Identification and characterization of a neutralization site within the second variable region of human immunodeficiency virus type 1 gp120.

Two monoclonal antibodies designated BAT085 and G3-136 were raised by immunizing BALB/c mice with gp120 purified from human immunodeficiency virus type 1 (HIV-1) IIIB-infected H9 cell extracts. Among three HIV-1 laboratory isolates (IIIB, MN, and RF), BAT085 neutralized only IIIB infection of CEM-SS cells, whereas G3-136 neutralized both IIIB and RF. These antibodies also neutralized a few primary HIV-1 isolates in the infection of activated human peripheral blood mononuclear cells. In indirect immunofluorescence assays, BAT085 bound to H9 cells infected with IIIB or MN, while G3-136 bound to H9 cells infected with IIIB or RF, but not MN. Using sequence-overlapping synthetic peptides of HIV-1 IIIB gp120, the binding site of BAT085 and G3-136 was mapped to a peptidic segment in the V2 region (amino acid residues 169 to 183). The binding of these antibodies to immobilized gp120 was not inhibited by the antibodies directed to the principal neutralization determinant in the V3 region or to the CD4-binding domain of gp120. In a competition enzyme-linked immunosorbent assay, soluble CD4 inhibited G3-136 but not BAT085 from binding to gp120. Deglycosylation of gp120 by endo-beta-N-acetylglucosaminidase H or reduction of gp120 by dithiothreitol diminished its reactivity with G3-136 but not with BAT085. These results indicate that the V2 region of gp120 contains multiple neutralization determinants recognized by antibodies in both a conformation-dependent and -independent manner.     

Probing the structure of the V2 domain of human immunodeficiency virus type 1 surface glycoprotein gp120 with a panel of eight monoclonal antibodies: human immune response to the V1 and V2 domains.

We have analyzed a panel of eight murine monoclonal antibodies (MAbs) that depend on the V2 domain for binding to human immunodeficiency virus type 1 (HIV-1) gp120. Each MAb is sensitive to amino acid changes within V2, and some are affected by substitutions elsewhere. With one exception, the MAbs were not reactive with peptides from the V2 region, or only poorly so. Hence their ability to bind recombinant strain IIIB gp120 depended on the preservation of native structure. Three MAbs cross-reacted with strain RF gp120, but only one cross-reacted with MN gp120, and none bound SF-2 gp120. Four MAbs neutralized HIV-1 IIIB with various potencies, and the one able to bind MN gp120 neutralized that virus. Peptide serology indicated that antibodies cross-reactive with the HxB2 V1 and V2 regions are rarely present in HIV-1-positive sera, but the relatively conserved segment between the V1 and V2 loops was recognized by antibodies in a significant fraction of sera. Antibodies able to block the binding of V2 MAbs to IIIB or MN gp120 rarely exist in sera from HIV-1-infected humans; more common in these sera are antibodies that enhance the binding of V2 MAbs to gp120. This enhancement effect of HIV-1-positive sera can be mimicked by several human MAbs to different discontinuous gp120 epitopes. Soluble CD4 enhanced binding of one V2 MAb to oligomeric gp120 but not to monomeric gp120, perhaps by inducing conformational changes in the oligomer.     

Generation and characterization of monoclonal antibodies to the putative CD4-binding domain of human immunodeficiency virus type 1 gp120.

