Virions proteins of an RSIV-type megalocytivirus from spotted knifejaw Oplegnathus punctatus (SKIV-ZJ07)

Production and characterization of monoclonal antibodies against mandarinfish ranavirus and first identification of pyloric caecum as the major target tissue

Mandarinfish ranavirus (MRV), also known as a variant of largemouth bass virus (LMBV), is an emerging pathogen in mandarinfish aquaculture. In this study, monoclonal antibodies (mAbs) against MRV were produced and characterized, and 7 mAbs were obtained through Western blotting screening and all 7 mAbs specifically recognized MRV/LMBV but not several piscine iridoviruses as ISKNV, GIV and TFV. By LC MS/MS analysis, the recognized viral proteins by seven mAbs were identified as MRV-pORF47L, MRV-pORF55R, MRV-pORF57L, MRV-pORF77L and MRV-pORF78L, respectively, and all five viral proteins are late expression structural proteins by Western blotting. Based on mAb 1C4, immuno-histochemistry and immuno-histo-fluorescence were performed to re-assess the tissue tropism of MRV. The result showed that abundant reactive signals were observed in infected spleen, kidney as well as intestine and pyloric caecum. Real-time quantitative PCR also demonstrated that spleen as well as pyloric caecum and intestines are the major target tissue upon MRV infection. In infected intestines and pyloric caecum, numerous enlarged, multinucleated cells with intracytoplasmic inclusions were identified as the target cells of MRV, suggesting that MRV serves as a digestive tract pathogen to mandarinfish, which may explain why acute infection of MRV can cause the typical clinicopathology featured by severe ascites.

Isolation, Characterization, and Transcriptome Analysis of an ISKNV-Like Virus from Largemouth Bass

Largemouth bass (Micropterus salmoides) is an important commercial fish farmed in China. Challenges related to diseases caused by pathogens, such as iridovirus, have become increasingly serious. In 2017, we detected iridovirus-infected diseased largemouth bass in Zunyi, Guizhou Province. The isolated virus was identified as an infectious spleen and kidney necrosis virus (ISKNV)-like virus (ISKNV-ZY). ISKNV-ZY induces a cytopathic effect after infecting mandarin fish brain (MFB) cells. Abundant hexagonal virus particles were observed in the cytoplasm of ISKNV-ZY-infected MFB cells, using electron microscopy. The whole genome of ISKNV-ZY contained 112,248 bp and 122 open reading frames. Phylogenetic tree analysis showed that ISKNV-ZY was most closely related to BCIV, indicating that it is an ISKNV-like megalocytivirus. ISKNV-ZY-infected largemouth bass started to die on day six and reached a death peak on days 7-8. Cumulative mortality reached 100% on day 10. Using RNA sequencing-based transcriptome analysis after ISKNV-ZY infection, 6254 differentially expressed unigenes (DEGs) were identified, of which 3518 were upregulated and 2673 downregulated. The DEGs were associated with endocytosis, thermogenesis, oxidative phosphorylation, the JAK-STAT signaling pathway, the MAPK signaling pathway, etc. These results contribute to understanding the molecular regulation mechanism of ISKNV infection and provide a basis for ISKNV prevention.

Analysis of the transcriptomic profiles of Mandarin fish (Siniperca chuatsi) infected with red sea bream iridovirus (RSIV)

