Microglial CX3CR1 is required for M2 polarization after intracerebral hemorrhage (INC1P.350)

Abstract Intracerebral hemorrhage (ICH) occurs when a blood vessel ruptures in the brain parenchyma, activating microglia. Once activated, microglia may polarize into an M1, pro-inflammatory phenotype, or an M2 phenotype associated with wound repair. Microglia express the chemokine receptor CX3CR1, which regulates microglial neurotoxicity. We have previously shown that mice with CX3CR1-null microglia have worse functional outcomes 14 days after ICH. We hypothesize that CX3CR1-null microglia fail to transition to an M2 phenotype and do not aid in tissue repair. C57BL/6 (WT) and CX3CR1GFP/GFP (CX3CR1-null) mice were irradiated and reconstituted with bone marrow from CD45.1 mice to make bone marrow chimeras. ICH was induced by injecting 20ul of WT whole blood into the right striatum. WT mice were sacrificed 1-14 days after ICH to examine changes in gene expression. Microglia were sorted and qRT-PCR was performed. Chimeras were sacrificed 14 days after ICH and perihematomal regions were harvested for intracellular staining (ICS). At 12 hours after ICH, microglia have high IL-6 gene expression (M1). However, M2 markers arginase-1, TGF-β and SOCS3 increase over time, suggesting microglia transition from M1 to M2 over the first 2 weeks after ICH. By ICS, CX3CR1-null microglia had significantly less CD206 (p<0.01) and BDNF (p<0.001) than WT microglia 14 days after ICH. Our data suggest that microglia transition from M1 to M2 after ICH and that CX3CR1 is required for M2 polarization.