(Circulation. 1999;99:299-304.)
© 1999 American Heart Association, Inc.
Basic Science Reports |
The Contribution of Activated Factor XIII to Fibrinolytic Resistance in Experimental Pulmonary Embolism
From Harvard School of Public Health (G.L.R., A.K.H.), Harvard Medical School, and Massachusetts General Hospital (G.L.R.), Boston, Mass.
Correspondence to Guy L. Reed, MD, Cardiovascular Biology Laboratory, Harvard School of Public Health, 677 Huntington Ave, Boston, MA 02115. E-mail reed{at}cvlab.harvard.edu
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Abstract |
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Background—The resistance of thrombi to fibrinolysis induced by plasminogen activators remains a major impediment to the successful treatment of thrombotic diseases. This study examines the contribution of activated factor XIII (factor XIIIa) to fibrinolytic resistance in experimental pulmonary embolism.
Methods and Results—The fibrinolytic effects of specific
inhibitors of factor XIIIa–mediated fibrin-fibrin
cross-linking and 
2-antiplasmin–fibrin cross-linking were measured
in anesthetized ferrets with pulmonary emboli. Five
experimental groups were treated with heparin (100 U/kg) and/or tissue
plasminogen activator (TPA, 1 mg/kg) and the
percent (mean±SD) lysis of emboli was determined: (1) control, normal
factor XIIIa activity (14.1±4.8% lysis); (2) inhibited factor XIIIa
activity (42.7±7.4%); (3) normal factor XIIIa activity+TPA
(32.3±7.7%); (4) inhibited factor XIIIa activity+TPA (76.0±11.9%);
and (5) inhibited 2-antiplasmin–fibrin cross-linking+TPA
(54.7±3.9%). Inhibition of factor XIIIa activity increased
endogenous lysis markedly (group 1 versus 2;
P<0.0001), to a level comparable to that achieved with
TPA (group 2 versus 3; P<0.05). Among groups receiving
TPA, selective inhibition of factor XIII-mediated
2-antiplasmin–fibrin cross-linking enhanced lysis (group 3 versus
5; P<0.0005). Complete inhibition of factor XIIIa also
amplified lysis (group 3 versus 4; P<0.0001) and had
greater effects than inhibition of 2-antiplasmin
cross-linking alone (group 4 versus 5; P<0.0005). No
significant fibrinogen degradation occurred in any group.
Conclusions—Factor XIIIa–mediated fibrin-fibrin and
2-antiplasmin–fibrin cross-linking both caused experimental
pulmonary emboli to resist endogenous and
TPA-induced fibrinolysis. This suggests that factor
XIIIa may play a critical role in regulating
fibrinolysis in human thrombosis.
Key Words: embolism • fibrinolysis • plasminogen activators • thrombus
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The success of plasminogen activators and adjunctive agents in the therapy of thrombotic disease remains limited by the intrinsic resistance of thrombi to fibrinolysis. This fibrinolytic resistance is evident in patients with acute thrombotic coronary occlusion: treatment with plasminogen activators results in full coronary reperfusion in only 33% to 55% of patients at 90 minutes.1 2 3 The resistance of thrombi to lysis by plasminogen activators may be even more marked in patients with venous thromboembolism. In deep venous thrombosis treated with tissue plasminogen activator (TPA), nearly two thirds of patients have minimal or no significant lysis evident on repeat venography at 24 hours.4 5 In patients with pulmonary embolism, TPA restores blood flow within 24 hours to only about a third of occluded lung segments, as judged by serial perfusion scanning.6 7 In grim contrast to the therapeutic successes seen in patients treated for acute myocardial infarction with TPA, those treated for pulmonary embolism with TPA show no proven reduction in mortality.6 7 Indeed, some experts suggest that the mortality of pulmonary embolism, despite more modern therapies, has remained unchanged for the past 30 years.8 9 Because improved thrombolysis could translate to reduced mortality and morbidity, there is a pressing need for a better understanding of the causes of fibrinolytic resistance.
In patients with venous thromboembolism, the standard therapy is to
inhibit new thrombus formation with heparin.10 Although
heparin accelerates inhibition of thrombin and factor Xa by
antithrombin, it does not accelerate endogenous
fibrinolysis, in vitro or in patients.5 6 7
Plasminogen activators are the only agents
currently available for accelerating the degradation of thrombi (ie,
causing clots to undergo lysis faster than they would dissolve by
endogenous fibrinolysis).
