Review

Mounting an immune response against foreign, extracellular material requires that this material be internalized by antigen presenting cells, processed to peptide fragments and then displayed on the cell surface for recognition by the T cell receptor on T helper cells. Such peptides, derived from internalized antigens are generally presented to T cells in association with Major Histocompatibility Complex (MHC) class II molecules. Recent work has identified subcompartments of the endosomal/lysosomal system that appear to be important sites for the generation of peptide-MHC class II complexes. These so-called MHC class II compartments receive antigenic fragments from endosomes and lysosomes and load them onto class II molecules. From these compartments, peptide-loaded class II molecules are transported to the plasma membrane, where they can trigger T cells bearing appropriate receptors. This review summarizes recent work characterizing MHC class II compartments as specialized organelles of the endosomal/lysosomal pathway. Interestingly, MHC class II compartments share many features common to organelles present in a wide variety of specialized cells with quite different functions. Modifying the endocytic pathway to serve the needs of specialized cells may be a common mechanism which allows very different cell types to carry out their disparate functions.