Circumventing tolerance to generate autologous monoclonal antibodies to the prion protein

Protective anti-prion antibodies in human immunoglobulin repertoires

Prion immunotherapy may hold great potential, but antibodies against certain PrP epitopes can be neurotoxic. Here, we identified > 6,000 PrP-binding antibodies in a synthetic human Fab phage display library, 49 of which we characterized in detail. Antibodies directed against the flexible tail of PrP conferred neuroprotection against infectious prions. We then mined published repertoires of circulating B cells from healthy humans and found antibodies similar to the protective phage-derived antibodies. When expressed recombinantly, these antibodies exhibited anti-PrP reactivity. Furthermore, we surveyed 48,718 samples from 37,894 hospital patients for the presence of anti-PrP IgGs and found 21 high-titer individuals. The clinical files of these individuals did not reveal any enrichment of specific pathologies, suggesting that anti-PrP autoimmunity is innocuous. The existence of anti-prion antibodies in unbiased human immunological repertoires suggests that they might clear nascent prions early in life. Combined with the reported lack of such antibodies in carriers of disease-associated PRNP mutations, this suggests a link to the low incidence of spontaneous prion diseases in human populations.

Novel regulators of PrPC expression as potential therapeutic targets in prion diseases

INTRODUCTION

Prion diseases are rare and fatal neurodegenerative disorders. The key molecular event in these disorders is the misfolding of the physiological form of the cellular prion protein, PrP^C, leading to the accumulation of a pathological isoform, PrP^Sc, with unique features. Both isoforms share the same primary sequence, lacking detectable differences in posttranslational modification, a major hurdle for their biochemical or biophysical independent characterization. The mechanism underlying the conversion of PrP^C to PrP^Sc is not completely understood, so finding an effective therapy to cure prion disorders is extremely challenging.

AREAS COVERED

This review discusses the strategies for decreasing prion replication and throws a spotlight on the relevance of PrP^C in the prion accumulation process.

EXPERT OPINION

PrP^C is the key substrate for prion pathology; hence, the most promising therapeutic approach appears to be the targeting of PrP^C to block the production of the infectious isoform. The use of RNA interference and antisense oligonucleotide technologies may offer opportunities for treatment because of their success in clinical trials for other neurodegenerative diseases.

Enhancement of binding avidity by bivalent binding enables PrPSc-specific detection by anti-PrP monoclonal antibody 132

Anti-prion protein (PrP) monoclonal antibody 132, which recognizes mouse PrP amino acids 119–127, enables us to reliably detect abnormal isoform prion protein (PrP^Sc) in cells or frozen tissue sections by immunofluorescence assay, although treatment with guanidinium salts is a prerequisite. Despite the benefit of this mAb, the mechanism of PrP^Sc-specific detection remains unclear. Therefore, to address this mechanism, we analyzed the reactivities of mono- and bivalent mAb 132 to recombinant mouse PrP (rMoPrP) by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). In ELISA, binding of the monovalent form was significantly weaker than that of the bivalent form, indicating that bivalent binding confers a higher binding stability to mAb 132. Compared with other anti-PrP mAbs tested, the reactivity of bivalent mAb 132 was easily affected by a decrease in antigen concentration. The binding kinetics of mAb 132 assessed by SPR were consistent with the results of ELISA. The dissociation constant of the monovalent form was approximately 260 times higher than that of the bivalent form, suggesting that monovalent binding is less stable than bivalent binding. Furthermore, the amount of mAb 132 that bound to rMoPrP decreased if the antigen density was too low to allow bivalent binding. If two cellular PrP (PrP^C) are close enough to allow bivalent binding, mAb 132 binds to PrP^C. These results indicate that weak monovalent binding to monomeric PrP^C diminishes PrP^C signals to background level, whereas after exposure of the epitope, mAb 132 binds stably to oligomeric PrP^Sc in a bivalent manner.

Recombinant PrP and Its Contribution to Research on Transmissible Spongiform Encephalopathies

The misfolding of the cellular prion protein (PrPC) into the disease-associated isoform (PrPSc) and its accumulation as amyloid fibrils in the central nervous system is one of the central events in transmissible spongiform encephalopathies (TSEs). Due to the proteinaceous nature of the causal agent the molecular mechanisms of misfolding, interspecies transmission, neurotoxicity and strain phenomenon remain mostly ill-defined or unknown. Significant advances were made using in vivo and in cellula models, but the limitations of these, primarily due to their inherent complexity and the small amounts of PrPSc that can be obtained, gave rise to the necessity of new model systems. The production of recombinant PrP using E. coli and subsequent induction of misfolding to the aberrant isoform using different techniques paved the way for the development of cell-free systems that complement the previous models. The generation of the first infectious recombinant prion proteins with identical properties of brain-derived PrPSc increased the value of cell-free systems for research on TSEs. The versatility and ease of implementation of these models have made them invaluable for the study of the molecular mechanisms of prion formation and propagation, and have enabled improvements in diagnosis, high-throughput screening of putative anti-prion compounds and the design of novel therapeutic strategies. Here, we provide an overview of the resultant advances in the prion field due to the development of recombinant PrP and its use in cell-free systems.

Prion protein-specific antibodies-development, modes of action and therapeutics application

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are lethal neurodegenerative disorders involving the misfolding of the host encoded cellular prion protein, PrP^C. This physiological form of the protein is expressed throughout the body, and it reaches the highest levels in the central nervous system where the pathology occurs. The conversion into the pathogenic isoform denoted as prion or PrP^Sc is the key event in prion disorders. Prominent candidates for the treatment of prion diseases are antibodies and their derivatives. Anti-PrP^C antibodies are able to clear PrP^Sc from cell culture of infected cells. Furthermore, application of anti-PrP^C antibodies suppresses prion replication in experimental animal models. Major drawbacks of immunotherapy are immune tolerance, the risks of neurotoxic side effects, limited ability of compounds to cross the blood-brain barrier and their unfavorable pharmacokinetic. The focus of this review is to recapitulate the current understanding of the molecular mechanisms for antibody mediated anti-prion activity. Although relevant for designing immunotherapeutic tools, the characterization of key antibody parameters shaping the molecular mechanism of the PrP^C to PrP^Sc conversion remains elusive. Moreover, this review illustrates the various attempts towards the development of anti-PrP antibody compounds and discusses therapeutic candidates that modulate PrP expression.

Molecular dynamics simulations of amyloid fibrils: an in silico approach

Amyloid fibrils play causal roles in the pathogenesis of amyloid-related degenerative diseases such as Alzheimer's disease, type II diabetes mellitus, and the prion-related transmissible spongiform encephalopathies. The mechanism of fibril formation and protein aggregation is still hotly debated and remains an important open question in order to develop therapeutic method of these diseases. However, traditional molecular biological and crystallographic experiments could hardly observe atomic details and aggregation process. Molecular dynamics (MD) simulations could provide explanations for experimental results and detailed pathway of protein aggregation. In this review, we focus on the applications of MD simulations on several amyloidogenic protein systems. Furthermore, MD simulations could help us to understand the mechanism of amyloid aggregation and how to design the inhibitors.

