Involvement of human leukocyte antigen class I molecules in human immunodeficiency virus infection of CD4-positive cells

Characterization and Role of Lentivirus-Associated Host Proteins

Enveloped viruses obtain their envelopes during the process of budding from infected cells. During this process, however, these viruses acquire parts of the host cell membranes and host cell-derived proteins as integral parts of their mature envelopes. These host-derived components of viral envelopes may subsequently exhibit various effects on the life cycle of the virus; virus cell interactions, especially host response to virus-incorporated self-proteins; and the pathogenesis of the disease induced by these viruses. Although it was known for some time that various viruses incorporate host cell-derived proteins, the issue of the role of these proteins has received increased attention, specifically in connection with human immunodeficiency virus (HIV) infection and development of acquired immunodeficiency syndrome (AIDS) in humans. The aim of this review is to summarize our current knowledge of the analysis and role of host-derived proteins associated with enveloped viruses, with emphasis on the potential role of these proteins in the pathogenesis of AIDS. Clearly, differences in the clinical outcome of those nonhuman primates infected with simian immunodeficiency virus (SIV) that are disease resistant compared with SIV-infected species that are disease susceptible provide a unique opportunity to determine whether differences in the incorporation of distinct sets of host proteins play a role with distinct clinical outcomes.

Infection of macaques with simian immunodeficiency virus induces a species-specific antibody response to major histocompatibility complex class I and class II molecules

Envelopes of retroviruses, including human immunodeficiency virus and simian immunodeficiency virus (SIV), contain host cell proteins that potentially represent novel targets for vaccine development. We show here that sera from rhesus macaques recognized simian major histocompatibility complex (MHC) molecules in response to infection with SIV. Antibodies from these animals did not cross-react with human MHC antigens on mitogen-activated peripheral blood mononuclear cells. The development of antibodies to MHC class I alpha-chain did not correlate with anti-SIV envelope antibody responses, suggesting that these antibodies did not arise through molecular mimicry. In contrast to the species-specific response in infected animals, sera from animals vaccinated with inactivated human cell-grown SIV reacted to both human and rhesus MHC class I and class II molecules.

Presence of host ICAM-1 in laboratory and clinical strains of human immunodeficiency virus type 1 increases virus infectivity and CD4(+)-T-cell depletion in human lymphoid tissue, a major site of replication in vivo

Human immunodeficiency virus type 1 (HIV-1) incorporates several host proteins. Earlier studies have indicated that such foreign constituents can modulate the virus life cycle, although the potential roles that these proteins might play in the viral pathology in vivo remain unclear. In an attempt to shed light on this issue, we first exposed explants of human lymphoid tissue to isogenic viruses except for the presence or absence of host-derived ICAM-1. Incorporation of ICAM-1 alone increased HIV-1 infectivity for human tonsillar tissue cultured ex vivo. This observation was made for viruses bearing distinct coreceptor utilization profiles. Conversion of LFA-1 to a high-affinity-high-avidity state for ICAM-1 further augmented the susceptibility of human tonsillar histocultures to infection by ICAM-1-bearing virions. A more massive depletion of CD4(+) T lymphocytes was seen with X4 ICAM-1/POS viruses than with isogenic ICAM-1/NEG virions. Exposure of X4 and R5 primary isolates of HIV-1 to a blocking anti-ICAM-1 antibody resulted in a decrease of virus infection. Finally, X4 and R5 virions derived from a natural human lymphoid tissue microenvironment incorporated high levels of ICAM-1. Altogether, these results indicate that the incorporation of host ICAM-1 can significantly modulate the biology of HIV-1 in a cellular milieu recognized as the major site of replication in vivo and suggest that host proteins found in HIV-1 particles may participate in the pathogenesis of this disease.

