Efficacy Assessment of Montanide <scp>GEL</scp> 02 <scp>PR</scp> and Liposome‐Based Encapsulation as Adjuvants for an <scp>FrpA</scp> Subunit Vaccine Against Photobacteriosis in <scp> <i>Solea senegalensis</i> </scp>

Photobacteriosis is a major threat to marine aquaculture, and the siderophore transporter FrpA represents a promising target for effective subunit vaccine development. In this study, we evaluated Montanide GEL 02 PR and phosphatidylcholine-based liposomes as aqueous and safer alternatives to Freund's adjuvant for an FrpA-based subunit vaccine in Solea senegalensis. Fish were immunized with recombinant FrpA formulated with either adjuvant, and induction of FrpA-specific IgM antibody levels was measured. Vaccine efficacy was subsequently assessed by analysis of innate immune gene expression following early pathogen exposure and by survival analysis after infection challenge. Vaccination with rFrpA-Montanide GEL 02 PR induced the highest levels of specific IgM antibodies and conferred strong protection against photobacteriosis, achieving survival rates comparable to those reported for bacterin-based vaccines. This formulation also promoted a rapid and coordinated up-regulation of immune-related genes, including transferrin (tf), lysozyme C1 (lysC1), and complement components C1q and C7. In contrast, rFrpA encapsulated in liposomes elicited weaker immune responses and limited protection. Overall, our results demonstrate that an FrpA-based subunit vaccine formulated with Montanide GEL 02 PR represents a promising strategy for controlling photobacteriosis in aquaculture.