Optic nerve as a source of activated retinal microglia post-injury

Using mice expressing green fluorescent protein (GFP) from a transgenic CD11c promoter we found that a controlled optic nerve crush (ONC) injury attracted GFP^hi retinal myeloid cells to the dying retinal ganglion cells and their axons. However, the origin of these retinal myeloid cells was uncertain. In this study we use transgenic mice in conjunction with ONC, partial and full optic nerve transection (ONT), and parabiosis to determine the origin of injury induced retinal myeloid cells. Analysis of parabiotic mice and fate mapping showed that responding retinal myeloid cells were not derived from circulating macrophages and that GFP^hi myeloid cells could be derived from GFP^lo microglia. Comparison of optic nerve to retina following an ONC showed a much greater concentration of GFP^hi cells and GFP^lo microglia in the optic nerve. Optic nerve injury also induced Ki67⁺ cells in the optic nerve but not in the retina. Comparison of the retinal myeloid cell response after full versus partial ONT revealed fewer GFP^hi cells and GFP^lo microglia in the retina following a full ONT despite it being a more severe injury, suggesting that full transection of the optic nerve can block the migration of responding myeloid cells to the retina. Our results suggest that the optic nerve can be a reservoir for activated microglia and other retinal myeloid cells in the retina following optic nerve injury.