J Clin Pathol 2000; 53:518-524
© 2000 Journal of Clinical
Pathology
Immunohistowax processing, a new fixation and embedding method for light
microscopy, which preserves antigen immunoreactivity and morphological
structures: visualisation of dendritic cells in peripheral organs
Bernard Pajak1, Thibaut De Smedt2,
Véronique Moulin1, Carl De Trez1,
Roberto Maldonado-López1, Georgette Vansanten1,
Emmanuel Briend3, Jacques Urbain1,
Oberdan Leo1 and Muriel Moser1
1 The Département de Biologie Moléculaire,
Université Libre de Bruxelles, B-1640 Rhode-Saint-Genése, Belgium
2 Immunex Corporation, Seattle, Washington 98101, USA
3 Cantab Pharmaceuticals Research LDT, Cambridge CB4 OWG, UK
Accepted for publication January 20, 2000 .
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Abstract
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Aims—To describe a new fixation and embedding method for
tissue samples, immunohistowax processing, which preserves both
morphology and antigen immunoreactivity, and to use this technique to
investigate the role of dendritic cells in the immune response in
peripheral tissues.
Methods—This technique was used to stain a population of
specialised antigen presenting cells (dendritic cells) that have the
unique capacity to sensitise naive T cells, and therefore to induce
primary immune responses. The numbers of dendritic cells in
peripheral organs of mice either untreated or injected with live
Escherichia coli were compared.
Results—Numbers of dendritic cells were greatly decreased
in heart, kidney, and intestine after the inoculation of bacteria.
The numbers of dendritic cells in the lung did not seem to be
affected by the injection of E coli. However, staining of lung
sections revealed that some monocyte like cells acquired morphological
and phenotypic features of dendritic cells, and migrated into
blood vessels.
Conclusions—These observations suggest that the injection
of bacteria induces the activation of dendritic cells in peripheral
organs, where they play the role of sentinels, and/or their movement
into lymphoid organs, where T cell priming is likely to occur.
Key Words: dendritic cell • Escherichia coli •
immunohistochemistry
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Introduction
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The immune response is the result of multiple interactions between
discrete cell populations of the immune system. Specialised cells
(antigen presenting cells (APCs)) present the antigen, in the form of
peptides in the groove of major histocompatibility complex (MHC)
molecules, to helper T cells, which in turn activate the
differentiation of effector cells, such as B cells or cytotoxic
effector cells. APCs are widely distributed in the body, in lymphoid
and non-lymphoid organs,1 whereas T cells
recirculate through the lymphoid organs, where they are located in
discrete sites. There is evidence that T cell priming occurs in the T
cell zones of lymphoid organs.2,3
Therefore, the first step of the immune response is likely to involve
the migration of APCs and their redistribution into T cell areas.
The population of APCs is heterogeneous and includes dendritic
cells, B cells, and macrophages. Among these cells, dendritic cells
appear to have the unique capacity to sensitise naive T cells and are
the APCs of the primary immune response.
The aim of our study was to analyse the movement of dendritic
cells in peripheral solid organs of mice injected with Gram negative
bacteria. To achieve this goal, a new immunohistochemistry processing
(immunohistowax processing) method, based on a proprietary fixation
and embedding medium, was developed and was shown to preserve both
morphology and antigen immunoreactivity. In this processing, protein
denaturation in the sample is minimised by the combined use of an
aldehyde free zinc salt solution and a new embedding wax
(Immunohistowax), which is liquid at low temperature (37°C). This
approach allowed us to identify dendritic cells by morphological as
well as phenotypic criteria. Our data show that most dendritic cells
disappear from kidney, heart, and intestine after the injection of
bacteria, whereas some lung dendritic cells become activated under
the same conditions.
