Immunocytochemical localization of the mannose receptor on ultrathin cryosections of chicken macrophages.

Emulphogene-solubilized chicken macrophages were used for the isolation of the mannose receptor by affinity chromatography on mannose-sepharose. From 5 X 10(9) cells 1 microgram protein was obtained, which was separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) into 2 bands with an approximate molecular weight of 130 and 170 kDa. The agglutinating activity was assayed with mannan-coated M. luteus cells. Agglutination was inhibited by D-mannose, L-fucose and D-N-acetylglucosamine. A rabbit antibody against the protein competed with mannan and mannosylated ferritin for the binding sites. The receptor was localized by immunolabelling on ultrathin frozen sections and the relative density of labelling/cell compartment was calculated. The receptor appeared randomly distributed on the surface. Labelling of coated pits was occasional. A higher density of the gold marker was found on surface infoldings (filopods, lamellopods). Subcellular membranous structures contained few labelled regions, with a relative increase from rough endoplasmatic reticulum to Golgi vacuoles. The highest average density was found on membranes of large vesicles near the surface, presumably derived from lamellopods which fuse at their tips to create an internalized vacuole. Fluorescence micrographs showed the complex folding of plasma processes, sometimes forming crater-like apertures. The particular fluorescence intensity of methanol-fixed cells, due to large vesicles, reflects the amount of receptor which is not exposed on the surface. The extent of receptor-rich membrane involved in formation of surface infoldings, craters and large vesicles indicates their role in receptor traffic in the absence of specific ligands.