Objective To study the immune regulation of Galectin-9 on the active CD4+T cells and demonstrate the mechanisms.Methods Lymphocytes were harvested from wild-type C57BL/6 mouse,from which na(i)ve CD4+T cells were separated via MACS and then stimulated with anti-CD3 antibody(Ab) (2.5 μg/ml),anti-CD28 Ab(5 μg/ml) and IL-2(100 ng/ml) for 3 days.The active CD4+T cells were divided into 3 groups:Control group,Galectin-9 group and Galectin-9+α-lactose group.We detected the cell proliferation level by CFSE fluorescence intensity and then dynamically observed the cell morphological changes.The proportion of CD4+CD69+T cell,Th1,Th2 and Th17 cell was valued; Meanwhile,ELISA was used to detect the cytokine levels of IFN-γ,IL-4,IL-10,IL-12,IL-17A and TGF-β1 secreted by lymphocytes.Also Western blot was used to observe the changes of T cell differentiation regulatory protein such as T-bet,GATA-3 and ROR-γt.Results Compared with control group and Galectin-9+α-lactose group,in Galectin-9 group,the cell morphology began to change at 2 h.Moreover the proportions of CD4+CD69+ T cell,Th1 and Th17 cells decreased (P＜0.05),but no significant differences in Th2 cells.The level of IFN-γ,IL-12,IL-17A and TGF-β1 from the supernatant decreased (P＜0.05),while Th2-type cytokines IL-4 and IL-1O did not change.In addition,the expressions of T-bet and ROR-γt were significantly down-regulated (P＜0.05).Conclusion Galectin-9 inhibited Th1 and Th17-type immune response,while had no effect on Th2-type immune response.The mechanism of the immune regulation may be related to affect the expression of Th1 and Th17 specific transcription factors at transcription level.