Immune surveillance for ERAAP dysfunction Peptide trimming is impaired in cells that lack ERAAP. Altered peptide-MHC I complexes are exported to the surface of these cells, forming an immunogenic pMHC I repertoire. WT CD8 T cells detect novel pMHC I complexes on ERAAP-deficient cells, presented by both classical MHC class Ia, and non-classical MHC class Ib, molecules. WT mice naturally have a large number of T cells, called QFL T cells, specific to the Qa-1-FL9 complex, expressed only on ERAAP-deficient cells. QFL T cells, and other pMHC I-specific T cells participate in immune surveillance for ERAAP dysfunction.