Scrapie prion protein accumulation by scrapie-infected neuroblastoma cells abrogated by exposure to a prion protein antibody

A single prion protein peptide can elicit a panel of isoform specific monoclonal antibodies

The main step in the pathogenesis of transmissible spongiform encephalopathies (TSE) is the conformational change of the normal cellular prion protein (PrP(C)) into the abnormal isoform, named prion (PrP(Sc)). Since PrP is a highly conserved protein, the production of monoclonal antibodies (mAbs) of high specificity and affinity to PrP is a difficult task. In the present study we show that it is possible to overcome the unresponsiveness of the immune system by immunizing wild-type BALB/c mice with a 13 amino acid PrP peptide from the C-terminal part of PrP, bound to the keyhole limpet hemocyanin (KLH). Immunization induced predominantly anti-PrP(Sc) humoral immune response. Furthermore, we were able to obtain a panel of mAbs of IgG class specific for different non-self-conformations of PrP, with anti-PrP(Sc)-specific mAbs being the most abundant.

Directed Evolution of an Anti-prion Protein scFv Fragment to an Affinity of 1 pM and its Structural Interpretation

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative prion disease affecting cattle that is transmissible to humans, manifesting as a variant of Creutzfeldt-Jakob disease (vCJD) likely following the consumption of meat contaminated with BSE prions. High-affinity antibodies are a prerequisite for the development of simple, highly sensitive and non-invasive diagnostic tests that are able to detect even small amounts of the disease-associated PrP conformer (PrP(Sc)). We describe here the affinity maturation of a single-chain Fv antibody fragment with a binding affinity of 1 pM to a peptide derived from the unstructured region of bovine PrP (BoPrP (90-105)). This is the tightest peptide-binding antibody reported to date and may find useful application in diagnostics, especially when PrP(Sc) is pretreated by denaturation and/or proteolysis for peptide-like presentation. Several rounds of directed evolution and off-rate selection with ribosome display were performed using an antibody library generated from a single PrP binder with error-prone PCR and DNA-shuffling. As the correct determinations of affinities in this range are not straightforward, competition biosensor techniques and KinExA methods were both applied and compared. Structural interpretation of the affinity improvement was performed based on the crystal structure of the original prion binder in complex with the BoPrP (95-104) peptide by modeling the corresponding mutations.

The intriguing prion disorders

Prion diseases are among the most intriguing illnesses. Despite their rare incidence, they have captured enormous attention from the scientific community and general public. One of the most hotly debated issues in these diseases is the nature of the infectious material. In recent years increasing evidence has emerged supporting the protein-only hypothesis of prion transmission. In this model PrPSc (the pathological isoform of the prion protein, PrPC) represents the sole component of the infectious particle. However, uncertainties about possible additional factors involved in the conversion of PrPC into PrPSc remain despite extensive attempts to isolate and characterize these elusive components. In this article, we review recent developments concerning the protein-only hypothesis as well as the possible involvement of cellular factors in PrPC to PrPSc conformational change and their influence on the pathogenesis of prion diseases.

The charge structure of helix 1 in the prion protein regulates conversion to pathogenic PrPSc

The prion diseases are transmissible neurodegenerative disorders linked to a pathogenic conformer (PrP(Sc)) of the normal prion protein (PrP(C)). Accumulation of PrP(Sc) occurs via a poorly defined process in which PrP(Sc) complexes with and converts endogenous PrP(C) to nascent PrP(Sc). Recent experiments have focused on the highly charged first alpha helix (H1) of PrP. It has been proposed that two putative asparagine-to-arginine intrahelical salt bridges stabilize H1 in PrP(C) yet form intermolecular ionic bonds with adjacent PrP molecules during conversion of PrP(C) to PrP(Sc) (M. P. Morrissey and E. I. Shakhnovich, Proc. Natl. Acad. Sci. USA 96:11293-11298, 1999). Subsequent work (J. O. Speare et al., J. Biol. Chem. 278:12522-12529, 2003 using a cell-free assay of PrP(Sc) conversion suggested that rather than promoting conversion, the salt bridges stabilize PrP(C) against it. However, the role of individual H1 charges in PrP(Sc) generation has not yet been investigated. To approach this question, we systematically reversed or neutralized each charged residue in H1 and tested the effect on conversion to PrP(Sc) in scrapie-infected murine neuroblastoma (ScN2a) cells. We find that replacements of charged H1 residues with like charges permit conversion, while charge reversals hinder it. Neutralization of charges in the N-terminal (amino acids 143 to 146) but not the C-terminal (amino acids 147 to 151) half of H1 permits conversion, while complete reversal of charge orientation of the putative salt bridges produces a nonconvertible PrP. Circular dichroism spectroscopy studies and confocal microscopy immunofluorescence localization studies indicated that charge substitutions did not alter the secondary structure or cell surface expression of PrP(C). These data support the necessity of specific charge orientations in H1 for a productive PrP(Sc)-PrP(C) complex.

Green fluorescent protein as a reporter of prion protein folding

Background

The amino terminal half of the cellular prion protein PrP^c is implicated in both the binding of copper ions and the conformational changes that lead to disease but has no defined structure. However, as some structure is likely to exist we have investigated the use of an established protein refolding technology, fusion to green fluorescence protein (GFP), as a method to examine the refolding of the amino terminal domain of mouse prion protein.

Results

Fusion proteins of PrP^c and GFP were expressed at high level in E.coli and could be purified to near homogeneity as insoluble inclusion bodies. Following denaturation, proteins were diluted into a refolding buffer whereupon GFP fluorescence recovered with time. Using several truncations of PrP^c the rate of refolding was shown to depend on the prion sequence expressed. In a variation of the format, direct observation in E.coli, mutations introduced randomly in the PrP^c protein sequence that affected folding could be selected directly by recovery of GFP fluorescence.

Conclusion

Use of GFP as a measure of refolding of PrP^c fusion proteins in vitro and in vivo proved informative. Refolding in vitro suggested a local structure within the amino terminal domain while direct selection via fluorescence showed that as little as one amino acid change could significantly alter folding. These assay formats, not previously used to study PrP folding, may be generally useful for investigating PrP^c structure and PrP^c-ligand interaction.

Prion diseases of humans and farm animals: epidemiology, genetics, and pathogenesis

Neuronal vacuolation (spongiosis), neuronal death, and pronounced glial reactions are the hallmarks of transmissible spongiform encephalopathies (TSEs), or prion diseases. A wealth of physical, biochemical, and immunological evidence indicates that the TSE agent, termed prion, does not contain agent-specific nucleic acid encoding its own constituents, as is the case for all other infectious pathogens. Also, no adaptive immune responses are elicited upon infection. A defining feature of TSEs is the deposition, mainly in the brain and lymphoreticular tissues, of an aggregated and structurally abnormal protein, designated PrP(Sc) or PrP-res, which represents a conformational isomer of the ubiquitous surface protein PrP(C). Biochemical and genetic evidence link PrP and its gene to the disease. Although TSEs are by definition transmissible, a growing number of Prnp-associated non-infectious neurodegenerative proteinopathies are now being recognized.

