Combined tissue factor pathway inhibitor and thrombomodulin deficiency produces an augmented hypercoagulable state with tissue-specific fibrin deposition.

BACKGROUND AND OBJECTIVE: Tissue factor pathway inhibitor (TFPI) and thrombomodulin (TM) are endothelial-associated anticoagulant proteins thought to control hemostasis in specific vascular beds. Here, we have examined the consequences of TFPI deficiency in the presence of a compounding procoagulant state caused by reduced TM function. METHODS AND RESULTS: TFPI(+/-)/TM(pro/pro) mice are born at less than expected frequency in either TFPI(+/-)/TM(pro/+) or TM(pro/pro) mothers but are born at near the expected frequency in TM(pro/+) mothers. Adult TFPI(+/-)/TM(pro/pro) mice have elevated thrombin-antithrombin complex and increased thrombus volume in an electrical injury model of venous thrombosis. In striking contrast to mice with single deficiency of TFPI or TM, TFPI(+/-)/TM(pro/pro) mice exhibit augmented fibrin deposition not only in the liver, but also in the cerebral microvasculature. CONCLUSIONS: TFPI(+/-)/TM(pro/pro) mice exhibit partial intrauterine lethality when carried by mothers with an underlying prothrombotic state, providing the first experimental evidence in an animal model that TFPI-dependent control of hemostasis in the vascular bed of the placenta fulfills a critical role for successful pregnancy outcome. In addition to the placenta, partial TFPI deficiency interacts with decreased TM function in an organ selective manner to produce fibrin deposition in other specific vascular beds, the liver and brain.

Factor XI-deficient mice exhibit increased bleeding after injury to the saphenous vein.

Essentials Factor XI (FXI) deficient mice have normal hemostasis in a tail transection model. The hemostatic capacity of FXI-/- mice was assessed in three different bleeding models. FXI-/- mice have increased saphenous vein bleeding. FXI-/- mice may be a useful experimental model to study bleeding associated with FXI deficiency. SUMMARY: Background Factor XI (FXI) is a key component of the intrinsic pathway of coagulation. It can be activated by either FXIIa or thrombin and amplifies thrombin generation during clot formation. Congenital FXI deficiency in humans (known as hemophilia C) is associated with bleeding after hemostatic challenge. However, to date there are no reports of excess bleeding in FXI-deficient mice. Objectives To determine if the absence of FXI in mice prolongs bleeding in different models. Methods We assessed the hemostatic capacity of FXI-/- mice in three different bleeding models: tail bleeding, surgical bleeding and saphenous vein bleeding. Results We found that tail bleeding and surgical bleeding of FXI-/- mice were similar to wild-type mice. However, FXI-/- mice had an impaired hemostatic capacity in the saphenous vein bleeding model compared with wild-type controls. Conclusions Our results indicate that FXI-/- mice have a mild hemostatic defect after injury to the saphenous vein but not after transection of the tail or vessels in the abdominal wall.

Human pancreatic tumors grown in mice release tissue factor-positive microvesicles that increase venous clot size.

Essentials Tumor-bearing mice have larger venous clots than controls. Human tissue factor is present in clots in tumor-bearing mice. Inhibition of human tissue factor reduces clot size in tumor-bearing mice. This new mouse model may be useful to study mechanisms of cancer-associated thrombosis. SUMMARY: Background Pancreatic cancer patients have a high rate of venous thromboembolism. Human pancreatic tumors and cell lines express high levels of tissue factor (TF), and release TF-positive microvesicles (TF+ MVs). In pancreatic cancer patients, tumor-derived TF+ MVs are present in the blood, and increased levels are associated with venous thromboembolism and decreased survival. Previous studies have shown that mice with orthotopic human or murine pancreatic tumors have circulating tumor-derived TF+ MVs, an activated clotting system, and increased incidence and mean clot weight in an inferior vena cava stenosis model. These results suggest that TF+ MVs contribute to thrombosis. However, the specific role of tumor-derived TF+ MVs in venous thrombosis in mice has not been determined. Objectives To test the hypothesis that tumor-derived TF+ MVs enhance thrombosis in mice. Methods We determined the contribution of TF+ MVs derived from human pancreatic tumors grown orthotopically in nude mice to venous clot formation by using an anti-human TF mAb. We used an inferior vena cava stasis model of venous thrombosis. Results Tumor-bearing mice had significantly larger clots than control mice. Clots from tumor-bearing mice contained human TF, suggesting the incorporation of tumor-derived MVs. Importantly, administration of an anti-human TF mAb reduced clot size in tumor-bearing mice but did not affect clot size in control mice. Conclusions Our results indicate that TF+ MVs released from orthotopic pancreatic tumors increase venous thrombosis in mice. This new model may be useful for evaluating the roles of different factors in cancer-associated thrombosis.

