CD4+ blood lymphocytes are rapidly killed in vitro by contact with autologous human immunodeficiency virus-infected cells

Changes in the cellular microRNA profile by the intracellular expression of HIV-1 Tat regulator: A potential mechanism for resistance to apoptosis and impaired proliferation in HIV-1 infected CD4+ T cells

HIV-1 induces changes in the miRNA expression profile of infected CD4+ T cells that could improve viral replication. HIV-1 regulator Tat modifies the cellular gene expression and has been appointed as an RNA silencing suppressor. Tat is a 101-residue protein codified by two exons that regulates the elongation of viral transcripts. The first exon of Tat (amino acids 1–72) forms the transcriptionally active protein Tat72, but the presence of the second exon (amino acids 73–101) results in a more competent regulatory protein (Tat101) with additional functions. Intracellular, full-length Tat101 induces functional and morphological changes in CD4+ T cells that contribute to HIV-1 pathogenesis such as delay in T-cell proliferation and protection against FasL-mediated apoptosis. But the precise mechanism by which Tat produces these changes remains unknown. We analyzed how the stable expression of intracellular Tat101 and Tat72 modified the miRNA expression profile in Jurkat cells and if this correlated with changes in apoptotic pathways and cell cycle observed in Tat-expressing cells. Specifically, the enhanced expression of hsa-miR-21 and hsa-miR-222 in Jurkat-Tat101 cells was associated with the reduced expression of target mRNAs encoding proteins related to apoptosis and cell cycle such as PTEN, PDCD4 and CDKN1B. We developed Jurkat cells with stable expression of hsa-miR-21 or hsa-miR-222 and observed a similar pattern to Jurkat-Tat101 in resistance to FasL-mediated apoptosis, cell cycle arrest in G₂/M and altered cell morphology. Consequently, upregulation of hsa-miR-21 and hsa-miR-222 by Tat may contribute to protect against apoptosis and to anergy observed in HIV-infected CD4+ T cells.

Modeling the Slow CD4+ T Cell Decline in HIV-Infected Individuals

The progressive loss of CD4+ T cell population is the hallmark of HIV-1 infection but the mechanism underlying the slow T cell decline remains unclear. Some recent studies suggested that pyroptosis, a form of programmed cell death triggered during abortive HIV infection, is associated with the release of inflammatory cytokines, which can attract more CD4+ T cells to be infected. In this paper, we developed mathematical models to study whether this mechanism can explain the time scale of CD4+ T cell decline during HIV infection. Simulations of the models showed that cytokine induced T cell movement can explain the very slow decline of CD4+ T cells within untreated patients. The long-term CD4+ T cell dynamics predicted by the models were shown to be consistent with available data from patients in Rio de Janeiro, Brazil. Highly active antiretroviral therapy has the potential to restore the CD4+ T cell population but CD4+ response depends on the effectiveness of the therapy, when the therapy is initiated, and whether there are drug sanctuary sites. The model also showed that chronic inflammation induced by pyroptosis may facilitate persistence of the HIV latent reservoir by promoting homeostatic proliferation of memory CD4+ cells. These results improve our understanding of the long-term T cell dynamics in HIV-1 infection, and support that new treatment strategies, such as the use of caspase-1 inhibitors that inhibit pyroptosis, may maintain the CD4+ T cell population and reduce the latent reservoir size.

The CD4+ T cell population within HIV-infected individuals declines slowly as disease progresses. When CD4+ cells drop to below 200 cells/ul, the infection is usually considered to enter the late stage, i.e., acquired immune deficiency syndrome (AIDS). CD4+ T cell depletion can take many years but the biological events underlying such slow decline are not well understood. Some studies showed that the majority of infected T cells in lymph nodes die by pyroptosis, a form of programmed cell death, which can release inflammatory signals attracting more CD4+ T cells to be infected. We developed mathematical models to describe this process and explored whether they can generate the long-term CD4+ T cell decline. We showed that pyroptosis induced cell movement can explain the slow time scale of CD4+ T cell depletion and that pyroptosis may also contribute to the persistence of latently infected cells, which represent a major obstacle to HIV eradication. The modeling prediction agrees with patient data in Rio de Janeiro, Brazil. These results suggest that a combination of current treatment regimens and caspase-1 inhibitor that can inhibit pyroptosis might provide a new way to maintain the CD4+ T cell population and eradicate the HIV latent reservoir.

HIV-1 subtype C unproductively infects human cardiomyocytes in vitro and induces apoptosis mitigated by an anti-Gp120 aptamer

HIV-associated cardiomyopathy (HIVCM) is of clinical concern in developing countries because of a high HIV-1 prevalence, especially subtype C, and limited access to highly active antiretroviral therapy (HAART). For these reasons, we investigated the direct and indirect effects of HIV-1 subtype C infection of cultured human cardiomyocytes and the mechanisms leading to cardiomyocytes damage; as well as a way to mitigate the damage. We evaluated a novel approach to mitigate HIVCM using a previously reported gp120 binding and HIV-1 neutralizing aptamer called UCLA1. We established a cell-based model of HIVCM by infecting human cardiomyocytes with cell-free HIV-1 or co-culturing human cardiomyocytes with HIV-infected monocyte derived macrophages (MDM). We discovered that HIV-1 subtype C unproductively (i.e. its life cycle is arrested after reverse transcription) infects cardiomyocytes. Furthermore, we found that HIV-1 initiates apoptosis of cardiomyocytes through caspase-9 activation, preferentially via the intrinsic or mitochondrial initiated pathway. CXCR4 receptor-using viruses were stronger inducers of apoptosis than CCR5 utilizing variants. Importantly, we discovered that HIV-1 induced apoptosis of cardiomyocytes was mitigated by UCLA1. However, UCLA1 had no protective effective on cardiomyocytes when apoptosis was triggered by HIV-infected MDM. When HIV-1 was treated with UCLA1 prior to infection of MDM, it failed to induce apoptosis of cardiomyocytes. These data suggest that HIV-1 causes a mitochondrial initiated apoptotic cascade, which signal through caspase-9, whereas HIV-1 infected MDM causes apoptosis predominantly via the death-receptor pathway, mediated by caspase-8. Furthermore the data suggest that UCLA1 protects cardiomyocytes from caspase-mediated apoptosis, directly by binding to HIV-1 and indirectly by preventing infection of MDM.

Plasma membrane signaling in HIV-1 infection

Plasma membrane is a multifunctional structure that acts as the initial barrier against infection by intracellular pathogens. The productive HIV-1 infection depends upon the initial interaction of virus and host plasma membrane. Immune cells such as CD4+ T cells and macrophages contain essential cell surface receptors and molecules such as CD4, CXCR4, CCR5 and lipid raft components that facilitate HIV-1 entry. From plasma membrane HIV-1 activates signaling pathways that prepare the grounds for viral replication. Through viral proteins HIV-1 hijacks host plasma membrane receptors such as Fas, TNFRs and DR4/DR5, which results in immune evasion and apoptosis both in infected and uninfected bystander cells. These events are hallmark in HIV-1 pathogenesis that leads towards AIDS. The interplay between HIV-1 and plasma membrane signaling has much to offer in terms of viral fitness and pathogenicity, and a better understanding of this interplay may lead to development of new therapeutic approaches. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

The presence of HIV-1 Tat protein second exon delays fas protein-mediated apoptosis in CD4+ T lymphocytes: a potential mechanism for persistent viral production

