Prion uptake in the gut: identification of the first uptake and replication sites

Immunology of Prion Protein and Prions

Many natural prion diseases are acquired peripherally, such as following the oral consumption of contaminated food or pasture. After peripheral exposure many prion isolates initially accumulate to high levels within the host's secondary lymphoid tissues. The replication of prions within these tissues is essential for their efficient spread to the brain where they ultimately cause neurodegeneration. This chapter describes our current understanding of the critical tissues, cells, and molecules which the prions exploit to mediate their efficient propagation from the site of exposure (such as the intestine) to the brain. Interactions between the immune system and prions are not only restricted to the secondary lymphoid tissues. Therefore, an account of how the activation status of the microglial in the brain can also influence progression of prion disease pathogenesis is provided. Prion disease susceptibility may also be influenced by additional factors such as chronic inflammation, coinfection with other pathogens, and aging. Finally, the potential for immunotherapy to provide a means of safe and effective prophylactic or therapeutic intervention in these currently untreatable diseases is considered.

Oral Prion Disease Pathogenesis Is Impeded in the Specific Absence of CXCR5-Expressing Dendritic Cells

After oral exposure, the early replication of certain prion strains upon stromal cell-derived follicular dendritic cells (FDC) in the Peyer's patches in the small intestine is essential for the efficient spread of disease to the brain. However, little is known of how prions are initially conveyed from the gut lumen to establish infection on FDC. Our previous data suggest that mononuclear phagocytes such as CD11c⁺ conventional dendritic cells play an important role in the initial propagation of prions from the gut lumen into Peyer's patches. However, whether these cells conveyed orally acquired prions toward FDC within Peyer's patches was not known. The chemokine CXCL13 is expressed by FDC and follicular stromal cells and modulates the homing of CXCR5-expressing cells toward the FDC-containing B cell follicles. Here, novel compound transgenic mice were created in which a CXCR5 deficiency was specifically restricted to CD11c⁺ cells. These mice were used to determine whether CXCR5-expressing conventional dendritic cells propagate prions toward FDC after oral exposure. Our data show that in the specific absence of CXCR5-expressing conventional dendritic cells the early accumulation of prions upon FDC in Peyer's patches and the spleen was impaired, and disease susceptibility significantly reduced. These data suggest that CXCR5-expressing conventional dendritic cells play an important role in the efficient propagation of orally administered prions toward FDC within Peyer's patches in order to establish host infection.IMPORTANCE Many natural prion diseases are acquired by oral consumption of contaminated food or pasture. Once the prions reach the brain they cause extensive neurodegeneration, which ultimately leads to death. In order for the prions to efficiently spread from the gut to the brain, they first replicate upon follicular dendritic cells within intestinal Peyer's patches. How the prions are first delivered to follicular dendritic cells to establish infection was unknown. Understanding this process is important since treatments which prevent prions from infecting follicular dendritic cells can block their spread to the brain. We created mice in which mobile conventional dendritic cells were unable to migrate toward follicular dendritic cells. In these mice the early accumulation of prions on follicular dendritic cells was impaired and oral prion disease susceptibility was reduced. This suggests that prions exploit conventional dendritic cells to facilitate their initial delivery toward follicular dendritic cells to establish host infection.

Pathways of Prion Spread during Early Chronic Wasting Disease in Deer

Among prion infections, two scenarios of prion spread are generally observed: (i) early lymphoid tissue replication or (ii) direct neuroinvasion without substantial antecedent lymphoid amplification. In nature, cervids are infected with chronic wasting disease (CWD) prions by oral and nasal mucosal exposure, and studies of early CWD pathogenesis have implicated pharyngeal lymphoid tissue as the earliest sites of prion accumulation. However, knowledge of chronological events in prion spread during early infection remains incomplete. To investigate this knowledge gap in early CWD pathogenesis, we exposed white-tailed deer to CWD prions by mucosal routes and performed serial necropsies to assess PrP^CWD tissue distribution by real-time quaking-induced conversion (RT-QuIC) and tyramide signal amplification immunohistochemistry (TSA-IHC). Although PrP^CWD was not detected by either method in the initial days (1 and 3) postexposure, we observed PrP^CWD seeding activity and follicular immunoreactivity in oropharyngeal lymphoid tissues at 1 and 2 months postexposure (MPE). At 3 MPE, PrP^CWD replication had expanded to all systemic lymphoid tissues. By 4 MPE, the PrP^CWD burden in all lymphoid tissues had increased and approached levels observed in terminal disease, yet there was no evidence of nervous system invasion. These results indicate the first site of CWD prion entry is in the oropharynx, and the initial phase of prion amplification occurs in the oropharyngeal lymphoid tissues followed by rapid dissemination to systemic lymphoid tissues. This lymphoid replication phase appears to precede neuroinvasion.IMPORTANCE Chronic wasting disease (CWD) is a universally fatal transmissible spongiform encephalopathy affecting cervids, and natural infection occurs through oral and nasal mucosal exposure to infectious prions. Terminal disease is characterized by PrP^CWD accumulation in the brain and lymphoid tissues of affected animals. However, the initial sites of prion accumulation and pathways of prion spread during early CWD infection remain unknown. To investigate the chronological events of early prion pathogenesis, we exposed deer to CWD prions and monitored the tissue distribution of PrP^CWD over the first 4 months of infection. We show CWD uptake occurs in the oropharynx with initial prion replication in the draining oropharyngeal lymphoid tissues, rapidly followed by dissemination to systemic lymphoid tissues without evidence of neuroinvasion. These data highlight the two phases of CWD infection: a robust prion amplification in systemic lymphoid tissues prior to neuroinvasion and establishment of a carrier state.

Sialylation Controls Prion Fate in Vivo

Prions or PrPSc are proteinaceous infectious agents that consist of misfolded, self-replicating states of a sialoglycoprotein called the prion protein or PrPC The current work tests a new hypothesis that sialylation determines the fate of prions in an organism. To begin, we produced control PrPSc from PrPC using protein misfolding cyclic amplification with beads (PMCAb), and also generated PrPSc with reduced sialylation levels using the same method but with partially desialylated PrPC as a substrate (dsPMCAb). Syrian hamsters were inoculated intraperitoneally with brain-derived PrPSc or PrPSc produced in PMCAb or dsPMCAb and then monitored for disease. Animals inoculated with brain- or PMCAb-derived PrPSc developed prion disease, whereas administration of dsPMCAb-derived PrPSc with reduced sialylation did not cause prion disease. Animals inoculated with dsPMCAb-derived material were not subclinical carriers of scrapie, as no PrPSc was detected in brains or spleen of these animals by either Western blotting or after amplification by serial PMCAb. In subsequent experiments, trafficking of brain-, PMCAb-, and dsPMCAb-derived PrPSc to secondary lymphoid organs was monitored in wild type mice. PrPSc sialylation was found to be critical for effective trafficking of PrPSc to secondary lymphoid organs. By 6 hours after inoculation, brain- and PMCAb-derived PrPSc were found in spleen and lymph nodes, whereas dsPMCAb-derived PrPSc was found predominantly in liver. This study demonstrates that the outcome of prion transmission to a wild type host is determined by the sialylation status of the inoculated PrPSc Furthermore, this work suggests that the sialylation status of PrPSc plays an important role in prion lymphotropism.