Attachment of human immunodeficiency virus-1 (HIV-1) particles bearing host-encoded B7-2 proteins leads to nuclear factor-kappa B- and nuclear factor of activated T cells-dependent activation of HIV-1 long terminal repeat transcription

Previous studies have shown that human immunodeficiency virus type-1 (HIV-1) can incorporate several surface proteins of host origin. Recent findings indicate that host-encoded cell surface constituents retain their functionality when found embedded into the viral envelope. The primary objective of the current study was to define whether interaction between some specific virion-bound host proteins with their natural cognate ligands present on target cells could mediate intracellular signaling cascade(s). For this purpose, we have generated a whole series of isogenic virus stocks (NL4-3 backbone) bearing or not bearing on their surface foreign CD28, CD54 (ICAM-1), CD80 (B7-1) or CD86 (B7-2) proteins. Our results indicate that incubation of human T lymphoid cells with virions bearing host-derived B7-2 proteins and anti-CD3 antibody can potently activate HIV-1 long terminal repeat-driven gene expression. This up-regulating effect necessitates the involvement of nuclear factor-kappa B (NF-kappa B) and nuclear factor of activated T cells (NFAT) as revealed by the use of vectors coding for dominant negative versions of both transcription factors (i.e. I kappa B alpha S32A/36A and dnNFAT) and band shift assays. The increase of NF-kappa B activity was abolished when infection with B7-2-bearing HIV-1 particles was performed in the presence of the fusion protein CTLA-4 Ig suggesting that the interaction between virally embedded B7-2 and CD28 on the target cell is responsible for the observed NF-kappa B induction. The findings presented here provide the first demonstration that host-encoded proteins acquired by HIV-1 can mediate signal transduction events.

Interaction between virion-bound host intercellular adhesion molecule-1 and the high-affinity state of lymphocyte function-associated antigen-1 on target cells renders R5 and X4 isolates of human immunodeficiency virus type 1 more refractory to neutralization

The oligomeric nature of the viral envelope proteins has been partly held responsible for the observed differences in neutralization sensitivity between primary and laboratory-adapted strains of human immunodeficiency virus type 1 (HIV-1). However, recent evidence suggests that host factors can also modify the sensitivity of HIV-1 particles to neutralization. Having previously demonstrated that the acquisition of host-encoded intercellular adhesion molecule (ICAM)-1 proteins by newly formed viruses has a functional significance for the life cycle of HIV-1, we investigated whether the acquisition of host-derived ICAM-1 by HIV-1 could affect the virus sensitivity to neutralization. In this study, we have first shown that the physical presence of host cell membrane ICAM-1 on HIV-1 was not modifying virus sensitivity to neutralization by either two different anti-gp120 monoclonal antibodies (0.5beta and 4.8D) or soluble CD4. However, the ability of the F105 anti-gp120 monoclonal antibody (specific for the CD4-binding site) to neutralize ICAM-1-bearing virions was diminished when target cells were pretreated with an lymphocyte function-associated antigen-1 (LFA-1)-activating antibody. Interestingly, ICAM-1/POS progeny viruses were found to be slightly more resistant to neutralization by individual human sera in target cells expressing a low-affinity form of LFA-1 than viruses devoid of host-encoded ICAM-1 proteins. This resistance was markedly enhanced when target cells expressed an activated LFA-1 form on their surface. These results suggest that the interaction between virally embedded host ICAM-1 and target cell surface LFA-1 should be considered a factor modulating neutralization sensitivity of HIV-1 by human sera from HIV-1-infected individuals.

Neutralization of HIV type 1 by alloimmune sera derived from polytransfused patients

Antibodies (Abs) against HLA and other cell surface molecules, which HIV-1 acquires during the budding process at the host cell surface, neutralize HIV-1 in vitro. Macaques were protected against infection by SIV grown in human cells after xenoimmunization with human MHC molecules. Besides the immune responses arising against xenogeneic antigens, the highly polymorphic character of the HLA antigens enables the induction of alloresponses after exposure to allogeneic HLA molecules. Since polytransfused (PT) patients develop alloresponses, including humoral anti-HLA responses, we assumed that sera derived from PT patients may neutralize HIV-1. In a model system two PT sera out of a panel of 12 PT and 6 normal control sera neutralized HIV IIIB in vitro. Neutralizing activity of the PT sera was comparable to the efficacy of anti-HIV sera. The neutralizing capacity coincided with strong IgG reactivity against (HIV-infected) cell lines, which were used for virus production, and recognition of cell-free viral particles. Active human complement enhanced HIV neutralization mediated by the sera. Our results suggest an IgG-mediated neutralization based on recognition of allogeneic HLA molecules expressed on the viral surface. A vaccination strategy based on alloimmunization appears conceivable and requires further investigation.