Neural stem cell model for prion propagation

The study of prion transmission and targeting is a major scientific issue with important consequences for public health. Only a few cell culture systems that are able to convert the cellular isoform of the prion protein into the pathologic scrapie isoform of the prion protein (PrP(Sc)) have been described. We hypothesized that central nervous system neural stem cells (NSCs) could be the basis of a new cell culture model permissive to prion infection. Here, we report that monolayers of differentiated fetal NSCs and adult multipotent progenitor cells isolated from mice were able to propagate prions. We also demonstrated the large influence of neural cell fate on the production of PrP(Sc), allowing the molecular study of prion neuronal targeting in relation with strain differences. This new stem cell-based model, which is applicable to different species and to transgenic mice, will allow thoughtful investigations of the molecular basis of prion diseases, and will open new avenues for diagnostic and therapeutic research.

Clearance and prevention of prion infection in cell culture by anti-PrP antibodies

Prion diseases are transmissible and invariably fatal neurodegenerative disorders associated with a conformational transformation of the cellular prion protein (PrP(C)) into a self-replicating and proteinase K (PK)-resistant conformer, scrapie PrP (PrP(Sc)). Humoral immunity may significantly prolong the incubation period and even prevent disease in murine models of prionoses. However, the mechanism(s) of action of anti-PrP monoclonal antibodies (Mabs) remain(s) obscure. The murine neuroblastoma N2a cell line, infected with the 22L mouse-adapted scrapie strain, was used to screen a large library of Mabs with similar binding affinities to PrP, to identify those antibodies which could clear established infection and/or prevent infection de novo. Three Mabs were found capable of complete and persistent clearing of already-infected N2a cells of PrP(Sc). These antibodies were 6D11 (generated to PK-resistant PrP(Sc) and detecting PrP residues 93-109), and 7H6 and 7A12, which were raised against recombinant PrP and react with neighbouring epitopes of PrP residues 130-140 and 143-155, respectively. Mabs were found to interact with PrP(Sc) formation both on the cell surface and after internalization in the cytosol. Treatment with Mabs was not associated with toxicity nor did it result in decreased expression of PrP(C). Both preincubation of N2a cells with Mabs prior to exposure to 22L inoculum and preincubation of the inoculum with Mabs prior to infecting N2a cells resulted in a significant reduction in PrP(Sc) levels. Information provided in these studies is important for the rational design of humoral immune therapy for prion infection in animals and eventually in humans.

Removal of the glycosylphosphatidylinositol anchor from PrP(Sc) by cathepsin D does not reduce prion infectivity

According to the protein-only hypothesis of prion propagation, prions are composed principally of PrP(Sc), an abnormal conformational isoform of the prion protein, which, like its normal cellular precursor (PrP(C)), has a GPI (glycosylphosphatidylinositol) anchor at the C-terminus. To date, elucidating the role of this anchor on the infectivity of prion preparations has not been possible because of the resistance of PrP(Sc) to the activity of PI-PLC (phosphoinositide-specific phospholipase C), an enzyme which removes the GPI moiety from PrP(C). Removal of the GPI anchor from PrP(Sc) requires denaturation before treatment with PI-PLC, a process that also abolishes infectivity. To circumvent this problem, we have removed the GPI anchor from PrP(Sc) in RML (Rocky Mountain Laboratory)-prion-infected murine brain homogenate using the aspartic endoprotease cathepsin D. This enzyme eliminates a short sequence at the C-terminal end of PrP to which the GPI anchor is attached. We found that this modification has no effect (i) on an in vitro amplification model of PrP(Sc), (ii) on the prion titre as determined by a highly sensitive N2a-cell based bioassay, or (iii) in a mouse bioassay. These results show that the GPI anchor has little or no role in either the propagation of PrP(Sc) or on prion infectivity.

Prion diseases: current understanding of epidemiology and pathogenesis, and therapeutic advances

The bovine spongiform encephalopathy (BSE) epidemic, along with the related threat to human health posed by the transmission of the BSE agent to humans, has highlighted the importance of prion diseases. These fatal neurodegenerative diseases are characterised by spongiform changes in the CNS, and comprise a wide spectrum of clinicopathological entities in humans and animals, such as Creutzfeldt-Jakob disease (CJD) and its emerging new variant (vCJD) in humans, and BSE and scrapie in animals. This article reviews the geographical distribution and the temporal trends of CJD and vCJD; the major events in the pathogenesis of prion diseases; the risk factors for sporadic CJD and vCJD; and the possible strategies for treating them. Worldwide statistics indicate that sporadic CJD has a stable incidence of one case per million people per year; in contrast, the incidence of vCJD appears to have increased exponentially from its characterisation in 1994 to a peak in 2000. As of December 2005, 183 definite or probable cases of vCJD had been reported worldwide. The crucial event in the pathogenesis of prion diseases is the conversion of the normally occurring cellular prion protein (PrP(c)) into a pathogenic form, called protease-resistant PrP (PrP(res)) or scrapie PrP (PrP(sc)). Pathogenetic studies in rodent models have shown that PrP(sc) is found in the enteric nervous system and in the gut-associated lymphoid tissue following oral scrapie ingestion. The role of the lymphoreticular system in the pathogenesis of TSE seems to be related to the strains of agents and the host genotype. Therapeutic approaches to vCJD are mainly based on the inhibition or prevention of the pathological change that creates PrP(sc). Derivatives of acridine (such as mepacrine [quinacrine]) and the phenothiazine psychotropics have been proposed as possible therapies because of their activity in cellular models; however, neither class was able to affect the protease resistance of preexisting PrP fibrils. More encouragingly, in animal models of prion disease, tetracyclines were found to reduce prion infectivity by direct inactivation of PrP(sc). While these findings are promising, the suitability of these compounds for clinical use is still limited by their low efficacy once symptoms are apparent. Treatments based on the vaccination approach have also produced positive results, but further investigations are necessary to establish their clinical application.

Antibody-mediated neuronal apoptosis: therapeutic implications for prion diseases

Neuronal cell death is considered to be a hallmark in prion diseases. These disorders are believed to result from the post-translational conversion of a normal cell membrane sialoglycoprotein PrPC, composed primarily of alpha-helical structure, into a disease specific isoform, PrPSc that is rich in beta-sheet and partially proteinase-resistant. Recent in vivo studies indicate that prion replication can be inhibited by anti-PrP monoclonal antibodies that led to the indefinite delay in the development of prion disease. The recent report by Solforosi and colleagues has increased the need to understand pathway(s) leading to prion-associated apoptosis and neuronal death thought to be the cause of death in transmissible spongiform encephalopathy (TSE) individuals. Furthermore, these reports increased momentum about the use of antibody-based therapy in prion diseases, although great caution should be exerted when using anti-prion antibodies directly into the central nervous system (CNS) with special emphasis on refined strategies such as specific targeting of regions of the prion protein thought not to be involved in signalling pathways.

Mouse-adapted scrapie infection of SN56 cells: greater efficiency with microsome-associated versus purified PrP-res

The process by which transmissible spongiform encephalopathy agents, or prions, infect cells is unknown. We employed a new differentiable cell line (SN56) susceptible to infection with three mouse-adapted scrapie strains to gain insight into the cellular infection process. The effect of disease-associated PrP (PrP-res) association with microsomal membranes on infection efficiency was examined by comparing sustained PrP-res production in cells treated with either scrapie brain microsomes or purified, detergent-extracted PrP-res. When normalized for quantity of input PrP-res, scrapie brain microsomes induced dramatically enhanced persistent PrP-res formation compared to purified PrP-res. Infected SN56 cells released low levels of PrP-res into the culture supernatant, which also efficiently initiated infection in recipient cells. Interestingly, microsomes labeled with a fluorescent marker were internalized by SN56 cells in small vesicles, which were subsequently found in neuritic processes. When bound to culture wells to reduce internalization during the infection process, scrapie microsomes induced less long-term PrP-res production than suspended microsomes. Long-term differentiation of infected SN56 cells was accompanied by a decrease in PrP-res formation. Our observations provide evidence that infection of cells is aided by the association of PrP-res with membranes and/or other microsomal constituents.

