Definition of the alpha 2 region of HLA-DR molecules involved in CD4 binding

HLA class II molecules present antigenic peptides to the T cell receptor of CD4+ T lymphocytes and interact with CD4 during the antigen recognition process. A major CD4 binding site encompassing amino acids (aa) 134-148 in the beta 2 domain of HLA-DR has been previously identified and residues located within the alpha 2 subunit of murine MHC class II I-Ad molecules have been shown to contribute to CD4-class II interaction. To characterize the alpha 2 region of HLA-DR molecules involved in the binding of CD4, we have synthesized overlapping linear and cyclic peptides derived from a region encompassing aa 121-143. We demonstrate that two linear peptides (aa 124-138 and 130-143) and a cyclic one (aa 121-138) specifically bind to CD4-sepharose affinity columns. Although cyclic analogues exhibit more ordered populations as detected by circular dichroism measurements, cyclization did not improve the activity of some peptides. Peptide sequence positioning in HLA-DR1 dimer model indicates that alpha 2 residues 124 to 136 form a solvent-exposed loop which faces the beta 2 loop delimited by residues 134-148. These data suggest that one CD4 molecule contacts both alpha 2 and beta 2 loops of the HLA-DR homodimer.