High level of surface CD4 prevents stable human immunodeficiency virus infection of T-cell transfectants

Separate cellular localizations of human T-lymphotropic virus 1 (HTLV-1) Env and glucose transporter type 1 (GLUT1) are required for HTLV-1 Env-mediated fusion and infection

Interaction of the envelope glycoprotein (Env) of human T-lymphotropic virus 1 (HTLV-1) with the glucose transporter type 1 (GLUT1) expressed in target cells is essential for viral entry. This study found that the expression level of GLUT1 in virus-producing 293T cells was inversely correlated with HTLV-1 Env-mediated fusion activity and infectivity. Chimeric studies between GLUT1 and GLUT3 indicated that the extracellular loop 6 (ECL6) of GLUT1 is important for the inhibition of cell-cell fusion mediated by Env. When GLUT1 was translocated into the plasma membrane from intracellular storage sites by bafilomycin A1 (BFLA1) treatment in 293T cells, HTLV-1 Env-mediated cell fusion and infection also were inhibited without the overexpression of GLUT1, indicating that the localization of GLUT1 in intracellular compartments rather than in the plasma membrane is crucial for the fusion activity of HTLV-1 Env. Immunoprecipitation and laser scanning confocal microscopic analyses indicated that under normal conditions, HTLV-1 Env and GLUT1 do not colocalize or interact. BFLA1 treatment induced this colocalization and interaction, indicating that GLUT1 normally accumulates in intracellular compartments separate from that of Env. Western blot analyses of FLAG-tagged HTLV-1 Env in virus-producing cells and the incorporation of HTLV-1 Env in virus-like particles (VLPs) indicate that the processing of Env is inhibited by either overexpression of GLUT1 or BFLA1 treatment in virus-producing 293T cells. This inhibition probably is due to the interaction of the Env with GLUT1 in intracellular compartments. Taken together, separate intracellular localizations of GLUT1 and HTLV-1 Env are required for the fusion activity and infectivity of HTLV-1 Env.

IMPORTANCE

The deltaretrovirus HTLV-1 is a causative agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Although HTLV-1 is a complex retrovirus that has accessory genes, no HTLV-1 gene product has yet been shown to regulate its receptor GLUT1 in virus-producing cells. In this study, we found that a large amount of GLUT1 or translocation of GLUT1 to the plasma membrane from intracellular compartments in virus-producing cells enhances the colocalization and interaction of GLUT1 with HTLV-1 Env, leading to the inhibition of cell fusion activity and infectivity. The results of our study suggest that GLUT1 normally accumulates in separate intracellular compartments from Env, which is indeed required for the proper processing of Env.

HIV-1 Nef dimerization is required for Nef-mediated receptor downregulation and viral replication

Nef, a human immunodeficiency virus type 1 (HIV-1) accessory factor capable of interaction with a diverse array of host cell signaling molecules, is essential for high-titer HIV replication and AIDS progression. Previous biochemical and structural studies have suggested that Nef may form homodimers and higher-order oligomers in HIV-infected cells, which may be required for both immune and viral receptor downregulation as well as viral replication. Using bimolecular fluorescence complementation, we provide the first direct evidence for Nef dimers within HIV host cells and identify the structural requirements for dimerization in vivo. Bimolecular fluorescence complementation analysis shows that the multiple hydrophobic and electrostatic interactions found within the dimerization interface of the Nef X-ray crystal structure are essential for dimerization in cells. Nef dimers localized to the plasma membrane as well as the trans-Golgi network, two subcellular localizations essential for Nef function. Mutations in the Nef dimerization interface dramatically reduced both Nef-induced CD4 downregulation and HIV replication. Viruses expressing dimerization-defective Nef mutants were disabled to the same extent as HIV that fails to express Nef in terms of replication. These results identify the Nef dimerization region as a potential molecular target for antiretroviral drug discovery.

