Bovine macrophage degradation of scrapie and BSE PrPSc

Extraneural infection route restricts prion conformational variability and attenuates the impact of quaternary structure on infectivity

Prions can exist as different strains that consist of conformational variants of the misfolded, pathogenic prion protein isoform PrPSc. Defined by stably transmissible biological and biochemical properties, strains have been identified in a spectrum of prion diseases, including chronic wasting disease (CWD) of wild and farmed cervids. CWD is highly contagious and spreads via direct and indirect transmission involving extraneural sites of infection, peripheral replication and neuroinvasion of prions. Here, we investigated the impact of infection route on CWD prion conformational selection and propagation. We used gene-targeted mouse models expressing deer PrP for intracerebral or intraperitoneal inoculation with fractionated or unfractionated brain homogenates from white-tailed deer, harboring CWD strains Wisc-1 or 116AG. Upon intracerebral inoculation, Wisc-1 and 116AG-inoculated mice differed in conformational stability of PrPSc. In brains of mice infected intraperitoneally with either inoculum, PrPSc propagated with identical conformational stability and fewer PrPSc deposits in most brain regions than intracerebrally inoculated animals. For either inoculum, PrPSc conformational stability in brain and spinal cord was similar upon intracerebral infection but significantly higher in spinal cords of intraperitoneally infected animals. Inoculation with fractionated brain homogenates resulted in lower variance of survival times upon intraperitoneal compared to intracerebral infection. In summary, we demonstrate that extraneural infection mitigates the impact of PrPSc quaternary structure on infection and reduces conformational variability of PrPSc propagated in the brain. These findings provide new insights into the evolution of stable CWD strains in natural, extraneural transmissions.

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.

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.

Human tonsil-derived follicular dendritic-like cells are refractory to human prion infection in vitro and traffic disease-associated prion protein to lysosomes

The molecular mechanisms involved in human cellular susceptibility to prion infection remain poorly defined. This is due, in part, to the absence of any well characterized and relevant cultured human cells susceptible to infection with human prions, such as those involved in Creutzfeldt-Jakob disease. In variant Creutzfeldt-Jakob disease, prion replication is thought to occur first in the lymphoreticular system and then spread into the brain. We have, therefore, examined the susceptibility of a human tonsil-derived follicular dendritic cell-like cell line (HK) to prion infection. HK cells were found to display a readily detectable, time-dependent increase in cell-associated abnormal prion protein (PrP(TSE)) when exposed to medium spiked with Creutzfeldt-Jakob disease brain homogenate, resulting in a coarse granular perinuclear PrP(TSE) staining pattern. Despite their high level of cellular prion protein expression, HK cells failed to support infection, as judged by longer term maintenance of PrP(TSE) accumulation. Colocalization studies revealed that exposure of HK cells to brain homogenate resulted in increased numbers of detectable lysosomes and that these structures immunostained intensely for PrP(TSE) after exposure to Creutzfeldt-Jakob disease brain homogenate. Our data suggest that human follicular dendritic-like cells and perhaps other human cell types are able to avoid prion infection by efficient lysosomal degradation of PrP(TSE).

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.

Phenotypic characterization of cells participating in transport of prion protein aggregates across the intestinal mucosa of sheep

The oral route is considered to be the main entry site of several transmissible spongiform encephalopathies or prion diseases of animals and man. Following natural and experimental oral exposure to scrapie, sheep first accumulate disease associated prion protein (PrP^d) in Peyer’s patch (PP) lymphoid follicles. In this study, recombinant ovine prion protein (rPrP) was inoculated into gut loops of young lambs and the transportation across the intestinal wall studied. In particular, the immunohistochemical phenotypes of cells bearing the inoculated prion protein were investigated. The rPrP was shown to be transported across the villi of the gut, into the lacteals and submucosal lymphatics, mimicking the transport route of PrP^d from scrapie brain inoculum observed in a previous intestinal loop experiment. The cells bearing the inoculated rPrP were mainly mononuclear cells, and multicolor immunofluorescence procedures were used to show that the rPrP bearing cells were professional antigen presenting cells expressing Major histocompatibility complex II (MHCII). In addition, the rPrP bearing cells labeled with CD205, CD11b and the macrophage marker CD68, and not with the dendritic cell markers CD11c and CD209. Others have reported that cells expressing CD205 and CD11b in the absence of CD11c have been shown to induce T cell tolerance or regulatory T cells. Based on this association, it was speculated that the rPrP and by extension PrP^d and scrapie infective material may exploit the physiological process of macromolecular uptake across the gut, and that this route of entry may have implications for immune surveillance.

