Enteric infection priming confers IL-17A–dependent protection from chemically-induced Colitis

Background and Aims Enteric infections trigger mucosal immune responses. However, whether such immune imprinting influences susceptibility to sterile inflammatory diseases like colitis remains unclear. The aims of this study were to investigate whether a resolved Citrobacter rodentium (CR) infection in mouse alters host susceptibility to chemically induced colitis and to identify the underlying immune mechanisms. Methods C57BL/6 mice were infected with wild-type CR or CR Δ map Δ espF , a mutant lacking tight junction-disrupting effectors. 3 weeks post clearance, mice were subjected to dextran sodium sulfate (DSS)-induced colitis. Disease severity was assessed by weight change, colon length, histopathology, and myeloperoxidase levels. Colonic immune cell populations were characterised by flow cytometry, and cytokine levels were measured from colon explants. Functional roles of IL-17A were evaluated using recombinant cytokine administration and neutralising antibody treatment. Results Mice that cleared CR infection fared better following DSS treatment compared to uninfected or Δ map Δ espF -infected mice. This protective phenotype was not directly dependent on microbiota, as confirmed by co-housing experiments. Protected mice displayed elevated numbers of colonic Th1 and Th17 cells and higher levels of IL-17A, IL-22, and IL-2 cytokines. Prophylactic treatment with IL-17A conferred protection in naïve mice, whereas IL-17A neutralisation in previously infected mice abrogated the benefit, identifying IL-17A as a key mediator of protection. Conclusions Resolved intestinal infection with CR confers long-term protection against colitis via persistent IL-17A-mediated immune reprogramming. These findings resonate with the “hygiene hypothesis” and highlight how prior microbial exposure can shape mucosal resilience.