Inflammatory monocytes but not microglia contribute to light-induced retinal degeneration

Abstract

Purpose : Activated resident microglia and infiltrating blood monocyte-derived macrophages have been implicated in several pre-clinical models of retinal degeneration. However, the relative contribution of microglia and circulating monocyte-derived macrophages in retinal inflammation are not well understood. Here we determined the contribution of inflammatory monocytes versus microglia to photoreceptor loss in an experimental model of light-induced retinal degeneration.

Methods : To deplete blood monocytes, male Balb/c mice were treated systemically with either clodronate-containing liposomes (1 mg i.v. daily starting 2 days before light exposure) or with IL34 and CSF1 neutralizing antibodies (200 μg i.p. three times per week starting 2 weeks before light exposure). Microglia depletion was achieved by feeding the male Balb/c mice with a CSF1R small molecule inhibitor (PLX3397, 290 mg/kg chow) for 21 days. The mice were exposed to fluorescent light (1200 lux) for 7 days while treated with depleting agents. Retinal degeneration was assessed by Spectral Domain Optical Coherence Tomography (SD-OCT) and flow cytometry. Blood monocytes and retinal microglia were quantified by flow cytometry and immunohistochemistry.

Results : Systemic treatment with clodronate or combined IL34 and CSF1 neutralizing antibodies depleted blood inflammatory (Ly6C^hi) monocytes by over 90% while retinal microglia numbers were not affected. Peripheral blood monocyte depletion with anti-IL34 and anti-CSF1 resulted in abrogation of Iba1⁺ myeloid cell accumulation in the subretinal space and choroidal capillary in mice exposed to light. Light-induced photoreceptor degeneration was significantly decreased in monocyte-depleted mice compared to control mice. Microglia numbers in the retina were reduced by over 90% in Balb/c mice treated with PLX3397 for 21 days while peripheral blood Ly6C^hi monocyte numbers remained unaffected. Surprisingly, microglia-depleted mice exhibited a similar degree of photoreceptor degeneration as the control mice upon light exposure.

Conclusions : Our results indicate that peripheral blood inflammatory monocytes but not resident microglia contribute to phototoxicity-induced retinal degeneration in mice, positioning circulating monocytes as a potential target for therapeutic intervention in retinal diseases.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.