-glucosaminidase, and subsequent development of renal failure remains to be determined.^[241] Nevertheless, many studies have documented an association between occupational exposure to lead and impairment of renal function. Loghman-Adham^[242] reviewed 20 studies of occupational and environmental lead exposure and concluded that such exposure results in abnormalities in renal function: Only 3 studies showed no change in renal function.

Diagnosis and Treatment

As the blood lead level only reflects recent lead exposure and is usually normal in patients with CRF due to their previously sustained low level of lead exposure, the diagnosis has to be based on measurement of the body lead burden. The test of choice is the EDTA mobilization test, which involves the administration of 2 g of EDTA intramuscularly in two divided doses 8 to 12 hours apart and collection of three consecutive 24-hour urine samples. A cumulative excretion of more than 600 mg is suggestive for high lead body burden. Renal failure in itself does not increase body lead load, but it does delay the excretion of lead.^[243] The diagnosis of lead nephropathy should be considered in any patient with progressive renal failure, mild-to-moderate proteinuria, significant hypertension, history of gout, and an appropriate history of exposure. Wedeen and co-workers^[244] treated eight industrially exposed patients, all having mild renal failure with GFR of around 50 mL/min before treatment, with EDTA injection thrice-weekly for 6 to 15 months—four patients improved with a 20% increase in their GFR. Lin and colleagues^[245]published a comprehensive study that skillfully attempts to bridge the clinical gap between association and causality with regard to patients with a high-normal body lead burden. There are two components to this work. A prospective, observational study was first conducted in a select group of patients with slowly progressive chronic renal insufficiency. The investigators did not enroll patients identified at baseline as having an overt elevation of body lead burden (>600 mg/72 hr). Patients were assessed for factors known to influence the progression of renal disease. Multivariate analyses identified increased baseline body lead burden as a prognostic factor. After 24 months of observation, the body lead burden was reassessed. EDTA mobilization tests identified 64 patients with a high-normal body lead burden (80 mg to <600 mg of urinary lead excretion over 72 hr) who then entered a 27-month intervention phase—a randomized, single-blind, placebo-controlled trial of EDTA chelation therapy. Remarkably, glomerular filtration was significantly improved in patients who received chelation therapy versus patients given placebo. The authors argue that chelation therapy slows the progression of renal insufficiency in patients with a mildly elevated body lead burden.^[245] One may argue that the results do not demonstrate that lead chelation therapy improves short-term renal function in humans. Rather, they show that EDTA chelation therapy does. EDTA has protean biochemical and cellular effects both in vitro and in vivo. This known facet of EDTA precludes a definitive statement that chelation of lead is the important mechanism. Although the final therapeutic answer toward the deleterious health effect of low-level lead exposure is still not given, the studies of the Taiwan group^[245] have provided the first solid observations on a potential efficient and safe treatment of lead exposure.

Cadmium

Cadmium is a metal that can cause severe toxicity for different organs in humans.^[246] Cadmium is a cumulative environmental pollutant that accumulates in the human body after inhalation or gastrointestinal absorption. Owing to its various applications and increased industrial production, the element's release in the environment increased considerably during the 1950s and onward, particularly in Belgium and Japan, which are among the most important cadmium-producing countries worldwide. However, the atmospheric emissions of cadmium from the zinc smelters have been reduced since the 1970s. At present, normal cadmium values are set at 0.1 to 0.8 mg/L (nonsmokers) in blood and 0.02 to 0.7 mg/g creatinine in urine. Cadmium is a highly toxic metal. The kidney is the element's most important target organ, and it has long been recognized that high-level exposure to cadmium after inhalation or ingetion can give rise to nephrotoxicity in humans and that this effect is usually considered to be the earliest and most important feature from the point of view of health. Cadmium induces a tubular proteinuria.^[247] The early mechanism of cadmium toxicity in proximal tubule cells may include a colchicine-like depolymerization of microtubules and impaired vesicle-dependent recycling of various brush-border membrane proteins. These processes may lead to a time-dependent loss of cell membrane components, resulting in reabsorptive and secretory defects that occur in cadmium-induced nephrotoxicity.^[248] Hence, when exposed to high levels of cadmium (cadmium in renal cortex > 100–400 mg/kg wet weight) in the workplace, workers have developed tubular proteinuria, renal glucosuria, aminoaciduria, hypercalciuria, phosphaturia, and polyuria, and in a few severe cases, (long-standing high exposure and urinary excretion > 20 mg/g creatinine and β2-microglobulin > 1500 mg/g creatinine) renal damage may progress to an irreversible reduction in glomerular filtration.^[240] Indeed, glomerular injury that can lead to decreased GFRs and possibly ESRD has been observed in workers exposed to cadmium.^[247] Uremia was a common cause of death among Japanese farmers suffering from Itai-Itai disease (characterized by multiple fractures, osteomalacia) and due to exposure to cadmium because of the use of contaminated river water for irrigation of rice fields.^[249]