Ablation of the cellular prion protein, PrPC, specifically on follicular dendritic cells has no effect on their maturation or function

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

Novel epitopes identified by anti-PrP monoclonal antibodies produced following immunization of Prnp0/0 Balb/cJ mice with purified scrapie prions

Prions, or infectious proteins, cause a class of uniformly fatal neurodegenerative diseases. Prions are composed solely of an aberrantly folded isoform (PrP(Sc)) of a normal cellular protein (PrP(C)). Shared sequence identity of PrP(Sc) with PrP(C) has limited the detection sensitivity of immunochemical assays, as antibodies specific for the disease-causing PrP(Sc) isoform have not been developed. Here we report the generation of three new monoclonal antibodies (MAbs) to PrP, which were isolated following immunization of Prnp(0/0) Balb/cJ mice with highly purified PrP(Sc) isolated from brain lipid rafts. Epitope mapping using synthetic PrP peptides revealed that the three MAbs bind different epitopes of PrP. The DRM1-31 MAb has a conformational epitope at the proposed binding site for the putative prion conversion co-factor "protein X." The DRM1-60 MAb binds a single linear epitope localized to the β2-α2 loop region of PrP, whereas DRM2-118 binds an epitope that includes sequences within the octarepeat region and near the site of N-terminal truncation of PrP(Sc) by proteinase K. Our novel anti-PrP MAbs with defined PrP epitopes may be useful in deciphering the conformational conversion of PrP(C) into PrP(Sc).

Direct evidence of generation and accumulation of β-sheet-rich prion protein in scrapie-infected neuroblastoma cells with human IgG1 antibody specific for β-form prion protein

We prepared β-sheet-rich recombinant full-length prion protein (β-form PrP) (Jackson, G. S., Hosszu, L. L., Power, A., Hill, A. F., Kenney, J., Saibil, H., Craven, C. J., Waltho, J. P., Clarke, A. R., and Collinge, J. (1999) Science 283, 1935-1937). Using this β-form PrP and a human single chain Fv-displaying phage library, we have established a human IgG1 antibody specific to β-form but not α-form PrP, PRB7 IgG. When prion-infected ScN2a cells were cultured with PRB7 IgG, they generated and accumulated PRB7-binding granules in the cytoplasm with time, consequently becoming apoptotic cells bearing very large PRB7-bound aggregates. The SAF32 antibody recognizing the N-terminal octarepeat region of full-length PrP stained distinct granules in these cells as determined by confocal laser microscopy observation. When the accumulation of proteinase K-resistant PrP was examined in prion-infected ScN2a cells cultured in the presence of PRB7 IgG or SAF32, it was strongly inhibited by SAF32 but not at all by PRB7 IgG. Thus, we demonstrated direct evidence of the generation and accumulation of β-sheet-rich PrP in ScN2a cells de novo. These results suggest first that PRB7-bound PrP is not responsible for the accumulation of β-form PrP aggregates, which are rather an end product resulting in the triggering of apoptotic cell death, and second that SAF32-bound PrP lacking the PRB7-recognizing β-form may represent so-called PrP(Sc) with prion propagation activity. PRB7 is the first human antibody specific to β-form PrP and has become a powerful tool for the characterization of the biochemical nature of prion and its pathology.

The application of transgenic mice for therapeutic antibody discovery

In 2006, panitumumab, the first fully human antibody generated from transgenic mice, was approved for clinical use by the US Food and Drug Administration (FDA). Since then, a further seven such antibodies have been approved. In this chapter, we discuss how transgenic mice technologies can provide a powerful platform for creating human therapeutic antibodies.

Construction of recombinant monoclonal antibodies from a chicken hybridoma line secreting specific antibody

The chicken is a useful animal for the development of the specificantibodies against the mammalian conserved proteins. We generated twotypes of recombinant chicken monoclonal antibodies (mAbs), using a phagedisplay technique from a chicken hybridoma HUC2-13 which secreted themAb to the N-terminal of the mammalian prion protein (PrP). Althoughthe mAb HUC2-13 is a useful antibody for the prion research, thehybridoma produces a low level of antibody production. In order to producea large amount of the mAb, we have constructed a single chain fragmentvariable region (scF(V)) mAb by using the variable heavy(V(H)) and light (V(L))genes which were amplified by using the two primer pairs and theflexible linker. The two phage display mAbs (HUC2p3 and HUC2p5)expressed on a M13 filamentous phage and their soluble type mAbs(HUC2s3 and HUC2s5) were reacted with the PrP peptide antigen in theELISA. In the Western blot analysis, the mAbs HUC2p3 and HUC2s3 wereas reactive to PrP(c) from mouse brains as the mAb HUC2-13 was. The nucleotide sequences of V(H) and V(L) genes from HUC2-13 and the two cloneswere identical except for only one residue. These results indicate that themethods presented here provide an effective tool for the improvement ofthe low levels of antibody production in the chicken hybridoma system.

PrP(Sc)-specific antibodies with the ability to immunodetect prion oligomers

The development of antibodies with binding capacity towards soluble oligomeric forms of PrPSc recognised in the aggregation process in early stage of the disease would be of paramount importance in diagnosing prion diseases before extensive neuropathology has ensued. As blood transfusion appears to be efficient in the transmission of the infectious prion agent, there is an urgent need to develop reagents that would specifically recognize oligomeric forms of the abnormally folded prion protein, PrPSc.

To that end, we show that anti-PrP monoclonal antibodies (called PRIOC mAbs) derived from mice immunised with native PrP-coated microbeads are able to immunodetect oligomers/multimers of PrPSc. Oligomer-specific immunoreactivity displayed by these PRIOC mAbs was demonstrated as large aggregates of immunoreactive deposits in prion-permissive neuroblastoma cell lines but not in equivalent non-infected or prn-p^0/0 cell lines. In contrast, an anti-monomer PrP antibody displayed diffuse immunoreactivity restricted to the cell membrane. Furthermore, our PRIOC mAbs did not display any binding with monomeric recombinant and cellular prion proteins but strongly detected PrPSc oligomers as shown by a newly developed sensitive and specific ELISA. Finally, PrioC antibodies were also able to bind soluble oligomers formed of Aβ and α-synuclein. These findings demonstrate the potential use of anti-prion antibodies that bind PrPSc oligomers, recognised in early stage of the disease, for the diagnosis of prion diseases in blood and other body fluids.