Inhibitors of the mitogen-activated protein kinase kinase 1/2 signaling pathway clear prion-infected cells from PrPSc

Prions represent a unique class of infectious agents in which the normal cellular prion protein (PrPC) is converted to an abnormal isoform (PrPSc), which accumulates in the brain and constitutes the major, if not the only, component of the infectious particle. Factors that still remain to be identified may facilitate the conversion of PrPC to PrPSc. In the present study, we first demonstrated that a growth factor of the neurotrophin family, brain-derived neurotrophic factor (BDNF), stimulates the formation of PrPSc in a gonadotropin-releasing hormone-secreting neuronal cell line (GT1-1 cells) infected with the Rocky Mountain Laboratory (RML) strain of scrapie as determined by Western blot analysis. We then observed that the prion-infected cells can be cleared from PrPSc by treatment with three inhibitors of mitogen-activated protein kinase kinase 1/2 (MEK1/2) [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene and 2-(2-amino-3-methyoxyphenyl)-4H-1-benzopyran-4-one, as well as alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl) benzeneacetonitrile, which passes the blood-brain barrier], a component of one of the intracellular signaling pathways activated by BDNF. The MEK1/2 inhibitors were also efficient in clearing PrPSc from prion-infected GT1-1 cells stimulated to accumulate high levels of PrPSc by enhanced serum concentrations in the medium or by the use of a serum-free neuron-specific neurobasal medium. PrPSc did not reappear in the cultures within 5 weeks after completion of treatment. We conclude that inhibitors of the MEK1/2 pathway can efficiently and probably irreversibly clear PrP(Sc) from prion-infected cells. The MEK pathway may therefore be a suitable target for therapeutic intervention in prion diseases.

Lymphocyte Toxicity of Prion Fragments

Prion protein fragments that are extracted from the brains of patients with Gerstmann-Straussler-Scheinker disease are known to have stimulating action on circulating leukocytes. In particular, the amyloidogenic hydrophobic prion peptide HuPrP (113-127) AGAAAAGAVVGGLGG has been reported to be associated with significant cellular toxicity. In this paper we show that the self assembled form of HuPrP (113-127) and its valine rich domains viz. GAVVGGLG [HuPrP (119-126)] and VVGGLGG [HuPrP (121-127)] are toxic to peripheral lymphocytes. To explore the cytotoxic mechanism of these fragments, we studied 3-(4,5-dimethylthiazol-2yl)-2-5-diphenyltetrazolium bromide (MTT) reduction, reactive oxygen species (ROS) generation, calcium influx and raft sequestration of' peptide treated lymphocytes. Langmuir monolayer studies on these peptides showed a maximum lipid perturbing property of HuPrP (121-127) as compared to the other two fragments. MTT reduction assays on lymphocytes treated with peptides indicated that the prion peptide fibrils are relatively more toxic than freshly solubilized peptide preparations. Lymphocytes treated with HuPrP (121-127), HuPrP (113-127) and HuPrP (119-126) fibrils underwent 60%, 30% and 40% cell death, respectively. Abeta(1-42), HuPrP (119-126) and HuPrP (121-127) fibrils caused 4 fold increases in intracellular ROS as compared with control cells. However, HuPrP (113-127) fibrils lacked such a significant ROS generating activity, indicating that a subtle difference in sequence leads to a difference in the toxic mechanism in the cell. HuPrP (119-126) and HuPrP (121-127) fibrils also produced maximum raft sequestration and calcium influx. Taken together, these data suggest that the assemblage of prion fragments has significant toxic activity on peripheral lymphocytes, a finding with implications for controlling reactive lymphocytes in prion infected subjects.

Prion protein-specific antibodies for therapeutic intervention of transmissible spongiform encephalopathies

Prion diseases, also called transmissible spongiform encephalopathies, are a group of fatal neurodegenerative conditions that affect humans and a wide variety of animals. There is no therapeutic or prophylactic approach against prion diseases available at present. The causative infectious agent is the prion, also termed PrPSc, which is a pathological conformer of the cellular prion protein PrPC. Passive immunisation studies with PrPC-specific antibodies indicated that immunotherapeutic strategies directed against PrPC can prevent prion disease. In this review, putative mechanisms of antibody-mediated prion inactivation, as well as active immunisation strategies, are discussed. Special attention is given to the problem of immunological self-tolerance against PrP.

Prion infection of mouse neurospheres

Only a few cell lines have been infected with prions, offering limited genetic diversity and sensitivity to several strains. Here we report that cultured neurospheres expressing cellular prion protein (PrP(C)) can be infected with prions. Neurosphere lines isolated from the brains of mice at embryonic day 13-15 grow as aggregates and contain CNS stem cells. We produced neurosphere cultures from FVB/NCr (FVB) mice, from transgenic (Tg) FVB mice that overexpress mouse PrP-A (Tg4053), and from congenic FVB mice with a targeted null mutation in the PrP gene (Prnp(0/0)) and incubated them with the Rocky Mountain Laboratory prion strain. While monitoring the levels of disease-causing PrP (PrP(Sc)) at each passage, we observed a dramatic rise in PrP(Sc) levels with time in the Tg4053 neurosphere cells, whereas the level of PrP(Sc) decayed to undetectable levels in cell cultures lacking PrP. PrP(Sc) levels in cultures from FVB mice initially declined but then increased with passage. Prions produced in culture were transmissible to mice and produced disease pathology. Intracellular aggregates of PrP(Sc) were present in cells from infected cultures. The susceptibility of neurosphere cultures to prions mirrored that of the mice from which they were derived. Neurosphere lines from Tg4053 mice provide a sensitive in vitro bioassay for mouse prions; neurosphere lines from other Tg mice overexpressing PrP might be used to assay prions from other species, including humans.

Surface Plasmon Resonance Analysis for the Screening of Anti-prion Compounds

The interaction of anti-prion compounds and amyloid binding dyes with a carboxy-terminal domain of prion protein (PrP121-231) was examined using surface plasmon resonance (SPR) and compared with inhibition activities of abnormal PrP formation in scrapie-infected cells. Most examined compounds had affinities for PrP121-231: antimalarials had low affinities, whereas Congo red, phthalocyanine and thioflavin S had high affinities. The SPR binding response correlated with the inhibition activity of abnormal PrP formation. Several drugs were screened using SPR to verify the findings: propranolol was identified as a new anti-prion compound. This fact indicates that drug screenings by this assay are useful.

Diagnostic approaches for viruses and prions in stem cell banks

Some stem cell lines may contain an endogenous virus or can be contaminated with exogenous viruses (even of animal origin) and may secrete viral particles or express viral antigens on their surface. Moreover, certain biotechnological products (e.g. bovine fetal serum, murine feeder cells) may contain prion particles. Viral and prion contamination of cell cultures and "feeder" cells, which is a common risk in all biotechnological products derived from the cell lines, is the most challenging and potentially serious outcome to address, due to the difficulty involved in virus and prion detection and the potential to cause serious disease in recipients of these cell products. Stem cell banks should introduce adequate quality assurance programs like the microbiological control program and can provide researchers with valuable support in the standardization and safety of procedures and protocols used for the viral and prion testing and in validation programs to assure the quality and safety of the cells.

