Reproducible and reliable immunohistochemical demonstration of thymidylate synthase in formalin-fixed, paraffin-embedded sections: Application of antigen retrieval in EDTA solution

Previous studies have suggested that high expression of thymidylate synthase (TS) is involved in the resistance of cancer cells to 5-fluorouracil (5-FU) and its derivatives. In order to overcome the difficulty in identifying TS immunoreactivity in formalin-fixed, paraffin-embedded sections, we determined the most suitable and reproducible condition of antigen retrieval for TS localization. Antigen retrieval methods examined included 0.2% trypsin, 0.002% proteinase K, and pressure cooking in 10 mM citrate buffer, pH 6.0 and pH 7.0, and 1 mM EDTA (ethylenediaminetetraacetic acid) solution, pH 8.0. Pressure cooking with EDTA was the best choice for retrieving the antigenicity of TS. Polyclonal antibody was superior to monoclonal antibody TS106. Preabsorption test demonstrated the specificity of the immunostaining. The staining sequence using routine paraffin sections can contribute to the establishment of the prognostic indicator and optimal 5-FU dosage in a variety of 5-FU-sensitive or -insensitive malignancies.