Purification and Properties of an Abnormal Blood Coagulation Factor IX (Factor IXBm)/Kinetics of Its Inhibition of Factor X Activation by Factor VII and Bovine Tissue Factor

An abnormal blood coagulation factor IX has been isolated from the blood of a hemophilia B patient with a variant of the disease (hemophilia Bm) characterized by a normal concentration of factor IX antigen, negligible factor IX coagulant activity, and a prolonged prothrombin time with bovine tissue factor. The isolated protein (factor IXBm) had the same apparent molecular weight as normal factor IX (55,000) and the same mobility on two dimensional immunoelectrophoresis as normal factor IX. Factor IXBm underwent limited proteolysis induced by activated factor XI, in the presence of Ca2+ ions, or induced by the reaction product of tissue factor, factor VII and Ca2+ ions. A timecourse study showed that activated factor XI cleaved factor IXBm and factor IX at similar rates. However, in contrast to normal factor IX, the limited protelysis of factor IXBm did not generate procoagulant activity.

In kinetic experiments purified factor IXBm behaved like a competitive inhibitor (Ki of 0.017 μM) of the activation of factor X by bovine tissue factor and factor VII. Normal factor IX was also found to inhibit the reaction but required a four-fold higher concentration to achieve the same inhibitory effects as factor IXBm.

Coagulation problems in liver disease

Three topics are addressed in this manuscript: (1) the causes of abnormal hemostasis in cirrhosis; (2) the evaluation of hemostasis in cirrhotic patients before invasive procedures or in the presence of bleeding; and (3) the assessment of the effect of recombinant activated factor VII (rFVIIa; NovoSeven, Novo Nordisk A/S, Bagsvaerd, Denmark) on hemostatic function in cirrhotic patients. Laboratory experiments are described that could enhance the understanding of the effect of rFVIIa on blood coagulation during hemostasis in cirrhotic patients.

A novel clotting assay for quantitation of plasma prothrombin (factor II) using Echis multisquamatus venom

Measuring plasma prothrombin activity seems useful for evaluating thrombotic risk and managing oral anticoagulant therapy as an adjunct to the international normalized ratio. Therefore, we designed a new plasma prothrombin assay based on the ability of Echis multisquamatus venom to activate prothrombin with only calcium as a cofactor. In this assay, 1 part of undiluted citrated plasma is added to 5 parts of a venom reagent and the clotting time is measured. The assay's advantages are that dilution of the test plasma is required only when prothrombin activity exceeds 100%, a single standard curve can be used over months for a given batch of stock reagent, and barium-adsorbed plasma is used for dilution of test plasma and construction of the standard curve, thus eliminating the need for prothrombin-deficient plasma. However, one should be aware of the following: (1) test samples must contain at least 200 mg/dL fibrinogen; and (2) when prothrombin concentrations were below 50%, the venom-based assay often gave values up to 10% higher than the thromboplastin-based assay. Values obtained in 262 plasma samples tested with the venom-based assay and with a thromboplastin-based prothrombin assay correlated well (r2 = 0.93).

Hemostatic factors in rabbit limb lymph: relationship to mechanisms regulating extravascular coagulation

Mechanisms regulating extravascular coagulation in interstitial fluids of peripheral tissues are poorly understood, since measurements of hemostatic factors in these fluids are unavailable. Because lymph from a body region reflects the composition of its interstitial fluid, we measured hemostatic factors in limb lymph of rabbits both as activity and as antigen. Mean lymph-to-plasma activity ratios were the following: fibrinogen, 0.28; prothrombin, 0.26; factor X, 0.27; factor VII, 0.17; and factors V and VIII, 0.08. All lymph fibrinogen was clottable; fibrin degradation products were absent. Lymph von Willebrand factor antigen was < 10% of plasma antigen and consisted primarily of lower molecular weight multimers. Mean lymph-to-plasma activity ratio for antithrombin was 0.38 and for tissue factor pathway inhibitor the ratio was 0.40. Low levels of antithrombin-factor Xa were measurable in lymph. The data are compatible with a basal factor VIIa-tissue factor-catalyzed extravascular activation of factor X that is prevented from progressing to generation of fibrin in limb interstitial fluid and lymph by low levels of factor VIII and factor V and by the inhibitory activity of antithrombin and tissue factor pathway inhibitor.

A Protac-based screening test for activated protein C-resistant factor Va and other defects of the protein C anticoagulant pathway

We have standardized a simple screening test for abnormalities of the protein C anticoagulant system. The test is basically a modified prothrombin time in which one aliquot of a test plasma is incubated for 3 min at 37 degrees C with Protac and another is incubated with buffer. During the incubation the Protac activates both protein C and factor V. The plasmas are then clotted with thromboplastin plus Ca2+, and the clotting time difference reflects the ability of the activated protein C (APC) to inactivate factor Va. With the use of Thromboplastin C Plus as the activator, clotting time differences found in 31 normal subjects (10.4 +/- 3.5 s, mean +/- 2SD) were distinct from clotting time differences found in 57 of 58 subjects with established APC-resistant factor Va (3.6 +/- 3.0 s). In addition, the Protac-based test detected six of seven patients with isolated protein C deficiency and 20 of 28 patients with isolated protein S deficiency. Because of the reported high prevalence of heterozygous APC-resistant factor Va in Caucasian populations, it should be particularly useful in determining whether this genetic risk is present in individuals who have experienced or are at increased environmental risk of venous thrombosis.

Correction of excessive anticoagulation with low-dose oral vitamin K1

BACKGROUND

Despite earlier acceptance of oral vitamin K1 (phytonadione) for the treatment of excessive anticoagulation, some recent guidelines do not recommend its use.

OBJECTIVE

To reevaluate the efficacy of oral vitamin K1 in correcting excessive anticoagulation.

DESIGN

Case series.

SETTING

Anticoagulation clinics at two university medical centers.

PATIENTS

81 outpatients who had an international normalized ratio (INR) greater than 5.0 but did not have significant bleeding.

INTERVENTIONS

Withholding 1 or 2 doses of warfarin, administering 2.5 mg of oral vitamin K1, measuring the INR after 24 to 48 hours, and adjusting the warfarin dose.

MEASUREMENTS

INRs were obtained from a portable capillary fingerstick monitor or from an automated photooptical coagulometer.

RESULTS

In 68 of 71 patients (96%), oral vitamin K1 lowered the INR from between 5.0 and 10.0 to less than 5.0 without inducing resistance to further anticoagulation.

CONCLUSIONS

Withholding 1 or 2 doses of warfarin and administering 2.5 mg of oral vitamin K1 is a reliable, safe, and inexpensive way to rapidly correct excessive anticoagulation (INR > 5.0) in patients who do not have serious bleeding episodes and have an INR of less than 10.0.

