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Previous studies have shown after the resolution of acute infection and viraemia, foot- and-mouth disease virus (FMDV) capsid proteins and/or genome are localised in the light zone of germinal centres of lymphoid tissue in cattle and African buffalo. The pattern of staining for FMDV proteins was consistent with the virus binding to follicular dendritic cells (FDCs). We have now demonstrated a similar pattern of FMDV protein staining in mouse spleens after acute infection and showed FMDV proteins are colocalised with FDCs. Blocking antigen binding to complement receptor type 2 and 1 (CR2/CR1) prior to infection with FMDV significantly reduced the detection of viral proteins on FDCs and FMDV genomic RNA in spleen samples. Blocking the receptors prior to infection also significantly reduced neutralising antibody titres. Therefore, the binding of FMDV to FDCs and sustained induction of neutralising antibody responses are dependent on FMDV binding to CR2/CR1 in mice.Author SummaryFoot and mouth disease virus causes a highly contagious acute vesicular disease, resulting in more than 50% of cattle, regardless of vaccination status, and almost 100% of African buffalo becoming persistently infected for long periods (months) of time. Yet, the mechanisms associated with establishment of persistent infections are still poorly understood. Infected animals are characterised by the presence of long-lived neutralising antibody titres, which contrast with the short-lived response induced by vaccination. We have used a mouse model to understand how foot and mouth disease virus is trapped and retained in the spleen for up to 28 days post infection and how the absence of antigen in the germinal centre prevents a sustainable neutralising antibody response, in the mouse. Our results highlight the importance of targeting antigen to FDCs to stimulate potent neutralising antibody responses after vaccination.

The early replication of some orally-acquired prion strains upon stromal-derived follicular dendritic cells (FDC) within the small intestinal Peyer’s patches is essential to establish host infection, and for the disease to efficiently spread to the brain. Factors that influence the early accumulation of prions in Peyer’s patches can directly influence disease pathogenesis. The host’s immune response to a gastrointestinal helminth infection can alter susceptibility to co-infection with certain pathogenic bacteria and viruses. Here we used the natural mouse small intestine-restricted helminth pathogen Heligmosomoides polygyrus to test the hypothesis that pathology specifically within the small intestine caused by a helminth co-infection would influence oral prion disease pathogenesis. When mice were co-infected with prions on d 8 after H. polygyrus infection the early accumulation of prions within Peyer’s patches was reduced and survival times significantly extended. Natural prion susceptible hosts such as sheep, deer and cattle are regularly exposed to gastrointestinal helminth parasites. Our data suggest that co-infections with small intestine-restricted helminth pathogens may be important factors that influence oral prion disease pathogenesis.

Prion diseases are transmissible, neurodegenerative disorders associated with misfolding of the prion protein. Previous studies show that reduction of microglia accelerates CNS prion disease and increases the accumulation of prions in the brain, suggesting that microglia provide neuroprotection by phagocytosing and destroying prions. In Csf1rΔFIRE mice, the deletion of an enhancer within Csf1r specifically blocks microglia development, however, their brains develop normally and show none of the deficits reported in other microglia-deficient models. Csf1rΔFIRE mice were used as a refined model in which to study the impact of microglia-deficiency on CNS prion disease. Although Csf1rΔFIRE mice succumbed to CNS prion disease much earlier than wild-type mice, the accumulation of prions in their brains was reduced. Instead, astrocytes displayed earlier, non-polarized reactive activation with enhanced synaptic pruning and unfolded protein responses. Our data suggest that rather than simply phagocytosing and destroying prions, the microglia instead provide host-protection during CNS prion disease and restrict the harmful activities of reactive astrocytes.Main pointsCNS prion disease is accelerated in mice completely lacking microglia. The rate of prion accumulation in the brain was unaltered in absence of microglia. Microglia provide host-protection during CNS prion disease independent of prion clearance.

Prion diseases are transmissible, neurodegenerative disorders to which there are no cures. Previous studies show that reduction of microglia accelerates prion disease and increases the accumulation of prions in the brain, suggesting that microglia provide neuroprotection by phagocytosing and destroying prions. In Csf1rΔFIRE mice, the deletion of an enhancer within Csf1r specifically blocks microglia development, however, their brains develop normally with none of the deficits reported in other microglia-deficient models. Csf1rΔFIRE mice were used as a refined model to study the impact of microglia-deficiency on CNS prion disease. Although Csf1rΔFIRE mice succumbed to prion disease much earlier than wild-type mice, this was not due to increased accumulation of prions in their brains. Instead, astrocytes displayed earlier, non-polarized reactive activation with enhanced synaptic pruning and unfolded protein responses. Our data suggest that rather than engulfing and degrading prions, the microglia instead provide neuroprotection and restrict the harmful activities of reactive astrocytes.

Prion diseases are a unique group of infectious chronic neurodegenerative disorders to which there are no cures. Although prion infections do not stimulate adaptive immune responses in infected individuals, the actions of certain immune cell populations can have a significant impact on disease pathogenesis. After infection, the targeting of peripherally-acquired prions to specific immune cells in the secondary lymphoid organs (SLO), such as the lymph nodes and spleen, is essential for the efficient transmission of disease to the brain. Once the prions reach the brain, interactions with other immune cell populations can provide either host protection or accelerate the neurodegeneration. In this review, we provide a detailed account of how factors such as inflammation, ageing and pathogen co-infection can affect prion disease pathogenesis and susceptibility. For example, we discuss how changes to the abundance, function and activation status of specific immune cell populations can affect the transmission of prion diseases by peripheral routes. We also describe how the effects of systemic inflammation on certain glial cell subsets in the brains of infected individuals can accelerate the neurodegeneration. A detailed understanding of the factors that affect prion disease transmission and pathogenesis is essential for the development of novel intervention strategies.