Plasminogen activators convert
plasminogen to plasmin, the fibrinolytic enzyme that
degrades the thrombus. The physical properties of thrombi (age, mass,
retraction, and composition) and local blood flow are thought to
influence net fibrinolysis by plasmin.11
Also, fibrinolytic resistance is likely to be mediated by specific
molecular factors such as 2-antiplasmin,12 13 14
plasminogen activator inhibitor-1
(PAI-1),15 16 thrombin-activated
fibrinolysis inhibitor,17 and
activated factor XIII (XIIIa). In vitro studies show that these
factors regulate fibrinolysis by inhibiting plasmin or
endogenous plasminogen activators
or by modifying the susceptibility of fibrin to proteolysis by plasmin.
Still, little is known about the relative importance of these factors
in regulating endogenous or pharmacological
fibrinolysis in vivo.
In particular, the contribution of coagulation factor XIIIa to
fibrinolytic resistance in vivo is poorly understood. Factor XIII is a
zymogen composed of 2 catalytic A subunits and 2 noncatalytic B
subunits.18 Factor XIII is activated to a
functional transglutaminase (factor XIIIa) when thrombin cleaves an
activation peptide from the amino terminus of the A
subunits.19 Factor XIIIa catalyzes the formation of
covalent bonds between glutamine and lysine residues in the 
and
chains of adjacent fibrin molecules, which markedly increase the
mechanical durability of the fibrin polymer.20 Factor
XIIIa also rapidly cross-links 2-antiplasmin, the fast-acting
plasmin inhibitor, to fibrin.21 In vivo, human
thromboemboli show evidence of extensive cross-linking by factor XIIIa,
and highly cross-linked thrombi are more resistant to lysis in
vitro.22 Conversely, genetic factor XIII deficiency in
humans is associated with rebleeding after trauma, suggesting that
thrombi in these patients may have an enhanced susceptibility to
fibrinolysis.23 The relative contribution
of fibrin-fibrin cross-linking or of 2-antiplasmin–fibrin
cross-linking to fibrinolytic resistance in vitro is still
debated.21 22 24 25 26 27 In this study, we examined the
contribution of factor XIIIa–mediated fibrin-fibrin cross-linking and
2-antiplasmin–fibrin cross-linking to the fibrinolytic resistance
of experimental pulmonary emboli.
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Methods |
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Materials were obtained from the following suppliers: aprotinin, Sigma; calcium chloride, Mallinckrodt; purified factor XIII and fibrinogen, American Diagnostica; goat anti-mouse antibody, Cappel Organon Technika; heparin (1000 U/mL), Elkins-Sinn Inc; fresh-frozen human plasma pooled from random donors, Massachusetts General Hospital; TPA with a specific activity of 580 000 IU/mg, Genentech; normal saline for intravenous use, Travenol Laboratories; sodium iodide, Aldrich Chemical Co; Na125I, Dupont-NEN; and bovine thrombin, Parke-Davis. Microcentrifuge tubes were obtained from National Scientific Supply Co.
Ferrets (
0.8 to 1 kg) were purchased from Marshall Farms (New York,
NY). Ketamine (100 mg/mL) was obtained from Fort Dodge
Laboratories and acepromazine maleate from Fermenta Animal Health Co.
The surgical instruments were from VWR and the tubing from Namic. Bard
Parker surgical blades were from Becton Dickinson; 4-0 silk sutures,
from American Cyanamid Co; Surflo IV catheter and 20-gauge 1- to 1/4-in
Venoject tubes with K3EDTA, from Terumo Medical
Corp; and sterile 3-way stopcocks, from Mallinckrodt Critical Care. An
autosyringe infusion pump (Baxter Health Care Corp) was used with
tubing and a microbore 60-in extension set obtained from McGaw of
Puerto Rico.