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Materials
and methods |
ANIMALS
Female Balb/c and C57BL/6 mice were purchased from IFFA-CREDO
(Brussels, Belgium) and maintained in our pathogen free facility.
escherichia coli inoculation
The K504 E coli strain was kindly provided by Dr E Van Driessche
(Laboratorium voor Chemie der Proteinen, Vrije Universiteit
Brussel, Belgium). Bacteria were grown in LB medium overnight at
37°C. Bacteria were pelleted by centrifugation at 3000
xg for 10 minutes, washed twice in
sterile phosphate buffered saline (PBS) and resuspended at 5
x 108 bacteria/ml in PBS.
A standard of absorbencies based on known colony forming units
(CFU) was used to calculate the inoculum concentration. Mice received
an intravenous injection of 200 µl of E coli suspension. An
E coli dose of 108 CFU was used in all experiments.
IMMUNOHISTOWAX PROCESSING
The tissue sample was fixed in a formaldehyde free zinc fixative
(Immunohistofix; Intertiles, Brussels, Belgium) for three days at
4°C. Sample thickness did not exceed 5 mm to allow optimal
infiltration of fixative and dehydrating agents. Dehydration was
performed according to two different protocols: the samples were
either dehydrated in a graded series of ethanol baths: 30%, 50%, 70%,
90%, and 100% for 30 minutes each at room temperature, or samples
were dehydrated in 100% acetone for six hours. The first protocol
seems to preserve tissue morphology more effectively, whereas the
second possibly preserved some antigens more efficiently.
Infiltration was performed at 37°C by means of three baths of
Immunohistowax for 20 minutes each. Tissue specimens were then
embedded in Immunohistowax and mounted on wooden blocks. Blocks were
stored at 4°C for at least one night before sectioning, and could be
kept for up to six months at room temperature. Blocks can become
difficult to cut at temperatures above 22°C, and were therefore kept
at 4°C and cut promptly. Sections of 3–5 µm were performed with a
sliding microtome and individual sections were transferred directly
using thin grids on a drop of water on gelatin precoated slides.
Floating on a water bath is unsuitable because of the slight
hydrophilicity of the wax. Slides were air dried at room temperature
and stored at room temperature for up to several months.
IMMUNOSTAINING
Immunohistowax processed sections were dewaxed in acetone for one to
five minutes and transferred to PBS. Immunohistochemical staining was
performed as follows.
Inhibition of endogenous peroxidases
If peroxidase was used for visualisation, the slides were first
treated with 3% H2O2 in PBS for 30 to 60 minutes to block
endogenous peroxidase; the use of methanol was avoided because it
might be detrimental for certain antigens, such as T cell markers.
Saturation step
We commonly use the blocking reagent (catalogue number, 1096176) from
Boehringer (Brussels, Belgium; 1% in PBS (PBS-BR)) to saturate
non-specific reaction sites because it gives a lower background than
bovine serum albumin, horse serum, or goat serum. Sections were
incubated in PBS-BR at room temperature for 30 minutes.
Single staining
Sections were washed in PBS and incubated with antibodies (5–25
µg/ml) for one to three hours at room temperature or overnight at 4°C
in PBS-BR. Primary monoclonal antibodies gave better results when
coupled to biotin or fluorescein isothiocyanate (FITC). Biotinylated
antibodies were visualised with ABC kits from Vector Laboratories
(Burlingame, California, USA; 1/100 in PBS-BR) for 30 minutes at room
temperature. Peroxidase was revealed using either diaminobenzidine
substrates with or without metal enhancer (Sigma, Bornem, Belgium),
TMB, Vector SG Substrate Kit, Vector VIP Substrate Kit (Vector), or
AEC (Sigma). Substrate kits from Vector Laboratories for alkaline
phosphatase were used according to the manufacturer's
recommendations. FITC conjugated antibodies were visualised by
incubation for 30 minutes with anti-FITC alkaline phosphatase or
peroxidase Fab fragment (Boehringer), diluted at 1/500 in PBS-BR.
Counterstaining
Single immunostained sections were counterstained with haematoxylin
or methyl green depending on the substrate colour.
When double or triple staining was performed with biotinylated
antibodies, excess biotin from the first antibody was blocked with
the Vector blocking kit. In the case of multiple peroxidase
stainings, enzymatic activity linked to the first antibody was
neutralised by incubating sections in H2O2 (3% in PBS) for
20–30 minutes.