Conditional expression of full-length humanized anti-prion protein antibodies in Chinese hamster ovary cells

Because of their high antigen specificity and metabolic stability, genetically engineered human monoclonal antibodies are on the way to becoming one of the most promising medical diagnostics and therapeutics. In order to establish an in vitro system capable of producing such biosimilar antibodies, we used human constant chain sequences to design the novel human antibody expressing vector cassette pMAB-ABX. A bidirectional tetracycline (tet)-controllable promotor was used for harmonized expression of immunoglobulin type G (IgG) heavy and light chains. As an example we used anti-prion protein (anti-PrP) IgGs. Therefore, the variable heavy (V(H)) and light chain (V(L)) sequences of anti-PrP antibodies, previously generated in our laboratory by DNA immunization of prion protein knock-out mice, were isolated from murine hybridoma cell lines and inserted into pMAB-ABX vector. After transfection of Chinese hamster ovary (CHO) cells, a number of stable antibody producing cell clones were selected. One cell line (pMAB-ABX-13F10/3B5) stably expressing the recombinant humanized antibody (rechuAb) 13F10/3B5 was selected for detailed characterization by Western blot, immunofluorescence, and flow cytometric analyses. The full-length recombinant humanized IgG antibody showed a high level of expression in the cytoplasm. In conclusion, the new cell system described here is a suitable tool to produce functional intact full-length humanized IgG antibodies.

Production and characterization of a panel of monoclonal antibodies against native human cellular prion protein

The human prion diseases, such as variant Creutzfeldt-Jakob disease (vCJD), are characterized by the conversion of the normal cellular prion protein (PrP(C)) into an abnormal disease associated form (PrP(Sc)). Monoclonal antibodies (MAbs) that recognize these different PrP isoforms are valuable reagents both in the diagnosis of these diseases and in prion disease research in general but we know of no attempts to raise MAbs against native human PrP(C). We immunized prion protein gene ablated (PrP(-/-)) mice with native human PrP(C) purified from platelets (pHuPrP) generating a predominantly IgG isotype anti-pHuPrP polyclonal antibody response in all mice. Following fusion of splenocytes from the immunized mice with SP2/0 myeloma cells, we were able to identify single cell clone and cryopreserve 14 stable hybridoma cell lines producing MAbs that reacted with pHuPrP. The properties of these MAbs (such as isotype, binding to native/denatured pHuPrP, and HuPrP epitopes recognized) are described. Furthermore, several of these MAbs showed a selectivity in their ability to immunoprecipitate disease associated PrP(Sc) and its corresponding protease resistant core (PrP(res)).

PrP-specific camel antibodies with the ability to immunodetect intracellular prion protein

Although there is currently no effective treatment for prion diseases, significant advances have been made in suppressing its progress, using antibodies that block the conversion of PrPC into PrPSc. In order to be effective in treating individuals that have prion diseases, antibodies must be capable of arresting disease in its late stages. This requires the development of antibodies with higher affinity for PrPSc and systems for effective translocation of antibodies across the blood–brain barrier in order to achieve high concentrations of inhibitor at the site of protein replication. An additional advantage is the ability of these antibodies to access the cytosol of affected cells. To this end, we have generated PrP-specific antibodies (known as PrioV) by immunization of camels with murine scrapie material adsorbed to immunomagnetic beads. The PrioV antibodies display a range of specificities with some recognizing the PrP27–30 proteinase K-resistant fragment, others specific for PrPC and a number with dual binding specificity. Independent of their PrP conformation specificity, one of the PrioV antibodies (PrioV3) was shown to bind PrPC in the cytosol of neuroblastoma cells. In marked contrast, conventional anti-PrP antibodies produced in mouse against similar target antigen were unable to cross the neuronal plasma membrane and instead formed a ring around the cells. The PrioV anti-PrP antibodies could prove to be a valuable tool for the neutralization/clearance of PrPSc in intracellular compartments of affected neurons and could potentially have wider applicability for the treatment of so-called protein-misfolding diseases.

Generation of antisera to purified prions in lipid rafts

Prion diseases are fatal neurodegenerative disorders caused by prion proteins (PrP). Infectious prions accumulate in the brain through a template-mediated conformational conversion of endogenous PrP(C) into alternately folded PrP(Sc). Immunoassays toward pre-clinical detection of infectious PrP(Sc) have been confounded by low-level prion accumulation in non-neuronal tissue and the lack of PrP(Sc) selective antibodies. We report a method to purify infectious PrP(Sc) from biological tissues for use as an immunogen and sample enrichment for increased immunoassay sensitivity. Significant prion enrichment is accomplished by sucrose gradient centrifugation of infected tissue and isolation with detergent resistant membranes from lipid rafts (DRMs). At equivalent protein concentration a 50-fold increase in detectable PrP(Sc) was observed in DRM fractions relative to crude brain by direct ELISA. Sequential purification steps result in increased specific infectivity (DRM <20-fold and purified DRM immunogen <40-fold) relative to 1% crude brain homogenate. Purification of PrP(Sc) from DRM was accomplished using phosphotungstic acid protein precipitation after proteinase-K (PK) digestion followed by size exclusion chromatography to separate PK and residual protein fragments from larger prion aggregates. Immunization with purified PrP(Sc) antigen was performed using wild-type (wt) and Prnp(0/0) mice, both on Balb/cJ background. A robust immune response against PrP(Sc) was observed in all inoculated Prnp(0/0) mice resulting in antisera containing high-titer antibodies against prion protein. Antisera from these mice recognized both PrP(C) and PrP(Sc), while binding to other brain-derived protein was not observed. In contrast, the PrP(Sc) inoculum was non-immunogenic in wt mice and antisera showed no reactivity with PrP or any other protein.

Simple method of monoclonal antibody production against mammalian cellular prion protein

Monoclonal antibodies (MAbs) against prion protein (PrP) are powerful tools for diagnosis and research in transmissible spongiform encephalopathies. Ten MAbs to recombinant/native cellular PrP (PrPc) in mammals were prepared with a simple method and identified in detail. Normal BALB/c mice were immunized with the recombinant bovine mature PrP (rbomPrP) and PrP27-30 (rboPrP27-30) expressed in Escherichia coli. The immunized splenocytes were fused with SP2/0 mouse myeloma cells, and positive hybridomas were selected by indirect enzyme-linked immunosorbent assay (ELISA). The characterizations of these MAbs, such as Ig, Ig subclass, titer, affinity index, specificity, epitopes recognized, and binding to recombinant/native PrPc of cattle, sheep, or human beings, were evaluated by Western blotting and indirect or sandwich ELISA. Ten MAbs could be divided into five groups depending on the results of indirect ELISA additivity test and their reaction to E. coli-expressed truncated-PrPs. Isotyping of the MAbs revealed that they belong to IgG1, IgG2a, and IgG2b subclass. Their indirect ELISA titers were between 10(3) and 10(6). Affinity constants were between 10(9) and 10(12) M(-1). Ten MAbs specifically reacted with the rbomPrP, without binding to prion-like protein Doppel and the lysates of E. coli. These MAbs could also respond to the recombinant mature PrP (rmPrP) of sheep and human beings. Also of interest was the ability of the MAbs to bind with dimer of rmPrP and PrP extracted from the brain tissue of cattle or sheep. We conclude that anti-PrP MAbs successfully prepared with a simple method could potentially be useful in mammalian prion research.

