Comparison of p53 immunoreactivity in fresh-cut versus stored slides with and without microwave heating

Because of technical problems in immunohistochemical staining of archival material, antigens can be masked or lost. A recent study reported diminished p53 immunoreactivity in slides that had been sectioned from parafinn-embedded tissue blocks and stored at room temperature. To investigate this issue, we performed immunohistochemical staining with use of a p53 monoclonal antibody (DO7) in 13 head and neck squamous cell carcinoma (HNSCC) and 13 non-small-cell lung carcinomas. The fresh-cut and stored slides were simultaneously stained with and without the use of a microwave heating (MWH) technique, and we compared the results of p53 immunostaining. The stored slides were sectioned from paraffin blocks 4 to 25 years old and kept at room temperature for 6 to 48 months. The slides were blindly evaluated for percentage of positivity and staining intensity. Twelve HNSCCs and six lung carcinomas showed p53 positivity. The stored slides showed a considerable decrease in staining intensity (P = 0.039), compared with fresh-cut slides. The difference in the percentage of positivity between the stored and fresh-cut slides was statistically significant, but the mean value of the difference was only 3.6%, which might not be meaningful for semiquantitation of immunostaining. MWH greatly enhanced staining intensity and percentage of positivity for both stored and fresh-cut slides. When MWH was applied, no significant difference in staining intensity (P = 0.063) was detected in fresh-cut versus stored slides, but the difference in the percentage of positivity was statistically significant (mean value, 3.1%). Individual cases showed a consistent p53 status regardless of the MWH treatment, storage duration, or age of the blocks. This study demonstrated a considerable decrease in p53 immunoreactivity in stored slides. Because the MWH successfully retrieved the p53 antigen without causing a change in p53 status, stored slides combined with an MWH antigen retrieval technique in a metal-containing solution should provide p53 immunostaining results similar to those from fresh-cut slides, as long as staining intensity is not a sole study parameter.