The glycoprotein genes and gene junctions of the fish rhabdoviruses spring viremia of carp virus and hirame rhabdovirus: analysis of relationships with other rhabdoviruses

Characterisation of putative novel tick viruses and zoonotic risk prediction

Tick-associated viruses remain a substantial zoonotic risk worldwide, so knowledge of the diversity of tick viruses has potential health consequences. Despite their importance, large amounts of sequences in public data sets from tick meta-genomic and -transcriptomic projects remain unannotated, sequence data that could contain undocumented viruses. Through data mining and bioinformatic analysis of more than 37,800 public meta-genomic and -transcriptomic data sets, we found 83 unannotated contigs exhibiting high identity with known tick viruses. These putative viral contigs were classified into three RNA viral families (Alphatetraviridae, Orthomyxoviridae and Chuviridae) and one DNA viral family (Asfarviridae). After manual checking of quality and dissimilarity towards other sequences in the data set, these 83 contigs were reduced to five contigs in the Alphatetraviridae from four putative viruses, four in the Orthomyxoviridae from two putative viruses and one in the Chuviridae which clustered with known tick-associated viruses, forming a separate clade within the viral families. We further attempted to assess which previously known tick viruses likely represent zoonotic risks and thus deserve further investigation. We ranked the human infection potential of 133 known tick-associated viruses using a genome composition-based machine learning model. We found five high-risk tick-associated viruses (Langat virus, Lonestar tick chuvirus 1, Grotenhout virus, Taggert virus and Johnston Atoll virus) that have not been known to infect human and two viral families (Nairoviridae and Phenuiviridae) that contain a large proportion of potential zoonotic tick-associated viruses. This adds to the knowledge of tick virus diversity and highlights the importance of surveillance of newly emerging tick-associated diseases.

Phylogenetic analysis of spring viraemia of carp virus isolated in Serbia

Spring viraemia of carp (SVC) is an infectious disease responsible for severe economic losses for various cyprinid species, particularly common carp (Cyprinus carpio carpio). The causative agent is the Rhabdovirus carpio or SVC virus (SVCV), a member of the Sprivivirus genus, within the Rhabdoviridae family. Phylogenetically, SVCV is divided into four genogroups (SVCV a, SVCV b, SVCV c and SVCV d), which have a reasonable correlation with the geographical distribution of the virus. In the late twentieth century, the disease was widespread in Serbian aquaculture and caused massive deaths in common carp. This study aimed to molecularly characterize the circulating SVCV isolates in Serbia over a 17-year period. The genetic relationships between 21 SVCV isolates from common carp and rainbow trout in Serbia between 1992 and 2009 were determined based on the partial nucleotide sequence of the glycoprotein gene (G gene). The phylogenetic analysis showed that the dominant SVCV isolates in Serbia belong to the SVCV d genogroup, with only one isolate belonging to genogroup SVCV b. The SVCV strains circulating in Serbia exhibited high homogeneity, as several isolates shared 100% similarity within these genogroups. Most Serbian isolates belonged to SVCV d1 and d2 subgroups, with one isolate notably different and included in a new subgroup SVCV d5. Understanding the SVCV genetic variants circulating in Serbia would be helpful in future epizootic investigations.

4'-(8-(4-Methylimidazole)-octyloxy)-arctigenin: The first inhibitor of fish rhabdovirus glycoprotein

Spring viraemia of carp virus (SVCV), a highly pathogenic rhabdovirus, could cause spring viraemia of carp (SVC) with up to 90% lethality. Like other rhabdoviruses, the entry of SVCV into susceptible cells was mediated by a single envelope glycoprotein G. Specific inhibitors targeting the glycoprotein were the most effective means to alleviate the epidemic. The programs including SWISS-MODEL, I-TASSER, Phyre2 and AlphaFold2 were used to build a three-dimensional structural model of glycoprotein. The structural comparison between SVCV-G and homology protein VSV-G revealed that the SVCV glycoprotein ectodomain (residues 19 to 466) folded into four distinct domains. Based on the potential small molecule binding sites on glycoprotein surfaces, virtual screening of the anti-SVCV drug libraries was performed using Autodock software and 4'-(8-(4-Methylimidazole)-octyloxy)-arctigenin (MOA) with a high binding affinity was identified. The solubility enhancer tags including trigger factor and maltose binding protein were fused with the ectodomain of glycoprotein, and the target protein with a purity of about 90% was successfully obtained. The interaction confirmation tests revealed that the fluorescence intensity of a characteristic peak induced by the endogenous chromophores in glycoprotein was decreased with the addition of MOA, indicating changes in the microenvironment of glycoprotein. Moreover, the interaction could cause a slight conformational change in glycoprotein, as shown by the content of β-turn, β-folding, and random coil of protein all increased with the decrease of α-helix content after the addition of MOA compound. These results demonstrated that MOA could act as a novel drug against fish rhabdovirus via direct targeting of glycoprotein.

