[Modern methods of diagnosis of thrombophylic states and complex treatment of patients with thrombotic complications of severe forms of varicose disease]

Actual issues of surgical treatment of patients, suffering complications of severe forms of varicose disease of the lower extremities (VDLE) are discussed. The causes of unsatisfactory results of treatment in patients, suffering varicothrombophlebitis (VTHPH), the main of which--absence of the only one tactics for operative treatment and anticoagulant therapy, were analyzed. The results of patients examination, suffering thrombotic complications of severe forms of VDLE, while its recurrent course, in conjunction of VTHPH and thrombosis of deep veins of the lower extremities, using diagnostic complex "PLR genetics thrombophilia", are adduced. Differential tactics of treatment in patients, suffering severe forms of VDLE, while various localization of thrombotic process, concerning the presence of thrombophilic states, is proposed.

Lack of secondary microthrombosis after thrombin-induced stroke in mice and non-human primates

BACKGROUND

Secondary microthrombosis is a major pathophysiologic mechanism leading to brain damage following transient mechanical vascular occlusion (TMVO), the most widely used experimental stroke model. Whether secondary microthrombosis also occurs in non-TMVO stroke models represents an important issue for clinical translation of antimicrothrombosis therapeutic strategies.

OBJECTIVES

To assess the occurrence and the pathogenic role of secondary microthrombosis in two thrombin-induced stroke models in mice and non-human primates (Macaca mulatta).

METHODS

Stroke was induced in mice and non-human primates by intra-arterial administration of recombinant thrombin. This method induces the formation of a fibrin-rich thrombus, which is spontaneously dissolved in the following hours by the endogenous fibrinolytic system. Perfusion-weighted imaging and fluorescent-lectin microangiography were performed after recanalization to detect secondary microthrombosis. Moreover, to investigate its pathogenic role, thrombin-induced stroke was induced in bradykinin receptor B1 (B1R) knockout mice, which are protected from the thromboinflammation responsible for secondary microthrombosis in TMVO models.

RESULTS

Reperfusion was stable and complete in all mice and non-human primates tested, revealing no secondary decrease in cerebral blood flow. No evidence of secondary microthrombosis was found in the two models. Accordingly, deficiency in B1R did not protect the mice from brain damage after thrombin-induced stroke.

CONCLUSIONS

Our data demonstrate that secondary microthrombosis does not occur after thrombin-induced stroke. In view of this, the pathophysiologic roles of hematologic players promoting or protecting against secondary microthrombosis (such as factor XII, von Willebrand factor, and T cells) deserve to be re-evaluated in non-TMVO stroke models.

Factor XII activation markers do not reflect FXII dependence of thrombin generation induced by polyvinylchloride

The role of factor XII (FXII) as the main trigger of the coagulation cascade during haemodialysis has been recently challenged. Polyvinylchloride (PVC) is the standard polymer for haemodialysis circuit tubings, but its interaction with FXII has not been extensively characterized. In a modified Chandler tubing loop model using heparinized fresh human whole blood we selectively inhibited coagulation factors VII, X or XII with monospecific antibodies. Contact of whole blood with PVC induced a strong thrombin generation [thrombin-antithrombin complexes (TAT) 64 ± 24 μg/l, before <1 μg/l]. Despite this, levels of FXII coagulation activity, free FXIIa or FXIIa-C1 inhibitor complexes remained unchanged. The anti-FXII antibody abolished thrombin generation (TAT 8 ± 5 μg/l, P < 0.05) and made the free FXIIa undetectable. Inhibition of FVII did not affect coagulation activation (TAT 68 ± 26 μg/l). Our data provide definitive evidence that PVC triggers the coagulation system via FXII. However, all FXII activation markers in plasma failed to detect contact activation.

