Attenuated <i>Salmonella</i> Typhimurium expressing <i>Salmonella</i> Paratyphoid A O-antigen induces protective immune responses against two <i>Salmonella</i> strains

<i>Salmonella enterica</i> serovar Paratyphi A is the main causative agent of paratyphoid fever in many Asian countries. As paratyphoid is spread by the fecal-oral route, the most effective means of controlling <i>S</i>. Paratyphi A infection is through the availability of clean water supplies and working sanitation services. Because sanitation facilities improve slowly in these poor areas and antibiotic resistance is severe, the development of a safe and effective vaccine remains a priority for controlling the spread of paratyphoid disease. In this study, we investigated the strategy of heterologous O-antigenic O2 serotype (<i>S</i>. Paratyphi A characterized) conversion in <i>S</i>. Typhimurium to prevent paratyphoid infections. A series of <i>S</i>. Typhimurium mutants were constructed with replacement of <i>abe, wzx</i>_B1 and <i>wbaV</i>_B1 genes with respective <i>prt-tyv</i>_A1, <i>wzx</i>_A1 and <i>wbaV</i>_A1, and the results showed that only three genes including <i>prt, wbaV_A1</i> and <i>wzx_A1</i> from <i>S</i>. Paratyphi A presence enable <i>S</i>. Typhimurium to sufficiently express O2 antigen polysaccharide. We also constructed a series of live attenuated <i>S</i>. Typhimurium vaccine candidates expressing heterologous O2 O-antigens, and a mouse model was used to evaluate the immunogenicity of live vaccines. ELISA data showed that vaccine candidates could induce a comparatively high level of <i>S</i>. Paratyphi A and/or <i>S</i>. Typhimurium LPS-specific IgG and IgA responses in murine model, and IgG2a levels were consistently higher than IgG1 levels. Moreover, the functional properties of serum antibodies were evaluated using <i>in vitro</i> C3 complement deposition and opsonophagocytic assays. Our work highlights the potential for developing <i>S</i>. Typhimurium live vaccines against <i>S</i>. Paratyphi A.