Development of a flow cytometry based method for rapid and sensitive detection of a novel marine fish iridovirus in cell culture

Megalocytivirus and Other Members of the Family Iridoviridae in Finfish: A Review of the Etiology, Epidemiology, Diagnosis, Prevention and Control

Aquaculture has expanded to become the fastest growing food-producing sector in the world. However, its expansion has come under threat due to an increase in diseases caused by pathogens such as iridoviruses commonly found in aquatic environments used for fish farming. Of the seven members belonging to the family Iridoviridae, the three genera causing diseases in fish comprise ranaviruses, lymphocystiviruses and megalocytiviruses. These three genera are serious impediments to the expansion of global aquaculture because of their tropism for a wide range of farmed-fish species in which they cause high mortality. As economic losses caused by these iridoviruses in aquaculture continue to rise, the urgent need for effective control strategies increases. As a consequence, these viruses have attracted a lot of research interest in recent years. The functional role of some of the genes that form the structure of iridoviruses has not been elucidated. There is a lack of information on the predisposing factors leading to iridovirus infections in fish, an absence of information on the risk factors leading to disease outbreaks, and a lack of data on the chemical and physical properties of iridoviruses needed for the implementation of biosecurity control measures. Thus, the synopsis put forth herein provides an update of knowledge gathered from studies carried out so far aimed at addressing the aforesaid informational gaps. In summary, this review provides an update on the etiology of different iridoviruses infecting finfish and epidemiological factors leading to the occurrence of disease outbreaks. In addition, the review provides an update on the cell lines developed for virus isolation and culture, the diagnostic tools used for virus detection and characterization, the current advances in vaccine development and the use of biosecurity in the control of iridoviruses in aquaculture. Overall, we envision that the information put forth in this review will contribute to developing effective control strategies against iridovirus infections in aquaculture.

Generation and characterization of aptamers against grass carp reovirus infection for the development of rapid detection assay

Grass carp reovirus (GCRV) causes devastating viral haemorrhagic disease in farmed grass carp (Ctenopharyngon idellus). As novel molecular probes, aptamers have been widely applied in rapid diagnosis and efficient therapies against virus or diseases. In this study, three single-stranded DNA (ssDNA) aptamers were selected against GCRV-infected CIK cells via SELEX (systematic evolution of ligands by exponential enrichment technology). Secondary structures predicted by MFOLD indicated that aptamers formed stem-loop structures, and GVI-11 had the lowest ΔG value of -30.84 KJ/mol. Three aptamers could specifically recognize GCRV-infected CIK cells, with calculated dissociation constants (Kd) of 220.86, 176.63 and 278.66 nM for aptamers GVI-1, GVI-7 and GVI-11, respectively, which indicated that they could serve as specific delivery system for antiviral therapies. The targets of aptamers GVI-1, GVI-7 and GVI-11 on the surface of GCRV-infected cells could be membrane proteins, which were trypsin-sensitive. Furthermore, FAM-labelled aptamer GVI-7 could be applied to detect GCRV infection in vivo. It is the first time to generate and characterize aptamers against GCRV-infected cells. These aptamers have great potentials in development of rapid diagnosis technology and antiviral agents against GCRV infection in aquaculture.

Isolation and Characterization of a Ranavirus Associated with Disease Outbreaks in Cultured Hybrid Grouper (♀ Tiger Grouper Epinephelus fuscoguttatus × ♂ Giant Grouper E. lanceolatus) in Guangxi, China

