Processing in the Endoplasmic Reticulum Generates an Epitope on the Insulin A Chain that Stimulates Diabetogenic CD8 T Cell Responses

RIP-B7.1 mice express the costimulator molecule B7.1 (CD80) on pancreatic β cells and are a well-established model for studying de novo induction of diabetogenic CD8 T cells. Immunization of RIP-B7.1 mice with preproinsulin (ppins)-encoding plasmid DNA efficiently induces experimental autoimmune diabetes (EAD). EAD is associated with an influx of CD8 T cells specific for the K b /A 12–21 epitope into the pancreatic islets and the subsequent destruction of β cells. In this study, we used this model to investigate how ppins-derived Ags are expressed and processed to prime diabetogenic, K b /A 12–21 -specific CD8 T cells. Targeting the K b /A 12–21 epitope, the insulin A chain, or the ppins to the endoplasmic reticulum (ER) (but not to the cytosol and/or nucleus) efficiently elicited K b /A 12–21 -specific CD8 T cell responses. The K b /A 12–21 epitope represents the COOH terminus of the ppins molecule and, hence, did not require COOH-terminal processing before binding its restriction element in the ER. However, K b /A 12–21 -specific CD8 T cells were also induced by COOH-terminally extended ppins-specific polypeptides expressed in the ER, indicating that the epitope position at the COOH terminus is less important for its diabetogenicity than is targeting the Ag to the ER. The K b /A 12–21 epitope had a low avidity for K b molecules. When epitopes of unrelated Ags were coprimed at the same site of Ag delivery, “strong” K b -restricted (but not D b -restricted) CD8 T cell responses led to the suppression of K b /A 12–21 -specific CD8 T cell priming and reduced EAD. Thus, direct expression and processing of the “weak” K b /A 12–21 epitope in the ER favor priming of autoreactive CD8 T cells.