Further characterization of Shigella sonnei live vaccine candidates WRSs2 and WRSs3-plasmid composition, invasion assays and Sereny reactions

Shigella sonnei live vaccine candidates WRSs2 and WRSs3 are principally attenuated by the loss of the virG(icsA) gene that prevents bacterial spread. In addition, both strains lack the enterotoxin SenA and a paralog SenB. WRSs3 also has a deletion in the gene encoding MsbB2. Both of these vaccine candidates are expected to be improved, safer versions of WRSS1 that only lacked the virG(icsA) gene and was reactogenic in volunteer trials. During the construction of WRSs2 and WRSs3, segments of the virulence plasmid spanning two FRT sites were inverted in orientation. These inversions have been analysed to show that the overall composition of the plasmid is unaltered. Furthermore, characterization of WRSs2 and WRSs3 in a cell culture and in an animal model have conclusively demonstrated that the segment inversions per se do not adversely affect the behavior of these vaccine candidates and pose no increased safety risk for human evaluation.