Comparative Study of Antigen Retrieval Heating Methods: Microwave, Microwave and Pressure Cooker, Autoclave, and Steamer

We present a study comparing the most popular beating methods currently used for antigen retrieval (AR) immunostaining: the microwave oven, microwave with pressure cooker, autoclave, and steamer heating. A panel of 21 antibodies was tested on formalin fixed, paraffin embedded sections using these heating methods and Tris-HC1 buffer, pH 9.5, plus 5% urea as the AR solution. Three observers independently evaluated the intensity of AR immunostaining. All heating methods yielded good results for AR immunostaining. There were only minor differences among the heating methods for AR when the optimal concentration of primary antibody for normal immunostaining was used; however, background staining may occasionally be troublesome if antibodies are not retitrated and diluted further for use on tissues after AR. Significant differences were observed only after further dilution of the primary antibodies; the microwave pressure cooker, extended microwave heating (5 min × 4) and autoclave heating then showed a similar intensity of staining that was stronger than results obtained with the steamer (20 min) or regular microwave heating (5 min × 2 J. Extension of the steamer heating time, however, yielded equivalent results. This study indicates that different heating methods can yield similar intensities of AR immunostaining if the heating times are adjusted appropriately. It is noteworthy that, in general, the adjusted conditions for maximal retrieval differ from those most widely cited in the literature, or recommended by manufacturers. That several heating devices may provide similar results permits the use of different AR heating methods according to the equipment available. This study also is an early step in standardizing the AR immunostaining protocol by providing uniform conditions for “maximal retrieval” as a common end point for all laboratories.     

MIB-1 immunostaining on cytological samples: a protocol without antigen retrieval.

The aim of the study was to evaluate the effect of fixation procedures on MIB-1 immunostaining on microwave-treated Papanicolaou-stained slides and to establish protocol for MIB-1 immunostaining on cytological samples without microwave pre-treatment. Cytospins for immunostaining and nuclear suspension for DNA measurement were prepared from human breast cancer cell line (MCF-7). Following fixation, the cytospins were either stained by Papanicolaou, stored in methanol or air-dried. Antigen retrieval by microwave was used before MIB-1 immunostaining only for Papanicolaou-stained cytospins. Air-dried cytospins and cytospins stored in methanol were immunostained without pre-treatment. The percentage of MIB-1 positive cells was compared with the S phase fraction of MCF-7 cells calculated from DNA histograms. Variations in fixation procedures used before Papanicolaou staining had no influence on the percentage of MIB-1 positive cells. The difference between the percentage of the MIB-1-positive cells on microwave-treated Papanicolaou-stained cytospins and on methanol-fixed cytospins without microwave pre-treatment was not significant. There was a strong correlation between the percentage of the MIB-1-positive cells and S phase fraction. Monoclonal antibody MIB-1 recognized Ki-67 antigen in Papanicolaou-stained cytospins treated by microwave as well as in cytospins that were fixed and stored in methanol without microwave pre-treatment.     

Antigen retrieval in formalin-fixed, paraffin-embedded tissues: an enhancement method for immunohistochemical staining based on microwave oven heating of tissue sections.

We describe a new approach for retrieval of antigens from formalin-fixed, paraffin-embedded tissues and their subsequent staining by immunohistochemical techniques. This method of antigen retrieval is based on microwave heating of tissue sections attached to microscope slides to temperatures up to 100 degrees C in the presence of metal solutions. Among 52 monoclonal and polyclonal antibodies tested by this method, 39 antibodies demonstrated a significant increase in immunostaining, nine antibodies showed no change, and four antibodies showed reduced immunostaining. In particular, excellent immunostaining results were obtained with a monoclonal antibody to vimentin as well as several different keratin antibodies on routine formalin-fixed tissue sections after pre-treatment of the slides with this method. These results showed that after antigen retrieval: (a) enzyme predigestion of tissues could be omitted; (b) incubation times of primary antibodies could be significantly reduced, or dilutions of primary antibodies could be increased; (c) adequate staining could be achieved in long-term formalin-fixed tissues that failed to stain by conventional methods; and (d) certain antibodies which were typically unreactive with formalin-fixed tissues gave excellent staining.     

