Foxp1 controls mature B cell survival and the development of follicular and B-1 B cells

Many patients with B cell lymphoma carry alterations in the gene coding for the transcription factor Foxp1. High Foxp1 expression has been linked to poor prognosis in those malignancies; however, the physiological functions of Foxp1 in mature B cells remain unknown. By employing genetic mouse models, we show that Foxp1 deletion results in reduced B cell numbers and impaired antibody production upon T cell-independent immunization. Foxp1-deficient mature B cells are impaired in survival and exhibit an increased proliferation capacity, and transcriptional analysis identified defective expression of the prosurvival Bcl-xl gene. Our results provide insight into the regulation of mature B cell survival by Foxp1 and have implications for understanding the role of Foxp1 in the development of B cell malignancies.

The transcription factor Foxp1 is critical for early B cell development. Despite frequent deregulation of Foxp1 in B cell lymphoma, the physiological functions of Foxp1 in mature B cells remain unknown. Here, we used conditional gene targeting in the B cell lineage and report that Foxp1 disruption in developing and mature B cells results in reduced numbers and frequencies of follicular and B-1 B cells and in impaired antibody production upon T cell-independent immunization in vivo. Moreover, Foxp1-deficient B cells are impaired in survival even though they exhibit an increased capacity to proliferate. Transcriptional analysis identified defective expression of the prosurvival Bcl-2 family gene Bcl2l1 encoding Bcl-xl in Foxp1-deficient B cells, and we identified Foxp1 binding in the regulatory region of Bcl2l1. Transgenic overexpression of Bcl2 rescued the survival defect in Foxp1-deficient mature B cells in vivo and restored peripheral B cell numbers. Thus, our results identify Foxp1 as a physiological regulator of mature B cell survival mediated in part via the control of Bcl-xl expression and imply that this pathway might contribute to the pathogenic function of aberrant Foxp1 expression in lymphoma.