Lupus is brought on when predisposing genetic factors activated by environmental factors, drugs, or infectious agents result in an abnormal immune response. This occurs when suppressor T cells fail to suppress, there are defects in cell signaling, there are defects in immune tolerance, apoptotic cells promote the creation of autoantibodies, and/or there is loss of regulatory cells that control autoreactivity. The latter results in the proliferation of B cells, leading to the formation of autoantibodies and immune complexes, which promote inflammation and tissue damage. Pathogenesis undergoes an often-gradual process consisting of several phases: predisposition, benign autoimmunity, prodrome, and clinical systemic lupus erythematosus. Only one person in 10 who possess lupus susceptibility genes ever develops full-blown lupus. Many individuals have “subclinical autoimmunity,” or undifferentiated connective tissue disorders where the process is attenuated.
Phase 1: Predisposition
Genetic Predisposing Factors
Unlike with cystic fibrosis, for example, there is no single lupus gene. It is a polygenic disorder. At least 30 susceptibility genes for SLE have been identified, and their presence varies widely depending on race, ethnicity, and geography.1 Genome scanning has shown that at least eight different chromosomal regions (especially on chromosome 1) contain susceptibility genes, especially in the presence of SSA or SSB. The risk for SLE is tenfold increased in monozy-gotic (identical) twins compared to dizygotic ones, and eight- to twentyfold in siblings of SLE patients compared to the healthy population. Most of the lupus-associated genes have odds ratios (relative risks) of less than 2.5 (1 would indicate no predisposition), and they are only of minimal clinical value. They predispose to autoimmunity because they determine which peptides can be presented to T-lymphocytes to activate help for autoantibody production and T-mediated immune responses. A handful of non-human leukocyte antigen (HLA) genes (e.g., C1q deficiency, DNAse, Trex1 mutations) have an odds ratio of 2.5 or greater, but are infrequently found. Other genes may be protective of lupus (e.g., a Toll-like receptor 5 [TLR5] polymorphism).
Epigenetics
Epigenetics refers to inherited or acquired modification of DNA without any changes in the DNA base sequence. These alterations occur via three mechanisms: DNA methylation, histone deacetylation, and microRNA. SLE is associated with hypomethylation, which leads to upregulated expression of surface molecules and T cell autoreactivity. Lupus-inducing drugs, ultraviolet light, and microRNA can promote hypomethylation. Alterations in histones conforma-tionally influence DNA transcription and repair. Deacetylation promotes au-toimmunity and alters DNA signaling. Agents that interfere with this pathway are in clinical trials for a variety of autoimmune disorders. MicroRNA (miRNA) are non-coding small RNAs (19–25 nucleotides in length) “gene silencing” sequences that regulate gene expression at post-transcriptional levels. Over 1000 have been described in humans.
The Influence of Gender
Given that 90% of lupus patients are female, hormones certainly play a role. Estrogens are thought to be permissive for autoimmunity, and androgens protective (although males with lupus usually have more severe disease). Estradiol prolongs the life of autoreactive Band T-lymphocytes. After childbirth, women may be exposed to a graft-versus-host reaction from their fetuses (microchime-rism), and their inactive X chromosome is enriched with hypomethylated genes (which can promote autoimmunity). A woman with lupus has a 2% risk of her son and a 10% risk of her daughter having lupus.
The Influence of the Environment
Ultraviolet light from the sun alters the structure of the dermis, which renders it more immunogenic, and kills skin cells, which induces apoptosis in kerati-nocytes (skin cells) and the formation of self antigen (Fig. 4.1). As only 28% of monozygotic twins both have SLE, environmental factors clearly play a role. Noninfectious agents associated with lupus include silica exposure; tobacco smoke; and possibly certain hair dyes, pesticides, allergens, foods, heavy metals, and solvents. Other than silica dust exposure (e.g., sandblasters, uranium mines) predisposing one to SLE, no other environmental agent, vocation, or other exposures have been proven to induce lupus.
Emotional stress clearly alters the immune system, and lupus has been infrequently temporally associated with routine vaccinations.
The Role of Infections
Lupus is often reported after a patient is subjected to an infectious process. Bacterial DNA serves as an adjuvant that can induce immune reactivity, and lupus patients have increased antibodies to retroviruses and viruses in the Epstein-Barr family. Molecular mimicry is thought to play a role in the process (e.g., a sequence of the Ro particle is similar to the Epstein-Barr nuclear antigen 1). There have been instances of lupus being “turned on” after a parvovirus B19 infection as well as other microbes. Infections can also cause a flare-up of preexisting lupus.
