Surface-displayed MCP-3 vaccine in Bacillus subtilis elicits protective immune responses against LMBV infection in largemouth bass.

Largemouth bass virus (LMBV) is a major pathogen threatening the aquaculture of largemouth bass, causing significant economic losses. Oral vaccination offers a practical and efficient strategy for disease prevention in farmed fish. In this study, a fusion CotC-MCP-3 gene was introduced into Bacillus subtilis 168 (Bs168) to enable surface display of the MCP-3 protein on bacterial spores through the anchoring protein CotC, thereby generating a candidate oral vaccine. At 28 days post-vaccination (dpv), qPCR analysis revealed pronounced upregulation of associated innate and adaptive immunity genes in the gut and head kidney of largemouth bass immunized with the MCP-3 vaccine. Vaccination markedly increased the abundance of IgM+ B cells in both the gut and head kidney, as well as total IgM levels in serum and gut mucus. Concomitantly, LMBV-specific IgM titers and neutralizing activities in both serum and gut mucus were significantly enhanced. Following the experimental LMBV challenge, fish in the MCP-3 vaccine group achieved a relative percent survival (RPS) of 68.7 %, accompanied by substantially lower viral loads in the head kidney, gut, and spleen, as well as notably attenuated histopathological lesions compared with the other groups. The Bs168-based MCP-3 oral vaccine provides strong protection against LMBV infection in largemouth bass, as demonstrated by these results, highlighting its potential as a promising strategy for managing viral diseases in aquaculture.