HIV-1 replication is efficiently controlled by the regulator protein Tat (101 amino acids) and codified by two exons, although the first exon (1-72 amino acids) is sufficient for this process. Tat can be released to the extracellular medium, acting as a soluble pro-apoptotic factor in neighboring cells. However, HIV-1-infected CD4(+) T lymphocytes show a higher resistance to apoptosis. We observed that the intracellular expression of Tat delayed FasL-mediated apoptosis in both peripheral blood lymphocytes and Jurkat cells, as it is an essential pathway to control T cell homeostasis during immune activation. Jurkat-Tat cells showed impairment in the activation of caspase-8, deficient release of mitochondrial cytochrome c, and delayed activation of both caspase-9 and -3. This protection was due to a profound deregulation of proteins that stabilized the mitochondrial membrane integrity, such as heat shock proteins, prohibitin, or nucleophosmin, as well as to the up-regulation of NF-κB-dependent anti-apoptotic proteins, such as BCL2, c-FLIPS, XIAP, and C-IAP2. These effects were observed in Jurkat expressing full-length Tat (Jurkat-Tat101) but not in Jurkat expressing the first exon of Tat (Jurkat-Tat72), proving that the second exon, and particularly the NF-κB-related motif ESKKKVE, was necessary for Tat-mediated protection against FasL apoptosis. Accordingly, the protection exerted by Tat was independent of its function as a regulator of both viral transcription and elongation. Moreover, these data proved that HIV-1 could have developed strategies to delay FasL-mediated apoptosis in infected CD4(+) T lymphocytes through the expression of Tat, thus favoring the persistent replication of HIV-1 in infected T cells.

Tits and bits of HIV Tat protein

INTRODUCTION

HIV-Tat protein displays an array of functions that are essential for HIV replication. The structural flexibility of Tat protein has been regarded as one of the unique features responsible for sustaining diverse functions, from facilitated membrane-crossing ability to strong affinity for RNA binding.

AREAS COVERED

RNA binding ability and presence of multiple interacting domains in the same protein are very important properties of HIV-Tat protein. Tat protein has shown great ability to influence cellular and viral gene expression. We discuss the functions of HIV Tat protein, describing its structural significance, secretion and uptake of HIV Tat protein by immune cells, post-translational modifications and role of HIV Tat protein in HIV pathogenesis.

EXPERT OPINION

Perturbation in expression of many cytokines and chemokines by HIV-Tat protein exhibits downstream immune suppressive function as well as activation of several apoptotic genes. This explains the massive death of immune cells due to bystander effect of HIV Tat protein among HIV-infected patients.

HIV-1 induces DCIR expression in CD4+ T cells

The C-type lectin receptor DCIR, which has been shown very recently to act as an attachment factor for HIV-1 in dendritic cells, is expressed predominantly on antigen-presenting cells. However, this concept was recently challenged by the discovery that DCIR can also be detected in CD4⁺ T cells found in the synovial tissue from rheumatoid arthritis (RA) patients. Given that RA and HIV-1 infections share common features such as a chronic inflammatory condition and polyclonal immune hyperactivation status, we hypothesized that HIV-1 could promote DCIR expression in CD4⁺ T cells. We report here that HIV-1 drives DCIR expression in human primary CD4⁺ T cells isolated from patients (from both aviremic/treated and viremic/treatment naive persons) and cells acutely infected in vitro (seen in both virus-infected and uninfected cells). Soluble factors produced by virus-infected cells are responsible for the noticed DCIR up-regulation on uninfected cells. Infection studies with Vpr- or Nef-deleted viruses revealed that these two viral genes are not contributing to the mechanism of DCIR induction that is seen following acute infection of CD4⁺ T cells with HIV-1. Moreover, we report that DCIR is linked to caspase-dependent (induced by a mitochondria-mediated generation of free radicals) and -independent intrinsic apoptotic pathways (involving the death effector AIF). Finally, we demonstrate that the higher surface expression of DCIR in CD4⁺ T cells is accompanied by an enhancement of virus attachment/entry, replication and transfer. This study shows for the first time that HIV-1 induces DCIR membrane expression in CD4⁺ T cells, a process that might promote virus dissemination throughout the infected organism.

The type II transmembrane protein DCIR belongs to the C-type lectin domain family receptor and is predominantly expressed in cells of the myeloid lineage. However recent evidence suggests that it can also be induced in CD4⁺ T cells placed under an inflammatory condition. We assessed the capacity of HIV-1 to promote DCIR expression in CD4⁺ T cells because the establishment of an inflammatory state is a hallmark of this retroviral infection in humans. We report here that a higher DCIR expression is detected not only in CD4⁺ T cells acutely infected with HIV-1 in vitro but also in clinical cell samples. Additional studies suggest a possible link between DCIR induction and apoptosis through both caspase-dependent and -independent intrinsic pathways. The greater expression of DCIR on the surface of CD4⁺ T cells results in more efficient virus attachment/entry, replication and transfer processes.

What does the structure-function relationship of the HIV-1 Tat protein teach us about developing an AIDS vaccine?

The human immunodeficiency virus type 1 (HIV-1) trans-activator of transcription protein Tat is an important factor in viral pathogenesis. In addition to its function as the key trans-activator of viral transcription, Tat is also secreted by the infected cell and taken up by neighboring cells where it has an effect both on infected and uninfected cells. In this review we will focus on the relationship between the structure of the Tat protein and its function as a secreted factor. To this end we will summarize some of the exogenous functions of Tat that have been implicated in HIV-1 pathogenesis and the impact of structural variations and viral subtype variants of Tat on those functions. Finally, since in some patients the presence of Tat-specific antibodies or CTL frequencies are associated with slow or non-progression to AIDS, we will also discuss the role of Tat as a potential vaccine candidate, the advances made in this field, and the importance of using a Tat protein capable of eliciting a protective or therapeutic immune response to viral challenge.

Tat-induced FOXO3a is a key mediator of apoptosis in HIV-1-infected human CD4+ T lymphocytes

The high mutation rate of HIV is linked to the generation of viruses expressing proteins with altered function whose impact on disease progression is unknown. We investigated how HIV-1 viruses lacking Env, Vpr, and Nef affect CD4(+) T cell survival. We found that in the absence of these proteins, HIV-1-infected CD4(+) primary T cells progress to the G(0) phase of the cell cycle and to cell death, indicating that viruses expressing inactive forms of these proteins can contribute to the CD4(+) T cell decline as the wild-type virus, suggesting that other HIV proteins are responsible for inducing apoptosis. Apoptosis in these cells is triggered by the alteration of the Egr1-PTEN-Akt (early growth response-1/phosphate and tensin homolog deleted on chromosome 10/Akt) and p53 pathways, which converge on the FOXO3a (Forkhead box transcription factor O class 3a) transcriptional activator. The FOXO3a target genes Fas ligand and TRAIL, involved in the extrinsic apoptotic pathway, and PUMA, Noxa, and Bim, which are part of the intrinsic apoptotic pathway, were also up-regulated, indicating that HIV infection leads to apoptosis by the engagement of multiple apoptotic pathways. RNAi-mediated knockdown of Egr1 and FOXO3a resulted in reduced apoptosis in HIV-infected HeLa and CD4(+) T cells, providing further evidence for their critical role in HIV-induced apoptosis and G(0) arrest. We tested the possibility that Tat is responsible for the T cell apoptosis observed with these mutant viruses. The induction of Egr1 and FOXO3a and its target genes was observed in Jurkat cells transduced by Tat alone. Tat-dependent activation of the Egr1-PTEN-FOXO3a pathway provides a mechanism for HIV-1-associated CD4(+) T cell death.

