Trafficking of spontaneously endocytosed MHC proteins

Class I MHC protein primarily presents endogenous antigen but also may present exogenous antigen. Here, we investigated the intracellular pathway of spontaneously internalized class I MHC protein by confocal microscopy. β(2)-microglobulin (β(2)m), labeled with a single fluorophore, was exchanged at the surface of B cell transfectants to specifically mark cell surface and endocytosed class I MHC protein. Intracellular β(2)m colocalized with fluorophore-conjugated transferrin, implying that class I MHC protein endocytosed into early endosomes. These endosomes containing fluorescent β(2)m were found close to or within the Golgi apparatus, marked by fluorescent ceramide. Even after 24 hr of incubation, very little fluorescent β(2)m was found in intracellular organelles stained by DiOC(6), marking the endoplasmic reticulum, or fluorophore-conjugated low density lipoprotein, marking late endosomes and lysosomes. Fluorophore-conjugated superantigens (staphylococcal enterotoxin A and B), presumed to enter cells bound to class II MHC protein, also were found to endocytose into β(2)m-containing early endosomes. Staining with mAb and use of transfectants expressing MHC protein attached to green fluorescent protein confirmed the presence of intracellular compartments rich in both class I and II MHC protein and demonstrated that class I and II MHC protein also colocalize in discrete microdomains at the cell surface. These cell surface microdomains also contained transferrin receptor and often were juxtaposed to cholesterol-rich lipid rafts. Thus, class I and II MHC protein meet in microdomains of the plasma membrane and endocytose into early endosomes, where both may acquire and present exogenous antigen.