A Lactococcus lactis-vectored oral vaccine induces protective immunity of mice against enterotoxigenic Escherichia coli lethal challenge

•A Lactococcus lactis co-expressing the fusion antigen STa-LAB-STb-F5 of ETEC and the M cell-targeting ligand OmpH was genetically engineered.•Oral vaccination with recombinant L. lactis-SLS-F5-OmpH induced antigen-specific mucosal, humoral, and cellular immunity.•Recombinant L. lactis-SLS-F5-OmpH can elicit effective immune protection against ETEC in mice model.•Recombinant L. lactis-SLS-F5-OmpH might be a promising candidate for the development of an oral vaccine against ETEC. Enterotoxigenic Escherichia coli (ETEC) is a global primary pathogenic bacterium causing diarrhoea in human and a wide variety of neonatal animals. Lactococcus lactis as non-pathogenic and food-grade lactic acid bacteria has already been explored as a vector for mucosal vaccine. Here, the current study was undertaken to evaluate the live recombinant L. lactis (rL. lactis) vaccine expressing the trivalent enterotoxin protein STa-LTB-STb and the F5 fimbrial antigen (SLS-F5) with OmpH of Yersinia enterocolitica in protection against ETEC. Western blot confirmed the expression of fusion protein SLS-F5-OmpH in nisin-controlled expression (NICE) system. Mice orally immunized with rL. lactis-SLS-F5-OmpH were observed to produce high levels of mucosal SIgA and serum IgG antibodies, while also inducing increases in the production of CD4+ and CD8+ T cells, lymphocyte proliferation, and secretion of cytokines. Moreover, orally immunized mice produced complete protection after ETEC challenge. The above results suggested that rL. lactis-SLS-F5-OmpH has the potential as a candidate for oral vaccine against ETEC.