Effects of antigen retrieval by microwave heating in formalin-fixed tissue sections on a broad panel of antibodies

Formaldehyde fixation of biopsy specimens for routine purposes has often been held responsible for the poor reproducibility of immunohistochemical studies. Recently, antigen retrieval (AGR) using microwave irradiation was described as a potential tool to enhance immunostaining. A comparison of conventional staining and staining after microwave heating was performed for 52 markers, using tissues fixed in formaldehyde for 24 h, 1 to 6 weeks and 3 years respectively, as well as consultant case material. After adequate duration of fixation (24 h), only a few markers (17%) showed better results after AGR, but this percentage was increased to 50% when tissues were fixed for longer periods. Maximal enhancement was obtained in the group of consultant cases (58% of tested markers demonstrated better staining results), in which the period of fixation and tissue processing was unknown. To achieve reliable enhancement with AGR, continuous heating (100° C) should not be shorter than 20 min. In conclusion, AGR may become the most important tool to simplify and equalize immunohistochemical techniques, if critically evaluated.     

Improved immunocytochemical staining through the use of Fab fragments of primary antibody, Fab-specific second antibody, and Fab-horseradish peroxidase

A modification of the unlabeled antibody method of immunocytochemistry is described here that offers increased immunoreagent penetration and greatly reduced background staining. The method involves the following alterations to the conventional technique; the use of Fab fragments of primary antibody, rather than whole immunoglobulin G (IgG) or serum; the use of a second, or link, anti-rabbit IgG serum that is Fab fragment-specific, rather than directed against the whole rabbit IgG molecule; the use of the Fab--horseradish peroxidase complex described by JR Slemmon, PM Salvaterra, and K Saito (J Histochem Cytochem 28:10, 1980), rather than peroxidase--antiperoxidase (PAP). Steps 2 and 3 alone brought about a significant reduction in background staining, but did not increase the depth of immunostaining, as compared to the PAP technique. When all three steps were combined, however, background staining was further reduced, and there was a five- to tenfold increase in the depth of immunostaining. These readily made changes should be useful in preembedding immunocytochemistry whenever enhanced reagent penetration is required.     

Non-specific Immunostaining by a Rabbit Antibody against Gustducin α Subunit in Mouse Brain

Gustducin is a guanosine nucleotide-binding protein functionally coupled with taste receptors and thus originally identified in taste cells of the tongue. Recently, bitter taste receptors and gustducin have been detected in the airways, digestive tracts and brain. The existing studies showing taste receptors and gustducin in the brain were carried out exclusively on frozen sections. In order to avoid the technical shortcomings associated with frozen sectioning, we performed immunofluorescence staining using vibratome-cut sections from mouse brains. Using a rabbit gustducin antibody, we could not detect neurons or astrocytes as reported previously. Rather, we found dense fibers in the nucleus accumbens and periventricular areas. We assumed these staining patterns to be specific after confirmation with conventional negative control staining. For the verification of this finding, we stained gustducin knockout mouse brain and tongue sections with the same rabbit gustducin antibody. Whereas negative staining was confirmed in the tongue, intensive fibers were constantly stained in the brain. Moreover, immunostaining with a goat gustducin antibody could not demonstrate the fibers in the brain tissue. The present study implies a cross immunoreaction that occurs with the rabbit gustducin antibody in mouse brain samples, suggesting that the conventional negative controls may not be sufficient when an immunostaining pattern is to be verified.     

