Monoclonal antibodies have been isolated from human immunodeficiency virus type 1 (HIV-1)-infected patients that recognize discontinuous epitopes on the gpl20 envelope glycoprotein, that block gpl20 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 gpl20 glycoprotein by three different monoclonal antibodies with these properties. These amino acids are located within seven discontinuous, conserved regions * Corresponding author.

We characterized the structural forms of the human immunodeficiency virus env-encoded proteins with a panel of monoclonal and polyclonal antibodies. Western blot (immunoblot) assays with antibodies specific for gp4l invariably recognized a major component of 160 kilodaltons and a less intense component of 120 kilodaltons in viral lysates. We demonstrated that these species are noncovalently associated tetramers and trimers of gp4l which represent the native form of this protein in virions. These complexes were stable when boiled in the presence of low concentrations of sodium dodecyl sulfate but were dissociated to gp4l monomers at high sodium dodecyl sulfate concentrations. Moreover, two human monoclonal antibodies preferentially recognized the oligomeric complexes over monomeric gp4l in Western blots, indicating the presence of epitopes recognized by the human immune system on the gp4l multimers which are not efficiently expressed by the dissociated monomers. The demonstration of the existence of multimeric env complexes and the enhanced and altered antigenicity of such multimers may be relevant to the design of subunit and recombinant human immunodeficiency virus env vaccines.

Antibody-mediated neutralization of human immunodeficiency virus type–1 (HIV-1) is thought to function by at least two distinct mechanisms: inhibition of virus–receptor binding, and interference with events after binding, such as virus–cell membrane fusion. Here we show, by the use of a novel virus–cell binding assay, that soluble CD4 and monoclonal antibodies to all confirmed glycoprotein (gp)120 neutralizing epitopes, including the CD4 binding site and the V2 and V3 loops, inhibit the adsorption of two T cell line–adapted HIV-1 viruses to CD4+ cells. A correlation between the inhibition of virus binding and virus neutralization was observed for soluble CD4 and all anti-gp120 antibodies, indicating that this is a major mechanism of HIV neutralization. By contrast, antibodies specific for regions of gp120 other than the CD4 binding site showed little or no inhibition of either soluble gp120 binding to CD4+ cells or soluble CD4 binding to HIV-infected cells, implying that this effect is specific to the virion–cell interaction. However, inhibition of HIV-1 attachment to cells is not a universal mechanism of neutralization, since an anti-gp41 antibody did not inhibit virus–cell binding at neutralizing concentrations, implying activity after virus–cell binding.

No abstract

Neutralizing antibodies that recognize the human immunodeficiency virus gpl20 exterior envelope glycoprotein and are directed against either the third variable (V3) loop or conserved, discontinuous epitopes overlapping the CD4 binding region have been described. Here we report several observations that suggest a structural relationship between the V3 loop and amino acids in the fourth conserved (C4) gpl20 region that constitute part of the CD4 binding site and the conserved neutralization epitopes. Treatment of the gpl20 glycoprotein with ionic detergents resulted in a V3 loop-dependent masking of both linear C4 epitopes and discontinuous neutralization epitopes overlapping the CD4 binding site. Increased recognition of the native gpl20 glycoprotein by an anti-V3 loop monoclonal antibody, 9284, resulted from single amino acid changes either in the base of the V3 loop or in the gpl20 C4 region. These amino acid changes also resulted in increased exposure of conserved epitopes overlapping the CD4 binding region. The replication-competent subset of these mutants exhibited increased sensitivity to neutralization by antibody 9284 and anti-CD4 binding site antibodies. The implied relationship of the V3 loop, which mediates post-receptor binding steps in virus entry, and components of the CD4 binding region may be important for the interaction of these functional gpl20 domains and for the observed cooperativity of neutralizing antibodies directed against these regions.

Two distinct regions or epitope clusters of human immunodeficiency virus type 1 (HIV-1) gp120 have been shown to elicit neutralizing antibodies: the V3 loop and the CD4-binding site. We have isolated neutralizing human monoclonal antibodies (HuMAbs) against conserved epitopes in both of these regions. In this study, we demonstrate that an equimolar mixture of two of these HuMAbs, one directed against the V3 loop and the other against the CD4-binding site, neutralizes HIV-1 at much lower concentrations than does either of the individual HuMAbs. Mathematical analysis of this effect suggests cooperative neutralization of HIV-1 by the two HuMAbs and demonstrates a high level of synergy, with combination indices (CIs) of 0.07 and 0.16 for 90% neutralization of the MN and SF-2 strains, respectively. The dose reduction indices (DRIs) for each of the two HuMAbs at 99% neutralization range approximately from 10 to 150. A possible mechanism for this synergism is suggested by binding studies with recombinant gp160 of the MN strain; these show enhanced binding of the anti-CD4 binding site HuMAb in the presence of the anti-V3 loop HuMAb. These results demonstrate the advantage of including both V3 loop and CD4-binding site epitopes in a vaccine against HIV-1 and indicate that combinations of HuMAbs against these two sites may be particularly effective in passive immunotherapy against the virus.

An anti-gpl20 monoclonal antibody (MAb), C108G (-yl, K), was isolated from a chimpanzee that had been infected with strain IIIB of human immunodeficiency virus type 1 (HIV-l111B) and subsequently immunized with the recombinant glycoprotein rgpl60MN. This MAb is specific for the IIIB strain of HIV-1 and related clones and exhibits very potent neutralization of these viruses; e.g., 100% neutralization of approximately 8 x 103 infectious units of HXB2 was achieved with 125 ng of C108G per ml. Commensurate with this potent

We have used a panel of anti-gp160 MAbs to construct anti-HIV immunotoxins by coupling antibodies to ricin A chain (RAC). The ability of the immunotoxins to kill HIV-1-infected cells and halt the spread of infection was tested in tissue culture on persistently and acutely infected cell lines and primary lymphocyte cultures stimulated with phytohemagglutinin (PHA blasts). Laboratory strains and clinical isolates of HIV both were tested. The constitution and antigen-binding capacity of the immunotoxins were confirmed by ELISA and indirect immunofluorescence. Immunotoxins that bind epitopes exposed on the cell surface effectively killed persistently infected cells, although killing was not directly proportional to binding of immunotoxin to cell. The activity of anti-gp41, but not anti-gp120, immunotoxins was markedly enhanced in the presence of soluble CD4 or peptides corresponding to the CDR3 region of CD4. CD4-mediated enhancement of anti-gp41 immunotoxin activity was observed for laboratory strains neutralized by sCD4 and for clinical isolates that were resistant to neutralization by sCD4. Immunotoxin action was potentiated by brefeldin A, bafilomycin A1, cortisone, and an amphipathic fusion peptide, but not by cytochalasin D, nocodazol, monodansyl cadaverine, or trans-retinoic acid. Anti-HIV immunotoxins are useful tool with which to study the functional expression of gp120/gp41 antigens on the surface of HIV-infected cells, as well as potential AIDS therapeutics. Because these studies relate to the accessibility of viral antigens to antibody-mediated attack, these studies also have relevance for vaccine development.