The use of proteolysis with ficin, for immunostaining of paraffin sections. A study of lymphoid, mesenchymal, and epithelial determinants in human tissues.

In order to maximize staining, modifications of immunostaining methods have included proteolytic enzyme digestion of tissue. The authors performed a study of the effect of ficin in 110 paraffinized specimens, including tonsil, lymph nodes, benign vascular and nerve sheath tumors, and various carcinomas and sarcomas. This agent was compared with pepsin and bromelain, as alternative proteases. A panel of monoclonal and polyclonal antibodies was used, with and without previous digestion by ficin, pepsin, and bromelain. A score was assigned to each stain, based on the number and intensity of reactive cells. Ficin enhanced staining markedly in immunostains with antibodies to keratin and Factor VIII-related antigen (F8RAG). Conversely, it abolished staining for LN-2 (a lymphoid marker) and weakened reactivity for S-100 in nerve sheath tumors. Bromelain produced similar results, except that it enhanced S-100. Pepsin was comparatively less active than ficin and bromelain overall but did produce the greatest amplification of vimentin staining in sarcomas. Digestion with any of the three enzymes failed to influence reactivities of leukocyte common antigen, UCHL-1 (a lymphoid marker), alpha-1-antichymotrypsin, carcinoembryonic antigen, epithelial membrane antigen, and blood group isoantigens. These results may reflect a dissimilar recognition of peptide targets in some antigenic proteins, by ficin, bromelain, and pepsin. Hence, one enzymatic agent is unlikely to produce optimal staining for all determinants. With this proviso, however, ficin appeared to be the best general enhancer for antigens known to require vigorous digestion (e.g., keratin; F8RAG) for optimal reactivity in paraffin sections.