CpG and LPS can interfere negatively with prion clearance in macrophage and microglial cells

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.

Distinctive Toll-like Receptors Gene Expression and Glial Response in Different Brain Regions of Natural Scrapie

Prion diseases are chronic and fatal neurodegenerative diseases characterized by the accumulation of disease-specific prion protein (PrP^Sc), spongiform changes, neuronal loss, and gliosis. Growing evidence shows that the neuroinflammatory response is a key component of prion diseases and contributes to neurodegeneration. Toll-like receptors (TLRs) have been proposed as important mediators of innate immune responses triggered in the central nervous system in other human neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. However, little is known about the role of TLRs in prion diseases, and their involvement in the neuropathology of natural scrapie has not been studied. We assessed the gene expression of ovine TLRs in four anatomically distinct brain regions in natural scrapie-infected sheep and evaluated the possible correlations between gene expression and the pathological hallmarks of prion disease. We observed significant changes in TLR expression in scrapie-infected sheep that correlate with the degree of spongiosis, PrP^Sc deposition, and gliosis in each of the regions studied. Remarkably, TLR4 was the only gene upregulated in all regions, regardless of the severity of neuropathology. In the hippocampus, we observed milder neuropathology associated with a distinct TLR gene expression profile and the presence of a peculiar microglial morphology, called rod microglia, described here for the first time in the brain of scrapie-infected sheep. The concurrence of these features suggests partial neuroprotection of the hippocampus. Finally, a comparison of the findings in naturallyinfected sheep versus an ovinized mouse model (tg338 mice) revealed distinct patterns of TLRgene expression.

Phagocytic Activities of Reactive Microglia and Astrocytes Associated with Prion Diseases Are Dysregulated in Opposite Directions

Phagocytosis is one of the most important physiological functions of the glia directed at maintaining a healthy, homeostatic environment in the brain. Under a homeostatic environment, the phagocytic activities of astrocytes and microglia are tightly coordinated in time and space. In neurodegenerative diseases, both microglia and astrocytes contribute to neuroinflammation and disease pathogenesis, however, whether their phagocytic activities are up- or downregulated in reactive states is not known. To address this question, this current study isolated microglia and astrocytes from C57BL/6J mice infected with prions and tested their phagocytic activities in live-cell imaging assays that used synaptosomes and myelin debris as substrates. The phagocytic uptake by the reactive microglia was found to be significantly upregulated, whereas that of the reactive astrocytes was strongly downregulated. The up- and downregulation of phagocytosis by the two cell types were observed irrespective of whether disease-associated synaptosomes, normal synaptosomes, or myelin debris were used in the assays, indicating that dysregulations are dictated by cell reactive states, not substrates. Analysis of gene expression confirmed dysregulation of phagocytic functions in both cell types. Immunostaining of animal brains infected with prions revealed that at the terminal stage of disease, neuronal cell bodies were subject to engulfment by reactive microglia. This study suggests that imbalance in the phagocytic activities of the reactive microglia and astrocytes, which are dysregulated in opposite directions, is likely to lead to excessive microglia-mediated neuronal death on the one hand, and the inability of astrocytes to clear cell debris on the other hand, contributing to the neurotoxic effects of glia as a whole.

All the Same? The Secret Life of Prion Strains within Their Target Cells

Prions are infectious β-sheet-rich protein aggregates composed of misfolded prion protein (PrP^Sc) that do not possess coding nucleic acid. Prions replicate by recruiting and converting normal cellular PrP^C into infectious isoforms. In the same host species, prion strains target distinct brain regions and cause different disease phenotypes. Prion strains are associated with biophysically distinct PrP^Sc conformers, suggesting that strain properties are enciphered within alternative PrP^Sc quaternary structures. So far it is unknown how prion strains target specific cells and initiate productive infections. Deeper mechanistic insight into the prion life cycle came from cell lines permissive to a range of different prion strains. Still, it is unknown why certain cell lines are refractory to infection by one strain but permissive to another. While pharmacologic and genetic manipulations revealed subcellular compartments involved in prion replication, little is known about strain-specific requirements for endocytic trafficking pathways. This review summarizes our knowledge on how prions replicate within their target cells and on strain-specific differences in prion cell biology.