Evaluation of immunohistochemical staining of human duodenal endocrine cells after microwave antigen retrieval

Summary The effect of microwave antigen retrieval on the immunostaining of human duodenal endocrine cells in formaldehyde-fixed, paraffin-embedded material was investigated. The sections were immunostained by the avidin-biot in complex (ABC) and immunogold-silver autometallography (IGSS) methods with and without prior microwave treatment. Dilutions of up to 1:30 000 of the following antisera/antibodies were used: anti-chromogranin A, anti-chromogranin AB, anti-secretin, anti-gastrin, anti-gastric inhibitory polypeptide, anti-somatostatin and anti-serotonin. The detection threshold for all the antibodies was lower after antigen retrieval, and the primary antibody could be used in higher dilutions. The dilutions varied for different antibodies and were between two and ten times the optimal dilution without antigen retrieval. At extremely high dilutions of, or without, the primary antibody, non-specific staining of some lymphocytes and the mucus of some goblet cells was observed when the avidin method was applied, but not with the immunogold technique. This phenomenon was not observed when optimal dilution or a lower dilution was used. This seems to have been caused by the binding of the avidin-biotin complex to epitopes in these structures unmasked by microwave treatment when competition with specific binding sites was absent. This revised version was published online in November 2006 with corrections to the Cover Date.     

Using microwave irradiation in Marchi's method for demonstrating degenerated myelin

Conventional methods for histological preparation of degenerated myelin are time-consuming and difficult. The purpose of our study was to shorten the time required for the procedure and to obtain better quality results for light microscopic demonstration of degenerated myelin in the central and peripheral nervous systems by using microwave irradiation. Rat brain and sciatic nerve were used for the study. The middle cerebral artery was occluded and the sciatic nerve was cut to produce myelin degeneration. Marchi's method was used for staining degenerated myelin. Fixation for light microscopy that would take two days using the conventional procedure was completed in 16.5-18.5 min using microwave irradiation. While staining of degenerated myelin requires 10 days for the conventional Marchi method, we decreased it to 7 h for brain tissue and 1 h for sciatic nerve by using the microwave oven. Moreover, a better quality preparation was achieved in the groups stained under microwave irradiation than those prepared by the conventional method.     

Using microwave irradiation in Marchi's method for demonstrating degenerated myelin.

Conventional methods for histological preparation of degenerated myelin are time-consuming and difficult. The purpose of our study was to shorten the time required for the procedure and to obtain better quality results for light microscopic demonstration of degenerated myelin in the central and peripheral nervous systems by using microwave irradiation. Rat brain and sciatic nerve were used for the study. The middle cerebral artery was occluded and the sciatic nerve was cut to produce myelin degeneration. Marchi's method was used for staining degenerated myelin. Fixation for light microscopy that would take two days using the conventional procedure was completed in 16.5-18.5 min using microwave irradiation. While staining of degenerated myelin requires 10 days for the conventional Marchi method, we decreased it to 7 h for brain tissue and 1 h for sciatic nerve by using the microwave oven. Moreover, a better quality preparation was achieved in the groups stained under microwave irradiation than those prepared by the conventional method.     

Using microwave irradiation in Marchi's method for demonstrating degenerated myelin

Conventional methods for histological preparation of degenerated myelin are time-consuming and difficult. The purpose of our study was to shorten the time required for the procedure and to obtain better quality results for light microscopic demonstration of degenerated myelin in the central and peripheral nervous systems by using microwave irradiation. Rat brain and sciatic nerve were used for the study. The middle cerebral artery was occluded and the sciatic nerve was cut to produce myelin degeneration. Marchi's method was used for staining degenerated myelin. Fixation for light microscopy that would take two days using the conventional procedure was completed in 16.5–18.5 min using microwave irradiation. While staining of degenerated myelin requires 10 days for the conventional Marchi method, we decreased it to 7 h for brain tissue and 1 h for sciatic nerve by using the microwave oven. Moreover, a better quality preparation was achieved in the groups stained under microwave irradiation than those prepared by the conventional method.     

