Spermidine protects against ETEC-induced intestinal injury by reshaping the gut microbiota and activating the AhR/IL-22 axis.

BACKGROUND: Enterotoxigenic Escherichia coli (ETEC) is a major cause of infectious diarrhea, characterized by excessive inflammatory responses and intestinal barrier disruption. Spermidine (SPD), a naturally occurring polyamine, has been implicated in intestinal homeostasis; however, its protective effects and underlying mechanisms against ETEC-induced intestinal injury remain unclear. METHODS: A mouse model of ETEC K88-induced intestinal injury was established to evaluate the protective effects of SPD. Disease severity, histopathological alterations, inflammatory responses, intestinal permeability, and epithelial barrier integrity were assessed. The protective mechanism of SPD was further investigated using IPEC-J2 cells, pharmacological inhibition of aryl hydrocarbon receptor (AhR), 16S rRNA gene sequencing, and fecal microbiota transplantation (FMT). RESULTS: SPD administration significantly alleviated ETEC-induced intestinal injury in mice, as evidenced by reduced body weight loss, diarrhea severity, histopathological damage, systemic inflammation, and intestinal permeability, together with restoration of tight junction integrity. In ETEC-challenged IPEC-J2 cells, SPD improved cell viability, suppressed pro-inflammatory cytokine production, and restored the expression of AhR, CYP1A1, ZO-1, and occludin. Mechanistically, inhibition of AhR signaling by CH-223191 markedly weakened the protective effects of SPD and reduced CYP1A1 and IL-22 expression. Furthermore, 16S rRNA sequencing revealed that SPD reshaped the gut microbiota, while FMT from SPD-treated donors transferred protective effects to recipient mice. AhR inhibition further attenuated the benefits mediated by microbiota transplantation. CONCLUSION: These findings demonstrate that SPD protects against ETEC-induced intestinal injury through coordinated regulation of gut microbiota remodeling and activation of the AhR/CYP1A1/IL-22 signaling axis. This study provides mechanistic insights into the role of SPD in maintaining intestinal barrier homeostasis and highlights its potential as a nutritional intervention strategy against enteric bacterial infections.