CD4 activation of HIV fusion

Strategic self-limiting production of infectious HIV particles by CRISPR in permissive cells

Post-translational glycosylation of the HIV-1 envelope protein involving precursor glycan trimming by mannosyl oligosaccharide glucosidase (MOGS) is critically important for morphogenesis of virions and viral entry. Strategic editing of the MOGS gene in T lymphocytes and myeloid origin cells harboring latent proviral DNA results in the production of non-infectious particles upon treatment of cells with latency reversal agents. Controlled activation of CRISPR-MOGS by rebound HIV-1 mitigates production of infectious particles that exhibit poor ability of the virus to penetrate uninfected cells. Moreover, exclusive activation of CRISPR in cells infected with HIV-1 alleviates concern for broad off-target impact of MOGS gene ablation in uninfected cells. Combination CRISPR treatment of peripheral blood lymphocytes prepared from blood of people with HIV-1 (PWH) tailored for editing the MOGS gene (CRISPR-MOGS) and proviral HIV-1 DNA (CRISPR-HIV) revealed a cooperative impact of CRISPR treatment in inhibiting the production of infectious HIV-1 particles. Our design for genetic inactivation of MOGS by CRISPR exhibits no detectable off-target effects on host cells or any deleterious impact on cell survival and proliferation. Our findings offer the development of a new combined gene editing-based cure strategy for the diminution of HIV-1 spread after cessation of antiretroviral therapy (ART) and its elimination.

Conformational and structural features of HIV-1 gp120 underlying the dual receptor antagonism by cross-reactive neutralizing antibody m18

We investigated the interaction between cross-reactive HIV-1 neutralizing human monoclonal antibody m18 and HIV-1YU-2 gp120 in an effort to understand how this antibody inhibits the entry of virus into cells. m18 binds to gp120 with high affinity (KD≈5 nM) as measured by surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC). SPR analysis further showed that m18 inhibits interactions of gp120 with both soluble CD4 and CD4-induced antibodies that have epitopes overlapping the coreceptor binding site. This dual receptor site antagonism, which occurs with equal potency for both inhibition effects, argues that m18 is not functioning as a mimic of CD4, in spite of the presence of a putative CD4-like loop formed by HCDR3 in the antibody. Consistent with this view, m18 was found to interact with gp120 in the presence of saturating concentrations of a CD4-mimicking small molecule gp120 inhibitor, suggesting that m18 does not require unoccupied CD4 Phe43 binding cavity residues of gp120. Thermodynamic analysis of the m18-gp120 interaction suggests that m18 stabilizes a conformation of gp120 that is unique from and less structured than the CD4-stabilized conformation. Conformational mutants of gp120 were studied for their impact on m18 interaction. Mutations known to disrupt the coreceptor binding region and to lead to complete suppression of 17b binding had minimal effects on m18 binding. This argues that energetically important epitopes for m18 binding lie outside the disrupted bridging sheet region used for 17b and coreceptor binding. In contrast, mutations in the CD4 region strongly affected m18 binding. Overall, the results obtained in this work argue that m18, rather than mimicking CD4 directly, suppresses both receptor binding site functions of HIV-1 gp120 by stabilizing a nonproductive conformation of the envelope protein. These results can be related to prior findings about the importance of conformational entrapment as a common mode of action for neutralizing CD4bs antibodies, with differences mainly in epitope utilization and the extent of gp120 structuring.

Neutralization of HIV by antibodies

Antibodies can neutralize HIV-1 with potency and cross-reactivity that varies widely and is related but not correlated to their antigen-binding affinity. Therefore, in addition to measuring binding affinity, an evaluation of the antibody neutralizing activity in tissue cultures is important for development of antibody-based therapeutics, design of candidate vaccine immunogens, and understanding the mechanisms of virus entry, neutralization, and evasion of immune responses. The development of a standardized assay for measurement of the in vitro neutralizing activities of the antibody has remained a challenging goal in the last two decades. There are two types of widely used assays, which vary in details between different laboratories--assays based on cell line/pseudovirus and assays based on infection of peripheral blood mononuclear cells (PBMCs). Here we describe in detail the PBMC-based assay, which is more laborious but in our opinion represents a closer approximation of the in vivo situation. As with all other in vitro assays the results of such measurements are only an indication of the antibody potency in vivo, and animal studies and ultimately clinical trials are needed for the development of such antibodies as potential prophylactics and therapeutics.

