Monitoring plasma processing steps with a sensitive Western blot assay for the detection of the prion protein

Determining the risk of transmissible spongiform encephalopathy (TSE) transmission by blood or plasma-derived products requires sensitive and specific assays for the detection of either infectivity or a reliable marker for infectivity. To this end, a Western blot assay that is both sensitive and reproducible for the detection of PrP(RES), a marker for TSE infectivity, was developed. Using the 263K strain of TSE as a model system, the Western blot assay proved to be sensitive, specific and quantitative over a 3-4 log dynamic range. Compared to the rodent bioassay, the assay was shown to detect PrP(RES) down to ~10^3.4 IU/ml, which is ~5-10 pg of PrP or ~10-20 ng brain equivalents. The Western blot was applied to monitor the partitioning of spiked PrP(Sc) through three plasma fractionation steps, cryoprecipitation, fraction I and fraction III, that are common to the purification of several human plasma-derived therapeutic products including albumin and immunoglobulins. The results from these studies demonstrated 1 log, 1 log and 4 logs of PrP(Sc) partitioning away from the effluent fraction for the cryoprecipitation, fraction I and fraction III steps, respectively.