Interaction with the CD4 receptor enhances the exposure on the human immunodeficiency type 1 gp120 exterior envelope glycoprotein of conserved, conformation-dependent epitopes recognized by the 17b and 48d neutralizing monoclonal antibodies. The 17b and 48d antibodies compete with anti-CD4 binding antibodies such as 15e or 21h, which recognize discontinuous gp120 sequences near the CD4 binding region. To characterize the 17b and 48d epitopes, a panel of human immunodeficiency virus type 1 gp120 mutants was tested for recognition by these antibodies in the absence or presence of soluble CD4. Single amino acid changes in five discontinuous, conserved, and generally hydrophobic regions of the gp120 glycoprotein resulted in decreased recognition and neutralization by the 17b and 48d antibodies. Some of these regions overlap those previously shown to be important for binding of the 15e and 21h antibodies or for CD4 binding. These results suggest that discontinuous, conserved epitopes proximal to the binding sites for both CD4 and anti-CD4 binding antibodies become better exposed upon CD4 binding and can serve as targets for neutralizing antibodies.

We have analyzed a panel of eight murine monoclonal antibodies (MAbs) that depend on the V2 domain for binding to human immunodeficiency virus type 1 (HlV-1) gpl20. Each MAb is sensitive to amino acid changes within V2, and some are affected by substitutions elsewhere. With one exception, the MAbs were not reactive with peptides from the V2 region, or only poorly so. Hence their ability to bind recombinant strain IIIB gpl20 depended on the preservation of native structure. Three MAbs cross-reacted with strain RF gpl20, but only one cross-reacted with MN gpl20, and none bound SF-2 gp120. Four MAbs neutralized HIV-1 IIIB with various potencies, and the one able to bind MN gpl20 neutralized that virus. Peptide serology indicated that antibodies cross-reactive with the HxB2 Vi and V2 regions are rarely present in HIV-1-positive sera, but the relatively conserved segment between the Vi and V2 loops was recognized by antibodies in a significant fraction of sera. Antibodies able to block the binding of V2 MAbs to IIIB or MN gpl20 rarely exist in sera from HIV-1-infected humans; more common in these sera are antibodies that enhance the binding of V2 MAbs to gpl20. This enhancement effect of HIV-1-positive sera can be mimicked by several human MAbs to different discontinuous gpl20 epitopes. Soluble CD4 enhanced binding of one V2 MAb to oligomeric gpl20 but not to monomeric gpl20, perhaps by inducing conformational changes in the oligomer.

We have probed the structure of the C4 and V3 domains of human immunodeficiency virus type 1 gpl20 by immunochemical techniques. Monoclonal antibodies (MAbs) recognizing an exposed gpl20 sequence, (E/ K)VGKAAMYAPP, in C4 were differentially sensitive to denaturation of gpl20, implying a conformational component to some of the epitopes. The MAbs recognizing conformation-sensitive C4 structures failed to bind to a gpl20 mutant with an alteration in the sequence of the V3 loop, and their binding to gpl20 was inhibited by both V3 and C4 MAbs. This implies an interaction between the V3 and C4 regions of gpl20, which is supported by the observation that the binding of some MAbs to the V3 loop was often enhanced by amino acid changes in and around the C4 region.