Boronic acid-modified bacterial cellulose microspheres as packing materials for enveloped virus removal

Viruses are the most abundant microorganisms on the earth, their existence in contaminated waters possesses a significant threat to humans. Waterborne viral infections could be fatal to sensitive population including young child, the elderly, and the immune-compromised. It is imperative to remove viruses during water treatment to better protect public health, especially in the light of evidence of detection of coronaviruses genetic fragments in raw sewage. We reported bench-scale experiments evaluating the extent and mechanisms of removal of a model virus (spring viremia of carp virus, SVCV) in water by adsorption. Microspheres made by boronic acid-modified bacterial cellulose with excellent mechanical strength were successfully fabricated as packing materials for the column to remove glycoproteins and enveloped viruses from water. The synthesized adsorbent was characterized by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Brunauer Emmett Teller (BET) measurement. The adsorption efficiency of glycoproteins was investigated by SDS-PAGE and the Broadford protein assay, while the binding capacity with the virus (spring viremia of carp virus) was monitored by cell culture to calculate the viral cytopathic effect and viral titer caused by the virus. The data obtained from the above experiments showed that ∼3-log removal of SVCV in 3 h, which significantly reduced the virus concentration from microspheres packed column. The present study provides substantial evidence to prove beyond doubt that material based on bacterial cellulose seems to have the potential for virus removal from water which can be extended to systems of significant importance.

Comparative virulence of spring viremia of carp virus (SVCV) genotypes in two koi varieties

Spring viremia of carp virus (SVCV), is a lethal freshwater pathogen of cyprinid fish, and Cyprinus carpio koi is a primary host species. The virus was initially described in the 1960s after outbreaks occurred in Europe, but a global expansion of SVCV has been ongoing since the late 1990s. Genetic typing of SVCV isolates separates them into 4 genotypes that are correlated with geographic origin: Ia (Asia), Ib and Ic (Eastern Europe), and Id (Central Europe). We compared infectivity and virulence of 8 SVCV strains, including 4 uncharacterized Chinese Ia isolates and representatives of genotypes Ia-d in 2 morphologically distinct varieties of koi: long-fin semi-scaled Beni Kikokuryu koi and short-fin fully scaled Sanke koi. Mortality ranged from 4 to 82% in the Beni Kikokuryu koi and 0 to 94% in the Sanke koi following immersion challenge. Genotype Ia isolates of Asian origin had a wide range in virulence (0-94%). Single isolates representing the European genotypes Ib and Ic were moderately virulent (38-56%). Each virus strain produced similar levels of mortality in both koi breeds, with the exception of the SVCV Id strain that appeared to have both moderate and high virulence phenotypes (60% in Beni Kikokuryu koi vs. 87% in Sanke koi). Overall SVCV strain virulence appeared to be a dominant factor in determining disease outcomes, whereas intraspecies variation, based on koi variety, had less of an impact. This study is the first side-by-side comparison of Chinese SVCV isolates and genotype Ia-d strain virulence in a highly susceptible host.

PEG-modified subunit vaccine encoding dominant epitope to enhance immune response against spring viraemia of carp virus

Spring viraemia of carp (SVC) caused by spring viraemia of carp virus (SVCV) can infect almost all fish of cyprinids, which bring huge economic losses to aquaculture. Glycoprotein (G), as the most important antigenic determinant protein of SVCV, is widely considered as an effective method against SVCV. In our previous study, we found that G3 (131 aa) is the potential dominant antigen epitope that induces strong immune responses similar to G protein (510 aa). Here, in order to further improve the immune effect, we reported a subunit vaccine (PEG-G3) constructed by PEG-modified dominant epitope protein (G3). The results of serum antibody production, enzyme activities and immune-related genes expression showed that PEG-G3 induces significantly stronger immune protective responses against SVCV than G3. PEG modification significantly increased the serum antibody level of the vaccine, which increased significantly after immunization and reached the peak at 21 day post-vaccination. T-AOC and AKP activities in the lowest concentration group (5 μg) of PEG-G3 were significantly higher than those in the highest concentration group (20 μg) of G3. In PEG-G3 group, the expression of almost all genes increased at least 4 times compared with the control group. After 14-day challenge, the RPS (relative percentage survival) of the highest concentration of PEG-G3 group was 53.6%, while that of G3 group is 38.9%. Therefore, this work shows that PEG modification and dominant epitope screening may be effective methods to improve the immune protective effect of vaccines and to resist the infection of aquatic animal viral diseases.