Prion diseases or transmissible spongiform encephalopathies are fatal, progressive, neurodegenerative, protein-misfolding disorders. Prion diseases may arise spontaneously, be inherited genetically or be acquired by infection and affect a variety of mammalian species including humans. Prion infections in the central nervous system (CNS) cause extensive neuropathology, including abnormal accumulations of misfolded host prion protein, vacuolar change resulting in sponge-like (spongiform) appearance of CNS tissue, neurodegeneration and reactive glial responses. Many different prion agent strains exist and these can differ based on disease duration, clinical signs and the targeting and distribution of the neuropathology in distinct brain areas. Reactive astrocytes are a prominent feature in the prion disease affected CNS as revealed by distinct morphological changes and upregulation of glial fibrillary acidic protein (GFAP). The CD44 antigen is a transmembrane glycoprotein involved in cell-cell interactions, cell adhesion and migration. Here we show that CD44 is also highly expressed in a subset of reactive astrocytes in regions of the CNS targeted by prions. Astrocyte heterogeneity revealed by differential CD44 upregulation occurs coincident with the earliest neuropathological changes during the pre-clinical phase of disease, and is not affected by the route of infection. The expression and distribution of CD44 was compared in brains from a large collection of 15 distinct prion agent strains transmitted to mice of different prion protein (Prnp) genotype backgrounds. Our data show that the pattern of CD44 upregulation observed in the hippocampus in each prion agent strain and host Prnp genotype combination was unique. Many mouse-adapted prion strains and hosts have previously been characterized based on the pattern of the distribution of the spongiform pathology or the misfolded PrP deposition within the brain. Our data show that CD44 expression also provides a reliable discriminatory marker of prion infection with a greater dynamic range than misfolded prion protein deposition, aiding strain identification. Together, our data reveal CD44 as a novel marker to detect reactive astrocyte heterogeneity during CNS prion disease and for enhanced identification of distinct prion agent strains.

More than 50 % of cattle (regardless of foot and mouth disease (FMD) vaccination) become persistently infected for long periods of time (years) after exposure to FMD virus (FMDV). The mechanisms associated with establishment of persistent infections are still poorly understood. I am testing the hypothesis that complement receptor 2 (CR2) on follicular dendritic cells (FDCs) located in the germinal centers of the lymphoid tissue, are involved in the trapping and long-term persistence of FMDV and that this persistence is essential for the maintenance of long-term antibody responses to FMDV. The aim of this study was to assess FMDV antigen retention and the generation of the specific immune response in vivo, in a mouse model. Groups of mice were treated with an anti-CR2 monoclonal antibody, named 4B2, which has been described previously to block CR2 long-term in vivo, blocking for 6 weeks after a single injection of 2 mg (Kulik et al., 2015). After treatment with 4B2, animals were infected with FMDV and lymphoid tissues and serum samples evaluated for the presence of antigen and the humoral immune response, respectively. The ability of 4B2 to block the binding of FMDV and PAP immune complexes (ICs) to FDCs in vitro as well as results about the role of FDC in trapping FMDV via CR2 in vivo is currently being investigated and will be discussed.

Prion diseases are a unique group of chronic neurodegenerative diseases that affect humans and certain domestic and free-ranging animal species. Many natural prion diseases are acquired peripherally, such as by ingestion of contaminated food or pasture. Although the pathology during prion disease appears to be restricted to the central nervous system, where it causes extensive neurodegeneration, some prion diseases accumulate to high levels within the secondary lymphoid tissues of the host's immune system as they make their journey from the site of infection to the brain. The replication of prions within these tissues is essential for the efficient spread of disease to the brain. Moreover, the immune system has a profound influence on the development of disease within the central nervous system. This chapter describes the interactions between prions and the host's immune system. Particular emphasis is given to studies which have helped to identify the key tissues, cells, and molecules which the prions exploit to facilitate their propagation from peripheral sites of exposure (such as the intestine) to the brain. This chapter also describes how prion disease pathogenesis and susceptibility may be influenced by inflammation, co-infection with other pathogens, and aging. A thorough understanding of the factors which influence prion disease susceptibility is important as it may help to identify important targets for therapeutic intervention and to help determine the risk of susceptibility to novel peripherally acquired prion diseases.

International audience; AbstractGastrointestinal disease caused by the apicomplexan parasite Cryptosporidium parvum is one of the most important diseases of young ruminant livestock, particularly neonatal calves. Infected animals may suffer from profuse watery diarrhoea, dehydration and in severe cases death can occur. At present, effective therapeutic and preventative measures are not available and a better understanding of the host–pathogen interactions is required. Cryptosporidium parvum is also an important zoonotic pathogen causing severe disease in people, with young children being particularly vulnerable. Our knowledge of the immune responses induced by Cryptosporidium parasites in clinically relevant hosts is very limited. This review discusses the impact of bovine cryptosporidiosis and describes how a thorough understanding of the host–pathogen interactions may help to identify novel prevention and control strategies.

Prion diseases are infectious neurodegenerative disorders characterized by accumulations of abnormally folded cellular prion protein in affected tissues. Many natural prion diseases are acquired orally, and following exposure, the early replication of some prion isolates upon follicular dendritic cells (FDC) within gut-associated lymphoid tissues (GALT) is important for the efficient spread of disease to the brain (neuroinvasion). Prion detection within large intestinal GALT biopsy specimens has been used to estimate human and animal disease prevalence. However, the relative contributions of the small and large intestinal GALT to oral prion pathogenesis were unknown. To address this issue, we created mice that specifically lacked FDC-containing GALT only in the small intestine. Our data show that oral prion disease susceptibility was dramatically reduced in mice lacking small intestinal GALT. Although these mice had FDC-containing GALT throughout their large intestines, these tissues were not early sites of prion accumulation or neuroinvasion. We also determined whether pathology specifically within the large intestine might influence prion pathogenesis. Congruent infection with the nematode parasite Trichuris muris in the large intestine around the time of oral prion exposure did not affect disease pathogenesis. Together, these data demonstrate that the small intestinal GALT are the major early sites of prion accumulation and neuroinvasion after oral exposure. This has important implications for our understanding of the factors that influence the risk of infection and the preclinical diagnosis of disease. IMPORTANCE Many natural prion diseases are acquired orally. After exposure, the accumulation of some prion diseases in the gut-associated lymphoid tissues (GALT) is important for efficient spread of disease to the brain. However, the relative contributions of GALT in the small and large intestines to oral prion pathogenesis were unknown. We show that the small intestinal GALT are the essential early sites of prion accumulation. Furthermore, congruent infection with a large intestinal helminth (worm) around the time of oral prion exposure did not affect disease pathogenesis. This is important for our understanding of the factors that influence the risk of prion infection and the preclinical diagnosis of disease. The detection of prions within large intestinal GALT biopsy specimens has been used to estimate human and animal disease prevalence. However, our data suggest that using these biopsy specimens may miss individuals in the early stages of oral prion infection and significantly underestimate the disease prevalence.