Cell biology of prion infection

The development of multiple cell culture models of prion infection over the last two decades has led to a significant increase in our understanding of how prions infect cells. In particular, new techniques to distinguish exogenous from endogenous prions have allowed us for the first time to look in depth at the earliest stages of prion infection through to the establishment of persistent infection. These studies have shown that prions can infect multiple cell types, both neuronal and nonneuronal. Once in contact with the cell, they are rapidly taken up via multiple endocytic pathways. After uptake, the initial replication of prions occurs almost immediately on the plasma membrane and within multiple endocytic compartments. Following this acute stage of prion replication, persistent prion infection may or may not be established. Establishment of a persistent prion infection in cells appears to depend upon the achievement of a delicate balance between the rate of prion replication and degradation, the rate of cell division, and the efficiency of prion spread from cell to cell. Overall, cell culture models have shown that prion infection of the cell is a complex and variable process which can involve multiple cellular pathways and compartments even within a single cell.

Inflammatory response of microglia to prions is controlled by sialylation of PrPSc

Neuroinflammation is recognized as one of the obligatory pathogenic features of neurodegenerative diseases including Alzheimer’s, Parkinson’s or prion diseases. In prion diseases, space and time correlations between deposition of disease-associated, pathogenic form of the prion protein or PrP^Sc and microglial-mediated neuroinflammation has been established. Yet, it remains unclear whether activation of microglia is triggered directly by a contact with PrP^Sc, and what molecular features of PrP^Sc microglia sense and respond to that drive microglia to inflammatory states. The current study asked the questions whether PrP^Sc can directly trigger activation of microglia and whether the degree of microglia response depends on the nature of terminal carbohydrate groups on the surface of PrP^Sc particles. PrP^Sc was purified from brains of mice infected with mouse-adapted prion strain 22L or neuroblastoma N2a cells stably infected with 22L. BV2 microglial cells or primary microglia were cultured in the presence of purified 22L. We found that exposure of BV2 cells or primary microglia to purified PrP^Sc triggered proinflammatory responses characterized by an increase in the levels of TNFα, IL6, nitric oxide (NO) and expression of inducible Nitric Oxide Synthase (iNOS). Very similar patterns of inflammatory response were induced by PrP^Sc purified from mouse brains and neuroblastoma cells arguing that microglia response is independent of the source of PrP^Sc. To test whether the microglial response is mediated by carbohydrate epitopes on PrP^Sc surface, the levels of sialylation of PrP^Sc N-linked glycans was altered by treatment of purified PrP^Sc with neuraminidase. Partial cleavage of sialic acid residues was found to boost the inflammatory response of microglia to PrP^Sc. Moreover, transient degradation of Iκβα observed upon treatment with partially desialylated PrP^Sc suggests that canonical NFκB activation pathway is involved in inflammatory response. The current study is the first to demonstrate that PrP^Sc can directly trigger inflammatory response in microglia. In addition, this work provides direct evidence that the chemical nature of the carbohydrate groups on PrP^Sc surface is important for microglial activation.

Prion strains depend on different endocytic routes for productive infection

Prions are unconventional agents composed of misfolded prion protein that cause fatal neurodegenerative diseases in mammals. Prion strains induce specific neuropathological changes in selected brain areas. The mechanism of strain-specific cell tropism is unknown. We hypothesised that prion strains rely on different endocytic routes to invade and replicate within their target cells. Using prion permissive cells, we determined how impairment of endocytosis affects productive infection by prion strains 22L and RML. We demonstrate that early and late stages of prion infection are differentially sensitive to perturbation of clathrin- and caveolin-mediated endocytosis. Manipulation of canonical endocytic pathways only slightly influenced prion uptake. However, blocking the same routes had drastic strain-specific consequences on the establishment of infection. Our data argue that prion strains use different endocytic pathways for infection and suggest that cell type-dependent differences in prion uptake could contribute to host cell tropism.