KDEL-tagged anti-prion intrabodies impair PrP lysosomal degradation and inhibit scrapie infectivity

Transmissible spongiform encephalopathy or prion diseases are fatal neurodegenerative disorders characterized by the conversion of the cellular prion protein (PrPC) into the infectious scrapie isoform (PrPSc). We have recently demonstrated that anti-prion intrabodies targeted to the lumen of the endoplasmic reticulum provide a simple and effective means to inhibit the transport of PrPC to the cell surface. Here, we report that they completely block the traffic of mature full-length PrPC molecules, impair prion lysosomal degradation, and interfere with the early phase of scrapie formation. Since anti-prion intrabodies efficiently block PrPSc accumulation in vitro, we investigated whether they could also antagonize scrapie infectivity in vivo. We found that mice intracerebrally injected with KDEL-8H4-NGF-differentiated PC12 cells infected with scrapie neither develop scrapie clinical signs nor brain damage. Furthermore, no protease-resistant PrPSc is detectable in brains of inoculated animals. These results indicate that anti-prion intrabody strategy may be effective against prion infection.

In response to pathogens, glial cells dynamically and differentially regulate Toll-like receptor gene expression

The mechanisms that mediate innate immune recognition of CNS infections are unknown. This study provides a comparison of Toll-like receptor (TLR) gene expression in resting and virus infected CNS cells. N2a neuroblastoma cells expressed TLR 3 but demonstrated no change in TLR gene expression in response to either LPS or virus infection. N9 microglia and differentiated primary astrocytes expressed most TLR genes. TLR 2 expression was highest in N9 microglia and TLR 7 in astrocytes. In both glial cell types, LPS stimulation upregulated pro-inflammatory cytokines, TLR 2 and TLR 3 gene expression but down-regulated other TLR genes. RNA virus infection substantially increased levels of type-I interferon (IFN) and TLR 3 transcripts and to a lesser extent TLR 9 transcripts. Microglia and astrocytes thus have the ability to discriminate between pathogens and elicit an appropriate response.

Skin-derived dendritic cells acquire and degrade the scrapie agent following in vitro exposure

The accumulation of the scrapie agent in lymphoid tissues following inoculation via the skin is critical for efficient neuroinvasion, but how the agent is initially transported from the skin to the draining lymph node is not known. Langerhans cells (LCs) are specialized antigen-presenting cells that continually sample their microenvironment within the epidermis and transport captured antigens to draining lymph nodes. We considered LCs probable candidates to acquire and transport the scrapie agent after inoculation via the skin. XS106 cells are dendritic cells (DCs) isolated from mouse epidermis with characteristics of mature LC cells. To investigate the potential interaction of LCs with the scrapie agent XS106 cells were exposed to the scrapie agent in vitro. We show that XS106 cells rapidly acquire the scrapie agent following in vitro exposure. In addition, XS106 cells partially degrade the scrapie agent following extended cultivation. These data suggest that LCs might acquire and degrade the scrapie agent after inoculation via the skin, but data from additional experiments demonstrate that this ability could be lost in the presence of lipopolysaccharide or other immunostimulatory molecules. Our studies also imply that LCs would not undergo maturation following uptake of the scrapie agent in the skin, as the expression of surface antigens associated with LC maturation were unaltered following exposure. In conclusion, although LCs or DCs have the potential to acquire the scrapie agent within the epidermis our data suggest it is unlikely that they become activated and stimulated to transport the agent to the draining lymph node.

The Murine B Cell Repertoire Is Severely Selected against Endogenous Cellular Prion Protein

Abs to the prion protein (PrP) can protect against experimental prion infections, but efficient Ab responses are difficult to generate because PrP is expressed on many tissues and induces a strong tolerance. We previously showed that immunization of wild-type mice with PrP peptides and CpG oligodeoxynucleic acid overcomes tolerance and induces cellular and humoral responses to PrP. In this study, we compared Ab and T cell repertoires directed to PrP in wild-type and PrP knockout (Prnp o/o) C57BL/6 mice. Animals were immunized with mouse PrP-plasmid DNA or with 30-mer overlapping peptides either emulsified in CFA or CpG/IFA. In Prnp o/o mice, Abs raised by PrP-plasmid DNA immunization recognized only N-terminal PrP peptides; analyses of Ab responses after PrP peptide/CFA immunization allowed us to identify six distinct epitopes, five of which were also recognized by Abs raised by PrP peptides/CpG. By contrast, in wild-type mice, no Ab response was detected after PrP-plasmid DNA or peptide/CFA immunization. However, when using CpG, four C-terminal peptides induced Abs specific for distinct epitopes. Importantly, immune sera from Prnp o/o but not from wild-type mice bound cell surface PrP. Abs of IgG1 and IgG2b subclasses predominated in Prnp o/o mice while the strongest signals were for IgG2b in wild-type mice. Most anti-PrP Th cells were directed to a single epitope in both Prnp o/o and wild-type mice. We conclude that endogenous PrPC expression profoundly affects the Ab repertoire as B cells reactive for epitopes exposed on native PrPC are strongly tolerized. Implications for immunotherapy against prion diseases are discussed.

Processing of ovine PrP(ARQ)C-EGFP chimeras containing Asn138 and Cys151 polymorphisms

Polymorphisms in the prion protein, PrP(C), affect the susceptibility of sheep to scrapie. Three rare polymorphisms, M137T, S138N, and R151C, have been found in Icelandic sheep. Observations suggest that R151C may be associated with lower scrapie susceptibility, whereas S138N is neutral. The effects of the S138N and R151C polymorphisms on the cellular processing of PrP(C) were examined in a model system consisting of the expression of ovine PrP(C)-EGFP (green fluorescent protein) chimeras in the mouse neuroblastoma cell line N2a. Chimeras with the haplotypes A136R154Q171 (ARQ), AN138RQ, and AC151RQ were compared. The chimeras did not differ regarding their translocation into the secretory system, glycosylation, and transport to the cell surface. However, the AC151RQ chimera differed from the other chimeras regarding disulfide bonding characteristics; furthermore, a slight difference was detected between AC151RQ and the other chimeras by limited proteolysis. The processing of the ARQ and AN138RQ chimeras was identical in the experiments performed consistent with observations that it is neutral.

An aggregation-specific enzyme-linked immunosorbent assay: detection of conformational differences between recombinant PrP protein dimers and PrP(Sc) aggregates

The conversion of the normal cellular prion protein, PrP(C), into the protease-resistant, scrapie PrP(Sc) aggregate is the cause of prion diseases. We developed a novel enzyme-linked immunosorbent assay (ELISA) that is specific for PrP aggregate by screening 30 anti-PrP monoclonal antibodies (MAbs) for their ability to react with recombinant mouse, ovine, bovine, or human PrP dimers. One MAb that reacts with all four recombinant PrP dimers also reacts with PrP(Sc) aggregates in ME7-, 139A-, or 22L-infected mouse brains. The PrP(Sc) aggregate is proteinase K resistant, has a mass of 2,000 kDa or more, and is present at a time when no protease-resistant PrP is detectable. This simple and sensitive assay provides the basis for the development of a diagnostic test for prion diseases in other species. Finally, the principle of the aggregate-specific ELISA we have developed may be applicable to other diseases caused by abnormal protein aggregation, such as Alzheimer's disease or Parkinson's disease.