Mounting
Slides were mounted in Aquatex (Merck, Overijse, Belgium) or
Polymount (Polysciences, Warrington, USA) depending on the solubility
of substrates.
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Results
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IMMUNOHISTOWAX PROCESSING
Morphological observations require thin tissue sections, which are
best obtained after the infiltration and embedding of specimens with
wax or plastic. However, because waxes are poorly soluble in water,
their use requires pretreatment with fixatives and dehydrating
solvents, which is known to affect protein structure. In addition,
fixation and/or embedding can also affect the immunodetection of many
cellular markers as a result of protein denaturation caused by
chemical and/or thermal injury.4–6
Although we have developed a novel wax that is liquid at near
physiological temperature (37°C), its poor solubility in water
required tissue dehydration before embedding. Dehydration was
performed with acetone or ethanol, which were fully miscible in the
liquid wax. Unfortunately, and as expected from previous reports,7
the dehydration step affected the staining of many antigens.
The data in table 1
show that dehydration prevented the staining of CD3, CD4, and CD8
antigens. By contrast, other antigens, such as MHC class II, CD11c,
B220, and Mac-1 were not altered by dehydration with one bath of 100%
acetone for six hours.
Because exposure to organic solvents before embedding could not be
avoided, we attempted to protect antigenic structures from chemical
denaturation by increasing protein stability before fixation.
Engineered metal chelation in proteins has been used successfully to
stabilise proteins against denaturation.8,9
We choose Zn2+ as a metal ion, based on its known ability to
interact with at least four amino acids (primarily histidine,
and to a lesser extent, aspartic acid, glutamic acid, and cysteine)
through binding to nitrogen, oxygen, and sulphur atoms.10,11
Its thermodynamic properties have been shown to promote favourable
entropic effects, which enhance the stability of secondary protein
structure.12 Ligand motifs that can be used
for metal binding are His-XXX-His for an
-helix,
His-X-His for a ß-strand,
and His-XX-His for a reverse type II ß-strand.13
In some cases, one or two histidine(s) might be replaced by aspartate
or cysteine. We analysed mouse protein sequences and found that
at least one motif was present in all sequences investigated, the
mean value being 3.5 motifs/sequence. Although structural data are
not available for most antigenic proteins, we assumed that a
sufficient number of these motifs was indeed able to bind a metal
ion.
Tissue specimens were pretreated with a zinc fixative (Immunohistofix),
dehydrated, and embedded in Immunohistowax. We compared the
staining of several antigens on tissue samples, pretreated or not
with zinc fixative. The data in table 1
show that the staining of CD3, CD4, and CD8 antigens was preserved by
pretreatment with the zinc fixative. We further tested several
monoclonal or polyclonal antibodies, fusion proteins, lectins, and
enzymes for the staining of membrane or intracellular proteins,
carbohydrate residues, surface receptors, and apoptotic cells. The
results in table 2
show that a large number of molecules expressed by T or B cells, NK
cells, macrophages, and dendritic cells could be detected on
Immunohistowax processed sections. In particular, determinants
expressed upon cell activation were stained using monoclonal
antibodies (specific for CD25, CD44, CD69, CD86, Ly-77, or DEC-205)
or a fusion protein (OX40 ligand–human IgG1). B cells specific for
the hapten arsonate were visualised on spleen sections using the
hapten coupled to bovine serum albumin (BSA), whereas T cells were
stained with a monoclonal antibody specific for their antigenic
receptor. Interleukin 2 (IL-2), IL-4, interferon
(ifn-),
and IL-10 were detected in the cytoplasm of activated T cells in
situ, using double staining with antibodies to T cells and to
lymphokines. Using this processing method, we previously identified
apoptotic cells in tissues by double staining with antibodies to cell
surface markers and the TUNEL (TdT mediated dUTP nick end labelling)
reaction.25 It should be noted that, among
all presently tested antibodies, only two did not work with this
technique: 2C11 (hamster antimouse CD3) and GK1.5 (rat antimouse
CD4).