Role of HIV Gp41 mediated fusion/hemifusion in bystander apoptosis

Mechanisms of HIV-mediated CD4+ T cell loss leading to immunodeficiency are amongst the most extensively studied yet unanswered questions in HIV biology. The level of CD4+ T cell depletion in HIV infected patients far exceeds the number of infected T cells, suggesting an indirect mechanism of HIV pathogenesis termed bystander cell death. Evidence is accumulating that the HIV envelope glycoprotein (Env) is a major determinant of HIV pathogenesis and plays a critical role in bystander cell death. The complex structure and function of HIV Env makes the determination of the mechanism of Env mediated apoptosis more complex than previously thought. This review will examine the complex relationship between HIV Env phenotype, coreceptor expression and immune activation in determining HIV pathogenesis. We review data here corresponding to the role of HIV Env hemifusion activity in HIV pathogenesis and how it interplays with other AIDS associated factors such as chemokine receptor expression and immune activation.

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.

The C terminus of HIV-1 Tat modulates the extent of CD178-mediated apoptosis of T cells

HIV infection and the progression to AIDS are characterized by the depletion of CD4(+) T cells through apoptosis of the uninfected bystander cells and the direct killing of HIV-infected cells. This is mediated in part by the human immunodeficiency virus, type 1 Tat protein, which is secreted by virally infected cells and taken up by uninfected cells and CD178 gene expression, which is critically involved in T cell apoptosis. The differing ability of HIV strains to induce death of infected and uninfected cells may play a role in the clinical and biological differences displayed by HIV strains. We chemically synthesized the 86-residue truncated short variant of Tat and its full-length form. We show that the trans-activation ability of Tat at the long terminal repeat does not correlate with T cell apoptosis but that the ability of Tat to up-regulate CD178 mRNA expression and induce apoptosis in T cells is critically dependent on the C terminus of Tat. Moreover, the greater 86-residue Tat-induced apoptosis is via the extrinsic pathway of CD95-CD178.

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.

Simian immunodeficiency virus Vpr/Vpx proteins kill bystander noninfected CD4+ T-lymphocytes by induction of apoptosis

The depletion of CD4+ T-lymphocytes central to the immunodeficiency in acquired immunodeficiency syndrome (AIDS) is largely mediated by apoptosis of both infected and uninfected cells, but the mechanisms involved and the viral proteins responsible are still poorly characterized. It has recently been suggested that, in human and simian immunodeficiency virus (HIV) and SIV, Vpr is a major modulator of apoptosis in infected cells. Recently, we have reported on a chimera of caprine arthritis-encephalitis virus (CAEV) carrying vpr/vpx genes from SIVmac239, which is replication competent in goat macrophages but not in lymphocytes or human cells. Despite infection being restricted to macrophages, inoculation of primary goat peripheral blood mononuclear cells (PBMCs) with this chimera induced apoptosis in the lymphocyte population. In addition, when infected goat synovial membrane (GSM) cells were co-cultured with human CD4+ T lymphocyte SupT1 cell line, these CD4+ T cells showed increased apoptosis. The parental CAEV induced no significant apoptosis in goat PBMC cultures or in co-cultures with human SupT1 lymphocytes. This indicates that SIV Vpr/Vpx proteins indeed mediate apoptosis of T-lymphocytes and, moreover, do so without the need for active infection of these cells. Moreover, this apoptosis was observed when SupT1s were cocultured in direct contact, but not in absence of contact with CAEV-pBSCAvpxvpr-infected GSM cells. In view of these data, we propose that SIV Vpx/Vpr activate cell-to-cell contact-dependent extracellular signaling pathways to promote apoptotic death of uninfected bystander T-lymphocytes. Understanding this mechanism might bring insight for intervening in the loss of CD4+ T lymphocytes in the SIV infection model and in human AIDS.

The glutamine-rich region of the HIV-1 Tat protein is involved in T-cell apoptosis

Human immunodeficiency virus (HIV) infection and the progression to AIDS are characterized by the depletion of CD4(+) T-cells. HIV-1 infection leads to apoptosis of uninfected bystander cells and the direct killing of HIV-infected cells. This is mediated, in part, by the HIV-1 Tat protein, which is secreted by virally infected cells and taken up by uninfected cells. We chemically synthesized two 86-residue subtype D Tat proteins, Ug05RP and Ug11LTS, from two Ugandan patients who were clinically categorized as either rapid progressor or long-term survivor, with non-conservative mutations located essentially in the glutamine-rich region. Structural heterogeneities were revealed by CD, which translate into differing trans-activational and apoptotic effects. CD data analysis and molecular modeling indicated that the short alpha-helix observed in subtype D Tat proteins from rapid progressor patients such as Tat Mal and Tat Ug05RP is not present in Ug11LTS. We show that Tat Ug05RP is more efficient than Tat Ug11LTS in its trans-activational role and in inducing apoptosis in binding tubulin via the mitochondrial pathway. The glutamine-rich region of Tat appears to be involved in the Tat-mediated apoptosis of T-cells.

Apoptosis of uninfected cells induced by HIV envelope glycoproteins

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. Although multiple mechanisms contribute to the gradual T cell decline that occurs in HIV-infected patients, programmed cell death of uninfected bystander T lymphocytes, including CD4+ and CD8+ T cells, is an important event leading to immunodeficiency. The HIV envelope glycoproteins (Env) play a crucial role in transducing this apoptotic signal after binding to its receptors, the CD4 molecule and a coreceptor, essentially CCR5 and CXCR4. Depending on Env presentation, the receptor involved and the complexity of target cell contact, apoptosis induction is related to death receptor and/or mitochondria-dependent pathways. This review summarizes current knowledge of Env-mediated cell death leading to T cell depletion and clinical complications and covers the sometimes conflicting studies that address the possible mechanisms of T cell death.

[HIV1-associated CD4 T lymphocyte apoptosis]

PURPOSE

Apoptosis during HIV infection has been evoked for ten years. The role of apoptosis during HIV infection have be confirmed by several authors but the exact relationships between viral replication, apoptosis and lymphocyte depletion remain to be clarified.

CURRENT KNOWLEDGE AND KEY POINTS

HIV may induce apoptosis of infected but also of uninfected bystander CD4+ lymphocytes. Those two types of HIV induced apoptosis lie on different pathways. While Fas and FasL are involved in apoptosis of bystander cells, mitochondrial pathway is required for apoptosis of infected cells. Cytokines but also anti HIV drugs may modulate HIV-induced lymphocyte apoptosis. Morever while protease inhibitor influence HIV replication and then secondary apotosis of infected cells, they can also interfere with spontaneous apoptosis of lymphocyte beside the context of HIV infection.

FUTURES AND PROJECTS

Apoptosis is thought to be one of the mechanism involved in CD4 T lymphocyte cell death during HIV infection. However relationships between apoptosis and HIV replication may be more complex. In fact it has been recently reported that while HIV replication induced lymphocyte apoptosis, apoptosis may in turn induced HIV replication in a loop amplification pathway

Intrinsic and extrinsic pathways signaling during HIV-1 mediated cell death

Infection with human immunodeficiency virus (HIV) is characterized by the gradual depletion of CD4+ T lymphocytes. The incorporation of the concept of apoptosis as a rationale to explain progressive T cell depletion has led to growing research in this field during the last 10 years. In parallel, the biochemical pathways implicated in programmed cell death have been extensively studied. Thus, the influence of mitochondrial control in the two major apoptotic pathways-the extrinsic and intrinsic pathways-is now well admitted. In this review, we summarized our current knowledge of the different pathways involved in the death of T cells in the course of HIV infection.

