Vascular Biology |
From the Institute for Biomedical Aging Research, Austrian Academy of Sciences (G.M., G.W.), and the Institute for General and Experimental Pathology (G.M., H.N., G.W.), the Department of Forensic Medicine (W.R.), the Department of Transplant Surgery (B.W.H.), the Department of Cardiac Surgery (D.H.), and the Department of Dermatology (N.R.), University of Innsbruck, Innsbruck, Austria.
Correspondence to Georg Wick, MD, Institute for General and Experimental Pathology, Fritz-Pregl-Strasse 3, 6020 Innsbruck, Austria. E-mail Georg.Wick{at}uibk.ac.at
| Abstract |
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Key Words: atherosclerosis intima arteries immunofluorescence dendritic cells
| Introduction |
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/ß
T cell receptor and a considerable number expressing
/
T
cell receptor as well), macrophages, dendritic cells (DCs),
some scattered mast cells, and a very few natural killer cells or B
cells.7 In analogy to the
mucosa-associated lymphoid tissue, we tentatively called these
accumulations of mononuclear cells in healthy children
"vascular-associated lymphoid tissue" and assumed a similar
function for these as a system of local defense of the vascular
system.8 During the last decade, great progress has been made in the field of DC research.9 Because it is now generally accepted that the immune system plays an important role in atherogenesis, it was deemed of interest to perform a critical study on the occurrence and distribution of DCs in the vascular system with special emphasis on the sites of the newly discovered vascular-associated lymphoid tissue.
One of the typical features of DCs is their presence at the borderline of the body to its environment. At these locations, they are present in an immature stage, characterized by high endocytotic activity and low T-cell stimulatory potential because of the lack of costimulatory molecules, such as CD40, CD54, and CD86.10 Thus, DCs are present in the skin,11 in the intestine (mainly in the Peyers patches),12 and in the respiratory tract.13 But only recently have there been suggestions that atherosclerotic lesions may also be populated by DCs.14 15 16 DCs in the atherosclerotic vessel wall express human lymphocyte antigen-DR, CD1a, and S-100 protein14 16 and are positive for intercellular adhesion molecule-117 and vascular cell adhesion molecule-1.18
Previous studies investigating vascular DCs concentrated only on atherosclerotically altered arteries, but there were no data available concerning the presence of DCs in the healthy intima, as we had demonstrated to be the case in earlier preliminary work.7 8
We have now focused our attention on the exact distribution and abundance of the vascular-associated DCs.
| Methods |
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The animals were euthanized under ketamine (25 mg/kg) and xylazine (5 to 10 mg/kg) anesthesia by heart puncture. The aortas were carefully removed from the surrounding tissue from the beginning of the aortic arch to the bifurcation into the 2 iliac arteries. Then, they were immersed in ice-cold PBS, pH 7.2, and immediately processed for further immunohistochemical or immunofluorescence studies.
Humans
Human arterial samples from 14 infants
and children aged 8 months to 16 years were obtained from the
Department of Forensic Medicine, University of Innsbruck, Medical
School. Causes of death were sudden infant death syndrome,
polytraumata, intoxication, drowning, pulmonary
embolism, and aspiration pneumonia.
For investigations on vascular DCs in adults, we used human arterial samples from the carotid arteries, aortas, and iliac arteries of 20 young adults between 17 and 34 years of age, whose death was due to suicide, homicide, or accident (obtained from the Department of Forensic Medicine, University of Innsbruck, Medical School) or who were candidates for organ explantation (obtained from the Department of Transplant Surgery, University of Innsbruck, Medical School) and did not suffer from any clinical manifestation of cardiovascular disease.
In 5 of the above-mentioned cases, veins were collected in addition to the arteries (either the jugular vein or the iliac vein as a pendant to the respective arterial sample).