Uptake and degradation of protease-sensitive and -resistant forms of abnormal human prion protein aggregates by human astrocytes

Sporadic Creutzfeldt-Jakob disease is the most common of the human prion diseases, a group of rare, transmissible, and fatal neurologic diseases associated with the accumulation of an abnormal form (PrP(Sc)) of the host prion protein. In sporadic Creutzfeldt-Jakob disease, disease-associated PrP(Sc) is present not only as an aggregated, protease-resistant form but also as an aggregated protease-sensitive form (sPrP(Sc)). Although evidence suggests that sPrP(Sc) may play a role in prion pathogenesis, little is known about how it interacts with cells during prion infection. Here, we show that protease-sensitive abnormal PrP aggregates derived from patients with sporadic Creutzfeldt-Jakob disease are taken up and degraded by immortalized human astrocytes similarly to abnormal PrP aggregates that are resistant to proteases. Our data suggest that relative proteinase K resistance does not significantly influence the astrocyte's ability to degrade PrP(Sc). Furthermore, the cell does not appear to distinguish between sPrP(Sc) and protease-resistant PrP(Sc), suggesting that sPrP(Sc) could contribute to prion infection.

A specific population of abnormal prion protein aggregates is preferentially taken up by cells and disaggregated in a strain-dependent manner

Prion diseases are characterized by the conversion of the soluble protease-sensitive host-encoded prion protein (PrP(C)) into its aggregated, protease-resistant, and infectious isoform (PrP(Sc)). One of the earliest events occurring in cells following exposure to an exogenous source of prions is the cellular uptake of PrP(Sc). It is unclear how the biochemical properties of PrP(Sc) influence its uptake, although aggregate size is thought to be important. Here we show that for two different strains of mouse prions, one that infects cells (22L) and one that does not (87V), a fraction of PrP(Sc) associated with distinct sedimentation properties is preferentially taken up by the cells. However, while the fraction of PrP(Sc) and the kinetics of uptake were similar for both strains, PrP(Sc) derived from the 87V strain was disaggregated more rapidly than that derived from 22L. The increased rate of PrP(Sc) disaggregation did not correlate with either the conformational or aggregate stability of 87V PrP(Sc), both of which were greater than those of 22L PrP(Sc). Our data suggest that the kinetics of disaggregation of PrP(Sc) following cellular uptake is independent of PrP(Sc) stability but may be dependent upon some component of the PrP(Sc) aggregate other than PrP. Rapid disaggregation of 87V PrP(Sc) by the cell may contribute, at least in part, to the inability of 87V to infect cells in vitro.

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.

Discovery of a novel, monocationic, small-molecule inhibitor of scrapie prion accumulation in cultured sheep microglia and Rov cells

Prion diseases, including sheep scrapie, are neurodegenerative diseases with the fundamental pathogenesis involving conversion of normal cellular prion protein (PrP^C) to disease-associated prion protein (PrP^Sc). Chemical inhibition of prion accumulation is widely investigated, often using rodent-adapted prion cell culture models. Using a PrP^Sc-specific ELISA we discovered a monocationic phenyl-furan-benzimidazole (DB772), which has previously demonstrated anti-pestiviral activity and represents a chemical category previously untested for anti-prion activity, that inhibited PrP^Sc accumulation and prion infectivity in primary sheep microglial cell cultures (PRNP 136VV/154RR/171QQ) and Rov9 cultures (VRQ-ovinized RK13 cells). We investigated potential mechanisms of this anti-prion activity by evaluating PrP^C expression with quantitative RT-PCR and PrP ELISA, comparing the concentration-dependent anti-prion and anti-pestiviral effects of DB772, and determining the selectivity index. Results demonstrate at least an approximate two-log inhibition of PrP^Sc accumulation in the two cell systems and confirmed that the inhibition of PrP^Sc accumulation correlates with inhibition of prion infectivity. PRNP transcripts and total PrP protein concentrations within cell lysates were not decreased; thus, decreased PrP^C expression is not the mechanism of PrP^Sc inhibition. PrP^Sc accumulation was multiple logs more resistant than pestivirus to DB772, suggesting that the anti-PrP^Sc activity was independent of anti-pestivirus activity. The anti-PrP^Sc selectivity index in cell culture was approximately 4.6 in microglia and 5.5 in Rov9 cells. The results describe a new chemical category that inhibits ovine PrP^Sc accumulation in primary sheep microglia and Rov9 cells, and can be used for future studies into the treatment and mechanism of prion diseases.

