IL33 (Interleukin 33)/ST2 (Interleukin 1 Receptor-Like 1) Axis Drives Protective Microglial Responses and Promotes White Matter Integrity After Stroke

Background and Purpose: Emerging evidence highlights the importance of IL33 (interleukin 33) and its receptor (ST2 [interleukin 1 receptor-like 1]) in normal brains and neurological disorders. This study explores the function of the IL33/ST2 signaling axis and a transcription factor STAT6 (signal transducer and activator of transcription 6) in white matter integrity and long-term recovery after stroke. Methods: Transient middle cerebral artery occlusion was induced in wild type, ST2 knockout, STAT6 knockout, and microglia/macrophage-depleted (by PLX5622 diet) mice. Sensorimotor and cognitive functions were evaluated. White matter integrity was measured by immunofluorescent staining, diffusion tensor imaging, electron microscopy, and electrophysiology. The death of oligodendrocytes and its precursor cells (OPC) and the microglia/macrophage responses were evaluated 3 days after stroke. Primary microglia-oligodendrocyte/OPC cocultures were used for mechanistic studies. Parametric tests (Student t test or ANOVA) or nonparametric Mann-Whitney U test were used for statistical analysis based on the numbers of groups, types of variables, and the structure of each data set. Results: ST2 deficiency exacerbates sensorimotor and cognitive deficits for 28 days after middle cerebral artery occlusion compared with wild-type mice, which was accompanied by deteriorated structural damages and impaired conduction of compound action potentials in white matter. ST2 knockout mice displayed increased death of oligodendrocytes and OPCs, and a concomitant exacerbation in neuroinflammation 3 days after stroke. Using microglia/macrophage-depleted mice and microglia-oligodendrocyte/OPC cocultures, we showed that IL33 protected oligodendrocytes and OPCs against ischemic injury in a microglia/macrophage dependent manner. Further mechanistic studies identified STAT6 as a molecule that mediates the protective effects of IL33/ST2 on oligodendrocytes in the ischemic brain. IL33 treatment failed to rescue oligodendrocytes and OPCs after stroke in STAT6 knockout mice. Conclusions: These results shed light on the IL33/ST2/STAT6 signaling as a potential immune regulatory mechanism to modulate microglia/macrophage activity, improve white matter integrity, and restore long-term neurological functions after stroke.