OBJECTIVE: To explore the mechanism of immunomodulatory activity of triptolide on primary immune thrombocytopenia (ITP)patients-derived plasmacytoid dendritic cells (pDCs). METHODS: pDCs in peripheral blood of ITP patients before therapy (group 1), ITP patients in complete response (ITP-CR, group 2) and healthy donors (group 3) were sorted by flow cytometry, then incubated with triptolide at 0, 5, 10 or 30 µg/L. After 24 hours, we collected the supernatants and then detected the concentrations of IFN-α, IL-6 and TNF-α using ELISA. After 5 days, the cultured cells were collected and CD11c, CD80 and CD86 expressions of myeloid dendritic cells (mDCs) were analyzed by flow cytometry, the morphology of mDC was observed by light microscope and electron microscope. RESULTS: After incubation with triptolide at 10 µg/L, the levels of IFN-α, IL-6 and TNF-α in group 1 \[(451.32 ± 85.77) ng/L, (105.68 ± 23.85) ng/L and (135.78 ± 30.62) ng/L\] and group 2 \[(391.71 ± 72.49) ng/L, (84.73 ± 17.77) ng/L and (108.16 ± 23.21) ng/L\] were significantly higher than those in group 3 \[(335.51 ± 67.54) ng/L, (73.62 ± 21.82) ng/L and (95.58 ± 32.85) ng/L\] (all P < 0.05); the levels of IFN-α, IL-6 and TNF-α in group 1 were significantly higher than those in group 2 (all P < 0.05) in a dose-dependent manner (P < 0.05). CD11c, CD80 and CD86 expressions of mDC in group1 and group 2 were significantly higher than those in group 3 (all P < 0.05); CD11c, CD80 and CD86 expressions of mDC in group 1 were significantly higher than those in group 2 (all P < 0.05) also in a dose-dependent manner (all P < 0.05). Triptolide could inhibit pDCs from differentiation into mDCs, the latter displayed more immature morphology than untreated-pDCs. CONCLUSION: Triptolide could decrease the immune function of pDCs from ITP, inhibit pDCs from differentiation and maturation.