Preclinical transmission of prions by blood transfusion is influenced by donor genotype and route of infection

Expression of the cellular prion protein by mast cells in the human carotid body

Prion diseases are fatal neurologic disorders that can be transmitted by blood transfusion. The route for neuroinvasion following exposure to infected blood is not known. Carotid bodies (CBs) are specialized chemosensitive structures that detect the concentration of blood gasses and provide feedback for the neural control of respiration. Sensory cells of the CB are highly perfused and densely innervated by nerves that are synaptically connected to the brainstem and thoracic spinal cord, known to be areas of early prion deposition following oral infection. Given their direct exposure to blood and neural connections to central nervous system (CNS) areas involved in prion neuroinvasion, we sought to determine if there were cells in the human CB that express the cellular prion protein (PrPC), a characteristic that would support CBs serving as a route for prion neuroinvasion. We collected CBs from cadaver donor bodies and determined that mast cells located in the carotid bodies express PrPC and that these cells are in close proximity to blood vessels, nerves, and nerve terminals that are synaptically connected to the brainstem and spinal cord.

Risk of variant Creutzfeldt-Jakob disease in a simulated cohort of Canadian blood donors

BACKGROUND AND OBJECTIVES

No transfusion-associated cases of variant Creutzfeldt-Jakob disease (vCJD) have occurred in more than 20 years. Yet, many countries have maintained blood donor deferral criteria for vCJD. We developed a risk simulation model to reassess the need for vCJD-related deferral criteria in Canada.

MATERIALS AND METHODS

The model provides results separately for Héma-Québec (HQ) and Canadian Blood Services (CBS). The model used a Monte Carlo simulation approach to estimate the risk of having a vCJD-contaminated blood donation ('risk of vCJD') in a simulated cohort of 10 million donors followed for up to 85 years. The model assumed current deferral criteria for vCJD were lifted, which would allow new 'at-risk' donors to give blood. The model accounted for disease prevalence, donors' travel/immigration history, PRNP genotype at codon 129, demographics and the type of labile blood product.

RESULTS

In the base case, the risk of vCJD was estimated at zero at both blood services. In the most pessimistic scenario, the risk of vCJD was 6.4 × 10-9 (i.e., 1 in 157 million donations) at HQ, or ≤1 in 77 million based on the upper bound of the 95% confidence interval (CI). At CBS, this risk was 4.8 × 10-8 (i.e., 1 in 21 million donations), or ≤1 in 16 million based on the upper bound of the 95% CI.

CONCLUSION

vCJD poses minimal risks to the Canadian blood supply. Current vCJD deferral criteria may, therefore, be lifted with virtually no impact on safety, while significantly expanding the donor base.

Transmission of Variant Creutzfeldt-Jakob Disease Through Blood Transfusion and Plasma-Derived Products: A Narrative Review of Observed and Modeled Risks

Secondary transmission of variant Creutzfeldt-Jakob disease (vCJD) can occur through blood transfusion or receipt of plasma-derived products. However, published reviews on this topic are outdated, focused on a single country or product type, or did not comprehensively review modeling studies on the risk of transfusion-transmission. We reviewed existing data on observed and modeled risks of transfusion-transmission of vCJD. To date, five patients are suspected to have acquired clinical vCJD or a vCJD infection after receiving a blood or plasma-derived product from a donor who later developed clinical vCJD. All of these cases received a nonleukodepleted blood-derived product in the United Kingdom between 1994 and 1999. Thus, all transfusion-associated cases occurred before the adoption of universal leukodepletion in 1999, which supports the preferential tropism of vCJD for leukocytes. In descriptive cohort studies, no cases of clinical vCJD were observed over ∼13 years of follow-up. In modeling studies, the risk of collecting a contaminated donation was generally <23 per million donations, that of infection was generally <10 per million transfusions or doses, and that of clinical vCJD was generally <2 per million transfusions or doses. These low risk estimates and the two-decade long absence of new cases of transfusion-associated vCJD suggest vCJD poses minimal risks to the safety of the blood supply. Furthermore, despite concerns of a second wave driven by individuals harboring a non-MM genotype at codon 129 of PRNP, there has been only 1 autopsy-confirmed case of clinical vCJD in an MV individual in 2016. The current trend to reassess or (in some countries) fully withdraw the blood donation criteria related to vCJD therefore seems justified, safe, and may significantly expand the donor base.

Abnormal prion protein, infectivity and neurofilament light-chain in blood of macaques with experimental variant Creutzfeldt-Jakob disease

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative infections. Variant Creutzfeldt-Jakob disease (vCJD) and sporadic CJD (sCJD) are human TSEs that, in rare cases, have been transmitted by human-derived therapeutic products. There is a need for a blood test to detect infected donors, identify infected individuals in families with TSEs and monitor progression of disease in patients, especially during clinical trials. We prepared panels of blood from cynomolgus and rhesus macaques experimentally infected with vCJD, as a surrogate for human blood, to support assay development. We detected abnormal prion protein (PrP^TSE) in those blood samples using the protein misfolding cyclic amplification (PMCA) assay. PrP^TSE first appeared in the blood of pre-symptomatic cynomolgus macaques as early as 2 months post-inoculation (mpi). In contrast, PMCA detected PrP^TSE much later in the blood of two pre-symptomatic rhesus macaques, starting at 19 and 20 mpi, and in one rhesus macaque only when symptomatic, at 38 mpi. Once blood of either species of macaque became PMCA-positive, PrP^TSE persisted through terminal illness at relatively constant concentrations. Infectivity in buffy coat samples from terminally ill cynomolgus macaques as well as a sample collected 9 months before clinical onset of disease in one of the macaques was assayed in vCJD-susceptible transgenic mice. The infectivity titres varied from 2.7 to 4.3 infectious doses ml^-1. We also screened macaque blood using a four-member panel of biomarkers for neurodegenerative diseases to identify potential non-PrP^TSE pre-symptomatic diagnostic markers. Neurofilament light-chain protein (NfL) increased in blood before the onset of clinical vCJD. Cumulatively, these data confirmed that, while PrP^TSE is the first marker to appear in blood of vCJD-infected cynomolgus and rhesus macaques, NfL might offer a useful, though less specific, marker for forthcoming neurodegeneration. These studies support the use of macaque blood panels to investigate PrP^TSE and other biomarkers to predict onset of CJD in humans.