Variants of human immunodeficiency virus type 1 that efficiently use CCR5 lacking the tyrosine-sulfated amino terminus have adaptive mutations in gp120, including loss of a functional N-glycan

By selecting the R5 human immunodeficiency virus type 1 (HIV-1) strain JR-CSF for efficient use of a CCR5 coreceptor with a badly damaged amino terminus [i.e., CCR5(Y14N)], we previously isolated variants that weakly utilize CCR5(Delta18), a low-affinity mutant lacking the normal tyrosine sulfate-containing amino-terminal region of the coreceptor. These previously isolated HIV-1(JR-CSF) variants contained adaptive mutations situated exclusively in the V3 loop of their gp120 envelope glycoproteins. We now have weaned the virus from all dependency on the CCR5 amino terminus by performing additional selections with HeLa-CD4 cells that express only a low concentration of CCR5(Delta18). The adapted variants had additional mutations in their V3 loops, as well as one in the V2 stem (S193N) and four alternative mutations in the V4 loop that eliminated the same N-linked oligosaccharide from position N403. Assays using pseudotyped viruses suggested that these new gp120 mutations all made strong contributions to use of CCR5(Delta18) by accelerating a rate-limiting CCR5-dependent conformational change in gp41 rather than by increasing viral affinity for this damaged coreceptor. Consistent with this interpretation, loss of the V4 N-glycan at position N403 also enhanced HIV-1 use of a different low-affinity CCR5 coreceptor with a mutation in extracellular loop 2 (ECL2) [i.e., CCR5(G163R)], whereas the double mutant CCR5(Delta18,G163R) was inactive. We conclude that loss of the N-glycan at position N403 helps to convert the HIV-1 envelope into a hair-trigger form that no longer requires strong interactions with both the CCR5 amino terminus and ECL2 but efficiently uses either site alone. These results demonstrate a novel functional role for a gp120 N-linked oligosaccharide and a high degree of adaptability in coreceptor usage by HIV-1.

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.

Determinants of human immunodeficiency virus type 1 resistance to gp41-derived inhibitory peptides

A synthetic peptide, DP178, containing amino acids 127 to 162 of the human immunodeficiency virus type 1 (HIV-1) gp41 Env glycoprotein, is a potent inhibitor of virus infection and virus mediated cell-to-cell fusion (C. Wild, T. Greenwell, and T. Matthews, AIDS Res. Hum. Retroviruses 9:1051-1053, 1993). In an effort to understand the mechanism of action of this peptide, we derived resistant variants of HIV-1(IIIB) and NL4-3 by serial virus passage in the presence of increasing doses of the peptide. Sequence analysis of the resistant isolates suggested that a contiguous 3-amino-acid sequence within the amino-terminal heptad repeat motif of gp41 was associated with resistance. Site-directed mutagenesis studies confirmed this observation and indicated that changes in two of these three residues were necessary for development of the resistant phenotype. Direct binding of DP178 to recombinant protein and synthetic peptide analogs containing the wild-type and mutant heptad repeat sequences revealed a strong correlation between DP178 binding and the biological sensitivity of the corresponding virus isolates to DP178. The results are discussed from the standpoints of the mechanism of action of DP178 and recent crystallographic information for a core structure of the gp41 ectodomain.

Identification of a synthetic peptide that mimics an HIV glycoprotein 120 envelope conformational determinant exposed following ligation of glycoprotein 120 by CD4

CD4 ligation of HIV envelope gp120 results in conformational changes in gp120 that lead to exposure of the gp41 fusogenic domain and fusion with the host cell membrane. One determinant at or near the CD4-binding site exposed on gp120 subsequent to CD4 binding is defined by two human MAbs termed 17b and 48d. These MAbs do not block CD4 binding to gp120; rather, their binding to gp120 is upregulated following CD4 binding. To determine if synthetic peptide mimetopes could be found that reflect conformational determinants on the surface of gp120, synthetic gp120 peptides from 10 divergent HIV isolates were screened for their ability to bind to 17b and 48d in ELISAs. Although MAb 48d binds to HIV IIIB recombinant gp120 protein, in our studies 48d selectively bound only to the HIV Can0A V3 peptide and not to HIV IIIB V3 peptide, whereas MAb 17b bound none of the peptides tested. Monoclonal antibody 48d bound to the HIV Can0A V3 peptide both in solid-phase ELISA and in solution in a competitive ELISA, but could not bind to HIV Can0A V3 peptide bound to human T cells. The HIV Can0A V3 peptide induced anti-HIV antibodies in rhesus monkeys that neutralized the laboratory-adapted HIV MN strain but did not induce antibodies that neutralized HIV IIIB/LAI, HIV SF-2, or HIV RF isolates, or that neutralized HIV primary isolates. These data suggested that the primary sequence of the HIV Can0A V3 loop exists in a conformer that mimicks a non-V3 determinant of native gp120 exposed subsequent to CD4 binding on the surface of gp120 of laboratory-adapted HIV strains. Structural studies of the Can0A V3 peptide and/or the 48d MAb may provide important information regarding the nature of gp120 conformational changes that occur following gp120 ligation by CD4.

