Characterization of activated retinal microglia following optic axotomy

Microglia are prominently involved in neural degenerative diseases of the CNS and the retina. In this study, we determined the activation and phagocytotic function of different subtypes of retinal microglial cells at 1 week and 1 month following optic axotomy. Fluorescent DiI crystals were placed at the stumps of the cut optic nerves of Lewis rats to retrolabel retinal ganglion cells. Microglial cells were indirectly labeled as they phagocytosed the dye particles in the dying ganglion cells. OX-42, 5D4, ED1, and OX-6 antibodies were used for immunohistochemical study. The OX-42- and 5D4-positive microglial cells were increased in the inner retinal layers after optic axotomy. The increase of OX-42-positive cells was considerably greater than that of 5D4-positive cells. The 5D4-positive cells were ramified in shape, whereas OX-42-positive cells were ameboid and ovoid. Both 5D4- and OX-42-positive cells phagocytosed dying ganglion cells at 1 week and 1 month after axotomy. Scattered ameboid ED1-positive cells were detected in the normal retina and showed phagocytotic activity at 1 month after optic axotomy. The number of ED1-positive cells in the retina was unchanged after axotomy. In optic axotomy, three types of microglial cells were activated, namely, 5D4-positive ramified cells and OX-42- and ED1-positive ameboid cells. All of them exhibited the phagocytosis of dying ganglion cells. Insofar as the blood-retinal barrier presumably remained intact in optic axotomy, the OX-42- and 5D4-positive cells might derive from resident microglial cells. The ED1-positive cells, presumably recently blood-borne macrophage in the CNS, remained the same number in the axotomized retina.