PrPSc with Seeding Activity Extensively Overlaps with Proteinase-Resistant PrPSc Rather than Infectious PrPSc

The disease-associated prion protein (PrPSc) has the ability to seed the conformational conversion of normal prion proteins into the amyloid fibril form. This prion seeding activity can be measured using an in vitro amplification assay termed real-time quaking-induced conversion (RT-QuIC). There is a strong correlation between RT-QuIC positivity and prion infection; however, the relationship between seeding activity and infectivity remains elusive. In this study, we used endpoint dilution RT-QuIC on the brain homogenates from wild-type mice with mouse-adopted bovine spongiform encephalopathy (mBSE) at defined intervals during the incubation period and evaluated the temporal relationship among prion seeding dose, levels of proteinase-resistant PrPSc (PrPres), and infectious titer. We found that the infectious titer reached a plateau by 100 days postinfection, whereas seeding dose and PrPres levels were continuously elevated. Our calculation showed that the doubling time (dt) for seeding dose from 40 to 100 days postinoculation was closer to the dt for PrPres levels than to the dt for prion titer. Although an uncoupling of seeding doses and PrPres levels was observed at end-stage disease in this model, our findings suggest that there is substantial but not complete overlap between PrPSc with seeding activity and PrPres rather than infectious PrPSc.