INJECTION OF GRAM NEGATIVE BACTERIA REDUCES THE NUMBER OF DENDRITIC
CELLS IN KIDNEY, HEART, AND INTESTINE
Dendritic cells are a trace population in most organs, usually
display dendrites, and express selected surface markers, such as MHC
class II molecules and/or CD11c. We stained sections of various
organs with CD11c or MHC class II specific monoclonal antibodies.
Dendritic cells in the kidney, heart, and intestine were found to be
negative for CD11c expression. In the heart, most class II positive
cells with a dendritic morphology (presumably dendritic cells) are
located in the pericardium (fig 1A),
whereas few cells were detected in the myocardium (not shown).
Dendritic cells in both sites decreased in numbers after the
injection of E coli (fig 1B
and data not shown). In the intestine, class II positive cells were
detected in the connective tissue (lamina propria) of the villi (fig
1C).
The injection of live bacteria resulted in the loss of most MHC class
II positive cells (fig 1D).
As shown in fig 1E,
MHC class II positive cells with a typical dendritic morphology could
be found in the renal cortex around the glomeruli in the kidney of
PBS treated mice. Intravenous inoculation of E coli led to a
reduction in the number of MHC class II positive cells (fig 1F).
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Figure 1 Immunostaining of major
histocompatibility complex (MHC) class II positive or CD11c positive
cells in organs of mice injected or not injected with bacteria. Heart (A
and B), intestine (C and D), kidney (E and F), and lung (G and H).
Sections from Balb/c mice injected 24 hours previously with phosphate
buffered saline (A, C, E, and G) or Escherichia coli (B, D, F, and H)
were stained with anti-I-Ed (A–F) or anti-CD11c (G and H)
monoclonal antibodies, and further counterstained with haemalun. Scale
bar, 50 µm (A, B, C, D, G, and H), 100 µm (E and F).
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INJECTION OF e coli induces phenotypic and morphological changes
in lung dendritic cells and their movement to blood vessels
By contrast, the numbers of CD11c positive cells in lung remained
unchanged after the inoculation of live E coli (compare fig 1G
and 1H).
Staining of lung sections with haemalun-eosin clearly shows
constriction of the alveoli (fig 2A and B)
and infiltration of the lung parenchyma by neutrophils (fig 2D and E),
starting one hour after the inoculation of live E coli. In
addition, immunostaining of sections revealed CD11c positive cells
with a globular shape within the alveoli (fig 3A).
These cells did not express MHC class II molecules (fig 3A)
and could be alveolar macrophages or dendritic cells at a very
immature stage (see below). Surprisingly, some of these CD11c
positive cells (approximately 20–25%) increased in size, acquired a
dendritic morphology, and upregulated the expression of I-E molecules
(fig 3B–E)
as early as one hour after the injection of live bacteria. Later on
(one to six hours after treatment), these dendritic like, CD11c
positive, MHC class II positive cells seem to migrate to the blood
vessels (fig 3C and D).
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Figure 2 Histopathology of the lung
after Escherichia coli injection. Lungs from Balb/c mice injected with
phosphate buffered saline (A and C) or E coli (B, D, and E) one hour
previously were fixed, dehydrated, and embedded according to the
Immunohistowax processing protocol and stained with haemalun-eosin. Note
the infiltration of neutrophils in alveoli (D) and in arterioles (E).
Scale bars, 250 µm (A and B); 25 µm (C–E).
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Figure 3 Phenotype and morphology of
lung dendritic cells after the inoculation of bacteria. Immunohistowax
processed lung sections from mice injected with phosphate buffered
saline (A) or live Escherichia coli (B–E) were double stained with
anti-CD11c in red and anti-I-Ed in blue. Note the CD11c
positive, major histocompatibility complex class II positive cells in
purple (B–E). Scale bars, 100 µm (A and B); 50 µm (C–E).
