As stated earlier, persistence of oncoviruses is one of the requirements for cancer development. To prevent this, rapid clearance of oncoviruses by the host immune system is crucial. Immunosuppressed patients like HIV-infected are more prone to the development of oncoviral-associated cancers. This chapter will review latest data on the immune responses in the recognition and clearance of oncoviruses. The activities of the immune system are divided into 2 steps: innate and adaptive immune responses; with the innate response been non-specific and rapid while the adaptive response is specific which requires activation. Both are required for effective clearance of oncoviruses.
2.1 Innate Immunity response to Oncoviruses
2.1.1 Innate response: The innate immune system is the first line of defense against invading pathogens. It includes the physical barriers, humorals barriers, and cellular components. The epithelium forms the physical barrier against invading pathogens and is impeamble to most infectious agents. In addition, the epithelium secretes certain chemicals such as lysozyme and phospholipase to limit the invasion of pathogens. The humoral component of the innate immunity includes complement system, coagulation system, antimicrobial peptides, chemokines, and cytokines. The invasion of a pathogen result in the epithelium cells releasing cytokines and chemokines that activate the immune cells present in the epithelium such as Langerhans cells (LCs) found in the skin and recruit immune cells to clear infection. The innate immune response depends on pattern recognition receptor (PRR) to enable it recognize Pathogen Associated Molecular Patterns (PAMPs) of all class of pathogens. There are four families of receptor. These include Toll-like receptors (TLRs), nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), retinoic acid inducible gene-I (RIG-1)-like receptors (RLRs) as well as family of Pyhin. The PRRs are also involved in the sensing of endogenous ligands, referred to as Danger Associated Molecular Patterns (DAMPs) released by cells in non-infectious condition such as injury or cell death. These receptors are localized in the membrane (e.g. TLR) or in the cytosol (e.g. RLR, NLR). Their localization is associated with either the PAMP/DAMP they sense. TLRs specialized in virus recognition through sensing of nucleic acid located within the endosomes. Aside the TLRs, two families of cytosolic innate sensors have been described that are involved in virus recognition; 1. RNA helicase and 2. Cytosolic DNA receptors which includes DAI, AIM2, and IFI16/p204. The cytoplasmic RNA helicase, RIG-1, and melanoma differentiation associated genes (MDA5) and DDXI/DDX21/DHX36 complex sense RNA or FNA viruses that replicate in the cytosol through an RNA intermediate. NLRs including NOD 1/ 2 are involved in bacterial recognition and the NLRP family members that sense various PAMP and DAMP. The binding of these receptors with their cognate ligands results in the recruitment of members of interleukin-1 receptor-associated kinase (IRAK) and tumor necrosis factor receptor-associated factor (TRAF) families resulting in the activation of transcription factors such as NF-kB, interferon regulation factors (IRFs) and the mitogen activated protein kinase (MAP kinase). The activation of these pathways depends on various adaptors downstream of the different PRRs. Triggering PRRs results in induction of proinflammatory cytokines, type 1 IFN and chemokines that collectively participate in the establishment of innate and adaptive immune response. Type I IFN is one of the most important cytokines involved in viral infections. It comprises of 13 subtypes. Type 1IFN binds to α-IFN receptor 1 and 2 (IFNAR1 and -2) which leads to receptor dimerization. In the classical pathway, receptor bound Janus kinases (Jaks) and non-receptor tyrosine kinase 2 (Tyk2) are activated which cross phosphorylates each other. The activated Jaks phosphorylate IFNAR1 and 2 which serves as Src homology 2 domain docking sites transducers of activated transcription 1 and 2 (STAT1 and 2). These are phosphorylated by Jak1 and Tyk2. The phosphorylated STATs heterodimer interacts with IFN regulatory factor 9 (IRF9) to form the active transcriptional factor complex IFN-stimulated gene factor 3 (ISG3) (Fig 1). This regulates expression of IFN stimulated genes.