The extent to which chronic low-level environmental exposure to cadmium affects renal function is much less clear. The Cadmibel study, in which a random sample of 1699 subjects was recruited from four areas of Belgium with varying degrees of cadmium pollution, showed that urinary excretion of retinol-binding protein, N-acetyl-b-glucosaminidase, β₂-microglobulin, amino acids, and calcium were significantly associated with urinary cadmium excretion. There was a 10% probability of these variables being abnormal when urinary cadmium exceeded 2 to 4 mg/24 hr. However, in a 5-year follow-up of a subcohort from the Cadmibel study, the so-called Pheecad study, in which 593 individuals with the highest urinary cadmium excretion were reexamined on average 5 years later, it was demonstrated that the subclinical tubular effects previously documented were not associated with deterioration in glomerular function.^[250] Hence, in the environmental cadmium-exposed population, the renal effects due to cadmium appear to be weak, stable, and even reversible . These findings in environmentally exposed subjects may reasonably be extrapolated to the current, moderately exposed, occupational population in whom, in various epidemiologic studies, increased cadmium levels/exposure have repeatedly been associated with disturbed levels of markers of early renal dysfunction, but without evidence for an accelerated progression toward CRF.

A number of studies have demonstrated an increased prevalence of kidney stones among individuals occupation-ally exposed to cadmium.^[251] Increased urinary excretion of calcium, resulting from tubular damage, is believed to be a primary determinant of stone formation in this setting. This fits with the recently developed concept of injury-induced phenotypic changes of the tubular epithelium cells followed by crystal adhesion-nephrocalcinosis.^[252] A recent epidemiologic study provides evidence for a link between renal tubular damage and dysfunction caused by environmental cadmium exposure and increased risk of high blood pressure.^[253] Minimizing exposure to cadmium is the most important therapeutic measure. Occupational exposure should be kept as low as technically feasible, preferably below 0.005 mg/m³. Exposure via food should be kept well below 30 mg cadmium per day.^[254] Other than general supportive therapy, no specific are methods available for treating acute cadmium poisoning. Although various chelating agents have been tried in animals, none has been shown to be efficient in humans.

Balkan Endemic Nephropathy

Balkan endemic nephropathy (BEN) is a chronic, familial, noninflammatory tubulointerstitial disease of the kidneys (see Table 33-3 ). A high frequency of urothelial atypia, occasionally culminating in tumors of the renal pelvis and urethra, is associated with this disorder.^[255] As the name suggests, BEN is most commonly seen southeastern Europe, including the areas traditionally considered to comprise the “Balkans”: Serbia, Bosnia and Herzegovina, Croatia, Romania, and Bulgaria. It is most likely to occur among those living along the confluence of the Danube River, a region in which the plains and low hills generally have high humidity and rainfall. There is a very high prevalence in endemic areas, with rates ranging between approximately 0.5% to 4.4%, increasing to as high as 20% if the disorder is suspected and carefully screened for among an at-risk population. A striking observation is that nearly all affected patients are farmers.

