CCR2 deficiency in monocytes impairs angiogenesis and functional recovery after ischemic stroke in mice

Abstract

Inflammatory Ly6C^hiCCR2⁺ monocytes infiltrate the brain after stroke but their functions are not entirely clear. We report that CCR2⁺ monocytes and CCR2⁺ lymphocytes infiltrate the brain after permanent ischemia. To underscore the role of CCR2⁺ monocytes, we generated mice with selective CCR2 deletion in monocytes. One day post-ischemia, these mice showed less infiltrating monocytes and reduced expression of pro-inflammatory cytokines, markers of alternatively macrophage activation, and angiogenesis. Accordingly, Ly6C^hi monocytes sorted from the brain of wild type mice 24 h post-ischemia expressed pro-inflammatory genes, M2 genes, and pro-angiogenic genes. Flow cytometry showed heterogeneous phenotypes within the infiltrating Ly6C^hiCCR2⁺ monocytes, including a subgroup of Arginase-1⁺ cells. Mice with CCR2-deficient monocytes displayed a delayed inflammatory rebound 15 days post-ischemia that was not found in wild type mice. Furthermore, they showed reduced angiogenesis and worse behavioral performance. Administration of CCR2^+/+ bone-marrow monocytes to mice with CCR2-deficient monocytes did not improve the behavioral performance suggesting that immature bone-marrow monocytes lack pro-reparative functions. The results show that CCR2⁺ monocytes contribute to acute post-ischemic inflammation and participate in functional recovery. The study unravels heterogeneity in the population of CCR2⁺ monocytes infiltrating the ischemic brain and suggests that pro-reparative monocyte subsets promote functional recovery after ischemic stroke.