Figure 1: Type 1 IFN signal transduction (Source: Google image)
Type 1 IFNs are produced by most of human cells during viral infections. Aside their antiviral properties, IFN also have angiogenic, anti-proliferative, and immunostimulatory properties. Type 1 IFN modulates innate immune response by contributing to NK cells homeostasis and activation. In addition, they modulate adaptive immune response by inducing the phenotypical and functional maturation of immature DCs such as the up regulation of CD40, CD80, CD83, CD86, MHC Class 1 and II, and increased ability to stimulate T cell proliferation. Type I IFN allows the DCs to present endogenous antigen to CD8+ T cells which allow for cross-priming of CD8+. This process is important for clearance of viral infection and cancer treatment. The direct antiviral activities of type 1 IFN depends on three major IFN responses; the protein kinase K (PKR), Mx protein, and 2’5’ oligoadenylate synthetase (2’5’OAS). The PKR is activated by dsRNA, caspases 3, 7, or 8 or polyanionic molecules. The main function of PKR after activation is to phosphorylate elf-2e resulting in the blocking of protein translation. Furthermore, PKR also improves the induction of IFNβ and apoptosis induced by RLR as a result of measles viral infection. The Mx proteins are GTPase and involved in the binding to nucelocapsid of some viruses to alter their intercellular transport thereby interfering with the viral polymerase activities during viral transcription. The 2’5’ OAS on the other hand activates RNAse due to the presence of dsRNA in order to degrade viral and cellular RNAs resulting in the blockage of viral infection. Certain cytokines have been found to play essential role in innate immunity; IL1β and IL18. These cytokine are found in pro-form then cleaved into active form by caspase-1. A multiprotein complex referred to as inflammasome is formed after the activation of NLRP-1 and -3 and triggering of DNA sensor AIMS. IL-1β is a proinflammatory cytokines involved in a number of inflammatory activities such as fever, stimulation of hepatic acute phase protein, hyperalgesia, and increase of bone marrow cell number. IL-18 on the other hand is essential for priming of NK cells which are critical for killing of tumor cells lacking MHC Class 1 molecules. In addition these two cytokines also stimulate the innate immune response by activating neutrophils and macrophages which engulf the pathogen and release of reactive oxygen species (ROS). The response by IL-1 and IL-18 are essential for host defenses against viruses or in the induction of innate immunity against tumors. Any breach or defect in their response results in enhancing the pathogenesis of viral infection or development of tumors.
2.1.2 Toll like Receptors (TLRs) in Oncoviral Infection: TLRs are homologue to the Toll receptor that was first described in the fruit fly Drosophilia. The Toll receptor plays a role in the development of dorso-ventral body axis in Drosophilia, acting as cell surface receptor for cytokine ligands. TLRs are type I transmembrane (TM) proteins made of extra-cellular leucine-rich receptors and are pattern recognition receptors expressed by various immune cells such as macrophages which recognize PAMP. The activation of the innate immune cells by TLRs initiated downstream signaling that leads to the up-regulation of pro-inflammatory cytokines and chemokines leading to the recruitment of effector cells to the local site of infection. TLRs plays essential role in the induction of innate immunity. TLRs have been described as essential for adaptive immune responses. This is based on initial data which showed that molecules recognizes as TLR ligands such as LPS enhances adaptive immune response. However, there is no compelling evidence to show that there is adaptive immune failure in the absence of TLR signaling. Rather, robust adaptive immune responses including antibody production have been reported after infection in animals lacking TLR signaling. TLR signaling drives DC maturation, antigen presentation, and CD8+ T –cytotoxic effector function. These are important for efficient antitumor immunity. So far about 13 mammalian TLRs have been described. Each TLR contain extracellular domain with leucin-rich repeats (LLR) motif that comprises a binding domain for recognition of their respective pathogens. The ligands for each TLR are different; for e.g. TLR3, TLR7, TLR8, and TLR9 recognizes nucleic acid from bacteria and virus while TLR2 recognizes bacteria lipoprotein, lipotechoic acid and fungal zymosans. TLR8 is required to sense HIV and activate inflammasome while TLR7 senses HCV. However aberrant TLR expression or chronic stimulation can alter homeostasis which has negative regulation of antitumor immunity as shown by data which described increased immune suppression function such