Ionising radiation has no influence on scrapie incubation period in mice

Optimization and evaluation of new decontamination procedures inactivating human prions

BACKGROUND

Prions are protein-only infectious agents for which no prophylactic or curative treatment exists. Infectivity bioassays based on hamster-263K prions allowed to identify processes capable of inactivating prions on medical devices. However, a 2016 publication study revealed that detergent formulations effective against hamster strain had poor efficacy against human strains. Shortly after, three probable cases of accidental Creutzfeldt-Jakob disease underscored the risk for scientists, health workers, and patients exposed to contaminated materials. The governmental guidelines were modified and emphasizing the need for formulations effective against human prions and robust in vitro and in vivo evaluation protocols. Here, we aimed to compare infectivity bioassays with those of their PMCA counterparts to propose a robust method for evaluating prionicide treatments against human prions.

METHODS

Stainless steel wires were contaminated with two humanized prion strains. The wires were then treated with different protocols based on a new formulation termed TFD Premium and WHO references. Residual prion seeding activity and infectivity on the wire and in wastewater were quantified using mb-PMCA and ad hoc bioassays. For vCJD, PMCA compared humanized prions and a human-derived prion isolate.

FINDINGS

TFD Premium proved more efficient at decontaminating humanized prions than 1 N NaOH for 1 hour at room temperature. Tg650-sCJD-VV2 were more resistant to inactivation than vCJD prions. For vCJD, strain from both sources shown similar resistant profile against TFD Premium. Finally, there was perfect alignment between the highly sensitive PMCA cell-free assay and the bioassays.

Rapid and sensitive determination of residual prion infectivity from prion-decontaminated surfaces

Prion diseases are untreatable fatal transmissible neurodegenerative diseases that affect a wide range of mammals, including humans, and are caused by PrPSc, the infectious self-templating conformation of the host-encoded protein, PrPC. Prion diseases can be transmitted via surfaces (e.g., forceps, EEG electrodes) in laboratory and clinical settings. Here, we use a combination of surface swabbing and real-time quaking-induced conversion (RT-QuIC) to test for residual surface-associated prions following prion disinfection. We found that treatment of several prion-contaminated laboratory and clinically relevant surfaces with either water or 70% EtOH resulted in robust detection of surface-associated prions. In contrast, treatment of surfaces with sodium hypochlorite resulted in a failure to detect surface-associated prions. RT-QuIC analysis of prion-contaminated stainless steel wires paralleled the findings of the surface swab studies. Importantly, animal bioassay and RT-QuIC analysis of the same swab extracts are in agreement. We report on conditions that may interfere with the assay that need to be taken into consideration before using this technique. Overall, this method can be used to survey laboratory and clinical surfaces for prion infectivity following prion decontamination protocols.IMPORTANCEPrion diseases can be accidentally transmitted in clinical and occupational settings. While effective means of prion decontamination exist, methods for determining the effectiveness are only beginning to be described. Here, we analyze surface swab extracts using real-time quaking-induced conversion (RT-QuIC) to test for residual prions following prion disinfection of relevant clinical and laboratory surfaces. We found that this method can rapidly determine the efficacy of surface prion decontamination. Importantly, examination of surface extracts with RT-QuIC and animal bioassay produced similar findings, suggesting that this method can accurately assess the reduction in prion titer. We identified surface contaminants that interfere with the assay, which may be found in clinical and laboratory settings. Overall, this method can enhance clinical and laboratory prion safety measures.

The immunobiology of prion diseases

Individuals infected with prions succumb to brain damage, and prion infections continue to be inexorably lethal. However, many crucial steps in prion pathogenesis occur in lymphatic organs and precede invasion of the central nervous system. In the past two decades, a great deal has been learnt concerning the cellular and molecular mechanisms of prion lymphoinvasion. These properties are diagnostically useful and have, for example, facilitated preclinical diagnosis of variant Creutzfeldt-Jakob disease in the tonsils. Moreover, the early colonization of lymphoid organs can be exploited for post-exposure prophylaxis of prion infections. As stromal cells of lymphoid organs are crucial for peripheral prion infection, the dedifferentiation of these cells offers a powerful means of hindering prion spread in infected individuals. In this Review, we discuss the current knowledge of the immunobiology of prions with an emphasis on how basic discoveries might enable translational strategies.

Prions and lymphoid organs: solved and remaining mysteries

Prion colonization of secondary lymphoid organs (SLOs) is a critical step preceding neuroinvasion in prion pathogenesis. Follicular dendritic cells (FDCs), which depend on both tumor necrosis factor receptor 1 (TNFR1) and lymphotoxin β receptor (LTβR) signaling for maintenance, are thought to be the primary sites of prion accumulation in SLOs. However, prion titers in RML-infected TNFR1 (-/-) lymph nodes and rates of neuroinvasion in TNFR1 (-/-) mice remain high despite the absence of mature FDCs. Recently, we discovered that TNFR1-independent prion accumulation in lymph nodes relies on LTβR signaling. Loss of LTβR signaling in TNFR1 (-/-) lymph nodes coincided with the de-differentiation of high endothelial venules (HEVs)-the primary sites of lymphocyte entry into lymph nodes. These findings suggest that HEVs are the sites through which prions initially invade lymph nodes from the bloodstream. Identification of HEVs as entry portals for prions clarifies a number of previous observations concerning peripheral prion pathogenesis. However, a number of questions still remain: What is the mechanism by which prions are taken up by HEVs? Which cells are responsible for delivering prions to lymph nodes? Are HEVs the main entry site for prions into lymph nodes or do alternative routes also exist? These questions and others are considered in this article.

