A mannose receptor is involved in retinal phagocytosis

A 175-kD mannose-specific receptor has been described in macrophages which appears to mediate pinocytosis and phagocytosis. A mannose-specific receptor has also been found on retinal pigment epithelial cells (RPE). Its role was examined in the phagocytosis of photoreceptor outer segments (ROS) by the RPE. All testing was done on cultured RPE cells challenged with fluorescein isothiocyanate (FITC)-labeled ROS for 4 hr at 37 degrees C. Total (internalized and bound) and phagocytized (internalized) ROS were quantified, and the effects of several experimental conditions on ROS phagocytosis were examined. Incubation of RPE cells in the presence of rabbit anti-serum (1:100) raised against the rat alveolar macrophage mannose receptor (anti-Mr) showed a 80% reduction in ROS phagocytosis compared with RPE in the presence of ROS alone. Anti-Mr preabsorbed with purified human mannose receptor protein (4 micrograms) showed no reduction in ROS phagocytosis. Incubation of RPE with preimmune serum showed no reduction in ROS phagocytosis. When FITC-labeled ROS (1.8 X 10(7)) were preincubated with purified mannose receptor, there was a 93% reduction in phagocytosis. Immunoblots of solubilized rat RPE microvillus membranes stained with the anti-Mr showed a single stained band at the 175-kD region, and immunolocalization studies showed specific labeling along RPE microvilli. These results suggest that a mannose-specific receptor is involved in retinal phagocytosis.