An ST2‐dependent role of bone marrow‐derived group 2 innate lymphoid cells in pulmonary fibrosis

Recent evidence supports that bone marrow (BM)‐derived hematopoietic progenitor cells play an important role in lung injury and fibrosis. While these cells give rise to multiple cell types, the ST2 (Il1rl1)‐expressing group 2 innate lymphoid cells (ILC2s) derived from BM progenitors have been implicated in tissue repair and remodeling, including in lung fibrosis. To further investigate the precise role of BM‐derived ILC2s in the pathogenesis of fibrotic lung disease, their importance in the bleomycin‐induced lung fibrosis model was evaluated by analyzing the effects of selective ST2 deficiency in the BM compartment. The results showed that while ST2‐sufficient control mice exhibited activation of lung IL‐33/ST2 signaling, ILC2 recruitment, IL‐13 induction, and fibrosis, these responses were significantly diminished in ST2‐deficient‐BM chimera mice, with selective loss of ST2 expression only in the BM. This diminished response to bleomycin was similar to that seen in ST2 global knockout mice, suggesting the predominant importance of ST2 from the BM compartment. In wild‐type mice, ILC2 recruitment to the lung was accompanied by a concomitant decrease in ST2⁺ BM cells. ST2‐deficient BM cells were unresponsive to IL‐33‐induced ILC2 maturation. Finally, lineage‐negative wild‐type, but not ST2‐deficient BM cells from bleomycin‐treated mice stimulated lung fibroblast type I collagen expression, which was associated with elevated TGFβ expression in the BM cells. Taken together, these findings suggested that the BM‐derived ILC2s were recruited to fibrotic lung through the IL‐33/ST2 pathway, and contributed to fibroblast activation to promote lung fibrosis. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.