A sensitive and specific ES-31 antigen detection based fluorometric assay for confirmation of Mycobacterium tuberculosis in cell culture

Confirmation of presence of M. tuberculosis bacilli on microscopic examination is very important in diagnosis of tuberculosis. The present study was undertaken to find the usefulness of mycobacterial ES-31 serine protease as a marker to detect tuberculosis bacilli using fluorescein isothiocyanate conjugated anti-ES-31 serine protease antibody. This immunofluorescence method was compared with Ziehl-Neelsen and auramine-O staining methods for detection of tuberculosis bacilli. Slides were prepared for each serially diluted tuberculosis H37Ra bacilli (1 x 10(7) bacilli/ml to 5 bacilli/ml). Slides for each dilution group were stained by ZN method, auramine-O and immunostaining methods using fluorescein isothiocyanate conjugated anti-ES-31 serine protease antibody. ZN staining method showed efficacy for detection of M. tuberculosis H37Ra upto 1 x 10(4) bacilli/ml while auramine-O method showed upto 1 x 10(2) bacilli/ml. The presence of bacilli was indicated by green fluorescence on immunostaining using anti-ES-31 antibody conjugate and this method was effective upto 10 bacilli/ml. The slides which were negative for ZN (1 x 10(3) cells/ml) and auramine-O (100 cells/ml) method showed positivity on restaining with immunofluorescent staining method. The results of this preliminary study showed that immunofluorescent staining method using specific anti-ES-31 antibody conjugate was more sensitive for detection of tuberculosis bacilli than ZN and auramine-O methods in samples of laboratory strain. The utility of this method will be studied further in clinical specimens.     

Method of Melanin Bleaching in MIB1-Ki67 Immunostaining of Pigmented Lesions: A Quantitative Evaluation in Malignant Melanomas

The effect of melanin bleaching on the immunoreactivity of the MIB1-Ki67 antigen in pigmented melanocytic lesions was investigated. Eight paired non-pigmented and heavily pigmented malignant melanomas (6 primary melanomas and 2 secondary melanomas) were selected. Avidin–biotin immunoperoxidase complex (ABC) and microwave antigen retrieval were used in immunostaining. Sections were incubated with 10% H2O2 for 24h before immunostaining with primary antibody MIB1, or after the completion of immunostaining. Non-bleached controls were obtained by conducting the identical staining but omitting the bleaching procedure. In all heavily pigmented lesions bleached by 10% H2O2 before or after immunostaining, the melanin was bleached effectively and MIB1-positively stained cells were clearly seen. Cell counting in the non-pigmented group found that there were no significant differences in the percentage of MIB1-positive melanoma cells (%MIB1) between non-bleached controls and those sections which had been bleached by 10% H2O2 either before or after the immunostaining. The results suggest that hydrogen peroxide can effectively bleach melanin in pigmented melanocytic lesions without significantly affecting MIB1-Ki67 immunolabelling.     

Cytokeratin demonstration in thick breast tissue slices

Tissue slices (500 to 1000 μm thick) of archival formalin-fixed, paraffin-embedded breast tissue were immunostained by a cytokeratin antibody (MNF116) using a streptavidin--biotin complex procedure. The technique requires prolonged exposure of tissue slices to the reagents. Use of the detergent Triton X-100 facilitated penetration of high molecular weight reagents through the tissue slices. Fifty of 58 slices 500 μm thick (86%) showed good to excellent immunostaining, and 13 of 20 slices 1000 μm thick (65%) showed similar staining. Omission of the primary antibody eliminated any immunostaining. Comparison with corresponding Haematoxylin staining of the thick slices (the conventional procedure for such breast tissue slices) showed that thick-slice cytokeratin immunostaining markedly improved visualization of the epithelial structure in normal lobules and invasive carcinomas. Although the immunohistochemical technique takes 33 days for completion, the quality of the epithelial images outweighs this disadvantage. This revised version was published online in November 2006 with corrections to the Cover Date.     

