The Adaptor Protein CARD9 Protects against Colon Cancer by Restricting Mycobiota-Mediated Expansion of Myeloid-Derived Suppressor Cells

The adaptor protein CARD9 links detection of fungi by surface receptors to the activation of the NF-κB pathway. Mice deficient in CARD9 exhibit dysbiosis and are more susceptible to colitis. Here we examined the impact of Card9 deficiency in the development of colitis-associated colon cancer (CAC). Treatment of Card9−/− mice with AOM-DSS resulted in increased tumor loads as compared to WT mice and in the accumulation of myeloid-derived suppressor cells (MDSCs) in tumor tissue. The impaired fungicidal functions of Card9−/− macrophages led to increased fungal loads and variation in the overall composition of the intestinal mycobiota, with a notable increase in C. tropicalis. Bone marrow cells incubated with C. tropicalis exhibited MDSC features and suppressive functions. Fluconazole treatment suppressed CAC in Card9−/− mice and was associated with decreased MDSC accumulation. The frequency of MDSCs in tumor tissues of colon cancer patients correlated positively with fungal burden, pointing to the relevance of this regulatory axis in human disease. [Display omitted] •Card9−/− mice have increased tumor burden upon AOM-DSS treatment compared to WT mice•CARD9-deficient macrophages exhibit impaired fungicidal abilities in the gut•Card9−/− mice have increased fungal loads and higher numbers of intestinal MDSCs•Antifungal treatment ameliorates colitis-associated cancer in Card9−/− mice The adaptor protein CARD9 plays a crucial role in anti-fungal immunity, linking detection of fungi by surface receptors to the activation of the NF-κB pathway. Wang et al. show that Card9−/− mice are more susceptible to colitis-associated cancer and outline a mechanism whereby fungal dysbiosis increases the frequency of myeloid-derived suppressor cells, thus contributing to tumorigenesis.