Red sea bream iridovirus (RSIV) belongs to the family Iridoviridae, genus Megalocytivirus, which could widely infect marine fish, causing diseases and huge economic losses. Now it has been reported that RSIV was also detected in diseased mandarin fish. Transmission electron microscopy and immunohistochemistry showed that spleen was the main target organ in mandarin fish infected with RSIV. To investigate the immune response mechanism of mandarin fish to RSIV infection, transcriptomics of RSIV-infected mandarin fish was analyzed. A total of 53,040 unigenes were obtained, and there were 21,576 and 17,904 unigenes had significant hit the Nr and SwissProt databases, respectively. In RSIV-infected and non-infected spleen tissues, there were 309 differentially expressed genes (DEGs), including 100 up-regulated genes and 209 down-regulated genes. Gene Ontology database (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways analysis were performed to reveal the function information and give a better understanding of the signal transduction pathways of DEGs. Further analysis of the cytokine-cytokine receptor interactions pathway exhibited that the expression of cytokines was widely activated after viral infection. In addition, ten DEGs were randomly selected and verified by quantitative real-time PCR, which revealed a similar expression tendency as the high-throughput sequencing data. These findings present valuable information that will benefit for better understanding of RSIV infection in mandarin fish.

Inactivated Whole Vaccine Inhibits Lethal Vibrio harveyi Infection in Oplegnathus punctatus

Aquaculture plays a key role in food production globally and provides a valuable source of protein and nutrition, addressing a worldwide growing demand. Oplegnathus punctatus (spotted knifejaw) is an economically important fish species with a high market value and demand. Previous studies on O. punctatus focused mainly on gonadal development, chromosomal microstructure, selective breeding, characterization of immune genes, and viral diseases. There is no published scientific research regarding vibriosis in this fish species. In this study, two potential pathogenic bacteria, Vibrio harveyi and Enterococcus gallinarum, were isolated from moribund cultured O. punctatus. The sequence of the universal 16S rDNA gene was used to identify potential pathogenic bacteria isolated from the moribund O. punctatus, and morphological assessments and API20E tests of the bacterial isolates were conducted to verify the identity and biochemical characteristics of the isolates. Injection of E. gallinarum did not lead to mortality in O. punctatus during the 21 days of observation. In contrast, fish died overnight when challenged with V. harveyi at 1.25 × 105 CFU/g body weight, suggesting that the cause of death of the cultured O. punctatus was V. harveyi infection. Antimicrobial sensitivity analyses revealed that the V. harveyi strain NTOU is sensitive to flumequine, doxycycline, oxolinic acid, and amoxycillin. Importantly, we demonstrated for the first time that intraperitoneal administration of an inactivated V. harveyi whole-cell vaccine resulted in a high level of protection against V. harveyi infection in O. punctatus.

Characterization of Ictalurid herpesvirus 1 Glycoprotein ORF59 and Its Potential Role on Virus Entry into the Host Cells

The channel catfish virus (CCV, Ictalurid herpesvirus 1) has caused sustained economic losses in the fish industry because of its strong infectivity and pathogenicity. Thus, it is necessary to determine the function of viral proteins in the CCV infection process. The present study aimed to characterize CCV glycoprotein ORF59 and explore its impact on virus infection in host cells. Firstly, its exclusive presence in the membrane fraction of the cell lysate and subcellular localization verified that CCV ORF59 is a viral membrane protein expressed at late-stage infection. A protein blocking assay using purified His6 tagged ORF59, expressed in sf9 insect cells using a baculovirus expression system, indicated a dose-dependent inhibitory effect of recombinant ORF59 protein on virus invasion. Knockdown of the ORF59 using a short hairpin (shRNA) showed that ORF59 silencing decreased the production of infectious virus particles in channel catfish ovary cells. The results of this study suggest that recombinant ORF59 protein might inhibit CCV entry into the host cells. These findings will promote future studies of the key functions of glycoprotein ORF59 during CCV infection.

Scale Drop Disease Virus Associated Yellowfin Seabream (Acanthopagrus latus) Ascites Diseases, Zhuhai, Guangdong, Southern China: The First Description

