IL1RL1 and idiopathic pulmonary fibrosis: Although studies assessing human [45] and murine cells [46] have demonstrated fibroblast responsiveness to IL-33, our results are consistent with those of Yagami and colleagues [47] and suggest that extracellular IL-33 cannot directly increase the pro-fibrotic activity of fibroblasts during the development of IPF as they lack the IL-33 receptor ST2.