Mapping PrPScPropagation in Experimental and Natural Scrapie in Sheep with Different PrP Genotypes

Abstract

Twenty-one orally inoculated and seven naturally infected sheep with scrapie were examined for PrP^Scin peripheral tissues and in the central nervous system (CNS), using immunohistochemistry. In the inoculated group, VRQ (valine at codon 136, arginine at codon 154 and glutamine at codon 171)/VRQ sheep generally had a greater accumulation of the pathologic form of prion protein (PrP^Sc) in peripheral tissues, as compared with VRQ/ARQ (alanine at codon 136, arginine at codon 154, and glutamine at codon 171) animals at corresponding time points after inoculation. PrP^Scwas not detected in the ileal Peyer's patch, the spleen, the superficial cervical lymph node, and peripheral nervous tissues of several inoculated VRQ/ARQ animals. All inoculated VRQ/VRQ sheep, but only one of eight inoculated VRQ/ARQ animals, were PrP^Sc-positive in the CNS. Thus, the propagation of PrP^Scseemed slower and more limited in VRQ/ARQ animals. Tissue and cellular localization of PrP^Scsuggested that PrP^Scwas disseminated through three different routes. PrP^Sc-positive cells in lymph node sinuses and in lymphatics indicated spreading by lymph. The sequential appearance of PrP^Scin the peripheral nervous system and the CNS, with satellite cells as early targets, suggested the periaxonal transportation of PrP^Scthrough supportive cells. Focal areas of vascular amyloid-like PrP^Scin the brain of five sheep, suggested the hematogenous dissemination of PrP^Sc. There was a poor correlation between the amount of PrP^Scin the CNS and clinical signs. One subclinically affected sheep showed widespread PrP^Scaccumulation in the CNS, whereas three sheep had early clinical signs without detectable PrP^Scin the CNS. A VV₁₃₆(homozygous for valine at codon 136) sheep inoculated with ARQ/ARR (alanine at codon 136, arginine at codon 154, and arginine at codon 171) tissue succumbed to disease, demonstrating successful heterologous transmission. Less susceptible sheep receiving VRQ/VRQ or ARQ/ARR material were PrP^Sc-negative by immunohistochemistry, enzyme-linked immunosorbent assay, and western blot.