Isolation of Exosomes and Microvesicles from Cell Culture Systems to Study Prion Transmission

Extracellular vesicles (EVs) are composed of microvesicles and exosomes. Exosomes are small membrane vesicles (40-120 nm sized) of endosomal origin released in the extracellular medium from cells when multivesicular bodies fuse with the plasma membrane, whereas microvesicles (i.e., shedding vesicles, 100 nm to 1 μm sized) bud from the plasma membrane. Exosomes and microvesicles carry functional proteins and nucleic acids (especially mRNAs and microRNAs) that can be transferred to surrounding cells and tissues and can impact multiple dimensions of the cellular life. Most of the cells, if not all, from neuronal to immune cells, release exosomes and microvesicles in the extracellular medium, and all biological fluids including blood (serum/plasma), urine, cerebrospinal fluid, and saliva contain EVs.Prion-infected cultured cells are known to secrete infectivity into their environment. We characterized this cell-free form of prions and showed that infectivity was associated with exosomes. Since exosomes are produced by a variety of cells, including cells that actively accumulate prions, they could be a vehicle for infectivity in body fluids and could participate to the dissemination of prions in the organism. In addition, such infectious exosomes also represent a natural, simple, biological material to get key information on the abnormal PrP forms associated with infectivity.In this chapter, we describe first a method that allows exosomes and microvesicles isolation from prion-infected cell cultures and in a second time the strategies to characterize the prions containing exosomes and their ability to disseminate the prion agent.

Increased Abundance of M Cells in the Gut Epithelium Dramatically Enhances Oral Prion Disease Susceptibility

Many natural prion diseases of humans and animals are considered to be acquired through oral consumption of contaminated food or pasture. Determining the route by which prions establish host infection will identify the important factors that influence oral prion disease susceptibility and to which intervention strategies can be developed. After exposure, the early accumulation and replication of prions within small intestinal Peyer's patches is essential for the efficient spread of disease to the brain. To replicate within Peyer's patches, the prions must first cross the gut epithelium. M cells are specialised epithelial cells within the epithelia covering Peyer's patches that transcytose particulate antigens and microorganisms. M cell-development is dependent upon RANKL-RANK-signalling, and mice in which RANK is deleted only in the gut epithelium completely lack M cells. In the specific absence of M cells in these mice, the accumulation of prions within Peyer's patches and the spread of disease to the brain was blocked, demonstrating a critical role for M cells in the initial transfer of prions across the gut epithelium in order to establish host infection. Since pathogens, inflammatory stimuli and aging can modify M cell-density in the gut, these factors may also influence oral prion disease susceptibility. Mice were therefore treated with RANKL to enhance M cell density in the gut. We show that prion uptake from the gut lumen was enhanced in RANKL-treated mice, resulting in shortened survival times and increased disease susceptibility, equivalent to a 10-fold higher infectious titre of prions. Together these data demonstrate that M cells are the critical gatekeepers of oral prion infection, whose density in the gut epithelium directly limits or enhances disease susceptibility. Our data suggest that factors which alter M cell-density in the gut epithelium may be important risk factors which influence host susceptibility to orally acquired prion diseases.

Early and Non-Invasive Detection of Chronic Wasting Disease Prions in Elk Feces by Real-Time Quaking Induced Conversion

Chronic wasting disease (CWD) is a fatal prion disease of wild and captive cervids in North America. Prions are infectious agents composed of a misfolded version of a host-encoded protein, termed PrP^Sc. Infected cervids excrete and secrete prions, contributing to lateral transmission. Geographical distribution is expanding and case numbers in wild cervids are increasing. Recently, the first European cases of CWD have been reported in a wild reindeer and two moose from Norway. Therefore, methods to detect the infection early in the incubation time using easily available samples are desirable to facilitate effective disease management. We have adapted the real-time quaking induced conversion (RT-QuIC) assay, a sensitive in vitro prion amplification method, for pre-clinical detection of prion seeding activity in elk feces. Testing fecal samples from orally inoculated elk taken at various time points post infection revealed early shedding and detectable prion seeding activity throughout the disease course. Early shedding was also found in two elk encoding a PrP genotype associated with reduced susceptibility for CWD. In summary, we suggest that detection of CWD prions in feces by RT-QuIC may become a useful tool to support CWD surveillance in wild and captive cervids. The finding of early shedding independent of the elk’s prion protein genotype raises the question whether prolonged survival is beneficial, considering accumulation of environmental prions and its contribution to CWD transmission upon extended duration of shedding.

Enteric glial cells: An emerging key player in intestinal homeostasis modulation under physiological and pathological conditions

The intestine contains multiple components including epithelial cells, microbiome as well as various neuroendocrine pathways, all of which are essential for maintaining dynamic mucosal homeostasis through complex interactions among different components in the gastrointestinal tract. Beyond the basic neurosupportive and neurotrophic effects, growing evidence reveals the key role of enteric glial cells (EGCs) in the modulation of bowel movement, nutrient absorption and secretion, intestinal immunity as well as barrier function. As well, abnormally activated EGCs are believed to be a vital player in the pathogenesis of a variety of diseases including inflammatory bowel disease, intestinal barrier dysfunction and infections. Here we provide a brief overview of recent research progress about the precise role and the molecule mechanisms of EGCs in modulating intestinal homeostasis, and highlight the critical role of EGC in various intestinal diseases.

The bowel and beyond: the enteric nervous system in neurological disorders

The enteric nervous system (ENS) is large, complex and uniquely able to orchestrate gastrointestinal behaviour independently of the central nervous system (CNS). An intact ENS is essential for life and ENS dysfunction is often linked to digestive disorders. The part the ENS plays in neurological disorders, as a portal or participant, has also become increasingly evident. ENS structure and neurochemistry resemble that of the CNS, therefore pathogenic mechanisms that give rise to CNS disorders might also lead to ENS dysfunction, and nerves that interconnect the ENS and CNS can be conduits for disease spread. We review evidence for ENS dysfunction in the aetiopathogenesis of autism spectrum disorder, amyotrophic lateral sclerosis, transmissible spongiform encephalopathies, Parkinson disease and Alzheimer disease. Animal models suggest that common pathophysiological mechanisms account for the frequency of gastrointestinal comorbidity in these conditions. Moreover, the neurotropic pathogen, varicella zoster virus (VZV), unexpectedly establishes latency in enteric and other autonomic neurons that do not innervate skin. VZV reactivation in these neurons produces no rash and is therefore a clandestine cause of gastrointestinal disease, meningitis and strokes. The gut-brain alliance has raised consciousness as a contributor to health, but a gut-brain axis that contributes to disease merits equal attention.

