Pathogenesis of mouse scrapie: distribution of agent in the pulp and stroma of infected spleens

Strain-Specific Targeting and Destruction of Cells by Prions

Prion diseases are caused by the disease-specific self-templating infectious conformation of the host-encoded prion protein, PrPSc. Prion strains are operationally defined as a heritable phenotype of disease under controlled conditions. One of the hallmark phenotypes of prion strain diversity is tropism within and between tissues. A defining feature of prion strains is the regional distribution of PrPSc in the CNS. Additionally, in both natural and experimental prion disease, stark differences in the tropism of prions in secondary lymphoreticular system tissues occur. The mechanism underlying prion tropism is unknown; however, several possible hypotheses have been proposed. Clinical target areas are prion strain-specific populations of neurons within the CNS that are susceptible to neurodegeneration following the replication of prions past a toxic threshold. Alternatively, the switch from a replicative to toxic form of PrPSc may drive prion tropism. The normal form of the prion protein, PrPC, is required for prion formation. More recent evidence suggests that it can mediate prion and prion-like disease neurodegeneration. In vitro systems for prion formation have indicated that cellular cofactors contribute to prion formation. Since these cofactors can be strain specific, this has led to the hypothesis that the distribution of prion formation cofactors can influence prion tropism. Overall, there is evidence to support several mechanisms of prion strain tropism; however, a unified theory has yet to emerge.

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.

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.

Exosome release from infected dendritic cells: a clue for a fast spread of prions in the periphery?

Prion diseases are incurable transmissible neurological disorders. In many natural and experimental prion diseases, infectious prions can be detected in the lymphoreticular system (LRS) long before they reach the brain where they cause a fatal rapidly progressive degeneration. Although major cell types that contribute to prion accumulation have been identified, the mode of prion dissemination in the LRS remains elusive. Recent evidence of a remarkably fast splenic prion accumulation after peripheral infection of mice, resulting in high prion titers in dendritic cells (DCs) and a release of prions from infected DCs via exosomes suggest that intercellular dissemination may contribute to rapid prion colonization in the LRS. A vast body of evidence from retroviral infections shows that DCs and other antigen-presenting cells (APCs) share viral antigens by intercellular transfer to warrant immunity against viruses if APCs remain uninfected. Evolved to adapt the immune response to evading pathogens, these pathways may constitute a portal for unimpeded prion dissemination owing to the tolerance of the immune system against host-encoded prion protein. In this review we summarize current paradigms for antigen-sharing pathways which may be relevant to better understand dissemination of rogue neurotoxic proteins.

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.

Plasmacytoid dendritic cells sequester high prion titres at early stages of prion infection

In most transmissible spongiform encephalopathies prions accumulate in the lymphoreticular system (LRS) long before they are detectable in the central nervous system. While a considerable body of evidence showed that B lymphocytes and follicular dendritic cells play a major role in prion colonization of lymphoid organs, the contribution of various other cell types, including antigen-presenting cells, to the accumulation and the spread of prions in the LRS are not well understood. A comprehensive study to compare prion titers of candidate cell types has not been performed to date, mainly due to limitations in the scope of animal bioassays where prohibitively large numbers of mice would be required to obtain sufficiently accurate data. By taking advantage of quantitative in vitro prion determination and magnetic-activated cell sorting, we studied the kinetics of prion accumulation in various splenic cell types at early stages of prion infection. Robust estimates for infectious titers were obtained by statistical modelling using a generalized linear model. Whilst prions were detectable in B and T lymphocytes and in antigen-presenting cells like dendritic cells and macrophages, highest infectious titers were determined in two cell types that have previously not been associated with prion pathogenesis, plasmacytoid dendritic (pDC) and natural killer (NK) cells. At 30 days after infection, NK cells were more than twice, and pDCs about seven-fold, as infectious as lymphocytes respectively. This result was unexpected since, in accordance to previous reports prion protein, an obligate requirement for prion replication, was undetectable in pDCs. This underscores the importance of prion sequestration and dissemination by antigen-presenting cells which are among the first cells of the immune system to encounter pathogens. We furthermore report the first evidence for a release of prions from lymphocytes and DCs of scrapie-infected mice ex vivo, a process that is associated with a release of exosome-like membrane vesicles.

