Production and characterization of monoclonal antibodies to a grouper iridovirus

A panel of six monoclonal antibodies (mAbs) against a grouper iridovirus (SGIV) were produced by immunization of Balb/c mice with purified virus preparations. Isotyping test revealed all the mAbs were IgG1. None of the mAbs possessed the ability to neutralize SGIV in cell cultures. Western blot showed that 4 mAbs reacted with 2 SGIV proteins at molecular mass of approximately 100 and 117 kDa in gradient-purified virus. Immunofluorescent studies showed that the two specific viral proteins VP100 and VP117 were localized within virus assembly sites in the cytoplasm of SGIV-infected grouper cells (GP). Fractionations of the iridovirus in a 20-60% sucrose gradient were detected successfully by all the six mAbs using immunodot blot. An antigen-capture enzyme-linked immunosorbent assay (ELISA) system, based on the use of mAb 7E11 for capture and a rabbit polyclonal antibody to SGIV for detection was developed. SGIV antigen was detected in gradient-purified virus and virus-infected grouper blood. These novel mAbs will facilitate the development of more specific and standardized diagnostic techniques for marine fish iridovirus.     

In situ hybridization of a marine fish virus, Singapore grouper iridovirus with a nucleic acid probe of major capsid protein

A DNA probe of 531 base pairs for Singapore grouper iridovirus (SGIV) was generated by polymerase chain reaction and labeled with nonradioactive digoxigenin. An in situ hybridization based method was developed to detect SGIV in formalin-fixed tissues from maricultured Malabar grouper, Epinephelus malabaricus Bloch and Schneider. The in situ hybridization detected SGIV in the kidney, spleen, liver, intestine, stomach and gills from naturally infected fish. Strong hybridization signals were obtained from the kidney and spleen tissues, while intermediate intensity signals were observed in the intestine and liver tissues. The weakest signals were obtained from the stomach and gills. The signals were located specifically within epithelial, endothelial and sub-endothelial hypertrophic cells in all tested tissues. The in situ hybridization procedure will provide an important diagnostic tool to complement histopathological methods, and contribute to epidemiological studies on the origin and distribution of iridovirus in mariculture.     

Development and characterization of a new tropical marine fish cell line from grouper, Epinephelus coioides susceptible to iridovirus and nodavirus

The development and characterization of a new tropical marine fish cell line (GS), derived from the spleen of orange spotted grouper, Epinephelus coioides is described. The GS cells grow well in Leibovitz's L-15 medium supplemented with 10% foetal bovine serum, and have been subcultured more than 200 times. The optimal growth temperature was 27 degrees C. The GS cell culture consisted of mostly fibroblastic cells. The modal diploid chromosome number was 48. GS cell cultures showed advanced cytopathic effects after infection with a pathogenic grouper iridovirus (Singapore grouper iridovirus, SGIV) or with a grouper nodavirus (Epinephelus tauvina nervous necrosis virus, ETNNV). Analysis by transmission electron microscopy showed a large number of SGIV and ETNNV particles in the cytoplasm of virus-infected cells, respectively, indicative of high sensitivity to these two viruses. Immunofluorescence microscopy showed that iridovirus-infected GS cells reacted strongly with monoclonal antibody against the grouper iridovirus. It is suggested that the GS cell line has good potential as a diagnostic tool for isolation and propagation of iridovirus and nodavirus. When the GS cells were transfected with pEGFP vector DNA, significant fluorescent signals were observed suggesting that the GS cell line can be used as a useful tool for transgenic and genetic manipulation studies.     

