IMMUNOHISTOCHEMICAL DETECTION OF PRION PROTEIN IN SHEEP WITH SCRAPIE

Prion protein (PrP), which is involved in the pathogenesis of scrapie, occurs in 2 forms. The form extracted from scrapie brain is protease resistant (PrP-res), whereas PrP from normal brain is protease sensiti ve (PrP-sen). This study examined whether PrP-res could be detected in brains of sheep with scrapie by immunohistochemistry (IHC). A suitabl e IHC procedure was developed using brain tissue from hamsters that ha d been inoculated with the transmissible mink encephalopathy agent. Ti ssue samples were fixed in PLP (periodate, lysine, paraformaldehyde) t hat contained paraformaldehyde at a concentration of 0.125%. Before ap plication of the IHC technique, tissue sections were deparaffinized an d treated with formic acid to simultaneously enhance PrP-res immunorea ctivity and degrade PrP-sen. Primary antibody was obtained from a rabb it immunized to PrP-res extracted from brains of mice with experimenta lly induced scrapie. Brains from 21 sheep with histopathologically con firmed scrapie were examined by IHC. In all 21 brains, PrP-res was wid ely distributed throughout the brain stem. Staining was particularly i ntense in neuronal cell bodies and around blood vessels, The IHC techn ique successfully detected PrP-res in brain samples that had been froz en or that were severely autolyzed before fixation in PLP. Brains from 11 scrapie-suspect sheep that were not considered histologically posi tive were also examined by IHC. PrP-res was found in 4 of these brains . Sections of brains from 14 clinically normal sheep did not have dete ctable PrP-res. Results of this study indicate that IHC detection of P rP-res is equivalent, and perhaps superior, to histopathology for the diagnosis of scrapie in sheep. Furthermore, IHC is applicable to tissu es that have autolytic changes or processing artifacts that prevent sa tisfactory histopathologic evaluation for lesions of scrapie.