IL-4 Is Required for Sex Differences in Vulnerability to Focal Ischemia in Mice

Background and Purpose-Interleukin (IL)-4 protects from middle cerebral artery occlusion in male mice. Females generally show less injury in response to the same ischemic challenge, but the underlying mechanisms are not fully understood. We tested the importance of IL-4 in female protection using IL-4 knockout (KO) mice. Methods-IL-4 KO and wild-type (WT) mice of both sexes were subjected to middle cerebral artery occlusion. Infarct volume was assessed by triphenyltetrazolium chloride staining and neurobehavioral outcome by neuroscore. T cell proliferation was assessed after Concanavalin A exposure. Ischemic brain immune cell populations were analyzed by fluorescence-activated cell sorting and immunostaining. Results-Infarction in WT females during estrus and proestrus phases was significantly smaller than in males; neurological score was better. Infarction volume was larger and neurological score worse in IL-4 KO compared with WT in both sexes, with no sex difference. Proliferation of T cells was inhibited in WT females with higher proliferation and no sex difference in IL-4 KO. Macrophage numbers and total T cells in the ischemic hemisphere were lower in WT females, and monocytes increased markedly in IL-4 KOs with no sex difference. The reduced macrophage infiltration in WT-females was predominantly M2. Loss of IL-4 increased CD68+ and iNOS+ cells and reduced YM1+ and Arg1+ cells in both sexes. Conclusions-IL-4 is required for female neuroprotection during the estrus phase of the estrus cycle. Protected WT females show a predominance of M2-activated microglia/macrophages and reduced inflammatory infiltration. Increasing macrophage M2 polarization, with or without added inhibition of infiltration, may be a new approach to stroke treatment.