Nef protein of HIV-1 induces apoptotic cytolysis of murine lymphoid cells independently of CD95 (Fas) and its suppression by serine/threonine protein kinase inhibitors

Implications of HIV-1 Nef for "Shock and Kill" Strategies to Eliminate Latent Viral Reservoirs

Finding a cure for HIV is challenging because the virus is able to integrate itself into the host cell genome and establish a silent state, called latency, allowing it to evade antiviral drugs and the immune system. Various “shock and kill” strategies are being explored in attempts to eliminate latent HIV reservoirs. The goal of these approaches is to reactivate latent viruses (“shock”), thereby exposing them to clearance by viral cytopathic effects or immune-mediated responses (“kill”). To date, there has been limited clinical success using these methods. In this review, we highlight various functions of the HIV accessory protein Nef and discuss their double-edged effects that may contribute to the limited effectiveness of current “shock and kill” methods to eradicate latent HIV reservoirs in treated individuals.

Human immunodeficiency virus-1 (HIV-1)-mediated apoptosis: new therapeutic targets

HIV has posed a significant challenge due to the ability of the virus to both impair and evade the host’s immune system. One of the most important mechanisms it has employed to do so is the modulation of the host’s native apoptotic pathways and mechanisms. Viral proteins alter normal apoptotic signaling resulting in increased viral load and the formation of viral reservoirs which ultimately increase infectivity. Both the host’s pro- and anti-apoptotic responses are regulated by the interactions of viral proteins with cell surface receptors or apoptotic pathway components. This dynamic has led to the development of therapies aimed at altering the ability of the virus to modulate apoptotic pathways. These therapies are aimed at preventing or inhibiting viral infection, or treating viral associated pathologies. These drugs target both the viral proteins and the apoptotic pathways of the host. This review will examine the cell types targeted by HIV, the surface receptors exploited by the virus and the mechanisms whereby HIV encoded proteins influence the apoptotic pathways. The viral manipulation of the hosts’ cell type to evade the immune system, establish viral reservoirs and enhance viral proliferation will be reviewed. The pathologies associated with the ability of HIV to alter apoptotic signaling and the drugs and therapies currently under development that target the ability of apoptotic signaling within HIV infection will also be discussed.

Validation of a novel secretion modification region (SMR) of HIV-1 Nef using cohort sequence analysis and molecular modeling

The HIV-1 accessory protein Nef plays an active role in the pathogenesis of AIDS by its numerous cellular interactions that facilitate the release of virus particles. This 27 kDa protein is required for maintenance of the viral replication in HIV, and is also known to contribute to immune evasion, blocking of apoptosis in virus-infected cells and enhancement of virus infectivity. Nef has been shown to be secreted and is present on the surface of virus-infected cells. Recent studies from our laboratory have shown that the Nef protein is secreted from nef-transfected and HIV-1-infected cells in small exosome-like vesicles (40-100 nm diam.) that do not contain virions. We have identified three amino-terminal domains of Nef as necessary for secretion: (i) the four arginine residues (17,19,21, 22) comprising the basic region; (ii) the phosphofurin acidic cluster sequence (PACS) composed of four glutamic acid residues (61-64); (iii) a previously unknown motif spanning amino acid residues 65-69 (VGFPV) which we named the secretion modification region (SMR). In this study, we have used population-based phylogeny data and sequence analysis to characterize the conservation of the Nef SMR domain that regulates vesicle secretion. We have performed in silico computational chemistry analysis involving molecular dynamic structure modeling of mutations in the SMR motif. Sequence analysis of Nef from HIV-1-infected patients, including slow progressors (SP), long term progressors (LTP) and long term non-progressors (LTNP) demonstrated 99 % conservation of the Nef SMR motif. Computational analysis including modeling of wild-type HIV-1 Nef and V66A Nef SMR mutant using structural homology and molecular dynamics of ligand-associated interactions indicated significant structural changes in the Nef mutant, thus supporting the importance of the SMR domain for mediating Nef vesicle secretion.

