Evaluation of Safety and Protective Efficacy of a waaJ and spiC Double Deletion Korean Epidemic Strain of Salmonella enterica Serovar Gallinarum

Safety of the Salmonella enterica serotype Dublin strain Sdu189-derived live attenuated vaccine—A pilot study

Salmonella enterica serovar Dublin (S. Dublin) is an important zoonotic pathogen with high invasiveness. In the prevention and control of the Salmonella epidemic, the live attenuated vaccine plays a very important role. To prevent and control the epidemic of S. Dublin in cattle farms, the development of more effective vaccines is necessary. In this study, we constructed two gene deletion mutants, Sdu189ΔspiC and Sdu189ΔspiCΔaroA, with the parental strain S. Dublin Sdu189. The immunogenicity and protective efficacy were evaluated in the mice model. First, both mutant strains were much less virulent than the parental strain, as determined by the 50% lethal dose (LD₅₀) for specific pathogen-free (SPF) 6-week-old female BALB/c mice. Second, the specific IgG antibody level and the expression level of cytokine TNF-α, IFN-γ, IL-4, and IL-18 were increased significantly in the vaccinated mice compared to the control group. In addition, the deletion strains were cleared rapidly from organs of immunized mice within 14 d after immunization, while the parental strain could still be detected in the spleen and liver after 21 d of infection. Compared with the parental strain infected group, no obvious lesions were detected in the liver, spleen, and cecum of the deletion strain vaccinated groups of mice. Immunization with Sdu189ΔspiC and Sdu189ΔspiCΔaroA both provided 100% protection against subsequent challenges with the wild-type Sdu189 strain. These results demonstrated that these two deletion strains showed the potential as live attenuated vaccines against S. Dublin infection. The present study established a foundation for screening a suitable live attenuated Salmonella vaccine.

Prospective lipid-A altered live attenuated Salmonella Gallinarum confers protectivity, DIVA capability, safety and low endotoxicity against fowl typhoid

The present study describes creating an attenuated Salmonella Gallinarum (SG) strain with reduced endotoxicity to prevent fowl typhoid. The strain was attenuated by deleting the lon, cpxR, and rfaL virulence-related genes. Endotoxicity was reduced by deleting the pagL open reading frame and replacing it with the lpxE gene derived from Francisella tularencis. Both events, (1) deletion of the pagL and (2) introduction of the lpxE genes, conferred reduced endotoxicity by detoxifying the lipid A structure. The detoxified SG strain (SGVSdt) was well tolerated in 7-day-old chicks when administered orally at 1 × 10⁸ CFU/bird and in 14-day-old birds administered 1 × 10⁷ CFU/bird subcutaneously. Parenteral immunization of detoxified vaccine strain was completely safe in birds and free of environmental contamination. Subcutaneous immunization conferred disease protection and induced humoral and cell-mediated immune responses marked by Th1-skewed patterns similar to those produced by the commercial SG9R vaccine strain. Compared with the SG9R-based vaccine, the SGVSdt construct generated significantly fewer inflammatory TNF-α responses while significantly inducing IFN-γ cytokine levels as an indication of an adaptive antibacterial response. The differentiating infected from vaccinated animals (DIVA) capability was on par with the predecessor SGVS. This study presents an appealing biological strategy to minimize lipid A-mediated endotoxicity without compromising protective efficacy against the SG challenge. Reduced endotoxicity permits the utilization of higher inoculation doses to maximize protection against fowl typhoid.