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

Abstract

Variant Creutzfeldt-Jakob disease (vCJD) is a human prion disease resulting from zoonotic transmission of bovine spongiform encephalopathy (BSE). Documented cases of vCJD transmission by blood transfusion necessitate on-going risk reduction measures to protect blood supplies, such as leucodepletion (removal of white blood cells, WBCs). This study set out to determine the risks of prion transmission by transfusion of labile blood components (red blood cells, platelets, plasma) commonly used in human medicine, and the effectiveness of leucodepletion in preventing infection, using BSE-infected sheep as a model. All components were capable of transmitting prion disease when donors were in the preclinical phase of infection, with the highest rates of infection in recipients of whole blood and buffy coat, and the lowest in recipients of plasma. Leucodepletion of components (<106 WBCs/unit) resulted in significantly lower transmission rates, but did not completely prevent transmission by any component. Donor PRNP genotype at codon 141, which is associated with variation in incubation period, also had a significant effect on transfusion transmission rates. A sensitive protein misfolding cyclic amplification (PMCA) assay, applied to longitudinal series of blood samples, identified infected sheep from 4 months post infection. However, in donor sheep (orally infected), the onset of detection of PrPSc in blood was much more variable, and generally later, compared to recipients (intravenous infection). This shows that the route and method of infection may profoundly affect the period during which an individual is infectious, and the test sensitivity required for reliable preclinical diagnosis, both of which have important implications for disease control. Our results emphasize that blood transfusion can be a highly efficient route of transmission for prion diseases. Given current uncertainties over the prevalence of asymptomatic vCJD carriers, this argues for the maintenance and improvement of current measures to reduce the risk of transmission by blood products.

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

“…PMCA analysis of serial tenfold dilutions of samples that tested positive showed that the limits of detection were reached at dilutions of 10 –1 –10 –4 (Table 2 ). In comparison, with tissues from pathology positive sheep, the limits of detection were reached at dilutions of 10 –9 –10 –10 for brain 15 and 10 –6 –10 –9 for PSLN (Fig. 1 ; Table S1 ).…”

“…Previous data suggested that the PRNP codon 141 polymorphism (L141F), which is associated with statistically significant differences in incubation period in BSE-infected sheep 17 , might also influence the extent of PrP Sc deposition in lymphoid tissues 15 . On average, 141LL sheep had the shortest incubation periods and the most widespread deposition of PrP Sc in lymphoid tissue, whilst 141LF sheep had the longest survival periods and more restricted distribution of PrP Sc in lymphoid tissues.…”

“…Group 3: P-CAPT experiment: recipients transfused with leucodepleted (LR) or leucodepleted and P-CAPT filtered (LR-PCAPT) red cell concentrate (RCC). Details of the experimental design and results for all groups can be found in previous publications 15 , 16 . …”

“…Based on our previously published data 15 , we reasoned that since blood samples from pathology negative subclinical recipient sheep tested negative by PMCA, they were unlikely to transmit BSE by blood transfusion. In order to test this, we carried out PMCA analysis on PSLN and brain samples from ten pathology negative secondary recipients.…”

“…In these experiments, blood collected from donor sheep orally infected with BSE was separated into different components (red cell concentrate, plasma and platelets), with each component being transfused into an individual recipient sheep. Whilst some infected donors transmitted infection to some or all of their respective recipients, a large number of recipient sheep did not develop clinical disease 15 , 16 during several years of follow-up, and showed no evidence of PrP Sc in brain or lymphoid tissues using conventional post mortem tests (immunohistochemistry, Western blot). We hypothesized that since these sheep were most likely exposed to a low dose of BSE via transfusion, a proportion of them might be subclinically infected, with PrP Sc levels below the threshold of detection for the conventional tests.…”