Dendritic Cell-Mediated-Immunization with Xenogenic PrP and Adenoviral Vectors Breaks Tolerance and Prolongs Mice Survival against Experimental Scrapie Martine Bruley Rosset 1,2 *, Antoine Sacquin 1,2 , Sylvie Lecollinet 3 , Thomas Chaigneau 1,2 , Micheline Adam 3 , Franc ¸ois Crespeau 4 , Marc Eloit 3 1 INSERM UMR 938, Paris, France, 2 UPMC Univ Paris 06, Ho ˆ pital Saint-Antoine, Saint-Antoine, Paris, France, 3 UMR1161 Virologie, INRA, AFSSA, ENVA, Ecole Nationale Ve ´te ´ rinaire d’Alfort, Maisons Alfort, France, 4 Laboratoire d’anatomopathologie, Ecole Nationale Ve ´te ´ rinaire d’Alfort, Maisons Alfort, France Abstract In prion diseases, PrP c , a widely expressed protein, is transformed into a pathogenic form called PrP Sc , which is in itself infectious. Antibodies directed against PrP c have been shown to inhibit PrP c to PrP Sc conversion in vitro and protect in vivo from disease. Other effectors with potential to eliminate PrPSc-producing cells are cytotoxic T cells directed against PrP- derived peptides but their ability to protect or to induce deleterious autoimmune reactions is not known. The natural tolerance to PrP c makes difficult to raise efficient adaptive responses. To break tolerance, adenovirus (Ad) encoding human PrP (hPrP) or control Ad were administered to wild-type mice by direct injection or by transfer of Ad-transduced dendritic cells (DCs). Control Ad-transduced DCs from Tg650 mice overexpressing hPrP were also used for immunization. DC- mediated but not direct administration of AdhPrP elicited antibodies that bound to murine native PrP c . Frequencies of PrP- specific IFNc-secreting T cells were low and in vivo lytic activity only targeted cells strongly expressing hPrP. Immunohistochemical analysis revealed that CD3 + T cell infiltration was similar in the brain of vaccinated and unvaccinated 139A-infected mice suggesting the absence of autoimmune reactions. Early splenic PrP Sc replication was strongly inhibited ten weeks post infection and mean survival time prolonged from 209 days in untreated 139A-infected mice to 246 days in mice vaccinated with DCs expressing the hPrP. The efficacy appeared to be associated with antibody but not with cytotoxic cell-mediated PrP-specific responses. Citation: Rosset MB, Sacquin A, Lecollinet S, Chaigneau T, Adam M, et al. (2009) Dendritic Cell-Mediated-Immunization with Xenogenic PrP and Adenoviral Vectors Breaks Tolerance and Prolongs Mice Survival against Experimental Scrapie. PLoS ONE 4(3): e4917. doi:10.1371/journal.pone.0004917 Editor: Neil Mabbott, University of Edinburgh, United Kingdom Received January 27, 2009; Accepted February 16, 2009; Published March 19, 2009 Copyright: ß 2009 Rosset 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 funds from INSERM and the Neuroprion network of excellence, European FP6. Antoine Sacquin received a PhD fellowship from ‘‘France Alzheimer Association’’. 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: martine.rosset@inserm.fr Introduction In prion diseases, cellular prion protein (PrP c ), a normal host protein present in the majority of tissues and highly expressed in the nervous system [1], is converted into a protease resistant form (PrPres), also called scrapie PrP (PrP Sc ), which is in itself infectious [2]. PrP c is a protease sensitive glycoprotein which is attached to the cell membrane by a glycophosphatidylinositol anchor. PrP Sc has a high b-sheet content and prone to aggregation. In the most widely accepted model, PrP Sc interacts with PrPc and converts it into PrP Sc [3]. Prion diseases are characterized by long asymptomatic periods of incubation. Yet, it has been reported that when PrP Sc accumulation is stopped, PrP Sc can be cleared, injury is reduced by compensatory neuronal mechanisms and synaptic function is restored [4], paving the way to therapeutics aimed at blocking PrP Sc accumulation. Obtaining an effective prophylactic or therapeutic vaccines against prion diseases combines the difficulties of developing vaccines against persistent infections, and those inherent to immunotherapy against self-antigens. Moreover, it remains possible that protection against challenge versus to cure established disease might involve different effectors. There is now good evidence showing that anti-PrP antibodies are able to inhibit PrP c to PrP Sc conversion in vitro [5–8] and to protect mice against prion disease in vivo providing they recognize native PrP c [9–12]. While the mechanism underlying their therapeutic effect has not yet been elucidated one explanation is that antibodies alter PrP trafficking and thereby inhibit the PrP c to PrP Sc conversion process [3]. However, it has been suggested that the main role of antibodies was to diminish the pool of GPI-anchored PrP c , thus leading to protection or to a delay in animal death [13]. Opsonisation of PrPSc by microglial cells may also be involved. Other effectors with potential to eliminate PrP Sc -producing cells are cytotoxic T cells (CTLs) directed against PrP-derived peptides but the repertoire of CD8 + T cells in wild-type (wt) mice and their ability to protect from prion disease or to induce deleterious autoimmune reactions have never been addressed. In prion infected mice, cells that replicate and accumulate PrP Sc are follicular dendritic cells (FDC) [14] while dendritic cells (DCs) uptake and transport PrP Sc [15] and participate in the process of neuroinvasion [16]. Both cells are MHC class I positive; FDC express naturally high level of PrP c that became much stronger after immune stimulation [17] and DCs upregulate its expression upon in vivo maturation [18] and therefore might be target of PLoS ONE | www.plosone.org 1 March 2009 | Volume 4 | Issue 3 | e4917