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

Prion protein E219K polymorphism: from the discovery of the KANNO blood group to interventions for human prion disease

KANNO is a new human blood group that was recently discovered. The KANNO antigen shares the PRNP gene with the prion protein and the prion protein E219K polymorphism determines the presence or absence of the KANNO antigen and the development of anti-KANNO alloantibodies. These alloantibodies specifically react with prion proteins, which serve as substrates for conversion into pathological isoforms in some prion diseases and may serve as effective targets for resisting prion infection. These findings establish a potential link between the KANNO blood group and human prion disease via the prion protein E219K polymorphism. We reviewed the interesting correlation between the human PRNP gene's E219K polymorphism and the prion proteins it expresses, as well as human red blood cell antigens. Based on the immune serological principles of human blood cells, the prion protein E219K polymorphism may serve as a foundation for earlier molecular diagnosis and future drug development for prion diseases.

Ligands binding to the prion protein induce its proteolytic release with therapeutic potential in neurodegenerative proteinopathies

The prion protein (PrPC) is a central player in neurodegenerative diseases, such as prion diseases or Alzheimer’s disease. In contrast to disease-promoting cell surface PrPC, extracellular fragments act neuroprotective by blocking neurotoxic disease-associated protein conformers. Fittingly, PrPC release by the metalloprotease ADAM10 represents a protective mechanism. We used biochemical, cell biological, morphological, and structural methods to investigate mechanisms stimulating this proteolytic shedding. Shed PrP negatively correlates with prion conversion and is markedly redistributed in murine brain in the presence of prion deposits or amyloid plaques, indicating a sequestrating activity. PrP-directed ligands cause structural changes in PrPC and increased shedding in cells and organotypic brain slice cultures. As an exception, some PrP-directed antibodies targeting repetitive epitopes do not cause shedding but surface clustering, endocytosis, and degradation of PrPC. Both mechanisms may contribute to beneficial actions described for PrP-directed ligands and pave the way for new therapeutic strategies against currently incurable neurodegenerative diseases.

Antibody Fragments as Tools for Elucidating Structure-Toxicity Relationships and for Diagnostic/Therapeutic Targeting of Neurotoxic Amyloid Oligomers

The accumulation of amyloid protein aggregates in tissues is the basis for the onset of diseases known as amyloidoses. Intriguingly, many amyloidoses impact the central nervous system (CNS) and usually are devastating diseases. It is increasingly apparent that neurotoxic soluble oligomers formed by amyloidogenic proteins are the primary molecular drivers of these diseases, making them lucrative diagnostic and therapeutic targets. One promising diagnostic/therapeutic strategy has been the development of antibody fragments against amyloid oligomers. Antibody fragments, such as fragment antigen-binding (Fab), scFv (single chain variable fragments), and VHH (heavy chain variable domain or single-domain antibodies) are an alternative to full-length IgGs as diagnostics and therapeutics for a variety of diseases, mainly because of their increased tissue penetration (lower MW compared to IgG), decreased inflammatory potential (lack of Fc domain), and facile production (low structural complexity). Furthermore, through the use of in vitro-based ligand selection, it has been possible to identify antibody fragments presenting marked conformational selectivity. In this review, we summarize significant reports on antibody fragments selective for oligomers associated with prevalent CNS amyloidoses. We discuss promising results obtained using antibody fragments as both diagnostic and therapeutic agents against these diseases. In addition, the use of antibody fragments, particularly scFv and VHH, in the isolation of unique oligomeric assemblies is discussed as a strategy to unravel conformational moieties responsible for neurotoxicity. We envision that advances in this field may lead to the development of novel oligomer-selective antibody fragments with superior selectivity and, hopefully, good clinical outcomes.

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.

Cell-based therapy against prion diseases

Despite multiple efforts to find treatments, prion diseases are still incurable. The currently available therapeutic strategies are mostly based on compounds to inhibit pathological PrP (PrPSc) accumulation, and cellular PrP (PrP^C) conversion into PrP^Sc. However, they cannot reverse the pathological changes already present in the brain. Cell-based therapeutic strategies could promote the repair of the pre-existing brain damage. The few available data come mostly from preclinical studies using neural stem cells, bone marrow-derived microglia and mesenchymal stem cells, as cell sources. Moreover, the benefits of cell-based therapeutic strategies could be linked not only to the replacement of damaged cells, but also to the secretion of trophic factors by the grafted cells that might modulate inflammation, cell death, or endogenous neurogenesis.