Localization of phosphophoryn in rat incisor dentin using immunocytochemical techniques

We studied the distribution of the phosphophoryn present in rat incisors by immunolocalization and histochemical techniques. The polyclonal antibody used reacts with both phosphorylated and de-phosphorylated phosphophoryn. Technical problems encountered in immunostaining and in preparing sections from mineralized dentin were resolved by use of peroxidase-conjugated protein A as the "second antibody" in indirect immunostaining reactions and by surface etching of partially demineralized sections. Staining with anti-rat incisor alpha-phosphophoryn antibody showed light staining over the odontoblasts and proximal odontoblastic processes, no stain over the predentin, dense staining over the intertubular dentin, and no stain over the mantle dentin. In the intertubular dentin the stain intensity was directly related to the distribution of mineral. These findings were directly corroborated by staining with Stains All. The mineralization of dentin and the distribution of phosphophoryn within the dentin may be much less uniform than previously supposed.     

Thymidylate synthase expression in patients with colorectal carcinoma using a polyclonal thymidylate synthase antibody in comparison to the TS 106 monoclonal antibody.

Colorectal cancer is one of the most common human cancers, for which 5-fluorouracil (5FU) is usually part of the treatment. Thymidylate synthase (TS), the target enzyme for 5FU, can be predictive for the outcome of 5FU-based therapy. TS levels in tumor samples can be determined with radiochemical enzyme assays, RT-PCR, and immunohistochemical staining. We validated TS immunohistochemistry with a polyclonal rabbit anti-human TS antibody using the avidin-biotin method. This antibody can be used on paraffin-embedded, formalin-fixed material using an antigen retrieval method with citrate buffer and microwave treatment. The antibody shows a granular cytosolic staining pattern. The reproducibility in cross-sections from colorectal tumors from 50 patients was 90% and the interobserver variability was acceptable with a kappa of 0.45. On Western blotting it detects purified TS at 36 kD, while in 5FU-treated cells the ternary complex between FdUMP, TS, and 5, 10-methylene-tetrahydrofolate is clearly visible at 38 kD, with no other interfering bands. In a separate set of tumors, immunostaining was compared with enzyme levels; Western blots correlated with enzyme levels. Because both this polyclonal antibody and the monoclonal antibody TS-106 are being used for large-scale studies, we also determined whether they could be used interchangeably. No differences were observed. This polyclonal antibody is specific and gives reproducible results. A study on a larger scale is ongoing to determine the role of TS as a predictive parameter in patients with colorectal cancer treated either with postoperative adjuvant 5FU/levamisole or with surgery only.     

Immunostaining of growth hormone and prolactin in paraffin-embedded and stored or previously stained materials.

In attempts to evaluate immunocytochemically autopsy and biopsy material previously obtained and processed for conventional histologic staining, we had to resort to immunostaining of tissues embedded years ago or even sections already stained with hematoxylin-eosin or aldehyde thionin-PAS-orange G. Hypophysial growth hormone and prolactin proved remarkably resistant to such prior treatment with regard to their antigenic properties, and could be readily immunostained in tissue embedded in paraffin 3-4 years earlier, and after destaining of sections prepared up to 7 years earlier. The results of such "retrospective" immunocytochemical evaluation of autopsy and biopsy materail is illustrated with the staining of "pregnancy cells" for prolactin in the hypophysis of a woman postpartum, the immunostaining for prolactin in the cells of adenomas associated with marked hyperprolactinemia, the staining for growth hormone in adenomas removed from children with gigantism, and the immunostaining for prolactin, growth hormone or both in several adenomas that were discovered at autopsy and not associated with a known clinical history of endocrine aberrations.     

