Construction and protective evaluation of a recombinant attenuated <i>Salmonella</i> vaccine delivering <i>Mycoplasma synoviae</i> antigens

<i>Mycoplasma synoviae</i> (<i>M. synoviae</i>) is a highly detrimental avian pathogen that has resulted in substantial economic losses within the global poultry industry. The existing vaccines, presently in use, fail to offer comprehensive protection. The objective of this study was to create recombinant attenuated <i>Salmonella</i> vaccine (RASV) strains, namely rSC0130(pS-rRS01790), rSC0130(pS-rBMP), rSC0130(pS-rGrpE), rSC0130(pS-rRS00900), and rSC0130(pS-rRS00275), expressing RS01790, BMP, GrpE, RS00900, and RS00275 proteins, and verify their protective effects. Assessed candidate vaccines for their growth characteristics, plasmid stability, and capability to synthesize exogenous antigens. The results demonstrated that the expression of exogenous antigens had no detrimental impact on the growth performance of the candidate vaccine strain. Additionally, the plasmid remained stable and continued to express through the 50th generation, confirming its long-term stability within the strain. Furthermore, all five antigens were successfully synthesized within the candidate vaccines. The immune protection results demonstrated that candidate vaccine immunization induced specific humoral, mucosal, and cellular immune responses. The growth performance of the rSC0130(pS-rRS01790), rSC0130(pS-rBMP), and rSC0130(pS-rGrpE) immunized groups exhibited significant recovery, and the air sac lesion scores were significantly lower than those of the challenge control group and the rSC0130(pS0018) control group. All candidate vaccines effectively reduced the colonization of <i>M. synoviae</i> in the pharynx and mitigated damage to the tracheal mucosa. These results underscore the potential of the RASV carrying the <i>M. synoviae</i> antigen as a promising approach for the development of vaccines against <i>M. synoviae</i> and suggest that RASV could serve as a novel strategy for prevention and control of <i>M. synoviae</i> infections.