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Accumulating evidence suggests that abnormal inflammation plays a critical role in the pathogenesis of pulmonary arterial hypertension (PAH). CD8+CD25+Foxp3+ T cell is a novel cell subtype, and its role in PAH is not yet investigated. Here, we observed that PAH patients presented a significant upregulation of CD8+CD25+Foxp3+ T cells and a downregulation of CD4+CD25+Foxp3+ T cells compared to healthy controls. Regardless, the total number of CD25+Foxp3+ T cells in PAH patients was still smaller than that in healthy controls. Compared to CD8+CD25- T cells, CD8+CD25+ T cells presented higher Foxp3 expression, lower interferon (IFN)-γ expression and higher transforming growth factor (TGF)-β expression, in both healthy and PAH individuals. The CD8+CD25+ T cells in PAH patients also demonstrated regulatory function by suppressing the proliferation of CD4+CD25- and CD8+CD25- effector T cells, albeit at lower efficiency than CD4+CD25+ T cells from PAH patients and healthy volunteers. CD8+CD25+ T cells from PAH responded to interleukin (IL)−2 supplement by expansion and upregulating Foxp3 expression. In PAH patients, IL-2-treated CD8+CD25+ T cells were more potent at inhibiting CD4+CD25- effector T cell proliferation than IL-2-untreated CD8+CD25+ T cells. Together, we found an upregulation of CD8+CD25+Foxp3+ T cells in PAH patients, and this T cell population presented suppressive activity that could be enhanced by IL-2 treatment.