Analytical validation of two RT-qPCR tests and detection of spring viremia of carp virus (SVCV) in persistently infected koi Cyprinus carpio

Spring viremia of carp virus (SVCV) ia a carp sprivivirus and a member of the genus Sprivivirus within the family Rhabdoviridae. The virus is the etiological agent of spring viremia of carp, a disease of cyprinid species including koi Cyprinus carpio L. and notifiable to the World Organisation for Animal Health. The goal of this study was to explore hypotheses regarding inter-genogroup (Ia to Id) SVCV infection dynamics in juvenile koi and contemporaneously create new reverse-transcription quantitative PCR (RT-qPCR) assays and validate their analytical sensitivity, specificity (ASp) and repeatability for diagnostic detection of SVCV. RT-qPCR diagnostic tests targeting the SVCV nucleoprotein (Q2N) or glycoprotein (Q1G) nucleotides were pan-specific for isolates typed to SVCV genogroups Ia to Id. The Q2N test had broader ASp than Q1G because Q1G did not detect SVCV isolate 20120450 and Q2N displayed occasional detection of pike fry sprivivirus isolate V76. Neither test cross-reacted with other rhabdoviruses, infectious pancreatic necrosis virus or co-localizing cyprinid herpesvirus 3. Both tests were sensitive with observed 50% limits of detection of 3 plasmid copies and high repeatability. Test analysis of koi immersed in SVCV showed that the virus could be detected for at least 167 d following exposure and that titer, prevalence, replicative rate and persistence in koi were correlated significantly with virus virulence. In this context, high virulence SVCV isolates were more prevalent, reached higher titers quicker and persisted in koi for longer periods of time relative to moderate and low virulence isolates.

Surface display of hirame novirhabdovirus (HIRRV) G protein in Lactococcus lactis and its immune protection in flounder (Paralichthys olivaceus)

Background

Hirame novirhabdovirus (HIRRV) can infect a wide range of marine and freshwater fish, causing huge economic losses to aquaculture industry. Vaccine development, especially oral vaccine, has become an effective and convenient way to control aquatic infectious diseases. HIRRV glycoprotein (G), an immunogenic viral protein is a potential vaccine candidate for prevention of the disease. Here, we aimed to construct a recombinant Lactococcus lactis strain expressing HIRRV-G on the cell surface as an oral vaccine to prevent HIRRV.

Results

Glycoprotein gene of HIRRV was successfully cloned and expressed in L. lactis NZ9000 in a surface-displayed form, yielding Ll:pSLC-G. An approximately 81 kDa recombinant G protein (containing LysM anchoring motif) was confirmed by SDS-PAGE, western blotting and mass spectrometry analysis. The surface-displayed G protein was also verified by immunofluorescence and flow cytometry assays. Furthermore, to evaluate the potential of Ll:pSLC-G as oral vaccine candidate, flounders were continuously fed with commercial diet pellets coated with 1.0 × 10⁹ cfu/g of induced Ll:pSLC-G for 1 week. Four weeks later, booster vaccination was performed with the same procedure. Compared with the controls, Ll:pSLC-G elicited significantly higher levels of specific IgM against HIRRV in flounder gut mucus at the second week and in serum at the fourth week (p < 0.05). Meanwhile, oral immunization with Ll:pSLC-G could provide 60.7% protection against HIRRV infection and a significantly lower virus load was detected than the controls on the third day post-challenge (p < 0.01). Moreover, on the first day post 1-week feeding, approximately 10⁴–10⁵ recombinant L. lactis cells were detected in every gram of foregut, midgut and hindgut of flounder, which were mainly localized at the bottom of gut mucus layer; and on day 21, 10²–10³L. lactis cells could still be recovered.

