Introduction
As a small but important component of the intestinal microbiota, the fungal community (mycobiota) has received increasing attention for its involvement in the pathogenesis of Crohn's disease (CD)1. In our previous study, we identified a significantly increased relative abundance of Exophiala dermatitidis (E. dermatitidis) in the gut of patients with CD, particularly those with active disease2. Caspase recruitment domain family member 9 (CARD9), a key adaptor molecule in antifungal immune responses3, has also been strongly associated with CD susceptibility4. In this study, we aimed to explore the potential role and underlying mechanisms of E. dermatitidis in intestinal inflammation.
Aims & Methods
To evaluate the impact of E. dermatitidis on intestinal inflammation, we generated Card9-knockout (KO) (Card9-/-) mice and constructed a dextran sulfate sodium (DSS)-induced colitis model colonized by E. dermatitidis. The underlying molecular mechanisms were investigated by a range of biochemical approaches. Flow cytometry was performed to analyze the proportions of myeloid cells, including CX3CR1+ macrophages, in the colonic lamina propria (LP).
Results
Compared to mice with DSS-induced colitis alone (WT-DSS group), the DSS-treated mice colonized by E. dermatitidis (WT-DSS + Ed group) showed a significant weight loss, a worsening of disease activity, a shorter length of the large intestine, and more severe intestinal inflammation characterized by increased inflammatory cell infiltration and crypt destruction in the histological examination of colon hematoxylin and eosin (H&E) staining. Card9-/- mice were used to further investigate whether E. dermatitis-mediated inflammation was dependent on the CARD9 signaling pathway. We observed that CARD9 deficiency protected against E. dermatitidis-exacerbated colitis, as measured by a higher trend of body weight, longer colon lengths, and amelioration of disease activity in the KO-DSS + Ed group. In addition, flow cytometric analysis of the colonic lamina propria revealed a higher percentage proportion of CX3CR1+ macrophages in the WT-DSS + Ed group and a markedly reduced proportion in the KO-DSS + Ed group. Furthermore, we identified the Mincle-Syk-CARD9-nuclear factor kappa-B (NF-κB) signaling pathway as playing an important role in mediating the inflammatory immune responses to E. dermatitidis.
Conclusion
E. dermatitidis exacerbates the DSS-induced colitis in mice through the Mincle-Syk-CARD9-NF-κB pathway with the participation of CX3CR1+ macrophages, providing a novel therapeutic strategy for CD targeting specific intestinal fungi.
References
1. Sokol H, Leducq V, Aschard H, et al. Fungal microbiota dysbiosis in IBD. Gut 2017;66:1039-1048.
2. Zeng L, Feng Z, Zhuo M, et al. Fecal fungal microbiota alterations associated with clinical phenotypes in Crohn's disease in southwest China. PeerJ 2022;10:e14260.
3. Hu A, Hu Z, Zou H, et al. CARD9 in host immunity to fungal, bacterial, viral, and parasitic infections: An update. Frontiers In Microbiology 2022;13:1021837.
4. Franke A, McGovern DPB, Barrett JC, et al. Genome-wide meta-analysis increases to 71 the number of confirmed Crohn's disease susceptibility loci. Nature Genetics 2010;42:1118-1125.