Species-specific immunocytochemical localization of Mycobacterium tuberculosis complex in fine needle aspirates of tuberculous lymphadenitis using antibody to 38 kDa immunodominant protein antigen

OBJECTIVE: To examine immunocytochemical localization of Mycobacterium tuberculosis (MTB) complex antigen in fine needle aspiration (FNA) smears of tuberculous lymphadenitis (TBLN) using species-specific monoclonal antibody MTSS to 38-kDa immnunodominant protein antigen as a diagnostic adjunct to conventional cytomorphology and its advantage over Ziehl-Neelsen (ZN) microscopy. Study Design FNA smears from 340 cases-174 TBLN; 34 negative controls from nontuberculous, positive controls of 13 known acid-fast bacilli (AFB)-positive sputum smears; 50 blind controls; and 69 other controls (smears from stock cultures of bacterial, atypical mycobacteria and fungal species) were subjected to ZN and immunocytochemical staining using MTSS by the streptavidin-biotin method. RESULTS: Immunocytochemical staining was positive in 59 of 61 (96.7%) archival and 110 of 113 (97.3%) fresh FNA smears; ZN positivity for AFB was observed in 27 of 61 (44.2%) archival and 48 of 113 (42.4%) fresh FNA smears of TBLN. CONCLUSION: The immunostaining using MTSS showed a definite advantage over conventional ZN staining for detection and specific diagnosis of TBLN in FNA smears with 0% false positive results. Immunostaining of cytosmears with species specific antibody to MTB would prove to be a good diagnostic adjunct to morphologic diagnosis.     

A practical approach to routine immunostaining of paraffin sections in the microwave oven

Summary The Streptavidin-Biotin Complex (SABC) Immunostaining method can be carried out by performing the rinsing and blocking steps and in addition the incubations with the primary and the secondary antibody sera in the microwave oven. Irradiation of the Streptavidin-Biotin Complex reduces stain activity due to destruction of horseradish peroxidase (HRP) (Boon & Kok, 1988). Therefore, we decided not to perform this step in the microwave oven. The microwave incubations can be performed using antisera dilutions of 1:1000 (instead of 1:50) in tissue fixed with Kryofix, allowing staining in staining racks. This keeps hands-on time low and simplifies the microwave steps. It is very difficult to obtain reproducible results in the microwave oven using droplet incubations due to problems with hot spots and antenna effects. These problems are avoided when cuvettes are used and air is blown through the solutions during microwave incubations. With effective temperature control the method is highly reproducible, and takes at least 100 min less than the conventional SABC method. It is, in particular, attractive when large series of slides must be routinely immunostained and when reproducible results are desired.     

Immunohistochemical localization of D-alanine to beta-cells in rat pancreas

A mouse monoclonal antibody against D-alanine (D-Ala) has been raised and the immunohistochemical localization of this D-amino acids in the rat pancreas is visualized. The obtained anti-D-Ala monoclonal antibody has no significant cross-reactivity to all proteinogenic L-amino acids and their D-enantiomers. Using this antibody, immunohistochemical staining was performed on the pancreas, and specific staining for d-Ala has been observed only in the Langerhans islets. To identify the types of D-Ala-immunopositive cells, double staining was carried out with antibodies against D-Ala and pancreatic hormones. Similar immunostaining patterns have been observed for D-Ala and insulin, while D-Ala is hardly co-localized with other hormones (glucagon, somatostatin, and pancreatic polypeptide). These results indicate for the first time that D-Ala is localized to insulin producing beta-cells in mammalian pancreas, suggesting that this D-amino acid would be involved in the regulation of the blood glucose level.     

Microwave irradiation for fixation and immunostaining of endothelial cells in situ

Using microwave irradiation during tissue fixation and immunostaining reduces sample preparation time and facilitates penetration of fixatives and antibody solutions into the tissues. This results in improved fixation and reduction of non-specific binding of antibodies, respectively. Experimental analyses of endothelial cells in blood vessels in situ have been limited because of the difficulty of tissue preparation. We report here a technique using intermittent microwave irradiation for blood vessel fixation and immunostaining the fixed tissues. Intermittent microwave irradiation during fixation reduced blood vessel contraction and resulted in well preserved morphology of blood vessels, especially the endothelial cells. Microwave irradiation also reduced non-specific binding of fluorescein-labeled antibodies. These microwave irradiation-assisted techniques are useful for analysis of endothelial cell function and for pathological study of blood vessels in situ.