Quantitative analysis of wet-heat inactivation in bovine spongiform encephalopathy

Kinetics of the reduction of Creutzfeldt-Jakob disease prion seeding activity by steam sterilization support the use of validated 134°C programmes

BACKGROUND

Prions are renowned for their distinct resistance to chemical or physical inactivation, including steam sterilization. Impaired efficacy of inactivation poses a risk to patients for iatrogenic transmission of Creutzfeldt-Jakob disease (CJD) via contaminated surgical instruments.

AIMS

Most established prion inactivation methods were validated against scrapie agents, although those were found to be generally less thermostable than human prions. Thus, knowledge gaps regarding steam-sterilization kinetics of CJD prions should be filled and current guidelines reviewed accordingly.

METHODS

Prion inactivation through widely recommended steam sterilization at 134°C was assessed for several holding times by analysing the residual prion seeding activity using protein misfolding cyclic amplification (PMCA).

FINDINGS

Scrapie 263K was found to be the least thermoresistant prion strain showing no seeding activity after 1.5 min at 134°C, while variant CJD was the most stable one demonstrating some seeding activity even after 18 min of steam sterilization. Sporadic CJD subtype VV2 exhibited residual seeding activity after 3 min, but no detectable activity after 5 min at 134°C.

CONCLUSION

Validated steam sterilization for 5 min at 134°C as previously recommended for the routine reprocessing of surgical instruments in contact with high-risk tissues is able to substantially reduce the seeding activity of CJD agents, provided that no fixating chemical disinfection has been performed prior to sterilization and that thorough cleaning has reduced the protein load on the surface to less than 100 μg per instrument.

Polyethyleneimine-modified resins effectively remove porcine circovirus and cellular prion protein

Polyethyleneimine (PEI) possesses various molecular weights (MWs), structures, and virus capture capacities. However, whether PEI can capture porcine circovirus (PCV) and animal cell-derived prion protein (PrP^C) that may contaminate source materials is unclear. Therefore, we conducted a feasibility study to assess the effectiveness of PEI in removing PCV and PrP^C as a model of pathogenic prions. The removal performance of PCV was evaluated by quantitative PCR using PEIs with various MWs, structures, and ion exchange capacities in Tris (pH 7.5) and acetate (pH 5.5) buffers under neutral (pH 7.5) to acidic (pH 5.5) conditions. Removal performances of PrP^C were also evaluated by western blotting using PEIs with various MWs and structures. Tris buffer did not affect the ability of PEI-modified resins to remove PCV, whereas acetate buffer affected removal performances, except those of PEI-10K-Br and PEI-70K-Br, which showed high ion-exchange capacities. PrP^C was captured by PEIs with high MWs, especially PEI-70K-Br, which was the most effective. The results of this feasibility study suggested that PEI-modified resin could remove PCV and PrP^C. PEI-70K-Br with an ion-exchange capacity of at least 0.3 meq/mL appears suitable as a PEI molecule for pathogen capture or removal of PCV or PrP^C from biological materials.

Eliminating transmissibility of bovine spongiform encephalopathy by dry-heat treatment

Bovine spongiform encephalopathy (BSE) prion is more resistant to heat inactivation compared to other prions, but the effect of heat inactivation has been reported to differ depending on the BSE-contaminated tissue state or heating type. We aimed to evaluate the secure level of inactivation of original BSE transmissibility by dry-heating. Cattle tissues affected with BSE were subjected to dry-heat treatment for 20 min at various temperatures ranging from 150 to 1000 °C. To assess the inactivation effect, we conducted protein misfolding cyclic amplification (PMCA) and follicular dendritic cell (FDC) assays in transgenic mice expressing bovine prion protein genes. Under dry-heating at 600 °C or higher, BSE cattle tissues lost their transmissibility in transgenic mice. In contrast, transmissibility was detected in the cattle tissues treated at temperatures of 400 °C or lower through the FDC assay combined with PMCA. In this study, we confirmed that transmissibility was eliminated in BSE-affected cattle tissues by dry-heating at 600 °C or higher.

