Abstract: Expression of IDO in Tumor Induced Myeloid-Derived Suppressor Cells and Its MechanismWeijiao DU, Jinpu YU, Hui LI, Runmei LI,Wenwen YU, Xiumei AN, Naining ZHANG, Shui CAO, Xiubao RENCorrespondence to: Shui CAO, E-mail: caoshui@yahoo.comDepartment of Biotherapy, Tianjin Medical University Cancer Institute and Hospital, Tianjin 300060, ChinaThis work was supported by National Natural Science Foundation of China (No. 81072159, 30972694) and Tianjin Funds for ScientificDevelopment of Colleges and Universities (No. 20090133)Abstract Objective: To set up an in vitro co-culture system of tumor cells and normal human CD33+ myeloid cells to simulatethe induction process of Myeloid-derived suppressor cells (MDSCs)， to observe the expression and immunosuppression of Indoleamine2, 3-dioxygenase (IDO) in MDSCs, and to explore the related mechanism of IDO expression in MDSCs. Methods: CD33+ cells werecollected from healthy donors by a magnetic cell sorting system. CD33+ cells were cultured alone as the control group. CD33 + cellswere co-cultured with MDA-MB-231 using Transwell plates as the co-culture group. Two days later, CD33+ cells were harvested to in-vestigate the phenotypes. The expression and activation of IDO and STAT3 mRNA were determined using real-time PCR and Westernblot, respectively. MDSCs-induced T cell apoptosis was detected using flow cytometry, and 1-MT was used to investigate the role ofIDO in MDSCs. Results: At two days after the co-culture of breast cancer cell line MDA-MB-231 and normal human CD33+ cells, apopulation of Lin-CD33+CD13+ CD14-CD15-MDSCs appeared, which could induce autoallergic T cell apoptosis and produce high ex-pression of IDO, stat3 and p-STAT3 protein. JAK2-STAT3 inhibitors JSI-124 treated MDSCs showed a lower level of IDO expression.In addition, IDO inhibitors 1-MT significantly reduced the MDSC-induced T cell apoptosis. Conclusion: The co-culture of tumor cellsand normal myeloid cells can induce MDSCs with specific phenotypes and immunosuppressive activities. Upregulation of IDO expres-sion resulting from the increase of STAT3 phosphorylation may be one of the major mechanisms of the MDSCs-induced immunosup-pression.Keywords Breast cancer; Myeloid-derived suppressor cells; Indoleamine 2, 3-dioxygenase; CD33+ cells