Mouse carotid artery ligation induces platelet-leukocyte-dependent luminal fibrin, required for neointima development

The relationship between platelet and leukocyte activation, coagulation, and neointima development was investigated in noninjured murine blood vessels subjected to blood stasis. The left common carotid artery of C57BL/6J mice was ligated proximal to the bifurcation. Tissue-factor expression in luminal leukocytes progressively increased over 2 weeks. On day 3 after ligation, in addition to infiltrated granulocytes, platelet microthrombi and platelet-covered leukocytes as well as tissue-factor-positive fibrin deposits lined the endothelium. Maximal neointima formation in carotid artery cross sections of control mice equaled 28+/-3.7% (n=11) and 42+/-5.1% (n=8) of the internal elastic lamina cross-sectional area 1 and 2 weeks after ligation. In FVIII(-/-) mice, stenosis was significantly lower 1 (11+/-3.6%, n=8) and 2 (21+/-4.7%, n=7) weeks after ligation (both P:<0.01 versus background-matched controls). In u-PA(-/-) mice, luminal stenosis was significantly higher 1 (38+/-7.0%, n=7) and 2 (77+/-5.6%, n=6) weeks after ligation (P:<0.05 and P:<0.01, respectively, versus matched controls). In alpha(2)-AP(-/-) mice, stenosis was lower at 1 week (14+/-2.6%, n=7, P:<0.01) but not at 2 weeks. Responses in tissue-type plasminogen activator or plasminogen activator inhibitor-1 gene-deficient mice equaled that in controls. Reducing plasma fibrinogen levels in controls with ancrod or inducing partial thrombocytopenia with busulfan resulted in significantly less neointima, but inflammation was inhibited only in busulfan-treated mice. We conclude that stasis induces platelet activation, leading to microthrombosis and platelet-leukocyte conjugate formation, triggering inflammation and tissue-factor accumulation on the carotid artery endothelium. Delayed coagulation then results in formation of a fibrin matrix, which is used by smooth muscle cells to migrate into the lumen.

Antagonism of vWF inhibits both injury induced arterial and venous thrombosis in the hamster

von Willebrand factor (vWF) is instrumental in arterial but has also been implicated in venous thrombogenesis. To address its role in venous thrombosis, experimental thrombosis was induced in the carotid artery and the femoral vein of hamsters, following which thrombus prevention by two different antagonists of vWF was studied. The first antagonist was the anti-human vWF monoclonal antibody AJvW-2, which inhibits the botrocetin and ristocetin induced aggregation of human blood platelets. AJvW-2 reacts with an epitope present in the A1 domain of vWF in very different species (human, pig, rabbit, dog, Guinea pig and rat). This epitope was found to be conformational and overlapping with vWF binding sites for aurin tricarboxylic acid (ATA), but not for botrocetin and heparin. AJvW-2 has affinities for vWF in the absence (Kd = 0.5 +/- 0.03 nmol/l in solution) and in the presence of shear stress (Kd = 3.3 +/- 0.6 nmol/l during perfusion at 1,300 s over subendothelial matrix associated vWF) sufficiently elevated to neutralize vWF. During perfusion of subendothelial matrix with anticoagulated human blood, the surface covered by adhering platelets was reduced by AJvW-2, with IC50s equal to 6.6 +/- 0.34 microg/ml at 1,300 s(-1) and to 1 +/- 0.01 microg/ml at 2,700 s(-1). As a second antagonist, molecular size gel filtered ATA was selected. Fractionated ATA inhibited platelet adhesion to matrix with IC50s equal to 0.27 +/- 0.09 mmol/l at 1,300 s(-1) and 0.16 +/- 0.008 mmol/l at 2,700 s(-1). When administered to hamsters, AJvW-2 prevented thrombosis in the injured carotid artery dose-dependently (ED50 = 0.15 +/- 0.01 mg/kg). Thrombosis in the similarly injured femoral vein was however also inhibited (ED50 = 0.37 +/- 0.06 mg/kg). Likewise, fractionated ATA completely inhibited carotid artery thrombosis (ED50 = 0.42 +/- 0.13 mg/kg), but also interfered with femoral vein thrombosis (apparent ED50 between 2 and 3 mg/kg). We conclude that antagonizing the vWF A1 domain by AJvW-2 and to a lesser extent also by fractionated ATA, inhibits thrombosis not only in the arterial but also in the venous circulation. Since venous thrombi were prevented at only 3-5-fold higher doses of antagonist, vWF participates in injury induced venous thrombosis.