Charged bipolar suramin derivatives induce aggregation of the prion protein at the cell surface and inhibit PrPSc replication

The conversion of the cellular prion protein (PrPc) into a pathogenic isoform (PrP(Sc)) is one of the underlying events in the pathogenesis of the fatal transmissible spongiform encephalopathies (TSEs). Numerous compounds have been described to inhibit prion replication and PrP(Sc) accumulation in cell culture. Among these, the drug suramin induces aggregation and re-targeting of PrPc to endocytic compartments. Plasma membrane and sites of conversion into PrP(Sc) are thereby bypassed. In the present study, a library of suramin analogues was tested as a potential class of new anti-prion compounds and the molecular mechanisms underlying these effects were analysed. Treatment of prion-infected neuroblastoma cells with compounds containing symmetrical aromatic sulfonic acid substitutions inhibited de novo synthesis of PrP(Sc) and induced aggregation and reduction of the half-life of PrPc without downregulating PrPc cell surface expression. Half-molecule compounds lacking the symmetrical bipolar structure or the anionic groups had no effect on PrP(Sc) synthesis or PrPc solubility. Cell surface expression of PrPc was necessary for the activity of effective compounds. Suramin derivatives did not induce aggregation of PrPc when transport along the secretory pathway was compromised, suggesting that their effects occur at a post trans-Golgi network (TGN) site, possibly close to the compartment of conversion into PrP(Sc). In vitro studies with recombinant PrP demonstrated that the inhibitory effect correlated with direct binding to PrP and induction of insoluble PrP aggregates. Our data reveal an anti-prion effect that differs from those characterising other sulphated polyanions and is dependent on the presence of the symmetrical anionic structure of these molecules.

Inhibition of prion propagation in scrapie-infected mouse neuroblastoma cell lines using mouse monoclonal antibodies against prion protein

We screened six mouse monoclonal antibodies (mAbs) against prion protein (PrP), which were previously established in our laboratory, for inhibitory activity against PrP(Sc)-accumulation in scrapie-infected cell lines and identified two mAbs, 3S9 and 2H9, as possessing this inhibitory activity. mAb 3S9 recognized an epitope including 154th tyrosine in the helix 1 region of PrP, while mAb 2H9 recognized a discontinuous region that included helix 1. In three scrapie-infected cell lines infected with different mouse-adapted scrapie strains, mAb 3S9 strongly inhibited accumulation of PrP(Sc), while mAb 2H9 moderately inhibited accumulation of PrP(Sc), indicating that inhibition of prion propagation by mAbs may be dependent on PrP(Sc) characteristics. Furthermore, mAb 3S9 completely excluded PrP(Sc) from these cell lines. These results suggest that mAbs 3S9 and 2H9 might be useful for clarifying the mechanisms of prion propagation and prevention by PrP-specific antibodies, and for tracing the conversion of PrP(C) to other PrP(Sc) isoforms.

Mucosal vaccination delays or prevents prion infection via an oral route

In recent years major outbreaks of prion disease linked to oral exposure of the prion agent have occurred in animal and human populations. These disorders are associated with a conformational change of a normal protein, PrP(C) (prion protein cellular), to a toxic and infectious form, PrP(Sc) (prion protein scrapie). None of the prionoses currently have an effective treatment. A limited number of active immunization approaches have been shown to slightly prolong the incubation period of prion infection. Active immunization in wild-type animals is hampered by auto-tolerance to PrP and potential toxicity. Here we report that mucosal vaccination with an attenuated Salmonella vaccine strain expressing the mouse PrP, is effective at overcoming tolerance to PrP and leads to a significant delay or prevention of prion disease in mice later exposed orally to the 139A scrapie strain. This mucosal vaccine induced gut anti-PrP immunoglobulin (Ig)A and systemic anti-PrP IgG. No toxicity was evident with this vaccination approach. This promising finding suggests that mucosal vaccination may be a useful method for overcoming tolerance to PrP and preventing prion infection among animal and potentially human populations at risk.

Paracrine inhibition of prion propagation by anti-PrP single-chain Fv miniantibodies

Prion diseases are characterized by the deposition of PrP(Sc), an abnormal form of the cellular prion protein PrP(C). A growing body of evidence suggests that antibodies to PrP(C) can antagonize deposition of PrP(Sc). However, host tolerance hampers the induction of immune responses to PrP(C), and cross-linking of PrP(C) by bivalent anti-PrP antibodies is neurotoxic. In order to obviate these problems, we explored the antiprion potential of recombinant single-chain antibody (scFv) fragments. scFv fragments derived from monoclonal anti-PrP antibody 6H4, flagged with c-myc and His6 tags, were correctly processed and secreted by mammalian RD-4 rhabdomyosarcoma cells. When cocultured with cells secreting anti-PrP scFv, chronically prion-infected neuroblastoma cells ceased to produce PrP(Sc), even if antibody-producing cells were physically separated from target cells in transwell cultures. Expression of scFv with irrelevant specificity, or of similarly tagged molecules, was not curative. Therefore, eukaryotically expressed scFv exerts a paracrine antiprion activity. The effector functions encoded by immunoglobulin constant domains are unnecessary for this effect. Because of their small size and their monovalent binding, scFv fragments may represent candidates for gene transfer-based immunotherapy of prion diseases.

Nitric oxide induces prion protein via MEK and p38 MAPK signaling

The prion diseases or transmissible spongiform encephalopathy, such as human Creutzfeldt-Jakob disease (CJD) and so-called mad cow disease, are attributed to the causative agent, the scrapie variant of prion protein (PrP(Sc)) which causes fatal neurodegeneration. To investigate if stresses such as nitric oxide (NO) induced the cellular isoform of prion protein (PrP(C)), lipopolysaccharide, and sodium nitroprusside were used to treat N2a and NT2 cells, which resulted in elevated levels of the PRNP mRNA and prion protein. The signaling pathway for the NO-induced PrP(C) production involved guanylyl cyclase, MEK, and p38 MAPK as shown by the effect of specific pharmacological inhibitors ODQ, PD98059, and SB203580, respectively. Knowing the PrP induction by the biologically existing stimulus, this study provides useful information about the possible cellular mechanism and strategies for the treatment of CJD.

No influence of amyloid-beta-degrading neprilysin activity on prion pathogenesis

Transmissible spongiform encephalopathies are characterized by the accumulation of PrPSc, a protease-resistant form of a host-derived protein termed PrPC. Substantial evidence indicates that PrPSc represents an essential component of the infectious agent, which is termed prion. The accumulation of PrPSc within the central nervous system of prion-infected organisms is a dynamic process that is regulated both by production and by clearance of PrPSc. Although several proteases have been implicated in proteolysis of PrPC, the mechanisms underlying proteolysis of PrPSc remain unclear. Here, it was investigated whether neprilysin, a metalloprotease known to degrade extracellular amyloidogenic proteins such as amyloid-beta, plays a role in prion pathogenesis in vivo. As neprilysin has a broad substrate specificity and is localized subcellularly in the vicinity of PrP, it represents a plausible candidate for prion degradation. Prions were therefore administered to mice lacking or overexpressing neprilysin in brain. However, the gene dosage of neprilysin did not modulate accumulation of PrPSc in brain. Also, incubation times and clinical course of prion disease, as well as brain infectivity titres at terminal stage, were unaffected. These data rule out neprilysin as a major modulator of PrPSc accumulation and prion pathogenesis.