Conditional modulation of membrane protein expression in cultured cells mediated by prion protein recognition of short phosphorothioate oligodeoxynucleotides

We demonstrate that the levels of native as well as transfected prion protein (PrP) are lowered in various cell lines exposed to phosphorothioate oligodeoxynucleotides (PS-DNA) and can be rapidly reverted to their normal amounts by removal of PS-DNA. This transient modulation was independent of the glycosylation state of PrP, and in addition, all three PrP glycoforms were susceptible to PS-DNA treatment. Deletion of the N-terminal domain (amino acids 23-99), but not of the other domains of PrP, abrogated its PS-DNA-mediated down-regulation. PrP versions localized in the mitochondria, cytoplasm, or nucleus were not modulated by PS-DNA, indicating that PrP surface exposure is required for executing this effect. Proteins that in their native forms were not responsive to PS-DNA, such as thymocyte antigen 1 (Thy1), Doppel protein (Dpl), green fluorescent protein (GFP), and cyan fluorescent protein (CFP), became susceptible to PS-DNA-mediated down-regulation following introduction of the N terminus of PrP into their sequence. These observations demonstrate the essential role of the N-terminal domain for promoting oligonucleotide-mediated reduction of the PrP level and suggest that transient treatment of cultured cells with PS-DNA may provide a general method for targeted modulation of the levels of desired surface proteins in a conditional and reversible manner.

Monitoring immune cells trafficking fluorescent prion rods hours after intraperitoneal infection

Presence of an abnormal form a host-encoded prion protein (PrPC) that is protease resistant, pathologic and infectious characterizes prion diseases such as Chronic Wasting Disease (CWD) of cervids and scrapie in sheep. The Prion hypothesis asserts that this abnormal conformer constitutes most or all of the infectious prion. The role of the immune system in early events in peripheral prion pathogenesis has been convincingly demonstrated for CWD and scrapie. Transgenic and pharmacologic studies in mice revealed an important role of the Complement system in retaining and replicating prions early after infection. In vitro and in vivo studies have also observed prion retention by dendritic cells, although their role in trafficking remains unclear. Macrophages have similarly been implicated in early prion pathogenesis, but these studies have focused on events occurring weeks after infection. These prior studies also suffer from the problem of differentiating between endogenous PrP(C) and infectious prions. Here we describe a semiquantitative, unbiased approach for assessing prion uptake and trafficking from the inoculation site by immune cells recruited there. Aggregated prion rods were purified from infected brain homogenate by detergent solubilization of non-aggregated proteins and ultracentrifugation through a sucrose cushion. Polyacrylamide gel electrophoresis, coomassie blue staining and western blotting confirmed recovery of highly enriched prion rods in the pelleted fraction. Prion rods were fluorochrome-labeled then injected intraperitoneally into mice. Two hours later immune cells from peritoneal lavage fluid, spleen and mediastinal and mesenteric lymph nodes were assayed for prion rod retention and cell subsets identified by multicolor flow cytometry using markers for monocytes, neutrophils, dendritic cells, macrophages and B and T cells. This assay allows for the first time direct monitoring of immune cells acquiring and trafficking prions in vivo within hours after infection. This assay also clearly differentiates infectious, aggregated prions from PrPC normally expressed on host cells, which can be difficult and lead to data interpretation problems in other assay systems. This protocol can be adapted to other inoculation routes (oral, intravenous, intranervous and subcutaneous, e.g.) and antigens (conjugated beads, bacterial, viral and parasitic pathogens and proteins, egg) as well.