A panel of seven monoclonal antibodies against the relatively conserved CD4-binding domain on human immunodeficiency virus type 1 (HIV-1) gp120 was generated by immunizing mice with purified gp120. These monoclonal antibodies reacted specifically with gp120 in an enzyme-linked immunosorbent assay and Western blots (immunoblots). By using synthetic peptides as antigens in the immunosorbent assay, the epitopes of these seven monoclonal antibodies were mapped to amino acid residues 423 to 437 of gp120. Further studies with radioimmunoprecipitation assays showed that they cross-reacted with both gp120 and gp160 of diverse HIV-1 isolates (HTLV-IIIB, HTLV-IIIRF, HTLV-IIIAL, and HTLV-IIIWMJ). They also bound specifically to H9 cells infected with HTLV-IIIB, HTLV-IIIRF, HTLV-IIIAL, HTLV-IIIZ84, and HTLV-IIIZ34 in indirect immunofluorescence studies. In addition, they blocked effectively the binding of HIV-1 to CD4+ C8166 cells. Despite the similarity of these properties, the monoclonal antibodies differed in neutralizing activity against HTLV-IIIB, HTLV-IIIRF, and HTLV-IIIAL, as demonstrated in both syncytium-forming assays and infectivity assays. Our findings suggest that these group-specific monoclonal antibodies to the putative CD4-binding domain on gp120 are potential candidates for development of therapeutic agents against acquired immunodeficiency disease syndrome.     

Immunization with a soluble CD4-gp120 complex preferentially induces neutralizing anti-human immunodeficiency virus type 1 antibodies directed to conformation-dependent epitopes of gp120.

Preservation of the conformation of recombinant gp120 in an adjuvant, enabling it to elicit conformation-dependent, epitope-specific, broadly neutralizing antibodies, may be critical for the development of any gp120-based human immunodeficiency virus type 1 (HIV-1) vaccine. It was hypothesized that recombinant gp120 complexed with recombinant CD4 could stabilize the conformation-dependent neutralizing epitopes and effectively deliver them to the immune system. Therefore, a soluble CD4-gp120 complex in Syntex adjuvant formulation was tested with mice for its ability to induce neutralizing anti-gp120 antibody responses. Seventeen monoclonal antibodies (MAbs) were generated and characterized. Immunochemical studies, neutralization assays, and mapping studies with gp120 mutants indicated that the 17 MAbs fell into three groups. Four of them were directed to what is probably a conformational epitope involving the C1 domain and did not possess virus-neutralizing activities. Another four MAbs bound to V3 peptide 302-321 and exhibited cross-reactive gp120 binding and relatively weak virus-neutralizing activities. These MAbs were very sensitive to amino acid substitutions, not only in the V3 regions but also in the base of the V1/V2 loop, implying a conformational constraint on the epitope. The last group of nine MAbs recognized conformation-dependent epitopes near the CD4 binding site of gp120 and inhibited the gp120-soluble CD4 interaction. Four of these nine MAbs showed broadly neutralizing activities against multiple laboratory-adapted strains of HIV-1, three of them neutralized only HIVIIIB, and the two lower-affinity MAbs did not neutralize any strain tested. Collectively, the results from this study indicate that immunization with the CD4-gp120 complex can elicit antibodies to conformationally sensitive gp120 epitopes, with some of the antibodies having broadly neutralizing activities. We suggest that immunization with CD4-gp120 complexes may be worth evaluating further for the development of an AIDS vaccine.     

Antibodies to discontinuous or conformationally sensitive epitopes on the gp120 glycoprotein of human immunodeficiency virus type 1 are highly prevalent in sera of infected humans.

