The transcription factors TCF1 and LEF1 promote B-1a cell homeostasis and regulatory function
B-1 cells are innate-like immune cells found abundantly in serosal cavities and their secreted antibodies are enriched for bacterial recognition1–5. The subset known as “B-1a” repress inflammation by production of regulatory molecules including IL-10 6–8. Unlike conventional B cells, which are produced daily in the bone marrow, B-1a cells are primarily generated from foetal precursors and maintained through self-renewal 9. The mechanisms regulating B-1a homeostasis and function are incompletely understood. Here we demonstrate that serosal cavity mouse and human B-1 cells as well as chronic lymphocytic leukaemia (CLL) B cells express the transcription factors TCF1 and LEF1, and circulating TCF1+LEF1+ B-1 cells increase significantly in patients with bacterial pleural infection. LEF1 is expressed from foetal B-1 progenitors, while TCF1 expression is higher in peripheral B-1a cells. TCF1 and LEF1 deficiency significantly reduces B-1a cells in mice. These transcription factors support B-1a turn-over regulating Myc targets and metabolic pathways. Upon B-1 cell activation, TCF1/LEF1 induce formation of a stem-like population, partly through IL-10 production. TCF/LEF1-deficient B-1 cells proliferate excessively, acquire an exhausted/senescent phenotype, express reduced IL-10 and PD-L1, and fail to suppress brain inflammation. These findings reveal a transcriptional program that links stemness and regulatory potential in B-1a cells. Overall design: B-1 progenitors from fetal liver and bone marrow, B-1 cells from peritoneal cavity and spleen of  TCF1WTLEF1WT mice were isolated by Fluorescence-activated cell sorting (FACS) followed by hashtage with Cite-seq antibodies. B-1 cells from peritoneal cavity of TCF1?LEF1? mice  were isolated by Fluorescence-activated cell sorting (FACS).