Nef stimulates human immunodeficiency virus type 1 replication in primary T cells by enhancing virion-associated gp120 levels: coreceptor-dependent requirement for Nef in viral replication

The Nef protein enhances human immunodeficiency virus type 1 (HIV-1) replication through an unknown mechanism. We and others have previously reported that efficient HIV-1 replication in activated primary CD4(+) T cells depends on the ability of Nef to downregulate CD4 from the cell surface. Here we demonstrate that Nef greatly enhances the infectivity of HIV-1 particles produced in primary T cells. Nef-defective HIV-1 particles contained significantly reduced quantities of gp120 on their surface; however, Nef did not affect the levels of virion-associated gp41, indicating that Nef indirectly stabilizes the association of gp120 with gp41. Surprisingly, Nef was not required for efficient replication of viruses that use CCR5 for entry, nor did Nef influence the infectivity or gp120 content of these virions. Nef also inhibited the incorporation of CD4 into HIV-1 particles released from primary T cells. We propose that Nef, by downregulating cell surface CD4, enhances HIV-1 replication by inhibiting CD4-induced dissociation of gp120 from gp41. The preferential requirement for Nef in the replication of X4-tropic HIV-1 suggests that the ability of Nef to downregulate CD4 may be most important at later stages of disease when X4-tropic viruses emerge.

Cell surface CD4 interferes with the infectivity of HIV-1 particles released from T cells

The CD4 protein is required for the entry of human immunodeficiency virus (HIV) into target cells. Upon expression of the viral genome, three HIV-1 gene products participate in the removal of the primary viral receptor from the cell surface. To investigate the role of surface-CD4 in HIV replication, we have created a set of Jurkat cell lines which constitutively express surface levels of CD4 comparable to those found in peripheral blood lymphocytes and monocytes. Expression of low levels of CD4 on the surface of producer cells exerted an inhibitory effect on the infectivity of HIV-1 particles, whereas no differences in the amount of cell-free p24 antigen were observed. Higher levels of cell surface CD4 exerted a stronger inhibitory effect on infectivity, and also affected the release of free virus in experiments where the viral genomes were delivered by electrotransfection. The CD4-mediated inhibition of HIV-1 infectivity was not observed in experiments where the vesicular stomatitis virus G protein was used to pseudotype viruses, suggesting that an interaction between CD4 and gp120 is required for interference. In contrast, inhibition of particle release by high levels of cell-surface CD4 was not overcome by pseudotyping HIV-1 with foreign envelope proteins. Protein analysis of viral particles released from HIV-infected Jurkat-T cells revealed a CD4-dependent reduction in the incorporation of gp120. These results demonstrate that physiological levels of cell-surface CD4 interfere with HIV-1 replication in T cells by a mechanism that inhibits envelope incorporation into viral membranes, and therefore provide an explanation for the need to down-modulate the viral receptor in infected cells. Our findings have important implications for the spread of HIV in vivo and suggest that the CD4 down-modulation function may be an alternative target for therapeutic intervention.

Generation of intersubtype human immunodeficiency virus type 1 recombinants in env gene in vitro: influences in the biological behavior and in the establishment of productive infections

The occurrence of human immunodeficiency virus type 1 (HIV-1) recombinant genomes belonging to different subtypes is a common event in regions where more than two subtypes cocirculate. Although there are accumulating data toward an increase in the number of intersubtype recombinants, little has been addressed about the biological behavior of such mosaic genomes. This work reports the biological characterization of engineered in vitro HIV-1 intersubtype recombinants in the gp120 region. The recombinants possess the entire gp120 of B or F Brazilian isolates in the Z6 (subtype D) backbone. Here we show that this type of recombinant structure results in profound impairment to the establishment of productive infections in CD4-positive cells. The characterization of biological properties of those recombinant viruses demonstrated viral production occurring only during a transient peak early on infection and that they are not able to down-regulate the expression of CD4 receptor on the cell surface. We also report the phenotype reversion of one recombinant virus studied here, after 62 days in culture. Two amino acid substitutions in highly constant gp120 regions (C1 and C4) were identified in the revertant virus. The mutation occurring in the C4 region is localized near two amino acid residues critical for gp120/CD4 interaction. Based on these data, we suggest that failure in CD4 down-modulation by recombinant viruses can be due to a structural dysfunction of gp160 protein unable to block CD4 at the endoplasmic reticule. The possibilities that the establishment of latent infections can be directly related to the continuous expression of CD4 on the infected cell surface and that the occurrence of mutations in amino acid nearby residues critical for gp120/CD4 interaction can restore the fully productive infectious process are discussed.

