Endogenous avidin-binding activity in human lymphoid tissue

When tissue antigens and antibodies get along: revisiting the technical aspects of immunohistochemistry--the red, brown, and blue technique

Once focused mainly on the characterization of neoplasms, immunohistochemistry (IHC) today is used in the investigation of a broad range of disease processes with applications in diagnosis, prognostication, therapeutic decisions to tailor treatment to an individual patient, and investigations into the pathogenesis of disease. This review addresses the technical aspects of immunohistochemistry (and, to a lesser extent, immunocytochemistry) with attention to the antigen-antibody reaction, optimal fixation techniques, tissue processing considerations, antigen retrieval methods, detection systems, selection and use of an autostainer, standardization and validation of IHC tests, preparation of proper tissue and reagent controls, tissue microarrays and other high-throughput systems, quality assurance/quality control measures, interpretation of the IHC reaction, and reporting of results. It is now more important than ever, with these sophisticated applications, to standardize the entire IHC process from tissue collection through interpretation and reporting to minimize variability among laboratories and to facilitate quantification and interlaboratory comparison of IHC results.

Pitfalls using tyramide signal amplification (TSA) in the mouse gastrointestinal tract: endogenous streptavidin-binding sites lead to false positive staining

Highly sensitive immunohistochemical detection systems such as tyramide signal amplification (TSA) are widely used, since they allow using two primary antibodies raised in the same species. Most of them are based on the streptavidin-biotin-peroxidase system and include streptavidin-coupled secondary antibodies. Using TSA in cryostat-sectioned tissues of mouse esophagus, we were puzzled by negative controls with unexpected staining mostly in the ganglionic areas. This prompted us to search for the causing agent and to include also other parts of the mouse gastrointestinal tract for comparison. Streptavidin-coupled antibodies bound to endogenous binding sites yet to be characterized, which are present throughout the mouse intestines. Staining was mainly localized around neuronal cell bodies of enteric ganglia. Thus, caution is warranted when applying streptavidin-coupled antibodies in the mouse gastrointestinal tract. The use of endogenous biotin-blocking kits combined with a prolonged post-fixation time could significantly reduce unintentional staining.

Endogenous avidin biotin activity (EABA) in thyroid pathology: immunohistochemical study

Background

Immunohistochemical methods based on the high affinity of avidin and biotin (e.g. ABC, LSAB) are characterized by high sensitivity and are widely used for detection of immunologic reaction. However, a non-specific reaction, observed in frozen tissues and in paraffin-embedded material, increasing after heat induced epitope retrieval (HIER), and caused either by endogenous biotin or any another chemical compound with high affinity for avidin, may lead to diagnostic mistakes. The aim of our investigation is to study presence of endogenous avidin biotin activity (EABA) in thyrocytes originating from various thyroid pathological lesions (neoplastic and non-neoplastic).

Materials and methods

The immunohistochemical study was performed on paraffin-embedded specimens of surgically resected thyroid tissue from 97 patients with thyroid diseases: 65 patients with papillary carcinoma (PTC), 11 patients with nodular goiter in whom features of benign papillary hyperplasia were found, 9 with lymphocytic thyroiditis (LT), 8 with follicular adenoma, and 4 patients with follicular carcinoma. In PTC immunohistochemical study was performed both in primary tumors and in lymph node metastases. After HIER, incubation with streptavidin from LSAB+ (DakoCytomation) kit was done.

Results

Strong cytoplasmic EABA was observed in 56 of 65 (87.5%) PTC and in oxyphilic cells in 8 of 9 cases of LT. Significant correlation between EABA in primary PTC tumor and EABA in lymph node metastases was stated. Normal surrounding thyroid tissues showed absence or weak EABA. Aberrant intranuclear localization of biotin was noted in morules of cribriform-morular variant of PTC. No statistically significant correlation between patient's age, sex, metastases presence and EABA was observed.

