Phocaeicola coprophilus-Derived 6-Methyluracil Attenuates Radiation-Induced Intestinal Fibrosis by Suppressing the IDO1-Kynurenine-AHR Axis.

Intestinal kynurenine levels are persistently elevated after radiation and correlate with fibrosis severity in both murine models and human rectal cancer samples.

• Kynurenine (Kyn) elevation was observed in murine radiation-induced intestinal fibrosis (RIF) models and in human rectal cancer samples.
• The correlation between Kyn levels and fibrosis severity was demonstrated across both experimental and clinical settings.
• Exogenous Kyn administration exacerbated RIF, providing functional evidence for its pathogenic role.

IDO1 inhibition attenuated radiation-induced intestinal fibrotic progression.

• Pharmacological inhibition of indoleamine 2,3-dioxygenase 1 (IDO1) reduced fibrotic progression in the murine RIF model.
• IDO1 is the enzyme responsible for the rate-limiting step in Kyn biosynthesis from tryptophan.
• This finding mechanistically links IDO1-driven Kyn production to fibrosis development after radiation.

Kynurenine activates the aryl hydrocarbon receptor (AHR) to promote fibroblast activation and fibrosis.

• The mechanistic pathway identified was IDO1-Kyn-AHR signaling.
• AHR activation downstream of Kyn promoted fibroblast activation.
• This axis was identified as the mechanistic link between elevated Kyn and fibrogenesis.

Antibiotic depletion of gut microbiota abrogated radiation-induced IDO1-Kyn upregulation and protected against RIF.

• Antibiotic treatment to deplete the gut microbiota prevented the radiation-induced increase in IDO1 and Kyn.
• Microbiota depletion also conferred protection against the development of RIF.
• This finding established that the gut microbiota is required for radiation-induced IDO1-Kyn pathway activation.

Fecal microbiota transplantation from irradiated mice recapitulated the elevated IDO1-Kyn phenotype in recipient animals.

• Transfer of fecal microbiota from irradiated donor mice to recipient mice was sufficient to reproduce elevated IDO1-Kyn signaling.
• This experiment confirmed that radiation-induced microbial changes are causally linked to IDO1-Kyn upregulation.
• The finding supports a donor microbiota-driven mechanism for fibrosis susceptibility.

Metagenomic analysis identified radiation-induced depletion of Phocaeicola coprophilus, whose abundance inversely correlated with kynurenine levels.

• P. coprophilus was identified as a radiation-depleted gut bacterium through metagenomic sequencing.
• The abundance of P. coprophilus showed an inverse correlation with intestinal Kyn levels.
• This inverse correlation implicated P. coprophilus as a suppressor of IDO1-Kyn signaling under normal conditions.

Supplementation with live P. coprophilus suppressed IDO1-Kyn signaling and ameliorated RIF.

• Administration of live P. coprophilus bacteria reduced IDO1 and Kyn levels after radiation.
• P. coprophilus supplementation led to attenuation of radiation-induced intestinal fibrosis.
• These results demonstrated that restoring P. coprophilus is sufficient to mitigate fibrotic outcomes.

Untargeted metabolomics showed that radiation reduces 6-methyluracil, a metabolite derived from P. coprophilus.

• Untargeted metabolomic profiling was used to identify radiation-induced metabolite changes.
• 6-methyluracil was identified as a P. coprophilus-derived metabolite that was reduced following radiation.
• The reduction in 6-methyluracil paralleled the depletion of P. coprophilus after radiation exposure.

Exogenous 6-methyluracil replenishment inhibited the IDO1-Kyn axis and mitigated radiation-induced intestinal fibrosis.

• Administration of exogenous 6-methyluracil suppressed IDO1-Kyn signaling after radiation.
• 6-methyluracil treatment attenuated the development of RIF in the experimental model.
• This finding identifies 6-methyluracil as the key effector metabolite mediating P. coprophilus's protective effects.

The study defines a microbiota-metabolite-host pathway in which radiation depletes P. coprophilus, causing loss of 6-methyluracil and derepression of the IDO1-Kyn-AHR axis to drive fibrogenesis.

• The complete pathway established was: radiation → depletion of P. coprophilus → loss of 6-methyluracil → derepression of IDO1-Kyn-AHR → fibroblast activation → fibrosis.
• Therapeutic options currently remain limited for RIF, providing clinical motivation for this mechanistic work.
• Restoration of either P. coprophilus or its metabolite 6-methyluracil was proposed as a promising therapeutic strategy.