Cell surface CD4 inhibits HIV-1 particle release by interfering with Vpu activity

One of the hallmarks of human immunodeficiency virus type I (HIV-1) infection is the rapid removal of the viral receptor CD4 from the cell surface. This remarkably efficient receptor interference requires the activity of three separate viral proteins: Env, Vpu, and Nef. We have investigated whether this unusually tight interference on cell surface CD4 expression had a more essential function during the viral life cycle than simply preventing superinfection. We now report that the removal of cell surface CD4 is required for optimal virus production by HIV-1. Indeed, maintenance of CD4 surface expression in infected cells lead to a 3-5-fold decrease in viral particle production. This effect was not due to the formation of intracellular complexes between CD4 and the gp160 viral envelope precursor but instead required the presence of CD4 at the cell surface and was specifically mediated by CD4 but not closely related plasma membrane receptors. The finding that CD4 had no significant effect on particle release by a Vpu-deficient variant indicates that CD4 acts by inhibiting the particle release-promoting activity of Vpu. Co-immunoprecipitation experiments further showed that CD4 and Vpu physically interact at the cell surface, suggesting that CD4 might inhibit Vpu activity by disrupting its oligomeric structure.

Inhibition of HIV-1 progeny virion release by cell-surface CD4 is relieved by expression of the viral Nef protein

BACKGROUND

The human immunodeficiency virus type 1 (HIV-1) Nef protein is required for efficient virus replication in vivo and displays a number of distinct and apparently unrelated biological activities in vitro. Of these, one of the most readily demonstrated is the efficient internalization and degradation of cell-surface CD4, the receptor for the HIV-1 envelope protein. The biological purpose of this internalization has, however, remained unclear.

RESULTS

Using human 293T cells expressing high levels of cell-surface CD4 or CD8, we demonstrate that CD4, but not CD8, can dramatically reduce the release of infectious virions bearing the HIV-1 envelope protein and induce a concomitant increase in the accumulation of cell-associated HIV-1 structural proteins. In contrast, CD4 had no effect on the release of HIV-1 bearing a heterologous envelope protein unable to bind CD4. Nef expression totally reversed CD4-mediated inhibition but only if the CD4 used remained susceptible to Nef-induced internalization.

CONCLUSIONS

These results support the hypothesis that cell-surface CD4 can interact with the envelope protein present on budding HIV-1 virions to inhibit their release. The internalization and degradation of cell-surface CD4 induced by the viral Nef protein can fully reverse this inhibition and is, therefore, likely to facilitate the spread of virus in vivo.

Cell-surface expression of CD4 reduces HIV-1 infectivity by blocking Env incorporation in a Nef- and Vpu-inhibitable manner

BACKGROUND

Human immunodeficiency virus-1 (HIV-1) infection decreases the cell-surface expression of its cellular receptor, CD4, through the combined actions of Nef, Env and Vpu. Such functional convergence strongly suggests that CD4 downregulation is critical for optimal viral replication, yet the significance of this phenomenon has so far remained a puzzle.

RESULTS

We show that high levels of CD4 on the surface of HIV-infected cells induce a dramatic reduction in the infectivity of released virions by the sequestering of the viral envelope by CD4. CD4 is able to accumulate in viral particles while at the same time blocking incorporation of Env into the virion. Nef and Vpu, through their ability to downregulate CD4, counteract this effect.

CONCLUSIONS

The CD4-mediated 'envelope interference' described here probably explains the plurality of mechanisms developed by HIV to downregulate the cell-surface expression of its receptor.

Signaling Through the Lymphotoxin-β Receptor Stimulates HIV-1 Replication Alone and in Cooperation with Soluble or Membrane-Bound TNF-α

The level of ongoing HIV-1 replication within an individual is critical to HIV-1 pathogenesis. Among host immune factors, the cytokine TNF-α has previously been shown to increase HIV-1 replication in various monocyte and T cell model systems. Here, we demonstrate that signaling through the TNF receptor family member, the lymphotoxin-β (LT-β) receptor (LT-βR), also regulates HIV-1 replication. Furthermore, HIV-1 replication is cooperatively stimulated when the distinct LT-βR and TNF receptor systems are simultaneously engaged by their specific ligands. Moreover, in a physiological coculture cellular assay system, we show that membrane-bound TNF-α and LT-α1β2 act virtually identically to their soluble forms in the regulation of HIV-1 replication. Thus, cosignaling via the LT-β and TNF-α receptors is probably involved in the modulation of HIV-1 replication and the subsequent determination of HIV-1 viral burden in monocytes. Intriguingly, surface expression of LT-α1β2 is up-regulated on a T cell line acutely infected with HIV-1, suggesting a positive feedback loop between HIV-1 infection, LT-α1β2 expression, and HIV-1 replication. Given the critical role that LT-α1β2 plays in lymphoid architecture, we speculate that LT-α1β2 may be involved in HIV-associated abnormalities of the lymphoid organs.

