Intracellular organelles involved in antigen processing and the binding of peptides to class II MHC molecules.

Nascent class II MHC (MHC class II) molecules are transported from the ER to a specialized late endocytic antigen processing compartment, termed MIIC (MHC class II compartment). This compartment, with some variations, has been defined in multiple studies of B cells, macrophages and dendritic cells. Though this compartment shares properties with both lysosomes and late endosomes, it is a specialized compartment that differs from these other endocytic compartments in important characteristics. The MIIC contains abundant MHC class II molecules and other molecules important for antigen processing, including lysosomal enzymes and HLA-DM. Biochemical and functional immunological assays have detected not only the presence but the initial formation of peptide-MHC class II complexes in MIIC during the processing of certain model antigens. However, MHC class II molecules are also present in other endocytic compartments, albeit at lower levels, and these compartments may also perform antigen processing functions.