An in vitro model of phagocytosis in bovine and human retinal pigment epithelium

Abstract

We have developed an in vitro system for studying phagocytosis in retinal pigment epithelium (RPE). Explants of bovine and human RPE and choroid have been maintained in organ culture, and phagocytosis of 1·1 μm diameter latex microspheres by the RPE has been studied over 24-hr periods. After a latent period of about 8 hr latex was rapidly phagocytized from the apical surface of the cell by more than 90% of the cells in the explant. Each latex bead was surrounded by a membrane derived from apical plasma membrane. Morphometric analysis of organellar area in electron micrographs of phagocytizing vs. fresh RPE cells showed that microvillus membrane is internalized during phagocytosis. A web of microfilaments was apparent near the plasma membrane where initial engulfment occurs, and many microtubules surrounded latex phagosomes deeper in the cytoplasm. Lysosomes fused with many latex phagosomes, but the number of lysosomes in bovine RPE cultured 22 hr exceeded that of fresh RPE. This suggests that either more lysosomes are assembled during acute phagocytic challenge, or that degradation o phagolysosomes occurs less rapidly when inert particles are phagocytized. Phagocytosis by bovine RPE was not affected by continual light or darkness. Dibutyryl cyclic adenosine monophosphate enhanced the total incorporation of latex, but dibutyryl cyclic guanosine monophosphate so altered the shape of cells that phagocytosis could not be assessed. Phagocytosis by human RPE in vitro is reported for the first time. Explants from rod-rich and cone-rich areas of human retina did not differ significantly in their phagocytic ability. The potential use of this explants preparation in the study of various aspects of the phagocytic mechanism in RPE is discussed.