U937 cells overexpressing bcl-xl are resistant to human immunodeficiency virus-1-induced apoptosis and human immunodeficiency virus-1 replication

Many viruses, including human immunodeficiency virus type 1 (HIV-1), induce apoptosis and are affected by cellular expression of antiapoptotic genes. We sought to examine the effect of antiapoptotic gene expression on HIV replication by transfecting the promyelomonocytic cell line U937 with the bcl-xl gene to obtain clones of U937 cells that overexpressed bcl-xl (designated U937bcl-xl), a negative control U937 clone transfected with vector alone (designated U937neo) and a clone overexpressing bcl-2 (designated U937bcl-2). After infection with HIV-1, U937neo cells underwent apoptosis four times as frequently as the U937bcl-xl cells. Furthermore, U937bcl-xl cells produced 5-fold less HIV-1 protein than U937neo, whereas U937bcl-2 produced at least 2-fold more p24 than the U937neo control. Transient coexpression of bcl-2 or bcl-xl decreased HIV production and transcription from the HIV LTR. To define the mechanism by which bcl-xl, but not bcl-2, inhibits HIV expression, we examined bcl-2 and bcl-xl expression after HIV infection and CD4 cross-linking. Although HIV-1 infection or cross-linking CD4 led to a decrease in expression of bcl-2, it had no effect on bcl-xl expression. These results provide a mechanism for the resistance of U937bcl-xl transfectants, but not U937bcl-2 transfectants, to HIV-1 replication in monocytic cells in vitro. Therapies that up-regulate bcl-xl expression potentially provide a novel means to decrease the destructiveness of HIV-1.

Virological importance of the protease-cleavage site in human immunodeficiency virus type 1 Nef is independent of both intravirion processing and CD4 down-regulation

The HIV-1 Nef protein is present within the virion and is processed there by the viral protease. Mutational analysis indicated that residues 54-60 in HIV-1 Nef were required for intravirion cleavage. When viruses were produced using T cell lines or primary lymphoblasts, these residues were also required for optimal viral infectivity. However, substitution of native Nef residues with those of a functional Gag cleavage site demonstrated that intravirion cleavage was insufficient for the virological function of this domain. Furthermore, the importance of certain cleavage site residues to infectivity was conditional on the producer cell type. In particular, a mutant containing a deletion of residues 54-57 was phenotypically nef defective when produced using T cells (CEM, A2.01, or primary lymphoblasts) but was minimally impaired when produced from 293 or HeLa cells. This mutant was cleavage resistant, indicating that proteolytic processing of Nef was dispensable for infectivity enhancement when virions were assembled in certain non-T cells. Residues 54-61 of the cleavage site, including 54-57, were also required for Nef-mediated down-regulation of CD4. However, the surface expression of CD4 on HeLa cells in amounts comparable to that on the surface of primary T lymphoblasts did not create a producer cell environment in which residues 54-57 acquired greater virological importance. Furthermore, these residues were required for optimal infectivity even during virion assembly in T cells (A2. 01) that expressed a CD4 molecule that is unable to respond to Nef. These data suggested that in producer T cells, certain cleavage site residues (54-57) contribute to a Nef-mediated virological effect that is unlikely to be linked causally to CD4 down-regulation. Conversely, in the context of 293 cells as viral producers, the Delta54-57 mutant separated genetically down-regulation of CD4 (for which it was defective) from enhancement of infectivity (for which it was functional). Together, these data indicate that the virological function of the cleavage site domain is both independent of intravirion proteolytic processing of Nef and independent of CD4 down-regulation.

