Mobilization of endogenous microglia is coordinated with systemic monocyte infiltration in a model of RPE cell injury

Abstract

Purpose : While interactions between immune cells and RPE cells have been documented in RPE injury models, how different immune cell populations participate in the overall process is incompletely understood. We examined the participation of endogenous microglia vs. exogenous recruited monocytes in a mouse model of RPE injury.

Methods : RPE cell injury was induced with sodium iodate (NaIO₃, single IP injection of 30mg/kg) in the following transgenic mouse strains: 1) CX3CR1^GFP/+ to mark myeloid cell populations, 2) CX3CR1^CreER/+: Rosa26-flox-STOP-flox-tdTomato (Cre-tdT) to mark endogenous microglia, 3) CCR2^RFP/+ to mark infiltrating monocytes. Adult Cre-tdT mice were transiently administered oral tamoxifen to induce tdT expression in all CX3CR1+ myeloid cells and kept then tamoxifen-free for 3 months to allow for turnover of labelled systemic monocytes prior to NaIO₃ administration. Eyes were harvested following NaIO₃ and analyzed with immunohistochemistry.

Results : NaIO₃-induced RPE injury resulted in early responses in inner retinal microglia which demonstrated deramification at 1d and then decreased markedly in number at 3d as they migrated to the subretinal space to injured RPE cells. Using Cre-tdT mice, we confirmed that subretinal CX3CR1+ cells originated primarily from the migration of endogenous microglia from the OPL and IPL. The decrease in the numbers of inner retinal microglia recovered prominently at 7d as a result of (1) infiltration of exogenous tdT- myeloid cells, and (2) proliferation of endogenous tdT+ microglia, which demonstrated Ki67-immunopositivity. The infiltration of exogenous monocytes to replace the depleted inner retina was confirmed by RFP-labeling in CCR2^RFP/+ mice. These infiltrating monocytes were initially round in morphology and were immunopositive for nestin, isolectin-B4, and BrdU in the RGC layer, but lost these markers as they migrated into the IPL and OPL and acquired more ramified morphologies.

Conclusions : Our findings reveal that RPE injury induces pan-retinal microglial responses involving both the migration of endogenous microglia and their replacement by monocyte infiltration and microglial proliferation. Microglial mobilization is coordinated with immune cell recruitment and proliferation, indicating the presence of intrinsic mechanisms that maintain immune cell homeostasis in the retina.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.