Neuroinvasion in prion diseases: the roles of ascending neural infection and blood dissemination

Prion disorders are infectious, neurodegenerative diseases that affect humans and animals. Susceptibility to some prion diseases such as kuru or the new variant of Creutzfeldt-Jakob disease in humans and scrapie in sheep and goats is influenced by polymorphisms of the coding region of the prion protein gene, while other prion disorders such as fatal familial insomnia, familial Creutzfeldt-Jakob disease, or Gerstmann-Straussler-Scheinker disease in humans have an underlying inherited genetic basis. Several prion strains have been demonstrated experimentally in rodents and sheep. The progression and pathogenesis of disease is influenced by both genetic differences in the prion protein and prion strain. Some prion diseases only affect the central nervous system whereas others involve the peripheral organs prior to neuroinvasion. Many experiments undertaken in different species and using different prion strains have postulated common pathways of neuroinvasion. It is suggested that prions access the autonomic nerves innervating peripheral organs and tissues to finally reach the central nervous system. We review here published data supporting this view and additional data suggesting that neuroinvasion may concurrently or independently involve the blood vascular system.

Prions: Protein Aggregation and Infectious Diseases

Transmissible spongiform encephalopathies (TSEs) are inevitably lethal neurodegenerative diseases that affect humans and a large variety of animals. The infectious agent responsible for TSEs is the prion, an abnormally folded and aggregated protein that propagates itself by imposing its conformation onto the cellular prion protein (PrPC) of the host. PrPCis necessary for prion replication and for prion-induced neurodegeneration, yet the proximal causes of neuronal injury and death are still poorly understood. Prion toxicity may arise from the interference with the normal function of PrPC, and therefore, understanding the physiological role of PrPCmay help to clarify the mechanism underlying prion diseases. Here we discuss the evolution of the prion concept and how prion-like mechanisms may apply to other protein aggregation diseases. We describe the clinical and the pathological features of the prion diseases in human and animals, the events occurring during neuroinvasion, and the possible scenarios underlying brain damage. Finally, we discuss potential antiprion therapies and current developments in the realm of prion diagnostics.

Generation of cell lines propagating infectious prions and the isolation and characterization of cell-derived exosomes

Prion-propagating cell lines are an efficient and useful means for studying the cellular and molecular mechanisms implicated in prion disease. Use of cell-based models has lead to the finding that prion protein (PrP(C)) and PrP(Sc) are released from cells in association with exosomes. Furthermore, exosomes have been shown to act as vehicles for infectivity, transferring PrP(Sc) between cell lines and providing a mechanism for prion spread between tissues. As a role for exosomes in prion disease is emerging, this chapter outlines a method for the generation of prion-infected cell lines and the isolation and characterization of PrP(C)- and PrP(Sc)-containing exosomes.

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.

Packaging of prions into exosomes is associated with a novel pathway of PrP processing

Prion diseases are fatal, transmissible neurodegenerative disorders associated with conversion of the host-encoded prion protein (PrP(C)) into an abnormal pathogenic isoform (PrP(Sc)). Following exposure to the infectious agent (PrP(Sc)) in acquired disease, infection is propagated in lymphoid tissues prior to neuroinvasion and spread within the central nervous system. The mechanism of prion dissemination is perplexing due to the lack of plausible PrP(Sc)-containing mobile cells that could account for prion spread between infected and uninfected tissues. Evidence exists to demonstrate that the culture media of prion-infected neuronal cells contain PrP(Sc) and infectivity but the nature of the infectivity remains unknown. In this study we have identified PrP(C) and PrP(Sc) in association with endogenously expressing PrP neuronal cell-derived exosomes. The exosomes from our prion-infected neuronal cell line were efficient initiators of prion propagation in uninfected recipient cells and to non-neuronal cells. Moreover, our neuronal cell line was susceptible to infection by non-neuronal cell-derived exosome PrP(Sc). Importantly, these exosomes produced prion disease when inoculated into mice. Exosome-associated PrP is packaged via a novel processing pathway that involves the N-terminal modification of PrP and selection of distinct PrP glycoforms for incorporation into these vesicles. These data extend our understanding of the relationship between PrP and exosomes by showing that exosomes can establish infection in both neighbouring and distant cell types and highlight the potential contribution of differentially processed forms of PrP in disease distribution. These data suggest that exosomes represent a potent pool of prion infectivity and provide a mechanism for studying prion spread and PrP processing in cells endogenously expressing PrP.