Tenidap inhibits replication of the human immunodeficiency virus-1 in cultured cells

Interleukin-6 (IL-6) may be important in the pathogenesis of HIV-1 because of its ability to induce HIV-1 expression in infected cells in vitro. Tenidap, a structurally and functionally novel antirheumatic drug affecting diverse biologic processes, has been shown to reduce IL-6 production by peripheral blood mononuclear cells stimulated with lipopolysaccharide. Tenidap also inhibits the activity of chloride-bicarbonate exchangers and causes acidification of the cytoplasmic compartment that is similar to the effect of the anion transport inhibitor UK5099. Furthermore, tenidap inhibits the cyclooxygenase-mediated pathway of arachidonic acid metabolism as do the nonsteroidal antiinflammatory drugs (NSAIDs). Here we show that tenidap decreased HIV-1 replication as measured by p24 core antigen in the acutely infected CD4+ T-lymphocyte lines H9 and Jurkat, in the acutely infected monocyte line U937, and in its chronically infected subclone U1.8/HIV. These effects were seen at concentrations in the range of 3 to 15 microM, well below those toxic to cells. The antiviral effects of tenidap may be independent of its ability to reduce IL-6 production based on the observations that these effects were as prominent in IL-6 nonresponsive lines as in IL-6 responsive lines and that the inhibition of p24 production was not reversed by exogenous IL-6.

Characterization of CD4-gp120 activation intermediates during human immunodeficiency virus type 1 syncytium formation

The mechanism by which cells expressing HIV envelope glycoproteins progress from binding CD4+ cells to syncytia formation is not entirely understood. The purpose of these investigations was to use physical and biochemical tools (temperature shifts, soluble CD4, protease inhibitors, and a battery of anti-CD4 monoclonal antibodies) to isolate discrete steps during syncytia formation. Previously (Fu et al., J Virol 1993;67:3818), we found that preincubation of cells stably expressing HIV-1 gp 160 (TF228.1.16) with CD4+ SupT1 cells at 16 degrees C, a temperature that is nonpermissive for syncytia formation, resulted in an increased rate of syncytia formation when the cocultures were shifted to the syncytia-permissive temperature of 37 degrees C. We have since found that syncytia formation is further enhanced by shifting the cocultures from 16 to 4 degrees C prior to incubation at 37 degrees C. Together, these data suggest that two discrete states, which we term the first and second activation intermediates (FAI and SAI), are involved in syncytia formation. We have found that acquisition of the FAI (by preincubation at 16 degree C) is sensitive to some serine protease inhibitors (PI), soluble CD4 (sCD4), shedding of gp120, and anti-CD4 monoclonal antibodies (MAb) directed toward the CDR-1/2 and CDR-3 regions of domain 1 on CD4. Expression of the FAI (formation of syncytia by shifting from 16 to 37 degrees C) remains sensitive to sCD4, shedding of gp120, and MAb directed toward CDR-1/2 but is less sensitive to MAb that bind CDR-3 and is insensitive to PI. Similarly, acquisition of the SAI (shifting cocultures from 16 to 4 degrees C), is sensitive to sCD4, shedding of gp120, and MAb directed toward CDR-1/2. In contrast, expression of the SAI (shifting cocultures from 16 to 4 to 37 degrees C) is sensitive only to MAb directed toward CDR-1/2 and cannot be blocked by sCD4, shedding of gp120, or PI. These data allow us to propose that syncytia formation, mediated by HIV-1 envelope glycoproteins, proceeds by a multistep cascade.

Canventol inhibits HIV-1 replication by Tat-induced Tar-independent mechanism

SUMMARY

Canventol (2-isopropyl-4-isopropyldencyclohex-2-ene-l-ol), a blocker of tumor necrosis factor alpha (TNF-alpha) release, inhibits human immunodeficiency virus type (HIV-1) production in chronically and acutely infected cells. This effect of Canventol on virus replication could be correlated with its inhibitory influence on necrosis factor (NF)-kappa B activation and HIV-1 long terminal repeat (LTR)-driven reporter gene expression in Jurkat cells and these could be overcome by the administration of TNF-alpha. Canventol inhibits activation of the promoter by the viral protein Tat through a TAR-independent mechanism. The HIV-1 promoter is synergistically upregulated when both the TAR-independent and the TAR-dependent modes of Tat action are in operation. Tat-induced downstream events, such as the production of cytokines like TNF-alpha and NF-kappa B activation, are central for this upregulation. Inhibitors of the respective modes of action of Tat downregulate HIV-1 LTR activation and virus replication.

