Complete Freund's adjuvant immunization prolongs survival in experimental prion disease in mice

Inflammasomes as Targets for Adjuvants

Inflammasomes are an essential part of the innate immune system. They are necessary for the development of a healthy immune response against infectious diseases. Inflammasome activation leads to the secretion of pro-inflammatory cytokines such as IL-1β and IL-18, which stimulate the adaptive immune system. Inflammasomes activators can be used as adjuvants to provide and maintain the strength of the immune response. This review is focused on the mechanisms of action and the effects of adjuvants on inflammasomes. The therapeutic and prophylaxis significance of inflammasomes in infectious diseases is also discussed.

Type I interferon protects neurons from prions in in vivo models

Infectious prions comprising abnormal prion protein, which is produced by structural conversion of normal prion protein, are responsible for transmissible spongiform encephalopathies including Creutzfeldt-Jakob disease in humans. Prions are infectious agents that do not possess a genome and the pathogenic protein was not thought to evoke any immune response. Although we previously reported that interferon regulatory factor 3 (IRF3) was likely to be involved in the pathogenesis of prion diseases, suggesting the protective role of host innate immune responses mediated by IRF3 signalling, this remained to be clarified. Here, we investigated the reciprocal interactions of type I interferon evoked by IRF3 activation and prion infection and found that infecting prions cause the suppression of endogenous interferon expression. Conversely, treatment with recombinant interferons in an ex vivo model was able to inhibit prion infection. In addition, cells and mice deficient in type I interferon receptor (subunit interferon alpha/beta receptor 1), exhibited higher susceptibility to 22L-prion infection. Moreover, in in vivo and ex vivo prion-infected models, treatment with RO8191, a selective type I interferon receptor agonist, inhibited prion invasion and prolonged the survival period of infected mice. Taken together, these data indicated that the interferon signalling interferes with prion propagation and some interferon-stimulated genes might play protective roles in the brain. These findings may allow for the development of new strategies to combat fatal diseases.

[Protective Role of the Host Innate Immune System in Prion Pathogenesis]

Prion diseases, including human Creutzfeldt-Jakob disease, are infectious, intractable central neurodegenerative diseases, which are also zoonoses that commonly infect not only higher organisms but also a wide variety of animals. Pathogenic prions induce abnormal prion protein (PrP), which is produced by structural conversion of normal PrP, a beta-sheet-rich structure with high aggregation propensity. Thus, it is believed that the host is immunotolerant against prions because there is no difference in the primary structure of normal and abnormal PrP, and prions do not induce a marked immune response. Recently, using mutated Toll-like receptor (TLR) 4-transgenic mice, a bioassay after prion inoculation has intriguingly found that the TLR4-signaling pathway may have a protective role against prion infection. Meanwhile, we reported that a transcription factor, interferon regulatory factor-3 (IRF-3), located downstream of TLR4 signaling, showed resistance to prions. IRF-3-inducing type I interferon (I-IFN) is a critical factor for the host defense against pathogen invasion. These findings indicate that the TLR-signaling pathway of the innate immune system might regulate prion invasion. However, the details have not been fully determined. In this symposium, we will introduce new findings including the relationship between I-IFN and prions.

Toll-like receptor 2 confers partial neuroprotection during prion disease

Neuroinflammation and neurodegeneration are common during prion infection, but the mechanisms that underlie these pathological features are not well understood. Several components of innate immunity, such as Toll-like receptor (TLR) 4 and Complement C1q, have been shown to influence prion disease. To identify additional components of innate immunity that might impact prion disease within the central nervous system (CNS), we screened RNA from brains of pre-clinical and clinical 22L-infected mice for alterations in genes associated with innate immunity. Transcription of several genes encoding damage-associated molecular pattern (DAMP) proteins and receptors were increased in the brains of prion-infected mice. To investigate the role of some of these proteins in prion disease of the CNS, we infected mice deficient in DAMP receptor genes Tlr2, C3ar1, and C5ar1 with 22L scrapie. Elimination of TLR2 accelerated disease by a median of 10 days, while lack of C3aR1 or C5aR1 had no effect on disease tempo. Histopathologically, all knockout mouse strains tested were similar to infected control mice in gliosis, vacuolation, and PrPSc deposition. Analysis of proinflammatory markers in the brains of infected knockout mice indicated only a few alterations in gene expression suggesting that C5aR1 and TLR2 signaling did not act synergistically in the brains of prion-infected mice. These results indicate that signaling through TLR2 confers partial neuroprotection during prion infection.

Toward Therapy of Human Prion Diseases

Three decades after the discovery of prions as the cause of Creutzfeldt-Jakob disease and other transmissible spongiform encephalopathies, we are still nowhere close to finding an effective therapy. Numerous pharmacological interventions have attempted to target various stages of disease progression, yet none has significantly ameliorated the course of disease. We still lack a mechanistic understanding of how the prions damage the brain, and this situation results in a dearth of validated pharmacological targets. In this review, we discuss the attempts to interfere with the replication of prions and to enhance their clearance. We also trace some of the possibilities to identify novel targets that may arise with increasing insights into prion biology.

Transgenic over-expression of mammalian heparanase delays prion disease onset and progression

Cellular heparan sulfate (HS) has a dual role in scrapie pathogenesis; it is required for PrP(Sc) (scrapie prion protein) formation and facilitates infection of cells, mediating cellular uptake of prions. We examined the involvement of heparanase, a mammalian endoglycosidase degrading HS, in scrapie infection. In cultured cells, heparanase treatment or over-expression resulted in a profound decrease in PrP(Sc). Moreover, disease onset and progression were dramatically delayed in scrapie infected transgenic mice over-expressing heparanase. Together, our results provide direct in vivo evidence for the involvement of intact HS in the pathogenesis of prion disease and the protective role of heparanase both in terms of susceptibility to infection and disease progression.

Immunotherapeutic approaches in prion disease: progress, challenges and potential directions

Therapeutic trials utilizing animal models of prion disease have explored a variety of compounds and a number of approaches with varying success, including several immunotherapeutic strategies, such as passive immunization through the delivery of viruses carrying nucleic acid inserts encoding prion protein-specific immunoglobulin. Targeted, organ-specific cellular production of therapeutic proteins is a relatively unexplored approach in the treatment of neurodegeneration despite many successful experimental outcomes in animal models and human trials of other diseases of the CNS. Emphasizing studies utilizing mouse models of disease, this review outlines developments and limitations of immunological approaches to the treatment of prion diseases. In addition, the authors discuss the potential of an experimental therapeutic strategy, utilizing hybridoma cells injected directly into the CNS to establish long-term production of anti-prion antibodies in vivo within the organ associated with the greatest pathogenic change in prion disease, the brain.

Protective role of MyD88-independent innate immune responses against prion infection

Despite recent progress in the understanding of prion diseases, little is known about the host-defense mechanisms against prion. Although it has long been thought that type I interferon (IFN-I) has no protective effect on prion infection, certain key molecules in innate immunity such as toll-like receptor (TLR) 4 seemed to be involved in the host response. For this reason we decided to focus on TLRs and investigate the role of a transcription factor, interferon regulatory factor 3 (IRF3), because the absence of MyD88, a major adaptor signaling molecule of TLRs, has no effect on the survival of prion infected mice. Intriguingly, survival periods of prion inoculated IRF3-knockout mice became significantly shorter than those of wild-type mice. In addition, IRF3 stimulation inhibited PrP^Sc replication in prion persistently-infected cells, and a de novo prion infection assay revealed that IRF3-overexpression could make host cells resistant to prion infection. Our work suggests that IRF3 may play a key role in innate immune responses against invasion of prion pathogens. Activated IRF3 could upregulate several anti-pathogen factors, including IFN-I, and induce sequential responses. Although the mechanism for the anti-prion effects mediated by IRF3 has yet to be clarified, certain interferon responsive genes might be involved in the anti-prion host-defense mechanism.