Single chain variable fragment against aβ expressed in baculovirus inhibits abeta fibril elongation and promotes its disaggregation

Alzheimer’s disease (AD) is the most common form of age-related dementia, and the most urgent problem is that it is currently incurable. Amyloid-β (Aβ) peptide is believed to play a major role in the pathogenesis of AD. We previously reported that an Aβ N-terminal amino acid targeting monoclonal antibody (MAb), A8, inhibits Aβ fibril formation and has potential as an immunotherapy for AD based on a mouse model. To further study the underlying mechanisms, we tested our hypothesis that the single chain fragment variable (scFv) without the Fc fragment is capable of regulating either Aβ aggregation or disaggregation in vitro. Here, a model of cell-free Aβ “on-pathway” aggregation was established and identified using PCR, Western blot, ELISA, transmission electron microscopy (TEM) and thioflavin T (ThT) binding analyses. His-tagged A8 scFvs was cloned and solubly expressed in baculovirus. Our data demonstrated that the Ni-NTA agarose affinity-purified A8 scFv inhibited the forward reaction of “on-pathway” aggregation and Aβ fibril maturation. The effect of A8 scFv on Aβ fibrillogenesis was markedly more significant when administered at the start of the Aβ folding reaction. Furthermore, the results also showed that pre-formed Aβ fibrils could be disaggregated via incubation with purified A8 scFv, which suggested that A8 scFv is involved in the reverse reaction of Aβ aggregation. Therefore, A8 scFv was capable of both inhibiting fibrillogenesis and disaggregating matured fibrils. Our present study provides valuable insight into the regulators of ultrastructural dynamics of cell-free “on-pathway” Aβ aggregation and will assist in the development of therapeutic strategies for AD.

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.

Potential candidate camelid antibodies for the treatment of protein-misfolding diseases

Protein-misfolding diseases (PMDs), including Alzheimer's disease would potentially reach epidemic proportion if effective ways to diagnose and treat them were not developed. The quest for effective therapy for PMDs has been ongoing for decades and some of the technologies developed so far show great promise. We report here the development of antibodies by immunization of camelids with prion (PrioV3) and Alzheimer's (PrioAD12, 13 & 120) disease-derived brain material. We show that anti-PrP antibody transmigration across the blood-brain barrier (BBB) was inhibited with phosphatidylinositol-specific phospholipase C (PIPLC). Our camelid anti-prion antibody was also shown to permanently abrogate prion replication in a prion-permissive cell line after crossing the artificial BBB. Furthermore, anti-Aβ/tau antibodies were able to bind their specific immunogens with ELISA and immunohistochemistry. Finally, both PrioV3 and PrioAD12 were shown to co-localize with Lamp-1, a marker of late endosomal/lysosomal compartments. These antibodies could prove to be a valuable tool for the neutralization/clearance of PrP(Sc), Aβ and tau proteins in cellular compartments of affected neurons and could potentially have wider applicability for the treatment of PMDs.

Therapeutic approaches for prion disorders

Prion diseases are lethal for both humans and animals, and affected individuals die after several months following a rapid disease progression. Although researchers have attempted for decades to develop effective therapeutics for the therapy of human prion disorders, until now no efficient drug has been available on the market for transmissible spongiform encephalopathy (TSE) treatment or cure. Approximately 200 patients worldwide have died or suffer from variant Creutzfeldt-Jakob disease (CJD). Incidences for sporadic and familial CJD are approximately 1.5-2 per million per year and one per 10 million per year, respectively, in Europe. This review summarizes classical and modern trials for the development of effective anti-TSE drugs, introduces potential effective delivery systems, such as lentiviral and adeno-associated virus systems for antiprion components, including antibodies and siRNAs, and presents vaccination trials. Most of the antiprion drugs target prion protein PrP(c) and/or PrP(Sc). Alternative targets are receptors and coreceptors for PrP, that is, the 37/67-kDa laminin receptor and heparan sulfate proteoglycanes. We review clinical trials for the treatment of TSEs and describe hindrances and chances for a breakthrough in therapy of prion disorders.

Single-chain fragment variable passive immunotherapies for neurodegenerative diseases

Accumulation of misfolded proteins has been implicated in a variety of neurodegenerative diseases including prion diseases, Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD). In the past decade, single-chain fragment variable (scFv) -based immunotherapies have been developed to target abnormal proteins or various forms of protein aggregates including Aβ, SNCA, Htt, and PrP proteins. The scFvs are produced by fusing the variable regions of the antibody heavy and light chains, creating a much smaller protein with unaltered specificity. Because of its small size and relative ease of production, scFvs are promising diagnostic and therapeutic reagents for protein misfolded diseases. Studies have demonstrated the efficacy and safety of scFvs in preventing amyloid protein aggregation in preclinical models. Herein, we discuss recent developments of these immunotherapeutics. We review efforts of our group and others using scFv in neurodegenerative disease models. We illustrate the advantages of scFvs, including engineering to enhance misfolded conformer specificity and subcellular targeting to optimize therapeutic action.

