Synthetic peptides representing discontinuous CD4 binding epitopes of HIV-1 gp120 that induce T cell apoptosis and block cell death induced by gp120.

A vaccine against HIV-1 virus would block initial infection and must target conserved residues. Since initial infection depends on binding of the viral envelope protein gp120 to CD4 on the cell surface, the CD4 binding site of gp120 is a target for vaccine design. To identify the optimal biologically active site, we synthesized a series of 32-mer peptides, based on conserved residues in the C3 and C4 regions of gp120. These included three of five sequence discontinuous residues known to be involved in CD4 binding, one or two of which were substituted with alanine. We also synthesized a 44-mer peptide with an additional branch to incorporate an extra C4 region sequence including a fourth CD4 binding residue. All these peptides used an oxidized Cys-X-Cys bridge to link the discontinuous sequence elements in a manner suggested by the known conserved disulfide bridges in gp120. Polyclonal sera raised to these peptides indicate that they all contain both B and T lymphocyte epitopes. Binding of the peptides to CD4-transfected HeLa cells reveals a hierarchy dependent on the number of relevant CD4 binding residues present. Furthermore, antibody cross-linking of peptides bound to the surface of human T cells results in apoptosis that is similar to the known properties of gp120. The peptide incorporating three CD4 binding residues competitively inhibited gp 120-induced T lymphocyte apoptosis. Thus, we have synthesized novel, branched peptides incorporating conserved discontinuous sequences from two different conserved domains of HIV-1 gp120 that contain T and B lymphocyte epitopes and mimic biological functions of the native protein. These synthetic peptides are candidates for future vaccine development.