Lymphotoxin, but not TNF, is required for prion invasion of lymph nodes

Neuroinvasion and subsequent destruction of the central nervous system by prions are typically preceded by a colonization phase in lymphoid organs. An important compartment harboring prions in lymphoid tissue is the follicular dendritic cell (FDC), which requires both tumor necrosis factor receptor 1 (TNFR1) and lymphotoxin β receptor (LTβR) signaling for maintenance. However, prions are still detected in TNFR1^−/− lymph nodes despite the absence of mature FDCs. Here we show that TNFR1-independent prion accumulation in lymph nodes depends on LTβR signaling. Loss of LTβR signaling, but not of TNFR1, was concurrent with the dedifferentiation of high endothelial venules (HEVs) required for lymphocyte entry into lymph nodes. Using luminescent conjugated polymers for histochemical PrP^Sc detection, we identified PrP^Sc deposits associated with HEVs in TNFR1^−/− lymph nodes. Hence, prions may enter lymph nodes by HEVs and accumulate or replicate in the absence of mature FDCs.

Prions are unique infectious agents thought to be composed entirely of an abnormal conformer of the endogenous prion protein. Prions cause a severe neurological disorder in humans and other animals known as prion disease. Though prion disease can arise spontaneously or from genetic mutations in the gene encoding the prion protein, many cases of prion disease arise due to peripheral exposure to the infectious agent. In these cases, prions must journey from the gastrointestinal tract and/or the bloodstream to the brain. Prions often colonize secondary lymphoid organs prior to invading the nervous system via neighboring peripheral nerves. Prion accumulation in follicular dendritic cells found in germinal centers of lymphoid organs is thought to be a crucial step in this process. However, prion colonization of lymph nodes, in contrast to spleen, does not depend on follicular dendritic cells, indicating that other mechanisms must exist. Here, we identify the signaling pathway required for follicular dendritic cell-independent prion colonization of lymph nodes, which also controls the differentiation of high endothelial venules, the primary entry point for lymphocytes into lymph nodes. Importantly, prions could be found within these structures, indicating that high endothelial venules are required for prion entry and accumulation in lymph nodes.

Pathogenesis of experimental scrapie

Most of our understanding of the pathogenesis of the unconventional slow infections comes from studies of experimental scrapie in mice and hamsters. After injection by non-neural peripheral routes, pathogenesis necessarily involves the lymphoreticular system (LRS) before the central nervous system (CNS). Available evidence indicates haematogenous spread from the site of injection to the scrapie replication sites in the LRS; later, infection spreads along visceral autonomic nerves from the LRS to the thoracic spinal cord, and thence to brain. The cells in the LRS which are important to scrapie pathogenesis are long lived. Neuroinvasion and spread of infection within the CNS probably involve neuronal pathways. We suggest that disease develops after infection has reached certain clinical target areas in the CNS but only when scrapie replication there has caused sufficient functional damage. Restriction of the replication process in both LRS and CNS is indicated by the occurrence of plateau concentrations of infectivity, especially in some long incubation scrapie models. A remarkable feature of these is that both neuroinvasion and clinical disease occur long after infectivity plateaux have been reached in the LRS and CNS, respectively. We propose that the slowness of scrapie is related to (1) limitations of cell-to-cell spread of infection from LRS to CNS, and (2) limitations on spread between neurons, coupled with restrictions on replication in brain.

A role for B lymphocytes in anti-infective prion therapies?

The deposition of proteins in the form of amyloid fibrils and plaques is the characteristic feature of a number of neurodegenerative conditions affecting the nervous system. These disorders include prion and Alzheimer's diseases and are of enormous importance for public health. It has become apparent over the last 20 years that specificity and application in prion diseases' diagnostic and therapeutic situations are the most important considerations in designing strategies for the generation of antiprion antibodies. Specific antiprion therapeutics have been suggested and the establishment of the 'proof-of-principle' that the use of epitope-specific antiprion antibodies leads to indefinite delay of disease onset, has increased momentum for its use, although caution should be exerted prior to the application of new therapeutic strategies in a clinical set up. Furthermore, in vivo stimulation of immune-competent cells to specifically recognize and neutralize the abnormally folded isoform should also be pursued.

Pathogenesis of prion diseases: current status and future outlook

The prion, a conformational variant of a host protein, is the infectious particle responsible for transmissible spongiform encephalopathy (TSE), a fatal neurodegenerative disease of humans and animals. The principal target of prion pathology is the brain, yet most TSEs also display prion replication at extra-cerebral locations, including secondary lymphoid organs and sites of chronic inflammation. Despite significant progress in our understanding of this infectious agent, many fundamental questions relating to the nature of the prion, including the mechanism of replication and the molecular events underlying brain damage, remain unanswered. Here we focus on the unresolved issues pertaining to prion pathogenesis, particularly on the role played by the immune system.

Prion diseases and the gastrointestinal tract

The gastrointestinal (GI) tract plays a central role in the pathogenesis of transmissible spongiform encephalopathies. These are human and animal diseases that include bovine spongiform encephalopathy, scrapie and Creutzfeldt-Jakob disease. They are uniformly fatal neurological diseases, which are characterized by ataxia and vacuolation in the central nervous system. Although they are known to be caused by the conversion of normal cellular prion protein to its infectious conformational isoform (PrPsc) the process by which this isoform is propagated and transported to the brain remains poorly understood. M cells, dendritic cells and possibly enteroendocrine cells are important in the movement of infectious prions across the GI epithelium. From there, PrPsc propagation requires B lymphocytes, dendritic cells and follicular dendritic cells of Peyer's patches. The early accumulation of the disease-causing agent in the plexuses of the enteric nervous system supports the contention that the autonomic nervous system is important in disease transmission. This is further supported by the presence of PrPsc in the ganglia of the parasympathetic and sympathetic nerves that innervate the GI tract. Additionally, the lymphoreticular system has been implicated as the route of transmission from the gut to the brain. Although normal cellular prion protein is found in the enteric nervous system, its role has not been characterized. Further research is required to understand how the cellular components of the gut wall interact to propagate and transmit infectious prions to develop potential therapies that may prevent the progression of transmissible spongiform encephalopathies.

