Figure 5. Crossing Boundaries; Various Models of Cross-Presentation by MHC I.
MHC I molecules can present exogenous antigens and antigens delivered in apoptotic bodies and other types of cell debris. For dendritic cells, this process can result in cross-priming of CD8+ T cells. MHC I can recycle through the endosomal pathway to acquire antigen fragments made by proteases such as insulin regulated aminopeptidase (IRAP)[129] for cross-presentation. Endosomes may also acquire TAP and other ER molecules that may help export antigens into the cytosol for proteasomal hydrolysis and the resulting peptides may be reimported into the endosomes for MHC I loading and recycling and/or be delivered in the normal antigen presentation pathway as shown in Figure 2.