Generation of monoclonal antibody that distinguishes PrPSc from PrPC and neutralizes prion infectivity

To establish PrP(Sc)-specific mAbs, we immunized Prnp(-/-) mice with PrP(Sc) purified from prion-infected mice. Using this approach, we obtained mAb 6H10, which reacted with PrP(Sc) treated with proteinase K, but not with PrP(Sc) pretreated with more than 3 M GdnHCl. In contrast, reactivity of pan-PrP mAbs increased with increasing concentrations of GdnHCl used for pretreatment of PrP(Sc). In histoblot analysis, mAb 6H10 showed a positive reaction on a non-denatured histoblot but reactivity was lower when the histoblot was pretreated by autoclaving. Epitope analysis suggested that the extreme C-terminus of PrP is likely to be part of the epitope for mAb 6H10. MAb 6H10 immunoprecipitated PrP(Sc) from brains of mice, sheep, and cattle infected with prions. Furthermore, pretreatment of purified PrP(Sc) with mAb 6H10 reduced the infectious titer more than 1 log. Taken together, these results suggest that mAb 6H10 recognizes a conformational epitope on PrP(Sc) that is related to prion infectivity.

Unswitched immunoglobulin M response prolongs mouse survival in prion disease

Several studies have failed to demonstrate the presence of immune responses to infectious prions during the course of prion disease, reflecting the identical primary structure of normal and disease-associated isoforms and the widespread expression of the normal cellular form of prion protein, PrP(C), leading to B- and/or T-cell tolerance of disease-associated isoforms and also possibly because antigen-presenting cells are unable to process the highly aggregated, detergent-insoluble, protease-resistant form, PrP(Sc). Under certain circumstances, PrP(Sc) can be revealed to the immune system in immunogenic form, and it has been shown previously that anti-PrP antibodies can be induced to prions immunoadsorbed to Dynabeads using specific anti-PrP monoclonal antibodies, even in PrP-sufficient mice. This study demonstrated in a murine scrapie model that PrP-Dynabeads effectively stimulated the immune system to produce anti-PrP IgM antibodies over prolonged periods after repeated immunization. It was also shown that these immune responses prolonged incubation times in murine scrapie.

[Production of monoclonal antibodies to the prion protein and their characterization]

Antibodies to the prion protein (PrP), particularly, monoclonal antibodies, are necessary tools in the diagnostics and study of prion diseases and potential means of their immunotherapy. For the production of monoclonal antibodies, BALB/c mice were immunized by a recombinant bovine PrP. Three stable hybridomas producing antibodies of IgM class were prepared. The antibodies were bound to PrP in a solid-phase enzyme immunoassay and immunoblotting. The epitope mapping accomplished with the use of synthetic peptides showed that an epitope located in region 25-36 of PrP corresponds to one antibody, and epitopes located in region 222-229, to the other two. The antibodies to fragment 222-229 purified by affinity chromatography recognized with a high specificity conglomerates of a pathogenic prion in the brain tissue of cows suffering from spongiform encephalopathy. Thus, in nontransgenic mice, PrP-specific monoclonal antibodies were produced, useful in studies and diagnostics of prion diseases.

The POM monoclonals: a comprehensive set of antibodies to non-overlapping prion protein epitopes

PrP^Sc, a misfolded and aggregated form of the cellular prion protein PrP^C, is the only defined constituent of the transmissible agent causing prion diseases. Expression of PrP^C in the host organism is necessary for prion replication and for prion neurotoxicity. Understanding prion diseases necessitates detailed structural insights into PrP^C and PrP^Sc. Towards this goal, we have developed a comprehensive collection of monoclonal antibodies denoted POM1 to POM19 and directed against many different epitopes of mouse PrP^C. Three epitopes are located within the N-terminal octarepeat region, one is situated within the central unstructured region, and four epitopes are discontinuous within the globular C-proximal domain of PrP^C. Some of these antibodies recognize epitopes that are resilient to protease digestion in PrP^Sc. Other antibodies immunoprecipitate PrP^C, but not PrP^Sc. A third group was found to immunoprecipitate both PrP isoforms. Some of the latter antibodies could be blocked with epitope-mimicking peptides, and incubation with an excess of these peptides allowed for immunochromatography of PrP^C and PrP^Sc. Amino-proximal antibodies were found to react with repetitive PrP^C epitopes, thereby vastly increasing their avidity. We have also created functional single-chain miniantibodies from selected POMs, which retained the binding characteristics despite their low molecular mass. The POM collection, thus, represents a unique set of reagents allowing for studies with a variety of techniques, including western blotting, ELISA, immunoprecipitation, conformation-dependent immunoassays, and plasmon surface plasmon resonance-based assays.

Monoclonal antibody fragment from combinatorial phage display library neutralizes alpha-latrotoxin activity and abolishes black widow spider venom lethality, in mice

Alpha-latrotoxin (alpha-ltx), a component of the venom of black widow spiders (BWSV), binds to higher vertebrates presynaptic nerve terminals, stimulating massive neurotransmitter release. This neurotoxic protein is responsible for most of the symptoms elicited in men by the bite of black widow spider (BWS), i.e. a neurological syndrome named latrodectism. By reasoning that targeting this single component would abrogate most of the effect of BWS envenomation, we took advantage of the antibody phage display technology to generate monoclonal Fab fragments able to bind and neutralize the alpha-ltx. To this aim, we immunized Balb/c mice with purified toxin and cloned their antibody repertoire in the pCombIII phage display vector. By combining a high-stringency affinity selection with a sensitive 45Ca(2+) uptake assay, we isolated a Fab fragment (FM1) able to bind the alpha-ltx in the low nM range and neutralize its ionophore activity, in vitro and in vivo. After the onset of overt symptomatology, administration of FM1 to experimentally envenomed mice induced remission of symptoms and prevented lethality. Since alpha-ltx is the only molecule responsible for the great toxicity of BWS bites in mammals, the FM1 Fab, highly effective in neutralizing the toxin in vivo, represents a promising immunotherapy reagent for treating latrodectic patients.