Factor XII promotes blood coagulation independent of factor XI in the presence of long-chain polyphosphates

BACKGROUND

Inorganic polyphosphates (polyP), which are secreted by activated platelets (short-chain polyP) and accumulate in some bacteria (long-chain polyP), support the contact activation of factor XII (FXII) and accelerate the activation of FXI.

OBJECTIVES

The aim of the present study was to evaluate the role of FXI in polyP-mediated coagulation activation and experimental thrombus formation.

METHODS AND RESULTS

Pretreatment of plasma with antibodies that selectively inhibit FXI activation by activated FXII (FXIIa) or FIX) activation by activated FXI (FXIa) were not able to inhibit the procoagulant effect of long or short-chain polyP in plasma. In contrast, the FXIIa inhibitor, corn trypsin inhibitor, blocked the procoagulant effect of long and short polyP in plasma. In a purified system, long polyP significantly enhanced the rate of FXII and prekallikrein activation and the activation of FXI by thrombin but not by FXIIa. In FXI-deficient plasma, long polyP promoted clotting of plasma in an FIX-dependent manner. In a purified system, the activation of FXII and prekallikrein by long polyP promoted FIX activation and prothombin activation. In an ex vivo model of occlusive thrombus formation, inhibition of FXIIa with corn trypsin inhibitor but not of FXI with a neutralizing antibodies abolished the prothrombotic effect of long polyP.

CONCLUSIONS

We propose that long polyP promotes FXII-mediated blood coagulation bypassing FXI. Accordingly, some polyp-containing pathogens may have evolved strategies to exploit polyP-initiated FXII activation for virulence, and selective inhibition of FXII may improve the host response to pathogens.

Inhibiting the intrinsic pathway of coagulation with a factor XII-targeting RNA aptamer

BACKGROUND

Exposure of the plasma protein factor XII (FXII) to an anionic surface generates activated FXII that not only triggers the intrinsic pathway of blood coagulation through the activation of FXI but also mediates various vascular responses through activation of the plasma contact system. While deficiencies of FXII are not associated with excessive bleeding, thrombosis models in factor-deficient animals have suggested that this protein contributes to stable thrombus formation. Therefore, FXII has emerged as an attractive therapeutic target to treat or prevent pathological thrombosis formation without increasing the risk for hemorrhage.

OBJECTIVES

Using an in vitro directed evolution and chemical biology approach, we sought to isolate a nuclease-resistant RNA aptamer that binds specifically to FXII and directly inhibits FXII coagulant function.

METHODS AND RESULTS

We describe the isolation and characterization of a high-affinity RNA aptamer targeting FXII/activated FXII (FXIIa) that dose dependently prolongs fibrin clot formation and thrombin generation in clinical coagulation assays. This aptamer functions as a potent anticoagulant by inhibiting the autoactivation of FXII, as well as inhibiting intrinsic pathway activation (FXI activation). However, the aptamer does not affect the FXIIa-mediated activation of the proinflammatory kallikrein-kinin system (plasma kallikrein activation).

CONCLUSIONS

We have generated a specific and potent FXII/FXIIa aptamer anticoagulant that offers targeted inhibition of discrete macromolecular interactions involved in the activation of the intrinsic pathway of blood coagulation.

Factor XII-independent activation of the bradykinin-forming cascade: Implications for the pathogenesis of hereditary angioedema types I and II

BACKGROUND

We have previously reported that prekallikrein expresses an active site when it is bound to high-molecular-weight kininogen (HK) and can digest HK to produce bradykinin. The reaction is stoichiometric and inhibited by C1 inhibitor (C1-INH) or corn trypsin inhibitor. Addition of heat shock protein 90 leads to conversion of prekallikrein to kallikrein in a zinc-dependent reaction.

OBJECTIVE

Our goal was to determine whether these reactions are demonstrable in plasma and distinguish them from activation through factor XII.

METHODS

Plasma was incubated in polystyrene plates and assayed for kallikrein formation. C1-INH was removed from factor XII-deficient plasma by means of immunoadsorption.