OC-04 - Tissue factor positive microvesicles activate platelets in vitro and in vivo and enhance thrombosis in mice.

INTRODUCTION: Cancer patients have a 4- to 7- fold increased risk of venous thromboembolism (VTE) compared with general population. Most tumor cells express tissue factor (TF) and constitutively release small membrane microvesicles called tumor microvesicles (TMVs). Clinical studies have shown that circulating MP-TF activity is associated with VTE in pancreatic cancer but not in other types of cancer. Thrombin is a potent platelet agonist and activates platelets via protease activated receptors (PARs). AIM: To determine the contribution of the TF+ TMV-thrombin-platelet pathway to cancer-associated thrombosis. MATERIALS AND METHODS: A human pancreatic adenocarcinoma cell line expressing high levels of TF (BxPc-3) was selected to study the effect of TF+ TMVs on platelet activation and thrombosis. RESULTS: TF+ TMVs induced platelet activation in vitro in a thrombin-dependent manner. The presence of orthotopically grown BxPc-3 tumors in mice was associated with increased levels of thrombin-antithrombin III complexes (TATc) and larger thrombi in an inferior vena cava stenosis model compared with control mice. Furthermore, injection of BxPc-3 TF+ TMVs into mice triggered platelet activation and enhanced venous thrombosis in a TF-dependent manner. Importantly, BxPc-3 TF+ TMV-enhanced thrombosis was reduced in Par4-deficient mice and wild-type mice treated with the platelet inhibitor clopidogrel, suggesting that platelet activation was required for the enhanced thrombosis. CONCLUSIONS: These studies suggest that platelet inhibitors may reduce thrombosis in cancer patients with elevated levels of TF+ TMVs.

Tissue factor-positive tumor microvesicles activate platelets and enhance thrombosis in mice.

UNLABELLED: ESSENTIALS: Cancer patients have a high rate of venous thrombosis (VT) but the underlying mechanisms are unknown. Tumor-derived, tissue factor-positive microvesicles in platelet activation in vitro and in vivo were studied. Tumor-derived, tissue factor-positive microvesicles enhanced VT in mice. Platelets may contribute to VT in some cancer patients, and this could be prevented with antiplatelet drugs. BACKGROUND: Cancer patients have an approximately 4-fold increased risk of venous thromboembolism (VTE) compared with the general population, and cancer patients with VTE have reduced survival. Tumor cells constitutively release small membrane vesicles called microvesicles (MVs) that may contribute to thrombosis in cancer patients. Clinical studies have shown that levels of circulating tumor-derived, tissue factor-positive (TF(+) ) MVs in pancreatic cancer patients are associated with VTE. Objectives We tested the hypothesis that TF(+) tumor-derived MVs (TMVs) activate platelets in vitro and in mice. MATERIALS AND METHODS: We selected two human pancreatic adenocarcinoma cell lines expressing high (BxPc-3) and low (L3.6pl) levels of TF as models to study the effect of TF(+) TMVs on platelets and thrombosis. RESULTS AND CONCLUSIONS: We found that both types of TF(+) TMVs activated human platelets and induced aggregation in vitro in a TF and thrombin-dependent manner. Further, injection of BxPc-3 TF(+) TMVs triggered platelet activation in vivo and enhanced thrombosis in two mouse models of venous thrombosis in a TF-dependent manner. Importantly, BxPc-3 TF(+) TMV-enhanced thrombosis was reduced in Par4-deficient mice and in wild-type mice treated with clopidogrel, suggesting that platelet activation was required for enhanced thrombosis. These studies suggest that TF(+) TMV-induced platelet activation contributes to thrombosis in cancer patients.

