GDF15 and idiopathic pulmonary fibrosis: We confirmed that GDF15 expression was elevated in IPF lung, while it was almost undetectable in healthy lung, and its major sources are alveolar epithelial cells and macrophages, as shown previously by others in bleomycin-treated murine lungs by IHC (23, 34) and in human IPF lungs by scRNA-Seq data sets (35).