CCR2+ monocytes contribute to hematoma clearance and long-term functional recovery after experimental intracerebral hemorrhage (HUM1P.324)

Abstract

Abstract CCR2+ inflammatory monocytes worsen early neurological injury in mouse models of intracerebral hemorrhage (ICH), but their potential roles during recovery are unclear. The purpose of this work was to determine how CCR2+ monocytes influence repair and recovery after ICH. All mice in this study were subjected to the blood injection ICH model, whereby 25 μl of blood were injected into the striatum, a brain region involved in motor coordination. To first determine if monocytes have the capacity to phagocytose debris in the brain, we added 1-μm carboxylate beads to the injected blood. Using an ImageStreamX MarkII imaging flow cytometer, we found that CCR2+ monocytes engulf more beads than resident microglia at day 1 (4.35 ± 2.23 vs. 0.23 ± 0.45 beads, p<0.0001), suggesting they are phagocytically active in the brain after ICH. Next, Ccr2-/- bone marrow was transplanted into CD45.1 hosts to create chimeras with normal microglia, but few circulating monocytes. Controls received C57BL6 marrow. Ccr2-/- chimeric brains contained significantly more residual hemoglobin 7 days after ICH than controls (8.92 ± 1.03 vs. 3.96 ± 2.80 μl, p<0.05), suggesting a role for CCR2+ monocytes in ICH clearance. Importantly, Ccr2-/- bone marrow chimeras exhibit severe motor deficits following an LPS challenge 14 days after ICH, whereas controls exhibit minimal deficits. Together, these data suggest CCR2+ monocytes are important regulators of hematoma clearance and are necessary for functional recovery.