Distinct Transmissibility Features of TSE Sources Derived from Ruminant Prion Diseases by the Oral Route in a Transgenic Mouse Model (TgOvPrP4) Overexpressing the Ovine Prion Protein

Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative diseases associated with a misfolded form of host-encoded prion protein (PrP). Some of them, such as classical bovine spongiform encephalopathy in cattle (BSE), transmissible mink encephalopathy (TME), kuru and variant Creutzfeldt–Jakob disease in humans, are acquired by the oral route exposure to infected tissues. We investigated the possible transmission by the oral route of a panel of strains derived from ruminant prion diseases in a transgenic mouse model (TgOvPrP4) overexpressing the ovine prion protein (A₁₃₆R₁₅₄Q₁₇₁) under the control of the neuron-specific enolase promoter. Sources derived from Nor98, CH1641 or 87V scrapie sources, as well as sources derived from L-type BSE or cattle-passaged TME, failed to transmit by the oral route, whereas those derived from classical BSE and classical scrapie were successfully transmitted. Apart from a possible effect of passage history of the TSE agent in the inocula, this implied the occurrence of subtle molecular changes in the protease-resistant prion protein (PrPres) following oral transmission that can raises concerns about our ability to correctly identify sheep that might be orally infected by the BSE agent in the field. Our results provide proof of principle that transgenic mouse models can be used to examine the transmissibility of TSE agents by the oral route, providing novel insights regarding the pathogenesis of prion diseases.     

Strain‐specific viral properties of variant Creutzfeldt–Jakob disease (vCJD) are encoded by the agent and not by host prion protein

Human CJD, endemic sheep scrapie, epidemic bovine spongiform encephalopathy (BSE), and other transmissible spongiform encephalopathies (TSEs), are caused by a group of related but molecularly uncharacterized infectious agents. The UK‐BSE agent infected many species, including humans where it causes variant CJD (vCJD). As in most viral infections, different TSE disease phenotypes are determined by both the agent strain and the host species. TSE strains are most reliably classified by incubation time and regional neuropathology in mice expressing wild‐type (wt) prion protein (PrP). We compared vCJD to other human and animal derived TSE strains in both mice and neuronal cultures expressing wt murine PrP. Primary and serial passages of the human vCJD agent, as well as the highly selected mutant 263K sheep scrapie agent, revealed profound strain‐specific characteristics were encoded by the agent, not by host PrP. Prion theory posits that PrP converts itself into the infectious agent, and thus short incubations require identical PrP sequences in the donor and recipient host. However, wt PrP mice injected with human vCJD brain homogenates showed dramatically shorter primary incubation times than mice expressing only human PrP, a finding not in accord with a PrP species barrier. All mouse passage brains showed the vCJD agent derived from a stable BSE strain. Additionally, both vCJD brain and monotypic neuronal cultures produced a diagnostic 19 kDa PrP fragment previously observed only in BSE and vCJD primate brains. Monotypic cultures can be used to identify the intrinsic, strain‐determining molecules of TSE infectious particles. J. Cell. Biochem. 106: 220–231, 2009. © 2008 Wiley‐Liss, Inc.     

Prions: where do we stand 20 years after the appearance of bovine spongiform encephalopathy?

Bovine spongiform encephalopathy (BSE) is a transmissible spongiform encephalopathy (TSE) identified twenty years ago in the British cattle herds. Creutzfeldt-Jakob disease (CJD) is a TSE that occurs in humans. In 1996, scientists found a possible link between BSE and a new variant of CJD (vCJD). The fact that the non conventional infectious agent of TSE, named prions, could cross the species barrier from cattle to human through meat consumption, raised a tremendous concern for public safety in Europe. This led to the development in the following two decades of substantial and expensive measures to contain BSE and prevent its transmission to humans. In parallel, scientific programs have been funded to progress through the comprehension of the physiopathology of these fatal disorders. In Europe, the BSE epidemics is now ending and the number of cases is decreasing thanks to the strict control of animal foodstuff that was the main source of prion contamination. Only a small number of vCJD have been detected, however, additional concerns have been raised recently for public safety as secondary transmission of CJD through medical procedure and blood transfusion is possible. In addition, the possibility that the BSE was transmitted to other animals including small ruminants is also worrisome. Research efforts are now focussing on decontamination and ante mortem diagnosis of TSE to prevent animal and human transmission. However, needs for fundamental research are still important as many questions remain to be addressed to understand the mechanism of prion transmission, as well as its pathogenesis.     

