Importance of conformation on the neutralizing antibody response to HIV-1 gp120

We have investigated the role of conformation of HIV-1 gp120 on its potential efficacy as a subunit vaccine. The questions that we set out to answer were: 1) Are there neutralizing antibodies directed to conformational epitopes in gp120? 2) If so, what is the spectrum of virus isolates neutralized by these antibodies? 3) Is a conformationally correct gp120 subunit more effective in the induction of neutralizing antibodies than a denatured subunit? 4) Does native gp120 subunit vaccination induce a broader neutralizing response than a gp120 antigen that cannot display conformational epitopes? To address these questions, we characterized the gp120-specific antibody response of HIV-1-infected humans and of experimental animals immunized with recombinant native and nonnative gp120 subunits. Two versions of recombining gp120 produced from the HIV-SF2 isolate of HIV-1 were employed in these studies: 1) a nonglycosylated, denatured version produced in genetically engineered yeast, which we presume is capable of presenting only linear determinants, and 2) a fully glycosylated, native version, produced in genetically engineered mammalian cells, that is capable of displaying linear as well as conformational epitopes. Antibodies directed exclusively to conformational epitopes in gp120 were purified from pooled HIV antibody-positive human sera using these two versions of HIV-SF2 gp120. These antibodies exhibited neutralizing activity, and this acivity was effective in the neutralization of a different, broader spectrum of HIV-1 isolates than that of antibodies to linear determinants of gp120 purified from the same serum pool. When these two versions of HIV-SF2 gp120 were used as subunit immunogens in baboons, clear differences in their abilities to elicit neutralizing antibodies were observed. The native version was more effective in the induction of neutralizing antibodies effective against HIV-SF2, the homologous virus isolate. The isolate specificity of the neutralizing response to these two versions of HIV-SF2 gp120 also differed. The nonglycosylated version induced neutralizing antibodies that were effective against only the isolate, or closely related isolates, from which the antigen was derived. In contrast, the native version induced a neutralizing response that was effective against a broad panel of HIV-1 isolates, including at least one isolate that one would not expect to be neutralized by antibodies to the PND of HIV-SF2 gp120.