The role of the CD4 receptor versus HIV coreceptors in envelope-mediated apoptosis in peripheral blood mononuclear cells

Broad Spectrum Mixed Lineage Kinase Type 3 Inhibition and HIV-1 Persistence in Macrophages

Mixed lineage kinases (MLKs) are a group of serine-threonine kinases that evolved in part to respond to endogenous and exogenous insults that result in oxidative stress and pro-inflammatory responses from innate immune cells. Human immunodeficiency virus type 1 (HIV-1) thrives in these conditions and is associated with the development of associated neurocognitive disorders (HAND). As part of a drug discovery program to identify new therapeutic strategies for HAND, we created a library of broad spectrum MLK inhibitors with drug-like properties. Serendipitously, the lead compound, URMC-099 has proved useful not only in reversing damage to synaptic architecture in models of HAND, but also serves to restore autophagy as a protective response when given in concert with nanoformulated antiretroviral therapy (nanoART) in persistently infected macrophages. These findings are reviewed in the context of MLK3 biology and cellular signaling pathways relevant to new HIV-1 therapies. Graphical abstract.

Metabolite-Sensing G Protein Coupled Receptor TGR5 Protects Host From Viral Infection Through Amplifying Type I Interferon Responses

The metabolite-sensing G protein–coupled receptors (GPCRs) bind to various metabolites and transmit signals that are important for proper immune and metabolic functions. However, the roles of metabolite-sensing GPCRs in viral infection are not well characterized. Here, we identified metabolite-sensing GPCR TGR5 as an interferon (IFN)-stimulated gene (ISG) which had increased expression following viral infection or IFN-β stimulation in a STAT1-dependent manner. Most importantly, overexpression of TGR5 or treatment with the modified bile acid INT-777 broadly protected host cells from vesicular stomatitis virus (VSV), newcastle disease virus (NDV) and herpes simplex virus type 1 (HSV-1) infection. Furthermore, VSV and HSV-1 replication was increased significantly in Tgr5-deficient macrophages and the VSV distribution in liver, spleen and lungs was increased in Tgr5-deficient mice during VSV infection. Accordingly, Tgr5-deficient mice were much more susceptible to VSV infection than wild-type mice. Mechanistically, TGR5 facilitates type I interferon (IFN-I) production through the AKT/IRF3-signaling pathway, which is crucial in promoting antiviral innate immunity. Taken together, our data reveal a positive feedback loop regulating IRF3 signaling and suggest a potential therapeutic role for metabolite-sensing GPCRs in controlling viral diseases.

HIV-1 Coreceptor CXCR4 Antagonists Promote Clonal Expansion of Viral Epitope-Specific CD8+ T Cells During Acute SIV Infection in Rhesus Monkeys In Vivo

BACKGROUND

The underlying molecular mechanisms and the kinetics of T cell receptor (TCR) repertoire selection during administration of CXCR4 or CCR5 inhibitors in infection of AIDS viruses in vivo have remained largely unexplored. Viral epitope-specific CD8(+) T lymphocytes play a dominant role in the control of HIV and simian immunodeficiency virus (SIV). We hypothesized that blockade of CXCR4 or CCR5 might influence the clonal expansion of epitope-specific CD8(+) T cells, contributing to antiviral immune responses in vivo.

METHODS

We measured frequencies of the dominant epitope p11C-specific CD8(+) T cells and analyzed the TCR repertoire of those cells in SIV-infected rhesus monkeys treated by CXCR4 or CCR5 inhibitors and vMIP-II, which binds multiple chemokine receptors.

RESULTS

A significantly increase in the levels of epitope-specific CD8(+) T cells was observed after blockade of CXCR4 or CCR5 compared with untreated control groups. Those CD8(+) T cells exhibited selected usage of TCR Vβ families and complementarity-determining region 3 (CDR3) segments. The clonal expansion of distinct Vβ populations could efficiently inhibit SIV replication in vitro, and CXCR4 inhibitor induced more expansion of epitope-specific CD8(+) T cells than CCR5 antagonist (P < 0.01), whereas vMIP-II treatment showed the most marked augmentation of p11C-specific CD8(+) T cells.

