Levels of infectivity in the blood throughout the incubation period of hamsters peripherally injected with scrapie

Time of Detection of Prions in the Brain by Nanoscale Liquid Chromatography Coupled to Tandem Mass Spectrometry Is Comparable to Animal Bioassay

Prions cause transmissible and inevitably fatal neurological diseases in agriculturally important animals, including bovine spongiform encephalopathy in domestic cattle, scrapie in sheep and goats, and chronic wasting disease in cervids. Because animals are largely asymptomatic throughout the course of the disease, early detection of prion disease is important. Hamsters were peripherally (ip) inoculated with hamster-adapted (Sc237) prions. By week 13 of a 14-week disease course, clinical signs appeared. A multiple-reaction-monitoring-based method was used to quantitate the amount of proteinase-K-digested prions (PrP 27-30) and the extent of methionine 213 oxidation present in the brains of infected hamsters. Detectable amounts of PrP 27-30 were present in all animals after 4 weeks. The extent of methionine 213 oxidation decreased over time. When we compared our quantitation results to those from other researchers using bioassay, we observed that consistent detection of PrP 27-30 by mass spectrometry occurs at a time when prions are reliably detected by bioassay.

Experimental inoculation of CD11c+ B1 lymphocytes, CD68+ macrophages, or platelet-rich plasma from scrapie-infected sheep into susceptible sheep results in variable infectivity

Many studies have demonstrated prion infectivity in whole blood and blood components in a variety of transmissible spongiform encephalopathies of livestock and rodents, and variant Creutzfeldt–Jakob disease in humans, as well as an association between pathogenic prion protein (PrP^Sc) and different immune cells (e.g. follicular dendritic cells, T and B lymphocytes, monocytes and tingible body macrophages). To further investigate the role of various blood components in prion disease transmission, we intracranially inoculated genetically susceptible VRQ/ARQ and ARQ/ARQ sheep with inocula composed of CD11c⁺ B1 lymphocytes, CD68 +macrophages, or platelet-rich plasma derived from clinically ill sheep infected with the US no. 13–7 scrapie agent. At the completion of the study, we found that VRQ/ARQ and ARQ/ARQ sheep inoculated with CD11c⁺ B1 lymphocytes and CD68⁺ macrophages developed scrapie with detectable levels of PrP^Sc in the central nervous system and lymphoreticular system, while those inoculated with platelet-rich plasma did not develop disease and did not have detectable PrP^Sc by immunohistochemistry or enzyme immunoassay. This study complements and expands on earlier findings that white blood cells harbour prion infectivity, and reports CD11c⁺ B1 lymphocytes and CD68⁺ macrophages as additional targets for possible preclinical detection of prion infection in blood.

Failure To Detect Prion Infectivity in Ticks following Prion-Infected Blood Meal

Chronic wasting disease (CWD) is an emerging and fatal contagious prion disease that affects cervids, including mule deer, white-tailed deer, black-tailed deer, red deer reindeer, elk, and moose. CWD prions are widely distributed throughout the bodies of CWD-infected animals and are found in the nervous system, lymphoid tissues, muscle, blood, urine, feces, and antler velvet. The mechanism of CWD transmission in natural settings is unknown. Potential mechanisms of transmission include horizontal, maternal, or environmental routes. Due to the presence of prions in the blood of CWD-infected animals, the potential exists for invertebrates that feed on mammalian blood to contribute to the transmission of CWD. The geographic range of the Rocky Mountain Wood tick, Dermancentor andersoni, overlaps with CWD throughout the northwest United States and southwest Canada, raising the possibility that D. andersoni parasitization of cervids may be involved in CWD transmission. We investigated this possibility by examining the blood meal of D. andersoni that fed upon prion-infected hamsters for the presence of prion infectivity by animal bioassay. None of the hamsters inoculated with a D. andersoni blood meal that had been ingested from prion-infected hamsters developed clinical signs of prion disease or had evidence for a subclinical prion infection. Overall, the data do not demonstrate a role for D. andersoni in the transmission of prion disease.IMPORTANCE Chronic wasting disease (CWD) is an emerging prion disease that affects cervids, including mule deer, white-tailed deer, black-tailed deer, red deer reindeer, elk, and moose. The mechanism of CWD transmission in unknown. Due to the presence of prions in the blood of CWD-infected animals, it is possible for invertebrates that feed on cervid blood to contribute to the transmission of CWD. We examined the blood meal of D. andersoni, a tick with a similar geographic range as cervids, that fed upon prion-infected hamsters for the presence of prion infectivity by animal bioassay. None of the D. andersoni blood meals that had been ingested from prion-infected hamsters yielded evidence of prion infection. Overall, the data do not support a role of D. andersoni in the transmission of prion disease.