Characterization of a mouse model for thrombomodulin deficiency.

Mutations in the gene encoding thrombomodulin (TM), a thrombin regulator, are suspected risk factors for venous and arterial thrombotic disease. We have previously described the generation of TM(Pro/Pro) mice carrying a TM gene mutation that disrupts the TM-dependent activation of protein C. Here, it is shown that inbred C57BL/6J TM(Pro/Pro) mice exhibit a hypercoagulable state and an increased susceptibility to thrombosis and sepsis. Platelet thrombus growth after FeCl(3)-induced acute endothelial injury was accelerated in mutant mice. Vascular stasis after permanent ligation of the carotid artery precipitated thrombosis in mutant but not in normal mice. Mutant mice showed increased mortality after exposure to high doses of endotoxin and demonstrated altered cytokine production in response to low-dose endotoxin. The severity of the hypercoagulable state and chronic microvascular thrombosis caused by the TM(Pro) mutation is profoundly influenced by mouse strain-specific genetic differences between C57BL/6 and 129SvPas mice. These data demonstrate that in mice, TM is a physiologically relevant regulator of platelet- and coagulation-driven large-vessel thrombosis and modifies the response to endotoxin-induced inflammation. The phenotypic penetrance of the TM(Pro) mutation is determined by as-yet-uncharacterized genetic modifiers of thrombosis other than TM.

A rat uterine horn model of genital tract wound healing.

A rat uterine horn model of genital tract wound healing is described. Healing was reflected by acquisition of strength and elasticity, measured by burst strength (BS) and extensibility (EX), respectively. A tensiometer (Instron Corp., Canton, MA) was used to assess these characteristics in castrated and estrogen-supplemented or nonsupplemented animals. While the horn weights (HW), BS, and EX of contralateral horns were not significantly different, the intra-animal variation of HW was 7.2%, BS was 17.7% and EX was 38.2%. In a second experiment, one uterine horn was divided and anastomosed, and the animal given estrogen supplementation or a placebo pellet. Estrogen administration was found to increase BS and EX of anastomosed horns prior to 14 days, but had no beneficial effect at 21 or 42 days. The data suggest that estrogen may be required for optimal early healing of genital tract wounds.

The de-endothelialized rat carotid arterial graft: a versatile experimental model for the investigation of arterial thrombosis.

A novel model of arterial thrombosis was developed. A mechanical endothelium-denuding injury was created (using a scalpel blade) on harvested, freezer-stored rat carotid arteries. Vessel length of 5 mm. were grafted into the femoral arteries of recipient Sprague-Dawley rats using microvascular anastomotic technique. Patency rates in untreated animals were compared with those in animals receiving systemic aspirin or heparin. The control group patency after 2 hours of flow was 15%, while grafts in aspirin- and heparin-treated animals achieved 35% and 95% patency rates, respectively. Uninjured non-frozen carotid grafts in untreated animals yielded a 95% patency rate, while frozen grafts achieved an 80% patency. Therapeutic levels of aspirin, heparin, and urokinase were confirmed through tail bleeding and whole blood clotting tests, as well as platelet aggregation studies and scanning electron microscopy of the graft lumenal surfaces. A long-term series using syngeneic grafts placed in recipients (Lewis-to-Lewis) and employing systemic heparinization demonstrated maintenance of patency for 4 weeks. Scanning electron microscopy revealed good re-endothelialization, well advanced by one week. Histology confirmed the regrowth of endothelial cells, but showed sparse cellular repopulation of medial and adventitial layers. The mechanical injury model was compared to enzymatic de-endothelialization (using trypsin or collagenase), for which patency rates were similar (10% and 0%, respectively). Trypsin de-endothelialized vessels were tested in vitro for the amount of active trypsin remaining bound to the lumenal surface; no detectable activity was found when trypsin inhibitor was applied following trypsin treatment. The versatility of allowing both in vitro evaluation and in vivo patency assessment demonstrates the uniqueness and value of this new model, offering an avenue toward more direct investigations of surface-mediated thrombotic processes.