HIV-1 Tat targets microtubules to induce apoptosis, a process promoted by the pro-apoptotic Bcl-2 relative Bim

Depletion of CD4(+) T cells is the hallmark of HIV infection and AIDS progression. In addition to the direct killing of the viral-infected cells, HIV infection also leads to increased apoptosis of predominantly uninfected bystander cells. This is mediated in part through the HIV-1 Tat protein, which is secreted by the infected cells and taken up by uninfected cells. Using an affinity-purification approach, a specific and direct interaction of Tat with tubulin and polymerized microtubules has been detected. This interaction does not affect the secretion and uptake of Tat, but is critical for Tat to induce apoptosis. Tat binds tubulin/microtubules through a four-amino-acid subdomain of its conserved core region, leading to the alteration of microtubule dynamics and activation of a mitochondria-dependent apoptotic pathway. Bim, a pro-apoptotic Bcl-2 relative and a transducer of death signals initiated by perturbation of microtubule dynamics, facilitates the Tat-induced apoptosis. Our findings reveal a strategy by which Tat induces apoptosis by targeting the microtubule network. Thus HIV-1 Tat joins a growing list of pathogen-derived proteins that target the cytoskeleton of host cells.

HIV-1 protease processes procaspase 8 to cause mitochondrial release of cytochrome c, caspase cleavage and nuclear fragmentation

Infection of T cells with HIV-1 induces apoptosis and modulates apoptosis regulatory molecules. Similar effects occur following treatment of cells with individual HIV-1 encoded proteins. While HIV-1 protease is known to be cytotoxic, little is known of its effect on apoptosis and apoptosis regulatory molecules. The ability of HIV-1 protease to kill cells, coupled with the degenerate substrate specificity of HIV-1 protease, suggests that HIV-1 protease may activate cellular factor(s) which, in turn, induce apoptosis. We demonstrate that HIV-1 protease directly cleaves and activates procaspase 8 in T cells which is associated with cleavage of BID, mitochondrial release of cytochrome c, activation of the downstream caspases 9 and 3, cleavage of DFF and PARP and, eventually, to nuclear condensation and DNA fragmentation that are characteristic of apoptosis. The effect of HIV-1 protease is not seen in T cell extracts which have undetectable levels of procaspase 8, indicating a specificity and requirement for procaspase 8.

Influence of mitochondrial control of apoptosis on the pathogenesis, complications and treatment of HIV infection

HIV infection is inexorably linked with disordered regulation of apoptosis, and consequent alterations in mitochondrial homeostasis, resulting in CD4 T cell death and enhanced susceptibility to opportunistic infections and malignancies. Effective treatment of HIV reverses the changes in mitochondrial homeostasis and apoptosis, and enhances immunocompetence. This review will summarize current knowledge of: i) the associations of apoptosis with HIV disease progression; ii) mechanisms of enhanced apoptosis in HIV infection; iii) putative role of apoptosis in HIV complications; iv) direct effects of HIV therapies on mitochondria and apoptosis; and finally v) treatment strategies for HIV based upon modifying the apoptotic response.

HIV-1 viral protein R compromises cellular immune function in vivo

HIV-1 viral protein R (Vpr) is a virion-associated gene product that profoundly affects T cell proliferation, induces apoptosis and can affect cytokine production in part through interfering with NF-kappa B-mediated transcription from host cells. Collectively, these effects support that Vpr could influence immune activation in vivo. However, this effect of Vpr has not been explored previously. Here we examined the effect of Vpr expression in an in vivo model system on the induction of antigen-specific immune responses using a DNA vaccine model. Vpr co-vaccination significantly altered the immune response to co-delivered antigen. Specifically, in the presence of Vpr, inflammation was markedly reduced compared to antigen alone. Vpr reduced antigen-specific CD8-mediated cytotoxic T lymphocyte activity and suppressed T(h)1 immune responses in vivo as evidenced by lower levels of IFN-gamma. In the presence of Vpr, there is a profound shift in isotype towards a T(h)2 response as determined by the IgG2a:IgG1 ratio. The data support that Vpr compromises antigen-specific immune responses and ultimately effector cell function, thus confirming a strong selective advantage to the virus at the expense of the host.

HIV-1 NL4-3, but not IIIB, inhibits JAK3/STAT5 activation in CD4(+) T cells

HIV-1 infection leads to T cell dysfunction and apoptosis in vivo and in vitro. The shared common gamma chain of IL-2R and its associated Janus kinase, JAK3, are indispensable for normal T cell function and survival. We have reported that CD4 ligation with HIV gp120 inhibits T cell receptor-induced activation and expression of JAK3. We have also shown that while some strains of HIV-1, such as NL4-3, induce apoptosis of infected CD4(+) T cells, other strains, such as HIV-1 IIIB, do not. Interestingly, we show here that infection of CD4(+) T cells with HIV-1 NL4-3, but not IIIB, inhibited activation and expression of JAK3. NL4-3-infected T cells were unable to upregulate JAK3 expression following stimulation through TCR/CD3. In addition, NL4-3, but not IIIB, inhibited tyrosine phosphorylation and expression of STAT5, a downstream target of JAK3. These data suggest a correlation between apoptosis of HIV-1-infected T cells and inhibition of the JAK3/STAT5 activation pathway.

Apoptotic effects in primary human umbilical vein endothelial cell cultures caused by exposure to virion-associated and cell membrane-associated HIV-1 gp120

During the course of HIV-1 infection, free virus, infected cells, and free HIV-1 proteins circulate within the host, exposing the host endothelium to these viral factors. We have previously presented evidence showing that soluble HIV-1 gp120 protein interacts with chemokine receptors on primary human endothelium and (through those interactions) induces apoptosis as well as other intracellular effects. The current study examines the effect of exposure of vascular endothelium to gp120 IIIb expressed on the surface of Jurkat cells and in the context of viral particles. Apoptosis was observed in human umbilical vein endothelial cell (HUVEC) cultures exposed to gp160-transfected Jurkat cells as well as to virion particles with gp120 on their surface. Additional experiments show that this apoptotic effect was caused by gp120 protein acting through chemokine receptors on the HUVEC surface, primarily the CXCR4 receptor. At higher concentrations of gp120, this lymphotrophic variant, which has been shown to interact predominantly with CXCR4, seems to interact with and induce apoptosis through the CCR5 receptor. Finally, this apoptotic effect in HUVEC cultures occurs at low levels of the inducing agent, gp120, on cell membranes or on virion particles. These results demonstrate that HIV-1 gp120 is capable of interacting with and killing vascular endothelial cells in multiple in vivo contexts.

Syncytium formation amplifies apoptotic signals: a new view on apoptosis in HIV infection in vitro

Infection of CD4+ cells with HIV in vitro causes extensive cytopathology. The mechanism that underlies this process is unclear and conflicting data exist regarding whether cytotoxicity is due to necrosis or apoptosis. It was previously reported and is shown here that the coculture of HIV glycoprotein-expressing cells with CD4+ cells results in apoptosis within several hours. This study demonstrates that apoptosis did not occur in single cells and was mediated neither by CD4 nor by coreceptor signaling, indicating that apoptosis was not induced by intra- or intercellular glycoprotein-receptor interaction. Detection of apoptosis required cell-to-cell fusion and undetectable levels of apoptotic cell death were substantially amplified upon syncytium formation. Similar results were obtained with syncytium-forming cultures of measles virus glycoprotein-expressing cells. These findings indicate that the apoptotic cell death observed in cultures of HIV and other syncytium-forming viruses is primarily due to amplification of background apoptosis in the wake of cell-to-cell fusion.

