Increasing the immunogenicity of a salmon louse immune dampening protein for DNA vaccine application in Atlantic salmon.

Salmon lice (Lepeophtheirus salmonis) cause major welfare issues and economic losses in salmon aquaculture, yet an effective vaccine remains elusive. DNA vaccines are promising but often elicit weak antibody responses in fish. Here, we evaluated different DNA vaccine designs aiming at increasing the immunogenicity of salmon louse immune-modulating labial gland protein 3 (LGP3), including tetrameric tandem repeats to increase the number of epitopes, and fusion or co-delivery with Moritella viscosa flagellin as a molecular adjuvant. Vaccine mRNA expression was detected at the injection site from 3 days until 8 weeks post vaccination (wpv), showing a varied expression among individuals with around 50-80 % of fish expressing antigen transcripts at 8 wpv. Flagellin-encoding constructs induced local inflammation at injection sites and an early upregulation of innate immune genes including tlr5s. Fusion of flagellin to monomeric LGP3 was, however, necessary to induce higher levels of antigen-specific Ig in plasma, whereas co-delivery on separate plasmids did not induce adequate antibody responses. Constructs encoding a tetrameric LGP3 tandem repeat also led to increased antibody levels; co-injection, and especially fusion with the flagellin adjuvant, further amplified this response. Both modifications result in lower but detectable levels of specific antibodies in mucus, showing some correlation to the systemic antibody level in plasma. Increasing the number of epitopes by tetramerization and incorporating flagellin through chimeric constructs, enhanced the immunogenicity of LGP3 in salmon. The results suggest that the applied principles have a broad application for improving DNA vaccine strategies in aquaculture, independent of the specific pathogen targeted.