The major findings of the present study are as follows: (i) Abx exposure before but not during DSS treatment deteriorates the immunoregulatory effects of Trp; (ii) Trp-enriched gut microbiota and microbial indole are essential for alleviating DSS-induced colonic inflammation after Abx -induced gut microbiota dysbiosis; (iii) the activation of HTR2B and the reduction of LBP by indole inhibit M1 macrophage polarization in colonic inflammation. This evidence concerns the gene LBP and colitis.