Isolation of functionally active murine follicular dendritic cells

Biochemical, genetic, and immunological studies of follicular dendritic cells (FDCs) have been hampered by difficulty in obtaining adequate numbers of purified cells in a functional state. To address this obstacle, we enriched FDCs by irradiating mice to destroy most lymphocytes, excised the lymph nodes, and gently digested the nodes with an enzyme cocktail to form single cell suspensions. The FDCs in suspension were selected using the specific mAb FDC-M1 with magnetic cell separation technology. We were able to get nearly a million viable lymph node FDCs per mouse at about 90% purity. When examined under light and transmission electron microscopy, the cytological features were characteristic of FDCs. Furthermore, the cells were able to trap and retain immune complexes and were positive for important phenotypic markers including FDC-M1, CD21/35, CD32, CD40, and CD54. Moreover, the purified FDCs exhibited classical FDC accessory activities including: the ability to co-stimulate B cell proliferation, augment antibody responses induced by mitogens or antigens, maintain B cell viability for weeks, and protect B lymphocytes from anti-FAS induced apoptosis. In short, this combination of methods made it possible to obtain a substantial number of highly enriched functional murine FDCs.

Prion diseases: current understanding of epidemiology and pathogenesis, and therapeutic advances

The bovine spongiform encephalopathy (BSE) epidemic, along with the related threat to human health posed by the transmission of the BSE agent to humans, has highlighted the importance of prion diseases. These fatal neurodegenerative diseases are characterised by spongiform changes in the CNS, and comprise a wide spectrum of clinicopathological entities in humans and animals, such as Creutzfeldt-Jakob disease (CJD) and its emerging new variant (vCJD) in humans, and BSE and scrapie in animals. This article reviews the geographical distribution and the temporal trends of CJD and vCJD; the major events in the pathogenesis of prion diseases; the risk factors for sporadic CJD and vCJD; and the possible strategies for treating them. Worldwide statistics indicate that sporadic CJD has a stable incidence of one case per million people per year; in contrast, the incidence of vCJD appears to have increased exponentially from its characterisation in 1994 to a peak in 2000. As of December 2005, 183 definite or probable cases of vCJD had been reported worldwide. The crucial event in the pathogenesis of prion diseases is the conversion of the normally occurring cellular prion protein (PrP(c)) into a pathogenic form, called protease-resistant PrP (PrP(res)) or scrapie PrP (PrP(sc)). Pathogenetic studies in rodent models have shown that PrP(sc) is found in the enteric nervous system and in the gut-associated lymphoid tissue following oral scrapie ingestion. The role of the lymphoreticular system in the pathogenesis of TSE seems to be related to the strains of agents and the host genotype. Therapeutic approaches to vCJD are mainly based on the inhibition or prevention of the pathological change that creates PrP(sc). Derivatives of acridine (such as mepacrine [quinacrine]) and the phenothiazine psychotropics have been proposed as possible therapies because of their activity in cellular models; however, neither class was able to affect the protease resistance of preexisting PrP fibrils. More encouragingly, in animal models of prion disease, tetracyclines were found to reduce prion infectivity by direct inactivation of PrP(sc). While these findings are promising, the suitability of these compounds for clinical use is still limited by their low efficacy once symptoms are apparent. Treatments based on the vaccination approach have also produced positive results, but further investigations are necessary to establish their clinical application.

Transgenic and knockout mice in research on prion diseases

Since the discovery of the prion protein (PrP) gene more than a decade ago, transgenetic investigations on the PrP gene have shaped the field of prion biology in an unprecedented way. Many questions regarding the role of PrP in susceptibility of an organism exposed to prions have been elucidated. For example mice with a targeted disruption of the PrP gene have allowed the demonstration that an organism that lacks PrPc is resistant to infection by prions. Reconstitution of these mice with mutant PrP genes allowed investigations on the structure-activity relationship of the PrP gene with regard to scrapie susceptibility. Unexpectedly, transgenic mice expressing PrP with specific amino-proximal truncations spontaneously develop a neurologic syndrome presenting with ataxia and cerebellar lesions. A distinct spontaneous neurologic phenotype was observed in mice with internal deletions in PrP. Using ectopic expression of PrP in PrP knockout mice has turned out to be a valuable approach towards the identification of host cells that are capable of replicating prions. Transgenic mice have also contributed to our understanding of the molecular basis of the species barrier for prions. Finally, the availability of PrP knockout mice and transgenic mice overexpressing PrP allows selective reconstitution experiments aimed at expressing PrP in neurografts or in specific populations of hemato- and lymphopoietic cells. Such studies have shed new light onto the mechanisms of prion spread and disease pathogenesis.

Progress in prion vaccines and immunotherapies

The transmissible spongiform encephalopathies have presented a challenge to physicians and scientists attempting to develop immunologically-based treatments. Self-tolerance has been one of the major obstacles to successfully raising antibodies against the prion protein (PrP), the host-encoded protein whose misfolded form (PrPSc) is linked to the protein-only infectious agent responsible for these disorders. Recently, it has been shown that antibodies directed against the normal cellular isoform of PrP (PrPC) can reduce or eliminate PrP isoform conversion in both in vitro and in vivo model systems. Similar studies with a PrPSc-specific epitope target are in progress. There is now rational hope that this devastating group of diseases may soon be amenable to immunotherapy and immunoprophylaxis.

