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Background Systemic sclerosis (SSc) is an autoimmune disease that has three major components:inflammation,fibrosis,and vasculopathy.T-helper 17 cell (Th17) and regulatory T cell (Treg) are considered to be critical for autoimmune disease pathogenesis.The role of Th17 and Treg in SSc is still unclear.The aim of this study was to detect the presence of Th17s and CD4+CD25+ Tregs in peripheral blood samples from SSc patients and to investigate the possible roles of these two T cell subsets in SSc pathogenesis.Methods Th17s (CD4 and IL-17 positive) and CD4+CD25+ Tregs (CD4,CD25 and Foxp3 positive) in the peripheral blood mononuclear cells of 53 SSc patients and 27 healthy controls were counted by flow cytometry.The differences between SSc and control patients were analyzed.Clinical parameters,including disease duration,duration of the second symptoms,Modified Rodnan Skin Score (MRSS),anti-topoisomerase I antibody,anti-U1 ribonucleoprotein (RNP) antibody,systemic involvements,pulmonary function test (PFT) and high resolution computed tomography (HRCT) score were prospectively collected following EUSTAR (EULAR scleroderma trial and research group) protocols.The correlations between the experimental and clinical data were investigated.Results The ratio of Th17 in SSc patients was significantly elevated compared to healthy controls (8.74％ vs.4.41％,P ＜0.001).The amount of Th17 was positively correlated with disease duration (R=0.531,P=0.013) and duration of the second symptoms (R=0.505,P=0.023).The ratio of CD4+CD25+ Treg in SSc patients also significantly differed from the healthy controls (3.04％ vs.2.24％,P=0.018).Elevated Tregs were more frequently observed in patients with a high interstitial lung disease (ILD) score on computed tomography (24/36) compared with patients with normal ILD scores (4/12,P=0.043).Elevated Tregs were also more often observed in patients with low carbon monoxide diffusing capacity (DLCO) (24/34) compared with patients with normal DLCO (4/11,P=0.042).Conclusions T cell abnormalities are remarkable in systemic sclerosis.Th17s proliferate and their numbers increase with lengthened disease duration.Th17s might participate in both inflammation and fibrosis by secreting IL-17.CD4+CD25+ Tregs also proliferate in SSc and may play important roles in promoting fibrosis.