Activation of aryl hydrocarbon receptor attenuates intestinal inflammation by enhancing IRF4-mediated macrophage M2 polarization

Background: Crohn's disease (CD) is characterized by immune cell dysregulation, with macrophages playing an indisputable role. Macrophages can exhibit opposing polarization under different conditions, resulting in pro- or anti-inflammatory effects. The aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor, is implicated in intestinal inflammation by regulating both innate and adaptive immune responses. However, the regulatory mechanism between AhR and macrophages in colitis has not been thoroughly investigated. Methods: Macrophage polarization in the colonic tissue of active CD patients was assessed. Following colitis induction in mice by 2,4,6-trinitro-benzenesulfonic acid (TNBS), an intraperitoneal injection of the natural AhR agonist 6-formylindolo[3,2-b]carbazole (FICZ) was administered. The severity of colitis was estimated, and macrophage polarization was detected. In an in vitro setting, bone marrow-derived macrophages (BMDMs) were polarized to the M2 phenotype in the presence or absence of FICZ. Interferon regulatory factor 4 (IRF4) siRNA was applied to knockdown IRF4 expression. M2-specific markers were quantified using quantitative real-time PCR (qRT-PCR), enzyme-linked immunosorbent assay (ELISA) and flow cytometry. Results: Compared with healthy controls, active CD patients exhibited a lower presence of M2 macrophages in colonic tissue. Experimentally, FICZ was found to protect mice against TNBS-induced colitis, as evidenced by reduced diarrhea, bloody stool, and weight loss. This effect was associated with an increase in M2 macrophages and the release of IL-10 in the intestine. In BMDMs, FICZ promoted the expressions of M2-specific markers, including Ym1, Fizz1, IL-10, and CD206. Furthermore, FICZ upregulated IRF4 expression. After IRF4 silencing with siRNA, the enhancement of macrophage M2 polarization by FICZ was significantly impaired. Conclusion: Activation of AhR appears to have a beneficial effect on intestinal inflammation by promoting macrophage M2 polarization. This effect is partially mediated by the upregulation of IRF4 expression and may lead to new insight into the pathogenesis of CD.