Scale drop disease virus (SDDV), an emerging piscine iridovirus prevalent in farmed Asian seabass Lates calcarifer in Southeast Asia, was firstly scientifically descripted in Singapore in 2015. Here, an SDDV isolate ZH-06/20 was isolated by inoculating filtered ascites from diseased juvenile yellowfin seabream into MFF-1 cell. Advanced cytopathic effects were observed 6 days post-inoculation. A transmission electron microscopy examination confirmed that numerous virion particles, about 140 nm in diameter, were observed in infected MFF-1 cell. ZH-06/20 was further purified and both whole genome and virion proteome were determined. The results showed that ZH-06/20 was composed of 131,122 bp with 135 putative viral proteins and 113 of them were further detected by virion proteome. Western blot analysis showed that no (or weak) cross-reaction was observed among several major viral proteins between ZH-06/20 and ISKNV-like megalocytivirus. An artificial challenge showed that ZH-06/20 could cause 100% death to juvenile yellowfin seabream. A typical sign was characterized by severe ascites, but not scale drop, which was considerably different from SDD syndrome in Asian seabass. Collectively, SDDV was confirmed, for the first time, as the causative agent of ascites diseases in farmed yellowfin seabream. Our study offers useful information to better understanding SDDV-associated diseases in farmed fish.

Identification of virion-associated transcriptional transactivator (VATT) of SGIV ICP46 promoter and their binding site on promoter

BACKGROUND

Iridoviruses are large DNA viruses that cause diseases in fish, amphibians and insects. Singapore grouper iridovirus (SGIV) is isolated from cultured grouper and characterized as a ranavirus. ICP46 is defined to be a core gene of the family Iridoviridae and SGIV ICP46 was demonstrated to be an immediate-early (IE) gene associated with cell growth control and could contribute to virus replication in previous research.

METHODS

The transcription start site (TSS) and 5'-untranslated region (5'-UTR) of SGIV ICP46 were determined using 5' RACE. The core promoter elements of ICP46s were analyzed by bioinformatics analysis. The core promoter region and the regulation model of SGIV ICP46 promoter were revealed by the construction of serially deleted promoter plasmids, transfections, drug treat and luciferase reporter assays. The identification of virion-associated transcriptional transactivator (VATT) that interact with SGIV ICP46 promoter and their binding site on promoter were performed by electrophoretic mobility shift assays (EMSA), DNA pull-down assays and mass spectrometry (MS).

RESULTS

SGIV ICP46 was found to have short 5'-UTR and a presumptive downstream promoter element (DPE), AGACA, which locates at + 36 to + 39 nt downstream of the TSS. The core promoter region of SGIV ICP46 located from - 22 to + 42 nt relative to the TSS. VATTs were involved in the promoter activation of SGIV ICP46 and further identified to be VP12, VP39, VP57 and MCP. A 10-base DNA sequence "ATGGCTTTCG" between the TSS and presumptive DPE was determined to be the binding site of the VATTs.

CONCLUSION

Our study showed that four VAATs (VP12, VP39, VP57 and MCP) might bind with the SGIV ICP46 promoter and be involved in the promoter activation. Further, the binding site of the VATTs on promoter was a 10-base DNA sequence between the TSS and presumptive DPE.

Assessment of fish iridoviruses using a novel cell line GS-1, derived from the spleen of orange-spotted grouper Epinephelus coioides (Hamilton) and susceptible to ranavirus and megalocytivirus

A new cell line (GS-1) was developed from the spleen tissue of the orange-spotted grouper, Epinephelus coioides applied for viral infection studies of fish ranavirus and megalocytivirus. The cells proficiently multiplied in Leibovitz’s L-15 medium supplemented with 10% fetal bovine serum at temperatures between 20°C and 32°C. Morphologically, the cell line comprised fibroblast-like cells, and this was confirmed by immunostaining with vimentin, fibronectin, and desmin antibodies. The optimal temperature for grouper iridovirus (GIV) and infectious spleen and kidney necrosis virus (ISKNV) proliferation in GS-1 cells was 25°C, and the highest titer of GIV was 10^8.4 TCID₅₀/ml, and the highest titer of ISKNV was 10^5.2 TCID₅₀/ml. Electron micrographs showed that the mean diameter of GIV virions was 180−220 nm, which was larger than ISKNV virions (160−200 nm). Negatively stained GIV particles possessed an envelope structure that was assembled by the three-layered structure with an inner electron-dense core surrounded by a lighter coat (mean diameter, 27 ± 3 nm). The highest GIV-induced mortality of groupers occurred at 25°C, whereas the highest ISKNV-induced mortality occurred at 30°C. In summary, GS-1 cell line is a valuable tool for isolating and investigating fish ranavirus and megalocytivirus in the same host system.

