Formulation and Immunological Assessment of <scp>DNA</scp> Vaccines Utilising In Vivo–Induced Antigens Against <scp> <i>Nocardia seriolae</i> </scp> in Hybrid Snakehead ( <scp> <i>Channa maculata</i> </scp> ♀ ×  <scp> <i>Channa argus</i> </scp> ♂)

Nocardia seriolae is the primary cause of fish nocardiosis, and it is urgent to develop an effective vaccine for controlling N. seriolae infection in aquaculture. In our previous study, two potential antigenic proteins, glutamate-cysteine ligase EgtA (EgtA) and hypothetical protein 6747 (hp6747), were determined via the in vivo-induced antigen technique (IVIAT). A DNA vaccine was aimed to be developed using these two in vivo-induced antigens as candidates for ligation with the pcDNA3.1-myc-his-A plasmid, and their immunological effects were evaluated in hybrid snakehead (Channa maculata ♀ × Channa argus ♂). Multiple epitopes of the EgtA and hp6747 proteins from N. seriolae were predicted on both T cells and B cells, and the transcription of the EgtA and hp6747 genes was found in the spleen, trunk kidney, muscle and liver in the fish after immunisation with pcDNA-EgtA and pcDNA-hp6747 DNA vaccines, respectively. Notably, the serum specific antibody (IgM) titres, as well as LYZ, ACP, AKP and SOD activities, were elevated in the vaccinated fish. Meanwhile, quantitative real-time PCR analysis demonstrated that the expression levels of several immune-related genes (TNFα, CD4, CD8α, IL-1β, MHCIIα and MHCIα) were significantly elevated across the aforementioned tissues in the vaccinated fish. Furthermore, they could provide the relative percent survival (RPS) at 61.04% and 43.73% against artificial challenge with N. seriolae in the fish after immunisation with pcDNA-EgtA and pcDNA-hp6747 DNA vaccines, respectively. Taken together, using these two in vivo-induced antigens of EgtA and hp6747 for vaccine development was the ideal strategy for controlling fish nocardiosis in aquaculture.