Interactions of human antibodies, epitope exposure, antibody binding and neutralization of primary isolate HIV-1 virions.

Objective: Development of an effective HIV vaccine has been limited because of the inherent structural properties of the HIV envelope on native virions and the failure of the immune system to respond in an effective manner. Identification of the interactions of human antibodies with virions resulting in neutralization will facilitate vaccine design.
Design: Combinations of human monoclonal antibodies (hMAb) were studied for binding to and neutralization of primary isolate virions.
Methods: Virion binding and neutralization were measured using primary isolate virions.
Results: Antibodies and combinations of antibodies to epitopes exposed upon CD4 binding (CD4i) and V3 loop antibodies resulted in additive binding and neutralization of R5X4 virus. Antibodies did not bind to or neutralize R5 virus as well. The combination of V3 loop antibody with 2G12 resulted in enhanced neutralization and binding to the R5X4 isolate but not the R5 isolate. Preincubation of the R5X4 isolate with F240, a non-neutralizing anti-gp41 antibody, significantly enhanced binding and neutralization by CD4i hMAb and 2F5. F240 also enhanced the binding of 2F5 to the R5 isolate and the neutralization of the R5 isolate mediated by 2G12.
Conclusions: Neutralizing epitopes are obscured on intact primary isolate virions and are dynamically exposed upon ligand (CD4) interactions. Interestingly, a non-neutralizing antibody to gp41 also increased binding and neutralizing activity of some hMAb that poorly neutralized R5 virus. These data suggest that non-neutralizing epitopes may be appropriate targets for vaccine design and epitope exposure should be considered in the development of immunotherapeutic strategies for HIV.