as enhanced regulatory T-cell proliferation. Chronic activation of TLRs promoted the process of carcinogenesis through pro-inflammatory response, anti-apoptotic and pro-fibrogenic signals in the tumor microenvironment and in the tumor cells. TLR signaling in addition to other inflammatory pathways play important roles in inflamed tumor microenvironment resulting in inflammation-drive disease progression as found in Helicobacter pyloriassociated gastric cancer. In HPV-associated cervical cancer for example, it has been suggested that TLRs plays significant role in the tumorigenesis process by modulating TLR expression and interfering with TLR signaling pathways leading to persistent viral infection and carcinogenesis. In a study by Zhang et al to investigate the expression of TLR8 and its relationship with Bcl-2 (regulators of apoptosis and autophagy) and VEGF (potent angiogenic factor in tumor angiogenesis) in cervical cancer reported of increased mRNA levels of TLR8 in cervical cancer cells as well as HeLa cells. In addition, there was positive correlation between the expression of TLR8, Bcl-2 and VEGF in cervical cancer. They therefore concluded that in patients with cervical cancer and HeLa cells, mRNA expression levels of TLR8 were up-regulated which was consistent with increased expression of Bcl-2 and VEGF. This study shows that high expression of TLR8 may strongly correlates with carcinogenesis and tumor invasion through the inhibition of TLR8 positive immune cells to recognize tumor or viral antigens that can influence anti-tumor immune response. More studies are needed to understand the role of this protein in cervical cancer-associated tumorigenesis and whether it can be used as a novel therapeutic target for HPV-associated cervical cancer. This is true with EBV-associated cancers where over expression of TLRs have been reported. The naïve B cell is the target of EBV infection in vivo. The human B-cell expresses cell surface TLR1,-2, and -6 in addition to endosomal TLR7,-9, and -10. Some studies showed that TLR6,-7,-9, and -10 are highly expressed more on memory B cell than on naïve B cell. Martin et al found that upon contact of EBV to B cell, a number of cell genes such as TLR7 and myeloid differentiation factor 88 (MyD88), a key component in TLR signal transduction are up-regulated. Another study reported of high expression of TLR3 in EBV-associated nasopharyngeal carcinoma through induction of inflammatory response. These data supports the suggestions that by hijacking the TLRs signaling pathway, oncoviruses could promote tumor growth through these pathway. A novel approach of inhibiting viral-associated tumors is utilizing the concept of TLR agonists. For example in HPV infection, approximately 15% of women that have high-risk HPV infection are not able to mount any effective immune response against HPV and persistence of high-risk HPV infection is one of the major factors in the development of cervical cancer. Slow clearance rate and lack of effective immune response means HPV is escaping immune detection. This means therapeutic interventions are needed to treat high-risk HPV infections. HPV infects Langerhans cells (LC) which are responsible for initiating an effective immune response against HPV. However, LC exposed to HPV does not induce specific T cell immune response. Utilizing TLR agonists would be effective in treating high-risk HPV infections. Fahey et al suggested that in HPV16 infection, TLR7 and -8 are expressed in human LC therefore using imidazoquinolines would activate LC exposed to HPV16 resulting in the induction of an HPV16-specific cell-mediated immune response. This author suggested in a systematic review that 5% imiquimod plus IM IFN can reduce the incidence of HPV-associated cervical cancer. Imidazoquinolines are TLR7 and/or -8 agonists making it potent innate immune modulators. More studies are needed to develop novel TLRs agonists due to the increasing cases of viral-associated cancers which are now global public health problem.
2.2 Inflammatory Response and Oncogenesis Process: The other arm of the immune response is the adaptive immune response which is a response of antigen-specific lymphocytes to antigen including the development of immunological memory. Adaptive immune responses are generated by clonal selection (an important paradigm in immunology) of lymphocytes. The adaptive immune response is mediated by B and T lymphocytes and data shows that adaptive immunity has some antitumorigenic effect. The major goal of any immune response is to eradicate pathogen mainly through the mechanism of inflammation. The details of adaptive immune response in relationship to oncoviral infections have been dealt with in a number of publications. This section will review the role of inflammation in the pathogenesis of oncoviral infections.
