Evaluation of the Effect of Promoter Type on the Immunogenicity of the Live Recombinant Salmonella Vaccines Expressing Escherichia Coli Heat-labile Enterotoxins (LTB)

Enterotoxigenic (ETEC)-induced diarrhoea is the second most common cause of death in children in the developing countries. Heat labile toxin (LT) is responsible for ETEC-induced diarrhoea. In the present study, a novel live ETEC vaccine based on subunit B of LT (LTB) expression in attenuated PhoP strain was developed. Herein, we aimed to compare the activity of promoters including constitutive , IPTG inducible and -inducible ( B and B78-23) in PhoP . Additionally, the ability of these recombinant PhoP /pLTBs to induce LTB-specific antibody responses in BALB/c mice after nasal immunization was evaluated. studies demonstrated that PhoP has the ability to produce rLTB. Furthermore, B promoter directed significantly more LTB expression in PhoP /pnirBLTB under anaerobic condition without induction compared to the amount of rLTB secreted by PhoP /ptrcLTB in bacterial soup under uninduced condition (6.06 ± 0.05 1.4 ± 0.46 μg/10 cfu, < 0.01). In addition, the constitutive rLTB expression from promoter was more than its expression from uninduced promoter in bacterial soup (4.2 ± 0.92 1.4 ± 0.46 (μg/10 cfu)) and pellet (27.4 ± 0.89 13.4 ± 1.42 (μg/10 cfu), < 0.0001). However, the mice immunized with PhoP /ptrcLTB elicited the superior anti-LTB responses among the PhoP containing the examined prompters, which were significantly higher than those induced by PhoP /pnirB78-23LTB and PhoP /pnirB, 6 weeks after the first immunization. Totally, it could be concluded that analysis of promoters for LTB expression in PhoP may not necessarily predict the recombinant PhoP immunogenicity.