Objective: The extent to which tryptophan (Trp) metabolism alterations explain or influence the outcome of inflammatory bowel diseases (IBDs) is still unclear. However, several Trp metabolism end-products are essential to intestinal homeostasis. Here, we investigated the role of metabolites from the kynurenine pathway. Design: Targeted quantitative metabolomics was performed in two large human IBD cohorts (1069 patients with IBD). Dextran sodium sulphate-induced colitis experiments in mice were used to evaluate effects of identified metabolites. In vitro, ex vivo and in vivo experiments were used to decipher mechanisms involved. Effects on energy metabolism were evaluated by different methods including Single Cell mEtabolism by profiling Translation inHibition. Results: In mice and humans, intestinal inflammation severity negatively correlates with the amount of xanthurenic (XANA) and kynurenic (KYNA) acids. Supplementation with XANA or KYNA decreases colitis severity through effects on intestinal epithelial cells and T cells, involving Aryl hydrocarbon Receptor (AhR) activation and the rewiring of cellular energy metabolism. Furthermore, direct modulation of the endogenous tryptophan metabolism, using the recombinant enzyme aminoadipate aminotransferase (AADAT), responsible for the generation of XANA and KYNA, was protective in rodent colitis models. Conclusion: Our study identified a new mechanism linking Trp metabolism to intestinal inflammation and IBD. Bringing back XANA and KYNA has protective effects involving AhR and the rewiring of the energy metabolism in intestinal epithelial cells and CD4⁺ T cells. This study paves the way for new therapeutic strategies aiming at pharmacologically correcting its alterations in IBD by manipulating the endogenous metabolic pathway with AADAT.

Secukinumab, ixekizumab and brodalumab are monoclonal antibody therapies that inhibit interleukin (IL)-17 activity and are widely used for the treatment of psoriasis, psoriatic arthritis and ankylosing spondylitis. The promising efficacy results in dermatology and rheumatology prompted the evaluation of these drugs in Crohn’s disease and ulcerative colitis, but the onset of paradoxical events (disease exacerbation after treatment with a theoretically curative drug) prevented their approval in patients with inflammatory bowel diseases (IBDs). To date, the pathophysiological mechanisms underlying these paradoxical effects are not well defined, and there are no clear guidelines for the management of patients with disease flare or new IBD onset after anti-IL-17 drug therapy. In this review, we summarise the literature on putative mechanisms, the clinical digestive effects after therapy with IL-17 inhibitors and provide guidance for the management of these paradoxical effects in clinical practice.

Triggering receptor expressed on myeloid cells-1 [TREM-1] is known to amplify inflammation in several diseases. Autophagy and endoplasmic reticulum [ER] stress, which activate the unfolded protein response [UPR], are closely linked and defects in these pathways contribute to the pathogenesis of inflammatory bowel disease [IBD]. Both autophagy and UPR are deeply involved in host-microbiota interactions for the clearance of intracellular pathogens, thus contributing to dysbiosis. We investigated whether inhibition of TREM-1 would prevent aberrant inflammation by modulating autophagy and ER stress and preventing dysbiosis. An experimental mouse model of colitis was established by dextran sulphate sodium treatment. TREM-1 was inhibited, either pharmacologically by LR12 peptide or genetically with Trem-1 knock-out [KO] mice. Colon tissues and faecal pellets of control and colitic mice were used. Levels of macroautophagy, chaperone-mediated autophagy [CMA], and UPR proteins were evaluated by western blotting. The composition of the intestinal microbiota was assessed by MiSeq sequencing in both LR12-treated and KO animals. We confirmed that inhibition of TREM-1 attenuates the severity of colitis clinically, endoscopically and histologically. We observed an increase in macroautophagy [ATG1/ULK-1, ATG13, ATG5, ATG16L1, and MAP1LC3-I/II] and in CMA [HSPA8 and HSP90AA1], whereas there was a decrease in the UPR [PERK, IRE-1α, and ATF-6α] protein expression levels in TREM-1 inhibited colitic mice. TREM-1 inhibition prevented dysbiosis. TREM-1 may represent a novel drug target for the treatment of IBD, by modulating autophagy activity and ER stress.