Objective:To evaluate the role of glutathione S-transferase μ1 (GSTM1) expression in mild hypothermia-induced mitigation of cerebral ischemia-reperfusion (I/R) injury and the relationship with microglial polarization.Methods:Eighty clean-grade healthy male Sprague-Dawley rats, aged 8 weeks, weighing 260-280 g, were divided into 4 groups ( n=20 each) using a random number table method: sham operation group (S group), cerebral I/R group (I/R group), mild hypothermia group (H group), and GSTM1 inhibitor + mild hypothermia group (IH group). The rat model of cerebral I/R injury was prepared using the filament occlusion method. The filament was removed to restore blood flow after the left middle cerebral artery was blocked for 2 h, and the rats′ brain and rectal temperature were maintained at 36-37 ℃ during the period. The vessels were only isolated and ligated without occlusion in S group. In H group, the entire body was wiped with 75% ethanol immediately after removing the filament, and the brain and rectal temperatures were maintained at 32-33 ℃ for 3 h, and the other procedures were the same as those previously described in I/R group. In IH group, GSTM1 inhibitor itaconic acid 8.6 mg/kg was intraperitoneally injected at 24 and 1 h before developing the model, and the other procedures were the same as those previously described in H group. Neurological deficits were evaluated using a modified neurological severity score (mNSS) at 24 h of reperfusion, and then the animals were sacrificed and the brains were removed for observation of cerebral infarction (by TTC staining) and for determination of the expression of GSTM1, M1-type microglial marker inducible nitric oxide synthase (iNOS), and M2-type microglial marker arginase-1 (Arg-1) (by Western blot), expression of GSTM1, iNOS and Arg-1 mRNA (quantitative real-time polymerase chain reaction) and contents of interleukin-6 (IL-6), IL-10, tumor necrosis factor-alpha (TNF-α) and transforming growth factor-beta (TGF-β) (by enzyme-linked immunosorbent assay). Results:Compared with S group, the mNSS and percentage of cerebral infarct size were significantly increased, and the expression of iNOS and Arg-1 protein and mRNA was up-regulated, the expression of GSTM1 and mRNA was down-regulated, and the contents of IL-6, TNF-α, IL-10 and TGF-β were increased in the other three groups ( P<0.05). Compared with I/R group and IH group, the mNSS and percentage of cerebral infarct size were significantly decreased, and the expression of iNOS protein and mRNA was down-regulated, the expression of Arg-1 protein and mRNA and GSTM1 was up-regulated, the contents of TNF-α and IL-6 were decreased, and the contents of TGF-β and IL-10 were increased in H group ( P<0.05). Conclusions:Up-regulated expression of GSTM1 is involved in mild hypothermia-induced mitigation of cerebral I/R injury, which is associated with inhibition of microglial polarization toward the M1 phenotype and promotion of polarization toward the M2 phenotype.