Original Contributions |
From the Divisions of Endocrinology and Metabolism (T.O'B.) and of Cardiovascular Disease (I.K.), the Division of Vascular Surgery (G.M., T.M., P.G., S.M.), and the Division of Anatomic Pathology (T.B.C.), The Mayo Clinic, Rochester, Minn; The Gladstone Institute of Cardiovascular Disease, University of California, San Francisco (J.T.); and the Department of Microbiology and Immunology, Pennsylvania State University, Hershey, Pa (D.S.).
Correspondence to Timothy O'Brien, MD, Division of Endocrinology, Mayo Clinic, 200 First St SW, Rochester, MN 55905. E-mail Obrien.timothy{at}mayo.edu
| Abstract |
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Key Words: macrophage colony stimulating factor monocytes atherosclerosis gene transfer smooth muscle cells
| Introduction |
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MCSF, an 85-kDa protein that is produced by cells of monocyte-macrophage lineage, endothelial cells, and fibroblasts, is known to stimulate the differentiation and proliferation of monocytic progenitor cells and may play a role in the differentiation of monocytes to macrophages in the arterial wall.7 MCSF activates various functions of mature macrophages through a specific receptor encoded by proto-oncogene c-fms.8 SMCs also express c-fms6 and thus, MCSF may also be involved in the migration and proliferation of these cells in atherosclerosis.
MCSF is expressed in atherosclerotic but not normal human arteries.9 Furthermore, MCSF deficiency results in significantly reduced atherogenesis in mouse models of atherosclerosis.10 The role of MCSF in atherogenesis may be addressed by overexpressing MCSF in the arterial wall by using gene transfer techniques. Recently, adenoviral vectors have been used to efficiently transduce cells of the vasculature.11 In this study, the effect of adenovirus-mediated gene transfer of the human MCSF gene to the rabbit carotid artery is examined.
| Methods |
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Generation of Recombinant Adenoviruses
In brief, cDNA for human MCSF or human apoE3 was subcloned into
pMAL123 (kindly provided by Dr Jeff O'Brian, Dupont Merck
Pharmaceuticals, Glenolden, Pa), which contains 358 bp of the left end
of adenovirus type 5, 241 bp of the major late promoter, 172 bp from
the tripartite leader sequence, a cloning polylinker, a simian virus 40
polyadenylation site (470 bp), and adenovirus DNA sequences 3329 to
5788 (Figure 1A
). pMAL123 is a
modification of pMCV212 obtained by inserting 3
linkers at EcoR1, Xho1-Bgl2, and
Bgl2 sites. The resulting plasmid was linearized and
cotransfected with Xba1-restricted adenovirus strain dl309
DNA into 293 cells by conventional calcium phosphate precipitation.
Recombinant vectors were generated by homologous recombination (Figure 1B
). Plaques containing recombinant adenovirus vectors were picked, and
viral DNA was isolated by a modified Hirt
extraction.13 Plaques were screened by
restriction mapping, and the virus was amplified in 293 cells from
plaques that contained inserts of the appropriate size. Positive
plaques underwent 2 more rounds of plaque purification. Plaques were
again picked and screened by restriction endonuclease mapping. The
resulting recombinant adenovirus vectors were expanded in 293 cells and
purified by double cesium-gradient ultracentrifugation
as previously described.14 The isolated viral
band was dialyzed against 140 mmol/L NaCl, 10 mmol/L HEPES
(pH 7.2), and 1 mmol/L MgCl2 and stored at
-70°C in 10% glycerol. The viral titer was determined by plaque
assay. Cells (293 and human umbilical vein endothelial)
transduced with AdMCSF expressed human MCSF as detected by RT-PCR (data
not shown). The adenoviral vectors encoding MCSF and apoE were under
the control of the major late promoter. An adenoviral vector encoding
ß-galactosidase under the control of the cytomegalovirus promoter was
used as a further control and as a means to examine the distribution of
transgene expression (AdßGal was a kind gift of Dr James M. Wilson,
University of Pennsylvania, Philadelphia).
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In Vivo Gene Transfer
All experimental protocols were approved by the Institutional
Animal Care and Use Committee and were performed in accordance with the
recommendations of the American Association for the Accreditation of
Laboratory Animal Care. Male New Zealand White rabbits weighing 3.0 to
3.5 kg were anesthetized with intramuscular ketamine
(68 mg/kg), xylazine (Rompun, 9 mg/kg), and acepromazine (2.3 mg/kg).
After bilateral paramedian incisions on the anterior neck, the common
carotid arteries were exposed by using atraumatic surgical technique.