Gene-based antibody strategies for prion diseases

Prion diseases or transmissible spongiform encephalopathies (TSE) are a group of neurodegenerative and infectious disorders characterized by the conversion of a normal cellular protein PrP^C into a pathological abnormally folded form, termed PrP^Sc. There are neither available therapies nor diagnostic tools for an early identification of individuals affected by these diseases. New gene-based antibody strategies are emerging as valuable therapeutic tools. Among these, intrabodies are chimeric molecules composed by recombinant antibody fragments fused to intracellular trafficking sequences, aimed at inhibiting, in vivo, the function of specific therapeutic targets. The advantage of intrabodies is that they can be selected against a precise epitope of target proteins, including protein-protein interaction sites and cytotoxic conformers (i.e., oligomeric and fibrillar assemblies). Herein, we address and discuss in vitro and in vivo applications of intrabodies in prion diseases, focussing on their therapeutic potential.

Effects of a brain-engraftable microglial cell line expressing anti-prion scFv antibodies on survival times of mice infected with scrapie prions

We first verified that a single chain Fv fragment against prion protein (anti-PrP scFv) was secreted by HEK293T cells and prevented prion replication in infected cells. We then stably expressed anti-PrP scFv in brain-engraftable murine microglial cells and intracerebrally injected these cells into mice before or after infection with prions. Interestingly, the injection before or at an early time point after infection attenuated the infection marginally but significantly prolonged survival times of the mice. These suggest that the ex vivo gene transfer of anti-PrP scFvs using brain-engraftable cells could be a possible immunotherapeutic approach against prion diseases.

Specific binding of the pathogenic prion isoform: development and characterization of a humanized single-chain variable antibody fragment

Murine monoclonal antibody V5B2 which specifically recognizes the pathogenic form of the prion protein represents a potentially valuable tool in diagnostics or therapy of prion diseases. As murine antibodies elicit immune response in human, only modified forms can be used for therapeutic applications. We humanized a single-chain V5B2 antibody using variable domain resurfacing approach guided by computer modelling. Design based on sequence alignments and computer modelling resulted in a humanized version bearing 13 mutations compared to initial murine scFv. The humanized scFv was expressed in a dedicated bacterial system and purified by metal-affinity chromatography. Unaltered binding affinity to the original antigen was demonstrated by ELISA and maintained binding specificity was proved by Western blotting and immunohistochemistry. Since monoclonal antibodies against prion protein can antagonize prion propagation, humanized scFv specific for the pathogenic form of the prion protein might become a potential therapeutic reagent.

Intrabody Expression in Mammalian Cells

The intracellular expression of antibodies or antibody fragments (intrabodies) in different compartments of mammalian cells allows to block or modulate the function of endogenous molecules. Intrabodies can alter protein folding, protein-protein, protein-DNA, protein-RNA interactions and protein modification. They can induce a phenotypic knockout and work as neutralizing agents by direct binding to the target antigen, by diverting its intracellular traffic or by inhibiting its association with binding partners. They have been largely employed as research tools and are emerging as therapeutic molecules for the treatment of human diseases as viral pathologies, cancer and misfolding diseases. The fast growing bio-market of recombinant antibodies provides intrabodies with enhanced binding specificity, stability and solubility, together with lower immunogenicity, for their use in therapy. This chapter describes the crucial aspects required to express intrabodies in different intracellular compartments of mammalian cells, their various modes of action and gives an update on the applications of intrabodies in human diseases.

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.

Virally and physically transgenized equine adipose-derived stromal cells as a cargo for paracrine secreted factors

BACKGROUND

Adipose-Derived Stromal Cells have been shown to have multiple lineage differentiation properties and to be suitable for tissues regeneration in many degenerative processes. Their use has been proposed for the therapy of joint diseases and tendon injuries in the horse. In the present report the genetic manipulation of Equine Adipose-Derived Stromal Cells has been investigated.

RESULTS

Equine Adipose-Derived Stromal Cells were successfully virally transduced as well as transiently and stably transfected with appropriate parameters, without detrimental effect on their differentiation properties. Moreover, green fluorescent protein alone, fused to neo gene, or co-expressed as bi-cistronic reporter constructs, driven by viral and house-keeping gene promoters, were tested. The better expressed cassette was employed to stably transfect Adipose-Derived Stromal Cells for cell therapy purposes. Stably transfected Equine Adipose-Derived Stromal Cells with a heterologous secreted viral antigen were able to immunize horses upon injection into the lateral wall of the neck.

CONCLUSION

This study provides the methods to successfully transgenize Adipose-Derived Stromal Cells both by lentiviral vector and by transfection using optimized constructs with suitable promoters and reporter genes. In conclusion these findings provide a working platform for the delivery of potentially therapeutic proteins to the site of cells injection via transgenized Equine Adipose-Derived Stromal Cells.

Progress in the development of therapeutic antibodies targeting prion proteins and beta-amyloid peptides

Prion diseases and Alzheimer's disease (AD) are characterized by protein misfolding, and can lead to dementia. However, prion diseases are infectious and transmissible, while AD is not. The similarities and differences between these diseases have led researchers to perform comparative studies. In the last 2 decades, progress has been made in immunotherapy using anti-prion protein and anti-beta-amyloid antibodies. In this study, we review new ideas and strategies for therapeutic antibodies targeting prion diseases and AD through conformation dependence.

