THE PHAGOCYTOSIS OF ROS BY RPE CELLS IS INHIBITED BY AN ANTISERUM TO RAT RPE CELL PLASMA-MEMBRANES

A polyclonal antiserum to a rat retinal pigment epithelium (RPE) plasma membrane-enriched fraction has been utilized to identify candidate receptor proteins which may be involved in the phagocytosis of rod outer segments (ROS) by the RPE. Immunoblots of RPE cell extracts show that the antiserum recognizes a number of glycoproteins, including two with Mrs of 174 and 75 kDa. The antiserum also recognizes their non-glycosylated counterparts, with Mrs of 169 and 65 kDa, respectively, which are synthesized after treatment of the cells with tunicamycin B2. Immuno-precipitation of [35S]-methionine-labeled RPE cell extracts also demonstrates the presence of antibodies to these same glycoproteins as well as to other proteins. The antiserum inhibits the binding of ROS to the RPE, which subsequently results in a decrease in the ingestion of ROS. ROS phagocytosis by the RPE is inhibited by 97% in the presence of a 1 : 10 dilution of the IgG fraction of the antiserum. Phagocytosis recovers to normal levels after 4-6 hr of chase in the absence of antibodies. After sequential adsorption of the IgG fraction to monolayers of fixed RPE cells, which removes RPE surface-specific IgGs, the extent of inhibition of ROS phagocytosis produced by the IgG fraction is reduced. Using immunoblotting we have identified a number of surface-specific immunoreactive bands which are adsorbed out of the antiserum, including the 174 and 75 kDa bands. These data give further support to the hypothesis that ROS phagocytosis is a receptor-mediated process, which occurs via specific cell surface glycoprotein receptors. © 1992.