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- gp120 antibody populations in HIV⁺ sera by using anti-Id mAb. One preparation of human anti-gp120 antibody (ES⁺ Ab) isolated on an anti-Id mAb (ES)-conjugated immunoabsorbent exhibited a novel neutralizing epitope specificity. The ES⁺ Ab bound only to the native form of recombinant gp120(SF2) and gp120(IIIB) 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 recombinant soluble CD4. In addition, flow cytometric analysis revealed a similar increase in the binding of ES⁺ Ab to the native form of gp120 expressed on the HIV-infected cells. The ES⁺ Ab competed with CD4 binding site- and V3- specific antibodies in binding to gp120, suggesting that the ES⁺ Ab epitope is located near the CD4 binding site epitope and the V3 region. The ES⁺ Ab neutralized six genetically distinct HIV-1 strains. The neutralizing activity of ES⁺ Ab on HIV(IIIB) was significantly increased in the presence of human anti-CD4 binding site mAb. These data suggest that the ES⁺ Ab epitope represents a conserved, conformational, neutralization target on gp120 that may be involved in viral infection in an event after the CD4-gp120 interaction and that is distinct from previously defined neutralizing epitopes of gp120. This finding may be important for the development of an AIDS vaccine and immunotherapy.