Submitted to: United States Animal Health Association Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: July 15, 2009
Publication Date: October 7, 2009
Citation: Kunkle, R.A., Hamir, A.N., Richt, J., Greenlee, J.J., Nicholson, E.M., Kehrli, Jr., M.E. 2009. Lack of Prion Accumulation in Lymphoid Tissues of Scrapie-affected Sheep with the AA136, QR171 Prion Protein Genotype [abstract]. American Association of Veterinary Laboratory Diagnosticians. Paper No. 186. Technical Abstract: Background: Sheep scrapie is a transmissible spongiform encephalopathy which can be transmitted horizontally through the shedding of an infectious conformer (PrP**Sc) of the normal cellular prion protein (PrP**c). Genetics profoundly influence the susceptibility of sheep to scrapie. PrP**c amino-acid polymorphisms A136V, R154H, Q171R, and Q171H are predictive for relative susceptibility (V136, R154, Q171) or resistance (A136, H154, R171) to classical scrapie in natural settings and in experimental oral inoculation studies. Objectives: To compare the clinical course and PrP**Sc tissue distribution in sheep with high resistance and high susceptibility genotypes after inoculation of scrapie-affected sheep brain homogenate directly into the brain. Methods: Five sheep each of genotype VRQ/VRQ, ARQ/ARQ(H), VRQ/ARQ, or ARQ/ARR were inoculated via intra-cerebral route with a 10% brain homogenate derived from a ARQ/ARQ sheep affected with scrapie. Sheep were euthanized in the terminal phase of scrapie development. Tissues collected at necropsy were examined by light microscopy, immunohistochemistry (IHC) and Western blot. Results: All inoculated sheep succumbed to scrapie. Clinical signs, microscopic lesions, and Western blot profiles were uniform across genotypes and consistent with manifestations of classical scrapie. Mean survival time differences were associated with the 171 polymorphic site with VRQ/VRQ and ARQ/ARQ sheep surviving 18 and 19 months, whereas VRQ/ARR and ARQ/ARR survived 56 and 60 months, respectively. Labeling of PrP**Sc by IHC revealed similar accumulations in central nervous system tissues across genotypes. Labeling of PrP**Sc in lymphoid tissue was consistently abundant in VRQ/VRQ, present in 4/5 ARQ/ARQ and VRQ/ARR, and totally absent in ARQ/ARR sheep. Discussion: The results of the study demonstrate the susceptibility of sheep with the high resistance genotype ARQ/ARR to scrapie by the intra-cerebral inoculation route, with attendant PrP**Sc accumulation in CNS tissues, and concurrent lack of PrP**Sc in lymphoid tissue. Our data suggest that genetic resistance to scrapie is associated with failure of PrP**Sc to accumulate in lymphoid tissue, and not associated with permissiveness of CNS tissue to PrP**Sc amplification.