Fusion from without directed by human immunodeficiency virus particles.

Fusion from without is the process through which particles of some enveloped viruses can direct fusion of target cells in the absence of viral replication. We demonstrate here that human immunodeficiency virus (HIV) particles can efficiently promote fusion from without. Using HeLa-CD4 cells carrying a Tat-inducible lacZ gene, we observed syncytia as early as 6 h after exposure to HIV particles, before HIV gene expression could be detected. Efficient syncytium formation could be obtained when cells were treated with zidovudine, which prevented HIV replication and expression but not cell-cell fusion. Fusion was also observed when cells were exposed to particles of a replication-defective HIV integrase mutant. Fusion from without by HIV particles could be blocked by a monoclonal antibody specific for the V3 loop of the HIV-1 envelope glycoprotein and by soluble CD4. This mechanism of cytopathicity, which can involve cells that do not actively replicate HIV and can be directed by replication-defective particles, could participate in the pathogenicity of the CD4 cell depletion that characterizes HIV infection.     

CD4, CXCR-4, and CCR-5 dependencies for infections by primary patient and laboratory-adapted isolates of human immunodeficiency virus type 1.

We have used a focal infectivity method to quantitatively analyze the CD4, CXCR-4, and CCR-5 dependencies for infections by diverse primary patient (PR) and laboratory-adapted (LA) isolates of human immunodeficiency virus type 1 (HIV-1). Infectivities of T-cell-tropic viruses were analyzed in a panel of HeLa-CD4 cell clones that have distinct quantities of CD4 and in human astroglioma U87MG-CD4 cells that express a large quantity of CD4 and become highly susceptible to infection after transfection with a CXCR-4 expression vector. The latter analysis indicated that PR as well as LA T-cell-tropic viruses efficiently employ CXCR-4 as a coreceptor in an optimal human cell line that contains abundant CD4. Previous uncertainties regarding coreceptor usage by PR T-cell-tropic HIV-1 isolates may therefore have derived from the assay conditions. As reported previously, unrelated LA and PR T-cell-tropic HIV-1 isolates differ in infectivities for the HeLa-CD4 clonal panel, with LA viruses infecting all clones equally and PR viruses infecting the clones in proportion to cellular CD4 quantities (D. Kabat, S. L. Kozak, K. Wherly, and B. Chesebro, J. Virol. 68:2570-2577, 1994). To analyze the basis for this difference, we used the HeLa-CD4 panel to compare a molecularly cloned T-cell-tropic PR virus (ELI1) with six of its variants that grow to different extents in CD4-positive leukemic cell lines and that differ only at specific positions in their gp120 and gp41 envelope glycoproteins. All mutations in gp120 or gp41 that contributed to laboratory adaptation preferentially enhanced infectivity for cells that had little CD4 and thereby decreased the CD4 dependencies of the infections. There was a close correlation between abilities of T-cell-tropic ELI viruses to grow in an expanded repertoire of leukemic cell lines, the reduced CD4 dependencies of their infections of the HeLa-CD4 panel, and their sensitivities to inactivation by soluble CD4 (sCD4). Since all of the ELI viruses can efficiently use CXCR-4 as a coreceptor, we conclude that an increase in viral affinity for CD4 rather than a switch in coreceptor specificity is principally responsible for laboratory adaption of T-cell-tropic HIV-1. Syncytium-inducing activities of the ELI viruses, especially when analyzed on cells with low amounts of CD4, were also highly correlated with their laboratory-adapted properties. Results with macrophage-tropic HIV-1 were strikingly different in both coreceptor and CD4 dependencies. When assayed in HeLa-CD4 cells transfected with an expression vector for CCR-5, macrophage-tropic HIV-1 isolates that had been molecularly cloned shortly after removal from patients were equally infectious for cells that had low or high CD4 quantities. Moreover, despite their substantial infectivities for cells that had only a trace of CD4, macrophage-tropic isolates were relatively resistant to inactivation by sCD4. We conclude that T-cell-tropic PR viruses bind weakly to CD4 and preferentially infect cells that coexpress CXCR-4 and large amounts of CD4. Their laboratory adaptation involves corresponding increases in affinities for CD4 and in abilities to infect cells that have relatively little CD4. In contrast, macrophage-tropic HIV-1 appears to interact weakly with CD4 although it can infect cells that coexpress CCR-5 and small quantities of CD4. We propose that cooperative binding of macrophage-tropic HIV-1 onto CCR-5 and CD4 may enhance virus adsorption and infectivity for cells that have only a trace of CD4.     

