Serpinb9 (Spi6)-deficient mice are impaired in dendritic cell-mediated antigen cross-presentation

Serpinb9 (Sb9, also called Spi6) is an intracellular inhibitor of granzyme B (GrB) that protects activated cytotoxic lymphocytes from apoptosis. We show here that the CD8(+) subset of splenic dendritic cells (DC), specialized in major histocompatibility complex class I (MHC I) presentation of exogenous antigens (cross-presentation), produce high levels of Sb9. Mice deficient in Sb9 are unable to generate a cytotoxic T-cell response against cell-associated antigen by cross-presentation, but maintain normal MHC-II presentation to helper T cells. This impaired cross-priming ability is autonomous to DC and is evident in animals deficient in both Sb9 and GrB, indicating that this role of Sb9 in DC is GrB-independent. In Sb9-deficient mice, CD8(+) DC develop normally, survive as well as wild-type DC after antigenic challenge, and exhibit unimpaired capacity to take up antigen. Although the core processing machinery is unaffected, Sb9-deficient DC appear to process antigen faster. Our results point to a novel, GrB-independent role for Sb9 in DC cross-priming.