Congo red and protein aggregation in neurodegenerative diseases

Congo red is a commonly used histological dye for amyloid detection. The specificity of this staining results from Congo red's affinity for binding to fibril proteins enriched in beta-sheet conformation. Unexpectedly, recent investigations indicate that the dye also possesses the capacity to interfere with processes of protein misfolding and aggregation, stabilizing native protein monomers or partially folded intermediates, while reducing concentration of more toxic protein oligomers. Inhibitory effects of Congo red upon amyloid toxicity may also range from blockade of channel formation and interference with glycosaminoglycans binding or immune functions, to the modulation of gene expression. Particularly, Congo red exhibits ameliorative effect in models of neurodegenerative disorders, such as Alzheimer's, Parkinson's, Huntington's and prion diseases. Another interesting application of Congo red analogues is the development of imaging probes. Based on their small molecular size and penetrability through blood-brain barrier, Congo red congeners can be used for both antemortem and in vivo visualization and quantification of brain amyloids. Therefore, understanding mechanisms involved in dye-amyloidal fibril binding and inhibition of aggregation will provide instructive guides for the design of future compounds, potentially useful for monitoring and treating neurodegenerative diseases.

Prions and peripheral nerves: A deadly rendezvous

The infectious particle causing transmissible spongiform encephalopathy (TSE), a fatal neurodegenerative disease of humans and animals, has been termed prion. Its major component is an aggregated variant of the cellular prion protein, PrP(C). The main target of prion pathology is the central nervous system (CNS), yet most prion diseases are initiated or accompanied by prion replication at extracerebral locations, including secondary lymphoid organs, muscle and, in some instances, blood. How do prions travel from the periphery into the CNS? Is this an active or a passive process and does neuronal prion transport explain the long incubation times in prion diseases? Alternatively, if prion infectivity arises spontaneously in the CNS, as believed from sporadic Creutzfeldt-Jakob patients, how do prions manage to travel from the CNS into the periphery (e.g., spleen, muscle) of the infected host? The mechanisms of neuronal prion transport from the periphery into the CNS or vice versa are heavily investigated and debated but poorly understood. Although research in the past has accumulated knowledge on prion progression from the periphery to the brain, we are far from understanding the molecular mechanisms of neuronal prion transport.

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.

Pathogenesis of prion diseases

Prion diseases are rare neurological disorders that may be of genetic or infectious origin, but most frequently occur sporadically in humans. Their outcome is invariably fatal. As the responsible pathogen, prions have been implicated. Prions are considered to be infectious particles that represent mainly, if not solely, an abnormal, protease-resistant isoform of a cellular protein, the prion protein or PrP(C). As in other neurodegenerative diseases, aggregates of misfolded protein conformers are deposited in the CNS of affected individuals. Pathogenesis of prion diseases comprises mainly two equally important, albeit essentially distinct, topics: first, the mode, spread, and amplification of infectivity in acquired disease, designated as peripheral pathogenesis. In this field, significant advances have implicated an essential role of lymphoid tissues for peripheral prion replication, before a likely neural spread to the CNS. The second is the central pathogenesis, dealing, in addition to spread and replication of prions within the CNS, with the mechanisms of nerve cell damage and death. Although important roles for microglial neurotoxicity, oxidative stress, and complement activation have been identified, we are far from complete understanding, and therapeutic applications in prion diseases still need to be developed.