Role of CD4 endocytosis in human immunodeficiency virus infection.

We have analyzed the role of CD4 endocytosis in human immunodeficiency virus (HIV) entry by measuring the infection of HeLa cells expressing various CD4 constructs with endocytosis rates of between 0.2 and 30%/min in a quantitative infectious focus assay. For a number of laboratory-adapted HIV-1 and HIV-2 strains, the highest levels of infection were found on cells with very limited CD4 endocytosis, while cells with efficient CD4 uptake were only poorly infectable, suggesting that CD4 internalization is not required for HIV entry. This was confirmed in a modified assay involving prebinding of HIV-1LAI to HeLa-CD4 cells at 4 degrees C, synchronized virus entry during warming to 37 degrees C, and neutralization of virions remaining at the cell surface with anti-V3 loop antibodies. Warming cells in hypertonic medium inhibited CD4 endocytosis but did not affect the rate or the extent of infection. These studies confirm that HIV infection does not require endocytosis and that laboratory-adapted virus strains can enter HeLa-CD4 cells by fusion at the plasma membrane.     

Specific reactions between purified HIV-1 particles and CD4+ cell membrane fragments in a cell-free system of virus fusion or entry

The initial step of human immunodeficiency virus type 1 (HIV-1) infection has been studied by Env-mediated fusion or entry assays with appropriate cells expressing CD4 or CXCR4/CCR5 receptors in cultures, where many factors underlying cellular activities likely regulate the fusion/entry efficiency. Here we attempted to develop a more simplified in vitro cell-free fusion/entry reaction that mimics HIV-1 infection in cultures. Membrane fragments of target cells and intact infectious HIV-1 particles were purified, mixed and incubated. The core p24 protein was released from the purified virions and detected by ELISA without detergents in the supernatant of the reaction mixtures. This release reaction proceeded temperature-dependently and in a dose-dependent manner between the virion and membrane fractions, and was specific for HIV-1 Env and CD4. Env-deleted or VSV-G-pseudotyped HIV-1 released little p24, if any. Pretreatment of the membrane fragments with anti-CD4 antibodies inhibited the p24 induction from both X4-tropic and R5-tropic HIV-1. Furthermore, X4 but not R5 HIV-1 reacted with the membrane prepared from intrinsically CXCR4-positive HeLa-CD4 cells, whereas both viruses reacted with that prepared from CCR5-transduced HeLa-CD4 cells, indicating that this cell-free reaction mimics coreceptor usage of HIV-1 infection. Therefore, a potent entry inhibitor of X4 HIV-1, SDF-1alpha, blocked the release from X4 but not R5 HIV-1. Inversely, a weak entry inhibitor of R5 HIV-1, MIP-1beta, partially affected only the release from R5 HIV-1. These results suggest that this cell-free reaction system provides a useful tool to study biochemical fusion/entry mechanisms of HIV-1 and its inhibitors.     

Multiple antiviral activities of cyanovirin-N: blocking of human immunodeficiency virus type 1 gp120 interaction with CD4 and coreceptor and inhibition of diverse enveloped viruses