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.

Murine Scrapie Infection Causes an Abnormal Germinal Centre Reaction in the Spleen

Follicular dendritic cells (FDCs) of the lymphoreticular system play a role in the peripheral replication of prion proteins in some transmissible spongiform encephalopathies (TSEs), including experimental murine scrapie models. Disease-specific PrP (PrPd) accumulation occurs in association with the plasmalemma and extracellular space around FDC dendrites, but no specific immunological response has yet been reported in animals affected by TSEs. In the present study, morphology (light microscopical and ultrastructural) of secondary lymphoid follicles of the spleen were examined in mice infected with the ME7 strain of scrapie and in uninfected control mice, with or without immunological stimulation with sheep red blood cells (SRBCs), at 70 days post-inoculation or at the terminal stage of disease (268 days). Scrapie infection was associated with hypertrophy of FDC dendrites, increased retention of electron-dense material at the FDC plasma membrane, and increased maturation and numbers of B lymphocytes within secondary follicles. FDC hypertrophy was particularly conspicuous in immune-stimulated ME7-infected mice. The electron-dense material was associated with PrP Napoli accumulation, as determined by immunogold labelling. We hypothesize that immune system changes are associated with increased immune complex trapping by hypertrophic FDCs expressing PrP Napoli molecules at the plasmalemma of dendrites, and that this process is exaggerated by immune system stimulation. Contrary to previous dogma, these results show that a pathological response within the immune system follows scrapie infection.

FDC-B1: a new monoclonal antibody directed against bovine follicular dendritic cells

Follicular dendritic cells (FDCs) are a unique population of accessory cells located in the light zone of the germinal centres of lymphoid follicles. Their involvement in the generation of humoral immune responses implies a potential role for these cells in many disorders. Indeed, in prion diseases, FDCs seem to be the major sites of extraneuronal cellular prion protein expression and the principal sites of the infectious agent accumulation in lymphoid organs. The identification of FDC is useful for the analysis of their distribution in reactive lymphoid tissue as well as in pathological conditions. The production and characterisation of a new mouse monoclonal antibody directed against bovine follicular dendritic cells (FDC-B1) is reported. The antigen detected by FDC-B1 is expressed exclusively on the surface of FDCs in ruminant lymphoid organs. The antigen has an approximate molecular weight of 28 kDa.

Phenotyping of protein-prion (PrPsc)-accumulating cells in lymphoid and neural tissues of naturally scrapie-affected sheep by double-labeling immunohistochemistry

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases characterized by amyloid deposition of protein-prion (PrPsc), the pathogenic isoform of the host cellular protein PrPc, in the immune and central nervous systems. In the absence of definitive data on the nature of the infectious agent, PrPsc immunohistochemistry (IHC) constitutes one of the main methodologies for pathogenesis studies of these diseases. In situ PrPsc immunolabeling requires formalin fixation and paraffin embedding of tissues, followed by post-embedding antigen retrieval steps such as formic acid and hydrated autoclaving treatments. These procedures result in poor cellular antigen preservation, precluding the phenotyping of cells involved in scrapie pathogenesis. Until now, PrPsc-positive cell phenotyping relied mainly on morphological criteria. To identify these cells under the PrPsc IHC conditions, a new, rapid, and highly sensitive PrPsc double-labeling technique was developed, using a panel of screened antibodies that allow specific labeling of most of the cell subsets and structures using paraffin-embedded lymphoid and neural tissues from sheep, leading to an accurate identification of ovine PrPsc-accumulating cells. This technique constitutes a useful tool for IHC investigation of scrapie pathogenesis and may be applicable to the study of other ovine infectious diseases.

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.

The transmissible spongiform encephalopathies: pathogenic mechanisms and strategies for therapeutic intervention

Primary neurodegenerative diseases tend to be intractable and largely affect the elderly. There is rarely the opportunity to identify individuals at risk and the appearance of clinical symptoms usually signifies the occurrence of irreversible neurological damage. This situation describes sporadic Creutzfeldt-Jakob disease which occurs world-wide, affecting one person per million per annum. The epidemic of bovine spongiform encephalopathy in the UK in the 1980s and the subsequent causal appearance of variant Creutzfeldt-Jakob disease in young UK residents in the 1990s has refocused attention on this whole group of diseases, known as the transmissible spongiform encephalopathies or prion diseases. The potentially lengthy incubation period of variant Creutzfeldt-Jakob disease, including perhaps an obligate peripheral phase, prior to neuroinvasion, marks variant Creutzfeldt-Jakob disease out as different from sporadic Creutzfeldt-Jakob disease. The formal possibility of detecting individuals infected with the bovine spongiform encephalopathy agent during this asymptomatic peripheral phase provides a strong incentive for the development of therapies for transmissible spongiform encephalopathies. This review focuses on recent advances in the understanding of the pathogenesis of these diseases, with particular reference to in vitro and animal model systems. Such systems have proved invaluable in the identification of potential therapeutic strategies that either specifically target the prion protein or more generally target peripheral pathogenesis. Furthermore, recent experiments in animal models suggest that even after neuroinvasion there may be pharmacological avenues to explore that might retard or even halt the degenerative process.

