Improved tolerability of a Salmonella Typhimurium live-attenuated vaccine strain 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 (I77 Δ guaBA Δ clpP ) was safe and immunogenic in rhesus macaques, but was shed for an undesirably long time post-immunization. In mice, occasional mortality post-vaccination was also noted (approximately 1 in every 15 mice). Herein 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 a S. Typhimurium Δ pipA mutant induced significantly less fluid accumulation in rabbit loops than the wild-type strain. New vaccine strain CVD 1926 (I77 Δ guaBA Δ clpP Δ pipA Δ 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 (109 colony forming units/dose) by oral gavage and at one or three months post-immunization mice were challenged with 700 or 100 LD50 (50% lethal dose), respectively, of wild-type strain I77. 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 immunogenic and protective as previous iterations in a mouse model.