Cyanovirin-N (CV-N) is a cyanobacterial protein with potent neutralizing activity against human immunodeficiency virus (HIV). CV-N has been shown to bind HIV type 1 (HIV-1) gp120 with high affinity; moreover, it blocks the envelope glycoprotein-mediated membrane fusion reaction associated with HIV-1 entry. However, the inhibitory mechanism(s) remains unclear. In this study, we show that CV-N blocked binding of gp120 to cell-associated CD4. Consistent with this, pretreatment of gp120 with CV-N inhibited soluble CD4 (sCD4)-dependent binding of gp120 to cell-associated CCR5. To investigate possible effects of CV-N at post-CD4 binding steps, we used an assay that measures sCD4 activation of the HIV-1 envelope glycoprotein for fusion with CCR5-expressing cells. CV-N displayed equivalently potent inhibitory effects when added before or after sCD4 activation, suggesting that CV-N also has blocking action at the level of gp120 interaction with coreceptor. This effect was shown not to be due to CV-N-induced coreceptor down-modulation after the CD4 binding step. The multiple activities against the HIV-1 envelope glycoprotein prompted us to examine other enveloped viruses. CV-N potently blocked infection by feline immunodeficiency virus, which utilizes the chemokine receptor CXCR4 as an entry receptor but is CD4 independent. CV-N also inhibited fusion and/or infection by human herpesvirus 6 and measles virus but not by vaccinia virus. Thus, CV-N has broad-spectrum antiviral activity, both for multiple steps in the HIV entry mechanism and for diverse enveloped viruses. This broad specificity has implications for potential clinical utility of CV-N.     

Establishment of a new system for determination of coreceptor usages of HIV based on the human glioma NP-2 cell line.

CD4 and one of the G-protein-coupled receptors (GPCRs) on the cell surface function as a receptor and a coreceptor, respectively, in infection of cells with human and simian immunodeficiency viruses (HIV/SIV). To determine which GPCRs can be coreceptors for HIV (HIV-1 and HIV-2) or SIV infection, several cell lines, including human osteosarcoma HOS-T4 cells and human glioma U87/CD4 cells, have been used. However, these cells often show susceptibilities to some HIV or SIV strains before transduction of GPCRs. The results of this study showed that a CD4-transduced human glioma cell line, NP-2/CD4, a human erythroleukemia cell line, K562/CD4, and a human ovarian cancer cell line, TYK/CD4, were completely resistant to the HIV-1 and HIV-2 strains tested. After transduction of several GPCRs into NP-2/CD4, K562/CD4, or TYK/CD4 cells, NP-2/CD4 cells but not K562/CD4 or TYK/CD4 cells mostly showed expected susceptibilities to several HIV strains. Therefore, an NP-2 cell system would be useful to determine the coreceptor usage of HIV isolates, to find a new coreceptor for HIV/SIV, and to analyze the early stages of HIV/SIV infection.     

Cytopathic variants of an attenuated isolate of human immunodeficiency virus type 2 exhibit increased affinity for CD4.

Naturally occurring isolates of human immunodeficiency virus (HIV) have been described which are deficient in their ability to fuse with and kill CD4+ target cells. Although the molecular basis for their attenuation has not yet been defined, several lines of evidence point toward the viral envelope gene as a key determinant of viral pathogenicity. In the present article, we report the biological characterization of two highly cytopathic variants derived by repeated cell-free passage of an attenuated isolate of HIV type 2 (HIV-2), termed HIV-2/ST. Unlike the parental virus, the cytopathic variants were found to infect Sup-T1 cells with great efficiency and to induce both cell fusion and profound killing in these cultures. To determine whether changes in the viral envelope gene were responsible for the observed phenotypic differences, we examined the CD4 binding affinity of these viruses using a novel assay designed to quantitate the binding of fluoresceinated CD4 to viral envelope in its native configuration on the cell surface. The results demonstrated that the affinity of parental HIV-2/ST envelope for CD4 was 2 orders of magnitude reduced, while the cytopathic variants exhibited a high CD4 binding affinity, comparable to that of cytopathic HIV-1 and HIV-2 isolates. From these data, we conclude that the cytopathic potential of HIV depends, at least in part, on its receptor-binding affinity. In addition, our study documents strong selection pressures for viruses with increased CD4 affinity during propagation in immortalized T-cell lines, thus emphasizing the need to study HIV envelope biology in natural target cells.     