Conclusion

Among thyroid lesions, false positive reactions are highly probable in papillary thyroid carcinoma and in lymphocytic thyroiditis if immunohistochemical detection is used on systems containing (strept)avidin. The most probable reason is the high endogenous biotin content.

Immunohistochemistry using antibodies to the cathelicidin LL37/hCAP18 in the tammar wallaby, Macropus eugenii

Antibodies to the human cathelicidin, hCAP18 have been used to examine epithelial tissues of adult and pouch young marsupials. Immunoreactivity was observed in skin, gastrointestinal tract, lung and mammary node of adults as well as skin, gastrointestinal tract, lung and bone marrow of pouch young. The locations of expression were similar to that reported in human tissues. Although the antibody to hCAP18 is primarily directed at the C-terminal antimicrobial peptide LL37, our observations suggest recognition of a common conserved element of this cathelicidin and lead us to conclude that the epithelial tissues of marsupials are sites of production of cathelicidin. This is consistent with observations in other mammals but is the first report of expression of these compounds in marsupials.

Technical pitfalls potentially affecting diagnoses in immunohistochemistry

Result of the immunohistochemical reactions routinely used in diagnostic surgical pathology should be properly interpreted, since false results, related to technical and interpretative pitfalls may lead to incorrect diagnosis. The main sources of such pitfalls are reviewed, analytically described and related to different steps (fixation, tissue processing and embedding, decalcification, antigen retrieval) which may affect the accuracy of immunohistochemistry. In addition, the presence of endogenous enzyme activity, improper binding of avidin to endogenous biotin, incorrect use of antibodies, chromogen and detection systems, as well as incorrect interpretation may produce unreliable data. The high frequency and extension of such pitfalls make mandatory the use of internal and external controls and adoption of cross-validation programmes. The present study, supported by an extensive review of the related literature, is intended as a guideline leading to proper interpretation of immunohistochemical data, an essential component of the diagnostic process. Experience on the antigen retrieval procedures for different antigens is also presented.

High endogenous avidin binding activity: an inexpensive and readily available marker for the differential diagnosis of kidney neoplasms

It has been documented that some tissues, such as salivary gland, liver, cardiac and skeletal muscles and kidney, have high level endogenous biotin or endogenous avidin binding activity (EABA). Limited data is available on EABA in renal cell neoplasms. A tissue microarray (TMA) was constructed that included oncocytoma (n=30), chromophobe renal cell carcinoma (RCC) (n=18), clear cell RCC (n=45), clear cell RCC with granular/eosinophilic (G/E) features (n=19), papillary RCC (n=21), papillary RCC with G/E features (n=29) and benign renal tissues (n=31). The TMA slides were stained with or without biotin blocker and analyzed using the automated cellular imaging system (ACIS(R)). Without biotin blocker, a high positive rate of EABA was found in oncocytoma (56/60, 93%) and normal renal tubules (46/60, 77%). A moderate positive rate of EABA was found in clear cell and papillary RCCs with G/E features (13/39, 33% and 19/55, 35%, respectively). Chromophobe RCC and RCC without G/E features had essentially no EABA. With biotin blocker, benign renal tissue and clear cell RCC were negative for EABA; but a significant number of renal oncocytoma (29/60, 48%) and a few papillary RCC with G/E features (5/52, 10%) remained positive for EABA. In conclusion, high EABA may be used to differentiate oncocytoma from chromophobe RCC, and the staining results must be interpreted with caution when avidin-biotin detection system is used in diagnosing renal neoplasms.

