Characterization of the interactions between MHC class I subunits: a systematic approach for the engineering of higher affinity variants of beta 2-microglobulin.

Human beta 2m (h beta 2m) binds to murine MHC I molecules with higher affinity than does murine beta2m and therefore can be used as a model system to define and dissect the interactions between beta2m and MHC I heavy chains that promote the stability of the complex. In the present study we compare three-dimensional crystal structures of human and murine MHC I molecules and use functional studies of chimeric human:murine beta 2m variants to define a region of beta2m that is involved in the higher affinity of h beta 2m for murine MHC I heavy chains. Further examination of the three-dimensional structure in this region revealed conformational differences between human and murine beta2m that affect the ability of an aspartic acid residue at position 53 (D53) conserved in both beta 2ms to form an ionic bond with arginine residues at positions 35 and 48 of the heavy chain. Mutation of residue D53 to either asparagine (D53N) or valine (D53V) largely abrogated the stabilizing effects of h beta 2m on murine MHC I expression in a predictable manner. Based on this observation a variant of h beta 2m was engineered to create an ionic bond between the heavy chain and beta 2m. This variant stabilizes cell surface H-2Dd heavy chains to a greater extent than wild-type h beta 2m. Studying these interactions in light of the growing database of MHC I crystal structures should allow the rational design of higher affinity h beta 2m variants for use in novel peptide-based vaccines capable of inducing cell-mediated immune responses to viruses and tumors.