Deletion of Tgfβ signal in activated microglia prolongs hypoxia-induced retinal neovascularization enhancing Igf1 expression and retinal leukostasis

Retinal neovascularization (NV) is the major cause of severe visual impairment in patients with ischemic eye diseases. While it is known that retinal microglia contribute to both physiological and pathological angiogenesis, the molecular mechanisms by which these glia regulate pathological NV have not been fully elucidated. In this study, we utilized a retinal microglia-specific Transforming Growth Factor-beta (Tgf beta) receptor knock out mouse model and human iPSC-derived microglia to examine the role of Tgf beta signaling in activated microglia during retinal NV. Using a tamoxifen-inducible, microglia-specific Tgf beta receptor type 2 (Tgf beta r2) knockout mouse [Tgf beta r2 KO (Delta MG)] we show that Tgf beta signaling in microglia actively represses leukostasis in retinal vessels. Furthermore, we show that Tgf beta signaling represses expression of the pro-angiogenic factor, Insulin-like growth factor 1 (Igf1), independent of Vegf regulation. Using the mouse model of oxygen-induced retinopathy (OIR) we show that Tgf beta signaling in activated microglia plays a role in hypoxia-induced NV where a loss in Tgf beta signaling microglia exacerbates and prolongs retinal NV in OIR. Using human iPSC-derived microglia cells in an in vitro assay, we validate the role of Transforming Growth Factor-beta 1 (Tgf beta 1) in regulating Igf1 expression in hypoxic conditions. Finally, we show that Tgf beta signaling in microglia is essential for microglial homeostasis and that the disruption of Tgf beta signaling in microglia exacerbates retinal NV in OIR by promoting leukostasis and Igf1 expression.