A site for CD4 binding in the beta 1 domain of the MHC class II protein HLA-DR1

Using a lymphocyte binding assay, we have previously demonstrated that the CD4 protein can mediate cell adhesion by direct interaction with MHC class II molecules. In this report, we have used this assay to test whether synthetic peptides, corresponding to DR beta sequences, could inhibit CD4-class II adhesion. A peptide derived from sequences within the beta1 domain (DR beta 41-55), as well as two peptides derived from sequences within the beta 2 domain (DR beta 121-135 and DR beta 141-155), were shown to inhibit CD4-class II adhesion. Inasmuch as a site for CD4 binding in the beta 2 domain had been previously documented, these studies were designed to investigate the role of the beta 1 domain as an additional site of interaction with CD4. Sixteen site-specific mutations were engineered within the beta1 domain of DR beta 1*0101. Several mutations were shown to disrupt CD4-dependent T cell activation. Based on these results, we propose a model for the molecular interaction of CD4 with MHC class II proteins in which both the beta 1 and beta 2 domains of class II interact with the two amino-terminal Ig-like domains of CD4.