Ablation of IL-17A leads to severe colitis in IL-10-deficient mice: implications of myeloid-derived suppressor cells and NO production

IL-10 is an immune regulatory cytokine and its genetic defect leads to gastrointestinal inflammation in humans and mice. Moreover, the IL-23/T(h)17 axis is known to be involved in these inflammatory disorders. IL-17A, a representative cytokine produced by T(h)17 cells, has an important role for the pathological process of inflammatory diseases. However, the precise function of IL-17A in inflammatory bowel disease (IBD) remains controversial. In this study, we evaluated the effect of IL-17A on colitis in IL-10-deficient (Il10(-/-)) mice. Mice lacking both IL-10 and IL-17A (Il10(-/-)Il17a(-/-)) suffered from fatal wasting and manifested more severe colitis compared with Il10(-/-)Il17a(+/-) mice. Moreover, we found that CD11b(+)Gr-1(+) myeloid-derived suppressor cells (MDSCs) accumulated in the bone marrow, spleen and peripheral blood of Il10(-/-)Il17a(-/-) mice. These MDSCs highly expressed inducible nitric oxide synthase (iNOS) (Nos2) and suppressed the T-cell response in vitro in a NOS-dependent manner. In correlation with these effects, the concentration of nitric oxide was elevated in the serum of Il10(-/-)Il17a(-/-) mice. Surprisingly, the severe colitis observed in Il10(-/-)Il17a(-/-) mice was ameliorated in Il10(-/-)Il17a(-/-)Nos2(-/-) mice. Our findings suggest that IL-17A plays suppressive roles against spontaneous colitis in Il10(-/-) mice in an iNOS-dependent manner and inhibits MDSC differentiation and/or proliferation.