Detection of Pathognomonic Biomarker PrPSc and the Contribution of Cell Free-Amplification Techniques to the Diagnosis of Prion Diseases

Transmissible spongiform encephalopathies or prion diseases are rapidly progressive neurodegenerative diseases, the clinical manifestation of which can resemble other promptly evolving neurological maladies. Therefore, the unequivocal ante-mortem diagnosis is highly challenging and was only possible by histopathological and immunohistochemical analysis of the brain at necropsy. Although surrogate biomarkers of neurological damage have become invaluable to complement clinical data and provide more accurate diagnostics at early stages, other neurodegenerative diseases show similar alterations hindering the differential diagnosis. To solve that, the detection of the pathognomonic biomarker of disease, PrP^Sc, the aberrantly folded isoform of the prion protein, could be used. However, the amounts in easily accessible tissues or body fluids at pre-clinical or early clinical stages are extremely low for the standard detection methods. The solution comes from the recent development of in vitro prion propagation techniques, such as Protein Misfolding Cyclic Amplification (PMCA) and Real Time-Quaking Induced Conversion (RT-QuIC), which have been already applied to detect minute amounts of PrP^Sc in different matrixes and make early diagnosis of prion diseases feasible in a near future. Herein, the most relevant tissues and body fluids in which PrP^Sc has been detected in animals and humans are being reviewed, especially those in which cell-free prion propagation systems have been used with diagnostic purposes.

Intraperitoneal Infection of Wild-Type Mice with Synthetically Generated Mammalian Prion

The prion hypothesis postulates that the infectious agent in transmissible spongiform encephalopathies (TSEs) is an unorthodox protein conformation based agent. Recent successes in generating mammalian prions in vitro with bacterially expressed recombinant prion protein provide strong support for the hypothesis. However, whether the pathogenic properties of synthetically generated prion (rec-Prion) recapitulate those of naturally occurring prions remains unresolved. Using end-point titration assay, we showed that the in vitro prepared rec-Prions have infectious titers of around 10⁴ LD₅₀ / μg. In addition, intraperitoneal (i.p.) inoculation of wild-type mice with rec-Prion caused prion disease with an average survival time of 210 – 220 days post inoculation. Detailed pathological analyses revealed that the nature of rec-Prion induced lesions, including spongiform change, disease specific prion protein accumulation (PrP-d) and the PrP-d dissemination amongst lymphoid and peripheral nervous system tissues, the route and mechanisms of neuroinvasion were all typical of classical rodent prions. Our results revealed that, similar to naturally occurring prions, the rec-Prion has a titratable infectivity and is capable of causing prion disease via routes other than direct intra-cerebral challenge. More importantly, our results established that the rec-Prion caused disease is pathogenically and pathologically identical to naturally occurring contagious TSEs, supporting the concept that a conformationally altered protein agent is responsible for the infectivity in TSEs.

The transmissible spongiform encephalopathies (TSEs) are a group of infectious neurodegenerative diseases affecting both humans and animals. The prion hypothesis postulates that prions are protein conformation based infectious agents responsible for TSE infectivity. Prions have been synthetically generated in vitro, but it remains unclear whether the properties of synthetically generated prion are the same as those of TSE agents and whether the disease caused by synthetically generated prion is identical to naturally occurring TSEs. In this study, we demonstrated that similar to the classical TSE agents, the synthetically generated prion has a titratable infectivity and is able to cause prion disease in wild-type mice via routes other than direct intra-cerebral inoculation. More importantly, we showed that the synthetically generated prion induced pathological changes, including the dissemination of disease-specific prion protein accumulation and the route and mechanism of neuroinvasion, were all typical of classical TSEs. These results demonstrate the similarity of synthetically generated prion to the infectious agent in TSEs, providing strong evidence supporting the prion hypothesis.

Re-assessment of PrP(Sc) distribution in sporadic and variant CJD

Human prion diseases are fatal neurodegenerative disorders associated with an accumulation of PrP^Sc in the central nervous system (CNS). Of the human prion diseases, sporadic Creutzfeldt-Jakob disease (sCJD), which has no known origin, is the most common form while variant CJD (vCJD) is an acquired human prion disease reported to differ from other human prion diseases in its neurological, neuropathological, and biochemical phenotype. Peripheral tissue involvement in prion disease, as judged by PrP^Sc accumulation in the tonsil, spleen, and lymph node has been reported in vCJD as well as several animal models of prion diseases. However, this distribution of PrP^Sc has not been consistently reported for sCJD. We reexamined CNS and non-CNS tissue distribution and levels of PrP^Sc in both sCJD and vCJD. Using a sensitive immunoassay, termed SOFIA, we also assessed PrP^Sc levels in human body fluids from sCJD as well as in vCJD-infected humanized transgenic mice (Tg666). Unexpectedly, the levels of PrP^Sc in non-CNS human tissues (spleens, lymph nodes, tonsils) from both sCJD and vCJD did not differ significantly and, as expected, were several logs lower than in the brain. Using protein misfolding cyclic amplification (PMCA) followed by SOFIA, PrP^Sc was detected in cerebrospinal fluid (CSF), but not in urine or blood, in sCJD patients. In addition, using PMCA and SOFIA, we demonstrated that blood from vCJD-infected Tg666 mice showing clinical disease contained prion disease-associated seeding activity although the data was not statistically significant likely due to the limited number of samples examined. These studies provide a comparison of PrP^Sc in sCJD vs. vCJD as well as analysis of body fluids. Further, these studies also provide circumstantial evidence that in human prion diseases, as in the animal prion diseases, a direct comparison and intraspecies correlation cannot be made between the levels of PrP^Sc and infectivity.