Background Staining of Visualization Systems in Immunohistochemistry

The aim of this study was to evaluate specific immunostaining and background staining in formalin-fixed, paraffin-embedded human tissues with the 2 most frequently used immunohistochemical detection systems, Avidin-Biotin-Peroxidase (ABC) and EnVision+. A series of fixed tissues, including breast, colon, kidney, larynx, liver, lung, ovary, pancreas, prostate, stomach, and tonsil, was used in the study. Three monoclonal antibodies, 1 against a nuclear antigen (Ki-67), 1 against a cytoplasmic antigen (cytokeratin), and 1 against a cytoplasmic and membrane-associated antigen and a polyclonal antibody against a nuclear and cytoplasmic antigen (S-100) were selected for these studies. When the ABC system was applied, immunostaining was performed with and without blocking of endogenous avidin-binding activity. The intensity of specific immunostaining and the percentage of stained cells were comparable for the 2 detection systems. The use of ABC caused widespread cytoplasmic and rare nuclear background staining in a variety of normal and tumor cells. A very strong background staining was observed in colon, gastric mucosa, liver, and kidney. Blocking avidin-binding capacity reduced background staining, but complete blocking was difficult to attain. With the EnVision+ system no background staining occurred. Given the efficiency of the detection, equal for both systems or higher with EnVision+, and the significant background problem with ABC, we advocate the routine use of the EnVision+ system.

Opisthorchis viverrini: ultrastructure and cytochemistry of the glycocalyx of the tegument

The ultrastructure and cytochemistry of the glycocalyx of the tegument of Opisthorchis viverrini during maturation from newly excysted juvenile to adult stages were investigated using colloidal iron, ruthenium red and lectin stainings. The results showed that the glycocalyx was intensely stained by the first two dyes, thus indicating the presence of relatively high amounts of negative charges. However, the thickness and intensity of the staining decreased during the fluke's maturation. Binding studies using lectin probes on the surface of adult parasites showed that binding sites for Canavaliaensiformis (Con A), Triticum vulgaris (WGA) and Ricinus communis I(RCA I) were present in relative large amounts on the glycocalyx of the adult tegument, whereas those for Dolichos biflorus (DBA) were relatively fewer in number, and those for Ulex europaeus I (UEA I) were absent. The binding patterns of Con A, WGA, RCA I and DBA were generally similar, and the reaction product was uniformly distributed over the dorsal and ventral surfaces of the parasite's body. These bindings, therefore, indicate the presence of D-mannose/D-glucose, N-acetyl-D-glucosamine/sialic acid, D-galactose and N-acetyl-D-galactosamine residues on the glycocalyx of the adult tegument.

Technical aspects of immunohistochemistry

Immunohistochemistry is an integral technique in many veterinary laboratories for diagnostic and research purposes. In the last decade, the ability to detect antigens (Ags) in tissue sections has improved dramatically, mainly by countering the deleterious effects of formaldehyde with antigen retrieval (AR) and increasing sensitivity of the detection systems. In this review, I address these topics and provide an overview of technical aspects of immunohistochemistry, including those related to antibodies (Abs) and Ags, fixation, AR, detection methods, background, and troubleshooting. Microarray technology and the use of rabbit monoclonal Abs in immunohistochemistry are also discussed.

Preparation of nonradioactive probes for in situ hybridization

In situ hybridization (ISH) enables the precise localization of RNA targets and provides an avenue to study the temporal and spatial patterns of expression of specific genes. ISH has evolved from being an esoteric technique to one that is routinely used by researchers in many areas of research. A major driving force has been the development of numerous nonisotopic labeling and signal detection methods. Historically, radioactive probes and autoradiography provided sensitivity that was unattainable with nonisotopic probes. But the long exposure times required for signal detection and the perceived dangers associated with radioactivity limit its use. Advances in nonisotopic detection systems have overcome many of the limitations associated with using radiolabeled probes. One of the most significant contributions from nonisotopic methods is the ability to discriminate between multiple nucleic acid sequences simultaneously.

