Background <p>Autoimmune inflammation results from dysregulated immune responses, with dysfunction of regulatory T cells (Tregs) being a key contributor due to their critical role in maintaining immune tolerance. The stability and function of Tregs are strongly influenced by the inflammatory microenvironment, yet the regulatory interactions between CD8⁺ T cells and CD4⁺Foxp3⁺ Tregs remain poorly understood.</p> Methods <p>We investigated the role of CD8⁺ T cells in regulating induced Tregs (iTregs) using in vitro T cell co-culture assays and two in vivo models of autoimmune disease. A naïve CD4⁺ T cell transfer colitis model was used to evaluate the suppressive function of iTregs in the presence or absence of CD8⁺ T cells, while an autoimmune arthritis model was employed to assess therapeutic efficacy. Flow cytometry, functional suppression assays, and mechanistic analyses were performed to define signaling pathways.</p> Results <p>CD8⁺ T cells promoted the differentiation of a CD39⁺ iTreg subset characterized by increased frequencies of CD103, CTLA-4, and Helios, leading to enhanced immunosuppressive capacity. In the colitis model, co-transfer of CD8⁺ naïve T cells alleviated disease by reinforcing iTreg-mediated suppression of Th1 and Th17 responses. Mechanistic studies revealed that CD8⁺ T cells regulated iTreg phenotype through a ROS/TGF-β signaling axis, with IRF4 in CD8⁺ T cells acting as a key mediator. Importantly, CD8⁺ T cell–primed iTregs showed superior therapeutic efficacy in the autoimmune arthritis model by suppressing pathogenic Th1/Th17 responses and supporting endogenous Treg homeostasis.</p> Conclusions <p>This study identifies a previously unrecognized role of CD8⁺ T cells in enhancing iTreg differentiation, stability, and suppressive function through the ROS/TGF-β–IRF4 pathway. These findings reveal a novel mechanism of immune regulation and suggest that harnessing CD8⁺ T cell–primed iTregs could represent a promising strategy to strengthen Treg-based therapies for autoimmune diseases.</p>