CONCLUSIONS

Antagonists of HIV coreceptors, particularly CXCR4, play an important role in the clonal expansion of SIV epitope-specific CD8(+) T cells in vivo, thus inhibitors of chemokine receptors such as CXCR4 or CCR5 may contribute to the ability of epitope-specific CD8(+) T cells to inhibit SIV or HIV infection.

Cocaine enhances HIV-1-induced CD4(+) T-cell apoptosis: implications in disease progression in cocaine-abusing HIV-1 patients

Substance abuse is a major barrier in eradication of the HIV epidemic because it serves as a powerful cofactor for viral transmission, disease progression, and AIDS-related mortality. Cocaine, one of the commonly abused drugs among HIV-1 patients, has been suggested to accelerate HIV disease progression. However, the underlying mechanism remains largely unknown. Therefore, we tested whether cocaine augments HIV-1-associated CD4(+) T-cell decline, a predictor of HIV disease progression. We examined apoptosis of resting CD4(+) T cells from HIV-1-negative and HIV-1-positive donors in our study, because decline of uninfected cells plays a major role in HIV-1 disease progression. Treatment of resting CD4(+) T cells with cocaine (up to 100 μmol/L concentrations) did not induce apoptosis, but 200 to 1000 μmol/L cocaine induced apoptosis in a dose-dependent manner. Notably, treatment of CD4(+) T cells isolated from healthy donors with both HIV-1 virions and cocaine significantly increased apoptosis compared with the apoptosis induced by cocaine or virions alone. Most important, our biochemical data suggest that cocaine induces CD4(+) T-cell apoptosis by increasing intracellular reactive oxygen species levels and inducing mitochondrial depolarization. Collectively, our results provide evidence of a synergy between cocaine and HIV-1 on CD4(+) T-cell apoptosis that may, in part, explain the accelerated disease observed in HIV-1-infected drug abusers.

HIV-1 Virological Synapse is not Simply a Copycat of the Immunological Synapse

The virological synapse (VS) is a tight adhesive junction between an HIV-infected cell and an uninfected target cell, across which virus can be efficiently transferred from cell to cell in the absence of cell-cell fusion. The VS has been postulated to resemble, in its morphology, the well-studied immunological synapse (IS). This review article discusses the structural similarities between IS and VS and the shared T cell receptor (TCR) signaling components that are found in the VS. However, the IS and the VS display distinct kinetics in disassembly and intracellular signaling events, possibly leading to different biological outcomes. Hence, HIV-1 exploits molecular components of IS and TCR signaling machinery to trigger unique changes in cellular morphology, migration, and activation that facilitate its transmission and cell-to-cell spread.

Controlling the HIV/AIDS epidemic: current status and global challenges

This review provides an overview of the current status of the global HIV pandemic and strategies to bring it under control. It updates numerous preventive approaches including behavioral interventions, male circumcision (MC), pre- and post-exposure prophylaxis (PREP and PEP), vaccines, and microbicides. The manuscript summarizes current anti-retroviral treatment options, their impact in the western world, and difficulties faced by emerging and resource-limited nations in providing and maintaining appropriate treatment regimens. Current clinical and pre-clinical approaches toward a cure for HIV are described, including new drug compounds that target viral reservoirs and gene therapy approaches aimed at altering susceptibility to HIV infection. Recent progress in vaccine development is summarized, including novel approaches and new discoveries.