Pathogenesis and Pathology

Although the etiology of BEN is unknown, many environmental and genetic factors have been evaluated as possible underlying causes.^[256]

Environmental Factors

Given that it is endemic to a specific geographic area, toxins, and/or environmental exposures that are unique to the Balkans have been investigated. However, no agent and/or general group of compounds or organisms, including trace elements (lead, cadmium, silica, selenium), viruses, fungus, and/or plant toxin, has yet been successfully identified. One intriguing possibility is that AA, a mutagenic and nephrotoxic alkaloid found in the plant Aristolochia clematis, may underlie both Chinese herbal nephropathy and BEN. Striking pathologic and clinical similarities exist between the progressive interstitial fibrosis observed in young women who have been on a slimming regimen including Chinese herbs (as well as other agents) and BEN, but this putative association between BEN and AA remains speculative.^[257] Two other environmental causes have to be considered: The mycotoxin hypothesis that considers that BEN is produced by achratoxin A and the pliocene lignite hypothesis, which proposes that the disease is caused by long-term exposure to polycyclic aromatic hydrocarbons and other toxic organic compounds leaching into the well drinking water from low-rank coals in the vicinity to the endemic settlements.^[258]

Genetic Factors

Support for a genetic etiology includes observations that the disease clearly affects particular families and that some ethnic populations who have lived in the areas for generations do not suffer from BEN. The exact mode of inheritance has not yet been established, and possible causative gene(s) have not been identified, but a locus in the region between 3q25 and 3q26 has been incriminated.^[259] By contrast, some observations are inconsistent with a genetic basis. First, BEN is observed in individuals who have immigrated into the Balkan area from regions without the disorder and in previously unaffected families who have lived for at least 15 years in endemic areas. Second, BEN does not develop in members from previously affected families who left endemic areas early in life or who spent less than 15 years in these areas.^[260] A unifying hypothesis may be that the disease most likely occurs in genetically predisposed individuals who are chronically exposed to a causative, as-yet-unidentified agent. In the early stages of the disease, renal histology reveals cortical focal tubular atrophy, interstitial edema, and peritubuloglomerular sclerosis with limited mononuclear cell infiltration. Narrowing and endothelial swelling of interstitial capillaries (e.g., capillarosclerosis) is also described. In advanced cases, marked tubular atrophy and interstitial fibrosis develop along with focal segmental glomerular changes and global sclerosis. There is an extremely high incidence of cellular atypia and urothelial carcinoma of the genitourinary tract.^[261]

Clinical Features

BEN is a slowly progressive tubulointerstitial disease that may culminate in ESRD. Clinical manifestations first appear in patients 30 and 50 years of age, with findings prior to the age of 20 being extremely rare. One of the first signs is tubular dysfunction, which is characterized by an increased excretion of low-molecular-weight proteins. Early tubular injury can also lead to renal glycosuria, aminoaciduria, and diminished ability to handle an acid (NH₄Cl) load.

Over more than 20 years, there is a progressive decrease in concentrating ability and in the GFR. Patients are usually without edema and are normotensive, hypertension only developing with end-stage disease. A normochromic normocytic anemia occurs with early disease, which becomes increasingly pronounced as the disorder progresses. Urinary tract infection is rarely observed. Kidneys are of normal size early in the course of the disease. A symmetrical reduction of kidney size with a smooth outline and normal pelvicaliceal system is subsequently observed in patients with late-stage disease. Intrarenal calcifications are not observed. BEN is also associated with the development of transitional cell carcinoma of the renal pelvis or ureter, with studies noting a wide range in incidence (2% to nearly 50%). These tumors are generally superficial and slow growing.

Diagnosis and Treatment

The diagnosis of BEN is based upon the presence of some combination of the following findings: symmetrically shrunken kidneys with absence of intrarenal calcifications ( Fig. 33-6 ); farmers living in the endangered villages; familial history positive for BEN; mild tubular proteinuria, hypostenuria, and glucosuria; normochromic hypochromic anemia occurring in patients with only slightly impaired renal function.^[262] As with many other chronic tubulointerstitial diseases of unclear origin, there is no specific prevention or treatment. Therapy is, therefore, largely supportive, with renal replacement therapy being initiated in patients with ESRD. The high incidence of cellular atypia of the genitourinary tract suggests that regular surveillance should be performed for abnormal urinary cytologies. Whether bilateral native nephroureterectomies are required, particularly in those undergoing renal transplantation, is unclear.

