The absence of a detectable immune response during transmissible spongiform encephalopathies is likely due to the fact that the essential component of infectious agents, the prion protein (PrP), is a self Ag expressed on the surface of many cells of the host. To overcome self-tolerance to PrP, we used 30-mer PrP peptides previously shown to be immunogenic in Prnp−/− mice, together with CFA or CpG-oligodeoxynucleotides (CpG) in IFA. Generation of anti-PrP T and B cell responses was analyzed in the spleen, lymph nodes, and serum of immunized C57BL/6 wild-type mice. Immunization with PrP peptides emulsified in CFA did not trigger an immune response to PrP. When CpG were used, vaccination with peptides P143–172 and P158–187 generated IFN-γ-secreting splenic T cells, and only P158–187 significantly stimulated IL-4-secreting T cells. Both peptides induced few Ab-producing B cells, and low and variable serum Ab titers. In contrast, immunization with peptide P98–127 did not induce significant levels of T cell responses but elicited specific peptide Abs. T cell epitope mapping, performed using 15-mer peptides covering PrP segment 142–182, revealed that an immunogenic motif lies between positions 156 and 172. These results demonstrate that T and B cell repertoires against PrP can be stimulated in C57BL/6 when adjuvant of the innate immunity such as CpG, but not CFA, is added to PrP peptides, and that the pattern of immune responses varies according to the epitope.

Recent reports suggest that immunity against the prion protein (PrP) retards transmissible spongiform encephalopathies progression in infected mice. A major obstacle to the development of vaccines comes from the fact that PrP is poorly immunogenic, as it is seen as self by the host immune system. Additional questions concern the immune mechanisms involved in protection and the risk of eliciting adverse reactions in the central nervous system of treated patients. Peptide-based vaccines offer an attractive strategy to overcome these difficulties. We have undertaken the identification of the immunogenic regions of PrP, which trigger helper T cells (Th) associated with antibody production. Our results identify two main regions, one between the structured and flexible portion of PrP (98-127) and a second between alpha 1 and alpha 2 helix (143-187). Peptides (30-mer) corresponding to these regions elicit class II-restricted Th cells and antibody production against native PrP and could therefore be of potential interest for a peptide-based vaccination.

Abs to the prion protein (PrP) can protect against experimental prion infections, but efficient Ab responses are difficult to generate because PrP is expressed on many tissues and induces a strong tolerance. We previously showed that immunization of wild-type mice with PrP peptides and CpG oligodeoxynucleic acid overcomes tolerance and induces cellular and humoral responses to PrP. In this study, we compared Ab and T cell repertoires directed to PrP in wild-type and PrP knockout (Prnpo/o) C57BL/6 mice. Animals were immunized with mouse PrP-plasmid DNA or with 30-mer overlapping peptides either emulsified in CFA or CpG/IFA. In Prnpo/o mice, Abs raised by PrP-plasmid DNA immunization recognized only N-terminal PrP peptides; analyses of Ab responses after PrP peptide/CFA immunization allowed us to identify six distinct epitopes, five of which were also recognized by Abs raised by PrP peptides/CpG. By contrast, in wild-type mice, no Ab response was detected after PrP-plasmid DNA or peptide/CFA immunization. However, when using CpG, four C-terminal peptides induced Abs specific for distinct epitopes. Importantly, immune sera from Prnpo/o but not from wild-type mice bound cell surface PrP. Abs of IgG1 and IgG2b subclasses predominated in Prnpo/o mice while the strongest signals were for IgG2b in wild-type mice. Most anti-PrP Th cells were directed to a single epitope in both Prnpo/o and wild-type mice. We conclude that endogenous PrPC expression profoundly affects the Ab repertoire as B cells reactive for epitopes exposed on native PrPC are strongly tolerized. Implications for immunotherapy against prion diseases are discussed.