Sandu, Gurpreet S

Background: Tissue factor pathway inhibitor (TFPI) is a key regulator of the tissue factor/factor VIIa complex. Systemic genetic disruption of TFPI exon 4 results in intrauterine lethality in mice. As such, the ability to study the biologic and tissue specific effects of TFPI deficiency has been limited. To define the tissue specific roles of TFPI we utilized a Cre-lox strategy and generated mice with flanking loxP sites surrounding exon 4 (encodes for the K1 domain). Homozygous floxed mice (TFPI Flox ) were crossed with the Tie2 Cre + or LysM Cre + mice and subsequently inbred to create homozygously floxed mice expressing Cre (TFPI Tie2 or TFPI LysM ). Methods and Results: The presence and efficiency of the deleted mRNA product was determined by RT-PCR. TFPI Tie2 and TFPI LysM mice were phenotypically similar to TFPI Flox littermates including weight, blood counts, and prothrombin and activated partial thromboplastin times. TFPI Tie2 mice have complete deletion in endothelial cells, blood cells and bone marrow cells. TFPI LysM mice show partial deletion in blood cells and complete deletion in macrophages. TFPI Tie2 mice had a 60% reduction (P < 0.0001) in circulating TFPI activity compared to TFPI Flox littermate controls. TFPI LysM mice did not have a significant reduction in circulating TFPI activity levels compared to their TFPI Flox counterparts. Neither TFPI Tie2 nor TFPI LysM mice had significant changes in hemostasis compared to TFPI Flox mice as determined by a tail bleeding assay. In an ADP-induced model of thromboembolism there were no significant changes in mortality between the TFPI Tie2 or TFPI LysM and their TFPI Flox littermates. In a ferric chloride model of arterial thrombosis, TFPI Tie2 mice had a significant reduction (P < 0.05) in time to occlusion while TFPI LysM mice did not differ compared to TFPI Flox mice. Conclusions: Deletion of TFPI from cells expressing Tie2 or LysM is consistent with normal development and hemostasis. TFPI Tie2 mice have decreased circulating levels of TFPI and an enhanced thrombotic response to ferric chloride. These data support an important regulatory role for EC TFPI in arterial thrombosis.