Prionemia and Leukocyte-Platelet-Associated Infectivity in Sheep Transmissible Spongiform Encephalopathy Models

Abstract

The dynamics of the circulation and distribution of transmissible spongiform encephalopathy (TSE) agents in the blood of infected individuals remain largely unknown. This clearly limits the understanding of the role of blood in TSE pathogenesis and the development of a reliable TSE blood detection assay. Using two distinct sheep scrapie models and blood transfusion, this work demonstrates the occurrence of a very early and persistent prionemia. This ability to transmit disease by blood transfusion was correlated with the presence of infectivity in white blood cells (WBC) and peripheral blood mononucleated cells (PBMC) as detected by bioassay in mice overexpressing the ovine prion protein PrP (tg338 mice) and with the identification of abnormal PrP in WBC after using protein misfolding cyclic amplification (PMCA). Platelets and a large variety of leukocyte subpopulations also were shown to be infectious. The use of endpoint titration in tg338 mice indicated that the infectivity in WBC (per ml of blood) was 10 6.5 -fold lower than that in 1 g of posterior brainstem sample. In both WBC and brainstem, infectivity displayed similar resistance to PK digestion. The data strongly support the concept that WBC are an accurate target for reliable TSE detection by PMCA. The presence of infectivity in short-life-span blood cellular elements raises the question of the origin of prionemia.

Bioassay of prion-infected blood plasma in PrP transgenic Drosophila

“…Collectively, these studies revealed that prion infectivity titres of blood samples harvested during the asymptomatic phase of prion disease were up to 10-fold less than those collected during the clinical phase [10,19,20,22]. Infectious prions were found to be associated with white and red blood cells, platelets and plasma, although interspecies variations existed with regard to the distribution of prion infectivity in these different blood fractions [10,[15][16][17][18][19][20][21]. Studies in ruminants have analysed bioassays of autologous whole or fractionated blood by the intravenous route [19,20].…”

“…Infectious prions were found to be associated with white and red blood cells, platelets and plasma, although interspecies variations existed with regard to the distribution of prion infectivity in these different blood fractions [10,[15][16][17][18][19][20][21]. Studies in ruminants have analysed bioassays of autologous whole or fractionated blood by the intravenous route [19,20]. These experimental transfusion studies have confirmed that all blood fractions prepared by protocols similar to those used in transfusion medicine can transmit prion disease [23,24].…”

“…Plasma samples from scrapie-infected sheep: these were obtained from classical scrapie-positive VRQ/VRQ sheep. Plasma samples from sheep experimentally infected with classical scrapie were prepared from animals challenged by oral inoculation with PG127-infected sheep brain homogenate as previously described [19]. Plasma samples from sheep naturally infected with classical scrapie were prepared from animals born and maintained in a flock endemic with the disease [32] and that were confirmed positive for PrPSc through routine statutory surveillance (data not shown).…”

“…To test this hypothesis, we used ovine PrP transgenic Drosophila to bioassay blood samples from sheep experimentally infected with PG127 scrapie and that are known to contain prion infectivity [19]. We chose to bioassay sheep plasma as this blood fraction is reported to contain a low level of prion infectivity and is notoriously difficult to assess by conventional prion bioassay [19].…”

“…The bioassay of blood-borne prion infectivity has been performed for a variety of different species, such as rodents, sheep, cervids and primates including humans, undergoing, collectively, either experimental or natural prion disease [10,[15][16][17][18][19][20][21]. These bioassays have typically involved intracerebral inoculation of blood, or blood fractions, into a recipient indicator species, usually rodents including mice with a PrP transgene autologous for the donor species.…”