HIV-induced apoptosis requires the CD4 receptor cytoplasmic tail and is accelerated by interaction of CD4 with p56lck

How the Discovery of the CD4/CD8-p56lck Complexes Changed Immunology and Immunotherapy

The past 25 years have seen enormous progress in uncovering the receptors and signaling mechanisms on T-cells that activate their various effecter functions. Until the late 1980s, most studies on T-cells had focused on the influx of calcium and the levels of cAMP/GMP in T-cells. My laboratory then uncovered the interaction of CD4 and CD8 co-receptors with the protein-tyrosine kinase p56^lck which are now widely accepted as the initiators of the tyrosine phosphorylation cascade leading to T-cell activation. The finding explained how immune recognition receptors expressed by many immune cells, which lack intrinsic catalytic activity, can transduce activation signals via non-covalent association with non-receptor tyrosine kinases. The discovery also established the concept that a protein tyrosine phosphorylation cascade operated in T-cells. In this vein, we and others then showed that the CD4- and CD8-p56^lck complexes phosphorylate the TCR complexes which led to the identification of other protein-tyrosine kinases such as ZAP-70 and an array of substrates that are now central to studies in T-cell immunity. Other receptors such as B-cell receptor, Fc receptors and others were also subsequently found to use src kinases to control cell growth. In T-cells, p56^lck driven phosphorylation targets include co-receptors such as CD28 and CTLA-4 and immune cell-specific adaptor proteins such as LAT and SLP-76 which act to integrate signals proximal to surface receptors. CD4/CD8-p56^lck regulated events in T-cells include intracellular calcium mobilization, integrin activation and the induction of transcription factors for gene expression. Lastly, the identification of the targets of p56^lck in the TCR and CD28 provided the framework for the development of chimeric antigen receptor (CAR) therapy in the treatment of cancer. In this review, I outline a history of the development of events that led to the development of the “TCR signaling paradigm” and its implications to immunology and immunotherapy.

Impact of vaginal microbiome communities on HIV antiretroviral-based pre-exposure prophylaxis (PrEP) drug metabolism

Despite the efficacy of antiretroviral-based pre-exposure prophylactics (PrEP) in men who have sex with men, studies in women have produced widely varying outcomes. Recent evidence demonstrates that vaginal microbial communities are associated with increased HIV acquisition risk and may impact PrEP efficacy. Here, we investigate the mechanisms underlying how vaginal bacteria alter PrEP drug levels and impact HIV infection rates ex vivo. Using cervicovaginal lavages (CVLs) from women with or without bacterial vaginosis (BV), we identified microbial metabolism of PrEP drugs in BV samples through LC-MS/MS analysis of soluble drug levels and metabolite formation in dual T-cell cultures. CVL samples were assessed for microbiome analysis using sequencing of bacterial 16S rRNA genes. We also observed non-Lactobacillus bacteria that are associated with BV may potentially impact PrEP efficacy through increased HIV infection rates in co-cultures containing Lactobacillus or BV bacteria, PrEP drugs, CEM-GFP cells, and HIV-1_LAI virus. Finally, we used these data to develop a novel predictive mathematical simulation modeling system to predict these drug interactions for future trials. These studies demonstrate how dysbiotic vaginal microbiota may impact PrEP drugs and provides evidence linking vaginal bacteria to PrEP efficacy in women.

HIV prevention strategies with antiretroviral drugs as pre-exposure prophylactics (PrEP) are not efficacious in women, in part due to biological factors such as the vaginal microbiome. Lactobacillus spp. prevent the colonization of dysbiotic bacteria (i.e. bacterial vaginosis), which is associated with increased HIV transmission. However, the role these microbes play in altering the pharmacokinetics of PrEP drugs is currently unknown. Here we show that primary bacteria from women with dysbiosis impact PrEP drug levels and kinetics. We found that dysbiotic microbes metabolize PrEP drugs faster than target cell uptake, and significantly alter HIV infection rates in vitro, and this can be predicted with novel computational modeling. Our results demonstrate that the vaginal microbiome may play a key role in HIV prevention through altering therapeutic drug levels via metabolism. Thus, better measurements and interventions for vaginal dysbiosis will be critical in improving the efficacy of HIV prevention efforts in women.

The inositol 5-phosphatase SHIP-1 and adaptors Dok-1 and 2 play central roles in CD4-mediated inhibitory signaling

CD4 functions to enhance the sensitivity of T cells to antigenic peptide/MHC class II. However, if aggregated in isolation, e.g. in the absence of T cell receptor (TCR), CD4 can transduce yet undefined signals that lead to T cell unresponsiveness to antigen and apoptosis. In Human Immunodeficiency Virus-1 (HIV-1) disease, CD4(+) T cell loss can result from gp120-induced CD4 signaling in uninfected cells. We show here that CD4 aggregation leads to Lck-dependent phosphorylation of the RasGAP adaptors Downstream of kinase-1/2 (Dok-1/2) and the inositol 5-phosphatase-1 (SHIP-1) and association of the two molecules. Studies using SHIP-1 shRNA, knockout mice and decoy inhibitors further indicate that CD4-mediated inhibition of TCR-mediated T cell activation is SHIP-1 and Dok-1/2 dependent, and involves SHIP-1 hydrolysis of Phosphatidylinositol 3,4,5-trisphosophate (PI(3,4,5)P3) needed for TCR signaling. Our studies provide evidence for a novel mechanism by which ill-timed CD4-mediated signals activated by ligands such as HIV-1 gp120 lead to disarmament of the immune system.

