Human immunodeficiency virus type 2 envelope glycoprotein: differential CD4 interactions of soluble gp120 versus the assembled envelope complex

Conserved Role of an N-Linked Glycan on the Surface Antigen of Human Immunodeficiency Virus Type 1 Modulating Virus Sensitivity to Broadly Neutralizing Antibodies against the Receptor and Coreceptor Binding Sites

HIV-1 establishes persistent infection in part due to its ability to evade host immune responses. Occlusion by glycans contributes to masking conserved sites that are targets for some broadly neutralizing antibodies (bNAbs). Previous work has shown that removal of a highly conserved potential N-linked glycan (PNLG) site at amino acid residue 197 (N7) on the surface antigen gp120 of HIV-1 increases neutralization sensitivity of the mutant virus to CD4 binding site (CD4bs)-directed antibodies compared to its wild-type (WT) counterpart. However, it is not clear if the role of the N7 glycan is conserved among diverse HIV-1 isolates and if other glycans in the conserved regions of HIV-1 Env display similar functions. In this work, we examined the role of PNLGs in the conserved region of HIV-1 Env, particularly the role of the N7 glycan in a panel of HIV-1 strains representing different clades, tissue origins, coreceptor usages, and neutralization sensitivities. We demonstrate that the absence of the N7 glycan increases the sensitivity of diverse HIV-1 isolates to CD4bs- and V3 loop-directed antibodies, indicating that the N7 glycan plays a conserved role masking these conserved epitopes. However, the effect of the N7 glycan on virus sensitivity to neutralizing antibodies directed against the V2 loop epitope is isolate dependent. These findings indicate that the N7 glycan plays an important and conserved role modulating the structure, stability, or accessibility of bNAb epitopes in the CD4bs and coreceptor binding region, thus representing a potential target for the design of immunogens and therapeutics.

IMPORTANCE

N-linked glycans on the HIV-1 envelope protein have been postulated to contribute to viral escape from host immune responses. However, the role of specific glycans in the conserved regions of HIV-1 Env in modulating epitope recognition by broadly neutralizing antibodies has not been well defined. We show here that a single N-linked glycan plays a unique and conserved role among conserved glycans on HIV-1 gp120 in modulating the exposure or the stability of the receptor and coreceptor binding site without affecting the integrity of the Env in mediating viral infection or the ability of the mutant gp120 to bind to CD4. The observation that the antigenicity of the receptor and coreceptor binding sites can be modulated by a single glycan indicates that select glycan modification offers a potential strategy for the design of HIV-1 vaccine candidates.

Broadly neutralizing antibody PGT121 allosterically modulates CD4 binding via recognition of the HIV-1 gp120 V3 base and multiple surrounding glycans

New broad and potent neutralizing HIV-1 antibodies have recently been described that are largely dependent on the gp120 N332 glycan for Env recognition. Members of the PGT121 family of antibodies, isolated from an African donor, neutralize ∼70% of circulating isolates with a median IC₅₀ less than 0.05 µg ml⁻¹. Here, we show that three family members, PGT121, PGT122 and PGT123, have very similar crystal structures. A long 24-residue HCDR3 divides the antibody binding site into two functional surfaces, consisting of an open face, formed by the heavy chain CDRs, and an elongated face, formed by LCDR1, LCDR3 and the tip of the HCDR3. Alanine scanning mutagenesis of the antibody paratope reveals a crucial role in neutralization for residues on the elongated face, whereas the open face, which accommodates a complex biantennary glycan in the PGT121 structure, appears to play a more secondary role. Negative-stain EM reconstructions of an engineered recombinant Env gp140 trimer (SOSIP.664) reveal that PGT122 interacts with the gp120 outer domain at a more vertical angle with respect to the top surface of the spike than the previously characterized antibody PGT128, which is also dependent on the N332 glycan. We then used ITC and FACS to demonstrate that the PGT121 antibodies inhibit CD4 binding to gp120 despite the epitope being distal from the CD4 binding site. Together, these structural, functional and biophysical results suggest that the PGT121 antibodies may interfere with Env receptor engagement by an allosteric mechanism in which key structural elements, such as the V3 base, the N332 oligomannose glycan and surrounding glycans, including a putative V1/V2 complex biantennary glycan, are conformationally constrained.