Comparison of nanofiltration efficacy in reducing infectivity of centrifuged versus ultracentrifuged 263K scrapie-infected brain homogenates in "spiked" albumin solutions

BACKGROUND

The safety of plasma-derived products is of concern for possible transmission of variant Creutzfeldt-Jakob disease. The absence of validated screening tests requires the use of procedures to remove or inactivate prions during the manufacture of plasma-derived products to minimize the risk of transmission. These procedures need proper validation studies based on spiking human plasma or intermediate fractions of plasma fractionation with prions in a form as close as possible to that present in blood.

STUDY DESIGN AND METHODS

Human albumin was spiked with low-speed or high-speed supernatants of 263K scrapie-infected hamster brain homogenates. Spiked albumin was then passed through a cascade of filters from 100 nm down to 20 to 15 nm. Residual infectivity was measured by bioassay.

RESULTS

The overall removal of infectivity spiked into albumin through serial nanofiltration steps was 4 to 5 logs using low-speed supernatant and 2 to 3 logs with high-speed supernatant.

CONCLUSION

These findings confirm the utility of nanofiltration in removing infectivity from plasma (or other products) spiked with scrapie brain homogenate supernatants. However, efficiency is diminished using supernatants that have been ultracentrifuged to reduce aggregated forms of the infectious agent. Thus, filtration removal data based on experiments using "standard" low-speed centrifugation supernatants might overestimate the amount of prion removal in plasma or urine-derived therapeutic products.

B cells and platelets harbor prion infectivity in the blood of deer infected with chronic wasting disease

Substantial evidence for prion transmission via blood transfusion exists for many transmissible spongiform encephalopathy (TSE) diseases. Determining which cell phenotype(s) is responsible for trafficking infectivity has important implications for our understanding of the dissemination of prions, as well as their detection and elimination from blood products. We used bioassay studies of native white-tailed deer and transgenic cervidized mice to determine (i) if chronic wasting disease (CWD) blood infectivity is associated with the cellular versus the cell-free/plasma fraction of blood and (ii) in particular if B-cell (MAb 2-104(+)), platelet (CD41/61(+)), or CD14(+) monocyte blood cell phenotypes harbor infectious prions. All four deer transfused with the blood mononuclear cell fraction from CWD(+) donor deer became PrP(CWD) positive by 19 months postinoculation, whereas none of the four deer inoculated with cell-free plasma from the same source developed prion infection. All four of the deer injected with B cells and three of four deer receiving platelets from CWD(+) donor deer became PrP(CWD) positive in as little as 6 months postinoculation, whereas none of the four deer receiving blood CD14(+) monocytes developed evidence of CWD infection (immunohistochemistry and Western blot analysis) after 19 months of observation. Results of the Tg(CerPrP) mouse bioassays mirrored those of the native cervid host. These results indicate that CWD blood infectivity is cell associated and suggest a significant role for B cells and platelets in trafficking CWD infectivity in vivo and support earlier tissue-based studies associating putative follicular B cells with PrP(CWD). Localization of CWD infectivity with leukocyte subpopulations may aid in enhancing the sensitivity of blood-based diagnostic assays for CWD and other TSEs.

Creutzfeldt-Jakob disease and blood transfusion: results of the UK Transfusion Medicine Epidemiological Review study

BACKGROUND AND OBJECTIVES

This paper reports the results to 1 March 2006 of an ongoing UK study, the Transfusion Medicine Epidemiological Review (TMER), by the National CJD Surveillance Unit (NCJDSU) and the UK Blood Services (UKBS) to determine whether there is any evidence that Creutzfeldt-Jakob disease (CJD), including sporadic CJD (sCJD), familial CJD (fCJD), and variant CJD (vCJD) is transmissible via blood transfusion.

MATERIALS AND METHODS

Sporadic CJD and fCJD cases with a history of blood donation or transfusion are notified to UKBS. All vCJD cases aged > 17 years are notified to UKBS on diagnosis. A search for donation records is instigated and the fate of all donations is identified by lookback. For cases with a history of blood transfusion, hospital and UKBS records are searched to identify blood donors. Details of identified recipients and donors are checked against the NCJDSU register to establish if there are any matches.

