Department of Immunobiology, The University of Arizona College of Medicine, Tucson, United States; School of Animal and Comparative Biomedical Sciences, University of Arizona, Tucson, United States; Cancer Biology Graduate Interdisciplinary Program and Genetics Graduate Interdisciplinary Program, The University of Arizona, Tucson, United States; The BIO-5 Institute, The University of Arizona, Tucson, United States; The University of Arizona Cancer Center, Tucson, United States
Department of Immunobiology, The University of Arizona College of Medicine, Tucson, United States; Cancer Biology Graduate Interdisciplinary Program and Genetics Graduate Interdisciplinary Program, The University of Arizona, Tucson, United States; The BIO-5 Institute, The University of Arizona, Tucson, United States; The University of Arizona Cancer Center, Tucson, United States; The Arizona Center on Aging, The University of Arizona College of Medicine, Tucson, United States
CD4+ T cells use T cell receptor (TCR)–CD3 complexes, and CD4, to respond to peptide antigens within MHCII molecules (pMHCII). We report here that, through ~435 million years of evolution in jawed vertebrates, purifying selection has shaped motifs in the extracellular, transmembrane, and intracellular domains of eutherian CD4 that enhance pMHCII responses, and covary with residues in an intracellular motif that inhibits responses. Importantly, while CD4 interactions with the Src kinase, Lck, are viewed as key to pMHCII responses, our data indicate that CD4–Lck interactions derive their importance from the counterbalancing activity of the inhibitory motif, as well as motifs that direct CD4–Lck pairs to specific membrane compartments. These results have implications for the evolution and function of complex transmembrane receptors and for biomimetic engineering.