FIGURE 33-6 CT scan without contrast media of a patient with Balkan endemic nephropathy, creatinine clearance of 15 mL/min, no hypertension, proteinuria less than 1 g/24 hr. Note the important bilateral atrophy of the kidneys and absence of intrarenal calcifications.

Hyperuricemia/Hyperuricosuria

Three different types of renal disease are induced by abnormal uric acid metabolism: acute uric acid nephropathy (which is not discussed here); chronic urate nephropathy; and uric acid stone disease. The kidneys are the major organs for the excretion of uric acid and a primary target organ affected in disorders of urate metabolism. Renal lesions result from crystallization of uric acid either in the urine outflow tract or in the renal parenchyma. The determinants of uric acid solubility are its concentration and the pH of the medium in which it is dissolved. Consequently, the supersaturation of the tubular fluid within the renal tubules as excreted uric acid becomes concentrated in the medulla and the acidification of the urine in the distal tubule are both conducive to the precipitation of uric acid. The major sites of urate deposition are the renal medulla, the collecting tubules, and the urinary tract. The pKa of uric acid is 5.4, and at the acid pH of the fluid in the distal tubule the bulk of filtered urate will be present in its nonionized form as uric acid, whereas at the more alkaline pH of the blood and interstitium, it is in its ionized form as urate salts.

Chronic Urate Nephropathy

The principle lesion of the chronic hyperuricemia is the deposition of microtophi of amorphous urate crystals in the interstitium, with a surrounding giant-cell reaction. This results in a secondary chronic inflammatory response similar to that seen with microtophus formation elsewhere in the body, potentially leading to interstitial fibrosis and CRF. Evidence linking CRF to gout is weak, and the long-standing notion that chronic renal disease is common in patients with hyperuricemia has been questioned in the light of prolonged follow-up studies of renal function in people with this condition. Renal dysfunction could be documented only when the serum urate concentration was more than 10 mg/dL (600 mmol/L) in women and more than 13 mg/dL (780 mmol/L) in men for prolonged periods. Furthermore, the deterioration of renal function in those with hyperuricemia of a lower magnitude has been attributed to the higher-than-expected occurrence of hypertension, diabetes mellitus, abnormal lipid metabolism, and nephrosclerosis.^[263] Nonetheless, it seems reasonable to prescribe allopurinol (in a dose appropriate to the level of renal function) to those very rare patients with biopsy evidence of “gouty nephropathy,” and possibly, to patients with CRF who have a grossly elevated serum urate.^[264] There is an association between severe lead intoxication, CRF, and gout (saturnine gout). It has also been suggested that there might be an association between renal disease and hyperuricemia in those with a past history of exposure to lead and consequent subclinical lead toxicity (saturnine nephropathy). Evidence for this association is not clear-cut, nor is the mechanism whereby lead exposure might aggravate hyperuricemia and renal failure. Recent evidence describing uric acid as an independent risk factor for cardiovascular death and major clinical events^[265] and even for its role in the development of hypertension and in the progression of renal failure came from Johnson's group.^[265] They showed that hyperuricemia induces endothelial dysfunction and that uric acid regulates critical proinflammatory pathways in vascular smooth muscle cells, possibly having a role in the vascular changes associated with hypertension and vascular disease. ^{[267] [268]}Recent studies have reported that mild hyperuricemia in normal rats induced by the uricase inhibitor oxonic acid results in hypertension, intrarenal vascular disease, and renal injury. They investigated the hypothesis that uric acid may contribute to progressive renal failure. Hyperuricemia accelerates renal progression in the remnant kidney model via a mechanism linked to high systemic blood pressure and COX-2–mediated, thromboxane-induced vascular disease. These studies provide direct evidence that uric acid may be a true mediator of renal disease and progression.^[268] It seems to be time to reevaluate the role of uric acid as a risk factor for cardiovascular-renal diseases and hypertension and to design human studies to address this controversy.

