Distinct Myeloid Cell Subsets Promote Angiogenesis and Damaged Tissue Clearance Following Mild Traumatic Brain Injury

Abstract

Abstract Mild traumatic brain injury (mTBI) leads to a robust immune response featuring resident microglia/macrophages and recruitment of peripherally-derived monocytes/macrophages. We sought novel insights into the dynamics and function of innate myeloid cells following brain injury by studying a closed-skull focal contusion model of mTBI that permits real-time imaging by intravital two-photon microscopy (TPM). TPM studies of CX3CR1GFP/+ CCR2RFP/+ reporter mice revealed an early microglia response followed by CCR2+ monocytes and CX3CR1+ macrophages that accumulated 1 to 7 days post-TBI. This transition coincided with dynamic changes in gene expression that shifted from pro-inflammatory to wound-healing. Early cell death and vascular destruction led to an impressive re-vascularization of the lesioned meninges, with 80–100% wound closure observed within 7 days of injury. We used histo-cytometry to define the anatomical position of the different myeloid subpopulations within the injured meninges during the repair phase. This revealed the presence of non-classical and resident macrophages that highly expressed the remodeling enzyme matrix metalloproteinase 2 (Mmp2) in the lesion core. Depletion of neutrophils and classical monocytes increased pathology due to inefficient dead cell clearance, but re-vascularization of the wound was not affected. Interestingly, vascular regeneration was significantly impaired only when non-classical monocytes and resident meningeal macrophages were depleted, resulting in reduced numbers of CD206+macrophages in the lesion. These studies demonstrate that neutrophils, monocytes, and resident macrophages participate in coordinated yet divergent wound-healing responses to mTBI.