Molecular analysis of the protease-resistant prion protein in scrapie and bovine spongiform encephalopathy transmitted to ovine transgenic and wild-type mice

The existence of different strains of infectious agents involved in scrapie, a transmissible spongiform encephalopathy (TSE) of sheep and goats, remains poorly explained. These strains can, however, be differentiated by characteristics of the disease in mice and also by the molecular features of the protease-resistant prion protein (PrP(res)) that accumulates into the infected tissues. For further analysis, we first transmitted the disease from brain samples of TSE-infected sheep to ovine transgenic [Tg(OvPrP4)] and to wild-type (C57BL/6) mice. We show that, as in sheep, molecular differences of PrP(res) detected by Western blotting can differentiate, in both ovine transgenic and wild-type mice, infection by the bovine spongiform encephalopathy (BSE) agent from most scrapie sources. Similarities of an experimental scrapie isolate (CH1641) with BSE were also likewise found following transmission in ovine transgenic mice. Secondly, we transmitted the disease to ovine transgenic mice by inoculation of brain samples of wild-type mice infected with different experimental scrapie strains (C506M3, 87V, 79A, and Chandler) or with BSE. Features of these strains in ovine transgenic mice were reminiscent of those previously described for wild-type mice, by both ratios and by molecular masses of the different PrP(res) glycoforms. Moreover, these studies revealed the diversity of scrapie strains and their differences with BSE according to labeling by a monoclonal antibody (P4). These data, in an experimental model expressing the prion protein of the host of natural scrapie, further suggest a genuine diversity of TSE infectious agents and emphasize its linkage to the molecular features of the abnormal prion protein.     

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.     

Intraspecies transmission of BASE induces clinical dullness and amyotrophic changes

The disease phenotype of bovine spongiform encephalopathy (BSE) and the molecular/ biological properties of its prion strain, including the host range and the characteristics of BSE-related disorders, have been extensively studied since its discovery in 1986. In recent years, systematic testing of the brains of cattle coming to slaughter resulted in the identification of at least two atypical forms of BSE. These emerging disorders are characterized by novel conformers of the bovine pathological prion protein (PrP(TSE)), named high-type (BSE-H) and low-type (BSE-L). We recently reported two Italian atypical cases with a PrP(TSE) type identical to BSE-L, pathologically characterized by PrP amyloid plaques and known as bovine amyloidotic spongiform encephalopathy (BASE). Several lines of evidence suggest that BASE is highly virulent and easily transmissible to a wide host range. Experimental transmission to transgenic mice overexpressing bovine PrP (Tgbov XV) suggested that BASE is caused by a prion strain distinct from the BSE isolate. In the present study, we experimentally infected Friesian and Alpine brown cattle with Italian BSE and BASE isolates via the intracerebral route. BASE-infected cattle developed amyotrophic changes accompanied by mental dullness. The molecular and neuropathological profiles, including PrP deposition pattern, closely matched those observed in the original cases. This study provides clear evidence of BASE as a distinct prion isolate and discloses a novel disease phenotype in cattle.     

Detection and localisation of PrP(Sc) in the liver of sheep infected with scrapie and bovine spongiform encephalopathy

Prions are largely contained within the nervous and lymphoid tissue of transmissible spongiform encephalopathy (TSE) infected animals. However, following advances in diagnostic sensitivity, PrP(Sc), a marker for prion disease, can now be located in a wide range of viscera and body fluids including muscle, saliva, blood, urine and milk, raising concerns that exposure to these materials could contribute to the spread of disease in humans and animals. Previously we demonstrated low levels of infectivity in the liver of sheep experimentally challenged with bovine spongiform encephalopathy. In this study we show that PrP(Sc) accumulated in the liver of 89% of sheep naturally infected with scrapie and 100% of sheep challenged with BSE, at both clinical and preclinical stages of the disease. PrP(Sc) was demonstrated in the absence of obvious inflammatory foci and was restricted to isolated resident cells, most likely Kupffer cells.     

