Prion diseases are presumed to be caused by the accumulation in the brain of a pathological protein called prion protein (PrP) scrapie which results from the transconformation of cellular PrP, a ubiquitous glycoprotein expressed in all mammals. Since all isoforms of PrP are perceived as self by the host immune system, a major problem in designing efficient immunoprophylaxis or immunotherapy is to overcome tolerance. The present study was aimed at investigating whether bone-marrow-derived dendritic cells (DCs) loaded with peptides previously shown to be immunogenic in PrP-deficient mice, can overcome tolerance in PrP-proficient wild-type mice and protect them against scrapie. Results show that, in such mice, peptide-loaded DCs elicit both lymphokine release by T cells and antibody secretion against native cellular PrP. Repeated recalls with peptide-loaded DCs reduces the attack rate of 139A scrapie inoculated intraperitoneally and retards disease duration by 40 days. Most interestingly, survival time in individual mice appears to be correlated with the level of circulating antibody against native cellular PrP. INTRODUCTIONPrion diseases, also called transmissible spongiform encephalopathies (TSEs), are fatal neurodegenerative disorders which progress slowly, induce severe neurological loss and are transmissible. The agent is most presumably the scrapie prion protein (PrP) or PrPSc, a misfolded protein, rich in beta-sheets, resulting from the transconformation of cellular PrP (PrPc). PrPc is a ubiquitous, membrane-anchored protein, which has been remarkably conserved through evolution, but whose function in health is still enigmatic. The presence of PrPSc in tissues of diseased individuals confirms the diagnosis TSE (Prusiner et al., 1998;Wisniewski et al., 1998).Prions do not spontaneously evoke antibody (Ab) or T-cell responses in diseased subjects, most likely because PrPSc, like PrPc, is not perceived as foreign or 'dangerous' (Matzinger, 1998) by the host immune system. In recent years, however, following encouraging immunotherapy results in Alzheimer's disease patients (Schenk et al., 1999), several teams have attempted to passively or actively generate adaptive immune responses against prions in wild-type (wt) mice and have shown that passive Ab treatment (Sigurdsson et al., 2003;White et al., 2003), constitutive transgenic secretion of anti-PrP IgM (Heppner et al., 2001) or active immunization using whole PrP (Sigurdsson et al., 2002), PrP peptides (Schwarz et al., 2003), PrP fragments (Bade et al., 2006) or DNA constructs encoding Prnp gene sequences (Fernandez-Borges et al., 2006) had beneficial effects on disease evolution.In order to identify major histocompatibility (MHC) class II-restricted T-cell epitopes in C57BL6 (B6)-H-2 b mice, we previously screened a bank of overlapping peptides of mouse PrP and identified two main epitopes. One is contained in peptide PrP , the other is shared by peptides PrP 143-172 and PrP . Immunization of mice made genetically deficient for PrP (Prnp 2/2 ) with PrP 97-1...

There is to date no effective way of preventing or curing neurodegenerative diseases such as Alzheimer disease or transmissible spongiform encephalopathies. The idea of treating those conditions by immunological approaches has progressively emerged over the last ten years. Encouraging results have been reported in Alzheimer disease and in peripheral forms of mouse prion diseases following passive injection of Abs or active immunization against the peptides or proteins presumably at the origin of those disorders. Still, major difficulties persist due to some characteristics of those conditions such as slow evolution, brain location, uncertainties regarding precise pathogenic pathways, and, above all, the fact that the target Ag is self, meaning that it is poorly immunogenic and potentially harmful if tolerance was transgressed. To analyze some of those difficulties, we are developing adoptive cell transfer approaches. In this study, lymphocytes sensitized against the prion protein in nontolerant Prnp−/− mice were transferred into histocompatible wild-type recipients which were partly or totally devoid of their own lymphocytes. Under such conditions, we found that the engrafted T lymphocytes resisted peripheral tolerance, remained reactive for several months against epitopes of the prion protein, and significantly attenuated the progression of prions in secondary lymphoid organs with subsequent delay in the evolution of the neurological disease. Interestingly, those protective T lymphocytes secreted lymphokines and migrated more readily into the host CNS but did not appear to be engaged in cooperation with host B cells for Ab production.