An outbreak of suspected iridovirus disease in cultured hybrid grouper (♀Tiger Grouper Epinephelus fuscoguttatus × ♂ Giant Grouper Epinephelus lanceolatus) occurred in the Guangxi Province in July, 2018. In this study, grouper iridovirus Guangxi (SGIV-Gx) was isolated from diseased hybrid grouper that were collected from Guangxi. Cytopathic effects were observed and identified in grouper spleen cells that were incubated with diseased tissue homogenates after 24 h, and the effects increased at 48 h postinfection. The transmission electron microscopy results showed that viral particles that were about 200 nm in diameter with hexagonal profiles were present in the cell cytoplasm of suspected virus-infected cells. The presence of SGIV-Gx (accession number: MK107821) was identified by polymerase chain reaction (PCR) and amplicon sequencing, which showed that this strain was most closely related to Singapore grouper iridovirus (AY521625.1). The detection of SGIV-Gx infection was further supported by novel aptamer (Q2c)-based detection technology. The effects of temperature and pH on viral infectivity were analyzed by using reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR) and cell culture. The results indicated that SGIV-Gx was resistant to exposure to pH levels 5, 7, and 7.5 for 1 h, but its infectivity was remarkably lower at pH levels 3 and 10 after 1 h. The analyses showed that SGIV-Gx was stable for 1 h at 4°C and 25°C but was inactivated after 1 h at 40, 50, and 60°C.

Quantitative Flow Cytometry to Measure Viral Production Using Infectious Pancreatic Necrosis Virus as a Model: A Preliminary Study

In recent decades, flow cytometry (FCM) has become an important tool in virology, due to its applications in viral replication and viral-cell interactions, as well as its capacity to quantify proteins (qFCM). In the present study, we have designed and evaluated a qFCM procedure for the in vitro analysis and quantification of fish viral proteins, using the infectious pancreatic necrosis virus (IPNV) as a model. We have also tested its use for viral titration and adapted the MARIS (method for analysing RNA following intracellular sorting) method for simultaneous quantification of viral RNA expression in infected cells. The procedure has proved to be repeatable and reproducible to an acceptable level, although to ensure reproducibility, the repetition of standard curves is inevitable. Regarding its use for viral quantification, a direct relationship (by a second-degree polynomial regression) between viral titres and Molecules of Equivalent Soluble Fluorochrome (MESF) was observed. Finally, the results support the use of this technology, not only for virus quantification, but also to study viral replication from a quantitative approach.

Development and characterization of aptamer-based enzyme-linked apta-sorbent assay for the detection of Singapore grouper iridovirus infection

AIMS

Singapore grouper iridovirus (SGIV) is a devastating aquaculture virus responsible for heavy economic losses to grouper, Epinephelus sp. aquaculture. The aim of this study was to develop a rapid and sensitive detection method for SGIV infections in infected groupers.

METHODS AND RESULTS

We previously generated DNA aptamers against SGIV-infected cells. In this study, we established and characterized a novel aptamer (Q3)-based enzyme-linked apta-sorbent assay (ELASA) for the detection of SGIV infection in Epinephelus coioides. The Q3-based ELASA could detect SGIV infection rapidly in vitro and in vivo, with high specificity and stability. Q3-based ELASA specifically recognized SGIV-infected cells, but not other-virus-infected cells or uninfected cells. Q3-based ELASA detected SGIV infection in a dose-dependent manner at Q3 concentrations as low as 125 nmol l(-1) . The results in relation to SGIV-infected cells (5 × 10(4) ), incubation time (1 min) and incubation temperature (37°C) demonstrated that Q3-based ELASA could detect SGIV infection quickly and stably, superior to antibody-based enzyme-linked immunosorbent assay. Q3-based ELASA could detect the presence of SGIV infection in kidney, liver and spleen samples in vivo, at dilutions of 1/50, 1/100 and 1/50 respectively. The complete detection process took 1-2 h.

CONCLUSIONS

Q3-based ELASA could be a useful tool for diagnosing SGIV infection.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first developed aptamer-based ELASA for detecting SGIV infection, and is widely applicable in grouper aquaculture industry in light of its rapidity, and high specificity and stability.

Flow cytometry assay for intracellular detection of Infectious Pancreatic Necrosis virus (IPNV) in Atlantic salmon (Salmo salar L.) leucocytes