Anti-Factor XIII Monoclonal Antibody Production and
Purification
The generation of monoclonal antibody 9C11 against the catalytic
A subunit of human factor XIII has been described.28 The
hybridoma producing 9C11 was cloned by limiting dilution28
and expanded into ascites in pristane-primed BALB/c mice. Antibody was
purified from filtered ascites by precipitation with 40% ammonium
sulfate. After resuspension and dialysis into 10 mmol/L
KH2PO4, pH 7.2, proteins
were absorbed on a DEAE–Affigel Blue Sepharose column and monoclonal
antibody 9C11 was eluted with a linear gradient spanning 0 to 100
mmol/L NaCl, as we have described.29 Eluted protein was
collected in fractions and analyzed by SDS-PAGE on 10%
gels.30
Inhibition of Fibrin-Fibrin Cross-Linking and
2-Antiplasmin–Fibrin Cross-Linking in Vitro
To determine the dose of factor XIII inhibitor or
2-antiplasmin peptide necessary to inhibit factor XIII–mediated
cross-linking, we mixed various concentrations of anti–factor XIII
antibody 9C11 (10 µL; 0 to 10 µg with fresh-frozen plasma (45
µL), bovine thrombin (100 U/mL; 3 µL), and calcium chloride (0.4
mol/L; 2.5 µL). The mixture was allowed to clot for 90 minutes at
37°C; the clots were then compressed and washed 3 times in 500 µL
of saline to remove unbound protein. The clots were solubilized in 95
µL of 9 mol/L urea and 5 µL of ß-mercaptoethanol at 37°C for 30
minutes. They were then mixed in 100 µL of SDS sample
buffer30 with 20 µL of bromphenol blue–glycerol
solution and incubated at 85°C for 5 minutes. Proteins were
electrophoresed on 6% SDS-polyacrylamide gels and
electroblotted to polyvinylidene membranes for
immunoblotting with a monoclonal antibody (4A5) to the
chain of fibrin30 31 and a polyclonal antibody to the
carboxy terminus of 2-antiplasmin, as we have
described.24
To inhibit the cross-linking of 2-antiplasmin to fibrin, we
synthesized a peptide
(NH2-Asn-Gln-Glu-Gln-Val-Ser-Pro-Leu-Thr-Leu-Leu-Lys) spanning the
cross-linking site on the amino terminus of
2-antiplasmin,32 as we have described.24
The purity of the peptide was analyzed by
high-performance liquid chromatography, and its
composition was verified by amino acid analysis on a Waters
Picotag system. The 2-antiplasmin peptide was then solubilized in
20 mmol/L Tris-HCl, and the pH was adjusted to 7.0. Various
concentrations of peptide (7.5 µL, 0 to 5 mmol/L final) were
mixed with 20 µL of plasma, 1.25 µL of calcium chloride (0.4
mmol/L), and 1.25 µL of thrombin (100 U/mL) and clotted for 90
minutes at 37°C as described above. The clots were solubilized and
analyzed by immunoblotting as described
above.24
Pulmonary Embolism Experiments
The protocol for studying pulmonary embolism in ferrets
has been described.29 In brief, male ferrets (1 kg)
were anesthetized with ketamine and acepromazine. After
the animals were fully anesthetized, the jugular vein and
carotid artery were exposed by an anterior midline incision and
cannulated with 20-gauge catheters. Pooled, citrated human plasma was
mixed with 125I-fibrinogen to 1 000 000
cpm/mL. Individual clots were formed by mixing
125I-fibrinogen–labeled plasma (45 µL) with
2.5 µL of bovine thrombin (100 U/mL) and 2.5 µL of calcium chloride
(0.4 mol/L). In some experiments, antibody 9C11 (3.1 µL; 10 µg) was
added to each mixture to inhibit factor XIII activity or
2-antiplasmin peptide (3 µL, 1.5 mmol/L final concentration)
was added to attenuate 2-antiplasmin cross-linking. After incubation
at 37°C for 90 minutes, the clots were compressed and washed 3 times
with saline to remove unbound protein. The radioactive content of the
clots was measured in a gamma counter immediately before injection.
Blood samples were drawn at baseline and at the end of the experiment.
Sodium iodide (10 mg) was injected to block thyroid uptake. Three clots
were embolized into the lungs by injection through the internal
jugular vein. Successful embolization was evinced by the accumulation
of radioactivity in the thorax.
All animals received weight-adjusted heparin at 100 U/kg (bolus), a dose sufficient to keep the activated partial thromboplastin time (aPTT) at >150 seconds throughout the procedure.29 TPA was given as a continuous infusion over 2 hours (1 mg/kg in 5 mL of normal saline). Animals were observed for a total of 4 hours after pulmonary embolization and then killed by lethal injection of anesthesia or CO2 inhalation. The thorax was dissected, and all intrathoracic structures were removed for gamma counting to detect residual thrombi. The percentage of clot lysis was determined for each ferret by dividing the total residual radioactivity in the thorax by that in the initial thrombi. A total of 28 animals were studied; 3 were excluded: 1 because of anesthesia-related death, 1 because of improper TPA infusion, and 1 because of failed embolism.
