Quantitative and qualitative defects of regulatory T-cells (Tregs) were described in RApatients. In this work we hypothesize the existence of senescent Tregs whichcontribute to the disease pathogenesis. For this reason, we examine the prevalence ofsenescent Tregs in patients with RA and healthy individuals. In functional assays wetest the mechanisms how senescent Tregs may influence immune regulation.A prospective study on 35 patients with RA and 25 healthy controls was performed todetermine prevalences of CD4+CD28+FoxP3+ and CD4+CD28-FoxP3+ T-cells aswell as the expression of surface markers CTLA-4 and PD-1 via FACS and theexpression of CD28 and the production of several cytokines (IL-2, IL-4, IL-6, IL-10, IL-17, TNF- and IFN-) of different Treg-subsets was measured. At last, the suppressiveactivity was determined in a functional CFSE-based assay. Elevated numbers ofCD4+CD28-FoxP3+ Tregs in peripheral blood from RA patients could be detected incomparison to healthy individuals. CD28-FoxP3+ cells expressed higher levels of theregulatory protein PD-1, whereas CTLA-4 expression was similar in both subsets.The exposure of CD4+CD25+CD127- Tregs to TNF- in an in vitro assay caused adownregulation of CD28 and an upregulation of CD25 and CD127 on TNF- treatedTregs in contrast to unstimulated Tregs could be detected. The expression of FoxP3,however, was similar in all groups. Regarding cytokine production treatment with IL-15caused an increased production of IL-4, IFN- as well as IL-17. At last the suppressiveactivity of TNF- treated Tregs was significantly increased compared to untreatedTregs. In this work, we discovered a novel T-cell subset which features both senescentas well as regulatory properties. These data, suggest an involvement of senescentregulatory T-cells in the pathogenesis of RA.