Abstract
Tissue factor pathway inhibitor-alpha (TFPI-α) is a Kunitz-type serine protease inhibitor,
which suppresses coagulation by inhibiting the tissue factor (TF)/factor VIIa complex
as well as factor Xa. In static plasma-phospholipid systems, TFPI-α thus suppresses
both factor Xa and thrombin generation. In this article, we used a microfluidics approach
to investigate how TFPI-α regulates fibrin clot formation in platelet thrombi at low
wall shear rate. We therefore hypothesized that the anticoagulant effect of TFPI-α
in plasma is a function of the local procoagulant strength—defined as the magnitude
of thrombin generation under flow, due to local activities of TF/factor VIIa and factor
Xa. To test this hypothesis, we modulated local coagulation by microspot coating of
flow channels with 0 to 100 pM TF/collagen, or by using blood from patients with haemophilia
A or B. For blood or plasma from healthy subjects, blocking of TFPI-α enhanced fibrin
formation, extending from a platelet thrombus, under flow only at <2 pM coated TF.
This enhancement was paralleled by an increased thrombin generation. For mouse plasma,
genetic deficiency in TFPI enhanced fibrin formation under flow also at 0 pM TF microspots.
On the other hand, using blood from haemophilia A or B patients, TFPI-α antagonism
markedly enhanced fibrin formation at microspots with up to 100 pM coated TF. We conclude
that, under flow, TFPI-α is capable to antagonize fibrin formation in a manner dependent
on and restricted by local TF/factor VIIa and factor Xa activities.
Keywords
fibrin clot formation - haemophilia - mouse - platelets - TFPI - thrombin generation
