Neutralizing antibodies from the sera of human immunodeficiency virus type 1-infected individuals bind to monomeric gp120 and oligomeric gp140

Antibodies that neutralize primary isolates of human immunodeficiency virus type 1 (HIV-1) appear during HIV-1 infection but are difficult to elicit by immunization with current vaccine products comprised of monomeric forms of HIV-1 envelope glycoprotein gp120. The limited neutralizing antibody response generated by gp120 vaccine products could be due to the absence or inaccessibility of the relevant epitopes. To determine whether neutralizing antibodies from HIV-1-infected patients bind to epitopes accessible on monomeric gp120 and/or oligomeric gp140 (ogp140), purified total immunoglobulin from the sera of two HIV-1-infected patients as well as pooled HIV immune globulin were selectively depleted of antibodies which bound to immobilized gp120 or ogp140. After passage of each immunoglobulin preparation through the respective columns, antibody titers against gp120 and ogp140 were specifically reduced at least 128-fold. The gp120- and gp140-depleted antibody fraction from each serum displayed reduced neutralization activity against three primary and two T-cell line-adapted (TCLA) HIV-1 isolates. Significant residual neutralizing activity, however, persisted in the depleted sera, indicating additional neutralizing antibody specificities. gp120- and ogp140-specific antibodies eluted from each column neutralized both primary and TCLA viruses. These data demonstrate the presence and accessibility of epitopes on both monomeric gp120 and ogp140 that are specific for antibodies that are capable of neutralizing primary isolates of HIV-1. Thus, the difficulties associated with eliciting neutralizing antibodies by using current monomeric gp120 subunit vaccines may be related less to improper protein structure and more to ineffective immunogen formulation and/or presentation.     

Sera from HIV-1 infected individuals in all stages of disease preferentially recognize the V3 loop of the prototypic macrophage-tropic glycoprotein gp120 ADA

The outer membrane glycoprotein gp120 and the transmembrane glycoprotein gp41 are predominant targets of the humoral immune response to infection by human immunodeficiency virus type 1. The third hypervariable region (V3 loop) is the principal neutralizing domain and is the primary target of neutralizing antibodies directed against the envelope proteins of HIV-1. The V3 loop is also the major determinant for HIV-1 cell-specific tropism. To further characterize the humoral immune response directed against the gp120 envelope proteins, we expressed two prototypic gp120 envelope proteins (LAI/HXB2 and ADA) and chimeric gp120 envelope proteins in stable transfected Drosophila melanogaster Schneider 2 cells. Sera from four infected adults over the course of infection [McNearney et al. (1992) Proc. natn. Acad. Sci. U.S.A. 89, p. 10,242] were assayed for reactivity with the respective envelope proteins. Sera obtained at early stages preferentially recognized the gp120 envelope protein ADA, whereas in later stages of infection the sera showed diminished reactivity with both gp120 LAI/HXB2 and gp120 ADA. Chimeric envelope proteins revealed that the humoral response was directed primarily against the V3 loop of gp120 ADA. Furthermore, 22 sera from HIV-1 infected individuals in different stages of the disease were tested. Reactivity of sera with the gp120 envelope protein ADA was seven-fold higher than with the gp120 envelope protein LAI/HXB2. Our results suggest that the humoral immune response is preferentially elicited against the V3 loop of the prototypic macrophage-tropic gp120 envelope protein ADA.     