Molecular neurology of prion disease

Prions are infectious pathogens principally composed of abnormal forms of a protein encoded in the host genome. They cause lethal neurodegenerative conditions including CJD, GSS, and kuru in humans and scrapie and bovine spongiform encephalopathy in domestic animals. Remarkably, distinct strains of prions occur despite absence of an agent-specific genome: misfolded proteins themselves may encode strain diversity--with wide implications in biology. The arrival of variant CJD, and the experimental confirmation that it is caused by infection with BSE-like prions, has focussed research on early diagnosis and treatment. Recent advances lead to considerable optimism that effective human therapies may now be developed. While several drugs have been tried in small numbers of patients, there is no clear evidence of efficacy of any agent and controlled clinical trials are urgently needed. Importantly, there is increasing recognition that fundamental processes involved in prion propagation--seeded aggregation of misfolded host proteins--are of far wider significance, not least in understanding the commoner neurodegenerative diseases that pose such a major and increasing challenge for healthcare in an ageing population.

[Prion diseases as zoonosis]

Prion diseases such as bovine spongiform encephalopathy (BSE) have been recognized as zoonosis since the existence of variant Creutzfeldt-Jakob disease (vCJD) was reported in 1996. BSE became a serious social problem even in Japan after the first BSE case was found in 2001. The incidence of BSE in EU and UK appears declining, and the vCJD incidence also shows a tendency to decrease. On the contrary, fears for the spread of BSE became actual problems: BSE occurrence outside of EU, transmission of vCJD by blood transfusion, and the first vCJD case in Japan. To prevent further spread and to reduce the risk of BSE, it is important to continue BSE screening/surveillance, removal of specified risk materials from food and feed chains, and effective feed regulation. For the disclosure and elimination of prion-contaminated blood, materials for medical and pharmaceutical products and so on, it is required to improve the sensitivity of prion detection methods. Furthermore, it is also important to establish therapeutics of human prion diseases.

Scratch-wounding renders cultivated cells less permissive to prion infection

Using permissive cell lines of epithelial or neuroglial origin, we found that scratch-wounding a small proportion of the recipient cells prior to prion exposure strongly reduced the cell culture's susceptibility to infection. We provide evidence suggesting that wound-triggered inhibition of prion infection was mediated by the release of nucleotides in the extracellular medium of injured cultures. While cell wounding or ATP treatment of unwounded target cells inhibited de novo infection, we found that they had no effect on steady-state infected cultures, indicating that these treatments affect the early stages of infection. These findings support the view that cells have the capacity to modulate their permissiveness to prion infection in response to external stimuli, such as a signalling molecule.

Decrease in pathology and progression of scrapie after immunisation with synthetic prion protein peptides in hamsters

Effective therapy for prion diseases is currently unavailable. Recently, vaccination was shown to be effective in mouse models of a particular neurodegenerative conditions: Alzheimer's disease (AD). Here, we report that vaccination with synthetic oligopeptides homologous to the hamster (Mesocricetus auratus) prion protein augments survival time in animals infected intraperitoneally with 263K scrapie agent. For each hamster included in the study, prion-specific serum antibodies as well as deposition of pathological prion protein (PrP(res)), glial fibrillary acidic protein (GFAP), and mRNA expression for cytokines (TNF alpha, IL-1beta, IL-10) in brain tissues were evaluated. In immunized animals, increased survival after challenge was associated with a reduction of cerebral lesion, PrP deposition and GFAP expression; in these animals, anti-prion protein peptide antibody levels were increased, and the expression of pro-inflammatory cytokines (TNF alpha and IL-1beta) was reduced. Vaccination could be an effective therapeutic approach to postpone disease onset.

Circumventing tolerance to the prion protein (PrP): vaccination with PrP-displaying retrovirus particles induces humoral immune responses against the native form of cellular PrP

Passive immunization with antibodies directed against the cellular form of the prion protein (PrPC) can protect against prion disease. However, active immunization with recombinant prion protein has so far failed to induce antibodies directed against native PrPC expressed on the cell surface. To develop an antiprion vaccine, a retroviral display system presenting either the full-length mouse PrP (PrP209) or the C-terminal 111 amino acids (PrP111) fused to the transmembrane domain of the platelet-derived growth factor receptor was established. Western blot analysis and immunogold electron microscopy of the retroviral display particles revealed successful incorporation of the fusion proteins into the particle membrane. Interestingly, retroviral particles displaying PrP111 (PrPD111 retroparticles) showed higher incorporation efficiencies than those displaying PrP209. Already 7 days after intravenous injection of PrPD111 retroparticles, PrPC-deficient mice (Prnp(o/o)) showed high immunoglobulin M (IgM) and IgG titers specifically binding the native PrPC molecule as expressed on the surface of T cells isolated from PrPC-overexpressing transgenic mice. More importantly, heterozygous Prnp(+/o) mice and also wild-type mice showed PrPC-specific IgM and IgG antibodies upon vaccination with PrPD111 retroparticles, albeit at considerably lower levels. Bacterially expressed recombinant PrP, in contrast, was unable to evoke IgG antibodies recognizing native PrPC in wild-type mice. Thus, our data show that PrP or parts thereof can be functionally displayed on retroviral particles and that immunization with PrP retroparticles may serve as a novel promising strategy for vaccination against transmissible spongiform encephalitis.

Update on Creutzfeldt-Jakob disease

PURPOSE OF REVIEW

Prion diseases are transmissible fatal neurodegenerative disorders in which infectivity is associated with the accumulation of PrP(Sc), a disease-related isoform of normal cellular prion protein. The recent emergence of variant Creutzfeldt-Jakob disease has led to major public health concerns, and the need for the development of effective treatments. As PrP(Sc) is associated both with pathology and infectivity, therapeutic approaches to date have largely aimed at preventing its accumulation, but this strategy has produced only modest results in animal models. The link between PrP(Sc) and neurotoxicity is unclear, and alternative pathological processes need to be considered. Here we focus on the latest progress in therapeutic strategies and potential mechanisms of prion neurotoxicity.

RECENT FINDINGS

Passive immunisation with anti-prion protein antibodies prevents peripheral prion replication and blocks progression to clinical disease in peripherally infected mice. A new approach, in which neuronal cellular prion protein is depleted in mice with established neuroinvasive prion infection, prevents the onset of clinical disease, blocks neuronal cell loss and reverses early spongiform pathology. This dramatic protective effect occurs despite the continued build-up of extraneuronal PrP(Sc) and continued replication of prion infectivity, effectively producing a sub-clinical state.

SUMMARY

New insights into the mechanisms of neurotoxicity in prion diseases support the concept that PrP(Sc) itself is not directly neurotoxic. They suggest that neuronal prion propagation results in the production of a toxic intermediate or depletion of a key constituent. Prevention of the formation of such a species rather than PrP(Sc) accumulation itself is a clear target for prion therapeutics.