Human Immunodeficiency Virus Type 1 Attachment to HeLa CD4 Cells Is CD4 Independent and gp120 Dependent and Requires Cell Surface Heparans

The binding of human immunodeficiency virus type 1 (HIV-1) (Hx10) virions to two different cell lines was analyzed by using a novel assay based on the detection, by anti-HLA-DR-specific antibodies, of HLA-DR⁺ virus binding to HLA-DR⁻ cells. Virion attachment to the CD4⁺-T-cell line A3.01 was highly CD4 dependent in that it was potently inhibited by CD4 monoclonal antibodies (MAbs), and little virus binding to the CD4⁻ sister A2.01 line was observed. By contrast, virion binding to HeLa cells expressing moderate or high levels of CD4 was equivalent to, or lower than, binding to wild-type CD4⁻ HeLa cells. Moreover, several CD4 MAbs did not reduce, but enhanced, HIV-1 attachment to HeLa-CD4 cells. CD4 was required for infection of HeLa cells, however, demonstrating a postattachment role for this receptor. MAbs specific for the V2 and V3 loops and the CD4i epitope of gp120 strongly inhibited virion binding to HeLa-CD4 cells, whereas MAbs specific for the CD4bs and the 2G12 epitopes enhanced attachment. Despite this, all gp120- and gp41-specific MAbs tested neutralized infectivity on HeLa-CD4 cells. HIV-1 attachment to HeLa cells was only partially inhibited by MAbs specific for adhesion molecules present on the virus or target cells but was completely blocked by polyanions such as heparin, dextran sulfate, and pentosan sulfate. Treatment of HeLa-CD4 cells with heparinases completely eliminated HIV attachment and infection, strongly implicating cell surface heparans in the attachment process. CD4 dependence for HIV-1 attachment to target cells is thus highly cell line specific and may be replaced by other ligand-receptor interactions.     

Differences in CD4 dependence for infectivity of laboratory-adapted and primary patient isolates of human immunodeficiency virus type 1.

CD4 is known to be an important receptor for human immunodeficiency virus type 1 (HIV-1) infection of T lymphocytes and macrophages. However, the limiting steps in CD4-dependent HIV-1 infections in vivo and in vitro are poorly understood. To address this issue, we produced a panel of HeLa-CD4 cell clones that express widely different amounts of CD4 and quantitatively analyzed their infection by laboratory-adapted and primary patient HIV-1 isolates. For all HIV-1 isolates, adsorption from the medium onto HeLa-CD4 cells was inefficient and appeared to be limiting for infection in the conditions of our assays. Adsorption of HIV-1 onto CD4-positive peripheral blood mononuclear cells was also inefficient. Moreover, there was a striking difference between laboratory-adapted and primary T-cell-tropic HIV-1 isolates in the infectivity titers detected on different HeLa-CD4 cells. Laboratory-adapted HIV-1 isolates infected all HeLa-CD4 cell clones with equal efficiencies regardless of the levels of CD4, whereas primary HIV-1 isolates infected these clones in direct proportion to cellular CD4 expression. Our interpretation is that for laboratory-adapted isolates, a barrier step that preceeds CD4 encounter was limiting and the subsequent CD4-dependent virus capture process was highly efficient, even at very low cell surface concentrations. In contrast, for primary HIV-1 isolates, the CD4-dependent steps appeared to be much less efficient. We conclude that primary isolates of HIV-1 infect inefficiently following contact with surfaces of CD4-positive cells, and we propose that this confers a selective disadvantage during passage in rapidly dividing leukemia cell lines. Conversely, in vivo selective pressure appears to favor HIV-1 strains that require large amounts of CD4 for infection.     

Detection of replication-competent and pseudotyped human immunodeficiency virus with a sensitive cell line on the basis of activation of an integrated beta-galactosidase gene.

We have constructed a HeLa cell line that both expresses high levels of CD4 and contains a single integrated copy of a beta-galactosidase gene that is under the control of a truncated human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). This cell line, called CD4-LTR/beta-gal, can be used to determine quantitatively the titer of laboratory-adapted HIV strains, and the method used to do so is as sensitive as the determination of viral titers in a T-cell line by end point dilution. Using this cell line as a titer system, we calculated that HIV-1 stocks contain only one infectious particle per 3,500 to 12,000 virions. Virus derived from a molecular clone of a macrophagetropic provirus will not infect this cell line. We have also cocultivated peripheral blood lymphocyte cultures from HIV-infected individuals with the CD4-LTR/beta-gal indicator cells. In a majority of primary isolates (five of eight), including isolates from asymptomatic patients, rare virus-infected cells that can activate the beta-galactosidase gene are present.     

