Mapping the Ig superantigen-binding site of HIV-1 gp120

The envelope glycoprotein, gp120, of HIV-1 has recently been identified as a member of the new family of Ig superantigens (Ig-SAg). This classification is based on the selective binding of gp120 to an unusually high proportion of endogenous, nonimmune Ig, and the selective activation of nonimmune B cells by gp120 in vitro. Many, if not all of the nonimmune Ig that bind to gp120 are members of the VH3 Ig gene family. The aim of this study was to determine the epitope on gp120 that was responsible for its Ig-SAg binding activity. To do this, we utilized a panel of 30 peptides derived from gp160 in a competition-binding assay. For five Igs that were tested, as well as for polyclonal serum IgM, two overlapping peptides (each 20 amino acids in length) were identified that were potent inhibitors of gp120 binding. Similarly, the 10 amino acid overlap region of these two peptides had inhibitory activity. Thus, this decamer sequence represented the optimal Ig-SAg epitope or mimotope. The amino acid residue at position 1 of the decamer, and to a lesser extent at position 10, was critical for peptide binding. In addition to this decamer peptide, other peptides that shared modest sequence homology were also selectively inhibitory for specific Ig samples. These findings provide the first definition of an Ig-SAg ligand at the peptide level and will facilitate further structural and biologic characterization of this new class of pathogenic Ags.