von Willebrand factor binds to native collagen VI primarily via its A1 domain

Collagen VI is abundant in the arterial subendothelium. To investigate its mechanism of interaction with von Willebrand factor (vWF), collagen VI was isolated from human placenta and from the extracellular matrix of the human lung fibroblast cell line MRC-5. Purified vWF bound to non-digested collagen VI with moderately high affinity (EC50 approximately 5 nM) and could be inhibited by the Hirudo medicinalis collagen inhibitor calin. The anti-(human vWF A1 domain) monoclonal antibody (AJvW-2), as well as aurin tricarboxylic acid (ATA), at concentrations that saturate the vWF A1 domain, also inhibited this binding. In contrast, the monoclonal anti-(human vWF A3 domain) antibody (82D6A3) inhibited vWF binding to collagens I, III and IV, but had no effect on vWF binding to collagen VI. Likewise, vWF binding to collagen VI was not inhibited by the recombinant vWF domain D4. Polyclonal anti-(collagen VI) antibodies, specifically neutralizing the binding of vWF to collagen VI, confirmed that in the intact endothelial cell extracellular matrix, collagen VI was accessible for interaction with vWF. This binding was only marginally affected by 82D6A3 but was dose-dependently inhibited by AJvW-2, ATA and the A1 domain analogue VCL (recombinant A1 domain of vWF), with IC50 values comparable to those found for the inhibition of vWF binding to isolated collagen VI. The weak interaction of isolated human platelets with collagen VI was mediated via the platelet collagen receptor (GPIa/IIa) and was competitively inhibited by vWF but not by VCL, suggesting that vWF and GPIa/IIa bind to neighbouring but distinct sites on collagen VI. We conclude that vWF binds to collagen VI primarily via its A1 domain, which distinguishes it from the vWF A3 domain-mediated binding to fibrillar collagens.

Characterisation of a novel series of aprotinin-derived anticoagulants. II. Comparative antithrombotic effects on primary thrombus formation in vivo

Upon vascular damage platelet activation and blood coagulation are initiated. Interference at the initial level of the activation of the coagulation cascade can result in effective inhibition of thrombus formation. The in vivo antithrombotic properties of a series of bovine pancreatic trypsin inhibitor mutants (BPTI, aprotinin) 4C2, 7L22, 5L15, 5L15-PEG, 6L15 and 5L84, as described in the accompanying paper, with a combined inhibitory activity on factor Xa, factor VIIa-tissue factor complex, factor XIa and plasma kallikrein were compared to rTAP, r-hirudin, heparin and enoxaparin in a platelet rich thrombosis model in hamsters. Platelet dependent thrombus deposition was quantified by dedicated image analysis after transillumination of the femoral vein to which a standardised vascular trauma was applied. After increasing intravenous bolus injections all tested agents, except for aprotinin, induced a dose dependent decrease of thrombus formation and a concomitant prolongation of the aPTT. From the linear correlation between these two parameters it was found that 5 out of the 6 tested aprotinin analogues, rTAP and r-hirudin completely inhibited thrombus formation at a therapeutical (2- to 3-fold) aPTT prolongation while 4C2, heparin and enoxaparin only inhibited thrombus formation for 40 to 50 percent at a 2-fold aPTT prolongation. Based on the calculated IC50 values for thrombus formation rTAP was found to be the most active compound in this model. It is concluded that acceptable interference at the initial level of the blood coagulation, e.g. within a therapeutical aPTT prolongation, can significantly inhibit platelet deposition at a site of vascular injury.

Calin from Hirudo medicinalis, an inhibitor of platelet adhesion to collagen, prevents platelet-rich thrombosis in hamsters

Interaction between exposed collagen and platelets and/or von Willebrand factor is believed to be one of the initiating events for thrombus formation at sites of damaged endothelium. Interference with this mechanism may provide an anti-thrombotic potential. Calin, a product from the saliva of the leech Hirudo medicinalis, was tested in vitro and for its in vivo activity in a thrombosis model in hamsters. Calin specifically and dose dependently (IC50:6.5 to 13 micrograms/mL) inhibited human platelet aggregation induced by collagen. In addition, specific platelet adhesion onto microtiter wells coated with collagen and detected with a monoclonal antiglycoprotein IIb/IIIa antibody-conjugated with horseradish peroxidase, could be completely prevented with Calin (IC50:22 micrograms/mL). A dose-response curve was constructed in groups of six hamsters in whom a standardized trauma was induced on the femoral vein. Thrombus formation was followed continuously using video recording and processing of the image obtained upon transillumination of the vessel. Intravenous Calin dose-dependently inhibited platelet-rich thrombus formation in this model with an ED50 of 0.07 mg/kg and complete inhibition with 0.2 mg/kg. No effects were seen on coagulation tests or bleeding times, whereas ex vivo aggregation induced by collagen was inhibited dose dependently. Local application of leech saliva, Calin, hirudin, or the combination of the latter two into the bleeding time wound of hamsters resulted in a mild prolongation of the bleeding time (twofold to threefold). A similar experiment in baboons did not cause any prolongation of the bleeding time. This is in sharp contrast with the long-lasting bleeding after a leech bite itself in both species. Calin from the leech Hirudo medicinalis is able, by binding to collagen, to effectively interfere with platelet-collagen interaction, which results in an antithrombotic effect observed in a platelet-rich thrombosis model in hamsters.