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.

Postbinding events mediated by human immunodeficiency virus type 1 are sensitive to modifications in the D4-transmembrane linker region of CD4

Evidence from both structural and functional studies of the CD4 molecule suggests that several domains, including the transmembrane (TM) domain and the adjoining extracellular region (D4-TM linker), contribute to the post-gp12O-binding events leading to human immunodeficiency virus-mediated membrane fusion. To investigate such a role in syncytium formation and cell-free infectivity, we generated several deletion and substitution mutations in the TM and D4-TM linker regions of the CD4 molecule. We found that while the TM domain of CD4 was dispensable for cell-cell and virus-cell interactions, modifications in the D4-TM linker led to perturbations in both processes. Deletion of the five amino acid residues linking D4 to the TM domain resulted in a delayed and reduced capacity to form syncytia, whereas replacement of the residues with the heterologous sequence from the CD8 molecule restored the kinetic profile to wild-type CD4 levels. On the other hand, both mutants of the CD4 D4-TM linker demonstrated delayed cell-free human immunodeficiency virus type 1 infectivity profiles. The defective fusion capacity may be linked to structural perturbations identified with anti-CD4 monoclonal antibodies in the D1-D2 interface and D3 domain of the deletion mutant yet absent in D1 and D4. While all cells were found to bind comparable levels of gp120, both D4-TM linker mutants appeared to induce a decrease in the V3 loop exposure of bound gp120. This underexposure may explain the delays in cell-free infectivities observed for both of these mutants. Together, these findings confirm a role for regions of the CD4 molecule located outside D1 in post-gp120-binding events and suggest that the D4-TM interface contributes to the conformational changes that direct the fusion process.

Exceptional fusogenicity of Chinese hamster ovary cells with murine retroviruses suggests roles for cellular factor(s) and receptor clusters in the membrane fusion process

Chinese hamster ovary (CHO) cells are naturally resistant to infection by amphotropic and ecotropic murine retroviruses, but they become susceptible after expressing corresponding receptors rRAM-1 and mCAT-1, respectively, and they then form abundant syncytia when exposed to these viruses. The fusogenic activities of CHO cell clones increase much more strongly with levels of receptor expression than do their susceptibilities to infection, suggesting that the assembly of receptor clusters may limit syncytium formation. However, other cell lines are not fusogenic, even if they express larger amounts of receptors. Our results suggest that a factor that is relatively abundant or active in CHO cells may functionally interact with rRAM-1 and mCAT-1 in a pathway that enables receptor-bearing membranes to fuse with membranes that contain viral envelope glycoproteins. In the case of CHO/rRAM-1 cells, syncytia form at foci of amphotropic 4070A virus infection by fusion-from-within of infected with uninfected cells. This fusogenic propensity is a sole property of the uninfected CHO/rRAM-1 cells, which fuse in cocultures with any cells infected with 4070A virus. With CHO/mCAT-1 cells, fusogenicity is even greater and involves fusion-from-without by ecotropic virion particles. In contrast to infection, which behaves as expected for a process limited by ecotropic virus attachment to single receptors, fusion-from-without increases dramatically for cells that express the highest levels of mCAT-1. We propose that infection and syncytium formation are limited at distinct steps of a common pathway that requires virus binding to a single receptor, assembly of multivalent virus-receptor complexes, structural changes in viral envelope glycoproteins, and membrane fusion. The limiting step in syncytium formation is a cellular process that depends on receptor clustering and is relatively active in CHO cells.

