Objective To investigate the role of T follicular helper ( Tfh) cells in the immuno-pathogenesis of persistent immune thrombocytopenic purpura ( pITP) .Methods Twenty children with pITP and twenty healthy controls were enrolled in this study .The proportion of CD4+CXCR5+ICOShigh PD-1high T ( cTfh) cells and the expression of ICOSL on CD 19+B cells in peripheral blood of the patients and healthy subjects were analyzed by flow cytometry .The expressions of Bcl-6, c-Maf, IL-21 and ICOSL at mRNA level were detected by real-time PCR.The plasma concentrations of IL-2, IL-6 and IL-21 were determined by ELISA.Results (1) Compared with the healthy controls , the proportions of cTfh cells increased signifi-cantly in patients with pITP [(17.45±9.04) %vs.(7.57±2.57) %, P〈0.05], but decreased with the treatment of dexamethasone (DEX) for 7 days [(5.93±1.64) %vs.(17.45±9.04) %, P〈0.05].(2) The expression of Bcl-6, c-Maf and IL-21 at mRNA level in patients with pITP were higher than those in healthy controls (P〈0.05).(3) Compared with healthy controls, the expression of ICOSL at mRNA and protein levels in CD19+B cells were significantly up-regulated in patients with pITP (P〈0.05), which showed no significant changes after treatment with DEX (P〉0.05).(4) The plasma concentration of IL-21 was remarkably elevated in patients with pITP , regardless of DEX treatment (P〈0.05).But the plasma concentrations of IL-2 and IL-6 showed no significant changes compared with control group (P〉0.05).Con-clusion The immunopathogenesis of persistent immune thrombocytopenic purpura might be associated with the hyper-activation of Tfh cells caused by excessive expression of ICOSL and IL-21.The persistent high expression of ICOSL and IL-21 might be one of the important factors resulting in the recurrence of pITP in children . Key words: T follicular helper cell; Bcl-6; ICOSL; IL-21 ; Immune thrombocytopenic purpura