Abstract: Objective To explore the role of 1,25-（OH）₂D₃ in inhibiting T helper cell 17（Th17） differentiation and the role of STAT5 in this process. Methods Sorted CD4⁺T cells were treated with 1,25-（OH）₂D₃ and/or STAT5 inhibitors. The Th17 cytokines （IL-17A,IL-22） levels in supernatants were detected by ELISA;the phosphorylation level of STAT5 was examined by cell immunofluorescence,and Western blot was used to detect STAT5 protein expression. Results The levels of IL-17A and IL-22 in 1,25-（OH）₂D₃-treated group were significantly lower than those in the control group [（12.5±0.5） ng/ml and （22.7±0.5） pg/ml vs. （48.5±0.9） ng/ml and（73.8±1.9） pg/ml,respectively,P<0.01],while in STAT5 inhibitor-treated group had higher levels of IL-17A [（33.5±0.7） ng/ml] and IL-22 [（89.1±1.4） pg/ml,P<0.05];IL-17A [（18.5±0.7） ng/ml] and IL-22 [（54.1±1.6） pg/ml] levels in cells treated with 1,25（OH）₂D₃ combined with STAT5 inhibitor were significantly higher than those in 1,25-（OH）₂D₃-treated alone,whereas lower than those in STAT5 inhibitor-treated cells （P<0.01 for both）;the expression of p-STAT5 was higher in 1,25-（OH）₂D₃-treated cells and lower in 1,25-（OH）₂D₃ combined with STAT5 inhibitor-treated cells,while in STAT5 inhibitor-treated cells had the lowest p-STAT5; the STAT5 protein expression was significantly higher in 1,25-（OH）₂D₃-treated cells,whereas lower in 1,25-（OH）₂D₃ combined with STAT5 inhibitor-treated cells or in STAT5 inhibitor-treated cells had the lowest STAT5 protein expression （P<0.01 for all）. Conclusion 1,25-（OH）₂D₃ might inhibit Th17 cell differentiation via STAT5 signaling pathway.

Key words: T helper cell 17, 1,25-（OH）₂D₃, STAT5, Cell differentiation, Mice