Characteristics of IL-25 and allergen-induced airway fibrosis in a murine model of asthma.

Interleukin (IL)-25 has been implicated in the pathogenesis of human asthma by inducing a Th2 cytokine response, but its possible role in the development of airway remodelling is less clear. We developed a murine surrogate of chronic airway inflammation induced by intranasal application of IL-25 alone. Comparison was with the 'classical' surrogate of ovalbumin (OVA) intranasal instillation into previously sensitized animals. Airway fibrotic biomarkers were analysed by immunohistochemistry and enzyme-linked immunosorbent assay. Additionally, proliferation assay and real-time polymerase chain reaction analysis were performed to assess IL-25's effects on primary human bronchial fibroblasts in vitro. In Balb/c mice, intranasal instillation of IL-25 alone induced florid airway fibrosis, including increased lay down of extracellular matrix proteins such as collagen I, III, V and fibronectin, increased numbers of fibroblasts/myofibroblasts, a profibrotic imbalance in matrix metalloproteinase/tissue inhibitor of metalloproteinase production and increased expression of profibrotic mediators including connective tissue growth factor and transforming growth factor-β1. These changes broadly reproduced those seen with classical intranasal OVA challenge in OVA-sensitized animals. Furthermore, IL-25 induced proliferation and expression of collagen I and III and smooth muscle α-actin in primary human lung fibroblasts. We conclude that chronic exposure of the airways to IL-25 alone is sufficient to cause functionally relevant airway remodelling, with the corollary that targeting of IL-25 may attenuate bronchial remodelling and fibrosis in human asthmatics.