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To further our understanding of the nature of HIV-1 immunogenicity, we injected mice with the virus envelope protein gp120 in different configurations: free, complexed with its receptor CD4, and as an immunocomplex with a monoclonal antibody directed against the V3 loop of the protein. Analyses of the polyclonal sera, as well as of monoclonal antibodies produced in each case, allowed us to conclude that the quality of the humoral immune response depended on the complexation state of the antigen. For the free gp120 and gp120-CD4 complex the responses were directed mainly toward conformational epitopes. However, gp120 immunocomplexed with anti-V3 loop Mab produced, in addition, numerous MAbs directed toward linear epitopes. Epitopes were mapped using immunoblots of gp120 cleaved with S. aureus V8 protease and a combinatorial epitope phage-display library. It was found that some of the linear epitopes had been previously identified as T cell epitopes. These results suggest that the immunocomplexed gp120 may be particularly well taken up by antigen-presenting cells, leading to the processing of the gp120 and the efficient presentation of T cell epitopes. Thus immunocomplexation should afford a means for enhancing the immunogenicity of gp120 and improving its presentation.

Antigen association to its corresponding binding site in the immunoglobulin molecule can elicit conformational rearrangements, generating novel epitopes termed metatopes. Such metatopes were characterized for the immunocomplex between the AIDS virus envelope protein, gp120, and M77, a mAb directed against the V3 loop. Five novel mAbs were described (GV1, GV3, GV7, GV8, and GV12). These mAbs were found to bind epitopes harbored in the M77 Fab fragment. Binding to the epitopes was shown to require the complexation of Fab with its antigen. The degree of this antigen requirement was found to be variable for the different mAbs and also for the state of IgG fragmentation. Binding of GV12 to its antigen increased the affinity of M77 for gp120. Moreover, in the presence of GV12, M77 acquired extended cross-reactivity for a second gp120 variant, namely BaL. These results could indicate a novel approach towards improving the performance of anti-HIV antibodies.

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