Proteolysis in MHC class II antigen presentation: who's in charge?

Antigen-presenting cells (APC) degrade proteins intracellularly to generate peptides, which are then bound by products of the major histocompatibility complex (MHC) and exposed on the surface of the APC for recognition by T cells. The supply of antigenic peptides and their association with MHC molecules requires the concerted action of a cohort of accessory molecules that includes chaperones, transporters of peptides, and the proteases that degrade the antigens.