Human immunodeficiency virus type 1 Nef protein inhibits activation pathways in peripheral blood mononuclear cells and T-cell lines

The balance between p53 isoforms modulates the efficiency of HIV-1 infection in macrophages

Several host factors influence HIV-1 infection and replication. The p53-mediated antiviral role in monocytes-derived macrophages (MDMs) was previously highlighted. Indeed, an increase in p53 level results in a stronger restriction against HIV-1 early replication steps through SAMHD1 activity. In this study, we investigated the potential role of some p53 isoforms in HIV-1 infection. Transfection of isoform-specific siRNA induces distinctive effects on the virus life cycle. For example, in contrast to a siRNA targeting all isoforms, a knockdown of Δ133p53 transcripts reduces virus replication in MDMs that is correlated with a decrease in phosphorylated inactive SAMHD1. Combination of Δ133p53 knockdown and Nutlin-3, a pharmacological inhibitor of MDM2 that stabilizes p53, further reduces susceptibility of MDMs to HIV-1 infection, thus suggesting an inhibitory role of Δ133p53 towards p53 antiviral activity. In contrast, p53β knockdown in MDMs increases the viral production independently of SAMHD1. Moreover, experiments with a Nef-deficient virus show that this viral protein plays a protective role against the antiviral environment mediated by p53. Finally, HIV-1 infection affects the expression pattern of p53 isoforms by increasing p53β and p53γ mRNA levels while stabilizing the protein level of p53α and some isoforms from the p53β subclass. The balance between the various p53 isoforms is therefore an important factor in the overall susceptibility of macrophages to HIV-1 infection, fine-tuning the p53 response against HIV-1. This study brings a new understanding of the complex role of p53 in virus replication processes in myeloid cells. Importance As of today, HIV-1 is still considered as a global pandemic without a functional cure, partly because of the presence of stable viral reservoirs. Macrophages constitute one of these cell reservoirs, contributing to the viral persistence. Studies investigating the host factors involved in cell susceptibility to HIV-1 infection might lead to a better understanding of the reservoir formation and will eventually allow the development of an efficient cure. Our team previously showed the antiviral role of p53 in macrophages, which acts by compromising the early steps of HIV-1 replication. In this study, we demonstrate the involvement of p53 isoforms, which regulates p53 activity and define the cellular environment influencing viral replication. In addition, the results concerning the potential role of p53 in antiviral innate immunity could be transposed to other fields of virology and suggest that knowledge in oncology can be applied to HIV-1 research.

HIV-1 infection of T cells and macrophages are differentially modulated by virion-associated Hck: a Nef-dependent phenomenon

The proline repeat motif (PxxP) of Nef is required for interaction with the SH3 domains of macrophage-specific Src kinase Hck. However, the implication of this interaction for viral replication and infectivity in macrophages and T lymphocytes remains unclear. Experiments in HIV-1 infected macrophages confirmed the presence of a Nef:Hck complex which was dependent on the Nef proline repeat motif. The proline repeat motif of Nef also enhanced both HIV-1 infection and replication in macrophages, and was required for incorporation of Hck into viral particles. Unexpectedly, wild-type Hck inhibited infection of macrophages, but Hck was shown to enhance infection of primary T lymphocytes. These results indicate that the interaction between Nef and Hck is important for Nef-dependent modulation of viral infectivity. Hck-dependent enhancement of HIV-1 infection of T cells suggests that Nef-Hck interaction may contribute to the spread of HIV-1 infection from macrophages to T cells by modulating events in the producer cell, virion and target cell.

HIV-1 Nef and T-cell activation: a history of contradictions

HIV-1 Nef is a multifunctional viral protein that contributes to higher plasma viremia and more rapid disease progression. Nef appears to accomplish this, in part, through modulation of T-cell activation; however, the results of these studies over the past 25 years have been inconsistent. Here, the history of contradictory observations related to HIV-1 Nef and its ability to modulate T-cell activation is reviewed, and recent reports that may help to explain Net's apparent ability to both inhibit and activate T cells are highlighted.

Chemokine responses are increased in HIV-infected Malawian children with invasive pneumococcal disease

BACKGROUND

Chemokines play an important role in the recruitment and regulation of leukocyte traffic during bacterial infection. The aims of this study were to investigate the chemokine response to invasive pneumococcal disease (IPD) and to examine the influence of HIV infection on the chemokine response, pneumococcal bacterial loads, and outcome.

METHODS

We prospectively studied 95 children with IPD, and blood and cerebrospinal fluid (CSF) samples were taken at admission for the determination of chemokines, interferon-gamma (IFNgamma), and pneumococcal bacterial loads.

RESULTS

Plasma CXCL8 and CCL2, CSF CXCL8 and CCL4, and IFNgamma were significantly higher in HIV-infected children than in HIV-uninfected children. Blood and CSF pneumococcal bacterial loads correlated with plasma and CSF chemokines, respectively, and were higher in HIV-infected children compared with HIV-uninfected children. Among HIV-infected children, plasma concentrations of CXCL8 and CCL2 were significantly higher in nonsurvivors than in survivors, but CCL5 was significantly lower. HIV-infected and HIV-uninfected children with IPD had higher concentrations of chemokines (except CCL5) than acutely ill HIV-infected and HIV-uninfected children with no detectable bacterial infection. Male gender and low plasma CCL2 concentrations were shown to be independently associated with survival.

CONCLUSIONS

Chemokines, in particular CCL2, are associated with survival in IPD and correlate with pneumococcal bacterial loads, disease presentation, and outcome.

HIV-1 Nef selectively activates Src family kinases Hck, Lyn, and c-Src through direct SH3 domain interaction

Nef is an HIV-1 virulence factor that promotes viral pathogenicity by altering host cell signaling pathways. Nef binds several members of the Src kinase family, and these interactions have been implicated in the pathogenesis of HIV/AIDS. However, the direct effect of Nef interaction on Src family kinase (SFK) regulation and activity has not been systematically addressed. We explored this issue using Saccharomyces cerevisiae, a well defined model system for the study of SFK regulation. Previous studies have shown that ectopic expression of c-Src arrests yeast cell growth in a kinase-dependent manner. We expressed Fgr, Fyn, Hck, Lck, Lyn, and Yes as well as c-Src in yeast and found that each kinase was active and induced growth suppression. Co-expression of the negative regulatory kinase Csk suppressed SFK activity and reversed the growth-inhibitory effect. We then co-expressed each SFK with HIV-1 Nef in the presence of Csk. Nef strongly activated Hck, Lyn, and c-Src but did not detectably affect Fgr, Fyn, Lck, or Yes. Mutagenesis of the Nef PXXP motif essential for SH3 domain binding greatly reduced the effect of Nef on Hck, Lyn, and c-Src, suggesting that Nef activates these Src family members through allosteric displacement of intramolecular SH3-linker interactions. These data show that Nef selectively activates Hck, Lyn, and c-Src among SFKs, identifying these kinases as proximal effectors of Nef signaling and potential targets for anti-HIV drug discovery.

