The dominant role of CD8+ dendritic cells in cross-presentation is not dictated by antigen capture

Mouse spleens contain three populations of conventional (CD11chigh) 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.