Highly Efficient Prion Transmission by Blood Transfusion

Abstract

It is now clearly established that the transfusion of blood from variant CJD (v-CJD) infected individuals can transmit the disease. Since the number of asymptomatic infected donors remains unresolved, inter-individual v-CJD transmission through blood and blood derived products is a major public health concern. Current risk assessments for transmission of v-CJD by blood and blood derived products by transfusion rely on infectious titers measured in rodent models of Transmissible Spongiform Encephalopathies (TSE) using intra-cerebral (IC) inoculation of blood components. To address the biological relevance of this approach, we compared the efficiency of TSE transmission by blood and blood components when administrated either through transfusion in sheep or by intra-cerebral inoculation (IC) in transgenic mice (tg338) over-expressing ovine PrP. Transfusion of 200 µL of blood from asymptomatic infected donor sheep transmitted prion disease with 100% efficiency thereby displaying greater virulence than the transfusion of 200 mL of normal blood spiked with brain homogenate material containing 10³ID₅₀ as measured by intracerebral inoculation of tg338 mice (ID₅₀ IC in tg338). This was consistent with a whole blood titer greater than 10^3.6 ID₅₀ IC in tg338 per mL. However, when the same blood samples were assayed by IC inoculation into tg338 the infectious titers were less than 32 ID per mL. Whereas the transfusion of crude plasma to sheep transmitted the disease with limited efficacy, White Blood Cells (WBC) displayed a similar ability to whole blood to infect recipients. Strikingly, fixation of WBC with paraformaldehyde did not affect the infectivity titer as measured in tg338 but dramatically impaired disease transmission by transfusion in sheep. These results demonstrate that TSE transmission by blood transfusion can be highly efficient and that this efficiency is more dependent on the viability of transfused cells than the level of infectivity measured by IC inoculation. In the UK, several v-CJD cases have been identified in patients that received blood or blood-derived products prepared from incubating asymptomatic donors. Since there is no screening test to identify infected donors, procedural risk reduction measures remain the only protection against v-CJD transfusion risk. These measures rely, in part, on the assumptions that (i) the level of infectivity in blood is low and (ii) the risk of blood borne transmission is directly correlated with the infectious titer of blood and blood products. Using a transmissible spongiform encephalopathy (TSE) animal model, we have provided evidence that despite a very low infectious titer in blood as measured by inoculation into brain, the transfusion of 0.2 mL of blood from asymptomatic infected donors is sufficient to transmit the disease with a 100% efficacy. We further demonstrated that this high efficiency of disease transmission is crucially dependant on the viability of the transfused white blood cells rather than on their infectious titer. These findings provide new insights into the pathogenesis of TSE diseases and require revision of some of the key assumptions of the v-CJD blood borne risk assessments.