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 c an 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 (P rPSc) can serve as a marker for TSE infectivity. STUDY DESIGN AND METHODS: Seven plasma protein-purification steps were perf ormed after the plasma intermediates were spiked with TSE-infected material . Resulting fractions were analyzed for PrPSc by using a Western blot assay and for TSE infectivity by using an animal bioassay. Western blots were qu antitated by an endpoint dilution analysis, and infectivity titers were cal culated by the Spearman-Karber method. RESULTS: PrPSc partitioning paralleled TSE infectivity partitioning, regard less of the nature of the protein-purification step. The detection ranges f or PrPSc and infectivity were 0 to 5.3 log and 1.1 to 8.9 log median infect ious dose per unit, respectively. Clearance of PrPSc and infectivity ranged from 1.0 to 6.0 log. CONCLUSION: Purification steps for isolating therapeutic proteins from huma n plasma showed the removal of both PrPSc and TSE infectivity. PrPSc partit ioning coincided with infectivity partitioning, which showed a close relati onship between PrPSc and TSE infectivity. By exploiting this association, t he in vitro Western blot assay for PrPSc was valuable for estimating the pa rtitioning of TSE infectivity during plasma protein purification.