Microglial phagocytosis of living photoreceptors contributes to inherited retinal degeneration

Abstract

Purpose: Microglia, the resident immune cells of the retina, have been implicated in the progression of photoreceptor degeneration in human and mouse models of retinal disease. We have previously characterized the distribution of retinal microglia during degeneration in the rd10 mouse model of retinitis pigmentosa (RP) and the molecular cues involved in guiding microglial activity and phagocytosis of rod photoreceptors. In order to determine the contribution of microglia to rod demise, we visualized live microglial dynamics and performed interventions to block microglial phagocytosis in rd10 mice.

Methods: rd10 homozygous mice were crossed with CX3CR1^+/GFP mice for live imaging experiments of retinal whole mounts. rd10 mice were crossed with CX3CR1-CreER x ROSA26-flox-STOP-flox-DTA mice in order to deplete microglia upon administration of tamoxifen. In vivo inhibition of microglial phagocytosis was accomplished with intravitreous injection of cyclic RGD peptide, a vitronectin receptor antagonist, into one eye and its inactive analogue, cyclic RAD, in the contralateral eye as an internal control. Immunohistochemistry and electroretinography were used to monitor anatomical and functional retinal degeneration.

Results: Ex vivo live imaging of retinal whole mounts revealed that microglia infiltrating the ONL repeatedly probe photoreceptor somata via dynamic process prior to the overt phagocytosis of rods. Interestingly, differential vital labeling of live and apoptotic cells with propidium iodide (PI) showed a disproportionately high number of live (PI-) photoreceptor cells undergoing phagocytosis compared to apoptotic (PI+) cells. Genetic ablation of retinal microglia during rod degeneration resulted in a significantly greater morphological and functional preservation of the ONL up to postnatal day (P)50. Injection of cRGD peptide to inhibit microglial phagocytosis decreased the number of infiltrating microglia and phagosomes per microglia, which in turn resulted in significantly greater ONL preservation at P26.

Conclusions: Our findings highlight the contribution of microglia to the rate of degeneration of abnormal rods in rd10 mice and implicate retinal microglia as a potential cellular target for therapies to prolong rod survival in RP.