The presence and absence of histocompatibility antigens in HIV type 1 produced by autologous blood-derived macrophages and peripheral blood lymphoblasts

Acquisition of cellular proteins by HIV-1 virions is known to alter the physiology of the virus in vitro. Reported studies of this aspect have been largely limited to transformed T cell lines. In this study, we investigated the incorporation of major histocompatibility antigens (HLAs) on a primary macrophage-tropic isolate, HIV-1ADA, grown from autologous monocyte-derived macrophages (MDMs) and peripheral blood mononuclear cells (PBMCs). A virus precipitation assay (VPA) demonstrated that HIV-1ADA grown from PBMCs incorporated substantial amounts of HLA class I (alpha chain and beta2m) and DR antigens, comparable with a laboratory strain, HIV-1MN, grown from the same host cells. HIV-1ADA, however, grown from MDMs incorporated significantly lower amounts of HLAI and -II antigens despite the fact that the infected MDMs were found to express significant amounts of HLA antigens. The lack of incorporation of these important immunomodulatory cell surface proteins may be yet another unique characteristic of macrophage-tropic isolates and suggests a possible role in their biology and or immunology.

Level of ICAM-1 surface expression on virus producer cells influences both the amount of virion-bound host ICAM-1 and human immunodeficiency virus type 1 infectivity

Using virions harvested from 293T cells stably expressing either low or high levels of surface ICAM-1, we determined that the number of virus-embedded host ICAM-1 proteins is positively influenced by the expression level of ICAM-1 on virus producer cells. Moreover, the increase in virion-bound host cell membrane ICAM-1 led to a concomitant enhancement of virus infectivity when a T-cell-tropic strain of human immunodeficiency virus type 1 (HIV-1) was used. The phenomenon was also seen when primary human cells were infected with virions pseudotyped with the envelope protein from a macrophage-tropic HIV-1 isolate, thus ruling out any envelope-specific effect. We also observed that target cells treated with NKI-L16, an anti-LFA-1 antibody known to increase the affinity of LFA-1 for ICAM-1, were markedly more susceptible to infection with HIV-1 particles bearing on their surfaces large numbers of host-derived ICAM-1 proteins. Given that cellular activation of leukocytes is known to modify the conformational state of LFA-1 and induce ICAM-1 surface expression, it is tempting to speculate that activation of virus-infected cells will lead to the production of HIV-1 particles bearing more host ICAM-1 on their surfaces and that such progeny virions will preferentially infect and replicate more efficiently in activated cells which are prevalent in lymphoid organs.

Human T-Cell Leukemia Virus Type II Directly Acts on CD34+ Hematopoietic Precursors by Increasing Their Survival Potential. Envelope-Associated HLA Class II Molecules Reverse This Effect

The role of human T-cell leukemia virus type II (HTLV-II) in human lymphoproliferative and hematopoietic abnormalities in which the retrovirus can be isolated is still elusive. Here we show that the C344 T-cell–derived lymphotropic HTLV-II type IIa Mo strain acts directly on CD34+ hematopoietic precursors by rescuing them from apoptosis induced by interleukin-3 (IL-3) deprivation. This effect is viral strain-specific, as it is not observed with the B-lymphotropic HTLV-II type IIb Gu strain, it does not require infection of the hematopoietic precursors, and, interestingly, it is strongly dependent on the infected cellular host from which the virus was derived. Indeed, growth adaptation of the Mo strain to the permissive B-cell line, BJAB, renders the virus no longer capable of mediating the antiapoptotic effect. However, pretreatment of the BJAB-adapted Mo strain with antibodies specific for HLA class II, but not class I, histocompatibility antigens restores the antiapoptotic potential of the virus. These results constitute the first evidence that HTLV-II retrovirus can directly influence the homeostasis of human progenitors, without infecting them, and that this crucial activity is strongly inhibited by the presence of host-derived envelope-associated HLA class II antigens.