Thrombosis of microvascular anastomoses in traumatized vessels: fibrin versus platelets.

Thrombosis is the end result of two closely interrelated processes: the coagulation cascade and the platelet aggregation process. To determine their relative contribution, we used pharmacologic agents that selectively block each process. The specific effect of each pharmacologic agent on either fibrin deposition or platelet activity was confirmed morphologically by scanning electron microscopy and was substantiated with ADP-induced platelet aggregation and blood clotting time determinations. Forty-two rats had both femoral arteries subjected to a standardized crush-avulsion injury. A total of 84 femoral microvascular anastomoses were subsequently performed. None of the 24 control anastomoses treated with saline remained patent, whereas 6 of 24 of the anastomoses treated with dazmagrel (a selective thromboxane synthetase and platelet aggregation inhibitor), 2.5 mg/kg IV, remained patent and 18 of 24 of those treated with a single dose of heparin, 200 U/kg IV, remained patent. All 12 anastomoses treated with both drugs remained patent but developed a 33 percent hematoma rate. We conclude that in this microvascular model, fibrin mesh deposition is a more significant factor than platelet aggregation in the pathogenesis of occlusional thrombosis within traumatized arteries. Its temporary inhibition with a single dose of heparin yielded a 75 percent improvement in patency rate.

A prospective study of microvascular free-flap surgery and outcome.

Over a 6-month period, 23 members of the International Microvascular Research Group participated in a prospective survey of their microvascular free-flap practice. Data were recorded with each case for 60 variables covering patient characteristics, surgical technique, pharmacologic treatment, and postoperative outcome. A total of 493 free flaps were reported with a representative demographic distribution for age, sex, indications for surgery, risk factors, flap type, surgical technique, and pharmacologic intervention. Mixed effects logistic regression modeling was used to determine predictors of flap failure and associated complications. The overall incidence of flap failure was 4.1 percent (20 of 493). Reconstruction of an irradiated recipient site and the use of a skin-grafted muscle flap were the only statistically significant predictors of flap failure, with increased odds of failure of 4.2 (p = 0.01) and 11.1 (p = 0.03), respectively. A postoperative thrombosis requiring re-exploration surgery occurred in 9.9 percent of the flaps. The incidence of this complication was significantly higher when the flap was transferred to a chronic wound and when vein grafts were needed, with increased odds of failure of 2.9 (p = 0.02) and 2.5 (p = 0.02), respectively. There was a lower incidence of postoperative thrombosis when rectus/transverse rectus abdominis muscle (TRAM) flaps were used, where odds of failure decreased by 0.36 (p = 0.04), and when subcutaneous heparin was administered in the postoperative period, where odds decreased by 0.27 (p = 0.04). There was an overall 69-percent salvage rate for flaps identified with a postoperative thrombosis. Intraoperative thrombosis occurred in 41 cases (8.3 percent) and was observed more frequently in myocutaneous flaps or when vein grafts were needed (5.5 and 5.0 greater odds, respectively; p < 0.001) but was not associated with higher flap failure (2 of 41 cases; 4.9-percent failure rate). The incidence of a hematoma and/or hemorrhage was increased in obese patients and when vein grafts were needed [2.7 (p = 0.02) and 2.6 (p = 0.03) greater odds, respectively], whereas this complication was significantly decreased in muscle flaps (myocutaneous or skin-grafted muscle), in tobacco users, when a heparinized solution was used for general wound irrigation, and when the attending surgeon performed the arterial anastomosis (in contrast to the resident or fellow on staff) (p < 0.05 for each factor). With the multivariable analysis, many factors were found not to have a significant effect on flap outcome, including the recipient site (e.g., head/neck, breast, lower limb, etc.); indications for surgery (trauma, cancer, etc.); flap transfer in extremes of age, smokers, or diabetics; arterial anastomosis with an end-to-end versus end-to-side technique; irrigation of the vessel without or with heparin added to the irrigation solution; and a wide spectrum of antithrombotic drug therapies. These results present a current baseline for free-flap surgery to which future advances and improvements in technique and practice may be compared.