The Role of Drugs
Pharmaceuticals have multifaceted actions ranging from exacerbating or aggravating an immune process, to inducing lupus. For example, a drug can alter DNA or render it immunogenic and lead to the production of autoantibodies (e.g., procainamide). Other agents promote autoreactive Tor B-lymphocytes (e.g., phenytoin). Hypomethylation of DNA results in altered DNA repair and autoantibody formation (e.g., fludarabine). Oxidized metabolites of certain agents in slow acetylators, for example, can induce an immune reaction (e.g., hydralazine, isonaizid). Certain drugs are sun-sensitizing (e.g., nonarylamine sulfa antibiotics, phenothiazines) and lead to a phototoxic inflammatory response. This sequence is summarized in Figure 4.2.
Figure 4.1 The role of ultraviolet light in lupus. Key: ICAM, intracellular adhesion molecule; IL, interleukin; TNF, tumor necrosis factor; UVA, ultraviolet A; UVB, ultraviolet B; VCAM, vascular cell adhesion molecule. Source: Wallace DJ, Hahn BH. Dubois’ Lupus Erythematosus, 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2007. Reprinted with permission.
Phase 2: Benign Autoimmunity
A study of U.S. Army recruits showed that lupus autoantibodies are present for up to nine years in as many as 85% of patients before clinical lupus is evident.2
The development of autoantibodies precedes the first symptoms of SLE by two to nine years. Antinuclear antibodies first form, followed by anti DNA, antiphospholipid antibodies, and finally antibodies to Sm and RNP. These autoantibodies are self-perpetuating where amino acid sequences are T cell determinants and peptides activate helper T cells, and ultimately antibodies are formed. Antigen-antibody combinations (immune complexes) become bound by complement receptors and Fc Receptorscyto (FcR) receptors on immuno-globulins and become fixed in tissue where inflammation ensues.
Phase 3: Prodrome—the Innate Immune System and Loss of Tolerance in the Adaptive Immune System
At some point, the body’s regulatory and tolerance systems are overwhelmed and patients begin to develop malaise and fatigue.3 Two major immune networks contribute to the generation of autoreactive T cells and autoantibodies in SLE: the innate immune system and the adaptive immune network.4 Infections, self-antigens, and other danger signals activate the immune system via dendritic cells located in tissues that sample the environment (Fig. 4.3). Toll-like receptors (TLR) in dendritic cells recognize molecular patterns in bacteria and viruses. Cytosine phosphate Guianine (CpG) DNA sequences, which are common in bacterial DNA but uncommon in mammalian DNA, are bound by TLR7 and TLR9 in dendritic cells and B lymphocytes. Lupus dendritic cells may be activated, because many autoantigens have similar molecular patterns to microbial DNA. Binding of TLR7 and TLR9 in plasmacytic dendritic cells results in the release of a-interferon and other cytokines. It has been shown that lupus cells and tissue have increased expression of a-interferon (the “interferon signature”).
Figure 4.2 Factors promoting the development of systemic lupus erythematosus. Source: Wallace DJ. The Lupus Book, 5th ed. New York: Oxford University Press; 2012. Used with permission.
Normal cells die via a mechanism known as apoptosis. Sometimes, debris from these dying cells becomes antigenic itself (e.g., nucleosomes, Ro in surface blebs, phosphatidyl serine in the outer cell membranes) which under the influence of oxidation, microorganisms, phosphorylation, and cleavage are processed by antigen-presenting cells. In the innate immune system, they are activated by DNA and RNA proteins complexed with TLR Toll via a process known as NETosis (neutrophil extracellular traps), which traps them and activates dendritic cells, cytokines, and interferon. The consequence of this is that effector T cells activate B cells (including plasmablasts) and form autoantibodies that deposit immunoglobulin and fix complement in tissues and promote inflammation. A similar pathway is initiated in adaptive immune responses.
Figure 4.3 Arbuckle, Development of Pathogenic Immunity, NEJM2
The adaptive immune body’s system of tolerance is lost via failed elimination of antigenic reactive lymphocytes in the spleen, lymph nodes, and bone marrow through inappropriate clonal deletion, receptor editing, or passive transfer. These mechanisms inhibit, delete, suppress, or ignore, which results in autoreactive cells that, when left untreated, can “break through.” The persistence of debris from damaged cells (apoptosis) further leads to autoantibody formation. The deposition of immune complexes and failure to adequately clear them produces damage from the activation of complement and other mediators of inflammation (e.g., chemotaxis for lymphocytes and phagocytic cells, cytokines, chemokines, proteolytic enzymes; see Fig. 4.4).