The antigenic structure of the HIV gp120 envelope glycoprotein

The human immunodeficiency virus HIV-1 establishes persistent infections in humans which lead to acquired immunodeficiency syndrome (AIDS). The HIV-1 envelope glycoproteins, gp120 and gp41, are assembled into a trimeric complex that mediates virus entry into target cells. HIV-1 entry depends on the sequential interaction of the gp120 exterior envelope glycoprotein with the receptors on the cell, CD4 and members of the chemokine receptor family. The gp120 glycoprotein, which can be shed from the envelope complex, elicits both virus-neutralizing and non-neutralizing antibodies during natural infection. Antibodies that lack neutralizing activity are often directed against the gp120 regions that are occluded on the assembled trimer and which are exposed only upon shedding. Neutralizing antibodies, by contrast, must access the functional envelope glycoprotein complex and typically recognize conserved or variable epitopes near the receptor-binding regions. Here we describe the spatial organization of conserved neutralization epitopes on gp120, using epitope maps in conjunction with the X-ray crystal structure of a ternary complex that includes a gp120 core, CD4 and a neutralizing antibody. A large fraction of the predicted accessible surface of gp120 in the trimer is composed of variable, heavily glycosylated core and loop structures that surround the receptor-binding regions. Understanding the structural basis for the ability of HIV-1 to evade the humoral immune response should assist in the design of a vaccine.     

Characterization of simian-human immunodeficiency virus envelope glycoprotein epitopes recognized by neutralizing antibodies from infected monkeys

We characterized human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein epitopes recognized by neutralizing antibodies from monkeys recently infected by molecularly cloned simian-human immunodeficiency virus (SHIV) variants. The early neutralizing antibody response in each infected animal was directed mainly against a single epitope. This primary neutralizing epitope, however, differed among individual monkeys infected by identical viruses. Two such neutralization epitopes were determined by sequences in the V2 and V3 loops of the gp120 envelope glycoprotein, while a third neutralization epitope, apparently discontinuous, was determined by both V2 and V3 sequences. These results indicate that the early neutralizing antibody response in SHIV-infected monkeys is monospecific and directed against epitopes composed of the gp120 V2 and V3 variable loops.     

Immunogenicity of synthetic HIV-1 gp120 V3-loop peptide-conjugate immunogens

A successful prophylactic human immunodeficiency virus type 1 (HIV-1) vaccine must elicit an immune response that will prevent establishment of the persistent viral infection. The only response shown to be effective in this regard is virus-neutralizing antibody directed against the viral gp120 hypervariable V3-loop region. Conjugate immunogens, containing cyclic peptides representing the V3 determinant covalently bound to a carrier protein, were capable of eliciting virus-neutralizing antibodies. The consistency of the response was related to peptide size. The smaller cyclic peptides, expressing relatively conserved sequences from the V3-loop apex, were poor inducers of neutralizing activity. In contrast, the largest cyclic peptides mediated neutralizing responses that were similar to those observed and previously reported for intact gp120 immunogens. A cyclic synthetic peptide expressing most of the prototypic HIV-1 MN variant V3 determinant warrants further study as a potentially effective vaccine immunogen.     

Comparative analysis of humoral immune responses to HIV type 1 envelope glycoproteins in mice immunized with a DNA vaccine, recombinant Semliki Forest virus RNA, or recombinant Semliki Forest virus particles

The Semliki Forest virus (SFV) system seems to be a useful new approach for generating effective immune responses against HIV-1 in animal models. We evaluated this system by comparing the humoral immune responses raised in mice immunized against the HIV-1 envelope with the SFV system, a DNA vaccine, and a recombinant Env glycoprotein. gp160 ELISA antibody titers (204,800) were highest in the sera from mice immunized with recombinant Semliki Forest virus particles. These sera contained antibodies to the CD4-binding site and recognized linear epitopes on gp120 and gp41 that were also recognized by a pool of sera from HIV1-infected individuals. This demonstrates that the HIV-1 envelope produced in vivo by the SFV system does not fold aberrantly. A low level of neutralizing antibodies against the HIV-1LAI strain was also detected in the serum of one mouse immunized with recombinant SFV particles, suggesting that booster injections should be given to achieve a more effective immune response. SFV recombinant particles induced the strongest humoral responses to the HIV-1 envelope of all the potential HIV env vaccines tested.     

