Expression of the type 1 human immunodeficiency virus Nef protein in T cells prevents antigen receptor-mediated induction of interleukin 2 mRNA

Differential production of IL-10 by T cells and monocytes of HIV-infected individuals: association of IL-10 production with CD28-mediated immune responsiveness

Immune unresponsiveness in HIV-1 infection can result from impaired signals delivered by the costimulatory CD28-B7 pathway and the altered production of immunoregulatory cytokines, in particular IL-10, whose production is altered in HIV-1 infection. In this study we investigate IL-10 regulation in T cells and monocytes from HIV+ individuals, and its association with CD28-mediated T cell proliferation. IL-10 production as analysed in T cell- and monocyte-depleted peripheral blood mononuclear cells (PBMC), and by intracellular staining at the single-cell level, reveals a defect in IL-10 production by CD4+ and CD8+ T cells, whereas monocytes constitute the major IL-10-producing cell type. To investigate the impact of IL-10 on immune responsiveness, CD28-mediated proliferative responses in HIV+ individuals were correlated with PHA-induced IL-10 production. CD4+ T cells expressed CD28, yet exhibited markedly reduced CD28-mediated cell proliferation. This CD28-mediated CD4+ T cell proliferation was found to be inversely associated with the levels of PHA-induced IL-10 production and could be restored, at least in part, by anti-IL-10 antibodies. These results suggest that IL-10 production is differentially regulated in T cells and monocytes of HIV+ individuals, and that IL-10 may have a role in inducing immune unresponsiveness by modulating the CD28-B7 pathway.

Interaction of HIV-1 Nef with the cellular dileucine-based sorting pathway is required for CD4 down-regulation and optimal viral infectivity

The HIV-1 Nef protein is important for pathogenesis, enhances viral infectivity, and regulates the sorting of at least two cellular transmembrane proteins, CD4 and major histocompatibility complex (MHC) class I. Although several lines of evidence support the hypothesis that the Nef protein interacts directly with the cellular protein sorting machinery, the sorting signal in HIV-1 Nef has not been identified. By using a competition assay that functionally discriminates between dileucine-based and tyrosine-based sorting signals, we have categorized the motif through which Nef interacts with the sorting machinery as dileucine-based. Inspection of diverse Nef proteins from HIV-1, HIV-2, and simian immunodeficiency virus revealed a well-conserved sequence in the central region of the C-terminal, solvent-exposed loop of Nef (E/DXXXLphi) that conforms to the consensus sequence of the dileucine-based sorting motifs found in cellular transmembrane proteins. This sequence in NefNL4-3, ENTSLL, functioned as an endocytosis signal when appended to the cytoplasmic tail of a heterologous protein. The leucine residues in this motif were required for the interaction of full-length Nef with the dileucine-based sorting pathway and were required for Nef-mediated down-regulation of CD4. These leucine residues were also required for optimal viral infectivity. These data indicate that a dileucine-based sorting signal in Nef is utilized to address the cellular sorting machinery. The data also suggest that an influence on the distribution of cellular transmembrane proteins may mechanistically unite two previously distinct properties of Nef: down-regulation of CD4 and enhancement of viral infectivity.

The human immunodeficiency virus type 1 Nef protein binds the Src-related tyrosine kinase Lck SH2 domain through a novel phosphotyrosine independent mechanism

Primate lentiviruses encode for an unique nef gene with an essential function in both viral replication and pathogenicity in the host. The molecular basis for this function remains however poorly defined. Several Nef-binding cellular proteins are thought to be instrumental in its function. Indeed, Nef contains a proline-rich motif implicated in the binding to the Src-like tyrosine kinase Hck and also to a Ser/Thr kinase of molecular weight 62 kDa. The disruption of this motif affects the binding to both these kinases as well as viral replication. Whereas Hck is expressed in the myeloid lineage and hence may account for the nef function in infected monocytes, we and others have reported previously that Nef also interacts with the T-lymphocyte Src-kinase Lck, leading to specific cell signaling impairment. This interaction occurs through the binding of Nef to both Lck SH2 and SH3 domains. Both the proline motif and phosphorylation of Nef on tyrosine residue were proposed to account for these interactions. Here, we investigate the mechanism of Lck SH2 binding by HIV-1 Nef. Using recombinant fusion proteins to precipitate lysates, we show that although SH2 binding is dependent on phosphorylation events, it occurs in a tyrosine independent manner because it requires neither tyrosine residues in Nef nor the phosphotyrosine binding pocket from the Lck SH2 domain, hence suggesting a role for a phosphoserine or a phosphothreonine residue. Further, we show that Hck SH2 does not interact with Nef, indicating that Hck SH3 binding is sufficient for Nef binding, whereas Lck SH2 cooperate together with SH3 to allow Nef binding to a level similar to Hck SH3. Together, our results establish different mechanisms for Hck and Lck binding by HIV-1 Nef protein, and identify a novel mechanism for Src-like tyrosine kinase targeting by a viral protein.

Mice transgenic for simian immunodeficiency virus nef are immunologically compromised

An intact nef gene is essential for rapid development of immunodeficiency in human immunodeficiency virus and simian immunodeficiency virus infections. To assess the role of nef in the immune response, mice transgenic for SIV nef were constructed and the humoral and cellular immune response to herpes simplex virus type-1 (HSV-1), measured. Mice transgenic for SIVmac239 nef exhibited a significantly increased mortality rate when challenged with HSV-1 and also showed unusual antibody kinetics in response to viral challenge. During a 32-week period following exposure to HSV, it was noted that IgG subclass titers continued to rise in the nef+ animals, while titers of nef- animals decreased. Additionally, following secondary challenge with HSV, nef- mice had a significantly greater rise in HSV-neutralizing antibody titers than nef+ mice. A decreased proliferative response to the T cell mitogen, PHA, was noted in the nef+ animals. These results suggest that the presence of nef+ is sufficient to induce immune dysfunction.

Binding of c-Raf1 kinase to a conserved acidic sequence within the carboxyl-terminal region of the HIV-1 Nef protein

Nef is a membrane-associated cytoplasmic phosphoprotein that is well conserved among the different human (HIV-1 and HIV-2) and simian immunodeficiency viruses and has important roles in down-regulating the CD4 receptor and modulating T-cell signaling pathways. The ability to modulate T-cell signaling pathways suggests that Nef may physically interact with T-cell signaling proteins. In order to identify Nef binding proteins and map their site(s) of interaction, we targeted a highly conserved acidic sequence at the carboxyl-terminal region of Nef sharing striking similarity with an acidic sequence at the c-Raf1-binding site within the Ras effector region. Here, we used deletion and site-specific mutagenesis to generate mutant Nef proteins fused to bacterial glutathione S-transferase in in vitro precipitation assays and immunoblot analysis to map the specific interaction between the HIV-1LAI Nef and c-Raf1 to a conserved acidic sequence motif containing the core sequence Asp-Asp-X-X-X-Glu (position 174-179). Significantly, we demonstrate that substitution of the nonpolar glycine residue for either or both of the conserved negatively charged aspartic acid residues at positions 174 and 175 in the full-length recombinant Nef protein background completely abrogated binding of c-Raf1 in vitro. In addition, lysates from a permanent CEM T-cell line constitutively expressing the native HIV-1 Nef protein was used to coimmunoprecipitate a stable Nef-c-Raf1 complex, suggesting that molecular interactions between Nef and c-Raf1, an important downstream transducer of cell signaling through the c-Raf1-MAP kinase pathway, occur in vivo. This interaction may account for the Nef-induced perturbations of T-cell signaling and activation pathways in vitro and in vivo.