Noumeavirus replication relies on a transient remote control of the host nucleus

Acanthamoeba are infected by a remarkable diversity of large dsDNA viruses, the infectious cycles of which have been characterized using genomics, transcriptomics and electron microscopy. Given their gene content and the persistence of the host nucleus throughout their infectious cycle, the Marseilleviridae were initially assumed to fully replicate in the cytoplasm. Unexpectedly, we find that their virions do not incorporate the virus-encoded transcription machinery, making their replication nucleus-dependent. However, instead of delivering their DNA to the nucleus, the Marseilleviridae initiate their replication by transiently recruiting the nuclear transcription machinery to their cytoplasmic viral factory. The nucleus recovers its integrity after becoming leaky at an early stage. This work highlights the importance of virion proteomic analyses to complement genome sequencing in the elucidation of the replication scheme and evolution of large dsDNA viruses.

Large dsDNA viruses either replicate in or disrupt the nucleus to gain access to host RNA polymerases, or they rely on virus-encoded, packaged RNA polymerases. Here, the authors show that Noumeavirus replicates in the cytoplasm and relies on a transient recruitment of nuclear proteins to initiate replication.

Detection of infectious spleen and kidney necrosis virus (ISKNV) and turbot reddish body iridovirus (TRBIV) from archival ornamental fish samples

Although infections caused by megalocytiviruses have been reported from a wide range of finfish species for several decades, molecular characterisation of the viruses involved has been undertaken only on more recent cases. Sequence analysis of the major capsid protein and adenosine triphosphatase genes is reported here from formalin-fixed, paraffin-embedded material from 2 archival ornamental fish cases from 1986 and 1988 in conjunction with data for a range of genes from fresh frozen tissues from 5 cases obtained from 1991 through to 2010. Turbot reddish body iridovirus (TRBIV) genotype megalocytiviruses, previously not documented in ornamental fish, were detected in samples from 1986, 1988 and 1991. In contrast, megalocytiviruses from 1996 onwards, including those characterised from 2002, 2006 and 2010 in this study, were almost indistinguishable from infectious spleen and kidney necrosis virus (ISKNV). Three of the species infected with TRBIV-like megalocytiviruses from 1986 to 1991, viz. dwarf gourami Trichogaster lalius (formerly Colisa lalia), freshwater angelfish Pterophyllum scalare and oscar Astronotus ocellatus, were infected with ISKNV genotype megalocytiviruses from 2002 to 2010. The detection of a TRBIV genotype isolate in ornamental fish from 1986 represents the index case, confirmed by molecular sequence data, for the genus Megalocytivirus.

Multiple Passages of Grunt Fin Cells Persistently Infected with Red Seabream Iridovirus (RSIV) at 15ºC or 30ºC to Yield Uninfected Cells

Red seabream iridovirus (RSIV), a member within genus Megalocytivirus (Iridoviridae), causes serious economic losses to marine fish aquaculture industry in East Asia. In this study, we established a Blue Striped Grunt Haemulon sciurus fin (grunt fin; GF) cell line persistently infected with RSIV (PI-GF^RSIV) by subculturing GF cells that survived RSIV inoculation. PI-GF^RSIV cells were morphologically indistinguishable from naive GF cells. They could stably produce RSIV at approximately 10^{4.9 ± 0.5} genomes per microliter after 24 passages over 18 months. The optimum temperature to produce RSIV in PI-GF^RSIV cells was 25°C. These cells also produced RSIV at 15, 20, and 30°C with multiple subcultures. The amount of RSIV yielded from PI-GF^RSIV cells decreased gradually by multiple subculturing at 15°C or 30°C. Red seabream iridovirus was no longer detected from PI-GF^RSIV cells after subcultures at these temperatures. These PI-GF^RSIV cells freed from RSIV infection exhibited a level of RSIV productivity similar to those of naive GF cells after inoculation with RSIV. Therefore, we consider that these PI-GF^RSIV cells were no longer infected with RSIV after multiple subculturing at 15°C or 30°C. Received October 15, 2015; accepted June 27, 2016.

