Beta-chain broadens range of CD8 recognition for MHC class I molecule

It is known that the alpha-chain of CD8 binds to a negatively charged loop composed of residues 223 to 229 on MHC class I Ag and that binding of CD8 alpha enhances Ag recognition of T cells. We have recently shown that the mouse CD8 alpha homodimer does not bind to either the HLA class I alpha 3 domain or a mutant of H-2Kb Ag containing a substitution of glutamine for methionine at residue 224, which brings this residue toward the human consensus. Here we report a complementary study of the CD8 beta-chain. The functional role of the CD8 beta-chain was analyzed by using four T cell hybridoma lines expressing mouse CD8 alpha and transfected with the mouse CD8 beta gene. As compared with the lines expressing only CD8 alpha, allorecognition of the chimeric H-2Kb Ag that contains the HLA class I alpha 3 domain was enhanced in lines expressing both CD8 alpha and -beta. This enhancement was blocked by either anti-CD8 mAb or anti-HLA class I alpha 3 domain mAb. In addition, we show that CD8 alpha beta binds the H-2Kb mutant Ag at residue 224. These results suggest that the beta-chain allows the CD8 alpha beta heterodimer to recognize the chimeric H-2Kb Ag. A model for the role of the beta-chain is presented.