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. IMPORTANCEShigella, 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.