RESULTS

CJD cases with donation history: 18/31 vCJD, 3/93 sCJD, and 3/5 fCJD cases reported as blood donors were confirmed to have donated labile components transfused to 66, 20, and 11 recipients respectively. Two vCJD recipients have appeared on the NCJDSU register as confirmed and probable vCJD cases. The latter developed symptoms of vCJD 6.5 years and 7.8 years respectively after receiving non-leucodepleted red blood cells (RBCs) from two different donors who developed clinical symptoms approximately 40 and 21 months after donating. A third recipient, given RBC donated by a further vCJD case approximately 18 months before onset of clinical symptoms, had abnormal prion protein in lymphoid tissue at post-mortem (5-years post-transfusion) but had no clinical symptoms of vCJD. CJD cases with history of transfusion: Hospital records for 7/11 vCJD and 7/52 sCJD cases included a history of transfusion of labile blood components donated by 125 and 24 donors respectively. Two recipients who developed vCJD were linked to donors who had already appeared on the NCJDSU register as vCJD cases (see above). No further links were established.

CONCLUSION

This study has identified three instances of probable transfusion transmission of vCJD infection, including two confirmed clinical cases and one pre- or sub-clinical infection. This study has not provided evidence, to date, of transmission of sCJD or fCJD by blood transfusion, but data on these forms of diseases are limited.

Prion-induced amyloid heart disease with high blood infectivity in transgenic mice

We investigated extraneural manifestations in scrapie-infected transgenic mice expressing prion protein lacking the glycophosphatydylinositol membrane anchor. In the brain, blood, and heart, both abnormal protease-resistant prion protein (PrPres) and prion infectivity were readily detected by immunoblot and by inoculation into nontransgenic recipients. The titer of infectious scrapie in blood plasma exceeded 10(7) 50% infectious doses per milliliter. The hearts of these transgenic mice contained PrPres-positive amyloid deposits that led to myocardial stiffness and cardiac disease.

Preparation of soluble infectious samples from scrapie-infected brain: a new tool to study the clearance of transmissible spongiform encephalopathy agents during plasma fractionation

BACKGROUND

Concern about the safety of blood, blood components, and plasma-derived products with respect to prions has increased since the report of two blood-related infections of variant Creutzfeldt-Jakob disease in the United Kingdom. Efforts were directed toward the development of procedures able to remove or inactivate prions from blood components or plasma-derived products with brain fractions of transmissible spongiform encephalopathy (TSE)-infected rodents as spiking materials. These spiking materials, however, are loaded with pathological prion protein (PrP(TSE)) aggregates that are likely not associated to blood infectivity. The presence of these aggregates may invalidate these studies.

STUDY DESIGN AND METHODS

Brains from 263K scrapie-infected hamsters were suspended in 10 percent phosphate-buffered saline. After low-speed centrifugation, the supernatant was collected and ultracentrifuged at 220,000 x g at 25 degrees C for 30 minutes. The high-speed supernatants (S(HS)) and pellets were collected; the proteinase-resistant PrP(TSE) was measured by Western blot and infectivity by intracerebral inoculation into weanling hamsters.

RESULTS

A substantial amount of prion infectivity (more than 10(5) LD(50) per mL of a 10% suspension of brain tissues) is present in the S(HS) fraction of 263K scrapie-infected hamster brains. Concomitantly, this fraction contains none or only traces of PrP(TSE) in its aggregate form.

CONCLUSION

This study describes a simple and fast protocol to prepare infectious material from 263K scrapie-infected brains that is not contaminated with PrP(TSE) aggregates. This S(HS) fraction is likely to be the most relevant material for endogenous spiking of human blood in validation experiments aimed at demonstrating procedures to remove or inactivate TSE infectious agents.

Managing the risk of transmission of variant Creutzfeldt Jakob disease by blood products

Whereas plasma-derived clotting factor concentrates now have a very good safety record for not being infectious for lipid enveloped viruses, concern has arisen about the possibility that prion diseases might be transmitted by blood products. There is epidemiological evidence that classical sporadic Creutzfeld Jakob disease (CJD) is not transmitted by blood transfusion. There is now good evidence that the abnormal prion associated with variant CJD can be transmitted by transfusion of fresh blood components and infect recipients. To reduce the risk of the pathological prion in the UK infecting recipients of clotting factor concentrates, these are now only manufactured from imported plasma collected from countries where there has not been bovine spongiform encephalopathy (BSE) in cattle and the risk of variant CJD in the population is, therefore, considered negligible. The safety of these concentrates is also enhanced because prion protein is, to an appreciable extent, excluded by the manufacturing process from the final product. To help reduce the chance of prion transmission by fresh blood products, donations are leucodepleted, there is increasing use of imported fresh frozen plasma (especially for treating children) and potential donors, who have been recipients of blood since 1980 (the beginning of the BSE epidemic in cattle) are deferred.

