Hypoxic-ischemic brain injury in neonatal mice sequentially recruits neutrophils with dichotomous phenotype and function.

Neonatal encephalopathy caused by hypoxia-ischemia (HI) leads to a strong neutrophil infiltration. The long-held assumption that neutrophils act exclusively as tissue-damaging cells, is challenged by increasing evidence of a profound neutrophil heterogeneity. Here, we uncovered a pronounced phenotypical and functional diversification of neutrophils in neonatal mice depending on the disease stage. Neutrophil infiltration was biphasic, peaking 1 and 7 days after HI. Early brain-infiltrating neutrophils displayed a hyperactivated phenotype, whereas neutrophils at day 7 exhibited an angiogenic phenotype with high Siglec-F expression. Acute neutrophil depletion protected against neural cell death, associated with decreased hyperactivity in adolescent animals. Delayed neutrophil depletion impaired vascular and oligodendrocyte regeneration, resulting in exacerbated alterations of anxiety-related behavior and myelination deficits. These findings suggest a divergent function of neutrophils, with early neutrophils aggravating tissue damage and late neutrophils contributing to neurological recovery. The disease stage-dependent neutrophil diversification offers new possibilities to identify disease-stage-specific therapeutic targets.