CD4-independent infection of two CD4(-)/CCR5(-)/CXCR4(+) pre-T-cell lines by human and simian immunodeficiency viruses

The infection of CD4-negative cells by variants of tissue culture-adapted human immunodeficiency virus type 1 (HIV-1) or HIV-2 strains has been shown to be mediated by the CXCR4 coreceptor. Here we show that two in vitro-established CD4(-)/CCR5(-)/CXCR4(+) human pre-T-cell lines (A3 and A5) can be productively infected by wild-type laboratory-adapted T-cell-tropic HIV-1 and HIV-2 strains in a CD4-independent, CXCR4-dependent fashion. Despite the absence of CCR5 expression, A3 and A5 cells were susceptible to infection by the simian immunodeficiency viruses SIVmac239 and SIVmac316. Thus, at least in A3 and A5 cells, one or more of the chemokine receptors can efficiently support the entry of HIV and SIV isolates in the absence of CD4. These findings suggest that to infect cells of different compartments, HIV and SIV could have evolved in vivo to bypass CD4 and to interact directly with an alternative receptor.     

CCR6 Functions as a New Coreceptor for Limited Primary Human and Simian Immunodeficiency Viruses

More than 12 chemokine receptors (CKRs) have been identified as coreceptors for the entry of human immunodeficiency virus type 1 (HIV-1), type 2 (HIV-2), and simian immunodeficiency viruses (SIVs) into target cells. The expression of CC chemokine receptor 6 (CCR6) on Th17 cells and regulatory T cells make the host cells vulnerable to HIV/SIV infection preferentially. However, only limited information is available concerning the specific role of CCR6 in HIV/SIV infection. We examined CCR6 as a coreceptor candidate in this study using NP-2 cell line-based in-vitro studies. Normally, CD4-transduced cell line, NP-2/CD4, is strictly resistant to all HIV/SIV infection. When CCR6 was transduced there, the resultant NP-2/CD4/CCR6 cells became susceptible to HIV-1HAN2, HIV-2MIR and SIVsmE660, indicating coreceptor roles of CCR6. Viral antigens in infected cells were detected by IFA and confirmed by detection of proviral DNA. Infection-induced syncytia in NP-2/CD4/CCR6 cells were detected by Giemsa staining. Amount of virus release through CCR6 has been detected by RT assay in spent culture medium. Sequence analysis of proviral DNA showed two common amino acid substitutions in the C2 envelope region of HIV-2MIR clones propagated through NP-2/CD4/CCR6 cells. Conversely, CCR6-origin SIVsmE660 clones resulted two amino acid changes in the V1 region and one change in the C2 region. The substitutions in the C2 region for HIV-2MIR and the V1 region of SIVsmE660 may confer selection advantage for CCR6-use. Together, the results describe CCR6 as an independent coreceptor for HIV and SIV in strain-specific manner. The alteration of CCR6 uses by viruses may influence the susceptibility of CD4+ CCR6+ T-cells and dendritic cell subsets in vivo and therefore, is important for viral pathogenesis in establishing latent infections, trafficking, and transmission. However, clinical relevance of CCR6 as coreceptor in HIV/SIV infections should be investigated further.     

An Endogenous Inhibitor of Human Immunodeficiency Virus in Human Lymphocytes Is Overcome by the Viral Vif Protein

The vif gene of human immunodeficiency virus type 1 (HIV-1) encodes a basic M_r 23,000 protein that is necessary for production of infectious virions by nonpermissive cells (human lymphocytes and macrophages) but not by permissive cells such as HeLa-CD4. It had been proposed that permissive cells may contain an unidentified factor that functions like the viral Vif protein. To test this hypothesis, we produced pseudotyped wild-type and vif-deleted HIV gpt virions (which contain the HIV-1 genome with the bacterial mycophenolic acid resistance gene gpt in place of the viral env gene) in permissive cells, and we used them to generate nonpermissive H9 leukemic T cells that express these proviruses. We then fused these H9 cells with permissive HeLa cells that express the HIV-1 envelope glycoprotein gp120-gp41, and we asked whether the heterokaryons would release infectious HIV gpt virions. The results clearly showed that the vif-deleted virions released by the heterokaryons were noninfectious whereas the wild-type virions were highly infectious. This strongly suggests that nonpermissive cells, the natural targets of HIV-1, contain a potent endogenous inhibitor of HIV-1 replication that is overcome by Vif.     