Conclusions

HIRRV-G protein was successfully expressed on the surface of L. lactis cells, which could trigger mucosal and humoral immune response of flounder and provide considerable immune protection against HIRRV. It suggests that genetically engineered L. lactis expressing G protein can be employed as a promising oral vaccine against HIRRV infection.

Addicted to sugar: roles of glycans in the order Mononegavirales

Glycosylation is a biologically important protein modification process by which a carbohydrate chain is enzymatically added to a protein at a specific amino acid residue. This process plays roles in many cellular functions, including intracellular trafficking, cell-cell signaling, protein folding and receptor binding. While glycosylation is a common host cell process, it is utilized by many pathogens as well. Protein glycosylation is widely employed by viruses for both host invasion and evasion of host immune responses. Thus better understanding of viral glycosylation functions has potential applications for improved antiviral therapeutic and vaccine development. Here, we summarize our current knowledge on the broad biological functions of glycans for the Mononegavirales, an order of enveloped negative-sense single-stranded RNA viruses of high medical importance that includes Ebola, rabies, measles and Nipah viruses. We discuss glycobiological findings by genera in alphabetical order within each of eight Mononegavirales families, namely, the bornaviruses, filoviruses, mymonaviruses, nyamiviruses, paramyxoviruses, pneumoviruses, rhabdoviruses and sunviruses.

Synthesis and in vitro activities evaluation of arctigenin derivatives against spring viraemia of carp virus

Spring viraemia of carp virus (SVCV) is a viral fish pathogen causing high mortality in several carp species and other cultivated fish. However, robust anti-SVCV drugs currently are extremely scarce. For the purpose of seeking out anti-SVCV drugs, here a total of 35 arctigenin derivatives were designed, synthesized and evaluated for their anti-viral activities. By comparing the inhibitory concentration at half-maximal activity (IC₅₀) of the 15 screened candidate drugs (max inhibitory response surpassing 90%) in epithelioma papulosum cyprini (EPC) cells infected with SVCV, 2Q and 6 A were chosen for additional validation studies, with an IC₅₀ of 0.077 μg/mL and 0.095 μg/mL, respectively. Further experiments revealed that 2Q and 6 A could significantly decrease SVCV-induced apoptosis and have a protective effect on cell morphology at 48 and 72 h post-infection. Moreover, the reactive oxygen species (ROS) induced upon SVCV infection could be obviously inhibited by 2Q and 6 A, while SVCV-infected cells were clearly observed. On account of these findings, 2Q and 6 A could have a promising application for the treatment of infection of SVCV and provide a considerable reference for novel antivirals in aquaculture.

Isolation, identification, and classification of a novel rhabdovirus from diseased Chinese rice-field eels (Monopterus albus)

In 2017, a clinical disease outbreak resulted in substantial mortality of adults and larvae of cultured Chinese rice-field eels (Monopterus albus) on a farm in Hubei, Central China. A rhabdovirus was isolated from moribund specimens, and typical clinical symptoms associated with an outbreak included an enlarged and swollen head. This differed from previous observations. Histological changes included necrosis and cavities of various sizes within the brain and kidney. Homogenized tissues of diseased Chinese rice-field eels were screened for viral isolation using six different fish cell lines. A rhabdovirus was isolated following observation of cytopathic effect (CPE) in a gibel carp brain (GiCB) cell line and confirmed by RT-PCR. Electron microscopy showed large numbers of rhabdovirus-shaped particles in the cytoplasm of the brain cells of the diseased Chinese rice-field eels and in the infected GiCB cell line. This virus has been named "Chinese rice-field eel rhabdovirus" (CrERV), and the complete nucleotide sequence of CrERV was cloned. This rhabdovirus is composed of 11,545 nucleotides with the following genomic organization: 3'-N-P-M-G-L-5'. The genes are separated by conserved gene junctions, and phylogenetic analysis of the L sequence revealed that CrERV forms a separate branch with Siniperca chuatsi rhabdovirus (SCRV) and hybrid snakehead rhabdovirus C1207 (HSHRV-C1207). This is the first report of the complete sequence of CrERV from the Chinese rice-field eel in China.