The molecular target of bicyclams, potent inhibitors of human immunodeficiency virus replication

Bicyclams are a novel class of antiviral compounds which act as potent and selective inhibitors of the replication of human immunodeficiency virus type 1 (HIV-1) and HIV-2. They block an early step in the viral life cycle following adsorption to the CD4 receptor and preceding reverse transcription. To identify the molecular target of these compounds, we genetically analyzed variants of the HIV-1 molecular clone NL4-3, which developed resistance against two structurally related bicyclams, JM2763 and the more potent SID791. The resistant strains were obtained after long-term passaging in MT-4 cells in the presence of progressively increasing compound concentrations. Recombinants between selected genes of the resistant strains and the parental NL4-3 provirus were generated by adapting the marker rescue technique to MT-4 cells. The bicyclam-resistant phenotype was rescued by transferring the envelope gp120 gene of bicyclam-resistant virus into the NL4-3 parental genetic background. In the gp120 genes of the resistant strains, we identified several mutations leading to amino acid substitutions in the V3 loop. Furthermore, two substitutions of highly conserved amino acids in close proximity to the disulfide bridges of the V3 and V4 loops were found in both SID791- and JM2763-resistant strains. Additional mutations in regions encoding V3, C4, V5, and C5 were present in SID791-resistant viruses. Recombination experiments with overlapping parts of the envelope gene indicated that most, if not all, of the mutations were necessary to develop the fully SID791 resistant phenotype. The mutations in the C-terminal part of gp120 downstream of the V3 loop sequence conferred partial resistance to JM2763 but did not significantly decrease susceptibility to SID791. The genetic data and the biological properties of the resistant viruses point to inhibition of entry and fusion as the mode of action of the HIV-inhibitory bicyclams. A possible mechanism of binding of bicyclams to gp120 leading to inhibition of unfolding of gp120 and its shedding from the gp41 fusion domain is discussed.

Antigenic analysis of HIV type 1 external envelope (Env) glycoprotein C2 region: implication for the structure of Env

To determine whether the amino acid sequence extending from residue 273 to residue 288 in the second conserved region C2 of the HIV-1 envelope glycoprotein represents a target for antibodies on monomeric and oligomerized HIV-1 gp120env, we characterized several antisera and monoclonal antibodies (MAb) raised against C2 synthetic peptides. A cross-reactive epitope was evidenced on HIV-1Lai and HIV-1Eli C2-derived peptides, but was not encountered on HIV-2 C2-derived peptide. This epitope was found to be expressed on the native monomeric gp120env but was not detected in the context of oligomeric Env, suggesting this region is sequestered in the oligomeric molecule. Preincubation of oligomeric Env with sCD4 apparently failed to expose this epitope. Our results suggest that the amino acid sequence extending from residue 273 to residue 288 in C2 of HIV-1 gp120env may be involved in intermolecular interaction within the oligomeric Env complex.

Characterization of retrovirus-induced SC-1 cell fusion

Virus-mediated cell-cell fusion with Moloney MLV and SC-1 cells was characterized. The level of fusion was highly dependent on the cell line used for propagation of the virus. Efficient fusion appeared to be very sensitive to negative charges on the cell surface and surroundings. Addition of polycations, removal of serum, and treatment with neuraminidase or hyaluronidase all stimulated fusion. Conversely, fusion was inhibited by fibronectin. Kinetic results and the time of action of inhibitors indicated that virus particles (or virus material) on the cell surface lead directly to fusion. The fusion then proceeded rapidly and required actin movement as shown by cytochalasin inhibition.

Direct interaction of complement factor H with the C1 domain of HIV type 1 glycoprotein 120

A protein that binds specifically to Env 105-119 (HEDIISLWDQSLKPC) was found in pools of normal human plasma when this peptide was used in affinity chromatography procedures. These samples represented the negative control in experiments aimed at the purification of putative human antibodies to the Env 105-119 region from AIDS sera. In this article we describe the biochemical characterization of this protein, which turned out to be complement factor H (CFH). We propose a functional role for this protein in the complex, early steps of CD4-dependent HIV infection.