The Siva protein is a novel intracellular ligand of the CD4 receptor that promotes HIV-1 envelope-induced apoptosis in T-lymphoid cells

In addition to its positive signaling function in the antigen presentation process, CD4 acts as the primary receptor for HIV-1. Contact between CD4 and the viral envelope leads to virus entry, but can also trigger apoptosis of uninfected CD4+ T-cells through a mechanism that is poorly understood. We show that Siva-1, a death domain-containing proapoptotic protein, associates with the cytoplasmic domain of CD4. This interaction is mediated by the cysteine-rich region found in the C-terminal part of the Siva-1 protein. Expression of Siva-1 specifically increases the susceptibility of both T-cell lines and unstimulated human primary CD4+ T-lymphocytes to CD4-mediated apoptosis triggered by the HIV-1 envelope, and results in activation of a caspase-dependent mitochondrial pathway. The same susceptibility is observed in T-cells expressing a truncated form of CD4 that is able to recruit Siva-1 but fails to associate with p56Lck, indicating that Siva-1 participates in a pathway independent of the p56Lck kinase activity. Altogether, these results suggest that Siva-1 might participate in the CD4-initiated signaling apoptotic pathway induced by the HIV-1 envelope in T-lymphoid cells.

Vaccine-delivered HIV envelope inhibits CD4(+) T-cell activation, a mechanism for poor HIV vaccine responses

The human immunodeficiency virus (HIV) causes impairment of the immune system in part by targeting CD4(+) T cells for infection and dysfunction. HIV envelope (Env) present on free virions and infected cells causes dysfunction of uninfected bystander CD4(+) T cells via interaction with both CD4 and coreceptors. Env is commonly used as part of a cocktail of HIV antigens in current vaccines. In DNA and viral vector vaccine approaches, antigen-presenting cells (APCs) and non-APCs in the vicinity of the vaccine delivery site and draining lymph node express vaccine-derived antigens. The studies here demonstrate that cell-surface expression of Env on APCs and non-APCs as part of the vaccine action causes an inhibition of antigen-induced CD4(+) T-cell activation and proliferation mediated by CD4 binding and suggests a potential mechanism for reduced activity of Env-containing HIV vaccines. Similar studies using a functional Env lacking CD4 binding circumvented suppression, suggesting an alternative and potentially superior approach to HIV vaccine design.

Physiological levels of virion-associated human immunodeficiency virus type 1 envelope induce coreceptor-dependent calcium flux

Human immunodeficiency virus (HIV) entry into target cells requires the engagement of receptor and coreceptor by envelope glycoprotein (Env). Coreceptors CCR5 and CXCR4 are chemokine receptors that generate signals manifested as calcium fluxes in response to binding of the appropriate ligand. It has previously been shown that engagement of the coreceptors by HIV Env can also generate Ca(2+) fluxing. Since the sensitivity and therefore the physiological consequence of signaling activation in target cells is not well understood, we addressed it by using a microscopy-based approach to measure Ca(2+) levels in individual CD4(+) T cells in response to low Env concentrations. Monomeric Env subunit gp120 and virion-bound Env were able to activate a signaling cascade that is qualitatively different from the one induced by chemokines. Env-mediated Ca(2+) fluxing was coreceptor mediated, coreceptor specific, and CD4 dependent. Comparison of the observed virion-mediated Ca(2+) fluxing with the exact number of viral particles revealed that the viral threshold necessary for coreceptor activation of signaling in CD4(+) T cells was quite low, as few as two virions. These results indicate that the physiological levels of virion binding can activate signaling in CD4(+) T cells in vivo and therefore might contribute to HIV-induced pathogenesis.

Innate immune dysfunction in HIV infection: effect of HIV envelope-NK cell interactions

We have previously described a number of NK cell dysfunctions in HIV-viremic individuals. In the present study, we performed DNA microarray analysis followed by phenotypic and functional characterization in an effort to investigate which HIV envelope glycoproteins (gp120) affect the physiologic functions of NK cells. Upon treatment of NK cells with HIV gp120, DNA microarray analyses indicated up-regulation of several categories of genes that are associated with apoptosis, suppression of both cellular proliferation and survival, as well as down-regulation of genes that play a vital role in cell proliferation, innate immune defense mechanism, and cell survival. Both subtypes of gp120 suppressed NK cell cytotoxicity, proliferation, and the ability to secrete IFN-gamma. NK cells exposed to X4-subtype HIV gp120 showed a significant decrease in the levels of CC chemokines, while exposure to R5-subtype HIV gp120 had minimal effect. Extended exposure to HIV gp120 resulted in apoptosis of NK cells, further validating the microarray data. Our data demonstrate that exposure of NK cells to HIV envelope proteins results in profound cellular abnormalities at the level of gene expression as well as generic cell functions. These findings are likely to be a consequence of a direct HIV gp120-mediated effect on NK cells. Identification of specific surface receptors on NK cells that interact with HIV envelope proteins might explain how HIV is capable of circumventing innate immune defense mechanisms and establishing infection in susceptible individuals.

