An LNP-encapsulated mRNA vaccine targeting the major capsid protein of largemouth bass virus confers protective immunity and reduces RIG-I pathway activation in Micropterus salmoides

Largemouth bass virus (LMBV) Infection poses a significant threat to largemouth bass farming in China, yet effective and safe vaccines remain scarce. In this study, we designed and evaluated four mRNA vaccine candidates against LMBV, all encoding the viral major capsid protein (MCP). The fully configured construct (LMUM) contained a 5' cap1, untranslated regions (UTRs), and a poly(A) tail, and was produced by in vitro transcription. Each mRNA was encapsulated into lipid nanoparticles (LNPs), yielding homogeneous particles (∼110 nm diameter, ∼54 mV zeta potential) with >96% encapsulation efficiency that provided substantial protection against RNase degradation (43% of mRNA remained intact in LNP-encapsulated groups versus 10% in unprotected controls). Intramuscular immunization of largemouth bass with LNP-encapsulated LMUM (0.2 μg/g body weight) induced MCP-specific serum antibodies after primary immunization, with a strong booster response after the second dose. In the LMUM-immunized group, spleen mRNA levels of IgM and CD4 were upregulated 3.4-fold and 3.2-fold, respectively, by day 14, while the groups lacking UTR or cap/poly(A) tail failed to induce these adaptive immune markers. Interestingly, the incomplete constructs triggered stronger activation of the RIG-I pathway (RIG-I, MAVS, TBK1) than the fully modified LMUM, suggesting that the combination of cap1, UTR, and poly(A) tail helps dampen excessive innate sensing. Following a lethal LMBV challenge, the LMUM vaccine conferred 45-50% relative survival after primary immunization, which increased to 53-57% after the booster dose. In contrast, the incomplete vaccine constructs and naked mRNA gave only 10-15% survival, and control groups experienced nearly complete mortality. These results demonstrate that a fully modified mRNA-LNP vaccine against LMBV induces robust humoral and cellular immune responses and provides significant protective efficacy in largemouth bass.