Rat retinal pigment epithelial cells show specificity of phagocytosis in vitro

Abstract

The retinal pigment epithelial (RPE) cell of the eye normally phagocytizes only retinal rod outer segments (ROS). The specificity of this phagocytic process was examined by incubating RPE cells with a variety of particle types. Confluent RPE cell cultures were incubated for 3 h at 37°C in the presence of rat ROS, rat red blood cells (RBC), algae, bacteria, or yeast. Other cell cultures were incubated with equal numbers of ROS and one other particle type. Quantitative scanning electron microscopy was used to determine the numbers and morphology of particles bound to RPE cells, while double immunofluorescence labeling (Chaitin, M.H., and M.O. Hall, 1983, Invest. Ophthalmol. Vis. Sci., 24:812-820) was used to quantitate particle binding and ingestion. Both assays demonstrated phagocytosis to be a highly specific process. RPE cells bound 40-250x more ROS than RBC, 30x more ROS than algae, and 5x more ROS than bacteria or yeast. Ingestion was more specific than binding; RPE cells ingested 970x more ROS than RBC, 140x more ROS than bacteria, and 35x more ROS than yeast. The phagocytic preference for ROS was maintained in competition experiments with other particle types. Serum was found to be essential for phagocytosis. This study demonstrates that both the binding and ingestion phases of phagocytosis are highly specific processes.