Activation/proliferation and apoptosis of bystander goat lymphocytes induced by a macrophage-tropic chimeric caprine arthritis encephalitis virus expressing SIV Nef

Caprine arthritis encephalitis virus (CAEV) is the natural lentivirus of goats, well known for its tropism for macrophages and its inability to cause infection in lymphocytes. The viral genome lacks nef, tat, vpu and vpx coding sequences. To test the hypothesis that when nef is expressed by the viral genome, the virus became toxic for lymphocytes during replication in macrophages, we inserted the SIVsmm PBj14 nef coding sequences into the genome of CAEV thereby generating CAEV-nef. This recombinant virus is not infectious for lymphocytes but is fully replication competent in goat macrophages in which it constitutively expresses the SIV Nef. We found that goat lymphocytes cocultured with CAEV-nef-infected macrophages became activated, showing increased expression of the interleukin-2 receptor (IL-2R). Activation correlated with increased proliferation of the cells. Interestingly, a dual effect in terms of apoptosis regulation was observed in exposed goat lymphocytes. Nef was found first to induce a protection of lymphocytes from apoptosis during the first few days following exposure to infected macrophages, but later it induced increased apoptosis in the activated lymphocytes. This new recombinant virus provides a model to study the functions of Nef in the context of infection of macrophages, but in absence of infection of T lymphocytes and brings new insights into the biological effects of Nef on lymphocytes.

Inhibition of human immunodeficiency virus envelope glycoprotein- mediated single cell lysis by low-molecular-weight antagonists of viral entry

The coexpression of human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins and receptors leads to the lysis of single cells by a process that is dependent upon membrane fusion. This cell lysis was inhibited by low-molecular-weight compounds that interfere with receptor binding or with receptor-induced conformational transitions in the envelope glycoproteins. A peptide, T20, potently inhibited cell-cell fusion but had no effect on single cell lysis mediated by the HIV-1 envelope glycoproteins. Thus, critical events in the lysis of single cells by the HIV-1 envelope glycoproteins occur in intracellular compartments accessible only to small inhibitory compounds.

Human Immunodeficiency virus type 1 Nef potently induces apoptosis in primary human brain microvascular endothelial cells via the activation of caspases

The lentiviral protein Nef plays a major role in the pathogenesis of human immunodeficiency virus type I (HIV-1) infection. Although the exact mechanisms of its actions are not fully understood, Nef has been shown to be essential for the maintenance of high-titer viral replication and disease pathogenesis in in vivo models of simian immunodeficiency virus infection of monkeys. Nef has also been suggested to play a pivotal role in the depletion of T cells by promoting apoptosis in bystander cells. In this context, we investigated the ability of extracellular and endogenously expressed HIV-1 Nef to induce apoptosis in primary human brain microvascular endothelial cells (MVECs). Human brain MVECs were exposed to baculovirus-expressed HIV-1 Nef protein, an HIV-1-based vector expressing Nef, spleen necrosis virus (SNV)-Nef virus (i.e., SNV vector expressing HIV-1 Nef as a transgene), and the HIV-1 strain ADA and its Nef deletion mutant, ADADeltaNef. We observed that ADA Nef, the HIV-1 vector expressing Nef, and SNV-Nef were able to induce apoptosis in a dose-dependent manner. The mutant virus with a deletion in Nef was able to induce apoptosis in MVECs to modest levels, but the effects were not as pronounced as with the wild-type HIV-1 strain, ADA, the HIV-1-based vector expressing Nef, or SNV-Nef viruses. We also demonstrated that relatively high concentrations of exogenous HIV-1 Nef protein were able to induce apoptosis in MVECs. Gene microarray analyses showed increases in the expression of several specific proapoptotic genes. Western blot analyses revealed that the various caspases involved with Nef-induced apoptosis are processed into cleavage products, which occur only during programmed cell death. The results of this study demonstrate that Nef likely contributes to the neuroinvasion and neuropathogenesis of HIV-1, through its effects on select cellular processes, including various apoptotic cascades.