An estimated 33 million adults and children currently live with the human immunodeficiency virus type 1 (HIV-1), which represents a global prevalence of 0.8%. In the absence of a cure, the development of a protective vaccine is the long sought-after goal in containment of the pandemic. HIV-1 Env is the sole viral surface glycoprotein and mediates viral engagement and entry into host cells, which constitutes the first step of the virus life cycle. Recently, a plethora of exciting new antibodies have been discovered that interact with HIV-1 Env and inhibit infection of target cells (i.e. neutralize the virus). Here, we structurally characterize the interaction of a recombinant HIV-1 Env with one class of such antibodies, namely antibodies of the PGT121 family. These studies have uncovered a novel mode of HIV-1 Env recognition. By interacting with key structural elements of HIV-1 Env near the apex at its membrane-distal end, these antibodies can interfere with binding to CD4, the receptor on T cells that is required for HIV-1 infection. These observations further delineate a glycan-dependent site of vulnerability on HIV-1 Env that can be used in vaccine design efforts.

Potent and broadly reactive HIV-2 neutralizing antibodies elicited by a vaccinia virus vector prime-C2V3C3 polypeptide boost immunization strategy

Human immunodeficiency virus type 2 (HIV-2) infection affects about 1 to 2 million individuals, the majority living in West Africa, Europe, and India. As for HIV-1, new strategies for the prevention of HIV-2 infection are needed. Our aim was to produce new vaccine immunogens that elicit the production of broadly reactive HIV-2 neutralizing antibodies (NAbs). Native and truncated envelope proteins from the reference HIV-2ALI isolate were expressed in vaccinia virus or in bacteria. This source isolate was used due to its unique phenotype combining CD4 independence and CCR5 usage. NAbs were not elicited in BALB/c mice by single immunization with a truncated and fully glycosylated envelope gp125 (gp125t) or a recombinant polypeptide comprising the C2, V3, and C3 envelope regions (rpC2-C3). A strong and broad NAb response was, however, elicited in mice primed with gp125t expressed in vaccinia virus and boosted with rpC2-C3. Serum from these animals potently neutralized (median 50% neutralizing titer, 3,200) six of six highly divergent primary HIV-2 isolates. Coreceptor usage and the V3 sequence of NAb-sensitive isolates were similar to that of the vaccinating immunogen (HIV-2ALI). In contrast, NAbs were not reactive on three X4 isolates that displayed major changes in V3 loop sequence and structure. Collectively, our findings demonstrate that broadly reactive HIV-2 NAbs can be elicited by using a vaccinia virus vector-prime/rpC2-C3-boost immunization strategy and suggest a potential relationship between escape to neutralization and cell tropism.

Kinetic studies of HIV-1 and HIV-2 envelope glycoprotein-mediated fusion

Background

HIV envelope glycoprotein (Env)-mediated fusion is driven by the concerted coalescence of the HIV gp41 N-helical and C-helical regions, which results in the formation of 6 helix bundles. Kinetics of HIV Env-mediated fusion is an important determinant of sensitivity to entry inhibitors and antibodies. However, the parameters that govern the HIV Env fusion cascade have yet to be fully elucidated. We address this issue by comparing the kinetics HIV-1_IIIB Env with those mediated by HIV-2 from two strains with different affinities for CD4 and CXCR4.

Results

HIV-1 and HIV-2 Env-mediated cell fusion occurred with half times of about 60 and 30 min, respectively. Binding experiments of soluble HIV gp120 proteins to CD4 and co-receptor did not correlate with the differences in kinetics of fusion mediated by the three different HIV Envs. However, escape from inhibition by reagents that block gp120-CD4 binding, CD4-induced CXCR4 binding and 6-helix bundle formation, respectively, indicated large difference between HIV-1 and HIV-2 envelope glycoproteins in their CD4-induced rates of engagement with CXCR4.

Conclusion

The HIV-2 Env proteins studied here exhibited a significantly reduced window of time between the engagement of gp120 with CD4 and exposure of the CXCR4 binding site on gp120 as compared with HIV-1_IIIB Env. The efficiency with which HIV-2 Env undergoes this CD4-induced conformational change is the major cause of the relatively rapid rate of HIV-2 Env mediated-fusion.

