IntroductionSince their discovery 15 years ago, 1 it is now well established that CD25 ϩ regulatory T cells (Tregs) are indispensable for immune homeostasis and self-tolerance. Tregs suppress the activation, proliferation, and effector functions of a wide range of immune cells via multiple mechanisms. 2 FOXP3 has been identified as a master transcription factor, controlling both Treg development and functionality. 3,4 In addition, human Tregs can be identified by high CD25 and low IL-7 receptor (CD127) expression. 5,6 A critical role of Tregs in controlling autoimmune responses is demonstrated in various animal models of autoimmune disease. 7 Furthermore, lack of functional Tregs leads to severe, systemic autoimmunity in humans. 8,9 Because of their unique function, Tregs are considered important for the treatment of autoimmune disease, and several strategies are now being explored to target these cells for therapeutic purposes. 10 However, there is still an ongoing debate whether the numbers and/or function of Tregs are changed in patients suffering from chronic autoimmune inflammation. 11 In rheumatoid arthritis (RA) and multiple sclerosis, similar Treg numbers,12,13 or even enhanced numbers in RA, 14 were observed in peripheral blood (PB) of patients compared with healthy controls (HCs). Thus, it appears that Treg numbers are not reduced in patients suffering from autoimmune inflammation. In addition, it remains unclear whether Treg function is impaired; some studies report reduced functioning of Tregs in PB of patients, 12,13,15 whereas others have found no difference. 14,16 In addition to these discrepancies concerning Treg numbers and function in the periphery, characterization of Tregs functionality at the site of autoimmune inflammation in humans is missing. High levels of Tregs have been found at the inflammatory sites in patients with arthritis and inflammatory bowel disease and these cells can suppress CD4 ϩ CD25 Ϫ effector cells in vitro. 17 Also at the site of inflammation in juvenile idiopathic arthritis (JIA), one of the most common childhood autoimmune diseases, we have previously shown that Tregs are present in high numbers and suppress proliferation of CD4 ϩ CD25 Ϫ effector cells in vitro. 18 However, in vivo inflammation persists despite the large numbers of Tregs present, suggesting that these cells are defective in their ability to control the ongoing autoimmune response. This may result from the local proinflammatory environment, because in vitro experiments have shown that pro-inflammatory cytokines can affect both Treg function 15,[19][20][21] as well as effector T-cell responses. 22,23 These data suggest that increasing Treg numbers or enhancing function for therapeutic purposes might be less effective in a chronic inflammatory environment. However, ex vivo data from patients with autoimmune disease are required to clarify the role of Tregs at the site of inflammation in humans.Here, we studied Treg function at the site of inflammation in patients with JIA and compared their inhibitory p...