Ovine plasma prion protein levels show genotypic variation detected by C-terminal epitopes not exposed in cell-surface PrP C}
Résumé
Ovine peripheral blood mononuclear cells (PBMCs) express PrPC and have the potential to harbour and release disease-associated forms of PrP during scrapie in sheep. Cell-surface PrPC expression by PBMCs, together with plasma PrPC levels, may contribute to the regulatory mechanisms that determine susceptibility and resistance to natural scrapie in sheep. Here we have correlated cell-surface PrPC expression on normal ovine PBMCs by FACS with the presence of PrPC in plasma measured by capture-detector immunoassay. FACS showed similar levels of cell-surface PrPC on homozygous ARR, ARQ and VRQ PBMCs. Cell-surface ovine PrPC showed modulation of N-terminal epitopes, which was more evident on homozygous ARR cells. Ovine plasma PrPC levels showed genotypic variation and the protein displayed C-terminal epitopes not available in cell-surface PrPC. Homozygous VRQ sheep showed the highest plasma PrPC level and homozygous ARR animals the lowest. For comparison, similar analyses were performed on normal bovine PBMCs and plasma. PrPC levels in bovine plasma were approximately four-fold higher than ovine homozygous ARQ plasma despite similar levels of PBMC cell-surface PrPC expression. Immunoassays using C-terminal-specific anti-PrP monoclonal antibodies as capture and detector reagents, revealed the highest level of PrPC in both ovine and bovine plasma, whilst lower levels were detected using N-terminal-specific monoclonal antibody FH11 as the capture reagent. This suggested that a proportion of plasma PrPC was N-terminally truncated. Our data indicate that the increased susceptibility to natural scrapie displayed by homozygous VRQ sheep correlates with a higher level of plasma PrPC.
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