Reliability of a modified tissue factor-dependent factor V assay for activated protein C resistant factor Va using a calcium-containing thromboplastin

The original tissue factor-dependent factor V assay for activated protein C resistant factor Va (Blood 1995; 85: 1704-1711) has been modified to use a calcium containing thromboplastin and to express results as an observed to expected ratio (Obs/Exp.). The latter permits establishing a normal range independent of variations due to differences in reagents. Comparing Obs/Exp ratios with DNA analysis in 72 persons revealed that an Obs/Exp ratio of 0.6 distinguished without overlap normals from heterozygotes for FV R506Q. Three homozygotes had a ratio of < 0.1. Application of this Obs/Exp cut-off ratio of 0.6 to a total of 226 plasma samples tested to date discriminated without overlap between normals and heterozygotes. We conclude that this assay-readily adaptable to any dedicated coagulation laboratory and capable of yielding reliable results in all clinical circumstances in which testing is indicated-can distinguish between normals and heterozygotes for the FV R506Q mutation without the need for confirmatory DNA analysis.

The effect of immunodepletion of antithrombin III on the response of rabbits to Russell's viper venom-induced activation of factor X

Many years ago it was shown that an infusion of tissue factor (TF) into rabbits causing only limited consumption of factor X and prothrombin resulted in extensive consumption of fibrinogen. More recently it was shown that an injection of a concentration of the factor X-activating fraction of Russell's viper venom (RVV-X) depleting rabbits of factor X resulted in only minimal consumption of both plasma prothrombin and fibrinogen. We report here experiments in which rabbits depleted of antithrombin III (ATIII) to different degrees were infused over 4 hours with a concentration of RVV-X, causing consumption of about 60% of plasma factor X. Similar minimal mean falls in plasma prothrombin and fibrinogen levels were observed in control rabbits given nonimmune goat IgG and in rabbits immunodepleted with goat anti-rabbit ATIII IgG to about 40% of normal plasma ATIII activity. However, if rabbits were immunodepleted to about 10% to 20% of normal plasma ATIII, then mean consumption of prothrombin was increased modestly and, more impressively, mean consumption of plasma fibrinogen was increased markedly. Whereas limited amounts of thrombin generated on the surface of phospholipid vesicles by factor VIIa/ TF can trigger extensive intravascular coagulation in rabbits with normal plasma ATIII levels, limited amounts of thrombin generated by reactions triggered by factor Xa formed in fluid phase did so only after plasma ATIII levels were markedly depleted. A possible reason for this difference is discussed.

Mechanism and effects of the binding of lupus anticoagulant IgG and prothrombin to surface phospholipid

We report here experiments on how lupus anticoagulant antibodies (LA IgG) that react with prothrombin bind to surface phospholipid and affect prothrombin's affinity for surface phospholipid and activation to thrombin. LA IgG was purified by protein A chromatography from the plasma of 16 patients of whom four had associated hypoprothrombinemia and 10 had experienced thrombosis. Many LA IgG bound, in the absence of phospholipid and calcium, not only to immobilized prothrombin but to both prothrombin 1 and fragment 1, which established at least an oligoclonal origin of LA IgG. No LA IgG bound to thrombin. Although prothrombin and Ca2+ were required to support binding of LA IgG to immobilized phosphatidylserine (PS), prothrombin at higher concentrations inhibited binding, presumably by competing with prothrombin/LA IgG complexes for PS binding sites. Prothrombin 1, which cannot bind to PS, also inhibited binding of many LA IgG to PS, presumably by forming competing soluble prothrombin 1/LA IgG complexes. Despite their ability to react with prothrombin independent of phospholipid, LA IgG enhanced binding of prothrombin to immobilized phospholipid and to cultured human umbilical vein endothelial cells. Prothrombin bound with LA IgG to the surface of endothelial cell monolayers could be activated to thrombin after supernatant prothrombin and LA IgG were washed away. The relation is discussed of these observations to a hypothesis that LA IgG mediated concentration of prothrombin on cell surface phospholipid represents a mechanism by which LA IgG could increase thrombotic risk.

Plasma PIVKA proteins in rabbits given warfarin

The presence of partially carboxylated forms of the vitamin K dependent coagulation factors (PIVKA) was evaluated in the plasma of rabbits treated with warfarin. Excess antigen over activity as measured in rabbit specific assays was taken as evidence for PIVKA. Our data confirm a previous report of the absence of plasma PIVKA prothrombin. In contrast, plasma PIVKA factors VII, IX, and X were demonstrable. A striking excess of plasma factor IX antigen over activity was measured and a large fraction of the factor IX antigen persisted in the plasma after its adsorption with barium citrate.

Studies of the activation of factor VII bound to tissue factor

Experiments were performed to evaluate activation of factor VII bound to relipidated tissue factor (TF) in suspension and to TF constitutively expressed on the surface of an ovarian carcinoma cell line (OC-2008). Activation was assessed by measuring cleavage of 125I-factor VII and by the ability of unlabeled factor VII to catalyze activation of a variant factor IX molecule that, after activation, cannot back-activate factor VII. Factor Xa was found to effectively activate factor VII bound to TF relipidated in either acidic or neutral phospholipid vesicles. Autoactivation of factor VII bound to TF in suspension was dependent on the preparation of TF apoprotein used and the technique of its relipidation. This highlights the need for caution in extrapolating data from TF in suspension to the activation of factor VII bound to cell surfaces during hemostasis. A relatively slow activation of factor VII bound to OC-2008 monolayers in the absence of added protease was observed consistently. Antithrombin in the presence or absence of heparin prevented this basal activation, whereas TF pathway inhibitor (TFPI/factor Xa complexes had only a limited inhibitory effect. Adding a substrate concentration of factor X markedly enhanced basal activation of factor VII, but both TFPI/factor Xa and antithrombin/heparin abolished this enhancement. Overall, our data are compatible with the hypothesis that not all factor VII/TF complexes formed at a site of tissue injury are readily activated to factor VIIa (VIIa)/TF complexes during hemostasis. The clinical significance of this is discussed.

Cells and the activation of factor VII

Binding of factor VII to tissue factor (TF) present on or released from cells initiates TF-dependent coagulation in vivo. Earlier data obtained with relipidated TF suspensions led to the hypothesis that all factor VII bound to limiting TF sites exposed after tissue injury could be rapidly activated to VIIa/TF complexes by low concentrations of factor Xa, IXa, and VIIa. However, newer data on the activation of factor VII bound to TF expressed on cultured cell monolayers support the hypothesis that not all VII/TF complexes formed at a site of tissue injury are readily activated to VIIa/TF complexes during hemostasis. This is possibly related to why recombinant factor VIIa can bypass impaired IXa/VIIIa/phospholipid complex function in hemostasis.