Detection of ultra-low levels of pathologic prion protein in scrapie infected hamster brain homogenates using real-time immuno-PCR

Pathologic prion protein (PrP(Sc)), implicated in transmissible spongiform encephalopathies, is detected by antibody-based tests or bioassays to confirm the diagnosis of prion diseases. Presently, the Western blot or an ELISA is officially used to screen the brain stem in cattle for the presence of PrP(Sc). The immuno-polymerase chain reaction (IPCR), a technique whereby the exponential amplification ability of PCR is coupled to the detection of proteins by antibodies in an ELISA format, was applied in a modified real-time IPCR method to detect ultra-low levels of prion protein. Using IPCR, recombinant hamster PrP(C) was consistently detected at 1 fg/mL and proteinase K (PK)-digested scrapie infected hamster brain homogenates diluted to 10(-8) (approximately 10-100 infectious units) was detected with a semi-quantitative dose response. This level of detection is 1 million-fold more sensitive than the levels detected by Western blot or ELISA and poises IPCR as a method capable of detecting PrP(Sc) in the pre-clinical phase of infection. Further, the data indicate that unless complete PK digestion of PrP(C) in biological materials is verified, ultrasensitive assays such as IPCR may inaccurately classify a sample as positive.

Prion Diseases and the Spleen

Transmissible spongiform encephalopathies are fatal neurodegenerative disorders that include Creutzfeldt-Jakob disease in humans, bovine spongiform encephalopathy and scrapie in sheep and goats. Transmissible spongiform encephalopathies are thought by some to result from changes in the conformation of a membrane glycoprotein called PrPC (prion protein) into a pathogenic form, PrPSc, which constitutes the major component of an unprecedented type of infectious particle supposedly devoid of nucleic acid. Although there is no primary immunological response to the infectious agent, several lines of evidence indicate an involvement of the lymphoreticular system in the development of prion diseases. Studies in rodents have shown that after peripheral infection, uptake of the scrapie agent is followed by an initial phase of replication in the lymphoreticular system, particularly the spleen and lymph nodes. Moreover, infectivity titers in lymphoreticular organs reach a maximum relatively quickly, well before those in the brain, and then maintain a plateau for the remainder of the disease progression. The presence of PrPSc in peripheral lymphoid organs of all cases of variant Creutzfeldt-Jakob disease strongly underscores the importance of the lymphoreticular system. Thus, a better understanding of the cells participating in PrPSc replication and dissemination into the central nervous system is of particular interest. This review will therefore discuss the present knowledge of the role of the spleen in transmissible spongiform encephalopathies as well as the participation of the different spleen cell types in the disease process.

Possible transmission of variant Creutzfeldt-Jakob disease by blood transfusion

BACKGROUND

Variant Creutzfeldt-Jakob disease (vCJD) is a novel human prion disease caused by infection with the agent of bovine spongiform encephalopathy (BSE). Epidemiological evidence does not suggest that sporadic CJD is transmitted from person to person via blood transfusion, but this evidence may not apply to vCJD. We aimed to identify whether vCJD is transmissible through blood transfusion.

METHODS

The national CJD surveillance unit reported all cases of probable or definite vCJD to the UK blood services, which searched for donation records at blood centres and hospitals. Information on named recipients and donors was provided to the surveillance unit to establish if any matches existed between recipients or donors and the database of cases of vCJD. Recipients were also flagged at the UK Office of National Statistics to establish date and cause of death.

FINDINGS

48 individuals were identified as having received a labile blood component from a total of 15 donors who later became vCJD cases and appeared on the surveillance unit's register. One of these recipients was identified as developing symptoms of vCJD 6.5 years after receiving a transfusion of red cells donated by an individual 3.5 years before the donor developed symptoms of vCJD.

INTERPRETATION

Our findings raise the possibility that this infection was transfusion transmitted. Infection in the recipient could have been due to past dietary exposure to the BSE agent. However, the age of the patient was well beyond that of most vCJD cases, and the chance of observing a case of vCJD in a recipient in the absence of transfusion transmitted infection is about 1 in 15000 to 1 in 30000.

What would T. H. Huxley have made of prion diseases?

T. H. Huxley was "Darwin's bulldog," and took the offensive in championing the cause of evolution against skeptical scientists and outraged theologians. As such, he took part in one of the great "paradigm shifts" of biology, at the end of the nineteenth century. Huxley was a rigorous scientist and wrote important articles on scientific method, as well as publishing extensively on a wide range of subjects in natural history. In the second half of the twentieth century, the "prion hypothesis" was put forward to explain the pathogenesis of a curious group of diseases known as the transmissible spongiform encephalopathies. This also involved a "paradigm shift" because the prion hypothesis postulated that biologically relevant information could be enciphered in protein conformation (rather than encoded in nucleic acid base sequences), and could be transmitted from one molecule to another, thereby causing infectious disease. This article examines a few of Huxley's remarks to speculate on how he might have responded to the scientific debate about prion disease had he lived a century later.

