A direct relationship between the partitioning of the pathogenic prion protein and transmissible spongiform encephalopathy infectivity during the purification of plasma proteins

BACKGROUND: Experimental evidence from rodent models indicates that blood can contain transmissible spongiform encephalopathy (TSE) infectivity, which suggests a potential risk for TSE transmission via proteins isolated from human plasma. Because methods that can reduce TSE infectivity typically are detrimental to protein function, infectivity must be removed to ensure the safety of these therapeutic proteins. Animal bioassays are conventionally used to detect infectivity, but the pathogenic form of the prion protein (PrP Sc ) can serve as a marker for TSE infectivity. STUDY DESIGN AND METHODS: Seven plasma protein-purification steps were performed after the plasma intermediates were spiked with TSE-infected material. Resulting fractions were analyzed for PrP Sc by using a Western blot assay and for TSE infectivity by using an animal bioassay. Western blots were quantitated by an endpoint dilution analysis, and infectivity titers were calculated by the Spearman-Karber method. RESULTS: PrP Sc partitioning paralleled TSE infectivity partitioning, regardless of the nature of the protein-purification step. The detection ranges for PrP Sc and infectivity were 0 to 5.3 log and 1.1 to 8.9 log median infectious dose per unit, respectively. Clearance of PrP Sc and infectivity ranged from 1.0 to 6.0 log. CONCLUSION: Purification steps for isolating therapeutic proteins from human plasma showed the removal of both PrP Sc and TSE infectivity. PrP Sc partitioning coincided with infectivity partitioning, which showed a close relationship between PrP Sc and TSE infectivity. By exploiting this association, the in vitro Western blot assay for PrP Sc was valuable for estimating the partitioning of TSE infectivity during plasma protein purification.