Does an inflammatory response have a role in cancer development? The data now shows that inflammatory condition is ideal for the development of cancer. The inflammatory response is a fundamental immune mechanism that involves a number of molecular and cellular components consisting of cytokines and chemokines that are released by proinflammatory cells. At the same time, some endogenous recruited components release anti-inhibitory mediators so that homeostasis is restored. The tools and strategies utilized by viruses to hijack the immune response is mostly associated with regulatory T-cells (Treg) that can inhibit inflammation and antiviral responses to other effector cells. Treg therefore appears to play dual role in cancer pathogenesis. In liver cancer, it is known that hepatocarcinogenesis is promoted by facilitated cellular turnover which is induced by chronic tissue damage and permanent cell regeneration as a result of chronic inflammation and sometimes after viral infections. After HBV and HCV infection, the human hepatocyte express and present the viral antigen to CD4+ T- and CD+ T-cells which result in the clearance of the virus by cytolytic and noncytolytic effector mechanisms. The CD8+ T cells play central role in the inhibition of viral replication mediated by cytokine and direct killing of infected hepatocyte while the CD4+ T cells activation may direct Th1 response with the secretion of IL2, TNFα, and IFN. Furthermore, the stimulation of CD4+ T cells may induce Th2 cells which secrete IL4, IL5, and IL10. A third cell Th17 has been described and associated with hepatic chronic inflammation following HBV and HCV infections. T17 are induced proinflammatory CD4+ T cells which secrete specific inflammatory cytokines such as IL17, IL21, Il22, IL6, and IL26. Further studies have shown that T17 exhibits either proinflammatory or protumor functions with high proportion reported in advanced tumor stages. A number of studies have reported that HCC patients exhibit specific inflammatory immune response and HBV-HCV-associated microenvironment is colonized by infiltrated inflammatory cells such as macrophage, NK, B, and T lymphocytes. From these data, the inflammatory products can be associated with viral hepatocarcinogenesis. Other reported the presence of these inflammatory cytokines in the activation of NFkB pathway. In most patients, the tumor progresses despite the mounted immune response. This strongly suggests that HCC escapes from the immune response probably as a result of HCC suppressive microenvironment which is associated with an essential role in tumor progression. In EBV-associated cancer, there is strong evidence linking EBV and nasopharyngeal carcinoma (NPC) and even Hodgkin’s lymphoma (HL) and both are associated with EBV latency II profile. IN NPC development, EBV does play crucial role but as to how and when, it is still not known. But we know that al NPC tumor cells bear EBV monoclonal viral gene. In addition, EBV is present in neoplastic cells of patients with HL which is characterized by the presence of malignant Hodgkin’s and Reed Sternbung (HRS) cells. Leukocytes infiltration is linked to tumor development and cancer progression. A number of studies explained that lack of efficiency of immune effector cells play a role in cancer pathogenesis process. One important feature of NPC is the presence of massive lymphoid infiltrate in primary tumor which might be supported by inflammatory cytokines produced by the malignant NPC cells. Studies have shown that most of the tumors infiltrating leucocytes (TIL) are CD3+ T cells but also CD4+ and CD8+ T-cells are also found in different population depending on the tumor specimen. Other cells such as NK, DC, IL1α, and CXCL10 have all been found in NPC cells. These findings support the argument that inflammatory responses are associated with tumor development and progression
2.3 Immune evasion by Oncoviruses: Viral infection triggers an array of immune response involving an early host response through the activation of PAMP as well as orchestrating an adaptive immunity. To allow infection, replication, and persistence, most oncoviruses employ a number of strategies to evade host immune response. Viral immune evasion range from modulation of cytokines and chemo-attractant expression to alteration of antigen presentation, and down-regulation of IFN-pathways and adherence molecules. A number of studies have shown the mechanisms employed by various oncoviruses to evade the immune system. Below is a description of some of the mechanisms employed by oncoviruses to evade host defense mechanism.