Isolation from cattle of a prion strain distinct from that causing bovine spongiform encephalopathy

To date, bovine spongiform encephalopathy (BSE) and its human counterpart, variant Creutzfeldt-Jakob disease, have been associated with a single prion strain. This strain is characterised by a unique and remarkably stable biochemical profile of abnormal protease-resistant prion protein (PrP(res)) isolated from brains of affected animals or humans. However, alternate PrP(res) signatures in cattle have recently been discovered through large-scale screening. To test whether these also represent separate prion strains, we inoculated French cattle isolates characterised by a PrP(res) of higher apparent molecular mass--called H-type--into transgenic mice expressing bovine or ovine PrP. All mice developed neurological symptoms and succumbed to these isolates, showing that these represent a novel strain of infectious prions. Importantly, this agent exhibited strain-specific features clearly distinct from that of BSE agent inoculated to the same mice, which were retained on further passage. Moreover, it also differed from all sheep scrapie isolates passaged so far in ovine PrP-expressing mice. Our findings therefore raise the possibility that either various prion strains may exist in cattle, or that the BSE agent has undergone divergent evolution in some animals.     

Sheep and goat BSE propagate more efficiently than cattle BSE in human PrP transgenic mice

A new variant of Creutzfeldt Jacob Disease (vCJD) was identified in humans and linked to the consumption of Bovine Spongiform Encephalopathy (BSE)-infected meat products. Recycling of ruminant tissue in meat and bone meal (MBM) has been proposed as origin of the BSE epidemic. During this epidemic, sheep and goats have been exposed to BSE-contaminated MBM. It is well known that sheep can be experimentally infected with BSE and two field BSE-like cases have been reported in goats. In this work we evaluated the human susceptibility to small ruminants-passaged BSE prions by inoculating two different transgenic mouse lines expressing the methionine (Met) allele of human PrP at codon 129 (tg650 and tg340) with several sheep and goat BSE isolates and compared their transmission characteristics with those of cattle BSE. While the molecular and neuropathological transmission features were undistinguishable and similar to those obtained after transmission of vCJD in both transgenic mouse lines, sheep and goat BSE isolates showed higher transmission efficiency on serial passaging compared to cattle BSE. We found that this higher transmission efficiency was strongly influenced by the ovine PrP sequence, rather than by other host species-specific factors. Although extrapolation of results from prion transmission studies by using transgenic mice has to be done very carefully, especially when human susceptibility to prions is analyzed, our results clearly indicate that Met129 homozygous individuals might be susceptible to a sheep or goat BSE agent at a higher degree than to cattle BSE, and that these agents might transmit with molecular and neuropathological properties indistinguishable from those of vCJD. Our results suggest that the possibility of a small ruminant BSE prion as vCJD causal agent could not be ruled out, and that the risk for humans of a potential goat and/or sheep BSE agent should not be underestimated.     

Seeded Fibrillation as Molecular Basis of the Species Barrier in Human Prion Diseases

Prion diseases are transmissible spongiform encephalopathies in humans and animals, including scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, chronic wasting disease (CWD) in deer, and Creutzfeldt-Jakob disease (CJD) in humans. The hallmark of prion diseases is the conversion of the host-encoded prion protein (PrP^C) to its pathological isoform PrP^Sc, which is accompanied by PrP fibrillation. Transmission is not restricted within one species, but can also occur between species. In some cases a species barrier can be observed that results in limited or unsuccessful transmission. The mechanism behind interspecies transmissibility or species barriers is not completely understood. To analyse this process at a molecular level, we previously established an in vitro fibrillation assay, in which recombinant PrP (recPrP) as substrate can be specifically seeded by PrP^Sc as seed. Seeding with purified components, with no additional cellular components, is a direct consequence of the “prion-protein-only” hypothesis. We therefore hypothesise, that the species barrier is based on the interaction of PrP^C and PrP^Sc. Whereas in our earlier studies, the interspecies transmission in animal systems was analysed, the focus of this study lies on the transmission from animals to humans. We therefore combined seeds from species cattle, sheep and deer (BSE, scrapie, CWD) with human recPrP. Homologous seeding served as a control. Our results are consistent with epidemiology, other in vitro aggregation studies, and bioassays investigating the transmission between humans, cattle, sheep, and deer. In contrast to CJD and BSE seeds, which show a seeding activity we can demonstrate a species barrier for seeds from scrapie and CWD in vitro. We could show that the seeding activity and therewith the molecular interaction of PrP as substrate and PrP^Sc as seed is sufficient to explain the phenomenon of species barriers. Therefore our data supports the hypothesis that CWD is not transmissible to humans.     