Protein conformation significantly influences immune responses to prion protein

In prion diseases, such as variant Creutzfeldt-Jakob disease normal cellular prion protein (PrPC), a largely alpha-helical structure is converted to an abnormal conformational isoform (PrPSc) that shows an increase in beta-sheet content. Similarly, the recombinant form of PrPC (ralpha-PrP) can be converted to a conformation dominated by beta-sheet (rbeta-PrP) by reduction and mild acidification in vitro, a process that may mimic in vivo conversion following PrPC internalization during recycling. Despite PrPSc accumulation and prion propagation in the lymphoreticular system before detectable neuroinvasion, no Ab response to PrP has been detected, probably due to immune tolerance. To investigate how the immune system may respond to alpha- and beta-PrP, we immunized Prnp(0/0) mice that are not tolerant of PrP with ralpha-PrP and rbeta-PrP. In this study, we show that although T cells stimulated by these differently folded conformers PrP recognize similar immunodominant epitopes (residues 111-130 and 191-210) the cytokine profile in response to ralpha- and rbeta-PrP was different. Challenge with ralpha-PrP elicited a strong response of IL-5 and IL-10, whereas rbeta-PrP led to an early increased production of IFN-gamma. In addition, immunization with ralpha-PrP led to production of predominantly IgG1 isotype Ab in the sera, whereas after immunization with rbeta-PrP, IgG2b was significantly produced. Thus, both humoral and cellular responses to these differently folded isoforms of the same protein are different, indicating a possible involvement of Th1 and Th2 pathway activation. These differences may be exploitable diagnostically and therapeutically for prion diseases, such as variant Creutzfeldt-Jakob disease.

Screening of 145 Anti-PrP Monoclonal Antibodies for Their Capacity to Inhibit PrPSc Replication in Infected Cells

Prion diseases are transmissible neurodegenerative disorders affecting humans and animals for which no therapeutic or prophylactic regimens exist. During the last three years several studies have shown that anti-PrP monoclonal antibodies (mAbs) can antagonize prion propagation in vitro and in vivo, but the mechanisms of inhibition are not known so far. To identify the most powerful mAbs and characterize more precisely the therapeutic effect of anti-PrP antibodies, we have screened 145 different mAbs produced in our laboratory for their capacity to cure cells constitutively expressing PrPSc. Our results confirm for a very large series of antibodies that mAbs recognizing cell-surface native PrPc can efficiently clean and definitively cure infected cells. Antibodies having a cleaning effect are directed against linear epitopes located in at least four different regions of PrP, suggesting an epitope-independent inhibition mechanism. The consequence of antibody binding is the sequestration of PrPc at the cell surface, an increase of PrPc levels recovered in cell culture medium, and an internalization of antibodies. Taken together these data suggest that the cleaning process is more likely due to a global effect on the PrP trafficking and/or transconformation process. Two antibodies, Sha31 and BAR236, show an IC50 of 0.6 nM, thus appearing 10-fold more efficient than previous antibodies described in the literature. Finally, five co-treatments were also tested, and only one of them, described previously (SAF34 + SAF61), lowered PrPSc levels in the cells synergistically.

Analysis of mammalian scrapie protein by novel monoclonal antibodies recognizing distinct prion protein glycoforms: an immunoblot and immunohistochemical study at the light and electron microscopic levels

The availability of specific monoclonal antibodies (mAbs) recognizing the aberrant form (PrP(Sc)) of the cellular prion protein (PrP(C)) in different mammalian species is important for molecular diagnostics, PrP(Sc) typing and future immunotherapy. We obtained a panel of anti-PrP monoclonal antibodies in PrP(0/0) knock-out mice immunized with recombinant human PrP(23-231). Two mAbs, recognizing PrP epitopes in the alpha-helix 1 (mAb SA65) and alpha-helix 2 (mAb SA21) regions, immunoreacted with PrP(C) and PrP(Sc) and its proteolytic product, PrP27-30, from human, murine, bovine, caprine and ovine brains by Western blot. Remarkably, mAb SA21 recognized unglycosylated and monoglycosylated PrP with the second site occupied by glycan moieties, but not monoglycosylated PrP with the first consensus site occupied or highly glycosylated species. Immunoblots with mAb SA21 disclosed that PrP glycosylated at the second site accounted for the slower migrating form of the customary monoglycosylated PrP doublet. mAb SA65 immunolabelled all PrP glycoforms by Western blot and was highly efficient in detecting tissue PrP by immunohistochemistry in light microscopy and in immunoelectron microscopy. These novel anti-PrP mAbs provide tools to investigate the subcellular site of PrP deposition in mammalian prion diseases and may also contribute to assess the role of different PrP glycoforms in human and animal prion diseases.

Testing the Possibility to Protect Bovine PrPC Transgenic Swiss Mice Against Bovine PrPSc Infection by DNA Vaccination Using Recombinant Plasmid Vectors Harboring and Expressing the Complete or Partial cDNA Sequences of Bovine PrPc

The objective of this study was to investigate the molecular mechanisms of neurobiological processes involved in the degeneration of the central nervous system. The bovine spongiform encephalopathy (BSE) was used as experimental model system for investigation of transmissible spongiform encephalopathy (TSE). The experimental strategy was to evaluate the possibility for protection of bovine PrP(C) transgenic mice against a bovine PrP(Sc) infection by DNA vaccination using the complete or partial cDNA sequences of the bovine prion protein. Three recombinant plasmids pCR3.1-EX-PrP-BSE-C20 (C20), pCR3.1-EX-PrP-BSE-90-235-C4 (C4), and pCR3.1-EX-PrP-BSE-106-131-C14 (C14) were constructed. These mammalian expression vectors harbor complete (C20) or partial (C4 and C14) cDNA sequences of the Bos taurus PrP(C) (BTPrP(C)) encoding for amino acid residues 1-264 (C20), 90-235 (C4), and 106-131 (C14) of the BTPrP(C). Transgenic mice harboring and expressing BTPrP(C) were generated using the donor strain C57/CBA, receptor strain Swiss mouse, and recombinant plasmid MoPrPXho-boPrP. Crossing of positive transgenic mice to bovine PrP and negative to murine PrP with 129/OLA (murine PrP-/-) and C57BL6x129/OLA (murine PrP+/-) mice was carried out to amplify the colony of transgenic mice termed bovine PrP(C) transgenic Swiss mice (BTPrP-TgM). The capabilities of C20, C4, and C14 to express the corresponding cDNA sequence of BTPrP(C) in vitro and in vivo were confirmed prior to DNA vaccination of the BTPrP-TgM using NIH 3T3 cells and BALB/c mice, respectively. In order to prove the capability of the constructed expression vectors to protect BTPrP-TgM in vivo against a BSE infection 80 female BTPrP-TgM were vaccinated intramuscularly and subcutaneously with DNA of the plasmids C20, C4, C14, and parental vector pCR3.1 (100 microg DNA corresponding to about 26-30 pmol DNA/animal and application) in four groups (each consists of 20 animals). DNA vaccination was followed by three additional boosters. The vaccinated animals (15 animals of each group) were challenged twice per oral with homogenates of brain material obtained from BSE cattle containing the infectious PrP(Sc) (100 microl/animal which corresponds to 15 mg of a 15% brain homogenate). The first and second challenge experiments were performed 76-83 and 181 days post DNA vaccination, respectively. A part of the vaccinated animals (3-5 animals of each group) that served as internal negative control were mock infected using the brain homogenate of healthy cattle or Phosphate saline buffer (PBS). A variety of symptoms and clinical pictures were observed during the monitoring of DNA vaccinated animals. However, the observed diseases seem to be similar in all experimental animal groups. After an observation period of 14 months post the second challenge experiment the remaining animals (some animals died or were sacrificed when moribund during the study) were sacrificed after expiration of the experimental schedule. The right hemisphere of the brain and a half of the spleen tissue of the individual animals were used for detection of PrP(Sc) by Western blot analysis. The misfolded bovine PrP(Sc) was not detected in the brain or spleen tissues of those animals that were vaccinated with DNA of C20, which was able to express the complete bovine PrP(C) protein in vitro and in vivo. In contrast, the bovine PrP(Sc) was detected in the brain or spleen tissues of animals that were DNA vaccinated with DNA of the parental vector pCR3.1, with DNA of C4, or with DNA of C14. The results of these studies underline that the constructed expression vector C20 possesses the protective capacity to inhibit the formation of misfolded bovine PrP(Sc) in BTPrP-TgM under the conditions used. A delay of occurrence of TSE-specific symptoms in the majority of the vaccinated animals seems to be due to the prolonged incubation time of BSE infection.