Ephrin-B2 expression critically influences Nipah virus infection independent of its cytoplasmic tail

Background

Cell entry and cell-to-cell spread of the highly pathogenic Nipah virus (NiV) requires binding of the NiV G protein to cellular ephrin receptors and subsequent NiV F-mediated fusion. Since expression levels of the main NiV entry receptor ephrin-B2 (EB2) are highly regulated in vivo to fulfill the physiological functions in axon guidance and angiogenesis, the goal of this study was to determine if changes in the EB2 expression influence NiV infection.

Results

Surprisingly, transfection of increasing EB2 plasmid concentrations reduced cell-to-cell fusion both in cells expressing the NiV glycoproteins and in cells infected with NiV. This effect was attributed to the downregulation of the NiV glycoproteins from the cell surface. In addition to the influence on cell-to-cell fusion, increased EB2 expression significantly reduced the total amount of NiV-infected cells, thus interfered with virus entry. To determine if the negative effect of elevated EB2 expression on virus entry is a result of an increased EB2 signaling, receptor function of a tail-truncated and therefore signaling-defective ΔcEB2 was tested. Interestingly, ΔcEB2 fully functioned as NiV entry and fusion receptor, and overexpression also interfered with virus replication.

Conclusion

Our findings clearly show that EB2 signaling does not account for the striking negative impact of elevated receptor expression on NiV infection, but rather that the ratio between the NiV envelope glycoproteins and surface receptors critically influence cell-to-cell fusion and virus entry.

An immunoglobulin fusion protein based on the gp120-CD4 receptor complex potently inhibits human immunodeficiency virus type 1 in vitro

Fusion proteins containing immunoglobulin Fc domains attached to bioactive moieties have been developed as therapeutic agents against several diseases. Here, we describe the development and characteristics of a novel fusion protein (FLSC R/T-IgG1) that targets CCR5, the major coreceptor for HIV-1 during primary infection. FLSC R/T-IgG1 was expressed from a synthetic gene that linked a single chain gp120-CD4 complex containing an R5 gp120 sequence with the hinge-CH2-CH3 portion of human immunoglobulin gamma subtype 1. Purified FLSC R/T-IgG1 exhibited a molecular mass of 189 kDa under reducing conditions, which matched the expected size of one polypeptide chain. Chemically crosslinked or untreated FLSC R/T-IgG1 exhibited a mass of a 360-kDa polypeptide under reducing and nonreducing conditions, which indicated that the molecule adopts a disulfide-linked bivalent structure. The chimeric molecule bound specifically to CCR5-expressing cells and to peptides derived from the CCR5 N-terminus. Such binding was more efficient than what was obtained with a monomeric single chain gp120-CD4 complex. FLSC R/T-IgG1 binding to CCR5 was blocked by preincubation of coreceptor-expressing cells with CCR5 ligands and by antibody to the coreceptor binding domain of gp120. Conversely, FLSC R/T-IgG1 blocked the binding of chemokine to CCR5. However, FLSC R/T-IgG1 did not trigger intracellular Ca2+ mobilization in peripheral blood mononuclear cells. FLSC R/T-IgG1 potently neutralized primary R5 HIV-1 in both a PBMC-based assay and cell line-based assays but did not affect the replication of X4 viruses. These findings suggest that FLSC R/T-IgG1 might be used as a possible therapeutic agent against HIV.

Acquired T-cell sensitivity to TRAIL mediated killing during HIV infection is regulated by CXCR4-gp120 interactions

BACKGROUND

Sensitivity towards apoptosis induced by ligation of the tumor necrosis factor family of death receptors is controlled in part by death receptor expression. Whereas cellular activation enhances Fas receptor expression and induces Fas sensitivity, such cellular activation neither alters TRAIL receptor expression nor induces TRAIL sensitivity. Cells infected by HIV acquire sensitivity to TRAIL induced death, although the mechanisms by which this is achieved are undefined.

OBJECTIVE

To define the mechanism by which cells from HIV infected patients acquire sensitivity to TRAIL mediated killing.

DESIGN

In vitro assessment of TRAIL receptor expression and TRAIL sensitivity.

METHODS

Treatment of Jurkat T cells, peripheral blood lymphocytes from HIV negative donors, or human osteogenic seroma (HOS) cells expressing CD4, CXCR4 or CCR5 with T tropic gp120, M tropic gp120, or agonistic antibodies against CD4, CXCR4 or CCR5. TRAIL receptors were measured by flow cytometry or reverse transcription-PCR and TRAIL sensitivity was assessed by incubation with recombinant TRAIL followed by Annexin V fluorescein isothiocyanate/Propidium Iodide (PI) staining.

RESULTS

Treatment of uninfected Jurkat T cells, as well as primary T cells with gp120 results in the upregulation of TRAIL death receptor expression and acquired sensitivity to TRAIL mediated cell death. The increase in TRAIL death receptor expression and acquisition of TRAIL sensitivity requires the chemokine coreceptor CXCR4 but not CCR5 or the CD4 receptor.

CONCLUSIONS

These results indicate that chemokine receptor interactions regulate TRAIL receptor expression and provide an explanation for the acquired T cell sensitivity to TRAIL mediated killing death during HIV infection.