This experimental protocol was approved by the Harvard Medical Area Standing Committee on Animals. The Harvard Medical School animal management program is accredited by the American Association of Laboratory Animal Care, and the procedures were conducted in accordance with National Institutes of Health standards, as set forth in the Guide for the Care and Use of Laboratory Animals (DHHS Publication No. [NIH] 85–23, revised 1985), the Public Health Service Policy on the Humane Care and Use of Laboratory Animals by Awardee Institutions, and the NIH Principles for the Utilization and Care of Vertebrate Animals Used in Testing, Research, and Training.
Fibrinogen Assays
Blood samples were collected on K3EDTA
(0.15% solution final) with aprotinin (50 U kallikrein per
milliliter). Platelet-poor plasma was obtained by
centrifugation of whole blood33 and
assayed for fibrinogen by the sodium sulfite method.34
Statistical Tests
The data were analyzed by a 1-way ANOVA followed by the
Bonferroni-Dunn procedure for testing multiple comparisons.
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Results |
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Inhibition of Factor XIII Activity During Clotting
The cross-linking by factor XIIIa of the
chains of fibrin is
one of the fastest catalytic processes mediated by the
enzyme.35 Indeed, in the presence of fibrin, the
activation of factor XIII by thrombin is accelerated.36 In
previous studies, we derived a monoclonal antibody (9C11) that fully
inhibits all factor XIIIa–mediated cross-linking in primate
plasmas.28 To determine the amount of 9C11 required for
inhibiting factor XIII activity in the present studies, we examined
its dose-related effects on fibrin-fibrin chain and
2-antiplasmin–fibrin cross-linking during clotting. In comparison
with clots formed in the absence of inhibitor or in the
presence of the nonspecific alkylating agent iodoacetamide, clots
formed with 9C11 at doses of 5 and 10 µg/clot showed no significant
fibrin-fibrin chain (Figure 1
, top
panel) or 2-antiplasmin–fibrin cross-linking (Figure 1, bottom panel). On the basis of these studies, we selected a 9C11 dose
of 10 µg for our experiments.
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Selective Inhibition of 2-Antiplasmin Cross-Linking to
Fibrin
We used a peptide inhibitor to examine the effect on
fibrinolytic resistance of factor XIIIa–mediated cross-linking of
2-antiplasmin to fibrin. This peptide represents the amino
terminal cross-linking site of 2-antiplasmin and competitively
inhibits the cross-linking of 2-antiplasmin to purified
fibrin.32 Figure 2 (top)
shows that increasing concentrations of the peptide quenched
2-antiplasmin–fibrin cross-linking, as reflected by a decrease in
the high-molecular-weight 2-antiplasmin immunoreactivity (indicated
by the arrow) and an increase in the lower-molecular-weight
un–cross-linked 2-antiplasmin, which migrates as a broad (heavily
glycosylated) protein at 70 kDa. Increasing concentrations of the
peptide had no apparent effect on the formation of fibrin -
cross-links, as assessed by immunoblotting (Figure 2, bottom). On the basis of these studies, we selected a peptide
concentration of 1.5 mmol/L for our studies.
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Role of Factor XIII Activity in Endogenous Lysis
To determine the importance of factor XIII in
endogenous fibrinolysis (ie, lysis caused
by the ferret's own fibrinolytic system), we compared the rates of
dissolution of pulmonary emboli in animals treated with and
without factor XIII inhibitor (Figure 3, left). All animals received heparin at
a weight-adjusted bolus of 100 U/kg; this dose was sufficient to keep
the aPTT at >150 seconds throughout the experiment. Lysis of
pulmonary emboli in the control group was 14.1±4.8%
(mean±SD), whereas that in the group treated with factor XIII
inhibitor was 3 times as much (42.7±7.4%;
P<0.0001). This experiment indicated that inhibition of
factor XIIIa activity markedly increased endogenous lysis
in these pulmonary emboli.
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Role of Factor XIIIa Activity in Pharmacological Lysis
We next examined the effect of factor XIII activity on the
fibrinolytic resistance of pulmonary emboli in ferrets treated
with TPA (1 mg/kg) over 2 hours, a regimen similar to that used to
treat pulmonary embolism in humans. As above, all animals were
treated with heparin, the standard therapy for human pulmonary
embolism.10 One experimental group received
pulmonary emboli with normal factor XIII levels, another
pulmonary emboli in which both factor XIIIa–mediated
fibrin-fibrin and 2-antiplasmin cross-linking had been quenched by
9C11, and a third pulmonary emboli in which the cross-linking
of 2-antiplasmin to fibrin had been selectively inhibited by the
2-antiplasmin peptide.
