ObjectivesTo identify the signaling pathways involved in the anti-inflammatory shift of the cytokine balance due to hypothermia during cardiopulmonary bypass.DesignExperimental animal study.SettingDepartment of experimental surgery of a university hospital.SubjectsYoung pigs.InterventionsAnimals underwent normothermic (37°C) or hypothermic (28°C) cardiopulmonary bypass (n = 6 each). Samples of liver tissue were taken before and 6 hrs after cardiopulmonary bypass.Measurements and Main ResultsIntrahepatic expression of tumor necrosis factor-&agr;, interleukin-10, inducible nitric oxide synthase, and suppressor of cytokine signaling-3 was detected by reverse transcriptase polymerase chain reaction and/or Western blotting. Concentrations of the inhibitory protein of nuclear factor-&kgr;B, I&kgr;B, and of the signal transducer and activator of transcription (STAT)-3 were measured by Western blotting. The DNA-binding activity of nuclear factor-&kgr;B and STAT-3 was assessed by electrophoretic mobility shift and supershift assays. Liver cell necrosis and apoptosis were assessed by histology and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay, respectively. Pigs operated on in hypothermia showed significantly higher intrahepatic concentrations of interleukin-10 and lower concentrations of tumor necrosis factor-&agr; than the others. They also showed a lower percentage of hepatic cell necrosis but not of apoptosis. This anti-inflammatory reaction observed in the hypothermic group was associated with a higher expression of suppressor of cytokine signaling-3 and with increased activation of STAT-3. Activation of nuclear factor-&kgr;B and expression of inducible nitric oxide synthase, however, were not significantly different between both groups.ConclusionOur results show that hypothermia during cardiopulmonary bypass up-regulates interleukin-10 via STAT-3 activation, which in turn leads to the attenuation of tumor necrosis factor-&agr; expression and to hepatic protection.