Human T-Cell Leukemia Virus Type II Directly Acts on CD34+ Hematopoietic Precursors by Increasing Their Survival Potential. Envelope-Associated HLA Class II Molecules Reverse This Effect

The role of human T-cell leukemia virus type II (HTLV-II) in human lymphoproliferative and hematopoietic abnormalities in which the retrovirus can be isolated is still elusive. Here we show that the C344 T-cell–derived lymphotropic HTLV-II type IIa Mo strain acts directly on CD34+ hematopoietic precursors by rescuing them from apoptosis induced by interleukin-3 (IL-3) deprivation. This effect is viral strain-specific, as it is not observed with the B-lymphotropic HTLV-II type IIb Gu strain, it does not require infection of the hematopoietic precursors, and, interestingly, it is strongly dependent on the infected cellular host from which the virus was derived. Indeed, growth adaptation of the Mo strain to the permissive B-cell line, BJAB, renders the virus no longer capable of mediating the antiapoptotic effect. However, pretreatment of the BJAB-adapted Mo strain with antibodies specific for HLA class II, but not class I, histocompatibility antigens restores the antiapoptotic potential of the virus. These results constitute the first evidence that HTLV-II retrovirus can directly influence the homeostasis of human progenitors, without infecting them, and that this crucial activity is strongly inhibited by the presence of host-derived envelope-associated HLA class II antigens.

T cells expressing activated LFA-1 are more susceptible to infection with human immunodeficiency virus type 1 particles bearing host-encoded ICAM-1

The incorporation of host-derived proteins in nascent human immunodeficiency virus type 1 (HIV-1) particles is a well-established phenomenon. We recently demonstrated that the physical presence of host-encoded ICAM-1 glycoproteins on HIV-1 leads to a significant increase in virus infectivity in an ICAM-1/LFA-1-dependent fashion (J.-F. Fortin, R. Cantin, G. Lamontagne, and M. Tremblay, J. Virol. 71:3588-3596, 1997). We show here that conversion of LFA-1 to high affinity for ICAM-1 with the use of anti-LFA-1 antibodies (clones NKI-L16 and MEM83) markedly enhances the susceptibility of different target T-lymphoid cell lines, as well as of primary peripheral blood mononuclear cells, to infection by ICAM-1-bearing HIV-1 particles (6- to 95-fold). It is known that T-cell receptor (TCR) cross-linking induces a transient increase in LFA-1 affinity for ICAM-1. Treatment of peripheral blood mononuclear cells with anti-TCR antibodies (clone OKT3) resulted in a transient increase in susceptibility to infection by ICAM-1-positive virions that parallels the previously reported kinetics of the LFA-1/ICAM-1 adhesion mechanism. Our results led us to postulate that the strong interaction taking place between virally incorporated ICAM-1 and cell surface-activated LFA-1 markedly enhances the efficiency of virus binding and entry, thus favoring greater infection by ICAM-1-bearing HIV-1 particles. In view of the knowledge that primary HIV-1 isolates harbor host-derived ICAM-1 on their surfaces, these results provide new information about the role of host-derived ICAM-1 in the life cycle of HIV-1 and how it could positively modulate the dynamics of the viral infection, mainly in cellular compartments, such as the lymphoid tissues, where the level of cellular activation is high and where the probability of encountering a T cell expressing the activated LFA-1 form is also elevated.

Naturally occurring IgG anti-HLA alloantibody does not correlate with HIV type 1 resistance in Nairobi prostitutes

In an effort to identify an immunological basis for natural resistance to HIV-1 infection, we have examined serum antibody responses to HLA class I antigens in female prostitutes of the Nairobi Sex Workers Study. Anti-HLA antibodies are known to block HIV infectivity in vitro and can be protective against SIV challenge in macaques immunized with purified class I HLA. Thus, it was postulated that broadly cross-reactive alloantibodies recognizing common HLA alleles in the client population might contribute to the prevention of heterosexual transmission of HIV. In fact, 12% of the women were found to have serum IgG antibodies against class I alloantigens. However, this alloantibody did not correlate with the HIV status of the women and was found in a similar proportion of HIV-positive and HIV-resistant women. The observed levels of alloantibody did not increase with HIV infection in susceptible individuals, suggesting that potential antigenic mimicry between HIV and host HLA class I antigens does not significantly increase levels of anti-class I antibodies. The lack of correlation between serum anti-allo-class I HLA antibodies and the risk of sexual transmission indicates that this humoral immune response is unlikely to be the natural mechanism behind the HIV-resistance phenotype of persistently HIV-seronegative women. This result, however, does not preclude the further investigation of alloimmunization as an artificial HIV immunization strategy.

