Abstract 5479: Endothelial Cell TFPI Regulates Murine Arterial Thrombosis but Not Hemostasis or Development
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 (TFPIFlox) were crossed with the Tie2 Cre+ or LysM Cre+ mice and subsequently inbred to create homozygously floxed mice expressing Cre (TFPITie2 or TFPILysM).
Methods and Results: The presence and efficiency of the deleted mRNA product was determined by RT-PCR. TFPITie2 and TFPILysM mice were phenotypically similar to TFPIFlox littermates including weight, blood counts, and prothrombin and activated partial thromboplastin times. TFPITie2 mice have complete deletion in endothelial cells, blood cells and bone marrow cells. TFPILysM mice show partial deletion in blood cells and complete deletion in macrophages. TFPITie2 mice had a 60% reduction (P < 0.0001) in circulating TFPI activity compared to TFPIFlox littermate controls. TFPILysM mice did not have a significant reduction in circulating TFPI activity levels compared to their TFPIFlox counterparts. Neither TFPITie2 nor TFPILysM mice had significant changes in hemostasis compared to TFPIFlox mice as determined by a tail bleeding assay. In an ADP-induced model of thromboembolism there were no significant changes in mortality between the TFPITie2 or TFPILysM and their TFPIFlox littermates. In a ferric chloride model of arterial thrombosis, TFPITie2 mice had a significant reduction (P < 0.05) in time to occlusion while TFPILysM mice did not differ compared to TFPIFlox mice.
Conclusions: Deletion of TFPI from cells expressing Tie2 or LysM is consistent with normal development and hemostasis. TFPITie2 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.