Prion diseases are a unique group of transmissible, typically sub-acute, neurodegenerative disorders. During central nervous system (CNS) prion disease, the microglia become activated and are thought to provide a protective response by scavenging and clearing prions. The mammalian intestine is host to a large burden of commensal micro-organisms, especially bacteria, termed the microbiota. The commensal microbiota has beneficial effects on host health, including through the metabolism of essential nutrients, regulation of host development and protection against pathogens. The commensal gut microbiota also constitutively regulates the functional maturation of microglia in the CNS, and microglial function is impaired when it is absent in germ-free mice. In the current study, we determined whether the absence of the commensal gut microbiota might also affect prion disease pathogenesis. Our data clearly show that the absence of the commensal microbiota in germ-free mice did not affect prion disease duration or susceptibility after exposure to prions by intraperitoneal or intracerebral injection. Furthermore, the magnitude and distribution of the characteristic neuropathological hallmarks of terminal prion disease in the CNS, including the development of spongiform pathology, accumulation of prion disease-specific protein (PrP), astrogliosis and microglial activation, were similar in conventionally housed and germ-free mice. Thus, although the commensal gut microbiota constitutively promotes the maintenance of the microglia in the CNS under steady-state conditions in naïve mice, our data suggest that dramatic changes to the abundance or complexity of the commensal gut microbiota are unlikely to influence CNS prion disease pathogenesis.

Prion diseases (or transmissible spongiform encephalopathies) are a unique group of fatal progressive neurodegenerative diseases of the central nervous system. The infectious agent is hypothesized to consist solely of a highly protease-resistant misfolded isoform of the host prion protein. Prions display a remarkable degree of resistance to chemical and physical decontamination. Many common forms of decontamination or neutralization used in infection control are ineffective against prions, except chaotropic agents that specifically disrupt proteins. Human cadaveric prosection or dissection for the purposes of teaching and demonstration of human anatomy has a distinguished history and remains one of the fundamentals of medical education. Iatrogenic transmission of human prion diseases has been demonstrated from the inoculation or implantation of human tissues. Therefore, although the incidence of human prion diseases is rare, restrictions exist upon the use of tissues from patients reported with dementia, specifically the brain and other central nervous system material. A current concern is the potential for asymptomatic variant Creutzfeldt-Jakob disease transmission within the UK population. Therefore, despite the preventative measures, the transmission of prion disease through human tissues remains a potential risk to those working with these materials. In this review, we aim to summarize the current knowledge on human prion disease relevant to those working with human tissues in the context of anatomical dissection.

Follicular dendritic cells (FDC) are situated in the primary follicles of lymphoid tissues where they maintain the structural integrity of the B-lymphocyte follicle, and help to drive immunoglobulin class-switch recombination, somatic hypermutation and affinity maturation during the germinal centre response. FDC can also provide a reservoir for pathogens that infect germinal centres including HIV and prions. FDC express high levels of the normal cellular form of the prion protein (PrP(C) ), which makes them susceptible to prion infection. The function of PrP(C) is uncertain and it is not known why FDC require such high levels of expression of a protein that is found mainly on cells of the central nervous system. In this study, the function of FDC was assessed in mice that had PrP(C) ablated specifically in their FDC. In mice with FDC-specific PrP(C) ablation, our analysis revealed no observable deficits in lymphoid follicle microarchitecture and FDC status. No effects on FDC ability to trap immune complexes or drive antigen-specific antibody responses and affinity maturation in B lymphocytes were observed. These data clearly demonstrate that PrP(C) expression is dispensable for the functional maturation of FDC and their ability to maintain antigen-specific antibody responses and affinity maturation.

The occurrence of variant Creutzfeldt–Jakob (vCJD) disease in humans was almost certainly the result of consumption of food contaminated with bovine spongiform encephalopathy (BSE) prions. Despite probable widespread exposure of the UK population to BSE-contaminated food in the 1980s, vCJD has been identified predominantly in young individuals, and there have been fewer cases of clinical disease than anticipated. The reasons for this are uncertain. Following peripheral exposure, many prions replicate within the lymphoid tissues before infecting the central nervous system. We have shown that the effects of host age on the microarchitecture of the spleen significantly impair susceptibility to mouse-adapted prions after peripheral exposure. The transmission of prions between different mammalian species is considered to be limited by the ‘species barrier’, which is dependent on several factors, including an intact immune system. Thus, cross-species prion transmission may be much less efficient in aged individuals. To test this hypothesis, we compared prion pathogenesis in groups of young (6–8 weeks old) and aged (600 days old) mice injected with primary BSE brain homogenate. We showed that prion pathogenesis was impaired dramatically in aged mice when compared with young animals. Whereas most young mice succumbed to clinical prion disease, all aged mice failed to develop clinical disease during their lifespans. However, the demonstration that prion accumulation was detected in the lymphoid tissues of some aged mice after injection with primary BSE brain homogenate, in the absence of clinical signs of prion disease, has important implications for human health.

Prion diseases are subacute neurodegenerative diseases that affect humans and certain domestic and free-ranging animals. The infectious prion agent is considered to comprise solely of abnormally folded isoforms of the cellular prion protein. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid tissues as they make their journey from the site of infection to the brain. The replication of these prions within secondary lymphoid tissues is important for the efficient spread of disease to the brain. This article provides an account of the key tissues, cells, and molecules involved in the propagation of prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain, and describes how additional factors such as inflammation and aging may influence susceptibility to peripherally acquired prion diseases.

