Two live attenuated Shigella flexneri 2a strains WRSf2G12 and WRSf2G15: A new combination of gene deletions for 2nd generation live attenuated vaccine candidates

► New, second-generation live attenuated Shigella flexneri 2a vaccine candidates. ► WRSf2G12 and WRSf2G15 are as invasive as parental strain 2457T in HeLa cells. ► WRSf2G12 and WRSf2G15 are Sereny negative and protective in guinea pigs. ► WRSf2G15 that lacks the MsbB2 gene has lower endotoxicity profile in macrophages. ► A 3 vs. 2 dose immunization regimen provides better protection in guinea pigs.