Sheep-Passaged Bovine Spongiform Encephalopathy Agent Exhibits Altered Pathobiological Properties in Bovine-PrP Transgenic Mice

Sheep can be experimentally infected with bovine spongiform encephalopathy (BSE), and the ensuing disease is similar to scrapie in terms of pathogenesis and clinical signs. BSE infection in sheep is an animal and human health concern. In this study, the transmission in BoPrP-Tg110 mice of prions from BSE-infected sheep was examined and compared to the transmission of original cattle BSE in cattle and sheep scrapie prions. Our results indicate no transmission barrier for sheep BSE prions to infect BoPrP-Tg110 mice, but the course of the disease is accelerated compared to the effects of the original BSE isolate. The shortened incubation period of sheep BSE in the model was conserved in subsequent passage in BoPrP-Tg110 mice, indicating that it is not related to infectious titer differences. Biochemical signature, lesion profile, and PrP(Sc) deposition pattern of both cattle and sheep BSE were similar. In contrast, all three sheep scrapie isolates tested showed an evident transmission barrier and further adaptation in subsequent passage. Taken together, those data indicate that BSE agent can be altered by crossing a species barrier, raising concerns about the virulence of this new prion towards other species, including humans. The BoPrP-Tg110 mouse bioassay should be considered as a valuable tool for discriminating scrapie and BSE in sheep

Elements modulating the prion species barrier and its passage consequences.

The specific characteristics of Transmissible Spongiform Encephalopathy (TSE) strains may be altered during passage across a species barrier. In this study we investigated the biochemical and biological characteristics of Bovine Spongiform Encephalopathy (BSE) after transmission in both natural host species (cattle, sheep, pigs and mice) and in transgenic mice overexpressing the corresponding cellular prion protein (PrPC) in comparison with other non-BSE related prions from the same species. After these passages, most features of the BSE agent remained unchanged. BSE-derived agents only showed slight modifications in the biochemical properties of the accumulated PrPSc, which were demonstrated to be reversible upon re-inoculation into transgenic mice expressing bovine-PrPC. Transmission experiments in transgenic mice expressing bovine, porcine or human-PrP revealed that all BSE-derived agents were transmitted with no or a weak transmission barrier. In contrast, a high species barrier was observed for the non-BSE related prions that harboured an identical PrP amino acid sequence, supporting the theory that the prion transmission barrier is modulated by strain properties (presumably conformation-dependent) rather than by PrP amino acid sequence differences between host and donor. As identical results were observed with prions propagated either in natural hosts or in transgenic mouse models, we postulate that the species barrier and its passage consequences are uniquely governed by the host PrPC sequence and not influenced by other host genetic factors. The results presented herein reinforce the idea that the BSE agent is highly promiscuous, infecting other species, maintaining its properties in the new species, and even increasing its capabilities to jump to other species including humans. These data are essential for the development of an accurate risk assessment for BSE

PrP^Sc proteinase K resistance ELISA test in BSE-derived prions.

<p>A) BSE-derived prions propagated in several host species (cattle-BSE, sheep-BSE, pig-BSE, mouse-BSE and human-vCJD); mouse-RML, sheep-scrapie and human-sCJD isolates are included for comparison purposes. B) BSE-derived prions propagated in transgenic mice expressing PrP^C of these species (BoTg-BSE, OvTg-BSE, PoTg-BSE, MoTga20-BSE and HuTg-BSE).</p

Transmission features of BSE-derived prions in comparison with non-BSE related prions in different mouse models (BoPrP-Tg110, PoPrP-Tg001 and HuPrP-Tg340).

a<p>Intracerebral inoculation with 2 mg brain tissue equivalent; n/n₀: diseased, PrP^res positive/inoculated animals; SEM: standard error of the mean.</p><p>ND: not determined.</p><p>Results from homologous transmissions (host and donor share the same species-PrP sequence) are marked in bold.</p

Glycoform ratios of PrP^res detected by Western blotting.

<p>PrP^res was detected with the Sha31 monoclonal antibody in the different BSE-derived prions propagated either in the natural hosts (A) or in transgenic mouse models over-expressing their corresponding PrP^C sequence (B). Atypical cattle-BSE H, sheep-scrapie, mouse-RML and human-sCJD isolates (C) are included for comparison purposes. Values are the normalized means from at least six repeated runs. Arrows indicate the reading direction of the axis.</p

Electrophoretic profiles and antibody labelling of PrP^res in BSE-derived prions.

<p>PrP^res was detected by western blot using the mAbs Sha31 (A) and 12B2 (B) in the different BSE-derived prions propagated either in natural hosts or in transgenic mouse models over-expressing their corresponding PrP^C sequence: cattle-BSE (lanes 1 and 11), BoTg-BSE (lanes 2 and 12), sheep-BSE (lane 3), OvTg-BSE (lane 4), pig-BSE (lane 5), PoTg-BSE (lane 6), mouse-BSE (lane 7), MoTga20-BSE (lane 8), human-vCJD (lane 13), HuTg-BSE (lane 14). Sheep-scrapie (lane 9), mouse-RML (lane 10), human-sCJD (lane 15) and atypical cattle-BSE H (lane 16) were included as control non-BSE related prions. Panels A and B were loaded with the same quantities of PrP^res extracted from each sample. MW, molecular weight in kilodaltons.</p

PrP^res in HuPrP-Tg340 mice.

<p>Electrophoretic profiles and antibody labelling of PrP^res as detected by mAbs Sha31 (A) and 12B2 (B) in brain extracts from HuPrP-Tg340 mice inoculated with the different BSE-derived prions: cattle-BSE (lane 1), BoTg-BSE (lane2), sheep-BSE (lane 3), OvTg-BSE (lane 4), pig-BSE (lane 5), PoTg-BSE (lane 6), mouse-BSE (lane 7), MoTga20-BSE (lane 8), human-vCJD (lane 9), HuTg-BSE (lane 10). Brain extract from HuPrP-Tg340 mice inoculated with human-sCJD isolate (lane 11) was included as a control non-BSE related prion propagated in the same mouse model. Panels A and B were loaded with the same quantities of PrP^res extracted from each samples. MW, molecular weight in kilodaltons.</p

PrP^res in BoPrP-Tg110 mice.

<p>Electrophoretic profiles and antibody labelling of PrP^res as detected by mAbs Sha31 (A) and 12B2 (B) in brain extracts from BoPrP-Tg110 mice inoculated with the different BSE-derived prions: cattle-BSE (lane 1), sheep-BSE (lane 2), OvTg-BSE (lane 3), pig-BSE (lane 4), PoTg-BSE (lane 5), mouse-BSE (lane 6), MoTga20-BSE (lane 7), human-vCJD (lane 8), HuTg-BSE (lane 9). Brain extracts from BoPrP-Tg110 mice inoculated with atypical cattle-BSE H (lane 10), sheep-scrapie (lane 11) and mouse-RML (lane 12) were included as a control non-BSE related prion propagated in the same mouse model. Panels A and B were loaded with the same quantities of PrP^res extracted from each sample. MW, molecular weight in kilodaltons.</p