The samples for frozen sections were collected, immersed in transport buffer as described by Michel et al,19 and transported to the laboratory on ice. Samples for the preparation of intimal sheets were immersed in ice-cold PBS instead of transport buffer. The surrounding connective tissue was removed, and each artery was filled with freezing medium (O.C.T. Tissue Tek, Miles Inc, Diagnostic Division), shock-frozen, and stored in liquid nitrogen until being cut into 4-µm-thick frozen sections.
Staining Procedures
Immunohistochemistry on Frozen Sections
Slides with frozen sections were air-dried for 30 to
60 minutes at room temperature. Blocking of nonspecific antibody
binding was achieved by incubation with 10% normal human serum (heat
inactivated at 56°C for 30 minutes) in blocking reagent
(No. 1096176, Boehringer-Mannheim) for 15 minutes. Excess serum
was blotted off; the primary antibody was diluted in Tris-buffered
saline (TBS), pH 7.4, and applied directly without any washing
procedure; and the sections were incubated for 30 minutes. Optimal
dilutions for all antibodies were determined in pilot studies.
Incubation took place in a humidified chamber at room temperature. The
sections were then rinsed 3 times in TBS, the secondary antibody was
applied, and the sections were incubated for another 30 minutes. They
were rinsed as described above, and incubation with the alkaline
phosphatase/antialkaline phosphatase (APAAP) complex followed at room
temperature for 30 minutes. The incubation step with the secondary
antibody and the APAAP complex was repeated to increase the staining
intensity. After 3 further changes of TBS, visualization with Fast Red
Naphthol (Sigma) and counterstaining with Mayers hemalaun was
performed. Slides were finally mounted in Kaysers glycerol gelatin
(Merck). The Table
shows the source and dilution of the antibodies
used.
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Double Staining on Frozen Sections
Double staining was achieved by combination of the
APAAP technique and the DAKO animal research kit. Immunohistochemistry
was performed as described above for single staining. After
visualization with Fast Red, the second antibody was treated with the
biotinylation reagent according to the manufacturers instructions and
developed with horseradish peroxidaselabeled streptavidin contained
in the test kit.
En Face Immunofluorescence
on Human or Rabbit Intimal Sheets
Fresh arterial samples were washed in
cold PBS to remove blood. They were opened longitudinally, cut into 2
halves, and dissected into small pieces of
0.7x0.7 cm, followed by
incubation in 0.5 mol/L NH4SCN (Merck) at 37°C
for 60 minutes.20 After a
wash in PBS at room temperature, the intima was gently lifted off from
the rest of the vessel wall with delicate forceps. After they were
rinsed in PBS for 30 minutes, the samples were ready to undergo the
staining procedure.
Sheets were acetone-fixed for 10 minutes at room temperature (with the exception of those that were to be stained for CD1a) and rinsed in PBS for 20 minutes thereafter. Then incubation in the optimally diluted primary antibody took place overnight at 4°C. Washing was performed in PBS/1% BSA (Sigma) for 4 hours at room temperature on a multiaxle rotator, and the sheet was incubated in diluted FITC-labeled secondary antibody for the next 4 hours at room temperature. Finally, the sheets were washed in PBS/1% BSA on the rotator at room temperature overnight and mounted the following morning. Incubation in NH4SCN, acetone fixation, and incubation with antibodies were performed in microtest tubes (No. 0030120.086, Eppendorf-Netherler-Hinz), whereas the washing took place in 50-mL tubes (No. 252070, Falcon, Becton-Dickinson Labware) to guarantee excess washing fluid. The specimens were examined with a the laser scanning confocal fluorescence microscope (Zeiss), with magnifications ranging between 100- and 630-fold.
Negative Controls
For all experiments, negative controls were carried
out either by omitting the first antibody or by using an irrelevant
isotype-matched antibody.
Photographic Documentation
Stained sections were examined by light microscopy
(Optiphon-2, Nikon), and pictures were taken on the same microscope
with use of automatic exposure equipment (UFX-DX, Nikon) and daylight
films (Fujichrome Velvia-50).