Chemokine-receptor activation by env determines the mechanism of death in HIV-infected and uninfected T lymphocytes

There is considerable confusion concerning the mechanism of lymphocyte death during HIV infection. During the course of HIV infection, M-tropic viruses (R5) that use CCR5 chemokine coreceptors frequently evolve to T-tropic viruses (X4) that use CXCR4 receptors. In this study we show that activation of the CD4 or CCR5 receptor by R5 HIVenv causes a caspase 8-dependent death of both uninfected and infected CD4 T cells. In contrast, CXCR4 activation by X4 HIVenv induces a caspase-independent death of both uninfected CD4 and CD8 T cells and infected CD4 cells. These results suggest that activation of the chemokine receptor by HIVenv determines the mechanism of death for both infected and uninfected T lymphocytes.

Involvement of protein kinase C in HIV-1 gp120-induced apoptosis in primary endothelium

We previously showed that HIV-1 gp120-induced apoptosis in primary human umbilical vein endothelial cell cultures (HUVEC), through CCR5 and CXCR4. Here, we have found that agonists of protein kinase C (PKC), basic fibroblast growth factor (bFGF), and short exposure to low concentrations of phorbol esters were found to block gp120-induced apoptosis in HUVEC cultures. PKC antagonists, sphingosine, H7, and extended exposure of cultures to high concentrations of phorbol esters were also found to block gp120-induced apoptosis in HUVEC cultures. A significant increase in the total amount of cellular PKC enzymatic activity was observed on exposure of HUVEC to gp120. No increase in total PKC activity was observed on exposure of HUVECs to the natural ligands SDF-1alpha, or regulated-on-activation normal T-expressed and secreted (RANTES) cells, and gp120-induced PKC induction was found to be totally blocked by CXCR4 antibodies and partially blocked by the caspase 3 inhibitor, DEVD-CHO. Alternatively, CXCR4 antibodies and DEVD-CHO totally blocked apoptosis. Finally, gp120-induced effects were found to be insensitive to pertussis toxin. Accumulated evidence suggests PKC involvement at multiple points in the gp120-induced apoptotic pathway; also suggests involvement of the CXCR4 receptor internalization pathway, and potentially suggests different downstream effects of gp120-receptor interactions and natural ligand-receptor interactions.

Preferential S phase entry and apoptosis of CD4(+) T lymphocytes of HIV-1-infected patients after in vitro cultivation

We have studied the relationship between spontaneous apoptosis and cell cycle perturbations in circulating peripheral blood lymphocytes of HIV-1-infected patients and healthy controls. PBMC obtained from HIV-1-infected patients and healthy controls were incubated in culture medium for 48 h. Cells were separated into CD4(+) and CD8(+) populations using immunomagnetic beads. Apoptosis and cell cycle phases were measured by propidium iodide staining and bromodeoxyuridine (BrdU) incorporation followed by flow cytometric analyses. In experiments using cells obtained from HIV-1-infected patients, spontaneous apoptosis was more frequent in CD4(+) T lymphocytes than in CD8(+) T lymphocytes (17.6% vs 9.5%, P < 0.005). Among healthy controls, spontaneous apoptosis in CD4(+) and CD8(+) T lymphocytes was comparable (4.5% vs 5.1%). Lymphocytes obtained from patients were more frequently in S phase than healthy controls' cells (2.2 +/- 0.9% vs 0.5 +/- 0.2%, P < 0.002) and patients' CD4(+) cells tended to enter S phase more frequently than controls' CD4(+) cells (4.2% +/- 3.5% vs 1.8% +/- 0.5% P < 0.04), whereas the frequency of S phase CD8(+) T cells was not different among patients (2.8% +/- 2.9%) and controls (1.8% +/- 0.5%) (P > 0.4). Kinetic analyses using BrdU and PI staining revealed that S phase cells were more likely to become apoptotic than resting (G(0)-G(1)) cells (28.4% +/- 10.3% vs 11.3% +/- 9.9% in patients, P < 0.04, and 15.3% +/- 2.8% vs 1.8% +/- 0.5% in controls, P < 0.003). Lymphocytes obtained from HIV-1-infected persons are activated in vivo to enter S phase and to undergo spontaneous apoptosis after brief in vitro cultivation. The present studies indicate that most apoptotic cells in this system are CD4(+) and kinetic analyses reveal that S phase cells are more likely to undergo spontaneous apoptosis than G(0)-G(1) cells. Accelerated cell death in HIV-1 disease may contribute to the failure of lymphocyte responsiveness to appropriate T cell receptor stimulation.

Mechanisms of HIV-associated lymphocyte apoptosis

Infection with the human immunodeficiency virus (HIV) is associated with a progressive decrease in CD4 T-cell number and a consequent impairment in host immune defenses. Analysis of T cells from patients infected with HIV, or of T cells infected in vitro with HIV, demonstrates a significant fraction of both infected and uninfected cells dying by apoptosis. The many mechanisms that contribute to HIV-associated lymphocyte apoptosis include chronic immunologic activation; gp120/160 ligation of the CD4 receptor; enhanced production of cytotoxic ligands or viral proteins by monocytes, macrophages, B cells, and CD8 T cells from HIV-infected patients that kill uninfected CD4 T cells; and direct infection of target cells by HIV, resulting in apoptosis. Although HIV infection results in T-cell apoptosis, under some circumstances HIV infection of resting T cells or macrophages does not result in apoptosis; this may be a critical step in the development of viral reservoirs. Recent therapies for HIV effectively reduce lymphoid and peripheral T-cell apoptosis, reduce viral replication, and enhance cellular immune competence; however, they do not alter viral reservoirs. Further understanding the regulation of apoptosis in HIV disease is required to develop novel immune-based therapies aimed at modifying HIV-induced apoptosis to the benefit of patients infected with HIV.

Mechanisms of HIV-associated lymphocyte apoptosis

Infection with the human immunodeficiency virus (HIV) is associated with a progressive decrease in CD4 T-cell number and a consequent impairment in host immune defenses. Analysis of T cells from patients infected with HIV, or of T cells infected in vitro with HIV, demonstrates a significant fraction of both infected and uninfected cells dying by apoptosis. The many mechanisms that contribute to HIV-associated lymphocyte apoptosis include chronic immunologic activation; gp120/160 ligation of the CD4 receptor; enhanced production of cytotoxic ligands or viral proteins by monocytes, macrophages, B cells, and CD8 T cells from HIV-infected patients that kill uninfected CD4 T cells; and direct infection of target cells by HIV, resulting in apoptosis. Although HIV infection results in T-cell apoptosis, under some circumstances HIV infection of resting T cells or macrophages does not result in apoptosis; this may be a critical step in the development of viral reservoirs. Recent therapies for HIV effectively reduce lymphoid and peripheral T-cell apoptosis, reduce viral replication, and enhance cellular immune competence; however, they do not alter viral reservoirs. Further understanding the regulation of apoptosis in HIV disease is required to develop novel immune-based therapies aimed at modifying HIV-induced apoptosis to the benefit of patients infected with HIV.