Differential autophagic effects triggered by five different vertebrate iridoviruses in a common, highly permissive mandarinfish fry (MFF-1) cell model

Autophagy of five vertebrate iridoviruses, including one megalocytivirus (infectious spleen and kidney necrosis virus, ISKNV) and four ranaviruses (Chinese giant salamander iridovirus, CGSIV; Tiger frog virus, TFV; Grouper iridovirus, GIV; and Largemouth bass virus, LMBV) were investigated in a common, highly permissive mandarinfish fry (MFF-1) cell model. The results showed marked autophagosome formation in GIV- and LMBV-infected cells but not in ISKNV-, CGSIV- and TFV-infected MFF-1 cells. Strong evidence for the autophagosomes was provided by transmission electron microscopy, the detection of mandarinfish microtubule-associated protein 1 light chain 3B (mLC3)-based fluorescent dot formation and mLC3-I/mLC3-II conversion was provided by Western blotting. Pharmacological tests indicated that autophagy plays an antiviral role during GIV or LMBV infection. Collectively, our data are the first to show that antiviral autophagic effects can be triggered by GIV and LMBV but not by ISKNV, TFV and CGSIV in a common susceptible cell model. These results suggest that differential host-virus interaction strategies may be utilized against different vertebrate iridoviruses; they also indicate the potential effectiveness of an antiviral treatment that modulates autophagy to control iridoviral infections, such as GIV and LMBV.

Identification and characterization of a novel FstK-like protein from spotted knifejaw iridovirus (genus Megalocytivirus)

Prokaryotes contain many DNA binding proteins with large molecular weights and multiple domains. DNA binding proteins are involved in DNA replication, transcription, and other physiological processes. In this study, a DNA binding protein, containing an Ftsk-like protein (FLP) domain, was cloned and characterized from SKIV-ZJ07, a member of the RSIV-type megalocytivirus, using bioinformatics and molecular biology approaches. SKIV-FLP is 3,762 base pairs long, encodes a viral protein of 1253 amino acid residuals, and contains an Ftsk (or EBV-NA3) and a Grx-2 domain. Virion localization indicated that SKIV-FLP is a major viral structural protein located below the major capsid protein. Laser confocal microscopy showed that SKIV-FLP is a cytoplasm-/nuclear-localized protein. However, the reconstruction experiments demonstrated that SKIV-FLP may contain three nuclear localization signals, each present in FLP-NT (1-380 aa), FtsK domain (380-880 aa), and Grx-2 domain (880-1253 aa). When SKIV-FLP was fused to the Gal-4 DNA-binding domain and co-transfected with L8G5-Luc, SKIV-FLP suppressed L8G5-Luc transcription. As a transcription inhibitor, SKIV-FLP also inhibited the transcription of NF-κB and IFN-γ (a type II IFN) promoter in HEK293T cells, suggesting that SKIV-FLP has a role in evading host immunity.