Recent developments in prion immunotherapy

Antibody-based immunotherapy may represent a realistic approach against prion diseases, given that antibodies to the cellular prion protein PrPC have been shown to antagonize deposition of the disease-associated prion protein (termed PrPSc) in in vitro assays and in laboratory animals. However, induction of protective antiprion immune responses in wild-type animals is difficult because of host tolerance to the endogenous PrPC. Several studies indicate that it might be possible to overcome tolerance to PrPC and induce immune responses to bacterially expressed, recombinant PrP. However, it is much more difficult to induce antibodies capable of recognizing native cell-surface PrPC, and there is reason to believe that the latter immune responses correlate with anti-prion protection. The difficulties involved in eliciting development of such anti-native PrPC immune responses may be partly intrinsic to B cells and, in addition, may reside in peripheral T helper tolerance.

Progress in prion vaccines and immunotherapies

The transmissible spongiform encephalopathies have presented a challenge to physicians and scientists attempting to develop immunologically-based treatments. Self-tolerance has been one of the major obstacles to successfully raising antibodies against the prion protein (PrP), the host-encoded protein whose misfolded form (PrPSc) is linked to the protein-only infectious agent responsible for these disorders. Recently, it has been shown that antibodies directed against the normal cellular isoform of PrP (PrPC) can reduce or eliminate PrP isoform conversion in both in vitro and in vivo model systems. Similar studies with a PrPSc-specific epitope target are in progress. There is now rational hope that this devastating group of diseases may soon be amenable to immunotherapy and immunoprophylaxis.

Neuroinvasion by scrapie following inoculation via the skin is independent of migratory Langerhans cells

Many natural transmissible spongiform encephalopathy (TSE) infections are likely to be acquired peripherally, and studies in mice show that skin scarification is an effective means of scrapie transmission. After peripheral exposure, TSE agents usually accumulate in lymphoid tissues before spreading to the brain. The mechanisms of TSE transport to lymphoid tissues are not known. Langerhans cells (LCs) reside in the epidermis and migrate to the draining lymph node after encountering antigen. To investigate the potential role of LCs in scrapie transportation from the skin, we utilized mouse models in which their migration was blocked either due to CD40 ligand deficiency (CD40L-/- mice) or after caspase-1 inhibition. We show that the early accumulation of scrapie infectivity in the draining lymph node and subsequent neuroinvasion was not impaired in mice with blocked LC migration. Thus, LCs are not involved in TSE transport from the skin. After intracerebral inoculation with scrapie, wild-type mice and CD40L-/- mice develop clinical disease with similar incubation periods. However, after inoculation via skin scarification CD40L-/- mice develop disease significantly earlier than do wild-type mice. The shorter incubation period in CD40L-/- mice is unexpected and suggests that a CD40L-dependent mechanism is involved in impeding scrapie pathogenesis. In vitro studies demonstrated that LCs have the potential to acquire and degrade protease-resistant prion protein, which is thought to be a component of the infectious agent. Taken together, these data suggest that LCs are not involved in scrapie transport to draining lymphoid tissues but might have the potential to degrade scrapie in the skin.

Rational targeting for prion therapeutics

Prions--pathogens that are lethal to humans and other animals--are thought to be conformational isomers of the cellular prion protein. Their unique biology, and the potential for a wider pathobiological significance of prion-like mechanisms, has motivated much research into understanding prion neurodegeneration. Moreover, concerns that extensive dietary exposure to bovine spongiform encephalopathy (BSE) prions might have infected many individuals--who might eventually develop its human counterpart, variant Creutzfeldt-Jakob disease (vCJD)--has focused much interest on therapeutics. The challenge of interrupting this aggressive, diffuse and uniformly fatal neurodegenerative process is daunting. However, the recent finding that the onset of clinical disease in established neuroinvasive prion infection in a mouse model can be halted and early pathology reversed is a source for considerable optimism. A therapeutic focus on the cellular prion protein, rather than prions themselves, which might not be directly neurotoxic, is suggested.

Approaches to Therapy of Prion Diseases

Devising approaches to the therapy of transmissible spongiform encephalopathies, or prion diseases, is beset by many difficulties. For one, the nature of the infectious agent, the prion, is understood only in outline, and its composition, structure, and mode of replication are still shrouded in mystery. In addition, the mechanism of pathogenesis is not well understood. Because clinical disease affects mainly the brain parenchyme, therapeutic agents must be able to traverse the brain-blood barrier (BBB) or have to be introduced directly into the cerebrospinal fluid or brain tissue. And finally, because the disease is usually recognized only after onset of severe clinical symptoms, the question arises as to whether the neurodegenerative processes can be reversed to any extent after a successful eradication of the agent.

Efficient Inhibition of Prion Replication by PrP-Fc2 Suggests that the Prion is a PrPSc Oligomer

Soluble dimeric prion protein (PrP-Fc(2)) binds to the disease-associated prion protein PrP(Sc), and inhibits prion replication when expressed in transgenic mice. Prion inhibition is effective even if PrP-Fc(2) is expressed at low levels, suggesting that its affinity for PrP(Sc) is higher than that of monomeric PrP(C). Here, we model prion accumulation as an exponential replication cycle of prion elongation and breakage. The exponential growth rate corresponding to this cycle is reflected in the incubation period of the disease. We use a mathematical model to calculate the exponential growth rate, and fit the model to in vivo data on prion incubation times corresponding to different levels of PrP(C) and PrP-Fc(2). We find an excellent fit of the model to the data. Surprisingly, targeting of PrP(Sc) can be effective at concentrations of PrP-Fc(2) lower than that of PrP(C), even if PrP-Fc(2) and PrP(C) have the same affinity for PrP(Sc). The best fit of our model to data predicts that the replicative prion consists of PrP(Sc) oligomers with a mean size of four to 15 units.

Concealment of epitope by reduction and alkylation in prion protein

Conversion of the cellular prion protein (PrP(C)) into its pathological isoform (PrP(Sc)), the key molecular event in the pathogenesis of prion diseases, is accompanied by a conformational transition of alpha-helix into beta-sheet structures involving alpha-helix 1 (alpha1) domain from residues 144 to 154 of the protein. Reduction and alkylation of PrP(C) have been found to inhibit the conversion of PrP(C) into PrP(Sc) in vitro. Here we report that while antibody affinity of epitopes in the N- and C-terminal domains remained unchanged, reduction and alkylation of the PrP molecule induced complete concealment of an epitope in alpha1 for anti-PrP antibody 6H4 that is able to cure prion infection in the cell model. Mass spectrometric analysis of recombinant PrP showed that the alkylation reaction takes place at reduced cysteines but no modification was observed in this cryptic epitope. Our study suggests that reduction and alkylation result in local or global rearrangement of PrP tertiary structure that is maintained in both liquid and solid phases. The implications in the conversion of PrP(C) into PrP(Sc) and the therapeutics of prion diseases are discussed.