Microglia Depletion Exacerbates Retinal Ganglion Cell Loss in a Mouse Model of Glaucoma

To test whether depletion of microglia in the optic nerve head has a beneficial effect on retinal ganglion cell numbers and function, we depleted microglia by oral administration of the CSF1R antagonist PLX5622. Then, ocular hypertension was induced by unilateral injection of magnetic microbeads into the anterior chamber. Visual function was assessed with pattern electroretinography and measurement of the optomotor reflex. Retinal ganglion cell bodies and axons were counted and gene expression patterns in optic nerve head astrocytes were tested on freshly dissociated astrocytes. PLX5622 efficiently depleted microglia in the retina and the optic nerve head, but about 20% of microglia persisted in the myelinated optic nerve proper even after prolonged exposure to the drug. PLX5622 did not affect ganglion cell function by itself. Elevation of the IOP for four weeks led to the expected decrease in visual acuity and pattern ERG amplitude. Microglia ablation did not affect these parameters. Ganglion cell and axon numbers were counted histologically post mortem. Mice in the microglia depletion group showed a moderate but significantly greater loss of ganglion cells than the control group. At four weeks post microbead injection, gene expression patterns in optic nerve head astrocytes are consistent with an A2 (or neuroprotective) pattern. Microglia depletion blunted the up-regulation of A2 genes in astrocytes. In conclusion, microglia depletion is unlikely to protect retinal ganglion cells in early glaucoma.