Repetitive immunization enhances the susceptibility of mice to peripherally administered prions

The susceptibility of humans and animals to prion infections is determined by the virulence of the infectious agent, by genetic modifiers, and by hitherto unknown host and environmental risk factors. While little is known about the latter two, the activation state of the immune system was surmised to influence prion susceptibility. Here we administered prions to mice that were repeatedly immunized by two initial injections of CpG oligodeoxynucleotides followed by repeated injections of bovine serum albumin/alum. Immunization greatly reduced the required dosage of peripherally administered prion inoculum necessary to induce scrapie in 50% of mice. No difference in susceptibility was observed following intracerebral prion challenge. Due to its profound impact onto scrapie susceptibility, the host immune status may determine disease penetrance after low-dose prion exposure, including those that may give rise to iatrogenic and variant Creutzfeldt-Jakob disease.

Prion-induced activation of cholesterogenic gene expression by Srebp2 in neuronal cells

Prion diseases are neurodegenerative diseases associated with the accumulation of a pathogenic isoform of the host-encoded prion protein. The cellular responses to prion infection are not well defined. By performing microarray analysis on cultured neuronal cells infected with prion strain 22L, in the group of up-regulated genes we observed predominantly genes of the cholesterol pathway. Increased transcript levels of at least nine enzymes involved in cholesterol synthesis, including the gene for the rate-limiting hydroxymethylglutaryl-CoA reductase, were detected. Up-regulation of cholesterogenic genes was attributable to a prion-dependent increase in the amount and activity of the sterol regulatory element-binding protein Srebp2, resulting in elevated levels of total and free cellular cholesterol. The up-regulation of cholesterol biosynthesis appeared to be a characteristic response of neurons to prion challenge, as cholesterogenic transcripts were also elevated in persistently infected GT-1 cells and prion-exposed primary hippocampal neurons but not in microglial cells and primary astrocytes. These results convincingly demonstrate that prion propagation not only depends on the availability of cholesterol but that neuronal cells themselves respond to prions with specific up-regulation of cholesterol biosynthesis.

Pathologic prion protein infects cells by lipid-raft dependent macropinocytosis

Transmissible spongiform encephalopathies, including variant-Creutzfeldt-Jakob disease (vCJD) in humans and bovine spongiform encephalopathies in cattle, are fatal neurodegenerative disorders characterized by protein misfolding of the host cellular prion protein (PrP(C)) to the infectious scrapie form (PrP(Sc)). However, the mechanism that exogenous PrP(Sc) infects cells and where pathologic conversion of PrP(C) to the PrP(Sc) form occurs remains uncertain. Here we report that similar to the mechanism of HIV-1 TAT-mediated peptide transduction, processed mature, full length PrP contains a conserved N-terminal cationic domain that stimulates cellular uptake by lipid raft-dependent, macropinocytosis. Inhibition of macropinocytosis by three independent means prevented cellular uptake of recombinant PrP; however, it did not affect recombinant PrP cell surface association. In addition, fusion of the cationic N-terminal PrP domain to a Cre recombinase reporter protein was sufficient to promote both cellular uptake and escape from the macropinosomes into the cytoplasm. Inhibition of macropinocytosis was sufficient to prevent conversion of PrP(C) to the pathologic PrP(Sc) form in N2a cells exposed to strain RML PrP(Sc) infected brain homogenates, suggesting that a critical determinant of PrP(C) conversion occurs following macropinocytotic internalization and not through mere membrane association. Taken together, these observations provide a cellular mechanism that exogenous pathological PrP(Sc) infects cells by lipid raft dependent, macropinocytosis.