Factors involved in entry of the human immunodeficiency virus type 1 into permissive cells: lack of evidence of a role for CD26

It has been proposed recently that the cell surface peptidase CD26 acts in concert with CD4, the human immunodeficiency virus (HIV) primary receptor molecule, to mediate HIV entry into permissive cells. We have failed to detect significant levels of CD26 cell surface expression and enzymatic activity in a number of commonly propagated human CD4+ cell lines, although CD26 mRNA was present at very low levels, as detected by reverse transcription PCR. No relationship existed between the expression of CD26 and the ability of these cells to be infected with HIV or to fuse to form syncytia. We have tested two inhibitors of CD26 enzymatic activity and several anti-CD26 monoclonal antibodies and found that they inhibit neither HIV infection nor HIV-induced syncytium formation. NIH 3T3 cells stably transfected with the cDNAs for human CD4 and CD26 expressed these molecules at the cell surface and had CD26 enzymatic activity. Inoculation of the double transfectants with HIV did not result in virus entry above the background level, as verified by PCR amplification of viral DNA. We were unable to recover infectious virus from the HIV-inoculated NIH 3T3 double transfectants either by transfer of supernatants or by cocultivation with human CD4+ indicator cells. Moreover, the transfectants did not fuse with HIV-infected cells to form syncytia, nor were syncytia observed in HIV-inoculated cultures. These results are inconsistent with the CD26 molecule being a cofactor for entry of HIV in CD4+ cells.

Conformational changes induced in the envelope glycoproteins of the human and simian immunodeficiency viruses by soluble receptor binding

We have investigated the molecular basis of biological differences observed among cell line-adapted isolates of the human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) and the simian immunodeficiency virus (SIV) in response to receptor binding by using a soluble form of CD4 (sCD4) as a receptor mimic. We find that sCD4 binds to the envelope glycoproteins of all of the HIV-1 isolates tested with affinities within a threefold range, whereas those of the HIV-2 and SIV isolates have relative affinities for sCD4 two- to eightfold lower than those of HIV-1. Treatment of infected cells with sCD4 induced the dissociation of gp120 from gp41 and increased the exposure of a cryptic gp41 epitope on all of the HIV-1 isolates. By contrast, neither dissociation of the outer envelope glycoprotein nor increased exposure of the transmembrane glycoprotein was observed when sCD4 bound to HIV-2- or SIV-infected cells. Moreover, immunoprecipitation with sCD4 resulted in the coprecipitation of the surface and transmembrane glycoproteins from virions of the HIV-2 and SIV isolates, whereas the surface envelope glycoprotein alone was precipitated from HIV-1. However, treatment of HIV-1-, HIV-2-, and SIV-infected cells with sCD4 did result in an increase in exposure of their V2 and V3 loops, as detected by enhanced antibody reactivity. This demonstrates that receptor binding to the outer envelope glycoprotein induces certain conformational changes which are common to all of these viruses and others which are restricted to cell line-passaged isolates of HIV-1.

Human and simian immunodeficiency viruses: virus-receptor interactions

The major cellular receptor for the primate immunodeficiency viruses is the CD4 molecule. As well as mediating virion attachment to the cell surface, CD4 is thought to activate the viral fusion pathway. CD4 is not, however, sufficient for viral entry; other molecules are probably involved, and in certain circumstances these may substitute for CD4. Viral tropism and cytopathogenicity are also influenced by receptor interactions.

Cell fusion mediated by interaction of a hybrid CD4.CD8 molecule with the human immunodeficiency virus type 1 envelope glycoprotein does occur after a long lag time

Several domains of CD4 have been suggested to play a critical role in events that follow its binding to the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (gp120-gp41). It has been reported previously that cells expressing a chimeric molecule consisting of the first 177 residues of human CD4 attached to residues from the hinge, transmembrane, and cytoplasmic domains of human CD8 did not form syncytia with HIV-1-infected cells (L. Poulin, L.A. Evans, S. Tang, A. Barboza, H. Legg, D.R. Littman, and J.A. Levy, J. Virol. 65: 4893-4901, 1991). In contrast, we found that the hybrid CD4.CD8 molecule expressed in human cells did render them susceptible to fusion with cells expressing HIV-1IIIB or HIV-1RF envelope glycoproteins encoded by vaccinia virus recombinants, but only after long lag times. The lag time of membrane fusion mediated by the hybrid CD4.CD8 molecule was fivefold longer than that for the wild-type CD4 molecule. However, the rate of binding to and the affinity of soluble gp120 for membrane-associated CD4.CD8 were the same as for CD4. Both molecules were laterally mobile, as determined by patching experiments. Coexpression of the CD4.CD8 chimera with wild-type CD4 did not lead to interference in fusion but had an additive effect. Therefore, the proximal membrane domains of CD4 play an important role in determining the kinetics of postbinding events leading to membrane fusion. We hypothesize that the long lag time is due to the inability of the CD4.CD8-gp120-gp41 complex to undergo the rapid conformational changes which occur during the fusion mediated by wild-type CD4.