Classical and nonclassical class I major histocompatibility complex molecules exhibit subtle conformational differences that affect binding to CD8αα

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 (TCR). Here we use surface plasmon resonance to study the binding of CD8αα to class I MHC molecules. CD8αα bound the classical MHC molecules HLA- A*0201, -A*1101, -B*3501, and -C*0702 with dissociation constants (K(d)) of 90-220 μM, a range of affinities distinctly lower than that of TCR/peptide-MHC interaction. We suggest such affinities apply to most CD8αα/classical class I MHC interactions and may be optimal for T-cell recognition. In contrast, CD8αα bound both HLA-A*6801 and B*4801 with a significantly lower affinity (≥1 mM), consistent with the finding that interactions with these alleles are unable to mediate cell-cell adhesion. Interestingly, CD8αα bound normally to the nonclassical MHC molecule HLA-G (K(d) ~ 150 μM), but only weakly to the natural killer cell receptor ligand HLA-E (K(d) ≥ 1 mM). Site-directed mutagenesis experiments revealed that variation in CD8αα binding affinity can be explained by amino acid differences within the α3 domain. Taken together with crystallographic studies, these results indicate that subtle conformational changes in the solvent exposed 3/4 domain loop (residues 223-229) can account for the differential ability of both classical and nonclassical class I MHC molecules to bind CD8.