A highly sensitive nonisotopic detection method for in situ hybridization

In situ hybridization is a technique that allows detection of specific DNA and RNA sequences in tissue sections. Nonisotopic techniques are fast and give a precise localization of the hybridization product, but a drawback is the low sensitivity. However, the sensitivity is dependent on the detection system used. To evaluate a sensitive in situ hybridization method with nonradioactive probes we compared three different detection systems, using biotin-labeled human papillomavirus (HPV) 16 probes. The three detection systems included (i) STAV-FITC method (streptavidin-fluorescein isothiocyanate/alkaline phosphatase anti-FITC), (ii) APAAP method (mouse anti-biotin/anti-mouse IgG/alkaline phosphatase mouse anti-alkaline phosphatase), and (iii) tyramide signal amplification (TSA) method (STAV-horseradish peroxidase (HRP)/biotinyl tyramide/STAV-HRP). The in situ hybridization methods were tested on CaSki and SiHa cells and two cervical carcinomas known to be HPV16 positive. The cells and tissues and been fixed in 4% buffered formalin and paraffin embedded. The three different detection systems gave satisfactory nuclear staining in CaSki cells (CaSki cells contain > 500 copies of HPV16 DNA) and the two cervical carcinomas. However, demonstration of HPV16 DNA in SiHa cells (SiHa cells contain one to two HPV16 genome copies) was possible only by use of the APAAP method. It was concluded that the APAAP method provides the best sensitivity among the nonisotopic detection systems and can detect single viral copies in formalin-fixed and paraffin-embedded material.

An immunohistochemical study of the distribution of biotin in tissues of pigs and chickens

An optimised indirect peroxidase-anti-peroxidase immunohistochemical technique was used to detect endogenous biotin in frozen tissue sections from biotin-supplemented and biotin-depleted pigs and chickens. A monoclonal anti-biotin antibody was used as primary antibody in this technique. Immunoreactive biotin was detected in many tissues of both species including liver, kidney, pancreas, adipose tissue, adrenal gland, testis, brain, choroid plexus, cardiac and skeletal muscle, epithelium of the respiratory and digestive systems, skin and lymphoid tissues. The specificity of immunostaining for biotin was confirmed by the finding of reduced staining intensities in tissues of biotin-depleted animals compared to those of biotin-supplemented animals. The results of this study suggest that biotin has metabolic functions in a wider range of tissues than previously known. They also indicate that endogenous tissue biotin should be considered as a source of false-positive staining when immunohistochemical or histochemical techniques which use avidin or streptavidin reagents or anti-biotin antibodies as components of the detection system, are applied to tissue sections.

Application of in situ hybridization with a novel phenytoin-labeled probe to conventional formalin-fixed, paraffin-embedded tissue sections

Non-isotopic in situ hybridization with a novel phenytoin (PHE)-labeled probe was developed. The mixture of cloned cytomegalovirus (CMV) DNA fragments was labeled by random primer technique using PHE-11(spacer)-dUTP, instead of dTTP. The tissue sections were treated with 0.2 N HCl and with proteinase K (1 microgram/ml), and then heated at 70 degrees C in the presence of 50 or 75% formamide. The sections were hybridized with PHE-labeled probe at 37 degrees C overnight. The hybridization signal was visualized by alkaline phosphatase-5-bromo-4-chloro-3-indolyl phosphate (BCIP)/4-nitroblue tetazolium (NBT) system. Strong hybridization signals were detected in sections of the small intestine and the placenta, even when denatured in the presence of 50% formamide. In the case of small intestine, CMV DNA was also detected in the endothelial cells of the mucosa where apparent infected cell was not observed histologically. In the sections of the submaxillary gland, the lung, the adrenal gland and the ovary, hybridization signal was not detected when denatured in the presence of 50% formamide, but detected after denaturation with 75% formamide. Thus, in situ hybridization with the novel PHE-labeled probe is applicable to conventional formalin-fixed, paraffin-embedded tissue sections.