RESULTS

We demonstrate that prekallikrein-HK will activate to kallikrein in phosphate-containing buffers and that the rate is further accelerated on addition of heat shock protein 90. Prolonged incubation of plasma deficient in both factor XII and C1-INH led to conversion of prekallikrein to kallikrein and cleavage of HK, as was seen in plasma from patients with hereditary angioedema but not plasma from healthy subjects.

CONCLUSIONS

These results indicate that C1-INH stabilizes the prekallikrein-HK complex to prevent HK cleavage either by prekallikrein or by prekallikrein-HK autoactivation to generate kallikrein. In patients with hereditary angioedema, kallikrein and bradykinin formation can occur without invoking factor XII activation, although the kallikrein formed can rapidly activate factor XII if it is surface bound.

Corn trypsin inhibitor coating attenuates the prothrombotic properties of catheters in vitro and in vivo

Catheters initiate coagulation by activating factor (f) XII, which can lead to catheter thrombosis. Fondaparinux, which only targets activated fX (fXa), is associated with more catheter thrombosis than heparin, which targets fXa and thrombin. To render catheters less thrombogenic and fondaparinux more effective, we examined whether coating catheters with corn trypsin inhibitor (CTI), which blocks fXIIa, attenuates catheter-induced clotting and promotes fondaparinux activity. Compared with unmodified catheters, CTI-coated catheters demonstrated (a) decreased adsorption of fibrinogen and fXII, (b) greater inhibition of fXIIa generated by catheter-induced autoactivation, (c) attenuated fXIIa-mediated activation of fXI and (d) longer plasma clotting times in the absence or presence of fondaparinux. In an accelerated catheter thrombosis model in rabbits, (a) the time to catheter occlusion was longer with CTI-coated catheters than with unmodified catheters and (b) an intravenous dose of fondaparinux that had no effect on the time to occlusion of unmodified catheters extended the time to occlusion of CTI-coated catheters. These findings support the concept that the prothrombotic activity of catheters reflects their capacity to activate fXII and identify CTI immobilization as a novel approach for rendering catheters and other blood-contacting medical devices less thrombogenic.

A comparison of blood factor XII autoactivation in buffer, protein cocktail, serum, and plasma solutions

Activation of blood plasma coagulation in vitro by contact with material surfaces is demonstrably dependent on plasma-volume-to-activator-surface-area ratio. The only plausible explanation consistent with current understanding of coagulation-cascade biochemistry is that procoagulant stimulus arising from the activation complex of the intrinsic pathway is dependent on activator surface area. And yet, it is herein shown that activation of the blood zymogen factor XII (Hageman factor, FXII) dissolved in buffer, protein cocktail, heat-denatured serum, and FXI deficient plasma does not exhibit activator surface-area dependence. Instead, a highly-variable burst of procoagulant-enzyme yield is measured that exhibits no measurable kinetics, sensitivity to mixing, or solution-temperature dependence. Thus, FXII activation in both buffer and protein-containing solutions does not exhibit characteristics of a biochemical reaction but rather appears to be a "mechanochemical" reaction induced by FXII molecule interactions with hydrophilic activator particles that do not formally adsorb blood proteins from solution. Results of this study strongly suggest that activator surface-area dependence observed in contact activation of plasma coagulation does not solely arise at the FXII activation step of the intrinsic pathway.

Immobilized transition metal ions stimulate contact activation and drive factor XII-mediated coagulation

BACKGROUND

Upon contact with an appropriate surface, factor XII (FXII) undergoes autoactivation or cleavage by kallikrein. Zn(2+) is known to facilitate binding of FXII and the cofactor, high molecular weight kininogen (HK), to anionic surfaces.

OBJECTIVES

To investigate whether transition metal ions immobilized on liposome surfaces can initiate coagulation via the contact pathway.

