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

Immunization with Genetically Modified Trypanosomes Provides Protection against Transmissible Spongiform Encephalopathies

Transmissible spongiform encephalopathies are incurable neurodegenerative diseases, associated with the conversion of the physiological prion protein to its disease-associated counterpart. Even though immunization against transmissible spongiform encephalopathies has shown great potential, immune tolerance effects impede the use of active immunization protocols for successful prophylaxis. In this study, we evaluate the use of trypanosomes as biological platforms for the presentation of a prion antigenic peptide to the host immune system. Using the engineered trypanosomes in an immunization protocol without the use of adjuvants led to the development of a humoral immune response against the prion protein in wild type mice, without the appearance of adverse reactions. The immune reaction elicited with this protocol displayed in vitro therapeutic potential and was further evaluated in a bioassay where immunized mice were partially protected in a representative murine model of prion diseases. Further studies are underway to better characterize the immune reaction and optimize the immunization protocol.

Immunotherapy against Prion Disease

The term "prion disease" encompasses a group of neurodegenerative diseases affecting both humans and animals. Currently, there is no effective therapy and all forms of prion disease are invariably fatal. Because of (a) the outbreak of bovine spongiform encephalopathy in cattle and variant Creutzfeldt-Jakob disease in humans; (b) the heated debate about the prion hypothesis; and (c) the availability of a natural prion disease in rodents, the understanding of the pathogenic process in prion disease is much more advanced compared to that of other neurodegenerative disorders, which inspired many attempts to develop therapeutic strategies against these fatal diseases. In this review, we focus on immunotherapy against prion disease. We explain our rationale for immunotherapy as a plausible therapeutic choice, review previous trials using either active or passive immunization, and discuss potential strategies for overcoming the hurdles in developing a successful immunotherapy. We propose that immunotherapy is a plausible and practical therapeutic strategy and advocate more studies in this area to develop effective measures to control and treat these devastating disorders.

Vaccination strategies

Currently all prion diseases are without effective treatment and are universally fatal. It is increasingly being recognized that the pathogenesis of many neurodegenerative diseases, such as Alzheimer disease (AD), includes "prion-like" properties. Hence, any effective therapeutic intervention for prion disease could have significant implications for other neurodegenerative diseases. Conversely, therapies that are effective in AD might also be therapeutically beneficial for prion disease. AD-like prion disease has no effective therapy. However, various vaccine and immunomodulatory approaches have shown great success in animal models of AD, with numerous ongoing clinical trials of these potential immunotherapies. More limited evidence suggests that immunotherapies may be effective in prion models and in naturally occurring prion disease. In particular, experimental data suggest that mucosal vaccination against prions can be effective for protection against orally acquired prion infection. Many prion diseases, including natural sheep scrapie, bovine spongiform encephalopathy, chronic wasting disease, and variant Creutzfeldt-Jakob disease, are thought to be acquired peripherally, mainly by oral exposure. Mucosal vaccination would be most applicable to this form of transmission. In this chapter we review various immunologically based strategies which are under development for prion infection.

The Good, the Bad, and the Ugly of Dendritic Cells during Prion Disease

Prions are a unique group of proteinaceous pathogens which cause neurodegenerative disease and can be transmitted by a variety of exposure routes. After peripheral exposure, the accumulation and replication of prions within secondary lymphoid organs are obligatory for their efficient spread from the periphery to the brain where they ultimately cause neurodegeneration and death. Mononuclear phagocytes (MNP) are a heterogeneous population of dendritic cells (DC) and macrophages. These cells are abundant throughout the body and display a diverse range of roles based on their anatomical locations. For example, some MNP are strategically situated to provide a first line of defence against pathogens by phagocytosing and destroying them. Conventional DC are potent antigen presenting cells and migrate via the lymphatics to the draining lymphoid tissue where they present the antigens to lymphocytes. The diverse roles of MNP are also reflected in various ways in which they interact with prions and in doing so impact on disease pathogenesis. Indeed, some studies suggest that prions exploit conventional DC to infect the host. Here we review our current understanding of the influence of MNP in the pathogenesis of the acquired prion diseases with particular emphasis on the role of conventional DC.

The immunobiology of prion diseases

Individuals infected with prions succumb to brain damage, and prion infections continue to be inexorably lethal. However, many crucial steps in prion pathogenesis occur in lymphatic organs and precede invasion of the central nervous system. In the past two decades, a great deal has been learnt concerning the cellular and molecular mechanisms of prion lymphoinvasion. These properties are diagnostically useful and have, for example, facilitated preclinical diagnosis of variant Creutzfeldt-Jakob disease in the tonsils. Moreover, the early colonization of lymphoid organs can be exploited for post-exposure prophylaxis of prion infections. As stromal cells of lymphoid organs are crucial for peripheral prion infection, the dedifferentiation of these cells offers a powerful means of hindering prion spread in infected individuals. In this Review, we discuss the current knowledge of the immunobiology of prions with an emphasis on how basic discoveries might enable translational strategies.

