The Inactivated ISKNV-I Vaccine Confers Highly Effective Cross-Protection against Epidemic RSIV-I and RSIV-II from Cultured Spotted Sea Bass Lateolabrax maculatus.

The genus Megalocytivirus of the family Iridoviridae is composed of two distinct species, namely, infectious spleen and kidney necrosis virus (ISKNV) and scale drop disease virus (SDDV), and both are important causative agents in a variety of bony fish worldwide. Of them, the ISKNV species is subdivided into three genotypes, namely, red seabream iridovirus (RSIV), ISKNV, and turbot reddish body iridovirus (TRBIV), and a further six subgenotypes, RSIV-I, RSIV-II, ISKNV-I, ISKNV-II, TRBIV-I, and TRBIV-II. Commercial vaccines derived from RSIV-I , RSIV-II and ISKNV-I have been available to several fish species. However, studies regarding the cross-protection effect among different genotype or subgenotype isolates have not been fully elucidated. In this study, RSIV-I and RSIV-II were demonstrated as the causative agents in cultured spotted seabass, Lateolabrax maculatus, through serial robust evidence, including cell culture-based viral isolation, whole-genome determination and phylogeny analysis, artificial challenge, histopathology, immunohistochemistry, and immunofluorescence as well as transmission electron microscope observation. Thereafter, a formalin-killed cell (FKC) vaccine generated from an ISKNV-I isolate was prepared to evaluate the protective effects against two spotted seabass original RSIV-I and RSIV-II. The result showed that the ISKNV-I-based FKC vaccine conferred almost complete cross-protection against RSIV-I and RSIV-II as well as ISKNV-I itself. No serotype difference was observed among RSIV-I, RSIV-II, and ISKNV-I. Additionally, the mandarin fish Siniperca chuatsi is proposed as an ideal infection and vaccination fish species for the study of various megalocytiviral isolates. IMPORTANCE Red seabream iridovirus (RSIV) infects a wide mariculture bony fish and has resulted in significant annual economic loss worldwide. Previous studies showed that the phenotypic diversity of infectious RSIV isolates would lead to different virulence characteristics, viral antigenicity, and vaccine efficacy as well as host range. Importantly, it is still doubted whether a universal vaccine could confer the same highly protective effect against various genotypic isolates. Our study here presented enough experimental evidence that a water in oil (w/o) formation of inactivated ISKNV-I vaccine could confer almost complete protection against RSIV-I and RSIV-II as well as ISKNV-I itself. Our study provides valuable data for better understanding the differential infection and immunity among different genotypes of ISKNV and RSIV isolates in the genus Megalocytivirus.

Establishment and characterization of a liver cell line, ALL, derived from yellowfin sea bream, Acanthopagrus latus, and its application to fish virology.

Yellowfin sea bream (Acanthopagrus latus) is an important economic fish, which is seriously threatened by various fish viruses. In this study, a cell line designated as ALL derived from the liver of yellowfin sea bream was developed and characterized. The cell line grew well in Dulbecco's modified Eagle's medium containing 10%-20% foetal bovine serum at 28°C. Amplification of the cytochrome B gene indicated that ALL cells originated from yellowfin sea bream. The modal chromosome number of ALL cells was 48. ALL cells were efficiently transfected with pEGFP-N3 plasmids, indicating the potential application of ALL cells in exogenous gene manipulation studies. ALL cells were susceptive to three main fish viruses, including viral haemorrhagic septicaemia virus (VHSV), red-spotted grouper nervous necrosis virus (RGNNV) and largemouth bass virus (LMBV). The replication of VHSV, RGNNV and LMBV in ALL cells was confirmed by quantitative real-time polymerase chain reaction, virus titre and transmission electron microscopy assays. Moreover, ALL cells could respond to VHSV, RGNNV and LMBV infections, as indicated by the differential expression of antiviral genes involving in the innate immune response. In conclusion, the newly established ALL cell line will be an excellent in vitro platform for the study of the virus-yellowfin sea bream interaction.

α-Lipoic Acid Exerts Its Antiviral Effect against Viral Hemorrhagic Septicemia Virus (VHSV) by Promoting Upregulation of Antiviral Genes and Suppressing VHSV-Induced Oxidative Stress.

Viral hemorrhagic septicemia virus (VHSV), belonging to the genus Novirhabdovirus, Rhabdoviridae family, is a causative agent of high mortality in fish and has caused significant losses to the aquaculture industry. Currently, no effective vaccines, Food and Drug Administration-approved inhibitors, or other therapeutic intervention options are available against VHSV. α-Lipoic Acid (LA), a potent antioxidant, has been proposed to have antiviral effects against different viruses. In this study, LA (CC50 = 472.6 µmol/L) was repurposed to exhibit antiviral activity against VHSV. In fathead minnow cells, LA significantly increased the cell viability post-VHSV infection (EC50 = 42.7 µmol/L), and exerted a dose-dependent inhibitory effect on VHSV induced-plaque, cytopathic effects, and VHSV glycoprotein expression. The time-of-addition assay suggested that the antiviral activity of LA occurred at viral replication stage. Survival assay revealed that LA could significantly upregulated the survival rate of VHSV-infected largemouth bass in both co-injection (38.095% vs. 1.887%, P < 0.01) and post-injection manner (38.813% vs. 8.696%, P < 0.01) compared with the control group. Additional comparative transcriptome and qRT-PCR analysis revealed LA treatment upregulated the expression of several antiviral genes, such as IRF7, Viperin, and ISG15. Moreover, LA treatment reduced VHSV-induced reactive oxygen species production in addition to Nrf2 and SOD1 expression. Taken together, these data demonstrated that LA suppressed VHSV replication by inducing antiviral genes expression and reducing VHSV-induced oxidative stress. These results suggest a new direction in the development of potential antiviral candidate drugs against VHSV infection.