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

Background Neuroinflammation plays an important role in neonatal hypoxic-ischemic encephalopathy (HIE). Although microglia are largely responsible for injury-induced inflammatory response, they play beneficial roles in both normal and disease states. However, the effects of microglial depletion on 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 to assess the effect of microglial depletion. The density of microglia was quantified using Iba-1 staining. Moreover, the proportion of resident microglia after the HI insult was analyzed using flow cytometric analysis. At P10, the HI insult was conducted using the Rice-Vannucci procedure at P10. The infarct size and apoptotic cells were analyzed at P13. Cytokine analyses were performed using quantitative polymerase chain reaction and enzyme-linked immunosorbent assay (ELISA) at P13. Results At P10, tamoxifen administration induced > 99% microglial depletion in DTA(+) mice. Following HI insult, there was persisted microglial depletion over 97% at P13. Compared to male DTA(-) mice, male DTA(+) mice exhibited significantly larger infarct volumes; however, there were no significant differences among females. Moreover, compared to male DTA(-) mice, male DTA(+) mice had a significantly higher density of TUNEL+ cells in the caudoputamen, cerebral cortex, and thalamus. Moreover, compared to female DTA(-) mice, female DTA(+) mice showed a significantly greater number of TUNEL+ cells in the hippocampus and thalamus. Compared to DTA(-) mice, ELISA revealed significantly lower IL-10 and TGF-beta levels in both male and female DTA(+) mice under both normal conditions and after HI (more pronounced). Conclusion We established a microglial depletion model that aggravated neuronal damage and apoptosis after the HI insult, which was predominantly observed in males.