Repair of calvarial defects with flap tissue: role of bone morphogenetic proteins and competent responding tissues.

Bone morphogenetic proteins 2 through 8 have the ability to induce the in vivo transformation of extraskeletal mesenchymal tissue into bone. The aims of this investigation were to determine the optimal responding tissue and the specificity of the inductive effect of bone morphogenetic protein 3. The optimal responding tissue was found to be skeletal muscle. The specificity of this response to bone morphogenetic protein 3 was compared with that of recombinant human basic fibroblast growth factor, recombinant platelet-derived growth factor, and recombinant insulin-like growth factor. Bone morphogenetic protein 3 was the only factor that induced de novo bone formation. This ability to transform muscle into bone was tested in 7 x 7 mm irradiated skull defects in the rat. After 1500 rads of exposure, these defects showed no significant signs of healing by 8 months. When these defects were treated with the microvascular transfer of a nonirradiated muscle flap, they had 8 percent healing at 4 months and 37 percent healing by 8 months. Defects treated with 30 micrograms bone morphogenetic protein 3 (without the muscle flap) achieved 50 percent healing by 4 months and 64 percent healing by 8 months. When the defects were treated with both the muscle flap and bone morphogenetic protein 3, there was 96 percent healing by 4 months and 100 percent healing by 8 months (p < 0.015, compared with bone morphogenetic protein 3 alone at both time points). At 8 months, the transplanted muscle was entirely transformed into bone and healed the skull defect with newly generated bone indistinguishable from the surrounding calvarial tissue. These findings suggest a potential clinical utility of bone morphogenetic protein 3-induced bone formation in skeletal reconstructions. Furthermore, they also show that there is a collaborative requirement for both the osteoinductive factor bone morphogenetic protein 3 and the presence of competent responsive cells in the well-perfused muscle.

A phase II trial of intraluminal irrigation with recombinant human tissue factor pathway inhibitor to prevent thrombosis in free flap surgery.

A multicenter, multinational, blinded, randomized, parallel-group, phase II study was conducted to investigate the use of recombinant human tissue factor pathway inhibitor (rhTFPI; SC-59735) as an antithrombotic additive to the intraluminal irrigating solution during microvascular anastomosis in free flap reconstructive surgery. A total of 622 patients undergoing free flap reconstruction were randomly assigned to three groups. For each group, a different intraluminal irrigating solution was administered at completion of the microvascular arterial and venous anastomoses and before blood flow to the flap was reestablished: rhTFPI at a concentration of 0.05 or 0.15 mg/ml (low-dose or high-dose group, respectively) or heparin at a concentration of 100 U/ml (current-standard-of-practice group). There were no other differences in treatment among the groups. Patient characteristics, risk factors, and surgical techniques used were similar among all three groups. Flap failure was lower (2 percent) in the low-dose rhTFPI group than in the high-dose rhTFPI (6 percent) and heparin (5 percent) groups, but this difference was not statistically significant (p = 0.069). There were no significant differences in the rate of intraoperative revisions of vessel anastomoses (11 percent, 12 percent, and 13 percent) or postoperative thrombosis (8 percent, 8 percent, and 7 percent) among the low-dose rhTFPI, high-dose rhTFPI, and heparin groups, respectively. The rate of postoperative wound hematoma was significantly lower in the low-dose rhTFPI group (3 percent) than in the high-dose rhTFPI (8 percent) and heparin (9 percent) groups (p = 0.040). There were no differences in blood chemistry or coagulation values among the three study groups. Other than hematomas, there were no differences in the incidence or severity of adverse reactions among the three groups. It is concluded that use of rhTFPI as an intraluminal irrigant during free flap reconstruction is safe, well tolerated, and as efficacious as use of heparin for preventing thrombotic complications during and after the operation. Furthermore, the lower dose of rhTFPI (0.05 mg/ml) may reduce the occurrence of postoperative hematoma and help prevent flap failure.

