Identification of the B cell superantigen-binding site of HIV-1 gp120

Previous studies showed that the gp120 envelope protein of HIV-1 is able to crosslink membrane IgM on normal human B cells and to induce their activation in a V(H)3 immunoglobulin gene-family-specific manner. Because this V(H) gene family is the largest in the human repertoire, this superantigen (SAg) property is thought to have deleterious consequences for the host, including a progressive decline of B cells with progression of the HIV-1-induced disease. Here, we have identified the sequence motifs on gp120 involved in SAg binding to normal Igs. We show that this SAg-binding activity is present in gp120s from highly divergent isolates of HIV-1 belonging to clades derived from various geographical origins, and that carbohydrate residues are not essential for its expression. The SAg-binding site is formed by protein sequences from two regions of the gp120 molecule. The core motif is a discontinuous epitope spanning the V4 variable domain and the amino-terminal region flanking the C4 constant domain. The most critical residues appear to be Leu(395)–Asp(397) and Ile(425)–Gln(427). Residues from the C2 constant domain (positions 252–272) also seem to play an accessory role in SAg binding of gp120 to normal human Igs. These findings are important in the design of a successful gp120-based vaccine against HIV-1.