MHC class II epitopes unfolded antigen MHC class II antigen processing Antigen processing and loading on to MHC class II molecules occurs in late endocytic/lysosomal vesicles, where epitopes which require extensive proteolytic processing are generated (Bryant & Ploegh 2003). These late endosome/lysosomal-like compartments are enriched in MHC II and are referred to as MIICs (MHC class II compartments) (Peters et al. 1991). A variety of lysosomal proteases like cathepsins S, D, B etc. and Asparaginyl endopeptidase (AEP) are suggested to be involved in the processing of antigens to generate CD4+ T cell epitopes (Deusing et al. 1998, Shi et al. 1999, Pluger et al. 2002, Watts et al. 2005). An initial cleavage by endopeptidases (AEP) would be necessary to unlock the antigen and allow further trimming of the ends by exopeptidases (Cathepsins). In the acidic environment of the lysosome, cathepsins are generally activated by autocatalytic cleavage of a propeptide which otherwise blocks the active site. Authored: Garapati, P V, 2012-02-21 EC Number: 3.4.22 Edited: Garapati, P V, 2012-02-21 Pubmed10072072 Pubmed10809954 Pubmed11813165 Pubmed12742663 Pubmed14734116 Pubmed16181339 Pubmed1847504 Pubmed9539769 Reactome Database ID Release 432130706 Reactome, http://www.reactome.org ReactomeREACT_120767 Reviewed: Neefjes, Jacques, 2012-05-14 has a Stoichiometric coefficient of 3