New perspectives on PKCtheta, a member of the novel subfamily of protein kinase C

Members of the protein kinase C (PKC) family of serine/threonine protein kinases have been implicated in numerous cellular responses in a large variety of cell types. Expression patterns of individual members and differences in their cofactor requirements and potential substrate specificity suggest that each isoenzyme may be involved in specific regulatory processes. The PKCtheta isoenzyme exhibits a relatively restricted expression pattern with high protein levels found predominantly in hematopoietic cells and skeletal muscle. PKCtheta was found to be expressed in T, but not B lymphocytes, and to colocalize with the T-cell antigen receptor (TCR) at the site of contact between the antigen-responding T cell and the antigen-presenting cell (APC). Colocalization of PKCtheta with the TCR was selective for this isoenzyme and occurred only upon antigen-mediated responses leading to T-cell activation and proliferation. PKCtheta was found to be involved in the regulation of transcriptional activation of early-activation genes, predominantly AP-1, and its cellular distribution and activation were found to be regulated by the 14-3-3 protein. Other findings indicated that PKCtheta can associate with the HIV negative factor (Nef) protein, suggesting that altered regulation of PKCtheta by Nef may contribute to the T-cell impairments that are characteristic of infection by HIV. PKCtheta is expressed at relatively high levels in skeletal muscle, where it is suggested to play a role in signal transduction in both the developing and mature neuromuscular junction. In addition, PKCtheta appears to be involved in the insulin-mediated response of intact skeletal muscle, as well as in experimentally induced insulin resistance of skeletal muscle. Further studies suggest that PKCtheta is expressed in endothelial cells and is involved in multiple processes essential for angiogenesis and wound healing, including the regulation of cell cycle progression, formation and maintenance of actin cytoskeleton, and formation of capillary tubes. Here, we review recent progress in the study of PKCtheta and discuss its potential role in various cellular responses.

Altered plaque formation by recombinant vaccinia virus expressing simian immunodeficiency virus Nef

The nef gene of primate lentiviruses encodes a myristoylated protein that is important for pathogenicity and the maintenance of high virus loads. A deletion in nef leads to a significant reduction of the pathogenicity of simian immunodeficiency virus (SIV) in macaques. At the cellular and biochemical levels, Nef has been shown to down-regulate CD4 and major histocompatibility complex class I molecules and to interact with cellular protein kinases. The importance of these activities for Nef function remains uncertain. We have prepared vaccinia virus recombinants expressing different alleles of SIV nef. When grown on TK- 143 cells, recombinants constructed with the nef allele from SIVmac1A11 produced typical plaques while recombinants expressing the nef allele from SIVmac239-R1 gave rise to plaques with altered morphology. By using chimeric Nef proteins and site-directed mutagenesis, the amino acid responsible for altered plaque formation was mapped to a leucine at residue 211. In vitro phosphorylation of immunoprecipitates prepared from cells infected with the vaccinia virus recombinants resulted in labeled proteins of 62 and 90 kDa. The recombinants differed in the ability to stimulate phosphorylation, and the leucine at residue 211 was again found to be the determining amino acid. These results might help elucidate the role of nef in the pathogenesis of SIV.

The SH3 domain-binding surface and an acidic motif in HIV-1 Nef regulate trafficking of class I MHC complexes

Nef, a regulatory protein of human and simian immunodeficiency viruses, downregulates cell surface expression of both class I MHC and CD4 molecules in T cells by accelerating their endocytosis. Fibroblasts were used to study alterations in the traffic of class I MHC complexes induced by Nef. We found that Nef downregulates class I MHC complexes by a novel mechanism involving the accumulation of endocytosed class I MHC in the trans-Golgi, where it colocalizes with the adaptor protein-1 complex (AP-1). This effect of Nef on class I MHC traffic requires the SH3 domain-binding surface and a cluster of acidic amino acid residues in Nef, both of which are also required for Nef to downregulate class I MHC surface expression and to alter signal transduction in T cells. Downregulation of class I MHC complexes from the surface of T cells also requires a tyrosine residue in the cytoplasmic domain of the class I MHC heavy chain molecule. The requirement of the same surfaces of the Nef molecule for downregulation of surface class I MHC complexes in T cells and for their accumulation in the trans-Golgi of fibroblasts indicates that the two effects of Nef involve similar interactions with the host cell machinery and involve a molecular mechanism regulating class I MHC traffic that is common for both of these cell types. Interestingly, the downregulation of class I MHC does not require the ability of Nef to colocalize with the adaptor protein-2 complex (AP-2). We showed previously that the ability of Nef to colocalize with AP-2 correlates with the ability of Nef to downregulate CD4 expression. Our observations indicate that Nef downregulates class I MHC and CD4 surface expression via different interactions with the protein sorting machinery, and link the sorting and signal transduction machineries in the regulation of class I MHC surface expression by Nef.

The proteolytic cleavage of human immunodeficiency virus type 1 Nef does not correlate with its ability to stimulate virion infectivity

The Nef protein of human immunodeficiency virus type 1 (HIV-1) promotes virion infectivity through mechanisms that are yet ill defined. Some Nef is incorporated into particles, where it is cleaved by the viral protease between amino acids 57 and 58. The functional significance of this event, which liberates the C-terminal core domain of the protein from its membrane-associated N terminus, is unknown. To address this question, we examined the modalities of Nef virion association and processing. We found that although significant levels of Nef were detected in HIV-1 virions partly in a cleaved form, cell-specific variations existed in the efficiency of Nef proteolytic processing. The virion association of Nef was strongly enhanced by myristoylation but did not require other HIV-1-specific proteins, since Nef was efficiently incorporated into and cleaved inside murine leukemia virus particles. Substituting alanine for tryptophan57 decreased the efficiency of Nef processing, while mutating leucine58 had little effect. In contrast, replacing both of these residues simultaneously almost completely prevented this process. However, when the resulting mutants were compared with a wild-type control in viral infectivity assays, no correlation was found between the levels of cleavage and the ability to stimulate virion infectivity. Furthermore, simian immunodeficiency virus Nef, which lacks the sequence recognized by the protease and as a consequence is not cleaved despite its incorporation into virions, could stimulate the infectivity of a nef-defective HIV-1 variant as efficiently as HIV-1 Nef. On these bases, we conclude that the proteolytic processing of Nef is not required for the ability of this protein to enhance virion infectivity.

Adenovirus-Mediated Human Immunodeficiency Virus-1 Nef Expression in Human Monocytes/Macrophages and Effect of Nef on Downmodulation of Fcγ Receptors and Expression of Monokines

To characterize the effect of human immunodeficiency virus-1 (HIV-1)nef expression in human monocytes/macrophage (HMØ) and U937 on the levels of FcγRs, HLA antigens, and monokines, elutriated HMØs and U937 cells were transfected with an adenovirus-mediated Nef expression system. Nef-expressing cells downmodulated FcγRI, FcγRII, and upregulated HLA class I molecules. Nef-expressing HMØs, treated with lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA), overexpressed tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and IL-10. However, IL-6 was induced by LPS and inhibited by PMA. Additionally, a subpopulation of Nef-expressing HMØs underwent apoptosis. Our data suggest that HIV-1 nefdownmodulated FcγRs in myeloid cells in a manner similar to that previously reported for its effect on CD4+ in T cells.

