CD8A and melanoma: Indole-3-acetic acid, a tryptophan metabolite produced by gut microbiota, was reported to activate tumor-associated macrophage to suppress the CD8+ T cell-mediated antitumor immunity in a pancreatic tumor model[55], while another one, indole-3-aldehyde, can activate CD8+ T cells to proliferate and release IFN-γ to enhance antitumor immunity in a melanoma model[56].