Multiple receptor-dependent steps determine the species specificity of HCV-229E infection

Human coronavirus (HCV)-229E causes disease only in humans and grows in human cells and in cells of other species that express recombinant human aminopeptidase N (hAPN), the receptor for HCV-229E. We compared the species specificity of HCV-229E infection with the species specificity of virus binding using immunofluorescence, assay of virus yields, fluorescence activated cell sorting and a monoclonal antibody directed against hAPN that blocks infection. We found that HCV-229E binds to intestinal brush border membranes (BBM) and to membranes of cell lines from cats, dogs, pigs, and humans, however the virus only infects two of these species. HCV-229E will not bind to BBM or to membranes from cell lines derived from hamster or mice. Animal coronaviruses related to HCV-229E, including FIPV, CCV, and TGEV bind to cell membranes from cats, dogs, cows, pigs and humans (but not mice), while each virus infects cells from only a subset of these species. Infectious genomic HCV-229E RNA, can infect cells of all of these species. These data suggest that the species-specificity of infection for this serogroup of coronaviruses is determined at the levels of virus binding and penetration. Since binding of viral spike glycoprotein to cellular receptors is not the only limiting factor, we suggest that one or more steps associated with virus penetration may determine the species specificity of infection with the HCV-229E serogroup of coronaviruses.

Indirect mechanisms of HIV pathogenesis: how does HIV kill T cells?

Although twelve years have passed since the identification of HIV as the cause of AIDS, we do not yet know how HIV kills its target, the CD4+ T cell, nor how this killing cripples the immune system. Prominent theories include direct killing of infected CD4+ T cells by the action or accumulation of cytopathic viral DNA, transcripts or proteins, or by virus-specific cytotoxic T lymphocytes, and indirect killing of uninfected CD4+ T cells (and other immune cells) by autoimmune mechanisms, cytokines, superantigens, or apoptosis. In the past year, studies have provided tantalizing clues as to why infected cells may not die and how these infected cells kill innocent bystander cells.

Optimal infectivity in vitro of human immunodeficiency virus type 1 requires an intact nef gene

The replication competence of human immunodeficiency virus type 1 genomes containing mutations in the nef open reading frame was evaluated in continuous cell lines. Mutants that contained a deletion in the nef open reading frame, premature termination codons, or missense mutations in the N-terminal myristoylation signal were constructed. The replication of these mutants was tested in three ways. First, plasmid genomes were used to transfect T-lymphoblastoid cells. Second, low-passage posttransfection supernatants were used to infect cells with a relatively low virus input. Third, high-titer virus stocks were used to infect cells with a relatively high virus input. These experiments demonstrated a 100- to 10,000-fold decrement in p24 production by the nef mutants compared with that by the wild-type virus. The greatest difference was obtained after infection with the lowest virus input. The myristoylation signal was critical for this positive effect of nef. To investigate the mechanism of the positive influence of nef, nef-positive and nef-minus viruses were compared during a single cycle of replication. These single-cycle experiments were initiated both by infection with high-titer virus stocks and by transfection with viral DNA. Single-cycle infection yielded a three- to fivefold decrement in p24 production by nef-minus virus. Single-cycle transfection yielded equal amounts of p24 production. These results implied that nef does not affect replication after the provirus is established. In support of these results, viral production from cells chronically infected with nef-positive or nef-minus viruses was similar over time. To determine whether the effect of nef was due to infectivity, end point titrations of nef-positive and nef-minus viruses were performed. nef-positive virus had a greater infectivity per picogram of HIV p24 antigen than nef-minus virus. These data indicated that the positive influence of nef on viral growth rate is due to an infectivity advantage of virus produced with an intact nef gene.

CD4 cell surface downregulation in HIV-1 Nef transgenic mice is a consequence of intracellular sequestration

The Nef gene product is a regulatory protein of HIV whose biological function is poorly understood. Nef has been thought to have a negative effect on viral replication in vitro but has been shown in studies with SIV to be necessary in the establishment of viraemia in vivo. In vitro studies in various human cell lines have shown that Nef downregulates the expression of cell surface CD4 and thus could have effects on the immune response. We have generated four transgenic mouse lines, with constructs containing two different Nef alleles under the control of CD2 regulatory elements to examine the interaction of Nef with the host immune system in vivo. In adult transgenic mice we have found marked downregulation in the level of CD4 on the surface of double positive thymocytes and a decrease in the number of CD4+ T cells in the thymus. Functional analyses have revealed a decrease in the total activation of transgenic thymocytes by anti-CD3 epsilon antibody. By specific intracellular staining of T cells in such mice we have found CD4 colocalizing with a Golgi-specific marker. These results strongly suggest a Nef mediated effect on developing CD4 thymocytes resulting from interference of Nef in the intracellular trafficking or post-translational modification of CD4.

