Future stem cell therapies may depend on a limited number of cell lines currently under development. These lines will have been extensively cultivated and exposed to a wide variety of human and animal-derived biological products, and in some cases exposed to other (feeder) human cells, before being used on a one-donor-to-many-recipients
basis. We have begun to investigate the potential for stem cell-mediated prion transmission by examining how self-renewing populations of human stem cells respond to transitory exposure to BSE or vCJD brain homogenates in vitro. Cellular uptake of PrPSc from culture medium is rapid, extensive and does not depend on species or codon 129 compatibility. It is most likely a non-specific uptake mechanism also involving brain components other than PrPSc (Fig. 8). The cells LDE225 do not appear to become infected as such; instead the majority of cells clear the exogenous PrPSc by as yet undetermined mechanisms. We do not know what the long-term consequences (if any) might be of transitory exposure find more of stem cells to prion infectivity, nor do we know what effect neuronal differentiation of pluripotent progenitors might have on prion replication in such cells and their derivatives. While the prospect of a major epidemic of vCJD in the UK and elsewhere seems to be receding, there remain a series of uncertainties surrounding the
eventual numbers of individuals that will suffer from this devastating condition. The issues include the effects of genotype on susceptibility and the possible existence of substantial numbers of asymptomatic infected
individuals PRKD3 that may pose risks of onward transmission. sCJD remains the most frequently occurring human prion disease and arguably the least well understood. Other idiopathic forms of human prion disease (such as VPSPr), characterized by protease-sensitive forms of the prion protein, also exist and their true prevalence may be hard to ascertain. The possible risks from newly described animal prion diseases and from emerging cellular therapies are currently poorly quantified. On a more theoretic level the prion hypothesis has provided a unifying conceptual framework for TSE research and provided a paradigm to interrogate the similarities and differences between the diverse neurodegenerative conditions involving prion-like mechanisms of molecular pathology. I would like to thank Professor Akiyoshi Kakita and Professor Hitoshi Takahashi for their generous invitation to attend the 53rd Annual Meeting of the Japanese Society of Neuropathology at Niigata. I would also like to acknowledge Japanese colleagues with whom it has been a pleasure to collaborate and spend time with over the years, including Akiko Iwaki, Akiyoshi Kakita, Katsumi Doh-ura, Kensuke Sasaki, Mari Tada, Masanori Morita, Masahito Yamada, Tetsuyuki Kitamoto, and last, but by no means least Toru Iwaki.