A Missing PD-L1/PD-1 Coinhibition Regulates Diabetes Induction by Preproinsulin-Specific CD8 T-Cells in an Epitope-Specific Manner

Coinhibitory PD-1/PD-L1 (B7-H1) interactions provide critical signals for the regulation of autoreactive T-cell responses. We established mouse models, expressing the costimulator molecule B7.1 (CD80) on pancreatic beta cells (RIP-B7.1 tg mice) or are deficient in coinhibitory PD-L1 or PD-1 molecules (PD-L1(−/−) and PD-1(−/−) mice), to study induction of preproinsulin (ppins)-specific CD8 T-cell responses and experimental autoimmune diabetes (EAD) by DNA-based immunization. RIP-B7.1 tg mice allowed us to identify two CD8 T-cell specificities: pCI/ppins DNA exclusively induced K(b)/A(12–21)-specific CD8 T-cells and EAD, whereas pCI/ppinsΔA(12–21) DNA (encoding ppins without the COOH-terminal A(12–21) epitope) elicited K(b)/B(22–29)-specific CD8 T-cells and EAD. Specific expression/processing of mutant ppinsΔA(12–21) (but not ppins) in non-beta cells, targeted by intramuscular DNA-injection, thus facilitated induction of K(b)/B(22–29)-specific CD8 T-cells. The A(12–21) epitope binds K(b) molecules with a very low avidity as compared with B(22–29). Interestingly, immunization of coinhibition-deficient PD-L1(−/−) or PD-1(−/−) mice with pCI/ppins induced K(b)/A(12–21)-monospecific CD8 T-cells and EAD but injections with pCI/ppinsΔA(12–21) did neither recruit K(b)/B(22–29)-specific CD8 T-cells into the pancreatic target tissue nor induce EAD. PpinsΔA(12–21)/(K(b)/B(22–29))-mediated EAD was efficiently restored in RIP-B7.1(+)/PD-L1(−/−) mice, differing from PD-L1(−/−) mice only in the tg B7.1 expression in beta cells. Alternatively, an ongoing beta cell destruction and tissue inflammation, initiated by ppins/(K(b)/A(12–21))-specific CD8 T-cells in pCI/ppins+pCI/ppinsΔA(12–21) co-immunized PD-L1(−/−) mice, facilitated the expansion of ppinsΔA(12–21)/(K(b)/B(22–29))-specific CD8 T-cells. CD8 T-cells specific for the high-affinity K(b)/B(22–29)- (but not the low-affinity K(b)/A(12–21))-epitope thus require stimulatory ´help from beta cells or inflamed islets to expand in PD-L1-deficient mice. The new PD-1/PD-L1 diabetes models may be valuable tools to study under well controlled experimental conditions distinct hierarchies of autoreactive CD8 T-cell responses, which trigger the initial steps of beta cell destruction or emerge during the pathogenic progression of EAD.