Anti-HLA alloantibody is found in children but does not correlate with a lack of HIV type 1 transmission from infected mothers

Searching for mechanisms of natural resistance to HIV infection with which to guide HIV vaccine design, we have examined antibody responses to HLA class I antigens in children of HIV-1-infected mothers. Anti-HLA antibodies are known to block HIV infectivity in vitro and can be protective against SIV challenge in macaques immunized with purified class I HLA. It was hypothesized that alloantibody to maternal HLA in children might contribute to the prevention of mother-to-child transmission of HIV-1. In fact, a surprisingly high proportion of the children examined, 22%, were found to have antibody against class I alloantigens. This alloantibody, however, did not correlate with the HIV status of the children and was found in a similar proportion of children of HIV-negative mothers. The HLA specificity of the antibody was not correlated with noninherited maternal HLA alleles and occurred with a higher frequency in older children. This result suggests environmental factors, rather than exposure to maternal cells, are involved in the formation of the alloantibody. The finding that anti-allo-class I HLA antibodies are not associated with a decreased risk of mother-to-child transmission indicates that this humoral immune response is unlikely to be the natural mechanism that accounts for the lack of transmission observed in many births. This result, however, does not preclude the further investigation of cellular alloimmune responses, or the use of alloimmunization as an artificial HIV immunization strategy.

Cellular proteins in HIV virions

Human immunodeficiency virus type-1 (HIV-1) virions contain both virus-encoded and cellular proteins. Recent advances in the detection, isolation, and functional characterisation of host proteins incorporated in the virion have begun to provide for new perspectives on the interactions between virus and cell. The acquisition of host proteins by HIV-1 may also influence viral pathology in vivo. This article reviews detection and analysis of host proteins found in HIV-1 particles and presents some potential roles that these proteins might play in the biology of this important virus. Copyright 1997 John Wiley & Sons, Ltd.

Transduction of activation signal that follows HIV-1 binding to CD4 and CD4 dimerization involves the immunoglobulin CDR3-like region in domain 1 of CD4

The role of CD4 during the human immunodeficiency virus type 1 (HIV-1) life cycle in T cells is not restricted to binding functions. HIV-1 binding to CD4 also triggers signals that lead to nuclear translocation of NF-kappaB and are important to the productive infection process. In addition to its cytoplasmic tail, in the ectodomain, the immunoglobulin (Ig) CDR3-like region of CD4 domain 1 seemed to play a role in this cascade of signals. We demonstrate in this work that the structural integrity of the CDR3-like loop is required for signal transduction. Substitutions of negatively charged residues by positively charged residues within the CDR3-like loop either inhibited NF-kappaB translocation after HIV-1 and gp120-anti-gp120 immune complexes binding to E91K,E92K mutants or induced its constitutive activation for E87K,D88K mutants. Moreover, A2.01-3B cells expressing the E91K,E92K mutant exhibited a lower HIV-1Lai replication. These cells, however, expressed p56(lck), demonstrated NF-kappaB translocation upon PMA stimulation, bound HIV-1Lai envelope glycoprotein with high affinity, and contained HIV-1 DNA 24 h after exposure to virus. E91K, E92K, and E87K,D88K mutant CD4 molecules were unable to bind a CD4 synthetic aromatically modified exocyclic, CDR3.AME-(82-89), that mimics the CDR3-like loop structure and binds to native cell surface CD4. This result together with molecular modeling studies indicates that the CDR3.AME-(82-89) analog binds to the CDR3-like loop of CD4 and strongly suggests that this region represents a site for CD4 dimerization. The negative charges on the CDR3-like loop thus appear critical for CD4-mediated signal transduction most likely related to CD4 dimer formation.

The Acquisition of Host-Derived Major Histocompatibility Complex Class II Glycoproteins by Human Immunodeficiency Virus Type 1 Accelerates the Process of Virus Entry and Infection in Human T-Lymphoid Cells