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 infection is facilitated in T cells by decreasing p56lck protein tyrosine kinase activity

Several studies have suggested an important role for the protein tyrosine kinase p56lck (Lck) in HIV infection; however, the exact nature of this role remains unclear. Using a series of well characterized Jurkat-derived cell lines having a wide range of Lck kinase activity, our results showed that, while the entry of HIV-1 into these cell lines was similar, the kinetics of virus production by these cells were very different. Cells expressing a kinase-inactive Lck showed accelerated viral replication, whereas, cells expressing Lck with normal or elevated enzymatic activity showed a delay in virus replication that was proportional to the initial level of endogenous Lck activity. The cell line having the highest initial Lck kinase activity showed the slowest rate of productive HIV-1 infection. Analysis of 2-LTR circles revealed that this inhibitory effect of Lck was not due to inhibition of reverse transcription of HIV-1 genome or migration of the proviral DNA into the nuclei. This affect of Lck was confirmed in additional studies that used either the S1T cell line lacking completely Lck or where the Lck activity was altered in Jurkat cells prior to infection. S1T cells showed a 3- to 12-fold increase in the level of infection compared to Jurkat cells despite similar CD4 and chemokine coreceptor expression and cell doubling times. Pretreatment of Jurkat with an antisense lck oligodeoxynucleotide inhibited the synthesis of functional Lck and facilitated the viral replication by the cells as did expressing a dominant-negative mutant Lck which increased the productive infection>3-fold. Conversely, whereas IL-16 had no affect on productive infection in S1T cells that lack Lck, IL-16 pretreatment of Jurkat cells resulted in an immediate (within 5 min) and sustained and gradual (over 5 h) increase in Lck activity that resulted in a reduction of HIV-1 replication that paralleled the increasing Lck kinase activity. These results show that the enzymatic activity of Lck kinase can affect viral replication, that a lack of, or decreased Lck activity facilitates viral replication. Conversely, Lck can mediate a delay in HIV-1 infection that is proportional to the initial endogenous Lck enzyme activity.

Mitochondria in HIV-1-induced apoptosis

It is now well admitted that HIV infection leading to AIDS is associated with an abnormal susceptibility of T cells to undergo apoptosis. Recent progress in research into programmed cell death has resulted in the identification of the principal pathways involved in this process. Thus the "extrinsic" as well as the "intrinsic" pathways converge to the mitochondria considered as the main sensor of programmed cell death. This review summarizes our knowledge of the influence of mitochondrial control on T cell death during HIV and SIV infections.

Cell surface receptors, virus entry and tropism of primate lentiviruses

Human immunodeficiency virus (HIV) exploits cell surface receptors to attach to and gain entry into cells. The HIV envelope spike glycoprotein on the surface of virus particles binds both CD4 and a seven-transmembrane coreceptor. These interactions trigger conformational changes in the envelope spike that induce fusion of viral and cellular membranes and entry of the viral core into the cell cytoplasm. Other cell surface receptors also interact with gp120 and aid attachment of virus particles. This review describes these receptors, their roles in HIV entry and their influence on cell tropism.

Productive HIV-1 infection of primary CD4+ T cells induces mitochondrial membrane permeabilization leading to a caspase-independent cell death

We have explored in vitro the mechanism by which human immunodeficiency virus, type 1 (HIV-1) induces cell death of primary CD4+ T cells in conditions of productive infection. Although HIV-1 infection primed phytohemagglutinin-activated CD4+ T cells for death induced by anti-CD95 antibody, T cell death was not prevented by a CD95-Fc decoy receptor, nor by decoy receptors of other members of the TNFR family (TNFR1/R2, TRAILR1/R2/OPG, TRAMP) or by various blocking antibodies, suggesting that triggering of death receptors by their cognate ligands is not involved in HIV-induced CD4 T cell death. HIV-1 induced CD4 T cell shrinkage, cell surface exposure of phosphatidylserine, loss of mitochondrial membrane potential (Deltapsim), and mitochondrial release of cytochrome c and apoptosis-inducing factor. A typical apoptotic phenotype (nuclear chromatin condensation and fragmentation) only occurred in around half of the dying cells. Treatment with benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, a broad spectrum caspase inhibitor, prevented nuclear chromatin condensation and fragmentation in HIV-infected CD4+ T cells and in a cell-free system (in which nuclei were incubated with cytoplasmic extracts from the HIV-infected CD4+ T cells). Nevertheless, benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone did not prevent mitochondrial membrane potential loss and cell death, suggesting that caspases are dispensable for HIV-mediated cell death. Our findings suggest a major role of the mitochondria in the process of CD4 T cell death induced by HIV, in which targeting of Bax to the mitochondria may be involved.

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.

Apoptosis and caspases

The expedition into the apoptosis signaling pathway, although it has just begun, has resulted in the discovery of a significant number of remarkable signaling molecules at all levels of this novel pathway After the pinnacle of this frenetic cloning effort has been reached, however, it is important to put this pathway and its constituents into a biological and pathophysiological context. It has become clear that cell death does not automatically mean activation of caspases. The recent discovery of a function of effector caspases of the apoptosis pathway outside of apoptosis is currently revolutionizing our view of these seemingly unrelated and rather counteracting processes, cell death and cell proliferation. It appears that caspases play a much more fundamental role in cells than originally expected.

HIV type 1-induced inhibition of CD45 tyrosine phosphatase activity correlates with disease progression and apoptosis, but not with anti-CD3-induced T cell proliferation

The tyrosine phosphatase CD45 is a key positive element in multiple lymphocyte signaling pathways. To understand the contribution of CD45 to HIV-1-induced T cell hyporesponsiveness and apoptosis we evaluated the CD45-associated tyrosine phosphatase activity of lymphocytes from patients with different stages of HIV-1 disease and compared it with CD45 expression, spontaneous and Fas-induced apoptosis, anti-CD3-induced T cell proliferation, distribution of CCR5 delta32/wt, and cytokine production. The proliferative response to anti-CD3 as well as the CD45-associated phosphatase activity were significantly reduced in progressors. In long-term nonprogressors (LTNPs) the proliferative response to anti-CD3 was also diminished, although to a lesser extent, while the tyrosine phosphatase activity was not significantly impaired. One-third of LTNPs were found positive for the 32-bp deletion of the CCR5 gene. This mutation had no effects on anti-CD3 proliferative response or CD45 phosphatase activity. A significant reduction in IL-2 and IFN-gamma was observed in both LTNPs and in normal progressors, whereas IL-4 production was significantly decreased only in progressors. Last, we observed a significant correlation between CD45 phosphatase activity and apoptosis. We therefore conclude that the impairment of CD45 tyrosine phosphatase activity correlates with disease progression and the level of T cell apoptosis, but not with anti-CD3-induced T cell proliferation. Moreover, we suggest that evaluation of CD45 tyrosine phosphatase activity may represent an additional tool with which to assess disease progression.