Preferential antibody recognition of structurally distinct HIV-1 gp120 molecules

We have developed an assay, using a biosensor matrix and surface plasmon resonance, that rapidly and reproducibly measures antibody reactivity to human immunodeficiency virus type 1 (HIV-1) gp120 in various structural conformations. In particular, antibodies displaying preferential reactivity to a CD4-binding competent ("native," rgp120) or CD4-binding incompetent ("reduced," rcmgp120) monomeric gp120 molecule were distinguished. This technique has advantages over conventional enzyme-linked immunosorbent assay (ELISA) methodology in which it is difficult to control the concentration of protein adsorbed to the ELISA wells and a significant disruption of protein structure occurs on adsorption. A population of gp120 molecules that lacked CD4 receptor binding capacity and bound antibodies specific for reduced gp120 was found in several native gp120 preparations. The relative amount of this CD4-binding incompetent population varied among the various preparations studied. This presence of CD4-binding incompetent molecules within various native recombinant gp120 preparations may have implications for HIV-1 envelope vaccine development. By measuring antibody-binding ratios, several monoclonal antibodies were identified, which, although elicited by immunization with various native gp120 preparations, bound specifically to reduced gp120. The ability to screen antibody specificity against HIV-1 envelope proteins with different conformations will assist in determining the quality of antibodies induced by various HIV-1 envelope vaccine candidates.     

Replication and neutralization of human immunodeficiency virus type 1 lacking the V1 and V2 variable loops of the gp120 envelope glycoprotein

A human immunodeficiency virus type 1 (HIV-1) mutant lacking the V1 and V2 variable loops in the gp120 exterior envelope glycoprotein replicated in Jurkat lymphocytes with only modest delays compared with the wild-type virus. Revertants that replicated with wild-type efficiency rapidly emerged and contained only a few amino acid changes in the envelope glycoproteins compared with the parent virus. Both the parent and revertant viruses exhibited increased sensitivity to neutralization by antibodies directed against the V3 loop or a CD4-induced epitope on gp120 but not by soluble CD4 or an antibody against the CD4 binding site. This result demonstrates the role of the gp120 V1 and V2 loops in protecting HIV-1 from some subsets of neutralizing antibodies.     

Determinants of human immunodeficiency virus type 1 envelope glycoprotein activation by soluble CD4 and monoclonal antibodies

Infection by some human immunodeficiency virus type 1 (HIV-1) isolates is enhanced by the binding of subneutralizing concentrations of soluble receptor, soluble CD4 (sCD4), or monoclonal antibodies directed against the viral envelope glycoproteins. In this work, we studied the abilities of different antibodies to mediate activation of the envelope glycoproteins of a primary HIV-1 isolate, YU2, and identified the regions of gp120 envelope glycoprotein contributing to activation. Binding of antibodies to a variety of epitopes on gp120, including the CD4 binding site, the third variable (V3) loop, and CD4-induced epitopes, enhanced the entry of viruses containing YU2 envelope glycoproteins. Fab fragments of antibodies directed against either the CD4 binding site or V3 loop also activated YU2 virus infection. The activation phenotype was conferred on the envelope glycoproteins of a laboratory-adapted HIV-1 isolate (HXBc2) by replacing the gp120 V3 loop or V1/V2 and V3 loops with those of the YU2 virus. Infection by the YU2 virus in the presence of activating antibodies remained inhibitable by macrophage inhibitory protein 1beta, indicating dependence on the CCR5 coreceptor on the target cells. Thus, antibody enhancement of YU2 entry involves neither Fc receptor binding nor envelope glycoprotein cross-linking, is determined by the same variable loops that dictate enhancement by sCD4, and probably proceeds by a process fundamentally similar to the receptor-activated virus entry pathway.     

