Conditions of Retinal Glial and Inflammatory Cell Activation after Irradiation in a GFP-Chimeric Mouse Model

PURPOSE. Microglia cells have been associated with immunologic defense and repair. The course of retinal disease after lethal irradiation for bone marrow depletion and substitution was evaluated with respect to macrophage and microglial involvement. METHODS. Lethal irradiation in C57BL/6 mice was conducted with a low-voltage radiation unit. The animals were randomized to shielded or unshielded radiation and subsequently received transplants of GFP(+) bone marrow cells (beta-actin promoter). The GFP transformation rate was analyzed by flow cytometry. GFP(+) cells in the retina were examined for colocalization with macrophage and dendritic cell markers at various time points between 1 and 7 months after irradiation. Clodronate liposomes were used to investigate the fate of migrated and residential microglia cells. Pathologic angiogenesis was investigated in laser-induced choroidal neovascularization (CNV) after unshielded and shielded irradiation. RESULTS. Flow cytometry revealed average transformation rates of 78.2% in unshielded and 64.1% in shielded group. Four weeks after transplantation, perfused flat mounts were virtually free of extravasal GFP(+) cells in both groups, whereas 4 months after irradiation, cluster cell infiltrations, preferentially in the peripheral retina, became apparent exclusively in the unshielded group. Cell morphology ranged from oval, to a few extensions, to dendritiform with long-branched extensions. Clodronate treatment resulted in a reduction of GFP(+) cells in the retinal tissue when applied 3 months after unshielded irradiation. Although GFP(+) cells accumulated in the choroidal scar after laser treatment, in both the shielded and unshielded groups, GFP(+) cells in the overlying retina were restricted to the unshielded group. CONCLUSIONS. Approximately 3 months after lethal full-body irradiation including the eye, bone marrow-derived leukocytes exhibit a wound-healing reaction, and unlike physiological turnover, infiltrate the retina and form microglial cells. (Invest Ophthalmol Vis Sci. 2010;51:4831 - 4839) DOI:10.1167/iovs.09-4923