Antigen-processing pathways in the APC Antigen-processing pathways in the APC. There are 3 pathways of antigen processing by the APC: exogenous, endogenous, and cross-presentation. The exogenous pathway processes proteins produced outside the APC and places their peptides on MHC class II for recognition by CD4+ T cells. Proteins derived from apoptotic bodies, bacteria, and particulate antigens are processed into peptides by the exogenous pathway. The exogenous pathway occurs in phagocytic cells, including B cells, macrophages, and dendritic cells. The endogenous pathway places cell-produced peptides into the peptide-binding groove of MHC class I for recognition by CD8+T cells. The endogenous pathway is responsible for immune recognition of viral peptides or self peptides. Peptides are cleaved from proteins in the cytosol by the proteasome. They then enter the endoplasmic reticulum (ER) in a process dependent on the transporter associated with antigen processing (TAP) and are loaded onto MHC class I, which is then shuttled to the cell surface. This pathway occurs in most cells, not just APCs, allowing sensing of viral infection in all cell types. The cross-presentation pathway also loads peptides onto MHC class I for recognition by CD8+T cells, but the proteins that are processed are not cell-intrinsic, and are instead taken up from the surrounding environment. This pathway is important for detecting viruses that infect cells other than APCs and tumor antigens taken up in the form of tumor apoptotic bodies. The cross-presentation pathway is most efficient in dendritic cells. Source: The Immune System and Immunotherapy, The Basic Science of Oncology, 5e Citation: Tannock IF, Hill RP, Bristow RG, Harrington L. The Basic Science of Oncology, 5e; 2016 Available at: http://hemonc.mhmedical.com/DownloadImage.aspx?image=/data/books/1791/tanbas5_ach21_f002.png&sec=124305647&BookID=1791&ChapterSecID=124305636&imagename= Accessed: November 15, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved