Development of a High-Hydrostatic-Pressure-Treated Recombinant Vaccine Targeting the Major Capsid Protein of Red Sea Bream Iridovirus

Red sea bream (<i>Pagrus major</i>) aquaculture represents one of the most economically important marine aquaculture industries in Japan and East Asia. However, viral diseases, particularly those caused by red sea bream iridovirus (RSIV), pose a serious threat to aquaculture production in this region. In this study, we applied high-hydrostatic-pressure (HHP) refolding technology to develop a recombinant vaccine targeting the RSIV major capsid protein (MCP). The recombinant MCP (RSIV-rMCP) expressed in <i>Escherichia coli</i> was insoluble; however, HHP treatment under alkaline (pH 10) conditions in the presence of arginine successfully solubilised the protein while preserving its structural integrity. The solubilised protein (HHP-RSIV-rMCP) induced strong RSIV-specific IgM responses and enhanced disease resistance in red sea bream. In contrast, sera from fish immunised with a commercial formalin-inactivated vaccine exhibited minimal reactivity to HHP-RSIV-rMCP but reacted significantly to formalin-treated HHP-RSIV-rMCP. These results indicate that the HHP-RSIV-rMCP vaccine induces conformation-specific IgM antibodies and that structural preservation is crucial for maintaining antigenicity. Collectively, our findings demonstrate that HHP refolding technology is an effective strategy for preparing structurally preserved antigens.