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Introduction: Inter-alpha inhibitor proteins (IAIPs) are immunomodulatory proteins that play a significant anti-inflammatory role in hypoxic ischemic (HI) brain injury. We have shown that administering IAIPs after HI improves histopathological brain injury, brain weight, and behavioral outcomes in neonatal rats. Neutrophils are specialized leukocytes known to infiltrate the brain parenchyma and exacerbate neuronal injury after HI. One molecular mechanism by which neutrophils exert damage on the blood-brain barrier (BBB) and brain tissue after ischemia is by the release of matrix metalloproteinase-9 (MMP9), an enzyme that breaks down the extracellular matrices of surrounding cells.Objective: To determine the effect of IAIPs on neutrophil infiltration and release of MMP9 in neonatal rats after HI.Methods: The Vannucci model was used to induce neonatal HI in postnatal day 7 rats that were assigned to a Non-ischemic sham-control group (Sham, n=12), right-sided carotid ligation with exposure to hypoxia (8% oxygen for 90 min) treated with placebo group (PL-HI, n=17), or an IAIP treated group (IAIP-HI, n=17). Rat sex was recorded. IAIP (30 mg/kg) or PL was given intraperitoneally at 0, 24 and 48 h after HI. We removed the rat brain after 72h and performed immunohistochemistry using MPO (neutrophil selective) and MMP9 fluorescent markers. We performed stereological analyses with the StereoInvestigator 10.0 Fractionator probe without knowledge of group assignment to quantify neutrophils and MMP9 positive cells present within the right hemisphere, cortex, corpus callosum, and hippocampus.Results: MPO positive cells were significantly reduced in male IAIP treated rats compared with PL-HI in the overall damaged hemisphere (p<0.01) and the corpus callosum (p<0.05). Further, we observed MPO and MMP9 co-localization, and IAIP treatment reduced the presence of MMP9 positive neutrophils in the cortex of male rats compared to placebo (P<0.05).