HIV-1 nef suppression by virally encoded microRNA

BACKGROUND

MicroRNAs (miRNAs) are 21 to approximately 25-nucleotides (nt) long and interact with mRNAs to trigger either translational repression or RNA cleavage through RNA interference (RNAi), depending on the degree of complementarity with the target mRNAs. Our recent study has shown that HIV-1 nef dsRNA from AIDS patients who are long-term non-progressors (LTNPs) inhibited the transcription of HIV-1.

RESULTS

Here, we show the possibility that nef-derived miRNAs are produced in HIV-1 persistently infected cells. Furthermore, nef short hairpin RNA (shRNA) that corresponded to a predicted nef miRNA (approximately 25 nt, miR-N367) can block HIV-1 Nef expression in vitro and the suppression by shRNA/miR-N367 would be related with low viremia in an LTNP (15-2-2). In the 15-2-2 model mice, the weight loss, which may be rendered by nef was also inhibited by shRNA/miR-N367 corresponding to suppression of nef expression in vivo.

CONCLUSIONS

These data suggest that nef/U3 miRNAs produced in HIV-1-infected cells may suppress both Nef function and HIV-1 virulence through the RNAi pathway.

Characterization of Nef-CXCR4 interactions important for apoptosis induction

The HIV-1 Nef protein was analyzed for apoptotic structural motifs that interact with the CXCR4 receptor and induce apoptosis in CD4(+) lymphocytes. Two apoptotic motifs were identified. One centered on Nef amino acids (aa) 50 to 60, with the overlapping 20-mer peptides retaining about 82% of the activity of the full Nef protein. The second centered on aa 170 to 180, with the overlapping 20-mer peptides retaining about 30% of the activity of the full protein. Significant apoptotic abilities were observed for 11-mer motif peptides spanning aa 50 to 60 and aa 170 to 180, with a scrambled version of the 11-mer motif peptide corresponding to aa 50 to 60 showing no apoptotic ability. Hallmarks of apoptosis, such as the formation of DNA ladders and caspase activation, that were observed with the full-length protein were equally evident upon exposure of cells to these motif peptides. A CXCR4 antibody and the endogenous ligand SDF-1alpha were effective in blocking Nef peptide-induced apoptosis as well as the physical binding of a fluorescently tagged Nef protein, while CCR5 antibodies were ineffective. The CXCR4-negative cell line MDA-MB-468 was resistant to the apoptotic peptides and became sensitive to the apoptotic peptides upon transfection with a CXCR4-expressing vector. A fluorescently tagged motif peptide and Nef protein displayed physical binding to CXCR4-transfected MDA-MB-468 cells, but not to CCR5-transfected cells. The removal of the apoptotic motif sequences from the full-length protein completely eliminated the ability of Nef to induce apoptosis. However, these modified Nef proteins still retained the ability to enhance viral infectivity. Thus, specific sequences in the Nef protein appear to be necessary for Nef protein-induced apoptosis as well as for physical interaction with CXCR4 receptors.

Extracellular Nef protein targets CD4+ T cells for apoptosis by interacting with CXCR4 surface receptors

The effects of soluble Nef protein on CD4(+) T cells were examined. CD4(+)-T-cell cultures exposed to soluble Nef were analyzed for apoptosis by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling and hallmarks of apoptosis including cytoplasmic shrinkage, nuclear fragmentation, DNA laddering, and caspase activation. We observed dose- and time-dependent inductions of apoptosis. DNA laddering and activated caspase 3 were also evident. Cells treated with Nef/protein kinase inhibitor complexes were protected from Nef-induced apoptosis, suggesting possible roles for protein kinases in the apoptosis pathway. Similarly, cells treated with Nef/anti-Nef antibody complexes were protected from Nef-induced apoptosis. The cellular receptor responsible for Nef-induced apoptosis was identified through antibody- and ligand-blocking experiments as a receptor commonly involved in viral entry. CXCR4 antibodies, as well as the endogenous ligand SDF-1alpha, were effective in blocking Nef-induced apoptosis, while CCR5 and CD4 antibodies were ineffective. Moreover, a CXCR4-deficient cell line, MDA-MB-468, which was resistant to Nef-induced apoptosis, became sensitive upon transfection with a CXCR4-expressing vector. This study suggests that extracellular Nef protein could contribute to the decline of CD4 counts prior to and during the onset of AIDS in patients with human immunodeficiency virus type 1 infections.