We have used an indirect-capture enzyme-linked immunosorbent assay to quantitate the reactivity of sera from human immunodeficiency virus type 1 (HIV-1)-infected humans with native recombinant gp120 (HIV-1 IIIB or SF-2) or with the gp120 molecule (IIIB or SF-2) denatured by being boiled in the presence of dithiothreitol with or without sodium dodecyl sulfate. Denaturation of IIIB gp120 reduced the titers of sera from randomly selected donors by at least 100-fold, suggesting that the majority of cross-reactive anti-gp120 antibodies present are directed against discontinuous or otherwise conformationally sensitive epitopes. When SF-2 gp120 was used, four of eight serum samples reacted significantly with the denatured protein, albeit with ca. 3- to 50-fold reductions in titer. Only those sera reacting with denatured SF-2 gp120 bound significantly to solid-phase-adsorbed SF-2 V3 loop peptide, and none bound to IIIB V3 loop peptide. Almost all antibody binding to reduced SF-2 gp120 was blocked by preincubation with the SF-2 V3 loop peptide, as was about 50% of the binding to native SF-2 gp120. When sera from a laboratory worker or a chimpanzee infected with IIIB were tested, the pattern of reactivity was reversed, i.e., there was significant binding to reduced IIIB gp120, but not to reduced SF-2 gp120. Binding of these sera to reduced IIIB gp120 was 1 to 10% that to native IIIB gp120 and was substantially decreased by preincubation with IIIB (but not SF-2) V3 loop peptide. To analyze which discontinuous or conformational epitopes were predominant in HIV-1-positive sera, we prebound monoclonal antibodies (MAbs) to IIIB gp120 and then added alkaline phosphatase-labelled HIV-1-positive sera. MAbs (such as 15e) that recognize discontinuous epitopes and compete directly with CD4 reduced HIV-1-positive sera binding by about 50%, whereas neutralizing MAbs to the C4, V2, and V3 domains of gp120 were either not inhibitory or only weakly so. Thus, antibodies to the discontinuous CD4-binding site on gp120 are prevalent in HIV-1-positive sera, antibodies to linear epitopes are less common, most of the antibodies to linear epitopes are directed against the V3 region, and most cross-reactive antibodies are directed against discontinuous epitopes, including regions involved in CD4 binding.     

Neutralization of multiple laboratory and clinical isolates of human immunodeficiency virus type 1 (HIV-1) by antisera raised against gp120 from the MN isolate of HIV-1.

Vaccines prepared from the envelope glycoprotein, gp120, of the common laboratory isolate of human immunodeficiency virus type 1 (HIV-1) (IIIB/LAV-1) elicit antibodies that neutralize the homologous virus but show little if any cross-neutralizing activity. This may be because the principal neutralizing determinant (PND) of gp120 is highly unusual in the IIIB/LAV-1 strain and is not representative of those found in the majority of field isolates. We have now examined the immunogenicity of recombinant gp120 prepared from the MN strain of HIV-1 (MN-rgp120), whose PND is thought to be representative of approximately 60% of the isolates in North America. Our results show that MN-rgp120 is a potent immunogen and elicits anti-gp120 titers comparable to those found in HIV-1-infected individuals. While both MN-rgp120 and IIIB-rgp120 induced antibodies able to block gp120 binding to CD4, strain-specific and type-common blocking antibodies were detected. Finally, antibodies to MN-rgp120 but not to IIIB-rgp120 were effective in neutralizing a broad range of laboratory and clinical isolates of HIV-1. These studies demonstrate that susceptibility or resistance to neutralization by antibodies to gp120 correlates with the PND sequence and suggest that the problem of antigenic variation may not be insurmountable in the development of an effective AIDS vaccine.     

The V3 region of the envelope glycoprotein of human immunodeficiency virus type 1 binds sulfated polysaccharides and CD4-derived synthetic peptides.

gp120 is the envelope glycoprotein found on the surface of human immunodeficiency virus type 1 (HIV-1), and it binds to human cell surface CD4 receptors to initiate the HIV-1 infection process. It is now well-established that synthetic peptides from the V3 region on gp120 elicit antibodies that block HIV-1 infection and HIV-1-mediated cell fusion. Here we show that synthetic peptides derived from similar V3 regions of several isolates of HIV-1 bind [3H]heparin, and we also demonstrate that [3H]heparin binds to recombinant gp120 IIIB. The binding could be blocked by unlabeled heparin, dextran sulfate, and by a highly anionic benzylated synthetic peptide derived from human CD4 (amino acids 81-92). The nonbenzylated peptides from the same region were considerably less active. Unlabeled heparin, dextran sulfate, and the CD4-derived peptides were able to compete with the binding of soluble gp120 to immobilized antibodies against fragments of the V3 from isolate IIIB, but they had no effect on the binding of gp120 to anti-peptide antibodies targeted against another unrelated region of gp120. Biotin conjugated to the benzylated CD4-peptide bound to gp120 and was blocked from this binding by anti-V3 antibodies. These results indicate that the three materials that have been demonstrated by others to block HIV-1 infection in vitro, sulfated polysaccharides, certain CD4-derived synthetic peptides, and anti-V3 antibodies, may be acting through a common mechanism that includes binding to the V3 region of gp120 on HIV-1.     