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.

SLP-76 Binding to p56lck: A Role for SLP-76 in CD4-Induced Desensitization of the TCR/CD3 Signaling Complex

Nonreceptor protein tyrosine kinases and associated substrates play a pivotal role in Ag receptor stimulation of resting cells and in the initiation of activation-induced cell death (AICD) of preactivated T cells. CD4-associated p56lck has been implicated not only in the activation of primary T cells, but also in the inhibition of T cell responses. We have previously shown that CD4+ T cell clones can be rescued from AICD when surface CD4 is engaged before the TCR stimulus. In this study, we show that prevention of AICD is associated with a CD4-dependent inhibition of TCR-triggered tyrosine phosphorylation of the Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76) and Vav. We provide evidence for a SLP-76 interaction with Src homology 3 domains of p56lck and identify amino acids 185–194 of SLP-76 as relevant docking site. In view of the multiple functions of p56lck and SLP-76/Vav in the initiation of TCR/CD3/CD4 signaling, we propose a model for the CD4-dependent inhibition of TCR signaling and AICD of preactivated T cells. Our data suggest that preformed activation complexes of adapter proteins and enzymes in the vicinity of the CD4/p56lck complex are no longer available for the TCR signal when CD4 receptors are engaged before TCR stimulation.

The implication of the chemokine receptor CXCR4 in HIV-1 envelope protein-induced apoptosis is independent of the G protein-mediated signalling

OBJECTIVE

The envelope glycoprotein complex (gp120/gp41)n of HIV-1 is one of the viral products responsible for increased apoptosis in HIV infection. Here the role of the chemokine receptor CXCR4 in HIV-1 envelope protein-induced apoptosis was investigated.

METHODS

Apoptosis occurring in cocultures of chronically HIV-1 IIIB-infected cells with CD4 target cells expressing the CXCR4 receptor was quantified by terminal deoxinucleotidyl transferase dUTP nick end labeling (TUNEL) or propidium iodide staining followed by fluorescent antibody cell sorting, which allows the evaluation of single-cell killing. Moreover global (single cell- and syncytium-associated) apoptosis was quantified by a new radioactive TUNEL-derived assay.

RESULTS

By using these different techniques it was shown that single and syncytium-forming CD4 T cells die by apoptosis upon contact with envelope protein expressing cells independently of viral replication. Moreover, both the CXCR4 agonist SDF-1alpha, and the antagonist AMD3100, showed inhibitory effects on HIV-1 envelope protein-induced apoptosis in the CD4 T-cell subset of peripheral blood mononuclear cells and CD4 cell lines. CXCR4 signalling-induced by HIV-1 envelope proteins in CD4 T cells was not detected. Furthermore, it was shown that envelope protein-induced apoptosis can occur after treating target cells with the Gi-protein inhibitor pertussis toxin.

CONCLUSIONS

Evidence is provided for a role of CXCR4 in the mechanisms of HIV envelope protein-induced pathogenesis, contributing to selective CD4 cell killing. The results suggest that CXCR4 is involved in HIV-1-induced apoptosis; however, this role does not appear to involve G-protein-mediated CXCR4 signalling.

MEK1 activation rescues Jurkat T cells from Fas-induced apoptosis

Although the protease cascade initiated by Fas (CD95, Apo-1) is well characterized, there remains little known about how kinase pathways may impact on Fas-mediated apoptosis. We recently observed that in T lymphocytes Fas strongly induced activation of JNK (c-Jun N-terminal kinase) but not of second messengers leading to activation of ERK (extracellular regulated kinase). Additionally, Fas-mediated apoptosis was significantly inhibited with PMA, a potent activator of the ERK signaling pathway. This suggested a model whereby activation of the ERK pathway might attenuate Fas-mediated apoptosis. This was confirmed in the current study by showing that activation of MEK1, the upstream regulator of ERK, reduces Fas-mediated apoptosis, whereas inhibition of MEK1 augments apoptosis by Fas. Furthermore, Fas-mediated apoptosis of Jurkat T cells is not affected by constitutively active or dominant negative variants that modulate the JNK pathway. These results demonstrate that Fas-induced JNK activation is not required for apoptosis by Jurkat T cells, but rather is more likely secondary to cell stress during the early phases of apoptosis. This is supported by the ability of the caspase blocker zVAD to inhibit both apoptosis and JNK activation by Fas.

The role of in vitro-induced lymphocyte apoptosis in feline immunodeficiency virus infection: correlation with different markers of disease progression

Human immunodeficiency virus infection is characterized by a progressive decline in the number of peripheral blood CD4(+) T lymphocytes, which finally leads to AIDS. This T-cell decline correlates with the degree of in vitro-induced lymphocyte apoptosis. However, such a correlation has not yet been described in feline AIDS, caused by feline immunodeficiency virus (FIV) infection. We therefore investigated the intensity of in vitro-induced apoptosis in peripheral blood lymphocytes from cats experimentally infected with a Swiss isolate of FIV for 1 year and for 6 years and from a number of long-term FIV-infected cats which were coinfected with feline leukemia virus. Purified peripheral blood lymphocytes were either cultured overnight under nonstimulating conditions or stimulated with phytohemagglutinin and interleukin-2 for 60 h. Under stimulating conditions, the isolates from the infected cats showed significantly higher relative counts of apoptotic cells than did those from noninfected controls (1-year-infected cats, P = 0.01; 6-year-infected cats, P = 0.006). The frequency of in vitro-induced apoptosis was inversely correlated with the CD4(+) cell count (P = 0. 002), bright CD8(+) cell count (P = 0.009), and CD4/CD8 ratio (P = 0. 01) and directly correlated with the percentage of bright major histocompatibility complex class II-positive peripheral blood lymphocytes (P = 0.004). However, we found no correlation between in vitro-induced apoptosis and the viral load in serum samples. Coinfection with feline leukemia virus enhanced the degree of in vitro-induced apoptosis compared with that in FIV monoinfected cats. We concluded that the degree of in vitro-induced apoptosis was closely related to FIV-mediated T-cell depletion and lymphocyte activation and could be used as an additional marker for disease progression in FIV infection.