The effect of viral regulatory protein expression on gene delivery by human immunodeficiency virus type 1 vectors produced in stable packaging cell lines.

We describe the generation of stable human immunodeficiency virus type 1 (HIV-1)-packaging lines that constitutively express high levels of HIV-1 structural proteins in either a Rev-dependent or a Rev-independent fashion. These cell lines were used to assess gene transfer by using an HIV-1 vector expressing the hygromycin B resistance gene and to study the effects of Rev, Tat, and Nef on the vector titer. The Rev-independent cell lines were created by using gag-pol and env expression vectors that contain the Mason-Pfizer monkey virus (MPMV) constitutive transport element (CTE). Vector titers approaching 10(4) CFU/ml were routinely obtained with these cell lines, as well as with the Rev-dependent cell lines, with HeLa-CD4 cells as targets. The presence of Nef and Tat in the producer cell each increased the vector titer 5- to 10-fold. Rev, on the other hand, was absolutely essential for gene transfer, unless the MPMV CTE was present in the vector. In that case, by using the Rev-independent cell lines for packaging, Rev could be completely eliminated from the system without a reduction in vector titer.     

Recombination in AIDS viruses

Recombination contributes to the generation of genetic diversity in human immunodeficiency viruses (HIV) but can only occur between viruses replicating within the same cell. Since individuals have not been found to be simultaneously coinfected with multiple divergent strains of HIV-1 or HIV-2, recombination events have been thought to be restricted to the rather closely related members of the quasispecies that evolves during the course of HIV infection. Here we describe examples of both HIV-1 and HIV-2 genomes that appear to be hybrids of genetically quite divergent viruses. Phylogenetic analyses were used to examine the evolutionary relationships among multiple HIV strains. Evolutionary trees derived from different genomic regions were consistent with respect to most of the viruses investigated. However, some strains of HIV-1 and HIV-2 exhibited significantly discordant branching orders indicative of genetic exchanges during their evolutionary histories. The crossover points of these putative recombination events were mapped by examining the distribution of phylogenetically informative sites supporting alternative tree topologies. A similar example of a recombinant simian immunodeficiency virus identified in West African green monkeys has also been described recently. These results indicate that coinfection with highly divergent viral strains can occur in HIV-infected humans and SIV-infected primates and could lead to the generation of hybrid genomes with significantly altered biological properties. Thus, future characterization of primate lentiviruses should include careful phylogenetic investigation of possible genomic mosaicism.Correspondence to: P.M. Sharp     

Sulfonated naphthyl porphyrins as agents against HIV-1

Sulfonated 5,10,15,20-tetra(1-naphthyl)porphyrin (T1NapS) and 5,10,15,20-tetra(2-naphthyl)porphyrin (T2NapS) and their copper and iron chelates show activity against the human immunodeficiency virus (HIV-1). The porphyrins were prepared by sulfonation of the parent structures with sulfuric acid. More highly sulfonated structures were prepared by sulfonation for longer times. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry showed species with as many as eight sulfonates. Some of the mass spectral peaks for the copper chelates were consistent with loss of water, apparently from intramolecular sulfone formation between two adjacent naphthalene rings that took place during copper insertion. The compounds could be separated using capillary electrophoresis; addition of beta- or gamma-cyclodextrin gave substantially better separation of the components. Activity against HIV was evaluated using an epithelial HeLa-CD4-CCR5 cell line; EC50 values for HIV-1 IIIB and HIV-1 JR-FL ranged from 1 to 15 microg/ml. The compounds exhibit low toxicity for human epithelial cells and have potential as microbicides which might be used to provide protection against sexual transmission of HIV.     