METHODS AND RESULTS

Liposomes containing a metal ion-chelating lipid, 1,2-dioleoyl-sn-glycero-3-{(N[5-amino-1-carboxypentyl]iminodiacetic acid)succinyl} ammonium salt (DOGS-NTA), were prepared by membrane extrusion (20% DOGS-NTA, 40% phosphatidylcholine, 10% phosphatidylserine, and 30% phosphatidylethanolamine). Ni(2+) immobilized on such liposomes accelerated clotting in normal plasma, but not factor XI (FXI)-deficient or FXII-deficient plasma. The results were similar to those obtained with a commercial activated partial thromboplastin time reagent. Charging such liposomes with other transition metal ions revealed differences in their procoagulant capacity, with Ni(2+) > Cu(2+) > Co(2+) and Zn(2+). Plasma could be depleted of FXI, FXII and HK by adsorption with Ni(2+) -containing beads, resulting in longer clot times. Consistent with this, FXI, FXII and HK bound to immobilized Ni(2+) or Cu(2+) with high affinity as determined by surface plasmon resonance. In the presence of Ni(2+) -bearing liposomes, K(m) and k(cat) values derived for autoactivation of FXII and prekallikrein, as well as for activation of FXII by kallikrein or prekallikrein by FXIIa, were similar to literature values obtained in the presence of dextran sulfate.

CONCLUSIONS

Immobilized Ni(2+) and Cu(2+) bind FXII, FXI and HK with high affinity and stimulate activation of the contact pathway, driving FXII-mediated coagulation. Activation of the contact system by immobilized transition metal ions may have implications during pathogenic infection or in individuals exposed to high levels of pollution.

Amorphous nanosilicas induce consumptive coagulopathy after systemic exposure

We previously reported that well-dispersed amorphous nanosilicas with particle size 70 nm (nSP70) penetrate skin and produce systemic exposure after topical application. These findings underscore the need to examine biological effects after systemic exposure to nanosilicas. The present study was designed to examine the biological effects. BALB/c mice were intravenously injected with amorphous nanosilicas of sizes 70, 100, 300, 1000 nm and then assessed for survival, blood biochemistry, and coagulation. As a result, injection of nSP70 caused fatal toxicity, liver damage, and platelet depletion, suggesting that nSP70 caused consumptive coagulopathy. Additionally, nSP70 exerts procoagulant activity in vitro associated with an increase in specific surface area, which increases as diameter reduces. In contrast, nSP70-mediated procoagulant activity was absent in factor XII-deficient plasma. Collectively, we revealed that interaction between nSP70 and intrinsic coagulation factors such as factor XII, were deeply related to nSP70-induced harmful effects. In other words, it is suggested that if interaction between nSP70 and coagulation factors can be suppressed, nSP70-induced harmful effects may be avoided. These results would provide useful information for ensuring the safety of nanomaterials (NMs) and open new frontiers in biological fields by the use of NMs.

Obesity-related derangements of coagulation and fibrinolysis: a study of obesity-discordant monozygotic twin pairs

