Early neutrophil infiltration is critical for inflammation-sensitized hypoxic-ischemic brain injury in newborns.

Neutrophils are the most abundant leukocytes and usually the first immune cell-type recruited to a site of infection or tissue damage. In asphyxiated neonates, elevated peripheral neutrophil counts are associated with poorer neurological outcomes. Induced neutropenia provides brain protection in animal models of neonatal hypoxic-ischemic (HI) injury, but the anti-neutrophil serum used in past studies heavily cross-reacts with monocytes, thus complicating the interpretation of results. Here we examined neutrophil influx and extravasation, and used a specific anti-Ly6G antibody for induced neutropenia against lipopolysaccharide (LPS)-pretreated HI injury in murine neonates, a model for inflammation-sensitized hypoxic-ischemic encephalopathy (HIE). As early as 6 h after the LPS/HI insult, the mRNAs for neutrophil-recruiting and mitogenic chemokines ascended in the ipsilateral hemisphere, coinciding with immuno-detection of neutrophils. However, neutrophils mainly resided within blood vessels, exhibiting signs for neutrophil extracellular traps (NETs), before 48 h post-LPS/HI. Prophylactic anti-Ly6G treatment blocked the brain infiltration of neutrophils, but not monocytes or lymphocytes, and markedly decreased LPS/HI-induced pro-inflammatory cytokines, matrix metalloproteinase 9 (MMP-9), and brain tissue loss. In contrast, anti-Ly6G treatment at 4 h post-LPS/HI failed to prevent the influx of neutrophils and brain damage. Together, these results suggest important pathological functions for early-arriving neutrophils in inflammation-sensitized HIE.