Human immunodeficiency virus type 1-associated CD4 downmodulation

This chapter discusses human immunodeficiency virus type 1 (HIV-1) associated with CD4 downmodulation. It also discusses the structure and function of CD4 and p56^lck and factors involved in hiv-1-associated cd4 downmodulation. There are, at present, at least three HIV-1 gene products known to be involved in cell surface CD4 downmodulation. These are Nef, Vpu, and gp160. Whereas Nef is expressed during the early phase of HIV-1 gene expression, both Vpu and gp160, which appear to act coordinately, are expressed during the late phase. This functional convergence of HIV-1 proteins on cell surface CD4 downmodulation, whether specific or nonspecific in activity, suggests that this event is of critical importance in the life cycle of HIV-1. Further elucidation of the mechanisms that underlie CD4 cell surface downmodulation may lead to the development of novel strategies aimed at preventing such events, and potentially to the development of new therapeutic approaches.

Physicochemical dissociation of CD4-mediated syncytium formation and shedding of human immunodeficiency virus type 1 gp120

The mechanism of CD4-mediated fusion via activated human immunodeficiency virus type 1 (HIV-1) gp41 and the biological significance of soluble CD4 (sCD4)-induced shedding of gp120 are poorly understood. The purpose of these investigations was to determine whether shedding of gp120 led to fusion activation or inactivation. BJAB cells (TF228.1.16) stably expressing HIV-1 envelope glycoproteins (the gp120-gp41 complex) were used to examine the effects of pH and temperature on sCD4-induced shedding of gp120 and on cell-to-cell fusion (syncytium formation) with CD4+ SupT1 cells. sCD4-induced shedding of gp120 was maximal at pH 4.5 to 5.5 and did not occur at pH 8.5. At physiologic pH, sCD4-induced shedding of gp120 occurred at 22, 37, and 40 degrees C but neither at 16 nor 4 degrees C. In contrast, syncytia formed at pH 8.5 (maximally at pH 7.5) but not at pH 4.5 to 5.5. At pH 7.5, syncytia formed at 37 and 40 degrees C but not at 22, 16, or 4 degrees C. Preincubation of cocultures of TF228.1.16 and SupT1 cells at 4, 16, or 22 degrees C before the shift to 37 degrees C resulted in similar, increased, or decreased syncytium formation, respectively, compared with the control. Furthermore, an activated intermediate of CD4-gp120-gp41 ternary complex may form at 16 degrees C; this intermediate rapidly executes fusion upon a shift to 37 degrees C but readily decays upon a shift to the shedding-permissive but fusion-nonpermissive temperature of 22 degrees C. These physicochemical data indicate that shedding of HIV-1 gp120 is not an integral step in the fusion cascade and that CD4 may inactivate the fusion complex in a process analogous to sCD4-induced shedding of gp120.

Downregulation of cell-surface CD4 expression by simian immunodeficiency virus Nef prevents viral super infection

The nef gene product encoded by the mac239 proviral clone of simian immunodeficiency virus (SIV) markedly enhances viral replication and pathogenesis in vivo. We have used this biologically active nef isolate to examine the phenotype of Nef in retrovirally transduced human T cells in culture. SIV Nef is shown to dramatically inhibit cell-surface expression of the CD4 glycoprotein without significantly affecting the total steady-state level of cellular CD4. This downregulation of the cell-surface CD4 receptor for human immunodeficiency virus type 1 (HIV-1) infection correlated with the acquisition of resistance to superinfection by HIV-1. However, SIV Nef did not affect the level of gene expression directed by the HIV-1 long terminal repeat. It is hypothesized that downregulation of cell-surface CD4 by Nef facilitates the efficient release of infectious progeny virions and, hence, viral spread in vivo.