Molecular and cellular analysis of human immunodeficiency virus-induced apoptosis in lymphoblastoid T-cell-line-expressing wild-type and mutated CD4 receptors

We have previously shown that the presence of the CD4 cytoplasmic tail is critical for human immunodeficiency virus (HIV)-induced apoptosis (J. Corbeil, M. Tremblay, and D. D. Richman, J. Exp. Med. 183:39-48, 1996). We have pursued our investigation of the role of the CD4 transduction pathway in HIV-induced apoptosis. To do this, wild-type and mutant forms of the CD4 cytoplasmic tail were stably expressed in the lymphoblastoid T-cell line A2.01. Apoptosis was prevented when CD4 truncated at residue 402 was expressed; however, cells expressing mutated receptors that do not associate with p56(lck) (mutated at the dicysteine motif and truncated at residue 418) but which conserved proximal domains of the cytoplasmic tail underwent apoptosis like wild-type CD4. The differences between wild-type and mutated receptors in the induction of apoptosis were not related to levels of p56(lck) or NF-kappaB activation. Initial signaling through the CD4 receptor played a major role in the sensitization of HIV-infected T cells to undergo apoptosis. Incubation of HIV-infected cells with monoclonal antibody (MAb) 13B8-2, which binds to CD4 in a region critical for dimerization of the receptor, prevented apoptosis without inhibiting HIV replication. Moreover, the apoptotic process was not related to Fas-Fas ligand interaction; however, an antagonistic anti-Fas MAb (ZB-4) enhanced apoptosis in HIV-infected cells without inducing apoptosis in uninfected cells. These observations demonstrate that CD4 signaling mediates HIV-induced apoptosis by a mechanism independent of Fas-Fas ligand interaction, does not require p56(lck) signaling, and may involve a critical region for CD4 dimerization.

Activation of E2F-mediated transcription by human T-cell leukemia virus type I Tax protein in a p16(INK4A)-negative T-cell line

The human T-cell leukemia virus type I (HTLV-I) is a causative agent of adult T-cell leukemia. Although the exact mechanism by which HTLV-I contributes to leukemogenesis is still unclear, the Tax protein is thought to play a major role in this process. This 40-kDa polypeptide is able to interact with the tumor suppressor p16(INK4A). Consequently, Tax can activate the signaling pathway that lead to the release of E2F that in turn induces expression of factors required for cell cycle progression. In this paper, we demonstrate that Tax can also activate E2F-mediated transcription independently of p16(INK4A). Indeed, when Tax is coexpressed with the E2F-1 transcription factor in CEM T-cells, which lack expression of p16(INK4A), it strongly potentiates the E2F-dependent activation of a reporter construct driven by a promoter containing E2F binding sites. This stimulation is abrogated by mutations affecting the E2F-binding sites. In addition, Tax also stimulates the transcription of the E2F-1 gene itself. Using Tax mutants that fail to activate either ATF- or NF-kappaB-dependent promoters and different 5' truncation mutants of the E2F-1 promoter, we show that the Tax-dependent transcriptional control of the E2F1 gene involves, at least in part, the ATF binding site located in the E2F-1 promoter.

Apoptosis induction by the binding of the carboxyl terminus of human immunodeficiency virus type 1 gp160 to calmodulin

The role of calmodulin (CaM) in apoptosis induced by gp160 of human immunodeficiency virus type 1 was investigated with cells undergoing single-cell killing. These cells were found to express, under the control of an inducible promoter, wild-type gp160 or mutant gp160 devoid of various lengths of the carboxyl terminus. Immunoprecipitation accompanied by immunoblotting revealed binding of CaM to wild-type gp160 but not to mutant gp160 bearing a carboxyl terminus with a deletion spanning more than five amino acid residues. A significant coenzyme activity was detected in the CaM bound to gp160 even in the presence of a Ca2+ chelater, EGTA. The cells forming this gp160-CaM complex exhibited an elevated intracellular Ca2+ level followed by DNA fragmentation, which is a hallmark of apoptosis, and finally cell killing, while the cells not forming this complex did not show any significant elevation in Ca2+ level or DNA fragmentation. These results thus indicated that CaM plays a key role in gp160-induced apoptosis.

