PU.1 Regulates Ig Light Chain Transcription and Rearrangement in Pre-B Cells during B Cell Development

B cell development and Ig rearrangement are governed by cell type-and developmental stage-specific transcription factors. PU.1 and Spi-B are E26-transformation-specific transcription factors that are critical for B cell differentiation. To determine whether PU. 1 and Spi-B are required for B cell development in the bone marrow, Spi1 (encoding PU. 1) was conditionally deleted in B cells by Cre recombinase under control of the Mb1 gene in Spib (encoding Spi-B)-deficient mice. Combined deletion of Spi1 and Spib resulted in a lack of mature B cells in the spleen and a block in B cell development in the bone marrow at the small pre-B cell stage. To determine target genes of PU.1 that could explain this block, we applied a gain-of-function approach using a PU.1/Spi-B-deficient pro-B cell line in which PU.1 can be induced by doxycycline. PU.1-induced genes were identified by integration of chromatin immunoprecipitation-sequencing and RNA-sequencing data. We found that PU. 1 interacted with multiple sites in the Igk locus, including Vk promoters and regions located downstream of V kappa second exons. Induction of PU.1 induced Igk transcription and rearrangement. Upregulation of Ig kappa transcription was impaired in small pre-B cells from PU.1/Spi-B-deficient bone marrow. These studies reveal an important role for PU.1 in the regulation of Igk transcription and rearrangement and a requirement for PU.1 and Spi-B in B cell development.