The prolonged culture of human immunodeficiency virus type 1 in primary lymphocytes increases its sensitivity to neutralization by soluble CD4

Primary strains of human immunodeficiency virus type 1 (HIV-1) are known to adapt to replication in cell lines in vitro by becoming sensitive to soluble CD4 (sCD4) and neutralizing antibodies (NAb). T-cell lines favor isolation of variants that use CXCR4 as a co-receptor, while primary isolates predominantly use CCR5. We have now studied how a primary R5 isolate, CC1/85, adapts to prolonged replication in primary human peripheral blood mononuclear cells (PBMC). After 19 passages, a variant virus, CCcon.19, had increased sensitivity to both sCD4 and NAb b12 that binds to a CD4-binding site (CD4BS)-associated epitope, but decreased sensitivity to anti-CD4 antibodies. CCcon.19 retains the R5 phenotype, its resistance to other NAbs was unaltered, its sensitivity to various entry inhibitors was unchanged, and its ability to replicate in macrophages was modestly increased. We define CCcon.19 as a primary T-cell adapted (PTCA) variant. Genetic sequence analysis combined with mutagenesis studies on clonal, chimeric viruses derived from CC1/85 and the PTCA variant showed that the most important changes were in the V1/V2 loop structure, one of them involving the loss of an N-linked glycosylation site. Monomeric gp120 proteins expressed from CC1/85 and the PTCA variant did not differ in their affinities for sCD4, suggesting that the structural consequences of the sequence changes were manifested at the level of the native, trimeric Env complex. Overall, the adaptation process probably involves selection for variants with higher CD4 affinity and hence greater fusion efficiency, but this also involves the loss of some resistance to neutralization by agents directed at or near to the CD4BS. The loss of neutralization resistance is of no relevance under in vitro conditions, but NAbs would presumably be a counter-selection pressure against such adaptive changes in vivo, at least when the humoral immune response is intact.

Inactivation of HIV in blood

BACKGROUND

The residual risk of HIV infection after HIV screening tests in combination with the risk of new emerging pathogens entering the blood supply has sparked research on the development of a technology for reduction of pathogens in RBCs.

STUDY DESIGN AND METHODS

HIV-1 was treated with PEN110 (INACTINE) and analyzed for the kinetics of virus reduction in RBC, the effect of PEN110 on nucleic acids, the integrity of the virus morphology and viral proteins, and the ability of the virus to bind HIV cell receptors and enter susceptible cells.

RESULTS

PEN110 effectively reduced HIV-1 to the limit of detection for a reduction factor of at least 5.57 log 50 percent tissue culture infectious dose per bulk test. The PEN110-treated virions maintained their morphology, protein integrity, and functionality. However, the PEN110-treated HIV-1 RNA genome was neither functional to serve as a template for RT-PCR amplification of about 1 kb nor able to support viral DNA synthesis in cell culture.

CONCLUSION

These results suggest that PEN110 inactivates HIV-1 by targeting the viral nucleic acid.

The stoichiometry of trimeric SIV glycoprotein interaction with CD4 differs from that of anti-envelope antibody Fab fragments

Human and simian immunodeficiency viruses infect host lymphoid cells by binding CD4 molecules via their gp160 envelope glycoproteins. Biochemical studies on recombinant SIVmac32H (pJ5) envelope ectodomain gp140 precursor protein show that the envelope is a trimer. Using size exclusion chromatography, quantitative amino acid analysis, analytical ultracentrifugation, and CD4-based competition assay, we demonstrate that the stoichiometry of CD4 receptor-oligomeric envelope interaction is 1:1. By contrast, Fab fragments of both neutralizing and non-neutralizing monoclonal antibodies bind at a 3:1 ratio. Thus, despite displaying equivalent CD4 binding sites on each of the three gp140 protomers within an uncleaved trimer, only one site binds the soluble 4-domain human CD4 extracellular segment. The anti-cooperativity and the faster k(off) of gp140 trimer:CD4 versus gp120 monomer:CD4 interaction suggest that CD4-induced conformational change is impeded in the intact envelope. The implications of these findings for immunity against human immunodeficiency virus and simian immunodeficiency virus are discussed.

