AHR and colitis: For the working mechanism of tryptophan metabolites, it was found that indole metabolites and kynurenine can interact with aryl hydrocarbon receptor (AHR) to induce T regulatory cells differentiation, confine Th17 and Th1 responses and produce anti-inflammatory mediators (296, 297); kynurenine reduces tumor-infiltrating CD8+ T cells and mediates immune evasion of tumor cells (298), while indole metabolites alleviate colitis and protect against colorectal cancer (299, 300).