Prion diseases or transmissible spongiform encephalopathies are a unique category of infectious protein-misfolding neurodegenerative disorders. Hypothesized to be caused by misfolding of the cellular prion protein these disorders possess an infectious quality that thrives in immune-competent hosts. While much has been discovered about the routing and critical components involved in the peripheral pathogenesis of these agents there are still many aspects to be discovered. Research into this area has been extensive as it represents a major target for therapeutic intervention within this group of diseases. The main focus of pathological damage in these diseases occurs within the central nervous system. Cells of the innate immune system have been proven to be critical players in the initial pathogenesis of prion disease, and may have a role in the pathological progression of disease. Understanding how prions interact with the host innate immune system may provide us with natural pathways and mechanisms to combat these diseases prior to their neuroinvasive stage. We present here a review of the current knowledge regarding the role of the innate immune system in prion pathogenesis.

Prion diseases are subacute neurodegenerative diseases that affect humans and a range of domestic and free-ranging animal species. These diseases are characterized by the accumulation of PrP (Sc), an abnormally folded isoform of the cellular prion protein (PrP (C)), in affected tissues. The pathology during prion disease appears to occur almost exclusively within the central nervous system. The extensive neurodegeneration which occurs ultimately leads to the death of the host. An intriguing feature of the prion diseases, when compared with other protein-misfolding diseases, is their transmissibility. Following peripheral exposure, some prion diseases accumulate to high levels within lymphoid tissues. The replication of prions within lymphoid tissue has been shown to be important for the efficient spread of disease to the brain. This article describes recent progress in our understanding of the cellular mechanisms that influence the propagation of prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. A thorough understanding of these events will lead to the identification of important targets for therapeutic intervention, or alternatively, reveal additional processes that influence disease susceptibility to peripherally-acquired prion diseases.

Infections with variant Creutzfeldt-Jakob disease (vCJD) have almost exclusively occurred in young patients, but the reasons for this age distribution are uncertain. Our data suggest that the pathogenesis of many peripherally acquired transmissible spongiform encephalopathy (TSE) agents is less efficient in aged individuals. Four vCJD cases linked to transfusion of vCJD-contaminated blood or blood products have been described. Three cases occurred in elderly patients, implying that intravenous exposure is more efficient in aged individuals than other peripheral routes. To test this hypothesis, young (6 to 8 weeks old) and aged (600 days old) mice were injected intravenously with a TSE agent. In aged and young mice, the intravenous route was more efficient than other peripheral routes of TSE agent exposure. However, in aged mice, disease pathogenesis was significantly reduced. Although most aged mice failed to develop clinical disease during their life spans, many showed histopathological signs of TSE disease in their brains. Thus, the effects of age on intravenous TSE pathogenesis may lead to significant levels of subclinical disease in the population. After peripheral exposure, many TSE agents accumulate upon follicular dendritic cells (FDCs) in lymphoid tissues before they infect the brain. In aged spleens, PrP C expression and TSE agent accumulation upon FDCs were reduced. Furthermore, the splenic marginal zone microarchitecture was substantially disturbed, adversely affecting the delivery of immune complexes to FDCs. This study is the first to suggest that the effects of aging on the microarchitecture and the function of the splenic marginal zone significantly influence the pathogenesis of an important pathogen.

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are neurological diseases that can be transmitted through a number of different routes. A wide range of mammalian species are affected by the disease. After peripheral exposure, some TSE agents accumulate in lymphoid tissues at an early stage of disease prior to spreading to the nerves and the brain. Much research has focused on identifying the cells and molecules involved in the transmission of TSE agents from the site of exposure to the brain and several crucial cell types have been associated with this process. The identification of the key cells that influence the different stages of disease transmission might identify targets for therapeutic intervention. This review highlights the involvement of mononuclear phagocytes in TSE disease. Current data suggest these cells may exhibit a diverse range of roles in TSE disease from the transport or destruction of TSE agents in lymphoid tissues, to mediators or protectors of neuropathology in the brain.

Following peripheral exposure, many transmissible spongiform encephalopathy (TSE) agents accumulate first in lymphoid tissues before spreading to the CNS (termed neuroinvasion) where they cause neurodegeneration. Early TSE agent accumulation upon follicular dendritic cells (FDCs) in lymphoid follicles appears critical for efficient neuroinvasion. Most clinical cases of variant Creutzfeldt-Jakob disease have occurred in young adults, although the reasons behind this apparent age-related susceptibility are uncertain. Host age has a significant influence on immune function. As FDC status and immune complex trapping is reduced in aged mice (600 days old), we hypothesized that this aging-related decline in FDC function might impair TSE pathogenesis. We show that coincident with the effects of host age on FDC status, the early TSE agent accumulation in the spleens of aged mice was significantly impaired. Furthermore, following peripheral exposure, none of the aged mice developed clinical TSE disease during their lifespans, although most mice displayed histopathological signs of TSE disease in their brains. Our data imply that the reduced status of FDCs in aged mice significantly impairs the early TSE agent accumulation in lymphoid tissues and subsequent neuroinvasion. Furthermore, the inefficient neuroinvasion in aged individuals may lead to significant levels of subclinical TSE disease in the population.

During transmissible spongiform encephalopathy (TSE) infections the accumulation of abnormal prion protein within the brain is often accompanied by severe neurodegeneration. Studies have implicated complement, including the membrane attack complex (MAC, C5b–C9), in inducing pathology in some neurodegenerative diseases. Recent studies show the MAC is localized on neurons in the brains of TSE patients implicating complement-mediated cell lysis in TSE neuropathology. To determine the role of the MAC in TSEs, we compared scrapie pathogenesis in C5-deficient and C5-sufficient mice. C5-deficient mice developed clinical scrapie with incubation periods similar to C5-sufficient mice. Furthermore, the severity of the neuropathology was not significantly different between C5-deficient and C5-sufficient mice. These data show that C5, and the MAC, are not involved in TSE pathogenesis.