Mediation of human immunodeficiency virus type 1 binding by interaction of cell surface heparan sulfate proteoglycans with the V3 region of envelope gp120-gp41.

The mechanism of heparan sulfate (HS)-mediated human immunodeficiency virus type 1 (HIV-1) binding to and infection of T cells was investigated with a clone (H9h) of the T-cell line H9 selected on the basis of its high level of cell surface CD4 expression. Semiquantitative PCR analysis revealed that enzymatic removal of cell surface HS by heparitinase resulted in a reduction of the amount of HIV-1 DNA present in H9h cells 4 h after exposure to virus. Assays of the binding of recombinant envelope proteins to H9h cells demonstrated a structural requirement for an oligomeric form of gp120/gp41 for HS-dependent binding to the cell surface. The ability of the HIV-1 envelope to bind simultaneously to HS and CD4 was shown by immunoprecipitation of HS with either antienvelope or anti-CD4 antibodies from 35SO4(2-)-labeled H9h cells that had been incubated with soluble gp140. Soluble HS blocked the binding of monoclonal antibodies that recognize the V3 and C4 domains of the envelope protein to the surface of H9 cells chronically infected with HIV-1IIIB. The V3 domain was shown to be the major site of envelope-HS interaction by examining the effects of both antienvelope monoclonal antibodies and heparitinase on the binding of soluble gp140 to H9h cells.     

Enhancement of human immunodeficiency virus type 1 envelope-mediated fusion by a CD4-gp120 complex-specific monoclonal antibody.

The entry of human immunodeficiency virus type 1 (HIV-1) into cells is initiated by binding of the viral glycoprotein gp120-gp41 to its cellular receptor CD4. The gp120-CD4 complex formed at the cell surface undergoes conformational changes that may allow its association with an additional membrane component(s) and the eventual formation of the fusion complex. These conformational rearrangements are accompanied by immunological changes manifested by altered reactivity with monoclonal antibodies specific for the individual components and presentation of new epitopes unique to the postbinding complex. In order to analyze the structure and function of the gp120-CD4 complex, monoclonal antibodies were generated from splenocytes of BALB/c mice immunized with soluble CD4-gp120 (IIIB) molecules (J. M. Gershoni, G. Denisova, D. Raviv, N. I. Smorodinsky, and D. Buyaner, FASEB J. 7:1185-1187 1993). One of those monoclonal antibodies, CG10, was found to be strictly complex specific. Here we demonstrate that this monoclonal antibody can significantly enhance the fusion of CD4+ cells with effector cells expressing multiple HIV-1 envelopes. Both T-cell-line-tropic and macrophage-tropic envelope-mediated cell fusion were enhanced, albeit at different optimal doses. Furthermore, infection of HeLa CD4+ (MAGI) cells by HIV-1 LAI, ELI1, and ELI2 strains was increased two- to fourfold in the presence of CG10 monoclonal antibodies, suggesting an effect on viral entry. These findings indicate the existence of a novel, conserved CD4-gp120 intermediate structure that plays an important role in HIV-1 cell fusion.     

Variant-specific monoclonal and group-specific polyclonal human immunodeficiency virus type 1 neutralizing antibodies raised with synthetic peptides from the gp120 third variable domain.