Antigenic characterization of a marine fish iridovirus from grouper,Epinephelus spp

Iridoviruses, recognized as causative agents of serious systemic diseases, have been identified from more than 20 fish species. Antigenic properties of a pathogenic iridovirus isolated from grouper, Epinephelus spp., in Singapore (SGIV) were investigated using rabbit IgG against the virus. Antisera were prepared by immunization of rabbit with purified virions. The rabbit IgG was purified from antiserum using a protein A-agarose column and adsorbed onto acetone-dried grouper (GP) cells. The viral surface-exposed antigens were visualized by a combination of immunogold transmission electron-microscopy and by indirect immunofluorescence, and the viral antigenic related proteins were discriminated by Western blot. The cross immunofluorescence assay showed that the grouper virus isolate was serologically close to viruses of the genus Ranavirus of family Iridoviridae. The viral antigens were detected from virus infected-cell cultures as early as 4 h of post infection using IFAT, and could be detectable in virus-infected fish blood as early as 3 days post infection. Immuno-dot assays revealed that the rabbit anti-SGIV IgG allowed sensitive detection of SGIV viral antigens. This study will facilitate the development of diagnostic techniques and vaccines for grouper iridovirus.     

Identification and characterization of a tumor necrosis factor receptor like protein encoded by Singapore grouper iridovirus

Virus encoded tumor necrosis factor receptors (TNFRs) have been demonstrated to facilitate virus to escape from apoptosis or other host immune response for viral replication. Singapore grouper iridovirus (SGIV), a large DNA virus which belongs to genus Ranavirus, is a major pathogen resulting in heavy economic losses to grouper aquaculture. Here, SGIV ORF096 (VP96) encoding a putative homolog of TNFR was identified and characterized. Multiple sequence alignment indicated that SGIV-VP96 contained two extracellular cysteine-rich domains (CRDs) with conserved four or six cysteine residues, but lacked the transmembrane domain at the C-terminus. SGIV-VP96 was identified as an early (E) gene and localized in the cytoplasm in transfected or infected cells. Overexpression of SGIV-VP96 in vitro enhanced cell proliferation, and improved cell survival against SGIV infection. Furthermore, virus infection induced apoptosis and caspase-3 activity were inhibited in SGIV-VP96 expressing FHM cells compared to the control cells. Taken together, our results suggested that SGIV might utilize virus encoded TNFR like genes to modulate the host apoptotic response for effective virus replication.     

Characterization of Singapore grouper iridovirus (SGIV) ORF086R,a putative homolog of ICP18 involved in cell growth control and virus replication

Singapore grouper iridovirus (SGIV), as a causative agent of serious systemic disease, causes significant economic losses in grouper aquaculture. In this study, a novel ICP18 homolog encoded by SGIV ORF086R was identified and characterized. Strikingly, ICP18 homologs can be found in all ranaviruses, but not in other sequenced large DNA viruses. SGIV ICP18 is an immediate-early gene and begins to be transcribed as early as 2 h post-infection (p.i.). Western blotting indicated that SGIV ICP18 is translated as early as 6 h p.i. and is a viral non-envelope protein. Subcellular localization analysis revealed that the SGIV ICP18 displays a finely punctate cytoplasmic pattern. Furthermore, overexpression of SGIV ICP18 can promote the growth of grouper embryonic cells (GP) and contribute to SGIV replication. These results should offer important insights into the pathogenesis of ranaviruses.     

Identification and characterization of a novel capsid protein encoded by Singapore grouper iridovirus ORF038L

Singapore grouper iridovirus (SGIV) is an important pathogen isolated from grouper, Epinephelus tauvina, and characterized as a novel ranavirus. To better understand the function of viral structural genes involved in SGIV infection and virus–host interactions, a candidate gene, VP38 (ORF038L), was investigated in this study. SGIV VP38 was found to encode a 170-aa peptide containing an RGD motif, and it showed significant identity only to members of the genus Iridovirus, family Iridoviridae, except megalocytivirus. The VP38 gene was identified by temporal expression pattern analysis and drug inhibition assay as a late (L) gene. Immunofluorescence localization revealed that P38 was distributed predominately in the cytoplasm and that association of VP38 with viral factories increased at the late stage of SGIV infection. Consistent results from immunoelectron microscopy (IEM) and western blot analysis revealed that SGIV VP38 is a viral capsid protein. Furthermore, antibodies specific for SGIV VP38 exhibited substantial SGIV-neutralizing activity in vitro, suggesting that VP38 might play an important role in SGIV infectivity.     

