Tissue factor pathway inhibitor (
TFPI) blocks
thrombin generation via the extrinsic blood coagulation pathway. Because the severe
bleeding in patients with
hemophilia occurs from deficiency of intrinsic blood coagulation pathway
factor VIII or IX, pharmacological agents that inactivate
TFPI and, therefore, restore
thrombin generation via the extrinsic pathway, are being developed for treatment of
hemophilia. Murine models of combined
TFPI and
factor VIII deficiency were used to examine the impact of
TFPI deficiency on
bleeding and clotting in
hemophilia. In breeding studies,
Factor VIII null (F8(-/-)) did not rescue the embryonic death of
TFPI null (
Tfpi(-/-)) mice.
Tfpi(+/-) did not alter the
bleeding phenotype of F8(-/-) mice. However, total inhibition of intravascular
TFPI through injection of anti-
TFPI antibody mitigated tail vein
bleeding. Interestingly, tail blood loss progressively decreased at doses greater than needed to totally inhibit plasma
TFPI, suggesting that inhibition of a sequestered pool of
TFPI released at the injury site mitigates
bleeding. Because
TFPI is sequestered within platelets and released following their activation, the function of platelet
TFPI was examined in F8(-/-) mice lacking hematopoietic cell
TFPI that was generated by fetal
liver transplantation. Blood loss after tail transection significantly decreased in
Tfpi(+/-);F8(-/-) mice with hematopoietic
Tfpi(-/-) cells compared with
Tfpi(+/-);F8(-/-) mice with
Tfpi(+/+) hematopoietic cells. Additionally, following femoral vein injury,
Tfpi(+/-);F8(-/-) mice with
Tfpi(-/-) hematopoietic cells had increased
fibrin deposition compared with identical-genotype mice with
Tfpi(+/+) hematopoietic cells. These findings implicate platelet
TFPI as a primary physiological regulator of
bleeding in
hemophilia.