An increased understanding of the importance of microbiota in shaping the host’s immune and metabolic activities has rendered fungal interactions with their hosts more complex than previously appreciated. The aryl hydrocarbon receptor (AhR) has a pivotal role in connecting tryptophan catabolism by microbial communities and the host’s own pathway of tryptophan metabolite production with the orchestration of T-cell function. AhR activation by a Lactobacillus-derived AhR ligand leads to the production of IL-22 to the beneﬁt of mucosal defense mechanisms, an activity upregulated in the absence of the host tryptophan catabolic enzyme, indoleamine 2,3-dioxygenase 1 (IDO1), which is required for protection from fungal diseases (“disease tolerance”). As AhR activation in turn leads to the activation—in a feedback fashion—of IDO1, the regulatory loop involving AhR and IDO1may have driven the coevolution of commensal fungi with themammalian immune systemand themicrobiota, to the beneﬁt of host survival and fungal commensal- ism. This review will discuss the essential help the microbiota provides in controlling the balance between the dual nature of the fungal–host relationship, namely, commensalism vs. infection.
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