Virulence of a chimeric recombinant infectious haematopoietic necrosis virus expressing the spring viraemia of carp virus glycoprotein in salmonid and cyprinid fish

Infectious haematopoietic necrosis virus (IHNV) and spring viraemia of carp virus (SVCV) are both rhabdoviruses of fish, listed as notifiable disease agents by the World Organization for Animal Health. Recombinant rhabdoviruses with heterologous gene substitutions have been engineered to study genetic determinants and assess the potential of these recombinant viruses for vaccine development. A recombinant IHNV (rIHNV), containing the full-length genome of a European IHNV strain, was modified by deleting the glycoprotein (G) gene and replacing it with a European SVCV G-gene to make the rIHNV-Gsvcv. The chimeric rIHNV-Gsvcv level of virulence in rainbow trout, common carp and koi was assessed, and its ability to induce a protective immune response in surviving koi against wild-type SVCV infection was tested. The rIHNV-Gsvcv infection of trout led to high mortality, ranging from 78% to 92.5%, after immersion. In contrast, no deaths occurred in juvenile common carp after infection with rIHNV-Gsvcv by either immersion or intraperitoneal (IP) injection. Similarly, koi infected with rIHNV-Gsvcv via IP injection had little to no mortality (≤9%). Koi that survived initial infection with a high dose of recombinant virus rIHNV-Gsvcv were protected against a virulent SVCV challenge resulting in a high relative per cent survival of 82.5%.

Susceptibility of Representative Great Lakes Fish Species to the North Carolina Strain of Spring Viremia of Carp Virus (SVCV)

Spring viremia of carp virus (SVCV) is a notifiable pathogen of the World Organization of Animal Health. Since SVCV was isolated in Lake Ontario in 2007, concern has grown about its spread in the Great Lakes basin and its potential negative impacts on fish species of importance in stock enhancement programs basinwide. The susceptibility of representative fish species from the families Cyprinidae (Fathead Minnow Pimephales promelas, Golden Shiner Notemigonus crysoleucas, Spotfin Shiner Cyprinella spiloptera, and Creek Chub Semotilus atromaculatus), Centrarchidae (Largemouth Bass Micropterus salmoides), Percidae (Walleye Sander vitreus), Salmonidae (Rainbow Trout Oncorhynchus mykiss), and Esocidae (Muskellunge Esox masquinongy) to SVCV was evaluated by experimental infection under laboratory conditions. Morbidity and mortality were recorded, and virus re-isolation, seminested reverse transcription PCR, and histopathological assessments were performed. Using intraperitoneal (i.p.) injection, Fathead Minnows and Golden Shiners were highly susceptible to SVCV (40-70% mortality). All dead or moribund and apparently healthy surviving Fathead Minnows and Golden Shiners were SVCV positive. The SVCV was also detected in challenged but healthy Spotfin Shiners (30%) and Creek Chub (5%). However, noncyprinid species exhibited no morbidity or mortality and were free of SVCV following an observation period of 30 d. In a follow-up experimental challenge, Fathead Minnows and Golden Shiners were SVCV challenged at 10³ and 10⁵ PFU/mL by means of waterborne immersion. After immersion, Fathead Minnows and Golden Shiners exhibited characteristic SVCV disease signs, but mortality was less (30% and 10% mortality, respectively) than that in fish with i.p. injections. The SVCV was detected in all mortalities and a subset of healthy Fathead Minnows and Golden Shiners. Necrotic changes were observed in the kidneys, liver, spleen, ovaries, and heart, and other histopathological lesions also occurred. These findings suggest that two of the four cyprinids tested are susceptible to SVCV-induced disease and that all four can act as potential carriers of SVCV in the Laurentian Great Lakes. Received January 11, 2017; accepted July 17, 2017.

Monoclonal antibody against M protein of spring viremia of carp virus

M protein of spring viremia of carp virus (SVCV) was expressed in Escherichia coli and then used to immunize BALB/c mice. One monoclonal antibody (5A1) against M protein was generated by fusion of mouse myeloma cell line SP2/0 and spleen lymphocytes from immunized mice. The characterizations of this MAb were confirmed by ELISA (enzyme linked immunosorbent assay), IFA (immunofluorescent assay), and Western blot analysis. All results indicate that MAb 5A1 was specific to SVCV M protein.