Prions efficiently cross the intestinal barrier after oral administration: Study of the bioavailability, and cellular and tissue distribution in vivo

Natural forms of prion diseases frequently originate by oral (p.o.) infection. However, quantitative information on the gastro-intestinal (GI) absorption of prions (i.e. the bioavailability and subsequent biodistribution) is mostly unknown. The main goal of this study was to evaluate the fate of prions after oral administration, using highly purified radiolabeled PrP(Sc). The results showed a bi-phasic reduction of PrP(Sc) with time in the GI, except for the ileum and colon which showed sustained increases peaking at 3-6 hr, respectively. Plasma and whole blood (125)I-PrP(Sc) reached maximal levels by 30 min and 3 hr, respectively, and blood levels were constantly higher than plasma. Upon crossing the GI-tract (125)I-PrP(Sc) became associated to blood cells, suggesting that binding to cells decreased the biological clearance of the agent. Size-exclusion chromatography revealed that oligomeric (125)I-PrP(Sc) were transported from the intestinal tract, and protein misfolding cyclic amplification showed that PrP(Sc) in organs and blood retained the typical prion self-replicating ability. Pharmacokinetic analysis found the oral bioavailability of (125)I-PrP(Sc) to be 33.6%. Interestingly, (125)I-PrP(Sc) reached the brain in a quantity equivalent to the minimum amount needed to initiate prion disease. Our findings provide a comprehensive and quantitative study of the fate of prions upon oral infection.

Intra-host mathematical model of chronic wasting disease dynamics in deer (Odocoileus)

Bioassays of native cervid hosts have established the presence of infectious chronic wasting disease (CWD) prions in saliva, blood, urine, and feces of clinically diseased and pre-clinical infected deer. The intra-host trafficking of prions from the time of initial infection to shedding has been less well defined. We created a discrete-time compartmentalized model to simulate the misfolding catalysis, trafficking, and shedding of infectious prions throughout the organ systems of CWD-infected cervids. Using parameter values derived from experimental infections of North American deer (Odocoileus spp.), the exponential-based model predicts prion deposition over time with: 1) nervous tissues containing the highest deposition of prions at 20 months post-infection and 2) excreted fluids containing low levels of prions throughout infection with the highest numbers of prions predicted to be shed in saliva and feces (as high as 10 lethal doses (1.34 × 10²⁹ prions) in 11–15 months). These findings are comparable to prion deposition described in literature as assayed by conventional and ultrasensitive amplification assays. The comparison of our model to published data suggests that highly sensitive assays (sPMCA, RT-QuIC, and bioassay) are appropriate for early prion detection in bodily fluids and secretions. The model provides a view of intra-host prion catalysis leading to pre-clinical shedding and provides a framework for continued development of antemortem diagnostic methods.

Multifaceted Role of Sialylation in Prion Diseases

Mammalian prion or PrP(Sc) is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of a sialoglycoprotein called the prion protein, or PrP(C). Sialylation of the prion protein N-linked glycans was discovered more than 30 years ago, yet the role of sialylation in prion pathogenesis remains poorly understood. Recent years have witnessed extraordinary growth in interest in sialylation and established a critical role for sialic acids in host invasion and host-pathogen interactions. This review article summarizes current knowledge on the role of sialylation of the prion protein in prion diseases. First, we discuss the correlation between sialylation of PrP(Sc) glycans and prion infectivity and describe the factors that control sialylation of PrP(Sc). Second, we explain how glycan sialylation contributes to the prion replication barrier, defines strain-specific glycoform ratios, and imposes constraints for PrP(Sc) structure. Third, several topics, including a possible role for sialylation in animal-to-human prion transmission, prion lymphotropism, toxicity, strain interference, and normal function of PrP(C), are critically reviewed. Finally, a metabolic hypothesis on the role of sialylation in the etiology of sporadic prion diseases is proposed.

The influence of the commensal and pathogenic gut microbiota on prion disease pathogenesis

Prion diseases are a unique group of transmissible, chronic, neurodegenerative disorders. Following peripheral exposure (e.g. oral), prions often accumulate first within the secondary lymphoid tissues before they infect the central nervous system (CNS). Prion replication within secondary lymphoid tissues is crucial for the efficient spread of disease to the CNS. Once within the CNS, the responses of innate immune cells within it can have a significant influence on neurodegeneration and disease progression. Recently, there have been substantial advances in our understanding of how cross-talk between the host and the vast community of commensal microorganisms present at barrier surfaces such as the gut influences the development and regulation of the host's immune system. These effects are evident not only in the mucosal immune system in the gut, but also in the CNS. The actions of this microbial community (the microbiota) have many important beneficial effects on host health, from metabolism of nutrients and regulation of host development to protection from pathogen infection. However, the microbiota can also have detrimental effects in some circumstances. In this review we discuss the many and varied interactions between prions, the host and the gut microbiota. Particular emphasis is given to the ways by which changes to the composition of the commensal gut microbiota or congruent pathogen infection may influence prion disease pathogenesis and/or disease susceptibility. Understanding how these factors influence prion pathogenesis and disease susceptibility is important for assessing the risk to infection and the design of novel opportunities for therapeutic intervention.

The Priority position paper: Protecting Europe's food chain from prions

Bovine spongiform encephalopathy (BSE) created a global European crisis in the 1980s and 90s, with very serious health and economic implications. Classical BSE now appears to be under control, to a great extent as a result of a global research effort that identified the sources of prions in meat and bone meal (MBM) and developed new animal-testing tools that guided policy. Priority ( www.prionpriority.eu ) was a European Union (EU) Framework Program 7 (FP7)-funded project through which 21 European research institutions and small and medium enterprises (SMEs) joined efforts between 2009 and 2014, to conduct coordinated basic and applied research on prions and prion diseases. At the end of the project, the Priority consortium drafted a position paper ( www.prionpriority.eu/Priority position paper) with its main conclusions. In the present opinion paper, we summarize these conclusions. With respect to the issue of re-introducing ruminant protein into the feed-chain, our opinion is that sustaining an absolute ban on feeding ruminant protein to ruminants is essential. In particular, the spread and impact of non-classical forms of scrapie and BSE in ruminants is not fully understood and the risks cannot be estimated. Atypical prion agents will probably continue to represent the dominant form of prion diseases in the near future in Europe. Atypical L-type BSE has clear zoonotic potential, as demonstrated in experimental models. Similarly, there are now data indicating that the atypical scrapie agent can cross various species barriers. More epidemiological data from large cohorts are necessary to reach any conclusion on the impact of its transmissibility on public health. Re-evaluations of safety precautions may become necessary depending on the outcome of these studies. Intensified searching for molecular determinants of the species barrier is recommended, since this barrier is key for important policy areas and risk assessment. Understanding the structural basis for strains and the basis for adaptation of a strain to a new host will require continued fundamental research, also needed to understand mechanisms of prion transmission, replication and how they cause nervous system dysfunction and death. Early detection of prion infection, ideally at a preclinical stage, also remains crucial for development of effective treatment strategies.