Human immunodeficiency virus type 1 Vpr protein does not modulate surface expression of the CD4 receptor

The CD4 receptor is required for the entry of human immunodeficiency virus (HIV) into target cells. It has long been known that Nef, Env, and Vpu participate in the removal of the viral receptor from the cell surface. Recently, it has been proposed that the HIV type 1 (HIV-1) Vpr protein may also play a role in the downmodulation of CD4 from the surfaces of infected cells (L. Conti, B. Varano, M. C. Gauzzi, P. Matarrese, M. Federico, W. Malorani, F. Belardelli, and S. Gessani, J. Virol. 74:10207-10211, 2000). To investigate the possible role of Vpr in the downregulation of the viral receptor Vpr alleles from HIV-1 and simian immunodeficiency virus were transiently expressed in transformed T cells and in 293T fibroblasts, and their ability to modulate surface CD4 was evaluated. All Vpr alleles efficiently arrested cells in the G(2) stage of the cell cycle. However, none of the tested Vpr proteins altered the expression of CD4 on the cell surface. In comparison, HIV-1 Nef efficiently downmodulated surface CD4 in all the experimental settings. Transformed T cells and primary lymphocytes were challenged with wild-type, Nef-defective, and Vpr-defective viruses. A significant reduction in the HIV-induced downmodulation of surface CD4 was observed in viruses lacking Nef. However, Vpr-deletion-containing viruses showed no defect in their ability to remove CD4 from the surfaces of infected cells. Our results indicate that Vpr does not play a role in the HIV-induced downmodulation of the CD4 receptor.     

Extracellular Vpr protein increases cellular permissiveness to human immunodeficiency virus replication and reactivates virus from latency.

The vpr gene product of human immunodeficiency virus (HIV) and simian immunodeficiency virus is a virion-associated regulatory protein that has been shown using vpr mutant viruses to increase virus replication, particularly in monocytes/macrophages. We have previously shown that vpr can directly inhibit cell proliferation and induce cell differentiation, events linked to the control of HIV replication, and also that the replication of a vpr mutant but not that of wild-type HIV type 1 (HIV-1) was compatible with cellular proliferation (D. N. Levy, L. S. Fernandes, W. V. Williams, and D. B. Weiner, Cell 72:541-550, 1993). Here we show that purified recombinant Vpr protein, in concentrations of < 100 pg/ml to 100 ng/ml, increases wild-type HIV-1 replication in newly infected transformed cell lines via a long-lasting increase in cellular permissiveness to HIV replication. The activity of extracellular Vpr protein could be completely inhibited by anti-Vpr antibodies. Extracellular Vpr also induced efficient HIV-1 replication in newly infected resting peripheral blood mononuclear cells. Extracellular Vpr transcomplemented a vpr mutant virus which was deficient in replication in promonocytic cells, restoring full replication competence. In addition, extracellular Vpr reactivated HIV-1 expression in five latently infected cell lines of T-cell, B-cell, and promonocytic origin which normally express very low levels of HIV RNA and protein, indicating an activation of translational or pretranslational events in the virus life cycle. Together, these results describe a novel pathway governing HIV replication and a potential target for the development of anti-HIV therapeutics.     

Expression of the human immunodeficiency virus type 1 (HIV-1) nef gene during HIV-1 production increases progeny particle infectivity independently of gp160 or viral entry.

The nef gene product of human immunodeficiency virus type 1 (HIV-1) promotes more-rapid kinetics of viral replication in primary peripheral blood mononuclear cells. We have previously shown that these enhancing effects of Nef on HIV-1 replication reflect an increase in viral infectivity detectable both in limiting dilution assays and through a single-cycle infection of the HeLa-CD4-long terminal repeat-beta-galactosidase indicator cell line. We now demonstrate that nef-defective HIV-1 can be rescued to near wild-type levels of infectivity by coexpressing Nef in trans in the cell line producing the virus. This observation indicates that HIV-1 virions produced in the presence of Nef are intrinsically different. However, we show that the major viral structural proteins are quantitatively similar in purified viral preparations. We also demonstrate the functional equivalence of the gp120-gp41 envelope glycoprotein complexes of Nef+ and Nef- HIV-1 through an assay for viral entry. Finally, we show that env-defective Nef+ HIV-1 pseudotyped with an amphotropic envelope is also more infectious than similarly pseudotyped Nef- HIV-1. Thus, the production of HIV-1 in the presence of Nef results in viral particles that are more infectious, and this increased infectivity is manifested at a stage after viral entry but prior to or coincident with HIV-1 gene expression.