Brain dendritic cells: biology and pathology

Dendritic cells (DC) are the professional antigen-presenting cells of the immune system. In their quiescent and mature form, the presentation of self-antigens by DC leads to tolerance; whereas, antigen presentation by mature DC, after stimulation by pathogen-associated molecular patterns, leads to the onset of antigen-specific immunity. DC have been found in many of the major organs in mammals (e.g. skin, heart, lungs, intestines and spleen); while the brain has long been considered devoid of DC in the absence of neuroinflammation. Consequently, microglia, the resident immune cell of the brain, have been charged with many functional attributes commonly ascribed to DC. Recent evidence has challenged the notion that DC are either absent or minimal players in brain immune surveillance. This review will discuss the recent literature examining DC involvement within both the young and aged steady-state brain. We will also examine DC contributions during various forms of neuroinflammation resulting from neurodegenerative autoimmune disease, injury, and CNS infections. This review also touches upon DC trafficking between the central nervous system and peripheral immune compartments during viral infections, the new molecular technologies that could be employed to enhance our current understanding of brain DC ontogeny, and some potential therapeutic uses of DC within the CNS.

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.

Detection and control of prion diseases in food animals

Transmissible spongiform encephalopathies (TSEs), or prion diseases, represent a unique form of infectious disease based on misfolding of a self-protein (PrP^C) into a pathological, infectious conformation (PrP^Sc). Prion diseases of food animals gained notoriety during the bovine spongiform encephalopathy (BSE) outbreak of the 1980s. In particular, disease transmission to humans, to the generation of a fatal, untreatable disease, elevated the perspective on livestock prion diseases from food production to food safety. While the immediate threat posed by BSE has been successfully addressed through surveillance and improved management practices, another prion disease is rapidly spreading. Chronic wasting disease (CWD), a prion disease of cervids, has been confirmed in wild and captive populations with devastating impact on the farmed cervid industries. Furthermore, the unabated spread of this disease through wild populations threatens a natural resource that is a source of considerable economic benefit and national pride. In a worst-case scenario, CWD may represent a zoonotic threat either through direct transmission via consumption of infected cervids or through a secondary food animal, such as cattle. This has energized efforts to understand prion diseases as well as to develop tools for disease detection, prevention, and management. Progress in each of these areas is discussed.

Immunotherapy for prion diseases: opportunities and obstacles

Transmissible spongiform encephalopathies (TSEs) represent a unique form of infectious disease based on the misfolding of a self-protein into a pathological conformation. While other human diseases are also attributed to protein misfolding, the TSEs are unique in their zoonotic potential and iatrogenic infectivity. These characteristics are of particular importance in the aftermath of the UK bovine spongiform encephalopathy (BSE) outbreak due to the dual concerns that a subpopulation of individuals exposed to the infectious agent may be serving as asymptomatic carriers, and that TSEs of other food animals may also threaten human health. These potentials, in addition to the ongoing baseline of familial and sporadic human prion diseases, necessitate development of effective treatment options. While TSEs represent a novel paradigm of infection, there is nevertheless the opportunity to apply traditional approaches of medicine for disease treatment and prevention, including vaccines for immunotherapy and immunoprophylaxis. However, vaccine development for TSEs is complicated by the challenges and potential dangers associated with induction of immune responses to a self-epitope, as well as the obstacles to treatment of a chronic infection through immunotherapy. The ongoing threat of TSEs to human health, together with the opportunity to apply information emerging from these investigations to other protein misfolding disorders, justifies the efforts required to overcome these obstacles.

Cell-based immunotherapy of prion diseases by adoptive transfer of antigen-loaded dendritic cells or antigen-primed CD(4+) T lymphocytes

Prion diseases are neurodegenerative conditions caused by the transconformation of a normal host glycoprotein, the cellular prion protein (PrPc) into a neurotoxic, self-aggregating conformer (PrPSc). TSEs are ineluctably fatal and no treatment is yet available. In principle, prion diseases could be attacked from different angles including: blocking conversion of PrPc into PrPSc, accelerating the clearance of amyloid deposits in peripheral tissues and brain, stopping prion progression in secondary lymphoid organs, reducing brain inflammation and promoting neuronal healing. There are many indications that adaptive and innate immunity might mediate those effects but so far, the achievements of immunointervention have not matched all expectations. Difficulties arise from the impossibility to diagnose TSE before substantial brain damage, poor accessibility of the CNS to immunological agents, deep immune tolerance to self-PrP and short term effects of many immune interventions contrasting with the slow progression of TSEs. Here, we discuss two approaches, inspired from cancer immunotherapy, which might overcome some of those obstacles. One is vaccination with antigen-pulsed or antigen-transduced dendritic cells to bypass self-tolerance. The other one is the adoptive transfer of PrP-sensitized CD4(+) T cells which can promote humoral, cell-mediated or regulatory responses, coordinate adaptive and innate immunity and have long lasting effects.

Mouse vaccination with dendritic cells loaded with prion protein peptides overcomes tolerance and delays scrapie

Prion diseases are presumed to be caused by the accumulation in the brain of a pathological protein called prion protein (PrP) scrapie which results from the transconformation of cellular PrP, a ubiquitous glycoprotein expressed in all mammals. Since all isoforms of PrP are perceived as self by the host immune system, a major problem in designing efficient immunoprophylaxis or immunotherapy is to overcome tolerance. The present study was aimed at investigating whether bone-marrow-derived dendritic cells (DCs) loaded with peptides previously shown to be immunogenic in PrP-deficient mice, can overcome tolerance in PrP-proficient wild-type mice and protect them against scrapie. Results show that, in such mice, peptide-loaded DCs elicit both lymphokine release by T cells and antibody secretion against native cellular PrP. Repeated recalls with peptide-loaded DCs reduces the attack rate of 139A scrapie inoculated intraperitoneally and retards disease duration by 40 days. Most interestingly, survival time in individual mice appears to be correlated with the level of circulating antibody against native cellular PrP.