Peyer's patches: organizing B-cell responses at the intestinal frontier

Secondary lymphoid tissues share the important function of bringing together antigens and rare antigen-specific lymphocytes to foster induction of adaptive immune responses. Peyer's patches (PPs) are unique compared to other secondary lymphoid tissues in their continual exposure to an enormous diversity of microbiome- and food-derived antigens and in the types of pathogens they encounter. Antigens are delivered to PPs by specialized microfold (M) epithelial cells and they may be captured and presented by resident dendritic cells (DCs). In accord with their state of chronic microbial antigen exposure, PPs exhibit continual germinal center (GC) activity. These GCs not only contribute to the generation of B cells and plasma cells producing somatically mutated gut antigen-specific IgA antibodies but have also been suggested to support non-specific antigen diversification of the B-cell repertoire. Here, we review current understanding of how PPs foster B-cell encounters with antigen, how they favor isotype switching to the secretory IgA isotype, and how their GC responses may uniquely contribute to mucosal immunity.

M-Cells Contribute to the Entry of an Oral Vaccine but Are Not Essential for the Subsequent Induction of Protective Immunity against Francisella tularensis

M-cells (microfold cells) are thought to be a primary conduit of intestinal antigen trafficking. Using an established neutralizing anti-RANKL (Receptor Activator of NF-κB Ligand) antibody treatment to transiently deplete M-cells in vivo, we sought to determine whether intestinal M-cells were required for the effective induction of protective immunity following oral vaccination with ΔiglB (a defined live attenuated Francisella novicida mutant). M-cell depleted, ΔiglB-vaccinated mice exhibited increased (but not significant) morbidity and mortality following a subsequent homotypic or heterotypic pulmonary F. tularensis challenge. No significant differences in splenic IFN-γ, IL-2, or IL-17 or serum antibody (IgG1, IgG2a, IgA) production were observed compared to non-depleted, ΔiglB-vaccinated animals suggesting complementary mechanisms for ΔiglB entry. Thus, we examined other possible routes of gastrointestinal antigen sampling following oral vaccination and found that ΔiglB co-localized to villus goblet cells and enterocytes. These results provide insight into the role of M-cells and complementary pathways in intestinal antigen trafficking that may be involved in the generation of optimal immunity following oral vaccination.

Enteric Glial Cells: A New Frontier in Neurogastroenterology and Clinical Target for Inflammatory Bowel Diseases

The word "glia" is derived from the Greek word "γλoια," glue of the enteric nervous system, and for many years, enteric glial cells (EGCs) were believed to provide mainly structural support. However, EGCs as astrocytes in the central nervous system may serve a much more vital and active role in the enteric nervous system, and in homeostatic regulation of gastrointestinal functions. The emphasis of this review will be on emerging concepts supported by basic, translational, and/or clinical studies, implicating EGCs in neuron-to-glial (neuroglial) communication, motility, interactions with other cells in the gut microenvironment, infection, and inflammatory bowel diseases. The concept of the "reactive glial phenotype" is explored as it relates to inflammatory bowel diseases, bacterial and viral infections, postoperative ileus, functional gastrointestinal disorders, and motility disorders. The main theme of this review is that EGCs are emerging as a new frontier in neurogastroenterology and a potential therapeutic target. New technological innovations in neuroimaging techniques are facilitating progress in the field, and an update is provided on exciting new translational studies. Gaps in our knowledge are discussed for further research. Restoring normal EGC function may prove to be an efficient strategy to dampen inflammation. Probiotics, palmitoylethanolamide (peroxisome proliferator-activated receptor-α), interleukin-1 antagonists (anakinra), and interventions acting on nitric oxide, receptor for advanced glycation end products, S100B, or purinergic signaling pathways are relevant clinical targets on EGCs with therapeutic potential.

Intestinal M cells

We have an enormous number of commensal bacteria in our intestine, moreover, the foods that we ingest and the water we drink is sometimes contaminated with pathogenic microorganisms. The intestinal epithelium is always exposed to such microbes, friend or foe, so to contain them our gut is equipped with specialized gut-associated lymphoid tissue (GALT), literally the largest peripheral lymphoid tissue in the body. GALT is the intestinal immune inductive site composed of lymphoid follicles such as Peyer's patches. M cells are a subset of intestinal epithelial cells (IECs) residing in the region of the epithelium covering GALT lymphoid follicles. Although the vast majority of IEC function to absorb nutrients from the intestine, M cells are highly specialized to take up intestinal microbial antigens and deliver them to GALT for efficient mucosal as well as systemic immune responses. I will discuss recent advances in our understanding of the molecular mechanisms of M-cell differentiation and functions.

Post-conversion sialylation of prions in lymphoid tissues

Sialylated glycans on the surface of mammalian cells act as part of a "self-associated molecular pattern," helping the immune system to recognize "self" from "altered self" or "nonself." To escape the host immune system, some bacterial pathogens have evolved biosynthetic pathways for host-like sialic acids, whereas others recruited host sialic acids for decorating their surfaces. Prions lack nucleic acids and are not conventional pathogens. Nevertheless, prions might use a similar strategy for invading and colonizing the lymphoreticular system. Here we show that the sialylation status of the infectious, disease-associated state of the prion protein (PrP(Sc)) changes with colonization of secondary lymphoid organs (SLOs). As a result, spleen-derived PrP(Sc) is more sialylated than brain-derived PrP(Sc). Enhanced sialylation of PrP(Sc) is recapitulated in vitro by incubating brain-derived PrP(Sc) with primary splenocytes or cultured macrophage RAW 264.7 cells. General inhibitors of sialyltranserases (STs), the enzymes that transfer sialic acid residues onto terminal positions of glycans, suppressed extrasialylation of PrP(Sc). A fluorescently labeled precursor of sialic acid revealed ST activity associated with RAW macrophages. This study illustrates that, upon colonization of SLOs, the sialylation status of prions changes by host STs. We propose that this mechanism is responsible for camouflaging prions in SLOs and has broad implications.

The Good, the Bad, and the Ugly of Dendritic Cells during Prion Disease

Prions are a unique group of proteinaceous pathogens which cause neurodegenerative disease and can be transmitted by a variety of exposure routes. After peripheral exposure, the accumulation and replication of prions within secondary lymphoid organs are obligatory for their efficient spread from the periphery to the brain where they ultimately cause neurodegeneration and death. Mononuclear phagocytes (MNP) are a heterogeneous population of dendritic cells (DC) and macrophages. These cells are abundant throughout the body and display a diverse range of roles based on their anatomical locations. For example, some MNP are strategically situated to provide a first line of defence against pathogens by phagocytosing and destroying them. Conventional DC are potent antigen presenting cells and migrate via the lymphatics to the draining lymphoid tissue where they present the antigens to lymphocytes. The diverse roles of MNP are also reflected in various ways in which they interact with prions and in doing so impact on disease pathogenesis. Indeed, some studies suggest that prions exploit conventional DC to infect the host. Here we review our current understanding of the influence of MNP in the pathogenesis of the acquired prion diseases with particular emphasis on the role of conventional DC.