Varying effects of temperature, Ca(2+) and cytochalasin on fusion activity mediated by human immunodeficiency virus type 1 and type 2 glycoproteins

We examined fusion mediated by the human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2) envelope glycoproteins under various experimental conditions. Incubation of HeLa cells expressing HIV-2(ROD) and HIV-2(SBL/ISY) envelope glycoproteins with HeLa-CD4 target cells resulted in fusion at temperatures >/=25 degrees C whereas fusion with cells expressing HIV-1(Lai) occurred only at >/=31 degrees C. HIV-2 envelope glycoprotein-mediated fusion proceeded in the absence of Ca(2+) in the culture medium, whereas HIV-1 fusion required Ca(2+) ions for fusion. In contrast to HIV-2 envelope glycoprotein fusion, incubations in the presence of the 0.5 microM cytochalasin B completely inhibited HIV-1 envelope glycoprotein-mediated fusion. Our results suggest that in contrast to HIV-2, HIV-1 fusion is dependent on dynamic processes in the target membrane.

An endoplasmic reticulum retrieval signal partitions human foamy virus maturation to intracytoplasmic membranes

Among all retroviruses, foamy viruses (FVs) are unique in that they regularly mature at intracytoplasmic membranes. The envelope glycoprotein of FV encodes an endoplasmic reticulum (ER) retrieval signal, the dilysine motif (KKXX), that functions to localize the human FV (HFV) glycoprotein to the ER. This study analyzed the function of the dilysine motif in the context of infectious molecular clones of HFV that encoded mutations in the dilysine motif. Electron microscopy (EM) demonstrated virion budding both intracytoplasmically and at the plasma membrane for the wild-type and mutant viruses. Additionally, mutant viruses retained their infectivity, but viruses lacking the dilysine signal budded at the plasma membrane to a greater extent than did wild-type viruses. Interestingly, this relative increase in budding across the plasma membrane did not increase the overall release of viral particles into cell culture media as measured by protein levels in viral pellets or infectious virus titers. We conclude that the dilysine motif of HFV imposes a partial restriction on the site of viral maturation but is not necessary for viral infectivity.

Human immunodeficiency virus type 2 preintegration complexes: activities in vitro and response to inhibitors

We have established an assay for the function of preintegration complexes (PICs) of human immunodeficiency virus type 2 (HIV-2) to investigate the integration mechanism and to develop additional methods for screening candidate integration inhibitors. We partially purified HIV-2 PICs and found that they were competent to integrate viral cDNA into target DNA in vitro. Analysis of the structure of integration products on Southern blots revealed forms consistent with those expected for authentic integration products and circular forms containing one and two long terminal repeats. To determine whether in vitro products had the detailed structure expected of integration products formed in vivo, we recovered product molecules and analyzed junctions between viral DNA and target DNA. In the integration junctions of all nine molecules examined, we observed the 5-bp duplication of target sequence characteristic of integration in vivo. We investigated the possible role in integration of Vpx, a protein present in HIV-2 but not HIV-1 and known to be present in viral cores. Although association of Vpx with viral cDNA was detectable, our studies revealed no obvious role of Vpx in integration since the activities of PICs from Vpx- virions were indistinguishable from those of wild type. We have also investigated the use of HIV-2 PICs as tools to screen candidate HIV inhibitors. Assays with HIV-2 PICs, like assays with HIV-1 PICs, were less sensitive to many small molecule inhibitors than were reactions with purified integrase only. Comparing results of assays with PICs from HIV-1 and HIV-2 may be particularly useful, since inhibitors active against both may be more widely useful and less vulnerable to escape mutants.

A sorting motif localizes the foamy virus glycoprotein to the endoplasmic reticulum

We recently identified an endoplasmic reticulum (ER) retrieval signal-the dilysine motif-in the glycoproteins of all five foamy viruses (FVs) for which sequences were available (P. A. Goepfert, G. Wang, and M. J. Mulligan, Cell 82:543-544, 1995). In the present study, expression of recombinant human FV (HFV) glycoprotein and analyses of oligosaccharide modifications and precursor cleavage indicated that the protein was localized to the ER. HFV glycoproteins encoding seven different dilysine motif mutations were then expressed. The results indicated that disruptions of the dilysine motif resulted in higher levels of forward transport of the HFV glycoprotein from the ER through the Golgi apparatus to the plasma membrane. We conclude that the dilysine motif is responsible for ER sorting of the FV glycoprotein. Signal-mediated ER localization has not previously been described for a retroviral glycoprotein.