Intimal sheets were analyzed by confocal microscopy, and pictures were stored and printed on a laser printer.
| Results |
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CD1a+ Cells Are
Present in Intima of Children
In children, DCs were found in all arterial
specimens of the intima in the subendothelial location,
showing the typical morphological features of DCs with long cytoplasmic
processes. They are found in higher density in areas of bifurcation and
more sparsely distributed in regions without a deviating flow pattern.
The best staining impressions were obtained in tangential sections of
the intima, displaying the intimal area more broadly
(Figure 1
).
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After demonstration of the presence of DCs in the intima of
healthy children by staining for CD1a, further analysis
concerning the characterization of the vascular-associated DCs was
performed to determine the subset of DCs. We stained for CD86 (B7.2) as
a marker for mature DCs, CD83, S-100
(Figure 2
), lag (a marker for Birbeck granules that are specific for
Langerhans cells), and CD31. von Willebrand factor as an
endothelial cellspecific molecule not expressed on
any subset of DCs was included as a
control.
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To exclude a type of cell showing smooth muscle cell21 or macrophage phenotype with additionally acquired DC markers, we also stained for smooth muscle cell myosin and CD68, respectively, in double-staining experiments or on serial sections. These experiments showed that vascular-associated DCs have the following phenotype: CD1a+ S-100+ lag+ CD31- CD68- CD83- CD86-, and they are negative for von Willebrand factor and smooth muscle cell myosin. This pattern of marker expression is similar to that of immature Langerhans cells in the epidermis.
En Face Immunofluorescence
on Intimal Sheets
Major Histocompatibility Complex Class
II+ Cells With DC Morphology Are Found
Abundantly in Intima of Normal Rabbits
Rabbit intimal sheets were analyzed by
immunofluorescence for Ia expression.
Ia+ cells were found spread over the intima
in all stained specimens. Considering the shapes of their cell bodies
and their long processes, they were primarily recognized as either
macrophages or DCs
(Figures 3
and 4
), and they formed a network in areas of
dense accumulation, particularly areas of
Ia+ cell accumulation, such as those parts
of the rabbit aorta at which arteries branched off. Ostia of branching
arteries were not surrounded by stained cells, but they were
concentrated at those parts of the branch at which the mechanical drag
is most severe
(Figure 5
). Optical sections in the laser scanning microscope
showed that all Ia+ cells were lying under
the endothelium and that none was adherent to the
endothelium. Endothelial cells did not
stain for Ia.
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Closer scrutiny showed that Ia+ cells were orientated longitudinally within the aorta except at those sites where turbulent predominates over laminar blood flow.
CD1a+ Cells Are
Found in Human Intima in Amounts Equivalent to Those in
Ia+ Cells in the Rabbit
Because no specific marker exists for rabbit DCs,
identification of these cells was performed on human
arterial intimal sheets from individuals without
macroscopic atherosclerotic lesions. CD1a+
cells were found in most parts of the arterial specimens
examined. As already shown in the rabbit by expression of major
histocompatibility complex (MHC) class II,
CD1a+ DCs are not distributed regularly, but
they aggregate in hemodynamically stressed areas, where
they form a network-like structure similar to Langerhans cells in the
skin, extending their processes in all directions
(Figure 6
).
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A very striking finding was that DCs do not exist in veins that can therefore be considered to be an integrated negative control for the staining procedures.
| Discussion |
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The highest density of vascular DCs was observed at areas in which turbulence predominates over laminar hemodynamics. This is true for human arteries as well as arteries from rabbits, which are a generally accepted experimental animal model for atherosclerosis research into the role of lipids as well as immunological processes. The location of vascular DCs corresponds exactly to the sites prone to the later development of atherosclerotic lesions. Turbulent flow results in complex secondary flows with flow reversal and dynamic stagnation points, resulting in prolonged endothelial resident times for large atherogenic particles (eg, LDLs) or blood cells. An increased contact time between leukocytes and endothelial cells could lead to increased transendothelial migration and, therefore, explains the higher density of all different cell types, including DCs, at these sites.
