Protective immunity induced by DNA vaccine encoding viral membrane protein against SGIV infection in grouper

Singapore grouper iridovirus (SGIV) is the main grouper-infecting virus in southern China that causes serious economic losses. However, there is no effective way to control this viral disease. In this study, SGIV ORF19R (SGIV-19R) encoding a viral membrane protein was constructed into pcDNA3.1-HA and then was used to evaluate the immune protective effects in grouper Epinephelus coioides. Subcellular localization showed that SGIV-19R distributed in the cytoplasm and co-localization analysis indicated the protein partially co-localized with the endoplasmic reticulum (ER). RT-PCR and Western blot analyses confirmed the expression of the vaccine plasmids in grouper muscle tissues. Moreover, the transcription levels of tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β), myxovirus resistance 1 (Mx1) and immunoglobulin M (IgM) genes were significantly up-regulated in the spleen, liver and kidney of vaccinated groupers. SGIV challenge experiments showed the relative percent survival (RPS) was significantly enhanced in fish with 49.9% at the DNA dose of 45 μg pcDNA3.1-19R, while 75.0% RPS when using 90 μg pcDNA3.1-19R. Meanwhile, vaccination with pcDNA3.1-19R significantly reduced the virus replication, evidenced by a low viral load in the spleen of survivals groupers after SGIV challenge. These results imply that pcDNA3.1-19R could induce protective immunity in grouper, and might be a potential vaccine candidate for controlling SGIV disease.