Evaluation of virulent and live Shigella sonnei vaccine candidates in a gnotobiotic piglet model

Newborn gnotobiotic (GB) piglets given virulent Shigella orally develop many of the clinical symptoms and gastrointestinal (GI) manifestations that mimic human shigellosis. Shigella sonnei virulent strain Moseley, a mutant ShET2-1,2, lacking enterotoxin SenA and its paralog SenB, and vaccine candidates WRSS1 and WRSs3 were evaluated in this model for rates of diarrhea, colonization and other GI symptoms and pathology. Moseley-infected piglets developed diarrhea from 1 to 7 days, with the highest rates seen on days 2-4 after inoculation. In contrast, WRSs3-infected piglets did not have diarrhea over the entire experimental period. Compared to the Moseley group, lower diarrheal rates were observed in the double enterotoxin mutant and significantly lower in the WRSS1 group. Moseley infection also caused marked mucosal damage in the GI tissues at PID1 to PID8, and induced predominantly proinflammatory cytokine secretion. IL-8 and to a lesser extent IL-6 and IL-1β were observed early after inoculation and IL-12 secretion could be measured till late in infection. The ShET2-1,2 mutant, WRSS1 and WRSs3 also colonized the GI tract in a manner similar to Moseley; however, both vaccine candidates developed milder histopathological indices and cytokine responses. WRSs3-infected animals showed the least pathology. Furthermore, unlike the other strains, WRSs3 was rarely detected in organs outside the gastrointestinal tract. These results support the development of the GB piglet model as a sensitive in vivo oral model for the evaluation of virulence of different Shigella strains which could be applied to other oral vaccine candidates.