Adenovirus-Mediated Human Immunodeficiency Virus-1 Nef Expression in Human Monocytes/Macrophages and Effect of Nef on Downmodulation of Fcγ Receptors and Expression of Monokines

To characterize the effect of human immunodeficiency virus-1 (HIV-1)nef expression in human monocytes/macrophage (HMØ) and U937 on the levels of FcγRs, HLA antigens, and monokines, elutriated HMØs and U937 cells were transfected with an adenovirus-mediated Nef expression system. Nef-expressing cells downmodulated FcγRI, FcγRII, and upregulated HLA class I molecules. Nef-expressing HMØs, treated with lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA), overexpressed tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and IL-10. However, IL-6 was induced by LPS and inhibited by PMA. Additionally, a subpopulation of Nef-expressing HMØs underwent apoptosis. Our data suggest that HIV-1 nefdownmodulated FcγRs in myeloid cells in a manner similar to that previously reported for its effect on CD4+ in T cells.

Human immunodeficiency virus (HIV) envelope binds to CXCR4 independently of CD4, and binding can be enhanced by interaction with soluble CD4 or by HIV envelope deglycosylation

Chemokine receptor CXCR4 (also known as LESTR and fusin) has been shown to function as a coreceptor for T-cell-tropic strains of human immunodeficiency virus type 1 (HIV-1). We have developed a binding assay to show that HIV envelope (Env) can interact with CXCR4 independently of CD4 but that this binding is markedly enhanced by the previous interaction of Env with soluble CD4. We also show that nonglycosylated HIV-1(SF-2) gp120 or sodium metaperiodate-treated oligomeric gp160 from HIV-1(451) bound much more readily to CXCR4 than their counterparts with intact carbohydrate residues did.

Co-localization of HIV-1 Nef with the AP-2 adaptor protein complex correlates with Nef-induced CD4 down-regulation

The nef gene of human and simian immunodeficiency viruses is critical for AIDS pathogenesis. Its function in vivo is unknown, but in vitro natural isolates of Nef down-regulate expression of the cell surface CD4 molecule, a component of the T cell antigen receptor and the viral receptor, by accelerating its endocytosis. We have used chimeric proteins comprised of the natural HIV-1 NA7 Nef fused to a strongly fluorescing mutant of green fluorescent protein (GFP) to correlate Nef function with intracellular localization in human CD4-positive Jurkat T cells. The NA7-GFP fusion protein co-localizes with components of the clathrin coat, including clathrin and the beta-subunit of the AP-2 adaptor protein complex, at discrete locations that are consistent with the normal cellular distribution of clathrin coats at the plasma membrane. The NA7-GFP protein is also found in the perinuclear region of the cell, which is likely to reflect the Golgi apparatus. Evidence from a CD4-negative fibroblast cell line indicates that co-localization of NA7-GFP with components of the clathrin coat does not require expression of the CD4 molecule. Analysis of a large panel of chimeric molecules containing mutant Nef moieties demonstrated that the N-terminal membrane targeting signal cooperates with additional element(s) in the disordered loops in the Nef molecule to co-localize the Nef protein with AP-2 adaptor complexes at the cell margin. This localization of NA7-GFP correlates with, but is not sufficient for, down-regulation of surface CD4 and at least one additional function of Nef is required. In T cells co-expressing CD4 and NA7-GFP, CD4 at the cell surface is redistributed into a discrete pattern that co-localizes with that of NA7-GFP. Our observations place NA7-GFP in physical proximity to AP-2-containing clathrin coat at the plasma membrane and imply that Nef interacts, either directly or indirectly, with a component of the AP-2-containing coat at this location. This evidence supports a model whereby Nef recruits CD4 to the endocytic machinery via AP-2-containing clathrin coats at the plasma membrane.

Mutational analysis of human immunodeficiency virus type 1 (HIV-1) accessory genes: requirement of a site in the nef gene for HIV-1 replication in activated CD4+ T cells in vitro and in vivo

Human immunodeficiency virus type 1 (HIV-1) accessory genes including nef, vif, and vpr are important factors that determine the replication and pathogenesis of HIV-1. The state of activation is also important for the replication of HIV-1. We evaluated the properties of nef-, vif-, and vpr-minus macrophage-tropic HIV-1(JR) CSF in primary CD4+ Th1- or Th2-like cell cultures which had been activated through CD3 molecules in the presence of interleukin-2 (IL-2) and IL-12 (Th1-like culture) or IL-4 (Th2-like culture), respectively. In activated Th1- or Th2-like cultures, replication of nef-minus HIV-1(JR-CSF) was markedly lower than that of wild-type HIV-1. Subsequent analysis by site-directed mutagenesis showed that (i) the presence of an acidic amino acid-rich domain (amino acid residues 72 to 75) in the Nef protein was critical for the enhancement of viral DNA synthesis, resulting in increased virus growth rate, and (ii) prolines that form part of Src homology 3 binding domain were not essential for viral replication. We also confirmed the importance of sites by using an HIV-1-infected animal model, the hu-PBL-SCID mouse system, representing HIV-1 replication and pathogenesis in activated CD4+ T cells in vivo. These results indicate that Nef accelerates viral replication in activated CD4+ T cells.

Lymphoproliferative immune function in the Sydney Blood Bank Cohort, infected with natural nef/long terminal repeat mutants, and in other long-term survivors of transfusion-acquired HIV-1 infection

OBJECTIVES

To assess T-helper cell immune function (proliferation) in members of the Sydney Blood Bank Cohort (SBBC) compared with other individuals with transfusion- and sexually acquired HIV-1 infection and with matched HIV-negative controls.

DESIGN AND METHODS

Decreasing CD4 counts and T-helper cell function are associated with disease progression. Peripheral blood mononuclear cells (PBMC) from study subjects were assayed for in vitro proliferative responses to HIV-1-derived antigens, recall antigens and alloantigen. T-helper cell function and CD4 counts in members of the SBBC were followed longitudinally.

RESULTS

Proliferative responses and CD4 counts from members of the SBBC were similar to or better than those of other transfusion- or sexually-acquired HIV-1-positive long-term non-progressors (LTNP), including the HIV-negative matched SBBC control groups. However, individuals with disease progression had reduced or undetectable proliferative responses to recall antigens but a conserved response to alloantigen; they also had low CD4 counts and low CD4:CD8 ratios. In the SBBC, these immune parameters were usually stable over time.

CONCLUSIONS

The unique SBBC with natural nef/long terminal repeat deletions in the HIV-1 genome were genuine LTNP without showing signs of disease progression. They appeared to be a group distinct from the tail-end of the normal distribution of disease progression rates, and may remain asymptomatic indefinitely. The SBBC virus may form the basis of a live attenuated immunotherapeutic or immunoprophylactic HIV vaccine.

