SFRP2 and interstitial lung disease: Moreover, among the 5 known human sFRPs, only sFRP2 is strongly upregulated by TGF-β1 (Supplemental Figure 4A), and inspection of several online scRNA-Seq files revealed that sFRP2 expression is specific to fibroblasts in ILD (7, 8, 17–19) and Rosas and Kropski online scRNA-Seq files at http://www.ipfcellatlas.com We next determined which subpopulations of fibroblasts expressed sFRP2 in fibrotic lungs.