NLRP3 inflammasome activation in macrophage cell lines by prion protein fibrils as the source of IL-1β and neuronal toxicity

Prion diseases are fatal transmissible neurodegenerative diseases, characterized by aggregation of the pathological form of prion protein, spongiform degeneration, and neuronal loss, and activation of astrocytes and microglia. Microglia can clear prion plaques, but on the other hand cause neuronal death via release of neurotoxic species. Elevated expression of the proinflammatory cytokine IL-1β has been observed in brains affected by several prion diseases, and IL-1R-deficiency significantly prolonged the onset of the neurodegeneration in mice. We show that microglial cells stimulated by prion protein (PrP) fibrils induced neuronal toxicity. Microglia and macrophages release IL-1β upon stimulation by PrP fibrils, which depends on the NLRP3 inflammasome. Activation of NLRP3 inflammasome by PrP fibrils requires depletion of intracellular K(+), and requires phagocytosis of PrP fibrils and consecutive lysosome destabilization. Among the well-defined molecular forms of PrP, the strongest NLRP3 activation was observed by fibrils, followed by aggregates, while neither native monomeric nor oligomeric PrP were able to activate the NLRP3 inflammasome. Our results together with previous studies on IL-1R-deficient mice suggest the IL-1 signaling pathway as the perspective target for the therapy of prion disease.

Prolongation of prion disease-associated symptomatic phase relates to CD3+ T cell recruitment into the CNS in murine scrapie-infected mice

Prion diseases are caused by the transconformation of the host cellular prion protein PrP(c) into an infectious neurotoxic isoform called PrP(Sc). While vaccine-induced PrP-specific CD4(+) T cells and antibodies partially protect scrapie-infected mice from disease, the potential autoreactivity of CD8(+) cytotoxic T lymphocytes (CTLs) received little attention. Beneficial or pathogenic influence of PrP(c)-specific CTL was evaluated by stimulating a CD8(+) T-cell-only response against PrP in scrapie-infected C57BL/6 mice. To circumvent immune tolerance to PrP, five PrP-derived nonamer peptides identified using prediction algorithms were anchored-optimized to improve binding affinity for H-2D(b) and immunogenicity (NP-peptides). All of the NP-peptides elicited a significant number of IFNγ secreting CD8(+) T cells that better recognized the NP-peptides than the natives; three of them induced T cells that were lytic in vivo for NP-peptide-loaded target cells. Peptides 168 and 192 were naturally processed and presented by the 1C11 neuronal cell line. Minigenes encoding immunogenic NP-peptides inserted into adenovirus (rAds) vectors enhanced the specific CD8(+) T-cell responses. Immunization with rAd encoding 168NP before scrapie inoculation significantly prolonged the survival of infected mice. This effect was attributable to a significant lengthening of the symptomatic phase and was associated with enhanced CD3(+) T cell recruitment to the CNS. However, immunization with Ad168NP in scrapie-incubating mice induced IFNγ-secreting CD8(+) T cells that were not cytolytic in vivo and did not influence disease progression nor infiltrated the brain. In conclusion, the data suggest that vaccine-induced PrP-specific CD8(+) T cells interact with prions into the CNS during the clinical phase of the disease.

Protective role of interferon regulatory factor 3-mediated signaling against prion infection

Abnormal prion protein (PrP(Sc)) generated from the cellular isoform of PrP (PrP(C)) is assumed to be the main or sole component of the pathogen, called prion, of transmissible spongiform encephalopathies (TSE). Because PrP is a host-encoded protein, acquired immune responses are not induced in TSE. Meanwhile, activation of the innate immune system has been suggested to partially block the progression of TSE; however, the mechanism is not well understood. To further elucidate the role of the innate immune system in prion infection, we investigated the function of interferon regulatory factor 3 (IRF3), a key transcription factor of the MyD88-independent type I interferon (IFN) production pathway. We found that IRF3-deficient mice exhibited significantly earlier onset with three murine TSE strains, namely, 22L, FK-1, and murine bovine spongiform encephalopathy (mBSE), following intraperitoneal transmission, than with wild-type controls. Moreover, overexpression of IRF3 attenuated prion infection in the cell culture system, while PrP(Sc) was increased in prion-infected cells treated with small interfering RNAs (siRNAs) against IRF3, suggesting that IRF3 negatively regulates PrP(Sc) formation. Our findings provide new insight into the role of the host innate immune system in the pathogenesis of prion diseases.

