S100A4 and idiopathic pulmonary fibrosis: Since previous studies demonstrated FSP-1 or α-smooth muscle actin (α-SMA) positive fibroblasts as derivatives from the lung residential alveolar epithetical cells, bone marrow mesenchymal cells [27], or endothelium [24], we cannot exclude any of these origins as the sources of ACE2 and TMPRSS2 expression in IPF lung tissues.