Sampling of Candida albicans and Candida tropicalis by Langerin-positive dendritic cells in mouse Peyer's patches

Members of the Candida genus, including C. albicans and C. tropicalis are opportunistic fungal pathogens that are increasingly associated with gastrointestinal infections and inflammatory bowel diseases. In healthy populations, however, C. albicans and C. tropicalis are considered benign members of the mycobiome, and are presumably kept in check by the mucosal immune system. In this study, we demonstrate in mice that C. albicans and C. tropicalis are sampled by Peyer's patch (PP) dendritic cells (DCs). Uptake into gut-associated lymphoid tissues occurred rapidly and was at least partly M cell-dependent. C. albicans and C. tropicalis preferentially localized in (and persisted within) a recently identified sub- population of Peyer's patch DCs distinguished by their expression of the C-type lectin receptor, Langerin. This study is the first to identify a subset of PP DCs capable of sampling Candida species.

The Gut-Associated Lymphoid Tissues in the Small Intestine, Not the Large Intestine, Play a Major Role in Oral Prion Disease Pathogenesis

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.

Effect of Q211 and K222 PRNP Polymorphic Variants in the Susceptibility of Goats to Oral Infection With Goat Bovine Spongiform Encephalopathy

BACKGROUND

The prion protein-encoding gene (PRNP) is one of the major determinants for scrapie occurrence in sheep and goats. However, its effect on bovine spongiform encephalopathy (BSE) transmission to goats is not clear.

METHODS

Goats harboring wild-type, R/Q211 or Q/K222 PRNP genotypes were orally inoculated with a goat-BSE isolate to assess their relative susceptibility to BSE infection. Goats were killed at different time points during the incubation period and after the onset of clinical signs, and their brains as well as several peripheral tissues were analyzed for the accumulation of pathological prion protein (PrP(Sc)) and prion infectivity by mouse bioassay.

RESULTS

R/Q211 goats displayed delayed clinical signs compared with wild-type goats. Deposits of PrP(Sc) were detected only in brain, whereas infectivity was present in peripheral tissues too. In contrast, none of the Q/K222 goats showed any evidence of clinical prion disease. No PrP(Sc) accumulation was observed in their brains or peripheral tissues, but very low infectivity was detected in some tissues very long after inoculation (44-45 months).

CONCLUSIONS

These results demonstrate that transmission of goat BSE is genotype dependent, and they highlight the pivotal protective effect of the K222 PRNP variant in the oral susceptibility of goats to BSE.

Syntenin and syndecan in the biogenesis of exosomes

Cells communicate with their environment in various ways, including by secreting vesicles. Secreted vesicles are loaded with proteins, lipids and RNAs that compose 'a signature' of the cell of origin and potentially can reprogram recipient cells. Secreted vesicles recently gained in interest for medicine. They represent potential sources of biomarkers that can be collected from body fluids and, by disseminating pathogenic proteins, might also participate in systemic diseases like cancer, atherosclerosis and neurodegeneration. The mechanisms controlling the biogenesis and the uptake of secreted vesicles are poorly understood. Some of these vesicles originate from endosomes and are called 'exosomes'. In this review, we recapitulate recent insight on the role of the syndecan (SDC) heparan sulphate proteoglycans, the small intracellular adaptor syntenin and associated regulators in the biogenesis and loading of exosomes with cargo. SDC-syntenin-associated regulators include the endosomal sorting complex required for transport accessory component ALG-2-interacting protein X, the small GTPase adenosine 5'-diphosphate-ribosylation factor 6, the lipid-modifying enzyme phospholipase D2 and the endoglycosidase heparanase. All these molecules appear to support the budding of SDC-syntenin and associated cargo into the lumen of endosomes. This highlights a major mechanism for the formation of intraluminal vesicles that will be released as exosomes.

Phenotypic Analysis of a Population of IgA+ Cells in the Follicle-Associated Epithelium of Mouse Peyer's Patches

The follicle-associated epithelium (FAE) selectively transports prions, viruses, pathogenic bacteria, commensal microflora, and even secretory IgA (SIgA)-immune complexes from the intestinal lumen to underlying gut-associated lymphoid tissues like Peyer’s patches. The FAE consists of a single layer of columnar epithelial cells that includes enterocytes and M (microfold) cells, intermingled with dendritic cells (DCs), macrophages, and naïve and memory B and T lymphocytes. In this report we describe a population of IgA⁺ cells that reside within and immediately below the FAE in mouse Peyer’s patches. Immunofluorescence microscopy analysis indicated that the FAE-associated IgA⁺ cells were negative for surface antigen markers specific for B cells (B220), T cells (CD3), DCs (CD11c), and plasma cells (CD138). The IgA⁺ cells were also negative Ki-67 and IRF4, indicating that they are not mature B cells or plasma cells. The IgA⁺ cells were, however, often found in close proximity to DCs, leading us to speculate that the population of IgA⁺ cells in the FAE constitutes an atypical subset of B cells involved in mucosal antigen surveillance and/or immune recall.

Prion strains are differentially released through the exosomal pathway

Cell-to-cell transfer of prions is a crucial step in the spreading of prion infection through infected tissue. At the cellular level, several distinct pathways including direct cell-cell contacts and release of various types of infectious extracellular vesicles have been described that may potentially lead to infection of naïve cells. The relative contribution of these pathways and whether they may vary depending on the prion strain and/or on the infected cell type are not yet known. In this study we used a single cell type (RK13) infected with three different prion strains. We showed that in each case, most of the extracellular prions resulted from active cell secretion through the exosomal pathway. Further, quantitative analysis of secreted infectivity indicated that the proportion of prions eventually secreted was dramatically dependent on the prion strain. Our data also highlight that infectious exosomes secreted from cultured cells might represent a biologically pertinent material for spiking experiments. Also discussed is the appealing possibility that abnormal PrP from different prion strains may differentially interact with the cellular machinery to promote secretion.

The enteric glia: identity and functions

Enteric glial cells were first described at the end of the 19th century, but they attracted more interest from researchers only in the last decades of the 20th. Although, they have a different embryological origin, the enteric GLIA share many characteristics with astrocytes, the main glial cell type of the central nervous system (CNS), such as in their expression of the same markers and in their functions. Here we review the construction of the enteric nervous system (ENS), with a focus on enteric glia, and also the main studies that have revealed the action of enteric glia in different aspects of gastrointestinal tract homeostasis, such as in the intestinal barrier, in communications with neurons, and in their action as progenitor cells. We also discuss recent discoveries about the roles of enteric glia in different disorders that affect the ENS, such as degenerative pathologies including Parkinson's and prion diseases, and in cases of intestinal diseases and injury.