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.

A novel anti-prion protein monoclonal antibody and its single-chain fragment variable derivative with ability to inhibit abnormal prion protein accumulation in cultured cells

mAbs T1 and T2 were established by immunizing PrP gene ablated mice with recombinant MoPrP of residues 121-231. Both mAbs were cross-reactive with PrP from hamster, sheep, cattle and deer. A linear epitope of mAb T1 was identified at residues 137-143 of MoPrP and buried in PrP(C) expressed on the cell surface. mAb T1 showed no inhibitory effect on accumulation of PrP(Sc) in cultured scrapie-infected neuroblastoma (ScN2a) cells. In contrast, mAb T2 recognized a discontinuous epitope ranged on, or structured by, residues 132-217 and this epitope was exposed on the cell surface PrP(C). mAb T2 showed an excellent inhibitory effect on PrP(Sc) accumulation in vitro at a 50% inhibitory concentration of 0.02 microg/ml (0.14 nM). The scFv form of mAb T2 (scFv T2) was secreted in neuroblastoma (N2a58) cell cultures by transfection through eukaryotic secretion vector. Coculturing of ScN2a cells with scFv T2-producing N2a58 cells induced a clear inhibitory effect on PrP(Sc) accumulation, suggesting that scFv T2 could potentially be an immunotherapeutic tool for prion diseases by inhibition of PrP(Sc) accumulation.

Developing intrabodies for the therapeutic suppression of neurodegenerative pathology

Many neurodegenerative diseases have misfolded proteins as a primary occurrence in pathogenesis. A combination of antibody and genetic engineering has emerged as a powerful tool for developing reagents that specifically target the misfolding process itself, and/or abnormal interactions of the misfolded protein species. This review focuses on the selection and testing of intracellular antibody fragments (intrabodies), with a particular focus on Huntington's disease (HD) and Parkinson's disease (PD), both of which show prominent intracellular protein aggregates in affected neurons. The most dramatic advances are in HD, where in vivo efficacy of intrabodies has been demonstrated. Targets in other neurodegenerative disorders, including Alzheimer's disease and prion diseases, are noted more briefly, with an emphasis on the potential for intracellular manipulations. Given the specificity and versatility of antibody-based reagents, the wide range of options for conformational and post-translationally-modified targets, and the recent improvement in gene delivery, this should be a fertile field for 21(st) century pharmacology.

Antibody-based immunotherapeutic attempts in experimental animal models of prion diseases

BACKGROUND

There has been a dramatic decrease in the risk of transmission of bovine spongiform encephalopathy to humans. In contrast, the risk of human-to-human transmission of variant Creutzfeldt-Jakob disease (vCJD) via medical treatments became potentially high since 4 vCJD cases were reported to be possibly transmitted through blood transfusion in the UK. However, no treatments are yet available for curing prion diseases.

OBJECTIVE

Conversion of the normal prion protein, PrP(C), to the amyloidogenic PrP, PrP(Sc), plays a pivotal role in the pathogenesis. Recently, certain anti-PrP or anti-37/67-kDa laminin receptor (LRP/LR) antibodies were shown to have the potential to cure chronically infected cells, clearing PrP(Sc) from the cells. This has raised the possibility of antibody based-immunotherapy for prion diseases. This article aims to introduce and discuss the recently published attempts of immunotherapy in prion diseases.

METHODS

Bibliographic research was carried out using the PubMed database. Patent literature was searched using the UK Intellectual Property Office website.

RESULTS/CONCLUSION

No satisfying consequences in animals could be detected with anti-PrP antibodies directly infused into the brains of animals by the intraventricular route or by anti-PrP or anti-LRP/LR single chain fragment antibodies directly delivered into the brain by virus vector-mediated gene transfer. This is probably because such delivery systems failed to deliver the antibodies to the neurons relevant for the treatments.

Single-chain Fv antibody fragments retain binding properties of the monoclonal antibody raised against peptide P1 of the human prion protein

Prion diseases are incurable neurodegenerative diseases that affect both humans and animals. The infectious agent is a pathogenic form of the prion protein that accumulates in brain as amyloids. Currently, there is neither cure nor reliable preclinical diagnostics on the market available. The growing number of reports shows that passive immunisation is one of the most promising strategies for prion disease therapy, where antibodies against prions may prevent and even cure the infection. Since antibodies are large molecules and, thus, might not be suitable for the therapy, different antibody fragments are a good alternative. Therefore, we have designed and prepared single-chain antibody fragments (scFvs) derived from the PrP(Sc)-specific murine monoclonal antibody V5B2. Using a new expression vector pMD204, we produced scFvs in two opposing chain orientations in the periplasm of Escherichia coli. Both recombinant antibody fragments retained the specificity of the parent antibody and one of these exhibited binding properties comparable to the corresponding murine Fab fragments with the affinity in nM range. Our monovalent antibody fragments are of special interest in view of possible therapeutic reagents for prion diseases as well as for development of a new generation of diagnostics.

