Microbiota-dependent citrulline metabolism alleviates sepsis by activating the aryl hydrocarbon receptor/IL-22 pathway to restore intestinal homeostasis

ABSTRACT This study investigated whether citrulline ameliorates sepsis-associated immune inflammation and intestinal barrier injury by modulating gut microbiota metabolites. Using an in vitro co-culture system and a cecal ligation and puncture mouse model, we found that citrulline fermentation products significantly increased the production of short-chain fatty acids and indole derivatives. These metabolic changes activated the aryl hydrocarbon receptor (AhR) pathway, upregulated interleukin-22 (IL-22) secretion, STAT3 phosphorylation, and restored the Treg/Th17 cell balance. Consequently, intestinal barrier function was improved, as evidenced by enhanced expression and membrane localization of tight junction proteins, and increased antimicrobial peptide production. Inhibition of AhR or STAT3 weakened the immunoregulatory and barrier-restorative effects of citrulline fermentation products in vitro. In vivo , citrulline treatment improved sepsis severity scores, reduced intestinal neutrophil infiltration, attenuated intestinal mucosal injury, increased goblet cell numbers, and decreased pro-inflammatory cytokines while increasing IL-10 levels in serum. Citrulline also alleviated CLP-induced lung and liver inflammatory injury. These protective effects were diminished by antibiotic treatment or AhR inhibition. RNA sequencing analysis confirmed that citrulline activates AhR and IL-22-responsive barrier gene programs while suppressing NF-κB-mediated inflammatory signaling. In conclusion, citrulline modulates the gut microbial metabolic environment to activate the AhR/IL-22/STAT3 axis, thereby rectifying immune imbalance, restoring intestinal integrity, and alleviating systemic inflammation in sepsis. These findings provide a novel theoretical foundation for nutritional and immune interventions in sepsis management. IMPORTANCE Sepsis, a life-threatening response to infection, often damages the gut, leading to a vicious cycle of inflammation and organ failure. While gut bacteria are known to influence health, how specific nutrients shape their protective effects in sepsis is unclear. This study reveals that the amino acid citrulline acts as a fuel for beneficial gut bacteria. When these bacteria metabolize citrulline, they produce molecules that act as signals to the body's immune system. These signals help calm harmful inflammation, repair the gut lining, and restore immune balance during sepsis. Our findings identify citrulline not just as a nutrient, but as a targeted regulator of the gut-immune axis, offering a promising new dietary strategy to support sepsis treatment by working with the body's microbial allies.