Calmodulin antagonists inhibit apoptosis of CD4+ T-cells from patients with AIDS

Recent studies indicate that Fas and Fas ligand are involved in apoptosis of T-cells in HIV-infected patients. We have demonstrated that calcium/calmodulin is involved in Fas-mediated apoptosis in human T-cell lines transfected with HIV recombinant cDNA. In the present study, we examined spontaneous apoptosis of T-cells in vitro in peripheral blood obtained from 11 patients with AIDS and 8 HIV-seronegative normal donors and the effect of the calmodulin antagonists, trifluoperazine (TFP) or tamoxifen (TMX), on apoptosis. The results show that: (1) levels of spontaneous apoptosis were higher in PBMCs obtained from patients with AIDS than HIV-negative normal controls and the levels of apoptosis correlated with the severity of disease. (2) The accelerated apoptosis occurred predominantly in CD4+ cells in patients with AIDS. (3) Calmodulin antagonists inhibited the spontaneous apoptosis of CD4+ T-cells from patients with AIDS, which resulted in an increase in the ratio of CD4+ to CD8+ T-cells. (4) The inhibitory effect of calmodulin antagonists on apoptosis was more significant in patients with advanced disease (CDC category C) compared to less severe disease (CDC category B). These results indicate that calmodulin antagonists inhibit HIV-associated apoptosis of CD4+ T-cells, and imply that the calcium/calmodulin play important roles in mediating apoptosis of CD4+ T-cells induced by HIV infection.

Anti-retroviral therapy reverses HIV-associated abnormalities in lymphocyte apoptosis

The objective of this study was to assess the role of anti-retroviral therapy (ART) on the susceptibility of peripheral blood lymphocytes (PBL) from HIV-1-infected individuals to activation-induced apoptosis and in comparison with changes in CD4 lymphocyte counts. Eleven symptomatic HIV+ patients were studied. Ex vivo apoptosis was measured in phytohaemagglutinin (PHA)-stimulated PBL and CD4 subsets by flow cytometry, at baseline and after 1 month (4-6 weeks) and 2/3 months of ART. Six patients had extended studies of the effects of therapy to a maximum of 21 months. Lymphocyte apoptosis was significantly elevated in HIV+ patients at baseline (median 22% compared with 7.5% in HIV- risk-matched controls; P < 0.05). This decreased to control levels on ART (7.4% at 4-6 weeks, P < 0.01, and 6.2% at 8-12 weeks, P < 0.05, compared with baseline). Similar changes occurred in the CD4+ subpopulation. The decrease in apoptosis was maintained for several months, but the effect was rapidly lost if ART was discontinued. CD4 counts showed a reciprocal relationship to changes in apoptosis. The association of changes in apoptosis with those in CD4 counts suggests a link between programmed cell death and lymphocyte depletion. Apoptosis reduced in some individuals without any reduction in viral load, suggesting apoptosis may be influenced by factors in addition to the overall extent of HIV replication.

The Effect of Deletion of the V3 Loop of gp120 on Cytotoxic T Cell Responses and HIV gp120-Mediated Pathogenesis

New strategies for improving the efficacy of HIV vaccines are of significant importance. In this study, we analyzed the effect of deletion of the hypervariable V3 loop of gp120 on envelope (env)-specific CTL responses in PBMC of HIV-infected individuals. We showed increased CTL activities against conserved epitopes of the env glycoprotein in cultures induced with the ΔV3 mutant compared with those stimulated with the full-length env gene products. In contrast to the wild-type env, the ΔV3 mutant-expressing cells were resistant to Ab-dependent cell-mediated cytotoxicity, formed no syncytia, and neither underwent nor induced apoptosis in CD4+ cells. Thus, the ΔV3 mutant may redirect immune responses toward conserved epitopes of gp160, has longer expression time due to increased resistance to Ab-dependent cell-mediated cytotoxicity, and does not trigger cytopathic effects associated with apoptosis and syncytium formation. This approach may apply to other Ags of HIV, where deletions of highly variable or immunosuppressive epitopes may improve the efficacy of HIV vaccines.

The Ick protein tyrosine kinase is not involved in antibody-mediated CD4 (CDR3-loop) signal transduction that inhibits HIV-1 transcription

Monoclonal antibodies (mAb) that bind to the immunoglobulin CDR3-like region in the D1 domain of the CD4 molecule can inhibit the HIV-1 life cycle in CD4-positive T cells and lymphoblastoid cell lines at the stage of transcription. This antiviral effect requires the integrity of the cytoplasmic tail of CD4 which is known to act as a signal transduction region through its association with the protein tyrosine kinase (PTK) p56lck. In this study, we investigated the putative role of this PTK in transducing inhibitory signals that act on HIV-1 replication after triggering by anti-CDR3-like region antibody treatment of infected T cell lines. CEM (CD4+/p56lck + inducible), MT2 (CD4+/p56lck - repressed), HSB-2 (CD4-/p56lck + constitutively), HSB-2 WTCD4 (CD4+/p56lck + constitutively), HSB-2 CD4.402 (CD4+ truncated form which lacks the cytoplasmic domain/p56lck + constitutively), and HSB-2 CD4mut (CD4+ unable to bind lck/p56lck + constitutively) were exposed to HIV-1 and cultured in medium supplemented with an anti-CDR3-like region-specific antibody or a control anti-CD4 mAb which does not inhibit HIV-1 transcription. We found that CDR3-loop-mediated inhibitory signals are efficiently transduced in CD4-positive cells which demonstrate a constitutive activation of p56lck or in CD4-positive cells lacking p56lck expression. Moreover, inhibitory signals were transduced in HSB-2 CD4mut cells expressing a cell surface CD4 with a double cysteine mutation in its cytoplasmic tail that renders the molecule unable to bind p56lck, but not HSB-2 CD4.402 cells expressing a truncated form of CD4 which lacks the cytoplasmic domain. These results indicate that the p56lck plays no direct role in this process and suggests the existence of another signaling partner for CD4.

Early modification of host cell gene expression induced by HIV-1

OBJECTIVE

Characterization of the effects of infection with HIV-1 on cellular gene expression.