Altered CD4+T Cell Phenotype and Function Determine the Susceptibility to Mucosal Candidiasis in Transgenic Mice Expressing HIV-1

The impairments of protective mucosal immunity which cause susceptibility to oropharyngeal candidiasis (OPC) in HIV infection remain undefined. This study used a model of OPC in CD4C/HIV MutA transgenic (Tg) mice expressing Rev, Env, and Nef of HIV-1 to investigate the role of transgene expressing dendritic cells (DCs) and CD4+ T cells in maintenance of chronic oral carriage of Candida albicans. DCs were depleted in the Tg mice and had an immature phenotype, with low expression of MHC class II and IL-12. CD4+ T cells were quantitatively reduced in the oral mucosa, cervical lymph nodes (CLNs) and peripheral blood of the Tg mice, and displayed a polarization toward a nonprotective Th2 response. Proliferation of CLN CD4+ T cells from infected Tg mice in response to C. albicans Ag in vitro was abrogated and the cells failed to acquire an effector phenotype. Coculture of C. albicans-pulsed DCs with CD4+ T cells in vitro showed that Tg expression in either or both of these cell populations sharply reduced the proliferation of CD4+ T cells and their production of IL-2. Finally, transfer of naive non-Tg CD4+ T cells into these Tg mice restored proliferation to C. albicans Ag and sharply reduced oral burdens of C. albicans. Overall, these results indicate that defective CD4+ T cells primarily determine the susceptibility to chronic carriage of C. albicans in these Tg mice.

HLA-A2 down-regulation on primary human macrophages infected with an M-tropic EGFP-tagged HIV-1 reporter virus

Multiple mechanisms are used by the human immunodeficiency virus type 1 (HIV-1) to interfere with host-cell immune effector functions. The 27-kD Nef protein has been shown to down-modulate specific genes of the major histocompatibility complex class I (MHC-I) on the surface of infected primary T cells, facilitating their escape from lysis by cytolytic T lymphocytes. Macrophages, as the other major immune cell type targeted by the virus, also contribute to the transmission, persistence, and pathogenesis of HIV-1. Yet, whether Nef modulates MHC-I expression on HIV-infected primary macrophages remains unclear. Currently available infectious HIV-1 molecular clones, which express a reporter gene, only infect T cells and/or do not express Nef. To overcome these limitations, we generated macrophage-tropic green fluorescent protein (GFP)-tagged HIV-1 viruses, which express the complete viral genome, and used these to assess the expression of human leukocyte antigen (HLA)-A2 on the surface of productively infected macrophages. The reporter viral genomes were replication-competent and stable, as Nef, p24 antigen, and GFP expression could be detected by immunostaining of infected, monocyte-derived macrophages (MDM) after more than 2 months postinfection. Fluorescence-activated cell sorter analyses of infected macrophages and T cells revealed that although wild-type reporter virus infection induced a statistically significant decrease in the density of surface HLA-A2, down-regulation of HLA-A2 was not seen in cells infected with reporter viruses encoding a frameshift or a single point mutation in Nef at prolines 74P and P80. The impact of Nef on HLA-A2 surface expression in MDM was also confirmed by confocal microscopy. These results suggest that the mechanisms of HLA-A2 down-modulation are similar in primary T cells and macrophages.

HIV-1 Nef disrupts MHC-I trafficking by recruiting AP-1 to the MHC-I cytoplasmic tail

To avoid immune recognition by cytotoxic T lymphocytes (CTLs), human immunodeficiency virus (HIV)-1 Nef disrupts the transport of major histocompatibility complex class I molecules (MHC-I) to the cell surface in HIV-infected T cells. However, the mechanism by which Nef does this is unknown. We report that Nef disrupts MHC-I trafficking by rerouting newly synthesized MHC-I from the trans-Golgi network (TGN) to lysosomal compartments for degradation. The ability of Nef to target MHC-I from the TGN to lysosomes is dependent on expression of the μ1 subunit of adaptor protein (AP) AP-1A, a cellular protein complex implicated in TGN to endolysosomal pathways. We demonstrate that in HIV-infected primary T cells, Nef promotes a physical interaction between endogenous AP-1 and MHC-I. Moreover, we present data that this interaction uses a novel AP-1 binding site that requires amino acids in the MHC-I cytoplasmic tail. In sum, our evidence suggests that binding of AP-1 to the Nef–MHC-I complex is an important step required for inhibition of antigen presentation by HIV.

In vivo mutational analysis of the N-terminal region of HIV-1 Nef reveals critical motifs for the development of an AIDS-like disease in CD4C/HIV transgenic mice

HIV-1 Nef is a critical determinant of pathogenicity in humans and transgenic (Tg) mice. To gain a better understanding of the molecular mechanisms by which Nef induces an AIDS-like disease in Tg mice, a mutational analysis of the N-terminal domain, involved in anchoring Nef to the plasma membrane, was carried out. The pathogenic effects of these Nef mutant alleles were evaluated in Tg mice by FACS analysis and by histopathological assessment. Mutation of the myristoylation site (G2A) completely abrogated the development of the AIDS-like organ disease in Tg mice, although partial downregulation of the CD4 cell surface protein and depletion of peripheral CD4+ T-cells, but not of CD4(+)CD8+ thymocytes, still occurred. Despite that, the peripheral CD4+ T cells expressing Nef(G2A) show normal spontaneous proliferation in vivo or after stimulation in vitro, including in an allogenic mixed leukocyte reaction (MLR). Three other internal deletion mutants of Nef, spanning amino acids 8-17 (Nef(Delta8-17)), 25-35 (Nef(Delta25-35)), and 57-66 (Nef(Delta57-66)), were also studied. Nef(Delta8-17) retained full pathogenic potential, although Nef(Delta25-35) and Nef(Delta57-66) Tg mice were free of organ disease. However, Nef(Delta25-35) Tg mice exhibited disorganization of thymic architecture and a partial depletion of peripheral CD4+ T cells. These data indicate that myristoylation and other regions at the N-terminus of Nef (aa 25-35 and 57-66) are involved in mediating severe T-cell phenotypes and organ disease, although residues 8-17 are dispensable for these Nef functions. In addition, these results indicate that at least some of the CD4+ T-cell phenotypes can develop independently of the other AIDS-like organ phenotypes. This apparent segregation of different Nef-mediated phenotypes suggests distinct mechanisms of Nef action in different populations of target cells, and may be relevant to human AIDS.