Monoclonal antibodies raised against covalently crosslinked complexes of human immunodeficiency virus type 1 gp120 and CD4 receptor identify a novel complex-dependent epitope on gp 120

The binding of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein, gp120, to its cell surface receptor, CD4, represents a molecular interaction involving distinct alterations in protein structure. Consequently, the pattern of epitopes presented on the gp120-CD4 complex should differ from those on free gp120. To investigate this concept, mice were immunized with covalently crosslinked complexes of viral HIV-1IIIBgp120 and soluble CD4. Two monoclonal antibodies (MoAbs) obtained from the immunized mice exhibited a novel epitope specificity. The MoAbs were marginally reactive with HIV-1IIIBgp120, highly reactive with gp120-CD4 complexes, and unreactive with soluble CD4. The same pattern of reactivity was seen in solid-phase assays using HIV-1(451)gp120. A similar specificity for complexes was evident in flow cytometry experiments, in which MoAb reactivity was dependent upon the attachment of gp120 to CD4-positive cells. In addition, MoAb reactivity was detected upon the interaction of CD4 receptors with purified HIV-1IIIB virions. Notably, seroantibodies from HIV-positive individuals competed for MoAb binding, indicating that the epitope is immunogenic in humans. The results demonstrated that crosslinked gp120-CD4 complexes elicit antibodies to cryptic gp120 epitopes that are exposed during infection in response to receptor binding. These findings may have important implications for the consideration of HIV envelope-receptor complexes as targets for virus neutralization.     

Antigenic implications of human immunodeficiency virus type 1 envelope quaternary structure: oligomer-specific and -sensitive monoclonal antibodies

A majority of monoclonal antibodies (mAbs) raised against soluble oligomeric human immunodeficiency virus type 1 isolate IIIB (HIV-1IIIB) envelope (env) glycoprotein reacted with conformational epitopes within the gp120 or gp41 subunits. Of 35 mAbs directed against gp41, 21 preferentially reacted with oligomeric env. A subset of these mAbs reacted only with env oligomers (oligomer-specific mAbs). In contrast, only 1 of 27 mAbs directed against the gp120 subunit reacted more strongly with env oligomers than with monomers, and none were oligomer-specific. However, 50% of anti-gp120 mAbs preferentially recognized monomeric env, suggesting that some epitopes in gp120 are partially masked or altered by intersubunit contacts in the native env oligomer. Two mAbs to oligomer-dependent epitopes in gp41 neutralized HIV-1IIIB and HIV-1SF2, and binding of these mAbs to env was blocked by preincubation with HIV-1-positive human serum. Thus, immunization with soluble, oligomeric env elicits antibodies to conserved, conformational epitopes including a newly defined class of neutralizing antibodies that bind to oligomer-specific epitopes in gp41, and may also minimize the production of antibodies that preferentially react with monomeric env protein.     

Sonolucent peripancreatic masses: differential diagnosis and related imaging

The third variable region (V3) of the HIV-1 gp120 envelope molecule appears to represent a target for naturally occurring neutralizing antibodies in HIV-1-infected individuals. In this report, we examined the extent of antibody cross-reactivity to a panel of V3-based synthetic peptides in six inbred strains of mice following repeated immunization with a baculovirus-derived recombinant gp160 (rgp160) preparation formulated with alum. The amino acid sequence of the rgp160 used in these immunizations was based upon the HIV-1 IIIB (LAI) isolate. Following five injections with rgp160, all six strains developed antibodies to the homologous IIIB-based V3 peptides, designated 304-321 and RP135. However, antibody cross-reactivity to the other nonhomologous V3 peptides was either undetectable or limited among the strains of mice examined. No in vitro neutralizing activity against HIV-1 was observed in sera from any of the six inbred strains of mice that were examined. These results suggest that repeated immunization of mouse strains with a rgp160/alum formulation leads to nonneutralizing antibodies directed against the V3 region which remain predominantly type specific.     