Receptor interference mediated by the envelope glycoproteins of various HIV-1 and HIV-2 isolates

The envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) plays a major role in the down-regulation of its receptor, CD4. This down-regulation, at least in part, is caused by the formation of gp160-CD4 intracellular complexes which fail to transport out of the endoplasmic reticulum (ER). In this report, we have evaluated the ability of envelope glycoproteins from various isolates to block CD4 transport within the endoplasmic reticulum. Using a recombinant vaccinia virus expression system in HeLa cells, we expressed different HIV-1 and HIV-2 envelope glycoproteins with CD4. Pulse-chase labeling followed by immunoprecipitation demonstrated that envelope glycoproteins from primary and lab-adapted isolates were capable of forming intracellular complexes with CD4, resulting in the partial inhibition of CD4 transport to the Golgi. Although the efficiency of CD4 modulation was variable, these differences did not correlate with the type of isolate from which the HIV-1 glycoprotein was derived. However, we did find that the HIV-2 ST envelope glycoprotein (gp150) was not as efficient at blocking CD4 as the glycoprotein (gp140) derived from HIV-2 ROD. The decreased ability of ST gp150 to block CD4 within the ER was associated with an increased efficiency of ST gp150 transport and cleavage. Thus, differences in the ability of HIV envelope glycoproteins to block CD4 transport do exist, and these differences may be determined by envelope glycoprotein transport kinetics.

The human immunodeficiency virus (HIV) type 2 envelope protein is a functional complement to HIV type 1 Vpu that enhances particle release of heterologous retroviruses

We have recently shown that the envelope glycoprotein of the ROD10 isolate of human immunodeficiency virus type 2 (HIV-2) has the ability to positively regulate HIV-2 viral particle release. The activity provided by the ROD10 Env was remarkably similar to that of the HIV-1 Vpu protein, thus raising the possibility that the two proteins act in a related fashion. We now show that the ROD10 Env can functionally replace Vpu to enhance the rate of HIV-1 particle release. When provided in trans, both Vpu and the ROD10 Env restored wild-type levels of particle release in a Vpu-deficient mutant of the NL4-3 molecular clone with indistinguishable efficiencies. This effect was independent of the presence of the HIV-1 envelope protein. The ROD10 Env also enhanced HIV-1 particle release in the context of HIV-2 chimeric viruses containing the HIV-1 gag-pol, indicating a lack of need for additional HIV-1 products in this process. In addition, we show for the first time that HIV-1 Vpu, as well as ROD10 Env, has the ability to enhance simian immunodeficiency virus (SIV) particle release. The effects of Vpu and ROD10 Env on SIV particle release were indistinguishable and were observed in the context of full-length SIVmac239 and simian-human immunodeficiency virus chimeras. These results further demonstrate that ROD10 Env can functionally complement Vpu with respect to virus release. In contrast, we found no evidence of a destabilizing activity of ROD10 Env on the CD4 molecule. HIV-1 and HIV-2 thus appear to have evolved genetically distinct but functionally similar strategies to resolve the common problem of efficient release of progeny virus from infected cells.

Human immunodeficiency virus type 2 glycoprotein enhancement of particle budding: role of the cytoplasmic domain

Previous studies have shown that the glycoprotein cytoplasmic domains of human immunodeficiency virus type 2 (HIV-2) or simian immunodeficiency virus of macaques modulate biological activities of the viral glycoprotein complex, including syncytium formation, exterior glycoprotein conformation, and glycoprotein incorporation into budding virus particles. We have now utilized a recombinant expression system to study interactions of full-length or truncated HIV-2 glycoproteins with coexpressed HIV-2 Gag proteins which self-assemble and bud as virus-like particles. Interestingly, budding of HIV-2 virus-like particles from cells was enhanced 5- to 24-fold when Gag was coexpressed with the full-length HIV-2 glycoprotein, compared with Gag expressed either alone or with a truncated HIV-2 glycoprotein. The results obtained in this model system indicate that an additional effect of the lengthy cytoplasmic domain of the glycoprotein of HIV-2 is enhancement of particle budding. We speculate that the cytoplasmic domain of the viral glycoprotein of HIV-2 enhances budding by (i) potentiation of Gag structure or function or (ii) membrane modulation.

The envelope glycoprotein of human immunodeficiency virus type 2 enhances viral particle release: a Vpu-like factor?