Evidence for an essential role of tissue factor dependent blood coagulation in the pathogenesis of the local Shwartzman reaction

Adherence and aggregation of leukocytes within the vessels of a prepared skin site has been shown to be essential for the pathogenesis of the local Shwartzman reaction (LSR) (Argenbright and Barton: J Clin Invest 89: 259, 1992). We have now performed experiments in rabbits to evaluate whether coagulation within the vessels of a prepared site is a second requirement for the reaction. Skin sites were prepared by an intradermal injection of endotoxin 24 hours before a provoking intravenous injection of endotoxin. Thirteen control rabbits all developed the LSR. Seven of 12 rabbits given warfarin to achieve anticoagulation approximating that used therapeutically in humans before the provoking injection were protected against the LSR (p = 0.003). Five of nine rabbits given anti-rabbit factor X IgG before the provoking injection to yield mean values in individual rabbits of between 7% and 18% plasma factor X activity were protected against the LSR (p = 0.009). Six of 11 rabbits given anti-rabbit factor VII IgG before the provoking injection to yield mean values in individual rabbits of between < 0.5% and 2.2% were protected against the LSR (p = 0.007). Four rabbits failed to develop the LSR at an endotoxin-prepared skin site when an infusion of tissue factor (TF) causing substantial intravascular coagulation was substituted for a provoking injection of endotoxin. It would appear that two events are required for the pathogenesis of the LSR provoked by endotoxin: formation of aggregated masses of WBC in the prepared skin vessels and deposition of fibrin due to TF-initiated coagulation.

Use of a generally applicable tissue factor--dependent factor V assay to detect activated protein C-resistant factor Va in patients receiving warfarin and in patients with a lupus anticoagulant

The original activated partial thromboplastin time-based assay for activated protein C (APC)-resistant factor Va (FVa) requires carefully prepared fresh plasma and cannot be used in patients receiving warfarin or in patients with antiphospholipid antibodies. A new test is described here that circumvents these limitations and distinguishes without overlap heterozygotes for APC-resistant FVa from persons with normal FV. A diluted test plasma is incubated with an FV-deficient substrate plasma and tissue factor and then clotted with Ca2+ or Ca2+ plus APC. Test results are independent of the FV level or the dilution of the test plasma used. Of 39 controls, 37 gave normal results. Two controls (5%) gave results indicative of APC resistant FVa and on DNA analysis were found to be heterozygous for FV R506Q. Twenty of 21 randomly selected patients receiving warfarin gave normal results. In the single patient with abnormal results, heterozygous FV R506Q was confirmed by DNA analysis. Two of 15 patients with protein S deficiency and 5 of 29 patients with a lupus anticoagulant had abnormal results. APC resistance caused by FV R506Q was confirmed in the five of these seven patients available for DNA analysis. APC-resistant FVa was also detected in 10 of 21 (46%) stored plasma from unrelated patients with venous thrombosis and negative earlier evaluation for a lupus anticoagulant or a deficiency of protein C, protein S, or antithrombin, which confirms a high incidence of this defect among patients with venous thrombosis.

Differences in the interactions of lupus anticoagulant IgG with human prothrombin and bovine prothrombin

Lupus anticoagulant (LA) IgGs have been reported to inhibit more effectively and consistently the Xa/Va/phospholipid complex-catalyzed activation of human prothrombin that the Xa/Va/phospholipid complex-catalyzed activation of bovine prothrombin. This led us to carry out studies to determine whether the ability to inhibit the activation of prothrombin of LA IgGs, separated from the plasma of 15 patients by protein A affinity chromatography, could be related to the ability of the LA IgGs to bind to prothrombin under various experimental conditions. Of 14 LA IgG preparations tested all prolonged to a variable but substantial extent the dilute Russell's viper venom time (dRVVT) of human plasma but only minimally prolonged the dRVVT of bovine plasma. In a purified prothrombin activation system with a rate limiting concentration of phospholipid, all 15 LA IgG preparations inhibited the activation of human prothrombin with the majority showing > 50% of inhibition. In contrast, only one LA IgG markedly inhibited (> 50%) the activation of bovine prothrombin and five others moderately inhibited (25-40%) the activation of bovine prothrombin. Nevertheless, the majority of LA IgG preparations bound to immobilized bovine prothrombin on a Western blot and also to immobilized bovine prothrombin on a microtiter well. In an ELISA in which phosphatidylserine (PS) was immobilized on microtiter wells, bovine prothrombin supported the binding of 10 of 15 LA IgG preparations to PS. However, the extent of binding was lower than that observed with human prothrombin.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence suggestive of activation of the intrinsic pathway of blood coagulation after injection of factor Xa/phospholipid into rabbits

The present study was carried out to extend an earlier observation from this laboratory that mean plasma factor X levels fell by about 15% after the injection into rabbits of a formed factor Xa/phospholipid complex that caused only minimal intravascular coagulation. We have now injected rabbits with formulations of factor Xa/phospholipid that caused considerable intravascular coagulation, as documented by substantial falls in fibrinogen, factor V, and factor VIII and a fall in plasma prothrombin activity of about 15% to 20% of the initial level. Mean plasma factor X activity fell by about 30% of the initial level. Factors participating in the intrinsic coagulation pathway--XII, XI, and IX--all fell by about 50% after injection of a complex made with 16.3 pmol factor Xa and 80 nmol phospholipid per 1 kg body wt and by about 35% after injection of a complex made with 32.6 pmol factor Xa and 40 nmol phospholipid per 1 kg body wt. In contrast, total plasma factor VII activity did not change, and specific plasma factor VIIa levels, which were lower than those measured in human plasma, did not rise after injection of factor Xa/phospholipid. The data are compatible with the hypothesis that factor Xa/phospholipid-induced generation of thrombin in vivo leads to factor XII-dependent activation of the intrinsic pathway of coagulation that results in significant activation of factor X. Further testing of this hypothesis appears warranted.