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.

Recent advances in prion biology

PURPOSE OF REVIEW

Prion diseases continue to present a diagnostic and therapeutic challenge to clinicians and researchers worldwide. Many important aspects of prion biology remain unclear, and we still do not understand the nature of the infectious agent, the mechanisms leading to central nervous system damage, and the physiological function of the cellular prion protein. The current diagnostic tools for prion infections are breathtakingly insensitive when compared with those of other infectious diseases. Finally, there are hardly any therapeutic strategies. However, not all is gloomy, and many recent developments have advanced our basic understanding of prion diseases.

RECENT FINDINGS

In most prion infections, the portal of entry is extraneural. Although we still do not understand all details, several molecules and cell types have been identified as key players in prion neuroinvasion. These include lymphotoxins and their receptors, follicular dendritic cells, and the autonomic nervous system. These advances in knowledge are spurring the exploration of strategies for postexposure prophylaxis.

SUMMARY

The prion phenomenon is, at the same time, the cause of horrible diseases, and a fascinating biological enigma. The scope of this review is to discuss a selection of novel findings in prion research.

Variant Creutzfeldt–Jakob disease: between lymphoid organs and brain

Prion diseases are often caused by peripheral uptake of the infectious agent. To reach their ultimate target, the central nervous system (CNS), prions enter their host, replicate in lymphoid organs and spread via peripheral nerves. Once the agent has reached the CNS disease progression is rapid, resulting in neurodegeneration and death. many of these mechanisms have been uncovered using genetically modified mice. A recently published study demonstrated the presence of pathological prion protein in sympathetic ganglia of patients suffering from variant Creutzfeldt-Jakob disease, suggesting that these mechanisms might apply to humans.

Prions and the immune system: a journey through gut, spleen, and nerves

For more than two decades it has been contended that prion infection does not elicit immune responses: transmissible spongiform encephalopathies do not go along with conspicuous inflammatory infiltrates, and antibodies to the prion protein are typically undetectable. Why is it, then, that prions accumulate in lymphoid organs, and that various states of immune deficiency prevent peripheral prion infection? This review revisits the current evidence of the involvement of the immune system in prion diseases, while attempting to trace the elaborate mechanisms by which peripherally administered prions invade the brain and ultimately cause damage. The investigation of these questions leads to unexpected detours, including the neurophysiology of lymphoid organs, and even the function of a prion protein homolog in male fertility.

Immune system and peripheral nerves in propagation of prions to CNS

Prions are not only unique in the way they replicate. Also the sequence of events triggered by peripheral prion infection, generically termed 'peripheral pathogenesis', sets prions aside from all other known pathogens. Whereas most bacteria, parasites, and viruses trigger innate and adaptive immune responses, the mammalian immune system appears to be remarkably oblivious to prions. Transmissible spongiform encephalopathies (TSEs) do not go along with inflammatory infiltrates, and antibodies to the prion protein are not typically raised during the course of the disease. On the other hand, there is conspicuous involvement of lymphoid organs, which accumulate sizeable concentrations of the infectious agent early during disease. Moreover, various states of immune deficiency can abolish peripheral pathogenesis and prevent 'take' of infection when prions are administered to peripheral sites. Here, we critically re-visit the current evidence for an involvement of the immune system in prion diseases, and will attempt to trace the elaborate mechanisms by which prions, upon entry into the body from peripheral sites, reach the brain.

