Gut Commensal Fungi Promote Th17 Polarization Through Dectin-1/Syk Signaling, Thereby Exacerbating the Immunopathology of Inflammatory Bowel Disease

Abstract

Although inflammation in colitis is attributed to intestinal fungal dysbiosis, we have not yet investigated how commensal fungi affect immune system immunopathology. To answer the question if fecal fungi promote an inflammation in intestinal mucosa by promoting Th17 cell polarisation via Dectin-1/Syk signalling, we study the immunopathological role of fungi in colritis using Dextran sulphate sodium (DSS) induced acute and chronic colitis animal models along with broad-spectrum antifungal clearance and specific symbiotic fungus (Candida albicans) recolonisation. We used Flow cytometry, immunofluorescence, fluorescence in situ hybridisation and 16S/ITS2 rRNA gene sequencing to study colonic lamina propria Th17 cells (IL-17A⁺CD4⁺) and intestinal mucosal fungal load and composition using Clec7a⁻/⁻ (Dectin1 knockout) mice and Syk-specific inhibitor R788. The direct effect of fungal ligand binding on Th17 development was confirmed in vitro by co-culturing naïve CD4⁚ T cells and bone marrow-derived dendritic cells (BMDCs). The gut mucosa of IBD patients and colitis mice is significantly increased by a large amount of fungel load and contents, while Candida growth is observed. Th17 infiltration, pathology and colon inflammation caused by DSS are all significantly reduced by the inclusion of funcolium clearance, but recolonised by C. This effect is reversed by albican. In vivo mechanistic studies showed that in vivo Clec7a⁻/⁻ and R7888 groups have significantly lower colitis, lower Th17 propria and mesenteric lymph nodes, and lower expression of IL-6 and IL-23 compared to wild-type mice. Fungal β-glucan stimulate BMDCs to produce IL-4 and IL-23 by activating Dectinus-1 / Syk/CARD9 signalling. Fitted innate immune cells promote th17 cell polarization through the Decttin-1 signaling pathway, and alter intestinal immune system by reprogramming innate immune cell responses via DCTN. In vitro mechanistic experiments suggest that Clec 822 and R8788 groups suffer much less serious colitis disorders, smaller Th17-type mouse proportions and lower expressions of IL6 and the IL-22 compared to a wild-like.