Measuring prions causing bovine spongiform encephalopathy or chronic wasting disease by immunoassays and transgenic mice

There is increasing concern over the extent to which bovine spongiform encephalopathy (BSE) prions have been transmitted to humans, as a result of the rising number of variant Creutzfeldt-Jakob disease (vCJD) cases. Toward preventing new transmissions, diagnostic tests for prions in livestock have been developed using the conformation-dependent immunoassay (CDI), which simultaneously measures specific antibody binding to denatured and native forms of the prion protein (PrP). We employed high-affinity recombinant antibody fragments (recFab) reacting with residues 95-105 of bovine (Bo) PrP for detection and another recFab that recognizes residues 132-156 for capture in the CDI. We report that the CDI is capable of measuring the disease-causing PrP isoform (PrP^Sc) in bovine brainstems with a sensitivity similar to that of end-point titrations in transgenic (Tg) mice expressing BoPrP. Prion titers were ∼10⁷ ID₅₀ units per gram of bovine brainstem when measured in Tg(BoPrP) mice, a figure ∼10 times greater than that determined by bioassay in cattle and ∼10, 000x greater than in wild-type mice. We also report substantial differences in BoPrP^Sc levels in different areas of the obex region, where neuropathology has been consistently observed in cattle with BSE. The CDI was able to discriminate between PrP^Sc from BSE-infected cattle and Tg(BoPrP) mice as well as from chronic wasting disease (CWD)-infected deer and elk. Our findings argue that applying the CDI to livestock should considerably reduce human exposure to animal prions.