Screening key protective antigens of largemouth bass virus and evaluation of their immunoprotective efficacy.

Precise identification of protective antigenic epitopes is critical not only for eliciting robust and specific immune responses but also for providing a vital foundation for vaccine development and monoclonal antibody engineering. In this study, we cloned six constructs, five truncated major capsid protein (MCP) of largemouth bass virus (LMBV) fragments (MCP-1, MCP-2, MCP-3, MCP-4, MCP-1&3) and the full-length MCP, into pET-32a expressed them in a prokaryotic system, and evaluated their antigenicity. Furthermore, the antigen protein was administered to fish in a four-week immunization trial. This study demonstrated that MCP-3 markedly enhanced both innate and adaptive immune responses. We also observed a pronounced expansion of IgM+ and IgT+ B cell populations. Notably, MCP-3 enhanced the titers of both serum and mucosal antigen-specific IgM and conferred the strongest neutralizing activity against LMBV. Challenge experiments further confirmed that the MCP-3 group achieved the highest relative percent survival (RPS = 88.89 %) and the lowest post-infection tissue viral loads. In summary, these results indicate that we have identified MCP-3 as a key protective antigen of LMBV, which may serve as a promising target for vaccine development to prevent and control LMBV infection.