Electron microscopic observations of a marine fish iridovirus isolated from brown-spotted grouper, Epinephelus tauvina.

The morphogenesis and the ultrastructure of a marine fish iridovirus isolated from diseased grouper, Epinephelus tauvina were studied by electron microscopy. The virus was grown on a marine fish cell line (GP) at 25 degrees C. After appearance of advanced cytopathic effect (CPE), various morphogenetic stages of virus amplification, maturation and assembly were detected in the cytoplasm of virus-infected cells. The matured nucleocapsids were probably formed by insertion of electron-dense core material into a partly forming empty capsid just before completely sealed. The nucleocapsids were located at the assembly sites as pseudocrystalline arrays or scattered individually. In the late phase of infection, the nucleocapsids were enveloped and released by budding from the plasma membrane. The budding virus particles could directly enter neighbouring cells by endocytosis to start the next round infection. Ultrastructure of the grouper iridovirus was studied using the methods of enzymatic digestions and detergent degradations. The purified iridovirus particles showed a three-layered membrane including an external lipoprotein envelope, an inner periodic protein capsid and a lipid-containing membrane. The regular array of surface capsid subunits was observed after degradation with detergent.     

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 β-propiolactone (BPL) inactivated virus at 4°C for 12h and formalin inactivated virus at 4°C for 12d, 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 4weeks 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.     

EB病毒感染对骨髓瘤细胞生物学特性的影响

运用B95-8细胞株培养上清中的EB病毒体外感染人多发性骨髓瘤(MM)细胞株RPMI8226,通过流式细胞仪(FCM)和Westernblot检测感染后病毒抗原LMP1和EBNA-2的表达。然后再采用FCM和RT-PCR方法,检测感染后细胞表面CD40L和CXCR4的表达。并且进一步通过细胞凋亡检测和Transwell,观察感染后细胞生物学功能的改变。结果显示,RPMI8226细胞感染后表达LMP1和EBNA-2;感染后细胞上调表达CD40L,细胞凋亡减少。而且,通过趋化性细胞因子受体CXCR4表达增高,感染细胞的迁移能力也得到增强。     

Cloning, characterization, and expression analysis of a thioredoxin from orange-spotted grouper (Epinephelus coioides)

Thioredoxins (TRXs) are a family of small, highly conserved proteins that are essential for the maintenance of cellular homeostasis. In this study, a thioredoxin gene was cloned from orange-spotted grouper, Epinephelus coioides (designated as Ec-TRX). The full-length cDNA of Ec-TRX was comprised of 767bp with a 327bp open reading frame that encodes a putative protein of 108 amino acids. Quantitative real-time PCR analysis revealed that the Ec-TRX mRNA was distributed abundantly in grouper, E. coioides skin and liver, and the expression in liver was up-regulated after viral challenge with Singapore grouper iridovirus (SGIV). Recombinant Ec-TRX (rEc-TRX) was expressed in Escherichia coli BL21 (DE3) and purified for mouse anti-Ec-TRX serum preparation. The rEc-TRX fusion protein was demonstrated to possess the expected redox activity in enzymatic analysis, and scavenge free radicals and protect supercoiled DNA from oxidative damage induced by a metal-ion catalyzed oxidation reaction. Subcellular localization revealed that Ec-TRX was distributed in both cytoplasm and nucleus. Overexpression of Ec-TRX in grouper spleen (GS) cells could promote the growth of GS cells and inhibit the replication of SGIV. These results suggest that Ec-TRX could function as an important antioxidant in a physiological context, and perhaps is involved in the responses to viral challenge.     