Studies of the mechanism for enhanced cell surface factor VIIa/tissue factor activation of factor X on fibroblast monolayers after their exposure to N-ethylmaleimide

Fibroblast monolayers constitutively expressing surface membrane tissue factor (TF) were treated with 0.1 mM N-ethylmaleimide (NEM) for 1 min to inhibit aminophospholipid translocase activity without inducing general cell damage. This resulted in increased anionic phospholipid in the outer leaflet of the cell surface membrane as measured by the binding of 125I-annexin V and by the ability of the monolayers to support the generation of prothrombinase. Specific binding of 125I-rVIIa to TF on NEM-treated monolayers was increased 3- to 4-fold over control monolayers after only brief exposure to 125I-rVIIa, but this difference progressively diminished with longer exposure times. A brief exposure of NEM-treated monolayers to rVIIa led to a maximum 3- to 4-fold enhancement of VIIa/TF catalytic activity towards factor X over control monolayers, but, in contrast to the binding studies, this 3- to 4-fold difference persisted despite increasing time of exposure to rVIIa. Adding prothrombin fragment 1 failed to diminish the enhanced VIIa/TF activation of factor X of NEM-treated monolayers. Moreover, adding annexin V, which was shown to abolish the ability of NEM to enhance factor X binding to the fibroblast monolayers, also failed to diminish the enhanced VIIa/TF activation of factor X. These data provide new evidence for a possible mechanism by which availability of anionic phospholipid in the outer layer of the cell membrane limits formation of functional VIIa/TF complexes on cell surfaces.

Antibody-induced acute factor X deficiency: clinical manifestations and properties of the antibody

A patient is described with serious bleeding due to a transient selective deficiency of factor X. Crossed immunoelectrophoresis of patient's plasma with anti-factor X antibody revealed an abnormal factor X arc suggestive of the presence of plasma factor X/anti-factor X immune complexes. A similar abnormal arc was obtained on adding the patient's IgG to normal plasma. Immunoblotting of factor X after reduced SDS-PAGE revealed that the patient's IgG bound to the light chain of intact factor X but not Gla-domainless factor X. The patient's IgG inhibited activation of factor X by VIIa/tissue factor (TF), by IXa/VIIIa/phospholipid complex, and by Russell's viper venom. The IgG failed to inhibit the proteolytic activity of factor Xa towards a chromogenic substrate. However, under reaction conditions of limited factor Xa availability, the IgG could be shown to impair hemostatic functions of factor Xa that require the participation of its light chain: activation of prothrombin by prothrombinase; activation of factor VII bound to TF; and inhibition of VIIa/TF activity by factor Xa/tissue factor pathway inhibitor complexes. A few earlier patients have been described with transient, selective factor X deficiency and serious bleeding, but in only one was evidence obtained of an antibody against factor X. It will be of interest to learn whether use of the techniques described in this report will permit the identification of immunoglobulin with similar binding and functional properties in future patients with this rare syndrome.

Functional and binding studies of the roles of prothrombin and beta 2-glycoprotein I in the expression of lupus anticoagulant activity

We present functional and binding data relevant to the reported roles for prothrombin and beta 2-glycoprotein I (beta 2GPI) in the expression of lupus anticoagulant activity. In a purified system containing human prothrombin, Xa, Va, and a rate-limiting concentration of phosphatidylserine (PS)/phosphatidylcholine (PC) vesicles, the preliminary incubation of vesicles with protein A separated IgG preparations from 10 lupus anticoagulant plasmas, calcium, and prothrombin enhanced the inhibitory effect of all IgG preparations upon thrombin generation. Experiments in a purified factor X activation system provided supporting data that a similar preliminary incubation with prothrombin enhanced the inhibitory effect of many of the IgG preparations upon factor X activation. However, we could not obtain unequivocal evidence that prothrombin was an obligatory cofactor for lupus anticoagulant IgG to inhibit procoagulant phospholipid function, because lupus anticoagulant IgG separated by protein A chromatography contained traces of prothrombin. The binding of many IgG preparations to immobilized PS was enhanced by prothrombin when calcium ions were present. beta 2GPI enhanced binding of many of the IgG preparations to immobilized PS both in the presence and absence of calcium, yet beta 2GPI failed to enhance the ability of the IgG preparations to inhibit phospholipid function in purified prothrombin and factor X assays. Moreover, the IgG preparations prolonged the dilute Russell's viper venom time (dRVVT) of beta 2GPI-depleted normal plasma. Nine of 10 IgG preparations bound to prothrombin on Western blots in the absence of calcium and phospholipid, whereas no preparation bound to beta 2GPI. Passage of five citrated lupus anticoagulant plasmas through a prothrombin affinity column in the absence of added calcium and phospholipid removed most of the activity prolonging the dRVVT of normal plasma, and IgG in the pass-through plasma no longer bound to PS in the presence of prothrombin and calcium ions. IgG in prothrombin column eluates had strikingly enhanced specific lupus anticoagulant activity and also specific PS binding activity in the presence of prothrombin and calcium ions. Thus, lupus anticoagulant plasmas were shown to contain IgG binding to prothrombin, in the absence of calcium ions and phospholipid, which could also, in the presence of calcium ions and prothrombin, bind to PS and express lupus anticoagulant activity.

The international normalized ratio (INR) for monitoring warfarin therapy: reliability and relation to other monitoring methods

OBJECTIVE

To enhance understanding of the reliability of the international normalized ratio (INR) for monitoring warfarin therapy and its relation to other monitoring techniques.

DESIGN

Prospective cohort study.

SETTING

A university hospital.

PATIENTS

79 patients attending an anticoagulation clinic.

MEASUREMENTS

International normalized ratios obtained with a portable capillary monitor (Coumatrak) and the following from a simultaneous plasma sample: INRs from prothrombin times done with six thromboplastins, prothrombin-proconvertin (P&P) test activity, specific prothrombin activity, and native prothrombin antigen.

RESULTS

Converting to INRs failed to standardize prothrombin time results obtained with high- and low-sensitivity thromboplastins. Coumatrak INRs correlated best with INRs obtained with high-sensitivity thromboplastins. The INR range of 2.0 to 3.0 corresponded to a P&P range of 30% to 13%, a native plasma prothrombin antigen range of 56 to 24 micrograms/mL, and a specific prothrombin activity range of 43% to 21%.

CONCLUSIONS

Low-sensitivity thromboplastins may give erroneously high INRs in the upper therapeutic range. Plasma prothrombin times should be done with a high-sensitivity thromboplastin, particularly in patients maintained at the upper limit of the therapeutic range. An INR so obtained correlated well with an INR obtained with a portable capillary blood monitor.

Studies of factor Xa/phospholipid-induced intravascular coagulation in rabbits. Effects of immunodepletion of tissue factor pathway inhibitor

In earlier studies from this laboratory evidence was obtained for a physiological function of tissue factor pathway inhibitor (TFPI) as a regulator of hemostasis capable of preventing thrombotic complications that might otherwise result from exposure of blood to trace amounts of tissue factor (TF). However, it was not possible to conclude that the protective effect of TFPI stemmed solely from inhibition of factor VIIa/TF catalytic activity, since TFPI neutralizes stoichiometric amounts of factor Xa in forming an inhibited factor Xa/TFPI/factor VIIa/TF complex. Therefore, we examined the effects of immunodepletion of TFPI on the extent of coagulation initiated in rabbits by exposure to factor Xa and phospholipid in the absence of TF. In one experimental approach, factor Xa was generated endogenously with the factor X-activating fraction of Russell's viper venom (0.33 microgram/kg) in rabbits receiving an infusion of phosphatidylcholine/phosphatidylserine (PCPS) vesicles, 1 mg/kg over 2 hours. In a second approach, rabbits were injected with a complex of factor Xa (0.75 microgram/kg) and PCPS (12.5 micrograms/kg). In contrast with the observed sensitization of TFPI-depleted rabbits to TF-induced coagulation, TFPI-depleted rabbits were not sensitized to coagulation initiated by factor Xa and phospholipid in the absence of TF. These data support the conclusion that the physiological function of TFPI in regulating TF-dependent coagulation stems primarily from its ability to inhibit factor VIIa/TF catalytic activity.

