The Murine B Cell Repertoire Is Severely Selected against Endogenous Cellular Prion Protein

Abstract

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.

Mouse vaccination with dendritic cells loaded with prion protein peptides overcomes tolerance and delays scrapie

“…1(d) suggests that B-and T-cell precursors are limiting, due to immune tolerance, and that the repertoire which escapes tolerance in wt mice is more difficult to activate. The fact that Abs to cell-surface PrPc are nevertheless generated in wt mice proves that helper Tand B-cell repertoires directed against PrPc epitopes are not totally purged as previously suggested (Gregoire et al, 2005;Polymenidou et al, 2004). …”

“…ELISA against a bank of plastic-immobilized peptides was performed as described previously (Gregoire et al, 2005). Sera were distributed as duplicates at a 1 : 50 dilution and left for 2 h at room temperature on antigen-coated polypropylene microtitre plates (Maxisorb).…”

Adoptive Transfer of T Lymphocytes Sensitized against the Prion Protein Attenuates Prion Invasion in Scrapie-Infected Mice

“…PrP-sufficient mice are totally unresponsive to recombinant DNA, thereby confirming that the expression of PrPc on host tissues induces a deep tolerance in both B and T cells (20,44,51). On the basis of these results, we have developed models in which the lymphocytes collected in DNA-immunized Prnp Ϫ/Ϫ mice were transferred into histocompatible C57BL/6 recipients that were totally or partially devoid of their own lymphocytes for better colonization.…”

“…6C) suggests that a few Ab-secreting B cells may have been transferred but did not persist in the long term. Several studies show that anti-PrP B lymphocytes are strictly controlled in PrP-sufficient wt mice (40,44,51). This would explain why imported B cells have a limited survival in RAG2…”

Contribution of Antibody and T Cell-Specific Responses to the Progression of 139A-Scrapie in C57BL/6 Mice Immunized with Prion Protein Peptides

“…Although numerous immunization procedures were developed using various Ag delivery approaches, high titer and affinity autoantibodies to native PrPc were difficult to obtain in wild-type (wt) mice because of the tolerance to endogenous PrPc (15)(16)(17)(18)(19)(20)(21)(22). In line with these data, we showed previously that the Ab repertoire to native PrPc is deeply tolerized and that only B cells reactive for burried epitopes produced Abs (23).…”