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 strain was based on the S. flexneri 2a vaccine T32. By homologous recombination exchange, the chromosomal asd gene of T32 was site-specifically inactivated, resulting in the strain unable to grow normally in LB broth, while another nsd gene of S. mutans was employed to construct an asd(+) complementary vector. This combination of asd(.)host/Asd(+) vector formed a balanced-lethal expression system in T32 strain. By use oi this system, two important protective antigen genes coding for S. sonnei Form I antigen and Shiga toxin B subunit were cloned and expressed in T32, which led to the construction of trivalent candidate vaccine FSM01. Experimental results showed that this strain was genetically stable, 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 three Shigella strains.