Development of antibody fragments for immunotherapy of prion diseases

Prions are infectious proteins responsible for a group of fatal neurodegenerative diseases called TSEs (transmissible spongiform encephalopathies) or prion diseases. In mammals, prions reproduce themselves by recruiting the normal cellular protein PrP(C) and inducing its conversion into the disease-causing isoform denominated PrP(Sc). Recently, anti-prion antibodies have been shown to permanently cure prion-infected cells. However, the inability of full-length antibodies and proteins to cross the BBB (blood-brain barrier) hampers their use in the therapy of TSEs in vivo. Alternatively, brain delivery of prion-specific scFv (single-chain variable fragment) by AAV (adeno-associated virus) transfer delays the onset of the disease in infected mice, although protection is not complete. We investigated the anti-prion effects of a recombinant anti-PrP (D18) scFv by direct addition to scrapie-infected cell cultures or by infection with both lentivirus and AAV-transducing vectors. We show that recombinant anti-PrP scFv is able to reduce proteinase K-resistant PrP content in infected cells. In addition, we demonstrate that lentiviruses are more efficient than AAV in gene transfer of the anti-PrP scFv gene and in reducing PrP(Sc) content in infected neuronal cell lines. Finally, we have used a bioinformatic approach to construct a structural model of the D18scFv-PrP(C) complex. Interestingly, according to the docking results, Arg(PrP)(151) (Arg(151) from prion protein) is the key residue for the interactions with D18scFv, anchoring the PrP(C) to the cavity of the antibody. Taken together, these results indicate that combined passive and active immunotherapy targeting PrP might be promising strategies for therapeutic intervention in prion diseases.

Complementarity determining regions of an anti-prion protein scFv fragment orchestrate conformation specificity and antiprion activity

The prion protein, PrP, exists in several stable conformations, with the presence of one conformation, PrP(Sc), associated with transmissible neurodegenerative diseases. Targeting PrP by high-affinity ligands has been proven to be an effective way of preventing peripheral prion infections. Here, we have generated bacterially expressed single chain fragments of the variable domains (scFv) of a monoclonal antibody in Escherichia coli, originally raised against purified PrP(Sc) that recognizes both PrP(C) and PrP(Sc). This scFv fragment had a dissociation constant (K(D)) with recombinant PrP of 2 nM and cleared prions in ScN2a cells at 4 nM, as demonstrated by a mouse prion bioassay. A peptide corresponding to the complementarity determining region 3 of the heavy chain (CDR3H) selectively bound PrP(Sc) but had lost antiprion activity. However, synthesis and application of an improved peptide mimicking side chain topology of CDR3H while exhibiting increased protease resistance, a retro-inverso d-peptide of CDR3H, still bound PrP(Sc) and reinstated antiprion activity. We conclude that (1) scFvW226 is so far the smallest polypeptide with bioassay confirmed antiprion activity, and (2) differential conformation specificity and bioactivity can be regulated by orchestrating the participation of different CDRs.

Immune-directed gene therapeutic development for Alzheimer's, prion, and Parkinson's diseases

The development of novel immune-based therapeutics for neurodegenerative diseases is an area of intense focus. Neurodegenerative diseases represent a particular challenge since in many cases the onset of symptoms occurs after considerable degeneration has ensued. Based on human genetic and histopathological evidence from patients with neurodegenerative diseases, animal models that recapitulate specific pathologic features have been developed. Utilizing these animal models in combination with viral vector-based gene therapeutics, specific epochs of disease can be targeted. One common feature of several neurodegenerative diseases is misfolded proteins. The mechanism by which these altered protein conformers lead to neurodegeneration is not completely understood but much effort has been put forward to either degrade aberrant protein or prevent the formation of misfolded conformers. In this review, we will summarize work that employs viral vector gene therapeutics to modulate the brain's response to misfolded proteins with a specific focus on neurodegeneration.

Targeting prion proteins in neurodegenerative disease

BACKGROUND

Spongiform neurodegeneration is the pathological hallmark of individuals suffering from prion disease. These disorders, whose manifestation is sporadic, familial or acquired by infection, are caused by accumulation of the aberrantly folded isoform of the cellular prion protein (PrP(c)), termed PrP(Sc). Although usually rare, prion disorders are inevitably fatal and transferrable by infection.

OBJECTIVE

Pathology is restricted to the central nervous system and premortem diagnosis is usually not possible. Yet, promising approaches towards developing therapeutic regimens have been made recently.

METHODS

The biology of prion proteins and current models of neurotoxicity are discussed and prophylactic and therapeutic concepts are introduced.

RESULTS/CONCLUSIONS

Although various promising drug candidates with antiprion activity have been identified, this proof-of-concept cannot be transferred into translational medicine yet.