Cytolysis by CCR5-using human immunodeficiency virus type 1 envelope glycoproteins is dependent on membrane fusion and can be inhibited by high levels of CD4 expression

T-tropic (X4) and dualtropic (R5X4) human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins kill primary and immortalized CD4(+) CXCR4(+) T cells by mechanisms involving membrane fusion. However, because much of HIV-1 infection in vivo is mediated by M-tropic (R5) viruses whose envelope glycoproteins use CCR5 as a coreceptor, we tested a panel of R5 and R5X4 envelope glycoproteins for their ability to lyse CCR5(+) target cells. As is the case for CXCR4(+) target cells, HIV-1 envelope glycoproteins expressed by single-round HIV-1 vectors killed transduced CD4(+) CCR5(+) cells in a membrane fusion-dependent manner. Furthermore, a CD4-independent R5 HIV-1 envelope glycoprotein was able to kill CD4-negative target cells expressing CCR5, demonstrating that CD4 is not intrinsically required for the induction of death. Interestingly, high levels of CD4 expression protected cells from lysis and syncytium formation mediated by the HIV-1 envelope glycoproteins. Immunoprecipitation experiments showed that high levels of CD4 coexpression inhibited proteolytic processing of the HIV-1 envelope glycoprotein precursor gp160. This inhibition could be overcome by decreasing the CD4 binding ability of gp120. Studies were also undertaken to investigate the ability of virion-bound HIV-1 envelope glycoproteins to kill primary CD4(+) T cells. However, neither X4 nor R5X4 envelope glycoproteins on noninfectious virions caused death in primary CD4(+) T cells. These results demonstrate that the interaction of CCR5 with R5 HIV-1 envelope glycoproteins capable of inducing membrane fusion leads to cell lysis; overexpression of CD4 can inhibit cell killing by limiting envelope glycoprotein processing.

Ethanol strongly potentiates apoptosis induced by HIV-1 proteins in primary human brain microvascular endothelial cells

Ethanol may have significant effects on human immunodeficiency virus type I (HIV-1) pathogenesis in vivo. As such, the effects of ethanol treatment were studied on the proapoptotic potential of various HIV-1 proteins in primary isolated human brain microvascular endothelial cells (MVECs), a major cellular component of the blood-brain barrier. Low-passage primary brain MVECs were treated with recombinant HIV-1 proteins Nef, Vpr, Tat and gp120 proteins from X4, R5, and X4R5 viral strains, with and without ethanol at various relevant concentrations. The apoptotic potential of each HIV-1 protein with and without ethanol was compared with cells treated with ethanol alone or GST protein as a control, under similar conditions. Specific HIV-1 proteins induced apoptosis in primary isolated human brain MVECs, which was potentiated on treatment with 0.1 and 0.3% (v/v) ethanol. Cotreatment with ethanol and specific HIV-1 proteins showed enhanced lactate dehydrogenase release, compared with MVECs treated with ethanol alone. The presence of ethanol in in vitro culture medium also enhanced HIV-1 protein-mediated tumor necrosis factor-alpha production, compared with cells treated with ethanol alone or GST protein. Thus, these studies demonstrate ethanol's potential for inducing apoptosis of human MVECs with relevant HIV-1-specific proteins and suggest a potential synergistic effect in augmenting HIV-1 neuroinvasion and neuropathogenesis in vivo.