Background : Transmissible spongiform encephalopathies (TSEs) are chronic infectious neurodegenerative diseases that are characterized by the accumulation in affected tissues of PrP Sc , an abnormal isoform of the host prion protein (PrP c ). Following peripheral exposure, PrP Sc usually accumulates on follicular dendritic cells (FDCS) in lymphoid tissues before neuroinvasion. Studies in mice have shown that TSE exposure through scarified skin is an effective means of transmission. Following inoculation via the skin, a functional immune system is critical for the transmission of scrapie to the brain as severe combined immunodeficiency (SCID) mice are refractory to infection. Until now, it was not known which components of the immune system are required for efficient scrapie neuroinvasion following skin scarification. Objective : To determine which cells are critical for the transmission of scrapie to the brain following inoculation via the skin. Methods : A chimeric mouse model was used, which had a mismatch in PrP c expression between FDCs and other bone marrow-derived cells within lymphoid tissues. These chimeric mice were challenged with scrapie by skin scarification to allow the separate roles of FDCs and lymphocytes in peripheral scrapie pathogenesis to be determined. Results : We show that mature FDCs are essential for the accumulation of scrapie within lymphoid tissues and the subsequent transmission of infection to the brain following TSE exposure by this route. Furthermore, we show that the accumulation of PrP Sc and infectivity in the spleen is independent of PrP expression by lymphocytes or other bone marrow-derived cells. Conclusion : Following inoculation with scrapie by skin scarification, replication in the spleen and subsequent neuroinvasion is critically dependent upon mature FDCs.

Prion diseases are sub-acute neurodegenerative diseases that affect humans and somedomestic and free-ranging animals. Infectious prion agents are considered to comprise solely ofabnormally folded isoforms of the cellular prion protein known as PrPSc. Pathology during priondisease is restricted to the central nervous system where it causes extensive neurodegeneration andultimately leads to the death of the host. The first half of this review provides a thorough accountof our understanding of the various ways in which PrPSc prions may spread between individualswithin a population, both horizontally and vertically. Many natural prion diseases are acquiredperipherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly alsovia the nasal cavity. Following peripheral exposure, some prions accumulate to high levels withinthe secondary lymphoid organs as they make their journey from the site of infection to the brain,a process termed neuroinvasion. The replication of PrPSc prions within secondary lymphoid organsis important for their efficient spread to the brain. The second half of this review describes the keytissues, cells and molecules which are involved in the propagation of PrPSc prions from peripheralsites of exposure (such as the lumen of the intestine) to the brain. This section also considers howadditional factors such as inflammation and aging might influence prion disease susceptibility.

Bovine spongiform encephalopathy, variant Creutzfeldt–Jakob disease (vCJD) and possibly also sheep scrapie are orally acquired transmissible spongiform encephalopathies (TSEs). TSE agents usually replicate in lymphoid tissues before they spread into the central nervous system. In mouse TSE models PrPc-expressing follicular dendritic cells (FDCs) resident in lymphoid germinal centres are essential for replication, and in their absence neuroinvasion is impaired. Disease-associated forms of PrP (PrPSc), a biochemical marker for TSE infection, also accumulate on FDCs in the lymphoid tissues of patients with vCJD and sheep with natural scrapie. TSE transport mechanisms between gut lumen and germinal centres are unknown. Migratory bone marrow-derived dendritic cells (DCs), entering the intestinal wall from blood, sample antigens from the gut lumen and carry them to mesenteric lymph nodes. Here we show that DCs acquire PrPSc in vitro, and transport intestinally administered PrPSc directly into lymphoid tissues in vivo. These studies suggest that DCs are a cellular bridge between the gut lumen and the lymphoid TSE replicative machinery.

Prion diseases are subacute neurodegenerative diseases that affect humans and animals. An abnormally folded isoform (PrP(Sc)) of the host cellular prion protein is considered to constitute the major, if not sole, component of the infectious prion. The occurrence of variant Creutzfeldt-Jakob disease in humans most likely arose due to consumption of food contaminated with bovine spongiform encephalopathy prions. The demonstration that some prion infections may have the capacity to transmit to other species, especially humans, has focused attention on the development of safe and effective vaccines against these invariably fatal and currently incurable diseases. Although much effort has been invested in the development of safe and effective anti-PrP vaccines, many important issues remain to be resolved.

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.

The transmissible spongiform encephalopathies are neurodegenerative disorders which include Creutzfeldt-Jakob disease in humans, and scrapie and bovine spongiform encephalopathy in animals. A major component of the infectious agent responsible for these diseases is considered to be a post-translationally modified form of a host-encoded glycoprotein PrPc, termed PrPSc. While PrPc is abundantly expressed in tissues of the central nervous system (CNS), little is known about its normal function. The expression of PrPc is not restricted to the CNS, as this protein can also be detected in the lymphoid tissues of mice and sheep. In this report we demonstrate that resting murine splenic lymphocytes express PrPc protein on their cell membranes. Furthermore, expression of PrPc was significantly enhanced following in vitro stimulation with the non-specific T-cell mitogen concanavalin A (Con A). Genetically engineered mice with an inactive PrPc gene (PrP-/- mice), were utilized to investigate the involvement of PrPc in lymphocyte activation. Experiments revealed that the Con A-induced proliferation of lymphocytes from PrP-/- mice was significantly reduced to approximately 50-80% that of wild-type (PrP+/+) mice 48 hr post-stimulation. These findings demonstrate an important role for PrPc in extra-neuronal tissues and suggest that PrPc is a lymphocyte surface molecule that participates in T-cell activation.

