CONFORMATIONAL-CHANGES AFFECTING THE V3 AND CD4-BINDING DOMAINS OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 GP120 ASSOCIATED WITH ENV PROCESSINGAND WITH BINDING OF LIGANDS TO THESE SITES

Two neutralizing human monoclonal antibodies (HuMAbs) directed against epitopes located near the tip of the V3 loop of human immunodeficienc y virus type 1 env protein recognized solubilized gPr160, but not gp12 0, in radioimmunoprecipitation assays. Efficient immunoprecipitation o f solubilized gp120 by these antibodies did occur in the presence of H uMAb 1125H, directed against a conformational epitope overlapping the CD4-binding site, or its F(ab'), fragment. In contrast to the inabilit y of the anti-V3 antibodies to immunoprecipitate solubilized gp120, th ese HuMAbs did bind to gp120 in intact virions; this level of binding increased severalfold in the presence of the F(ab')2 fragment of 1125H . These results demonstrate that neutralization epitopes in the V3 loo p are sequestered in soluble gp120 but partly exposed in gPr160 and in virion-associated gp120 and that binding of antibodies to the discont inuous CD4-binding site leads to conformational changes that result in the exposure of V3 epitopes in soluble gp120 and their enhanced acces sibility in gPr160 and in virion-associated gp120. Enhanced binding of suboptimal concentrations of 1125H to soluble gp120 was also induced by the presence of an anti-V3 HuMAb, indicating the occurrence of reci procal allosteric interactions between the V3 loop and the CD4-binding site. It is likely that these effects contribute to the synergistic n eutralization of human immunodeficiency virus type 1 previously report ed for antibodies directed against these two regions.