According to a recent study, a tryptophan metabolite, indole-3-carboxaldehyde (3-IAld) reduces immune checkpoint inhibitor (ICI)-induced colitis in mice by influencing the structure and function of the mouse gut microbiota, resulting in an increase in sugar-fermenting bacteria and SCFA-producing bacteria and regulating Aryl hydrocarbon receptor (Ahr)/IL-22, and IL-10+ Treg cells (Renga et al., 2022; Figure 1). Here, AHR is linked to colitis.