Localization of MHC class II molecules in murine bone marrow-derived macrophages.

The localization of MHC class II molecules (Ia) was studied by ultrastructural immunocytochemistry in murine bone marrow-derived macrophages stimulated by recombinant interferon-gamma (rIFN-gamma). Ia molecules were detected on the plasma membrane and on the limiting membrane and internal structures of vesicular acidic compartments. Some of these vesicules also contained cathepsin B and/or cathepsin D. The use of BSA-gold, a marker of fluid phase endocytosis, allowed the identification of Ia-positive organelles as endocytic compartments. The first to be labelled with BSA-gold also contained the cation-independent mannose 6-phosphate receptor (MPR) but not the 120,000 molecular weights lysosomal glycoprotein (lgp 120). Later on, BSA-gold appeared in Ia+, MPR+, lgp 120+ compartments. Collectively these data suggest that intracellular Ia molecules are mainly present in early and late endosomes.