Prions: from neurografts to neuroinvasion

Spongiform encephalopathies are infectious neurodegenerative diseases caused by pathogens that seem to be devoid of any informational nucleic acids. Histopathologically, these diseases are characterized by spongiform degeneration of the central nervous system. Although the main pathological changes during the course of the disease occur in the brain, the infectious agent accumulates early in lymphoid tissue. The consecutive development of clinical disease depends on the presence of an intact immune system including mature B-cells and follicular dendritic cells. In this article we review the state of knowledge on the routes of neuroinvasion used by the infectious agent in order to gain access to the central nervous system upon entry into extracerebral sites.

Follicular dendritic cells in scrapie pathogenesis

Scrapie pathogenesis was studied in chimaeric mice that carried the prion protein (PrP) gene only in particular cells of the immune system. These mice were produced by grafting bone marrow from PrP expressing donors into PrP deficient recipients and vice versa. As follicular dendritic cells are not replaced significantly from the bone marrow in adult mice, this procedure resulted in a mismatch in PrP genotype between these cells and bone marrow derived cells such as lymphocytes. Using these models we obtained strong evidence that follicular dendritic cells produced high levels of the normal form of PrP in uninfected mice. Furthermore, the replication of a mouse-passaged scrapie strain in the spleen depended only on the presence of PrP expressing follicular dendritic cells. PrP expression by lymphocytes or other bone marrow derived cells had no influence on replication in spleen or on neuroinvasion in these models. These results indicate that the follicular dendritic cell is a potential target for prophylactic or therapeutic intervention in transmissible spongiform encephalopathies.

Cellular and sub-cellular localisation of PrP in the lymphoreticular system of mice and sheep

Using immunocytochemistry or immunogold electron microscopy, abnormal PrP accumulation was found in lymphoreticular tissues of Suffolk sheep naturally exposed to scrapie and in the spleens of ME7 infected C57 BL mice at 70 days after infection and at the terminal stage of disease at 170 days. Clinically diseased scrapie affected sheep show widespread PrP accumulation within tingible body macrophages (TBMs) and follicular dendritic cells (FDCs) of secondary lymphoid follicles. Serial tonsillar biopsies taken from 171 ARQ/ARQ sheep at 4 months of age did not contain abnormal PrP accumulations but 80% of biopsies were positive by 14 months. In contrast, whole body necropsies of sheep not previously biopsied failed to detect PrP in the tonsil of sheep at 4, 8, 12 or 16 months of age. These findings suggest that the biopsy procedure of susceptible sheep but not resistant sheep may induce tonsillar infection. In spleen of mice both at 70 and 170 dpi, accumulations of PrP were found within lysosomes of TBMs and also at the plasma-lemma of FDCs. In the light zone of follicles of terminally diseased mice, all FDC dendrites were arranged in the form of highly reactive or hyperplastic labrynthine glomerular complexes. PrP was consistently seen between FDC dendrites in association with abundant electron dense antigen-antibody complexes. At 70 days after challenge, labrynthine complexes were rare and invariably labelled for PrP. However, sparse PrP labelling was also seen on simple FDC dendrites at this stage. These observations suggests that scrapie infected FDCs continually release PrP from the cell surface where it accumulates in excess in association with trapped immune complexes and dendritic extension. It is likely that TBMs acquire lysosomal PrP following phagocytosis of effete FDC processes or from the extracellular space. We suggest that the normal function of PrP may involve cell process extension or immune complex trapping.

Prions: pathogenesis and reverse genetics

Spongiform encephalopathies are a group of infectious neurodegenerative diseases. The infectious agent that causes transmissible spongiform encephalopathies was termed prion by Stanley Prusiner. The prion hypothesis states that the partially protease-resistant and detergent-insoluble prion protein (PrPsc) is identical with the infectious agent, and lacks any detectable nucleic acids. Since the latter discovery, transgenic mice have contributed many important insights into the field of prion biology. The prion protein (PrPc) is encoded by the Prnp gene, and disruption of Prnp leads to resistance to infection by prions. Introduction of mutant PrPc genes into PrPc-deficient mice was used to investigate structure-activity relationships of the PrPc gene with regard to scrapie susceptibility. Ectopic expression of PrPc in PrPc knockout mice proved a useful tool for the identification of host cells competent for prion replication. Finally, the availability of PrPc knockout and transgenic mice overexpressing PrPc allowed selective reconstitution experiments aimed at expressing PrPc in neurografts or in specific populations of hemato- and lymphopoietic cells. The latter studies helped in elucidating some of the mechanisms of prion spread and disease pathogenesis.

Prions and the lymphoreticular system

Following intracerebral or peripheral inoculation of mice with scrapie prions, infectivity accumulates first in the spleen and only later in the brain. In the spleen of scrapie-infected mice, prions were found in association with T and B lymphocytes and to a somewhat lesser degree with the stroma, which contains the follicular dendritic cells (FDCs) but not with non-B, non-T cells; strikingly, no infectivity was found in lymphocytes from blood of the same mice. Transgenic PrP knockout mice expressing PrP restricted to either B or T lymphocytes show no prion replication in the lymphoreticular system. Therefore, splenic lymphocytes either acquire prions from another source or replicate them in dependency on other PrP-expressing cells. The essential role of FDCs in prion replication in spleen was shown by treating mice with soluble lymphotoxin-beta receptor, which led to disappearance of mature FDCs from the spleen and concomitantly abolished splenic prion accumulation and retarded neuroinvasion following intraperitoneal scrapie inoculation.

Follicular dendritic cells in TSE pathogenesis

The pathogenesis of transmissible spongiform encephalopathies (TSEs) often includes a replication phase in lymphoid tissues before infection spreads to the central nervous system. Recent studies show that the follicular dendritic cells of the germinal centres are critical for this replication. These cells are therefore potential targets for therapy or prophylaxis in natural TSEs, such as variant Creutzfeldt-Jakob disease.

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.