Lymph nodal prion replication and neuroinvasion in mice devoid of follicular dendritic cells

Variant Creutzfeldt-Jakob disease and scrapie are typically initiated by extracerebral exposure to prions, and exhibit early prion accumulation in germinal centers. Follicular dendritic cells (FDCs), whose development and maintenance in germinal centers depends on tumor necrosis factor (TNF) and lymphotoxin (LT) signaling, are thought to be indispensable for extraneural prion pathogenesis. Here, we administered prions intraperitoneally to mice deficient for TNF and LT signaling components. LT alpha(-/-), LT beta(-/-), LT betaR(-/-), and LT alpha(-/-) x TNFalpha(-/-) mice resisted infection and contained no infectivity in spleens and lymph nodes (when present). However, TNFR1(-/-), TNFR2(-/-), and some TNFalpha(-/-) mice developed scrapie similarly to wild-type mice. High prion titers were detected in lymph nodes, but not spleens, of TNFR1(-/-) and TNF alpha(-/-) mice despite absence of FDCs and germinal centers. Transfer of TNFR1(-/-) fetal liver cells into lethally irradiated Prnp(0/0) mice restored infectivity mainly in lymph nodes. Prion protein (PrP) colocalized with a minority of macrophages in tumor necrosis factor receptor (TNFR) 1(-/-) lymph nodes. Therefore, prion pathogenesis can be restricted to lymphoreticular subcompartments, and mature follicular dendritic cells are dispensable for this process. Macrophage subsets are plausible candidates for lymphoreticular prion pathogenesis and neuroinvasion in the absence of FDCs, and may represent a novel target for postexposure prophylaxis.

Transmission of prion disease

The transmission of bovine spongiform encephalopathy to humans as variant Creutzfeldt-Jakob disease (vCJD) has focused public attention on how prion diseases are transmitted and how prions reach the brain after exposure. Prion diseases are characterised by transmissibility and neuropathological features of gliosis, neuronal loss and microscopic vacuoles, termed spongiosis. The principal component of prions is the glycoprotein PrP(Sc), which is a conformational modified isoform of the normal membrane protein PrP(C). How are prions transmitted and how do prions find their way once they have been ingested? Prion models in mouse and hamster point to lymphoreticular cells which support an early replication phase of prions before reaching the central nervous system via peripheral nerves. Whilst some key players seem to have been identified so far, the mechanisms of prion propagation to the brain are still not fully understood. Seemingly contradictory results have led to some confusion and have provoked discussion.

Infected splenic dendritic cells are sufficient for prion transmission to the CNS in mouse scrapie

Transmissible spongiform encephalopathies display long incubation periods at the beginning of which the titer of infectious agents (prions) increases in peripheral lymphoid organs. This "replication" leads to a progressive invasion of the CNS. Follicular dendritic cells appear to support prion replication in lymphoid follicles. However, the subsequent steps of neuroinvasion remain obscure. CD11c(+) dendritic cells, an unrelated cell type, are candidate vectors for prion propagation. We found a high infectivity titer in splenic dendritic cells from prion-infected mice, suggesting that dendritic cells carry infection. To test this hypothesis, we injected RAG-1(0/0) mice intravenously with live spleen cell subsets from scrapie-infected donors. Injection of infected dendritic cells induced scrapie without accumulation of prions in the spleen. These results suggest that CD11c(+) dendritic cells can propagate prions from the periphery to the CNS in the absence of any additional lymphoid element.

Efficient lymphoreticular prion propagation requires PrP(c) in stromal and hematopoietic cells

In most prion diseases, infectivity accumulates in lymphoreticular organs early after infection. Defects in hematopoietic compartments, such as impaired B-cell maturation, or in stromal compartments, such as abrogation of follicular dendritic cells, can delay or prevent lymphoreticular prion colonization. However, the nature of the compartment in which prion replication takes place is controversial, and it is unclear whether this compartment coincides with that expressing the normal prion protein (PrP(c)). Here we studied the distribution of infectivity in splenic fractions of wild-type and fetal liver chimeric mice carrying the gene that encodes PrP(c) (Prnp) solely on hematopoietic or on stromal cells. We fractionated spleens at various times after intraperitoneal challenge with prions and assayed infectivity by bioassay. Upon high-dose challenge, chimeras carrying PrP(c) on hematopoietic cells accumulated prions in stroma and in purified splenocytes. In contrast, after low-dose challenge ablation of Prnp in either compartment prevented splenic accumulation of infectivity, indicating that optimal prion replication requires PrP(c) expression by both stromal and hematopoietic compartments.