2.3.1 Interfering with interferon: Innate immune sensing of viral infection leads to the production of Type 1 IFN, especially IFN-α and IFN-β which are produced by cell types depending on the viral infection and promote an antiviral state in surrounding cells by the induction of IFN-stimulated expression. Oncoviruses develops mechanisms that results in the inhibition of IFN activities. In chronic HCV infection, Lee et al found that excess production of IFNα receptor 2a (IFNαR2a) is associated with interference of IFNα. This is in conformity with the findings that elevated levels of soluble IFNα2Ra in serum and urine was associated with hairy cell leukemia and adenocarcinoma which resulted in high resistance of IFN therapy. Another mechanism is the suppressing the function of TLR. A study by Hasan et al reported that infection of human epithelial cells with HPV16 promotes the function of an inhibitory transcriptional complex containing NF-kBp50-p65 and ERα induced by E7 oncoprotein which led to the downregulation of TLR9. Other mechanisms are alteration with pre-, post-entry and pre-integration events in which IFN play active roles. More studies are needed to elucidate the molecular and cellular mechanism of this interference.
2.3.2 Molecular mimicry (MM): MM is defined as the theoretical possibility that sequence similarities between pathogen and host peptide are sufficient to lead to cross-activation of auto reactive T and B cells by pathogen-derived peptides. This means molecular mimicry represents a shared immunologic epitope with a microbe and the host
With viral infection, studies have shown that it is sometimes associated with the initiation or exacerbation of autoimmune disease. Although the mechanism underlying this is still unclear, it is believed that one of the mechanisms is viral determinate mimicking host antigen resulting in triggering of self-reactive T cells clones to destroy host tissue. Molecular mimicry is associated with the induction of autoimmune diseases. One good example is in rheumatic fever where autoimmune disease can develop as a result of infection with group A beta-hemolytic streptococci infection. The mechanism underlying this has been described by Zabriskine and Cummingham. In a study Zhao et al described the role of molecular mimicry in HSV-1 infection. Their study showed that an epitope expressed by a coat protein of HSV-1 KOS strain was recognized by autoreactive T cells that targeted corneal antigens in a murine model of autocrine herpes stromal keratitis. The mutants HSV-1 viruses which did not express this epitope did not induce autoimmune disease. Thus, the researcher concluded that expression of molecular mimics can influence the development of autoimmune disease after viral infection. Data on molecular mimicry in oncoviral infection is scarce however few researchers have attempted to explore the role of molecular mimicry in relation to the pathogenesis of oncoviruses. It has been shown that Hepatitis B virus polymerase shared an immunologic epitope with myelin basic protein (MBP). When viral peptide was injected into rabbits, some of the animals developed an experimental autoimmune encephalomyelitis (EAE)-like disease, developed antibodies to MBP, and had T cell reactivity. Four virus proteins similar to two human macrophage inflammatory protein (MIP) chemokines, IL-6, and interferon regulatory protein (IRF) are encoded by KSHV genome. Similarly, in HIV-1 transmission, studies have shown that vMIP-1 is similar to human MIP chemokines in its ability to inhibit replication of HIV-1 strains which are dependent on the CCR co-receptor. This means that these genes might have a role in KSHV and HIV-1 interaction. Others have associated molecular mimicry to atherosclerosis. With our knowledge on molecular mimicry increasing it is highly essential for us to explore of their role in chronic infection especially in relationship to oncoviruses.
2.3.3 MHC Downregulation: T-cell-mediated response is very crucial in the defense against intracellular pathogens. The MHC class 1 molecule is a common target used by most oncoviruses to evade the immune system. The mechanism of action of MHC has been described earlier. For evasion of the immune system, KSHV encodes K3 and K5 zinc finger membrane protein that removes the MHC class 1 molecule from the cell surface thereby downregulating class 1 molecule. In EBV infection, the virus enters a state of latency in the B lymphocyte. This is characterized by EBNA-1 expression that is involved in the maintenance of viral DNA episome. The glycine-alanine repeat domain of EBNA-1 confers the virus with the ability of evading the immune system. This inhibits MHC class 1-restricted presentation of EBNA-1 epitopes linked in cis. When the EBV enters replicative phase of infection, about 80 genes are expressed which induces strong immune response but still the virus replicates. A number of studies have shown that this is due to the activities of several proteins which interfere with different stages of MHC class 1 and class II antigen presentation. These includes: inhibition of TAP by BNLF2a, which prevents peptide loading by MHC class 1, and inhibition of MHC class II antigen presentation by gp42/gH Ig L. These mechanisms confer EBV lytic proteins with the ability to interfere with CD4+ and CD8+ immune response thereby allowing viral replication.
