Ultrastructural localization of RPE-associated epitopes recognized by monoclonal antibodies in human RPE and their induction in human fibroblasts by vitreous

Electron microscopic immunocytochemistry was performed to localize the epitopes recognized by monoclonal antibodies RPE15 and RPE9, reported to specifically stain retinal pigment epithelial (RPE) cells by light microscopy, and to evaluate the usefulness of these antibodies for recognizing phenotypically altered, pathological RPE cells. The labeling patterns of the two antibodies were indistinguishable, and in human eyes positivity was limited to RPE cells. In in situ and vitreous-cultured human RPE cells the epitopes were localized to the surface and intracellular membranes and to the cytoplasm. In vitreous culture many RPE cells developed processes containing filaments which reacted with either antibody. Human retinal glial cells were negative. Some human fibroblasts in vitreous culture showed labeling of the same structures as RPE cells with either antibody, limiting the usefulness of these antibodies for distinguishing RPE cells from fibroblasts, which can assume similar morphologies when in contact with vitreous; however, they may be useful adjuncts to anti-cytokeratin antibodies for RPE cell identification in various pathological conditions.