Therapeutic approaches to prion diseases

Prion diseases are unique in that they comprise sporadic, genetic, and iatrogenically or environmentally acquired forms. When disease is acquired by peripheral route, neuroinvasion occurs via at least two different neural pathways (vague and splanchnic nerves) and is usually preceded by prion propagation in secondary lymphoid organs. Conversely, in the other etiologic forms, PrPSc formation occurs within, and is apparently limited to, the CNS. Longitudinal studies on experimental scrapie indicate that substantial neuropathologic changes (i.e., glial activation and nerve cell degeneration) already are present before the onset of symptoms and are topographically related to PrPSc deposits. Accordingly, any effective intervention should start during the preclinical stage of disease, and be aimed at preventing neuroinvasion or PrPSc propagation in the CNS. Unfortunately, no tests are available currently to detect presymptomatic individuals, except for carriers of pathogenic mutations of the PRNP gene. Inhibition of PrPSc formation can be achieved through (1) abrogation of PrPC synthesis or prevention of its transport to the cell surface; (2) stabilization of the PrPC structure to make its conformational change unfavorable; (3) sequestration of PrPSc; (4) reversion of PrPSc to a protease-sensitive form; or (5) interference with the interaction between PrPC, PrPSc, and other macromolecules that feature in the conversion process. The compounds that have some effectiveness in in vitro, cell culture, or animal models of prion disease seem to operate through one of these mechanisms (see Table 1); however, even the most effective drugs only work when administered at the time of infection or very short thereafter, and these conditions are incurable at present. The heterogeneity and complexity of the etiopathogenesis of prion diseases suggest that various strategies and a combination of several compounds with different modes of actions are likely necessary for prevention and treatment. Major efforts should be focused on the development of preclinical diagnostic tests in conjunction with immunization strategies for diseases acquired by peripheral route and identification of more effective compounds for the other etiological forms.

A possible role of keratinocytes of skin and mucous membranes in prion propagation and transmission

Prion diseases or transmissible spongiform encephalopathies are lethal neurodegenerative diseases caused by proteinaceous agents that consist of an abnormal form of a host protein designated PrP and are devoid of nucleic acids. In laboratory settings these diseases are usually transmitted by intracerebral or peripheral inoculation. In the field they have been shown to be transmitted by uptake of contaminated food but in most instances the route of transmission remains obscure. Both nervous and lymphatic tissues in peripheral organs have been implicated in the spread and propagation of prions. The exact sites of uptake and initial propagation of the infectious agents have not yet been determined, however. As the expression of PrPc is required for the propagation of the infectious agent the search for peripheral cells positive for PrPc may reveal potential routes of entry and transmission. Recently epidermal and mucosal keratinocytes have been found to express PrPc. These data together with the recent finding that epithelial cells are able to support prion replication in vitro suggest that keratinocytes might play a role in the pathogenesis and/or transmission of prion diseases.

Factor VIII and transmissible spongiform encephalopathy: the case for safety

Haemophilia A is the most common inherited bleeding disorder, caused by a deficiency in coagulation factor VIII (FVIII). Current treatment of haemophilia A is based on repeated infusions of plasma-derived FVIII concentrate or of recombinant FVIII, which may be exposed to plasma-derived material of human or animal origin used in its tissue culture production process. We review epidemiological and experimental studies relevant to blood infectivity in the transmissible spongiform encephalopathies (TSEs, or 'prion' diseases), and evaluate the hypothetical risk of TSE transmission through treatment with plasma-derived or recombinant FVIII.

The transmissible spongiform encephalopathies (prion diseases): a review for dental surgeons

The transmissible spongiform encephalopathies (prion diseases) are a fatal group of neurological diseases characterised by the accumulation of an abnormal form of prion protein in the brain. In humans, these disorders occur in sporadic, acquired and familial forms. Outbreaks of bovine spongiform encephalopathy, predominantly in the United Kingdom, and the emergence of a clinically and pathologically distinct human prion disease, variant CJD, has generated much interest in the transmissible spongiform encephalopathies. As the agent is detectable in lymphoid and neural tissue in variant CJD, clinicians should be aware of the possibility of cross infection of the causative agent. This is particularly important because the abnormal prion protein is resistant to routine sterilisation procedures. This article reviews the transmissible spongiform encephalopathies, and summarises guidelines concerning prevention of crossinfection when treating patients with or at risk of developing prion disease.