In the group with normal factor XIII activity, TPA caused more lysis
(Figure 3
, TPA, 32.3±7.7%) than was seen in the group that
received no plasminogen activator (control,
14.1±4.8%; P<0.005). Also, there was a nonsignificant
trend (evaluated by the Dunn-Bonferroni correction) to more
fibrinolysis in the group that received the factor XIII
inhibitor but no TPA (Figure 3, F13-I) in comparison
with the group with normal factor XIII activity that received TPA
(42.7±7.4% versus 32.3±7.7%; P<0.05). Overall, factor
XIIIa activity was an important determinant of
fibrinolysis in animals treated with TPA because the
factor XIII inhibitor group (Figure 3, TPA+F13I)
showed significantly more lysis than the group with normal factor XIII
activity (76.0±11.9% versus 32.3±7.7%; P<0.0001). In
particular, factor XIIIa–mediated cross-linking of 2-antiplasmin to
fibrin made a specific contribution to fibrinolytic resistance because
selective inhibition of this cross-linking also significantly
accelerated lysis by TPA (Figure 3, TPA+2AP-I) in comparison
with lysis in animals with normal factor XIIIa activity (54.7±3.9%
versus 32.3±7.7%; P<0.0005). Still, selective inhibition
of 2-antiplasmin–fibrin cross-linking was less effective at
amplifying lysis than was inhibition of all factor XIIIa–mediated
cross-linking (54.7±3.9% versus 76.0±11.9%; P<0.0005),
suggesting that fibrin-fibrin cross-linking also contributed to
fibrinolytic resistance.
Effects on Fibrinogen Levels
We then determined whether the inhibition of total factor XIIIa
activity or the selective inhibition of 2-antiplasmin–fibrin
cross-linking enhanced the systemic degradation of the clotting factor
fibrinogen during fibrinolysis. Fibrinogen levels were
measured for all animals before and after the experiment. Figure 4 compares the residual fibrinogen levels
at the end of the study for each group, expressed as a percentage of
the initial fibrinogen value. There was no significant decrease in
fibrinogen levels (below 100%) for any of the experimental groups.
This indicated that nonspecific degradation of fibrinogen did not occur
when TPA was administered alone, in combination with inhibition of
factor XIII, or in combination with inhibition of 2-antiplasmin
cross-linking.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The intrinsic resistance of thrombi to endogenous and pharmacological fibrinolysis remains an important cause of morbidity and mortality in patients with thrombotic disease. In the experiments described here, we examined the contribution of factor XIIIa–mediated fibrin-fibrin cross-linking and
2-antiplasmin–fibrin cross-linking to the fibrinolytic resistance
of pulmonary emboli. These studies required potent and specific
inhibitors of the 2 factor XIIIa–mediated processes.
We used a monoclonal antibody (9C11), which is capable of quenching all
factor XIIIa–mediated cross-linking,28 and verified that
during clotting it completely inhibited both fibrin -chain
cross-linking and 2-antiplasmin–fibrin cross-linking. To
selectively inhibit only factor XIIIa–mediated
2-antiplasmin–fibrin cross-linking, we used a peptide corresponding
to the amino terminus of human 2-antiplasmin, which contains the
cross-linking site.32 We also verified that this peptide
attenuated 2-antiplasmin–fibrin cross-linking without significantly
perturbing factor XIIIa–mediated fibrin -chain cross-linking.
We analyzed the effect of these 2 inhibitors on the
fibrinolysis of formed pulmonary emboli in an
established model of pulmonary embolism in
ferrets.29 To simulate the standard therapy for humans
with pulmonary embolism, and to inhibit the accretion of new
thrombus on these emboli, all animals were given doses of heparin
sufficient to significantly prolong the aPTT (>150 seconds) throughout
the experiment.29 Inhibition of factor XIIIa activity
tripled the rate of endogenous fibrinolysis
(42.7±7.4% versus 14.1±9.8%). This was a profound effect, because
the amount of endogenous lysis in clots with inhibited
factor XIIIa activity was as much as, or perhaps slightly more than,
the amount induced by TPA in clots with normal factor XIIIa activity
(32.2±7.7%). A parallel enhancement was seen in the lysis of
pulmonary emboli by TPA (1 mg/kg): inhibition of both factor
XIIIa–fibrin-fibrin and 2-antiplasmin–fibrin cross-linking
substantially increased lysis (76.0±11.9%) over that seen with the
same dose of TPA alone (32.3±7.7%). In addition, selective inhibition
of 2-antiplasmin–fibrin cross-linking amplified TPA-induced lysis
(54.7±3.9%) in comparison with that induced by TPA alone
(32.3±7.7%). This effect underscores the inhibitory role
played by the cross-linking of 2-antiplasmin to fibrin during
initiation of fibrinolysis. That even higher
fibrinolysis was achieved with TPA when factor
XIIIa–mediated fibrin-fibrin cross-linking and
2-antiplasmin–fibrin cross-linking were both inhibited fully
(76.0±11.9%) suggests that both cross-linking processes are important
inhibitors of fibrinolysis induced by TPA.
