Outer membrane vesicles of Shigella boydii type 4 induce passive immunity in neonatal mice

Like most other Gram-negative bacteria, Shigella releases outer membrane vesicles (OMVs) into the surrounding environment during growth. In this study, we have exploited OMVs of Shigella as a protective immunogen in a mice model against Shigellosis. Distinctive vesicle secretion was noticed from different Shigella strains. Among them, Shigella boydii type 4 (BCH612) was secreting relatively higher amounts. We immunized female adult mice orally with 32 μg of purified Shigella boydii type 4 (BCH612) OMVs four times at 1-week intervals. Antibodies against these vesicles were detected in immunized sera until 120 days, indicating a persistent immune response. To observe whether the passive immunity had been transferred to the neonates, the immunized female mice were mated and the offspring were challenged orally, with wild-type homologous and heterologous Shigella strains. All offspring of immunized mothers survived the challenge with homologous strain BCH612 and up to 81% protective efficacy was noted against heterologous strains Shigella dysenteriae 1, Shigella flexneri 2a, Shigella flexneri 3a, Shigella flexneri 6 and Shigella sonnei. Our results exhibited for the first time that oral immunization of adult female mice with purified OMVs of Shigella, without any adjuvant, conferred passive protection to their offspring against shigellosis. These findings will contribute to the future development of a potential non-living vaccine candidate against shigellosis.