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Effective vaccines are needed to combat diarrheal diseases due to Shigella. Two live oral S. sonnei vaccine candidates, WRSs2 and WRSs3, attenuated principally by the lack of spreading ability, as well as the loss of enterotoxin and acyl transferase genes, were tested for safety and immunogenicity. Healthy adults 18-45 years of age, assigned to 5 cohorts of 18 subjects each (WRSs2 (n = 8), WRSs3 (n = 8) or placebo (n = 2)) were housed in an inpatient facility and administered a single oral dose of study agent 5 min after ingestion of oral bicarbonate. Ascending dosages of vaccine (from 10 3 CFU to 10 7 CFU) were evaluated. On day 8, treatment with ciprofloxacin (500 mg BID for 3 days) was initiated and subjects were discharged home 2 days after completing antibiotics. Subjects returned for outpatient visits on day 14, 28 and 56 post-vaccination for monitoring and collection of stool and blood samples. Both WRSs2 and WRSs3 were generally well tolerated and safe over the entire dose range. Among the 80 vaccinees, 11 subjects developed diarrhea, 8 of which were mild and did not affect daily activities. At the 10 7 CFU dose, moderate diarrhea occurred in one WRSs2 subject while at the same dose of WRSs3, 2 subjects had moderate or severe diarrhea. Vaccinees mounted dose-dependent mucosal and systemic immune responses that appeared to correlate with fecal shedding. S. sonnei vaccine candidates WRSs2 and WRSs3 are safe and immunogenic over a wide dose range. Future steps will be to select the most promising candidate and move to human challenge models for efficacy of the vaccine., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Live, attenuated Shigella vaccine candidates, such as Shigella sonnei strain WRSS1, Shigella flexneri 2a strain SC602, and Shigella dysenteriae 1 strain WRSd1, are attenuated principally by the loss of the VirG(IcsA) protein. These candidates have proven to be safe and immunogenic in volunteer trials and in one study, efficacious against shigellosis. One drawback of these candidate vaccines has been the reactogenic symptoms of fever and diarrhea experienced by the volunteers, that increased in a dose-dependent manner. New, second-generation virG(icsA)-based S. sonnei vaccine candidates, WRSs2 and WRSs3, are expected to be less reactogenic while retaining the ability to generate protective levels of immunogenicity seen with WRSS1. Besides the loss of VirG(IcsA), WRSs2 and WRSs3 also lack plasmid-encoded enterotoxin ShET2-1 and its paralog ShET2-2. WRSs3 further lacks MsbB2 that reduces the endotoxicity of the lipid A portion of the bacterial LPS. Studies in cell cultures and in gnotobiotic piglets demonstrate that WRSs2 and WRSs3 have the potential to cause less diarrhea due to loss of ShET2-1 and ShET2-2 as well as alleviate febrile symptoms by loss of MsbB2. In guinea pigs, WRSs2 and WRSs3 were as safe, immunogenic and efficacious as WRSS1., (Published by Elsevier Ltd.)