Expression of fibroblast activation protein-α in human deep venous thrombus

Background Fibroblast activation protein-α (FAP), a type-II transmembrane serine protease, is expressed during wound healing, in cancer-associated fibroblasts, and in chronic fibrosing diseases. However, its expression in deep vein thrombus (DVT) remains unclear. Therefore, in this study, we investigated FAP expression and localization in DVT. Methods First, we pathologically accessed aspirated thrombi of patients with DVT (n=14), classifying thrombotic areas as follows; fresh, cellular lysis, endothelialization, fibroblastic reaction. Endothelialization and fibroblastic reaction were defined as organizing reactions. We immunohistochemically examined FAP-expressed areas and expressed cells. Second, we analyzed FAP expression in cultured dermal fibroblasts. Results All the aspirated thrombi showed a mixture of at least three of the four thrombotic areas. Specifically, 83 % of aspirated thrombi showed fresh and organizing reactions within each thrombus. Immunohistochemical expression of FAP was restricted in organizing area. Further, FAP expression in the thrombi was mainly found in vimentin-positive or α-smooth muscle actin-positive fibroblasts in double immunofluorescence. Some CD163-positive macrophages also showed FAP expression. FAP mRNA and protein levels significantly increased in cultured fibroblasts with low- proliferative activity under 0.1% fetal bovine serum (FBS) than that in fibroblasts under 10% FBS. Fibroblasts cultured in 10% FBS showed a significant decrease in FAP mRNA levels following supplementation with hemin, but not with thrombin. Conclusions The heterogeneous composition of the venous thrombi suggests the existence of a multistep thrombus formation process in human DVT. Further, fibroblasts or myofibroblasts may express FAP in organizing process in DVT. FAP expression may increase in fibroblasts with low-proliferative activity. ### Competing Interest Statement The authors have declared no competing interest. * CAF : cancer-associated fibroblast DMEM : Dulbecco’s Modified Eagle medium DVT : deep vein thrombosis FAP : fibroblast activation protein-α FBS : fetal bovine serum GP IIb/IIIa : glycoprotein IIb/IIIa SMA : smooth muscle actin