Multiple human immunodeficiency virus type 1 Nef functions contribute to efficient replication in primary human macrophages

The human immunodeficiency virus type 1 (HIV-1) Nef protein has been shown to accelerate viral growth kinetics in primary human T-lymphocytes and macrophages; however, the specific function(s) of Nef responsible for this phenotype in macrophages is unknown. To address this issue, mutants of a molecularly cloned macrophage-tropic isolate, HIV-1(SF162), were generated expressing single point mutations that abrogate the ability of Nef to interact with cellular kinases or mediate CD4 down-regulation. Infection of primary monocyte-derived macrophages (MDM) with these mutant viruses revealed that residues in the PXXP motif contribute to efficient replication. Interestingly, viruses expressing alleles of Nef defective in CD4 down-modulation activity retain wild-type levels of infectivity in single-round assays but exhibited delayed replication kinetics and grew to lower titres compared to the wild-type virus in MDM. These data suggest that efficient HIV-1 replication is dependent on the ability of Nef to interact with cellular kinases and remove CD4 from the surface of infected macrophages.

CD4+ T cells from CD4C/HIVNef transgenic mice show enhanced activation in vivo with impaired proliferation in vitro but are dispensable for the development of a severe AIDS-like organ disease

The cellular and molecular mechanisms of dysfunction and depletion of CD4+ T lymphocytes over the course of human immunodeficiency virus type 1 (HIV-1) infection are still incompletely understood, but chronic immune activation is thought to play an important role in disease progression. We studied CD4+ T-cell biology in CD4C/HIV transgenic (Tg) mice, in which Nef expression is sufficient to induce a severe AIDS-like disease including a preferential decrease of CD4+ T cells. We show here that Nef-expressing Tg CD4+ T cells exhibit an activated/memory-like phenotype which appears to be independent of antigenic stimulation, as documented in experiments involving breeding with AD10 TcR Tg mice. In addition, in vivo bromodeoxyuridine incorporation showed that a larger proportion of Tg than non-Tg CD4+ T cells entered the S phase. However, in vitro, Tg CD4+ T cells were found to have a very limited capacity to divide in response to stimulation with anti-CD3 and anti-CD28 or in allogeneic mixed leukocyte reactions. Interestingly, despite these observations, the deletion of Tg CD4+ T cells had little impact on the development of other AIDS-like organ phenotypes. Thus, the Nef-induced chronic activation of CD4+ T cells may exhaust the T-cell pool and may contribute to the thymic atrophy and the low number of CD4+ T cells observed in these Tg mice.

Determinants of increased replicative capacity of serially passaged simian immunodeficiency virus with nef deleted in rhesus monkeys

Most rhesus macaques infected with simian immunodeficiency virus SIVmac239 with nef deleted (either Delta nef or Delta nef Delta vpr Delta US [Delta 3]) control viral replication and do not progress to AIDS. Some monkeys, however, develop moderate viral load set points and progress to AIDS. When simian immunodeficiency viruses (SIVs) recovered from two such animals (one Delta nef and the other Delta 3) were serially passaged in rhesus monkeys, the SIVs derived from both lineages were found to consistently induce moderate viral loads and disease progression. Analysis of viral sequences in the serially passaged derivatives revealed interesting changes in three regions: (i) an unusually high number of predicted amino acid changes (12 to 14) in the cytoplasmic domain of gp41, most of which were in regions that are usually conserved; these changes were observed in both lineages; (ii) an extreme shortening of nef sequences in the region of overlap with U3; these changes were observed in both lineages; and (iii) duplication of the NF-kappa B binding site in one lineage only. Neither the polymorphic gp41 changes alone nor the U3 deletion alone appeared to be responsible for increased replicative capacity because recombinant SIVmac239 Delta nef, engineered to contain either of these changes, induced moderate viral loads in only one of six monkeys. However, five of six monkeys infected with recombinant SIVmac239 Delta nef containing both TM and U3 changes did develop persisting moderate viral loads. These genetic changes did not increase lymphoid cell-activating properties in the monkey interleukin-2-dependent T-cell line 221, but the gp41 changes did increase the fusogenic activity of the SIV envelope two- to threefold. These results delineate sequence changes in SIV that can compensate for the loss of the nef gene to partially restore replicative and pathogenic potential in rhesus monkeys.

HIV-1 Nef control of cell signalling molecules: multiple strategies to promote virus replication

HIV-1 has at its disposal numerous proteins encoded by its genome which provide the required arsenal to establish and maintain infection in its host for a considerable number of years. One of the most important and enigmatic of these proteins is Nef. The Nef protein of HIV-1 plays a fundamental role in the virus life cycle. This small protein of approximately 27 kDa is required for maximal virus replication and disease progression. The mechanisms by which it is able to act as a positive factor during virus replication is an area of intense research and although some controversy surrounds Nef much has been gauged as to how it functions. Its ability to modulate the expression of key cellular receptors important for cell activation and control signal transduction elements and events by interacting with numerous cellular kinases and signalling molecules, including members of the Src family kinases, leading to an effect on host cell function is likely to explain at least in part its role during infection and represents a finely tuned mechanism where this protein assists HIV-1 to control its host.

Direct binding of human immunodeficiency virus type 1 Nef to the major histocompatibility complex class I (MHC-I) cytoplasmic tail disrupts MHC-I trafficking

Nef, an essential pathogenic determinant for human immunodeficiency virus type 1, has multiple functions that include disruption of major histocompatibility complex class I molecules (MHC-I) and CD4 and CD28 cell surface expression. The effects of Nef on MHC-I have been shown to protect infected cells from cytotoxic T-lymphocyte recognition by downmodulation of a subset of MHC-I (HLA-A and -B). The remaining HLA-C and -E molecules prevent recognition by natural killer (NK) cells, which would otherwise lyse cells expressing small amounts of MHC-I. Specific amino acid residues in the MHC-I cytoplasmic tail confer sensitivity to Nef, but their function is unknown. Here we show that purified Nef binds directly to the HLA-A2 cytoplasmic tail in vitro and that Nef forms complexes with MHC-I that can be isolated from human cells. The interaction between Nef and MHC-I appears to be weak, indicating that it may be transient or stabilized by other factors. Supporting the fact that these molecules interact in vivo, we found that Nef colocalizes with HLA-A2 molecules in a perinuclear distribution inside cells. In addition, we demonstrated that Nef fails to bind the HLA-E tail and also fails to bind HLA-A2 tails with deletions of amino acids necessary for MHC-I downmodulation. These data provide an explanation for differential downmodulation of MHC-I allotypes by Nef. In addition, they provide the first direct evidence indicating that Nef functions as an adaptor molecule able to link MHC-I to cellular trafficking proteins.

