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 complex (MHC) class I molecules to CD8⁺ T cells in a process referred to as cross-priming. Cross-priming is essential for the induction of CD8⁺ T cell responses directed towards antigens not expressed in professional APCs. Although in vitro experiments have shown that dendritic cells (DCs) and macrophages 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 β2-microglobulin–deficient splenocytes and show that they indeed present cell-associated antigens in the context of MHC class I molecules. This process is transporter associated with antigen presentation (TAP) dependent, suggesting an endosome to cytosol transport. To determine whether a specific subset of splenic DCs is involved in this cross-presentation, we negatively and positively selected for CD8⁻ and CD8⁺ DCs. Only the CD8⁺, and not the CD8⁻, DC subset demonstrates cross-priming ability. FACS^® 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 cell-associated antigens.