The third variable (V3) domain of the human immunodeficiency virus type 1 (HIV-1) external membrane glycoprotein gp120 is of crucial importance in eliciting neutralizing antibodies in infected persons. Polyclonal (PAb) and monoclonal (MAb) antibodies directed against selected epitopes in the V3 domain are valuable tools for analysis of the involvement of such sequences in neutralization and for definition of the relation between amino acid variability and immunological cross-reactions. The aim of this study was to obtain such site-specific antibodies. By using synthetic peptides derived from the V3 domain, a group-specific neutralizing PAb, two high-affinity HIV-1 IIIB neutralizing MAb, and two nonneutralizing MAb were raised. A 15-amino-acid peptide overlapping the tip of the V3 domain of HIV-1 MN was used to produce a rabbit PAb (W0/07). This PAb inhibited syncytium formation induced by HIV-1 IIIB and four field isolates. A similar IIIB-derived peptide was used to generate two murine immunoglobulin G1 (IgG1) MAb (IIIB-V3-13 and IIIB-V3-34). Pepscan analysis mapped the binding site of IIIB-V3-34 to the sequence IRIQRGPGR. The Kds of IIIB-V3-13 and IIIB-V3-34 for gp120 were 6.8 x 10(-11) and 1.6 x 10(-10) M, respectively. These MAb neutralized IIIB but not MN and inhibited syncytium formation induced by IIIB. They are applicable in enzyme-linked immunosorbent assays, immunocytochemistry, and flow cytometry. A peptide covering the left base of the V3 domain was used to generate two murine IgG1 MAb (IIIB-V3-21 and IIIB-V3-26). The binding site of IIIB-V3-21 was mapped to the sequence INCTRPN. These MAb did not neutralize HIV-1 and did not inhibit syncytium formation. This study supports the notion that HIV-1 neutralizing antibodies suitable for multiassay performance can be obtained with synthetic peptides and that high-affinity MAb can be generated. Such site-specific antibodies are useful reagents in the analysis of HIV-1 neutralization. In addition, the cross-neutralization of different viral strains by PAb generated through single-peptide immunization is directly relevant to vaccine development.     

Discontinuous, conserved neutralization epitopes overlapping the CD4-binding region of human immunodeficiency virus type 1 gp120 envelope glycoprotein.

Monoclonal antibodies have been isolated from human immunodeficiency virus type 1 (HIV-1)-infected patients that recognize discontinuous epitopes on the gp120 envelope glycoprotein, that block gp120 interaction with the CD4 receptor, and that neutralize a variety of HIV-1 isolates. Using a panel of HIV-1 gp120 mutants, we identified amino acids important for precipitation of the gp120 glycoprotein by three different monoclonal antibodies with these properties. These amino acids are located within seven discontinuous, conserved regions of the gp120 glycoprotein, four of which overlap those regions previously shown to be important for CD4 recognition. The pattern of sensitivity to amino acid change in these seven regions differed for each antibody and also differed from that of the CD4 glycoprotein. These results indicate that the CD4 receptor and this group of broadly neutralizing antibodies recognize distinct but overlapping gp120 determinants.     

Humoral response to oligomeric human immunodeficiency virus type 1 envelope protein.

The humoral immune response to human immunodeficiency virus type 1 (HIV-1) is often studied by using monomeric or denatured envelope proteins (Env). However, native HIV-1 Env complexes that maintain quaternary structure elicit immune responses that are qualitatively distinct from those seen with monomeric or denatured Env. To more accurately assess the levels and types of antibodies elicited by HIV-1 infection, we developed an antigen capture enzyme-linked immunosorbent assay using a soluble, oligomeric form of HIV-1IIIB Env (gp140) that contains gp120 and the gp41 ectodomain. The gp140, captured by various monoclonal antibodies (MAbs), retained its native oligomeric structure: it bound CD4 and was recognized by MAbs to conformational epitopes in gp120 and gp41, including oligomer-specific epitopes in gp41. We compared the reactivities of clade B and clade E serum samples to captured Env preparations and found that while both reacted equally well with oligomeric gp140, clade B seras reacted more strongly with monomeric gp120 than did clade E samples. However, these differences were minimized when gp120 was captured by a V3 loop MAb, which may lead to increased exposure of the CD4 binding site. We also measured the ability of serum samples to block binding of MAbs to epitopes in gp120 and gp41. Clade B serum samples consistently blocked binding of oligomer-dependent MAbs to gp41 and, to a slightly lesser extent, MAbs to the CD4 binding site in gp120. Clade E serum samples showed equivalent or greater blocking of oligomer-dependent gp41 antibodies and considerably less blocking of CD4-binding-site MAbs. Finally, we found that < 5% of the antibodies in clade B sera bound to epitopes present only in monomeric gp120, 30% bound to epitopes present in both monomeric gp120 and oligomeric gp140, and 70% bound to epitopes present in oligomeric gp140, which includes gp41. Thus, captured oligomeric Env closely reflects the antigenic characteristics of Env protein on the surface of virions and infected cells, retains highly conserved epitopes that are recognized by antibodies raised against different clades, and makes it possible to detect a much greater fraction of total anti-HIV-1 Env activity in sera than does native monomeric gp120.     

