Tryptophan metabolite FICZ improved symptoms of AD-like dermatitis, decreased TEWL, restored filaggrin expression, reduced the number of infiltrated mast cells, and reduced expression of IL-22 and IFN-γ genes in a mouse modelFICZ upregulated expression and abundance of filaggrin in HaCaT and NHEKs cells. Here, IL22 is linked to dermatitis.