Modulatory Effect of Inducing Iridovirus DNA Vaccine Combining Different CpG ODN Adjuvant in the Systemic Immune Mechanism Underlying the Protection Against Grouper Iridovirus of Asian Seabass (Lates calcarifer)

Grouper iridovirus (GIV) is a major causative agent of disease in Asian seabass (Lates calcarifer), often leading to high mortality. However, information on vaccine development against GIV infection remains limited. This study investigated the potential of DNA vaccines combined with unmethylated cytosine-phosphate-guanine oligodeoxynucleotides (CpG ODNs) to enhance the immune response of Asian seabass against GIV. A DNA vaccine was constructed by inserting the cloned GIV major capsid protein (MCP) gene into the pCDNA3.1 plasmid, which was subsequently introduced into Escherichia coli DH5α for propagation. Juvenile Asian seabass were intramuscularly injected with four treatments: PBS sterile (Control), DNA vaccine alone (V1), DNA vaccine combined with 20 nM CpG ODN (V2), and DNA vaccine combined with 2 µM CpG ODN (V3). Results showed that groups V1 and V3 exhibited strong protective effects against GIV infection. Both groups significantly enhanced the expression of several innate and adaptive immune-related genes compared to the control group. A separate challenge experiment further confirmed that vaccinated fish in the V1 and V3 groups had markedly higher survival rates than the control group. Notably, the V3 vaccine demonstrated superior efficacy, enhancing innate immunity, adaptive responses, and survival without any negative impact on growth performance. This study represents the first successful report of a DNA vaccine combined with CpG ODN adjuvant against GIV infection in Asian seabass. These findings highlight the potential of CpG-adjuvanted DNA vaccines as a promising strategy for improving disease resistance in aquaculture.