Replication of HIV-1 deleted Nef mutants in chronically immune activated human T cells

Lymphocytes (PBMC) obtained from blood of HIV-sera negative Ethiopian immigrants (ETH) were highly susceptible to HIV-1 infection in vitro with no need for stimulation by mitogens. As the HIV nef gene product has been shown to enhance viral replication in stimulated primary lymphocytes, we investigated in this work the role of Nef in viral replication in the ETH cells. Lymphocytes obtained from ETH individuals supported high replication of wild-type HIV-1 and low but significant replication level of the two deleted Nef mutants (encode truncated Nef proteins consisting only of either the first 35 or the first 86 amino acids of Nef). In contrast, no replication was observed in nonactivated cells obtained from non-ETH individuals. After activation of the PBMC from ETH individuals with PHA, replication of both wild-type strains and the two deleted Nef mutant viruses further increased. The CD4(+) T cells of ETH individuals exhibited elevated levels of the surface activation markers CD45RO and HLA-DR, compared with T cells derived from non-ETH group. Likewise, expression of the chemokine receptors CCR5 and CXCR4 on these cells was higher in the ETH group than in the non-ETH group. Replication of HIV-1 wild-type and the isogenic-deleted Nef mutants was significantly correlated with the proportion of ETH cells expressing CD45RO and the chemokine receptors. This study suggests that HIV-1 may respond differently to several activation states characteristic of T cells. One activation state, defined by chronically activated lymphocytes from ETH individuals, is permissive to the wild-type HIV-1 and, to a lesser degree, to the Nef mutants. Further activation of these cells by exogenous stimuli enhances replication of the virus. Our results support the notion that Nef enhances the basal level of T cell activation and consequently, viral replication.

The expression of HIV-1 tat and nef genes induces cell-specific changes in growth properties and morphology of different types of rat cells

Among the viral regulatory genes the tat and nef genes of HIV-1 encode the proteins playing a central role in viral replication and exerting pleiotropic effects on the survival and growth of the cells. These effects differ in various cell types, possibly due to the use of genes from different HIV-1 isolates. In this work, we studied the effects of the tat and nef genes on three types of cultured rat cells: primary embryo fibroblasts, pseudonormal Rat-2, and pheochromocytoma PC12. Both genes affected growth properties and morphology of cells, the effects being cell-specific. The proliferative activity of both Rat-2 and PC12 cells was considerably increased after transfection with the tat gene. In primary rat embryo fibroblasts the tat gene induced multilayered foci. More importantly, it was shown that the efficiency of transformation was higher in cells coexpressing tat and nef. The nef gene caused considerable suppression of Rat-2 cell proliferation, but no changes in their morphology. The nef gene transfection of PC12 cells also led to suppression of their proliferative activity. In addition, cellular agglomerates which were morphologically similar to multinuclear syncytial cells were detected in these cells for the first time.

Interaction with simian Hck tyrosine kinase reveals convergent evolution of the Nef protein from simian and human immunodeficiency viruses despite differential molecular surface usage

Simian and human immunodeficiency virus type 1 (SIV and HIV-1) Nef proteins are thought to use different molecular surfaces to mediate the protein-protein interactions required for their otherwise similar functions. This genetically separable function suggests convergent evolution of primate lentiviruses and/or structural differences between human and nonhuman primate cellular target proteins. However, such comparative molecular analyses have not been undertaken so far using the respective natural host-derived cellular targets. We cloned simian Src family kinase Hck and analyzed structurally and biochemically its interaction with SIV Nef.

Human immunodeficiency virus type 1 Nef binds to tumor suppressor p53 and protects cells against p53-mediated apoptosis

The nef gene product of human immunodeficiency virus type 1 (HIV-1) is important for the induction of AIDS, and key to its function is its ability to manipulate T-cell function by targeting cellular signal transduction proteins. We reported that Nef coprecipitates a multiprotein complex from cells which contains tumor suppressor protein p53. We now show that Nef interacts directly with p53. Binding assays showed that an N-terminal, 57-residue fragment of Nef (Nef 1-57) contains the p53-binding domain. Nef also interacted with p53 during HIV-1 infection in vitro. As p53 plays a critical role in the regulation of apoptosis, we hypothesized that Nef may alter this process. Nef inhibited UV light-induced, p53-dependent apoptosis in MOLT-4 cells, with Nef 1-57 being as effective as its full-length counterpart. The inhibition by Nef of p53 apoptotic function is most likely due its observed ability to decrease p53 protein half-life and, consequently, p53 DNA binding activity and transcriptional activation. These data show that HIV-1 Nef may augment HIV replication by prolonging the viability of infected cells by blocking p53-mediated apoptosis.

Nef of HIV-1 interacts directly with calcium-bound calmodulin

It was recently found that the myristoyl group of CAP-23/NAP-22, a neuron-specific protein kinase C substrate, is essential for the interaction between the protein and Ca(2+)-bound calmodulin (Ca(2+)/CaM). Based on the N-terminal amino acid sequence alignment of CAP-23/NAP-22 and other myristoylated proteins, including the Nef protein from human immunodeficiency virus (HIV), we proposed a new hypothesis that the protein myristoylation plays important roles in protein-calmodulin interactions. To investigate the possibility of direct interaction between Nef and calmodulin, we performed structural studies of Ca(2+)/CaM in the presence of a myristoylated peptide corresponding to the N-terminal region of Nef. The dissociation constant between Ca(2+)/CaM and the myristoylated Nef peptide was determined to be 13.7 nM by fluorescence spectroscopy analyses. The NMR experiments indicated that the chemical shifts of some residues on and around the hydrophobic clefts of Ca(2+)/CaM changed markedly in the Ca(2+)/CaM-Nef peptide complex with the molar ratio of 1:2. Correspondingly, the radius of gyration determined by the small angle X-ray scattering measurements is 2-3 A smaller that of Ca(2+)/CaM alone. These results demonstrate clearly that Nef interacts directly with Ca(2+)/CaM.

Effects of HIV-1 Nef on cellular gene expression profiles

The early human immunodeficiency virus (HIV) accessory protein Nef makes an important contribution to virulence, but the mechanisms by which Nef influences pathogenesis remain unclear. Many well-studied effects of Nef, like CD4 and class I MHC downregulation, occur posttranslationally. However, Nef has the potential to affect gene expression by interfering with cell signaling pathways and by virtue of structural features such as the Pro-X-X-Pro motif, which may interact with src homology region-3 domains of src-like kinases. We used a cDNA microarray screening strategy to identify cellular genes whose steady state transcriptional levels may be affected by Nef. We generated HeLa cell lines expressing wild-type or mutant HIV-1 nef protein sequences. Using cDNA microarray technology, we compared the patterns of cellular gene expression in the various cell lines to the pattern in non-Nef-expressing HeLa cells. By matching the patterns of cellular gene expression in HeLa cell lines expressing various Nefs with that of parental HeLa cells, we identified several cellular genes whose expression was modulated differentially by Nef and its mutants. We confirmed the differential expression of selected genes by RNA filter blotting. Genes expressed at higher levels included proteases, transcription factors, protein kinases, nuclear import/export proteins, adaptor molecules and cyclins, some of which have previously been implicated as being important for HIV replication and pathogenesis. The results indicate that Nef expression can alter the expression of cellular genes and suggest that this alteration in cellular gene expression may serve to optimize the cell to support the subsequent stages of viral replication.