Anti-PrP antibodies detected at terminal stage of prion-affected mouse

The causative agent of prion diseases is the pathological isoform (PrPSc) of the host-encoded cellular prion protein (PrPC). PrPSc has an identical amino acid sequence to PrPC; thus, it has been assumed that an immune response against PrPSc could not be found in prion-affected animals. In this study, we found the anti-prion protein (PrP) antibody at the terminal stage of mouse scrapie. Several sera from mice in the terminal stage of scrapie reacted to the recombinant mouse PrP (rMPrP) molecules and brain homogenates of mouse prion diseases. These results indicate that mouse could recognize PrPC or PrPSc as antigens by the host immune system. Furthermore, immunization with rMPrP generates high titers of anti-PrP antibodies in wild-type mice. Some anti-PrP antibodies immunized with rMPrP prevent PrPSc replication in vitro. The mouse sera from terminal prion disease have several wide epitopes, although mouse sera immunized with rMPrP possess narrow epitopes.

Repetitive immunization enhances the susceptibility of mice to peripherally administered prions

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

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.

Dendritic cell-mediated-immunization with xenogenic PrP and adenoviral vectors breaks tolerance and prolongs mice survival against experimental scrapie

In prion diseases, PrP^c, a widely expressed protein, is transformed into a pathogenic form called PrP^Sc, which is in itself infectious. Antibodies directed against PrP^c have been shown to inhibit PrP^c to PrP^Sc conversion in vitro and protect in vivo from disease. Other effectors with potential to eliminate PrPSc-producing cells are cytotoxic T cells directed against PrP-derived peptides but their ability to protect or to induce deleterious autoimmune reactions is not known. The natural tolerance to PrP^c makes difficult to raise efficient adaptive responses. To break tolerance, adenovirus (Ad) encoding human PrP (hPrP) or control Ad were administered to wild-type mice by direct injection or by transfer of Ad-transduced dendritic cells (DCs). Control Ad-transduced DCs from Tg650 mice overexpressing hPrP were also used for immunization. DC-mediated but not direct administration of AdhPrP elicited antibodies that bound to murine native PrP^c. Frequencies of PrP-specific IFNγ-secreting T cells were low and in vivo lytic activity only targeted cells strongly expressing hPrP. Immunohistochemical analysis revealed that CD3⁺ T cell infiltration was similar in the brain of vaccinated and unvaccinated 139A-infected mice suggesting the absence of autoimmune reactions. Early splenic PrP^Sc replication was strongly inhibited ten weeks post infection and mean survival time prolonged from 209 days in untreated 139A-infected mice to 246 days in mice vaccinated with DCs expressing the hPrP. The efficacy appeared to be associated with antibody but not with cytotoxic cell-mediated PrP-specific responses.

Accelerated prion disease pathogenesis in Toll-like receptor 4 signaling-mutant mice

Prion diseases such as scrapie involve the accumulation of disease-specific prion protein, PrP(Sc), in the brain. Toll-like receptors (TLRs) are a family of proteins that recognize microbial constituents and are central players in host innate immune responses. The TLR9 agonist unmethylated CpG DNA was shown to prolong the scrapie incubation period in mice, suggesting that innate immune activation interferes with prion disease progression. Thus, it was predicted that ablation of TLR signaling would result in accelerated pathogenesis. C3H/HeJ (Tlr4(Lps-d)) mice, which possess a mutation in the TLR4 intracellular domain preventing TLR4 signaling, and strain-matched wild-type control (C3H/HeOuJ) mice were infected intracerebrally or intraperitoneally with various doses of scrapie inoculum. Incubation periods were significantly shortened in C3H/HeJ compared with C3H/HeOuJ mice, regardless of the route of infection or dose administered. At the clinical phase of disease, brain PrP(Sc) levels in the two strains of mice showed no significant differences by Western blotting. In addition, compared with macrophages from C3H/HeOuJ mice, those from C3H/HeJ mice were unresponsive to fibrillogenic PrP peptides (PrP residues 106 to 126 [PrP(106-126)] and PrP(118-135)) and the TLR4 agonist lipopolysaccharide but not to the TLR2 agonist zymosan, as measured by cytokine production. These data confirm that innate immune activation via TLR signaling interferes with scrapie infection. Furthermore, the results also suggest that the scrapie pathogen, or a component(s) thereof, is capable of stimulating an innate immune response that is active in the central nervous system, since C3H/HeJ mice, which lack the response, exhibit shortened incubation periods following both intraperitoneal and intracerebral infections.