Membrane-fusing capacity of the human immunodeficiency virus envelope proteins determines the efficiency of CD+ T-cell depletion in macaques infected by a simian-human immunodeficiency virus

The mechanism of the progressive loss of CD4+ T lymphocytes, which underlies the development of AIDS in human immunodeficiency virus (HIV-1)-infected individuals, is unknown. Animal models, such as the infection of Old World monkeys by simian-human immunodeficiency virus (SHIV) chimerae, can assist studies of HIV-1 pathogenesis. Serial in vivo passage of the nonpathogenic SHIV-89.6 generated a virus, SHIV-89.6P, that causes rapid depletion of CD4+ T lymphocytes and AIDS-like illness in monkeys. SHIV-KB9, a molecularly cloned virus derived from SHIV-89.6P, also caused CD4+ T-cell decline and AIDS in inoculated monkeys. It has been demonstrated that changes in the envelope glycoproteins of SHIV-89.6 and SHIV-KB9 determine the degree of CD4+ T-cell loss that accompanies a given level of virus replication in the host animals (G. B. Karlsson et. al., J. Exp. Med. 188:1159-1171, 1998). The envelope glycoproteins of the pathogenic SHIV mediated membrane fusion more efficiently than those of the parental, nonpathogenic virus. Here we show that the minimal envelope glycoprotein region that specifies this increase in membrane-fusing capacity is sufficient to convert SHIV-89.6 into a virus that causes profound CD4+ T-lymphocyte depletion in monkeys. We also studied two single amino acid changes that decrease the membrane-fusing ability of the SHIV-KB9 envelope glycoproteins by different mechanisms. Each of these changes attenuated the CD4+ T-cell destruction that accompanied a given level of virus replication in SHIV-infected monkeys. Thus, the ability of the HIV-1 envelope glycoproteins to fuse membranes, which has been implicated in the induction of viral cytopathic effects in vitro, contributes to the capacity of the pathogenic SHIV to deplete CD4+ T lymphocytes in vivo.

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.

Importance of membrane fusion mediated by human immunodeficiency virus envelope glycoproteins for lysis of primary CD4-positive T cells

In established T-cell lines, the membrane-fusing capacity of the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins mediates cytopathic effects, both syncytium formation and single-cell lysis. Furthermore, changes in the HIV-1 envelope glycoproteins are responsible for the increased CD4(+) T-cell-depleting ability observed in infected monkeys upon in vivo passage of simian-human immunodeficiency virus (SHIV) chimeras. In this study, a panel of SHIV envelope glycoproteins and their mutant counterparts defective in membrane-fusing capacity were expressed in primary human CD4(+) T cells. Compared with controls, all of the functional HIV-1 envelope glycoproteins induced cell death in primary CD4(+) T-cell cultures, whereas the membrane fusion-defective mutants did not. Death occurred almost exclusively in envelope glycoprotein-expressing cells and not in bystander cells. Under standard culture conditions, most dying cells underwent lysis as single cells. When the cells were cultured at high density to promote syncytium formation, the envelope glycoproteins of the passaged, pathogenic SHIVs induced more syncytia than those of the respective parental SHIV. These results demonstrate that the HIV-1 envelope glycoproteins induce the death of primary CD4(+) T lymphocytes by membrane fusion-dependent processes.

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.

Nef protein induces differential effects in CD8+ cells from HIV-1-infected patients

BACKGROUND

The Nef protein of HIV-1 is suspected to play a role in the depletion of uninfected CD4+ lymphocytes that leads to AIDS. By contrast its effect on CD8+ cells, whose functions are also deregulated during HIV-1 infection, is presently unclear. Here we describe a number of derangements induced in vitro by Nef in CD8+ cells from HIV-1-infected patients.

DESIGN

Peripheral lymphocytes from 16 HIV-1+ subjects and 9 uninfected individuals were cultivated on a Nef-transfected mouse fibroblast layer exposing the carboxyl-terminal region of the viral protein on cell membrane. The cultures were then measured for both apoptosis and proliferation by subdiploid DNA content and Ki67 expression, respectively, whereas the molecular analysis of purified CD8+ cells investigated the Fas-L mRNA levels in Nef-treated CTLs. In addition, we evaluated the Nef-induced variation in the extent of CD8+/HLA-DR+ subset, which includes non cytotoxic cells secreting T-cell antiviral factor (CAF) and a soluble factor inhibiting the HIV-1 replication.

