Improvement of supersensitive immunohistochemistry with an autostainer: a simplified catalysed signal amplification system.

The ImmunoMax/catalysed signal amplification (CSA) system is a supersensitive method of paraffin immunohistochemistry. It incorporates antigen retrieval, the streptavidin-biotin complex (sABC) method, and the catalysing reporter deposition/catalysing biotinylated tyramide reaction. Strong, non-specific cytoplasmic reaction in the ImmunoMax/CSA is due to endogenous biotin unmasked in the antigen retrieval step. We examined procedures to diminish this non-specific immunoreaction and improved the ImmunoMax/CSA. Antigen retrieval in a hot water bath yielded a smaller endogenous biotin immunoreaction than antigen unmasking in an autoclave. Post-antigen retrieval fixation in buffered 10% formalin solution suppressed the biotin immunoreaction but masked the target antigen, Ki67. Post-reaction washing with 0.1% Tween 20 in Tris-HCl buffer at 35 degrees C did not diminish the endogenous biotin immunoreaction. Animal serum also did not suppress the non-specific immunoreactivity of biotin and antibodies. Because endogenous biotin is detected by duplicated biotin-streptavidin reactions in the ImmunoMax/CSA, we replaced the sABC step with a labelled polymer secondary antibody (the EnVision system)--a simplified CSA system--because the sensitivity of the EnVision system was the same as that of the sABC method. The non-specific immunoreaction induced by the EnVision system was masked competitively by blocking protein. By using an antibody against Ki67 antigen that can react only with the nucleus, we were able to evaluate the non-specific cytoplasmic immunoreaction induced by the detection system. We believe that the simplified CSA system will open up the field of supersensitive paraffin immunohistochemistry.