Experimental transmission of bovine spongiform encephalopathy to European red deer (<Emphasis Type="Italic">Cervus elaphus elaphus</Emphasis>)

Background

Bovine spongiform encephalopathy (BSE), a member of the transmissible spongiform encephalopathies (TSE), primarily affects cattle. Transmission is via concentrate feed rations contaminated with infected meat and bone meal (MBM). In addition to cattle, other food animal species are susceptible to BSE and also pose a potential threat to human health as consumption of infected meat products is the cause of variant Creutzfeldt-Jakob disease in humans, which is invariably fatal. In the UK, farmed and free ranging deer were almost certainly exposed to BSE infected MBM in proprietary feeds prior to legislation banning its inclusion. Therefore, although BSE has never been diagnosed in any deer species, a possible risk to human health remains via ingestion of cervine products. Chronic wasting disease (CWD), also a TSE, naturally infects several cervid species in North America and is spreading rapidly in both captive and free-ranging populations.

Results

Here we show that European red deer (Cervus elaphus elaphus) are susceptible to intra-cerebral (i/c) challenge with BSE positive cattle brain pool material resulting in clinical neurological disease and weight loss by 794–1290 days and the clinical signs are indistinguishable to those reported in deer with CWD. Spongiform changes typical of TSE infections were present in brain and accumulation of the disease-associated abnormal prion protein (PrP^d) was present in the central and peripheral nervous systems, but not in lymphoid or other tissues. Western immunoblot analysis of brain material showed a similar glycosylation pattern to that of BSE derived from infected cattle and experimentally infected sheep with respect to protease-resistant PrP isoforms. However, the di-, mono- and unglycosylated bands migrated significantly (p < 0.001) further in the samples from the clinically affected deer when compared to BSE infected brains of cattle and sheep.

Conclusion

This study shows that deer are susceptible to BSE by intra-cerebral inoculation and display clinical signs and vacuolar pathology that are similar to those of CWD. These findings highlight the importance of preventing the spread to Europe of CWD from North America as this may necessitate even more extensive testing of animal tissues destined for human consumption within the EU. Although the absence of PrP^d in lymphoid and other non-neurological tissues potentially limits the risk of transmission to humans, the replication of TSE agents in peripheral tissues following intra-cerebral challenge is often limited. Thus the assessment of risk posed by cervine BSE as a human pathogen or for environmental contamination should await the outcome of ongoing oral challenge experiments.

     

Subclinical prion infection

Prion diseases are transmissible neurodegenerative disorders that include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle and Creutzfeldt–Jakob disease (CJD) in humans. The principal component of the infectious agent responsible for these diseases appears to be an abnormal isoform of the host-encoded prion protein (PrP), designated PrP^Sc. Prion diseases are transmissible to the same or different mammalian species by inoculation with, or dietary exposure to, infected tissues. Although scrapie in sheep has been recognized for over 200 years, it is the recent epidemic of BSE that has centred much public and scientific attention on these neurodegenerative diseases. The occurrence of variant CJD in humans and the experimental confirmation that it is caused by the same prion strain as BSE has highlighted the need for intensive study into the pathogenesis of these diseases and new diagnostic and therapeutic approaches. The existence and implications of subclinical forms of prion disease are discussed.     

Susceptibility of Young Sheep to Oral Infection with Bovine Spongiform Encephalopathy Decreases Significantly after Weaning

