Originally published as JHC exPRESS on October 18, 2005. doi:10.1369/jhc.4A6514.2005
Volume 54 (4): 385-395, 2006 Copyright ©The Histochemical Society, Inc. Immunohistochemical Expression of Endothelial Markers CD31, CD34, von Willebrand Factor, and Fli-1 in Normal Human Tissues
Institute of Pathology, Lausanne, Switzerland Correspondence to: Marc P. Pusztaszeri, Institute of Pathology, Rue du Bugnon 25 1011, Lausanne, Switzerland. E-mail: Marc.Pusztaszeri{at}chuv.ch
Few systematic studies have been published comparing the expression and distribution of endothelial cell (EC) markers in different vascular beds in normal human tissues. We investigated by immunohistochemistry the expression of CD31, CD34, von Willebrand factor (vWF), and Fli-1 in EC of the major organs and large vessels. Tissue samples obtained from autopsies and biopsy specimens were routinely processed and stained immunohistochemically for CD31, CD34, and vWF. Biopsy material was also stained immunohistochemically for Fli-1, D2-40, and Lyve-1. The expression pattern of the markers was heterogeneous in some of the organs studied. In the kidney, fenestrated endothelium of the glomeruli strongly expressed CD31 and CD34 but was only focally positive or completely negative for vWF. Alveolar wall capillaries of the lung strongly stained for CD31 and CD34 but were usually negative for vWF. The staining intensity for vWF increased gradually with the vessel caliber in the lung. Sinusoids of the spleen and liver were diffusely positive for CD31. They were negative for CD34 in the spleen and only expressed CD34 in the periportal area in the liver. Fli-1 was expressed in all types of EC but also in lymphocytes. D2-40 stained lymphatic endothelium only. Lyve-1 immunostaining was too variable to be applied to routinely processed tissues. The expression of EC markers CD31, CD34, and vWF in the vascular tree is heterogeneous with a specific pattern for individual vessel types and different anatomic compartments of the same organ. D2-40 labels lymphatic EC only. (J Histochem Cytochem 54:385395 2006)
Key Words: platelet endothelial cell adhesion molecule 1 CD34 von Willebrand factor Fli-1 endothelial cells heterogeneity microvasculature
ENDOTHELIAL CELLS (EC) have many functions and play a central role in the control of coagulation, thrombolysis, vascular tone, permeability, inflammation, tissue repair, and angiogenesis. They constitute a heterogeneous cell population in the human body. Functions and, as a consequence, molecular characteristics of EC vary along the vascular tree and in the same organ between different vessels (Cines et al. 1998
The few studies published have mostly been restricted to the lung. Müller et al. (2002a)
CD31, a member of the immunoglobulin superfamily, is a 130-kDa transmembrane glycoprotein also designated as PECAM-1 (platelet endothelial cell adhesion molecule 1). It is present on the surface of platelets, monocytes, macrophages, and neutrophils and is a constituent of the endothelial intercellular junction. It plays a major role in the adhesion cascade between EC and the inflammatory cells during inflammation in facilitating leucocyte migration and between EC during angiogenesis. It has recently been recognized for its angiogenic role (DeLisser et al. 1997
CD34 is a 110-kDa transmembrane glycoprotein present on leukemic cells, EC, and stem cells. In addition, it is localized on cells of the splenic marginal zone, dendritic interstitial cells around vessels, nerves, hair follicles, muscle bundles, and sweat glands in a variety of tissues and organs. Its function is still unclear. It is used for leukemia diagnosis and subclassification and for diagnosis of vascular tumors. Antibodies to CD34 also strongly label gastrointestinal stromal tumors, and the antigen is invariably found in solitary fibrous tumor and dermatofibrosarcoma protuberans (Kutzner 1993
vWF (factor VIII-related antigen) is a glycoprotein that mediates platelet adhesion to subendothelium at sites of vascular injury and binds and stabilizes factor VIII in the circulation (Alles and Bosslet 1988
Fli-1 is a nuclear transcription factor involved in cellular proliferation and tumorigenesis. It appears to be a relatively sensitive and specific marker for Ewing sarcoma and primitive neuroectodermal tumors that are characterized in 90% of cases by a specific translocation t(11;22), resulting in the fusion of the EWS gene on chromosome 22 to the Fli-1 gene on chromosome 11. The Fli-1 immunoreactivity in vascular tumors appears to equal or exceed that for CD31, CD34, and vWF (Folpe et al. 2001
The present study was undertaken to immunohistochemically compare the expression of commonly used cytoplasmic EC markers (CD31, CD34, vWF) and the new nuclear EC markers like Fli-1 in different vascular beds of normal human tissues. To provide a comprehensive picture, we did not restrict the study to one particular organ but included different parenchymatous organs and large vessels. Moreover, knowing that the expression of CD31, CD34, vWF, and Fli-1 is not restricted to blood vessels, we also used D2-40, a monoclonal antibody that has recently been recognized to be highly specific for lymphatic endothelium (Fukunaga 2005
Normal tissues were obtained from biopsies and autopsy cases. Tissues were formalin fixed, paraffin embedded, and stained immunohistochemically for CD31, CD34, and vWF. In addition, biopsy specimens were stained for Fli-1 protein and D2-40. The use of human material for this study was approved by the local medical ethics committee.
