Intracellular traffic to compartments for MHC class II peptide loading: signals for endosomal and polarized sorting

In this review we focus on the traffic of MHC class II and endocytosed anti gens to intracellular compartments where antigenic peptides are loaded. We also discuss briefly the nature of the peptide loading compartment and the sorting signals known to direct antigen receptors and MHC class II and asso ciated molecules to this location. MHC class II molecules are expressed on a variety of polarized epithelial and endothelial cells, and polarized cell s are thus potentially important for antigen presentation. Here we review s ome cell biological aspects of polarized sorting of MHC class II and the as sociated invariant chain and the signals that are involved in the sorting p rocess to the basolateral domain. The molecules involved in sorting and loa ding of peptide may modulate antigen presentation, and in particular we dis cuss how invariant chain may change the cellular phenotype and the kinetics of the endosomal pathway.