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Discussion
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Our report describes a new fixation and embedding technique, called
Immunohistowax processing, that permits immunostaining of a large
variety of antigens and preserves the morphology of all tissues
tested. To achieve minimal denaturation of proteins during
dehydration and embedding, the protein structures were stabilised by
pretreatment with an aldehyde free zinc fixative before dehydration
with ethanol or acetone. Tissue specimens were then embedded in an
inert wax at 37°C. This processing allowed antigen immunodetection
and morphological analysis of thin sections (3–5 µm).
The Immunohistowax processing technique allowed us to detect a
trace population of APCs—cells of the dendritic family—in peripheral
organs using phenotypic and morphological criteria.
It is generally believed that dendritic cells play the role of
sentinels in the periphery, and upon the encounter of an appropriate
signal (signal of danger?) are redistributed to T cell areas, where
they probably prime T cells.26 The major
role of dendritic cells in inducing primary immune responses
correlates with some specialisation of function over time and space.
In non-lymphoid tissues, dendritic cells are present in an immature
state: well equipped to capture and process antigens but unable to
sensitise T cells optimally. In secondary lymphoid organs, dendritic
cells are mature; that is, they poorly capture and process proteins
but have the capacity to prime naive T cells. The data presented here
indicate that the inoculation of live bacteria induces the
disappearance of most dendritic cells from the heart, kidney, and
intestine. Their loss could be attributed to migration from these
organs or death by apoptosis. We and others have shown previously
that the injection of lipopolysaccharide or toxoplasma extracts
provoked the migration of splenic dendritic cells from the marginal
zone between the red and white pulp to the areas where T cells are
located.27,28 These
observations, together with those of Roake et al,29
suggest that dendritic cells that have encountered bacteria might
migrate to T cell zones in lymph nodes. Experiments are under way to
test whether new migrant cells, bearing microbial antigens, can be
detected in draining lymph nodes.
In the pulmonary alveoli, some CD11c positive cells acquire a
dendritic morphology, upregulate the expression of MHC class II
molecules, and migrate into proximal capillaries. These observations
are reminiscent of a report by Randolph et al,30
showing that monocytes differentiate into dendritic cells in vitro,
after migration across the endothelium in the subluminal to lumenal
direction, a phenomenon that is potentiated by an additional
stimulus, such as lipopolysaccharide of zymosan particles. It is
therefore tempting to speculate that, in the lung, MHC class II
negative, CD11c positive monocytes are induced to differentiate into
dendritic cells through crossendothelial migration and exposure to
microorganisms. Additional work will be required to test this
hypothesis.
In conclusion, Immunohistowax processing seems to allow optimal
conservation of morphology and immunoreactivity. Indeed, almost all
antigens tested were detected in Immunohistowax processed sections,
including antigens that were not stained in paraffin wax embedded
sections or cryosections. Of note, this method only requires standard
equipment and does not rely on the antigen retrieval technique. We
believe that Immunohistowax processing will be useful to identify in
situ the cell populations that secrete various cytokines and to
define more accurately the spatial and temporal organisation of the
immune response.
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Acknowledgments |
We thank Drs Ralph Steinman (Rockefeller Institute, New York, USA),
Gerry Klaus (National Institute for Medical Research, London, UK),
Hervé Bazin (Université Catholique de Louvain, Bruxelles, Belgium),
and John Shields (Cantab Pharmaceuticals, Cambridge, UK) for
providing useful reagents; G Dewasme, M Swaenepoel, F Tielemans, and
P Veirman for technical assistance; and D Nolan for editorial
assistance. The laboratory of animal physiology was supported by
grants of the Fonds National de la Recherche Scientifique
(FNRS)/Télévie, the Fonds de la Recherche Fondamentale Collective,
the European Commission (CEC TMR Network Contract FMRX-CT96–0053),
and the Belgian Programme on Interuniversity Poles of Attraction
initiated by the Belgian State, Prime Minister's Office, Science
Policy Programming. TDS, CDT, RM-L, and MM are supported by the FNRS.
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