Tissue factor in coagulation: Which? Where? When?

Abstract

Tissue factor (TF) is an integral membrane protein, normally separated from the blood by the vascular endothelium, which plays a key role in the initiation of blood coagulation. With a perforating vascular injury, TF becomes exposed to blood and binds plasma factor VIIa. The resulting complex initiates a series of enzymatic reactions leading to clot formation and vascular sealing. In some pathological states, circulating blood cells express TF as a result of exposure to an inflammatory stimulus leading to intravascular clotting, vessel occlusion, and thrombotic pathology. Numerous controversies have arisen related to the influence of structural features of TF, its presentation, and its function. There are contradictory reports about the synthesis and presentation of TF on blood cells and the presence (or absence) of functionally active TF circulating in normal blood either on microparticles or as a soluble protein. In this review we discuss TF structure-function relationships and the role of TF during various phases of the blood coagulation process. We also highlight controversies concerning the expression/presence of TF on various cells and in blood in normal and pathological states.

Figure 1

The structure of various TF species. Indicated molecular weights were determined from the amino acid composition (AA), gel electrophoresis (SDS) and mass-spectroscopy (MALDI-TOF). (This figure was originally published in Surgery108).

Figure 2

TF titrations in contact pathway inhibited whole blood (A) and plasma (B) from healthy individuals. Black and white bars represent two healthy donors. (This figure was originally published in Blood64).

Figure 3

Flow cytometric analyses of resting and calcium ionophore A23187-treated platelets. Resting (A) or A23187-treated platelets (B) were immunostained with anti-TF-5 monoclonal antibody. A23187-treated platelets were also treated with an anti-P-selectin antibody (C). An irrelevant, isotype-matched mouse IgG was used in the control experiments. (This figure was originally published in Blood64).

Figure 4

Termination of TF and factor VIIa activity in TF (5 pM) initiated thrombin generation in Synthetic Coagulation Proteome. Anti-TF and anti-factor VIIa inhibitory antibodies were added at 0 s (✖), 10 s (●), 60 s (▲) and 240 s (■) after the initiation of the reaction or not added at all (◆). Arrows indicate antibody addition time-points. (This figure was originally published in J Biol Chem98).

Figure 5

Resupply of the synthetic coagulation proteome—the effect of factor VIII on the stability of the response. A 5 pM TF-initiated reaction mixture was subdivided after 20 min (A), and the eight separate aliquots subsequently resupplied at different times with an equal volume of a mixture containing 1.4 µM prothrombin/3.4 µM antithrombin/2 µM phospholipids either without factor VIII (closed symbols) or with 0.7 nM factor VIII (open symbols). The resulting time courses of thrombin generation are presented. Resupply with the mixture without factor VIII was conducted immediately (20 min → t=0, (◆) and 45 (●), 90 (■) and 110 (▲) min later. Resupply with the mixture supplemented with factor VIII was conducted at 15 (□), 55 (△) and 100 (○) min after the subdivision of the TF-initiated reaction. Thrombin levels for the final 5 min of the TF-initiated episode are also shown (◊). Thrombin levels are expressed as total picomoles of active thrombin to normalize for the volume change. An arrow indicates the resupply time for each aliquot. Reproduced with permission from Orfeo et al.

Figure 6

Schema of a two compartment model of the regulation of TF-initiated blood coagulation. A cross section of a blood vessel showing the luminal space, endothelial cell layer and extravascular region is presented at the site of a perforation. The blood coagulation process in response is depicted in four stages. Tissue factor-factor VIIa complex, TF•VIIa; prothrombinase complex, Xa•Va; intrinsic factor Xase, VIIIa•IXa; ATIII-endothelial cell heparan sulfate proteoglycan complex bound to thrombin or factor Xa, HS•ATIII•(IIa or Xa); protein C bound to thrombomodulin-thrombin, TM•IIa•PC. Stage 1. Perforation results in delivery of blood, and with it circulating factor VIIa and platelets, to an extravascular space rich in membrane bound TF. Platelets adhere to collagen and von Willebrand factor associated with the extravascular tissue, and TF binds factor VIIa , initiating the process of factor IX and factor X activation. Factor Xa activates small amounts of prothrombin to thrombin that activates more platelets and converts factor V and factor VIII to factor Va and factor VIIIa. Stage 2. The reaction is propagated by platelet-bound intrinsic factor Xase and prothrombinase with the former being the principle factor Xa generator. Initial clotting occurs and fibrin begins to fill in the void in cooperation with activated platelets. Stage 3. A barrier composed of activated platelets ladened with procoagualant complexes and enmeshed in fibrin scaffolding is formed. The reaction in the now filled perforation is terminated by reagent consumption attenuating further thrombin generation but functional procoagulant enzyme complexes persist because they are protected from the dynamic inhibitory processes found on the intravascular face. Stage 4. View downstream of the perforation. Enzymes escaping from the plugged perforation are captured by antithrombin-heparan complexes and the protein C system is activated by residual thrombin binding to endothelial cell thrombomodulin, initiating the dynamic anticoagulant system. These intravascular processes work against occlusion of the vessel despite the continuous resupply of reactants across the intravascular face of the thrombus. (This figure was originally published in J Biol Chem98).