Cytotoxic activity of the amino-terminal region of HIV type 1 Nef protein

Myristoylated 21- and 25-residue N-terminal peptides of the Nef protein of HIV-1 lysed human erythrocytes and were cytotoxic toward a human CD4+ T cell line, CEM, and primary human peripheral blood mononuclear cells (PBMCs). The corresponding nonmyristoylated N-terminal peptides were only very weakly hemolytic and cytotoxic. A myristoylated peptide consisting of residues 31-50 of Nef was neither hemolytic nor cytotoxic. Alteration of the tryptophan residue at position 13 to a serine did not change the hemolytic and cytotoxic activity. Studies of the ultraviolet fluorescence of the tryptophan at position 5 in the peptide, using an artificial membrane system and fluorescence-quenching agents that inserted into the bilayer at different levels, suggested that myristoylation results in this residue being brought into contact with the upper hydrocarbon region of the lipid bilayer of the cell membrane. This tryptophan is flanked by a number of polar residues that would maintain it in this position, resulting in a considerable increase in disorder in the upper regions of the lipid bilayer, leading to its destabilization and to lysis. The cytotoxic activity of the myristoylated Nef fragments may, in part, explain the killing and deletion of cells, especially in lymphoid tissues, during HIV infection.

A novel acyl-CoA thioesterase enhances its enzymatic activity by direct binding with HIV Nef

In addition to playing a crucial role in the pathogenesis of AIDS, HIV nef induces down-regulation of CD4 expression and TCR signaling and also regulates the sorting pathway in host T cells. To elucidate the Nef function in HIV progression, we searched for a cellular component which interacts with Nef. A human cDNA encoding a novel acyl-CoA thioesterase (hACTE-III) was isolated as an HIV nef-binding protein by yeast two-hybrid system. hACTE-III is homologous to E. coli thioesterase II but to none of the mammalian thioesterases and therefore belongs to a new type. hACTE-III exhibits enzymatic specificity for a broad range of fatty acyl-CoAs. The hACTE-III-binding region within Nef is localized in the central region (amino acids 109-152). hACTE-III greatly enhances its enzymatic activity upon direct binding to Nef. Considering that either Nef-overexpression or impaired fatty acid regulation induces alteration of subcellular morphology, the augmented hACTE-III function by Nef-binding might induce dysfunction of T cells.

Intravirion generation of the C-terminal core domain of HIV-1 Nef by the HIV-1 protease is insufficient to enhance viral infectivity

Wild-type HIV-1 is more infectious than nef-deleted HIV-1 in both limiting dilution and single-cycle infectivity assays. Moreover, Nef expression from a separate plasmid in the virus-producing cells is capable of restoring the infectivity of genetically nef-deficient HIV-1. These observations indicate that the virion itself is altered by Nef expression to promote viral infectivity. Sucrose gradient-purified HIV-1 virions contain full-length Nef protein and its inclusion is dependent on N-terminal myristylation of Nef. As myristylation-defective mutants of Nef do not enhance infectivity, incorporation of Nef into virions may mediate the enhanced infectivity. Studies with recombinant Nef have further shown that HIV-1 protease can cleave Nef into two polypeptides, a 20-kDa C-terminal core domain and a small N-terminal domain. Our analysis of purified HIV-1 virions also showed a 20-kDa form of Nef. The generation of this 20-kDa form of Nef was inhibited by an HIV-1 protease inhibitor, and its C-terminal core domain identity was confirmed through epitope-tagging. Immunoblots of virions demonstrated that 60-80% of the incorporated Nef is cleaved by the HIV-1 protease. This finding raised the possibility that the Nef core domain, which may no longer be tethered to the membrane due to absence of an N-terminal myristyl anchor, might mediate the enhanced infectivity. Therefore, a panel of mutants surrounding the proteolytic cleavage site in Nef were analyzed for effects on cleavage and enhancement of viral infectivity. Although some Nef mutants both failed to cleave and did not enhance viral infectivity, other mutants proved discordant in these functions. Specifically, two mutants that contained point mutations in the N-terminal domain cleaved normally, hence generating wild-type Nef core domain, yet failed to enhance infectivity. Thus, although the majority of the Nef protein in HIV-1 virions is cleaved by the viral protease into a 20-kDa C-terminal core domain, generation of this core domain of Nef appears insufficient to enhance HIV-1 infectivity. These findings suggest that protease cleavage of the Nef protein in virions is irrelevant for the infectivity function of Nef.

A role for natural simian immunodeficiency virus and human immunodeficiency virus type 1 nef alleles in lymphocyte activation

A T-lymphoid cell line termed 221 was derived from a rhesus monkey infected with herpesvirus saimiri. Growth of 221 cells was dependent on the addition of interleukin-2 (IL-2) to the culture medium. In the absence of IL-2, 221 cells arrested in G0-G1 but did not die. Simian immunodeficiency virus (SIV) replicated efficiently in IL-2-stimulated 221 cells whether or not the nef gene was present. In the absence of IL-2, nef-containing SIV replicated 8 to 100 times more efficiently in 221 cells than did the same virus lacking nef. nef-containing virus preferentially stimulated the production of IL-2 from 221 cells. HIV-1 nef and v-ras genes, but not the c-ras gene, were shown to substitute functionally for SIV nef when tested as recombinant viruses in this assay system. These results demonstrate a role for natural nef in causing lymphoid cell activation, and they provide a system for delineating the biochemical mechanisms responsible for this activation.

Protein kinase C-mediated phosphorylation of HIV-I nef in human cell lines

Stable human cell lines expressing the human immunodeficiency virus type I (HIV-I) Nef protein from inducible promoters were used to analyze the phosphorylation status of Nef in vivo. Nef phosphorylation in both HeLa and Jurkat cells was stimulated by phorbol ester treatment. Phosphoamino acid analysis revealed a predominance of phosphoserine with a small proportion of phosphothreonine. Treatment of cells with selective protein kinase inhibitors revealed that Nef phosphorylation was markedly reduced by bisindolylmaleimide, an inhibitor of protein kinase C, but was unaffected by inhibitors of mitogen-activated protein kinase kinase or cAMP-dependent kinase. These data implicate protein kinase C in Nef phosphorylation in vivo, and thus confirm and extend earlier in vitro data. Phosphorylation of a nonmyristoylated Nef mutant was impaired, suggesting that membrane targeting of Nef was required for phosphorylation. This was expected given that activated protein kinase C translocates from the cytosol to the plasma membrane. However, analysis of the subcellular localization of phosphorylated wild-type Nef revealed that both the cytosolic and membrane-associated pools of Nef were phosphorylated to an equivalent extent. Thus the significance of myristoylation for Nef function may be in influencing protein conformation, although these data could be explained by a transient and dynamic interaction between myristoylated Nef and the plasma membrane.

Human immunodeficiency virus type 1 Nef does not alter T-cell sensitivity to antigen-specific stimulation

We have developed an in vitro model to study the influence that human immunodeficiency virus type 1 (HIV-1) may have on the ability of T cells to respond to antigenic challenge. We have examined consequences of HIV-1 gene expression on T-cell activation in antigen-dependent T cells that have stably integrated copies of replication-defective proviral HIV-1. Virus production by HIV-infected, antigen-dependent T cells was induced in response to antigenic stimulation and then decreased as infected cells returned to a state of quiescence. Contrary to the predictions of models proposing that Nef alters signal transduction pathways in T lymphocytes and thereby alters cellular activation, Nef expression in antigen-dependent T-cell clones did not influence their proliferative responses to low or intermediate concentrations of antigen and did not affect other measures of T-cell activation, such as induction of interleukin 2 receptor alpha-chain expression and cytokine production. In addition, we found no evidence for alteration of T-cell responsiveness to antigen by the gag, pol, vif, tat, or rev gene of HIV-1.