Dual role of the HIV-1 vpr protein in the modulation of the apoptotic response of T cells

We investigated the effect of vpr, physiologically expressed during the course of an acute HIV-1 infection, on the response of infected cells to apoptotic stimuli as well as on the HIV-induced apoptosis. At 48 h after infection, Jurkat cells exhibited a lower susceptibility to undergo apoptosis with respect to uninfected cells. This effect was not observed following infection with either a vpr-mutated virus or a wild-type strain in the presence of antisense oligodeoxynucleotides targeted at vpr mRNA. Single-cell analysis, aimed at simultaneously identifying apoptotic and infected cells, revealed that resistance to apoptosis correlated with productive infection. Notably, vpr-dependent protection from induced apoptosis was also observed in HIV-1-infected PBMC. In contrast, at later stages of infection, a marked increase in the number of cells spontaneously undergoing apoptosis was detected in infected cultures. This virus-induced apoptosis involved vpr expression and predominantly occurred in productively infected cells. These results indicate that HIV-1 vpr can exert opposite roles in the regulation of apoptosis, which may depend on the level of its intracellular expression at different stages of HIV-1 infection. The dual function of vpr represents a novel mechanism in the complex strategy evolved by HIV to influence the turnover of T lymphocytes leading to either viral persistence or virus release and spreading.

Exposure of normal monocyte-derived dendritic cells to human immunodeficiency virus type-1 particles leads to the induction of apoptosis in co-cultured CD4+ as well as CD8+ T cells

The depletion of immune T cells by human immunodeficiency virus type-1 (HIV-1) infection is a major mechanism involved in the pathogenesis of AIDS. Here, we examined a possible effector function of blood monocyte-derived dendritic cells (DCs) to induce apoptosis in bystander CD4+ and CD8+ T cells. The DCs were generated by culturing monocytes in the presence of granulocyte-macrophage colony-stimulating factor and interleukin-4. The DCs exposed to HIV-1 particles were co-cultured with healthy donor-derived blood T cells at a ratio of 1:20. Analyses by percent cell mortality, staining with propidium iodide and reactivity with Annexin V revealed the induction of apoptosis in both CD4+ and CD8+ target T cells. Further, this apoptosis occurred without stimulation with mitogens when the cell cycle of target T cells shifted from G0 to G1, probably due to the mitogenic effect of the DCs. Thus, induction of apoptosis in both CD4+ and CD8+ T cells occurred via interaction with DCs adsorbed with HIV-1 particles.

Destruction of primary CD4(+) T cells by cell-cell interaction in human immunodeficiency virus type 1 infection in vitro

Infection of CD4(+) T lymphocytes with human immunodeficiency virus (HIV) in vitro is accompanied by extensive cytopathicity. The mechanism of cell death is unclear, but may be related to expression of the viral envelope glycoprotein. Here, it is demonstrated that T cell destruction in primary T cells occurs upon contact of infected with uninfected lymphocytes. Cell death was due to the interaction of the envelope glycoprotein with CD4 and subsequent fusion of the cells. Agents that interfered with cell-to-cell fusion such as a monoclonal antibody to CD4 and the peptide T20 prevented T cell death and depletion. In contrast, single-cell lysis due to expression and intracellular processing of the envelope glycoprotein was insignificant. These results suggest that cell-to-cell fusion and concomitant rapid cell death promote the depletion of T cells in HIV-infected individuals.

Caspase-dependent apoptosis of cells expressing the chemokine receptor CXCR4 is induced by cell membrane-associated human immunodeficiency virus type 1 envelope glycoprotein (gp120)

Human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins interact with CD4 and chemokine receptors on T cells to deliver signals that trigger either activation, anergy, or apoptosis. However, the molecular mechanisms driving these responses remain poorly understood. In this study we demonstrate that apoptosis is induced upon HIV-1 envelope binding to the chemokine receptor CXCR4. Cells expressing a mutant form of CXCR4 with a C-terminal deletion were also sensitive to HIV-1 envelope-mediated apoptosis, indicating that the cytoplasmic tail of CXCR4 is not required to induce the apoptotic pathway. The specificity of this process was analyzed using several inhibitors of gp120-CD4-CXCR4 interaction. Monoclonal antibodies directed against the gp120-binding site on CD4 (ST4) and against CXCR4 (MAB173) prevented the apoptotic signal in a dose-dependent manner. The cell death program was also inhibited by SDF-1alpha, the natural ligand of CXCR4, and by suramin, a G protein inhibitor that binds with a high affinity to the V3 loop of HIV-1 gp120 envelope protein. These results highlight the role played by gp120-binding on CXCR4 to trigger programmed cell death. Next, we investigated the intracellular signal involved in gp120-induced apoptosis. This cell death program was insensitive to pertussis toxin and did not involve activation of the stress- and apoptosis-related MAP kinases p38(MAPK) and SAPK/JNK but was inhibited by a broad spectrum caspase inhibitor (z-VAD.fmk) and a relatively selective inhibitor of caspase 3 (z-DEVD.fmk). Altogether, our results demonstrate that HIV induces a caspase-dependent apoptotic signaling pathway through CXCR4.

Could Nef and Vpr proteins contribute to disease progression by promoting depletion of bystander cells and prolonged survival of HIV-infected cells?

A growing body of literature suggests that the HIV accessory proteins Nef and Vpr could be involved in depletion of CD4(+) and non-CD4(+) cells and tissue atrophy, and in delaying the death of HIV-infected cells. Cell depletion is likely to be predominantly a bystander effect because the number of cells dying far outnumbers HIV-infected cells and is not confined to CD4(+) cells. The myristylated N-terminal region of Nef has severe membrane disordering properties, and when present in the extracellular medium causes rapid lysis in vitro of a wide range of CD4(+) and non-CD4(+) cells, suggesting a role for extracellular Nef in the depletion of bystander cells. A direct role for HIV-1 Nef in cytopathicity is supported by studies in HIV-infected Hu Liv/Thy SCID mice, in transgenic mice expressing nef gene alone, and in rhesus macaques infected with SIV/HIV chimeric virus containing HIV-1 nef. The N-terminal region of Nef has been directly implicated in development of simian AIDS. Extracellular Vpr and C-terminal fragments of Vpr cause membrane permeabilization and apoptosis of a wide range of CD4(+) and non-CD4(+) cells, and could also contribute to depletion of bystander cells. A direct in vivo role for Vpr in thymocyte depletion, thymic atrophy, and nephropathy is suggested in studies with vpr transgenic mice. Intracellular Nef and Vpr could help HIV-infected cells evade cell death by inhibiting apoptosis of infected cells and by avoiding virus-specific CTL response. Nef and Vpr are potential targets for therapeutic intervention and vaccine development, and strategies that prevent the death of bystander cells while promoting the early death of HIV-infected cells could arrest or retard progression to AIDS.

Primary HIV-1 infection of human CD4+ T cells passaged into SCID mice leads to selection of chronically infected cells through a massive fas-mediated autocrine suicide of uninfected cells

We have recently shown that a human CD4+ T cell line (CEM-SS) acquires the permissiveness to M-tropic strains and primary isolates of HIV-1 after transplantation into SCID mice. This permissiveness was associated with the acquisition of a memory (CD45RO+) phenotype as well as of a functional CCR5 coreceptor. In this study, we have used this model for invest-igating in vivo the relationships between HIV-1 infection, apoptosis and T cell differentiation. When an in vivo HIV-1 infection was performed, the CEM cell tumors grew to a lower extent than the uninfected controls. CEM cells explanted from uninfected SCID mice (ex vivo CEM) underwent a significant level of spontaneous apoptosis and proved to be CD45RO+, Fas+ and Fas-L+, while Bcl-2 expression was significantly reduced as compared to the parental cells. Acute HIV-1 infection markedly increased apoptosis of uninfected ex vivo CEM cells, through a Fas/Fas-L-mediated autocrine suicide/fratricide, while parental cells did not undergo apoptosis following viral infection. The susceptibility to apoptosis of ex vivo CEM cells infected with the NSI strain of HIV-1, was progressively lost during culture, in parallel with the loss of Fas-L and marked changes in the Bcl-2 cellular distribution. On the whole, these results are strongly reminiscent of a series of events possibly occurring during HIV-1 infection. After an initial depletion of bystander CD4+ memory T cells during acute infection, latently or chronically infected CD4+ T lymphocytes are progressively selected and are protected against spontaneous apoptosis through the development of an efficient survival program. Studies with human cells passaged into SCID mice may offer new opportunities for an in vivo investigation of the mechanisms involved in HIV-1 infection and CD4+ T cell depletion.

