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

Abstract

RIP-B7.1 mice express the costimulator molecule B7.1 (CD80) on pancreatic beta 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 beta 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.