Comparative evaluation of safety and efficacy of a live Salmonella gallinarum vaccine candidate secreting an adjuvant protein with SG9R in chickens

We previously reported JOL916, a live attenuated Salmonella gallinarum (SG), as a vaccine candidate for protection from fowl typhoid (FT). In the present study, we evaluated JOL1355, an SG that secretes heat-labile enterotoxin B subunit protein, for safety, immunogenicity, and protective efficacy against FT. In a single intramuscular inoculation, live attenuated SG (JOL916) and commercial live attenuated SG9R immunized groups showed gross lesions and bacterial persistence up to 21 days post-immunization. However, the JOL1355 immunized group showed gross lesions and bacterial persistence up to only 3 and 7 days post-immunization, respectively. In addition, several birds in the JOL916 and SG9R immunized group shown clinical signs after immunization, while JOL1355 immunized birds did not show any adverse effects. In a subsequent study, birds were primed and boosted at 4 and 8 weeks of age, respectively, and compared with control birds inoculated with sterile phosphate-buffered saline. The immunized groups B (JOL916), C (SG9R), and D (JOL1355) exhibited significantly higher humoral and cellular immune responses compared to those in the unimmunized control group A. In addition, the birds of each group were challenged with virulent SG at 11 weeks of age, and significantly increased survival rates were observed in all immunized groups compared with the control group. These results indicated that JOL1355 was able to efficiently induce an acquired immune response to protect birds after challenge, and may be safer than JOL916 and the commercial vaccine SG9R in chickens.