Early Pro-inflammatory Microglia Activation After Inflammation-Sensitized Hypoxic-Ischemic Brain Injury in Neonatal Rats

Abstract

<b>Background:</b> Perinatal asphyxia, leading to neonatal encephalopathy, is one of the leading causes for child mortality and long-term morbidities. Neonatal encephalopathy rates are significantly increased in newborns with perinatal infection. Therapeutic hypothermia is only neuroprotective in 50% of cooled asphyxiated newborns. As shown experimentally, cooling has failed to be neuroprotective after inflammation-sensitized hypoxic ischemic (HI) brain injury. Microglia are thought to be key players after inflammation-sensitized HI brain injury. We performed this study investigating early microglia phenotype polarization in our newborn animal model of inflammation-sensitized HI brain injury, better understanding the underlying pathophysiological processes. <b>Methods:</b> Seven days old Wistar rat pups were injected with either vehicle (NaCl 0.9%) or E. coli lipopolysaccharide (LPS), followed by left carotid ligation combined with global hypoxia inducing a mild unilateral hypoxic-ischemic injury. Pups were randomized to (1) Sham group (<i>n</i> = 41), (2) LPS only group (<i>n</i> = 37), (3) Veh/HI group (<i>n</i> = 56), and (4) LPS/HI group (<i>n</i> = 79). On postnatal days 8 and 14 gene-expression analysis or immunohistochemistry was performed describing early microglia polarization in our model. <b>Results:</b> We confirmed that LPS pre-sensitization significantly increases brain area loss and induced microglia activation and neuronal injury after mild hypoxia-ischemia. Additionally, we show that microglia upregulate pro-inflammatory genes involving <i>NLRP-3</i> inflammasome gene expression 24 h after inflammation-sensitized hypoxic-ischemic brain injury. <b>Conclusion:</b> These results demonstrate that microglia are early key mediators of the inflammatory response following inflammation-sensitized HI brain injury and that they polarize into a predominant pro-inflammatory phenotype 24 h post HI. This may lead to new treatment options altering microglia phenotype polarization early after HI brain injury.