Recombinant human immunodeficiency Pr55gag virus-like particles presenting chimeric envelope glycoproteins induce cytotoxic T-cells and neutralizing antibodies

Very recently, we demonstrated that the replacement of the human immunodeficiency virus type-1 (HIV-1) gp41 transmembrane protein by an Epstein-Barr virus gp220/350-derived membrane anchor resulted in the incorporation of chimeric envelope (Env) oligomers into Pr55gag virus-like particles (VLPs), exceeding that of wild-type gp160 by a factor of 10. In this study, we examined the immunostimulatory properties of Pr55gag VLPs to both (i) chimeric HIV-1 gp120 external envelope proteins and (ii) full-length gp160 presented on the outer surface of the particles. Immunization studies carried out with VLPs presenting different derivatives of the chimeric and wild-type Env proteins elicited a consistent anti-Pr55gag as well as anti-Env antibody response in complete absence of additional adjuvants. In both cases, the immune sera exhibited an in vitro neutralizing activity against homologous HIV-1 infection in MT4 cells. Noteworthy, these VLPs were also capable of inducing a strong CD8+ cytotoxic T-cell (CTL) response in immunized BALB/c mice that was directed toward a known CTL epitope in the third variable domain V3 of the gp120 external glycoprotein. However, the induction of V3-loop-specific CTLs critically depended on the amounts of Env proteins that were presented by the Pr55gag VLPs. Moreover, the CD8+ CTL response was not significantly altered by adsorbing the VLPs to alum or by repeated booster immunizations. These results illustrate that Pr55gag VLPs provide a safe and effective means of enhancing neutralizing humoral responses to particle-entrapped gp120 proteins and are also capable of delivering these proteins to the MHC class I antigen processing and presentation pathway. Therefore, antigenically expanded Pr55gag VLPs represent an attractive approach in the design of vaccines for which specific stimulation of neutralizing antibodies and cytotoxic effector functions to complex glycoproteins is desired.     

Specificity of antibodies produced against native or desialylated human immunodeficiency virus type 1 recombinant gp160

In a previous report we have shown that, in contrast to antibodies produced against native or fully deglycosylated human immunodeficiency virus type 1 (HIV-1) gp160 in rabbits, antibodies raised against desialylated HIV-1 gp160 also recognize gp140 from HIV-2 at high titers. Here, we characterize the fine specificity of these cross-reactive antibodies. Inhibition assays with a panel of synthetic peptides as competitors showed that cross-reactivity to gp140 was due to antibodies that were specific for the region encompassing HIV-1 gp41 immunodominant epitope, mimicked by peptide P39 (residues 583 to 609), the latter being able to totally inhibit the formation of complexes between radiolabeled HIV-2 gp140 and antibodies elicited by desialylated HIV-1 gp160. In addition, anti-desialylated gp160 antibodies retained on a P39 affinity column still bound HIV-2 gp140. Fine mapping has enabled us to localize the cross-reactive epitope within the N-terminal extremity of the gp41 immunodominant region. Interestingly, this cross-reactive antibody population did not recognize glycosylated or totally deglycosylated simian immunodeficiency virus gp140 despite an amino acid homology with HIV-1 within this region that is comparable to that of HIV-2. This cross-reactivity between HIV-1 and HIV-2 did not correlate with cross-neutralization. These results illustrate the influence of carbohydrate moieties on the specificity of the antibodies produced and clearly indicate that such procedures may be an efficient way to raise specific immune responses that are not type specific. Moreover, this cross-reactivity might explain the double-positive reactivity observed, in some human sera, against both HIV-1 and HIV-2 envelope antigens.     

