Thrombosis |
From the Developmental Research Laboratories, Pharmaceutical Research Laboratories, Ajinomoto Co, Inc, Kawasaki, Japan.
Correspondence to Dr Hiroshi Yamamoto, Developmental Research Laboratories, Pharmaceutical Research Laboratories, Ajinomoto Co, Inc, 1-1 Suzuki-cho, Kawasaki-ku, Kawasaki-shi 210-8681, Japan. E-mail hiroshia_yamamoto{at}ajinomoto.com
| Abstract |
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Key Words: AJvW-2 von Willebrand factor thrombus angioplasty antibodies
| Introduction |
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30% of patients.1 2 3
Pathophysiologically, restenosis is
characterized by several components, such as smooth muscle cells
(SMCs), extracellular matrix, and organized thrombus, in the
neointimal lesion; however, the precise interplay between
these components remains unclear.4 5 6 Immediately after angioplasty, platelets adhere to the injured vessel wall and release various mitogens, such as platelet-derived growth factor (PDGF), leading to SMC migration, proliferation, and excessive extracellular matrix synthesis. The interaction of von Willebrand factor (vWF) with platelet membrane glycoprotein (GP) Ib plays a crucial role in platelet adhesion to the injured vessel wall7 8 9 and in the subsequent development of an arterial thrombus.10 11 12
AJvW-2 is a murine monoclonal antibody against human vWF with a specific inhibitory effect on the GPIb-vWF axis.13 In addition, AJvW-2 cross-reacts well with vWF of other species, and this antibody inhibits thrombus formation in guinea pigs and has a lower bleeding risk than has been observed with the GPIIb/IIIa blockers.13 In addition, thrombus inhibition was more efficient in arterial thrombosis than in venous thrombosis in hamsters, compatible with a shear stressdependent inhibitory action.14
In the present study, we evaluated the inhibitory effect of AJvW-2 on thrombus deposition onto the vessel wall and the subsequent neointima formation after balloon injury in guinea pigs by investigating the involvement of thrombus generation in neointima formation.
| Methods |
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Angioplasty
Hartley male guinea pigs, weighing 350 to 550 g, were
anesthetized with intraperitoneal sodium
pentobarbital (40 mg/kg), and the left common carotid artery was
exposed. A 2F Fogarty balloon catheter (Baxter) was inserted through
the external carotid artery and advanced into the thoracic aorta. The
balloon was inflated and advanced back and forth twice. The catheter
was withdrawn, and the external carotid artery was ligated. In the
present study, heparin (Novo Nordisk) was intravenously
administered to all animals via the ear vein at dose of 100 U/kg 30
minutes before angioplasty. In addition, either saline or AJvW-2 was
administered 20 minutes before angioplasty.
Fourteen days after balloon injury, the animals were anesthetized, and both common carotid arteries were removed, rinsed with saline, and fixed with 10% buffered formalin. Each artery was divided into 2 sections (5-mm length) and vertically embedded in the same paraffin block. Two consecutive cross sections (4-µm thickness) were cut at 1-mm intervals and stained with hematoxylin and eosin for the measurement of neointimal thickness and with elastic-Masson for the measurement of the organized thrombus area.
Immunohistochemistry
All procedures were performed according to the labeled
streptavidin biotin method. Formalin-fixed paraffin-embedded sections
were pretreated with 3%
H2O2. After treatment with
goat serum (Dako) for 20 minutes, sections were incubated with either
rabbit polyclonal antibody for vWF (Dako) or for fibrin(ogen) (Dako)
for 1 hour. After 3 washes in PBS, sections were incubated with
biotinylated goat anti-rabbit immunoglobulins (Dako) for 20 minutes. A
peroxidase-conjugated streptavidin was applied for 10 minutes, and
bound peroxidase was detected with diaminobenzidine as a
chromogenic substrate. Slides were counterstained with
hematoxylin. Antigen retrieval was needed by treatment with 0.1%
trypsin for vWF immunostaining. In addition, to avoid
nonspecific binding, biotinylated goat anti-rabbit immunoglobulin
(Dako) was adsorbed with normal guinea pig plasma at room temperature
before usage.