Prevention of microvascular thrombosis by topical application of recombinant tissue factor pathway inhibitor.

Tissue factor pathway inhibitor is a naturally occurring protein inhibitor of factor X and the tissue factor-factor VII complex of the extrinsic pathway of coagulation. The potential of tissue factor pathway inhibitor as a topical antithrombotic agent was evaluated in a rabbit model of thrombosis that combined intimal injury, anastomosis, and a twisted pedicle. In 207 rabbit ears, a near-complete amputation was performed, preserving the central ear artery and vein. The central ear artery was transected, the intima was removed mechanically over a 1-cm length, the artery was anastomosed, and the ear was twisted 360 degrees, wrapping the intact vein around the artery. Before recirculation, the lumen was irrigated on a blinded, randomized basis with either hirudin (100 or 500 units/ml), heparin (50 or 100 units/ml), tissue factor pathway inhibitor (10, 40, 125, or 250 microgram/ml), heparin and tissue factor pathway inhibitor together, or vehicle (control). Upon arterial reflow, the ears were observed for 7 days. Patency rates after 7 days were as follows: hirudin, 30 and 55 percent; heparin, 43 and 50 percent; tissue factor pathway inhibitor, 75 and 90 percent; heparin and tissue factor pathway inhibitor, 75 percent; and vehicle, 6 percent. The higher concentrations of tissue factor pathway inhibitor led to significantly higher patency rates than heparin, hirudin, or control solutions. Electron microscopic evaluation of specimens irrigated with gold- labeled tissue factor pathway inhibitor revealed the inhibitor bound to the injured intimal surface for at least 3 days postoperatively. Coagulation studies showed no change in the clotting profile upon intravascular infusion with tissue factor pathway inhibitor even at the highest dose used topically. We conclude that tissue factor pathway inhibitor is a more effective topical antithrombotic agent than either heparin or hirudin.

Investigation in the use of topical solutions for management or prevention of thrombosis in traumatized vessels.

Systemic or local infusion of anticoagulants, platelet antiaggregating agents, and/or fibrinolytic agents for preventing thrombosis carries an inherent risk of generalized or localized hemorrhage. When the thrombotic stimulus is confined to a small region of traumatized vasculature, a new avenue of therapeutic approach [correction of aproach] is suggested which is achieved by topical irrigation of a compound specifically designed to bind to the lumenal surface and prevent surface-mediated thrombosis. Several agents have been developed and tested in animal models of simulated vascular trauma. Small peptides homologous to regions of the fibrinogen molecule that bind to platelet receptors and/or participate in fibrin strand cross-linkage can inhibit the incidence of thrombotic occlusion. Exogenous tissue factor pathway inhibitor can also prevent trauma-induced thrombosis. When vessels are surgically exposed during vascular reconstruction, the antithrombotic agent can be irrigated across the injured surface without the need for systemic infusion. This creates a thrombus-resistant surface in the regions of injured vasculature that receive direct application of the topical agent. This new approach to antithrombotic therapy may reduce the risk of spontaneous bleeding in trauma patients undergoing vessel repair and reconstruction. These new agents hold promise for site-specific targeting of antithrombotic therapy.

Lentivirus-mediated platelet gene therapy of murine hemophilia A with pre-existing anti-factor VIII immunity.

