• INFLAMMATORY BOWEL DISEASE
• COLONIC MUCOSAL METABOLISM
• GASTROINTESTINAL IMMUNE RESPONSE
• Tryptophan

The concept that the cellular metabolism of immune and tissue-resident cells controls immune responses in and beyond the gut has emerged over the last decades.1 In the gut mucosa, cell metabolism is shaped by our diet and the commensal microbiota.2 The essential amino acid tryptophan, which is enriched in animal-based protein derived from, for example, egg, red and white meat, fish and milk, is metabolised by the gut microbiota and host cells generating metabolites with immune-modulatory functions. Microbes metabolise tryptophan into indole metabolites generating ligands for aryl hydrocarbon receptor (AhR) signalling at mucosal surfaces of mammals.3 It has been demonstrated that the commensal gut microbiota of mammals controls mucosal T-cell responses by the generation of indoles4 and tryptophan metabolism controls inflammatory cytokine production.5 In the early 2000’s, tryptophan has gained increasing attention for the treatment of inflammatory conditions, partly because experimental autoimmune encephalomyelitis (phenocopying aspects of multiple sclerosis) was ameliorated by treatment with a synthetic tryptophan metabolite in mice.6 Mammalian metabolism of tryptophan in gut epithelium and in mucosal immune cells involves the enzyme indoleamine 2,3-dioxygenase 1 (IDO1), which catalyses the generation of bioactive compounds such as kynurenines.7 Kynurenines serve a wide range of physiological functions, maintaining health or promoting disease, …