Immunogenicity and protective effects of inactivated Singapore grouper iridovirus (SGIV) vaccines in orange-spotted grouper, Epinephelus coioides.

Vaccination is one of the best methods against viral diseases. In this study, experimental inactivated Singapore grouper iridovirus (SGIV) vaccines were prepared, and immunogenicity and protection against virus infection of the vaccines were investigated in orange-spotted grouper, Epinephelus coioides. Two kinds of vaccines, including ß-propiolactone (BPL) inactivated virus at 4°C for 12 h and formalin inactivated virus at 4°C for 12 d, was highly protective against the challenge at 30-day post-vaccination and produced relative percent of survival rates of 91.7% and 100%, respectively. These effective vaccinations induced potent innate immune responses mediated by pro-inflammatory cytokines and type I interferon (IFN)-stimulated genes (ISGs). It is noteworthy that ISGs, such as Mx and ISG15, were up-regulated only in the effective vaccine groups, which suggested that type I IFN system may be the functional basis of early anti-viral immunity. Moreover, effective vaccination also significantly up-regulated of the expression of MHC class I gene and produced substantial amount of specific serum antibody at 4 weeks post-vaccination. Taken together, our results clearly demonstrated that effective vaccination in grouper induced an early, nonspecific antiviral immunity, and later, a specific immune response involving both humoral and cell-mediated immunity.

Selection and identification of Singapore grouper iridovirus vaccine candidate antigens using bioinformatics and DNA vaccination.

In this study, we described a rapid and efficient method which integrated the bioinformatic prediction and DNA vaccine technology to identify vaccine candidates against Singapore grouper iridovirus (SGIV). The 162 previously defined open reading frames (ORFs) of SGIV were subjected to extensive sequence similarity searches, as well as motif, cellular location, and domain prediction. Based on our analysis, 13 genes were chosen and cloned into the eukaryotic expression vector pcDNA 3.1. In vitro and in vivo expression of these DNA vaccine constructs was examined in Epinephelus akaara spleen cells (EAGS) and immunized fish by Western blot and RT-PCR analysis, respectively. Three weeks after the second booster, immunized fish were challenged with SGIV and the level of protection and survival was assessed. Fish vaccinated with plasmid DNA encoding viral ORF072, ORF039 and ORF036 (designated as pcDNA-72, pcDNA-39 and pcDNA-36, respectively) exhibited 66.7%, 66.7% and 58.3% relative percent survival rates, respectively, in comparison with the control fish. These three DNA vaccines induced innate immune responses, raising significantly high level of Mx expression relative to the fish vaccinated with the empty plasmid at 3 days post-vaccination. Furthermore, recombinant protein from ORF072 was also used to immunize another set of fish and similar protective effect was obtained. Taken together, our results validated the applicability of bioinformatics in genome mining, resulting in the identification of three protective antigens. The promising results obtained in the present study have prompted further testing to improve the immunogenicity of these potential DNA vaccines.

Molecular cloning and characterization of two novel hepcidins from orange-spotted grouper, Epinephelus coioides.

Orange-spotted grouper, Epinephelus coioides is one of the most important economic species of marine-cultured fish in China and Southeast Asia countries. However, very little information of the innate immune mechanisms against microbial pathogens is available in grouper, Epinephelus sp. Hepcidin, as an antimicrobial peptide (AMP), is a very important component in the innate immune system and widespread in fish. In this study, two novel types of hepcidin gene (designated EC-hepcidin1 and EC-hepcidin2) were cloned from E. coioides. They consist of open reading frames (ORFs) of 267 bp and 263 bp encoding the putative peptides of 88 and 87 amino acids, respectively. The homologous identity of deduced amino acid sequences between EC-hepcidin1 and EC-hepcidin2 is up to 79%, and predicted mature regions of both them have four cysteines residues. Genomic DNAs of both EC-hepcidin1 and EC-hepcidin2 consist of three exons and two introns. RT-PCR results showed that EC-hepcidin1 transcript was most abundant in liver and less in stomach. However, the transcript of EC-hepcidin2 was only detected in liver. The expressions of both EC-hepcidins were up-regulated by microbial and viral challenges, and iron overload, respectively, and EC-hepcidin1 was more responsive. The growth of Gram-negative bacterium of Vibrio vulnificus and Gram-positive bacterium of Staphylococcus aureus was inhibited by synthetic EC-hepcidins, and EC-hepcidin1 displayed stronger antimicrobial activity. The replication of Singapore grouper iridovirus (SGIV) was inhibited in the EC-hepcidin1 and EC-hepcidin2 over-expressed stable transfected fish cell lines (GS/pcDNA-Hep1, GS/pcDNA-Hep2) indicative of the antiviral activity of EC-hepcidins. These data should offer important information on the antimicrobial and antiviral roles of EC-hepcidins, and will be help to the better understanding of molecular mechanisms of grouper innate immunity.

Identification and characterization of Singapore grouper iridovirus (SGIV) ORF162L, an immediate-early gene involved in cell growth control and viral replication.

Singapore grouper iridovirus (SGIV) is a major pathogen resulting in heavy economic losses to grouper aquaculture. In this study, SGIV ORF162L encoding a putative homolog of ICP46 was identified and characterized. Interestingly, ICP46 could be found in all sequenced iridoviruses and is considered as a core gene of the family Iridoviridae. SGIV ICP46 was classified as an immediate-early (IE) gene during in vitro infection using drug inhibition analysis, reverse transcription polymerase chain reaction and Western blot analysis. Subcellular localization revealed that SGIV ICP46 was distributed predominantly in the cytoplasm. Furthermore, SGIV ICP46 proved to be a structural protein of the nucleocapsid; its overexpression could promote the growth of grouper embryonic cells and contribute to SGIV replication. This is the first report of the characterization of a putative ICP46 homolog and these results should offer important insights into the pathogenesis of iridoviruses.