Outer Membrane Vesicles Derived from Salmonella Typhimurium Can Deliver Shigella Flexneri 2a O-polysaccharide Antigen to Prevent Lethal Pulmonary Infection in Mice

The threat to health from shigellosis has become quite serious in many developing countries, causing severe diarrhea. Shigella flexneri 2a ( S. flexneri 2a) is the principal species responsible for this endemic disease. Although there have been multiple attempts to design a vaccine against Shigellosis, one that is effective has not yet been developed. Lipopolysaccharide (LPS) is both an essential virulence factor and a protective antigen against Shigella , due to its outer domain, termed O-polysaccharide antigen. In the present study, S. flexneri 2a O-polysaccharide antigen was innovatively bio-synthesized in Salmonella and attached to core-lipid A by the ligase WaaL, and thus purified outer membrane vesicles (OMVs) were used as a vaccine for subsequent research. Here, we identified the expression of the heterologous polysaccharide antigen and described the isolation, characterization, and immune protection efficiency of the OMV vaccine. The expression of S. flexneri 2a did not affect the formation of Salmonella OMVs or their cytotoxicity. Furthermore, the results of the animal experiments indicated that immunization of the mice both intranasally and intraperitoneally with the OMV vaccine induced significant specific anti-Shigella LPS antibodies in both vaginal secretions and fluid from bronchopulmonary lavage, in addition to within sera. The OMV vaccine immunized by both routes of administration provided significant protection against virulent S. flexneri 2a infection, as judged by a serum bactericidal assay (SBA), opsonization assay, challenge experiment, and pathological analysis. The present study firstly indicates that the proposed vaccination strategy represents a novel and improved approach to control Shigellosis by the combination of bioconjugation of Salmonella glycosyl carrier lipid and OMV. In addition, the strategy of genetic manipulation described here is ideally suited for designs based on other Shigella serotypes, allowing the development of a Shigella vaccine with broad-protection.