Infection by human immunodeficiency virus type 1 (HIV-1) results in a progressive depletion of CD4+ T lymphocytes, leading to fatal immunodeficiency. The mechanisms causing the marked loss of CD4+ T lymphocytes are incompletely understood. However, several lines of evidence indicate that direct cytopathology mediated by HIV-1 is a key element in such CD4+ T-cell depletion. In this study, we investigated whether the previously reported incorporation of host-derived major histocompatibility class II glycoproteins (MHC-II) on HIV-1 can alter its replicative capacity. To achieve this goal, virus stocks were produced in parental MHC-II–expressing RAJI cells and in MHC-II–negative RAJI mutants (RM3), both of which have been stably transfected with human CD4 cDNA to allow productive infection with HIV-1. An enhancement of the rate/efficiency of virus entry was seen after infection with normalized amounts of virions carrying host-derived MHC-II on their surface as compared with inoculation with virions devoid of cellular MHC-II. Data from time-course and infectivity experiments showed that the kinetics of infection were more rapid for virions bearing host-derived MHC-II glycoproteins than for MHC-II–free HIV-1 particles. These results suggest that virally embedded cellular MHC-II glycoproteins are functional and can have a positive effect on early events in the virus replicative cycle. Therefore, we show that the acquisition of cellular MHC-II glycoproteins by HIV-1 can modify its biologic properties and might, consequently, influence the pathogenesis of this retroviral disease.

The Acquisition of Host-Derived Major Histocompatibility Complex Class II Glycoproteins by Human Immunodeficiency Virus Type 1 Accelerates the Process of Virus Entry and Infection in Human T-Lymphoid Cells

Infection by human immunodeficiency virus type 1 (HIV-1) results in a progressive depletion of CD4+ T lymphocytes, leading to fatal immunodeficiency. The mechanisms causing the marked loss of CD4+ T lymphocytes are incompletely understood. However, several lines of evidence indicate that direct cytopathology mediated by HIV-1 is a key element in such CD4+ T-cell depletion. In this study, we investigated whether the previously reported incorporation of host-derived major histocompatibility class II glycoproteins (MHC-II) on HIV-1 can alter its replicative capacity. To achieve this goal, virus stocks were produced in parental MHC-II–expressing RAJI cells and in MHC-II–negative RAJI mutants (RM3), both of which have been stably transfected with human CD4 cDNA to allow productive infection with HIV-1. An enhancement of the rate/efficiency of virus entry was seen after infection with normalized amounts of virions carrying host-derived MHC-II on their surface as compared with inoculation with virions devoid of cellular MHC-II. Data from time-course and infectivity experiments showed that the kinetics of infection were more rapid for virions bearing host-derived MHC-II glycoproteins than for MHC-II–free HIV-1 particles. These results suggest that virally embedded cellular MHC-II glycoproteins are functional and can have a positive effect on early events in the virus replicative cycle. Therefore, we show that the acquisition of cellular MHC-II glycoproteins by HIV-1 can modify its biologic properties and might, consequently, influence the pathogenesis of this retroviral disease.

Host-derived ICAM-1 glycoproteins incorporated on human immunodeficiency virus type 1 are biologically active and enhance viral infectivity

Human immunodeficiency virus type 1 (HIV-1) acquires several host cell membrane proteins when it buds from infected cells. To study the effect of virally incorporated host-derived ICAM-1 glycoproteins on the biology of HIV-1, we have developed a transient expression system that has enabled us to produce virus particles differing only in the absence or the presence of virion-bound ICAM-1. By using a single-round infection assay based on an ICAM-1-negative target T-cell line stably transfected with an HIV-1 long terminal repeat driven luciferase gene construct, we have been able to demonstrate that the acquisition of host-derived ICAM-1 by HIV-1 has functional significance, since it leads to a pronounced increase in viral infectivity (4.6- to 9.8-fold) in an ICAM-1/LFA-1-dependent fashion, as shown by blocking with anti-ICAM-1 and -LFA-1 antibodies. The same potentiating effect on viral infectivity was also observed with monocytoid cells. Studies of the kinetics of infection revealed that the positive effect mediated by virally embedded host cell membrane ICAM-1 is due to an increase in the efficiency of early steps in the viral life cycle. These results provide new insights into how incorporation of host proteins can modulate the biological properties of HIV-1. Our findings have direct clinical relevance, considering that ICAM-1 is expressed on the surface of virus-infected cells and, more importantly, that host-derived ICAM-1 has been shown to be acquired by clinical HIV-1 isolates grown on primary mononuclear cells. These data justify a more complete analysis of the other putative role(s) that virally incorporated ICAM-1 may play in the life cycle of HIV-1, for example, at the level of neutralization sensitivity.