Use of Phaseolus vulgaris leukoagglutinating lectin in histochemical and blotting techniques: a comparison of digoxigenin- and biotin-labelled lectins

An increase in the number of beta 1,6 branches of the trimannosyl core of asparagine-linked oligosaccharides has been shown to be directly correlated with the metastatic potential of cultured tumour cells. The Phaseolus vulgaris leukoagglutinating lectin (PHA-L) binds to beta 1,6 branches of tri- and tetra-antennary oligosaccharides. We have applied digoxigenin- and biotin-conjugated PHA-L to establish a non-radioactive detection system for beta 1,6 branches, which can be used in lectin blotting as well as light and electron microscopic cytochemistry. For this purpose the HCT116 human colon carcinoma cell line and colon carcinoma tissue were investigated. Digoxigenin-conjugated PHA-L in conjunction with alkaline phosphatase-conjugated anti-digoxigenin antibodies was superior to biotin-conjugated PHA-L in lectin blotting with respect to sensitivity and specificity. Similarly, the digoxigenin conjugated PHA-L in conjunction with gold-labelled anti-digoxigenin antibodies resulted in more intense specific staining and lower background compared to biotin-conjugated PHA-L visualized with a streptavidin immunogold complex. The specificity of lectin binding in blotting and cytochemical studies was demonstrated by the absence of staining when the lectin was omitted or preabsorbed with glycoprotein, and following pretreatment of the cellular homogenates or tissue sections by N-glycosidase F. Our results demonstrate that digoxigenin-conjugated PHA-L provides high sensitivity and specificity for histochemical and blotting techniques and is amenable for quantification. The technique should have applications in tumour research.

Pancreatic morphogenesis and extracellular matrix organization during rat development

We investigated the rat pancreatic morphology at various developmental stages ranging from 12 days of gestation to the neonatal stage, with special emphasis on alterations in extracellular matrix organization in vivo. The rat pancreatic development in utero could be divided into four representative stages as follows: (1) initial epithelial buds (12 days of gestation), (2) elongated and branching epithelium (13-14 days), (3) tubular structure (15-16 days), and (4) acinar structure (17 days or more). Ultrastructurally, the fetal and neonatal pancreata were almost constantly encompassed by continuous basal lamina, except for the earliest stage, in which minute disruptions of basal lamina were observed. Through the disruption, the direct epithelial-mesenchymal contact was formed between an endocrine cell and an adjacent mesenchymal cell, which implied epithelial-mesenchymal interactions in processes of endocrine cell differentiation. Collagen fibrils were frequently accumulated at the cleft (branchpoint) of the branching epithelium during the second and third stages mentioned above. Immunohistochemically, fibronectin and collagen type-I were localized particularly beside the neck (narrow part) or cleft of the pancreatic epithelium at these stages, although continuous linear localization of these matrices was noted around the initial pancreatic bud. This was in contrast to invariable linear localization of laminin and collagen type-IV at the epithelial/mesenchymal interface throughout the pancreatic development. Diffuse fibrillar localization of fibronectin and collagen type-I in the mesenchyme was pronounced at the later stages and after birth. Collagen type-III was only focally detectable around the pancreatic epithelium from the second stage, and its distinct localization was noted in the interlobular connective tissue after birth. Thus, chronological changes in extracellular matrix organization seemed to be closely related to morphogenetic processes of the rat pancreas, especially in the branching epithelial morphogenesis, and the major alterations appeared prior to distinct acinar cell differentiation.

In situ detection of human Ig light-chain mRNA on formalin-fixed and paraffin-embedded tissue sections using digoxigenin-labelled RNA probes

Digoxigenin-labelled RNA probes complementary to human immunoglobulin (Ig) kappa and lambda light-chain mRNAs were produced by in vitro transcription. Using these probes, several existing in situ hybridization protocols were studied. By modifying and optimizing pretreatment procedures, which include hybridization, stringency washings and probe detection, a simplified non-radioactive in situ hybridization method for Ig light-chain mRNAs was developed. The light-chain signals were consistently identified in plasma cells, germinal centrocytes, centroblasts and immunoblasts in formalin-fixed and paraffin-embedded sections of lymphoid tissues. Monotypic light-chain mRNA was demonstrated in archival cases of kappa or lambda light-chain-restricted B-cell lymphoma. Background staining was found to be negligible in all the tissues tested. These results indicate that the in situ hybridization methodology described in this study is specific and sensitive for the detection of Ig light-chain mRNAs and has practical value in routine histology.