Infectious Pancreatic Necrosis virus (IPNV) is traditionally detected in adherent leucocytes using immunofluorescence labelled specific antibodies, PCR or by further cultivation of infected cell material in cell lines. We present a flow cytometry (FCM) assay for detection of intracellular IPNV in salmon leucocytes, where each single cell is analysed for presence of virus. The method is established using in vitro challenge of salmon leucocytes and CHSE-214 cells. For detection of intracellular virus antigen the Cytofix/Cytoperm kit from BD is optimal compared with paraformaldehyde or acetone/methanol for cell permeabilisation. This is combined with labelling procedures allowing both internal virus antigen labelling and external antibody labelling of cell markers to identify B-cells and neutrophils. The secondary antibodies were Alexa Fluor 647 for the internal labelling and RPE for the external labelling of bound cell subtype specific antibodies. The presences of virus within cells are also demonstrated by confocal and light microscopy of infected cells. IPNV is successfully detected in blood and head kidney leucocyte samples. IPNV is found both in B-cells and neutrophils as well as in other types of leucocytes that could not be identified due to lack of cell-specific antibodies. Serial samples from cultivation of in vitro infected leucocytes and CHSE-214 cells analysed by flow cytometry showed that number of infected cells increased with increasing number of days. The flow cytometry protocol for detection of intracellular IPNV is verified using CHSE-214 cells persistently infected with IPNV. These analyses are compared with virus titre and virus infected naive CHSE-214 cells. The detection of IPNV in persistently infected cells indicates that carrier fish can be analysed, as such cells are considered to have virus titres similar to carriers.

Establishment and characterization of a new cell line derived from kidney of grouper, Epinephelus akaara (Temminck & Schlegel), susceptible to Singapore grouper iridovirus (SGIV)

A marine fish cell line derived from the kidney of red-spotted grouper, Epinephelus akaara, designated as EAGK was established and characterized. The EAGK cells multiplied well in Leibovitz's L-15 medium containing 10% foetal bovine serum at 25 °C and have been subcultured for more than 90 passages. Karyotyping, chromosomal typing and ribosomal RNA (rRNA) genotyping analysis revealed that EAGK had a modal diploid chromosome number of 82 and was a fibroblast cell line originated from grouper. A severe cytopathic effect was observed in EAGK cells incubated with Singapore grouper iridovirus (SGIV), but not with soft-shelled turtle iridovirus, viral nervous necrosis virus or spring viraemia of carp virus. SGIV replication was further confirmed by immunofluorescence, electron microscopy and virus titre determination. Bright fluorescence was observed after transfection with fluorescent protein reporter plasmids, indicating that EAGK cells can be used to identify gene functions in vitro. In addition, the cell organelles including mitochondria and endoplasm reticulum changed and aggregated around virus factories after SGIV infection, suggested that the EAGK cell line could be an important tool for investigation of iridovirus-host interactions.

Detection of cutaneous antibodies in excised skin explants from grass carp, Ctenopharyngodon idella (Valenciennes), immune to Scophthalmus maximus rhabdovirus

This study determined whether cutaneous antibodies were present in excised skin explants of grass carp, Ctenopharyngodon idella, immune to Scophthalmus maximus rhabdovirus (SMRV). Culture fluid from immune skin explants were assayed by indirect enzyme-linked immunosorbent assay (iELISA), Western blot, indirect immunofluorescent assay (IFA) and flow cytometry (FCM). iELISA showed that cutaneous antibody titres were much lower (1:12) than antiserum titres (1:1458) from intraperitoneally immunized grass carp. The phosphoprotein and matrix protein antigens of purified SMRV proteins were recognized by cutaneous antibodies from skin culture fluid using Western blot. The skin culture fluid produced staining signals in viral assembly sites and cytoplasm of SMRV-infected epithelioma papulosum cyprini (EPC) cells by IFA. FCM showed that 4.39% SMRV-infected EPC cells were detected, while non-specific reaction was seen in 2% of control cells. This is the first description of cutaneous antibodies against SMRV in grass carp.

