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Validation of prion inactivation processes for medical devices relies on in-vivo experimental protocols. However, bioassays are costly, long (1-2 years) and ethically disputable. Additionally, results obtained with one prion strain - for example, 263K (hamster-adapted strain originating from sheep scrapie) - cannot be easily extrapolated to relevant human prion strains, further questioning the utility of bioassays. Over the past two decades, cell-free prion amplification assays have emerged as potential alternatives to bioassays. Rather than measuring prion infectivity, they quantify prion seeding activity (i.e. the capacity to convert the normal prion protein into the disease-associated isoform). The results obtained from an optimized cell-free assay termed 'miniaturized-bead protein misfolding cyclic amplification' (mb-PMCA) with four processes using three different prion strains - 263K and two human prions derived from variant and sporadic Creutzfeldt-Jakob disease - were compared with published bioassays using the same three strains and processes, when available. Tests performed on reference processes (steam, sodium hydroxide, sodium hypochlorite) and low temperature H 2 O 2 sterilization (STERRAD NX TM Advanced cycle) showed perfect alignment between mb-PMCA and available bioassays. STERRAD NX TM Advanced cycle was efficacious against all three prion strains. These data confirm that PMCA, particularly mb-PMCA, is a relevant alternative to animal bioassays for the assessment of prion inactivation processes, and highlight the interest of some low temperature H 2 O 2 sterilization cycles., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Mammalian prions are neurotropic pathogens formed from PrP Sc assemblies, a misfolded variant of the host-encoded prion protein PrP C . Multiple PrP Sc conformations or strains self-propagate in host populations or mouse models of prion diseases, exhibiting distinct biological and biochemical phenotypes. Constrained interactions between PrP Sc and PrP C conformations confer species specificity and regulate cross-species transmission. The pathogenicity of fibrillar assemblies derived from bacterially expressed recombinant PrP (rPrP) has been instrumental in demonstrating the protein-only nature of prions. Yet, their ability to encode different strains and transmit between species remains poorly studied, hampering their use in exploring structure-to-strain relationships. Fibrillar assemblies from rPrP with hamster, mouse, human, and bovine primary structures were generated and tested for transmission and adaptation in tg7 transgenic mice expressing hamster PrP C . All assemblies, except the bovine ones, were fully pathogenic on the primary passage, causing clinical disease, PrP Sc brain deposition, and spongiform degeneration. They exhibited divergent adaptation processes and strain properties upon subsequent passage. Assemblies of hamster origin propagated without apparent need for adaptation, those of mouse origin adapted abruptly, and those of human origin required serial passages for optimal fitness. Molecular analyses revealed the presence of endogenously truncated PrP Sc species in the resulting synthetic strains that lack the 90-140 amino acid region considered crucial for infectivity. In conclusion, rPrP assemblies provide a facile means of generating novel prion strains with adaptative/evolutive properties mimicking genuine prions. The PrP amino acid backbone is sufficient to encode different strains with specific adaptative properties, offering insights into prion transmission and strain diversity., (© 2024 The Author(s). The FEBS Journal published by John Wiley & Sons Ltd on behalf of Federation of European Biochemical Societies.)

Background: The manufacturing processes of plasma products include steps that can remove prions. The efficacy of these steps is measured in validation studies using animal brain-derived prion materials called spikes. Because the nature of the prion agent in blood is not known, the relevance of these spikes, particularly with steps that are based on retention mechanisms such as nanofiltration, is important to investigate., Study Design and Methods: The aggregation and sizes of PrP res assemblies of microsomal fractions (MFs) extracted from 263K-infected hamster brains were analyzed using velocity gradients. The separated gradient fractions were either inoculated to Tg7 mice expressing hamster-PrP c to measure infectivity or used in Protein Misfolding Cyclic Amplification for measuring seeding activity. The collected data allowed for reanalyzing results from previous nanofiltration validation studies., Results: A significant portion of MFs was found to be composed of small PrP res assemblies, estimated to have a size ≤24 mers (~22-528 kDa), and to contain a minimum of 20% of total prion infectivity. With this data we could calculate reductions of 4.10 log (15 N), 2.53 log (35 N), and 1.77 log (35 N) from validation studies specifically for these small PrP res objects., Conclusion: Our gradient data provided evidence that nanofilters can remove the majority of the smallest PrP res entities within microsomes spikes, estimated to be in a size below 24 mers, giving insight about the fact that, in our conditions, size exclusion may not be the only mechanism for retention nanofiltration., (© 2024 AABB.)

It is commonly accepted that the prion replicative propensity and strain structural determinant (SSD) are encoded in the fold of PrP Sc amyloid fibril assemblies. By exploring the quaternary structure dynamicity of several prion strains, we revealed that all mammalian prion assemblies exhibit the generic property of spontaneously generating two sets of discreet infectious tetrameric and dimeric species differing significantly by their specific infectivity. By using perturbation approaches such as dilution and ionic strength variation, we demonstrated that these two oligomeric species were highly dynamic and evolved differently in the presence of chaotropic agents. In general, our observations of seven different prion strains from three distinct species highlight the high dynamicity of PrP Sc assemblies as a common and intrinsic property of mammalian prions. The existence of such small infectious PrP Sc species harboring the SSD indicates that the prion infectivity and the SSD are not restricted only to the amyloid fold but can also be encoded in other alternative quaternary structures. Such diversity in the quaternary structure of prion assemblies tends to indicate that the structure of PrP Sc can be divided into two independent folding domains: a domain encoding the strain structural determinant and a second domain whose fold determines the type of quaternary structure that could adopt PrP Sc assemblies., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)