Classical and nonclassical class I major histocompatibility complex molecules exhibit subtle conformational differences that affect binding to CD8 alpha alpha

The cell surface molecules CD4 and CD8 greatly enhance the sensitivity of T -cell antigen recognition, acting as "co-receptors" by binding to the same major histocompatibility complex (MHC) molecules as the T-cell receptor (TC R), Here we use surface plasmon resonance to study the binding of CD8 alpha alpha to class I MHC molecules. CD8 alpha alpha bound the classical MHC mo lecules HLA-A*0201, -A*1101, -B*3501, and -C*0702 with dissociation constan ts (K-d) of 90-220 mu M, a range of affinities distinctly lower than that o f TCR/peptide-MHC interaction. We suggest such affinities apply to most CD8 alpha alpha/classical class I MHC interactions and may be optimal for T-ce ll recognition. In contrast, CD8 alpha alpha bound both HLA-A*6801 and B*48 01 with a significantly lower affinity (greater than or equal to 1 mM), con sistent with the finding that interactions with these alleles are unable to mediate cell-cell adhesion. Interestingly, CD8 alpha alpha bound normally to the nonclassical MHC molecule HLA-G (K-d similar to 150 mu M), but only weakly to the natural killer cell receptor ligand HLA-E (K-d greater than o r equal to 1 mM). Site-directed mutagenesis experiments revealed that varia tion in CD8 alpha alpha binding affinity can be explained by amino acid dif ferences within the alpha 3 domain. Taken together with crystallographic st udies, these results indicate that subtle conformational changes in the sol vent exposed alpha 3 domain loop (residues 223-229) can account for the dif ferential ability of both classical and nonclassical class I MHC molecules to bind CD8.