Ovine infection with the agents of scrapie (CH1641 isolate) and bovine spongiform encephalopathy: immunochemical similarities can be resolved by immunohistochemistry

Immunochemical ("rapid") tests, which recognize a partly protease-resistant conformer of the prion protein (PrP(res)) are now widely used in Europe for the diagnosis of transmissible spongiform encephalopathies (TSEs). Some of these tests can be used to distinguish natural scrapie from experimental bovine spongiform encephalopathy (BSE) in sheep, on the basis of migration pattern differences of PrP(res) in Western immunoblots. However, PrP(res) from sheep inoculated with CH1641 scrapie gives an immunoblot profile similar to that of sheep inoculated with BSE. Therefore, field scrapie strains similar to CH1641 might be misclassified as ovine BSE in the rapid tests currently employed. This study confirmed that the Western blot similarities (size of the unglycosylated band and distinct reactivity with 6H4 and P4 antibodies) between CH1641 and BSE remained consistent regardless of the PrP genotype of the sheep, but the two infections resulted in accumulation of disease-associated PrP (PrP(d)) that could easily be distinguished by the immunohistochemical "peptide mapping" method. This method, which reveals conformational differences of PrP(d) by the use of a panel of antibodies, indicated that PrP(d) from the CH1641 isolate was truncated further upstream in the N terminus than was PrP(d) from other ovine TSEs, including experimental BSE. In addition, the immunohistochemical "PrP(d) profile method", which defines the phenotype of PrP(d) accumulation in the brain of affected sheep, showed that CH1641 infection leads to much more intra-neuronal and considerably less extracellular PrP(d) than does experimental BSE. The overall results demonstrate that a combined Western blotting and immunohistochemical approach is required to discriminate between different TSE strains in sheep.