Human monoclonal antibodies to the V3 loop of HIV-1 gp120 mediate variable and distinct effects on binding and viral neutralization by a human monoclonal antibody to the CD4 binding site.

Interactive effects between human monoclonal antibodies specific for the V3 loop (257-D and 447-D) and an epitope within the CD4 binding site (F105) of HIV-1 gp120 were evaluated for neutralization of viral cytopathogenicity and binding to HIV-infected cells. Regardless of antibody pair, only additive effects were observed in neutralization of MN and SF2 virus though each antibody alone had potent neutralizing activity on these strains. Significant cooperativity was observed between F105 and 447-D in neutralization of RF. Relatively high concentrations (> 100 micrograms/ml) of each individual antibody are required for partial neutralization (25--40%) of RF. Coincubation with 10 micrograms/ml of each antibody increased neutralization activity 3--4-fold more than predicted for additive effects alone. No enhancement was seen upon coincubation of F105 with 257-D which does not neutralize RF. Antibody interactions with native antigen on HIV-infected cells was measured by flow cytometry. Results were consistent with neutralization results in the majority of flow cytometry experiments; however, enhanced binding did not necessarily predict enhanced neutralization. These data support the notion that either a conformational change occurs with binding of V3 loop antibodies which enhances the binding and neutralizing activity of antibodies directed to the CD4 binding site of gp120 or vice versa, or new antigenic sites are exposed by the V3 loop antibodies on cell surfaces and virions. Of importance, cooperativity is observed even at very low antibody concentrations.