Non-suppressive regulatory T cell subset expansion in pulmonary arterial hypertension

Regulatory T cells (Tregs) have been reported to play a pivotal role in the vascular remodeling of pulmonary arterial hypertension (PAH). Recent studies have revealed that Tregs are heterogeneous and can be characterized by three phenotypically and functionally different subsets. In this study, we investigated the roles of Treg subsets in the pathogenesis of PAH in eight patients with PAH and 14 healthy controls. Tregs and their subsets in peripheral blood samples were analyzed by flow cytometry. Treg subsets were defined as CD4⁺CD45RA⁺FoxP3^low resting Tregs (rTregs), CD4⁺CD45RA⁻FoxP3^high activated Tregs (aTregs), and CD4⁺CD45RA⁻FoxP3^low non-suppressive Tregs (non-Tregs). The proportion of Tregs among CD4⁺ T cells was significantly higher in PAH patients than in controls (6.54 ± 1.10 vs. 3.81 ± 0.28 %, p < 0.05). Of the three subsets, the proportion of non-Tregs was significantly elevated in PAH patients compared with controls (4.06 ± 0.40 vs. 2.79 ± 0.14 %, p < 0.01), whereas those of rTregs and aTregs were not different between the two groups. Moreover, the expression levels of cytotoxic T lymphocyte antigen 4, a functional cell surface molecule, in aTregs (p < 0.05) and non-Tregs (p < 0.05) were significantly higher in PAH patients compared with controls. These results suggested the non-Treg subset was expanded and functionally activated in peripheral lymphocytes obtained from IPAH patients. We hypothesize that immunoreactions involving the specific activation of the non-Treg subset might play a role in the vascular remodeling of PAH.