IL-18 cytokine levels modulate innate immune responses and cryptosporidiosis in mice.

IL-18 is known to play a key role limiting Cryptosporidium parvum infection. In this study, we show that IL-18 depletion in SCID mice significantly exacerbates C. parvum infection, whereas, treatment with recombinant IL-18 (rIL-18), significantly decreases the parasite load, as compared to controls. Increases in serum IFN-γ levels as well as the up-regulation of the antimicrobial peptides, cathelicidin antimicrobial peptide and beta defensin 3 (Defb3) were observed in the intestinal mucosa of mice treated with rIL-18. In addition, C. parvum infection significantly increased mRNA expression levels (> 50 fold) of the alpha defensins, Defa3 and 5, respectively. Interestingly, we also found a decrease in mRNA expression of IL-33 (a recently identified cytokine in the same family as IL-18) in the small intestinal tissue from mice treated with rIL-18. In comparison, the respective genes were induced by IL-18 depletion. Our findings suggest that IL-18 can mediate its protective effects via different routes such as IFN-γ induction or by directly stimulating intestinal epithelial cells to increase antimicrobial activity.