When I started my postdoctoral career in September 1995, fresh off the completion of my PhD thesis on the effects of African trypanosomes on the host immune system, I was relishing the opportunity to move to a new city, study a new pathogen, and begin my career as an immunologist. However, I was immediately surprised that my scientific friends and contemporaries, as well as members of the public, didn’t seem to share this enthusiasm. When I mentioned I was studying prions, or transmissible spongiform encephalopathies (TSEs), most people would simply respond, “Oh,” and continue whatever it was they were doing before they attempted to make small talk with a scientist. Before you do the same, I should quickly mention that prions are a group of pathogens that cause substantial damage to the brains of infected animals and humans. These diseases are currently untreatable, and they are unique in that they are caused by the simplest pathogens known to science. Prions are devoid of DNA, and they were proposed, by the Nobel Laureate Professor Stanley Prusiner, to be made up of just one molecule, the host-produced prion protein, which can mis-fold into the deadly, infectious prion particles. I remember attending a prize lecture given by Prof. Prusiner, in which he mentioned how luck can have a big role in bringing your work to prominence. Of course, to do so, one also has to be able to recognise the opportunities, however they may arise, and have the insight to run with them. I mention this because, within six months of starting my postdoctoral career, the United Kingdom Secretary for Health made a formal statement in the Houses of Parliament about the likely link between “mad cow disease” in cattle (bovine spongiform encephalopathy; BSE) and a new human prion disease (variant Creutzfeldt-Jakob disease; vCJD). The assumption was that consumption of BSE-contaminated food may have been the source of this new prion disease in humans. All the more concerning was that this new disease was predominantly observed in young people. Ever since that announcement, it was striking to note that now, when members of the public found out what I did for a living, they didn’t walk away! Everyone had heard about prions and had their own opinions about where they came from, whether we should eat beef, and what I, as a scientist, should be doing about it all. Importantly, I had the impression that the public understood why more research was critical. This alarming recognition that a prion disease could transmit from animals to humans brought with it many opportunities to pursue basic research into prions and prion diseases. During the intervening years, my research has been trying to answer the same simple question: How do prions spread from the gut to the brain? Addressing this issue is important because we may be able to use this information to design therapies to block the spread of prions to the brain and, in doing so, block prion disease and prevent the irreversible nerve damage it causes. Work from my own laboratory and many others around the world has made substantial progress in improving our understanding of this aspect of the disease process. Despite this, a reliable and effective therapy still eludes us. Approximately 229 cases of vCJD have been diagnosed worldwide, with the majority of these occurring in the UK. When considered in the context of other important human diseases, such as cancer and Alzheimer’s disease, the incidence of vCJD is fortunately very rare. One may therefore question why it was—and why it continues to be, in my opinion—necessary to invest in prion disease research. However, without the advances made in basic science over the years, this incidence could have been much higher. For example, data showing that prions appear to hijack cells in the body’s own immune system in order to establish disease led to the suggestion that white blood cells may harbour prions in an infected individual and might spread the disease to others via blood transfusions. Indeed, a small number of vCJD cases in the UK have since been linked to accidental transmission via infected whole blood. Based on the scientific evidence, the removal of white blood cells (leukodepletion) from blood for transfusion was introduced in the UK to reduce the possibility of accidental blood-borne prion transmission. This intervention may also have helped reduce the accidental spread of other blood-borne infectious diseases, such as viruses. Prions are also incredibly resistant to standard decontamination methods, and their potential to persist on the surfaces of surgical instruments used on infected appendixes, tonsils, etc. was highlighted. Measures have since been put in place to avoid the accidental transmission of prions via used surgical instruments. Not all basic research discoveries will inform policy makers as to how best to reduce the incidence of prion diseases in populations, or will translate into new therapies or diagnostic tests. However, prion disease research has often led to unexpected benefits elsewhere. Early on in my career, it was evident that the prions were exploiting a rare, under-studied, population of cells in the immune system termed follicular dendritic cells. While this in itself was particularly frustrating, since little was known of their biology and few tools were available to study them, this also provided an excellent opportunity to learn more. We now understand much more about the biology of this rare population of cells—not only as they relate to prion disease but also their importance to immune function and the impact aging has on them. While the relatively rare incidence of prion diseases may be a blessing, this does mean that, should a promising therapy or anti-prion vaccine be discovered, the economic returns are unlikely to be huge. Are other animal prion diseases (such as chronic wasting disease in deer and elk in North America, or novel strains of BSE and sheep scrapie) also able to transmit to humans? An important challenge will therefore be to engage with the pharmaceutical industry to encourage them to help to quickly turn our basic science advances into cheap, safe, and reliable anti-prion therapies for use in humans and domestic animals. Image 1 Neil A. Mabbott.

Many prion diseases are acquired by peripheral exposure, and skin lesions are an effective route of transmission. Following exposure, early prion replication, upon FDCs in the draining LN is obligatory for the spread of disease to the brain. However, the mechanism by which prions are conveyed to the draining LN is uncertain. Here, transgenic mice were used, in which langerin+ cells, including epidermal LCs and langerin+ classical DCs, were specifically depleted. These were used in parallel with transgenic mice, in which nonepidermal CD11c+ cells were specifically depleted. Our data show that prion pathogenesis, following exposure via skin scarification, occurred independently of LC and other langerin+ cells. However, the depletion of nonepidermal CD11c+ cells impaired the early accumulation of prions in the draining LN, implying a role for these cells in the propagation of prions from the skin. Therefore, together, these data suggest that the propagation of prions from the skin to the draining LN occurs via dermal classical DCs, independently of langerin+ cells.

Prion diseases are characterised by the accumulation of PrPSc, an abnormally folded isoform of the cellular prion protein (PrPC), in affected tissues. Following peripheral exposure high levels of prion-specific PrPSc accumulate first upon follicular dendritic cells (FDC) in lymphoid tissues before spreading to the CNS. Expression of PrPC is mandatory for cells to sustain prion infection and FDC appear to express high levels. However, whether FDC actively replicate prions or simply acquire them from other infected cells is uncertain. In the attempts to-date to establish the role of FDC in prion pathogenesis it was not possible to dissociate the Prnp expression of FDC from that of the nervous system and all other non-haematopoietic lineages. This is important as FDC may simply acquire prions after synthesis by other infected cells. To establish the role of FDC in prion pathogenesis transgenic mice were created in which PrPC expression was specifically “switched on” or “off” only on FDC. We show that PrPC-expression only on FDC is sufficient to sustain prion replication in the spleen. Furthermore, prion replication is blocked in the spleen when PrPC-expression is specifically ablated only on FDC. These data definitively demonstrate that FDC are the essential sites of prion replication in lymphoid tissues. The demonstration that Prnp-ablation only on FDC blocked splenic prion accumulation without apparent consequences for FDC status represents a novel opportunity to prevent neuroinvasion by modulation of PrPC expression on FDC., Author Summary Prion diseases are infectious neurological disorders and are considered to be caused by an abnormally folded infectious protein termed PrPSc. Soon after infection prions accumulate first upon follicular dendritic cells (FDC) in lymphoid tissues before spreading to the brain where they cause damage to nerve cells. Cells must express the normal cellular prion protein PrPC to become infected with prions. However, whether FDC are infected with prions or simply acquire them from other infected cells is unknown. To establish the role of FDC in prion disease PrPC expression was specifically “switched on” or “off” only on FDC. We show that PrPC-expressing FDC alone are sufficient to sustain prion replication in the spleen. Furthermore, prion replication is blocked in the spleen when PrPC-expression is switched off only on FDC. These data definitively demonstrate that FDC are the essential sites of prion replication in lymphoid tissues.

