BCR and TLR-induced IRF1 promotes T cell independent B cell responses by supporting Notch2 dependent Marginal Zone B cell development

The prototypic interferon (IFN)-inducible transcription factor, IRF1, not only controls expression of IFN genes but also regulates T cell and macrophage fate specification and function. We show that IRF1 expression in B cells is required for marginal zone B (MZB) cell development and T cell independent antibody responses [80% B6.129S2-Ighmtm1Cgn/J (MT) + B6.129S2-Irf1tm1Mak/J (Irf1-/-), B-Irf1-/- chimeras]. While IFNs can induce IRF1 expression in MZB precursors, IFN signaling is not required for MZB cell development [C57BL/6J (B6), B6.129S7-Ifngr1tm1Agt/J (Ifngr1-/-), B6(Cg)-Ifnar1tm1Agt/J (Ifnar1-/-)]. Instead, BCR and TLR signals, which influence MZB commitment, promote IRF1 expression and nuclear translocation in MZB cell precursors. In turn, IRF1 is required for Notch-dependent gene expression in transitional B cells and development of the MZB cell compartment. Thus, T independent MZB-driven antibody responses are regulated by BCR and TLR signals that initiate IRF1-dependent Notch programming of transitional B cells and subsequent commitment of these cells to the MZB lineage.