Mutations in human immunodeficiency virus type 1 gp41 affect sensitivity to neutralization by gp120 antibodies.

Three closely related molecular human immunodeficiency virus type 1 (HIV-1) clones, with differential neutralization phenotypes, were generated by cloning of an NcoI-BamHI envelope (env) gene fragment (HXB2R nucleotide positions 5221 to 8021) into the full-length HXB2 molecular clone of HIV-1 IIIB. These env gene fragments, containing the complete gp120 coding region and a major part of gp41, were obtained from three different biological clones derived from a chimpanzee-passaged HIV-1 IIIB isolate. Two of the viruses thus obtained (4.4 and 5.1) were strongly resistant to neutralization by infection-induced chimpanzee and human polyclonal antibodies and by HIV-1 IIIB V3-specific monoclonal antibodies and weakly resistant to soluble CD4 and a CD4-binding-site-specific monoclonal antibody. The third virus (6.8) was sensitive to neutralization by the same reagents. The V3 coding sequence and the gp120 amino acid residues important for the discontinuous neutralization epitope overlapping the CD4-binding site were completely conserved among the clones. However, the neutralization-resistant clones 4.4 and 5.1 differed from neutralization-sensitive clone 6.8 by two mutations in gp41. Exchange experiments confirmed that the 3' end of clone 6.8 (nucleotides 6806 to 8021; amino acids 346 to 752) conferred a neutralization-sensitive phenotype to both of the neutralization-resistant clones 4.4 and 5.1. From our study, we conclude that mutations in the extracellular portion of gp41 may affect neutralization sensitivity to gp120 antibodies.     

Dextran sulfate blocks antibody binding to the principal neutralizing domain of human immunodeficiency virus type 1 without interfering with gp120-CD4 interactions.

The mechanism of the antiviral activity of sulfated polysaccharides on human immunodeficiency virus type 1 (HIV-1) was investigated by determining the effect of dextran sulfate on the binding of CD4 and several anti-gp120 monoclonal antibodies to both recombinant and cell surface gp120. Dextran sulfate did not interfere with the binding of sCD4 to rgp120 on enzyme-linked immunosorbent assay (ELISA) plates or in solution and did not block sCD4 binding to HIV-1-infected cells expressing gp120 on the cell surface. Dextran sulfate had minimal effects on rgp120 binding to CD4+ cells at concentrations which effectively prevent HIV replication. In contrast, it potently inhibited the binding of both rgp120 and cell surface gp120 to several monoclonal antibodies directed against the principal neutralizing domain of gp120 (V3). In an ELISA format, dextran sulfate enhanced the binding of monoclonal antibodies against amino-terminal regions of gp120 and had no effect on antibodies directed to other regions of gp120, including the carboxy terminus. The inhibitory effects of polyanionic polysaccharides on viral binding, viral replication, and formation of syncytia therefore appear mediated by interactions with positively charged amino acids concentrated in the V3 region. This high local positive charge density, unique to the V3 loop, leads us to propose that this property is critical to the function of the V3 region in mediating envelope binding and subsequent fusion between viral and cell membranes. The specific interaction of dextran sulfate with this domain suggests that structurally related molecules on the cell surface, such as heparan sulfate, may be additional targets for HIV binding and infection.     