Effect of intraventricular infusion of anti-prion protein monoclonal antibodies on disease progression in prion-infected mice

It is well known that anti-prion protein (PrP) monoclonal antibodies (mAbs) inhibit abnormal isoform PrP (PrPSc) formation in cell culture. Additionally, passive immunization of anti-PrP mAbs protects the animals from prion infection via peripheral challenge when mAbs are administered simultaneously or soon after prion inoculation. Thus, anti-PrP mAbs are candidates for the treatment of prion diseases. However, the effects of mAbs on disease progression in the middle and late stages of the disease remain unclear. This study carried out intraventricular infusion of mAbs into prion-infected mice before and after clinical onset to assess their ability to delay disease progression. A 4-week infusion of anti-PrP mAbs initiated at 120 days post-inoculation (p.i.), which is just after clinical onset, reduced PrPSc levels to 70-80 % of those found in mice treated with a negative-control mAb. Spongiform changes, microglial activation and astrogliosis in the hippocampus and thalamus appeared milder in mice treated with anti-PrP mAbs than in those treated with a negative-control mAb. Treatment with anti-PrP mAb prolonged the survival of mice infected with Chandler or Obihiro strain when infusion was initiated at 60 days p.i., at which point PrPSc is detectable in the brain. In contrast, infusion initiated after clinical onset prolonged the survival time by about 8 % only in mice infected with the Chandler strain. Although the effects on survival varied for different prion strains, the anti-PrP mAb could partly prevent disease progression, even after clinical onset, suggesting immunotherapy as a candidate for treatment of prion diseases.

Vaccine approaches to prevent and treat prion infection : progress and challenges

Prion diseases are transmissible neurodegenerative diseases of humans and animals. The prion agent consists of a misfolded protein, PrPSc (prion protein, scrapie form), of a glycosylphosphatidylinositol-anchored host protein, PrPC (PrP cellular form) of unknown function. During prion replication, PrPSc induces host PrPC to adopt its pathogenic conformation. Some PrPSc may aggregate to microscopically visible, extracellular prion plaques that stain for amyloid. The development of antiprion vaccines presents some challenges. While there is strong self-tolerance to an endogenous antibody response to PrPC and PrPSc, highly potent monoclonal antibodies (mAbs) have been raised in mice in which the prion protein gene has been deleted by gene targeting. These mAbs have been demonstrated to be antiprion-active in permanently scrapie-infected neuroblastoma (ScN2a) cells, primarily when bound to one of four epitopes (the octarepeat region, the region around codons 90-110, helix 1 region codons 145-160, and the extreme C-terminal codons 210-220). The mAbs directed against codon regions 90-110 or 145-160 are also antiprion-active in vivo, but only after intraperitoneal infection with prions, not intracerebral infection, suggesting their blood-brain barrier (BBB) impermeability. The challenge will be to make antibodies, or recombinant derivatives thereof, BBB permeable; this is preferably achieved by monovalent antibody fragments since divalent ones were found to be neurotoxic. Self-tolerance of wild-type animals to PrP immunizations was found to be of extrathymic origin. Even though antibodies raised in wild-type mice were found to display antiprion activity in ScN2a cells, these mice did not have significant extensions of incubation times when challenged intraperitoneally with prions. A general low affinity of these antibody responses to native surface-bound PrPC may account for this. Since wild-type mice were found to develop sufficient T-cell responses to codon regions 145-160 and 210-220, we believe that there is a theoretical chance of a successful vaccination therapy. The influence of the way the immunogen is presented has already been shown to be of major importance for the ensuing immune response, in that presentation of PrP with CpG oligodeoxynucleotides as adjuvant or viral packaging improved antibody responses. Major progress for active immunizations may therefore be expected in this field. Eradication programs will be one of the most important uses of active immunization protocols. For this purpose, vaccines will have to be inexpensive, easy to handle, and effective. In the short term, passive immunizations will likely be most promising for therapy of prion disease, including for human medical interventions. Active immunization protocols are less likely to succeed quickly, and will take years if not decades to be validated for domestic or free-ranging animals.

CNS delivery of vectored prion-specific single-chain antibodies delays disease onset

A unifying characteristic of prion diseases is the conversion of a normal cellular protein (PrP(c)) to an abnormal pathogenic conformation, designated PrP(sc). Antibodies directed against PrP(c), when added to scrapie-infected cell cultures or passively administered in vivo, can result in elimination of PrP(sc) or prevent its replication, respectively. In our efforts to develop an approach with potential prophylactic utility we employed a recombinant adeno-associated vector type 2 (rAAV2) viral vector platform to express PrP(c)-specific single-chain fragment variable (scFv) antibodies within the central nervous system (CNS) of susceptible mice that were subsequently inoculated peripherally with infectious prions. Vector expressed scFvs delayed onset of prion pathogenesis as evidenced by improvements in clinical signs and rotarod performance, in extended incubation periods, and in decreased PrP(sc) burden in the CNS. This novel antibody delivery platform enables the in vivo translation of prion prophylactics to other species afflicted by transmissible spongiform encephalopathies (TSEs) and which also has relevance to the development of therapeutics for other protein-misfolding diseases such as Alzheimer's or Parkinson's disease.