Induction of immune responses to HIV-1 by canarypox virus (ALVAC) HIV-1 and gp120 SF-2 recombinant vaccines in uninfected volunteers. NIAID AIDS Vaccine Evaluation Group

OBJECTIVE: To determine the ability of live attenuated canarypox virus expressing HIV antigens to induce CD8+ cytotoxic T-cell responses and to prime for neutralizing antibody responses to boosting with purified recombinant gp120 subunit vaccine. DESIGN: A prospective, double-blind, randomized, immunogenicity and safety study was conducted in healthy adults at low risk for acquiring HIV infection and who were seronegative for HIV. METHODS: CD8+ cytotoxic T-cells directed against Env or Gag expressing target cells were measured after live recombinant canarypox-HIV-1 vaccine priming (vaccine given at days 0, 7, 14 and 21). Neutralizing antibodies were measured after subunit boosting (vaccine given at days 28 and 84). RESULTS: CD8+ CTL were induced in 64% of volunteers by the live recombinant canarypox-HIV-1 vaccine. All volunteers who received two doses of subunit vaccine after live recombinant canarypox priming developed neutralizing antibodies directed against laboratory strains of HIV-1 and seven out of eight volunteers tested developed neutralizing antibodies to the primary isolate, BZ167, but to none of eight other primary isolates. Unprimed controls had low or absent neutralizing antibodies after two doses of subunit vaccine. CONCLUSIONS: The live canarypox vector was safe, stimulated cytotoxic T-cells and primed for a vigorous neutralizing antibody response upon boosting with subunit gp120 vaccine. This vaccine combination should be evaluated further for inducing protection against HIV infection.     

Human studies in the development of human immunodeficiency virus vaccines

The safety and immunogenicity of candidate human immunodeficiency virus type 1 (HIV-1) vaccines have been studied in > 1500 healthy, seronegative (HIV-1-uninfected) subjects. HIV-1 envelope proteins, gp160 and gp120, have been the most extensively investigated. A live virus vector construct, vaccinia with insertion of the HIV-1 env gene, has also been studied. HIV-1 candidate vaccines have been well tolerated, with no acute or longer-term serious toxicity. Intramuscular multidose gp120 vaccines induce neutralizing antibodies, lymphoproliferative responses, and anti-HIV-1 CD4 cytotoxic T cell (CTL) activity. Immunization with the vaccinia-env construct, followed by a boost with an envelope protein, also induces neutralizing antibodies, and anti-HIV-1 CTL activity (CD8, major histocompatibility complex class I-restricted) has been observed. To date, serum from vaccinees can neutralize laboratory-adapted HIV-1 strains in vitro but not primary isolates; the significance of this observation is unknown. Additional approaches to vaccination against HIV-1 are in development.     

Contribution of virion ICAM-1 to human immunodeficiency virus infectivity and sensitivity to neutralization

Incorporation of the intercellular adhesion molecule ICAM-1 into human immunodeficiency virus type 1 (HIV-1) particles increased virus infectivity on peripheral blood mononuclear cells (PBMCs) by two- to sevenfold. The degree of ICAM-1-mediated enhancement was greater for viruses bearing envelope glycoproteins derived from primary HIV-1 isolates than for those bearing envelope glycoproteins from laboratory-adapted strains. Treatment of target PBMCs with an antibody against LFA-1, a principal ICAM-1 receptor, was able to nullify the ICAM-1-mediated enhancement. The incorporation of ICAM-1 rendered HIV-1 virions less susceptible to neutralization by a monoclonal antibody directed against the viral envelope glycoproteins. Surprisingly, an antibody against ICAM-1 completely neutralized infection by ICAM-1-containing viruses, reducing the efficiency of virus entry by almost 100-fold. Thus, HIV-1 neutralization by an ICAM-1-directed antibody involves more than an inhibition of the contribution of ICAM-1 to virus entry.     

