2), and Irf (3) families have been well studied in the context of the immune system in part due to their clear and established association with the upstream signaling cascades. In contrast, although the phosphatidylinositol 3-kinase (PI3K) pathway is critically important for the regulation of immune cells, its downstream transcription factors are still unclear except for the Foxo family (4). In this study, we investigated the possibility that the transcription factor Bach2 might be regulated in B cells by the PI3K pathway. Salient features of Bach2 relevant to this study are as follows.Bach2 regulates the immune cells at multiple points. During the development of B cells from the progenitor cells, Bach2, together with its related factor Bach1, represses the expression of myeloid genes. The repression of the myeloid program is critical for the progenitor cells to be committed to the B cell fate (5). At the stage of pre-B cells, the successful completion of antibody heavy chain gene rearrangement is monitored by the lack or presence of pre-B cell receptor (pre-BCR 2 checkpoint). Bach2 plays a critical role in negative selection (i.e. elimination of cells unsuccessful in the rearrangement) at the pre-BCR checkpoint (6). In mature B cells, Bach2 is required for the class switch recombination (CSR) and somatic hypermutation that diversify the effector function and antigen affinity, respectively, of antibody molecules in response to antigen and other stimulation (7). Bach2 promotes CSR by delaying the expression of Blimp-1, the master regulator of plasma cell differentiation, and thereby securing a time window for CSR before the terminal differentiation to plasma cells (8 -10). A reduction in the Bach2 expression in memory B cells is involved in their rapid plasma cell differentiation upon antigen re-exposure (11). An integral view of the Bach2 functions in B cells has been proposed as a gene regulatory network (GRN) consisting of Bach2 and other * This work was supported by Grants-in-aid 15H02506, 25670156, 24390066, 23116003, 21249014, 17054028, and 25291042

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The transcription factor Bach2 regulates both acquired and innate immunity at multiple steps, including antibody class switching and regulatory T cell development in activated B and T cells, respectively. However, little is known about the molecular mechanisms of Bach2 regulation in response to signaling of cytokines and antigen. We show here that mammalian target of rapamycin (mTOR) controls Bach2 along B cell differentiation with two distinct mechanisms in pre-B cells. First, mTOR complex 1 (mTORC1) inhibited accumulation of Bach2 protein in nuclei and reduced its stability. Second, mTOR complex 2 (mTORC2) inhibited FoxO1 to reduce Bach2 mRNA expression. Using expression profiling and chromatin immunoprecipitation assay, the Ccnd3 gene, encoding cyclin D3, was identified as a new direct target of Bach2. A proper cell cycle was lost at pre-B and mature B cell stages in Bach2-deficient mice. Furthermore, AZD8055, an mTOR inhibitor, increased class switch recombination in wild-type mature B cells but not in Bach2-deficient cells. These results suggest that the mTOR-Bach2 cascade regulates proper cell cycle arrest in B cells as well as immunoglobulin gene rearrangement.