Detecting the temporal status of blood-borne prions in transmissible spongiform encephalopathy-infected hosts

Abstract

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are infectious, fatal neurodegenerative diseases with a protracted subclinical disease state spanning months to years. Prion diseases develop when the normal cellular prion protein (PrPC) undergoes a conformational change into an aberrant, disease-causing, isoform (PrPSc/PrPres/PrPD), which aggregates into amyloid fibrils. Prions are unique from all other infectious diseases in that they lack nucleic acid. Prion diseases are known to naturally occur in cattle, sheep, mink, cervids, and humans; however, the exact mechanisms of transmission are unknown. Sufficient infectious prions to transmit and cause disease are known to be present in tissues and bodily fluids of all TSE-infected mammals during clinical and subclinical stages of disease. Extensive extraneural PrPSc-deposition has been observed in chronic wasting disease (CWD)-infected cervids and transmissible mink encephalopathy (TME)-infected hamsters and is very similar to what has been described for variant Creutzfeldt-Jacob disease (vCJD)-infected humans. Importantly, blood taken from humans and animals lacking overt clinical symptoms is capable of transmitting disease through transfusion. In this thesis we set out to answer questions regarding hematogenous prions: 1) How long does it take for prions to enter the blood after initial TSE exposure? 2) Are hematogenous prions present in all animals infected with CWD and TME? and 3) Does the route of prion entry affect the temporal status of hematogenous prions. To answer these questions, we analyzed longitudinally-collected whole blood samples from TSE-exposed animals by a modified version of the highly sensitive in vitro amyloid-amplification assay "real-time quaking-induced conversion" (RT-QuIC) we termed whole blood (wb) RT-QuIC. Longitudinal whole blood samples (15 minutes post exposure-terminal disease) were collected from experimental CWD-exposed (oral, aerosol, and intravenous inoculation) white-tailed and Reeves' muntjac deer and TME-exposed (extranasal inoculation) Syrian hamsters. We detected PrP conversion-competent amyloid in the blood of 100% of infected animals as early as 15 minutes post inoculation throughout terminal clinical TSE disease. These results were observed for all inoculation routes. Furthermore, we observed the presence of prions in the blood in two phases--a primary and secondary prionemia. The results of this work suggest that: 1) inoculated prions traverse mucosal barriers and enter the blood within 15 minutes of exposure; 2) the route of inoculation has little effect on the temporal status of prions in the blood; 3) there are two distinct phases of prionemia representing the initial inoculum (primary prionemia) and de novo host-generated prions (secondary prionemia); and 4) the observed characteristics of prionemia can be recapitulated in various TSE-host combinations and may recapitulated the extraneural pathogenesis of human TSEs.