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.

Anti-prion activity generated by a novel vaccine formulation

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE) of domestic and wild cervids in North America. To address possible prevention regimens for CWD, we have used a mouse model system and the Rocky Mountain Laboratory (RML) mouse-adapted scrapie prion strain to screen efficacy of potential vaccine candidates. Three peptides derived from the primary amino acid sequence of the prion protein were conjugated to blue carrier protein (BCP) and formulated in an adjuvant containing M. avium subsp. avium. CL57/BL6 mice were vaccinated and boosted with 50 microg of the carrier protein-peptide conjugate formulation; all vaccines produced a humoral immune response as measured by ELISA. Disease challenge with the RML scrapie prion strain revealed anti-prion activity was generated by the vaccine formulations as measured by a delay in clinical disease onset and prolonged survivorship.

Insights into prion strains and neurotoxicity

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases that are caused by prions and affect humans and many animal species. It is now widely accepted that the infectious agent that causes TSEs is PrP(Sc), an aggregated moiety of the host-derived membrane glycolipoprotein PrP(C). Although PrP(C) is encoded by the host genome, prions themselves encipher many phenotypic TSE variants, known as prion strains. Prion strains are TSE isolates that, after inoculation into distinct hosts, cause disease with consistent characteristics, such as incubation period, distinct patterns of PrP(Sc) distribution and spongiosis and relative severity of the spongiform changes in the brain. The existence of such strains poses a fascinating challenge to prion research.

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.

Antibodies to a Nonconjugated Prion Protein Peptide 95-123 Interfere with PrP Sc Propagation in Prion-Infected Cells

1. Vaccination-induced anti-prion protein antibodies are presently regarded as a promising approach toward treatment of prion diseases. Here, we investigated the ability of five peptides corresponding to three different regions of the bovine prion protein (PrP) to elicit antibodies interfering with PrP(Sc) propagation in prion-infected cells.2. Rabbits were immunized with free nonconjugated peptides. Obtained immune sera were tested in enzyme-linked immunosorbent assay (ELISA) and immunoblot for their binding to recombinant PrP and cell-derived pathogenic isoform (PrP(Sc)) and normal prion protein (PrP(c)), respectively. Sera positive in all tests were chosen for PrP(Sc) inhibition studies in cell culture.3. All peptides induced anti-peptide antibodies, most of them reacting with recombinant PrP. Moreover, addition of the serum specific to peptide 95-123 led to a transient reduction of PrP(Sc) levels in persistently prion-infected cells.4. Thus, anti-PrP antibodies interfering with PrP(Sc) propagation were induced with a prion protein peptide nonconjugated to a protein carrier. These results point to the potential application of the nonconjugated peptide 95-123 for the treatment of prion diseases.

The Human Prion Protein Residue 129 Polymorphism Lies Within a Cluster of Epitopes for T Cell Recognition

T cell immune responses to central nervous system-derived and other self-antigens are commonly described in both healthy and autoimmune individuals. However, in the case of the human prion protein (PrP), it has been argued that immunologic tolerance is uncommonly robust. Although development of an effective vaccine for prion disease requires breaking of tolerance to PrP, the extent of immune tolerance to PrP and the identity of immunodominant regions of the protein have not previously been determined in humans. We analyzed PrP T cell epitopes both by using a predictive algorithm and by measuring functional immune responses from healthy donors. Interestingly, clusters of epitopes were focused around the area of the polymorphic residue 129, previously identified as an indicator of susceptibility to prion disease, and in the C-terminal region. Moreover, responses were seen to PrP peptide 121-134 containing methionine at position 129, whereas PrP 121-134 [129V] was not immunogenic. The residue 129 polymorphism was also associated with distinct patterns of cytokine response: PrP 128-141 [129M] inducing IL-4 and IL-6 production, which was not seen in response to PrP 128-141 [129V]. Our data suggest that the immunogenic regions of human PrP lie between residue 107 and the C-terminus and that, like with many other central nervous system antigens, healthy individuals carry responses to PrP within the T cell repertoire and yet do not experience deleterious autoimmune reactions.

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

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

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

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

Recent developments in prion immunotherapy

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

Progress in prion vaccines and immunotherapies

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

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.

