Isolation and characterization of a mannose receptor from human pigment epithelium.

Recent work demonstrated that a mannose receptor is involved in the phagocytosis of rod outer segments by the rat retinal pigment epithelium (RPE). In this study the binding of soluble mannose-containing ligands by human RPE explants is described. In addition, the authors report the isolation of a mannose receptor from human RPE and describe its relationship to the macrophage mannose receptor. Epithelial explants bound the soluble ligand 125I-mannose bovine serum albumin (BSA) by a mannose-specific process. The protein involved in mannose recognition was extracted from human tissue and purified using ligand-affinity chromatography. The protein that bound to the affinity column had a molecular weight of 175 kD by sodium dodecyl sulfate gel electrophoresis and migrated with the same mobility as the human macrophage mannose receptor. Antibodies directed against the macrophage receptor crossreacted with the mannose receptor from human RPE by immunoblot analysis. Binding specificity studies demonstrated that mannose and mannan inhibited ligand binding to the purified receptor by 65% and 90%, respectively; galactose had no effect. Using immunogold labeling of human RPE cells in explant culture, antimacrophage mannose receptor was localized at the apical plasma membrane. These results suggest that human RPE expresses a mannose receptor on its apical surface (as does the rat RPE) and that this receptor is similar to the human macrophage mannose receptor.