R5 and X4 HIV envelopes induce distinct gene expression profiles in primary peripheral blood mononuclear cells

HIV envelope binds to and signals through its primary cellular receptor, CD4, and through a coreceptor, either CC chemokine receptor 5 (CCR5) or CXC chemokine receptor 4 (CXCR4). Here, we evaluate the response of peripheral blood mononuclear cells to a panel of genetically diverse R5 and X4 envelope proteins. Modulation of gene expression was evaluated by using oligonucleotide microarrays. Activation of transcription factors was evaluated by using an array of oligonucleotides encoding transcription factor binding sites. Responses were strongly influenced by coreceptor specificity. Treatment of cells from CCR5delta32 homozygous donors with glycoprotein (gp)120 derived from an R5 virus demonstrated that the majority of responses elicited by R5 envelopes required engagement of CCR5. R5 envelopes, to a greater extent than X4 envelopes, induced the expression of genes belonging to mitogen-activated protein kinase signal transduction pathways and genes regulating the cell cycle. A number of genes induced by R5, but not X4, envelopes were also up-regulated in the resting CD4+ T cell population of HIV-infected individuals. These results suggest that R5 envelope facilitates replication of HIV in the pool of resting CD4+ T cells. Additionally, signaling by R5 gp120 may facilitate the transmission of R5 viruses by inducing a permissive environment for HIV replication.

NK cells in HIV infection: Paradigm for protection or targets for ambush

Natural killer cells are a crucial component of the innate immune response to certain tumours and to various viruses, fungi, parasites and bacteria. HIV has infected more than 60 million people worldwide and has led to more than 23 million deaths. At present, there are approximately 40 million people who are living with HIV infection, and there were 5 million new infections in 2004. As part of the innate immune system, natural killer cells might have an important role in host defence against HIV infection, as well as in the control of HIV replication in vivo. In this regard, it is important to understand how natural killer cells and HIV interact. This Review focuses on the role of natural killer cells in controlling HIV infection and on the impact of HIV and HIV-viraemia-induced immune activation on natural-killer-cell function.

Thymic pathogenicity of an HIV-1 envelope is associated with increased CXCR4 binding efficiency and V5-gp41-dependent activity, but not V1/V2-associated CD4 binding efficiency and viral entry

We previously described a thymus-tropic HIV-1 envelope (R3A Env) from a rapid progressor obtained at the time of transmission. An HIV-1 molecular recombinant with the R3A Env supported extensive replication and pathogenesis in the thymus and did not require Nef. Another Env from the same patient did not display the same thymus-tropic pathogenesis (R3B Env). Here, we show that relative to R3B Env, R3A Env enhances viral entry of T cells, increases fusion-induced cytopathicity, and shows elevated binding efficiency for both CD4 and CXCR4, but not CCR5, in vitro. We created chimeric envelopes to determine the region(s) responsible for each in vitro phenotype and for thymic pathogenesis. Surprisingly, while V1/V2 contributed to enhanced viral entry, CD4 binding efficiency, and cytopathicity in vitro, it made no contribution to thymic pathogenesis. Rather, CXCR4 binding efficiency and V5-gp41-associated activity appear to independently contribute to thymic pathogenesis of the R3A Env. These data highlight the contribution of unique HIV pathogenic factors in the thymic microenvironment and suggest that novel mechanisms may be involved in Env pathogenic activity in vivo.

