The study investigates the metabolic requirements and regulatory pathways controlling the differentiation of regulatory B (Breg) cells. Key findings include: 1. **Metabolic Requirements for Breg Cell Differentiation**: Breg cells rely on mitochondrial electron transport and homeostatic levels of reactive oxygen species (ROS) for differentiation. Single-cell RNA sequencing revealed that the metabolic redox protein thioredoxin (Trx) is highly expressed in Breg cells, while its inhibitor, thioredoxin-interacting protein (TXNIP), is downregulated. 2. **Pharmacological and Genetic Inhibition of Trx**: Inhibiting Trx results in mitochondrial membrane depolarization and increased ROS levels, selectively suppressing Breg cell differentiation and function while promoting pro-inflammatory B cell differentiation. 3. **SLE Patients and Breg Cell Deficiencies**: Patients with systemic lupus erythematosus (SLE) exhibit B cell mitochondrial membrane depolarization, elevated ROS levels, and reduced Trx+ B cells. Exogenous Trx restoration in SLE B cells restores Breg cell differentiation and mitochondrial membrane polarization. 4. **Role of Trx in Breg Cell Differentiation**: Trx is crucial for maintaining mitochondrial health and low ROS levels in Breg cells. Inhibition of Trx impairs Breg cell differentiation and suppressive function, leading to increased production of pro-inflammatory cytokines. 5. **Therapeutic Implications**: The findings suggest that targeting Trx-dependent pathways may represent a novel therapeutic approach to modulate Breg activity in autoimmune diseases and cancer. These results highlight the importance of metabolic regulation, particularly through the thioredoxin system, in the differentiation and function of Breg cells, providing new insights into the underlying mechanisms and potential therapeutic strategies.The study investigates the metabolic requirements and regulatory pathways controlling the differentiation of regulatory B (Breg) cells. Key findings include: 1. **Metabolic Requirements for Breg Cell Differentiation**: Breg cells rely on mitochondrial electron transport and homeostatic levels of reactive oxygen species (ROS) for differentiation. Single-cell RNA sequencing revealed that the metabolic redox protein thioredoxin (Trx) is highly expressed in Breg cells, while its inhibitor, thioredoxin-interacting protein (TXNIP), is downregulated. 2. **Pharmacological and Genetic Inhibition of Trx**: Inhibiting Trx results in mitochondrial membrane depolarization and increased ROS levels, selectively suppressing Breg cell differentiation and function while promoting pro-inflammatory B cell differentiation. 3. **SLE Patients and Breg Cell Deficiencies**: Patients with systemic lupus erythematosus (SLE) exhibit B cell mitochondrial membrane depolarization, elevated ROS levels, and reduced Trx+ B cells. Exogenous Trx restoration in SLE B cells restores Breg cell differentiation and mitochondrial membrane polarization. 4. **Role of Trx in Breg Cell Differentiation**: Trx is crucial for maintaining mitochondrial health and low ROS levels in Breg cells. Inhibition of Trx impairs Breg cell differentiation and suppressive function, leading to increased production of pro-inflammatory cytokines. 5. **Therapeutic Implications**: The findings suggest that targeting Trx-dependent pathways may represent a novel therapeutic approach to modulate Breg activity in autoimmune diseases and cancer. These results highlight the importance of metabolic regulation, particularly through the thioredoxin system, in the differentiation and function of Breg cells, providing new insights into the underlying mechanisms and potential therapeutic strategies.