2.3.4 Generation of escape mutants: Oncoviruses have the ability of evading the immune system by rapidly mutating. Viruses with RNA genome or RNA replicative intermediate utilizes low-fidelity polymerase to generate mutants that are antigenically difference resulting in immune system evasion. In HBV infection for example, replication takes place through reverse transcription of an RNA intermediate. The prospect of generating mutant viruses is high. Selective pressure to evade the host immune clearance readily selects out escape mutants. The cytotoxic T cells mediate the clearance of HBV from the liver and contribute to liver damage. Most of the acutely infected adults resolve all their clinical symptoms however mutation in the viral genes may result in lack of response to the viral antigens. With HCV, an RNA virus, the intrinsic infidelity of the HCV RNA polymerase generates many quasispecies that might be associated with immune evasion. In HTLV-1-associated HAM/TSP, the virus persist in the host despite a vigorous cellular and antibody response which suggest that the virus has developed effective mechanism to counteract the host immune surveillance. The open reading frame-1 (orf-1) has 2 products P12 and P8 that increase the activity of NFAT. Mutation in the orf-1 gene results in the development of mutant virus that is associated with immune evasion, viral replication, and persistence.
2.4 Advancing Oncoviruses-associated cancer immunotherapy with checkpoint immune inhibitors: Immunotherapy is a promising therapeutic intervention in cancer cases; with immune checkpoints inhibitors gaining interest in the therapeutic development for cancer treatment. Immune checkpoints refers to excess number of inhibitory pathways directly connected into the immune system and are crucially important for maintaining self-tolerance and attenuating excessive immune reaction which is important for maintaining homeostasis and minimizing collateral tissue damage. Immune checkpoint inhibitors are also known as co-inhibitory molecules or co-stimulatory molecules expressed on T-cells; thus immune checkpoints mediates either positive or negative signals that modify MHC-TCR signaling pathways. These signals each regulate T-cell survival, proliferation, differentiation, or responses to cognate antigens. Therefore the net effect is dependent on the balance among the signals. T-cell activation requires co-stimulatory signals. If the antigen get connected with the co-stimulatory signals on APC, T-cell remain in a state of anergy. The co-inhibitory molecules induce T-cell dysfunction or apoptosis. When the immune system utilizes this inhibitory pathway, it can attenuate excessive immune reactions and ensure self-tolerance. These function involves programme cell death protein-1 (PD-1), programme cell death-1 ligand1/2 (PD-L1/2), cytotoxic T lymphocytes antigen 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), T-cell immunoglobulin mucin-3 (TIM-3), and B and T lymphocytes attenuator (BTLA). Tumor cells can bring together these suppressing effects as one of their immunoediting mechanisms. A number of studies have shown that immune checkpoint inhibitor with monoclonal antibody promoted endogenous antitumor activities of immune cells. This section will review some of the checkpoint inhibitors available and the prospect of such inhibitors in oncoviral infection.
A number of immune checkpoints inhibitors have been developed which are in clinical trials. PD-1 and PD-Li inhibitors induces higher response rate across a wide range of tumors than other immunotherapy. It therefore provides a novel therapeutic intervention strategy in oncoviral infections. They have lower rate of high-grade toxicities, mainly immune-mediated side effects. In addition, it is less labor intense to administer than some other types of immunotherapy. It does not require personal preparation for each patient as in dendritic cell vaccine such as Provenge and Chimeric antigen receptor (CAM) T-cell therapy and result in a longer duration of response.