Inactivation of Scrapie Prions by the Electrically Charged Disinfectant CAC-717

Previous studies have revealed that the electrically charged disinfectant CAC-717 has strong virucidal and bactericidal effects but is safe for humans and animals. In this study, CAC-717 was further evaluated for its potential effects as a disinfectant against scrapie prions. Western blotting showed that CAC-717 reduced the amount of the abnormal isoform of prion protein (PrP^Sc) in prion-infected cell (ScN2a) lysates. Furthermore, the reduction of prion transmissibility was confirmed by a mouse bioassay, in which mice injected with scrapie prions pre-treated with CAC-717 survived longer than those injected with untreated scrapie prions. Lastly, to evaluate the seeding activity of ScN2a cell lysates treated with CAC-717, quantitative protein misfolding cyclic amplification (PMCA) was performed directly on ScN2a cell lysates treated with CAC-717, which showed that the median dose of PMCA (PMCA₅₀) dropped from log9.95 to log5.20 after CAC-717 treatment, indicating more than a 4 log reduction. This suggests that the seeding activity of PrP^Sc is decreased by CAC-717. Collectively, these results suggest that CAC-717 has anti-prion activity, reducing both PrP^Sc conversion activity and prion transmissibility; thus, CAC-717 will be useful as a novel disinfectant in prion diseases.

Evaluation of the Application for new alternative biodiesel production process for rendered fat of Cat 1 (BDI-RepCat process, AT)

A new alternative method for the production of biodiesel from rendered fat of all categories of animal by‐products was assessed. The process was compared to the approved biodiesel production process described in Chapter IV Section 2 D of Annex IV of Commission Regulation (EU) 142/2011. Tallow derived from Category 1 material is treated according to Method 1 from the same Regulation (133°C, 20 min, 3 bar) and subsequently mixed with 15% methanol, heated to reaction temperature (220°C) in several heat exchangers and transferred into the continuous conversion reactor by means of a high pressure pump (80 bar) for 30 min. In the conversion phase, there is an exposure to methanol in the absence of alkaline or acidic conditions. The impact of this procedure on the thermostability of transmissible spongiform encephalopathy (TSE) has not been assessed in the literature. After the reaction, the biodiesel/glycerol mixture is distilled under vacuum at a minimum temperature of 150°C and a maximum pressure of 10 mbar, which is equivalent to the distillation step in the approved biodiesel production process, for which a 3 log₁₀ reduction factor in PrP27–30 was obtained. Therefore, a similar level of TSE infectivity reduction could be expected for that phase of the method. A previous EFSA Opinion established that a reduction of 6 log₁₀ in TSE infectivity should be achieved by any proposed alternative method in order to be equivalent to the approved processing method. This level of reduction has not been shown with experimental trials run under conditions equivalent to the ones described for the RepCat process. It was not possible to conclude whether or not the level of TSE infectivity reduction in the RepCat process is at least of 6 log₁₀. Therefore, it was also not possible to conclude about the equivalence with the approved biodiesel production process.

Selective propagation of mouse-passaged scrapie prions with long incubation period from a mixed prion population using GT1-7 cells

In our previous study, we demonstrated the propagation of mouse-passaged scrapie isolates with long incubation periods (L-type) derived from natural Japanese sheep scrapie cases in murine hypothalamic GT1-7 cells, along with disease-associated prion protein (PrPSc) accumulation. We here analyzed the susceptibility of GT1-7 cells to scrapie prions by exposure to infected mouse brains at different passages, following interspecies transmission. Wild-type mice challenged with a natural sheep scrapie case (Kanagawa) exhibited heterogeneity of transmitted scrapie prions in early passages, and this mixed population converged upon one with a short incubation period (S-type) following subsequent passages. However, when GT1-7 cells were challenged with these heterologous samples, L-type prions became dominant. This study demonstrated that the susceptibility of GT1-7 cells to L-type prions was at least 105 times higher than that to S-type prions and that L-type prion-specific biological characteristics remained unchanged after serial passages in GT1-7 cells. This suggests that a GT1-7 cell culture model would be more useful for the economical and stable amplification of L-type prions at the laboratory level. Furthermore, this cell culture model might be used to selectively propagate L-type scrapie prions from a mixed prion population.

Rapid and Highly Sensitive Detection of Variant Creutzfeldt-Jakob Disease Abnormal Prion Protein on Steel Surfaces by Protein Misfolding Cyclic Amplification: Application to Prion Decontamination Studies

