Mapping PrPSc Propagation in Experimental and Natural Scrapie in Sheep with Different PrP Genotypes

Twenty-one orally inoculated and seven naturally infected sheep with scrapie were examined for PrPSc in 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 (PrPSc) 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. PrPSc was 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 PrPSc-positive in the CNS. Thus, the propagation of PrPSc seemed slower and more limited in VRQ/ARQ animals. Tissue and cellular localization of PrPSc suggested that PrPSc was disseminated through three different routes. PrPSc-positive cells in lymph node sinuses and in lymphatics indicated spreading by lymph. The sequential appearance of PrPSc in the peripheral nervous system and the CNS, with satellite cells as early targets, suggested the periaxonal transportation of PrPSc through supportive cells. Focal areas of vascular amyloid-like PrPSc in the brain of five sheep, suggested the hematogenous dissemination of PrPSc. There was a poor correlation between the amount of PrPSc in the CNS and clinical signs. One subclinically affected sheep showed widespread PrPSc accumulation in the CNS, whereas three sheep had early clinical signs without detectable PrPSc in 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 PrPSc-negative by immunohistochemistry, enzyme-linked immunosorbent assay, and western blot.