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Background:Indoleamine 2,3-dioxygenase (IDO),an enzyme for tryptophan metabolism through the kynurenine pathway,exhibits an immunosuppressive effect and induces immune tolerance in tumor cells.The effects of IDO on pancreatic cancer are poorly understood.This study aimed to investigate the expression and prognostic significance of IDO in pancreatic cancer.Methods:We evaluated the protein expression of IDO in PANC-1,CFPAC-1,and BxPC-3 cell lines with or without 48 h treatment by 500 U/ml interferon-γ (IFN-γ).We performed immunohistochemical staining and West blot analysis for IDO expression in both pancreatic cancer and normal pancreas tissues obtained from Chinese PLA General Hospital from July 2012 to December 2013.Survival analysis was performed to correlate IDO expression and histopathologic parameters with overall survival.The Kaplan-Meier method and Cox proportional hazards regression model were conducted.Results:PANC-1,CFPAC-1,and BxPC-3 cell lines expressed IDO at the protein level,and the relative expression amount increased after stimulation with 500 U/ml IFN-γ.Immunohistochemical analysis results revealed that high IDO expression was observed in 59％ of pancreatic adenocarcinoma tissues.Compared with normal pancreatic tissues,pancreatic adenocarcinoma showed significantly higher IDO expression levels,especially among patients with high tumor node metastasis (TNM) stages (x2 =4.550,P =0.030),poor histological differentiation (x2 =5.690,P =0.017),and lymph node metastasis (x2 =4.340 P =0.037).Kaplan-Meier survival curves showed that high IDO expression was correlated with low survival rates (hazard ratio [HR] =0.49 P =0.009).Multivariate analysis using Cox proportional hazards model indicated that lymph node metastasis (HR =0.35 P =0.010) and IDO expression (HR =0.42 P =0.020)were two independent prognostic predictors of pancreatic adenocarcinoma.Conclusions:The study confirmed that high IDO expression in pancreatic adenocarcinoma was related to poor prognosis of patients.These findings provided evidence that IDO was involved in pancreatic adenocarcinoma progression and might serve as a relevant therapeutic target.