TMPRSS4 and idiopathic pulmonary fibrosis: Sequential slides of a single IPF biopsy stained with TMPRSS4 specific antibody and toluidine blue suggested that the TMPRSS4-expressing interstitial cells could be mast cells and to confirm this finding, we co-localized TMPRSS4 with tryptase and chymase, two proteases present in the granular content of mast cells that are commonly used as markers of these cells.