T cells from individuals in advanced stages of HIV-1 infection do not proliferate but express activation antigens in response to HIV-1-specific antigens

Like T cells from healthy subjects, those of HIV-1-infected patients are capable of expressing activation antigens on their surface after antigenic or mitogenic stimulation, but their proliferative activity is strongly reduced or even absent, especially in patients with advanced stages of the disease. The characteristic of expressing activation antigens in response to different stimuli in the absence of cell proliferation is shared by CD4+ and CD8+ T-cell subsets from HIV-1-infected patients. The number of T cells capable of expressing CD25 and CD71 in response to HIV-1-related antigens but not of proliferating increased significantly with the progression of the disease, but the number of T cells capable of expressing the two activation antigens in response to the classic tetanus toxoid recall antigen decreased. The higher numbers of T cells capable of responding to HIV-1-related antigens in conjunction with a reduction in the number of T cells responding to recall antigens may explain the occurrence of different infections, including opportunistic microorganisms, during the more advanced stages of HIV-1 infection. Because the increase in the number of HIV-1 antigen-responding T cells (defined by CD25 and CD71 activation antigen expression) is a characteristic of symptomatic HIV-1-infected patients, expression (by flow cytometry) of these activation antigens on T cells in response to HIV-1 antigens could be used as a new marker of disease progression.

Preferential replication of HIV-1 in the CD45RO memory cell subset of primary CD4 lymphocytes in vitro

The ability of HIV-1 to establish an infection and replicate to high copy number in CD4 lymphocytes is dependent on both the activation state of the cell and virus-encoded regulatory proteins that modulate viral gene expression. To study these required virus-cell interactions, we have used an in vitro cell model of acute HIV infection of quiescent, primary CD4 lymphocytes and subsequent induction of T cell activation and virus replication by lectin or CD3 receptor cross-linking. Experiments were done to determine if the capacity of HIV to establish infection and complete replication was impacted by the maturational state of the CD4 cell target or the specific signal induction pathway engaged during activation. Primary CD4 cells were FACS-sorted into the major phenotypic subsets representative of memory (CD45RO) and naive (CD45RA) cells. Levels of virus replication were compared between infection with wild-type NL4-3 virus and an isogenic mutant containing a deletion in nef regulatory gene. PHA mitogen stimulation was compared with anti-CD3, with and without anti-CD28 costimulation, for induction of cell proliferation and virus replication. In both infected and uninfected cells, the RA cell subset exhibited significantly greater response to CD3/CD28 stimulation than did the RO cell subset. In contrast, the majority of virus replication occurred consistently in the RO cell subset. Deletion of HIV nef function caused a severe reduction in viral replication, especially in the RA naive cell subset after CD3 induction. PCR analysis of viral DNA formation, during infection of quiescent cells, demonstrated that the observed differences in HIV replication capacity between RO and RA cell subsets were not due to inherent differences in cell susceptibility to infection. Our results indicate that HIV replication is enhanced selectively in CD45RO memory phenotype cells through the probable contribution of specialized cellular factors which are produced during CD3-initiated signal transduction.

The N-terminus of Nef from HIV-1/SIV associates with a protein complex containing Lck and a serine kinase

The Nef protein of human and primate lentiviruses is a key factor in HIV/SIV pathogenesis. Here we report that Nef associates with two different kinases, forming a multiprotein complex at the far N-terminus of the viral protein. One of the kinases was identified as Lck, whereas the second protein was found to be a serine kinase that phosphorylated Nef and Lck in vitro and could be discriminated from the serine kinase identified previously. The Nef-associated kinase complex (NAKC) was demonstrated in COS cells, in HIV-infected cells, and in vitro using recombinant Lck and Nef proteins. Deletion of a short amphipathic alpha-helix in the N-terminus, which was found to be conserved in all Nef proteins, inhibited association of the NAKC and significantly reduced virion infectivity.

The nef Gene of SIVmac239 Is Necessary for Efficient Growth in H9 Cells

AIDS viruses require an intact functional nef gene in order to induce disease. The nonpathogenic molecular cloned virus SIVmac239nef-deletion encodes a truncated nef gene. This attenuated reading frame is expressed both in vitro and in a virus-infected animal in vivo. Encoding the first 58 amino acids of Nef, the reading frame retained its ability to down-modulate CD4 from the surface of T cells. CD4-down-modulated stable cell lines expressing full-length and truncated nef genes were significantly less infected by SIV. SIVmac239nef-open and SIVmacnef-deletion encoding a truncated nef clearly differed in replication kinetics in H9 cells and H9-derived cell lines. SIVmac239nef-deletion replication was delayed in H9. Copyright 1996 S. Karger AG, Basel

Human immunodeficiency virus type 1 Nef binds directly to Lck and mitogen-activated protein kinase, inhibiting kinase activity

It is now well established that human immunodeficiency virus type I (HIV-1) Nef contributes substantially to disease pathogenesis by augmenting virus replication and markedly perturbing T-cell function. The effect of Nef on host cell activation could be explained in part by its interaction with specific cellular proteins involved in signal transduction, including at least a member of the src family kinase, Lck, and the serine/threonine kinase, mitogen-activated protein kinase (MAPK). Recombinant Nef directly interacted with purified Lck and MAPK in coprecipitation experiments and binding assays. A proline-rich repeat sequence [(Pxx)4] in Nef occurring between amino acid residues 69 to 78 is highly conserved and bears strong resemblance to a defined consensus sequence identified as an SH3 binding domain present in several proteins which can interact with the SH3 domain of various signalling and cytoskeletal proteins. Binding and coprecipitation assays with short synthetic peptides corresponding to the proline-rich repeat sequence [(Pxx)4] of Nef and the SH2, SH3, or SH2 and SH3 domains of Lck revealed that the interaction between these two proteins is at least in part mediated by the proline repeat sequence of Nef and the SH3 domain of Lck. In addition to direct binding to full-length Nef, MAPK was also shown to bind the same proline repeat motif. Nef protein significantly decreased the in vitro kinase activity of Lck and MAPK. Inhibition of key members of signalling cascades, including those emanating from the T-cell receptor, by the HIV-1 Nef protein undoubtedly alters the ability of the infected T cell to respond to antigens or cytokines, facilitating HIV-1 replication and contributing to HIV-1-induced disease pathogenesis.

Human immunodeficiency virus type 1 Nef associates with a member of the p21-activated kinase family

Although human immunodeficiency virus (HIV) Nef is essential for the induction of AIDS, its biochemical function has remained an enigma. In this study, HIV Nef protein is shown to associate with a serine-threonine kinase that recognizes histone H4 as a substrate, is serologically related to rat p21-activated kinase (PAK), and is specifically activated by Rac and Cdc42. These characteristics define the Nef-associated kinase as belonging to the PAK family. PAKs initiate kinase cascades in response to environmental stimuli, and their identification as a target of Nef implicates these signaling molecules in HIV pathogenesis and provides a novel target for clinical intervention.