Mechanism of the anticoagulant effect of warfarin as evaluated in rabbits by selective depression of individual procoagulant vitamin K-dependent clotting factors

We have evaluated the contribution of depression of individual procoagulant vitamin K-dependent clotting factors to the ability of warfarin to protect rabbits against tissue factor-induced coagulation. Mean activities of individual procoagulant factors were determined, in assays with rabbit substrates, for a group of rabbits achieving a protective degree of anticoagulation with warfarin. Values were: factor VII, 12%; factor IX, 7%; factor X, 14%, and prothrombin, 13%. The effect upon tissue factor-induced coagulation of selective immunodepletion of each factor to a comparable level was then evaluated. Immunodepletion of plasma factor X or prothrombin, but not of factor VII or factor IX, protected otherwise normal rabbits against tissue factor-induced coagulation. Next, we determined the effect upon the protection in warfarin-treated rabbits of selectively restoring factor X or prothrombin before infusing tissue factor. When either factor was selectively restored, warfarin's protective effect was abolished. Moreover, selective restoration of prothrombin sensitized warfarin-treated rabbits to coagulation more severe than observed in nontreated control rabbits. One may extrapolate from these data that depression of both factor X and prothrombin are required for warfarin's clinical antithrombotic efficacy and that depression of plasma prothrombin is particularly important.

Binding of factor VIIa to tissue factor permits rapid antithrombin III/heparin inhibition of factor VIIa

Because free factor VIIa is inactivated only very slowly by a plasma concentration of antithrombin III (AT III) even in the presence of heparin, it has been assumed that AT III plays no significant role in regulating the initiation of tissue factor-dependent blood coagulation. However, in the present study, we present evidence that factor VIIa bound to tissue factor, unlike free factor VIIa, is readily inactivated by AT III in the presence of heparin. In a reaction mixture containing calcium ions and approximately equimolar concentrations of relipidated tissue factor (8.9 nmol/L) and factor VIIa (10 nmol/L), AT III (100 micrograms/mL) plus heparin (1 U/mL) inhibited 50% of the factor VIIa coagulant activity of the reaction mixture within 5 minutes. AT III/heparin was also shown to inhibit the catalytic activity towards factor X of factor VIIa/tissue factor complexes formed on monolayers of an ovarian carcinoma cell line (OC-2008) that constitutively expresses surface membrane tissue factor. AT III, even in the absence of exogenously added heparin, substantially inhibited the functional activity of factor VIIa/cell surface tissue factor complexes on intact monolayers. AT III alone and AT III/heparin, to a greater extent, also inhibited factor VIIa on "nonfunctional" factor VIIa/tissue factor complexes on intact monolayers, with resultant inhibition of their expression of factor VIIa/tissue factor catalytic activity toward factor X after cell lysis. The potential physiologic significance of these findings is discussed.

Trousseau's syndrome

We report 4 cases of Trousseau's syndrome, in which spontaneous recurrent or migratory venous thromboses, arterial emboli caused by nonbacterial thrombotic endocarditis, or both, develop in a patient with a recognized or occult malignant tumor. The clinical course of 3 of the patients emphasizes a key point: The occurrence for no known reason of thromboses preventable by anticoagulation therapy with heparin but not with warfarin sodium should alert a physician to focus diagnostic efforts on uncovering an underlying malignant lesion. Thromboses may occur months to years before the tumor is discovered, and a thorough negative initial examination does not obviate the need for a continuing search. Patients with Trousseau's syndrome have persistent low-grade intravascular coagulation, and therapy with heparin should be continued indefinitely. Stopping heparin therapy for even a day may permit a new thrombosis to develop. Immunostaining a biopsy specimen from 1 patient provided evidence that 2 properties of a neoplastic lesion are required for the syndrome to develop: The malignant cells express surface membrane tissue factor, and structural features of the tumor permit the malignant cells or vesicles it sheds to be exposed to circulating blood.

Heparin-releasable and platelet pools of tissue factor pathway inhibitor in rabbits

Earlier studies from this laboratory have established that tissue factor pathway inhibitor (TFPI) functions as a natural anticoagulant protecting rabbits from intravascular coagulation triggered by the exposure of blood to small amounts of tissue factor. In addition to the TFPI circulating in plasma, humans have been shown to have heparin-releasable and platelet pools of TFPI. In order better to extrapolate from studies carried out in rabbits to an understanding of human hemostasis, we have examined the presence and extent of heparin-releasable and platelet pools of TFPI in rabbits. We find that in the rabbit the heparin-releasable pool of TFPI activity, as measured in a capacity assay, may be smaller relative to the plasma pool than in humans; that the platelet pool of TFPI activity is comparable to that of humans; and that rabbit TFPI, unlike human TFPI, has the same apparent molecular mass in all vascular pools. These studies extend our understanding of the properties of TFPI in rabbits and the appropriateness of using the rabbit for studies of TFPI relevant to human hemostasis.

Blood coagulation and its alterations in hemorrhagic and thrombotic disorders

Clinical observations have added to the understanding of basic mechanisms of blood coagulation and its alterations in certain hemorrhagic and thrombotic states. Much clinical evidence exists for concluding that the exposure of blood to tissue factor (thromboplastin) on tissue cells represents the key event initiating fibrin clot formation after tissue injury. This then results in the formation of activated factor VII (VIIa)-tissue factor complexes, which must activate both factor X and factor IX for normal hemostasis. I describe the possible clinical consequences of an aberrant function of the natural anticoagulants regulating blood coagulation--antithrombin, protein C, and tissue factor pathway inhibitor. Understanding the physiologic function of tissue factor pathway inhibitor can illuminate why hemophilic patients bleed, but many other questions remain. I briefly review the four causes for acquired disorders of the blood coagulation reactions--vitamin K deficiency, hepatocellular disease, antibodies to clotting factors, and disseminated intravascular coagulation--but limit my comments to the mechanisms that trigger the formation of antibodies to clotting factors and how these antibodies can deplete the blood of clotting factor activities. Finally, heparin is able to potentiate tissue factor pathway inhibitor function, which is a possible reason why the use of heparin but not warfarin can prevent the numerous thrombotic episodes of the Trousseau's syndrome.

