Apoptosis, or programmed cell death, is a key event in biologic homeostasis but is also involved in the pathogenesis of many human diseases including human immunodeficiency virus (HIV) infection. HIV infection usually leads to progressive decline in functionality and number of CD4+ T lymphocytes, resulting in AIDS development. Not only does the HIV genome encode pro-apoptotic proteins, which kill both infected and uninfected lymphocytes through either members of the TNF (Tumor-Necrosis Factor) family or the mitochondrial pathway, but it also creates a state of chronic immune activation that is responsible for the exacerbation of physiological mechanisms of clonal deletion (Ref.1).HIV directly modulates cell death using various strategies in which several viral proteins, in particular the envelope glycoproteins (Env), play an essential role. Importantly, Env, expressed on infected cells, triggers autophagy in uninfected CD4 T cells, leading to their apoptosis (Ref.2).
The adaptive immune response against HIV1 and other pathogenic viruses requires members of MHC-I (Major Histocompatibility Complex Class-I) to present viral antigens on the surface of infected cells. HIV1 uses its multifunctional early gene products to attenuate MHC-I cell surface expression by endocytosis of MHC-I molecules from the plasma membrane to the trans-Golgi network (Ref.3). HIV-infection activates the two main pathways of cell death in lymphocytes: AICD (Activation-Induced Cell Death) through the TNF family of death receptors and ACAD (Activated T-Cell Autonomous Death) through BCL2 (B-Cell CLL/Lymphoma-2)-related proteins and mitochondrion. The intensity of cell death is correlated with the pathogenicity of the infection and with disease progression and it is closely linked to HIV-driven generalized immune activation. Among the HIV1 proteins that have been implicated in regulating apoptosis are GP120 (Glycoprotein), Vpr, Nef, and Tat. These HIV-encoded products are involved in the control of cell death in the host, either by triggering the death receptor pathways— upregulation of Fas(CD95)–FasL (Fas Ligand, also known as CD95L) and TNFR–TNF/TRAIL (TNF-Related Apoptosis-Inducing Ligand) pathways or by acting through the mitochondrial pathway— phosphorylation of p53, upregulation of expression of BAX (BCL2-Associated X protein), dissipation of mitochondrial transmembrane potential and Cyto C (Cytochrome-C) release, and activation of caspases. Tat is essential for viral replication because of its function in transactivating the viral LTR (Long Terminal Repeat) (Ref.4). Mechanisms by which Tat has been reported to induce apoptosis include up-regulation of the apoptosis effector molecule FasL,inhibition of expression of manganese-dependent superoxide dismutase,and activation of cyclin-dependent kinases (Ref.5). Vpr, in addition to transporting the preintegration virus RNA complex, also serve to activate IKK (I-KappaB Kinases), NIK (NF-KappaB-Inducing Kinase) and transcription factors such as NF-kappaB (Nuclear Factor-kappaB) complexed with inhibitory I-KappaB (Inhibitor of Kappa Light Chain Gene Enhancer in B-Cells) proteins and allow for an initial burst of HIV transcription (Ref.6). Phosphorylation of I-kappaBs leads to their dissociation and degradation from active NF-kappaB. Nef down regulates cell surface expression of the primary HIV1 receptor—CD4—by increasing endocytosis of cell surface CD4 and delivering the receptor to lysosomes, where it is degraded. It assembles with and inhibits the enzymatic activity of ASK1 (Apoptosis-Signal Regulating Kinase-1). This kinase plays a central role in death signaling initiated through both the human Fas and TNFR1. Further Nef inhibits Fas-and TNF-mediated induction of MKK7 (Mitogen-Activated Protein Kinase Kinase-1)-JNK (c-Jun N-terminal kinase-1) activity and significantly suppresses the induction of apoptosis by these agonists. It also accelerates the development of AIDS in HIV-infected individuals (Ref.4).