Case Selection
Autopsy Specimens
Immunohistochemistry
The 4-µm paraffin sections were deparaffinized in xylol and rehydrated in graded alcohol series. Endogenous peroxidase was inhibited using 3% H2O2 in methanol. The sections were then washed in distilled water and heated in a microwave oven (in citrate buffer 10 mM, pH 6, for CD31, CD34, Fli-1, and Lyve 1 and EDTA 1 mM, pH 7.5, for factor VIII) 15 min for epitope retrieval. No pretreatment was needed for D2-40. Slides were incubated first with normal horse serum (1/30 avidin 10%) for CD31 and CD34 and with normal goat serum (10% avidin) for factor VIII and Fli-1 for 5 min and then in biotin for 10 min. Endogenous biotin was inhibited with a Vector Blocking kit (Vector Laboratories; Burlingame, CA). The slides were then incubated at 20C for 40 min with monoclonal antibodies for CD31, CD34, and D2-40 and polyclonal antibody for factor VIII and Fli-1. For CD31 and Lyve-1, the slides were incubated overnight at 8C. The slides were incubated with anti-mouse/rabbit biotinylated bridging antibodies (dilution 1/200) for 30 min. Sections were then washed and incubated with standard avidinbiotin complex (ABC; DakoCytomation, Glostrup, Denmark) for 30 min. Antibody binding was revealed using H2O2 as a substrate and diaminobenzidine as chromogen. Counterstaining was performed with hematoxylin.
The expression pattern for CD31, CD34, vWF, and Fli-1 in the different tissues we studied was strikingly heterogeneous. The differences concerned especially the small vascular beds including continuous and fenestrated capillaries and sinusoids. Major differences were seen in the parenchymatous organs and are summarized in Table 2 . There were no significant differences between the autopsy and the biopsy tissues.
Kidney The fenestrated endothelium of the glomeruli always strongly expressed CD31 and CD34 but was only focally positive or completely negative for vWF (Figures 1A 1C). In one biopsy case, the glomeruli were diffusely positive for vWF. In the three autopsy cases from hypertensive patients, the glomeruli were weakly or moderately positive for vWF as compared with the autopsy cases from patients without hypertension. More interstitial capillaries stained for CD34 than for CD31 or vWF. Fli-1 usually stained the nuclei of EC in the glomeruli (Figure 1D), making it easier to differentiate between EC and other types of cells also present in the glomeruli, such as podocytes or mesangial cells. In addition, EC of arterioles, venules, and interstitial capillaries were stained. There were, however, some cases in which the interpretation was difficult as a result of important background staining.
Lungs Capillaries in the alveolar wall strongly stained for CD31 and CD34 but were usually negative for vWF (Figures 2A 2C). The staining intensity for vWF also increased gradually with the vessel caliber. Small arterioles and venules showed light staining but larger arteries and veins showed intense staining. The staining intensity did not vary according to vessel type or caliber for CD31 and CD34. Alveolar macrophages were positive for CD31 with a membranous pattern of staining but were negative for CD34 and vWF. Fli-1 diffusely stained the nuclei of EC of the different vessel types in the lungs (Figure 2D). However, the nuclei of lymphocytes were also stained, as well as the cytoplasm of some alveolar wall cells probably corresponding to mastocytes.
Spleen Sinusoidal EC were diffusely positive for CD31 but negative for CD34 (Figures 3A and 3B). In the red pulp, CD34 only marked capillaries.
vWF staining was similar to that for CD31 but slightly less intense (Figure 3C). There were no differences in the staining pattern of the central arteries, veins, and other vessels present in the spleen. Fli-1 stained the nuclei of EC present in the sinusoids but also the lymphocytes present in the white pulp. The larger vessels including the central arteries also stained (Figure 3D).
Liver
Heart The staining pattern of the intramyocardial capillaries was different between CD31 and CD34. More capillaries showed CD34 than CD31 immunoreactivity. The pattern of staining for vWF was similar to that for CD31. The endocardial endothelium strongly stained for CD31 but irregularly and less intensely for CD34 and vWF. Capillaries in fat tissue were positive for CD34. EC from interstitial capillaries, arterioles, and venules present in the myocardium also stained for Fli-1. A diffuse cytoplasmic background staining of cardiomyocytes was also seen with Fli-1.
