Aggravated brain injury after neonatal hypoxic ischemia in microglia depleted male mice

Abstract Background: Neuroinflammation plays important roles in neonatal hypoxic–ischemic encephalopathy (HIE). Microglia are largely responsible for the injury-induced inflammatory response, but they also play beneficial roles in both normal and disease states. Nevertheless, the effects of microglial depletion in neonatal HIE remain unclear. Methods: Tamoxifen was administered to Cx3cr1 CreER/+ Rosa26 DTA/+ (microglia-depleted model) and Cx3cr1 CreER/+ Rosa26 DTA/- (control) mice at P8 and P9 in order to assess the effect of microglial depletion. The density of microglia was quantified using Iba-1 staining and the proportion of resident microglia after the hypoxic–ischemic (HI) insult was also analyzed using flow cytometric analysis. The HI insult employed the Rice-Vannucci procedure at P10. Infarct size and apoptotic cells were analyzed at P13. Cytokine analysis was performed by quantitative polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA) at P13. Results: Tamoxifen administration induced over 99% microglial depletion at P10 in DTA + mice. Microglial depletion over 97% persisted at P13 following the HI insult. Male DTA + mice exhibited significantly larger infarct volumes than did male DTA - mice, but there was no significant difference in females. The density of TUNEL + cells in DTA + mice was significantly higher than that in DTA - mice in the caudoputamen, cerebral cortex, and thalamus in males. Females showed significantly greater numbers of TUNEL + cells in the hippocampus and thalamus in DTA + mice compared to DTA - mice. ELISA revealed that IL-10 and TGF-β levels were significantly lower in both male and female DTA + mice compared to DTA - mice, both under normal conditions and, more pronounced, after HI. Conclusion: We established a model of microglial depletion that aggravated neuronal damage and apoptosis after the HI insult, an effect most predominant in males.