COMPETITION OF HLA-DR AND A BETA-2 DOMAIN PEPTIDE FOR HIV ENVELOPE GLYCOPROTEIN GP120 BINDING TO CD4

HLA class II molecules and the HIV envelope glycoprotein gp120 are lig ands of CD4. Reciprocal interaction sites have been well characterized for gp120 and CD4, but require further definition for HLA class II an d CD4. A major CD4 binding site encompassing amino acids 134-148 in th e beta 2 domain of HLA-DR has been previously identified. Recently, we have shown, by extensive characterization of mutated CD4 molecules ex pressed in COS cells, that HLA class II antigens interact mainly throu gh the HIV gp120 binding site and possibly through a second minor inte raction site mapping on the same face of the molecule. Based on the di rect binding in vitro of iodinated affinity-purified HLA-DR1 molecules to polystyrene immobilized human lgG3-CD4, we now report on reciproca l binding inhibition of gp120, HLA-DR1 and the DR beta 2 synthetic pep tide to CD4. The results strongly suggest that gp120 and the beta 2 re gion (amino acids 134-148) of HLA-DR1 bind mainly to the same part of CD4 domain 1 and that the CD4 binding site of HLA-DR requires the exis tence of a class II homodimer. In that case, alpha 2 chain residues mi ght interact with CD4 residues different from those involved in the bi nding of gp120 but located close to them in the first domain. Converse ly, no direct evidence was found of a direct interaction between CD4 a nd the DR beta 1 domain sequence 35-46, suggesting that the DR beta 1 synthetic peptide which had been shown to inhibit CD4-class II-depende nt T cell functions should act by a distinct mechanism such as the pre vention of HLA class II dimerization.