Fast and ultrasensitive method for quantitating prion infectivity titre

Bioassay by end-point dilution has been used for decades for routine determination of prion infectivity titre. Here we show that the new protein misfolding cyclic amplification with beads (PMCAb) technique can be used to estimate titres of the infection-specific forms of the prion protein with a higher level of precision and in 3–6 days as opposed to 2 years, when compared with the bioassay. For two hamster strains, 263 K and SSLOW, the median reactive doses 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 median infectious dose (ID50) values measured by bioassay. The 102- to 103-fold differences between ID50 and PMCAb50 values could be due 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 infection-specific prion protein seeds in animals versus PMCAb reactions. A well-calibrated PMCAb reaction can be an efficient and cost-effective method for the estimation of infection-specific prion protein titre.