The prevalence of variant Creutzfeldt-Jakob disease (vCJD) in the population remains uncertain, although it has been estimated that 1 in 2000 people in the United Kingdom are positive for abnormal prion protein (PrP^TSE) by a recent survey of archived appendix tissues. The prominent lymphotropism of vCJD prions raises the possibility that some surgical procedures may be at risk of iatrogenic vCJD transmission in healthcare facilities. It is therefore vital that decontamination procedures applied to medical devices before their reprocessing are thoroughly validated. A current limitation is the lack of a rapid model permissive to human prions. Here, we developed a prion detection assay based on protein misfolding cyclic amplification (PMCA) technology combined with stainless-steel wire surfaces as carriers of prions (Surf-PMCA). This assay allowed the specific detection of minute quantities (10⁻⁸ brain dilution) of either human vCJD or ovine scrapie PrP^TSE adsorbed onto a single steel wire, within a two week timeframe. Using Surf-PMCA we evaluated the performance of several reference and commercially available prion-specific decontamination procedures. Surprisingly, we found the efficiency of several marketed reagents to remove human vCJD PrP^TSE was lower than expected. Overall, our results demonstrate that Surf-PMCA can be used as a rapid and ultrasensitive assay for the detection of human vCJD PrP^TSE adsorbed onto a metallic surface, therefore facilitating the development and validation of decontamination procedures against human prions.

Subcritical Water Hydrolysis Effectively Reduces the In Vitro Seeding Activity of PrPSc but Fails to Inactivate the Infectivity of Bovine Spongiform Encephalopathy Prions

The global outbreak of bovine spongiform encephalopathy (BSE) has been attributed to the recycling of contaminated meat and bone meals (MBMs) as feed supplements. The use of MBMs has been prohibited in many countries; however, the development of a method for inactivating BSE prions could enable the efficient and safe use of these products as an organic resource. Subcritical water (SCW), which is water heated under pressure to maintain a liquid state at temperatures below the critical temperature (374°C), exhibits strong hydrolytic activity against organic compounds. In this study, we examined the residual in vitro seeding activity of protease-resistant prion protein (PrP^Sc) and the infectivity of BSE prions after SCW treatments. Spinal cord homogenates prepared from BSE-infected cows were treated with SCW at 230–280°C for 5–7.5 min and used to intracerebrally inoculate transgenic mice overexpressing bovine prion protein. Serial protein misfolding cyclic amplification (sPMCA) analysis detected no PrP^Sc in the SCW-treated homogenates, and the mice treated with these samples survived for more than 700 days without any signs of disease. However, sPMCA analyses detected PrP^Sc accumulation in the brains of all inoculated mice. Furthermore, secondary passage mice, which inoculated with brain homogenates derived from a western blotting (WB)-positive primary passage mouse, died after an average of 240 days, similar to mice inoculated with untreated BSE-infected spinal cord homogenates. The PrP^Sc accumulation and vacuolation typically observed in the brains of BSE-infected mice were confirmed in these secondary passage mice, suggesting that the BSE prions maintained their infectivity after SCW treatment. One late-onset case, as well as asymptomatic but sPMCA-positive cases, were also recognized in secondary passage mice inoculated with brain homogenates from WB-negative but sPMCA-positive primary passage mice. These results indicated that SCW-mediated hydrolysis was insufficient to eliminate the infectivity of BSE prions under the conditions tested.

Rapid assessment of bovine spongiform encephalopathy prion inactivation by heat treatment in yellow grease produced in the industrial manufacturing process of meat and bone meals

BACKGROUND

Prions, infectious agents associated with transmissible spongiform encephalopathy, are primarily composed of the misfolded and pathogenic form (PrPSc) of the host-encoded prion protein. Because PrPSc retains infectivity after undergoing routine sterilizing processes, the cause of bovine spongiform encephalopathy (BSE) outbreaks are suspected to be feeding cattle meat and bone meals (MBMs) contaminated with the prion. To assess the validity of prion inactivation by heat treatment in yellow grease, which is produced in the industrial manufacturing process of MBMs, we pooled, homogenized, and heat treated the spinal cords of BSE-infected cows under various experimental conditions.

RESULTS

Prion inactivation was analyzed quantitatively in terms of the infectivity and PrPSc of the treated samples. Following treatment at 140°C for 1 h, infectivity was reduced to 1/35 of that of the untreated samples. Treatment at 180°C for 3 h was required to reduce infectivity. However, PrPSc was detected in all heat-treated samples by using the protein misfolding cyclic amplification (PMCA) technique, which amplifies PrPScin vitro. Quantitative analysis of the inactivation efficiency of BSE PrPSc was possible with the introduction of the PMCA50, which is the dilution ratio of 10% homogenate needed to yield 50% positivity for PrPSc in amplified samples.

CONCLUSIONS

Log PMCA50 exhibited a strong linear correlation with the transmission rate in the bioassay; infectivity was no longer detected when the log PMCA50 of the inoculated sample was reduced to 1.75. The quantitative PMCA assay may be useful for safety evaluation for recycling and effective utilization of MBMs as an organic resource.