Incunabular immunological events in prion trafficking

While prions probably interact with the innate immune system immediately following infection, little is known about this initial confrontation. Here we investigated incunabular events in lymphotropic and intranodal prion trafficking by following highly enriched, fluorescent prions from infection sites to draining lymph nodes. We detected biphasic lymphotropic transport of prions from the initial entry site upon peripheral prion inoculation. Prions arrived in draining lymph nodes cell autonomously within two hours of intraperitoneal administration. Monocytes and dendritic cells (DCs) required Complement for optimal prion delivery to lymph nodes hours later in a second wave of prion trafficking. B cells constituted the majority of prion-bearing cells in the mediastinal lymph node by six hours, indicating intranodal prion reception from resident DCs or subcapsulary sinus macrophages or directly from follicular conduits. These data reveal novel, cell autonomous prion lymphotropism, and a prominent role for B cells in intranodal prion movement.

The diverse roles of mononuclear phagocytes in prion disease pathogenesis

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.

Exosome-producing follicle associated epithelium is not involved in uptake of PrPd from the gut of sheep (Ovis aries): an ultrastructural study

In natural or experimental oral scrapie infection of sheep, disease associated prion protein (PrP(d)) often first accumulates in Peyer's patch (PP) follicles. The route by which infectivity reaches the follicles is unknown, however, intestinal epithelial cells may participate in intestinal antigenic presentation by delivering exosomes as vehicles of luminal antigens. In a previous study using an intestinal loop model, following inoculation of scrapie brain homogenate, inoculum associated PrP(d) was detected by light microscopy shortly (15 minutes to 3.5 hours) after inoculation in the villous lacteals and sub-mucosal lymphatics. No PrP(d) was located within the follicle-associated epithelium (FAE), sub-FAE domes or the PP follicles. To evaluate this gut loop model and the transportation routes in more detail, we used electron microscopy (EM) to study intestinal tissues exposed to scrapie or control homogenates for 15 minutes to 10 days. In addition, immuno-EM was used to investigate whether exosomes produced in the FAE may possess small amounts of PrP(d) that were not detectable by light microscopy. This study showed that the integrity of the intestinal epithelium was sustained in the intestinal loop model. Despite prominent transcytotic activity and exosome release from the FAE of the ileal PP in sheep, these structures were not associated with transportation of PrP(d) across the mucosa. The study did not determine how infectivity reaches the follicles of PPs. The possibility that the infectious agent is transported across the FAE remains a possibility if it occurs in a form that is undetectable by the methods used in this study. Infectivity may also be transported via lymph to the blood and further to all other lymphoid tissues including the PP follicles, but the early presence of PrP(d) in the PP follicles during scrapie infection argues against such a mechanism.

The effects of lysosomal and proteasomal inhibitors on abnormal forms of prion protein degradation in murine macrophages

It has been reported that macrophages degrade infectious forms of prion protein (PrP(Sc) ). In order to investigate the mechanisms underlying PrP(Sc) degradation in macrophages, the effects of lysosomal and proteasomal inhibitors on macrophage cell lines which were incubated with scrapie-affected brain homogenate were studied. PrP(Sc) degradation was inhibited in the presence of both proteasomal and lysosomal inhibitors. Indirect fluorescence assays to determine the cellular localization of PrP(Sc) were undertaken. PrP(Sc) colocalized with the lysosomal membrane protein Lamp-1 and ubiquitin, a protein that is related to the proteasome. The present data indicate that macrophages might degrade PrP(Sc) via the lysosomal and proteasomal pathways.

Transcytosis of murine-adapted bovine spongiform encephalopathy agents in an in vitro bovine M cell model

Transmissible spongiform encephalopathies (TSE), including bovine spongiform encephalopathy (BSE), are fatal neurodegenerative disorders in humans and animals. BSE appears to have spread to cattle through the consumption of feed contaminated with BSE/scrapie agents. In the case of an oral infection, the agents have to cross the gut-epithelial barrier. We recently established a bovine intestinal epithelial cell line (BIE cells) that can differentiate into the M cell type in vitro after lymphocytic stimulation (K. Miyazawa, T. Hondo, T. Kanaya, S. Tanaka, I. Takakura, W. Itani, M. T. Rose, H. Kitazawa, T. Yamaguchi, and H. Aso, Histochem. Cell Biol. 133:125-134, 2010). In this study, we evaluated the role of M cells in the intestinal invasion of the murine-adapted BSE (mBSE) agent using our in vitro bovine intestinal epithelial model. We demonstrate here that M cell-differentiated BIE cells are able to transport the mBSE agent without inactivation at least 30-fold more efficiently than undifferentiated BIE cells in our in vitro model. As M cells in the follicle-associated epithelium are known to have a high ability to transport a variety of macromolecules, viruses, and bacteria from gut lumen to mucosal immune cells, our results indicate the possibility that bovine M cells are able to deliver agents of TSE, not just the mBSE agent.