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

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.

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.

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-β levels in both male and female DTA⁺ mice under both normal conditions and after HI (more pronounced).

We established a microglial depletion model that aggravated neuronal damage and apoptosis after the HI insult, which was predominantly observed in males.