Quantification
All quantification was performed with a computerized image
graphic analyzer (Mac Scope). The areas of the intima, media,
and organized thrombus were measured, and neointimal
thickness was expressed as the ratio of intimal to medial area (I/M
ratio). Four sections per artery were evaluated, and the highest score
of I/M ratio was selected for each animal. The degree of organized
thrombus was expressed as the ratio of thrombus (red-stained region
with elastic-Masson) to the medial area (Th/M ratio). For the
measurement of vWF and fibrin(ogen) deposition, positively stained
areas and medial or intimal areas were measured in each segment, and
deposition was calculated by the following formula: [vWF- or
fibrin(ogen)-positive area]/(medial or intimal area)x100 (%). Two
sections per artery were evaluated (n=5 in each group).
Ex Vivo Platelet Aggregation
Hartley guinea pigs, weighing 300 to 500 g, were used.
Either saline or AJvW-2 (0.6 or 1.8 mg/kg) was
intravenously administered via the ear vein (n=4 each).
After 5 minutes and at 1, 2, and 3 days, blood was collected from the
aorta, anticoagulated with trisodium citrate (0.38% final
concentration), and centrifuged at 1100 rpm for 10 minutes to
obtain platelet-rich plasma and additionally at 2700 rpm for 10
minutes to obtain platelet-poor plasma. The platelet count was
measured with a Sysmex E-2000 (Toa Medical Electronics) and adjusted to
250 000/µL. Platelet aggregation was induced by ristocetin
(Sigma Chemical Co) at a final concentration of 2.0 mg/mL, and the
percentage of maximum light transmission was measured with an
aggregometer (NBS Hematracer 801, Niko Bioscience, Inc).
Cell Culture
SMC clones were isolated from the carotid arteries of each
guinea pig by an explant method (n=3). Isolated cells
(1x104 cells per well) from each clone were
seeded in 24-well dishes and cultured in DMEM containing 10% FBS at
37°C in a humidified atmosphere of 5% CO2 in
air. After 24 hours, antibody was added into the cell culture media.
Cells were detached with trypsin and counted with a hemocytometer every
day for 3 days.
Statistics
All data are expressed as mean±SEM. ANOVA followed by a Dunnett
test or unpaired t test was used for validation of the I/M
ratio and platelet aggregation. The Mann-Whitney U test
was used for vWF and fibrin(ogen) depositions. In addition, a Fisher
exact probability test was performed for the incidence of organized
thrombus, and a Spearman rank test was performed for the correlation
between the I/M ratio and Th/M ratio. A value of P<0.05 was
considered statistically significant.
| Results |
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Neointima Formation
Four cases of thrombus-occluded vessels were observed 14 days
after angioplasty and were discarded from the present study (2
cases in the control group and 2 cases in the 0.6 mg/kg AJvW-2treated
group). AJvW-2 dose-dependently prevented neointima
formation by a bolus injection 14 days after balloon injury (Figure 2
). I/M ratios were 0.83±0.11,
0.56±0.14, and 0.29±0.07 in groups treated with saline, 0.6 mg/kg
AJvW-2, and 1.8 mg/kg AJvW-2, respectively. A significant inhibition
was observed at 1.8 mg/kg (65% reduction), although the lower dose
(0.6 mg/kg) showed a trend toward inhibition (33% reduction).
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Correlation Between Organized Thrombus and Neointima
Formation
Figure 3A
shows that arteries were
completely occluded by thrombus 3 days after injury and that red blood
cells penetrated into the medial tear. Figure 3B
shows that
organized thrombi were incorporated into the neointima 14
days after injury. Organized thrombi were observed in 10 of 42 sections
in the control group. AJvW-2 markedly reduced the incidence of
organized thrombus formation (1 of 40 sections) at 1.8 mg/kg
(P<0.01). A significant positive correlation was observed
between the organized thrombus area (Th/M ratio) and the I/M ratio
(r2=0.465, P=0.025).
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In addition, we compared the inhibitory effect of 1.8 mg/kg AJvW-2 on neointima formation between the sections with and without organized thrombus. In the sections with organized thrombi, the I/M ratios were 0.54 in one animal treated with AJvW-2 (n=1) and 1.62±0.33 in the control group (n=10). Whereas in the sections without organized thrombi, the I/M ratios were 0.25±0.03 in the AJvW-2 group (n=39) and 0.44±0.04 in the control group (n=32, P<0.01). Although AJvW-2 prevented neointima formation in either case, the inhibition was more pronounced in the arteries with thrombus (67% versus 43% in the arteries without thrombus).