DESIGN AND METHODS

Differential RNA display was applied to compare uninfected and HIV-1LAI-infected CEM cells 24 h post-inoculation. Differential bands were selected, cloned and several clones per band were sequenced. RNase protection assay was used to confirm differential display findings in HIV-1LAI-infected CEM cells as well as in another T-cell line (H9) infected with a different strain (HIV-1 SF33)

RESULTS

Twelve differentially expressed bands, six up- and six downregulated in HIV-infected cells compared with controls, were selected. Four of the six upregulated bands were HIV transcripts. RNase protection assay of the remaining eight bands confirmed differential expression of four genes, including induction of a mariner transposase and moesin as well as suppression of alpha-nascent polypeptide-associated complex and mitochondrial heat shock protein 75 in HIV-1-infected cell cultures. Furthermore, a significant increase of glioma pathogenesis-related protein was found by RNase protection assay.

CONCLUSIONS

Based on this initial limited differential display analysis, it was estimated that expression of 3% of the host genes was altered by HIV-1. Amongst the identified gene modifications, the induction of a mariner transposase may alter cellular gene expression itself, whilst the enhanced expression of glioma pathogenesis-related protein suggests a role in the host cell response to viral infection. The increase in moesin may facilitate viral budding and uptake. Furthermore, the suppression of alpha-nascent polypeptide-associated complex may promote translocation of HIV-1 polypeptides into the endoplasmic reticulum, whereas the downregulation of mitochondrial heat shock protein 75 may contribute to a cytopathic effect on mitochondria and possibly impairs antigen presentation.

Delayed human immunodeficiency virus type 1-induced apoptosis in cells expressing truncated forms of CD4

It has been reported previously that cells expressing a truncated form of CD4 which lacks the cytoplasmic tail of the molecule (truncation at position 402) were not sensitive to human immunodeficiency virus type 1 (HIV-1)-induced apoptosis in an acute-phase model of infection (J. Corbeil, M. Tremblay, and D. D. Richman, J. Exp. Med. 183:39-48, 1996). The role played by the cytoplasmic domain of CD4 in HIV-1-induced apoptosis was reexamined here with clones of A2.01 cells expressing different forms of CD4 and the DNA intercalant YOPRO-1 assay. Six days after virus exposure, we found evidence of apoptosis in A2.01 cells expressing the wild-type CD4 (A2.01/CD4), whereas enhanced apoptosis remained absent in cultures of A2.01/CD4.401 and A2.01/CD4.403 cells (A2.01 cells which express CD4.401 and CD4.403 molecules with truncations at positions 401 and 403, respectively). However, cell death by apoptosis measured with YOPRO-1 was found in cultures of A2.01/CD4.401 and A2.01/CD4.403 cells 15 days after virus exposure. This result was confirmed with a terminal dUTP nick end-labeling assay and propidium iodide staining. The long lag time postinfection required for apoptosis to be observed in cultures of infected cells expressing truncated forms of CD4 was due to the delayed viral replication in these cells, as shown by monitoring of the viral reverse transcriptase activity and HIV-1 p24gag antigen expression. These results emphasize the relationship between virus replication and cell death by apoptosis.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

HIV-induced apoptosis of activated primary CD4+ T lymphocytes is not mediated by Fas-Fas ligand

OBJECTIVE

To investigate the role of the Fas-Fas ligand (FasL) interaction in HIV-1-induced apoptosis of primary CD4+ T lymphocytes.

DESIGN

Activated CD4+ T lymphocytes are the main target of HIV, and T-cell activation leads to the expression of Fas-FasL and enhances HIV-mediated apoptosis. Phytohemagglutinin-activated primary CD4+ T cells were infected with HIV; the process of cell death was examined, and whether the dying and dead cells were the productively infected cells. The modulation of Fas and FasL expression and its role in HIV-induced cell death was also investigated.

METHODS

The number of viable and dead cells was determined by trypan blue exclusion. Apoptosis was quantified using an enzyme-linked immunosorbent assay measuring the release of cytoplasmic histone-associated DNA fragments. The percentage of HIV-infected cells was determined by FACS analysis, and viral production was assessed by a p24 core antigen assay. The following three markers, HIV-gp-120, annexin-V and 7-AAD, were used to monitor the apoptotic process in HIV-negative and positive cells. Fas and FasL expression was analyzed at the RNA level by reverse transcription polymerase chain reaction and at the protein level by flow cytometry. The contribution of Fas-FasL interactions to apoptosis was examined by blocking experiments using the antagonist ZB4 anti-Fas antibody.

RESULTS

HIV-induced apoptosis in activated purified CD4+ T lymphocytes required infectious virus and was dose-dependent. Apoptosis in HIV-infected cultures was mostly confined to productively infected cells. The expression of Fas and FasL was not significantly modulated by infection and blocking Fas-FasL interactions did not reduce the extent of apoptosis.

CONCLUSIONS

HIV-induced apoptosis of activated CD4+ T cells in vitro is confined to productively infected cells and is not mediated by a Fas-FasL interaction.

Increased Enzymatic Activity of the T-Cell Antigen Receptor-Associated Fyn Protein Tyrosine Kinase in Asymptomatic Patients Infected With the Human Immunodeficiency Virus

