Nfkb1Activation by the E26 Transformation-Specific Transcription Factors PU.1 and Spi-B Promotes Toll-Like Receptor-Mediated Splenic B Cell Proliferation

Abstract

Generation of antibodies against T-independent and T-dependent antigens requires Toll-like receptor (TLR) engagement on B cells for efficient responses. However, the regulation of TLR expression and responses in B cells is not well understood. PU.1 and Spi-B (encoded bySfpi1andSpib, respectively) are transcription factors of the E26 transformation-specific (ETS) family and are important for B cell development and function. It was found that B cells from mice knocked out for Spi-B and heterozygous for PU.1 (Sfpi1+/−Spib−/−[PUB] mice) proliferated poorly in response to TLR ligands compared to wild-type (WT) B cells. The NF-κB family member p50 (encoded byNfkb1) is required for lipopolysaccharide (LPS) responsiveness in mice.PUBB cells expressed reducedNfkb1mRNA transcripts and p50 protein. TheNfkb1promoter was regulated directly by PU.1 and Spi-B, as shown by reporter assays and chromatin immunoprecipitation analysis. Occupancy of theNfkb1promoter by PU.1 was reduced inPUBB cells compared to that in WT B cells. Finally, infection ofPUBB cells with a retroviral vector encoding p50 substantially restored proliferation in response to LPS. We conclude thatNfkb1transcriptional activation by PU.1 and Spi-B promotes TLR-mediated B cell proliferation.