Coagulation and fibrinolytic activities are under strong genetic control. We studied the effects of acquired obesity, independent of genetic factors on coagulation and fibrinolysis activities in obesity-discordant healthy monozygotic (MZ) twin pairs. Fourteen obesity-discordant (BMI within-pair difference >3 kg/m(2)) and 10 concordant (BMI difference <2 kg/m(2)) MZ twin pairs were identified from the nationwide FinnTwin16 study. Body composition (dual-energy x-ray absorptiometry), abdominal fat distribution (magnetic resonance imaging), liver fat (magnetic resonance spectroscopy), high sensitivity C-reactive protein, insulin sensitivity (euglycemic hyperinsulinemic clamp), and a panel of different markers of blood coagulation and fibrinolysis in the fasting state were measured. Strong resemblance was observed in most coagulation factors within all twin pairs, with the intraclass correlations ranging from 0.73 to 0.97, P < 0.03. However, the activities of fibrinogen and FIX, FXI, and FXII, and plasminogen activator inhibitor-1 (PAI-1) activities were increased in the obese co-twins (P < 0.05) and strongly correlated with the measures of adiposity, inflammation, and insulin resistance (r = 0.32-0.73, P < 0.05) among the twin individuals. Intrapair differences in fibrinogen and PAI-1 correlated with those in BMI, adiposity, and fasting insulin levels (r = 0.40-0.58, P < 0.05) indicating the independent effect of obesity. Derangements of blood coagulation and fibrinolysis are present already in early adulthood in obese subjects. Acquired obesity, independent of genetic factors, increases the activities of fibrinogen and activities of FIX, FXI, FXII, and PAI-1. This study confirms the mechanisms of simultaneous activities of intrinsic coagulation factors and impaired fibrinolysis predisposing obese subjects to thrombosis.

Factor XI and XII as antithrombotic targets

PURPOSE OF REVIEW

Arterial and venous thrombosis are major causes of morbidity and mortality, and the incidence of thromboembolic diseases increases as a population ages. Thrombi are formed by activated platelets and fibrin. The latter is a product of the plasma coagulation system. Currently available anticoagulants such as heparins, vitamin K antagonists and inhibitors of thrombin or factor Xa target enzymes of the coagulation cascade that are critical for fibrin formation. However, fibrin is also necessary for terminating blood loss at sites of vascular injury. As a result, anticoagulants currently in clinical use increase the risk of bleeding, partially offsetting the benefits of reduced thrombosis. This review focuses on new targets for anticoagulation that are associated with minimal or no therapy-associated increased bleeding.

RECENT FINDINGS

Data from experimental models using mice and clinical studies of patients with hereditary deficiencies of coagulation factors XI or XII have shown that both of these clotting factors are important for thrombosis, while having minor or no apparent roles in processes that terminate blood loss (hemostasis).

SUMMARY

Hereditary deficiency of factor XII (Hageman factor) or factor XI, plasma proteases that initiate the intrinsic pathway of coagulation, impairs thrombus formation and provides protection from vascular occlusive events, while having a minimal impact on hemostasis. As the factor XII-factor XI pathway contributes to thrombus formation to a greater extent than to normal hemostasis, pharmacological inhibition of these coagulation factors may offer the exciting possibility of anticoagulation therapies with minimal or no bleeding risk.

Differential protein expression in serum of abdominal aortic aneurysm patients - a proteomic approach

OBJECTIVE

The aim of the study is to investigate the differential expression of proteins in serum of abdominal aortic aneurysm (AAA) patients in relation to aneurysm size (D(max)) and progression.

METHODS

Two-dimensional differential in-gel electrophoresis (2D-DIGE) together with tandem mass spectrometry (MS/MS) was used to analyse the serum proteome from patients with small (D(max) 30-54 mm) AAA, either stable (increase D(max) <5 mm year⁻¹; n = 8) or progressive (increase D(max) ≥5 mm year⁻¹; n = 8), and large (D(max) ≥ 55 mm; n = 8) AAA. The identified proteins were quantitatively validated in a larger population (n = 80).

RESULTS

Several proteins were differentially expressed in serum of small stable, small progressive and large AAA. Three validated proteins (immunoglobulin G (IgG), α1-antitrypsin (α1-AT) and Factor XII activity) showed strong correlation with D(max). Size combined with either Factor XII activity or α1-antitrypsin had minimal effect on the prognostic value in predicting aneurysm progression compared with size alone (area under the curve (AUC), 0.85; 95% confidence interval (CI), 0.73-0.97; p < 0.001 and AUC, 0.85; 95% CI, 0.72-0.98; p < 0.001 vs. AUC, 0.83; 95% CI, 0.71-0.96; p < 0.001, respectively).