The immune system of patients infected with human immunodeficiency virus (HIV) is in a state of chronic activation; however, the nature of HIV-related immune activation is unknown. As normal T-cell activation involves early tyrosine phosphorylation induced by the T-cell antigen receptor-associated src-family protein tyrosine kinase p59fyn(T) (Fyn), we examined a potential role for this kinase in HIV-related immune dysfunction. We determined the relative specific kinase activity of Fyn in lysates of peripheral blood mononuclear cells from 47 normal control individuals tested negative for HIV-1 and -2, human T-cell lymphotropic virus Type I, hepatitis B virus (HBV), hepatitis C virus (HCV), and syphilis; 14 asymptomatic HIV-infected patients having near-normal CD4+ T-cell counts (350 to 980 CD4+ cells/μL); 4 patients with symptomatic acquired immunodeficiency syndrome (AIDS) (<30 CD4+ cells/μL); 13 patients having chronic infection with HBV (6 patients) or HCV (7 patients); and 6 patients with systemic lupus erythematosis (SLE). All patients with asymptomatic HIV disease were shown to have a profound increase (mean increase of 19-fold; range threefold to 56-fold increase; p = 1.33 × 10−9) in the relative specific kinase activity of Fyn compared to uninfected controls or patients with hepatitis or SLE. In contrast, patients with AIDS had an Fyn-specific kinase activity that was much less affected (mean increase of threefold; range onefold to sevenfold increase; p = 1.30 × 10−5). It was further shown that HIV infection affects the Fyn-specific kinase activity in CD8+-enriched cells, suggesting abnormal Fyn activity in both CD8+ as well as CD4+ T lymphocytes. Initial results implicate a role for the CSK protein tyrosine kinase as responsible for the abnormal Fyn kinase activity observed in HIV-infected patients. These data indicate early and chronic activation of Fyn as a unique HIV-related effect that has the potential to be diagnostic for early HIV infection and/or may serve as a prognostic indicator for advancement to full-blown AIDS. More importantly, sustained activation of the protein tyrosine kinase associated with T-cell antigen receptor function may result in, or contribute to, the immunopathogenic effects associated with HIV infection.

A new reporter cell line to monitor HIV infection and drug susceptibility in vitro

Determination of HIV infectivity in vitro and its inhibition by antiretroviral drugs by monitoring reduction of production of p24 antigen is expensive and time consuming. Such assays also do not allow accurate quantitation of the number of infected cells over time. To develop a simple, rapid, and direct method for monitoring HIV infection, we generated a stable T-cell line (CEM) containing a plasmid encoding the green fluorescent protein (humanized S65T GFP) driven by the HIV-1 long terminal repeat. Clones were selected that displayed low constitutive background fluorescence, but a high level of GFP expression upon infection with HIV. HIV-1 infection induced a 100- to 1,000-fold increase in relative fluorescence of cells over 2 to 4 days as monitored by fluorescence microscopy, cytofluorimetry, and flow cytometry. Addition of inhibitors of reverse transcriptase, protease, and other targets at different multiplicities of infection permitted the accurate determination of drug susceptibility. This technique also permitted quantitation of infectivity of viral preparations by assessment of number of cells infected in the first round of infection. In conclusion, the CEM-GFP reporter cell line provides a simple, rapid, and direct method for monitoring HIV infectivity titers and antiretroviral drug susceptibility of syncytium-inducing strains.

The inhibition of pro-apoptotic ICE-like proteases enhances HIV replication

Accelerated programmed cell death, or apoptosis, contributes to the CD4+ T-cell depletion characteristic of infection by human immunodeficiency virus (HIV). It has therefore been proposed that limiting apoptosis may represent a therapeutic modality for HIV infection. We found, however, that T leukemia cells or peripheral blood mononuclear cells (PBMCs) exposed to HIV-1 underwent enhanced viral replication in the presence of the cell death inhibitor, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-AVD-fmk). Furthermore, z-VAD-fmk, which targets the pro-apoptotic interleukin-1 beta-converting enzyme (ICE)-like proteases, stimulated endogenous virus production in activated PBMCs derived from HIV-1-infected asymptomatic individuals. These findings suggest that programmed cell death may serve as a beneficial host mechanism to limit HIV spread and that strategies to inhibit it may have deleterious consequences for the infected host.

Gene activating and proapoptotic potential are independent properties of different CD4 epitopes

CD4 engagement triggers an early signaling cascade which initiates late events such as transcription factor activation. The outcome of CD4 engagement is T-cell commitment to alternative, dramatically different fates, such as activation and apoptosis. We have tested a panel of anti-CD4 mAbs specific for different CD4 epitopes, as well as HIV-1 gp120, for the capacity to activate crucial early events such as enhancement of p56(lck) kinase activity and Shc phosphorylation. The same CD4 epitopes were characterized for their capacity both to deliver a gene activating signal and to program T-cells to activation dependent death. No correlation could be found between capacity of specific CD4 epitopes to deliver a gene activating signal and capacity to prime T-cells to apoptosis, suggesting that gene activating and proapoptotic potential are independent functions of CD4 epitopes. Furthermore, while triggering of the calcium pathway appears critical in NF-AT activation, optimal p56(lck) activation and Shc phosphorylation might be required for initiation of the apoptotic pathway.

Fas (CD95) expression and death-mediating function are induced by CD4 cross-linking on CD4+ T cells

The CD4 receptor contributes to T-cell activation by coligating major histocompatibility complex class II on antigen presenting cells with the T-cell receptor (TCR)/CD3 complex, and triggering a cascade of signaling events including tyrosine phosphorylation of intracellular proteins. Paradoxically, CD3 cross-linking prior to TCR stimulation results in apoptotic cell death, as does injection of anti-CD4 antibodies in vivo of CD4 ligation by HIV glycoprotein (gp) 120. In this report we investigate the mechanism by which CD4 cross-linking induces cell death. We have found that CD4 cross-linking results in a small but rapid increase in levels of cell surface Fas, a member of the tumor necrosis factor receptor family implicated in apoptotic death and maintenance of immune homeostasis. Importantly, CD4 cross-linking triggered the ability of Fas to function as a death molecule. Subsequent to CD4 cross-linking, CD4+ splenocytes cultured overnight became sensitive to Fas-mediated death. Death was Fas-dependent, as demonstrated by cell survival in the absence of plate-bound anti-Fas antibody, and by the lack of CD4-induced death in cells from Fas-defective lymphoproliferative (lpr) mice. We demonstrate here that CD4 regulates the ability of Fas to induce cell death in Cd4+ T cells.