If full heparinization did not completely prevent the absorption and
activation of ferret factor XIII onto these clots after embolization,
some degree of cross-linking may have occurred in the thrombi of all
experimental groups. The effect of this cross-linking would be to blunt
the increased fibrinolysis attributed to the factor
XIII inhibitors. This would not change the conclusion that
factor XIIIa cross-linking is a major cause of fibrinolytic resistance,
but it would imply that its role may be even larger than was observed
in these experiments.
By providing new in vivo evidence about the role of factor XIIIa in the
regulation of fibrinolysis, these studies should help
resolve past controversy. Although in vitro studies indicate that clots
in which factor XIII is deficient (or inhibited) undergo lysis at
accelerated rates,27 28 the magnitude and molecular causes
of this effect have been debated. In vitro studies suggest that
fibrin-cross-linked 2-antiplasmin is an important determinant of the
susceptibility of plasma clots to
fibrinolysis.21 25 27 In fact, Jansen et
al27 concluded that while factor XIIIa–mediated
cross-linking inhibited fibrinolysis, its major
inhibitory effect was mediated through the cross-linking of
2-antiplasmin to fibrin. In contrast, Francis and
Marder,26 in studies of
supraphysiological doses of factor XIII, concluded
that fibrin-fibrin cross-linking in and of itself contributes to
fibrinolytic resistance. Their assertions were given credence by the
finding that coronary fibrinolysis in dogs was
accelerated by a weak, nonspecific inhibitor that prevented
only the slowest factor XIIIa–mediated process, fibrin -chain
cross-linking.37 The use of 2 potent, selective
inhibitors of factor XIIIa–mediated cross-linking in the
present study permitted us to establish that fibrin-fibrin
cross-linking and fibrin-2-antiplasmin cross-linking both contribute
to fibrinolytic resistance.
In addition to factor XIIIa, it is likely that other molecular factors
in the thrombus (eg, PAI-1 and 2-antiplasmin) cause the fibrinolytic
resistance seen in thrombotic diseases such as pulmonary
embolism. Both PAI-1 and 2-antiplasmin are detected readily in human
thrombi.38 Several studies in vivo have shown that PAI-1
contributes to the resistance of thrombi to endogenous
fibrinolysis.39 40 41 42 43 Whether PAI-1
contributes to the resistance of thrombi to pharmacological
fibrinolysis with full-dose plasminogen
activators remains to be settled; high doses of TPA appear
to overwhelm the inhibitory capacity of PAI-1, and agents
like streptokinase are impervious to it.44 However, there
is clear evidence from the present study, and from other studies in
which inhibitory antibodies were tested, that
2-antiplasmin causes resistance to endogenous as well as
pharmacological fibrinolysis in venous thromboemboli in
vivo.29 45
In humans, pulmonary emboli appear to develop from the fragmentation of propagating thrombi in the deep venous system. Anticoagulants that interfere with the activity of thrombin prevent thrombus propagation by inhibiting the new deposition of fibrin.10 Still, despite effective anticoagulation, the inherent fibrinolytic resistance of formed thrombi prevents optimal treatment of patients with thrombotic disease. In these studies of heparinized animals, we found that factor XIIIa–mediated cross-linking played a critical role in limiting the endogenous and pharmacological fibrinolysis of formed experimental pulmonary emboli; this argues that factor XIII inhibitors eventually may be of potential use in the treatment of thrombotic disease. By virtue of its inhibitory effects on endogenous fibrinolysis, it is also likely that factor XIIIa facilitates the growth of new or forming thrombi, a hypothesis that merits investigation in suitable models of developing venous thrombosis.
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Acknowledgments |
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This work was supported in part by National Institutes of Health grant HL-57314 (to G.L.R.). We gratefully acknowledge the editorial assistance of Thomas McVarish. This report is dedicated to the memory of Edgar Haber, MD.
Received March 26, 1998; revision received September 4, 1998; accepted September 9, 1998.
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