Small-ruminant lentivirus enhances PrPSc accumulation in cultured sheep microglial cells

Sheep scrapie is the prototypical transmissible spongiform encephalopathy (prion disease), which has a fundamental pathogenesis involving conversion of normal cellular prion protein (PrP(C) [C superscript stands for cellular]) to disease-associated prion protein (PrP(Sc) [Sc superscript stands for sheep scrapie]). Sheep microglial cell cultures, derived from a prnp 136VV/171QQ near-term fetal brain, were developed to study sheep scrapie in the natural host and to investigate potential cofactors in the prion conversion process. Two culture systems, a primary cell culture and a cell line transformed with the large T antigen of simian virus 40, were developed, and both were identified as microglial in origin as indicated by expression of several microglial phenotype markers. Following exposure to PrP(Sc), sheep microglial cells demonstrated relatively low levels (transformed cell line) to high levels (primary cell line) of PrP(Sc) accumulation over time. The accumulated PrP(Sc) demonstrated protease resistance, an inferred beta-sheet conformation (as determined by a commercial enzyme-linked immunosorbent assay), specific inhibition by anti-PrP antibodies, and was transmissible in a dose-dependent manner. Primary microglia coinfected with a small-ruminant lentivirus (caprine arthritis encephalitis virus-Cork strain) and PrP(Sc) demonstrated an approximately twofold increase in PrP(Sc) accumulation compared to that of primary microglia infected with PrP(Sc) alone. The results demonstrate the in vitro utility of PrP(Sc)-permissive sheep microglial cells in investigating the biology of natural prion diseases and show that small-ruminant lentiviruses enhance prion conversion in cultured sheep microglia.

Contribution of antibody and T cell-specific responses to the progression of 139A-scrapie in C57BL/6 mice immunized with prion protein peptides

Prion diseases are associated with the conversion of the normal host cellular prion protein to an abnormal protease-resistant (PrPres) associated with infectivity. No specific immune response against prions develops during infection due to the strong tolerance to cellular prion protein. We examined the protective potential on prion diseases of immune responses elicited in C57BL/6 mice with PrP peptides 98-127 (P5) or 158-187 (P9) with CpG. After immunization, P5-treated mice developed high titer and long-lasting Abs, and P9-treated mice developed transient IFN-gamma secreting T cells and poor and variable Ab responses. Both treatments impaired early accumulation of PrPres in the spleen and prolonged survival of mice infected with 139A scrapie. Additional P9 boosts after 139A infection sustained the T cell response and partially inhibited PrPres early accumulation but did not improve the survival. Surprisingly, when P9 injections were started 1 mo after infection and repeated subsequently, specific T cell and Ab responses were impaired and no beneficial effect on prion disease was observed. After a single injection of P9, the number of IFN-gamma secreting CD4+ T cells was also reduced in mice 8- to 10-wk postinfection compared with healthy mice. In vivo and in vitro removal of CD4+CD25+ T cells restored the T cell response to P9 in infected mice. In conclusion, CD4+ T cells as well as Abs might participate to the protection against scrapie. Of importance, the peripheral accumulation of PrPres during infection negatively interferes with the development of T and B cell responses to PrP and regulatory T cells might contribute to this phenomenon.

Treatment by CpG or Flt3-ligand does not affect mouse susceptibility to BSE prions

Dendritic cells (DC) have been suspected to play an important role in prion diseases. We evaluated the role of DC in a murine model of Bovine Spongiform Encephalopathy (BSE) by the use of the growth factor Flt3 ligand, which stimulates DC generation, and CpG oligodeoxynucleotides, which induce DC maturation. We observed that pre-treatments or treatments with Flt3-L or CpG alter neither the time course of prion disease nor the accumulation of the protease-resistant prion protein in intraperitoneally infected mice.