Phase 4: Sustaining Clinical Lupus—the Adaptive Immune System and Loss of Immune Regulation
Immune complexes and apoptotic cells circulate in the bloodstream and need to be disposed of so they do not settle in tissues (which causes inflammation) or release chemicals (e.g., cytokines, chemokines) which also promote inflammation. In SLE, this clearance fails due to a variety of mechanisms: defective phagocytosis, altered transport by complement receptors, defective regulation of T helper cells by regulatory T cells, inadequate production or function of regulatory cells that kill or suppress autoreactive B cells, low production of interleukin 2 by T cells, and defects in apoptosis that permit the survival of effector T and autoreactive B cells (Fig. 4.5). When activated, T and B cells produce cytokines and autoantibodies. When underactivated, cells fail to undergo apoptosis, and B and T cells become autoreactive. Both phenomena occur in lupus. Defects in immune tolerance permit prolonged survival of B and T cells, which leads to activated B cells, memory B cells, and plasma cell formation, and ultimately autoreactive B cells. These are further influenced by B cell surface antigen receptors, soluble BlyS (B lymphocyte stimulation—which is what is blocked by the drug belimumab), genetic polymorphisms (variations) affecting B cell receptor signaling, and the intracellular mobilization of calcium. Regulatory T cells (T reg) suppress inflammation, and their function is diminished in lupus. T cell receptor activation is altered, and T reg mechanisms fail, which allow the activation of the pro-inflammatory cytokine interleukin-17. When there is a defect in T suppressor apoptosis, autoreactive B cells form.
Figure 4.4 Innate immune response in systemic lupus erythematosus (SLE). Key: DC, dendritic cells; IFNα, interferon alpha; IL, interleukin; ICAM, intracellular adhesion molecule; MHC, major histocompatibility complex; TLR, Toll-like receptor. Reprinted with permission.
Phase 5: Tissue Damage
Tissue damage is produced by the deposition of circulating immune complexes into tissue, which in turn activates endothelial cells, cytokines, and chemokines. In the kidneys, this produces inflammation, followed by proliferation and ultimately fibrosis (scarring). Complement activation, overloading of the Complement Receptor 1 (CR1) transport system, antibodies to complement components (anti Complement 1 qC1q), and congenital or acquired deficiency in complement components also lead to tissue inflammation and damage. Lupus is also characterized by accelerated atherosclerosis. This results from circulating immune complexes and complement split products activating endothelial cells in coronary arteries, which leads to the release of chemokines, cytokines, and activated monocytes. A nidus of plaque forms, which, in combination with oxidized LDL (bad cholesterol), forms “foam” cells that produce damage to coronary arteries. Chronicity of this process results in tissue and organ damage (see Fig. 4.6).
Figure 4.5 A model of the pathogenesis of systemic lupus erythematosus that implicates the products of disease-associated polymorphic genes. Crow, A Model of the Pathogenesis of SLE, NEJM5
Figure 4.6 Adaptive immunity in systemic lupus erythematosus (SLE). Key. APC, anaphase-promoting complex; Ag, antigen. Reprinted with permission.
Table 4.1 Explanations for the development of systemic lupus erythematosus in genetically predisposed individuals with autoantibodies
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1. Critical “dose” of susceptibility genes 2. Environment: infection, ultraviolet light, drugs, chemicals, hormones, stress 3. Antigens: alteration to more immunogenic forms, sustained exposure to high quantities of apoptotic cells, molecular mimicry, autoantigen-induced epitope spreading 4. Band T-cell abnormalities: activation with less antigen, more reactivity, abnormal receptor engagement, resistance to apoptosis, cytokine interactions 5. Pathogenic autoantibody subsets: based on charge, avidity, idiotypes, persistence of immune complexes 6. Targeted tissues more permissive to antibodies |
Summary
When individuals with susceptibility genes are exposed to environmental factors, chemicals, certain microbes, or drugs, a milieu is created wherein the regulatory and suppressor T cells are overwhelmed and malfunction. Defects in tolerance, cell signaling, and apoptosis result in an increase in B cells and, ultimately, autoantibody formation. The inability of the reticuloendothelial system to adequately clear circulating immune complexes leads to further inflammation and the deposition of these complexes into tissue5,6 (Table 4.1).
References
1. Croker JA, Kimberly RA. Genetics of susceptibility and severity in systemic lupus erythematosus. Curr Opin Rheumatol. 2005;17:529–537.
2. Arbuckle MR, McClain MT, Rubertone MB. Development of autoantibodies before the clinical onset of systemic lupus erythematosus. N Engl J Med. 2003;349:1526–1533.
3. Hahn BB, Ebling F, Singh RR, et al. Cellular and molecular mechanisms of autoantibody production in lupus. Ann N Y Acad Sci. 2005. 115.40–417.
4. Christensen SR, Shlomchik MJ. Regulation of lupus-related autoantibody production and clinical disease by Toll-like receptors. Semin Immunol. 2007;19:11–23.
5. Crow MK. Collaboration, genetic associations and lupus erythematosus. N Engl J Med. 2008;358:956–961.
6. Tsokos GC. Systemic lupus erythematosus. N Engl J Med. 365;2110–2011.