BACKGROUND: The development of inhibitory antibodies, referred to as inhibitors, against exogenous factor VIII in a significant subset of patients with hemophilia A remains a persistent challenge to the efficacy of protein replacement therapy. Our previous studies using the transgenic approach provided proof-of-principle that platelet-specific expression could be successful in treating hemophilia A in the presence of inhibitory antibodies. OBJECTIVE: To investigate a clinically translatable approach for platelet gene therapy of hemophilia A with pre-existing inhibitors. METHODS: Platelet FVIII expression in preimmunized FVIII(null) mice was introduced by transplantation of lentivirus-transduced bone marrow or enriched hematopoietic stem cells. FVIII expression was determined with a chromogenic assay. The transgene copy number per cell was quantitated with real-time PCR. Inhibitor titer was measured with the Bethesda assay. Phenotypic correction was assessed by the tail clipping assay and an electrolytically induced venous injury model. Integration sites were analyzed with linear amplification-mediated PCR. RESULTS: Therapeutic levels of platelet FVIII expression were sustained in the long term without evoking an anti-FVIII memory response in the transduced preimmunized recipients. The tail clip survival test and the electrolytic injury model confirmed that hemostasis was improved in the treated animals. Sequential bone marrow transplants showed sustained platelet FVIII expression resulting in phenotypic correction in preimmunized secondary and tertiary recipients. CONCLUSIONS: Lentivirus-mediated platelet-specific gene transfer improves hemostasis in mice with hemophilia A with pre-existing inhibitors, indicating that this approach may be a promising strategy for gene therapy of hemophilia A even in the high-risk setting of pre-existing inhibitory antibodies.

Thrombosis risk modification in transgenic mice containing the human fibrinogen thrombin-binding gamma' chain sequence.

BACKGROUND AND OBJECTIVES: Thrombin binding activity in murine fibrin (Antithrombin I) is restricted to its E domains inasmuch as murine gamma' chains (mu-gamma') do not bind thrombin. This feature prompted us to produce a 'gain-of-function' transgenic mouse in which the wild-type (WT) C-terminal mu-gamma' chain fibrinogen sequence had been replaced with the C-terminal thrombin-binding human gamma' sequence. RESULTS: This procedure resulted in a murine fibrinogen species containing chimeric hu-gamma' chains (hu-gamma' fibrinogen). As anticipated, thrombin bound to WT fibrin at a single class of sites, whereas thrombin binding to heterodimeric hu-gamma'-containing fibrin was increased, reflecting its content of hu-gamma' chains. In an electrolytically-induced femoral vein thrombosis injury model, we found no differences in the volume of thrombus generation between WT and heterozygous hu-gamma' mice. However, heterozygous factor (F) V Leiden (FVL(+/-)) mice developed greater thrombus volumes than did WT controls (P < 0.01). In doubly heterozygous FVL(+/-), hu-gamma' mice, thrombus formation was reduced to WT levels (P < 0.05). CONCLUSIONS: Murine hu-gamma' fibrinogen down-regulates venous thrombosis in the presence of another known thrombosis risk factor, FV Leiden. This finding indicates that hu-gamma' chain-containing fibrinogen is a thrombosis risk modifier.

Heat-induced tissue fusion for microvascular anastomosis.

Laser tissue welding was compared with a crude method of bipolar coagulator-generated heat application for achieving the same heat-induced welding effect in rat microarterial anastomoses. Rat femoral arteries were anastomosed with three triangulated stay sutures and subsequent laser welding or bipolar coagulator application between each pair of stitches. Control (non-welded) vessels received nine stitches placed circumferentially. Laser-welded vessel patency at 1 or more days postoperatively was 90% (65/72) for vessels treated with 0.1-second laser pulses, not significantly different from controls (100%; 16/16) or coagulator-welded anastomoses (88%; 14/16). Pseudoaneurysm rates were higher in the welded vessels (9% and 14% for laser- and coagulator-treated vessels, respectively) than in controls (0%). Histologic and electron microscopic evaluation revealed good healing with no apparent differences between laser- and coagulator-welded repairs. These findings suggest that laser application for microvascular tissue welding is similar to poorly controlled welding with a bipolar coagulator.