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

Authors

• MV Russo
• DB McGavern

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 of the response by intravital two-photon microscopy (TPM). We combined intravital imaging with meningeal whole-mount immunohistochemistry and cell-specific depletions to obtain a better understanding of how myeloid cells contribute to inflammation and tissue repair following focal brain injury. TPM studies of CX3CR1GFP/+ CCR2RFP/+ dual reporter mice revealed a substantial involvement of myeloid cells during mTBI. Resident microglia tended to the damaged glial limitans almost immediately after injury. This was followed by recruitment of inflammatory CCR2+ monocytes that swarmed the meninges and an accumulation of CX3CR1+ macrophages over the ensuing 4-7 days. This transition coincided with dynamic changes in gene expression that shifted from pro-inflammatory to wound-healing over time. 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) around the lesion core. Cell-specific depletions revealed distinct contributions of myeloid cell subsets to the mTBI wound-healing response. While peripheral depletion of neutrophils and classical monocytes increased pathology due to inefficient dead cell clearance, 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.

Scientific Focus Area: Immunology

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