Development and characterization of monoclonal antibodies to spring viraemia of carp virus

Five monoclonal antibodies (mAbs) against spring viraemia of carp (SVCV0504, isolated from common carp in China) were produced from mice immunized with purified virus preparations. The virion of SVCV contains five structural proteins, representing the nucleoprotein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G) and RNA-dependent RNA polymerase (L). Western blotting analysis revealed that three mAbs (1H5, 1E10, and 1H7) recognized specifically to a single protein of 47kDa (N), the mAb 3G4 reacted with two SVCV0504 proteins of 69kDa (G) and 47kDa (N), while the mAb 1A9 reacted with three SVCV0504 proteins of 69kDa (G), 50kDa (P), and 47kDa (N). By indirect ELISA, two mAbs (1H5 and 1H7) showed cross-reactivity with pike fry rhabdovirus (PFRV), but no cross-reactions with the Siniperca chuatsi rhabdovirus (SCRV), Scophthalmus maximus rhabdovirus (SMRV), Paralichthys olivaceus rhabdovirus (PoRV) were demonstrated with the five mAbs. Indirect immunofluorescence showed intense fluorescence in the cytoplasm of the SVCV0504-infected epithelioma papulosum cyprini (EPC) cells in areas corresponding to the location of granular structures. The sucrose gradient-purified SVCV0504 particles could be detected successfully by these mAbs using immunodot blotting. mAb 1A9 could completely neutralize 100 TCID(50) (50% tissue culture infective dose) of SVCV0504 at a dilution of 1:8. This is the first report of development of the neutralizing mAbs against SVCV. The mAb 1A9 was analyzed further and could be used to successfully detect viral antigens in the infected-EPC cell cultures or in cryosections from experimentally infected crucian carp (Carassius auratus) by immunohistochemistry assay. Furthermore, a flow cytometry procedure for the detection and quantification of cytoplasmic SVCV0504 in cell cultures was developed with mAb 1A9. At 28h after inoculation with the virus (0.01PFU/cell), 10.12% of infected cells could be distinguished from the uninfected cells. These mAbs will be useful in diagnostic test development and pathogenesis studies for fish rhabdovirus.

Rapid and sensitive detection of Singapore grouper iridovirus by loop-mediated isothermal amplification

AIMS

The aim of this paper was to develop a loop-mediated isothermal amplification (LAMP) method for rapid, sensitive and inexpensive detection of Singapore grouper iridovirus (SGIV) in grouper (GP), Epinephelus sp.

METHODS AND RESULTS

A set of six specific primers was designed by targeting the SGIV ORF-014L. With Bst DNA polymerase large fragment, the target DNA can be amplified as early as 20 min at 65 degrees C in a simple water bath. The detection limit is about 0.02 fg (equivalent to 6.3 copies) of plasmid ORF-014L. LAMP products could be judged with three different methods. There were no cross-reactions with seven other aquatic animal viruses indicating high specificity of the LAMP. The LAMP method was applied to detect SGIV in virus-infected GP cells and GP tissues effectively.

CONCLUSIONS

The LAMP described in this study is a cheap, sensitive, specific and rapid protocol for the detection of SGIV in cells and in GP tissues.

SIGNIFICANCE AND IMPACT OF THE STUDY

The developed LAMP method can be simply applied both in field condition and in laboratory operation for specific detection of SGIV infection.

Characterization and Application of Monoclonal Antibodies against Turbot (Scophthalmus maximus) Rhabdovirus

Five monoclonal antibodies (mAbs), 1G8, 1H9, 2D2, 2D3, and 2F5, against Scophthalmus maximus rhabdovirus (SMRV) were prepared. Characterization of the mAbs included indirect enzyme-linked immunosorbent assay, isotyping, viral inhibition assay, immunofluorescence staining of virus-infected cell cultures, and Western blot analysis. Isotyping revealed that 1G8 and 1H9 were of the IgG2b subclass and that the other three were IgM. 2D2, 2D3, and 2F5 partially inhibited SMRV infection in epithelioma papulosum cyprinid (EPC) cell culture. Western blotting showed that all five mAbs could react with two SMRV proteins with molecular masses of approximately 30 kDa (P) and 26 kDa (M). These two proteins were localized within the cytoplasm of SMRV-infected EPC cells by immunofluorescence assay. Also, progressive foci of viral replication in cell cultures were monitored from 6 to 24 h, using mAb 2D3 as the primary antibody. A flow cytometry procedure was used to detect and quantify SMRV-infected (0.01 PFU/cell) EPC cells with mAb 2D3, and 10.8% of cells could be distinguished as infected 36 h postinfection. Moreover, mAb 2D3 was successfully applied for the detection of viral antigen in cryosections from flounder tissues by immunohistochemistry tests.