Practical suggestions for successful immunoenzyme double-staining experiments

Many methodologies exist to perform an immunoenzyme double staining. Hence, the practical problem arises as to which of these methods is optimal for one's own experimental design. A process of selection is described which is derived from our own practical experience. First, a general strategy is outlined for the handling of tissue sections to be used for multiple staining methods. Secondly, the selection of an appropriate immunoenzyme double-staining concept is made using a flow chart. Thereafter we give criteria for the definitive selection of an immunoenzyme double-staining protocol based on the characteristics of the tissue or cell type under study. Particular attention is given to the selection of appropriate detection systems, applying enzymes or gold particles, and good contrasting colour combinations. The problems of visualizing co-localization using immunoenzyme double staining are dealt with, and suggestions are made to adapt the method, if necessary, in order to optimize it.

Detection of cytomegalovirus by in situ hybridization using a digoxigenin-tailed oligonucleotide

A non-isotopic in situ hybridization procedure was used to detect cytomegalovirus (CMV) sequences within routinely fixed tissue. A digoxigenin-tailed oligonucleotide was hybridized to sections of specimens obtained at autopsy from 2 patients with CMV infection. Hybrids were revealed by an alkaline phosphatase-conjugated anti-digoxigenin antibody. Serial sections were also assayed for the presence of CMV by in situ hybridization with a biotin-labelled cDNA probe and by immunohistochemistry and routinely stained for morphological evaluation. Results show that the two in situ hybridization procedures are equally sensitive but superior to the immunohistochemical detection of the viral antigen. Most cells positive for CMV DNA had the cytopathological features characteristic of CMV infection. A minor population of infected cells lacking morphological changes was also found. We recommend the routine application of the oligonucleotide-based assay because it is specific, easy and less expensive than other similar procedures.

Chemiluminescence dot blot hybridization assay for detection of B19 parvovirus DNA in human sera

A chemiluminescence dot blot hybridization assay was used for the detection of B19 parvovirus DNA in human sera by using digoxigenin-labeled probes. The probes were revealed immunoenzymatically by use of anti-digoxigenin Fab fragments conjugated with alkaline phosphatase. The chemiluminescence signal was obtained by reacting the labeled probe-target complex with an enzyme-triggerable dioxetane substrate. The emitted photons were detected with instant photographic films. In the search for B19 parvovirus DNA, 2,808 serum samples were analyzed.

In situ hybridisation in perspective

In the introduction to this review two questions were posed: is the technology associated with ISH ready for general use, and will the method become an important investigative tool? With the exception of the demonstration of some single and low copy sequences, non-radioactive ISH is now sufficiently developed and simplified to make it a routine technique. It is also clear that ISH will continue to have an important research role. In diagnostic pathology the technique is already providing valuable information and the present decade should see the development of many more diagnostic applications.

Sensitivity of digoxigenin and biotin labelled probes for detection of human papillomavirus by in situ hybridisation

The sensitivity of digoxigenin and biotin labelled DNA probes for the detection of human papillomavirus (HPV) by dot blotting and in situ hybridisation was compared in tissues from cervical, laryngeal, and anogenital neoplasia. Probes were either labelled with digoxigenin by the random primer technique and detected with anti-digoxigenin antibody, or labelled with biotin by nick translation and detected with streptavidin, both methods having a common final visualisation procedure using alkaline phosphatase. Digoxigenin labelled probes proved two to 10-fold more sensitive by quantitative dot blotting and four-fold more sensitive in detecting HPV 16 DNA in a series of 31 anal carcinomas, compared with biotinylated probes. The digoxigenin method also produced less non-specific background staining of tissue sections than biotin labelled probes. It is concluded that digoxigenin DNA labelling and detection provides a simple, reliable, and efficient alternative to the use of biotin or radioactive isotopes for the detection of HPV DNA by in situ hybridisation. Digoxigenin labelled probes also offer the possibility of double labelling in situ hybridisation procedures when used with biotin labelled probes to provide simultaneous identification of different DNA sequences.