vWF and Fibrin(ogen) Deposition
For the purpose of evaluating the effect of AJvW-2 on initial
thrombus formation after balloon injury, immunohistochemistry was
performed with polyclonal antibodies against vWF and fibrin(ogen) as a
thrombus marker. Representative photomicrographs are
shown in Figure 4
and Figure
I (which can
be accessed online at www.ahajournals.org). In normal carotid arteries,
only endothelial cells were slightly immunoreactive for
vWF, and no immunoreactivity for fibrin(ogen) was observed (online
Figure
IA and IB). One day after injury, despite heparin treatment,
both vWF and fibrin(ogen) were deposited in the injured medial tears,
where no living cell was observed, coinciding with mural thrombi
(Figure 4A
and 4B
). The distribution of vWF was
consistent with that of fibrin(ogen) in the injured artery.
Deposition of both markers on day 1 was markedly decreased in the
AJvW-2treated group compared with the control group (Figure 4C
and 4D
). On day 3, deposition was still observed and slightly
organized (data not shown). On day 7, although less reactivity for both
adhesive proteins was observed in the media, the modest
neointima was prominently immunoreactive for vWF but not
for fibrin(ogen) (data not shown). On day 14, further thickening of
intima was observed, and vWF deposits were still present diffusely
or prominently in the luminal part of the intima, just beneath the
endothelial cells, whereas no immunoreactivity for
fibrin(ogen) was observed (online Figure
IC and ID).
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The summarized results are shown in the
Table
. Deposition in the media was
gradually decreased in the control group for both markers. vWF and
fibrin(ogen) deposition in the media was significantly inhibited by a
bolus injection of 1.8 mg/kg AJvW-2 (91.5% and 54.5% reduction,
respectively) on day 1, although no inhibition was observed on day 3.
On the contrary, the deposition of both markers in the intima was not
affected by an initial bolus injection of AJvW-2.
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SMC Proliferation
The direct effect of AJvW-2 on the proliferation of cultured SMCs
isolated from guinea pig carotid arteries was investigated. AJvW-2 did
not affect the cell growth up to 150 µg/mL, which was 3 times as high
as the maximum calculated plasma concentrations achieved by a bolus
injection of 1.8 mg/kg (2.75±0.58x104 versus
2.52±0.55x104 cells per well in the control
group on day 3). In addition, immunohistochemistry for proliferating
cell nuclear antigen showed that AJvW-2 (1.8 mg/kg) slightly reduced
the rate of proliferating SMCs in the media as well as the intima, but
not significantly (data not shown).
| Discussion |
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Various animal models have been established to investigate the inhibitory effect of pharmacological agents on neointima formation. Although numerous agents, such as heparin, statins, and angiotensin-converting enzyme inhibitors, have shown a significant inhibitory effect on neointima formation in rats, none of them was successful in clinical trials.15 16 17 18 SMC proliferation was suggested to contribute to neointima formation mainly in rats. However, other components, such as organized thrombi and extracellular matrix changes, were also observed in clinical restenotic lesions, pointing to the study limitation of a rat model of balloon injury. We chose guinea pigs for the present study because they are relatively thrombogenic animals compared with rats. In addition, they are small, easy to handle, and appropriate for the first evaluation of this antibody. Previous studies indicated that thrombosis after balloon injury was a frequent event (33% occlusive thrombi) in guinea pigs, although that was virtually never observed in rats.19 This difference between species might be attributed to the difference of prostacyclin production by the arterial wall.20 Actually, in our preliminary study, occlusive thrombi and large mural thrombi after balloon injury were observed in 77% and in 19% of injured animals, respectively. Therefore, we used heparin to prevent acute thrombotic occlusion. We also used it because heparin has been routinely used in clinical practice during PTCA procedures.
In this model, some organized thrombi were detected in severely
stenosed vessel walls on day 14, which positively correlated with
neointimal thickness. In addition, vWF and fibrin(ogen)
depositions were observed in the injured medial tears during the early
phase. Both adhesive proteins may function as a mediator of
platelet-platelet or platelet-vascular interactions and may
also be incorporated into
-granules in platelets. These results
suggest that immediately after balloon injury, platelet-mediated
thrombus was deposited into the injured medial tear. Thereafter, the
mural thrombi organized and resulted in the severe luminal narrowing
that was due to SMC migration and proliferation in the presence of
organized thrombi. Mural thrombi may contribute to
neointima formation not only through the growth factors or
cytokines released from activated platelets but
also by functioning as a scaffold for SMC migration and
proliferation.
