Phagocytosis by human retinal glial cells in culture

Under a variety of pathologic conditions, glial cells of the retina are capable of phagocytosis. Although phagocytosis may play a role in retinal pathobiology, the regulation of the phagocytic activity of retinal glial cells is poorly understood. We used a culture system to study phagocytosis by human retinal glial cells. The cultured cells were obtained from adult postmortem eyes and were immunoreactive to antibodies for glial fibrillary acidic protein and Muller cells. Electron microscopy demonstrated that the glial cells in culture were capable of phagocytosing fragments of retinal cells as well as latex beads. To rapidly quantitate phagocytosis, flow cytometry was used to detect glial cells that had internalized fluorescein-labeled microspheres. We found that reducing the extracellular calcium concentration decreased the phagocytic activity of the retinal glia. An inhibitory effect by nifedipine, a calcium channel blocker, on phagocytosis suggests a role for these ion channels in mediating a phagocytic response by retinal glial cells. A possible modulatory action of cyclic AMP was indicated by a decrease in phagocytic activity with exposure to 8-bromo-cyclic AMP. In addition to identifying conditions that reduce phagocytosis, we found that vitamin D3 can stimulate the phagocytic activity of retinal glia. Our experiments establish that phagocytosis by human retinal glial cells can be studied in a culture system and demonstrate that certain molecules can regulate the phagocytic activity of these cells.