Bach2 promotes B cell receptor-induced proliferation of B lymphocytes and represses cyclin-dependent kinase inhibitors

Bach2 (BTB and CNC homology-2) is a transcriptional repressor which is required for the formation of the germinal center (GC) and reactions including class switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes in B cells within the GC. While BCR-induced proliferation is essential for GC reactions, the function of Bach2 in regulating B cell proliferation has not been elucidated. In this study, we demonstrate that Bach2 is required to sustain high levels of B cell proliferation in response to BCR signaling. Following BCR engagement in vitro, B cells from Bach2-deficient (Bach2-/-) mice showed lower incorporation of 5-bromo-2'-deoxyuridine (BrdU) and reduced cell cycle progression compared with wild-type (WT) cells. Bach2-/- B cells also underwent increased apoptosis as evidenced by an elevated frequency of sub-G1 cells and early apoptotic cells. Transcriptome analysis of BCR-engaged B cells from Bach2-/- mice revealed reduced expression of the anti-apoptotic gene Bcl2l1 encoding Bcl-XL, and an elevated expression of cyclin-dependent kinase inhibitor (CKI) family genes including Cdkn1a, Cdkn2a and Cdkn2b. Reconstitution of Bcl-XL expression partially rescued the proliferation defect of Bach2-/- B cells. Chromatin imunoprecipitation (ChIP) experiments showed that Bach2 bound to the CKI family genes, indicating that these genes are direct repression targets of Bach2. These findings identify Bach2 as a requisite factor for sustaining high levels of BCR-induced proliferation, survival and cell cycle progression, and promotes expression of Bcl-XL and repression of CKI genes, respectively. BCR-induced proliferation defects may contribute to impaired GC formation observed in Bach2-/- mice. In order to reveal the molecular mechanism of prolifrerative difects of Bach2-/- follicular zone B cells in response to B cell receptor signaling, gene expression profiles of Bach2-/- follicular zone B cells were compared to its of wild-type follicular zone B cells before and after B cell receptor stimulation.