purpose. To determine whether primate embryonic stem (ES) cell-derived pigment epithelial cells (ESPEs) have the properties and functions of retinal pigment epithelial (RPE) cells in vitro and in vivo.

methods. Cynomolgus monkey ES cells were induced to differentiate into pigment epithelial cells by coculturing them with PA6 stromal cells in a differentiating medium. The expanded, single-layer ESPEs were examined by light and electron microscopy. The expression of standard RPE markers by the ESPEs was determined by RT-PCR, Western blot, and immunocytochemical analyses. The ESPEs were transplanted into the subretinal space of 4-week-old Royal College of Surgeons (RCS) rats, and the eyes were analyzed immunohistochemically at 8 weeks after grafting. The effect of the ESPE graft on the visual function of RCS rats was estimated by optokinetic reflex.

results. The expanded ESPEs were hexagonal and contained significant amounts of pigment. The ESPEs expressed typical RPE markers: ZO-1, RPE65, CRALBP, and Mertk. They had extensive microvilli and were able to phagocytose latex beads. When transplanted into the subretinal space of RCS rats, the grafted ESPEs enhanced the survival of the host photoreceptors. The effects of the transplanted ESPEs were confirmed by histologic analyses and behavioral tests.

conclusions. The ESPEs had morphologic and physiological properties of normal RPE cells, and these findings suggest that these cells may provide an unlimited source of primate cells to be used for the study of pathogenesis, drug development, and cell-replacement therapy in eyes with retinal degenerative diseases due to primary RPE dysfunction.