Regulation of factor VIIa/tissue factor functional activity in an umbilical vein model

Activation of factor IX in an umbilical vein model was established to result solely from factor VIIa/tissue factor (TF) activity generated within the umbilical vein wall, and the model was then used to study regulation of such extravascular factor VIIa-TF complexes. Vein segments were filled with a reaction mixture containing factor VIIa, Ca2+, a substrate, either [3H]factor IX or [3H]factor X, and a test material. Subsamples were assayed for activation peptide release. Test materials included defibrinated plasma or recombinant protein as a source of TF pathway inhibitor (TFPI), recombinant factor VIIa to 10 times plasma factor VII concentrations, and annexin V. A plasma concentration of TFPI inhibited but did not totally suppress factor VIIa/TF activity. Reducing the TFPI concentration by 50% markedly reduced the inhibition. A 10-fold increase in the factor VIIa concentration in reaction mixtures failed to accelerate factor Xa generation. Annexin V, in contrast to its inhibition of factor VIIa/TF formed with TF reconstituted into mixed phospholipid vesicles, failed to inhibit factor VIIa-TF complexes formed within the vessel wall. We conclude that 1) moderate variation in plasma TFPI concentration or activity may affect TFPI's ability to inhibit factor VIIa/TF activity during hemostasis, 2) a plasma concentration of factor VII suffices to saturate TF sites exposed in a vessel after tissue injury, and 3) the resistance of factor VIIa-TF complexes to inhibition by annexin V suggests that they are formed in the umbilical vein model primarily on cell surfaces.

Relations between factor VIIa binding and expression of factor VIIa/tissue factor catalytic activity on cell surfaces

The kinetics of the binding of rVIIa to cell surface tissue factor (TF) and the resultant expression of VIIa/TF activity were studied. Binding of 125I-rVIIa (10 nM) to cell surface TF required 30-60 min for saturation, whereas VIIa/TF activity was fully expressed toward factor X (F X) on intact monolayers after only 1 min of incubation. At the time only 10-20% of the total VIIa TF complexes present at saturation had formed. Freeze-thawing the monolayers before assay increased VIIa/TF activity up to 30-fold, and the time course of its expression was similar to that of TF-specific binding of VIIa to the monolayers. Equilibrium binding revealed a single high affinity binding class of TF sites on intact monolayers for rVIIa with a Kd of 1.6 nM. Experiments with active-site inhibited rVIIa yielded evidence for two populations of VIIa. TF complexes on intact monolayers: (1) a minor population (less than 20%) that formed within 1 min of incubation and accounted for all VIIa/TF activity toward F X present on the intact monolayers, and (2) a major population that was inactive toward F X on intact monolayers but which was fully active after the monolayers were lysed. Tissue factor pathway inhibitor (TFPI).F Xa complexes inhibited the VIIa/TF activity of the first population, i.e. of the complexes active on intact monolayers, half maximally at a concentration of 0.2 nM TFPI. TFPI/Xa also bound to the second population of VIIa.TF complexes on intact monolayers and inhibited their expression of VIIa/TF activity following cell lysis with a half-maximal inhibitory concentration of 2.0 nM. The potential physiologic implications of these findings are discussed.

Mechanisms of binding of recombinant extrinsic pathway inhibitor (rEPI) to cultured cell surfaces. Evidence that rEPI can bind to and inhibit factor VIIa-tissue factor complexes in the absence of factor Xa

Extrinsic pathway inhibitor plays a key role in modulating tissue factor-dependent blood coagulation. We have studied binding of radioiodinated recombinant extrinsic pathway inhibitor (rEPI) to cultured cell surfaces. rEPI in the absence of added reactants bound to a limited extent to three cell lines studied. Binding of rEPI to two cell lines possessing surface tissue factor, but not to a cell line lacking surface tissue factor, was markedly increased in the presence of both factor VIIa and factor Xa, and calcium ions. Moreover, some increased tissue factor-dependent binding was also demonstrated with factor VIIa alone. Binding isotherms of rEPI to factor VIIa-tissue factor obtained with an ovarian carcinoma cell line were hyperbolic. Scatchard plots indicated the following: a Kd value of 4.5 +/- 1.5 nM and 335,000 +/- 84,000 sites/cell when factor Xa was present; a Kd value of 11.9 +/- 3.5 nM and 236,000 +/- 68,000 sites/cell when factor Xa was absent. In functional studies, high concentrations of rEPI, e.g. 27-67.5 nM, were found to inhibit factor VIIa-tissue factor-catalyzed release of activation peptide from tritiated factor IX in the absence of factor Xa. Whereas factor Xa was thus shown not to be required for rEPI to inhibit factor VIIa-tissue factor catalytic activity, its presence markedly enhanced rEPI's inhibitory function. Since the local concentration of extrinsic pathway inhibitor achieved at a site of tissue injury is unknown, the physiologic significance of the observation of extrinsic pathway inhibitor-induced inhibition of factor VIIa-tissue factor activity in the absence of factor Xa is not clear. However, factor Xa-independent inhibition could play a significant role when large doses of rEPI are administered in experimental studies of thrombosis.

Clinical significance of antibodies to bovine and human thrombin and factor V after surgical use of bovine thrombin

Two patients exposed during surgery to bovine thrombin developed antibodies reacting with both bovine and human thrombin and Factor V. Their thrombin times were markedly prolonged with bovine thrombin and modestly prolonged with human thrombin. High titer anti-bovine Factor V created diagnostic confusion in one patient by neutralizing bovine Factor V in a prothrombin assay substrate. Although weaker, antibody activity against human Factor V led to postoperative factor V deficiency in both patients. Such cross-reacting antibodies, recognizable by their higher titer against bovine than human Factor V, should be suspected when a patient surgically exposed to bovine thrombin develops a Factor V anticoagulant after operation. Crossed immunoelectrophoresis of adsorbed bovine plasma with each patient's plasma as antibody revealed many precipitin arcs indicative of immunization of the patients to additional proteins in a commercial thrombin preparation.