Evidence for intact CD28 signaling in T cell hyporesponsiveness induced by the HIV-1 nef gene

Infection by human immunodeficiency virus (HIV)-1 is associated with quantitative and qualitative T cell alterations that severely impair the host's immune defense system. The molecular basis for this immunosuppression remains unclear. Peripheral blood mononuclear cells (PBMC) isolated from patients show markedly decreased interleukin (IL)-2 secretion but unaffected or even increased T helper (Th)2 cytokine production. T cell functional defects were recently reported to correlate more with T cell receptor (TcR) signaling, whereas signals provided by ligation of co-receptors CD27 and CD28 appeared to be preserved. Among the various mechanisms proposed to be involved in HIV-1-induced T cell dysfunction, we and others have reported that the nef gene product exhibited significant immunosuppressive activity. By using an inducible stably integrated nef gene, we demonstrated that Nef specifically down-regulated IL-2 and interferon (IFN)-gama produced upon TcR triggering. Here, using the same experimental system, we extended our initial observations to additional mitogenic signals, and investigated the co-stimulatory function of CD28. Nef down-regulated IL-2, but not IL-4 produced upon induction by combinations of mitogens that mimicked TcR signals together with CD28 mAb or CD28's natural ligand (CD80 and CD86). However, the co-signals provided by CD28 to up-regulate IL-2 induction were unaffected by Nef, since IL-2 produced by nef-transfected cells was proportionally enhanced to the same extent as that of control cells, either upon stimulation by the CD28 mAb or CD80 and CD86. In addition, phosphatidylinositol-3 kinase recruitment induced upon CD28 triggering was also found to be unaltered by nef expression. Together with the observation that similar levels of the Nef protein were detected in nef-transfected cells and upon infection of PBMC, these data suggest a selective immunosuppression induced by nef in human T cells by altering TcR signaling without detectable impact on CD28 co-receptor function. These data agree with the T cell defects observed in PBMC isolated from HIV-infected individuals.

The HIV nef protein associates with protein kinase C theta

Expression of the human immunodeficiency virus (HIV) Nef protein has been linked to both decreased cell surface expression of CD4 and an impairment of signal transduction. The recently reported association of Nef with an unidentified serine kinase provides a clue as to how Nef might exert its effects. Considering the key role of protein kinase C (PKC) in T cell activation, we investigated the possibility that Nef interacts with PKC. Our results, using two approaches for detecting interactions between Nef and PKC isozymes in Jurkat cells, show that Nef interacts preferentially with thetaPKC. The interaction of Nef and thetaPKC is independent of calcium, enhanced by phospholipid activators of PKC and not affected by a PKC pseudosubstrate peptide. Phorbol 12-myristate 13-acetate and phytohemagglutinin stimulation of Jurkat cells expressing Nef fails to produce the usual translocation of thetaPKC from the cytosol to the particulate fraction; translocation of betaPKC and epsilonPKC was unaffected. Indeed, there appears to be a net loss of thetaPKC in Nef-expressing cells following stimulation. The loss of thetaPKC, which may be a result of inhibition of its binding to RACKs due to Nef binding, could contribute to the various impairments of T cell function associated with HIV infection and Nef expression.

Modulation of CD3-gamma gene expression after HIV type 1 infection of the WE17/10 T cell line is progressive and occurs in concert with decreased production of viral p24 antigen

HIV-1 infection of WE17/10, an IL-2-dependent CD4+ human T cell line, abrogates T cell receptor (TCR)/CD3 expression due to a transcription level defect in the CD3-gamma chain gene. Kinetic examination of surface receptor density reveals that these complexes are progressively reduced early after HIV-1 infection as the cells transition from TCR/CD3hi-->TCR/CD3lo-->TCR/CD3-. The passage from TCR/CD3hi reversible TCR/CD3lo is characterized by a steady decrease in receptor density from 100 to 50% of control values with similar kinetic for all of the viral variants tested. This first phase in TCR/CD3 downmodulation was found to occur in concert with a decrease in viral p24 antigen production. The switch from TCR/CD3- is distinguished by the conversion of individual cells to the receptor negative phenotype. Although broad kinetic differences in this second phase were observed between viral variants, its onset was consistently accompanied by a further reduction in virus production. In some of the HIV-1-infected WE17/10 cell lines, surface receptor expression was spontaneously upregulated during the second phase of infection, reversing the progression from TCR/CD3(-)-->TCR/CD3lo-->TCR/CD3hi. Thus, in HIV-1-infected WE17/10 cells, changes in CD3-gamma gene transcription are accompanied by altered viral p24 antigen production and the resulting modulation of surface receptor expression can be summarized by the formula: TCR/CD3hi reversible TCR/CD3lo reversible TCR/CD3-.

The HIV-1 nef protein interferes with phosphatidylinositol 3-kinase activation 1

nef is a human immunodeficiency virus (HIV) gene encoding a 27-kDa myristoylated protein with structural features of a signal transducing molecule, but whose functions are largely unknown. We studied the interactions of Nef with the signal transduction pathways triggered by the platelet-derived growth factor (PDGF) receptor. The association of phosphatidylinositol (PI) 3-kinase with the activated receptor was severely impaired by nef expression. Conversely, PDGF-induced receptor tyrosine phosphorylation, binding to phospholipase C-gamma and to Ras-GAP were not modified. Microtubule-associated protein kinase activation and intracellular calcium influx in response to PDGF were either unaffected or only slightly enhanced. Nef significantly reduced the proliferative response to the growth factor, while the chemotactic response was unchanged. These data show that Nef affects selectively the PI 3-kinase signaling pathway and suggest that this interference results in some of the HIV adverse effects on host cell functions.

Physical and functional interaction of Nef with Lck. HIV-1 Nef-induced T-cell signaling defects

The nef gene is unique to the primate lentiviruses and encodes a cytoplasmic membrane-associated protein that affects T-cell signaling and is essential for both maintenance of a high virus load in vivo and for disease progression. Here we investigated the perturbation of cell signaling by Nef in T-cells and found that Nef interacts with the T-cell restricted Lek tyrosine kinase both in vitro and in vivo. The molecular basis for this interaction was analyzed. We show that cell-derived Nef is precipitated in a synergistic manner by the recombinant Src homology 2 (SH2) and SH3 domains from Lck. A functional proline-rich motif and the tyrosine phosphorylation of Nef were evidenced as likely participants in this interaction. The precipitation of Nef by the Lck recombinant proteins was specific, since neither Fyn, Csk, p85 phosphatidylinositol 3-kinase nor phospholipase Cgamma SH2 domains coprecipitated Nef from T-cells. Finally, depressed Lck kinase activity resulted from the presence of Nef, both in vitro and in intact cells, and nef expression resulted in impairment of both proximal and distal Lck-mediated signaling events. These results provide a molecular basis for the Nef-induced T-cell signaling defect and its role in AIDS pathogenesis.

Interleukin-2 and human immunodeficiency virus infection: pathogenic mechanisms and potential for immunologic enhancement

A hallmark of human immunodeficiency virus (HIV) infection is the progressive loss of CD4+ T lymphocytes; however, qualitative defects in immune responses occur prior to the precipitous drop CD4+ T cell numbers. One of the first immunologic defects to be described in HIV-infected individuals is a deficiency in interleukin (IL)-2 production. The addition of IL-2 in vitro to cultures of mononuclear cells from HIV-infected individuals partially or completely restored certain defective cellular immune responses. However, production of or addition of IL-2 has also been associated with increased viral replication in infected T cells. These observations underscore the pernicious correlation between immune activation and HIV replication. However, recent in vitro and in vivo studies have provided promising preliminary results suggesting that, at least at certain stages of disease, the benefits of IL-2 mediated immune enhancement may outweigh or override the inductive effects of this cytokine on HIV production.