In situ detection of cytomegalovirus DNA in biopsies of AIDS patients using a hybrido-immunocytochemical assay

An in situ hybrido-immunocytochemical assay, with a digoxigenin-labelled probe, was used to show the presence of cytomegalovirus DNA in both paraffin and frozen sections from tissue blocks of 5 AIDS patients. The hybridization probe was constructed by using two different DNA fragments of the repeated sequences of the CMV genome. The CMV DNA probe hybridized in situ was immunocytochemically visualized by anti-digoxigenin Fab fragments labelled with alkaline phosphatase. This hybridization procedure proved to be sensitive, specific, and provided good resolving power. Thus, it might effectively be employed in immunohistological and virological laboratories for the diagnosis of CMV infections in AIDS patients; indeed it might even be applied further in the virological context.

A hybrido-immunocytochemical assay for the in situ detection of cytomegalovirus DNA using digoxigenin-labeled probes

A non-radioactive hybrido-immunocytochemical assay for the detection of cytomegalovirus (CMV) DNA in infected cells was developed. Two different DNA fragments belonging to the repeated sequences of CMV genome were used to construct the hybridization probe. The probe was constructed by incorporating deoxyuridine triphosphate labeled with digoxigenin. The in situ hybridized CMV DNA probe was immunocytochemically visualized by anti-digoxigenin. Fab fragments labeled with alkaline phosphatase. This procedure permitted the DNA detection, in the nuclei of infected cells fixed at 48 h after infection, of the Towne CMV reference strain and 21 different laboratory-isolated CMV strains. Our assay demonstrated a high specificity, sensitivity and reproducibility.

Detection of viral genomes in the liver by in situ hybridization using 35S-, bromodeoxyuridine-, and biotin-labeled probes

Methods employing 35S-, biotin-, and bromodeoxyuridine (BrdUrd)-labeled DNA probes were compared for the detection of hepatitis B virus (HBV) and cytomegalovirus (CMV) in the liver. The results demonstrate that: 1) HBV can be detected reliably only by the use of radiolabeled probes, whereas methods employing nonradioactive probes obviously are not sensitive enough for this virus. The use of 35S-labeled probes shortens the exposure times considerably in comparison to tritiated probes. 2) Biotin-labeled probes are of limited value for in situ hybridization on liver tissues because the presence of endogenous avidin-binding activity often leads to false positive results. 3) Brd-Urd-labeled probes are a useful alternative to biotinylated probes for the detection of CMV. In comparison with biotinylated probes, BrdUrd-labeled probes produce a specific signal of similar staining intensity in the absence of background staining in the liver.

Multiple immunoenzyme staining techniques. Use of fluoresceinated, biotinylated and unlabelled monoclonal antibodies

Simultaneous detection of multiple tissue epitopes with an overlapping distribution pattern by monoclonal antibodies is sometimes needed for routine immunohistological evaluations. Therefore, multistep double and triple immunoenzymatic methods using antibodies from the same species or Ig (sub)class have been developed. Since only commercially available monoclonal antibodies (either unlabelled, biotinylated or as fluorescein conjugate) have been used, the techniques may be regarded as generally applicable. The staining protocol for double staining consists of six incubation steps: (1) unlabelled monoclonal antibody 1; (2) enzyme I-conjugated anti-mouse Ig; (3) normal mouse serum--for blocking; (4) fluoresceinated monoclonal antibody 2; (5) rabbit anti-fluorescein isothiocyanate--employing the fluorochrome as hapten; (6) enzyme II-conjugated anti-rabbit Ig. For enzymes I and II, peroxidase, alkaline phosphatase and beta-galactosidase can be applied; excellent results were obtained with the following colour combinations: peroxidase activity in red/alkaline phosphatase in blue and beta-galactosidase in green/alkaline phosphatase in violet. Moreover, this double staining method can be extended to provide an immunoenzyme triple staining technique by mixing biotinylated monoclonal antibody 3 and avidin-biotin enzyme III complex with the steps 4 and 5 reagents, respectively. In this way three tissue epitopes can simultaneously be detected clearly and selectively in green (beta-galactosidase), blue (alkaline phosphatase) and red (peroxidase).