Chronic Hypokalemia

The cause of potassium depletion is malnutrition (anorexia nervosa, vomiting, and/or abuse of laxatives and/or diuretics). Several renal abnormalities, most of which are reversible with potassium repletion, can be induced by hypokalemia.^[269] Vasopressin-resistant impairment of the ability to concentrate the urine, increased renal ammonia production (linked to intrarenal complement activation), enhanced bicarbonate reabsorption, altered sodium reabsorption, and hyperkalemia nephropathy have been described. Persistent hypokalemia can induce a variety of changes in renal function, impairing tubular transport, and possibly inducing chronic tubulointerstitial disease and cyst formation.^[270] Hypokalemic nephropathy in humans produces characteristic vacuolar lesions in the epithelial cells of the proximal tubule and, occasionally, the distal tubule. More severe changes, including interstitial fibrosis, tubular atrophy, and cyst formation that is most prominent in the renal medulla, occur if prolonged hypokalemia is maintained. Renal cyst formation as a consequence of chronic hypokalemia has now been reported in a patient with Bartter's syndrome resulting from a gene defect in CLC-KC.^[271] Renal growth accelerates when rats are placed on a potassium-deficient diet, and within 8 days, there is a 25% increase in kidney mass.^[272] The changes are most prominent in the outer medulla, especially the inner stripe, where hyperplastic, enlarged collecting duct cells form cellular outgrowths that project into the lumen, causing partial obstruction. If the potassium-deficient state persists, then cellular infiltrates appear in the renal interstitial compartment and tubulointerstitial fibrosis develops. It has been proposed that some pathologic changes may be initiated by the high levels of ammonia generated in potassium deficiency and may be mediated through the activation of the alternate complement pathway . In support of this thesis is the finding that bicarbonate supplementation sufficient to suppress renal ammoniagenesis attenuates the renal enlargement and tubulointerstitial disease; against it are reports that increased renal ammoniagenesis induced by acid loading causes renal enlargement without cellular proliferation or interstitial disease.^[273] A recent paper provides results consistent with a sustained role for IGF-1 in promoting the marked tubular epithelial cell hypertrophy and hyperplasia that occur in the inner stripe of the outer medulla of the kidney in chronic potassium depletion.^[274] The same study also showed that potassium depletion causes a selective increase in the renal expression of TGF-β in the hypertrophied, nonhyperplastic, thick ascending limb, but—unlike IGF-1—it is absent from the hyperplastic collecting duct cells. This might be responsible for preventing the conversion of the mitogenic stimulus of IGF-1 into a hypertrophic one. It is possible that TGF-β causes the prominent interstitial infiltrate that develops in chronic hypokalemia, because this “growth factor” is a well-known chemoattractant for macrophages. A recent study by Suga and co-workers^[275] showed that AT1-receptor blockade ameliorates tubulointerstitial injury induced by chronic potassium deficiency. The same authors showed that endothelin-1 can mediate hypokalemic renal injury in two different ways: by directly stimulating endothelin-A receptors and by locally promoting endogenous endothelin-1 production via endothelin-B receptors. Hence, endothelin-A and -B receptor blockade may be renoprotective in hypokalemic nephropathy.^[276]

Sarcoidosis

Sarcoidosis is a multisystem disorder of unknown etiology characterized by the accumulation in many tissues of T lymphocytes, mononuclear phagocytes, and noncaseating granulomas. Recently, indigenous Propionibacterium acnes seems to play a role in the pathogenesis of the formation of granulomatous lesions in the lungs.^[277] Clinically important renal involvement is an occasional problem—hypercalciuria and hypercalcemia are most often responsible, although granulomatous interstitial disease, glomerular disease, obstructive uropathy, and rarely, ESRD may also occur.^[278] Most affected patients have clear evidence of diffuse active sarcoidosis, although some patients present with an isolated elevation in the plasma creatinine concentration and no or only minimal extrarenal manifestations. ^{[280] [281]} The true incidence of renal involvement in sarcoidosis remains unknown, but several small series of renal biopsies suggest that some degree of renal involvement occurs in approximately 35% of patients with sarcoidosis.