Sympathetic innervation of lymphoreticular organs is rate limiting for prion neuroinvasion

Transmissible spongiform encephalopathies are commonly propagated by extracerebral inoculation of the infectious agent. Indirect evidence suggests that entry into the central nervous system occurs via the peripheral nervous system. Here we have investigated the role of the sympathetic nervous system in prion neuroinvasion. Following intraperitoneal prion inoculation, chemical or immunological sympathectomy delayed or prevented scrapie. Prion titers in spinal cords were drastically reduced at early time points after inoculation. Instead, keratin 14-NGF transgenic mice, whose lymphoid organs are hyperinnervated by sympathetic nerves, showed reduction in scrapie incubation time and, unexpectedly, much higher titers of prion infectivity in spleens. We conclude that sympathetic innervation of lymphoid organs is rate limiting for prion neuroinvasion and that splenic sympathetic nerves may act as extracerebral prion reservoirs.

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.

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.

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.

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.

Immune system-dependent and -independent replication of the scrapie agent

Using the severe combined immunodeficiency (SCID) mouse model, we investigated the requirement of the immune system for the development of scrapie after peripheral inoculation. A total of 33% of SCID mice, all but one immunologically reconstituted SCID mice (93%), and all CB17 control mice developed the disease. PrPres was detectable in the brains of all diseased animals and in the spleens of reconstituted SCID and CB17 control mice but not of the diseased non-immunologically reconstituted SCID mice. The immune system appears to be a primary target in the pathogenesis of scrapie, but direct spread to the central nervous system from the peritoneum via visceral nerve fibers can probably also occur.

Interaction of scrapie agent and cells of the lymphoreticular system

The current study focused on the role of lymphoid elements of the lymphoreticular system in scrapie pathogenesis. In the first experiment, adherent and non-adherent splenocytes from mice infected with the 139A scrapie strain were prepared. The level of infectivity on a per cell basis was significantly higher in the adherent cell population. In a second set of experiments, thymocytes, unfractionated splenocytes, T-cell enriched and T-cell depleted fractions of splenocytes were infected in vitro with ME7 scrapie strain. There was no evidence of replication of scrapie in ME7-exposed cells in any of the preparations during the first 5-14 days post-exposure. In assays done 5 days after infection, most of the infectivity was cell-associated. These data suggest that lymphoid cells are not involved in scrapie replication. The level of IgA in the serum of 139A-infected mice was markedly reduced compared to the levels in mice injected with normal mouse brain homogenate or with the ME7 scrapie strain. The reduction in IgA levels in 139A-infected mice was evident at each of the 4 time points tested. The final experiment dealt with the question of scrapie replication in the lymphoreticular organs in mouse strains with different incubation periods for 139A after intraperitoneal injection. The results in this experiment suggest that the difference in incubation periods is related to differences in time of access of infection to the central nervous system rather than to differences in the ability of agent to replicate in spleen.

PrP protein is associated with follicular dendritic cells of spleens and lymph nodes in uninfected and scrapie-infected mice

Abnormal forms of a host protein, PrP, accumulate in the central nervous system in scrapie-affected animals. Here, PrP protein was detected immunocytochemically in tissue sections of spleen, lymph node, Peyer's patches, thymus, and pancreas from uninfected mice and from mice infected with a range of mouse-passaged scrapie strains and bovine spongiform encephalopathy (BSE). In the spleen, lymph node and Peyer's patches, PrP-positive cells were identified as follicular dendritic cells (FDC) by their location, appearance, and immune complex trapping function, whereas in the thymus they appeared to be two types of stromal cells: interdigitating cells (IDC) and cortical epithelial cells. In pancreas, PrP-containing cells were confined to the islets of Langerhans. Although the distribution of PrP immunolabelling was the same in tissues from scrapie-affected and uninfected mice, there was evidence that PrP accumulated in abnormal forms in FDC of infected mice. If, as is likely, PrP is essential for agent replication, our results suggest that FDC are the site of scrapie and BSE replication in the spleen and lymph node.