Viral hijacking of G-protein-coupled-receptor signalling networks

Viruses use a surprising diversity of approaches to hijack G-protein-coupled receptors and harness their activated intracellular signalling pathways. All of these approaches ultimately function to ensure viral replicative success and often contribute to their pathogenesis. Indeed, a single virus might deploy a repertoire of these strategies to regulate key intracellular survival, proliferative and chemotactic pathways. Understanding the contribution of these biochemical routes to viral pathogenesis might facilitate the development of effective target-specific therapeutic strategies against viral diseases.

Feline immunodeficiency virus envelope glycoprotein mediates apoptosis in activated PBMC by a mechanism dependent on gp41 function

Feline Immunodeficiency Virus (FIV) is a lentivirus that causes immunodeficiency in cats, which parallels HIV-1-induced immunodeficiency in humans. It has been established that HIV envelope (Env) glycoprotein mediates T cell loss via a mechanism that requires CXCR4 binding. The Env glycoprotein of FIV, similar to HIV, requires CXCR4 binding for viral entry, as well as inducing membrane fusion leading to syncytia formation. However, the role of FIV Env in T cell loss and the molecular mechanisms governing this process have not been elucidated. We studied the role of Env glycoprotein in FIV-mediated T cell apoptosis in an in vitro model. Our studies demonstrate that membrane-expressed FIV Env induces apoptosis in activated feline peripheral blood mononuclear cells (PBMC) by a mechanism that requires CXCR4 binding, as the process was inhibited by CXCR4 antagonist AMD3100 in a dose-dependent manner. Interestingly, studies regarding the role of CD134, the recently identified primary receptor of FIV, suggest that binding to CD134 may not be important for induction of apoptosis in PBMC. However, inhibiting Env-mediated fusion post CXCR4 binding by FIV gp41-specific fusion inhibitor also inhibited apoptosis. Under similar conditions, a fusion-defective gp41 mutant was unable to induce apoptosis in activated PBMC. Our findings are the first report suggesting the potential of FIV Env to mediate apoptosis in bystander cells by a process that is dependent on gp41 function.

CD25(+)CD4(+) regulatory T cells from the peripheral blood of asymptomatic HIV-infected individuals regulate CD4(+) and CD8(+) HIV-specific T cell immune responses in vitro and are associated with favorable clinical markers of disease status

Human immunodeficiency virus (HIV) disease is associated with loss of CD4⁺ T cells, chronic immune activation, and progressive immune dysfunction. HIV-specific responses, particularly those of CD4⁺ T cells, become impaired early after infection, before the loss of responses directed against other antigens; the basis for this diminution has not been elucidated fully. The potential role of CD25⁺CD4⁺ regulatory T cells (T reg cells), previously shown to inhibit immune responses directed against numerous pathogens, as suppressors of HIV-specific T cell responses was investigated. In the majority of healthy HIV-infected individuals, CD25⁺CD4⁺ T cells significantly suppressed cellular proliferation and cytokine production by CD4⁺ and CD8⁺ T cells in response to HIV antigens/peptides in vitro; these effects were cell contact dependent and IL-10 and TGF-β independent. Individuals with strong HIV-specific CD25⁺ T reg cell function in vitro had significantly lower levels of plasma viremia and higher CD4⁺: CD8⁺ T cell ratios than did those individuals in whom this activity could not be detected. These in vitro data suggest that CD25⁺CD4⁺ T reg cells may contribute to the diminution of HIV-specific T cell immune responses in vivo in the early stages of HIV disease.

