Emergence of Neutrophil-Like Monocyte Subset with Distinct Single-Cell Transcriptomic Signature Controls Neuroinflammation in Brain Injury

Monocyte/macrophages represent key cellular elements that contribute to acute tissue damage in the brain. Strategies aimed at their sequestration have revealed critical insight into the injury response that warrants further investigation. We show that combining pre-depletion of circulating blood monocytes with murine brain injury, augmented the release of CD115+/Ly6Chi monocytes. This correlated with tissue and blood-brain barrier protection as well as improved cerebral blood flow. We reveal that this shift resulted in a reduced population of neutrophils but increased presence of monocytes in the brain, with a predominate anti-inflammatory phenotype that correlates with neuroprotection. Moreover, we identified that the circulating monocytes that emerge under these conditions display a morphology and gene expression profile consistent with a neutrophil-like subset common during bone marrow (BM) emergency monopoiesis. Single-cell RNA sequencing of the circulating monocytes showed distinct changes in monocyte clusters including an increased classical type subset that demonstrate higher expression of neutrophil-specific genes compared to control. Lastly, reconstitution of pre-depleted mice with ex vivo expanded BMDMs attenuated neuroprotection, however, mice receiving resident BMDMs retained protection. These findings suggest that targeting surrogate monocyte subsets may help quell the neuroimmune response to brain injury and curb tissue damage.