The Vpu protein is a human immunodeficiency virus type 1 (HIV-1)-specific accessory protein that is required for the efficient release of viral particles from infected cells. Even though HIV-2 does not encode Vpu, we found that this virus is nevertheless capable of efficiently releasing virus particles. In fact, the rate of virus release from HeLa cells transfected with a full-length molecular clone of HIV-2, ROD10, was comparable to that observed for the vpu+ HIV-1 NL4-3 isolate and was not further enhanced by expression of Vpu in trans. However, consistent with previous observations showing that HIV-2 particle release is Vpu responsive in the context of HIV-1/HIV-2 chimeric constructs; exchanging the gag-pol region of NL4-3 with the corresponding region from pROD10 rendered the resulting chimeric virus Vpu responsive. Our finding that the responsiveness of HIV-2 particle release to Vpu is context dependent suggested the presence of a Vpu-like factor(s) encoded by HIV-2. Using chimeric proviruses encoding HIV-2 gag and pol in the context of the HIV-1 provirus that were coexpressed with subgenomic HIV-2 constructs, we found that the HIV-2 envelope glycoprotein had the ability to enhance HIV-2 particle release with an efficiency comparable to that of the HIV-1 Vpu protein. Conversely, inactivation of the HIV-2 env gene in the original ROD10 clone resulted in a decrease in the rate of viral particle release to a level that was comparable to that of Vpu-deficient HIV-1 isolates. Providing the wild-type envelope in trans rescued the particle release defect of the ROD10 envelope mutant. Thus, unlike HIV-1, which encodes two separate proteins to regulate virus release or to mediate viral entry, the HIV-2 Env protein has evolved to perform both functions.

Increase in soluble CD4 binding to and CD4-induced dissociation of gp120 from virions correlates with infectivity of human immunodeficiency virus type 1

Mutations in the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins gp120 and gp41, previously shown to confer an enhanced replicative capacity and broadened host range to the ELI1 strain of HIV-1, were analyzed for their biochemical effects on envelope structure and function. The tendency of purified virions to release their extracellular gp120 component, either spontaneously or after interacting with soluble CD4 (CD4-induced shedding) was assessed. A single amino acid substitution in part of the CD4 binding site of gp120 (Gly-427 to Arg) enhanced both spontaneous and CD4-induced shedding of gp120 from virions, while a single change in the fusogenic region of gp41 (Met-7 to Val) affected only CD4-induced shedding. Although each codon change alone conferred increased growth ability, virus with both mutations exhibited the most rapid replication kinetics. In addition, when both of these mutations were present, virions had the highest tendency to shed gp120, both spontaneously and after exposure to soluble CD4. Analysis of CD4 binding to virion-associated gp120 showed that the changes in both gp120 and gp41 contributed to increased binding. These results demonstrated that the increased replicative capacity of the ELI variants in human CD4+ cell lines was associated with altered physical and functional properties of the virion envelope glycoproteins.

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.

Role of HIV-1 envelope V3 loop cleavage in cell tropism

The envelope protein is an important determinant of HIV-1 cell-specific tropism. The gp160 envelope precursor proteins from macrophage-tropic or T lymphoid cell line-tropic strains of HIV-1 were expressed in recombinant vaccinia virus-infected cell lines or primary lymphocytes or macrophages. No significant differences in the kinetics of synthesis of gp160, processing into gp120 and gp41 proteins, N-linked glycosylation, or release of gp120 into the medium were noted with the different envelope proteins. However, gp120 envelope protein shed into the medium was found to be at least partially cleaved at a site within the V3 loop. The gp120 envelope proteins from macrophage-tropic isolates exhibited lower rates of cleavage than those from lymphoid cell line-tropic strains in all cell types examined. Cell-free protease digestion studies also demonstrated relative resistance of the envelopes from macrophage-tropic compared to lymphoid cell line-tropic strains. All recombinant envelope proteins were recognized by monoclonal antibodies directed at gp41 or the C-terminal gp120 epitopes, and no differences in binding to CD4 were noted.

Mapping the determinants of human immunodeficiency virus 2 for infectivity, replication efficiency, and cytopathicity

Human immunodeficiency virus 2 (HIV-2) ISY and the newly derived HIV-2KR are infectious molecular clones that yield viruses differing markedly in their abilities to infect and/or induce syncytia in various T- and monocytoid-cell lines. Chimeric viruses were constructed from these two viral genomes to localize the genetic determinants of some of these properties. Envelope sequences, particularly those spanning the CD4 binding site, appear to be critical for the ability of HIV-2KR to infect MOLT-4 clone 8 and SupT1 cells and to efficiently infect the H9 cell line. On the other hand, multiple determinants may contribute to cytopathicity (gp41 and nef) in H9 cells and replication efficiency in monocytic (THP-1) cells.