Replication and pathogenicity of human immunodeficiency virus type 1 accessory gene mutants in SCID-hu mice

The functional roles of the human immunodeficiency virus type 1 (HIV-1) accessory genes (nef, vpr, vpu, and vif) are as yet unclear. Using the SCID-hu model system, we have examined the infectivity, replicative capacity, and pathogenicity of strains of the molecular clone HIV-1NL4-3 that contain deletion mutations in these individual accessory genes. We determined that deletion of these genes had differential effects on both infectivity and pathogenicity. Deletion of vpr had little or no effect on viral infectivity, replication, and pathogenicity; however, deletion of vpu or vif had a significant effect on infectivity and moderate effects on pathogenicity. nef-minus strains were the most attenuated in this system, demonstrating significantly lower levels of infectivity and pathogenicity. However, deletion of these individual genes attenuated but did not abrogate the pathogenic properties of HIV-1. Mutant viruses still retained the ability to induce thymocyte depletion to various degrees if implants were infected with higher doses of virus or observed for longer periods of time. The relative contributions of these genes to in vivo pathogenic potential should be taken into consideration when one is contemplating a live attenuated vaccine for HIV-1.

HIV type 1 Nef perturbs eye lens development in transgenic mice

HIV transgenic mice often display lens cataracts as a consequence of viral-specific expression of HIV gene products in the developing lens. Cataractous mouse lines encoding either HIV-1 proviral DNA, HIV delta Gag/Pol] proviral DNA, or the HIV-1 nef gene alone were examined to ascertain the effect of Nef on murine lens development. Ocular disease was characterized by a progressive architectural disorganization within the lens fiber cell compartment developing in 100% of HIV-positive mice in five reported transgenic lines. Late embryonic stage transgenic lenses featured a mild microphthalmia, pyknotic nuclei within the lens fiber department, ballooning lens fiber cells, and elongated lens epithelial cells. Increased DNA fragmentation was evident in transgenic embryonic lenses, suggesting that cell death occurred by apoptosis. As studied in HIV delta Gag/Pol] transgenic mice, HIV transcription was developmentally linked to alpha A- and alpha B-crystallin gene expression, preceded disease development (in E14.5-E16.5 embryos), and persisted for weeks after birth. HIV-1 Nef was the predominant HIV gene product detected in the lens fiber cells of this line and was expressed almost to the exclusion of other HIV gene products. Nef was implicated as a major determinant of disease because (1) cataracts developed in mice transgenic for Nef alone and (2) the expression of other HIV gene products in wild-type HIV provirus transgenic mice occurred without a concomitant change in lens pathology.

Repair and evolution of nef in vivo modulates simian immunodeficiency virus virulence

Experimental evidence from the simian immunodeficiency virus (SIV) model of AIDS has shown that the nef gene is critical in the pathogenesis of AIDS. Consequently, nef is of considerable interest in both antiviral drug and vaccine development. Preliminary findings in two rhesus macaques indicated that a deletion of only 12 bp found in the overlapping nef/3' long terminal repeat (LTR) region (9501 to 9512) of the SIVmacC8 molecular clone was associated with reduced virus isolation frequency. We show that this deletion can be repaired in vivo by a sequence duplication event and that sequence evolution continues until the predicted amino acid sequence of the repair is virtually indistinguishable from that of the virulent wild type. These changes occurred concomitantly with reversion to virulence, evidenced by a high virus isolation frequency and load, decline in anti-p27 antibody, substantial reduction in the CD4/CD8 ratio, and development of opportunistic infections associated with AIDS. These findings clearly illustrate the capacity for repair of small attenuating deletions in primate lentiviruses and also strongly suggest that the region from 9501 to 9512 in the SIV nef/3' LTR region is of biological relevance. In addition, the ability of attenuated virus to revert to virulence raises fundamental questions regarding the nature of superinfection immunity.

Dissociation of the CD4 downregulation and viral infectivity enhancement functions of human immunodeficiency virus type 1 Nef

Recent evidence indicates that the nef gene of human immunodeficiency virus type 1 augments rather than inhibits viral replication in both cell culture and in vivo models. In addition, nef alters various normal cellular processes, including the display of CD4 on the cell surface. However, it remains unknown whether the enhancement of infectivity and the downregulation of CD4 represent linked or independent biologic properties of this single protein. In the present studies, mutational analyses were performed to define structure-function relationships within the Nef protein that mediate these effects. To assess the functional consequences of these mutations, sensitive and reliable assays were developed to quantitate the viral infectivity enhancement and CD4 downregulation functions of Nef. The results indicate that membrane-targeting sequences at the N terminus of Nef are important for both functions of Nef, while certain other conserved regions are dispensable for both functions. A conserved proline-X-X repeat segment in the central core of the protein, which is reminiscent of an SH3-binding domain, is critical for the enhancement of infectivity function but is dispensable for CD4 downregulation. However, the downregulation of CD4 by Nef appears to involve a two-step process requiring the initial dissociation of p56lck from CD4 to permit engagement of the endocytic apparatus by CD4. Together, these findings demonstrate that the infectivity enhancement and CD4 downregulation activities of human immunodeficiency virus type 1 Nef can be dissociated. Thus, these processes may be independent of one another in the viral replication cycle.

A conserved domain and membrane targeting of Nef from HIV and SIV are required for association with a cellular serine kinase activity

Among the primate lentiviruses (human immunodeficiency virus (HIV) -1, HIV-2, and simian immunodeficiency virus (SIV), the nef gene is highly conserved and encodes a myristylated protein of approximately 27 kDa (HIV-1) or approximately 34 kDa (HIV-2, SIV). Previously, we found Nef expressed either as a CD8-Nef fusion protein or as a native protein in virally infected T cell lines associates with a cellular serine kinase. This kinase activity phosphorylated two proteins of 62 and 72 kDa that coimmunoprecipitate with Nef in in vitro kinase assays. Using transient expression, various Nef alleles and mutants have been analyzed for association with the cellular kinase activity. The ability of Nef to associate with the kinase activity is conserved among several alleles of HIV-1 as well as SIVmac239 and is observed in non-lymphoid cell lines of simian and murine origins. Two separate regions of HIV-1SF2 Nef are critical for the associated kinase activity. One domain overlaps with a central highly conserved region found in all primate lentivirus nef genes and has been provisionally mapped to amino acids 45-127. Because membrane localization of Nef is important for the associated cellular kinase activity, the second domain represents a membrane targeting signal. Moreover, point mutations within the central region that abrogate the Nef-associated kinase activity in HIV-1SF2 Nef have the same effect when introduced into SIVmac239open Nef.

