CONSTRUCTION OF A TRIVALENT CANDIDATE SHIGELLA VACCINE STRAIN WITH HOST-VECTOR BALANCED-LETHAL SYSTEM

A trivalent live Shigella vaccine candidate FSD01 against S. flexneri 2a, S. sonnei and S. dysenteriae I was constructed. This candidate str ain was based on the S. flexneri 2a vaccine T32. By homologous recombi nation exchange, the chromosomal asd gene of T32 was site-specifically inactivated, resulting in the strain unable to grow normally in LB br oth, while another nsd gene of S. mutans was employed to construct an asd(+) complementary vector. This combination of asd(.)host/Asd(+) vec tor formed a balanced-lethal expression system in T32 strain. By use o i this system, two important protective antigen genes coding for S. so nnei Form I antigen and Shiga toxin B subunit were cloned and expresse d in T32, which led to the construction of trivalent candidate vaccine FSM01. Experimental results showed that this strain was genetically s table, but its recombinant plasmid was non-resistant. Moreover, it was able to effectively express trivalent antigens in one host and induce protective responses in mice. against the challenges of the above thr ee Shigella strains.