Alternative pathways for MHC class I presentation: a new function for autophagy.

The classical view that endogenous antigens are processed by the proteasome and loaded on MHC class I molecules in the endoplasmic reticulum, while exogenous antigens taken up by endocytosis or phagocytosis are degraded and loaded on MHC class II in lysosome-derived organelles, has evolved along with the improvement of our understanding of the cell biology of antigen-presenting cells. In recent years, evidence for alternative presentation pathways has emerged. Exogenous antigens can be processed by the proteasome and loaded on MHC class I through a pathway called cross-presentation. Moreover, endogenous antigens can be targeted to lytic organelles for presentation on MHC class II through autophagy, a highly conserved cellular process of self-eating. Recent evidence indicates that the vacuolar degradation of endogenous antigens is also beneficial for presentation on MHC class I molecules. This review focuses on how various forms of autophagy participate to presentation of these antigens on MHC class I.