References
PMID:22790179, PMID:14508489
The presentation of exogenous antigens on MHC class I molecules, known as cross-presentation, is essential for the initiation of CD8+ T cell responses. In vivo, cross-presentation is mainly carried out by specific dendritic cell (DC) subsets through an adaptation of their endocytic and phagocytic pathways.
After phagocytosis, antigens are exported into the cytosol and degraded by the proteasome4, 5, 6. The resulting peptides are thought to be translocated into the lumen of the endoplasmic reticulum (ER) by specific transporters associated with antigen presentation (TAP), and loaded onto MHC class I molecules by a complex ???loading machinery??? (which includes tapasin, calreticulin and Erp57)