This process involves two distinct pathways for processing of antigens from an organism's own (self) proteins or intracellular pathogens (e.g. viruses), or from phagocytosed pathogens (e.g. bacteria); subsequent presentation of these antigens on class I or class II major histocompatibility complex (MHC) molecules is dependent on which pathway is used.
Both MHC class I and II are required to bind antigens before they are stably expressed on a cell surface.
While the joint distinction between the two pathways is useful, there are instances where extracellular-derived peptides are presented in the context of MHC class I and cytosolic peptides are presented in the context of MHC class II (this often happens in dendritic cells).
The endogenous pathway is used to present cellular peptide fragments on the cell surface on MHC class I molecules.
The partially folded MHC class I molecule then interacts with TAP via tapasin (the complete complex also contains calreticulin and Erp57 and, in mice, calnexin).
Proteins are endocytosed and degraded by acid-dependent proteases in endosomes; this process takes about an hour.
The invariant chain also facilitates MHC class II's export from the ER in a vesicle.
An MHC class II-like structure, HLA-DM, removes CLIP and replaces it with a peptide from the endosome.
[2] In Cross-presentation, peptides derived from extracellular proteins are presented in the context of MHC class I.
Nef from some HIV strains enhance the movement of MHC molecules back into the cytoplasm, preventing them from presenting antigens.
The antigen is digested into peptide fragments by various proteasomes and is then displayed at the cell surface attached along with a class II histocompatibility molecule.