Sites of prion protein accumulation in scrapie-infected mouse spleen revealed by immuno-electron microscopy

Prion protein (PrP) from the brains of animals with transmissible spongiform encephalopathies is partially protease resistant (PrP(res)) compared with fully sensitive PrP (PrP(sen)) from uninfected brains. In most experimental models, PrP(res) is a reliable indicator of infectivity. Light microscopic studies have suggested that both PrP(sen) and disease-specific accumulations of PrP are associated with follicular dendritic cells (FDCs). Using immunogold electron microscopy, this study has demonstrated disease-specific accumulation of PrP in the spleens of C57 BL mice, 70 days after intracerebral infection with the ME7 strain of scrapie and at the terminal stage of disease at 170 days. At both stages, tingible body macrophages contained PrP within lysosomes and PrP was also detected at the plasmalemma of FDCs. In the light zone of follicles of terminally diseased mice, all FDC dendrites were arranged in the form of highly reactive or hyperplastic labyrinthine glomerular complexes, within which PrP was consistently seen between FDC processes in association with abundant electron dense material, interpreted as antigen-antibody complexes. Within some glomeruli, fibrillar forms of PrP consistent with amyloid were seen. At 70 days after challenge, large or hyperplastic labyrinthine complexes were rare and invariably labelled for PrP. However, sparse PrP labelling was also seen on simple FDC processes at this stage. The ubiquitous accumulation of extracellular PrP in complex glomerular dendrites of FDCs in spleens from terminally affected mice, contrasted with simple FDC profiles, sparse PrP and limited electron dense deposits in all but a few FDCs of 70-day post-infected mice. This suggests that FDCs continually release PrP from the cell surface, where it is associated with trapped antigen-antibody complexes and dendritic extension. It is likely that tingible body macrophages acquire PrP following phagocytosis of PrP within iccosomes or from the extracellular space around FDC dendrites. These studies would not support an intracellular phase of PrP accumulation in FDCs but show that PrP is produced in excess by scrapie-infected cells from where it is released into the extracellular space. We suggest that PrP(sen) is involved in dendritic extension or in the process of antibody-antigen trapping, perhaps as part of the binding mechanism for antigen-antibody complexes. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Detection of PrP in extraneural tissues

Transmissible spongiform encephalopathies (TSEs) or "prion diseases" are a group of unconventional fatal diseases. TSEs are characterised by the accumulation of a modified form of the normal host glycoprotein, PrP (PrP(c)). In the course of infection PrP(c) is converted to an abnormally protease resistant form, PrP(Sc). The exact nature of the infectious agent responsible for these diseases remains controversial. While there is compelling evidence that TSE agents contain an informational molecule, possibly a nucleic acid, some believe that the infectious agent or "prion" is solely composed of PrP(Sc). Nevertheless, PrP is required for TSE pathogenesis, as mice devoid of the PrP gene (PrP(-/-)) remain healthy when challenged with TSE isolates and are unable to replicate infectivity within the central nervous system (CNS) or in other tissues. In recent years immunocytochemistry has been used to pinpoint which cells are associated with abnormal accumulations of PrP, providing important information on the cellular targeting of TSE infection. In uninfected and scrapie-infected mice, PrP protein is found in the CNS and in extraneural tissues such as spleen and lymph nodes. In the peripheral lymphoid system, PrP is associated with follicular dendritic cells that are known to be important for replication of infectivity for at least one TSE strain. This review will focus on current methods for the immunocytochemical detection of PrP in murine extraneural tissues, mainly lymphoid tissues, and will discuss recent findings on the role of the peripheral lymphoid system in TSE pathogenesis.

Impaired prion replication in spleens of mice lacking functional follicular dendritic cells

In scrapie-infected mice, prions are found associated with splenic but not circulating B and T lymphocytes and in the stroma, which contains follicular dendritic cells (FDCs). Formation and maintenance of mature FDCs require the presence of B cells expressing membrane-bound lymphotoxin-alpha/beta. Treatment of mice with soluble lymphotoxin-beta receptor results in the disappearance of mature FDCs from the spleen. We show that this treatment abolishes splenic prion accumulation and retards neuroinvasion after intraperitoneal scrapie inoculation. These data provide evidence that FDCs are the principal sites for prion replication in the spleen.

Peripheral pathogenesis of prion diseases

Prions are infectious pathogens that cause a group of neurodegenerative diseases characterized by spongiform degeneration of the central nervous system. Prions appear to lack any informational nucleic acid. The most notable prion diseases include bovine spongiform encephalopathy, scrapie in sheep and Creutzfeldt-Jakob disease of humans. Transmission is thought to be achieved through conversion of a normal host protein into a pathological isoform. Although the main pathological changes during the course of the disease occur in the brain, the infectious agent accumulates early in lymphoid tissue. The subsequent development of clinical disease depends on the presence of an intact immune system including mature B-cells. In this article we review the state of knowledge on the routes of neuroinvasion used by the infectious agent in order to gain access to the central nervous system upon entry into extracerebral sites.

