IL13 and fibrosis: Moreover, IL-13 is a pivotal inducer of bronchial sub-epithelial fibrosis due to fibroblast proliferation and collagen production, and these effects are at least in part mediated by IL-13-dependent activation of transforming growth factor-β1 (TGF-β1) (Lee et al., 2001; Firszt et al., 2014).