Human immunodeficiency virus type I Nef independently affects virion incorporation of major histocompatibility complex class I molecules and virus infectivity

We have recently reported that HIV-1 Net down-regulates the cell surface expression of major histocompatibility complex class I (MHC-I) molecules. MHC-I molecules are one of the predominant cellular proteins associated with HIV-1 virions. Wild-type or nef-mutated HIV-1 virions were analyzed by immunoelectronic microscopy and Western blot for particle-associated MHC-I molecules. The number of MHC-I molecules was significantly higher in HIV-1 virions produced in the absence of Nef than in wild-type virions, indicating that Nef affects the incorporation of MHC-I molecules into virions. Wild-type HIV particles have been shown to be more infectious than Nef- viruses. This difference was maintained when Nef+ and Nef virions devoid of MHC-I molecules were produced in Daudi-CD4 cells. Therefore, the enhancement of virion infectivity and the down-regulation of MHC-I represent independent biological properties of Nef.

Binding of human immunodeficiency virus type 1 to CD4 induces association of Lck and Raf-1 and activates Raf-1 by a Ras-independent pathway

We have analyzed CD4-mediated signaling during the early stages of human immunodeficiency virus type 1 (HIV-1) infection. Binding of purified HIV-1 virions or recombinant HIV-1 glycoprotein gp120 to CD4 receptors resulted in association and tyrosine phosphorylation and activation of tyrosine kinase Lck and serine/threonine kinase Raf-1. The association between Lck and Raf-1 was mediated by stimulation of the CD4 receptors, since it was abolished by preincubation of the virus with soluble CD4 and was not detected in CD4-negative A201 T cells. However, the Lck-Raf-1 association was restored in A201 cells permanently transfected with human CD4 cDNA and stimulated with anti-CD4 antibodies. In addition, a catalytically active Lck was required for the association of Lck and Raf-1. Surprisingly, the CD4-mediated signaling, induced by the HIV-1 binding, did not result in stimulation of the Ras GTP-binding activity or its association with Raf-1, indicating that the signaling pathway generated by the HIV-1 binding is not identical to the classical Ras/Raf-1 pathway. Furthermore, overexpression of activated Raf-1 in Jurkat T cells stimulated the HIV long terminal repeat promoter activity and significantly enhanced HIV-1 replication. This suggests that the Lck-Raf-1 pathway, rapidly stimulated by the binding of HIV-1 or gp120 to CD4 receptors, may play an essential role in the transcriptional activation of the integrated HIV-1 provirus as well as in its pathogenicity.

Inhibition of human immunodeficiency virus type 1 production in infected peripheral blood mononuclear cells by human leukocyte antigen class I-specific antibodies: evidence for a novel antiviral mechanism

A well-characterized mechanism by which anti-HLA class I monoclonal antibodies (MAb) inhibit human immunodeficiency virus type 1 (HIV-1) propagation in in vitro cell cultures is the neutralization of the virus through interactions with HLA molecules associated with the virion envelope. Yet, the possibility that another mechanism of inhibition might affect a postbinding stage of the virus life cycle has been strongly suggested by our previous investigations. To demonstrate that the interaction of MAb B1-1G6 with the light chain of cell surface-expressed HLA class I molecules inhibits a postbinding step of the HIV-1 life cycle, peripheral blood mononuclear cells (PBMCs) were exposed to viruses grown in HLA class I-negative, CD4-positive cells (these viruses, which did not carry HLA class I molecules, cannot be neutralized by anti-HLA MAb during the first round of infection), and PCR was used at various times postexposure to search for the different forms of HIV-1 DNA and RNA in virus-exposed PBMCs cultured in either the presence or [correction of] absence of MAb B1-1G6. Although viral DNA was found in MAb B1-1G6-treated cells, spliced HIV-1 mRNA could not be detected in those cells. In contrast, HIV-1 gene expression was found in HIV-1-infected PBMCs treated with B9-12-1, another HLA class I-specific MAb which prevents infection of cells by cell-free viruses but which fails to inhibit cell-to-cell transmission of HIV-1. These results highlight a second antiviral mechanism by which anti-HLA MAb inhibit in vitro HIV-1 propagation.