Neutralizing antibodies to an immunodominant envelope sequence do not prevent gp120 binding to CD4.

Animals immunized with the human immunodeficiency virus type 1 gp160 glycoprotein or certain recombinant envelope components develop potent virus-neutralizing activity. This activity is principally due to antibodies directed toward a hypervariable region of gp120 between cysteine residues 302 and 337 and is virus isolate specific. These antisera, as well as two neutralizing monoclonal antibodies directed against the same hypervariable sequence, do not appreciably block gp120 from binding CD4. In contrast, serum samples from infected humans possess high titers of antibodies that block gp120-CD4 binding; these titers approximately correlate with the serum neutralization titers. Our results suggest that there are at least two targets on the envelope glycoprotein for virus neutralization. The target responsible for the broader neutralizing activity of human serum may be a conserved region of gp120 involved in CD4 binding. The antibodies directed at the hypervariable region of the envelope inhibit a different step in virus infection which is subsequent to receptor binding. The extent to which these two different epitopes of gp120 may be involved in protection against human immunodeficiency virus infection is discussed.     

Differential level in co-down-modulation of CD4 and CXCR4 primed by HIV-1 gp120 in response to phorbol ester, PMA, among HIV-1 isolates

HIV-1 enters cells through interacting with cell surface molecules such as CD4 and chemokine receptors. We generated recombinant soluble gp120s derived from T-cell line-tropic (T-tropic) and macrophage-tropic (M-tropic) HIV-1 strains using a baculovirus expression system and investigated the association of CD4-gp120 complex with the chemokine receptor and/or other surface molecule(s). For monitoring the co-down-modulations of the CD4-gp120 complex, a cytoplasmic domain deletion mutant (tailless CD4), which is not capable of undergoing down-modulation by itself in response to phorbol ester PMA, was used. Our studies revealed both cell-type and HIV-1 strain-specific differences. We found that T-tropic gp120s were capable of priming co-down-modulation with tailless CD4 by interacting with CXCR4, whereas M-tropic SF162 gp120 could not after PMA treatment even in the presence of CCR5. Among the T-tropic HIV-1 envelopes, IIIB gp120 was the most potent. Furthermore, the ability of gp120 to prime the PMA induced co-down-modulation of tailless CD4 appeared to be dependent on the concentration of the principal coreceptor CXCR4. Nevertheless, the observation that IIIB gp120 strongly primed tailless CD4 co-down-modulation on human osteosarcoma HOS cells that express undetectable levels of surface CXCR4 raised the possibility that membrane component(s) other than those recently identified can be involved in down-modulation of the CD4/gp120 complexes.     

The HIV-1 gp120 inhibits the binding of adenosine deaminase to CD26 by a mechanism modulated by CD4 and CXCR4 expression

HIV-1 external envelope glycoprotein gp120 inhibits adenosine deaminase (ADA) binding to its cell surface receptor in lymphocytes, CD26, by a mechanism that does not require the gp120-CD4 interaction. To further characterize this mechanism, we studied ADA binding to murine clones stably expressing human CD26 and/or human CD4, and transiently expressing human CXCR4. In this heterologous model, we show that both recombinant gp120 and viral particles from the X4 HIV-1 isolate IIIB inhibited the binding of ADA to wild-type or catalytically inactive forms of CD26. In cells lacking human CXCR4 expression, this gp120-mediated inhibition of ADA binding to human CD26 was completely dependent on the expression of human CD4. In contrast, when cells were transfected with human CXCR4 the inhibitory effect of gp120 was significantly enhanced and was not blocked by anti-CD4 antibodies. These data suggest that the interaction of gp120 with CD4 or CXCR4 is required for efficient inhibition of ADA binding to CD26, although in the presence of CXCR4 the interaction of gp120 with CD4 may be dispensable.     