Single chain Fv antibodies directed against the 37 kDa/67 kDa laminin receptor as therapeutic tools in prion diseases

Transmissible spongiform encephalopathies are a group of neurological disorders associated with the deposition of PrP(Sc), an abnormal form of the cellular prion protein PrP(c). The 37 kDa/67 kDa laminin receptor (LRP/LR) has been identified as a prion receptor and several lines of evidence strongly suggest that this protein plays a role during prion pathogenesis. Here we report the selection of recombinant single chain antibodies (scFvs) directed against LRP from naïve and synthetic phage scFv libraries for therapeutic application. Western blotting and FACS analysis confirmed a specific LRP/LR recognition pattern of the two selected scFvs S18 and N3. Both scFvs specifically interfered with the PrP/LRP interaction in vitro. High yield production of the scFvs of approx. 1mg/l of culture medium was achieved in E. coli. Passive immunotransfer of the scFv S18 antibody reduced PrP(Sc) levels by approx. 40% in the spleen of scrapie infected C57BL/6 mice 90 days post scFv injection, suggesting that scFv S18 interferes with peripheral PrP(Sc) propagation, without a significant prolongation of incubation and survival times.

Peptide Aptamers Expressed in the Secretory Pathway Interfere with Cellular PrPSc Formation

Prion diseases are rare and obligatory fatal neurodegenerative disorders caused by the accumulation of a misfolded isoform (PrPSc) of the host-encoded prion protein (PrPc). Prophylactic and therapeutic regimens against prion diseases are very limited. To extend such strategies we selected peptide aptamers binding to PrP from a combinatorial peptide library presented on the Escherichia coli thioredoxin A (trxA) protein as a scaffold. In a yeast two-hybrid screen employing full-length murine PrP (aa 23-231) as a bait we identified three peptide aptamers that reproducibly bind to PrP. Treatment of prion-infected cells with recombinantly expressed aptamers added to the culture medium abolished PrPSc conversion with an IC50 between 350 and 700 nM. For expression in eukaryotic cells, peptide aptamers were fused to an N-terminal signal peptide for entry of the secretory pathway. The C terminus was modified by a glycosyl-phosphatidyl-inositol-(GPI) anchoring signal, a KDEL retention motif and the transmembrane and cytosolic domain of LAMP-I, respectively. These peptide aptamers retained their binding properties to PrPc and, depending on peptide sequence and C-terminal modification, interfered with endogenous PrPSc conversion upon expression in prion-infected cells. Notably, infection of cell cultures could be prevented by expression of KDEL peptide aptamers. For the first time, we show that trxA-based peptide aptamers can be targeted to the secretory pathway, thereby not losing the affinity for their target protein. Beside their inhibitory effect on prion conversion, these molecules could be used as fundament for rational drug design.

Toxic effects of intracerebral PrP antibody administration during the course of BSE infection in mice

The absence of specific immune response is a hallmark of prion diseases. However, in vitro and in vivo experiments have provided evidence that an anti-PrP humoral response could have beneficial effects. Prophylactic passive immunization performed at the time of infection delayed or prevented disease. Nonetheless, the potential therapeutic effect of PrP antibodies administered shortly before the clinical signs has never been tested in vivo. Moreover, a recent study showed the potential toxicity of PrP antibodies administered intracerebrally. We aimed at evaluating the effect of a prolonged intracerebral anti-PrP antibody administration at the time of neuroinvasion in BSE infected Tg20 mice. Unexpectedly, despite a good penetration of the antibodies in the brain parenchyma, the treatment was not protective against the development of BSE. Instead, it led to an extensive neuronal loss, strong astrogliosis and microglial activation. Since this effect was observed after injection of anti-PrP antibodies as whole IgGs, F(ab')(2) or Fab fragments, the toxicity was directly related to the ability of the antibodies to recognize native PrP and to the intracerebral concentration achieved, and not to the Fc portion or the divalence of the antibodies. This experiment shows that a prolonged treatment with anti-PrP antibodies by the intracerebral route can induce severe side-effects and calls for caution with regard to the use of similar approaches for late therapeutic interventions in humans.