Productive infection of double-negative T cells with HIV in vivo

HIV induces CD4 down-regulation from the surface of infected cells by several independent mechanisms, suggesting an important biological role for this phenomenon. In vitro CD4 down-regulation generates T cells with a double-negative (DN) CD4(-)CD8(-) T cell receptor-alphabeta(+) phenotype. However, evidence that this down-regulation occurs in vivo in HIV-infected subjects is lacking, and viral load or viral production assays invariably focus on CD4(+) T cells. We show here that HIV infection can often be detected in sorted DN cells from peripheral blood and lymph nodes, even when plasma viral load is undetectable. DN T cells infected with HIV represented up to 20% of the cellular viral load in T cells, as determined by DNA PCR. In patients on successful highly active antiretroviral therapy, the viral load decreased in the plasma in CD4(+) and in DN T cells, suggesting that infected DN cells, like CD4(+) cells, contribute to viral production and are sensitive to highly active antiretroviral therapy. Indeed, HIV unspliced and multispliced RNAs were often detectable in DN T cells in spite of the small size of this subset. Infectious virus from DN T cells was transmitted efficiently in coculture experiments with uninfected T cell lymphoblasts, even when viral DNA in the DN cells was barely detectable. We conclude that a discrete population of infected DN T cells exists in HIV-positive subjects, even when the plasma viral load is undetectable. These cells may represent an important source of infectious virus.

Effect of extracellular human immunodeficiency virus type 1 glycoprotein 120 on primary human vascular endothelial cell cultures

During the course of an HIV-1 infection, free infectious and noninfectious virus particles, and free HIV-1 proteins, circulate within the host, exposing the host endothelium to these viral factors, even if the endothelium is not infected. This suggests that extracellular HIV-1 proteins could influence endothelial cell function, leading to pathogenesis. In light of this, we have used primary cultured human vascular endothelial cells (HUVECs) to screen for effects of the HIV-1 protein gp120 on endothelial cell function. The results of this study show that short exposure of HUVEC cultures to this protein causes significant levels of cytotoxicity. Further, using several different assays, we have shown that this cytotoxic effect on HUVECs appears to be due to induction of an apoptotic program. The biphasic nature of gp120 titration curves suggests that multiple cellular factors are mediating these gp120-induced effects. Competition studies appear to confirm this by showing that the apoptotic effect is mediated through two cell surface receptors on HUVECs, CCR5 and CXCR4. Alternatively, competition studies examining CD4 receptors suggests that CD4 played no role in gp12O-induced effects on HUVECs.

Activation of Bcl-2 promoter-directed gene expression by the human immunodeficiency virus type-1 Tat protein

Human immunodeficiency virus type 1 (HIV-1) Tat transcriptionally activates expression from a number of viral and cellular promoters. Recent studies demonstrate the ability of Tat to differentially modulate cellular responses to apoptotic signaling. The antiapoptotic effects of Tat appear to correlate with increased expression of Bcl-2, a cellular protein that enhances cellular survival. Here, endogenous expression of HIV-1 Tat in HeLa and Jurkat cells elevates levels of Bcl-2. Transient expression assays performed in HeLa cells demonstrate that Tat directly or indirectly enhances Bcl-2 promoter-directed gene expression by more than 10-fold. Analyses of Tat mutants demonstrate that two noncontiguous regions in the N- and C-termini of Tat mediate maximal transactivation of the Bcl-2 promoter. The requirement for C-terminal sequences contrasts with transactivation of the HIV-1 long terminal repeat in which the N-terminal 57 amino acids are required but downstream residues are not. Bcl-2 promoter analyses suggest that sequences required for Tat responsiveness are located upstream of P1 and between the P1 and P2 promoter units. Results from these studies reveal effects of HIV-1 Tat on Bcl-2 expression and provide a putative mechanism by which endogenously expressed Tat affects cellular survival through the up-regulation of Bcl-2.

B7 cosignal potentiates apoptosis of uninfected CD4+ T lymphocytic cell lines primed by HIV envelope proteins

In lymphoid organs, follicular dendritic cells (FDCs), monocytes, and macrophages are targets for HIV infection and reservoirs for infectious virus. Strikingly, the apoptotic cells in these sites are essentially uninfected CD4+ T lymphocytes, but lie in close proximity to infected cells or FDCs carrying trapped HIV virions. To decipher this apoptotic pathway, we have established a two-step experimental system that reproduces in vitro the HIV envelope protein-mediated apoptosis restricted to uninfected CD4+ T lymphocytic cell lines. In this assay, uninfected CD4+ T cell targets undergo apoptosis following an initial priming step on HeLa cells expressing functional HIV envelope proteins at their plasma membrane and a second and necessary stimulation step via the CD3-TCR complex. The CD4+ T lymphocytic cells susceptible to apoptosis are, in contrast, resistant to cell fusion mediated by HIV envelope protein and express SDF-1. FDCs and macrophages are known to be high B7 expressors. Thus in lymph nodes, the cells that have trapped HIV particles in immune complexes at the plasma membrane present both HIV envelope proteins and B7.1 at their surface. We mimicked this situation in vitro by priming CD4+ T lymphocytes on cells expressing the costimulatory molecule B7 in addition to HIV envelope proteins, and show that it resulted in an acceleration and a twofold increase in apoptosis. Finally, we characterized two enzymes, PI3Kinase and PI-PLC, which are both downstream effectors of the CD4 (HIV envelope protein receptor) and CD28 (B7 receptor) activation pathways, and that participated in the early steps of priming for apoptosis.

Differential expression profiles of apoptosis-affecting genes in HIV-infected cell lines and patient T cells

OBJECTIVE

To clarify the molecular mechanisms of HIV-induced apoptosis.

DESIGN

The assessment of expression patterns for genes affecting the interrelated cell cycle and apoptosis processes in HIV-1LAI-infected T lymph oblastoid (CEM) cells, as well as CD4 and CD8 cells from HIV-infected individuals and controls.

METHODS

The kinetics of HIV infection in CEM cells were defined by flow cytometry of green fluorescent protein expression from a reporter vector. Apoptosis of CEM cells was measured by propidium iodine staining and flow cytometry. Gene expression levels were determined by a multiprobe RNase protection assay.

RESULTS

The infection and apoptosis of CEM cells were associated with enhanced expression of Bax, Bcl-2, Bcl-X(L) and caspase 1 (ICE). There was increased expression of Bcl-2 and caspase 1 and decreased expression of cyclin-dependent kinase inhibitor p21CIP1 in CD4 cells of HIV-infected individuals compared with uninfected controls. The CD8 cells of HIV-infected individuals exhibited increased expression of Bcl-2, Bcl-X(L), Bax and caspase 1 but, in contrast to the CD4 subset, they showed elevated expression of p21CIP1 and p16INK4a compared with controls.