Use of commercially available monoclonal antibodies for immunoenzyme double staining

An immunoenzyme double-staining method for the simultaneous detection of two cellular epitopes, using commercially available mouse monoclonal antibodies, is described. The method employs a combination of the suppression of endogenous biotin and two successive indirect techniques with a blocking step in between. The first indirect method involves an unlabelled monoclonal antibody followed by an alkaline phosphatase-conjugated goat anti-mouse immunoglobulin. After a blocking step with normal mouse serum, the second indirect method is applied using a biotinylated monoclonal antibody followed by the visualization of this antibody by avidin-biotinylated peroxidase complex (ABC) or rabbit anti-biotin and peroxidase-conjugated swine anti-rabbit immunoglobulin in successive steps. Using these methods in combination with the introduction of dioctyl sodium sulphosuccinate and tetramethylbenzidine as chromogens for peroxidase activity, two cellular epitopes could be distinguished clearly in tissue sections by the green- and violet-stained peroxidase and alkaline phosphatase activities, respectively. The expression of two epitopes on the same cellular constituent is outlined by the coappearance of both enzyme activities as a bluish-purple colour. This method allows for the simultaneous identification, localization and enumeration of two cellular epitopes. These can serve as parameters for a number of pathological processes.

Detection of viral DNA by in situ hybridization using bromodeoxyuridine-labeled DNA probes

Detection of viral DNA in infected tissues is demonstrated by in situ nucleic acid hybridization employing bromodeoxyuridine (BrdUrd)-labeled probes. BrdUrd is used alternatively to biotinylated nucleotides in the preparation of DNA probes and, after hybridization, developed with a monoclonal anti-BrdUrd antibody and the alkaline phosphatase-anti-alkaline phosphatase technique. The sensitivity using both BrdUrd- and biotin-labeled probes is similar. Furthermore, the simultaneous detection of viral coinfections will be possible by means of this technique in conjunction with biotin-labeled DNA probes.

The utilization of biotin-antibiotin interaction for the detection of antibody to hepatitis B surface antigen (anti-HBs)

A biotin-antibiotin solid-phase enzyme-linked immunoassay for the detection and quantitation of the antibody to hepatitis B surface antigen (anti-HBs) is described. The assay utilizes hepatitis B surface antigen as a solid-phase 'capture' reagent and a mixture of biotinylated HBsAg and antibiotin-conjugated horseradish peroxidase as a detector reagent. The assay was compared to a commercial enzyme immunoassay (AUSAB EIA) which used the biotin-avidin system for anti-HBs detection. The two assays were found to measure the same molecules and to correlate well regarding anti-HBs titers.

Simultaneous inhibition of endogenous avidin-binding activity and peroxidase applicable for the avidin-biotin system using monoclonal antibodies

The use of the avidin-biotin technique in immunoperoxidase staining provides a simple and highly sensitive method for detecting the localization of antigens defined by monoclonal antibodies. However, endogenous biotin, which is widely distributed in tissues, often causes non-specific staining by binding to avidin [endogenous avidin-binding activity (EABA)]. Endogenous peroxidase activity (EPA) also makes the estimation of specific staining difficult. In the present study, several methods for the inhibition of EABA and/or EPA were examined using the avidin-biotin technique and monoclonal antibodies against murine Mac-1 and Ia antigen. Of these, the overnight incubation of sections in 40% methanol in phosphate-buffered saline containing 0.3% hydrogen peroxide gave the best result, as it inhibited EABA and EPA simultaneously without denaturating of the antigenic determinants recognized by the monoclonal antibodies.