Influence of preimmunization with tetanus toxoid on immune responses to tetanus toxin fragment C-guest antigen fusions in a Salmonella vaccine carrier.

We have previously described a new system for the delivery of recombinant antigens in live Salmonella vaccines as genetic fusions to the C terminus of fragment C of tetanus toxin (TetC) driven by the anaerobically inducible nirB promoter. It has been reported that preimmunization with tetanus toxoid (TT) can suppress the antibody response to peptides chemically coupled to TT (epitope-specific suppression) in both animals and humans, which could interfere with efficacy of the Salmonella-TetC delivery system. We report that preimmunization of BALB/c mice with TT in alum did not suppress the response to either of two protective antigens of Schistosoma mansoni, the full-length S. mansoni P28 glutathione S-transferase (P28) and a construct consisting of eight tandem copies of the protective peptide comprising amino acids 115 to 131 of P28. The guest antigens were expressed in the aroA Salmonella typhimurium SL3261 vaccine strain as fusions to TetC. Preimmunization with TT 10 weeks before administration of the recombinant salmonellae did not alter the antibody response to the full-length P28, whereas the response to the peptide comprising amino acids 115 to 131 was increased by preimmunization with TT, with the increase seen mainly in the immunoglobulin G1 isotype. The antitetanus response was increased by preimmunization with TT in all groups receiving salmonellae expressing TetC. The results could be important when one is considering the use of the Salmonella-TetC delivery system in populations preimmunized with TT.