Dendritic cells (DCs) comprise several subsets, and their roles in the presentation of antigens derived from pathogens, vaccines and self tissues are now beginning to be elucidated. Differences in location, life cycle and intrinsic abilities to capture, process and present antigens on their MHC class I and class II molecules enable each DC subset to have distinct roles in immunity to infection and in the maintenance of self tolerance. Unexpected interactions among DC subsets have also been revealed. These interactions, which allow the integration of the intrinsic abilities of different DC types, enhance the ability of the DC network to respond to multiple scenarios of infection.

We demonstrate that functional and phenotypic equivalents of mouse splenic CD8+ and CD8− conventional dendritic cell (cDC) subsets can be generated in vitro when bone marrow is cultured with fms-like tyrosine kinase 3 (flt3) ligand. In addition to CD45RAhigh plasmacytoid DC, two distinct CD24high and CD11bhigh cDC subsets were present, and these subsets showed equivalent properties to splenic CD8+ and CD8− cDC, respectively, in the following: 1) surface expression of CD11b, CD24, and signal regulatory protein-α; 2) developmental dependence on, and mRNA expression of, IFN regulatory factor-8; 3) mRNA expression of TLRs and chemokine receptors; 4) production of IL-12 p40/70, IFN-α, MIP-1α, and RANTES in response to TLR ligands; 5) expression of cystatin C; and 6) cross-presentation of exogenous Ag to CD8 T cells. Furthermore, despite lacking surface CD8 expression, the CD24high subset contained CD8 mRNA and up-regulated surface expression when transferred into mice. This culture system allows access to bona fide counterparts of the splenic DC subsets.

Mouse spleens contain three populations of conventional (CD11c high ) dendritic cells (DCs) that play distinct functions. The CD8 ؉ DC are unique in that they can present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. It is unclear whether this special ability is because only the CD8 ؉ DC can capture the antigens used in cross-presentation assays, or because this is the only DC population that possesses specialized machinery for cross-presentation. To solve this important question we examined the splenic DC subsets for their ability to both present via MHC class II molecules and cross-present via MHC class I using four different forms of the model antigen ovalbumin (OVA). These forms include a cell-associated form, a soluble form, OVA expressed in bacteria, or OVA bound to latex beads. With the exception of bacterial antigen, which was poorly cross-presented by all DC, all antigenic forms were cross-presented much more efficiently by the CD8 ؉ DC. This pattern could not be attributed simply to a difference in antigen capture because all DC subsets presented the antigen via MHC class II. Indeed, direct assessments of endocytosis showed that CD8 ؉ and CD8 ؊ DC captured comparable amounts of soluble and bead-associated antigen, yet only the CD8 ؉ DC cross-presented these antigenic forms. Our results indicate that cross-presentation requires specialized machinery that is expressed by CD8 ؉ DC but largely absent from CD8 ؊ DC. This conclusion has important implications for the design of vaccination strategies based on antigen targeting to DC.antigen presentation ͉ mice ͉ endocytosis ͉ ovalbumin ͉ vaccines D endritic cells (DC) possess several mechanisms that make them highly efficient antigen-presenting cells. DC can endocytose a large variety of exogenous antigens for presentation via MHC class II molecules and for cross-presentation via MHC class I (1). The cross-presenting capacity of DC is unusual, because most other cell types are only able to present endogenous antigens (i.e., antigens synthesized by the antigenpresenting cells themselves) on their MHC class I molecules. Thus, DC possess specialized machinery, as yet not fully defined, that allows delivery of exogenous antigens into the MHC class I presentation pathway for cross-presentation (2, 3).On the other hand, several populations of DC have been described (4, 5), and it is unclear whether all these DC types can cross-present (6). Mouse spleens contain three ''conventional'' (CD11c high ) DC subsets: CD8 ϩ DC, CD4 ϩ DC, and CD4 Ϫ CD8 Ϫ [double negative (DN)] DC. Previous studies have shown that cell-associated antigen was cross-presented by CD8 ϩ but not CD8 Ϫ DC (7-10). The unique capacity of the CD8 ϩ DC at cross-presenting this form of antigen was attributed to their ability to capture dead cells (7-9, 11, 12) because antigens in soluble or immunocomplexed form, or associated to bacteria, were reportedly cross-presented by both CD8 ϩ and CD8 Ϫ DC (8, 9, 13). These findings led to the hypothesis that all DC can...

The importance of conventional dendritic cells (cDCs) in the processing and presentation of antigen is well established, but the contribution of plasmacytoid dendritic cells (pDCs) to these processes, and hence to T cell immunity, remains unclear. Here we showed that unlike cDCs, pDCs continued to synthesize major histocompatibility complex (MHC) class II molecules and the MHC class II ubiquitin ligase MARCH1 long after activation. Sustained MHC class II-peptide complex formation, ubiquitination and turnover rendered pDCs inefficient in the presentation of exogenous antigens but enabled pDCs to continuously present endogenous viral antigens in their activated state. As the antigen-presenting abilities of cDCs and pDCs are fundamentally distinct, these two cell types may activate largely nonoverlapping repertoires of CD4(+) T cells.