Thioredoxin (Trx) is a metabolic regulator that controls the differentiation and function of regulatory B cells (Breg). This study shows that Breg cells rely on mitochondrial electron transport and homeostatic levels of reactive oxygen species (ROS) for their development, unlike non-Breg cells. Single-cell RNA sequencing revealed that Trx, a metabolic redox protein, is highly expressed in Breg cells, while its inhibitor, TXNIP, is downregulated. Pharmacological inhibition or gene silencing of Trx led to mitochondrial membrane depolarization and increased ROS levels, selectively suppressing Breg cell differentiation and function, while favoring pro-inflammatory B cell differentiation. Patients with systemic lupus erythematosus (SLE), characterized by Breg cell deficiencies, exhibit B cell mitochondrial membrane depolarization, elevated ROS, and fewer Trx+ B cells. Exogenous Trx stimulation restored Breg cells and mitochondrial membrane polarization in SLE B cells to healthy levels, indicating Trx insufficiency underlies Breg cell impairment in SLE patients. Breg cells limit immunopathology by producing IL-10, IL-35, and TGFβ. They maintain peripheral tolerance through various suppressive mechanisms. Breg cells are reduced in number and function in autoimmunity, including SLE, and their restoration is associated with better clinical outcomes. Conversely, Breg cells support a pro-tumorigenic response in cancer models and are associated with poor prognosis. In humans, Breg cells account for 1–2% of the total B cell population in peripheral blood, but they can increase to 15–20% upon activation. The metabolic demands of B cells adjust according to their functional needs. Naive B cells are metabolically quiescent, while activated B cells use glycolysis, fatty acid oxidation, and oxidative phosphorylation (OXPHOS) to meet their energy demands. In mice, activation of naive B cells induces glycolysis upregulation, with a progressive reliance on OXPHOS during plasma cell differentiation. Mature plasma cells rely heavily on glucose uptake and mitochondrial pyruvate import for antibody glycosylation. The thioredoxin system, including Trx, thioredoxin reductase (TrxR), and NADPH, regulates multiple cellular processes, including gene expression, antioxidant response, apoptosis, and proliferation. Trx and Trx2 function as an antioxidant system that maintains thiol-related redox status by neutralizing excess ROS. Dysregulation of the Trx system affects cellular functions and cell fate, leading to diseases like cancer and autoimmunity. The study shows that Breg cells use OXPHOS to meet their metabolic needs. Inhibition of OXPHOS with rotenone or antimycin A ablated B cell IL-10 expression and secretion, confirming the dependency of Breg cells on OXPHOS metabolism. High concentrationsThioredoxin (Trx) is a metabolic regulator that controls the differentiation and function of regulatory B cells (Breg). This study shows that Breg cells rely on mitochondrial electron transport and homeostatic levels of reactive oxygen species (ROS) for their development, unlike non-Breg cells. Single-cell RNA sequencing revealed that Trx, a metabolic redox protein, is highly expressed in Breg cells, while its inhibitor, TXNIP, is downregulated. Pharmacological inhibition or gene silencing of Trx led to mitochondrial membrane depolarization and increased ROS levels, selectively suppressing Breg cell differentiation and function, while favoring pro-inflammatory B cell differentiation. Patients with systemic lupus erythematosus (SLE), characterized by Breg cell deficiencies, exhibit B cell mitochondrial membrane depolarization, elevated ROS, and fewer Trx+ B cells. Exogenous Trx stimulation restored Breg cells and mitochondrial membrane polarization in SLE B cells to healthy levels, indicating Trx insufficiency underlies Breg cell impairment in SLE patients. Breg cells limit immunopathology by producing IL-10, IL-35, and TGFβ. They maintain peripheral tolerance through various suppressive mechanisms. Breg cells are reduced in number and function in autoimmunity, including SLE, and their restoration is associated with better clinical outcomes. Conversely, Breg cells support a pro-tumorigenic response in cancer models and are associated with poor prognosis. In humans, Breg cells account for 1–2% of the total B cell population in peripheral blood, but they can increase to 15–20% upon activation. The metabolic demands of B cells adjust according to their functional needs. Naive B cells are metabolically quiescent, while activated B cells use glycolysis, fatty acid oxidation, and oxidative phosphorylation (OXPHOS) to meet their energy demands. In mice, activation of naive B cells induces glycolysis upregulation, with a progressive reliance on OXPHOS during plasma cell differentiation. Mature plasma cells rely heavily on glucose uptake and mitochondrial pyruvate import for antibody glycosylation. The thioredoxin system, including Trx, thioredoxin reductase (TrxR), and NADPH, regulates multiple cellular processes, including gene expression, antioxidant response, apoptosis, and proliferation. Trx and Trx2 function as an antioxidant system that maintains thiol-related redox status by neutralizing excess ROS. Dysregulation of the Trx system affects cellular functions and cell fate, leading to diseases like cancer and autoimmunity. The study shows that Breg cells use OXPHOS to meet their metabolic needs. Inhibition of OXPHOS with rotenone or antimycin A ablated B cell IL-10 expression and secretion, confirming the dependency of Breg cells on OXPHOS metabolism. High concentrations