Regulated Delayed 2a O-antigen Synthesis in Live Recombinant Serovar Typhimurium Induces Comparable Levels of Protective Immune Responses with Constitutive Antigen Synthesis System.

(), a leading cause of bacillary dysentery, is a major public health concern particularly affecting children in developing nations. We have constructed a novel attenuated vaccine system based on the regulated delayed antigen synthesis (RDAS) and regulated delayed expression of attenuating phenotype (RDEAP) systems for delivering the 2a (Sf2a) O-antigen. The new vaccine platform was constructed through chromosomal integration of the P and P cassettes, resulting in a gradual depletion of WbaP enzyme. An expression vector, encoding Sf2a O-antigen biosynthesis under the control of the LacI-repressible P promoter, was maintained in the vaccine strain through antibiotic-independent selection. Mice immunized with the vaccine candidates were evaluated for cell-mediate and humoral immune responses. In the presence of exogenous arabinose, the vaccine strain synthesized native LPS as a consequence of WbaP expression. Moreover, arabinose supported LacI expression, thereby repressing Sf2a O-antigen production. In the absence of arabinose , native LPS synthesis is repressed whilst the synthesis of the Sf2a O-antigen is induced. Murine immunization with the vaccine strain elicited robust Sf2a-specific protective immune responses together with long term immunity. These findings demonstrate the protective efficacy of recombinant Sf2a O-antigen delivered by a vaccine platform.