The molecular basis of class II MHC allelic control of T cell responses.
To identify the molecular basis for the effects of MHC molecule polymorphism on T cell responses, we have combined functional T cell response testing with measurements of peptide binding to the class II MHC molecules on transfected cells. Our studies identify a small subset of spatially localized polymorphic residues of the E alpha E beta dimer (strand residue beta 29, and helix residues beta 72 and beta 75) regulating cytochrome c peptide presentation by two distinct mechanisms. The first effect is on quantitative control of net peptide binding. The replacement of the valine found at position beta 29 in E beta k with the glutamic acid found in E beta b results in a selective loss of pigeon cytochrome peptide but not moth cytochrome peptide binding to the resultant mutant E alpha E beta k molecule. Reciprocally, the replacement of glutamic acid at beta 29 in E beta b with valine results in a gain of pigeon peptide binding. These changes in binding parallel changes in T cell responses in vitro to these peptide-E alpha E beta combinations and mirror the in vivo immune response gene phenotypes of mice expressing E alpha E beta k and E alpha E beta b. E alpha E beta s molecules, which have a beta 29 glutamic acid, are nevertheless able to bind and present pigeon cytochrome peptides, and this is due to changes in helix residues beta 72 and beta 75 that compensate for the negative effect of the beta 29 glutamic acid. The second activity is a critical change in the conformation of the peptide bound to the same extent by distinct MHC molecules, as revealed by changes in T cell responses to moth cytochrome peptides presented by two E alpha E beta molecules differing only at position beta 29. Both of these effects can be ascribed to a single polymorphic residue modeled to be inaccessible to TCR contact (beta 29), providing a striking demonstration of how MHC molecule polymorphism can modify T cell-dependent immune responses without direct physical participation in the receptor recognition event.