Following peripheral exposure to transmissible spongiform encephalopathies (TSEs), infectivity usually accumulates in lymphoid tissues before neuroinvasion. The host prion protein (PrPc) is critical for TSE agent replication and accumulates as an abnormal, detergent insoluble, relatively proteinase-resistant isoform (PrPSc) in diseased tissues1, 2. Early PrPSc accumulation takes place on follicular dendritic cells (FDCs) within germinal centers in lymphoid tissues of patients with variant Creutzfeldt−Jakob disease3 (vCJD), sheep with natural scrapie4 or rodents following experimental peripheral infection with scrapie5, 6, 7. In mouse scrapie models, the absence of FDCs blocks scrapie replication and PrPSc accumulation in the spleen, and neuroinvasion is significantly impaired6, 7, 8, 9. The mechanisms by which the TSE agent initially localizes to lymphoid follicles and interacts with FDCs are unknown. Antigens are trapped and retained on the surface of FDCs through interactions between complement and cellular complement receptors10, 11. Here we show that in mice, both temporary depletion of complement component C3 or genetic deficiency of C1q significantly delays the onset of disease following peripheral infection, and reduces the early accumulation of PrPSc in the spleen. Thus, in the early stages of infection, C3 and perhaps C1q contribute to the localization of TSE infectivity in lymphoid tissue and may be therapeutic targets.

Transmissible spongiform encephalopathies (TSEs) or prion diseases are infectious neurological disorders of man and animals, characterised by abnormal disease-associated prion protein (PrP(d)) accumulations in the brain and lymphoreticular system (LRS). Prior to neuroinvasion, TSE agents often accumulate to high levels within the LRS, apparently without affecting immune function. However, our analysis of scrapie-affected sheep shows that PrP(d) accumulations within the LRS are associated with morphological changes to follicular dendritic cells (FDCs) and tingible body macrophages (TBMs). Here we examined FDCs and TBMs in the mesenteric lymph nodes (MLNs) of scrapie-affected mice by light and electron microscopy. In MLNs from uninfected mice, FDCs could be morphologically categorised into immature, mature and regressing forms. However, in scrapie-affected MLNs this maturation cycle was adversely affected. FDCs characteristically trap and retain immune complexes on their surfaces, which they display to B-lymphocytes. In scrapie-affected MLNs, some FDCs were found where areas of normal and abnormal immune complex retention occurred side by side. The latter co-localised with PrP(d) plasmalemmal accumulations. Our data suggest this previously unrecognised morphology represents the initial stage of an abnormal FDC maturation cycle. Alterations to the FDCs included PrP(d) accumulation, abnormal cell membrane ubiquitin and excess immunoglobulin accumulation. Regressing FDCs, in contrast, appeared to lose their membrane-attached PrP(d). Together, these data suggest that TSE infection adversely affects the maturation and regression cycle of FDCs, and that PrP(d) accumulation is causally linked to the abnormal pathology observed. We therefore support the hypothesis that TSEs cause an abnormality in immune function.

Peripheral accumulation of abnormal prion protein (PrP) in variant Creutzfeldt-Jakob disease and some animal models of transmissible spongiform encephalopathies (TSEs) may occur in the lymphoreticular system. Within the lymphoid tissues, abnormal PrP accumulation occurs on follicular dendritic cells (FDCs). Clusterin (apolipoprotein J) has been recognized as one of the molecules associated with PrP in TSEs, and clusterin expression is increased in the central nervous system where abnormal PrP deposition has occurred. We therefore examined peripheral clusterin expression in the context of PrP accumulation on FDCs in a range of human and experimental TSEs. PrP was detected immunohistochemically on tissue sections using a novel highly sensitive method involving detergent autoclaving pretreatment. A dendritic network pattern of clusterin immunoreactivity in lymphoid follicles was observed in association with the abnormal PrP on FDCs. The increased clusterin immunoreactivity appeared to correlate with the extent of PrP deposition, irrespective of the pathogen strains, host mouse strains or various immune modifications. The observed co-localization and correlative expression of these proteins suggested that clusterin might be directly associated with abnormal PrP. Indeed, clusterin immunoreactivity in association with PrP was retained after FDC depletion. Together these data suggest that clusterin may act as a chaperone-like molecule for PrP and play an important role in TSE pathogenesis.

Many natural infections with transmissible spongiform encephalopathy (TSE) agents are likely to be acquired peripherally for example, following ingestion of contaminated feed. Following peripheral exposure TSE agents accumulate in lymphoid tissues before spreading to the brain. Many studies have attempted to identify the cells and their components that are required for the delivery of the TSE agent from the site of inoculation to the brain, a process termed neuroinvasion. In the lymphoid tissues of TSE-affected hosts these agents, as identified by disease-specific prion protein accumulations, usually accumulate on follicular dendritic cells (FDCs). Studies of mouse TSE models have shown that mature FDCs are critical for replication of infection in lymphoid tissues and subsequent neuroinvasion. Although examples of FDC-independent neuroinvasion have been described, treatments that interfere with the integrity or function of FDCs reduce TSE susceptibility by blocking the spread of disease to the brain. For example, temporary depletion of FDCs before oral inoculation with TSE agents blocks the accumulation of disease-specific PrP in Peyer’s patches and mesenteric lymph nodes, and prevents neuroinvasion.

