Follicular Dendritic Cell-Specific Prion Protein (PrP c ) Expression Alone Is Sufficient to Sustain Prion Infection in the Spleen Laura McCulloch 1 , Karen L. Brown 1 , Barry M. Bradford 1 , John Hopkins 1 , Mick Bailey 2 , Klaus Rajewsky 3 , Jean C. Manson 1 , Neil A. Mabbott 1 * 1 The Roslin Institute & Royal (Dick) School of Veterinary Sciences, University of Edinburgh, Midlothian, United Kingdom, 2 Division of Veterinary Pathology, Infection and Immunity, School of Clinical Veterinary Science, University of Bristol, Avon, United Kingdom, 3 Program in Cellular and Molecular Medicine, Children’s Hospital, and Immune Disease Institute, Harvard Medical School, Boston, Massachusetts, United States of America Abstract Prion diseases are characterised by the accumulation of PrP Sc , an abnormally folded isoform of the cellular prion protein (PrP C ), in affected tissues. Following peripheral exposure high levels of prion-specific PrP Sc accumulate first upon follicular dendritic cells (FDC) in lymphoid tissues before spreading to the CNS. Expression of PrP C is mandatory for cells to sustain prion infection and FDC appear to express high levels. However, whether FDC actively replicate prions or simply acquire them from other infected cells is uncertain. In the attempts to-date to establish the role of FDC in prion pathogenesis it was not possible to dissociate the Prnp expression of FDC from that of the nervous system and all other non-haematopoietic lineages. This is important as FDC may simply acquire prions after synthesis by other infected cells. To establish the role of FDC in prion pathogenesis transgenic mice were created in which PrP C expression was specifically ‘‘switched on’’ or ‘‘off’’ only on FDC. We show that PrP C -expression only on FDC is sufficient to sustain prion replication in the spleen. Furthermore, prion replication is blocked in the spleen when PrP C -expression is specifically ablated only on FDC. These data definitively demonstrate that FDC are the essential sites of prion replication in lymphoid tissues. The demonstration that Prnp-ablation only on FDC blocked splenic prion accumulation without apparent consequences for FDC status represents a novel opportunity to prevent neuroinvasion by modulation of PrP C expression on FDC. Citation: McCulloch L, Brown KL, Bradford BM, Hopkins J, Bailey M, et al. (2011) Follicular Dendritic Cell-Specific Prion Protein (PrP c ) Expression Alone Is Sufficient to Sustain Prion Infection in the Spleen. PLoS Pathog 7(12): e1002402. doi:10.1371/journal.ppat.1002402 Editor: Jason Bartz, Creighton University, United States of America Received June 15, 2011; Accepted October 11, 2011; Published December 1, 2011 Copyright: ß 2011 McCulloch et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by project (Grant numbers BB/526741-1 and BBS/E/R/00001813) and Institute Strategic Grant funding from the UK Biotechnology and Biological Sciences Research Council. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: neil.mabbott@roslin.ed.ac.uk Introduction Prion diseases (Transmissible spongiform encephalopathies; TSE) are sub-acute neurodegenerative diseases that affect both humans and animals. Many prion diseases, including natural sheep scrapie, bovine spongiform encephalopathy, chronic wasting disease in mule deer and elk, and kuru and variant Creutzfeldt- Jakob disease in humans, are acquired by peripheral exposure (eg: orally or via lesions to skin or mucous membranes). After peripheral exposure prions accumulate first upon follicular dendritic cells (FDC) as they make their journey from the site of infection to the CNS (a process termed, neuroinvasion) [1–7]. FDC are a unique subset of stromal cells resident within the primary B cell follicles and germinal centres of lymphoid tissues [8]. Prion accumulation upon FDC is critical for efficient disease pathogen- esis as in their absence neuroinvasion are impaired [1–4]. From the lymphoid tissues prions invade the CNS via the peripheral nervous system [9]. During prion disease aggregations of PrP Sc , an abnormally folded isoform of the cellular prion protein (PrP C ) accumulate in affected tissues. Prion infectivity co-purifies with PrP Sc [10] and is considered to constitute the major, if not sole, component of infectious agent [11]. Host cells must express cellular PrP C to sustain prion infection [12] and FDC appear to express high levels of PrP C on the cell membrane in uninfected mice [13,14]. Although prion neuroinvasion from peripheral sites of exposure is dependent upon the presence of FDC in lymphoid tissues, it is not known whether FDC actually replicate prions themselves. FDC characteristically trap and retain native antigen on their surfaces for long periods in the form of immune complexes, consisting of antigen-antibody and/or complement components. Prions are also considered to be acquired by FDC as complement-opsonized immune complexes [15–18]. Thus, during prion infection FDC might simply trap and retain PrP Sc -containing immune complexes on their surfaces following synthesis by other infected cells such as neurones. Many cell types including classical DC, lymphocytes, mast cells, platelets, reticulocytes and epithelial cells secrete membrane vesicles termed exosomes that are enriched in cell-specific protein [19,20]. Although the functions of exosomes are uncertain FDC can bind them on their surfaces. These microvesicles permit FDC to passively acquire and display proteins on their surfaces that they do not express at the mRNA level [21]. Studies have shown that prions only accumulated in the spleens of mice in which the FDC- PLoS Pathogens | www.plospathogens.org 1 December 2011 | Volume 7 | Issue 12 | e1002402