The pathogenicity of human immunodeficiency virus (HIV) type 1 Nef in CD4C/HIV transgenic mice is abolished by mutation of its SH3-binding domain, and disease development is delayed in the absence of Hck

The human immunodeficiency virus type 1 (HIV-1) Nef protein is an important determinant of AIDS pathogenesis. We have previously reported that HIV-1 Nef is responsible for the induction of a severe AIDS-like disease in CD4C/HIV transgenic (Tg) mice. To understand the molecular mechanisms of this Nef-induced disease, we generated Tg mice expressing a mutated Nef protein in which the SH3 ligand-binding domain (P(72)XXP(75)XXP(78)) was mutated to A(72)XXA(75)XXQ(78). This mutation completely abolished the pathogenic potential of Nef, although a partial downregulation of the CD4 cell surface expression was still observed in these Tg mice. We also studied whether Hck, one of the effectors previously found to bind to this PXXP motif of Nef, was involved in disease development. Breeding of Tg mice expressing wild-type Nef on an hck(-/-) (knockout) background did not abolish any of the pathological phenotypes. However, the latency of disease development was prolonged. These data indicate that an intact PXXP domain is essential for inducing an AIDS-like disease in CD4C/HIV Tg mice and suggest that interaction of a cellular effector(s) with this domain is required for the induction of this multiorgan disease. Our findings indicate that Hck is an important, but not an essential, effector of Nef and suggest that another factor(s), yet to be identified, may be more critical for disease development.

Anergy and human immunodeficiency virus infection

Human immunodeficiency virus (HIV), the retrovirus associated with acquired immune deficiency syndrome (AIDS), induces a spectrum of immune abnormalities including a state of anergy in the host. This state is due to the binding of HIV envelope glycoprotein moieties to CD4 molecules and chemokine receptors. Resulting decrease in antigen presenting cell function and the interference with functioning of positive and negative regulatory molecules involved in signal transduction have an anergizing effect on the immune system. This effect is exemplified by diminished production of interleukin-2 (IL-2) and interferon-gamma and reduced expression of IL-2 receptor by CD4 helper cells of HIV patients. These immune abnormalities lead to clinically relevant immunological phenomena such as Type-1 to Type-2 switch, decrease in delayed-type hypersensitivity dermal reaction, etc. Insight into these interesting phenomena could pave the path for favorably altering the immunological milieu for drug and vaccine trials.

Using death to one's advantage: HIV modulation of apoptosis

Infection by human immunodeficiency virus (HIV) is associated with an early immune dysfunction and progressive destruction of CD4+ T lymphocytes. This progressive disappearance of T cells leads to a lack of immune control of HIV replication and to the development of immune deficiency resulting in the increased occurrence of opportunistic infections associated with acquired immune deficiency syndrome (AIDS). The HIV-induced, premature destruction of lymphocytes is associated with the continuous production of HIV viral proteins that modulate apoptotic pathways. The viral proteins, such as Tat, Env, and Nef, are associated with chronic immune activation and the continuous induction of apoptotic factors. Viral protein expression predisposes lymphocytes, particularly CD4+ T cells, CD8+ T cells, and antigen-presenting cells, to evolve into effectors of apoptosis and as a result, to lead to the destruction of healthy, non-infected T cells. Tat and Nef, along with Vpu, can also protect HIV-infected cells from apoptosis by increasing anti-apoptotic proteins and down-regulating cell surface receptors recognized by immune system cells. This review will discuss the validity of the apoptosis hypothesis in HIV disease and the potential mechanism(s) that HIV proteins perform in the progressive T cell depletion observed in AIDS pathogenesis.

Anergy and acquired immune deficiency syndrome

Human immunodeficiency virus (HIV), the retrovirus associated with acquired immune deficiency syndrome (AIDS), acts as a super-antigen by binding to the variable region of the beta (V beta) chain of T-cell receptor (TCR). It's binding to CD4 molecules and chemokine receptors induces a spectrum of immune abnormalities including 'a state of anergy' in the host. This state is due to a defective function of T-helper cell-1 (Th-1), a reduction in production of lymphokines required for signal transduction, an impaired cytotoxic cell activation and a decrease in antigen presenting function of monocyte-macrophage cell lineage. These immune abnormalities form the basis for severe opportunistic infections and malignancies in the host. Malnutrition, micronutrient abnormalities, concomitant infections and genetic factors, etc., are some of the compounding co-factors that further contribute to 'the state of anergy'.

Apoptosis enhancement by the HIV-1 Nef protein

The HIV-1 nef gene, essential for AIDS pathogenesis, encodes a 27-kDa protein (Nef) whose biochemical and biological functions are unclear. It has been suggested that Nef expression contributes to the T cell depletion observed during the disease by promoting their apoptosis. We report that in CD4(+) human lymphoblastoid cell lines transfected with the nef cDNA obtained from three different HIV-1 strains, expression of the Nef protein enhances and accelerates the response to four unrelated apoptotic agents (staurosporine, anisomycin, camptothecin, and etoposide) but not to an anti-Fas agonist Ab. Nef reduces the expression of the anti-apoptotic proteins Bcl-2 and Bcl-X(L) and induces a striking enhancement of apoptotic hallmarks, including mitochondrial depolarization, exposure of phosphatidylserine on the cell surface, activation of caspase-3, and cleavage of the caspase target poly(ADP-ribose) polymerase. Interestingly, the peptide Z-Val-Ala-DL-Asp-fluoromethylketone (a broad-spectrum caspase inhibitor) reduces, but does not abolish, phosphatidylserine exposure, suggesting that Nef also activates a caspase-independent apoptotic pathway. Surprisingly, Nef expression increases DNA degradation but without causing oligonucleosomal fragmentation. An increased apoptotic response and down-modulation of Bcl-2/Bcl-X(L) following Nef expression are observed also in NIH-3T3 fibroblasts. These data show that Nef enhances programmed cell death in different cell types by affecting multiple critical components of the apoptotic machinery independently from the Fas pathway.

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.