R5 HIV gp120-mediated cellular contacts induce the death of single CCR5-expressing CD4 T cells by a gp41-dependent mechanism

The use of CXC chemokine receptor 4 (CXCR4) and CC chemokine receptor 5 (CCR5) by X4 and R5 human immunodeficiency virus (HIV) envelopes (Env) influences HIV cytopathicity. Here, we have evaluated the role of CCR5 and gp41 in Env-induced cell death occurring during the contacts of uninfected, primary cells with MOLT cells infected with different R5 and X4 HIV isolates. As reported for X4-Env, R5 HIV-infected cells destroyed CD4 T cells expressing the appropriate coreceptor by inducing the formation of syncytia and the death of single target cells. Therefore, only the small (<10%) CCR5+ subset of primary CD4 T cells was sensitive to cellular presentation of R5-Env, and CCR5-CD4 T cells showed complete resistance to R5-Env-mediated cell death. X4- and R5-infected cells killed single primary cells by a common mechanism that was dependent on gp41 function and induced a rapid loss of mitochondrial membrane potential and plasma membrane integrity in target cells. Single-cell death was not affected by the blockade of HIV replication in target cells or G-protein signaling through CXCR4/CCR5. In contrast, caspase inhibition (Z-Val-Ala-Asp-fluoromethylketone) profoundly changed the outcome of cell-to-cell contacts by reducing the number of single dead CD4 T cells and increasing the rate of syncytium formation. In conclusion, X4 and R5 HIV Env share a common gp41-dependent mechanism to kill CD4 T cells during cellular contacts. Env tropism and coreceptor expression but not differential killing mechanisms seem to govern the extent of cytopathic effects induced by HIV infection.

Apoptosis of bystander T cells induced by human immunodeficiency virus type 1 with increased envelope/receptor affinity and coreceptor binding site exposure

Apoptosis of uninfected bystander CD4(+) T cells contributes to T-cell depletion during human immunodeficiency virus type 1 (HIV-1) pathogenesis. The viral and host mechanisms that lead to bystander apoptosis are not well understood. To investigate properties of the viral envelope glycoproteins (Env proteins) that influence the ability of HIV-1 to induce bystander apoptosis, we used molecularly cloned viruses that differ only in specific amino acids in Env. The ability of these strains to induce bystander apoptosis was tested in herpesvirus saimiri-immortalized primary CD4(+) T cells (CD4/HVS), which resemble activated primary T cells. Changes in Env that increase affinity for CD4 or CCR5 or increase coreceptor binding site exposure enhanced the capacity of HIV-1 to induce bystander apoptosis following viral infection or exposure to nonreplicating virions. Apoptosis induced by HIV-1 virions was inhibited by CD4, CXCR4, and CCR5 antibodies or by the CXCR4 inhibitor AMD3100, but not the fusion inhibitor T20. HIV-1 virions with mutant Envs that bind CXCR4 but are defective for CD4 binding or membrane fusion induced apoptosis, whereas CXCR4 binding-defective mutants did not. These results demonstrate that HIV-1 virions induce apoptosis through a CXCR4- or CCR5-dependent pathway that does not require Env/CD4 signaling or membrane fusion and suggest that HIV-1 variants with increased envelope/receptor affinity or coreceptor binding site exposure may promote T-cell depletion in vivo by accelerating bystander cell death.

The stromal cell-derived factor-1alpha/CXCR4 ligand-receptor axis is critical for progenitor survival and migration in the pancreas

The SDF-1α/CXCR4 ligand/chemokine receptor pair is required for appropriate patterning during ontogeny and stimulates the growth and differentiation of critical cell types. Here, we demonstrate SDF-1α and CXCR4 expression in fetal pancreas. We have found that SDF-1α and its receptor CXCR4 are expressed in islets, also CXCR4 is expressed in and around the proliferating duct epithelium of the regenerating pancreas of the interferon (IFN) γ–nonobese diabetic mouse. We show that SDF-1α stimulates the phosphorylation of Akt, mitogen-activated protein kinase, and Src in pancreatic duct cells. Furthermore, migration assays indicate a stimulatory effect of SDF-1α on ductal cell migration. Importantly, blocking the SDF-1α/CXCR4 axis in IFNγ-nonobese diabetic mice resulted in diminished proliferation and increased apoptosis in the pancreatic ductal cells. Together, these data indicate that the SDF-1α–CXCR4 ligand receptor axis is an obligatory component in the maintenance of duct cell survival, proliferation, and migration during pancreatic regeneration.