Skin
Bone Marrow
Lymph Nodes
Large Vessels Large Arteries (Thoracic and Abdominal Aorta, Pulmonary Artery) The staining pattern of the endothelium was similar for CD31 and CD34 in the various large arteries we investigated. Due to loss of endothelial cell integrity, staining of the endothelium was often discontinuous for CD31 and CD34. This was not the case with vWF, which showed a continuous pattern of staining with the regions depleted of EC still being stained for vWF. CD34 also stained some components of the media, probably corresponding to smooth muscle cells.
Middle-sized Arteries (Renal Artery, Femoral Artery)
Large Veins (Inferior Vena Cava, Pulmonary Vein) and Middle-sized Veins (Femoral Vein)
Lymphatic Endothelium EC in arteries and veins were always negative for D2-40. CD31, CD34, vWF, and Fli-1 were occasionally and irregularly expressed in lymphatic endothelium, but the staining intensity was usually lower than in vascular endothelium and was very weak when compared with D2-40. Hence, D2-40 immunostaining gives a mirror image when compared with the other EC markers (Figures 6A 6D). Mesothelial cells from the pleura or spleen capsule were occasionally positive for D2-40. This was also the case with osteocytes in the bone and bronchial wall chondrocytes, as well as basal cells from sebaceous glands and hair follicle cells, in a pattern similar to that of CD34. In bone marrow, hematopoietic stem cells including megakaryocytes were negative for D2-40.
This study confirms that EC in different vessel types of various organs express EC antigens heterogeneously. Variations in expression patterns were most striking in the small capillaries of the continuous, fenestrated, and sinusoid type. We could confirm studies in humans (Gorog et al. 1998
In large vessels, discontinuous immunolabeling was found for CD31 and CD34, in spite of continuous labeling for vWF. This might be explained through loss of integrity of the EC lining, which was suggested by the presence of EC positive for CD31, CD34, and vWF inside the vascular lumen. Subendothelial matrix-bound vWF could explain the continuous staining seen with vWF (Rand et al. 1991
CD34 also stained some components in the media of large vessels. Indeed, it has been shown in earlier studies that aortic smooth muscle cells and some fibroblasts also stain for CD34 (Miettinen et al. 1994
We found D2-40 to be a sensitive and relatively specific marker for lymphatic endothelium in all parenchymatous organs. CD31, CD34, vWF, and Fli-1 did not discriminate between vascular and lymphatic EC and are therefore panvascular EC markers. However, other cells (including mesothelial cells) also stained for D2-40. This is in agreement with a study showing that D2-40 is expressed in normal and reactive mesothelial cells and in most (9396%) mesotheliomas (Ordonez 2005; Ordonez in press
An important application of EC marker immunohistochemistry is the classification of human vascular tumors. CD31 is generally regarded as the single best marker of EC differentiation because it is expressed in
Under pathological conditions, EC can modify their antigen expression. In the liver, capillarization of hepatic sinusoids is a well-recognized phenomenon that occurs in long-standing liver disease and hepatic cirrhosis as well as in hepatocellular carcinoma (HCC). EC of the sinusoids that usually do not express CD34 can alter their phenotype and express this marker. On the contrary, Lyve-1, thought to be restricted to lymphatic vessels, is expressed in liver sinusoids but downregulated in liver cirrhosis and HCC (Carreira et al. 2001 For diagnostic purposes, vWF is a problematic marker because it is not expressed by all EC and is retained in the adjacent extracellular matrix, which limits its reliability for identification of the presence of endothelial cells. In vascular tumors, it is expressed in only 5075% of cases. In our study, Fli-1 was expressed in all types of EC but was also expressed by other cell types, especially lymphocytes. The expression of Fli-1 did not change among different vessel types or organs. This marker can thus be a valuable tool in diagnosing both benign and malignant vascular tumors, especially when used in conjunction with membranous EC markers such as CD31 or CD34. The cytoplasmic reactivity seen in hepatocytes, renal tubular cells, and cardiomyocytes could be explained either by cross-reactivity with a cytoplasmic protein or by the presence of Fli-1 protein in the cytoplasm of these cells.
In conclusion, this study confirms that EC antigen expression varies among different vessel subtypes and different anatomic compartments of the same organ. The antigen expression pattern in a given organ or vascular bed is dynamic and can change due to a variety of pathological processes or simply in senescence (Müller et al. 2002b
We thank Magalie Cornavon, Christine Brueger, Solange Gros, and Marie-Martine Bertholet for expert immunochemistry support.
Received for publication August 31, 2004; accepted September 28, 2005
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