Bioassay by end-point dilution has been employed for decades for routine determination of prion infectivity titer. Here we show that the new Protein Misfolding Cyclic Amplification with Teflon beads (PMCAb) can be used to estimate titers of the misfolded version of the prion protein (PrPSc) with a higher level of precision and in 3 to 6 days as opposed to two years, when compared with bioassay. For two hamster strains 263K and SSLOW, median infective doses (ID50) determined by PCMAb (PMCAb50) were found to be 1012.8 and 1012.2 per gram of brain tissue, which are 160- and 4,000-fold higher than the corresponding ID50 values measured by bioassay. These 102-103-fold differences could be attributed to a large excess of PMCAb-reactive prion protein seeds with little or no infectivity. Alternatively, the differences between ID50 and PMCAb50 could be due to higher rate of clearance of PrPSc seeds in animals versus PMCAb reactions. A well calibrated PMCAb reaction can be an efficient and cost effective method for the estimation of PrPSc titer.
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