Improved Tolerability of a Salmonella enterica Serovar Typhimurium Live-Attenuated Vaccine Strain Achieved by Balancing Inflammatory Potential with Immunogenicity

A notable proportion of Salmonella-associated gastroenteritis in the United States is attributed to Salmonella enterica serovar Typhimurium. We have previously shown that live-attenuated S. Typhimurium vaccine candidate CVD 1921 (177 Delta guaBA Delta clpP) was safe and immunogenic in rhesus macaques but was shed for an undesirably long time postimmunization. In mice, occasional mortality postvaccination was also noted (approximately 1 in every 15 mice). Here we describe a further attenuated vaccine candidate strain harboring deletions in two additional genes, htrA and pipA. We determined that S. Typhimurium requires pipA to elicit fluid accumulation in a rabbit ileal loop model of gastroenteritis, as an S. Typhimurium Delta pipA mutant induced significantly less fluid accumulation in rabbit loops than the wildtype strain. New vaccine strain CVD 1926 (177 Delta guaBA Delta clpP Delta pipA Delta htrA) was assessed for inflammatory potential in an organoid model of human intestinal mucosa, where it induced less inflammatory cytokine production than organoids exposed to the precursor vaccine, CVD 1921. To assess vaccine safety and efficacy, mice were given three doses of CVD 1926 (10(9) CFU/dose) by oral gavage, and at 1 or 3 months postimmunization, mice were challenged with 700 or 100 LD50 (50% lethal doses), respectively, of wild-type strain 177. CVD 1926 was well tolerated and exhibited 47% vaccine efficacy following challenge with a high inoculum and 60% efficacy after challenge with a low inoculum of virulent S. Typhimurium. CVD 1926 is less reactogenic yet equally as immunogenic and protective as previous iterations in a mouse model.