CONCLUSION

The present study indicates that both Factor XII and α1-antitrypsin are found in increased amounts in the serum of patients with expanding AAA. However, combination of either Factor XII or α1-antitrypsin with aneurysm diameter had little effect on prediction of aneurysm progression versus diameter alone.

Factor XII activation is essential to sustain the procoagulant effects of particulate matter

BACKGROUND

Particulate matter (PM) is a key component of ambient air pollution and has been associated with an increased risk of thrombotic events and mortality. The underlying mechanisms remain unclear.

OBJECTIVES

 To study the mechanisms of PM-driven procoagulant activity in human plasma and to investigate mainly, the coagulation driven by ultrafine particles (UFPs; < 0.1 μm) in genetically modified mice.

METHODS

Thrombin generation in response to PM of different sizes was assessed in normal human platelet-poor plasma, as well as in plasmas deficient in the intrinsic pathway proteases factors XII (FXII) or XI (FXI). In addition, UFPs were intratracheally instilled in wild-type (WT) and FXII-deficient (FXII(-/-) ) mice and plasma thrombin generation was analyzed in plasma from treated mice at 4 and 20 h post-exposure.

RESULTS

In normal human plasma, thrombin generation was enhanced in the presence of PM, whereas PM-driven thrombin formation was completely abolished in FXII- and FXI-deficient plasma. UFPs induced a transient increase in tissue factor (TF)-driven thrombin formation at 4 h post-instillation in WT mice compared with saline instillation. Intratracheal instillation of UFPs resulted in a procoagulant response in WT mice plasma at 20 h, whereas it was entirely suppressed in FXII(-/-) mice.

CONCLUSIONS

 Overall, the data suggest that PM promotes its early procoagulant actions mostly through the TF-driven extrinsic pathway of coagulation, whereas PM-driven long lasting thrombogenic effects are predominantly mediated via formation of activated FXII. Hence, FXII-driven thrombin formation may be relevant to an enhanced thrombotic susceptibility upon chronic exposure to PM in humans.

Hageman factor, platelets and polyphosphates: early history and recent connection

  Platelet activation and blood coagulation are essential for hemostasis and contribute to a variety of other biological processes such as inflammation, complement activation and tissue repair. Factor (F)XII, originally called Hageman factor, plays an important role in the kallikrein-kinin system by activating prekallikrein. In the 1960s, a platelet activity that promoted FXII activation was identified but its biochemical nature remained unknown. Inorganic polyphosphates (poly P) are polymers that consist of many phosphate residues linked by phosphoanhydride bonds. These polymers exist in all living organisms. In bacteria, poly P is important for growth and survival. Recently, poly P has been identified in human platelet dense granules. Studied have shown that upon platelet activation and secretion, poly P activates FXII, indicating that it is most likely the elusive platelet FXII activator. Poly P also regulates coagulation and fibrinolysis. In this review, we focus on early studies of FXII and the identification of platelet FXII activation activity, and discuss recent findings of poly P in FXII activation and coagulation.

Monitoring thrombin generation: is addition of corn trypsin inhibitor needed?

Thrombin generation monitoring has the potential to be used as a clinical diagnostic tool in the near future. However, robust preanalytical conditions may be required, and one factor that has been reported is in-vitro contact activation that might influence in-vitro measurements of thrombin generation and thereby act as an unpredictable pre-analytical variable. The aim of the current study was to investigate the influence of contact activation and the necessity of corn trypsin inhibitor (CTI) to abolish contact activation in thrombin generation measurements at low tissue factor (TF) concentrations. Thrombin generation was performed using the calibrated automated thrombinoscopy (CAT), thereby determining the endogenous thrombin potential (ETP), peak height, and the lag time, in plasma obtained from healthy volunteers. Addition of CTI after plasma preparation had no significant influence on thrombin generation triggered with 0.5 pM TF or higher, as demonstrated by unaltered ETP and lag time values between analyses with and without CTI. Addition of CTI before blood collection reduced thrombin generation triggered with 0.5 pM TF: both the ETP and peak height were significantly reduced compared to no CTI addition. In contrast, thrombin generation remained unaltered at a 1 pM TF trigger or above. This study demonstrates that addition of CTI after plasma separation is not necessary when triggering with TF concentrations of 0.5 pM and higher. Furthermore, it was demonstrated that it is not needed to pre-fill blood collecting tubes with CTI when measuring thrombin generation at TF concentrations of >/=1 pM.