Rana grylio virus (RGV) envelope protein 2L: subcellular localization and essential roles in virus infectivity revealed by conditional lethal mutant

Rana grylio virus (RGV) is a pathogenic iridovirus that has resulted in high mortality in cultured frog. Here, an envelope protein gene, 2L, was identified from RGV and its possible role in virus infection was investigated. Database searches found that RGV 2L had homologues in all sequenced iridoviruses and is a core gene of iridoviruses. Western blotting detection of purified RGV virions confirmed that 2L protein was associated with virion membrane. Fluorescence localization revealed that 2L protein co-localized with viral factories in RGV infected cells. In co-transfected cells, 2L protein co-localized with two other viral envelope proteins, 22R and 53R. However, 2L protein did not co-localize with the major capsid protein of RGV in co-transfected cells. Meanwhile, fluorescence observation showed that 2L protein co-localized with endoplasmic reticulum, but did not co-localize with mitochondria and Golgi apparatus. Moreover, a conditional lethal mutant virus containing the lac repressor/operator system was constructed to investigate the role of RGV 2L in virus infection. The ability to form plaques and the virus titres were strongly reduced when expression of 2L was repressed. Therefore, the current data showed that 2L protein is essential for virus infection. Our study is the first report, to our knowledge, of co-localization between envelope proteins in iridovirus and provides new insights into the understanding of envelope proteins in iridovirus.

Field trial tests of FKC vaccines against RSIV genotype Megalocytivirus in cage-cultured mandarin fish (Siniperca chuatsi) in an inland reservoir

Megalocytiviruses are one of the most important causative agents in finfish industry in China, Japan and South East Asia. The viruses are mainly composed of ISKNV, RSIV and TRBIV genotypes. Among them, ISKNV genotype isolate is the most important causative agent in mandarin fish industry in South China. Since its first occurrence in mid-1990s in China, no effective drug has been developed to prevent and control this virus until our recent work. In this study, unusual RSIV genotype Megalocytivirus was validated as the causative agent in natural mass mortality of cage-cultured mandarin fish in an inland reservoir. One isolate was obtained using MFF-1 cells from natural mass mortality of mandarin fish and designated as Megalocyti-LJ2012. Based on two previous megalocytiviral isolates, formalin-killed cell (FKC) vaccines were prepared to immunize 2000 and 9000 cage-cultured mandarin in October 2011 and August 2012, respectively. As results, greater than 70% protective effects were observed in vaccination group in both individual field tests. Adjuvant-emulsified FKC vaccine provided even greater than 99% protective effect (N = 1000). In contrast, almost all fish died in non-vaccination group (N = 1000). Immuno-protection test under laboratory condition showed that 100% relative percent survival was obtained in surviving fish from vaccination group after challenge with Megalocyti-LJ2012 at 4 months post vaccination. Taken together, the present study shows that FKC vaccine is also efficient in preventing RSIV genotype Megalocytivirus in cage-cultured mandarin fish in two field tests.

Cloning of a new fibroblast cell line from an early primary culture from mandarin fish (Siniperca chuatsi) fry for efficient proliferation of megalocytiviruses

Megalocytiviruses are important emerging pathogens in both freshwater and marine finfish aquaculture. However, a limited number of piscine cell lines are persistently susceptible to these viruses, which greatly limits the study of megalocytiviruses. In this study, a new fibroblast-like cell line was established from an early primary culture from mandarin fish fry by a single cell cloning and was designated as MFF-8C1. The MFF-8C1 cells grow well in Dulbecco's modified Eagle's medium supplemented with 10 % fetal bovine serum and had been subcultured more than 60 passages since the initial recovery culture in October 2009. Chromosomal analysis revealed that 91 % of the MFF-8C1 cells maintained a normal diploid chromosome number (2n = 48) in the 46th passage. Infection experiments showed that both freshwater-borne and marine-borne megalocytiviruses induce severe cytopathic effects in infected MFF-8C1 cells characterized by the rounding and enlargement of cells, which are highly consistent with the previous description of the infection in other susceptible cells with megalocytivirus. Megalocytivirus infections were further confirmed by a transmission electron microscopy. Furthermore, the MFF-8C1-cultured megalocytiviral suspension was highly virulent to infected mandarin fish. In summary, a new fibroblast cell line from mandarin fish fry that was highly permissive to megalocytiviruses was established. The MFF-8C1 cell line is a promising cellular substrate candidate for cell-cultured vaccine production of megalocytivirus.