Development of a novel live attenuated vaccine based on epitope-fused OmpU1-LptDEP antigen for vibriosis control in aquaculture.

The global expansion of aquaculture has been accompanied by escalating bacterial disease outbreaks, particularly vibriosis, leading to substantial economic losses. To address the limitations of conventional vaccines, including cumbersome administration and suboptimal efficacy, we developed a novel live attenuated vaccine against Vibrio pathogens using an epitope fusion strategy. By recombinant fusion of the epitope peptides of LptD (LptDEP) with a truncated OmpU (OmpU1) and using the attenuated V. parahaemolyticus VP3 strain (deficient in haemolysin and type III secretion system T3SS1/2) as a delivery vector, two candidate vaccines, VP5p (plasmid-based expression) and VP6 (chromosomal integration), were constructed. Both candidate vaccines exhibited excellent genetic stability and safety. Immunological evaluations revealed that a single immersion immunization effectively induced both innate and adaptive immune responses, with significant upregulation of mhc1α, il1b, cd4-1, tnfa, tgfb and igm in spleen and liver tissues, along with increased serum acid phosphatase (ACP) and alkaline phosphatase (AKP) activities. Challenge trials with wild-type V. parahaemolyticus demonstrated strong protection, with VP5p and VP6 achieving relative percent survival (RPS) values of 75 % and 60 %, respectively (compared to 45 % for parental VP3 controls), alongside a 1-2 log reduction in bacterial load. This study establishes a proof-of-concept for epitope fusion live vaccines that combine enhanced immunoprotection with genetic stability, offering a promising paradigm for sustainable vibriosis control in commercial aquaculture.