Two MHC surface amino acid differences distinguish foreign peptide recognition from autoantigen specificity

KRN T cells can recognize two self MHC alleles with differing biological co nsequences. They respond to the foreign peptide RN(42-56) bound to I-A(k) o r alternatively initiate autoimmune arthritis by interacting with a self Ag , GPI(282-294), on I-A(g7), Five surface amino acid differences between the two MHC molecules collectively alter which peptide side chains are recogni zed by the KRN TCR, In this study, it is shown that mutation of only two of these residues, alpha 65 and beta 78, in I-A(k) to their I-A(g7) counterpa rts is sufficient to allow recognition of the TCR contacts from GPI(282-294 ). To provide a detailed mechanism for the specificity change, the distinct contributions of each of these two mutations to the global effect on pepti de specificity were analyzed. The alpha 65 mutation is shown to broaden the spectrum of amino acids permissible at P8 of the peptide. In contrast, the beta 78 mutation alone blocks KRN TCR interaction with I-A(k) and requires the simultaneous presence of the alpha 65 mutation to preserve recognition . In the presence of the alpha 65 mutation, the beta 78 residue broadens pe ptide recognition at P3 and prevents recognition of the P8 L in RN(42-56), thus producing the observed specificity shift. These results localize the f unctionally relevant differences between the surfaces of two self-restricte d MHC molecules to two residues that have counterbalanced positive and nega tive contributions to interaction with a single TCR, They highlight how sub tle structural distinctions attributable to single amino acids can stand at the interface between foreign Ag responsiveness and pathogenic autoreactiv ity.