The CCR5 and CXCR4 coreceptors--central to understanding the transmission and pathogenesis of human immunodeficiency virus type 1 infection

In this review, we will discuss what is known, what is suspected, and what still remains obscure about the central role played by coreceptor expression and usage in the transmission and pathogenic consequences of human immunodeficiency virus type 1 (HIV-1) infection. An emphasis will be on the HIV-1 phenotypic variants that are defined by their usage of the CCR5 or CXCR4 coreceptors, and how the different cellular tropism of these variants influences how and where HIV-1 replicates in vivo. We will also review what might happen when coreceptor antagonists are used clinically to treat HIV-1 infection.

Chemokine-induced cell death in CCR5-expressing neuroblastoma cells

CCR5 is expressed in neurons but its function in this cellular context is hitherto poorly understood. We have generated CCR5-expressing SH-SY5Y neuroblastoma cells. CCR5 ligands induced cell death in these cells, but not in control neuroblastoma cells or in CCR5-expressing fibroblasts. CCR5-dependent killing of neuroblastoma cells occurred through apoptosis, since it was accompanied by caspase-3 activation and could be prevented by a caspase-3 inhibitor. Finally, cell killing by activated microglia was more rapid and extensive in CCR5-expressing neuroblastoma cells than in control cells. In summary, CCR5 may act as a death receptor in cells of neuronal lineage and therefore be involved in inflammatory neurodegeneration.

HIV receptors on lymphocytes

Human immunodeficiency virus infection, despite tremendous efforts in research and prevention, is spreading at an increasing speed, especially in developing countries. Currently available therapeutic approaches significantly extend the lifespan of HIV-infected people, but their use is associated with a severe drug regimen, several undesirable side effects, and high cost. Therefore, the scientific community is steadfastly pursuing novel strategies for inhibiting viral replication, promoting a better immune response, and developing an effective vaccine. Recent research on HIV receptors has introduced new concepts in the field, showing that expression of receptors, although necessary for virus entry, is subordinate to quality of expression, so that efficient infection occurs when receptors are properly colocalized. In addition, intracellular signaling triggered by HIV receptors has been shown to play important roles in pathogenesis by inducing apoptosis of bystander cells. Induction of some pathways of intracellular signaling, however, can instead suppress HIV replication, so that modulation of these pathways constitutes an additional target to be exploited for therapy or vaccines. This article reviews the most exciting aspects of these novel findings and discusses their practical application in the fight against HIV infection.

Mechanisms of CD4+ T lymphocyte cell death in human immunodeficiency virus infection and AIDS

AIDS, caused by the retroviruses human immunodeficiency virus type 1 and type 2 (HIV-1 and HIV-2), has reached pandemic proportions. Therefore, it is critical to understand how HIV causes AIDS so that appropriate therapies can be formulated. Primarily, HIV infects and kills CD4(+) T lymphocytes, which function as regulators and amplifiers of the immune response. In the absence of effective anti-retroviral therapy, the hallmark decrease in CD4(+) T lymphocytes during AIDS results in a weakened immune system, impairing the body's ability to fight infections or certain cancers such that death eventually ensues. The major mechanism for CD4(+) T cell depletion is programmed cell death (apoptosis), which can be induced by HIV through multiple pathways. Death of HIV-infected cells can result from the propensity of infected lymphocytes to form short-lived syncytia or from an increased susceptibility of the cells to death. However, the apoptotic cells appear to be primarily uninfected bystander cells and are eradicated by two different mechanisms: either a Fas-mediated mechanism during activation-induced cell death (AICD), or as a result of HIV proteins (Tat, gp120, Nef, Vpu) released from infected cells stimulating apoptosis in uninfected bystander cells. There is also evidence that as AIDS progresses cytokine dysregulation occurs, and the overproduction of type-2 cytokines (IL-4, IL-10) increases susceptibility to AICD whereas type-1 cytokines (IL-12, IFN-gamma) may be protective. Clearly there are multiple causes of CD4(+) T lymphocyte apoptosis in AIDS and therapies that block or decrease that death could have significant clinical benefit.