Multiple effects of CD4 CDR3-related peptide derivatives showing anti-HIV-1 activity on HIV-1 gp120 functions

The interaction of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 with CD4 CDR3-related peptide derivatives showing anti-HIV-1 activity has been studied. Conformational changes in gp120, which could affect its interaction with CD4 and its shedding from virions, were detected by fluorescence spectrum analysis of tryptophan residues after addition of peptide representative of the CD4 CDR3-related region, but not the CD4 CDR2-related region. Interestingly, the addition of scrambled peptide, S1 (with altered amino acid sequence compared with the native CDR3-related peptide but unaltered overall composition), which we recently showed to have stronger anti-HIV-1 activity than the original CDR3-related peptide, had no effects on the conformational change in gp120 or on its interaction with CD4 and its shedding from HIV-1 virions. However, all of the CDR3-related peptides, including S1, showed blocking effects on the binding of antibodies against gp120 V3 loop and C-terminus regions. Thus, we concluded that there were at least two separable activities of the CDR3-related peptides in anti-HIV-1 activity, i.e. induction of conformational changes in gp120, which could affect its binding to CD4 and to gp41 (as observed in native CDR3-related peptides), and inactivation of V3 loop and C-terminus regions in gp120 (as observed in all of the CDR3-related peptides, including S1).     

CD4-Induced conformational changes in the human immunodeficiency virus type 1 gp120 glycoprotein: consequences for virus entry and neutralization

Human immunodeficiency virus type 1 (HIV-1) entry into target cells involves sequential binding of the gp120 exterior envelope glycoprotein to CD4 and to specific chemokine receptors. Soluble CD4 (sCD4) is thought to mimic membrane-anchored CD4, and its binding alters the conformation of the HIV-1 envelope glycoproteins. Two cross-competing monoclonal antibodies, 17b and CG10, that recognize CD4-inducible gp120 epitopes and that block gp120-chemokine receptor binding were used to investigate the nature and functional significance of gp120 conformational changes initiated by CD4 binding. Envelope glycoproteins derived from both T-cell line-adapted and primary HIV-1 isolates exhibited increased binding of the 17b antibody in the presence of sCD4. CD4-induced exposure of the 17b epitope on the oligomeric envelope glycoprotein complex occurred over a wide range of temperatures and involved movement of the gp120 V1/V2 variable loops. Amino acid changes that reduced the efficiency of 17b epitope exposure following CD4 binding invariably compromised the ability of the HIV-1 envelope glycoproteins to form syncytia or to support virus entry. Comparison of the CD4 dependence and neutralization efficiencies of the 17b and CG10 antibodies suggested that the epitopes for these antibodies are minimally accessible following attachment of gp120 to cell surface CD4. These results underscore the functional importance of these CD4-induced changes in gp120 conformation and illustrate viral strategies for sequestering chemokine receptor-binding regions from the humoral immune response.     

The humoral immune response to human T-cell lymphotropic virus type 1 envelope glycoprotein gp46 is directed primarily against conformational epitopes

Individuals infected with human T-cell lymphotropic virus type 1 (HTLV-1) develop a robust immune response to the surface envelope glycoprotein gp46 that is partially protective. The relative contribution of antibodies to conformation-dependent epitopes, including those mediating virus neutralization as part of the humoral immune response, is not well defined. We assess in this report the relationship between defined linear and conformational epitopes and the antibodies elicited to these domains. First, five monoclonal antibodies to linear epitopes within gp46 were evaluated for their ability to abrogate binding of three human monoclonal antibodies that inhibit HTLV-1-mediated syncytia formation and recognize conformational epitopes. Binding of antibodies to conformational epitopes was unaffected by antibodies to linear epitopes throughout the carboxy-terminal half and central domain of HTLV-1 gp46. Second, an enzyme-linked immunoadsorbent assay was developed and used to measure serum antibodies to native and denatured gp46 from HTLV-1-infected individuals. In sera from infected individuals, reactivity to denatured gp46 had an average of 15% of the reactivity observed to native gp46. Third, serum antibodies from 24 of 25 of HTLV-1-infected individuals inhibited binding of a neutralizing human monoclonal antibody, PRH-7A, to a conformational epitope on gp46 that is common to HTLV-1 and -2. Thus, antibodies to conformational epitopes comprise the majority of the immune response to HTLV-1 gp46, and the epitopes recognized by these antibodies do not appear to involve sequences in previously described immunodominant linear epitopes.