Mechanism of action of N-butyl deoxynojirimycin in inhibiting HIV-1 infection and activity in combination with nucleoside analogs

The effects on HIV-1 infection of a glucosidase inhibitor, N-butyl deoxynojirimycin (N-buDNJ), were examined. The combinations of N-buDNJ and nucleoside analogs dideoxyinosine (DDI), dideoxycytidine (DDC), or azidothymidine (AZT) were examined in an acute infection assay. The combination of N-buDNJ and nucleoside analog reduced the yield of reverse transcriptase activity more than did either agent alone, and the effects on the number of infectious virus particles were additive or synergistic. In studies of the mechanism whereby N-buDNJ alters HIV-1 envelope fusion activity, no effects on CD4 binding were detected. However, cleavage within the V3 loop of gp120 was reduced by N-buDNJ treatment, possibly reflecting an altered conformation of this region of the envelope protein.

Transdominant inhibition of wild-type human immunodeficiency virus type 2 replication by an envelope deletion mutant

The envelope glycoprotein of human immunodeficiency virus type 2 (HIV-2) is primarily responsible for virus attachment and entry into the target cell population. We constructed an HIV-2 mutant virus containing an in-frame deletion within the putative CD4-binding sequences of the envelope glycoprotein and confirmed that the mutant envelope is unable to bind CD4 and that the mutant virus is noninfectious. To investigate whether this mutant could dominantly interfere with wild-type replication, we coexpressed proviral DNAs of both wild-type and mutant viruses in cells and assayed the production of infectious HIV-2 virions. Interference with virus replication was indeed observed with mutant DNA, and a maximal effect was achieved with 10-fold excess mutant DNA over wild-type DNA in the cotransfection experiments. The transdominant effect on virus replication does not appear to be at the level of wild-type envelope expression or gp120-CD4 interaction. Rather, the interference may be at the level of mixed-oligomer formation during progeny virus assembly and may occur by either destabilizing the multimeric structure of gp120 or forming a defective mixed multimeric gp120 which is unable to complete the receptor binding and/or postbinding events needed for infection.

Sulfation of the human immunodeficiency virus envelope glycoprotein

Sulfation is a posttranslational modification of proteins which occurs on either the tyrosine residues or the carbohydrate moieties of some glycoproteins. In the case of secretory proteins, sulfation has been hypothesized to act as a signal for export from the cell. We have shown that the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein precursor (gp160) as well as the surface (gp120) and transmembrane (gp41) subunits can be specifically labelled with 35SO42-. Sulfated HIV-1 envelope glycoproteins were identified in H9 cells infected with the IIIB isolate of HIV-1 and in the cell lysates and culture media of cells infected with vaccinia virus recombinants expressing a full-length or truncated, secreted form of the HIV-1 gp160 gene. N-glycosidase F digestion of 35SO4(2-)-labelled envelope proteins removed virtually all radiolabel from gp160, gp120, and gp41, indicating that sulfate was linked to the carbohydrate chains of the glycoprotein. The 35SO42-label was at least partially resistant to endoglycosidase H digestion, indicating that some sulfate was linked to complex carbohydrates. Brefeldin A, a compound that inhibits the endoplasmic reticulum to Golgi transport of glycoproteins, was found to inhibit the sulfation of the envelope glycoproteins. Envelope glycoproteins synthesized in cells treated with chlorate failed to incorporate 35SO42-. However, HIV glycoproteins were still secreted from cells in the presence of chlorate, indicating that sulfation is not a requirement for secretion of envelope glycoproteins. Sulfation of HIV-2 and simian immunodeficiency virus envelope glycoproteins has also been demonstrated by using vaccinia virus-based expression systems. Sulfation is a major determinant of negative charge and could play a role in biological functions and antigenic properties of HIV glycoproteins.

