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The prototypic IFN-inducible transcription factor, IRF1, not only controls inflammatory gene expression but also regulates T cell and macrophage fate specification and function. Using bone marrow chimeras (80% B6.129S2-Ighm^tm1Cgn/J [µMT] + 20% B6.129S2-Irf1^tm1Mak/J [Irf1^−/−]), we show that IRF1 expression in B cells is required for marginal zone B (MZB) cell development and T cell–independent Ab responses. Although IFNs can induce IRF1 expression in MZB precursors, deletion of the IFN-γR (C57BL/6J [B6], B6.129S7-Ifngr1^tm1Agt/J) or IFN-αR (B6[Cg]-Ifnar1^tm1Agt/J) did not affect MZB cell development. Instead, BCR and TLR signals promote IRF1 expression and nuclear translocation in MZB cell precursors. In turn, IRF1 is required for Notch2-dependent gene expression in BCR- and TLR-stimulated transitional B cells and development of the MZB cell compartment. Thus, IRF1 regulates MZB-driven T cell–independent Ab responses by regulating Notch programming in MZB precursors and facilitating commitment of these cells to the MZB lineage.

Aberrant intestinal inflammation plays a critical role in the development of colitis-associated colorectal cancer (CAC), yet the mechanisms controlling tumor development by the myeloid immune compartment are not fully understood. Although altered microRNA expression is observed in CAC, it is also unclear how myeloid-specific microRNAs impact the inflammatory process that underpins the continuum from ulcerative colitis to tumorigenesis. In this study, we report that miR-223 acts to limit myeloid-driven inflammation in the azoxymethane (AOM)–dextran sodium sulfate (DSS) model of CAC in mice. In this model, miR-223^–/y mice present with significantly larger tumors with an enhanced proliferative signature. Immunoprofiling showed that miR-223^–/y mice have significantly increased colonic myeloid immune infiltrate (neutrophils, monocytes, and macrophages) following AOM-DSS. This was accompanied by an increased inflammatory chemokine and cytokine signature for monocytes and neutrophils. Bone marrow chimera studies demonstrate that myeloid-expressed miR-223 is responsible for the enhanced tumor proliferation and inflammatory response. RNA sequencing identified several pathways that could be contributing to the development of CAC in miR-223^–/y mice, including the IL-6/IL-17a cytokine family and STAT3 signaling. Lastly, neutrophil depletion with an anti-GR1 Ab (Ly6G/Ly6C) during the initial phase of the AOM-DSS model reduced the tumor burden in miR-223^–/y mice. Collectively, our data indicate that miR-223 is an important regulator of mucosal inflammation and acts to constrain the progression from ulcerative colitis to CAC by limiting myeloid-associated inflammation.