Microglial cells invade the outer retina as photoreceptors degenerate in Royal College of Surgeons rats.

PURPOSE: Photoreceptor degeneration is accompanied by the invasion of phagocytic cells into the outer retina of Royal College of Surgeons (RCS) rats. Previous studies suggested that these mononuclear phagocytes were blood-borne macrophages and not retinal pigment epithelial cells nor Müller glia. Thus, immunospecific markers were used to identify these cells and to determine their distribution in the dystrophic retina.
METHODS: Retinas from RCS and control (RCS-rdy+) rats were processed for immunocytochemistry using antibodies against phosphotyrosine, which labels both microglial cells and peripheral macrophages, and ED2, which labels peripheral macrophages only. As a positive control to demonstrate ED2-labeling of peripheral macrophages that enter the retina during injury, experiments were performed using needle-punctured Long Evans rat retinas.
RESULTS: In normal animals, process-bearing, phosphotyrosine-reactive cells were restricted to the inner retinal layers and the outer plexiform layer. In early dystrophic retinas, phosphotyrosine-reactive cells also were observed in the outer retinal layers. The number of phosphotyrosine-labeled cells in the outer retina increased substantially in later stages of dystrophy. ED2-reactive cells were observed in normal or dystrophic retinas only after needle puncture.
CONCLUSIONS: These findings suggest that phagocytic cells during the early stages of dystrophy in RCS rat retinas are derived from resident microglial cells, not from peripheral macrophages. The migration of microglial cells into the outer retina when photoreceptor cells begin to degenerate further suggests that they may play a major role in photoreceptor cell death in the dystrophic retina.