Objective To investigate expression level of interleukin-27 (IL-27)and its impact on virus replication in lung after infected with respiratory syncytial virus (RSV). Methods In vivo,7-week aged female C57 mice were infected with RSV and sacrificed on day 0,0.5,1,2,4,6,8 (n=5).The left lung was extracted for RNA,and then real-time PCR was used to detect the mRNA levels of IL-27p28,IL-27EBI3, RSV-M protein and interferon-β (IFN-β).The right lung was used for HE staining.In vitro, human lung epithelial cells (A549)were infected with RSV,and stimulated with different concentrations of recombinant human interleukin 27 (rhIL-27)in doses of 0,1ng/mL,10ng/mL,and 50ng/mL.After 24 hours,the mRNA expression of interferon induced transmembrane protein 3 (IFITM3),IFN-β,and RSV-M protein were determined by real-time PCR.IFITM3 and STAT1/pSTAT1 protein expression levels were detected by Western blot. Results The viral load in the lungs of mice peaked on 4 day post infection (DPI),then gradually decreased,and almost returned to normal on 8 DPI.IFN-β increased transiently 12 hours after infection,and then quickly returned to normal baseline.IL-27 p28/EBI3 levels peaked on 1 DPI,then gradually decreased to normal on 4 DPI.Stimulating RSV-infected A549 cells with rhIL-27 of 10ng/mL and 50ng/mL significantly inhibited viral replication,enhanced IFTIM3 mRNA expression,and induced STAT1 phosphorylation.However,rhIL-27 stimulation did not affect IFN-β mRNA expression. Conclusions The IL-27 mRNA expression increases after RSV infection.The inhibition of IL-27 on RSV replication might be related to up-regulation of STAT1 phosphorylation and IFITM3 protein expression.