Antithrombotic effects and bleeding time prolongation with synthetic platelet GPIIb/IIIa inhibitors in animal models of platelet-mediated thrombosis

Cyclic Arg-Gly-Asp (RGD) containing synthetic peptides such as L-cysteine, N-(mercaptoacetyl)-D-tyrosyl-L-arginylglycyl-L-alpha-aspartyl- cyclic (1-->5)-sulfide, 5-oxide (G4120) and acetyl-L-cysteinyl-L-asparaginyl-L-propyl-L-arginyl-glycyl-L-alpha- aspartyl-[0-methyltyrosyl]-L-arginyl-L-cysteinamide, cyclic 1-->9-sulfide (TP9201) bind with high affinity to the platelet GPIIb/IIIa receptor. The relationship between antithrombotic effect, ex vivo platelet aggregation and bleeding time prolongation with both agents was studied in hamsters with a standardized femoral vein endothelial cell injury predisposing to platelet-rich mural thrombosis, and in dogs with a carotid arterial eversion graft inserted in the femoral artery. Intravenous administration of G4120 in hamsters inhibited in vivo thrombus formation with a 50% inhibitory bolus dose (ID50) of approximately 20 micrograms/kg, ex vivo ADP-induced platelet aggregation with ID50 of 10 micrograms/kg, and bolus injection of 1 mg/kg prolonged the bleeding time from 38 +/- 9 to 1,100 +/- 330 s. Administration of TP9201 in hamsters inhibited in vivo thrombus formation with ID50 of 30 micrograms/kg, ex vivo platelet aggregation with an ID50 of 50 micrograms/kg and bolus injection of 1 mg/kg did not prolong the template bleeding time. In the dog eversion graft model, infusion of 100 micrograms/kg of G4120 over 60 min did not fully inhibit platelet-mediated thrombotic occlusion but was associated with inhibition of ADP-induced ex vivo platelet aggregation and with prolongation of the template bleeding time from 1.3 +/- 0.4 to 12 +/- 2 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Synergistic antithrombotic properties of G4120, a RGD-containing synthetic peptide, and argatroban, a synthetic thrombin inhibitor, in a hamster femoral vein platelet-rich thrombosis model

The synergistic antithrombotic properties of G4120, a synthetic Arg-Gly-Asp (RGD) containing peptide which strongly inhibits platelet aggregation, and of Argatroban, a synthetic thrombin inhibitor, were examined in a reproducible quantitative hamster femoral vein platelet-rich mural thrombosis model. Bolus injections of G4120 and Argatroban inhibit thrombus formation in a dose-dependent way; 50% inhibition (ID50) is obtained with 11 micrograms/kg G4120 and with 2 mg/kg Argatroban. Combined bolus injections of 3 micrograms/kg G4120 with 0.5, 0.75 or 1 mg/kg Argatroban and of 1 mg/kg Argatroban with 1.5 or 3 micrograms/kg G4120 caused linear dose-dependent inhibition of thrombus formation, whereas 3 micrograms/kg G4120 or 1 mg/kg Argatroban alone had very little effect (less than 20% inhibition). ID50 was obtained with the combination of 3 micrograms/kg G4120 and 0.5 mg/kg Argatroban, corresponding to an equi-effective fractional combination of 0.62 with a 95% confidence interval of 0.50 to 0.74. Alternatively the ID50 was obtained with the combination of 1 mg/kg Argatroban and 1.3 micrograms/kg G4120, corresponding to an equi-effective fractional combination of 0.52 with a 95% confidence interval of 0.18 to 0.86. In both instances these results are indicative of a significant synergistic interaction. Bolus injection of 10 mg/kg aspirin, 100 U/kg heparin or the combination did not inhibit thrombus formation. The synergistic effect of the combination of platelet inhibiting RGD-peptides and synthetic thrombin inhibitors could be useful in the prevention of arterial occlusion with platelet-rich thrombus in patients with ischemic heart disease following thrombolytic therapy or angioplasty, although this combination is not expected to reverse platelet thrombus formation.