Sun, R.; Liu, Y.; Wang, Y.; Xu, N.; Ma, X.; Lv, Q.; Zhao, X.; Vallee, I.; Boireau, P.; Wu, Z.; Liu, M.; Liu, X.; Jin, X.

Helminths systemically suppress host immunity, yet whether they impose immune tolerance by rewiring host-associated microbial metabolism remains unclear. Here we show that Trichinella spiralis infection remodels intestinal tryptophan metabolism to generate an AhR-dependent regulatory immune state. T. spiralis infection enriched the commensal bacterium Ligilactobacillus murinus, which converted tryptophan into indole-3-lactic acid (ILA), a microbial metabolite that directly engaged the aryl hydrocarbon receptor. Antibiotic-mediated microbiota depletion abolished infection-induced ILA accumulation, AhR activation and Treg/Th17 rebalancing, whereas fecal microbiota transplantation from infected donors or supplementation with L. murinus or ILA restored these effects. Pharmacological blockade or genetic deletion of AhR eliminated the ability of T. spiralis, L. murinus and ILA to restrain LPS-induced cytokine-storm-like lung inflammation, establishing AhR as an essential host node in this circuit. Extending these findings to viral inflammatory disease, oral ILA improved survival and reduced pulmonary immunopathology in SARS-CoV-2-infected K18-hACE2 mice. Re-analysis of human COVID-19 metabolomic data further revealed reduced circulating ILA in severe disease. These findings define a helminth-remodeled microbial tryptophan metabolic pathway that promotes disease tolerance and identify the ILA-AhR axis as a candidate postbiotic strategy for limiting hyperinflammatory tissue injury.