Tumor necrosis factor alpha-deficient, but not interleukin-6-deficient, mice resist peripheral infection with scrapie

In most peripheral infections of rodents and sheep with scrapie, infectivity is found first in lymphoid tissues and later in the central nervous system (CNS). Cells within the germinal centers (GCs) of the spleen and lymph nodes are important sites of extraneural replication, from which infection is likely to spread to the CNS along peripheral nerves. Here, using immunodeficient mice, we investigate the identity of the cells in the spleen that are important for disease propagation. Despite possessing functional T and B lymphocytes, tumor necrosis factor alpha-deficient (TNF-alpha(-/-)) mice lack GCs and follicular dendritic cell (FDC) networks in lymphoid tissues. In contrast, lymphoid tissues of interleukin-6-deficient (IL-6(-/-)) mice possess FDC networks but have impaired GCs. When the CNSs of TNF-alpha(-/-), IL-6(-/-), and wild-type mice were directly challenged with the ME7 scrapie strain, 100% of the mice were susceptible, developing disease after closely similar incubation periods. However, when challenged peripherally (intraperitoneally), most TNF-alpha(-/-) mice failed to develop scrapie up to 503 days postinjection. All wild-type and IL-6(-/-) mice succumbed to disease approximately 300 days after the peripheral challenge. High levels of scrapie infection and the disease-specific isomer of the prion protein, PrP(Sc), were detectable in spleens from challenged wild-type and IL-6(-/-) mice but not from TNF-alpha(-/-) mice. Histopathological analysis of spleen tissue demonstrated heavy PrP accumulations in direct association with FDCs in challenged wild-type and IL-6(-/-) mice. No PrP(Sc) accumulation was detected in spleens from TNF-alpha(-/-) mice. We conclude that, for the ME7 scrapie strain, mature FDCs are critical for replication in lymphoid tissues and that in their absence, neuroinvasion following peripheral challenge is impaired.

Role of spleen macrophages in the clearance of scrapie agent early in pathogenesis

The involvement of spleen macrophages in the early stages of scrapie pathogenesis was studied by applying the 'macrophage-suicide technique' to scrapie-infected mice. This method comprises critically the intravenous administration to mice of dichloromethylene disphosphonate encapsulated into liposomes. Depletion of spleen macrophages before scrapie infection induced an increased amount of scrapie inoculum in the spleen, consequently leading to accelerated scrapie agent replication in the early phase of pathogenesis, as followed by PrPres accumulation, a specific hallmark of scrapie. The same effect was observed when spleen macrophages were depleted just before the beginning of scrapie agent replication. These findings suggest that macrophages may partly control scrapie infection in peripheral tissues by sequestration of the scrapie inoculum and may thus impair early scrapie agent replication in the spleen. In addition to macrophages, most follicular dendritic cells and B lymphocytes, which are thought to support scrapie agent replication, were also transiently depleted by dichloromethylene disphosphonate administration. This suggests that a compensatory mechanism is sufficient to ensure the persistence of infection in these early stages of pathogenesis.

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.

Scrapie replication in lymphoid tissues depends on prion protein-expressing follicular dendritic cells

The immune system is central in the pathogenesis of scrapie and other transmissible spongiform encephalopathies (TSEs) or 'prion' diseases. After infecting by peripheral (intraperitoneal or oral) routes, most TSE agents replicate in spleen and lymph nodes before neuroinvasion. Characterization of the cells supporting replication in these tissues is essential to understanding early pathogenesis and may indicate potential targets for therapy, for example, in 'new variant' Creutzfeldt-Jakob disease. The host 'prion' protein (PrP) is required for TSE agent replication and accumulates in modified forms in infected tissues. Abnormal PrP is detected readily on follicular dendritic cells (FDCs) in lymphoid tissues of patients with 'new variant' Creutzfeldt-Jakob disease, sheep with natural scrapie and mice experimentally infected with scrapie. The normal protein is present on FDCs in uninfected mice and, at lower levels, on lymphocytes. Studies using severe combined immunodeficiency (SCID) mice, with and without bone marrow (BM) grafts, have indicated involvement of FDCs and/or lymphocytes in scrapie pathogenesis. To clarify the separate roles of FDCs and lymphocytes, we produced chimeric mice with a mismatch in PrP status between FDCs and other cells of the immune system, by grafting bone marrow from PrP-deficient knockout mice into PrP-expressing mice and vice versa. Using these chimeric models, we obtained strong evidence that FDCs themselves produce PrP and that replication of a mouse-passaged scrapie strain in spleen depends on PrP-expressing FDCs rather than on lymphocytes or other bone marrow-derived cells.

Molecular biology of prion propagation

The occurrence of new variant Creutzfeldt-Jakob disease and the experimental confirmation that it is caused by the same prion strain as BSE has dramatically highlighted the need for a precise understanding of the molecular basis of prion propagation. The molecular basis of prion-strain diversity, previously a major challenge to the protein-only model, is now becoming clearer. The conformational change thought to be central to prion propagation, from a predominantly alpha-helical fold to one predominantly comprising beta-structure, can now be reproduced in vitro, and the ability of beta-PrP to form fibrillar aggregates provides a plausible molecular mechanism for prion propagation. These and other advances in the fundamental biology of prion propagation are leading to prion diseases becoming arguably the best understood of the neurodegenerative conditions and strategies for the development of rational therapeutics are becoming clearer.

PrP-dependent association of prions with splenic but not circulating lymphocytes of scrapie-infected mice

An intact immune system, and particularly the presence of mature B lymphocytes, is crucial for mouse scrapie pathogenesis in the brain after peripheral exposure. Prions are accumulated in the lymphoreticular system (LRS), but the identity of the cells containing infectivity and their role in neuroinvasion have not been determined. We show here that although prion infectivity in the spleen is associated with B and T lymphocytes and to a lesser degree with the stroma, no infectivity could be detected in lymphocytes from blood. In wild-type mice, which had been irradiated and reconstituted with PrP-deficient lymphohaematopoietic stem cells and inoculated with scrapie prions, infectivity in the spleen was present in the stroma but not in lymphocytes. Therefore, splenic B and T lymphocytes can either synthesize prions or acquire them from another source, but only when they express PrP.