Upregulation of the apoptosis-associated protein Grb3-3 in HIV-1-infected human CD4(+) lymphocytes

The mechanism(s) by which HIV-1 infection contributes to depletion of CD4(+) T cell is not well understood. In this report, we investigated whether a recently identified isoform of growth factor receptor bound protein (Grb2), named Grb3-3, a signaling molecule that is associated with the MAP kinase pathway and with apoptosis could be involved. We find that Grb3-3 is markedly up-regulated following HIV-1 infection of CD4(+) peripheral blood mononuclear cells undergoing apoptosis. Although IL-2 deprived CD4(+) cells also undergo apoptosis to a similar extent, Grb3-3 upregulation is not detected under these experimental conditions. Transient overexpression of Grb3-3 in Jurkat T-cells also causes apoptosis. Upon staurosporine stimulation, Grb3-3 predisposes Sup-T1 cell to apoptosis. Finally, analysis of the HIV-1 genes responsible for Grb3-3 expression demonstrates that Tat and Nef can independently induces its expression, suggesting these two earliest viral gene products of HIV-1 may share some common pathway(s) in up-regulating Grb3-3 expression.

HIV-Nef enhances interleukin-2 production and phosphatidylinositol 3-kinase activity in a human T cell line

OBJECTIVE

The Nef protein has a major influence on disease pathogenesis in HIV-infected individuals. The objective of the present study was to examine the effects of Nef on T lymphocyte activation and associated signalling events.

DESIGN

A recombinant vaccinia expression system was used to express Nef in a human T cell line. Stimulation of these cells with anti-CD28 antibody, and either phorbol 12-myristate 13-acetate (PMA) or anti-CD3, activates signal transduction pathways and results in IL-2 production and IL-2 receptor alpha-chain (CD25) expression. Cellular responses were examined in cells expressing either Nef or an irrelevant control protein.

METHODS

Activation of signalling was assessed by immunoblot analysis, or by in-vitro phosphatidylinositol 3-kinase (PI3K) assays. IL-2 production was measured by enzyme-linked immunosorbent assay, and CD25 cell surface expression was examined using flow cytometry.

RESULTS

Infection of cells with recombinant vaccinia expressing HIV-nef resulted in a marked increase in the production of IL-2 when cells were activated. The enhanced IL-2 response was accompanied by an increase in the level of PI3K activity. IL-2 production remained sensitive to inhibition with the PI3K competitive inhibitor Ly294002, and to the fungal macrolide, rapamycin. In contrast, CD25 expression was not affected, and there were no measurable changes to nuclear factor kappaB (NFkappaB) activation pathways.

CONCLUSION

Enhanced IL-2 production in stimulated T cells expressing HIV-Nef is associated with increased activation of PI3K-dependent signalling pathways. The results support a model in which Nef affects HIV disease progression by distorting T cell responses.

Stable expression of Epstein-Barr virus BZLF-1–encoded ZEBRA protein activates p53-dependent transcription in human Jurkat T-lymphoblastoid cells

Interaction between viral proteins and tumor suppressor p53 is a common mechanism of viral pathogenesis. The Epstein-Barr virus (EBV) BZLF-1 ORF-encoded ZEBRA protein (also denoted EB1, Z, Zta) binds to p53 in vitro and has been associated with the altered transcription of p53-regulated genes in B lymphocytes and epithelial cells. In this work, Jurkat T-lymphoblastoid cells that express ZEBRA were characterized by the use of transiently transfected p53 and p53 reporter genes. Stable expression of ZEBRA was associated with the activation of p53-dependent transcription and increased p53 dependent apoptotic cell death. In Jurkat cell lines, stably expressed ZEBRA protein was apparently localized to the cell cytoplasm, in contrast to the typical nuclear localization of this protein in other cell types. Previous studies have suggested that EBV infection of T lymphocytes may contribute to the malignant transformation of T cells and the increased replication of human immunodeficiency virus. Our observations suggest a mechanism through which ZEBRA protein expressed in human T lymphocytes could alter T-cell proliferation and apoptosis during EBV infection.

Stable expression of Epstein-Barr virus BZLF-1–encoded ZEBRA protein activates p53-dependent transcription in human Jurkat T-lymphoblastoid cells

Interaction between viral proteins and tumor suppressor p53 is a common mechanism of viral pathogenesis. The Epstein-Barr virus (EBV) BZLF-1 ORF-encoded ZEBRA protein (also denoted EB1, Z, Zta) binds to p53 in vitro and has been associated with the altered transcription of p53-regulated genes in B lymphocytes and epithelial cells. In this work, Jurkat T-lymphoblastoid cells that express ZEBRA were characterized by the use of transiently transfected p53 and p53 reporter genes. Stable expression of ZEBRA was associated with the activation of p53-dependent transcription and increased p53 dependent apoptotic cell death. In Jurkat cell lines, stably expressed ZEBRA protein was apparently localized to the cell cytoplasm, in contrast to the typical nuclear localization of this protein in other cell types. Previous studies have suggested that EBV infection of T lymphocytes may contribute to the malignant transformation of T cells and the increased replication of human immunodeficiency virus. Our observations suggest a mechanism through which ZEBRA protein expressed in human T lymphocytes could alter T-cell proliferation and apoptosis during EBV infection.

Synergistic activation of NFAT by HIV-1 nef and the Ras/MAPK pathway

Nef is a lentiviral protein involved in pathogenesis of AIDS, but its molecular mechanisms of action remain incompletely understood. Here we report a novel effect of Nef on lymphocyte signaling, which is mediated via a T cell receptor (TCR)-independent contribution of Nef to induction of nuclear factor of activated T cells (NFAT), a transcription factor that plays a central role in coordinating T cell activation. Expression of Nef did not significantly alter the basal level of NFAT activity in Jurkat cells nor the increased activity following T cell receptor stimulation by anti-CD3 or anti-CD3 + anti-CD28. We also mimicked NFAT induction by TCR triggering by simultaneous activation of the Ras and calcium signaling pathways with phorbol 12-myristate 13-acetate and ionomycin, respectively. Strikingly, whereas activation of either of these pathways individually did not induce NFAT activity in control cells, in Nef-expressing cells phorbol 12-myristate 13-acetate treatment alone resulted in a 100-fold increase in NFAT-directed gene expression. Experiments with different dominant negative mutant signaling proteins, inhibitory chemicals, and Lck-deficient Jurkat cells revealed that this effect was mediated via activation of calcineurin by Nef-induced changes in calcium metabolism, but was independent of TCR-associated signaling events. This ability of Nef to substitute for triggering of the calcium pathway in induction of NFAT could promote activation of human immunodeficiency virus (HIV)-infected T cells in response to stimuli mediated via TCR or other cell surface receptors under conditions when activation of Ras rather than calcium signaling would otherwise predominate.