Induced fit in HIV-neutralizing antibody complexes: evidence for alternative conformations of the gp120 V3 loop and the molecular basis for broad neutralization

Human monoclonal antibody (mAb) 447-52D neutralizes a broad spectrum of HIV-1 isolates, whereas murine mAb 0.5beta, raised against gp120 of the X4 isolate HIV-1(IIIB), neutralizes this strain specifically. Two distinct gp120 V3 peptides, V3(MN) and V3(IIIB), adopt alternative beta-hairpin conformations when bound to 447-52D and 0.5beta, respectively, suggesting that the alternative conformations of this loop play a key role in determining the coreceptor specificity of HIV-1. To test this hypothesis and to better understand the molecular basis underlying an antibody's breadth of neutralization, the solution structure of the V3(IIIB) peptide bound to 447-52D was determined by NMR. V3(IIIB) and V3(MN) peptides bound to 447-52D exhibited the same N-terminal strand conformation, while the V3(IIIB) peptide revealed alternative N-terminal conformations when bound to 447-52D and 0.5beta. Comparison of the three known V3 structures leads to a model in which a 180 degrees change in the orientation of the side chains and the resulting one-residue shift in hydrogen bonding patterns in the N-terminal strand of the beta-hairpins markedly alter the topology of the surface that interacts with antibodies and that can potentially interact with the HIV-1 coreceptors. Predominant interactions of 447-52D with three conserved residues of the N-terminal side of the V3 loop, K312, I314, and I316, can account for its broad cross reactivity, whereas the predominant interactions of 0.5beta with variable residues underlie its strain specificity.     

Alternative conformations of HIV-1 V3 loops mimic beta hairpins in chemokines,suggesting a mechanism for coreceptor selectivity

The V3 loop of the HIV-1 envelope glycoprotein gp120 is involved in binding to the CCR5 and CXCR4 coreceptors. The structure of an HIV-1(MN) V3 peptide bound to the Fv of the broadly neutralizing human monoclonal antibody 447-52D was solved by NMR and found to be a beta hairpin. This structure of V3(MN) was found to have conformation and sequence similarities to beta hairpins in CD8 and CCR5 ligands MIP-1alpha, MIP-1beta, and RANTES and differed from the beta hairpin of a V3(IIIB) peptide bound to the strain-specific murine anti-gp120(IIIB) antibody 0.5beta. In contrast to the structure of the bound V3(MN) peptide, the V3(IIIB) peptide resembles a beta hairpin in SDF-1, a CXCR4 ligand. These data suggest that the 447-52D-bound V3(MN) and the 0.5beta-bound V3(IIIB) structures represent alternative V3 conformations responsible for selective interactions with CCR5 and CXCR4, respectively.     

Fusion proteins of HIV-1 envelope glycoprotein gp120 with CD4-induced antibodies showed enhanced binding to CD4 and CD4 binding site antibodies

Development of successful AIDS vaccine immunogens continues to be a major challenge. One of the mechanisms by which HIV-1 evades antibody-mediated neutralizing responses is the remarkable conformational flexibility of its envelope glycoprotein (Env) gp120. Some recombinant gp120s do not preserve their conformations on gp140s and functional viral spikes, and exhibit decreased recognition by CD4 and neutralizing antibodies. CD4 binding induces conformational changes in gp120 leading to exposure of the coreceptor-binding site (CoRbs). In this study, we test our hypothesis that CD4-induced (CD4i) antibodies, which target the CoRbs, could also induce conformational changes in gp120 leading to better exposed conserved neutralizing antibody epitopes including the CD4-binding site (CD4bs). We found that a mixture of CD4i antibodies with gp120 only weakly enhanced CD4 binding. However, such interactions in single-chain fusion proteins resulted in gp120 conformations which bound to CD4 and CD4bs antibodies better than the original or mutagenically stabilized gp120s. Moreover, the two molecules in the fusion proteins synergized with each other in neutralizing HIV-1. Therefore, fusion proteins of gp120 with CD4i antibodies could have potential as components of HIV-1 vaccines and inhibitors of HIV-1 entry, and could be used as reagents to explore the conformational flexibility of gp120 and mechanisms of entry and immune evasion.