Bovine spongiform encephalopathy (BSE) is a transmissible spongiform encephalopathy (TSE) (or prion disease) that is readily transmissible to sheep by experimental infection and has the shortest incubation period in animals with the ARQ/ARQ PRNP genotype (at codons 136, 154, and 171). Because it is possible that sheep in the United Kingdom could have been infected with BSE by being fed contaminated meat and bone meal supplements at the same time as cattle, there is considerable interest in the responses of sheep to BSE inoculation. Epidemiological evidence suggests that very young individuals are more susceptible to TSE infection; however, this has never been properly tested in sheep. In the present study, low doses of BSE were fed to lambs of a range of ages (∼24 h, 2 to 3 weeks, 3 months, and 6 months) and adult sheep. The incidence of clinical BSE disease after inoculation was high in unweaned lambs (∼24 h and 2 to 3 weeks old) but much lower in older weaned animals The incubation period was also found to be influenced by the genotype at codon 141 of the PRNP gene, as lambs that were LF heterozygotes had a longer mean incubation period than those that were homozygotes of either type. The results suggest that sheep in the United Kingdom would have been at high risk of BSE infection only if neonatal animals had inadvertently ingested contaminated supplementary foodstuffs.     

Exploring the zoonotic potential of animal prion diseases

Following the discovery of a causal link between bovine spongiform encephalopathy (BSE) in cattle and variant Creutzfeldt–Jakob disease (vCJD) in humans, several experimental approaches have been used to try to assess the potential risk of transmission of other animal transmissible spongiform encephalopathies (TSEs) to humans. Experimental challenge of non-human primates, humanised transgenic mice and cell-free conversion systems have all been used as models to explore the susceptibility of humans to animal TSEs. In this review we compare and contrast in vivo and in vitro evidence of the zoonotic risk to humans from sheep, cattle and deer prions, focusing primarily on chronic wasting disease and our own recent studies using protein misfolding cyclic amplification.     

Changing a single amino acid in the N-terminus of murine PrP alters TSE incubation time across three species barriers

The PrP gene of the host exerts a major influence over the outcome of transmissible spongiform encephalopathy (TSE) disease, but the mechanism by which this is achieved is not understood. We have introduced a specific mutation into the endogenous murine PrP gene using gene targeting to produce transgenic mice with a single amino acid alteration (proline to leucine) at amino acid position 101 in their PrP protein (P101L). The effect of this alteration on incubation time, targeting and PrP(Sc) formation has been studied in TSE-infected animals. Transgenic mice carrying the P101L mutation in PrP have remarkable differences in incubation time and targeting of central nervous system pathology compared with wild-type littermates, following inoculation with infectivity from human, hamster, sheep and murine sources. This single mutation can alter incubation time across three species barriers in a strain-dependent manner. These findings suggest a critical role for the structurally 'flexible' region of PrP in agent replication and targeting of TSE pathology.     

Transmission barriers for bovine, ovine, and human prions in transgenic mice

Transgenic (Tg) mice expressing full-length bovine prion protein (BoPrP) serially propagate bovine spongiform encephalopathy (BSE) prions without posing a transmission barrier. These mice also posed no transmission barrier for Suffolk sheep scrapie prions, suggesting that cattle may be highly susceptible to some sheep scrapie strains. Tg(BoPrP) mice were also found to be susceptible to prions from humans with variant Creutzfeldt-Jakob disease (CJD); on second passage in Tg(BoPrP) mice, the incubation times shortened by 30 to 40 days. In contrast, Tg(BoPrP) mice were not susceptible to sporadic, familial, or iatrogenic CJD prions. While the conformational stabilities of bovine-derived and Tg(BoPrP)-passaged BSE prions were similar, the stability of sheep scrapie prions was higher than that found for the BSE prions but lower if the scrapie prions were passaged in Tg(BoPrP) mice. Our findings suggest that BSE prions did not arise from a sheep scrapie strain like the one described here; rather, BSE prions may have arisen spontaneously in a cow or by passage of a scrapie strain that maintains its stability upon passage in cattle. It may be possible to distinguish BSE prions from scrapie strains in sheep by combining conformational stability studies with studies using novel Tg mice expressing a chimeric mouse-BoPrP gene. Single-amino-acid substitutions in chimeric PrP transgenes produced profound changes in incubation times that allowed us to distinguish prions causing BSE from those causing scrapie.     

Strain-specific barriers against bovine prions in hamsters

We investigated the susceptibilities of Syrian golden hamsters to transmissible spongiform encephalopathy agents from cattle. We report efficient transmission of the L-type atypical bovine spongiform encephalopathy (BSE) agent into hamsters. Importantly, hamsters were also susceptible to the transmissible mink encephalopathy agent from cattle, which has molecular features similar to those of the L-type BSE agent, as also shown in bovinized transgenic mice. In sharp contrast, hamsters could not be infected with classical or H-type BSE agents from cattle. However, previous adaptation of the classical BSE agent in wild-type mice led to efficient transmission. Thus, this study demonstrates the existence of distinct "strain barriers" upon the transmission of bovine prions in hamsters.     