Extracellular vesicles--Their role in the packaging and spread of misfolded proteins associated with neurodegenerative diseases

Many cell types, including neurons, are known to release small membranous vesicles known as exosomes. In addition to their protein content these vesicles have recently been shown to contain messenger RNA (mRNA) and micro RNA (miRNA) species. Roles for these vesicles include cell-cell signalling, removal of unwanted proteins, and transfer of pathogens (including prion-like misfolded proteins) between cells, such as infectious prions. Prions are the infectious particles that are responsible for transmissible neurodegenerative diseases such as Creutzfeldt-Jakob disease (CJD) of humans or bovine spongiform encephalopathy (BSE) of cattle. Exosomes are also involved in processing the amyloid precursor protein (APP), which is associated with Alzheimer's disease (AD). As exosomes can be isolated from circulating fluids such as serum, urine, and cerebrospinal fluid (CSF), they provide a potential source of biomarkers for neurological conditions. Here, we review the roles these vesicles play in neurodegenerative disease and highlight their potential in diagnosing these disorders through analysis of their RNA content.

Role of cell death in the propagation of PrP(Sc) in immune cells

A number of studies have suggested that macrophages, dendritic cells, and follicular dendritic cells play an important role in the propagation of PrP(Sc). Both accumulation and proteolysis of PrP(Sc) have been demonstrated in peripheral macrophages. Macrophages may act as reservoirs for PrP(Sc) particles if the cells die during transient PrP(Sc) propagation. However, whether cell death plays a role in PrP(Sc) propagation in macrophages remains unclear. In this study, we investigated the possibility of propagation and transmission of PrP(Sc) between dead immune cells and living neural cells. We found that under specific conditions, transient PrP(Sc) propagation occurs in dead cells, indicating that interaction between PrP(C) and PrP(Sc) on plasma membrane lipid rafts might be important for PrP(Sc) propagation. Co-culturing of killed donor PrP(Sc)-infected macrophages with recipient N2a-3 neuroblastoma cells accelerated PrP(Sc) transmission. Our results suggest that cell death may play an important role in PrP(Sc) propagation, whereas transient PrP(Sc) propagation in macrophages has little effect on PrP(Sc) transmission.

Peripheral prion disease pathogenesis is unaltered in the absence of sialoadhesin (Siglec-1/CD169)

Prions are a unique group of pathogens, which are considered to comprise solely of an abnormally folded isoform of the cellular prion protein. The accumulation and replication of prions within secondary lymphoid organs is important for their efficient spread from the periphery to the brain where they ultimately cause neurodegeneration and death. Mononuclear phagocytes (MNP) play key roles in prion disease pathogenesis. Some MNP appear to facilitate the propagation of prions to and within lymphoid tissues, whereas others may aid their clearance by phagocytosis and by destroying them. Our recent data show that an intact splenic marginal zone is important for the efficient delivery of prions into the B-cell follicles where they subsequently replicate upon follicular dendritic cells before infecting the nervous system. Sialoadhesin is an MNP-restricted cell adhesion molecule that binds sialylated glycoproteins. Sialoadhesin is constitutively expressed upon splenic marginal zone metallophilic and lymph node sub-capsular sinus macrophage populations, where it may function to bind sialylated glycoproteins, pathogens and exosomes in the blood and lymph via recognition of terminal sialic acid residues. As the prion glycoprotein is highly sialylated, we tested the hypothesis that sialoadhesin may influence prion disease pathogenesis. We show that after peripheral exposure, prion pathogenesis was unaltered in sialoadhesin-deficient mice; revealing that lymphoid sequestration of prions is not mediated via sialoadhesin. Hence, although an intact marginal zone is important for the efficient uptake and delivery of prions into the B-cell follicles of the spleen, this is not influenced by sialoadhesin expression by the MNP within it.

Foodborne transmission of bovine spongiform encephalopathy to non-human primates results in preclinical rapid-onset obesity

Obesity has become one of the largest public health challenges worldwide. Recently, certain bacterial and viral pathogens have been implicated in the pathogenesis of obesity. In the present study, we retrospectively analyzed clinical data, plasma samples and post-mortem tissue specimens derived from a risk assessment study in bovine spongiform encephalopathy (BSE)-infected female cynomolgus monkeys (Macaca fascicularis). The original study design aimed to determine minimal infectious doses after oral or intracerebral (i.c.) infection of macaques to assess the risk for humans. High-dose exposures resulted in 100% attack rates and a median incubation time of 4.7 years as described previously. Retrospective analyses of clinical data from high-dosed macaques revealed that foodborne BSE transmission caused rapid weight gain within 1.5 years post infection (β = 0.915; P<0.0001) which was not seen in age- and sex-matched control animals or i.c. infected animals. The rapid-onset obesity was not associated with impaired pancreatic islet function or glucose metabolism. In the early preclinical phase of oral transmission associated with body weight gain, prion accumulation was confined to the gastrointestinal tract. Intriguingly, immunohistochemical findings suggest that foodborne BSE transmission has a pathophysiological impact on gut endocrine cells which may explain rapid weight gain. To our knowledge, this is the first experimental model which clearly demonstrates that foodborne pathogens can induce obesity.

Close interactions between sympathetic neural fibres and follicular dendritic cells network are not altered in Peyer's patches and spleen of C57BL/6 mice during the preclinical stage of 139A scrapie infection

During preclinical stage of prion diseases, secondary lymphoid organs seem to play an important role in prion amplification prior the invasion of the associated peripheral nervous system. In mice, it was shown that the relative positioning of follicular dendritic cells (FDC) and sympathetic nervous system (SNS) affects the velocity of neuroinvasion following scrapie inoculation. In this study, we checked if scrapie infection, by oral or intraperitoneal route, could influence this neuroimmune interface between FDC and tyrosine hydroxylase (TH) positive neural fibres within Peyer's patches (PP) and spleen of the C57BL/6 mouse strain. We concluded that, in vivo, scrapie 139A and ME7 strains do not modify FDC-SNS neuroimmune interface. However, age seems to alter this neuroimmune interface and thus could influence the neuroinvasion in prion pathogenesis.

Silver nanoparticle behavior, uptake, and toxicity in Caenorhabditis elegans: effects of natural organic matter

Significant progress has been made in understanding the toxicity of silver nanoparticles (Ag NPs) under carefully controlled laboratory conditions. Natural organic matter (NOM) is omnipresent in complex environmental systems, where it may alter the behavior of nanoparticles in these systems. We exposed the nematode Caenorhabditis elegans to Ag NP suspensions with or without one of two kinds of NOM, Suwannee River and Pony Lake fulvic acids (SRFA and PLFA, respectively). PLFA rescued toxicity more effectively than SRFA. Measurement of total tissue silver content indicated that PLFA reduced total organismal (including digestive tract) uptake of ionic silver, but not of citrate-coated Ag NPs (CIT-Ag NPs). The majority of the CIT-Ag NP uptake was in the digestive tract. Limited tissue uptake was detected by hyperspectral microscopy but not by transmission electron microscopy. Co-exposure to PLFA resulted in the formation of NOM-Ag NP composites (both in medium and in nematodes) and rescued AgNO3- and CIT-Ag NP-induced cellular damage, potentially by decreasing intracellular uptake of CIT-Ag NPs.

