The fate of immunoreactive opsin following phagocytosis by pigment epithelium in human and monkey retinas.

Polyclonal and monoclonal antibodies to human rhodopsin were used to identify and localize this principal glycoprotein of the photoreceptor outer segment discs on thin sections of human and monkey retinal pigment epithelium (RPE) and on immunoblots of RPE subcellular fractions following gel electrophoresis. Antiopsin was visualized with protein A-gold labeling by electron microscopy or peroxidase-linked second antibody on immunoblots. In immunocytochemical studies using polyclonal antibodies, the rod outer segments (ROS) were heavily labeled whereas cone outer segments labeling was variable and more sparse. Phagosomes and other small bodies in the RPE, interpreted as secondary lysosomes, were labeled. In contrast, lipofuscin granules, osmiophilic residual bodies of the lysosomal system of the RPE, were negative. No reactive sites were found in Bruch's membrane or in drusen. A monoclonal antibody (MAB) specific for the amino terminus of human opsin and another MAB specific for the carboxy terminal region of bovine opsin produced labeling patterns similar to, but about one-half the density obtained with polyclonal antibodies. In the immunoblot analyses, the lipofuscin granule fraction from a sucrose density gradient of human RPE homogenates was positive for rhodopsin only in those specimens that were found, upon ultrastructural examination, to contain recognizable phagosomes. When phagosomes were lacking and therefore did not contaminate the lipofuscin granule fraction, the immunoblots were negative for opsin. Melanolipofuscin granule fractions were uniformly negative for opsin. We conclude that the superficial hydrophilic antigen binding sites on the opsin molecule for which the antibodies are specific have been altered or destroyed by lysosomal enzyme digestion within the phagolysosomal system of the RPE prior to formation of definitive lipofuscin granules. Thus, these antibodies are of limited value in revealing the ultimate fate of the whole rhodopsin molecule, eg, the hydrophobic sequences that are the most likely residues in lipofuscin granules.