Introduction:
Intracerebral hemorrhage (ICH) results in the activation of microglia, the resident immune cells of the central nervous system. Microglia may polarize into an M1, pro-inflammatory phenotype, or an M2 phenotype associated with repair. CX3CR1 is a chemokine receptor on microglia and monocyte subsets. CX3CR1-null microglia have been shown to have dysregulated inflammation. We hypothesize that CX3CR1-null microglia have a prolonged M1 phenotype, contributing to worse functional outcome after ICH.
Methods:
ICH was modeled by injection of 20μl of blood into the right striatum. Neurological deficit was quantified using digital gait analysis, cylinder test, and beam walking. Mice were sacrificed 14 days after ICH; brains were harvested for flow cytometry and immunohistochemistry (IHC). C57BL/6 (WT) and CX3CR1
GFP/GFP
(CX3CR1-null)
mice were irradiated and reconstituted with bone marrow from WT mice carrying the congenic marker CD45.1 to generate bone marrow chimeras (CD45.1WT or CD45.1CX3CR1-null). M1 microglia were identified as expressing MHCII and M2 microglia with CD206.
Results:
The CD45.1CX3CR1-null mice show worse functional outcome 14 days after ICH by cylinder test (p=0.002), beam walking (p=<0.001) and gait analysis (p=0.02). By flow cytometry, few peripheral leukocytes remain in the brain at 14 days, indicating that F4/80
+
and CD11b
+
cells visualized by IHC are likely microglia, not peripheral macrophages. By IHC, CD45.1 CX3CR1-null mice have significantly more amoeboid F4/80
+
MHCII
+
cells per field (M1 microglia) than CD45.1WT mice (p=0.02). CD45.1 CX3CR1-null mice have significantly fewer CD11b
+
CD206
+
cells per field (M2 microglia) compared to CD45.1WT mice (p=0.04).
Conclusions:
Our results suggest microglial CX3CR1 signaling is necessary for microglia to transition from M1 to M2 and contribute to recovery after ICH.
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