Ipilimumab is a humanized IgG1 monoclonal antibody that inhibits CTLA-4. A number of clinical studies have been undertaken to evaluate its effect in patients with different malignancies such as melanoma, non-Hodgkin lymphoma, renal cell carcinoma, and prostate cancer. In a phase I study to evaluate a single 3mg/kg dose of Ipilimumab in patients with metastatic hormone-refractory prostate cancer, it was shown that 2 (14%) of the 14 patients showed ≥50% decline in prostate specific antigen. A patient developed grade 3 rash which required systemic administration of corticosteroid. A phase II trial compared 3 doses: 0.3, 3, or 10 mg/kg administered every 3 weeks for a total of 4 doses. Eligible patients were allowed to receive reinduction or maintenance therapy. The overall response rate (ORR) in the 10mg/kg arm was superior to those in the other arms but immune-related adverse events (irAEs) were higher in the 10mg/kg arm. The US FDA approved the use of Ipilimumab after a phase III randomized controlled trials (RCTs) showed survival benefit. Another agent: Pembrolizumab, a humanized IgG-4k antibody that block PD-1. A phase 1 dose escalation study evaluated 3 doses level: 1, 3, and 10mg/kg which were administered every 2 weeks in patients with multiple solid tumors. All dose levels were found safe but the maximum tolerated dose was not identified. Clinical response was observed at all dose levels. In 2014, Pembrolizumab received approval for the treatment of patients with advanced melanoma by the FDA after reviewing the drug under its Accelerated Approval program. Other immune checkpoints inhibitors being evaluated include BMS 936559 for a number of cancers such as melanoma and ovarian cancer, MPDL3280A as monotherapy for advance melanoma, and Nivolumab for melanoma and non-small cell carcinoma of the lungs (NSCLC), etc. There are no data on evaluating any immune checkpoint inhibitors in tumor virology but taking into consideration that most of these oncoviruses are known to use signaling pathway to in the course of the pathogenesis, for e.g. in chronic HIV infection, molecule of B7:CD28 family PD-1, CTLA-4 and their ligands play active and reversible role in virus-specific T-cell exhaustion associated with HIV infection in human and SIV models in macaques. CTLA-4 was found to be moderately over expressed in CD4 population with progressive HIV infection and its expression was inversely correlated with CD4 count. CTLA-4 was also reported to be strongly expressed in HIV-specific CD 4 T cells at the time of acute HIV infection. A study reported that in HIV infected subjects at different stages of HIV infection, CTLA-4 was upregulated on HIV-specific CD4 in all categories of HIV-infected individuals, with the exception of controllers who controlled viremia in the absence of ART therapy. With PD-L1, its role in CTL exhaustion was shown initially in murine LCMV. A study by Barber et al found that PA-1 was expressed on early effector CD8 T cells after infection with both a LCMV strain that leads to chronic infection and persistent viremia. It was reported that in HIV infection, high level of PD-1 and its ligand PD-L1 and L2 are expressed which are found on hematopoietic and non hematopoietic cells. All these studies highlight the importance of these pathways in the HIV pathogenesis. This means inhibiting PD-L1 expression by a specific drug will be a novel strategy to manage chronic HIV infection and HIV-associated cancer. However data shows that many cancer patients do not respond to therapeutic immune checkpoints intervention because of lack of tumor-infiltrating effector T-cells. Therefore the novel option in this regard is combinational intervention with vaccine. Cancer vaccine may prepare patients for treatment with checkpoint inhibitors by inducing effector T-cell infiltration into the tumor and checkpoint signals. Therefore the combination of cancer vaccine and an immune checkpoint inhibitor may function simultaneously to induce more effective antitumor responses. Others have however proposed combinational therapy involving two or more different checkpoint inhibitors such as PD-1 plus PD-L1 which is being tried by Astra Zeneca. The rationale for this is that while both therapies block the interaction between PD-1 and PD-L1, PD-1 inhibitors additionally block the interaction between PD-1 and PD-L2, another ligand of the receptor, while PD-L1 inhibitor additionally block the interaction between PD-L1 and B7.1 ( also a ligand for CTLA-4). Analysis in pre-clinical models showed that this combination strategy has some synergy.
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