Identification of an epitope on the recombinant bovine PrP that is able to elicit a prominent immune response in wild-type mice

The main cause for the development of transmissible spongiform encephalopathies (TSE) is the conformational change of prion protein from the normal cellular isoform (PrP(C)) into the abnormal isoform, named prion (PrP(Sc)). The two isoforms have the same primary structure, and with PrP being highly conserved among different species, no immune response to PrP(Sc) has been observed in infected humans or other mammals so far. The problem of inducing immune response was encountered when producing monoclonal antibodies against PrP, therefore mice lacking a functional Prnp gene were predominantly used for the immunization. In the present paper we report that by immunizing wild-type BALB/c mice with chemically unmodified recombinant bovine PrP a potent humoral immune response was achieved. Furthermore, we were able to isolate the monoclonal antibody (mAb) E12/2 and few other mAbs, all reacting specifically with bovine and human PrP, but not with PrP from several other mammals. The epitope of mAb E12/2 is located at the C-terminal end of helix 1, with His155 being crucial for binding. It has been proven that mAb E12/2 is useful for human and bovine TSE research as well as for diagnostics. Our results show that there are sufficient structural differences between mouse and bovine PrP to provoke a prominent humoral immune response.

Phosphorothioate oligonucleotides reduce PrP levels and prion infectivity in cultured cells

Prions are composed solely of the disease-causing prion protein (PrPSc) that is formed from the cellular isoform PrPC by a posttranslational process. Here we report that short phosphorothioate DNA (PS-DNA) oligonucleotides diminished the levels of both PrPC and PrPSc in prion-infected neuroblastoma (ScN2a) cells. The effect of PS-DNA on PrP levels was independent of the nucleotide sequence. The effective concentration (EC50) of PS-DNA required to achieve half-maximal diminution of PrPSc was approximately 70 nM, whereas the EC50 of PS-DNA for PrPC was more than 50-fold greater. This finding indicated that diminished levels of PrPSc after exposure to PS-DNA are unlikely to be due to decreased PrPC levels. Bioassays in transgenic mice demonstrated a substantial diminution in the prion infectivity after ScN2a cells were exposed to PS-DNAs. Whether PS-DNA will be useful in the treatment of prion disease in people or livestock remains to be established.

Phosphorothioate oligonucleotides reduce PrP levels and prion infectivity in cultured cells

Prions are composed solely of the disease-causing prion protein (PrPSc) that is formed from the cellular isoform PrPC by a posttranslational process. Here we report that short phosphorothioate DNA (PS-DNA) oligonucleotides diminished the levels of both PrPC and PrPSc in prion-infected neuroblastoma (ScN2a) cells. The effect of PS-DNA on PrP levels was independent of the nucleotide sequence. The effective concentration (EC50) of PS-DNA required to achieve half-maximal diminution of PrPSc was approximately 70 nM, whereas the EC50 of PS-DNA for PrPC was more than 50-fold greater. This finding indicated that diminished levels of PrPSc after exposure to PS-DNA are unlikely to be due to decreased PrPC levels. Bioassays in transgenic mice demonstrated a substantial diminution in the prion infectivity after ScN2a cells were exposed to PS-DNAs. Whether PS-DNA will be useful in the treatment of prion disease in people or livestock remains to be established.

Generation of genetic engineering monoclonal antibodies against prion protein

Two strains of Fab monoclonal antibodies (mAbs) against prion protein, designated as IV-66 and IV-78, were selected from the phage display libraries. The gene sequences encoding the light kappa chain and heavy Fd chain of IV-78 were inserted into a baculovirus expression cassette vector for mouse IgG expression. Western blot, Dot-ELISA and immunoprecipitation confirmed that these Fab and IgG mAbs reacted well with the recombinant hamster and human PrP proteins expressed in prokaryotic and in mammalian cells and PrP(Sc) from scrapie-infected hamsters. It demonstrates that mAbs against prion protein are successfully generated by phage-display technique.

CpG oligodeoxynucleotide-enhanced humoral immune response and production of antibodies to prion protein PrPSc in mice immunized with 139A scrapie-associated fibrils

Prion diseases are characterized by conversion of the cellular prion protein (PrP(C)) to a protease-resistant conformer, the srapie form of PrP (PrP(Sc)). Humoral immune responses to nondenatured forms of PrP(Sc) have never been fully characterized. We investigated whether production of antibodies to PrP(Sc) could occur in PrP null (Prnp(-/-)) mice and further, whether innate immune stimulation with the TLR9 agonist CpG oligodeoxynucleotide (ODN) 1826 could enhance this process. Whether such stimulation could raise anti-PrP(Sc) antibody levels in wild-type (Prnp(+/+)) mice was also investigated. Prnp(-/-) and Prnp(+/+) mice were immunized with nondenatured 139A scrapie-associated fibrils (SAF), with or without ODN 1826, and were tested for titers of PrP-specific antibodies. In Prnp(-/-) mice, inclusion of ODN 1826 in the immunization regime increased anti-PrP titers more than 13-fold after two immunizations and induced, among others, antibodies to an N-terminal epitope, which were only present in the immune repertoire of mice receiving ODN 1826. mAb 6D11, derived from such a mouse, reacts with the N-terminal epitope QWNK in native and denatured forms of PrP(Sc) and recombinant PrP and exhibits a K(d) in the 10(-)(11) M range. In Prnp(+/+) mice, ODN 1826 increased anti-PrP levels as much as 84% after a single immunization. Thus, ODN 1826 potentiates adaptive immune responses to PrP(Sc) in 139A SAF-immunized mice. These results represent the first characterization of humoral immune responses to nondenatured, infectious PrP(Sc) and suggest methods for optimizing the generation of mAbs to PrP(Sc), many of which could be used for diagnosis and treatment of prion diseases.

Manufacturing immunity to disease in a test tube: the magic bullet realized

Although it took over one hundred years, Ehrlich's concept of the magic bullet is now a reality. Today, therapeutic antibodies are, arguably, the most important class of new drugs for the treatment of illnesses ranging from Alzheimer's disease to cancer. The emergence of therapeutic antibodies had to wait for advances in immunochemistry that allowed construction of antibodies in vitro. The centerpiece of the new technology is the combinatorial antibody library, which essentially allows one to synthesize an artificial immune system with a diversity that exceeds that of the natural repertoire. The construction of such libraries was perceived to be difficult because, if the natural immune system was to be used as the starting material, construction of the libraries would entail protocols that are the opposite of usual cloning. In gene cloning one starts with complexity and reduces it to a singularity. In the generation of diversity by construction of combinatorial antibody libraries, one starts with a collection of clones, randomly expands their complexity, and then returns them to recoverable singularities. The methods developed to accomplish this seemingly formidable task now allow construction of antibodies in a test tube to any antigen. These synthetic antibodies may be qualitatively and quantitatively superior to those of nature.

