Loss of retinal microglia exacerbates vascular outcomes in mouse oxygen-induced retinopathy model

Abstract

PURPOSE: Involvement of retinal microglia in the mouse model of oxygen-induced retinopathy (OIR) has been noted but not well documented. We applied an inhibitory strategy on retinal microglia to investigate whether these cells are beneficial or detrimental for retinal vasculature in mouse OIR model. METHODS: Mouse pups with their nursing mother were exposed to 75% oxygen on postnatal (P) day 7 for 5 days, and returned back to room air on P12, to induce OIR. On P5 or P12, retinal microglia depletion was achieved by intravitreal injection of 1 μl of liposomes containing clodronate (CLD) to one eye while injection of liposomes containing PBS to the other eye served as treatment control. Retinae from pups stayed in room air were used as normal controls. Retinal vasculature and microglia were examined in P12 and P17 OIR retinae. Electroretinogram (ERG) was performed on P17 to assess retinal function. RESULTS: Retinae injected with CLD showed significantly reduced retinal microglia when compared with PBS-treated ones. When injected on P5, CLD-treated OIR retinae showed an increased avascular zone compared with PBS-treated OIR retinae (PBS: 38.0±4.0% vs. CLD: 52.8±2.9% of total retinal area) on P12. On P17, larger avascular zone was still observed in CLD-treated retinae (PBS: 24.0±1.1% vs. CLD: 31.0±1.5% of total retinal area); moreover, increased neovascularization was observed in CLD-treated retinae (PBS: 15.4±1.1% vs. CLD: 22.3±1.5% of total retinal area). When injected on P12, CLD-treated OIR retinae showed increased neovascularization compared with PBS-treated ones (PBS: 8.8±0.9% vs. CLD: 11.5±0.6% of total retinal area). In ERG functional assays, OIR treatment significantly suppressed retina ERG response compared with the normal control retinae that stayed in room air; however, no significant difference was observed between CLD- and PBS-treated retinae, regardless of the time of injections. CONCLUSIONS: Loss of retinal microglia in either hyperoxic or hypoxic stage of OIR exacerbates retinal vascular consequences in OIR retinae. Yet, difference in functional changes after retinal microglia loss are not obvious between CLD- and PBS-treated OIR retinae when neovascularization peaks on P17. Retinal microglia might play a protective role in retinal vasculature maintenance in the OIR process.