Distinct pathways of antigen uptake and intracellular routing in CD4 and CD8 T cell activation.

The mechanisms that allow antigen-presenting cells (APCs) to selectively present extracellular antigen to CD8+ effector T cells (cross-presentation) or to CD4+ T helper cells are not fully resolved. We demonstrated that APCs use distinct endocytosis mechanisms to simultaneously introduce soluble antigen into separate intracellular compartments, which were dedicated to presentation to CD8+ or CD4+ T cells. Specifically, the mannose receptor supplied an early endosomal compartment distinct from lysosomes, which was committed to cross-presentation. These findings imply that antigen does not require intracellular diversion to access the cross-presentation pathway, because it can enter the pathway already during endocytosis.