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Technical Advance |








From the Institutes of Clinical Pathology*
and
Neurology,
University of Vienna, Vienna,
Austria; the Department of Biology,
University of Konstanz, Konstanz, Germany; and the Department of
Immunology and Cell Biology,
Forschungsinstitut Borstel, Borstel, Germany
| Abstract |
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| Introduction |
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Recently, monoclonal antibody MIB-5 was suggested as a proliferation marker for murine tissues.9,10 MIB-5 has been raised against bacterially expressed parts of the human Ki-67 cDNA,10,11 and reacts with the Ki-67 protein in proliferating cells of several mammalian species.9,10 Ki-67 protein is expressed as alternative splice products of 360 kd and 320 kd. Both forms are bound to chromatin and seem to perform structural roles of unknown nature.12 Ki-67 protein is rapidly degraded once mammalian cells cease to proliferate, but although documented in numerous studies, the physiological meaning of this rapid Ki-67 degradation has yet to be established.12 However, the validity of MIB-5 as a proliferation marker for murine tissues has not yet been thoroughly tested.
Antibodies against MCM3 have also been suggested as proliferation markers.13 The MCM family is highly conserved among eukaryotes and consists of at least six different nuclear proteins that are expressed in all eukaryotic cells where they perform essential functions in the regulation of chromatin replication.13,14 MCM3, a 105-kd protein, is expressed in proliferating cells where it is bound to chromatin before S phase, but gradually released during S phase and dispersed throughout the cell during mitosis.15 After growth arrest and differentiation, MCM3 disappears from cells with an estimated half life of 24 hours and is no longer synthesized in growth-arrested, differentiated cells.13 Anti-MCM3 antibodies stain cells in paraffin-embedded tissues of various species.3,13
In our study we tested the applicability of MIB-5 and anti-MCM3 antibodies as proliferation markers for paraffin-embedded nonneoplastic and neoplastic murine tissues as compared to anti-BrdU immunolabeling as the current gold standard.
| Materials and Methods |
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We analyzed nonneoplastic tissues sampled from four wild-type C57BL/6 mice (two males, two females). Wild-type tissues included brain, gut, heart, thymus, lungs, kidney, spleen, liver, ovary, testes, skin, and skull and spinal column with bone marrow. We further analyzed 15 pituitary adenomas in RB-1 hemizygous mice carrying in addition the GFAP-v-src transgene (RB-1+/- GFAP-v-src mice), 7 astrocytomas in TP53 hemizygous mice carrying in addition the GFAP-v-src transgene (TP53+/- GFAP-v-src mice) or RB-1+/- GFAP-v-src mice, and 16 GFAP-expressing nonglial tumors of soft tissue and bone and 1 lymphoma of the spleen in 15 TP53+/- GFAP-v-src mice.16 The gut of one male white laboratory rat was also studied. All mice were injected 16 and 4 hours before sacrifice intraperitoneally with 0.2 ml of a 5 mg/ml BrdU solution. Mice were killed by cervical dislocation. After removal, tissues were fixed in formalin for 24 hours, and embedded in paraffin. Skull and spinal column were decalcified in 0.5 mol/L of ethylenediaminetetraacetic acid, pH 8.0, for 10 days, before paraffin-embedding. For comparison of slowly and fast proliferating tissues, paraffin blocks were prepared that contained both brain (harbors postmitotic nerve cells) and gut (harbors briskly proliferating epithelium). Institutional review board approval for this study was obtained.
