Immunological responses of Penaeus monodon to DNA vaccine and its efficacy to protect shrimp against white spot syndrome virus (WSSV)

White spot disease is an important viral disease caused by white spot syndrome virus (WSSV) and is responsible for huge economic losses in the shrimp culture industry worldwide. The VP28 gene encoding the most dominant envelope protein of WSSV was used to construct a DNA vaccine. The VP28 gene was cloned in the eukaryotic expression vector pcDNA3.1 and the construct was named as pVP28. The protective efficiency of pVP28 against WSSV was evaluated in Penaeus monodon by intramuscular challenge. In vitro expression of VP28 gene was confirmed in sea bass kidney cell line (SISK) by fluorescence microscopy before administering to shrimp. The distribution of injected pVP28 in different tissues of shrimp was studied and the results revealed the presence of pVP28 in gill, head soft tissue, abdominal muscle, hemolymph, pleopods, hepatopancreas and gut. RT-PCR and fluorescence microscopy analyses showed the expression of pVP28 in all these tissues examined. The results of vaccination trials showed a significantly higher survival rate in shrimp vaccinated with pVP28 (56.6-90%) when compared to control groups (100% mortality). The immunological parameters analyzed in the vaccinated and control groups revealed that the vaccinated shrimp showed significantly high level of prophenoloxidase and superoxide dismutase (SOD) when compared to the control groups. The high levels of prophenoloxidase and superoxide dismutase (SOD) might be responsible for developing resistance against WSSV in DNA vaccinated shrimp.