Vaccination with prion peptide-displaying papillomavirus-like particles induces autoantibodies to normal prion protein that interfere with pathologic prion protein production in infected cells

Prion diseases are fatal neurodegenerative disorders caused by proteinaceous infectious pathogens termed prions (PrP(Sc)). To date, there is no prophylaxis or therapy available for these transmissible encephalopathies. Passive immunization with monclonal antibodies recognizing the normal host-encoded prion protein (PrP(C)) has been reported to abolish PrP(Sc) infectivity and to delay onset of disease. Because of established immunologic tolerance against the widely expressed PrP(C), active immunization appears to be difficult to achieve. To overcome this limitation, papillomavirus-like particles were generated that display a nine amino acid B-cell epitope, DWEDRYYRE, of the murine/rat prion protein in an immunogenic capsid surface loop, by insertion into the L1 major capsid protein of bovine papillomavirus type 1. The PrP peptide was selected on the basis of its previously suggested central role in prion pathogenesis. Immunization with PrP-virus-like particles induced high-titer antibodies to PrP in rabbit and in rat, without inducing overt adverse effects. As determined by peptide-specific ELISA, rabbit immune sera recognized the inserted murine/rat epitope and also cross-reacted with the homologous rabbit/human epitope differing in one amino acid residue. In contrast, rat immune sera recognized the murine/rat peptide only. Sera of both species reacted with PrP(C) in its native conformation in mouse brain and on rat pheochromocytoma cells, as determined by immunoprecipitation and fluorescence-activated cell sorting analysis. Importantly, rabbit anti-PrP serum contained high-affinity antibody that inhibited de novo synthesis of PrP(Sc) in prion-infected cells. If also effective in vivo, PrP-virus-like particle vaccination opens a unique possibility for immunologic prevention of currently fatal and incurable prion-mediated diseases.

Novel single chain antibodies to the prion protein identified by phage display

A well defined structure is available for the carboxyl half of the cellular prion protein (PrP(c)), while the structure of the amino terminal half of the molecule remains ill defined. The unstructured nature of the polypeptide has meant that relatively few of the many antibodies generated against PrP(c) recognise this region. To circumvent this problem, we have used a previously characterised and well expressed fragment derived from the amino terminus of PrP(c) as bait for panning a single chain antibody phage (scFv-P) library. Using this approach, we identified and characterised 1 predominant and 3 additional scFv-Ps that contained different V(H) and V(L) sequences and that bound specifically to the PrP(c) target. Epitope mapping revealed that all scFv-Ps recognised linear epitopes between PrP(c) residues 76 and 156. When compared with existing monoclonal antibodies (MAb), the binding of the scFvs was significantly different in that high level binding was evident on truncated forms of PrP(c) that reacted poorly or not at all with several pre-existing MAbs. These data suggest that the isolated scFv-Ps bind to novel epitopes within the amino-central region of PrP(c). In addition, the binding of MAbs to known linear epitopes within PrP(c) depends strongly on the endpoints of the target PrP(c) fragment used.

The role of the cellular prion protein in the immune system

Prion protein (PrP) plays a key role in the pathogenesis of prion diseases. However, the normal function of the protein remains unclear. The cellular isoform (PrP(C)) is expressed widely in the immune system, in haematopoietic stem cells and mature lymphoid and myeloid compartments in addition to cells of the central nervous system. It is up-regulated in T cell activation and may be expressed at higher levels by specialized classes of lymphocyte. Furthermore, antibody cross-linking of surface PrP modulates T cell activation and leads to rearrangements of lipid raft constituents and increased phosphorylation of signalling proteins. These findings appear to indicate an important but, as yet, ill-defined role in T cell function. Although PrP(-/-) mice have been reported to have only minor alterations in immune function, recent work has suggested that PrP is required for self-renewal of haematopoietic stem cells. Here, we consider the evidence for a distinctive role for PrP(C) in the immune system and what the effects of anti-prion therapeutics may be on immune function.

Development of a monoclonal antibody-based immunohistochemistry method for BSE surveillance in China

Five hybridoma cell lines secreting anti-PrP antibodies were established from the fusion between mouse myeloma Sp2/0 and spleen cells from mice immunized with recombinant Chinese Luxi yellow cattle (Bos taurus, Luxi) PrP (24-234) or recombinant Chinese small-tailed Han sheep PrP (94-227). According to their Western blot reactivity, five monoclonal antibodies (mAbs) could be divided into two groups. Group A, mAbs 1H2, 4C6, and 4C11 recognized re-PrP, PrP(C), and PrP(Sc) from both bovine and sheep. Group B, mAbs 2H3 and 4H10 only recognized re-PrP and PrP(Sc) of sheep, and especially, these two mAbs could not recognize PrP(C) from both bovine and sheep. In immunohistochemistry (IHC) test, mAb 4C11 immunostained the PrP(Sc) accumulation in tissue sections from BSE cattle and Scrapie sheep, and compared with mAb 6H4, it had the same immunohistochemical pattern. An IHC method based on mAb 4C11 for the detection of BSE was established and had been applied for the long-term surveillance of BSE in China. From 2001 to 2004, 12,692 samples from the whole country had been tested and all had negative results.

Copper induces conformational changes in the N-terminal part of cell-surface PrPC

Prion diseases are caused by misfolding of the cellular prion protein, PrPC. In vitro studies have shown that PrP binds copper via the octarepeat region lying within the unstructured N-terminal segment of the protein, but the significance of copper in PrP metabolism remains unclear. Here, six specific antibodies recognizing different epitope regions of PrP were used to measure the effect of copper on the conformation of the molecule at the cell surface. Binding of an antibody, E149, to an epitope within the octarepeat domain of PrP is halved in the presence of copper, whereas binding of antibodies recognizing epitope motifs C-terminal to residue 90 of PrP remain relatively unaltered under equivalent conditions. These experiments strongly suggest that copper induces localized conformational change within the N-terminal portion of cell-surface PrPC.

Detection of prion particles in samples of BSE and scrapie by fluorescence correlation spectroscopy without proteinase K digestion

A characteristic feature of prion diseases such as bovine spongiform encephalopathy (BSE) is the accumulation of a pathological isoform of the host-encoded prion protein, PrP. In contrast to its cellular isoform PrP(C), the pathological isoform PrP(Sc) forms insoluble aggregates. All commercial BSE tests currently used for routine testing are based on the proteinase K (PK) resistance of PrP, but not all pathological PrP is PK-resistant. In the present study, single prion particles were counted by fluorescence correlation spectroscopy (FCS). The property of PK resistance is not required, i.e., both the PK-resistant and the PK-sensitive parts of the prion particles are detectable. PrP aggregates were prepared from the brains of BSE-infected cattle, as well as from scrapie-infected hamsters, by the NaPTA precipitation method without PK digestion. They were labeled using two different PrP-specific antibodies for FCS measurements in the dual-color mode (2D-FIDA). Within the limited number of samples tested, BSE-infected cattle and scrapie-infected hamsters in the clinical stage of the disease could be distinguished with 100% specificity from a control group. Thus, a diagnostic tool for BSE detection with complete avoidance of PK treatment is presented, which should have particular advantages for testing animals in the preclinical stage.