HIV1 typically enters host cells through the interaction of the viral envelope protein, GP120, with CD4 and chemokine receptors CCR5 (CC-motif Receptor-5) and CXCR4 (Chemokine (C-X-C motif) Receptor-4) on the surface of the host cells. The relative expression of these coreceptors determines the relative susceptibility of cells to HIV1 infection (Ref.7). External apoptotic stimuli, as well as signals generated from within the cell, result in the activation of signal transduction pathways that converge on a caspase cascade resulting in the execution of apoptotic cell death. Signaling through the death receptor, Fas, results in recruitment of an adaptor protein, FADD (Fas-Associated via Death Domain). The binding of FADD to Fas recruits the Caspase8, which results in autoproteolytic activation of Caspase8 (Ref.8). Caspase8 is the most proximal caspase to the death receptors and is responsible for initiating the stepwise activation of multiple caspases, resulting in the signaling cascade that leads to the apoptotic death of the cell. Tat increases the levels of Caspase8 RNA, with a resulting increase in Caspase8 protein.The extrinsic pathway is initiated by the binding of TNF-family death-receptor ligands to their cognate receptors. Through their DDs (TRADD, (TNFR-Associated Death Domain), multimerized receptors interact with the DDs of adaptor proteins and RIP(Receptor-Interacting Protein), which also contain DEDs (Death-Effector Domains) and TRAF1/TRAF2 (TNF Receptor-Associated Factors) that facilitate binding to pro-caspase8 and/or pro-caspase10 to form the DISC (Death-Inducing Signal Complex). As part of the DISC, the pro-caspases are cleaved into their active forms and initiate the intrinsic pathway of apoptosis. BID (BH3-Interacting Domain death agonist) is then cleaved to produce tBID (truncated) and the effector caspase cascade is activated. Death-receptor-induced apoptosis can be blocked by FLIP (FLICE-Like Inhibitory Protein), which inhibits the proteolytic processing of caspase8. HIV-encoded proteins GP120, Nef and Tat upregulate the expression of Fas and FasL. In addition, Tat upregulates the expression of TRAIL. GP120, Tat and protease upregulate Caspase8; and GP120 and Nef increase the activity of Caspase3. The intrinsic apoptotic pathway is initiated by internal sensors, such as p53, which activate BH3-containing proteins and mediate the assembly of pro-apoptotic members of the BCL2 family, including BAX and BAK (BCL2-Antagonist/Killer), into hetero-oligomeric pores in the mitochondrial membrane. This results in the release of pro-apoptotic factors like Cyto C, SMAC (Second Mitochondria-Derived Activator of Caspase) and HTRA2 (High Temperature Requirement Protein-A2)/Omi—into the cytoplasm. This is associated with the loss of mitochondrial membrane potential, which can be blocked by anti-apoptotic proteins BCL2/BCL-XL. Release of Cyto C promotes the formation of the apoptosome, which includes APAF1 (Apoptotic Protease Activating Factor-1) and pro-caspase9. Autolytic activation of Caspase9 initiates the effector caspase cascade, which activates ICAD (DNA Fragmentation Factor) leading to DNA fragmentation. Caspase activation is negatively regulated by IAPs (Inhibitor Of Apoptosis Proteins), which are counterbalanced by SMAC and HTRA2/Omi. HIV-encoded proteins also trigger the mitochondrial pathway: GP120 induces the phosphorylation of p53; Tat and GP120 promote BAX insertion into the mitochondrial membrane and subsequent release of Cyto C; Vpr has a direct effect on the mitochondrial PTP (Permeability Transition Pore); GP120, Tat and Nef inhibit expression of BCL2, whereas the protease cleaves it, and Nef inhibits the expression of BCL-XL.
Chronic activation of the TNF-Alpha-signaling pathway plays a pivotal role in HIV1 pathogenesis, leading to increased HIV1 transcription, induction of mononuclear cell apoptosis, and suppression hematopoiesis. The HIV1 GP120 directly induces TNF-Alpha secretion by peripheral blood mononuclear cells and directly upregulates viral replication in an autocrine and paracrine manner in chronically infected cell lines as well as in peripheral blood mononuclear cells. Thus, the importance of chronic induction of the HIV1 LTR by TNF-Alpha in HIV1 pathogenesis may, in part, explain why HIV1 is more pathogenic than HIV2. Aberrant expression of inflammatory cytokines observed during progression of HIV1 disease has been implicated in the pathogenicity of AIDS (Ref.9). The functional consequence of HIV-driven apoptosis is the impairment of HIV-specific immunity, which results from the apoptosis of virus-specific CD4+ T Helper cells, subsequent decreased synthesis of Type-1 cytokines and altered differentiation and activity of HIV-specific CD8+ cytotoxic T lymphocytes. Suppression of virus replication by potent anti-retroviral therapy can partly restore CD4+ T-Cell numbers and functions. Quantitative restoration involves several phases, including decreased HIV-driven lymphocyte apoptosis, homeostatic proliferation and reconstitution of the naive T-Cell pool by increased thymic output. Qualitative restoration is incomplete and new strategies, such as immune-based therapy, are presently under investigation to boost HIV-specific immunity.