Recombinant Lactococcus lactis surface-displaying the major capsid protein of sea perch iridovirus confers effective immune protection in Lateolabrax japonicus.

Sea perch iridovirus (SPIV) causes severe mortality in aquaculture. Lactococcus lactis M48, a strain isolated from the intestine of sea perch, exhibits tolerance to acidic conditions, bile salts, and hyperosmotic stress, along with excellent intestinal colonization capacity and high safety. In this study, a recombinant strain surface-displaying the SPIV major capsid protein (MCP) was constructed based on L. lactis M48, designated RE SPIV MCP M48. Fluorescence microscopy and Western blot analyses confirmed that L. lactis M48 successfully expressed the pgsA-anchored MCP-eGFP fusion protein. Oral immunization with RE SPIV MCP M48 significantly upregulated the expression of immune-related genes (IgM, MHC II, IL-1β, and IL-8) and induced the production of specific anti-SPIV MCP serum antibodies. Following SPIV challenge, the survival rate of the control group was only 40%, whereas the RE SPIV MCP M48 group achieved a survival rate of 90% with a relative percent survival (RPS) of 83.3%. Oral immunization with the recombinant strain attenuated histopathological damage of sea perch during SPIV infection and notably suppressed viral replication in the liver, spleen, and head kidney. These results demonstrate that recombinant L. lactis surface-displaying SPIV MCP can effectively induce protective immunity against SPIV infection, representing a promising oral vaccine candidate for sea perch aquaculture.