Enhanced enteric invasion of scrapie agents into the villous columnar epithelium via maternal immunoglobulin

The Effects of Immune System Modulation on Prion Disease Susceptibility and Pathogenesis

Prion diseases are a unique group of infectious chronic neurodegenerative disorders to which there are no cures. Although prion infections do not stimulate adaptive immune responses in infected individuals, the actions of certain immune cell populations can have a significant impact on disease pathogenesis. After infection, the targeting of peripherally-acquired prions to specific immune cells in the secondary lymphoid organs (SLO), such as the lymph nodes and spleen, is essential for the efficient transmission of disease to the brain. Once the prions reach the brain, interactions with other immune cell populations can provide either host protection or accelerate the neurodegeneration. In this review, we provide a detailed account of how factors such as inflammation, ageing and pathogen co-infection can affect prion disease pathogenesis and susceptibility. For example, we discuss how changes to the abundance, function and activation status of specific immune cell populations can affect the transmission of prion diseases by peripheral routes. We also describe how the effects of systemic inflammation on certain glial cell subsets in the brains of infected individuals can accelerate the neurodegeneration. A detailed understanding of the factors that affect prion disease transmission and pathogenesis is essential for the development of novel intervention strategies.

Altered distribution, aggregation, and protease resistance of cellular prion protein following intracranial inoculation

Prion protein (PrP^C) is a protease-sensitive and soluble cell surface glycoprotein expressed in almost all mammalian cell types. PrP^Sc, a protease-resistant and insoluble form of PrP^C, is the causative agent of prion diseases, fatal and transmissible neurogenerative diseases of mammals. Prion infection is initiated via either ingestion or inoculation of PrP^Sc or when host PrP^C stochastically refolds into PrP^Sc. In either instance, the early events that occur during prion infection remain poorly understood. We have used transgenic mice expressing mouse PrP^C tagged with a unique antibody epitope to monitor the response of host PrP^C to prion inoculation. Following intracranial inoculation of either prion-infected or uninfected brain homogenate, we show that host PrP^C can accumulate both intra-axonally and within the myelin membrane of axons suggesting that it may play a role in axonal loss following brain injury. Moreover, in response to the inoculation host PrP^C exhibits an increased insolubility and protease resistance similar to that of PrP^Sc, even in the absence of infectious prions. Thus, our results raise the possibility that damage to the brain may be one trigger by which PrP^C stochastically refolds into pathogenic PrP^Sc leading to productive prion infection.

How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of abnormally folded isoforms of the cellular prion protein known as PrP^Sc. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. The first half of this review provides a thorough account of our understanding of the various ways in which PrP^Sc prions may spread between individuals within a population, both horizontally and vertically. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid organs as they make their journey from the site of infection to the brain, a process termed neuroinvasion. The replication of PrP^Sc prions within secondary lymphoid organs is important for their efficient spread to the brain. The second half of this review describes the key tissues, cells and molecules which are involved in the propagation of PrP^Sc prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how additional factors such as inflammation and aging might influence prion disease susceptibility.

Transmission of prions within the gut and towards the central nervous system

The prion protein is a glycoprotein characterized by a folded α-helical structure that, under pathological conditions, misfolds and aggregates into its infectious isoform as β-sheet rich amyloidic deposits. The accumulation of the abnormal protein is responsible for a group of progressive and fatal disorders characterized by vacuolation, gliosis, and spongiform degeneration. Prion disorders are characterized by a triple aetiology: familial, sporadic or acquired, although most cases are sporadic. The mechanisms underlying prion neurotoxicity remain controversial, while novel findings lead to hypothesize intriguing pathways responsible for prion spreading. The present review aims to examine the involvement of the gastrointestinal tract and hypothesizes the potential mechanisms underlying cell-to-cell transmission of the prion protein. In particular, a special emphasis is posed on the mechanisms of prion transmission within the gut and towards the central nervous system. The glycation of prion protein to form advanced glycation end-products (AGE) interacting with specific receptors placed on neighboring cells (RAGE) represents the key hypothesis to be discussed.

Blocking of FcR suppresses the intestinal invasion of scrapie agents

Prion diseases are a family of neurodegenerative zoonotic foodborne disorders. Although prions can be transmitted orally, the mechanism by which prions are incorporated into the intestine remains unclear. Our previous studies have shown that an abnormal isoform of prion protein (PrP^Sc), which is the main component of prions, was efficiently incorporated into the intestine in suckling mice but not in weaned mice. Furthermore, suckling SCID mice lacking maternal antibodies showed decreased uptake of PrP^Sc into the intestine compared with suckling wild-type mice, while the lack of PrP^Sc uptake into the intestine of suckling SCID mice was rescued by the oral administration of IgG. These findings raise the possibility that the neonatal Fc receptor (nFcR), which contributes to the uptake of maternal antibodies into the intestine, plays a role in PrP^Sc incorporation into the intestine. The present immunohistochemical study further showed that the FcR blocker Z-ε-aminocaproic acid (ZAA) inhibited PrP^Sc incorporation into the intestinal villi of suckling mice, supporting the above mentioned concept. Therefore, our findings provide strong evidence that nFcR and maternal antibodies are involved in PrP^Sc incorporation into the intestine before the weaning period.