A crucial role for B cells in neuroinvasive scrapie

Although prions are most efficiently propagated via intracerebral inoculation, peripheral administration has caused kuru [Gajdusek et al, 1966], iatrogenic Creutzfeldt-Jakob disease (CJD) [Gibbs et al, 1997], bovine spongiform encephalitis (BSE), and new variant CJD [Hill et al, 1997; Bruce et al, 1997]. Neurological disease after peripheral inoculation depends on prion expansion within cells of the lymphoreticular system (LRS) [Lasmezas et al. 1996; Wilesmith et al, 1992]. In order to identify the nature of the latter cells, we inoculated a panel of immune deficient mice with prions intraperitoneally. While defects affecting only T lymphocytes had no apparent effect, all mutations affecting differentiation and responses of B lymphocytes prevented development of clinical scrapie. Since absence of B cells and of antibodies correlates with severe defects in follicular dendritic cells (FDCs), the lack of any of these three components may prevent clinical scrapie. Yet, mice expressing immunoglobulins exclusively of the M subclass without detectable specificity for PrPc, and mice with differentiated B cells but lacking functional FDCs, developed scrapie after peripheral inoculation: therefore, differentiated B cells appear to play a crucial role in neuroinvasion of scrapie regardless of B-cell receptor specificity.

Investigation of variant Creutzfeldt-Jakob disease and other human prion diseases with tonsil biopsy samples

BACKGROUND

Prion diseases are associated with the accumulation of an abnormal isoform of cellular prion protein (PrPSc), which is the principal constituent of prions. Prions replicate in lymphoreticular tissues before neuroinvasion, suggesting that lymphoreticular biopsy samples may allow early diagnosis by detection of PrPSc. Variant Creutzfeldt-Jakob disease (variant CJD) is difficult to distinguish from common psychiatric disorders in its early stages and definitive diagnosis has relied on neuropathology. We studied lymphoreticular tissues from a necropsy series and assessed tonsillar biopsy samples as a diagnostic investigation for human prion disease.

METHODS

Lymphoreticular tissues (68 tonsils, 64 spleens, and 40 lymph nodes) were obtained at necropsy from patients affected by prion disease and from neurological and normal controls. Tonsil biopsy sampling was done on 20 patients with suspected prion disease. Tissues were analysed by western blot to detect and type PrPSc, by PrP immunohistochemistry, or both.

FINDINGS

All lymphoreticular tissues obtained at necropsy from patients with neuropathologically confirmed variant CJD, but not from patients with other prion diseases or controls, were positive for PrPSc. In addition, PrPSc typing revealed a consistent pattern (designated type 4t) different from that seen in variant CJD brain (type 4) or in brain from other CJD subtypes (types 1-3). Tonsil biopsy tissue was positive in all eight patients with an adequate biopsy sample and whose subsequent course has confirmed, or is highly consistent with, a diagnosis of variant CJD and negative in all patients subsequently confirmed to have other diagnoses.

INTERPRETATION

We found that if, in the appropriate clinical context, a tonsil biopsy sample was positive for PrPSc, variant CJD could be diagnosed, which obviates the need for a brain biopsy sample to be taken. Our results also show that variant CJD has a different pathogenesis to sporadic CJD.

Prion Protein Expression in Human Leukocyte Differentiation

The cellular isoform of the prion protein (PrPC) is a small glycoprotein attached to the outer leaflet of the plasma membrane by a glycosylphosphatidylinositol anchor. This molecule is involved in the pathogenesis of prion diseases in both humans and animals. We have characterized the expression patterns of PrPC during human leukocyte maturation by flow cytometry with monoclonal antibodies to PrPC, the glycan moiety CD15, and the stem cell marker CD34. We observe that prion protein is present on CD34+bone marrow (BM) stem cells. Although lymphocytes and monocytes maintain PrPC expression throughout their differentiation, PrPC is downregulated upon differentiation along the granulocyte lineage. In vitro retinoic acid–induced differentiation of the premyeloid line HL-60 into granulocyte-like cells mimics the suppression of PrPC in granulocyte differentiation, as both PrPC mRNA and protein are downregulated. These data suggest that selected BM cells and peripheral mononuclear cells may support prion agent replication, because this process is dependent on availability of PrPC. Additionally, retinoic acid–induced extinction of PrPC expression in HL-60 cells provides a potential model to study PrP gene regulation and protein function. Finally, these data suggest the existence of cell-specific glycoforms of PrPC that may determine cellular susceptibility to infection by the prion agent.

Prion Protein Expression in Human Leukocyte Differentiation

The cellular isoform of the prion protein (PrPC) is a small glycoprotein attached to the outer leaflet of the plasma membrane by a glycosylphosphatidylinositol anchor. This molecule is involved in the pathogenesis of prion diseases in both humans and animals. We have characterized the expression patterns of PrPC during human leukocyte maturation by flow cytometry with monoclonal antibodies to PrPC, the glycan moiety CD15, and the stem cell marker CD34. We observe that prion protein is present on CD34+bone marrow (BM) stem cells. Although lymphocytes and monocytes maintain PrPC expression throughout their differentiation, PrPC is downregulated upon differentiation along the granulocyte lineage. In vitro retinoic acid–induced differentiation of the premyeloid line HL-60 into granulocyte-like cells mimics the suppression of PrPC in granulocyte differentiation, as both PrPC mRNA and protein are downregulated. These data suggest that selected BM cells and peripheral mononuclear cells may support prion agent replication, because this process is dependent on availability of PrPC. Additionally, retinoic acid–induced extinction of PrPC expression in HL-60 cells provides a potential model to study PrP gene regulation and protein function. Finally, these data suggest the existence of cell-specific glycoforms of PrPC that may determine cellular susceptibility to infection by the prion agent.