Side branches, 5 to 8 mm away from their origin, were coagulated
or tied off with 50 silk. The adventitia was cleaned off at the
proximal and distal ends of the exposed segment. After the
administration of heparin (100 U/kg) and placement of a purse-string
monofilament suture (80 ethilon) at the proximal end of the isolated
segment, the common carotid arteries were clamped proximally and
distally and a 24-gauge catheter was inserted proximally through the
purse string. The blood was removed by gently massaging the artery with
a wet cotton swab and a gauze wick at the open end of the catheter. One
hundred microliters of the adenoviral vectorcontaining solution
(2x1010 plaque-forming units per milliliter) or
the same volume of diluent alone (PBS with 0.5% BSA) was instilled
into the vessel lumen, the catheter was removed, and the purse-string
suture tied to close the defect in the arterial wall.
Twenty minutes later, flow was restored by removing the clamps. The
skin incisions were closed in 2 layers and the animal allowed to
recover. Five or 21 days later, the animals were anesthetized
and the transduced carotid arteries were excised. The sites of
arterial clamping and cannulation were not harvested.
Seventeen animals were used for the 5-day experiments. Twelve animals
were transduced with AdMCSF on 1 side. Ten of these were transduced
with AdApoE and 2 with AdßGal in the contralateral carotid artery. In
5 additional animals, AdßGal was used on 1 side and PBS with 0.5%
BSA on the other. Five animals were used for the 21-day studies.
Evaluation of Transgene Expression
Detection of ß-Galactosidase Activity
Vessels harvested for 5-bromo-4-chloro-3-indolyl
ß-D-galactopyranoside (X-Gal) staining were fixed with
2% paraformaldehyde and 0.2%
glutaraldehyde in PBS, pH 7.4, for 30 minutes. Vessels
were immersed in X-Gal reagent (Boehringer Mannheim Corp) for
90 minutes. Samples were rinsed with PBS, photographed, placed in 10%
buffered formalin, and embedded in paraffin by using standard
histological techniques. Five-micron-thick sections
taken at multiple levels through the block were lightly stained with
eosin only and examined by light microscopy.
Detection of MCSF Expression by RT-PCR
Recombinant MCSF expression was analyzed by RT-PCR. In 2
animals, total cellular RNA was isolated from arterial
segments with RNeasy as per the manufacturer's instructions. After
pretreatment with amplification-grade DNAse 1 (GIBCO BRL), RNA was
reverse transcribed using SuperScript II reverse transcriptase (GIBCO)
and random hexamers (GIBCO). cDNA was amplified in a thermal cycler
with human MCSFspecific primers (sense,
5'-GAA-CAG-TTG-AAA-GAT-CCA-GTG-3'; antisense,
5'-TCG-GAC-GCA-GGC-CTT-GTC-ATG-3') and the following
parameters: 1 minute at 95°C, 1 minute at 55°C, and 1.5
minutes at 72°C, for 40 cycles. For RT-PCR, samples were
analyzed in the presence or absence of reverse
transcriptase.
Detection of MCSF by Immunofluorescence
From 2 animals, fresh segments of artery were snap-frozen in
LN2 and OCT and stored at -70°C.
Five-micron-thick sections were fixed with acetone for 10 minutes.
After nonspecific antibodybinding was blocked with 5% BSA, the
tissue sections were incubated with a monoclonal mouse anti-MCSF
antibody (Genetics Institute HM7/7.7.10, diluted 1:100 in 1% BSA-PBS)
for 60 minutes. TRITC-conjugated anti-mouse IgG (T2402, Sigma Chemical
Co, diluted 1:64) was then applied for 20 minutes. Slides were mounted
with PBS-glycerol (1:1, vol/vol) and examined with a
fluorescence microscope (Nikon).
Morphometric Analysis of Intimal Infiltrates
Carotid arteries transduced with AdMCSF, AdApoE, AdßGal, or
PBS-A were analyzed for the presence of an intimal infiltrate 5
days after transduction (n=5 for each). In addition, AdMCSF- and
AdßGal-transduced vessels were analyzed 21 days after
transduction (n=5). Common carotid arteries were perfusion fixed in
situ with 2% paraformaldehyde for 5 minutes at 80
mm Hg and harvested. Segments were snap-frozen in OCT. Fifteen
5-µm-thick sections were cut at intervals of 100 µm, postfixed
in acetone for 10 minutes, and stained with hematoxylin-eosin. The
thickness of the intima and the underlying media was measured by light
microscopy and computer-assisted morphometry (DIGICEL, ASK). Intimal
thickness as a percentage of medial thickness was calculated.