Immunodepletion of extrinsic pathway inhibitor sensitizes rabbits to endotoxin-induced intravascular coagulation and the generalized Shwartzman reaction

We have reported earlier that immunodepletion of extrinsic pathway inhibitor (EPI) sensitizes rabbits to disseminated intravascular coagulation (DIC) induced by infusing a low concentration of tissue factor (TF). We now describe the effect of immunodepletion of EPI in rabbits administered endotoxin. Cortisone-treated rabbits were administered anti-rabbit EPI immunoglobulin (IgG) or Fab fragments or were administered control nonimmune material before an injection of endotoxin. In four of seven rabbits administered anti-EPI, plasma EPI activity levels were reduced by 70% to 80% of initial levels for 6 to 8 hours. In these rabbits the endotoxin induced extensive DIC, as evidenced by substantial decreases in fibrinogen, factor V, factor VIII, and platelets, and gross hemorrhagic necrosis of the kidneys due to massive deposition of fibrin in the glomerular microcirculation (the generalized Shwartzman reaction). In three rabbits administered anti-EPI, plasma EPI levels were only transiently reduced. In these rabbits and in four rabbits administered nonimmune IgG or Fab, endotoxin induced minimal to moderate intravascular clotting and deposits of fibrin were not found in the glomerular capillaries. Because it is believed that TF expressed on monocytes triggers endotoxin-induced coagulation, these data are taken as evidence that EPI functions as a natural anticoagulant that can regulate factor VIIa/TF activity expressed on cell surfaces in vivo. They support a hypothesis that EPI prevents thrombotic complications that might otherwise result from exposure of blood to cytokine-induced generation of small amounts of TF on cell surfaces in many inflammatory and infectious disease states.

Mechanism for diminished tissue factor expression by endothelial cells cultured with heparin binding growth factor-1 and heparin

We have extended our earlier observation that growing primary cultures of human umbilical vein endothelial cells (HUVEC) with heparin binding growth factor 1 (HBGF-1) 20 micrograms/mL and heparin 12 U/mL inhibits expression of tissue factor (TF) activity on HUVC monolayers perturbed with thrombin. TF activity was measured as the ability of monolayers or cell lysates to support FVIIa-catalyzed activation peptide release from 3H-FX. TF antigen in HUVEC extracts was measured in an enzyme-linked immunosorbent assay (ELISA) that uses a double-antibody sandwich technique with rabbit and goat antibodies to human TF. TF-mRNA was measured by Northern blot hybridization with a 32P-TF cDNA probe. Cells growth with HBGF-1/heparin had both decreased surface and total TF activity as compared with HUVEC from the same endothelial cell pool grown without HBGF-1/heparin. Means +/- SD for TF antigen for four primary cultures were 4.4 +/- 0.9 ng/10(6) cells without HBGF-1/heparin and 0.6 +/- 0.3 ng/10(6) cells with HBGF-1/heparin. TF mRNA 4 hours after incubation with thrombin of HUVEC grown without HBGF-1/heparin was about sevenfold higher than TF mRNA of HUVEC grown with HBGF-1/heparin. These data establish that growing primary cultures of HUVEC with HBGF-1/heparin impairs their ability to synthesize TF apoprotein after perturbation. This may be part of a generalized response of endothelial cells to HBGF-1/heparin facilitating migration during angiogenesis.

A sulfated rabbit endothelial cell glycoprotein that inhibits factor VIIa/tissue factor is functionally and immunologically identical to rabbit extrinsic pathway inhibitor (EPI)

Colburn and Buonassisi (In Vitro Cell Dev. Biol. 24, 1133-1136, 1988) have isolated a single chain sulfated glycoprotein inhibitor of factor VIIa/tissue factor-catalyzed activation of factor X from conditioned media of an established rabbit endothelial cell line. We report herein that their endothelial cell-derived inhibitor and extrinsic pathway inhibitor (EPI) isolated from rabbit plasma have identical functional properties with respect to their interactions with factor Xa and with factor VIIa/tissue factor. In addition, the endothelial cell inhibitor and rabbit plasma EPI migrate with the same apparent molecular weights on non-reduced SDS-PAGE and contain similar amounts of N-linked carbohydrate. Like the endothelial cell inhibitor the EPI of rabbit plasma exists as a single chain molecule. Furthermore, the endothelial cell inhibitor is recognized and neutralized by a polyclonal antibody raised against rabbit plasma EPI. We therefore conclude that cultured rabbit endothelial cells produce an inhibitor of factor VIIa/tissue factor activity that is functionally and immunologically identical to rabbit plasma EPI.

Depletion of extrinsic pathway inhibitor (EPI) sensitizes rabbits to disseminated intravascular coagulation induced with tissue factor: evidence supporting a physiologic role for EPI as a natural anticoagulant

Although in vitro experiments have established that extrinsic pathway inhibitor (EPI) is the only known plasma inhibitor of factor VIIa-tissue factor (TF) catalytic activity of potential physiologic significance, evidence of its function in vivo has been lacking. TF-induced intravascular coagulation may occur in patients despite normal plasma levels of EPI and, in our earlier studies, normal plasma EPI levels did not protect rabbits from intravascular coagulation induced by an infusion of purified TF (1 microgram/kg). Studies have now been carried out in which plasma EPI levels were reduced in rabbits to below 20% of the initial level by injection of anti-rabbit EPI IgG. Infusion into such animals of purified rabbit TF apoprotein (0.25 microgram/kg) reconstituted into phospholipid vesicles induced substantial disseminated intravascular coagulation. Infusion of control saline or phospholipid vesicles not containing TF was without significant effect as was infusion of TF (0.25 microgram/kg) into animals injected with nonimmune goat IgG. These data establish that EPI can dampen TF-induced intravascular coagulation in rabbits. They support the hypothesis that EPI plays a significant role in regulating coagulation resulting from the exposure of blood to trace concentrations of TF during the illnesses and minor injuries of normal existence.

A sensitive, accurate assay for extrinsic pathway inhibitor (EPI) activity in rabbit plasma: paradoxical effect of excess exogenous factor X

A sensitive assay is described for the measurement of rabbit plasma EPI activity in experimental studies of induced hypercoagulable states in this species. It is based upon the ability of a dilution of rabbit plasma to inhibit human factor VIIa/rabbit brain tissue factor (TF) catalyzed activation of human factor IX (tritiated activation peptide release assay). Addition of 3H-factor IX to the reaction mixture is delayed for 45 minutes to allow full inhibition by EPI/factor Xa complex before the residual catalytic activity of factor VIIa/TF is measured. Although the diluted rabbit plasma test sample contains both EPI and factor X, supplemental factor X is added to the reaction mixture to assure that only EPI content of the test sample affects the assay result. However, the final concentration of factor X in the reaction mixture is critical. Too high a concentration of factor X diminishes the sensitivity of the assay. The reason for this phenomenon, which was observed with both human and rabbit factor X preparations, is unknown.

