AbstractLevels of regulatory T cells (Tregs) are increased in different cancer types as well as in inflammatory diseases, such as rheumatoid arthritis. Treg accumulation may result from aberrant proliferation and trafficking as well as greater resilience to oxidative stress compared with conventional T cells. This enhanced antioxidative capacity of Tregs possibly serves as feedback inhibition during inflammation and prevents uncontrolled immune reactions by favoring survival of suppressor rather than effector cells. In this study, we demonstrate that human Tregs express and secrete higher levels of thioredoxin-1, a major antioxidative molecule. Thioredoxin-1 has an essential role in maintaining their surface thiol density as the first line of antioxidative defense mechanisms and is sensitive to proinflammatory stimuli, mainly tumor necrosis factor-α, in a nuclear factor-κB-dependent fashion. The antiapoptotic and oncogenic potential of (secreted) Trx-1 suggests that it may exert effects in Tregs beyond redox regulation.