Subclinical infection occurs frequently following low dose exposure to prions by blood transfusion

Infectious prion diseases have very long incubation periods, and the role that subclinical infections play in transmission, persistence and re-emergence of these diseases is unclear. In this study, we used a well-established model of vCJD (sheep experimentally infected with bovine spongiform encephalopathy, BSE) to determine the prevalence of subclinical infection following exposure by blood transfusion from infected donors. Many recipient sheep survived for years post-transfusion with no clinical signs and no disease-associated PrP (PrP^Sc) found in post mortem tissue samples by conventional tests. Using a sensitive protein misfolding cyclic amplification assay (PMCA), we found that the majority of these sheep had detectable PrP^Sc in lymph node samples, at levels approximately 10⁵-10⁶ times lower than in equivalent samples from clinically positive sheep. Further testing revealed the presence of PrP^Sc in other tissues, including brain, but not in blood samples. The results demonstrate that subclinical infection is a frequent outcome of low dose prion infection by a clinically relevant route for humans (blood transfusion). The long term persistence of low levels of infection has important implications for prion disease control and the risks of re-emergent infections in both humans and animals.

Risk of variant Creutzfeldt-Jakob disease transmission by blood transfusion in Australia

Background and Objectives

Most of the 233 worldwide cases of variant Creutzfeldt–Jakob disease (vCJD) have been reported in the United Kingdom and 3 have been associated with transfusion‐transmission. To mitigate the potential vCJD risk to blood safety, Australian Red Cross Lifeblood imposes restrictions on blood donation from people with prior residency in, or extended travel to, the United Kingdom during the risk period 1980–1996. We have modified a previously published methodology to estimate the transfusion‐transmission risk of vCJD associated with fresh component transfusion in Australia if the UK residence deferral was removed.

Materials and Methods

The prevalence of current pre‐symptomatic vCJD infection in the United Kingdom by age at infection and genotype was estimated based on risk of exposure to the bovine spongiform encephalopathy agent for the period 1980–1996. These results were used to estimate the age‐specific prevalence of undiagnosed, pre‐symptomatic vCJD in the Australian population in the current year due to prior UK residency or travel. The primary model outputs were the 2020 vCJD risks/unit of vCJD contamination, transfusion‐transmission (infections) and clinical cases.

Results

The overall (prior UK residency in and travel to United Kingdom, 1980–1996) mean risk of contamination per unit was 1 in 29,900,000. The risks of resulting vCJD transmission (infection) and clinical case were 1 in 389,000,000 and 1 in 1,450,000,000, respectively.

Conclusion

Our modelling suggests that removing the Lifeblood donation deferral for travel to, or UK residence, would result in virtually no increased risk of vCJD transfusion‐transmission and would be a safe and effective strategy for increasing the donor base.

Prion disease modelled in Drosophila

Prion diseases are fatal neurodegenerative conditions of humans and various vertebrate species that are transmissible between individuals of the same or different species. A novel infectious moiety referred to as a prion is considered responsible for transmission of these conditions. Prion replication is believed to be the cause of the neurotoxicity that arises during prion disease pathogenesis. The prion hypothesis predicts that the transmissible prion agent consists of PrPSc, which is comprised of aggregated misfolded conformers of the normal host protein PrPC. It is important to understand the biology of transmissible prions and to identify genetic modifiers of prion-induced neurotoxicity. This information will underpin the development of therapeutic and control strategies for human and animal prion diseases. The most reliable method to detect prion infectivity is by in vivo transmission in a suitable experimental host, which to date have been mammalian species. Current prion bioassays are slow, cumbersome and relatively insensitive to low titres of prion infectivity, and do not lend themselves to rapid genetic analysis of prion disease. Here, we provide an overview of our novel studies that have led to the establishment of Drosophila melanogaster, a genetically well-defined invertebrate host, as a sensitive, versatile and economically viable animal model for the detection of mammalian prion infectivity and genetic modifiers of prion-induced toxicity.

Body-first Parkinson's disease and variant Creutzfeldt-Jakob disease - similar or different?

In several neurodegenerative disorders, proteins that typically exhibit an α-helical structure misfold into an amyloid conformation rich in β-sheet content. Through a self-templating mechanism, these amyloids are able to induce additional protein misfolding, facilitating their propagation throughout the central nervous system. This disease mechanism was originally identified for the prion protein (PrP), which misfolds into PrP^Sc in a number of disorders, including variant Creutzfeldt-Jakob disease (vCJD) and bovine spongiform encephalopathy (BSE). More recently, the prion mechanism of disease was expanded to include other proteins that rely on this self-templating mechanism to cause progressive degeneration, including α-synuclein misfolding in Parkinson's disease (PD). Several studies now suggest that PD patients can be subcategorized based on where in the body misfolded α-synuclein originates, either the brain or the gut, similar to patients developing sporadic CJD or vCJD. In this review, we discuss the human and animal model data indicating that α-synuclein and PrP^Sc misfolding originates in the gut in body-first PD and vCJD, and summarize the data identifying the role of the autonomic nervous system in the gut-brain axis of both diseases.