Clinical Features

Hypercalciuria, hypercalcemia, nephrolithiasis, granulomatous interstitial nephritis, glomerular disease, and urinary tract disorders can all be observed in patients with sarcoidosis. Macrophages in a sarcoid granuloma contain a 1α-hydroxylase enzyme, but not a 24-hydroxylase enzyme, capable of converting vitamin D to its active form. The resultant increase in the absorption of calcium from the gut, which occurs in up to 50% of those with sarcoidosis, leads to hypercalciuria and, in roughly 2.5% to 20% of cases, to hypercalcemia. Most patients remain asymptomatic, but nephrolithiasis, nephrocalcinosis, renal insufficiency, and polyuria are important complications.^[281] Nephrolithiasis occurs in approximately 1% to 14% of patients with sarcoidosis and may be the presenting feature. Nephrocalcinosis, observed in over half of those with renal insufficiency, is the most common cause of CRF in sarcoidosis.^[282] The increase in urine output associated with hypercalcemia and hypercalciuria is due to a reduced responsiveness to antidiuretic hormone.

An interstitial nephritis with granuloma formation is common in sarcoidosis, but the development of clinical disease manifested by renal insufficiency is unusual. A survey of all renal biopsies over a 6-year period at three general hospitals found clinically significant sarcoid granulomatous interstitial nephritis in only four cases. Most affected patients have clear evidence of diffuse active sarcoidosis, although some present with an isolated elevation in the plasma creatinine concentration and no or only minimal renal manifestations. Renal biopsy reveals normal glomeruli, interstitial infiltration mostly with mononuclear cells, noncaseating granulomas in the interstitium, tubular atrophy, and with more chronic disease, interstitial fibrosis.^[283] Granulomatosis interstitial nephritis is also seen in other diseases, including allergic interstitial nephritis (mainly drug-induced, caused by NSAIDs and 5-ASA), Wegener's granulomatosis, beryliosis, and tuberculosis. The urinary manifestations of granulomatous interstitial nephritis are relatively nonspecific comparable with other chronic tubulointerstitial diseases, being normal or showing only sterile pyuria or mild proteinuria.

Glomerular involvement is rare in sarcoidosis. A variety of different lesions have been described in isolated cases, including membranous nephropathy, a proliferative or crescentic glomerulonephritis, and focal glomerulosclerosis. The presence of heavy proteinuria or red cell casts tends to differentiate these glomerulopathies from interstitial nephritis. Occasionally, retroperitoneal lymph node involvement, retroperitoneal fibrosis, or renal stones may produce ureteral obstruction.

Diagnosis and Treatment

Sarcoid nephropathy should be considered in any patient with unexplained renal failure and hypercalcemia, renal tubular defect, nephrocalcinosis, or increased immunoglobulins. These patients often have signs and symptoms of pulmonary, ocular, and/or dermal involvement with sarcoidosis. The presence of granulomas on renal biopsy, although not specific to sarcoidosis, should strongly suggest this diagnosis in an appropriate setting. Granulomatous interstitial nephritis can be treated effectively with glucocorticoids, typically prednisolone 1 to 1.5 mg/kg initially, tapered off following signs and symptoms of disease activity. Patients often respond quickly with an improvement in renal function, but this depends greatly on the extent and severity of inflammation and fibrosis before treatment was initiated. There are no controlled trials regarding the dose or length of the treatment.

The hypercalcemic/hypercalciuric syndrome also responds quickly to corticosteroids: In general, the dose needed to treat this complication is significantly lower than that required to treat granulomatous interstitial nephritis and can be as low as 35 mg of prednisolone daily. Chloroquine, by decreas-ing the level of 1,25-dihydroxycholecalciferol, is an effec-tive therapy for the hypercalcemic/hypercalciuric syndrome. Ketoconazole, an inhibitor of steroidogenesis, has been used in a single patient who could not tolerate corticosteroids and was effective in decreasing the level of active vitamin D as well as serum and urinary calcium.^[284] A recent report describes a successful treatment of sarcoid nephritis with infliximab.^[280] Although uncommon in patients with sarcoidosis, ESRD requiring renal replacement therapy is most often due to hypercalcemic nephropathy rather than granulomatous nephritis. Graft loss due to disease recurrence has been reported exceptionally.

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