RESULTS

The viral protein induced in peripheral blood lymphocytes (PBL) a moderate tendency to proliferate, as measured by the increment of Ki67 antigen, particularly on the CD8+ subset of HIV-1 infected individuals (P < 0.05). This profile was particularly evident in cultures from patients with severe CD4+ lymphopenia and paralleled an apparent expansion of the CD8+/CD57+ suppressor cell subset. Molecular analysis of purified CD8+ cells revealed a defective expression of Fas-L mRNA in Nef-cultured CTLs, whereas the viral protein exerted a down modulatory effect on the CD8+/HLA-DR+ subset (P < 0.05), thus suggesting a potential inhibition of CAF.

CONCLUSIONS

These results support a potential role of Nef in the progression of HIV-1 infection as a number of cellular functions are affected in the CD8+ subset. In particular, the defective functions of CD8+ cells induced by the viral protein could contribute, at least partly, to the escape of HIV-1 from the immune control of these cells.

The cell death-inducing ability of glycoprotein 120 from different HIV strains correlates with their ability to induce CD4 lateral association with CD95 on CD4+ T cells

CD4 cross-linking by HIV gp120 triggers CD4+ T cell death. Several authors have suggested that this effect is mediated by CD95, but this possibility is debated by other authors. In a previous work, we found by co-capping that gp120(451) and gp120MN, but not gp120(IIIB), induce lateral association of CD4 with CD95 on the T cell surface. In this work, we used fluorescence resonance energy transfer to confirm that CD4/CD95 lateral association is induced by gp120(451), but not gp120(IIIB). Moreover, we found that gp120 ability to induce the CD4/CD95 association correlates with ability to induce cell death, since gp120(451) and gp120MN induced higher levels of cell death than did gp120(IIIB) in PHA-derived CD4+ T cell lines. CD95 involvement in gp120-induced cell death was confirmed by showing that gp120(451) and gp120MN did not induce death in CD4+ T cells derived from patients with autoimmune/lymphoproliferative disease (ALD) and decreased CD95 function. Cell death induced by gp120MN was inhibited by a recombinant CD95/IgG.Fc molecule blocking the CD95/CD95L interaction. However, inhibition was late and only partial. These data suggest that the gp120-induced CD4/CD95 association exerts a dual effect: an early effect that is independent of CD95L and may be due to direct triggering of CD95 by gp120, and a late effect that may be due to sensitization of CD95 to triggering by CD95L. In line with the former effect, cell treatment with gp120MN activated caspase 3 in the presence of Fas/IgG.Fc, which shows that cell death induced by gp120MN independently of CD95L uses the same pathway as CD95.

Human Immunodeficiency Virus Type 1 Nef Protein Sensitizes CD4+ T Lymphoid Cells to Apoptosis via Functional Upregulation of the CD95/CD95 Ligand Pathway

Many viruses have evolved genes encoding proteins that regulate cell death by apoptosis. The human immunodeficiency virus type 1 (HIV-1) Nef protein alters T-cell development and signaling and is required for optimal viral replication and pathogenicity in vivo. To analyze the interference of Nef with cell survival, we used both regulated and constitutively expressed nef alleles in stably transfected T-cell lines. Nef-expressing cells were sensitized to cell death by apoptosis, which was specifically exacerbated by an anti-CD95 IgM monoclonal antibody (MoAb). Flow cytometric analysis showed that the surface expression of both CD95 and CD95 ligand (CD95L) was upregulated by endogenous Nef expression. Nef-mediated apoptosis was almost completely suppressed by the addition in culture of an anti-CD95 Fab′ IgG MoAb, which specifically blocks CD95/CD95L interactions. Lastly, mutation of a proline motif in the core region of the nef gene, which disrupts its ability to interact with cellular kinases and reduces HIV-1 replication in vitro, completely abrogated the Nef-mediated induction of apoptosis as well as its ability to upregulate surface CD95 and CD95L. These findings may provide molecular insight into the role of endogenous Nef in the T-cell depletion observed in vivo, particularly HIV-specific cytotoxic CD8+ T cells.