Identification and functional characterization of an interferon regulatory factor 7-like (IRF7-like) gene from orange-spotted grouper, Epinephelus coioides

Interferon regulatory factor (IRF) 7 plays a crucial role in modulating cellular responses to viral infection and cytokines, including interferons (IFNs). In the present study, a novel IRF7 gene (designated as EcIRF7) was cloned and characterized from orange-spotted grouper, Epinephelus coioides. The full-length EcIRF7 cDNA is composed of 2089 bp and encodes a polypeptide of 433 amino acids with 81% identity to IRF7 of Siniperca chuatsi, and the genomic DNA of EcIRF7 consists of 9 exons and 8 introns, with a length of approximately 5629 bp. EcIRF7 contains three conserved domains including a DNA-binding domain (DBD), an IRF associated domain (IAD) and a serine-rich domain, all of which are highly conserved across species. Recombinant EcIRF7 was expressed in Escherichia coli BL21 (DE3) and purified for mouse anti-EcIRF7 serum preparation. Realtime quantitative PCR (RT-qPCR) analysis revealed a broad expression of EcIRF7, with a relative strong expression in spleen, kidney, skin and intestine. The expression of EcIRF7 was differentially up-regulated after stimulation with Vibrio vulnificus, Staphylococcus aureus and Singapore grouper iridovirus (SGIV). EcIRF7 showed similar intracellular localization pattern to those of mammalian and chicken, and translocated into nucleus after SGIV infection. Further more, EcIRF7 was proved to be capable of activating zebrafish type I IFN promoter and inhibiting the replication of SGIV in grouper spleen (GS) cells. These results suggest that EcIRF7 is potentially involved in grouper immune responses to invasion of viral and bacterial pathogens.     

Lipidomic study of intracellular Singapore grouper iridovirus

Singapore grouper iridoviruses (SGIV) infected grouper cells release few enveloped extracellular viruses by budding and many unenveloped intracellular viruses following cell lysis. The lipid composition and function of such unenveloped intracellular viruses remain unknown. Detergent treatment of the intracellular viruses triggered the loss of viral lipids, capsid proteins and infectivity. Enzymatic digestion of the viral lipids with phospholipases and sphingomyelinase retained the viral capsid proteins but reduced infectivity. Over 220 lipid species were identified and quantified from the viruses and its producer cells by electrospray ionization mass spectrometry. Ten caspid proteins that dissociated from the viruses following the detergent treatments were identified by MALDI-TOF/TOF-MS/MS. Five of them were demonstrated to be lipid-binding proteins. This is the first research detailing the lipidome and lipid-protein interactions of an unenveloped virus. The identified lipid species and lipid-binding proteins will facilitate further studies of the viral assembly, egress and entry.     

Persistent infection of pig embryo kidney cell cultures caused by a variant of tick-borne encephalitis virus

The results of modelling and study of persistent infection of pig embryo kidney cell cultures (PEK) with MF variant of tick-borne encephalitis (TBE) virus are presented. In the life of chronically infected cultures periods of formation and stabilization were observed which were characterized by different types of relationships between the persisting virus and the cells. By the characters under study the persisting virus did not differ significantly from the initial MF variant. Inoculation of PEK and BHK-21 cell monolayers with samples of DNA from chronically infected cultures permitted to isolate transfected agents. Their identification by the neutralization tests and the indirect immunofluorescence procedure confirmed that they belonged to TBE virus. It is suggested that the mechanism of TBE virus persistence in PEK-MF system is based on the integration of TBE virus genome with DNA of chronically infected PEK cells.     

Modulation by cell trypsinization of Sendai virus expression in African green monkey kidney cells: first infection and establishment of a carrier state.

