To provide the basis for the development of a vaccine against prion disease, B and T cell tolerance against PrP has to be analyzed. Therefore, transgenic mice expressing a PrP-specific B cell receptor in the presence or absence of the endogenous or of transgenic PrP will be analyzed. The obtained data will reveal tolerization thresholds for individual PrP-specificities depending on the level and site of expression of the self-antigen PrPC. Since considerable numbers of B-cells appear not to be intrinsically tolerant to PrP, it is probably T helper cell tolerance that renders the immune system unresponsive against the ubiquitously expressed self-antigen PrPC, finally resulting in the inhibition of an immunity to prions in wild-type mice. To overcome T cell tolerance in normal mice and to provide the antigen in a highly B cell immunogenic form, recombinant rhabdovirus-like particles expressing the full length PrP or parts of it on the surface, will be generated. The rational to use rhabdovirus was based on the fact (i) that recombinant rhabdovirus-like particles are replication deficient, (ii) that rhabdovirus does induce strong T helper cell responses directed against the internal nucleoprotein, and (iii) that rhabdovirus is a very strong B cell activator. Recombinant virus-like particles will be used to exploit vaccination regimens. The objective is (i) to work out optimal conditions to induce strong anti-PrP immunoglobulin (Ig) titers and (ii) to analyze respective anti-PrP immunoglobulin responses for their putative protective effect against prions.
In addition, a new mouse model (termed inducible switch mouse) will be developed allowing the antigen-independent induction of transgenic anti-PrP Ig that can switch the subclass from IgM/D to all other Ig-subclasses (i.e. IgA, IgG etc.) upon appropriate stimulation. The mouse model is based on a gene targeting approach replacing the endogenous J segments of the immunoglobulin heavy chain locus by an inverted loxP flanked cassette containing two variable regions in opposing orientations, one conferring an irrelevant specificity and the other conferring the PrP-specificity. Upon hormone treatment of mb-1.Cre mice (kindly provided by Dr. M. Reth), Cre-recombinase activity will be induced during early B cell development inverting the variable region cassette and thus bringing the PrP conferring variable region in the correct position to be expressed. Since IgG - unlike IgM - often is the causative reagent of autoimmune disease, inducible switch mice will provide a useful tool allowing for (i) simple and constant production of PrP-specific immunoglobulins including IgG and, consequently, (ii) studying the potential risk of inducing autoagression by constant anti-PrP IgG production. Thus, by means of (i) recombinant virus and (ii) transgenic mice, vaccine efficacy and safety - two major issues of establishing an applicable vaccination regimen in human and animals - can be studied extensively. These results will not only be of great importance in designing an anti-prion vaccine, but will also have implications on other immuno-therapeutic approaches that involve immune responses against self-epitopes/proteins, e.g. in Alzheimer’s disease (anti-plaque immune responses), in cancer (anti-hormone, anti-cytokine immune responses), in contraception (anti-hormone, anti-cytokine immune responses), and others. Moreover, the value and efficiency of rhabdovirus-like particles as a new and future technology for developing vaccines other than prionostatic ones will be assessed.
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