Abstract 114: CX3CR1-null Microglia Fail to Transition to an M2 Phenotype after Intracerebral Hemorrhage

Introduction: Intracerebral hemorrhage (ICH) results in the activation of microglia. Microglia can polarize into either a pro-inflammatory (M1), or a reparative (M2) phenotype; the effect of microglial polarization after ICH is unknown. Microglia express the chemokine receptor CX3CR1, which has been shown to regulate microglial neurotoxicity. CX3CR1 +/GFP mice have a functional CX3CR1 and are frequently used to identify microglia. We previously reported that mice with CX3CR1-null microglia do not show functional recovery 14 days after ICH. We hypothesize that 1) microglia transition from M1 to M2 phenotypes in the acute to subacute period after ICH and 2) CX3CR1-null microglia fail to make this transition, inhibiting recovery after ICH. Methods: ICH was modeled by injecting 20ul of WT blood into the right striatum. Microglia were sorted from male CX3CR1 +/GFP mice 0.5, 1, 3, 7, and 14 days after ICH in order to specifically study microglia phenotypes. RNA was extracted and qRT-PCR was performed to analyze changes in gene expression. To study microglial CX3CR1 function, C57BL/6 (WT) and CX3CR1 GFP/GFP (CX3CR1-null) mice were irradiated and reconstituted with WT bone marrow (CD45.1) to generate bone marrow chimeras (CD45.1->WT or CD45.1->CX3CR1-null). Brains were harvested for flow cytometry 14 days after ICH. Results: 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 (Fig. 1), suggesting microglia transition from M1 to M2 over the first 2 weeks after ICH. By flow cytometry, CX3CR1-null microglia had significantly less SIRPα, a receptor involved in phagocytosis of erythrocytes, and CD206, an M2 marker, than WT microglia 14 days after ICH. Conclusions: Our data show WT microglia transition from an M1 to an M2 phenotype after ICH. Our results also suggest microglial CX3CR1 is necessary for transition toward an M2 phenotype after ICH and this transition is required for recovery after ICH.