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Background While our understanding and use of treatments for Clostridioides difficile infection (CDI) has improved, initial CDI still carries significant morbidity and mortality owing to heterogeneity in host immune responses. Further, host immunity is a critical modulator of fecal microbiota transplantation (FMT) success in CDI. Thus, understanding the host immune response during CDI is essential. Aims To assess the cellular immune responses that trigger the onset and resolution of injury and inflammation in CDI. Methods Colonic injury and inflammation triggered by CDI was modelled in mice using intrarectal installation of C. difficile toxins A and B (TcdA/B). Colonic tissue was collected at various timepoints following TcdA/B exposure to assess gene expression (qPCR), cytokine production (ELISA) and immune cell responses (flow cytometry). Knockout mice and neutralizing antibodies were used to deplete cytokines or cells. Results Examinion of colonic gene expression at different times following TcdA/B exposure found a dominant transcriptional signature related to neutrophil adhesion and diapedesis. In addition to the typical neutrophil chemokines Cxcl1 and Cxcl2, TcdA/B exposure also increased expression of neutrophil effector genes including Elane (neutrophil elastase). Neutrophil influx in response to TcdA/B was a critical driver of intestinal injury as antibody-mediated depletion of neutrophils lead to significantly less damage in the colon following TcdA/B exposure. Along with neutrophil influx, there were high levels of antimicrobial gene expression in the colon after TcdA/B exposure including RegIIIγ, S100a8, and Socs3, all genes regulated by IL-22. Upon further investigation, IL-22 was a significant mediator in the host response to TcdA/B exposure as it was upregulated &amp;gt;150-fold in the colon and originated from type 3 innate lymphoid cells (ILC3). Further, TcdA/B exposure in IL-22-/- mice lead to significantly more colonic damage compared to wildtype (WT) mice. Subsequent screening of previously published RNAseq data from IL-22-treated mouse colonic organoids identified various upregulated proteins involved in immune regulation, including the gene Slpi that encodes a protein (secretory leukocyte peptidase inhibitor) that inhibits leukocyte proteases, including neutrophil elastase. While TcdA/B challenge robustly induced the expression of Slpi in the colon of WT mice, IL-22-/- mice failed to express increased levels of Slpi and had greater levels of neutrophil elastase activity in the colon. Conclusions Together these data suggest a stepwise immune response to TcdA/B where ILC3 produce IL-22 to induce epithelial release of SLPI that attenuates the damaging effects of early neutrophil responses. Strategies to upregulate IL-22 may help control damage triggered by CDI and promote resolution of injury. Funding Agencies Lloyd Sutherland Chair in GI Research, Canadian Research Chair

journal of the canadian association of gastroenterology

A12 STEPWISE COORDINATION OF COLONIC NEUTROPHILS AND INNATE LYMPHOID CELLS IN THE ONSET AND RESOLUTION OF CLOSTRIDIOIDES DIFFICILE TOXIN-INDUCED INJURY