V3-SPECIFIC NEUTRALIZING ANTIBODIES IN SERA FROM HIV-1 GP160-IMMUNIZED VOLUNTEERS BLOCK VIRUS FUSION AND ACT SYNERGISTICALLY WITH HUMAN MONOCLONAL-ANTIBODY TO THE CONFORMATION-DEPENDENT CD4 BINDING-SITE OF GP120

Sera from 11 volunteers immunized with a recombinant HIV-1 gp160-expre ssing vaccinia virus (HIVAC-1e; Oncogen/Bristol-Myers Squibb, Seattle, WA) and boosted with baculovirus-derived rgp160 (VaxSyn; MicroGeneSys , Inc., Meriden, CT) were evaluated for functional serum antibodies an d their epitopes. Sera obtained prior to boosting had undetectable HIV -I-specific IgG and neutralizing activity, and did not block HIV-1 fro m binding or fusing to CD4+ MT-2 cells. 14 d after boosting, sera from each volunteer contained HIV-1-specific IgG titers of 1:40 to 1:1,280 . Five of these sera also contained neutralizing antibodies, where mos t or all neutralizing activity was blocked by a synthetic peptide corr esponding to amino acids 307-330 of the V3 loop of gp120, indicating t hat neutralizing antibodies were mostly V3 loop-specific. All sera obt ained after boosting contained HIV-1 binding / fusion-inhibition antib odies, and a significant portion of their activity was blocked by the V3 loop peptide, a result consistent with the presence of antibodies a gainst the region of the V3 loop that participates in fusion. Three se ra with V3 loop-specific neutralizing and fusion-inhibition antibodies were studied further. In competitive antibody binding experiments, an tibodies reactive with the conformation-dependent, CD4 binding site of gp120 were undetectable in each serum. When evaluated in combination with a monoclonal antibody to the CD4 binding site of gp120, two sera demonstrated synergism in neutralizing assays, and all three sera demo nstrated synergism in binding/fusion-inhibition assays, further indica ting that the functional antibodies were primarily V3 loop-specific. T he synergism also suggests that a vaccine that elicits strong serum an tibody responses to both regions of gp120 may improve the potential fo r inducing protective immunity.