IL-22 mediates host defense against an intestinal intracellular parasite in the absence of IFN-γ at the cost of Th17-driven immunopathology.

The roles of Th1 and Th17 responses as mediators of host protection and pathology in the intestine are the subjects of intense research. In this study, we investigated a model of intestinal inflammation driven by the intracellular apicomplexan parasite Eimeria falciformis. Although IFN-γ was the predominant cytokine during E. falciformis infection in wild-type mice, it was found to be dispensable for host defense and the development of intestinal inflammation. E. falciformis-infected IFN-γR(-/-) and IFN-γ(-/-) mice developed dramatically exacerbated body weight loss and intestinal pathology, but they surprisingly harbored fewer parasites. This was associated with a striking increase in parasite-specific IL-17A and IL-22 production in the mesenteric lymph nodes and intestine. CD4(+) T cells were found to be the source of IL-17A and IL-22, which drove the recruitment of neutrophils and increased tissue expression of anti-microbial peptides (RegIIIβ, RegIIIγ) and matrix metalloproteinase 9. Concurrent neutralization of IL-17A and IL-22 in E. falciformis-infected IFN-γR(-/-) mice resulted in a reduction in infection-induced body weight loss and inflammation and significantly increased parasite shedding. In contrast, neutralization of IL-22 alone was sufficient to increase parasite burden, but it had no effect on body weight loss. Treatment of an E. falciformis-infected intestinal epithelial cell line with IFN-γ, IL-17A, or IL-22 significantly reduced parasite development in vitro. Taken together, to our knowledge these data demonstrate for the first time an antiparasite effect of IL-22 during an intestinal infection, and they suggest that IL-17A and IL-22 have redundant roles in driving intestinal pathology in the absence of IFN-γ signaling.