Nanopore analysis reveals differences in structural stability of ovine PrP(C) proteins corresponding to scrapie susceptible (VRQ) and resistance (ARR) genotypes

Species, as well as individuals within species, have unique susceptibilities to prion infection that are likely based on sequence differences in cellular prion protein (PrP(C)). Species barriers to transmission also reflect PrP(C) sequence differences. Defining the structure-activity relationship of PrP(C)/PrP(Sc) with respect to infectivity/susceptibility will benefit disease understanding and assessment of transmission risks. Here, nanopore analysis is employed to investigate genotypes of sheep PrP(C) corresponding to differential susceptibilities to scrapie infection. Under non-denaturing conditions scrapie resistant (ARR) and susceptible (VRQ) genotypes display similar, type I (bumping) predominant event profiles, suggesting a conserved folding pattern. Under increasingly denaturing conditions both proteins shift to type II (intercalation/translocation) events but with different sensitivities to unfolding. Specifically, when pre-incubated in 2M Gdn-HCl, the VRQ variant had more of type II events as compared with the ARR protein, suggesting a more flexible unfolding pattern. Addition of PrP(Sc)-specific polyclonal antibody (YML) to the ARR variant, pre-incubated in 2M Gdn-HCl, reduced the number of type II events with no clear intercalation/translocation peak, whereas for VRQ, type II events above blockades of 90 pA bound YML. A second PrP(Sc)-specific antibody (SN6b) to a different cryptic epitope reduced type II events for VRQ but not the ARR variant. Collectively, the event patterns associated with sequential denaturation, as well as interactions with PrP(Sc)-specific antibodies, support unique patterns and/or propensities of misfolding between the genotypes. Overall, nanopore analysis identifies intermediate conformations that occur during the unfolding pathways of ARR and VRQ genotypes and may help to understand the correlation of structural properties that induce protein misfolding.

Mucus properties and goblet cell quantification in mouse, rat and human ileal Peyer's patches

Peyer's patches (PPs) are collections of lymphoid follicles in the small intestine, responsible for scanning the intestinal content for foreign antigens such as soluble molecules, particulate matter as well as intact bacteria and viruses. The immune cells of the patch are separated from the intestinal lumen by a single layer of epithelial cells, the follicle-associated epithelium (FAE). This epithelium covers the dome of the follicle and contains enterocyte-like cells and M cells, which are particularly specialized in taking up antigens from the gut. However, the presence and number of goblet cells as well as the presence of mucus on top of the FAE is controversial. When mouse ileal PPs were mounted in a horizontal Ussing-type chamber, we could observe a continuous mucus layer at mounting and new, easily removable mucus was released from the villi on the patch upon stimulation. Confocal imaging using fluorescent beads revealed a penetrable mucus layer covering the domes. Furthermore, immunostaining of FAE from mice, rats and humans with a specific antibody against the main component of intestinal mucus, the MUC2 mucin, clearly identify mucin-containing goblet cells. Transmission electron micrographs further support the identification of mucus releasing goblet cells on the domes of PPs in these species.

Plasma membrane invaginations containing clusters of full-length PrPSc are an early form of prion-associated neuropathology in vivo

During prion disease, cellular prion protein (PrP(C)) is refolded into a pathogenic isoform (PrP(Sc)) that accumulates in the central nervous system and causes neurodegeneration and death. We used immunofluorescence, quantitative cryo-immunogold EM, and tomography to detect nascent, full-length PrP(Sc) in the hippocampus of prion-infected mice from early preclinical disease stages onward. Comparison of uninfected and infected brains showed that sites containing full-length PrP(Sc) could be recognized in the neuropil by bright spots and streaks of immunofluorescence on semi-thin (200-nm) sections, and by clusters of cryo-immunogold EM labeling. PrP(Sc) was found mainly on neuronal plasma membranes, most strikingly on membrane invaginations and sites of cell-to-cell contact, and was evident by 65 days postinoculation, or 54% of the incubation period to terminal disease. Both axons and dendrites in the neuropil were affected. We hypothesize that closely apposed plasma membranes provide a favorable environment for prion conversion and intercellular prion transfer. Only a small proportion of clustered PrP immunogold labeling was found at synapses, indicating that synapses are not targeted specifically in prion disease.

Role of palatine tonsils as a prion entry site in classical and atypical experimental sheep scrapie

Atypical and classical scrapie-infected sheep brain tissue was monolaterally injected into the tonsils of lambs to investigate their role as a prion entry point. We first detected classical PrP(Sc) within the inoculated tonsil and in the ipsilateral retropharyngeal lymph node at 3 months postinoculation (p.i.). At 7 months p.i., PrP(Sc) colonized other lymphoid tissues bilaterally, including ileal Peyer's patches. The earliest PrP(Sc) deposition within the brain was ipsilaterally observed at 9 months p.i. in the substantia reticularis of the medulla oblongata. At 12 months p.i., PrP(Sc) deposition was present bilaterally in the nucleus parasympathicus nervi vagi, as well as in the intermediolateral cell column of the thoracolumbar spinal cord. No PrP(Sc) was detected in the lambs inoculated with atypical scrapie. These findings suggest that neuroinvasion may naturally occur from the tonsil after a widespread prion replication within the lymphoid tissues during classical scrapie only, thus mimicking the pathogenesis after oral ingestion.

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.

Characterization of human thymic exosomes

Exosomes are nanosized membrane-bound vesicles that are released by various cell types and are capable of carrying proteins, lipids and RNAs which can be delivered to recipient cells. Exosomes play a role in intercellular communication and have been described to mediate immunologic information. In this article we report the first isolation and characterization of exosomes from human thymic tissue. Using electron microscopy, particle size determination, density gradient measurement, flow cytometry, proteomic analysis and microRNA profiling we describe the morphology, size, density, protein composition and microRNA content of human thymic exosomes. The thymic exosomes share characteristics with previously described exosomes such as antigen presentation molecules, but they also exhibit thymus specific features regarding surface markers, protein content and microRNA profile. Interestingly, thymic exosomes carry proteins that have a tissue restricted expression in the periphery which may suggest a role in T cell selection and the induction of central tolerance. We speculate that thymic exosomes may provide the means for intercellular information exchange necessary for negative selection and regulatory T cell formation of the developing thymocytes within the human thymic medulla.

Microfold (M) cells: important immunosurveillance posts in the intestinal epithelium

The transcytosis of antigens across the gut epithelium by microfold cells (M cells) is important for the induction of efficient immune responses to some mucosal antigens in Peyer's patches. Recently, substantial progress has been made in our understanding of the factors that influence the development and function of M cells. This review highlights these important advances, with particular emphasis on: the host genes which control the functional maturation of M cells; how this knowledge has led to the rapid advance in our understanding of M-cell biology in the steady state and during aging; molecules expressed on M cells which appear to be used as "immunosurveillance" receptors to sample pathogenic microorganisms in the gut; how certain pathogens appear to exploit M cells to infect the host; and finally how this knowledge has been used to specifically target antigens to M cells to attempt to improve the efficacy of mucosal vaccines.