CONCLUSIONS

The Bax increase in CEM cells appears to be a direct effect associated with a high frequency of infection and apoptosis, because it was not found in the CD4 cells of patients. In contrast, the increase of Bax in the CD8 cells of patients seems to be an indirect effect. Increases in Bcl-2, Bcl-X(L) and caspase 1 in HIV-infected CEM cells may be caused by both direct and indirect mechanisms, because they also occurred in CD4 and CD8 cells of HIV-infected individuals. In addition, the low expression of p21CIP1 in the CD4 subset of HIV-infected individuals could promote apoptosis, whereas the high expression of p21CIP1 and p16INK4a in the CD8 subset may lead to a state of anergy, akin to replicative senescence.

CXCR4 and CD4 mediate a rapid CD95-independent cell death in CD4(+) T cells

AIDS is characterized by a progressive decrease of CD4(+) helper T lymphocytes. Destruction of these cells may involve programmed cell death, apoptosis. It has previously been reported that apoptosis can be induced even in noninfected cells by HIV-1 gp120 and anti-gp120 antibodies. HIV-1 gp120 binds to T cells via CD4 and the chemokine coreceptor CXCR4 (fusin/LESTR). Therefore, we investigated whether CD4 and CXCR4 mediate gp120-induced apoptosis. We used human peripheral blood lymphocytes, malignant T cells, and CD4/CXCR4 transfectants, and found cell death induced by both cell surface receptors, CD4 and CXCR4. The induced cell death was rapid, independent of known caspases, and lacking oligonucleosomal DNA fragmentation. In addition, the death signals were not propagated via p56(lck) and Gialpha. However, the cells showed chromatin condensation, morphological shrinkage, membrane inversion, and reduced mitochondrial transmembrane potential indicative of apoptosis. Significantly, apoptosis was exclusively observed in CD4(+) but not in CD8(+) T cells, and apoptosis triggered via CXCR4 was inhibited by stromal cell-derived factor-1, the natural CXCR4 ligand. Thus, this mechanism of apoptosis might contribute to T cell depletion in AIDS and might have major implications for therapeutic intervention.

A specific T-cell subset with CD4+/CD38- markers derived from HIV-1 carriers induces apoptosis in healthy donor-derived T-lymphocytes

Apoptosis is an important mechanism of human immunodeficiency virus type 1 (HIV-1)-induced T-cell depletion. Our recent findings revealed mitogenic stimulation-dependent apoptosis induction in healthy donor-derived peripheral blood T-lymphocytes after adsorption with defective HIV-1 particles through acquirement by a subset of CD4+/CD38- cells of specific killer function. Based on these in vitro observations, we have extended the significance of this killing activity of CD4+/CD38- cells directly derived from HIV-1 carriers. The CD4+/CD38- cells from HIV-1-positive individuals showed significantly higher cell-killing activities than those from HIV-1-negative donors by co-culture with allogeneic resting T-cells after mitogenic stimulation. Furthermore, most of the samples induced apoptosis in a Fas-dependent manner. Thus, it is suggested that HIV-1 infection-related apoptosis is triggered by inappropriate activation of a certain resting T-cell subset, presumably due to adsorption with HIV-1 particles.

Characterization of the cytotoxic factor(s) released from thymic dendritic cells upon human immunodeficiency virus type 1 infection

We previously demonstrated that infection of primary human thymic dendritic cells (DCs) with laboratory strains of HIV leads to the release of soluble factor(s) which induced thymocyte killing. In the present paper, we extend the characterization of this process. Our results reveal that primary HIV-1 isolates are similarly able to induce the production of cytotoxic factor(s) from thymic DCs and that the release of such factor(s) is dependent on viral infection. Interestingly, we observed that CD4+ and CD8+ purified thymocyte subsets, and activated PBMCs are susceptible to the cytotoxic activity, whereas freshly isolated resting PBMCs are resistant to this effect. Cycloheximide treatment prevents the killing of thymocytes exposed to HIV-infected DC supernatant, revealing that this form of cell death is an active biological process requiring protein synthesis. Finally, our data suggest that FasL and TNF alpha could both participate in the killing process. These in vitro observations provide a plausible model, whereby HIV-infected DCs can play a role in vivo in the induction of uninfected thymocyte killing.

The HIV-1 vpr protein acts as a negative regulator of apoptosis in a human lymphoblastoid T cell line: possible implications for the pathogenesis of AIDS

Although apoptosis is considered one of the major mechanisms of CD4(+) T cell depletion in HIV-infected patients, the virus-infected cells somehow appear to be protected from apoptosis, which generally occurs in bystander cells. Vpr is an auxiliary HIV-1 protein, which, unlike the other regulatory gene products, is present at high copy number in virus particles. We established stable transfectants of CD4+ T Jurkat cells constitutively expressing low levels of vpr. These clones exhibited cell cycle characteristics similar to those of control-transfected cells. Treatment of control clones with apoptotic stimuli (i.e., cycloheximide/tumor necrosis factor alpha (TNF-alpha), anti-Fas antibody, or serum starvation) resulted in a massive cell death by apoptosis. In contrast, all the vpr-expressing clones showed an impressive protection from apoptosis independently of the inducer. Notably, vpr antisense phosphorothioate oligodeoxynucleotides render vpr-expressing cells as susceptible to apoptosis induced by cycloheximide and TNF-alpha as the control clones. Moreover, the constitutive expression of HIV-1 vpr resulted in the upregulation of bcl-2, an oncogene endowed with antiapoptotic activities, and in the downmodulation of bax, a proapoptotic factor of the bcl-2 family. Altogether, these results suggest that low levels of the endogenous vpr protein can interfere with the physiological turnover of T lymphocytes at early stages of virus infection, thus facilitating HIV persistence and, subsequently, viral spread. This might explain why apoptosis mostly occurs in bystander uninfected cells in AIDS patients.

Interferon-alpha and -gamma differentially reduce rapid immature T-cell death by contact with HIV-1 carrier cell clones in vitro

The non-antigen specific rapid cytotoxic (CT) death of immature TdT+CD4+CD8+ T cells due to contact with HIV-1 carrier T-cell clones we have found recently is a novel phenomenon. The effects of interferons (IFN) on this CT reaction were studied in vitro. Treatment of the HIV-1 carrier clones, referred to as "effectors," with IFN-alpha but not IFN-gamma, or of the susceptible immature TdT+CD4+CD8+ T cells, referred to as "targets," with IFN-gamma but not IFN-alpha, for 24 hr prior to CT testing was found to reduce the CT reaction. Simultaneously, a down-regulated CD8 expression and an up-regulated antigen expression of both major histocompatibility antigen complex class I (MHC-I) and HIV-1 gp120/gp160 in the IFN-alpha treated effector (gp120+CD8+ HPB-ALL/HIV), and/or simultaneously up-regulated antigen expression of both CD8 and MHC-I in the IFN-gamma treated target (CD4+CD8+ HPB-ALL) were found to be associated with reduced CT reaction. However, altered antigen expression in the IFN-gamma treated effectors or IFN-alpha treated targets did not affect the ultimate degree of CT reaction. This study thus suggests a possible therapeutic efficacy of IFN by reducing the direct elimination of the T-cell precursors in HIV-1 infection.

CD26 as a positive regulator of HIV envelope-glycoprotein induced apoptosis in CD4+ T cells

The membrane-expressed HIV-1 envelope glycoprotein complex, gp120/gp41, has been shown to be responsible for the initiation of cell killing by apoptosis in CD4+ T cells. By using two experimental approaches we demonstrate that CD26, independent of its DPP IV activity, appears to be implicated in this function of the gp120/gp41 complex to initiate apoptosis.