Generation of antibodies against bovine recombinant prion protein in various strains of mice

Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, belong to a group of neurodegenerative disorders affecting humans and animals. To date, definite diagnosis of prion disease can only be made by analysis of tissue samples for the presence of protease-resistant misfolded prion protein (PrP(Sc)). Monoclonal antibodies (MAbs) to the prion protein provide valuable tools for TSE diagnosis, as well as for basic research on these diseases. In this communication, the development of antibodies against recombinant bovine prion protein (brecPrP) in four strains of mice (BALB/c, ND4, SJL, and NZB/NZW F(1)) is described. Immunization of autoimmunity-prone NZB/NZW F(1) and SJL mice with brecPrP was applied to overcome self-tolerance against the prion protein. ND4 and SJL mice did not develop an immune response to brecPrP. BALB/c mice produced antibody titers of 1:1,000 to 1:1,500 in an enzyme-linked immunosorbent assay (ELISA), while NZB/NZW F(1) mice responded with titers of 1:7,000 to 1:11,000. A panel of 71 anti-brecPrP MAbs recognizing continuous and discontinuous epitopes was established from BALB/c and NZB/NZW F(1) mice. Seven anti-brecPrP MAbs reacted with both the cellular form of PrP and protease K-resistant PrP(Sc) from sheep brain in Western blot assays. The epitope specificity of these MAbs was determined, and applicability to immunohistochemical detection of prions was studied. The MAbs generated will be useful tools in the development of TSE immunochemical diagnosis and for research. This is the first report of the development of anti-PrP MAbs by use of autoimmune NZB/NZW F(1) mice as an alternative approach for the generation of PrP-specific MAbs.

Assembly of natural and recombinant prion protein into fibrils

The conversion of the alpha-helical, cellular isoform of the prion protein (PrP C ) to the insoluble, beta-sheet-rich, infectious, disease-causing isoform (PrP Sc ) is the fundamental event in the prion diseases. The C-terminal fragment of PrP Sc (PrP 27-30) is formed by limited proteolysis and retains infectivity. Unlike full-length PrP Sc , PrP 27-30 polymerizes into rod-shaped structures with the ultra-structural and tinctorial properties of amyloid. To study the folding of PrP, both with respect to the formation of PrP Sc from PrP C and the assembly of rods from PrP 27-30, we solubilized Syrian hamster (sol SHa) PrP 27-30 in low concentrations (0.2%) of sodium dodecyl sulfate (SDS) under conditions previously used to study the structural transitions of this protein. Sol SHaPrP 27-30 adopted a beta-sheet-rich structure at SDS concentrations between 0.02% and 0.04% and remained soluble. Here we report that NaCl stabilizes SHaPrP 27-30 in a soluble, beta-sheet-rich state that allows fibril assembly to proceed over several weeks. Under these conditions, fibril formation occurred not only with sol PrP 27-30, but also with native SHaPrP C . Addition of sphingolipids seems to increase fibril growth. When recombinant (rec) SHaPrP(90-231) was exposed to low concentrations of SDS, similar to those used to polymerize sol SHaPrP 27-30 in the presence of 250 mM NaCl, fibril formation occurred regularly. When fibrils formed from PrP 27-30 or PrP C were bioassayed in transgenic mice overexpressing full-length SHaPrP, no infectivity was obtained, whereas amyloid fibrils formed of rec mouse PrP(89-230) were infectious. At present, it cannot be determined whether the lack of infectivity is caused by a difference in the structure of the fibrils or in the bioassay conditions.

Breaking immune tolerance to the prion protein using prion protein peptides plus oligodeoxynucleotide-CpG in mice

The absence of a detectable immune response during transmissible spongiform encephalopathies is likely due to the fact that the essential component of infectious agents, the prion protein (PrP), is a self Ag expressed on the surface of many cells of the host. To overcome self-tolerance to PrP, we used 30-mer PrP peptides previously shown to be immunogenic in Prnp(-/-) mice, together with CFA or CpG-oligodeoxynucleotides (CpG) in IFA. Generation of anti-PrP T and B cell responses was analyzed in the spleen, lymph nodes, and serum of immunized C57BL/6 wild-type mice. Immunization with PrP peptides emulsified in CFA did not trigger an immune response to PrP. When CpG were used, vaccination with peptides P143-172 and P158-187 generated IFN-gamma-secreting splenic T cells, and only P158-187 significantly stimulated IL-4-secreting T cells. Both peptides induced few Ab-producing B cells, and low and variable serum Ab titers. In contrast, immunization with peptide P98-127 did not induce significant levels of T cell responses but elicited specific peptide Abs. T cell epitope mapping, performed using 15-mer peptides covering PrP segment 142-182, revealed that an immunogenic motif lies between positions 156 and 172. These results demonstrate that T and B cell repertoires against PrP can be stimulated in C57BL/6 when adjuvant of the innate immunity such as CpG, but not CFA, is added to PrP peptides, and that the pattern of immune responses varies according to the epitope.

Identification of two immunogenic domains of the prion protein-PrP-which activate class II-restricted T cells and elicit antibody responses against the native molecule

Recent reports suggest that immunity against the prion protein (PrP) retards transmissible spongiform encephalopathies progression in infected mice. A major obstacle to the development of vaccines comes from the fact that PrP is poorly immunogenic, as it is seen as self by the host immune system. Additional questions concern the immune mechanisms involved in protection and the risk of eliciting adverse reactions in the central nervous system of treated patients. Peptide-based vaccines offer an attractive strategy to overcome these difficulties. We have undertaken the identification of the immunogenic regions of PrP, which trigger helper T cells (Th) associated with antibody production. Our results identify two main regions, one between the structured and flexible portion of PrP (98-127) and a second between alpha 1 and alpha 2 helix (143-187). Peptides (30-mer) corresponding to these regions elicit class II-restricted Th cells and antibody production against native PrP and could therefore be of potential interest for a peptide-based vaccination.

Antigenic characterization of an abnormal isoform of prion protein using a new diverse panel of monoclonal antibodies

We established a panel of monoclonal antibodies (mAbs) against prion protein (PrP) by immunizing PrP gene-ablated mice with the pathogenic isoform of prion protein (PrPSc) or recombinant prion protein (rPrP). The mAbs could be divided into at least 10 groups by fine epitope analyses using mutant rPrPs and pepspot analysis. Seven linear epitopes, lying within residues 56-90, 119-127, 137-143, 143-149, 147-151, 163-169, and 219-229, were defined by seven groups of mAbs, although the remaining three groups of mAbs recognized discontinuous epitopes. We attempted to examine whether any of these epitopes are located on the accessible surface of PrPSc. However, no mAbs reacted with protease-treated PrPSc purified from scrapie-affected mice, even when PrPSc was dispersed into a detergent-lipid protein complex, to reduce the size of PrPSc aggregates. In contrast, denaturation of PrPSc by guanidine hydrochloride efficiently exposed all of the epitopes. This suggests that any epitope recognized by this panel of mAbs is buried within the PrPSc aggregates. Alternatively, if the corresponding region(s) are on the surface of PrPSc, the region(s) may be folded into conformations to which the mAbs cannot bind. The reactivity of a panel of mAb also showed that the state of PrPSc aggregation influenced the denaturation process, and the sensitivity to denaturation appeared to vary between epitopes. Our results demonstrate that this new panel of well-characterized mAbs will be valuable for studying the biochemistry and biophysics of PrP molecules as well as for the immuno-diagnosis of prion diseases.

