A modular and combinatorial view of the antigen cross-presentation pathway in dendritic cells

Major histocompatibility complex class I (MHC I) presentation of exogenous antigens (cross-presentation) by dendritic cells (DC) is essential for CD8 T-cell immunity. Most cells use MHC I molecules to present peptides derived from endogenous proteins processed in the cytosol by the proteasome. The resulting peptides are translocated into the endoplasmic reticulum for loading onto MHC I molecules, and these complexes are then transported to the cell surface. In cross-presenting DC, these steps have been proposed to occur along two major tracks. In the 'endocytic' track, exogenous antigen processing and loading occur within endosomal compartments, using MHC I molecules recycled from the plasma membrane and transported back to the surface. In the 'cytosolic' track, antigens are translocated from endosomes to the cytosol, accessing the endogenous MHC I presentation pathway. This dichotomy now appears too simplistic. Some steps may occur in locations belonging to the endosomal track and others in the cytosolic track, or in hybrid compartments combining features of both. We propose a 'modular' view of cross-presentation, whereby processing, loading and MHC I transport represent modules that can occur in one or more locations. Cross-presentation of each MHC I-peptide complex may result from combining one or more options for each of these modules.