Abstract W MP95: CCR2+ Monocytes Contribute to Hematoma Clearance After Experimental Intracerebral Hemorrhage

Abstract

Background: Inflammation is a key contributor to injury and recovery after intracerebral hemorrhage (ICH). CCR2+ monocytes were recently identified as mediators of early injury in a rodent ICH model, but their potential roles in ICH recovery have not been addressed. The purpose of this study was to determine how CCR2+ monocytes influence repair and recovery after ICH Methods: Ccr2-/- bone marrow was transplanted into wild type congenic mice to create chimeras with normal microglia, but few circulating monocytes. Control mice received wild type bone marrow. Mice were subjected to the blood injection ICH model, whereby 20 μl of blood were injected into the striatum. The resulting motor weakness was quantified using the cylinder and beam balance tests. Residual hemoglobin was quantified in brains at day 7 using Drabkin’s reagent. In wild type mice, 1-μm fluorescent beads were injected with the blood as a phagocytosis substrate. An imaging flow cytometer was used to visualize phagocytic cells. Results: Mice with Ccr2-/- bone marrow have less severe motor deficits up to day 3, but both genotypes were equally recovered at day 7. Despite normal behavior, mice with Ccr2-/- bone marrow had significantly more residual hemoglobin in their brains (8.92 ± 1.03 vs. 3.96 ± 2.80 μl, p<0.05; Fig.), suggesting a role for monocytes in ICH clearance. To determine if monocytes have the capacity to phagocytose debris after ICH, we injected fluorescent beads with the ICH blood. CCR2+ monocytes engulfed more beads than resident microglia at day 1 (4.35 ± 2.23 vs. 0.23 ± 0.45 beads, p<0.0001; Fig.), suggesting a more active role in phagocytosis. Conclusions: Hematoma clearance has been attributed mainly to microglia, but these results indicate that CCR2+ monocytes are also involved. Interestingly, the lack of a motor deficit at day 7 indicates that the presence of a hematoma does not inhibit functional recovery in the absence of CCR2+ inflammatory monocytes.