Role of HLA class I in HIV type 1-induced syncytium formation

Neutralization of HIV-1 in vitro by anti-HLA class I antibodies suggests that class I molecules are involved in HIV-1 infection. HIV-infected cells can fuse with uninfected cells in a process that leads to the formation of multinucleated syncytia, involving an interaction between host and viral antigens expressed at the cell surfaces. We used a syncytium assay between the 8E5 cell line chronically infected with a pol-defective variant of LAV IIIb, and the CD4-positive cell line MOLT3, to study the role of HLA class I in HIV-1-induced cell fusion. By probing cells with a panel of anti-HLA monoclonal antibodies (MABs) we demonstrated that the fusion process is modulated specifically by C alleles of HLA class I expressed on uninfected cells but not by that on already infected cells. Addition of beta 2-microglobulin to the cocultures resulted in a dose-dependent enhancement in both the number and size of syncytia, whereas exogenous HLA-C-restricted peptides inhibited syncytium formation, implying that only certain conformational states of HLA class I are permissive for syncytium formation. Treatment of cocultures with HLA-Cw4-restricted peptides containing amino acid substitutions in the anchor residues showed that syncytium inhibition was dependent on conventional binding of the peptide inside the groove. The data indicate that HLA class I, in a conformation free of peptide but associated with beta 2-microglobulin, can directly influence virus-induced cell fusion.

The cytoplasmic tail of CD4 is required for inhibition of human immunodeficiency virus type 1 replication by antibodies that bind to the immunoglobulin CDR3-like region in domain 1 of CD4

Monoclonal antibodies (MAb) directed against the immunoglobulin complementary determining region 3 (CDR3)-like region of the CD4 molecule inhibit human immunodeficiency virus type 1 (HIV-1) transcription. We report here data showing that the cytoplasmic tail of CD4 is required for such inhibition to be achieved. To this aim, we studied the effect of MAb 13B8-2 treatment on (i) HIV-1 production in A2.01 cells, which express different forms of the CD4 gene, (ii) Tat-induced HIV-1 promoter activation, and (iii) mitogen-activated protein kinase (MAPK) activation, which is induced in CD4-positive cells by HIV-1 cross-linking of CD4. Inhibition of HIV production by 13B8-2 MAb treatment was consistently observed in cells expressing wild-type CD4 and cells expressing a hybrid CD4-CD8 molecule (amino acids 1 to 177 of CD4 fused to the hinge, transmembrane, and cytoplasmic domains of CD8). However, no delay in HIV-1 production was observed in cells expressing a truncated CD4 which lacks the cytoplasmic domain (CD4.401). Chloramphenicol acetyltransferase assays demonstrated that Tat-dependent activation of the HIV-1 long terminal repeat promoter was inhibited by MAb 13B8-2 in A2.01/CD4 and A2.01/CD4-CD8 but not in A2.01/CD4.401 cells. Finally, we found that MAb 13B8-2 treatment inhibited the activation of MAPK induced in A2.01/CD4 and A2.01/CD4-CD8 following cross-linking of CD4 by HIV-1.

Role of virion-associated glycosylphosphatidylinositol-linked proteins CD55 and CD59 in complement resistance of cell line-derived and primary isolates of HIV-1

This study investigates whether cell-derived glycosylphosphatidylinositol-linked complement control proteins CD55 and CD59 can be incorporated into HIV-1 virions and contribute to complement resistance. Virus was prepared by transfection of cell lines with pNL4-3, and primary isolates of HIV-1 were derived from patients' PBMCs. Virus was tested for sensitivity to complement-mediated virolysis in the presence of anti-gp160 antibody. Viral preparations from JY33 cells, which lack CD55 and CD59, were highly sensitive to complement. HIV-1 preparations from H9 and U937 cells, which express low levels of CD55 and CD59, had intermediate to high sensitivity while other cell line-derived viruses and primary isolates of HIV-1 were resistant to complement-mediated virolysis. Although the primary isolates were not lysed, they activated complement as measured by binding to a complement receptor positive cell line. While the primary isolates were resistant to lysis in the presence of HIV-specific antibody, antibody to CD59 induced lysis. Likewise, antibody to CD55 and CD59 induced lysis of cell line-derived virus. Western blot analysis of purified virus showed bands corresponding to CD55 and CD59. Phosphatidylinositol-specific phospholipase C treatment of either cell line-derived or primary isolates of HIV-1 increased sensitivity to complement while incubation of sensitive virus with purified CD55 and CD59 increased resistance to complement. These results show that CD55 and CD59 are incorporated into HIV-1 particles and function to protect virions from complement-mediated destruction, and they are the first report of host cell proteins functioning in protection of HIV-1 from immune effector mechanisms.