Prions and blood products

The transmission of Creutzfeldt-Jakob disease (CJD) by human pituitary-derived growth hormone has led to concerns that blood products might also provide a route for the iatrogenic transmission of CJD. A number of actions have been implemented by regulatory authorities to address such concerns, and numerous studies have been undertaken to determine whether or not there is a risk of CJD being transmitted in this manner. To date, no excess risk has been identified, leading to a growing consensus that there is little or no risk of long established forms of CJD being transmitted to recipients of blood products. This opinion does not extend to new variant CJD (vCJD) which is found predominantly in the UK and is believed to have resulted from the transmission of bovine spongiform encephalopathy (BSE) to humans. Unlike that of CJD, the prevalence of vCJD is not known. In addition, the detection of abnormal prion protein in the tonsils of vCJD-infected individuals has led to speculation that blood infectivity may be greater than in patients with CJD. A number of precautionary measures have been taken to address the possibility that vCJD may be transmissible by blood products; however, further scientific advances are needed to enable this risk to be defined. A suitable screening test is required to identify any infected blood donors, particularly where cellular blood components are being derived from populations believed to be at risk from BSE infection. Recent experimental data suggest that process operations used in the manufacture of plasma products may be capable of removing prion agents to a significant extent. However, further work is required to confirm these observations and to determine whether or not all potential vCJD infectivity would be removed by these means.

Different levels of prion protein (PrPc) expression on hamster, mouse and human blood cells

The host prion protein, PrPc, and its conformationally changed isoform, PrPsc, play an essential role in the transmissible spongiform encephalopathy (TSE) infections. The prion hypothesis postulates that PrPsc is the TSE infectious agent and that it serves as a template to convert host PrPc to additional PrPsc. Blood of experimentally TSE-infected rodents has been shown to contain TSE infectivity. If blood-borne TSE infectivity requires association with PrPc, differences in the distribution of PrPc in blood could affect the amount and distribution of blood-borne infectivity in different hosts. We have compared the distribution of PrPc on the peripheral blood cells of humans, hamsters and mice using quantitative flow cytometry. Human lymphocytes, monocytes and platelets displayed much greater quantities of PrPc than corresponding mouse cells. Mouse platelets did not express any detectable PrPc. A similar low level of PrPc was found on both human and mouse red blood cells. None of the hamster peripheral blood cells displayed detectable amounts of PrPc. If PrPc contributes to the propagation or transport of TSE infectivity in blood, the species differences in PrPc distribution reported here need to be considered when extrapolating the results of rodent TSE transmission studies with blood and blood components to humans.

Prion diseases and the immune system

Transmissible spongiform encephalopathies are caused by unusual infectious agents that are purported to contain a single type of macromolecule, a modified host glycoprotein. The term prion has been applied to this group of agents. Surprisingly, the immune system appears to behave as a Trojan's horse rather than a protective fortification during prion infections. Because prions seem to be essentially composed of a protein, PrP(Sc), identical in sequence to a host encoded protein, PrP(C), the specific immune system displays a natural tolerance. However, lymphoid organs are strongly implicated in the preclinical stages of the disease. Certain immunodeficient animals are resistant to prions after peripheral inoculation. In normal subjects, cells of the immune system support the replication of prions and/or allow neuroinvasion. A better understanding of these aspects of prion diseases could lead to immunomanipulation strategies aimed at preventing the spread of infectious agents to the central nervous system.

Risk of transmission of Creutzfeldt-Jakob disease by transfusion of blood, plasma, and plasma derivatives

Studies in experimental animals and case-reports of transmission of Creutzfeldt-Jakob Disease (CJD) by blood transfusion or by albumin products have raised the possibility that CJD may be transmitted by transfusion. The risk of transmission of CJD by transfusion remains theoretical, since no confirmed case of CJD has ever been causally attributed to the receipt of a blood transfusion, no confirmed case of CJD has developed in recipients of clotting factor concentrates, and no cluster of CJD cases has been reported following the administration of a pooled plasma derivative to which a donor who subsequently developed CJD had contributed. However, based on a review of the hitherto available data, it is impossible to conclude at this time that CJD is not transmitted by blood or plasma transfusion or by the administration of pooled plasma derivatives. This review discusses the findings of the animal experiments and the human studies that investigated the potential for transmission of CJD among humans by transfusion, and explains the statistical difficulties associated with proving the negative hypothesis that CJD is not transmitted by transfusion.

[Prion diseases and blood transfusion]

Prion diseases are lethal disorders, some of which are transmissible by infectious route. Experimental data concerning neuroinvasion indicate that there is a viremia during the migration of the prion agent to the central nervous system. The possibility of accidental transmission via blood products and therefore potential transfusion risk thus arises. The analysis of experimental and epidemiological data available at present contributes to the following conclusion: the potential and theoretical risk for contamination from blood products is not null but mathematically very low, there is no indisputable experimental proof for that risk via systemic route and no case is definite and the risk is probably linked to leukocytes, and especially B lymphocytes. These conclusions are reassuring but nevertheless justify strict epidemiological survey and a reasonable discussion for each transfusion. Some groups of people have to be excluded from blood donors.