PrPTSE distribution in a primate model of variant, sporadic, and iatrogenic Creutzfeldt-Jakob disease

Human prion diseases, such as Creutzfeldt-Jakob disease (CJD), are neurodegenerative and fatal. Sporadic CJD (sCJD) can be transmitted between humans through medical procedures involving highly infected organs, such as the central nervous system. However, in variant CJD (vCJD), which is due to human contamination with the bovine spongiform encephalopathy (BSE) agent, lymphoreticular tissue also harbors the transmissible spongiform encephalopathy-associated prion protein (PrP(TSE)), which poses a particularly acute risk for iatrogenic transmission. Two blood transfusion-related cases are already documented. In addition, the recent observation of PrP(TSE) in spleen and muscle in sCJD raised the possibility that peripheral PrP(TSE) is not limited to vCJD cases. We aimed to clarify the peripheral pathogenesis of human TSEs by using a nonhuman primate model which mimics human diseases. A highly sensitive enzyme-linked immunosorbent assay was adapted to the detection of extraneural PrP(TSE). We show that affected organs can be divided into two groups. The first is peripheral organs accumulating large amounts of PrP(TSE), which represent a high risk of iatrogenic transmission. This category comprises only lymphoreticular organs in the vCJD/BSE model. The second is organs with small amounts of PrP(TSE) associated with nervous structures. These are the muscles, adrenal glands, and enteric nervous system in the sporadic, iatrogenic, and variant CJD models. In contrast to the first set of organs, this low level of tissue contamination is not strain restricted and seems to be linked to secondary centrifugal spread of the agent through nerves. It might represent a risk for iatrogenic transmission, formerly underestimated despite previous reports of low rates of transmission from peripheral organs of humans to nonhuman primates (5, 10). This study provides an additional experimental basis for the classification of human organs into different risk categories and a rational re-evaluation of current risk management measures.     

PrP(c) mRNA, but not PrP(Sc) is found in the salivary glands of scrapie-infected sheep

Transmission studies in transmissible spongiform encephalopathies (TSEs) have become increasingly important due to the possible transmission of bovine spongiform encephalopathy to humans resulting in new variant Creutzfeldt-Jacob disease. The horizontal transmission of scrapie, a TSE of sheep, is poorly understood. Possible sources of horizontal transmission are the submandibular and parotid salivary glands. TSEs like natural sheep scrapie are characterized by the conversion of a normal protease sensitive prion protein, PrP(c), to an abnormal protease resistant prion protein, PrP(Sc). Since the presence of PrP(Sc) is an indicator of disease, the salivary glands of scrapie-infected sheep were examined for the presence of PrP(Sc). Although PrP(c) mRNA was detected in the salivary glands, PrP(Sc) was not found in the salivary glands of scrapie-infected sheep. These data suggest that the salivary glands are unlikely sources of horizontal transmission of natural sheep scrapie.     

Entry versus blockade of brain infection following oral or intraperitoneal scrapie administration: role of prion protein expression in peripheral nerves and spleen

Naturally occurring transmissible spongiform encephalopathy (TSE) diseases such as bovine spongiform encephalopathy in cattle are probably transmitted by oral or other peripheral routes of infection. While prion protein (PrP) is required for susceptibility, the mechanism of spread of infection to the brain is not clear. Two prominent possibilities include hematogenous spread by leukocytes and neural spread by axonal transport. In the present experiments, following oral or intraperitoneal infection of transgenic mice with hamster scrapie strain 263K, hamster PrP expression in peripheral nerves was sufficient for successful infection of the brain, and cells of the spleen were not required either as a site of amplification or as transporters of infectivity. The role of tissue-specific PrP expression of foreign PrP in interference with scrapie infection was also studied in these transgenic mice. Peripheral expression of heterologous PrP completely protected the majority of mice from clinical disease after oral or intraperitoneal scrapie infection. Such extensive protection has not been seen in earlier studies on interference, and these results suggested that gene therapy with mutant PrP may be effective in preventing TSE diseases.