Mechanism of interleukin-25 (IL-17E)-induced pulmonary inflammation and airways hyper-reactivity

Background IL-25, a novel member of the IL-17 cytokine family, promotes CD4(+) T-helper 2 lymphocyte-like (Th type-2) inflammatory responses in the lung. Although IL-25 upregulates IL-13 in the lung, the contribution of this and other type 2 cytokine signalling pathways to the induction and persistence of air-ways hyper-reactivity (AHR) and allergic inflammation are unclear. Objective To determine the downstream factors employed by IL-25 to induce Th type-2 pulmonary inflammation and AHR. Methods IL-25 was delivered to the airways of BALB/c mice by intra-tracheal (i.t.) instillation and AHR and Th type-2 inflammatory responses were characterized in wild type (V\rT) and Th type-2-cytokine and -signalling pathway-deficient (-/-) mice. Results IL-25 treatment resulted in AHR, eosinophilic inflammation, mucus hypersecretion and a progressive increase in the production of Th type-2 cytokines in the lungs. Levels of arginase-I (arg-I) and eotaxin were also elevated by IL-25 treatment. A significant reduction in AHR, and attenuation of mucus production was observed in IL-25-treated IL-13(-/-), IL-4 receptor alpha (IL-4R alpha(-/-))- and signal-transducer-and-activator-of-transcription-factor-6 (STAT6(-/-))-deficient mice. AHR was also inhibited in IL-4(-/-) - and IL-5/eotaxin(1)(-/-) deficient mice treated with 11-25, however, mucus hypersecretion was not completely ablated. IL-25 promoted Th type-2 responses by directly acting on naive T cells. Conclusion IL-25 potently (single dose) induces sustained AHR and acute pulmonary inflammation with eosinophilia. IL-25-induced AHR is dependent on the production of Th type-2 cytokines, and removal of IL-13 and its signal transduction pathway prevents IL-25-induced airways inflammation and AHR. IL-25 potently induces inflammatory cascades that may exacerbate allergic airways inflammation by promoting Th type-2 cytokine responses in conjunction with the up-regulation of factors (eotaxin and arg-I) that can amplify inflammation associated with allergic disorders. Dysregulation in IL-25 production may predispose to features of allergic airways disease.