Human Immunodeficiency Virus Type 1 Nef Protein Sensitizes CD4+ T Lymphoid Cells to Apoptosis via Functional Upregulation of the CD95/CD95 Ligand Pathway

Many viruses have evolved genes encoding proteins that regulate cell death by apoptosis. The human immunodeficiency virus type 1 (HIV-1) Nef protein alters T-cell development and signaling and is required for optimal viral replication and pathogenicity in vivo. To analyze the interference of Nef with cell survival, we used both regulated and constitutively expressed nef alleles in stably transfected T-cell lines. Nef-expressing cells were sensitized to cell death by apoptosis, which was specifically exacerbated by an anti-CD95 IgM monoclonal antibody (MoAb). Flow cytometric analysis showed that the surface expression of both CD95 and CD95 ligand (CD95L) was upregulated by endogenous Nef expression. Nef-mediated apoptosis was almost completely suppressed by the addition in culture of an anti-CD95 Fab′ IgG MoAb, which specifically blocks CD95/CD95L interactions. Lastly, mutation of a proline motif in the core region of the nef gene, which disrupts its ability to interact with cellular kinases and reduces HIV-1 replication in vitro, completely abrogated the Nef-mediated induction of apoptosis as well as its ability to upregulate surface CD95 and CD95L. These findings may provide molecular insight into the role of endogenous Nef in the T-cell depletion observed in vivo, particularly HIV-specific cytotoxic CD8+ T cells.

High levels of HIV-1 replication show a clear correlation with downmodulation of Bcl-2 protein in peripheral blood lymphocytes of HIV-1-seropositive subjects

Peripheral blood lymphocytes (PBLs) from 51 HIV-1-seropositive subjects with different levels of HIV-1 replication and 20 healthy blood donors were examined for the expression of the antiapoptotic Bcl-2 protein. All the plasma samples from HIV-1 patients were characterized for the presence of HIV-1 p24 and HIV, RNA viral load. Bcl-2 protein expression in fresh peripheral blood lymphocytes was studied by different tests, including Western blot and indirect immunofluorescence techniques. Direct immunofluorescence staining, revealed by flow cytometry, was applied to quantify the number of specific anti-Bcl-2 antibody epitope binding sites, thus extrapolating the relative number of Bcl-2 into the cells. The results indicate that the expression of Bcl-2 protein is significantly lower in peripheral blood lymphocytes of HIV-1-seropositive patients showing high levels of viral replication, detected by means of HIV-1 p24 and RNA viral load, with respect to HIV-1 patients with low levels of virus replication and healthy blood donors. The clear-cut inverse correlation between viral replication and Bcl-2 expression reinforces the view that HIV-1-mediated apoptosis probably represents a key mechanism in AIDS pathogenesis.

Inhibition of HIV-1 Nef-induced apoptosis of uninfected human blood cells by serine/threonine protein kinase inhibitors, fasudil hydrochloride and M3

The Nef protein of HIV-1 binds to and induces apoptotic cytolysis of uninfected but activated human peripheral blood mononuclear cells (PBMC) and various cell line cells derived from CD4+ T, CD8+ T and B lymphocytes, macrophages, and neutrophils. The Nef-induced apoptosis also occurs with blood cells not expressing CD95 (Fas). The Nef-induced apoptosis as well as Fas-mediated apoptosis was inhibited by acetyl-Try-Val-Ala-Asp-CHO, an IL-1beta converting enzyme (ICE) inhibitor. On the other hand, serine/threonine protein kinase (PK) inhibitors, H-7, fasudil hydrochloride and M3, inhibited the Nef-induced apoptosis, and not the Fas-mediated one, without affecting the cell-binding activity of Nef and Nef-binding capacity of the activated cells. Preincubation of the cells with the drugs before being bound by Nef was required for the inhibition of apoptosis. These results suggest that the PK inhibitors specifically act on a cellular protein involved in the upper stream of signal transduction pathway of the Nef-induced apoptosis, which is different from the Fas-mediated pathway but meets it upstream of ICE. In addition, the drugs suppressed the cellular activation-associated cell surface expression of a putative Nef-binding protein in PBMC, although they had no influence on its expression in cell line cells. These findings suggest the feasibility of clinical use of the PK inhibitors to prevent the development of AIDS by inhibiting the Nef-induced apoptosis of uninfected blood cells.