Prion diseases are neurodegenerative conditions that cause extensive damage to nerve cells within the brain and can be fatal. Some prion disease agents accumulate first in lymphoid tissues, as they make their journey from the site of infection, such as the gut, to the brain. Studies in mouse models have shown that this accumulation is obligatory for the efficient delivery of prions to the brain. Indeed, if the accumulation of prions in lymphoid tissues is blocked, disease susceptibility is reduced. Therefore, the identification of the cells and molecules that are involved in the delivery of prions to the brain might identify targets for therapeutic intervention. This review describes the current understanding of the mechanisms involved in the delivery of prions to the brain.

Transmissible spongiform encephalopathies (TSEs) are a group of subacute infectious neurodegenerative diseases that are characterized by the accumulation in affected tissues of PrP(Sc), an abnormal isoform of the host prion protein (PrPc). Following peripheral exposure, TSE infectivity and PrP(Sc) usually accumulate in lymphoid tissues prior to neuroinvasion. Studies in mice have shown that exposure through scarified skin is an effective means of TSE transmission. Following inoculation via the skin, a functional immune system is critical for the transmission of TSEs to the brain, but until now, it has not been known which components of the immune system are required for efficient neuroinvasion. Temporary dedifferentiation of follicular dendritic cells (FDCs) by treatment with an inhibitor of the lymphotoxin-beta receptor signalling pathway (LTbetaR-Ig) 3 days before or 14 days after inoculation via the skin, blocked the early accumulation of PrP(Sc) and TSE infectivity within the draining lymph node. Furthermore, in the temporary absence of FDCs before inoculation, disease susceptibility was reduced and survival time significantly extended. Treatment with LTbetaR-Ig 14 days after TSE inoculation also significantly extended the disease incubation period. However, treatment 42 days after inoculation did not affect disease susceptibility or survival time, suggesting that the infection may have already have spread to the nervous system. Together these data show that FDCs are essential for the accumulation of PrP(Sc) and infectivity within lymphoid tissues and subsequent neuroinvasion following TSE exposure via the skin.

The complement system is an essential component of the innate immune system that aids the recognition and destruction of pathogens. Despite the potent cytolytic activity of this system, some pathogens have turned interactions with complement to their advantage. Transmissible spongiform encephalopathies (TSEs), or prion diseases, also seem to use the complement system to facilitate target cell infection in lymphoid tissues. TSEs are associated with the accumulation of disease-specific prion protein in the brain, which is accompanied by progressive neurodegeneration. Unregulated activation of complement can cause host tissue damage, as observed in some neurodegenerative disorders, and active complement components have been detected in the brains of hosts with TSEs. Whether complement inhibitors will be useful to treat TSEs remains to be determined.

Transmissible spongiform encephalopathies (TSEs) are often acquired peripherally, for example by ingestion or iatrogenic exposure. After entry, TSE agents, as identified by disease-specific protein accumulation, usually accumulate on follicular dendritic cells (FDCs) in lymphoid tissues long before infection spreads to the brain. Neuroinvasion of TSE agents is significantly impaired in the absence of mature FDCs. Treatments that interfere with the integrity or function of FDCs extend survival time by blocking replication in lymphoid tissues and spread to the brain. The identification of FDCs as critical for TSE pathogenesis provides a cellular target to which therapies can be specifically directed.

Infection of tissues outside the nervous system occurs in a number of prion diseases, but sporadic Creutzfeldt-Jakob disease in humans has not been considered one of them. A clinical study of Swiss patients with sporadic disease reconsiders this assumption, and a second study examines the spread of infection between follicular dendritic cells and nerves.

After oral exposure, prions are thought to enter Peyer's patches via M cells and accumulate first upon follicular dendritic cells (FDCs) before spreading to the nervous system. How prions are actually initially acquired from the gut lumen is not known. Using high-resolution immunofluorescence and cryo-immunogold electron microscopy, we report the trafficking of the prion protein (PrP) toward Peyer's patches of wild-type and PrP-deficient mice. PrP was transiently detectable at 1 day post feeding (dpf) within large multivesicular LAMP1-positive endosomes of enterocytes in the follicle-associated epithelium (FAE) and at much lower levels within M cells. Subsequently, PrP was detected on vesicles in the late endosomal compartments of macrophages in the subepithelial dome. At 7–21 dpf, increased PrP labelling was observed on the plasma membranes of FDCs in germinal centres of Peyer's patches from wild-type mice only, identifying FDCs as the first sites of PrP conversion and replication. Detection of PrP on extracellular vesicles displaying FAE enterocyte-derived A33 protein implied transport towards FDCs in association with FAE-derived vesicles. By 21 dpf, PrP was observed on the plasma membranes of neurons within neighbouring myenteric plexi. Together, these data identify a novel potential M cell-independent mechanism for prion transport, mediated by FAE enterocytes, which acts to initiate conversion and replication upon FDCs and subsequent infection of enteric nerves., Author Summary Prion diseases are orally transmissible, but how the abnormally folded isoform of the prion protein (PrPSc) transits from the gastrointestinal tract to infect neural tissues is not known. Here we demonstrate that in contrast to the current literature, PrPSc enters Peyer's patches primarily through specialised enterocytes with much lower levels trafficking through M cells. Proteins from homogenized PrPSc infected brain tissue are transcytosed across the follicle-associated epithelium and delivered to macrophages and follicular dendritic cells, which appear to serve as the primary site of PrP conversion and replication following oral exposure to PrPSc before infecting the enteric nerves.

Although the transmissible spongiform encephalopathies (TSEs) are neurological diseases, TSE agents usually replicate in lymphoid tissues long before infection spreads to the brain. Studies of a mouse scrapie TSE model have demonstrated that mature follicular dendritic cells (FDCs) expressing the host prion protein (PrPc) are essential for replication of infection in lymphoid tissues1 and subsequent spread of infection to the nervous system2. Abnormal forms of PrP (PrPSc) accumulate on FDCs in scrapie-infected mice2, 3, and in patients with variant Creutzfeldt-Jakob disease (ref. 4). Here we confirm that, as predicted, treatment that interferes with the integrity of FDCs also interferes with TSE pathogenesis.