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*
Department of Immunology, Duke University Medical Center, Durham, NC 27710;
Blood Research Institute, The Blood Center, Milwaukee, WI; and
Department of Structural Biology, St. Jude Childrens Research Hospital, Memphis, TN
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 DRß sequences, could inhibit CD4-class II adhesion. A peptide derived from sequences within the ß1 domain (DRß4155), as well as two peptides derived from sequences within the ß2 domain (DRß121135 and DRß141155), were shown to inhibit CD4-class II adhesion. Inasmuch as a site for CD4 binding in the ß2 domain had been previously documented, these studies were designed to investigate the role of the ß1 domain as an additional site of interaction with CD4. Sixteen site-specific mutations were engineered within the ß1 domain of DRß1*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 ß1 and ß2 domains of class II interact with the two amino-terminal Ig-like domains of CD4.
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