A CONSERVATIVE MUTATION IN A CLASS-I MHC MOLECULE OUTSIDE THE PEPTIDEBINDING GROOVE STIMULATES RESPONSES TO SELF PEPTIDES

A transgenic mouse has been made that expresses a mutant MHC class I H -2K(b) molecule with glutamic acid at position 65 (E65) in place of gl utamine. The side chain at position 65, on the outward face of the alp ha-helix of the alpha1 domain of the class I molecule, interacts with the TCR, and not with the peptide binding groove. The transgenic mouse , on a DBA/2 background, mounts K(b,E65)-restricted Ag-specific respon ses to conventional K(b)-restricted Ag such as OVA and vesicular stoma titis virus, and shows strong alloreactivity to wild-type K(b). The tr ansgenic mouse also mounts a primary in vitro alloreactive response di rected to a mutant molecule with aspartic acid at position 65 (D65). T his response is relatively weak, probably because of the structural si milarities between aspartic and glutamic acid side chains; both have c arboxylic termini, and the aspartic acid side chain is shorter by a si ngle secondary carbon. The alloreactive CTL lines elicited by this con servative change are cross-reactive among several position-65 variants of H-2K(b). Individual CTL clones are specific for self peptides that can be extracted from cells expressing K(b,E65), and from purified wi ld-type K(b) molecules, and that are recognized in the context of the D65 residue. Thus, the smallest variance from self in a class I molecu le, even outside the peptide binding groove, can be antigenic.