Immunohistologic Analysis of Intimal Infiltrates
Five-micron-thick frozen sections were fixed with acetone for 10
minutes. Endogenous peroxidase was blocked with 0.1%
NaN3 and 3%
H2O2 for 10 minutes.
Nonspecific antibody binding was blocked with 5% normal goat serum and
0.05% Tween 20 for 10 minutes. Primary antibodies were diluted (1:50,
vol/vol) in 1% normal goat serum0.05% Tween and used for 30
minutes. RAM11 (Dako) and SMC
-actin (Dako) primary antibodies were
used. Biotinylated rabbit anti-mouse IgG F(ab')2
(E0413, Dako) (1:300, vol/vol) for 20 minutes, peroxidase-labeled
streptavidin (P0397, Dako) (1:300, vol/vol) for 20 minutes,
and 0.4% of 3-amino-9-ethylcarbazole (A6424, Sigma) substrate solution
were sequentially applied. Slides were counterstained with hematoxylin,
mounted with glycerol gelatin (Sigma), and examined with a light
microscope.
Statistical Analysis
Statistical analysis was performed by Kruskal-Wallis
ANOVA using the Newman-Keuls test. A value of P<0.05 was
considered statistically significant.
| Results |
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Detection of ß-Galactosidase in AdßGal-Transduced Carotid
Arteries
To characterize the distribution of adenovirus-mediated gene
expression in the rabbit carotid artery in vivo, an adenoviral vector
encoding ß-galactosidase was used. Efficient gene transfer limited to
the endothelium was evident 5 days after transduction
(Figure 2
).
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Detection of MCSF Expression In Vivo by RT-PCR
Expression of recombinant MCSF was detected by RT-PCR 5 days after
transduction. Recombinant MCSF was detected in AdMCSF- but not in
AdApoE-transduced vessels (Figure 3
). The
reaction was negative in the absence of reverse transcriptase. MCSF
expression was also detected at 21 days in AdMCSF- but not in
AdßGal-transduced vessels (data not shown).
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Detection of Recombinant Protein Expression In Vivo by
Immunohistochemistry
Expression of recombinant MCSF protein was analyzed in
transduced arterial segments by
immunofluorescence using a monoclonal antibody to
MCSF. MCSF was not detected in AdApoE-transduced vessels. In contrast,
AdMCSF-transduced vessels demonstrated immunoreactive protein in the
endothelium (Figure 4
).
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Morphometric Analysis of Intimal Infiltrates
To evaluate the cellular response of the arterial wall
to expression of MCSF, artery segments transduced with
AdMCSF, AdßGal, AdApoE, or diluent alone were examined at 5
days by light microscopy. An intimal cellular infiltrate was
present in AdMCSF- but not in AdßGal- (Figure 5
), AdApoE- or sham-transduced
vessels (n=5). The mean intima to media percentage was significantly
greater in AdMCSF-transduced carotid arteries than in any of the three
negative controls (Figure 6
)
(P<0.05 versus controls). In AdMCSF-transduced vessels, a
cellular intimal infiltrate with an intima to media percentage of
19.9±6.1 was observed. In arteries transduced with AdßGal, AdApoE,
or diluent alone there was no intimal infiltrate or only a minimal,
characteristically 1-cell-layer-thick infiltrate, with intima to media
percentages of 3.69±1.7, 0.4±0.3, and 0.7±0.6, respectively (Figure 6
). In addition, AdMCSF- and AdßGal-transduced carotid arteries were
examined 21 days after transduction. At this time, the intima to media
percentage was still greater in AdMCSF-transduced vessels, although
this difference was no longer statistically significant (15.89±12.4%
versus 6.04±3.4%, P=NS) (Figure 6
).
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Immunohistologic Analysis of Intimal Infiltrates
Immunohistochemical staining for rabbit macrophages with
RAM11 antibody demonstrated that the cellular infiltrates in
AdMCSF-transduced vessels contained small numbers of rabbit
macrophages (Figure 7
).
Immunohistochemical staining for SMCs with an
-actin antibody
demonstrated that the cellular infiltrates were composed predominantly
of SMCs (Figure 8
). In contrast, intimal
staining for
-actin was not present in control vessels.
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| Discussion |
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Adenoviral vectors are capable of mediating efficient gene transfer to the vascular wall in vivo.10 In the current study, an adenoviral vector encoding human MCSF was generated. In vivo experiments involved luminal administration of the vector to an isolated segment of the rabbit carotid artery. Previous experiments in the rat model have demonstrated that this mode of gene delivery results in endothelium-specific gene transfer.15 In our study, we found recombinant protein expression in the endothelium of AdMCSF- and AdßGal-transduced vessels, suggesting that luminal administration of adenoviral vectors may be used to obtain gene transfer to the endothelium of the rabbit carotid artery.
