Modulation of immune responses dictate the severity of Clostridium difficile infection

Clostridium difficile infection (CDI) occurs when the microbiome is disrupted, most often through the use of antibiotics. Previous studies have demonstrated that increased inflammatory markers, such as IL-8, are better at predicting poor patient outcome than enhanced bacterial burden. This finding, coupled with the neutrophilia observed in CDI patients, supports the hypothesis that the severity of disease may be correlated with intensity of host response. Thus, we aim to elucidate mediators in the immune response, which may regulate a healthy immune response. We have established a pathogenic role for IL-23 in CDI through the observation that mortality is decreased in two distinct mouse models where IL-23 signaling is absent. The absence of IL-23 resulted in decreased expression of downstream type 17 cytokines IL-22, IL-17A and neutrophilia. The role of these mediators remains to be examined, yet mice lacking IL-23 signaling had enhanced integrity of the epithelial barrier suggesting IL-23 drives tissue pathology.
Moving forward, our goal was to determine immune mediators that regulate beneficial inflammation during CDI. Previous studies have demonstrated an inverse relationship in the signaling of cytokines IL-23 and IL-25. Additionally, IL-25 expression is decreased in antibiotic treated and germ free mice. The ability of C. difficile to cause active infection after antibiotic treatment stresses the beneficial role of commensal bacteria. Despite this knowledge, the role of the microbiota to modulate disease severity by influencing the host immune response has yet to be explored. As a result, we hypothesized that IL-25 was protective during infection. Here, we show that IL-25 production, a cytokine regulated by the microbiota, is suppressed during CDI in humans and in a mouse model. Administration of IL-25 protein protected mice from CDI-associated mortality without affecting the amount of C. difficile bacteria or toxin in the gut. We identified eosinophils as the cellular mechanism by which IL-25 protects using two distinct mouse models lacking eosinophils. In both instances, IL-25 was unable to reduce mortality or morbidity or prevent gut barrier disruption in the absence of eosinophils. Thus, our study provides a novel mechanism by which induction of innate immune responses, specifically microbiota-regulated IL-25 actions on eosinophils, can reduce the severity of CDI and protect the gut barrier.