Development of recombinant S. Typhimurium, as a model for S. Typhi-based vaccine vectors
Dunstan SJ., Simmons CP., Strugnell RA.
© 1998, Faculty of Medicine, Universitas Indonesia. All rights reserved. The successfull testing and development of a new live, attenuated mutant (s) of S. typhi as a typhoid vaccine creates an opportunity for developing novel bivalent or multivalent human vaccines. Development of such vaccines will require careful analysis of optimal expression system (gene dosage, promoter strength,…) to ensure that the recombinant S. typhi elicit a maximal response against the co-expressed heterologous antigen. We have modelled the construction of bivalent vaccines in S. typhimurium using the murine model of typhoid fever. The model antigen used in these studies, the C fragment of tetanus toxin (TT), is highly immunogenic in mice when expressed as a recombinant protein from live, attenuated S. typhimurium. Parameters including background mutation, plasmid copy numbers and stability, promoter strength/regulation and translation effects were examined in this study. Isogenic pur A, aroA, aroA/aroD, htrA and ompR mutants of S. typhimurium SL1344 were compared with the cya/crp mutant of SR-11 for in vivo growth and penetration into central lymphoid organs, and immunisation against C fragment. Protective levels of TT-specific antibody were elicited by all recombinant Salmonellae except the purA mutant. Immunogenicity did not correlate with penetration into the spleen or specific splenic T cell responses, but did correlate with Peyer’s patch colonisation. Maximal immunogenicity of plasmid constructs of C fragment correlated with plasmid stability, and low copy, but stable replicons (eg.pRSF 10 JO) were effective in vaccine delivery. C fragment expressed from Col EI plasmids pBR322 and its derivative pAT153, induced very high levels of TT-specific antibody whereas the high copy derivative pIC20H did not elicit TT- specific immune response and was highly unstable in vivo. In vivo-regulated promoters (ppagC, pnirB andpkatG) were compared with the constitutive promoter ptrc for in vivo expression of the reporter antigen firefly luciferase, and in vivo immunogenicity of C fragment constructs. The strongest in vivo promoter (ptrc) was less effective at immunisation than the in vivo regulated promoter ppagC. Information from these studies will be used to optimally express other heterologous antigens from recombinant S. typhimurium mutants, with the ultimate aim of developing novel bivalent S. typhi vaccines.