A Recombinant Shigella flexneri Strain Expressing ETEC Heat-Labile Enterotoxin B Subunit Shows Promise for Vaccine Development via OMVs

Diarrheal diseases caused by <i>Shigella</i> and enterotoxigenic <i>Escherichia coli</i> (ETEC) are significant health burdens, especially in resource-limited regions with high child mortality. In response to the lack of licensed vaccines and rising antibiotic resistance for these pathogens, this study developed a recombinant <i>Shigella flexneri</i> strain with the novel incorporation of the <i>eltb</i> gene for the heat-labile enterotoxin B (LTB) subunit of ETEC directly into <i>Shigella</i>'s genome, enhancing stability and consistent production. This approach combines the immunogenic potential of LTB with the antigen delivery properties of <i>S. flexneri</i> outer membrane vesicles (OMVs), aiming to provide cross-protection against both bacterial pathogens in a stable, non-replicating vaccine platform. We confirmed successful expression through GM1-capture ELISA, achieving levels comparable to ETEC. Additionally, proteomic analysis verified that the isolated vesicles from the recombinant strains contain the LTB protein and the main outer membrane proteins and virulence factors from <i>Shigella</i>, including OmpA, OmpC, IcsA, SepA, and Ipa proteins, and increased expression of Slp and OmpX. Thus, our newly designed <i>S. flexneri</i> OMVs, engineered to carry ETEC's LTB toxin, represent a promising strategy to be considered as a subunit vaccine candidate against <i>S. flexneri</i> and ETEC.