Scrapie protein degradation by cysteine proteases in CD11c+ dendritic cells and GT1-1 neuronal cells

Dendritic cells (DC) of the CD11c(+) myeloid phenotype have been implicated in the spread of scrapie in the host. Previously, we have shown that CD11c(+) DC can cause a rapid degradation of proteinase K-resistant prion proteins (PrP(Sc)) in vitro, indicating a possible role of these cells in the clearance of PrP(Sc). To determine the mechanisms of PrP(Sc) degradation, CD11c(+) DC that had been exposed to PrP(Sc) derived from a neuronal cell line (GT1-1) infected with scrapie (ScGT1-1) were treated with a battery of protease inhibitors. Following treatment with the cysteine protease inhibitors (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane (E-64c), its ethyl ester (E-64d), and leupeptin, the degradation of PrP(Sc) was inhibited, while inhibitors of serine and aspartic and metalloproteases (aprotinin, pepstatin, and phosphoramidon) had no effect. An endogenous degradation of PrP(Sc) in ScGT1-1 cells was revealed by inhibiting the expression of cellular PrP (PrP(C)) by RNA interference, and this degradation could also be inhibited by the cysteine protease inhibitors. Our data show that PrP(Sc) is proteolytically cleaved preferentially by cysteine proteases in both CD11c(+) DC and ScGT1-1 cells and that the degradation of PrP(Sc) by proteases is different from that of PrP(C). Interference by protease inhibitors with DC-induced processing of PrP(Sc) has the potential to modify prion spread, clearance, and immunization in a host.

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.

Immunotherapy as a therapeutic treatment for neurodegenerative disorders

Human neurodegenerative illnesses such as Alzheimer's disease and Creutzfeldt-Jakob disease exact an enormous cost on individuals, families and society. For these and related disorders, current treatment is largely symptomatic without influencing the underlying disease process. Until recently, the development of immunotherapeutic approaches to neurodegenerative disorders had been almost completely ignored despite growing successes against other non-infectious diseases such as cancer. However, since Schenk and colleagues described the antibody-mediated clearance of amyloid plaques in a transgenic mouse model of Alzheimer's disease, a number of studies have confirmed the feasibility of this strategy for several neurodegenerative disorders including Huntington's disease and prion diseases. These reports offer the exciting prospect that either the immune system or its derivative components can be harnessed to fight the misfolded and/or aggregated proteins that accumulate in many neurodegenerative illnesses. If the remarkable power of clonal expansion, specificity and efficiency of the immune system can successfully inactivate these abnormal proteins, real hope exists that effective immunotherapeutic treatments for neurodegenerative illnesses may be available in the near future.

Prion Diseases: From Molecular Biology to Intervention Strategies

Prion diseases are fatal neurodegenerative infectious disorders for which no therapeutic or prophylactic regimens exist. Understanding the molecular process of conformational conversion of the cellular prion protein (PrP(c)) into its pathological isoform (PrP(Sc)) will be necessary to devise effective antiprion strategies. In recent years, new findings in the cell biology of PrP(c), in the molecular pathogenesis of PrP(Sc), and in the cellular quality control mechanisms involved in these scenarios have accumulated. A function of the prion protein in signalling, the possible impact of the proteasome, and aggresomes as intracellular waste deposits have been described. Here, important pathogenetic similarities with the more frequent neurodegenerative disorders are evident. The need for therapeutic, postexposure, and prophylactic possibilities was drastically illustrated by the emergence of variant Creutzfeldt-Jakob disease (vCJD), a new human prion disease caused by bovine spongiform encephalopathy (BSE) derived prions. Although prion infectivity in humans is usually restricted to the central nervous system, in vCJD patients prions are present in the lympho-reticular system, posing a theoretical risk of accidental human-to-human transmission. A variety of chemical antiprion substances have been reported in in vitro and cell culture based assays or in animal studies. Occasionally, they have also made their way into the first human trials. In addition, various promising interference strategies have been devised in transgenic models, although they are usually hard to transfer into nontransgenic in vivo situations. New findings in the fields of peripheral prion pathogenesis and immune system involvement fuelled the search for antiprion strategies formerly considered to be entirely impossible. This opened the door towards classical immunological interference techniques. Remarkably, passive and even active vaccination approaches now seem to be realistic goals.