Inasmuch as our experimental rabbits were normocholesterolemic, these results emphasize that predisposed areas can be recognized from their cellular makeup even without any lipid deposition.
The absence of vascular DCs in veins could be another explanation (besides the difference in blood pressure) of why we develop arteriosclerosis and not venosclerosis. Another striking finding from the present study concerns the localization of DCs in the arterial intima. They are orientated longitudinally with the blood stream in areas of laminar flow conditions, but they change their distribution pattern in areas of turbulent flow. So far, we can only speculate that extracellular matrix proteins in the intima are involved in this process. Further studies on intimal extracellular matrix patterns will show whether they are distributed differentially in areas subjected to different hemodynamic stress.
Surprisingly, en face staining for DCs revealed a network formed by vascular DCs similar to that seen with Langerhans cells in the skin, and by this technique, we were able to show that the vessel wall seems to be protected from blood-borne pathogens in the same manner as other surfaces of the body in close contact with the environment. The fact that the vascular wall is a huge surface and a possible site of antigen processing and presentation and our present demonstration of DCs in the arterial intima open new perspectives in this respect. This prospect is certainly due to the general belief that blood is a sterile fluid not requiring immunosurveillance, because possible noxious substances have already passed the gate control of the skin or the mucosa. However, a closer look shows that even in the blood, invasions of bacteria can occur. Bacteremia not only occurs during septic diseases but also is an intermittent physiological condition in healthy organs, triggered by minitraumata (eg, tooth brushing),24 turning pathological only when it lasts longer than a few hours.
This amount of professional antigen-presenting cells in the arterial intima opens new perspectives to immune reactions taking place in the vessel wall itself.
To date, it has been speculated that initialization of the
immune response leading to development of
atherosclerosis might take place in the para-aortic
lymph nodes, but our studies implicate local initiation of the priming
of naive lymphocytes. Macrophages, which are always found in
atherosclerotic lesions, were proposed as possible candidates because
they are highly positive for MHC class II
molecules.25
Endothelial cells are not able to present antigens
in the normal unaffected intima because they do not express MHC class
II molecules constitutively. Endothelial cells are MHC
II positive essentially only in those areas in which activated
T cells have accumulated subendothelially but not in
the normal intima.26 This
phenomenon can be explained by the release of interferon-
from
infiltrating T
cells.27 28 29
For this reason, endothelial cells can be excluded as
initiators of an immune reaction by presenting antigen to
lymphocytes, whereas they could play a role in the promotion of immune
reactions and in the activation of memory T cells. We now disclose the
possibility that resident intimal DCs may be effective as local
antigen-presenting cells and could perhaps also initiate
(auto)immune processes in the vessel wall. Their
subendothelial position is very advantageous to this
scenario, inasmuch as the DCs can come in close contact with
blood-borne antigens as well as with lymphocytes that adhere to the
endothelial layer and migrate through the vessel wall.
However, whether arteries need be of certain size to be populated by
DCs or whether DCs are present even in arterioles remains to be
investigated. The arterial surface is a huge area when
calculated for the whole body; therefore, our data identify a new
"immunological space" at which important immunological reactions
could take place and influence physiological as
well as pathological processes.
In the present study, we describe the natural habitat of vascular DCs; functional investigations will follow on the basis of the present morphological data.
From the present study, it is not yet clear whether these DCs play an atherosclerosis-promoting or -inhibiting role. Both mechanisms are possible, ie, induction of tolerance or stimulation of an immune response to the antigen triggering atherogenesis. However, our morphological data suggest an atherosclerosis-enhancing function by the accumulation of DCs in areas of the vascular tree that are predisposed to later atherosclerotic development.
| Acknowledgments |
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Received November 28, 2000; accepted January 23, 2001.
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