Immunohistochemistry
After removal of paraffin with xylene, endogenous peroxidase was blocked in all tissues with 3% H2O2 in methanol. All tissue sections were pretreated in a microwave oven in citrate buffer at pH 6.0 for 10 minutes at 100°C at 300 W. Unspecific binding sites were blocked using washing buffer 1 (no. K5006; DAKO, Glostrup, Denmark) before incubation with the primary antibody. Slides were incubated for 1 hour at room temperature with the primary antibodies. As primary antibodies, we used monoclonal antibody MIB-5 (antibody concentration 0.2 mg/ml, no. 2093; Immunotech, Marseille, France ) (this antibody is now available from DAKO, Glostrup, Denmark), a biotinylated monoclonal anti-BrdU antibody (no. MD 5215; Caltag Laboratories, Burlingame, CA) (antibody concentration 0.2 mg/ml), a monoclonal anti-MCM3 antibody (antibody concentration 2.5 mg/ml), and a polyclonal anti-MCM3 antibody (antibody concentration 0.2 mg/ml). Preparation of the polyclonal anti-MCM3 antibody has been described previously.17 Monoclonal anti-MCM3 was prepared using a standard protocol against bacterially expressed human MCM3 protein (data to be published). To avoid background because of secondary antibody binding to mouse immunoglobulins in tissue, MIB-5 was biotinylated using the Animal Research Kit Peroxidase (no. K3954; DAKO) according to the manufacturers instructions before immunostaining. As a detection system, a streptavidin-peroxidase complex included in the kit was used. After microwave treatment, specimens were digested for 5 minutes with 0.03% proteinase K (no. P-6556; Sigma-Aldrich, St. Louis, MO). Detection of anti-BrdU binding was performed using an avidin-peroxidase system (no. A-3151; Sigma-Aldrich). To minimize background staining of the monoclonal anti-MCM3 antibody, the Mouse on Mouse (M.O.M.) kit (no. PK-2200; Vector, Burlingame, CA) was used as a detection system. For detection of polyclonal anti-MCM3 binding, an anti-rabbit secondary antibody (no. Z0196; DAKO) and a peroxidase-anti-peroxidase system (no. Z0113; DAKO) were used. Diaminobenzidine was used as chromogen for all immunostainings, counterstaining was performed with hemalaun. Omission or substitution of MIB-5 or monoclonal anti-MCM3 antibody by isotype-matched nonspecific immunoglobulins was used as negative controls. Binding specificity of polyclonal MCM3 antibody was controlled by replacing the antibody with nonspecific rabbit serum.
Analysis
In a first step, the percentages of immunostained cells were
determined in samples of small intestine of two wild-type mice and in
pituitary adenomas of two RB-1+/- GFAP-v-src
mice. Immunostaining was performed with anti-BrdU, MIB-5, and
monoclonal/polyclonal anti-MCM3 as primary antibodies on subsequent
serial sections. In the small intestine, the fraction of immunostained
epithelial cells was determined on cross-cut villi showing the
epithelium as monolayer. Epithelial cells were counted from the lowest
point of the crypt to the tip of the villus and the percentage of
positive cells was calculated. The mean value of the percentages of
decorated cells in three villi was used for further calculations. In
pituitary adenomas, the fraction of immunostained cells was determined
using an eyeshield and counting tumor cells at a magnification of x400
in three subsequent microscopic fields in the tissue area showing the
highest density of immunostained cells. The percentage of decorated
cells was calculated for each field and the mean value of the three
fields was used for further calculations. Immunostaining and assessment
of decorated cells was performed for all primary antibodies at
dilutions of 1:10, 1:20, 1:50, 1:100, 1:200, 1:400, 1:800, 1:1500,
1:3000, 1:4000, 1:8000, and 1:16,000 (Figure 1, A and B)
. In addition to conventional
light microscopic examination, specimens were also investigated by
computerized image analysis using the Advanced Cellular Imaging System
(ACIS; Chromavision, San Juan Capistrano, CA) with the Ki-67 module
according to the manufacturers instructions. For assessment of
significant differences of cells stained by anti-BrdU, MIB-5, or
monoclonal/polyclonal anti-MCM3 in small intestine or pituitary
adenomas we used the paired Friedman test with antibody dilution as
pairing factor. Significant results were further investigated by the
sign tests with adjustment according to Bonferroni for multiple
testing. A P value
5% was considered as significant.
