Thioredoxin is a metabolic rheostat controlling regulatory B cells.

Metabolic programming is important for B cell fate, but the bioenergetic requirement for regulatory B (B reg ) cell differentiation and function is unknown. Here we show that B reg cell differentiation, unlike non-B reg cells, relies on mitochondrial electron transport and homeostatic levels of reactive oxygen species (ROS). Single-cell RNA sequencing analysis revealed that TXN, encoding the metabolic redox protein thioredoxin (Trx), is highly expressed by B reg cells, unlike Trx inhibitor TXNIP which was downregulated. Pharmacological inhibition or gene silencing of TXN resulted in mitochondrial membrane depolarization and increased ROS levels, selectively suppressing B reg cell differentiation and function while favoring pro-inflammatory B cell differentiation. Patients with systemic lupus erythematosus (SLE), characterized by B reg cell deficiencies, present with B cell mitochondrial membrane depolarization, elevated ROS and fewer Trx + B cells. Exogenous Trx stimulation restored B reg cells and mitochondrial membrane polarization in SLE B cells to healthy B cell levels, indicating Trx insufficiency underlies B reg cell impairment in patients with SLE.