The key must fit: macrophages transport prion infection to the central nervous system and may determine the sites of infection within it

It is suggested that the agent for transmissible spongiform encephalopathies is transferred from an original peripheral site of infection into the brain by recruited and selected circulating macrophages/monocytes. It is because of this selection that strains of disease appear to be different when infecting separate species, but retain characteristics when infecting a single species.

A matter for debate: the risk of bovine spongiform encephalopathy to humans posed by blood transfusion in the UK

If human infection with bovine spongiform encephalopathy (BSE) were to occur, donated peripheral blood from humans that might have become infected from eating adequate quantities of food containing BSE should, until evidence is available to the contrary, be assumed to contain the human form of the disease. The chance of disease transfer to a blood recipient in 1995, which might in turn cause clinical disease with an incubation period of 20 years, is calculated. Transfusion is calculated to be a potential cause of a maximum of only 0.2% of clinical cases of Creutzfeldt-Jakob disease (CJD) in the UK population if the BSE epidemic were to spread to humans. Prospective epidemiological techniques would be unlikely to demonstrate any such minor contribution that blood transfusion might make to CJD incidence.

Experimental drug treatment of scrapie: a pathogenetic basis for rationale therapeutics

Pharmacological treatment with polyanions or amphotericin B in hamsters with experimental scrapie reveals that it is possible to delay the appearance of the disease only when the drug is given before the invasion of the agent into the clinical target areas of the brain. We suggest such early treatment may be possible for individuals at high risk of acquiring the disease, such as healthy mutation-positive relatives of patients with familial Creutzfeldt-Jakob disease or Gerstmann-Sträussler syndrome, or recipients of potentially contaminated pituitary-extracted human growth hormone.

Molecular location of a species-specific epitope on the hamster scrapie agent protein

Scrapie is a transmissible neurodegenerative disease of sheep and goats. An abnormal host protein, Sp33-37, is the major protein component of the scrapie agent and the only known disease- or agent-specific macromolecule. Two monoclonal antibodies (MAbs), 4H8 (immunoglobulin G2b [IgG2b]) and 6B11 (IgG1), produced by immunizing mice with the intact hamster 263K scrapie agent protein, Sp33-37Ha, were found to have species specificity similar to that reported previously for MAb 3F4 (IgG2a), which was produced by using PrP-27-30 as the immunogen (R. J. Kascsak, R. Rubenstein, P. A. Merz, M. Tonna-DeMasi, R. Fersko, R. I. Carp, H. M. Wisniewski, and H. Diringer, J. Virol. 61:3688-3693, 1987). These antibodies all bound to Sp33-37 derived from hamster but not from mouse cells. Competitive binding assays demonstrated that all three MAbs bound to the same or overlapping sites on Sp33-37Ha. The molecular location of the epitope for these antibodies was determined to within 10 residues by using an antigen competition enzyme-linked immunosorbent assay in which synthetic peptides spanning Sp33-37Ha residues 79 to 93 or 84 to 93 specifically inhibited binding of these antibodies to plates coated with purified Sp33-37Ha. A synthetic peptide with the mouse-specific sequence (83 to 92) that differed from the hamster sequence by substitution at two positions (MetHa-87----LeuMo-86 and MetHa-90----ValMo-89) did not inhibit antibody binding to Sp33-37Ha. MAb 3F4 binding to hamster Sp33-37 was eliminated by chemical modification of Sp33-37Ha with diethylpyrocarbonate or succinic anhydride and by cleavage with CNBr or trypsin. The effect of diethylpyrocarbonate on MAb 3F4 binding was not reversed by hydroxylamine treatment. MAb 3F4 binding was not affected by prolonged exposure of Sp33-37Ha to 70% formic acid or by boiling in sodium dodecyl sulfate. We conclude that the epitope for these MAbs is a linear determinant that includes Met-87, Lys-88, and Met-90 and that Met-90 is probably the major species-specific determinant.

"Life, Jim, but not as we know it"? Transmissible dementias and the prion protein

The spongiform encephalopathies are unusual in several respects. Firstly, they are transmissible, and in some cases inheritable. Secondly, variants of these disorders occur in many species and can be transmitted by consumption of infected material; this has led to concern as to the potential risk from eating contaminated animal products. Thirdly, increasing evidence suggests that a 'prion' protein is central to their aetiology and pathogenesis, and that no nucleic acid is involved in the infective process. The role of the prion gene and its protein is outlined and proposed as the basis for an improved classification of the transmissible dementias.