Mutation of a conserved residue (D123) required for oligomerization of human immunodeficiency virus type 1 Nef protein abolishes interaction with human thioesterase and results in impairment of Nef biological functions

Nef is a myristoylated protein of 27 to 35 kDa that is conserved in primate lentiviruses. In vivo, Nef is required for high viral load and full pathological effects. In vitro, Nef has at least four activities: induction of CD4 and major histocompatibility complex (MHC) class I downregulation, enhancement of viral infectivity, and alteration of T-cell activation pathways. We previously reported that the Nef protein from human immunodeficiency virus type 1 interacts with a novel human thioesterase (hTE). In the present study, by mutational analysis, we identified a region of the Nef core, extending from the residues D108 to W124, that is involved both in Nef-hTE interaction and in Nef-induced CD4 downregulation. This region of Nef is located on the oligomer interface and is in close proximity to the putative CD4 binding site. One of the mutants carrying a mutation in this region, targeted to the conserved residue D123, was also found to be defective in two other functions of Nef, MHC class I downmodulation and enhancement of viral infectivity. Furthermore, mutation of this residue affected the ability of Nef to form dimers, suggesting that the oligomerization of Nef may be critical for its multiple functions.

Characterization and molecular basis of the oligomeric structure of HIV-1 nef protein

The Nef protein of human immunodeficiency virus type I (HIV-1) is an important determinant for the onset of AIDS disease. The self-association properties of HIV-1 Nef are analyzed by chemical cross-linking, dynamic light scattering, equilibrium analytical ultracentrifugation, and NMR spectroscopy. The experimental data show that the HIV-1 Nef core domain forms stable homo-dimers and trimers in solution, but not higher oligomers. These Nef homomers are not covalently linked by disulfide bridges, and the equilibrium between these forms is dependent on the Nef concentration. We further provide the molecular basis for the Nef core dimers and trimers obtained by analysis of crystallographic models. Oligomerization of biological polypeptides is a common tool used to trigger events in cellular signaling and endocytosis, both of which are targeted by Nef. The quaternary structure of Nef may be of physiological importance and may help to connect its cellular targets or to increase affinity of the viral molecule for its ligands. The herein described models for Nef dimers and trimers will allow further mutational studies to elucidate their role in vivo. These results provide novel insight into the structural and functional relationships of this important viral protein. Moreover, the oligomer interface may represent a novel target for the design of antiviral agents.

Nef modulation of HIV type 1 gene expression and cytopathicity in tissues of HIV transgenic mice

Transgenic mice bearing HIV-1 proviral DNA deleted in the gag/pol region (HIVd1443 mice) model a chronic, nonproductive form of viral gene expression in various cell types including macrophages. They display a disease phenotype that includes HIV-associated nephropathy (HIVAN), congenital cataracts, papillomatosis, and growth failure. The role of HIV-1 Nef in viral gene regulation and the development of disease was explored in mice bearing an isogenic HIV transgene in which nef was mutated by frameshift mutation. Like its Nef+ counterpart, HIVd1443[Nef-] mice expressed HIV gene products in the skin, muscle, kidney, and peritoneal macrophages. While these mice did not develop cataracts, papillomatous skin lesions, or display any apparent growth defect, they did develop HIVAN. Nef expression was introduced to HIVd1443[Nef-] mice through breeding to mice bearing an HIV LTR-linked nef transgene. Nef-complemented HIVd1443[Nef-] mice had reduced levels of viral gene products in the muscle and kidney. In contrast, HIV gene expression in the skin of these mice remained high and papillomatous lesions emerged that were more severe than those on wild-type HIVd1443 mice. Still, Nef had a negative effect on LPS-induced viral gene expression in visibly normal skin. In comparisons of peritoneal macrophages, viral RNA expression was significantly reduced in resident macrophages of Nef+ mice. HIV inflammatory macrophages expressed viral genes and displayed an altered FACS profile. In particular, Nef+ populations were marked by an increased proportion of F4/80med/Mac-1-cells as well as fewer Mac-1 cells and reduced F4/80 staining. This HIV proviral transgenic model has demonstrated the capacity of HIV-1 Nef to contribute to HIV cytopathicity by altering cellular maturation and viral gene expression in vivo.

Simian immunodeficiency virus containing mutations in N-terminal tyrosine residues and in the PxxP motif in Nef replicates efficiently in rhesus macaques

SIVmac Nef contains two N-terminal tyrosines that were proposed to be part of an SH2-ligand domain and/or a tyrosine-based endocytosis signal and a putative SH3-ligand domain (P(104)xxP(107)). In the present study, we investigated the effects of combined mutations in these tyrosine and proline residues on simian immunodeficiency virus (SIV) Nef interactions with the cellular signal transduction and endocytic machinery. We found that mutation of Y(28)F, Y(39)F, P(104)A, and P(107)A (FFAA-Nef) had little effect on Nef functions such as the association with the cellular tyrosine kinase Src, downregulation of cell surface expression of CD4 and class I major histocompatibility complex, and enhancement of virion infectivity. However, mutations in the PxxP sequence reduced the ability of Nef to stimulate viral replication in primary lymphocytes. Three macaques infected with the SIVmac239 FFAA-Nef variant showed high viral loads during the acute phase of infection. Reversions in the mutated prolines were observed between 12 and 20 weeks postinfection. Importantly, reversion of A(107)-->P, which restored the ability of Nef to coprecipitate a 62-kDa phosphoprotein in in vitro kinase assays, did not precede the development of a high viral load. The Y(28)/Y(39)-->F(28)/F(39) substitutions did not revert. In conclusion, mutations in both the tyrosine residues and the putative SH3 ligand domain apparently do not disrupt major aspects of SIV Nef function in vivo.

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.

The Nef protein of HIV-1 associates with rafts and primes T cells for activation

The Nef protein is an important virulence factor of primate lentiviruses, yet the mechanisms by which it exerts this influence are imperfectly understood. Here, using an inducible system, we demonstrate that Nef increases IL-2 secretion from T cells stimulated via CD3 or CD28. This effect requires the conservation of the Nef myristoylation signal and SH3-binding proline-based motif. Together with several proteins involved in the initiation and propagation of T cell signaling, Nef associates with membrane microdomains known as rafts. The Nef-mediated superinduction of IL-2 reflects the activation of both NFAT and NFkappaB. Accordingly, Nef also enhances HIV-1 transcription in response to CD3 or CD28 stimulation. Nef-induced IL-2 hyperresponsiveness is also observed in primary CD4 lymphocytes. Overall, these data suggest that Nef acts at the level of rafts to prime T cells for activation. Likely consequences of this effect are the promotion of HIV-1 replication and the facilitation of virus spread.

