Abstract : Prion diseases are caused by the transconformation of the host cellular prion protein PrPc into an infectious neurotoxic isoform called PrPSc. While vaccine-induced PrP-specific CD4+ T cells and antibodies partially protect scrapie-infected mice from disease, the potential autoreactivity of CD8+ cytotoxic T lymphocytes (CTLs) received little attention. Beneficial or pathogenic influence of PrPc-specific CTL was evaluated by stimulating a CD8+ T-cell-only response against PrP in scrapie-infected C57BL/6 mice. To circumvent immune tolerance to PrP, five PrP-derived nonamer peptides identified using prediction algorithms were anchored-optimized to improve binding affinity for H-2Db and immunogenicity (NP-peptides). All of the NP-peptides elicited a significant number of IFNy secreting CD8+ T cells that better recognized the NP-peptides than the natives; three of them induced T cells that were lytic in vivo for NP-peptide-loaded target cells. Peptides 168 and 192 were naturally processed and presented by the 1C11 neuronal cell line. Minigenes encoding immunogenic NP-peptides inserted into adenovirus (rAds) vectors enhanced the specific CD8+ T-cell responses. Immunization with rAd encoding 168NP before scrapie inoculation significantly prolonged the survival of infected mice. This effect was attributable to a significant lengthening of the symptomatic phase and was associated with enhanced CD3+ T cell recruitment to the CNS. However, immunization with Ad168NP in scrapie-incubating mice induced IFNy-secreting CD8+ T cells that were not cytolytic in vivo and did not influence disease progression nor infiltrated the brain. In conclusion, the data suggest that vaccine-induced PrP-specific CD8+ T cells interact with prions into the CNS during the clinical phase of the disease.