GDF15 and idiopathic pulmonary fibrosis: IPF fibroblasts were treated with eight human recombinant cytokines (all at 10 ng/ml), IL-17A, osteopontin, ENA-78, GM-CSF, GDF-15, GRO-α, IL-6, and IL-8, or with PGE2 (1–1,000 nM).