A T helper epitope was expressed in three innovative delivery vehicles recently developed in our laboratories and based respectively, on the filamentous bacteriophage fd, the E2 protein from the PDH complex of Bacillus stearothermophilus and the protein CotC of Bacillus subtilis spores. Studies of antigenicity and immunogenicity were performed by using a specific T cell hybridoma and by priming mononuclear cells isolated from the venous blood of human donors. The results indicate that the E2 system is the best suited for inducing a specific immune response towards a CD4 T cell epitope. Importantly, TCR clonal analysis demonstrated the persistence over years of a previously described antigen specific clonotype and its presence correlates with the immunogenic strength of the antigen delivery system.