The mRNA encoding the scrapie agent protein is present in a variety of non-neuronal cells

PrP 27-30, a unique protease-resistant protein associated with scrapie infectivity, derives from the proteolytic cleavage of a larger precursor encoded by a host gene. To identify sites of PrP biosynthesis, in situ hybridization was done using cloned PrP cDNA as a probe. In rodent brain, PrP mRNA was expressed in neurons, ependymal cells, choroid plexus epithelium, astrocytes, pericytes, endothelial cells and meninges of both scrapie-infected and uninfected animals. PrP mRNA was also detected in vitro in isolated brain microglia cells. Pulmonary cells and heart muscle cells contained high levels of this mRNA. Hybridization was not detected in spleen, confirming earlier RNA blot experiments indicating extremely low levels of PrP mRNA in this tissue. Results indicate that PrP mRNA is a normal component in a variety of non-neuronal tissues and may explain the origin of the amyloid plaques present in the subependymal region of scrapie-infected brain.

Intraperitoneal infection with scrapie is established within minutes of injection and is non-specifically enhanced by a variety of different drugs

Single intraperitoneal (i.p.) doses of 16 different drugs were given to mice 2 h before injecting scrapie i.p. Scrapie was injected as serial ten-fold dilutions of standard inocula and the effective titres obtained were used as a measure of the relative efficiency of infection in treated compared to saline injected mice. Despite the wide variety of drugs tested, most of them increased, non-specifically, the efficiency of infection by 0.6 to 2.1 log10 i.p. LD50 units (i.e., 4 to 126-fold), but only when both drug and scrapie were given i.p. The effect was greatest with a 2 h or a 6 h interval suggesting an involvement either of resident peritoneal cells or of elicited cells such as polymorphonuclear neutrophils. There was no increase in the efficiency of infection after intervals of 2 or 7 days when induced macrophages would predominant. The reverse sequence of injections (scrapie-0.5 h-drug) had no effect despite the persistence of high scrapie titre in the peritoneum at the time of drug injection. However, the effect was restored by a second injection of scrapie in the sequence, scrapie-drug-scrapie. It is concluded that scrapie infection is established within minutes of injection but much of the inoculum is associated with peritoneal cells which are irrelevant to pathogenesis. Drugs may enhance the infection of relevant peritoneal cells or their targeting to the visceral lymphoreticular tissues where early replication takes place.

Polyclonal increase in certain IgG subclasses in mice persistently infected with the 87V strain of scrapie

Eight different combinations of seven strains of scrapie agent and the three known Sinc genotypes of mice were screened for changes in the concentration of IgG in serum. A single radial immunodiffusion assay was used to measure IgG throughout the incubation period which in different models ranged from an average of 125 days to longer than the maximum observation period of about 600 days. The only major changes occurred with the 87V strain of scrapie injected intracerebrally (i.c.) or intraperitoneally (i.p.) into mouse strains of the Sinc genotype p7p7. IgG concentration reached 1.5 to 2.0 times the control values in i.c. infected mice, which developed clinical disease after 270 to 320 days and also in i.p. infected mice, which did not develop the disease within the 600-day observation period. At very high IgG concentration, the increase was polyclonal; it involved the IgG 1 subclass more than the others and was accompanied by an increased rate of IgG clearance from serum. It is suggested that some scrapie infections of mice (and sheep) may upset the control of IgG production. The underlying mechanism may involve cell-pathogen interactions which are common to all scrapie infections, but only lead to gross changes in IgG in some combinations of agent strain and host genotype.