Discordant fibrin formation in hemophilia

BACKGROUND

The conversion of fibrinogen to fibrin and its crosslinking to form a stable clot are key events in providing effective hemostasis.

OBJECTIVES

To evaluate the relationship of fibrinopeptide (FP) release and factor (F) XIII activation in whole blood from hemophiliacs.

PATIENTS/METHODS

We investigated FPA and FPB release, FXIII activation and fibrin mass in tissue factor-initiated coagulation in whole blood from individuals with hemophilia and healthy subjects.

RESULTS

In hemophiliacs, the rates of fibrin formation were delayed as compared to healthy individuals. FPA/FPB release and FXIII activation were decreased in hemophiliacs vs. healthy individuals: 5.4 +/- 0.7 microM min(-1) to 1.7 +/- 0.4 microM min(-1) (P = 0.003), 2.3 +/- 0.6 microM min(-1) to 0.5 +/- 0.1 microM min(-1) (P = 0.025), and 12.1 +/- 0.7 nM min(-1) to 3.1 +/- 0.7 nM min(-1) (P < 0.0005), respectively. More FPA was released in hemophiliacs (6.6 +/- 1.2 microM) prior to clot time (CT) than in healthy individuals (2.6 +/- 0.4 microM, P = 0.013), whereas FPB and activated FXIII levels remained comparable. FXIII activation, which normally coincides with FPA release, was delayed in hemophiliacs. At CT in normal blood, the FPA concentration was 2.6-fold higher than that of FPB (P = 0.003), whereas in hemophiliacs this ratio was increased to 6.6-fold (P = 0.001).

CONCLUSIONS

These data suggest that essential dynamic correlations exist between the presentations of fibrin I, fibrin II, and FXIIIa. The 'discordance' of fibrin formation in hemophiliacs results in clots that are more soluble than normal (43% lower mass; P = 0.02). The resulting poor physical clot strength probably plays a crucial role in the pathology of hemophilia.

Prevention of vascular graft occlusion and thrombus-associated thrombin generation by inhibition of factor XI

The protease thrombin is required for normal hemostasis and pathologic thrombogenesis. Since the mechanism of coagulation factor XI (FXI)-dependent thrombus growth remains unclear, we investigated the contribution of FXI to thrombus formation in a primate thrombosis model. Pretreatment of baboons with a novel anti-human FXI monoclonal antibody (aXIMab; 2 mg/kg) inhibited plasma FXI by at least 99% for 10 days, and suppressed thrombin-antithrombin (TAT) complex and beta-thromboglobulin (betaTG) formation measured immediately downstream from thrombi forming within collagen-coated vascular grafts. FXI inhibition with aXIMab limited platelet and fibrin deposition in 4-mm diameter grafts without an apparent increase in D-dimer release from thrombi, and prevented the occlusion of 2-mm diameter grafts without affecting template bleeding times. In comparison, pretreatment with aspirin (32 mg/kg) prolonged bleeding times but failed to prevent graft occlusion, supporting the concept that FXI blockade may offer therapeutic advantages over other antithrombotic agents in terms of bleeding complications. In whole blood, aXIMab prevented fibrin formation in a collagen-coated flow chamber, independent of factor XII and factor VII. These data suggest that endogenous FXI contributes to arterial thrombus propagation through a striking amplification of thrombin generation at the thrombus luminal surface.