Compensatory Mechanisms Allow Undersized Anchor-deficient Class I MHC Ligands to Mediate Pathogenic Autoreactive T Cell Responses

Self-reactive T cells must escape thymic negative selection to mediate pathogenic autoimmunity. In the NOD mouse model of autoimmune diabetes, several β cell–cytotoxic CD8 T cell populations are known, with the most aggressive of these represented by AI4, a T cell clone with promiscuous Ag-recognition characteristics. We identified a long-elusive β cell–specific ligand for AI4 as an unusually short H-2Db–binding 7-mer peptide lacking a C-terminal anchor residue and derived from the insulin A chain (InsA14–20). Crystallography reveals that compensatory mechanisms permit peptides lacking a C-terminal anchor to bind sufficiently to the MHC to enable destructive T cell responses, yet allow cognate T cells to avoid negative selection. InsA14–20 shares two solvent-exposed residues with previously identified AI4 ligands, providing a structural explanation for AI4’s promiscuity. Detection of AI4-like T cells, using mimotopes of InsA14–20 with improved H-2Db–binding characteristics, establishes the AI4-like T cell population as a consistent feature of the islet infiltrates of NOD mice. Our work establishes undersized peptides as previously unrecognized targets of autoreactive CD8 T cells and presents a strategy for their further exploration as Ags in autoimmune disease.