The MCSF vector used in this study was generated by standard techniques of homologous recombination in 293 cells as previously described.16 In this vector, transgene expression is driven by the major late promoter. For analysis of the effect of transgene expression at 5 days, one set of control vessels was transduced by an adenoviral vector encoding the human apoE gene, which, like the MCSF vector, was also under the control of the major late promoter. Another set of control vessels was transduced by an adenoviral vector encoding ß-galactosidase. In contrast to AdMCSF and AdApoE, transgene expression in AdßGal was driven by the cytomegalovirus promoter. This vector is used to identify the efficiency of gene transfer and the cellular distribution of transgene expression. As a final control, vessels were transduced by diluent alone. We thus had a series of controls for the vector itself, the promoter used, and the effect of surgery per se.
One major difficulty associated with the use of adenovirus-mediated gene transfer to study the effects of vascular wall gene expression is vector-induced cytotoxicity. Newman et al17 have described prolonged vascular cell activation, inflammation, and neointimal hyperplasia as a result of adenoviral transduction in the rabbit iliofemoral artery. These finding were minimal at 3 days and extensive at 10 and 30 days after viral transduction. Thus, interpretation of the effects of adenovirus-mediated gene transfer on vessel wall morphology must be interpreted with caution. In the current experiment, an intimal infiltrate was demonstrated 5 days after transduction of the rabbit carotid artery with AdMCSF. We chose 5 days in these experiments because the vessel wall changes in the study by Newman et al were not present until 10 days after transduction. To exclude the possibility that surgery or the adenoviral vector was inducing the cellular infiltrate, vessels were harvested from animals that had undergone sham surgery or whose vessels had been transduced with AdßGal or AdApoE. A minimal infiltrate was observed in a small number of sections from vessels harvested 5 days after viral transduction. Thus, 3 sets of controls yielded minimal evidence of intimal infiltrate 5 days after transduction.
The observation that the intimal infiltrate associated with MCSF overexpression was composed predominantly of SMCs was not expected. It is of interest, however, that MCSF exerts profound effects on recruitment and terminal differentiation of pluripotent cells in other systems.18 19 MCSF is known to induce proliferation of intimal but not medial SMCs.6 The former are known to express the MCSF receptor c-fms. Induction of c-fms expression in vascular SMCs by a number of growth factors, including platelet-derived growth factor, epidermal growth factor, and basic fibroblast growth factor, has been demonstrated.20
Our results demonstrate that MCSF expression in the endothelium leads to intimal accumulation of predominantly SMCs and suggests that MCSF expression in the vessel wall may result in infiltration of the intima by both macrophages and SMCs. The absence of a cellular infiltrate in the intima of control vessels suggests that the infiltrate observed in AdMCSF-transduced vessels was due to MCSF overexpression and was not vector induced or the result of surgery per se.
Transduced rabbit carotid arteries were also examined 21 days after transduction. In this series of experiments, vessels were transduced with AdMCSF or AdßGal. Interestingly, as with the 5-day animals, extensive intimal infiltration was not observed in the AdßGal-transduced vessels at this time. A minimal infiltrate not more than 1 cell thick was observed in some sections. In contrast to the observation at 5 days, the intima to media thickness ratio was not significantly different in the AdMCSF- and AdßGal-transduced carotid arteries at 21 days. Therefore, the extent of the intimal infiltrate did not appear to increase with time. Although MCSF expression was still detectable by RT-PCR at 21 days after transduction, it should be noted that adenovirus-mediated transgene expression has been reported to markedly diminish with time after gene transfer. Therefore, the results of the time-course experiment suggest that expression of MCSF in the rabbit carotid artery resulted in an intimal infiltrate that did not increase with time. This may be due to the fact that adenovirus-mediated gene transfer results in transient transgene expression.
In summary, we have described the generation of an adenoviral vector encoding MCSF. Vascular wall cells were successfully transduced with this vector in vivo. Adenovirus-mediated gene transfer of MCSF to the rabbit carotid artery in vivo resulted in recombinant MCSF expression and a cellular infiltrate composed of SMCs and small numbers of macrophages. These findings suggest that adenovirus-mediated transfer of the MCSF gene to the vessel wall may result in monocyte recruitment and the induction of SMC migration and proliferation. However, the role of the current generation of vectors in studying the role of MCSF in vascular wall pathology may be limited by the inflammatory response elicited by the vector at later times. Therefore, before further studies of this issue are undertaken, improvements in adenoviral vector technology will be necessary.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received January 28, 1997; accepted February 6, 1998.
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