Extracellular vesicles: exosomes, microvesicles, and friends

Cells release into the extracellular environment diverse types of membrane vesicles of endosomal and plasma membrane origin called exosomes and microvesicles, respectively. These extracellular vesicles (EVs) represent an important mode of intercellular communication by serving as vehicles for transfer between cells of membrane and cytosolic proteins, lipids, and RNA. Deficiencies in our knowledge of the molecular mechanisms for EV formation and lack of methods to interfere with the packaging of cargo or with vesicle release, however, still hamper identification of their physiological relevance in vivo. In this review, we focus on the characterization of EVs and on currently proposed mechanisms for their formation, targeting, and function.

Prion disease and the innate immune system

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.

Antigen sampling in the small intestine

Active sampling of intestinal antigen initiates regulated immune responses that ensure intestinal homeostasis. Several specialized mechanisms transport luminal antigen across the gut epithelium. Epithelium overlying lymphoid compartments is equipped with transcytotic microfold (M) cells that transport particulate material either directly or with the help of dendritic cells (DCs). By contrast, normal villous epithelium transports antigen by means of antigen-shuttling receptors together with phagocytes that scan the gut epithelium and potentially the gut lumen. Here, we examine recent insights into the nature of the epithelial and immune cell types involved in antigen uptake and describe how the process of antigen transport has been visualized by intravital microscopy. These new findings might help optimize antigen delivery systems for mucosal vaccination.

Identification of novel genes selectively expressed in the follicle-associated epithelium from the meta-analysis of transcriptomics data from multiple mouse cell and tissue populations

The follicle-associated epithelium (FAE) overlying the Peyer's patches and the microfold cells (M cells) within it are important sites of antigen transcytosis across the intestinal epithelium. Using a meta-analysis approach, we identified a transcriptional signature that distinguished the FAE from a large collection of mouse cells and tissues. A co-expressed cluster of 21 FAE-specific genes was identified, and the analysis of the transcription factor binding site motifs in their promoter regions indicated that these genes shared an underlying transcriptional programme. This cluster contained known FAE- (Anxa10, Ccl20, Psg18 and Ubd) and M-cell-specific (Gp2) genes, suggesting that the others were novel FAE-specific genes. Some of these novel candidate genes were expressed highly by the FAE and M cells (Calcb, Ces3b, Clca2 and Gjb2), and others only by the FAE (Ascl2, Cftr, Fgf15, Gpr133, Kcna1, Kcnj15, Mycl1, Pgap1 and Rps6kl). We also identified a subset of novel FAE-related genes that were induced in the intestinal epithelium after receptor activator of nuclear factor (NF)-κB ligand stimulation. These included Mfge8 which was specific to FAE enterocytes. This study provides new insight into the FAE transcriptome. Further characterization of the candidate genes identified here will aid the identification of novel regulators of cell function in the FAE.

Rapid transepithelial transport of prions following inhalation

Prion infection and pathogenesis are dependent on the agent crossing an epithelial barrier to gain access to the recipient nervous system. Several routes of infection have been identified, but the mechanism(s) and timing of in vivo prion transport across an epithelium have not been determined. The hamster model of nasal cavity infection was used to determine the temporal and spatial parameters of prion-infected brain homogenate uptake following inhalation and to test the hypothesis that prions cross the nasal mucosa via M cells. A small drop of infected or uninfected brain homogenate was placed below each nostril, where it was immediately inhaled into the nasal cavity. Regularly spaced tissue sections through the entire extent of the nasal cavity were processed immunohistochemically to identify brain homogenate and the disease-associated isoform of the prion protein (PrP(d)). Infected or uninfected brain homogenate was identified adhering to M cells, passing between cells of the nasal mucosa, and within lymphatic vessels of the nasal cavity at all time points examined. PrP(d) was identified within a limited number of M cells 15 to 180 min following inoculation, but not in the adjacent nasal mucosa-associated lymphoid tissue (NALT). While these results support M cell transport of prions, larger amounts of infected brain homogenate were transported paracellularly across the respiratory, olfactory, and follicle-associated epithelia of the nasal cavity. These results indicate that prions can immediately cross the nasal mucosa via multiple routes and quickly enter lymphatics, where they can spread systemically via lymph draining the nasal cavity.

Peyer's patch M cells derived from Lgr5(+) stem cells require SpiB and are induced by RankL in cultured "miniguts"

Peyer's patches consist of domains of specialized intestinal epithelium overlying gut-associated lymphoid tissue (GALT). Luminal antigens reach the GALT by translocation through epithelial gatekeeper cells, the so-called M cells. We recently demonstrated that all epithelial cells required for the digestive functions of the intestine are generated from Lgr5-expressing stem cells. Here, we show that M cells also derive from these crypt-based Lgr5 stem cells. The Ets family transcription factor SpiB, known to control effector functions of bone marrow-derived immune cells, is specifically expressed in M cells. In SpiB(-/-) mice, M cells are entirely absent, which occurs in a cell-autonomous fashion. It has been shown that Tnfsf11 (RankL) can induce M cell development in vivo. We show that in intestinal organoid ("minigut") cultures, stimulation with RankL induces SpiB expression within 24 h and expression of other M cell markers subsequently. We conclude that RankL-induced expression of SpiB is essential for Lgr5 stem cell-derived epithelial precursors to develop into M cells.

Prion pathogenesis and secondary lymphoid organs (SLO): tracking the SLO spread of prions to the brain

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.

Transcytosis of HTLV-1 across a tight human epithelial barrier and infection of subepithelial dendritic cells

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia/lymphoma and HTLV-1-associated myelopathy/tropical spastic paraparesis. In addition to blood transfusion and sexual transmission, HTLV-1 is transmitted mainly through prolonged breastfeeding, and such infection represents a major risk for the development of adult T-cell leukemia/lymphoma. Although HTLV-1-infected lymphocytes can be retrieved from maternal milk, the mechanisms of HTLV-1 transmission through the digestive tract remain unknown. In the present study, we assessed HTLV-1 transport across the epithelial barrier using an in vitro model. Our results show that the integrity of the epithelial barrier was maintained during coculture with HTLV-1-infected lymphocytes, because neither morphological nor functional alterations of the cell monolayer were observed. Enterocytes were not susceptible to HTLV-1 infection, but free infectious HTLV-1 virions could cross the epithelial barrier via a transcytosis mechanism. Such virions were able to infect productively human dendritic cells located beneath the epithelial barrier. Our data indicate that HTLV-1 crosses the tight epithelial barrier without disruption or infection of the epithelium to further infect target cells such as dendritic cells. The present study provides the first data pertaining to the mode of HTLV-1 transport across a tight epithelial barrier, as can occur during mother-to-child HTLV-1 transmission during breastfeeding.