CD4 activation of HIV fusion

The primary cellular receptor for the human immunodeficiency viruses type 1 (HIV-1) and type 2 (HIV-2) is the CD4 antigen. HIV infection of CD4+ cells is initiated by binding of the virus to the cell surface, via a high affinity interaction between CD4 and the HIV outer envelope glycoprotein, gp120. The development of model systems using soluble recombinant forms of CD4 (sCD4) has allowed kinetic and thermodynamic analyses of CD4 binding to gp120, and study of the post-binding events leading to virus-cell membrane fusion. It has thus been demonstrated that the affinity of sCD4 for gp120 on virions or HIV-infected cells depends on both the primary sequence and the tertiary structure of gp120 in the membrane. With cell-line adapted isolates of HIV-1, sCD4 binding induces conformational changes in gp120, leading to the complete dissociation of gp120 from the transmembrane glycoprotein, gp41, and exposing cryptic epitopes of gp41. Similar observations have been made with cell-anchored CD4; exposure of cryptic gp41 epitopes occurs at the fusion interface between clusters of CD4-expressing and HIV-infected cells. Thus, for HIV-1, CD4 induces exposure of fusogenic components of gp41 which triggers virus-cell membrane coalescence. This is termed receptor-mediated activation of fusion. With primary isolates of HIV-1 and the related lentiviruses, HIV-2 and simian immunodeficiency virus (SIV), the CD4-induced molecular rearrangements in gp120 are more subtle, implying that there is a spectrum of responses to sCD4 binding. The high-affinity binding site on CD4 for gp120 is necessary and probably sufficient for activation of HIV fusion, although other regions of CD4 may indirectly influence viral entry. There are two regions on the envelope glycoproteins which are recognized as playing a role in HIV entry: the N-terminus of gp41 and the gp120 V3 loop. The roles of these domains are discussed.

Lack of correlation between soluble CD4-induced shedding of the human immunodeficiency virus type 1 exterior envelope glycoprotein and subsequent membrane fusion events

The noncovalent association of the gp120 and gp41 envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) is disrupted by soluble CD4 binding, resulting in shedding of the gp120 exterior envelope glycoprotein. This observation has led to the speculation that interaction of gp120 with the CD4 receptor triggers shedding of the exterior envelope glycoprotein, allowing exposure of gp41 domains necessary for membrane fusion steps involved in virus entry or syncytium formation. To test this hypothesis, a set of HIV-1 envelope glycoprotein mutants were used to examine the relationship of soluble CD4-induced shedding of the gp120 glycoprotein to envelope glycoprotein function in syncytium formation and virus entry. All mutants with a threefold or greater reduction in CD4-binding ability exhibited marked decreases in gp120 shedding in response to soluble CD4, even though several of these mutants exhibited significant levels of envelope glycoprotein function. Conversely, most fusion-defective mutants with wild-type gp120-CD4 binding affinity, including those with changes in the V3 loop, efficiently shed gp120 following soluble CD4 binding. Thus, soluble CD4-induced shedding of gp120 is not a generally useful marker for conformational changes in the HIV-1 envelope glycoproteins necessary for the virus entry or syncytium formation processes. Some gp120 mutants, despite being expressed on the cell surface and capable of efficiently binding soluble CD4, exhibited decreased gp120 shedding. These mutants were still sensitive to neutralization by soluble CD4, indicating that, for envelope glycoproteins exhibiting high affinity for soluble CD4, competitive inhibition may be more important than gp120 shedding for the antiviral effect.

Cytoplasmic domain truncation enhances fusion activity by the exterior glycoprotein complex of human immunodeficiency virus type 2 in selected cell types

To investigate the glycoprotein determinants of viral cytopathology, we constructed chimeric env genes between a noncytopathic strain of human immunodeficiency virus type 2 (HIV-2), designated HIV-2/ST, and a highly fusogenic and cytopathic variant derived from this virus. Expression of the resulting chimeric glycoproteins indicated that efficient syncytium formation in the human T-cell line Sup T1 mapped to the C-terminal region of the transmembrane (TM) glycoprotein subunit. In this region, the wild-type and cytopathic ST glycoproteins differed by only four amino acids and by the presence of a premature termination codon in the cytopathic variant. Subsequent site-directed mutagenesis indicated that the cytoplasmic domain truncation was responsible for the enhanced fusion activity. This modification, however, increased the fusion activity of the glycoprotein only in Sup T1 cells (in which the ST variant arose) but not in Molt 4 clone 8 or peripheral blood mononuclear cells. These observations indicate that the length of the cytoplasmic domain of the HIV-2 glycoprotein modulates the fusion activity of the exterior glycoprotein complex in a cell-specific manner. Such adaptability appears to permit the emergence of fusogenic variants during HIV-2 passage in vitro and may also regulate viral growth or cytopathic effects in selected cell types during natural infection in vivo.