Impact of vector-priming on the immunogenicity of a live recombinant Salmonella enterica serovar typhi Ty21a vaccine expressing urease A and B from Helicobacter pylori in human volunteers

Orally administered recombinant Salmonella vaccines represent an attractive option for mass vaccination programmes against various infectious diseases. Therefore, it is crucial to gather knowledge about the possible impact of preexisiting immunity to carrier antigens on the immunogenicity of recombinant vaccines. Thirteen volunteers were preimmunized with Salmonella typhi Ty21a in order to evaluate the effects of prior immunization with the carrier strain. Then, they received three doses of 1-2 x 10(10) viable organisms of either the vaccine strain S. typhi Ty21a (pDB1) expressing subunits A and B of recombinant Helicobacter pylori urease (n = 9), or placebo strain S. typhi Ty21a (n = 4). Four volunteers were preimmunized and boosted with the vaccine strain S. typhi Ty21a (pDB1). No serious adverse effects were observed in any of the volunteers. Whereas none of the volunteers primed and boosted with the vaccine strain responded to the recombinant antigen, five of the nine volunteers preimmunized with the carrier strain showed cellular immune responses to H. pylori urease (56%). This supports the results of a previous study in non-preimmunized volunteers where 56% (five of nine) of the volunteers showed a cellular immune response to urease after immunisation with S. typhi Ty21a (pDB1).