Two African green monkey kidney (AGMK) cell lines, 37RC (interferon-producing) and Vero (non-interferon-producing), were infected by egg-grown Sendai virus passaged in eggs at high and low m.o.i. The appearance of haem-adsorption, and cytopathic effect (c.p.e.) as well as the presence of haemagglutinating virions in the supernates were much more pronounced with a virus seed obtained with 10(-3) diluted passages than with a seed obtained with undiluted inoculum. They were also independent of interferon production (data obtained in 37RC and Vero cells were almost superimposable). In studies carried out with the virus seed prepared at high dilution the establishment of infection was maximal when monolayers were infected as soon as 5 h after trypsinization and cell seeding, regardless of cell density. Virus in supernates obtained from cultures infected 5, 20 or 50 h after seeding exhibited a greatly reduced infectivity for monkey cells, but not apparently for chick embryos. Trypsin treatment of the virus supernates restored their infectivity for AGMK cells more efficiently for virus released from cells infected 5 h after seeding than for virus released from cells infected later after seeding. In keeping with these observations, virus in supernates from cultures infected 5, 20 or 50 h after seeding contained increasing amounts of auto-interfering virions. The decreased infectivity obtained in cell supernates was not accounted for by major differences in virus RNA synthesis. Similarly, the optimum infection established in cells seeded only for 5 h was not due to increased virus adsorption. Several lines of cells surviving first infection with egg-grown Sendai virus have been obtained and kept in cultures for 3 to 18 months. Virus release and c.p.e. apparently become reduced in the cells surviving the first infection until the newly repopulated monolayers must undergo trypsinization. Weekly protease treatments of the cells reactivate all parameters of virus infection which again will tend to disappear slowly and then reappear following each trypsinization ('intermittent' carrier state). The establishment and the 'intermittent' reactivation of these lines were not prevented by the inclusion in the medium of anti-Sendai virus serum. Temperature-sensitive ts functions do not seem to play an important role in this virus-host relationship.     

Improved yields of cell-free varicella-zoster virus.

Systematic studies on the replication of varicella-zoster virus in infected human fetal diploid lung cells have defined more optimal conditions for infection and harvesting of cultures and have led to the production of cell-free virus preparations with infectivity titers of greater than or equal to 10(6) plaque-forming units per ml. The highest yields of cell-free virus were obtained by (i) sonic treatment of the cellular phase of cultures inoculated with trypsin-dispersed infected cells at ratios of 1 infected cell to 6 to 10 uninfected cells in the monolayer and (ii) harvesting cells after 24 to 36 h of incubation at 36 degrees C. At this time the cultures showed minimal viral cytopathic effect. Spread of infectivity occurred much more rapidly in cultures inoculated with whole infected cells than in those infected with cell-free virus. Complement-fixing antigens with improved titers of greater than or equal to 1:128 were prepared from varicella-zoster virus-infected cell cultures in the same manner as cell-free virus, but harvested after 3 to 4 days of incubation when the cultures showed an advanced cytopathic effect.     

Biological and biochemical characterization of a latent subacute sclerosing panencephalitis (SSPE) virus infection in tissue culture

The present investigation describes the biological and biochemical properties of a persistent SSPE virus infection. Persistently infected cells were derived by cocultivation of infected brain cells and uninfected Vero cells, and cultures were maintained by normal subculturing methods. No infectious virus was ever released from these cultures, and all attempts to induce infectious virus release were unsuccessful. Biological assays showed that infected cells contained nucleocapsid and salt-dependent hemagglutinin antigens, whereas the normal hemagglutinin appeared not to be present. Electron microscopic examination demonstrated the presence of both intranuclear and cytoplasmic nucleocapsids together with the release of virus particles (defective?) from the cell membrane. Biochemical analysis demonstrated that approximately 90% of the intracellular genomic RNA was defective or subgenomic although a small quantity of infectious genomes was present. It is proposed that the large quantities of defective genomes in the infected cells are the major factor in the maintenance of this persistent infection.     

Enhanced infectivity of bluetongue virus in cell culture by centrifugation.

The effects of centrifugation of the infection of cell culture with bluetongue virus (BTV) were investigated. Baby hamster kidney cells were infected with BTV with or without centrifugation. Viral antigen was detected by immunofluorescence at 24 h in both centrifuged and noncentrifuged cultures. However, after 24 h of infection, the production of PFU in centrifuged cell cultures was 10- to 20-fold greater than that seen in cultures not centrifuged. In addition, centrifugation enhanced the direct detection of PFU from blood samples collected from a sheep experimentally infected with BTV.