Correlation between Infectivity and Disease Associated Prion Protein in the Nervous System and Selected Edible Tissues of Naturally Affected Scrapie Sheep

The transmissible spongiform encephalopathies (TSEs) or prion diseases are a group of fatal neurodegenerative disorders characterised by the accumulation of a pathological form of a host protein known as prion protein (PrP). The validation of abnormal PrP detection techniques is fundamental to allow the use of high-throughput laboratory based tests, avoiding the limitations of bioassays. We used scrapie, a prototype TSE, to examine the relationship between infectivity and laboratory based diagnostic tools. The data may help to optimise strategies to prevent exposure of humans to small ruminant TSE material via the food chain. Abnormal PrP distribution/accumulation was assessed by immunohistochemistry (IHC), Western blot (WB) and ELISA in samples from four animals. In addition, infectivity was detected using a sensitive bank vole bioassay with selected samples from two of the four sheep and protein misfolding cyclic amplification using bank vole brain as substrate (vPMCA) was also carried out in selected samples from one animal. Lymph nodes, oculomotor muscles, sciatic nerve and kidney were positive by IHC, WB and ELISA, although at levels 100–1000 fold lower than the brain, and contained detectable infectivity by bioassay. Tissues not infectious by bioassay were also negative by all laboratory tests including PMCA. Although discrepancies were observed in tissues with very low levels of abnormal PrP, there was an overall good correlation between IHC, WB, ELISA and bioassay results. Most importantly, there was a good correlation between the detection of abnormal PrP in tissues using laboratory tests and the levels of infectivity even when the titre was low. These findings provide useful information for risk modellers and represent a first step toward the validation of laboratory tests used to quantify prion infectivity, which would greatly aid TSE risk assessment policies.

传染性海绵状脑病（transmissible spongiform encephalopathies, TSEs），又称朊病毒病，是一类致死性神经退行性疾病，其病理特征为宿主蛋白的异常构型亚型——朊蛋白（prion protein, PrP）的蓄积。异常PrP检测技术的验证是高通量实验室检测方法得以应用的核心前提，可有效规避生物测定法的固有局限性。本研究以痒病——一种典型的TSE——为模型，探究感染性与实验室诊断工具间的关联，本数据集可为优化防控策略提供数据支撑，以阻断人类通过食物链接触小反刍动物TSE病原的风险。研究人员通过免疫组织化学（immunohistochemistry, IHC）、蛋白质免疫印迹（Western blot, WB）及酶联免疫吸附试验（ELISA），对4只绵羊的样本中异常PrP的分布与蓄积情况进行了评估。此外，研究人员选取4只绵羊中的2只的部分样本，采用敏感性较高的田鼠生物测定法检测感染性；同时选取其中1只动物的部分样本，开展以田鼠脑组织为底物的蛋白质错误折叠循环扩增（vPMCA）实验。淋巴结、动眼肌、坐骨神经与肾脏经IHC、WB及ELISA检测均呈阳性，尽管其异常PrP水平较脑组织低100~1000倍，且生物测定法可检出其中的感染性病原。经生物测定法证实无感染性的组织，在包括PMCA在内的所有实验室检测中均呈阴性结果。尽管在异常PrP水平极低的组织中观察到检测结果存在不一致，但IHC、WB、ELISA与生物测定法的整体相关性良好。最为关键的是，即便感染滴度较低，实验室检测对组织中异常PrP的检出结果与感染性水平之间仍存在良好相关性。本研究结果可为风险建模人员提供有价值的参考数据，同时为验证用于定量检测朊病毒感染性的实验室检测方法迈出了关键一步，这将极大助力TSE风险评估政策的制定与实施。