Monoclonal Antibodies Raised against Covalently Crosslinked Complexes of Human Immunodeficiency Virus Type 1 gp120 and CD4 Receptor Identify a Novel Complex-Dependent Epitope on gp120

Abstract

The binding of the human immunodeficiency virus type 1(HIV-1) envelope glycoprotein, gp120, to its cell surface receptor, CD4, represents a molecular interaction involving distinct alterations in protein structure. Consequently, the pattern of epitopes presented on the gp120-CD4 complex should differ from these on free gp120. To investigate this concept, mice were immunized with covalently crosslinked complexes of vital HIV-1IIIB gp120 and soluble CD4. Two monoclonal antibodies (MoAbs) obtained from the immunized mice exhibited a novel epitope specificity. The MoAbs were marginally reactive with HIV-1IIIB gp120, highly reactive with gp120-CD4 complexes, and unreactive with soluble CD4. The same pattern of reactivity was seen in solid-phase assays using HIV-1451 gp120. A similar specificity for complexes was evident in flow cytometry experiments. in which MoAb reactivity was dependent upon the attachment of gp120 to CD4-positive cells. In addition, MoAb reactivity was detected upon the interaction of CD4 receptors with purified HIV-1IIIB virions. Notably, seroantibodies from HIV-positive individuals competed for MoAb binding, indicating that the epitope is immunogenic in humans. The results demonstrated that crosslinked gp120-CD4 complexes elicit antibodies to cryptic gp120 epitopes that are exposed during infection in response to receptor binding. These findings may have important implications for the consideration of HIV envelope-receptor complexes as targets for virus neutralization. © 1995 Academic Press. All rights reserved.