The role of the spleen in the neuroinvasion of scrapie in mice

The pathogenesis of 139A scrapie has been studied in CW mice infected intraperitoneally (i.p.), intravenously (i.v.) or subcutaneously (s.c.). In mice splenectomised before i.p. infection, the evidence points to a neuroinvasive pathway from visceral lymph nodes (and other sites of scrapie replication in the peritoneum) to the thoracic spinal cord. However, in non-splenectomised mice, the major neuroinvasive pathway is clearly from spleen to thoracic cord because i.p. incubation periods are shorter and replication in the thoracic cord starts correspondingly earlier than in splenectomised mice. Studies of splenectomy at different times after i.p. infection show that pathogenesis becomes independent of the spleen once infection has initiated scrapie replication in the spinal cord. The simplest interpretation of all the evidence favours the spread of scrapie infection along splenic nerve fibres to the thoracic spinal cord. The same neuroinvasive pathway is suggested by the findings using the s.c. and i.v. routes of infection. In addition it was found that the 100-fold greater efficiency of infection by the i.v. compared to the i.p. route was entirely dependent on the spleen, because splenectomy before i.v. infection reduced its efficiency to the same as that found in i.p. infected (non-splenectomised) mice.

Ionising radiation has no influence on scrapie incubation period in mice

Scrapie has an early non-clinical stage when replication of agent occurs in lymphoreticular organs. Whole-body irradiation failed to alter the incubation or neuropathology of the disease. Many experiments were carried out with different strains of scrapie agent and host, doses and timing of irradiation. The results suggest that mitotically quiescent cells are involved in agent replication.

Isolation of a cDNA clone encoding the leader peptide of prion protein and expression of the homologous gene in various tissues

We have isolated a hamster cDNA clone representing the coding sequences for the entire precursor of prion protein (PrP) 27-30. This clone encodes a protein of 254 residues and contains an in-frame ATG codon 42 bases upstream from the one previously reported. Analysis of the predicted amino acid sequence suggests that the PrP precursor protein contains an amino-terminal signal sequence, and a membrane-spanning domain in the carboxyl terminus. Cleavage of the signal peptide would produce a mature protein of 232 amino acids. Sequences homologous to the hamster PrP cDNA were detected in hamster, mouse, sheep, human, and rabbit genomes. A related 2.5-kilobase transcript was present in the brain of normal and scrapie-infected rodents. Two homologous transcripts, 2.5 and 1.1 kilobases, were detected in the lung and heart of normal animals. No PrP mRNA was detected in spleen stroma, a tissue known to contain high titers of scrapie. Antisera raised to the 27- to 30-kDa polypeptide detected the PrP in both normal and infected brains but failed to detect this protein in either normal or infected spleens. Homologous mRNA species were detected in human, sheep, and rabbit brain, even though the latter is resistant to scrapie infection. Our data suggest that PrP is not a necessary component of the infectious agent.

Suppression of scrapie infection in mice by heteropolyanion 23, dextran sulfate, and some other polyanions

Studies of polyanions that suppress scrapie have been done to pinpoint the cell types in the lymphoreticular system which are important in pathogenesis and to suggest possible prophylactic or therapeutic strategies for the unconventional slow viruses. A regime of three daily injections of the inorganic heteropolyanion HPA-23 reduced the effective scrapie dose by more than 99%; i.e., some mice survived peripherally injected doses of 100 50% lethal dose units. The effect was greatest when the first dose of HPA-23 was given 4 h after injecting scrapie, but it declined rapidly as this interval was increased, and there was virtually no effect 2 days after infection. A single dose of high-molecular-weight organic polyanions such as carrageenan or dextran sulfate (DS-500) greatly reduced (i.e., greater than 99%) the efficiency of scrapie infection. In contrast to HPA-23, DS-500 was equally effective whether given 4 days before or 8 h after the time of infection. The antiscrapie effect of DS-500 appeared to be independent of its activity as a B-cell mitogen and of its ability to produce a cytotoxic blockade of phagocytic cells. DS-500 probably caused the aggregation and loss from blood of scrapie inoculum which was present immediately after injection, but it had additional effects on scrapie at later times.