Properties of factor VIIa/tissue factor complexes in an umbilical vein model

An umbilical vein model was designed in which washed vein segments are filled with a reaction mixture containing factor VIIa, Ca(+)+, and a substrate, either 3H-factor IX or 3H-factor X. The vein wall provides the tissue factor (TF) for factor VIIa/TF complexes that activate the substrates as measured by activation peptide release. The model was developed to study TF induced on venous endothelium in situ. However, unlike previous studies with TF expressed on cultured umbilical vein endothelial cells, factors IX and X were activated without first having to expose the vein wall to a perturbing stimulus. Histologic studies revealed that washing the vein and mixing the reaction mixture before subsampling had disrupted the endothelium. Immunostaining with anti-TF antibodies revealed no staining of endothelium but intense staining in extensions of Wharton's jelly penetrating fenestrations of the muscularis media of the vein. Thus, the model provided data on factor VIIa/TF formed, not on endothelium, but within the mucoid connective tissue of Wharton's jelly. It is known that factor VIIa/TF formed with TF in suspension or with TF expressed on the surface of cultured cells activates factor X more rapidly than factor IX. In contrast, in the umbilical vein model, when each substrate was present in an 88 nmol/L concentration, factors IX and X were activated at equivalent rates (mean activation rate for factor IX, 18.8 +/- 3.6 nmol/L/h; for factor X, 17.8 +/- 2.9 nmol/L/h; n = 9 paired vein segments). These data strengthen the evidence that factor VIIa/TF activation of factor IX represents a key initial reaction of coagulation in tissues. These results also show that data obtained with factor VIIa/TF complexes formed on the surface of cultured cells need not hold for factor VIIa/TF complexes formed in extracellular matrix.

Localization of human tissue factor antigen by immunostaining with monospecific, polyclonal anti-human tissue factor antibody

Tissue factor, the cofactor for factor VIIa-catalyzed activation of factors IX and X, plays an important role in the initiation of hemostasis. However, the distribution of tissue factor in the body has not been defined until recently. In the present study frozen sections of non-malignant human tissues were immunostained using polyclonal, monospecific rabbit anti-human tissue factor antibodies. Specificity of the anti-tissue factor antibody was established by Western blotting. Sensitivity of the immunostaining technique for tissue factor antigen was confirmed by correlating staining of non-perturbed and perturbed cultured human umbilical vein endothelial cells with their surface membrane tissue factor coagulant activity. Brain, lung and placenta, all known to possess large amounts of tissue factor procoagulant activity, stained strongly for tissue factor, as did peripheral nerves and autonomic ganglia. Epithelium of skin, mucosa, and glomeruli also stained; however, epithelium lining excretory ducts failed to stain. Skeletal muscle did not stain, but cardiac muscle stained faintly. Smooth muscle also did not stain except for the muscularis mucosa of the esophagus, which stained brightly. Fibroblasts varied in stainability; those found in the adventitia of vessels stained strongly. The endothelium, tunica intima and tunica media of blood vessels consistently failed to stain. The distribution of tissue factor antigen as demonstrated by immunostaining supports the hypothesis that maintenance of a physical barrier between tissue factor activity and blood is key to the normal regulation of hemostasis.

Disseminated intravascular coagulation in rabbits induced by administration of endotoxin or tissue factor: effect of anti-tissue factor antibodies and measurement of plasma extrinsic pathway inhibitor activity

Rabbits were given polyclonal anti-tissue factor (TF) immunoglobulin G (IgG) before an injection of endotoxin to test the hypothesis that TF triggers disseminated intravascular coagulation (DIC) after endotoxin. The rabbits had been prepared with cortisone to develop DIC after one injection of endotoxin. Anti-TF IgG substantially reduced the falls in fibrinogen, factors V and VIII, and platelets noted in control rabbits given preimmune IgG before endotoxin. At autopsy 24 hours later, fibrin was present in glomerular capillaries of 4 of 5 control rabbits, but in none of 11 rabbits given anti-TF IgG. DIC was also induced in a second group of rabbits by the infusion, over 4 hours, of 1 microgram/kg of purified, reconstituted rabbit brain TF. This resulted in striking falls in plasma fibrinogen, factors V, and VIII that were diminished, but not prevented by prior treatment with anti-TF IgG. Circulating activated factor VII, induced by either TF infusion or endotoxin, could not be detected after DIC. Mean plasma extrinsic pathway inhibitor (EPI) activity did not fall significantly after endotoxin, and only to about 65% of the preinfusion after infusion of TF. Thus, DIC induced by both agents proceeded despite nearly normal plasma EPI levels. Because EPI neutralizes factor VIIa/TF in vitro only after a short lag period, the DIC that persisted for up to 6 hours after injection of endotoxin suggests that TF activity continued to be generated during this period on cells to which the circulating blood was exposed. All animals given endotoxin became ill with cyanosis, tachypnea, cold ears, and diarrhea, regardless of whether they had received anti-TF IgG to attenuate DIC. Infusion of TF caused some animals to die acutely with pulmonary arterial thromboses, but surviving animals did not appear ill. The findings support the hypothesis that exposure of blood to TF triggers DIC after endotoxin, but is not important for the pathogenesis of endotoxin-induced shock.

Factor VIIa-catalyzed activation of factor X independent of tissue factor: its possible significance for control of hemophilic bleeding by infused factor VIIa

Infusing factor VIIa (FVIIa) has been reported to control bleeding in hemophilic patients with factor VIII (FVIII) inhibitors. This is difficult to attribute to an enhanced FVIIa/tissue factor (TF) activation of factor X, since in vitro studies suggest that infusion of FVIIa should neither increase substantially the rate of formation of FVIIa/TF complexes during hemostasis (Proc Natl Acad Sci USA 85:6687, 1988) nor bypass the dampening of TF-dependent coagulation by the extrinsic pathway inhibitor (EPI) (Blood 73:359, 1989). Partial thromboplastin times have also been reported to shorten after infusion of FVIIa. The experiments reported herein establish that shortening of partial thromboplastin times after adding FVIIa to hemophilic plasma in vitro stems from an FVIIa-catalyzed activation of factor X independent of possible trace contamination of reagents with TF. Experiments in purified systems confirmed that FVIIa can slowly activate factor X in a reaction mixture containing Ca2+ and phospholipid but no source of TF. The rate of activation was sufficient to account for the shortening of partial thromboplastin times observed. EPI, which turned off continuing FVIIa/TF activation of factor X, was unable to prevent continuing FVIIa/phospholipid activation of factor X. Because circulating plasma contains only a trace, if any, free FVIIa, such a reaction could never occur physiologically. However, infusing FVIIa creates a nonphysiologic circumstance in which a continuing slow FVIIa/phospholipid catalyzed activation of factor X could conceivably proceed in vivo unimpeded by EPI. Such a mechanism of factor X activation might compensate for an impaired factor IXa/FVIIIa/phospholipid activation of factor X during hemostatis, and therefore control bleeding in a hemophilic patient.