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

Human immunodeficiency virus type 1 (HIV-1) Nef protein causes the loss of cell surface CD4 and interleukin-2 (IL-2) receptor (Tac) from peripheral blood mononuclear cells (PBMC) and CD4+ T-cell lines. As both CD4 and the IL-2 receptor play crucial roles in antigen-driven helper T-cell signalling and T-cell proliferation, respectively, the role of Nef in the viral life cycle may be to perturb signalling pathways emanating from these receptors. However, the intracellular targets for Nef that result in receptor down-regulation are unknown. Using a recombinant glutathione S-transferase-full-length 27 kDa Nef (Nef27) fusion protein, produced in Escherichia coli by translation from the first start codon of HIV-1 nef clone pNL4-3, as an affinity reagent to probe cytoplasmic extracts of MT-2 cells and PBMC, we have shown interaction with at least seven host cell protein species ranging from 24 to 75 kDa. Immunoblotting identified four of these proteins as p56lck, CD4, p53, and p44mapk/erk1, all of which are intimately involved in intracellular signalling. To assess the relevance of these interactions and further define the biochemical activity of Nef in signal transduction pathways, highly purified Nef27 protein was introduced directly into PBMC by electroporation. Nef27-treated PBMC showed reduced proliferative responsiveness to exogenous recombinant IL-2. Normally, stimulation of T-cells by IL-2 or phorbol 12-myristate 13-acetate provokes both augmentation of p56lck activity and corresponding posttranslational modification of p56lck. These changes were also inhibited by treatment of PBMC with Nef, suggesting that Nef interferes with activation of p56lck and as a consequence of signalling via the IL-2 receptor. Further evidence for Nef interfering with cell proliferation was the decreased production of the proto-oncogene c-myb, which is required for cell cycle progression, in Nef-treated MT-2 cells. In contrast to the binding characteristics and biological effects of Nef27, the alternate 25-kDa isoform of Nef (Nef25) produced by translation from the second start codon of HIV nef pNL4-3 (57 nucleotide residues downstream) was shown to interact with only three cellular proteins of approximately 26, 28, and 56 kDa from PBMC and MT-2 cells, one of which was identified as p56lck. Also, proliferation and posttranslational modification of p56lck in response to IL-2 stimulation were not profoundly affected by treatment of PBMC with Nef25 compared with Nef27.(ABSTRACT TRUNCATED AT 400 WORDS)

The human immunodeficiency virus type 1 (HIV-1) CD4 receptor and its central role in promotion of HIV-1 infection

Interactions between the viral envelope glycoprotein gp120 and the cell surface receptor CD4 are responsible for the entry of human immunodeficiency virus type 1 (HIV-1) into host cells in the vast majority of cases. HIV-1 replication is commonly followed by the disappearance or receptor downmodulation of cell surface CD4. This potentially renders cells nonsusceptible to subsequent infection by HIV-1, as well as by other viruses that use CD4 as a portal of entry. Disappearance of CD4 from the cell surface is mediated by several different viral proteins that act at various stages through the course of the viral life cycle, and it occurs in T-cell lines, peripheral blood CD4+ lymphocytes, and monocytes of both primary and cell line origin. At the cell surface, gp120 itself and in the form of antigen-antibody complexes can trigger cellular pathways leading to CD4 internalization. Intracellularly, the mechanisms leading to CD4 downmodulation by HIV-1 are multiple and complex; these include degradation of CD4 by Vpu, formation of intracellular complexes between CD4 and the envelope precursor gp160, and internalization by the Nef protein. Each of the above doubtless contributes to the ultimate depletion of cell surface CD4, although the relative contribution of each mechanism and the manner in which they interact remain to be definitively established.

Constitutive expression of human immunodeficiency virus type 1 tat gene inhibits interleukin 2 and interleukin 2 receptor expression in a human CD4+ T lymphoid (H9) cell line

Human immunodeficiency virus (HIV-1) tat, a trans-activator of the HIV long terminal repeat, is essential for HIV replication and causes inhibition of antigen-mediated T cell proliferation. To understand the mechanism of inhibition of T cell proliferation, we have investigated the regulation of IL-2 production and its receptor expression on a human CD4+ T lymphoid cell line (H9) transfected with HIV-1 tat gene. When cells were activated by mitogens, as compared to control cells, a significant decrease in both IL-2 mRNA and protein was observed in tat-transfected cells. Similarly, mitogen-induced IL-2R alpha and IL-2R beta mRNA and surface expression of IL-2R alpha and IL-2R beta chains were also significantly decreased in tat-transfected cells compared to control cells. Only IL-2 receptor density was decreased; the affinity of the ligand for the receptor appeared to be unchanged. In contrast to our previous studies with B-lymphoblastoid cell line (Puri RK and Aggarwal BB: Cancer Res 1992; 52:3787-3790), IL-4R expression was unaltered by HIV tat transfection in the H9 T cell line, indicating a cell type-specific phenomenon. Owing to the central role of IL-2 immunoregulation, our data suggest that immunosuppressive effects of HIV-1 tat may be mediated at least in part through the inhibition of both IL-2 production and IL-2 receptor expression.

Oligomerization of the HIV type 2 Nef protein: mutational analysis of the heptad leucine repeat motif and cysteine residues

The human immunodeficiency virus type 2 (HIV-2) Nef protein expressed in Escherichia coli forms highly stable homooligomeric complexes in vitro. Similarly, the native protein synthesized in the persistently infected H9 T cell line also forms stable homooligomers in vivo. To determine whether homooligomer formation is mediated by the leucine zipper-type sequence located in the middle region of the protein, site-directed mutagenesis was used to introduce double and triple point mutations at heptad leucine positions L1, L2, and L4 within the HIV-2NIHZ Nef protein sequence. Here, we show that substitution of a serine residue for the L1 (residue 108) and L2 (residue 115) heptad leucines, and a glutamine residue for the L4 (residue 129) heptad leucine, did not prevent Nef homooligomer formation in vitro. However, a more drastic substitution of alpha-helix-breaking proline residue for the L2 and L4 heptad leucines significantly abrogated ability of the protein to form stable homooligomers. In addition, because significantly higher levels of the Nef oligomers were consistently observed under the nonreducing SDS-PAGE condition, site-specific mutagenesis was also used to examine the role of cysteine residues in generating disulfide-linked Nef dimers in vitro. Here, we also show that single cysteine-to-glycine substitutions at positions 28, 32, or 55 drastically reduced covalent Nef dimer formation and thermal stability of the Nef protein in vitro. Therefore, these results demonstrate that the leucine zipper-type motif in the HIV-2 Nef protein mediates stable homooligomer formation in vitro, and also establish a role for covalent disulfide bonds in the formation of linked Nef dimers and thermal stability of the monomer Nef in vitro.

Characterization of nef sequences in long-term survivors of human immunodeficiency virus type 1 infection

Studies with the simian immunodeficiency virus have shown that nef deletion results in a low level of viremia and a lack of disease progression in monkeys. Given the similarity of this clinical profile to that observed in long-term survivors of human immunodeficiency virus type 1 (HIV-1) infection, we sought to examine the nef gene in 10 patients who are clinically healthy and immunologically normal despite 12 to 15 years of infection. PCR and DNA sequencing were used to determine nef sequences in peripheral blood mononuclear cells obtained from long-term survivors. We found that there is no gross deletion within nef in the cases studied; most nef sequences (91.1%) obtained from 10 subjects contained a full-length and intact open reading frame. In addition, at the protein level, there were no discernible differences between the Nef consensus sequences derived from long-term survivors and those from patients with AIDS. We therefore conclude that deletion of or gross sequence abnormality within nef is not likely to be a common explanation for the well-being of long-term survivors of HIV-1 infection. Moreover, phylogenetic analysis of nef sequences suggests that HIV-1 strains found in our study subjects do not have a common origin.