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| Results |
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Computerized image analysis of MIB-5-stained slides yielded similar
values as compared to conventional light microscopic investigation
(data not shown), whereas cytoplasmic background of some of the
anti-BrdU and anti-MCM3 immunostained sections did not allow
computerized image analysis. Therefore, quantitative assessment of
immunostained cells was performed by means of conventional light
microscopic analysis. Figure 1, a and b
, shows compiled data of
quantitative assessment and illustrates for all antibodies the
percentages of decorated cells depending on antibody dilution. Figures 2 and 3
show samples of immunostaining.
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Analyzing a series of tumors, MIB-5-decorated cells ranged from 5.8 to
47.2% in astrocytomas (n = 7), from 12.1 to
78.1% in tumors of soft tissue and bone (n =
16), and from 0.3 to 8.8% in pituitary adenomas
(n = 15). [Labeling of up to 8.8% of the
murine pituitary adenoma cells is rather high as compared to human
pituitary adenomas, in which MIB-1 labeling has been reported to be
<5%.19
Interspecies differences and differences of the
genetic basis of human pituitary adenomas and our murine pituitary
adenomas are possible explanations for the slight difference of MIB-1
and MIB-5 labeling.] In the case of lymphoma, 29.4% of cells showed
MIB-5 binding. The number of mitotic figures/10 HPF ranged from 4 to 35
in astrocytomas, from 12 to 55 in tumors of soft tissue and bone, and
from 0 to 13 in pituitary adenomas. The case of lymphoma showed 22
mitotic figures/10 HPF. There was a strong correlation between the
percentage of MIB-5-decorated cells and number of mitotic figures/10
HPF (r = 0.93, P < 0.001,
Pearsons coefficient of correlation) (Figure 4)
.
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| Discussion |
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In our study, we tested systematically the validity of MIB-5 and
anti-MCM3 antibodies as cell proliferation markers on routinely
processed, paraffin-embedded murine tissues as an alternative to
anti-BrdU staining. All of the antibodies labeled nuclei of
proliferating cells of various tissues and of tumor cells, but no
neuronal nuclei. At different dilutions MIB-5 labeled similar cell
fractions only in proliferating tissue zones and the results closely
correlated with anti-BrdU staining. MIB-5 proved thus as a reliable and
reproducible tool to detect the fraction of proliferating cells. In
contrast, anti-MCM3 antibodies labeled significantly higher percentages
of cells as compared to anti-BrdU and MIB-5 staining. In the small
intestine, anti-MCM3 stained some of the postmitotic
epithelial cells on the villous surface, in addition to proliferating
epithelial cells in the crypt (Figure 2)
. With increasing antibody
dilutions, the percentage of anti-MCM3-immunostained cells showed a
linear decrease. Anti-MCM3 immunostaining seems to parallel the
situation in assessment of proliferation rate using immunostaining for
proliferating cell nuclear antigen, where the number of decorated cells
depends strongly on the sensitivity of the immunocytochemical system
used,22
thus causing little reproducibility of staining
results after repeated immunostaining or when using different staining
protocols. The presence of MCM3 in a cell fraction that is larger than
the fraction containing either BrdU or Ki-67 may be explained by a
relatively long half-life of MCM3 protein. MCM3 protein has been shown
to disappear more slowly after initiation of differentiation than the
Ki-67 antigen.13
Thus, MCM3 protein may still be
detectable for some time in early G0 phase, after
the cell has accomplished its cycle. However, even at low dilutions,
anti-MCM3 antibodies did not stain neuronal cell nuclei in the brain.
In conclusion, our results show that anti-MCM3 antibodies are not useful for quantitative assessment of proliferating cells, although they might be useful for detecting the fraction of cells remaining competent for proliferation.23 In contrast, monoclonal antibody MIB-5 shows staining results in murine tissues, equivalent to MIB-1 staining of human tissues. Assessment of cell proliferation with the MIB5 antibody allows bypassing the methodological drawbacks of anti-BrdU staining. Thus, we propose antibody MIB-5 as the first-choice cell proliferation marker for routinely processed formalin-fixed and paraffin-embedded murine tissues.
| Acknowledgements |
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| Footnotes |
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Accepted for publication February 27, 2001.
| References |
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