The antithrombotic surface of endothelium is regulated in a coordinated manner. Tissue factor pathway inhibitor (TFPI) localized at the endothelial cell surface regulates the production of FXa by inhibiting the TF/VIIa complex. Systemic homozygotic deletion of the first Kunitz (K1) domain of TFPI results in intrauterine lethality in mice. Here we define the cellular sources of TFPI and their role in development, hemostasis, and thrombosis using TFPI conditional knockout mice. We used a Cre-lox strategy and generated mice with a floxed exon 4 (TFPI Flox ) which encodes for the TFPI-K1 domain. Mice bred into Tie2-Cre and LysM-Cre lines to delete TFPI-K1 in endothelial (TFPI Tie2 ) and myelomonocytic (TFPI LysM ) cells resulted in viable and fertile offspring. Plasma TFPI activity was reduced in the TFPI Tie2 (71% ؎ 0.9%, P < .001) and TFPI LysM (19% ؎ 0.6%, P < .001) compared with TFPI Flox littermate controls. Tail and cuticle bleeding were unaffected. However, TFPI Tie2 mice but not TFPI LysM mice had increased ferric chlorideinduced arterial thrombosis. Taken together, the data reveal distinct roles for endothelial-and myelomonocytic-derived TFPI. (Blood. 2010;116(10):1787-1794)
IntroductionThe endothelium provides an antithrombotic interface with circulating blood which is generated in part by the coordinated expression of endothelial-derived anticoagulants. The regulated expression of these endothelial-derived proteins may account in part for differences in thrombotic phenotype among vessels. 1 Tissue factor pathway inhibitor (TFPI) is a Kunitz-type serine protease inhibitor expressed in endothelial cells and regulates the initiation of coagulation by inhibiting tissue factor (TF)/factor VIIa activation of factor X.TFPI, originally known as lipoprotein associated coagulation inhibitor, was isolated from a hepatoma cell line. 2 TFPI circulates at low (nmol/L) levels in humans largely associated with lipoproteins. 3 Infusion of heparin increases circulating levels of TFPI in humans, and this increase has been attributed to displacement of TFPI from glycosoaminoglycans on the surface of endothelial cells. 4,5 As such, the endothelium has been thought to be the dominant source of circulating TFPI. However, TFPI is also expressed in platelets, vascular smooth muscle, cardiac myocytes, and monocyte/macrophages. 4,[6][7][8][9][10] The physiologic importance of TFPI is confirmed in that no known human deficiencies of TFPI have been reported. Homozygotic deletion of exon 4 in mice (which encodes the Kunitz 1 domain) resulted in embryonic lethality. 11 Heterozygotic deletion results in an increased response to acute and chronic vascular injury. [12][13][14] Conversely, vasculardirected overexpression of TFPI attenuates this response. 15 To better define the cellular sources of TFPI and their role in development, hemostasis, and thrombosis, we generated mice with endothelial and monocytic-restricted deletion of exon 4, which encodes for the TFPI-K1 domain. We also used bone marrow transplantation as a means to generate ...