Establishment of a Chicken Monoclonal Antibody Panel Against Mammalian Prion Protein

A panel of chicken monoclonal antibodies (mAbs) was developed against prion protein (PrP), the sequence of which is a highly conserved molecule among mammals. A portion of the splenocytes from chickens immunized with recombinant mouse PrP was fused with the chicken B cell line, MuH1. The remaining splenocytes were used to generate the recombinant mAbs by phage display. A total of 36 anti-PrP mAbs, 2 from cell fusion and 34 from phage display were established. The specificity of these mAbs was determined by Western blot and ELISA using various PrP antigens including recombinant PrPs, synthetic PrP peptides and PrPs from brains or scrapie-infected neuroblastoma cell line. These mAbs were classified into three main groups, protease K (PK)-sensitive (Group I), PK cleavage site proximal (Group II) and PK-resistant (Group III), based on their abilities to recognize PrP following PK-treatment. Some mAbs were found to selectively recognize different glycoforms of PrP as well as the metabolic fragments of PrP. Furthermore, we found that PrP recognition by chickens differed from that by PrP-knockout mouse. These results indicate that these newly generated PrP antibodies from chickens will help to research the PrP and to establish the diagnosis of prion disease.

Cytosolic prion protein in neurons

Localizing the cellular prion protein (PrPC) in the brain is necessary for understanding the pathogenesis of prion diseases. However, the precise ultrastructural localization of PrPC still remains enigmatic. We performed the first quantitative study of the ultrastructural localization of PrPC in the mouse hippocampus using high-resolution cryoimmunogold electron microscopy. PrPC follows the standard biosynthetic trafficking pathway with a preferential localization in late endosomal compartments and on the plasma membrane of neurons and neuronal processes. PrPC is found with the same frequency within the synaptic specialization and perisynaptically, but is almost completely excluded from synaptic vesicles. Unexpectedly, PrP is also found in the cytosol in subpopulations of neurons in the hippocampus, neocortex, and thalamus but not the cerebellum. Cytosolic PrP may have altered susceptibility to aggregation, suggesting that these neurons might play a significant role in the pathogenesis of prion diseases, in particular those mammals harboring mutant PrP genes.

Cytosolic prion protein in neurons

Localizing the cellular prion protein (PrPC) in the brain is necessary for understanding the pathogenesis of prion diseases. However, the precise ultrastructural localization of PrPC still remains enigmatic. We performed the first quantitative study of the ultrastructural localization of PrPC in the mouse hippocampus using high-resolution cryoimmunogold electron microscopy. PrPC follows the standard biosynthetic trafficking pathway with a preferential localization in late endosomal compartments and on the plasma membrane of neurons and neuronal processes. PrPC is found with the same frequency within the synaptic specialization and perisynaptically, but is almost completely excluded from synaptic vesicles. Unexpectedly, PrP is also found in the cytosol in subpopulations of neurons in the hippocampus, neocortex, and thalamus but not the cerebellum. Cytosolic PrP may have altered susceptibility to aggregation, suggesting that these neurons might play a significant role in the pathogenesis of prion diseases, in particular those mammals harboring mutant PrP genes.

Generation of Antibodies Against Prion Protein by Scrapie-Infected Cell Immunization of PrP0/0Mice

Four monoclonal antibodies (MAbs) specific for prion protein (PrP) were generated by using PrP-knockout mice immunized with a scrapie-infected mouse neuroblastoma cell line (N2a/22L). The MAbs reacted with both the cellular form (PrP(C)) and the protease K-treated form (PrP(Sc)) on Western blotting. Of the four MAbs, three recognized mouse and hamster PrP, while the remaining MAb recognized mouse, sheep, and bovine PrPs. In addition, these MAbs were shown to react only with the unglycosylated and monoglycoslated forms of PrP(Sc) in N2a/22L, but reacted with all glycosylated forms of PrP(C) and PrP(Sc) from mouse brain. This study was the first to report the development of anti-PrP MAbs using scrapie-infected cells as an immunogen and provides one approach for the generation of PrP-specific MAbs.

Comparison of PrP transcription and translation in two murine myeloma cell lines

The generation of monoclonal antibodies (MAbs) to the prion protein, PrP, is important in order to establish a large repertoire of useful reagents for the diagnosis of transmissible spongiform encephalopathies (TSE), or prion diseases. However, the presence of PrP on the surface of all mammalian cells (PrP(C)) causes self-recognition, thereby restricting the ability of mice to produce an immune response to the PrP immunogen. Although this problem has been alleviated with the generation and use of PrP-knockout mice, the production of MAbs has continued to be severely hampered presumably since the fusion partner for spleen-derived lymphocytes was PrP(C)-containing myeloma cell lines. The availability of a mouse myeloma cell line expressing little or no PrP(C) on the surface would therefore be useful for MAb generation. Our data indicate that cells differ in their levels of PrP(C) expression and suggest that not all murine myeloma cell lines are equally useful for obtaining hybridomas secreting anti-PrP MAbs.

Polyclonal anti-PrP auto-antibodies induced with dimeric PrP interfere efficiently with PrPSc propagation in prion-infected cells

Prion diseases are neurodegenerative infectious disorders for which no prophylactic regimens are known. In order to induce antibodies/auto-antibodies directed against surface-located PrP(c), we used a covalently linked dimer of mouse prion protein expressed recombinantly in Escherichia coli. Employing dimeric PrP as an immunogen we were able to effectively overcome autotolerance against murine PrP in PrP wild-type mice without inducing obvious side effects. Treatment of prion-infected mouse cells with polyclonal anti-PrP antibodies generated in rabbit or auto-antibodies produced in mice significantly inhibited endogenous PrP(Sc) synthesis. We show that polyclonal antibodies are binding to surface-located PrP(c), thereby interfering with prion biogenesis. This effect is much more pronounced in the presence of full IgG molecules, which, unlike Fab fragments, seem to induce a significant cross-linking of surface PrP. In addition, we found immune responses against different epitopes when comparing antibodies induced in rabbits and PrP wild-type mice. Only in the auto-antibody situation in mice an immune reaction against a region of PrP is found that was reported to be involved in the PrP(Sc) conversion process. Our data point to the possibility of developing means for an active immunoprophylaxis against prion diseases.