Lactobacillus casei displaying MCP2α and FlaC delivered by PLA microspheres effectively enhances the immune protection of largemouth bass (Micropterus salmoides) against LMBV infection.

Largemouth bass ranavirus (LMBV) seriously affects the development of largemouth bass (Micropterus salmoides) industry and causes huge economic losses. Oral vaccine can be a promising method for viral disease precaution. In this study, MCP2α was identified as a valuable epitope region superior to MCP and MCP2 of LMBV by neutralizing antibody experiments. Then, recombinant Lactobacillus casei expressing the fusion protein MCP2αC (MCP2α as antigen, C represents flagellin C from Aeromonas hydrophila as adjuvant) on surface was constructed and verified. Further, PLA microsphere vaccine loading recombinant MCP2αC L. casei was prepared. The PLA microspheres vaccine were observed by scanning electron microscopy and showed a smooth, regular spherical surface with a particle size distribution between 100 and 200 μm. Furthermore, we evaluated the tolerance of PLA-MCP2αC vaccine in simulated gastric fluid and simulated intestinal fluid, and the results showed that PLA-MCP2αC can effectively resist the gastrointestinal environment. Moreover, the protective effect of PLA-MCP2αC against LMBV was evaluated after oral immunization and LMBV challenge. The results showed that PLA-MCP2αC effectively up-regulated the activity of serum biochemical enzymes (T-SOD, T-AOC, LZM, complement C3) and induced the mRNA expression of representative immune genes (IL-1β, TNF-α, IFN-γ, MHC-IIα, Mx, IgM) in spleen and head kidney tissues. The survival rate of largemouth bass vaccinated with PLA-MCP2αC increased from 24 % to 68 %. Meanwhile, PLA-MCP2αC inhibited the LMBV burden in spleen, head kidney and liver tissues and attenuated tissue damage in spleen. These results suggested that PLA-MCP2αC can be used as a candidate oral vaccine against LMBV infection in aquaculture.