CD8(+) but not CD8(-) dendritic cells cross-prime cytotoxic T cells in vivo

Bone marrow-derived antigen-presenting cells (APCs) take up cell-associated antigens and present them in the context of major histocompatibility compl ex (MHC) class I molecules to CD8(+) T cells in a process referred to as cr oss-priming. Cross-priming is essential for the induction of CD8(+) T cell responses directed towards antigens not expressed in professional APCs. Alt hough in vitro experiments have shown that dendritic cells (DCs) and macrop hages are capable of presenting exogenous antigens in association with MHC class I, the cross-presenting cell in vivo has not been identified. We have isolated splenic DCs after in vivo priming with ovalbumin-loaded beta2-mic roglobulin-deficient splenocytes and show that they indeed present cell-ass ociated antigens in the context of MHC class I molecules. This process is t ransporter associated with antigen presentation (TAP) dependent, suggesting an endosome to cytosol transport. To determine whether a specific subset o f splenic DCs is involved in this cross-presentation, we negatively and pos itively selected for CD8(-) and CD8(+) DCs. Only the CD8(+), and not the CD 8(-), DC subset demonstrates cross-priming ability. FACS((R)) studies after injection of splenocytes. loaded with fluorescent beads showed that 1 and 0.6% of the CD8(+) and the CD8(-) DC subsets, respectively, had one or more associated beads. These results indicate that CD8(+) DCs play an important role in the generation of cytotoxic T lymphocyte responses specific for ce ll-associated antigens.