Our findings were supported by the following previous reports: Recchia et al6 showed that the degree of luminal stenosis was greater in the presence of lesions containing thrombus than in those without thrombus in hypercholesterolemic pigs. Bosmans et al21 suggested that fibrin(ogen) and vWF depositions were associated with intimal thickening after balloon angioplasty of the rabbit carotid artery. Furthermore, there was a clinical observation indicating that the presence of angiographically identifiable thrombus at the time of the angioplasty procedure was associated with the higher rate of restenosis.22
AJvW-2, a murine monoclonal antibody against human vWF, prevented neointima formation 14 days after balloon injury. Significant decreases in vWF and fibrin(ogen) deposition in the injured media were observed on day 1 in the AJvW-2 group. In addition, AJvW-2 markedly decreased the incidence of organized thrombus formation. AJvW-2 did not affect the proliferation of cultured SMCs or the rate of proliferating cell nuclear antigenpositive SMCs after injury. These results suggest that AJvW-2 prevented neointima formation mainly because of the inhibition of initial platelet-mediated thrombus formation, leading to PDGF release and fibrin formation, which induced SMC migration.
Prominent deposits of vWF, but not fibrin(ogen), in the intima were also observed at 7 and 14 days after injury in the present study. Previous reports have suggested that vWF deposition is associated with neointima formation after balloon injury or collar placement in normocholesterolemic rabbits as well as with atherosclerotic plaque formation in cholesterol-fed rabbits.21 23 Although the role of vWF deposition in the intima remains unclear, the presence of vWF was not affected by the initial injection of AJvW-2.
Previous studies have demonstrated that the GPIb-vWF axis might be involved in neointima formation in experimental animal models. The recombinant vWF A1 domain fragment (VCL) prevented neointima formation in rats by an intravenous infusion for 72 hours.24 In addition, a fractionated aurintricarboxylic acid prevented neointima formation in hamsters by an intravenous infusion for 7 days.25 However, aurintricarboxylic acid is a multimeric molecular mixture that binds to vWF and has also been reported to prolong the coagulation parameters,26 thus complicating the understanding of its effect on neointima formation. In addition, VCL is an antagonist of GPIb but not of vWF, and it is an unstable molecule. In the present study, we showed that the specific and stable blockade of the GPIb-vWF axis with AJvW-2 led to the prevention of neointima formation with no changes in platelet count, prothrombin time, activated partial thromboplastin time, and plasma vWF antigen level (data not shown).
The significant inhibition of neointima formation was observed with a single bolus injection of AJvW-2, which would be a great advantage in clinical application. In the present study, the significant inhibition of platelet aggregation persisted for 2 days by injection of the effective dose (1.8 mg/kg). These results suggest that the blockade of the GPIb-vWF axis might be required for at least 2 days after balloon injury to show a significant inhibition of neointima formation in guinea pigs. No inhibitory effects on the deposition of vWF and fibrin(ogen) were observed on day 3, when the antiplatelet effect of AJvW-2 disappeared, and a slight increase was observed in only the AJvW-2 group. These results suggest that a more potent inhibitory effect on neointima formation would be expected if a longer duration of the antithrombotic effect of AJvW-2 is achieved.
However, the effective dose (1.8 mg/kg) in the present study is already very high, because AJvW-2 has previously been shown to produce a marked inhibition against acute thrombosis at 0.3 mg/kg in guinea pigs.13 Because AJvW-2 might be rapidly cleared (eg, as a result of its immunogenicity for guinea pigs), an excessive bolus dose might be required to maintain an effective plasma concentration for 2 days. In another study, the repetitive administration of 0.6 mg/kg for an initial 4 days (from 1 day before to 2 days after injury) also significantly prevented neointima formation (data not shown).
In conclusion, the anti-vWF monoclonal antibody AJvW-2 prevented neointima formation by a single bolus injection in guinea pigs. This inhibitory action might be due to the inhibition of initial thrombus deposition (thus avoiding thrombus organization and severe luminal narrowing) rather than the direct inhibition of SMC proliferation. These results suggest that AJvW-2 may be a therapeutic agent for the prevention of acute thrombosis as well as restenosis in clinical practice.
Received April 19, 2000; accepted June 27, 2000.
| References |
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