IDENTIFICATION OF A NEW NEUTRALIZING EPITOPE CONFORMATIONALLY AFFECTED BY THE ATTACHMENT OF CD4 TO GP120

We have developed a strategy to purify and characterize various anti-g p120 antibody populations in HIV+ sera by using anti-Id mAb. One prepa ration of human anti-gp120 antibody (ES+Ab) isolated on an anti-Id mAb (ES)-conjugated immunoabsorbent exhibited a novel neutralizing epitop e specificity. The ES+Ab bound only to the native form of recombinant gp120SF2 and gp1201IIIB and not to the third hypervariable region (V3) loop peptide. In contradistinction to other CD4-gp120-inhibiting and V3-specific neutralizing antibodies, ES+Ab exhibited a dose-dependent enhancement of binding to recombinant gp120 in the presence of recombi nant soluble CD4. In addition, flow cytometric analysis revealed a sim ilar increase in the binding of ES+Ab to the native form of gp120 expr essed on the HIV-infected cells. The ES+Ab competed with CD4 binding s ite- and V3-specific antibodies in binding to gp120, suggesting that t he ES+Ab epitope is located near the CD4 binding site epitope and the V3 region. The ES+Ab neutralized six genetically distinct HIV-1 strain s. The neutralizing activity of ES+Ab on HIV(IIIB) was significantly i ncreased in the presence of human anti-CD4 binding site mAb. These dat a suggest that the ES+Ab epitope represents a conserved, conformationa l, neutralization target on gp120 that may be involved in viral infect ion in an event after the CD4-gp120 interaction and that is distinct f rom previously defined neutralizing epitopes of gp120. This finding ma y be important for the development of an AIDS vaccine and immunotherap y.