While it was found that the microbial tryptophan metabolite indole-3-aldehyde promotes ICI therapy in murine melanoma via engagement of the aryl hydrocarbon receptor in T cells, activation of AhR signaling in tumor-associated macrophages by microbiota-derived indoles restrained anti-tumor T cell responses in murine models of pancreatic ductal adenocarcinoma [46], [47]. This evidence concerns the gene AHR and pancreatic ductal adenocarcinoma.