Selective re-routing of prion protein to proteasomes and alteration of its vesicular secretion prevent PrPScformation

Conversion of the cellular prion protein (PrP(C)) into the abnormal scrapie isoform (PrP(Sc)) is the hallmark of prion diseases, which are fatal and transmissible neurodegenerative disorders. ER-retained anti-prion recombinant single-chain Fv fragments have been proved to be an effective tool for inhibition of PrP(C) trafficking to the cell surface and antagonize PrP(Sc) formation and infectivity. In the present study, we have generated the secreted version of 8H4 intrabody (Sec-8H4) in order to compel PrP(C) outside the cells. The stable expression of the Sec-8H4 intrabodies induces proteasome degradation of endogenous prion protein but does not influence its glycosylation profile and maturation. Moreover, we found a dramatic diverting of PrP(C) traffic from its vesicular secretion and, most importantly, a total inhibition of PrP(Sc) accumulation in NGF-differentiated Sec-8H4 PC12 cells. These results confirm that perturbing the intracellular traffic of endogenous PrP(C) is an effective strategy to inhibit PrP(Sc) accumulation and provide convincing evidences for application of intracellular antibodies in prion diseases.

Potential of active and passive immunizations for the prevention and therapy of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies are fatal neurodegenerative disorders that affect humans and certain animals and are caused by prions. In most cases, infection occurs by ingestion of prions. Their long-time persistence in the environment creates a reservoir of potentially infectious matter that renders the eradication of the disease problematic. Unfortunately, no cure is available to date. Yet, for both the treatment of infected and the protection of uninfected individuals, active and passive immunizations have been shown to have a beneficial effect on the course of the disease. The current review provides an overview of such antibody-based approaches and assesses their feasibility and potential in prophylaxis and therapy of transmissible spongiform encephalopathies.

Immunisation strategies against prion diseases: prime-boost immunisation with a PrP DNA vaccine containing foreign helper T-cell epitopes does not prevent mouse scrapie

Vaccination against prion diseases constitutes a promising approach for the treatment and prevention of the disease. Passive immunisation with antibodies binding to the cellular prion protein (PrP(C)) can protect against prion disease. However, immunotherapeutic strategies with active immunisation are limited due to the immune tolerance against the self-antigen. In order to develop an anti-prion vaccine, we designed a novel DNA fusion vaccine composed of mouse PrP and immune stimulatory helper T-cell epitopes of the tetanus toxin that have previously been reported to break tolerance to other self-antigens. This approach provoked a strong PrP(C)-specific humoral and cellular immune response in PrP null mice, but only low antibody titres were found in vaccinated wild-type mice. Furthermore, prime-boost immunisation with the DNA vaccine and recombinant PrP protein increased antibody titres in PrP null mice, but failed to protect wild-type mice from mouse scrapie.

Reversal of misfolding: prion disease behavioral and physiological impairments recover following postnatal neuronal deletion of the PrP gene

The prionoses are fatal neurodegenerative diseases caused by a pathogenic protein, PrP scrapie, that derives from misfolding of a normal form, PrP(c). These diseases progress through stages. A new study by Mallucci et al. in this issue of Neuron shows that prion disease may be reversed in mice by selective removal of the gene in neurons after early physiological, cognitive, and pathological features have developed.

Expression and detection strategies for an scFv fragment retaining the same high affinity than Fab and whole antibody: Implications for therapeutic use in prion diseases

Since antibodies currently constitute the most rapidly growing class of human therapeutics, the high-yield production of recombinant antibodies and antibody fragments is a real challenge. Using as model a monoclonal antibody directed against the human prion protein that we prepared previously and tested for its therapeutic value, we describe here experimental conditions allowing the production of large quantities (up to 35 mg/l of bacterial culture) of correctly refolded and totally functional single chain fragment variable (scFv). These quantities were sufficient to characterize the binding properties of this small recombinant fragment through in vitro and ex vivo approaches. Interestingly, this scFv retains full binding capacity for its antigen, i.e. the human prion protein, when compared with the corresponding Fab or whole antibody, and recognizes soluble, solid-phase-adsorbed, and membrane-bound prion protein. This strongly suggests that from the mAb cloning step to the refolding of the recombinant fragment, each stage is well controlled, leading to almost 100% functional scFv. These results are of interest not only in view of possible immunotherapy for prion diseases, but also more generally in emphasizing the great promise of these small recombinant molecules in the context of targeted therapies.

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.

Novel aspects of prions, their receptor molecules, and innovative approaches for TSE therapy

1. Prion diseases are a group of rare, fatal neurodegenerative diseases, also known as transmissible spongiform encephalopathies (TSEs), that affect both animals and humans and include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep, chronic wasting disease (CWD) in deer and elk, and Creutzfeldt-Jakob disease (CJD) in humans. TSEs are usually rapidly progressive and clinical symptoms comprise dementia and loss of movement coordination due to the accumulation of an abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(c)). 2. This article reviews the current knowledge on PrP(c) and PrP(Sc), prion replication mechanisms, interaction partners of prions, and their cell surface receptors. Several strategies, summarized in this article, have been investigated for an effective antiprion treatment including development of a vaccination therapy and screening for potent chemical compounds. Currently, no effective treatment for prion diseases is available. 3. The identification of the 37 kDa/67 kDa laminin receptor (LRP/LR) and heparan sulfate as cell surface receptors for prions, however, opens new avenues for the development of alternative TSE therapies.