Outer Membrane Vesicles Derived from Salmonella enterica Serotype Typhimurium Can Deliver Shigella flexneri 2a O-Polysaccharide Antigen To Prevent Shigella flexneri 2a Infection in Mice.

Shigellosis has become a serious threat to health in many developing countries due to the severe diarrhea it causes. Shigella flexneri 2a is the principal species responsible for this endemic disease. Despite multiple attempts to design a vaccine against shigellosis, no effective vaccine has been developed yet. Lipopolysaccharide (LPS) is both an essential virulence factor and an antigen protective against Shigella, due to its outer domain, termed O-polysaccharide antigen. In the present study, S. flexneri 2a O-polysaccharide antigen was innovatively biosynthesized in Salmonella and attached to core-lipid A via the ligase WaaL, with purified outer membrane vesicles (OMVs) utilized as vaccine vectors. Here, we identified the expression of the heterologous O-antigen and have described the isolation, characterization, and immune protection efficiency of the OMV vaccine. Furthermore, the results of animal experiments indicated that immunization of mice with the OMV vaccine induced significant specific anti-Shigella LPS antibodies in the serum, with similar trends in IgA levels from vaginal secretions and fluid from bronchopulmonary lavage, both intranasally and intraperitoneally. The OMV vaccine derived from both routes of administration provided significant protection against virulent S. flexneri 2a infection, as judged by a serum bactericidal assay, opsonization assay, and challenge test. This vaccination strategy represents a novel and improved approach to control shigellosis by the combination of Salmonella glycosyl carrier lipid bioconjugation with OMVs. IMPORTANCE Shigella, the cause of shigellosis or bacillary dysentery, is a major public health concern, especially for children in developing countries. An effective vaccine would control the spread of the disease to some extent. However, no licensed vaccine against Shigella infection in humans has so far been developed. The Shigella O-antigen polysaccharide is effective in stimulating the production of protective antibodies and so could represent a vaccine antigen candidate. In addition, bacterial outer membrane vesicles (OMVs) have been used as antigen delivery platforms due to their nanoscale properties and ease of antigen delivery to trigger an immune response. Therefore, the present study provides a new strategy for vaccine design, combining a glycoconjugated vaccine with OMVs. The design concept of this strategy is the expression of Shigella O-antigen via the LPS synthesis pathway in recombinant Salmonella, from which the OMV vaccine is then isolated. Based on these findings, we believe that the novel vaccine design strategy in which polysaccharide antigens are delivered via bacterial OMVs will be effective for the development and clinical application of an effective Shigella vaccine.