The HIV Nef protein alters Ca(2+) signaling in myelomonocytic cells through SH3-mediated protein-protein interactions

Human immunodeficiency virus Nef plays an important role in AIDS pathogenesis. In addition to the well known down-regulation of cell surface receptors (CD4, MHCI), Nef is able to alter cellular signaling. Of particular interest for this study is the ability of Nef to bind with a very high affinity to SH3 domains of myelomonocyte-specific protein-tyrosine kinases of the Src family (Src-like PTK). We have therefore investigated Ca(2+) signaling in HL60 cells retrovirally transduced with wild type Nef or with a Nef mutant deficient in the SH3-interacting proline-rich motif (Nef((PXXP)4(-))). In differentiated HL60 cells, Nef markedly altered cellular Ca(2+) signaling; the amount of intracellularly stored Ca(2+) was increased, and as a consequence, store-operated Ca(2+)-influx was decreased. This effect was not observed in undifferentiated HL60 cells or in CEM T-lymphocytes and correlated with the differentiation-induced up-regulation of Src-like PTK. The Nef effect on Ca(2+) signaling depended entirely on the integrity of its PXXP motif. The Src-like PTK p56/59(hck) co-immunoprecipitated with both Nef and with the inositol 1,4,5-trisphosphate receptor, providing a possible mechanistic link between the viral protein and intracellular Ca(2+) stores of the host cell. Collectively, our results demonstrate that the human immunodeficiency virus 1 Nef protein manipulates intracellular Ca(2+) stores through SH3-mediated interactions in myelomonocytic cells.

Activation of the PAK-related kinase by human immunodeficiency virus type 1 Nef in primary human peripheral blood lymphocytes and macrophages leads to phosphorylation of a PIX-p95 complex

Human immunodeficiency virus type 1 (HIV-1) Nef enhances virus replication in both primary T lymphocytes and monocyte-derived macrophages. This enhancement phenotype has been linked to the ability of Nef to modulate the activity of cellular kinases. We find that despite the reported high-affinity interaction between Nef and the Src kinase Hck in vitro, a Nef-Hck interaction in the context of HIV-1-infected primary macrophages is not detectable. However, Nef binding and activation of the PAK-related kinase and phosphorylation of its substrate could be readily detected in both infected primary T lymphocytes and macrophages. Furthermore, we show that this substrate is a complex composed of the recently characterized PAK interacting partner PIX (PAK-interacting guanine nucleotide exchange factor) and its tightly associated p95 protein. PAK and PIX-p95 appear to be differentially activated and phosphorylated depending on the intracellular environment in which nef is expressed. These results identify the PIX-p95 complex as a novel effector of Nef in primary cells and suggest that the regulation of the PAK signaling pathway may differ in T cells and macrophages.

The conserved core of human immunodeficiency virus type 1 Nef is essential for association with Lck and for enhanced viral replication in T-lymphocytes

The Nef protein of the primate lentiviruses, including human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV), is a myristylated protein associated with increased viral replication and enhanced pathogenicity. Both the potentiation of T-lymphocyte activation and the enhanced serine-phosphorylation of HIV-1 capsid by Nef correlate with increased viral replication. We report the functional interactions of the Nef proteins with Src kinases. The Nef proteins from HIV-1 and SIV bind to Lck as well as Hck, Lyn, and Fyn. The SH3 and SH2 domains of Lck are sufficient for coprecipitation with non-tyrosine-phosphorylated Nef proteins. The conserved core region of HIV-1 Nef is essential for the interaction with Lck and is also important for enhanced HIV-1 replication in T-lymphocytes. In addition, we show that SIV and HIV-1 Nef proteins are differentially tyrosine-phosphorylated. The kinase-active Lck tyrosine-phosphorylates SIVmac239 Nef but does not phosphorylate HIV-1 Nef. These data suggest that the association of Nef and Lck is central to the enhanced viral replication of HIV-1 and SIV in T-lymphocytes.

A portrait of the Bcl-2 protein family: life, death, and the whole picture

The Bcl-2 family of proteins are important regulators of cell death. They are comprised of two opposing factions, the proapoptotic versus the antiapoptotic members. Both are required for normal development and cellular homeostasis of the immune system and other tissues. However, in certain circumstances they may participate in the development of disease.

Lack of negative influence on the cellular transcription factors NF-kappaB and AP-1 by the nef protein of human immunodeficiency virus type 1

In order to investigate the molecular mechanism of the reported negative effect of the Nef protein of human immunodeficiency virus type 1 (HIV-1) on the cellular transcription factors NF-kappaB and AP-1, human T cell lines (both populations and subclones) expressing the nef gene from HIV-1 clone pNL432 were constructed. Functional expression of the nef gene was confirmed by downregulation of CD4 and MHC class I proteins on the cell surface as measured by fluorescence-activated cell sorter analysis. However, contrary to previous reports, no significant difference was found in the induced level of NF-kappaB and AP-1 activity between nef(+) and nef(-) cell lines upon stimulation by phorbol 12-myristate 13-acetate and phytohaemagglutinin, as measured by transient transfection and electromobility shift assays. These data indicate that the Nef protein does not have a negative effect on the induction of NF-kappaB and AP-1.

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 human immunodeficiency virus type 1 nef gene can to a large extent replace simian immunodeficiency virus nef in vivo

The nef gene of the pathogenic simian immunodeficiency virus (SIV) 239 clone was replaced with primary human immunodeficiency virus type 1 (HIV-1) nef alleles to investigate whether HIV-1 Nef can substitute for SIV Nef in vivo. Initially, two rhesus macaques were infected with the chimeric viruses (Nef-SHIVs). Most of the nef alleles obtained from both animals predicted intact open reading frames. Furthermore, forms containing upstream nucleotide substitutions that enhanced expression of the inserted gene became predominant. One animal maintained high viral loads and slowly progressed to immunodeficiency. nef long terminal repeat sequences amplified from this animal were used to generate a second generation of Nef-SHIVs. Two macaques, which were subsequently infected with a mixture of cloned chimeric viruses, showed high viral loads and progressed to fatal immunodeficiency. Five macaques received a single molecular clone, named SHIV-40K6. The SHIV-40K6 nef allele was active in CD4 and class I major histocompatibility complex downregulation and enhanced viral infectivity and replication. Notably, all of the macaques inoculated with SHIV-40K6 showed high levels of viral replication early in infection. During later stages, however, the course of infection was variable. Three animals maintained high viral loads and developed immunodeficiency. Of the remaining two macaques, which showed decreasing viral loads after the acute phase of infection, only one efficiently controlled viral replication and remained asymptomatic during 1.5 years of follow-up. The other animal showed an increasing viral load and developed signs of progressive infection during later stages. Our data demonstrate that HIV-1 nef can, to a large extent, functionally replace SIVmac nef in vivo.