Recombinant lactobacillus expressing G protein of spring viremia of carp virus (SVCV) combined with ORF81 protein of koi herpesvirus (KHV): A promising way to induce protective immunity against SVCV and KHV infection in cyprinid fish via oral vaccination

Spring viremia of carp virus (SVCV) and koi herpesvirus (KHV) are highly contagious and pathogenic to cyprinid fish, causing enormous economic losses in aquaculture. Although DNA vaccines reported in recent years could induce protective immune responses in carps against these viruses via injection, there are a number of consequences and uncertainties related to DNA vaccination. Therefore, more effective and practical method to induce protective immunity such as oral administration would be highly desirable. In this study, we investigated the utilities of a genetically engineered Lactobacillus plantarum (L. plantarum) coexpressing glycoprotein (G) of SVCV and ORF81 protein of KHV as oral vaccine to induce protective immunity in carps via oral vaccination. The surface-displayed recombinant plasmid pYG-G-ORF81 was electroporated into L. plantarum, giving rise to LP/pYG-G-ORF81, where expression and localization of G-ORF81 fusion protein from the LP/pYG-G-ORF81 was identified by SDS-PAGE, Western blotting and immunofluorescence assay. Bait feed particles containing the LP/pYG-G-ORF81 were used as vaccine to immunize carps via gastrointestinal route. Compared to control groups, the carps orally immunized with the LP/pYG-G-ORF81 were induced significant levels of immunoglobulin M (IgM), and its immunogenicity was confirmed by viral loads reduction detected by PCR assay after virus challenge followed by an effective protection rate 71% in vaccinated carps and 53% in vaccinated koi until at days 65 post challenge, respectively. Our study here demonstrates, for the first time, the ability of recombinant L. plantarum as oral vaccine against SVCV and KHV infection in carps, suggesting a practical multivalent strategy for the control of spring viremia of carp and koi herpesvirus disease.

Genomic evidence of homologous recombination in spring viremia of carp virus: a negatively single stranded RNA virus

A new strain of spring viraemia of carp virus, denominated SVCV-265, was isolated from an ornamental common carp (Cyprinus carpio) in Shanghai, China, 2013. The isolate could produce obvious cytopathic effects on EPC cells, while was shown to be of low virulence for juvenile koi. Complete genome sequencing revealed the genome of the SVCV-265 strain is 11,029 nucleotides in length and phylogenetic analysis showed the isolate was clustered within Asia clade but was divergent from Chinese A1, A2 and BJ0505-2 strains. Previous report indicated that the G and P gene of SVCV shared similar topologies of evolutionary trees. In this study, phylogenetic analysis based on the P gene sequences showed the SVCV-265 was clustered into Iai subgroup and divergent from Chinese isolates A1, A2 and BJ0505-2, which were clustered into Iaii group. However, sequence alignment of the G gene showed the SVCV-265 has a close relationship with A1, A2 and BJ0505-2 isolates. Recombination analysis of all the whole sequences of SVCV available revealed isolates A2 and BJ0505-2 were likely the homologous recombination descendants of the A1 and SVCV-265. The crossover regions were located between 3845-6387nt for A2 and 3573-6444 nt for BJ0505-2, respectively. Phylogenetic analysis of the crossover region further confirmed these findings. This current study describes the molecular characterization of the new isolate SVCV-265 from China and is the first report of homologous recombination in SVCV.

First isolation of hirame rhabdovirus from freshwater fish in Europe

A rhabdovirus was isolated in cell culture inoculated with tissue material from diseased grayling, Thymallus thymallus (L.), originating from a fish farm affected by a mortality episode in Poland. Diagnostics tests showed that the virus was not related to novirhabdoviruses known in Europe, nor to vesiculovirus-like species, except perch rhabdovirus (PRhV) with which it shared moderate serological relations. However, RT-PCR with PRhV probes gave negative results. To identify the virus, a random-priming sequence-independent single primer amplification was adopted. Surprisingly, two of the obtained sequences exhibited a high identity (>99%) with hirame rhabdovirus (HIRRV), a novirhabdovirus usually found in fish in marine Asiatic countries, for instance Japan, China and Korea. The full-length sequence of the phosphoprotein gene (P) demonstrated a higher identity of the present isolate with HIRRV from China compared with the Korean isolate. An identical viral sequence was also found in brown trout, Salmo trutta trutta L., affected by mortalities in a second farm in the same region, after a likely contamination from the grayling farm. To our knowledge, this is the first report of HIRRV in Europe, and in two hosts from fresh water that have not been described before as susceptible species.

Selection and characterization of single-chain recombinant antibodies against spring viraemia of carp virus from mouse phage display library

Antibody-displaying phage library was selected after three rounds of panning against spring viraemia of carp virus (SVCV) by phage display technology. Eight positive clones which could produce soluble single-chain fragment variable (scFv) antibody induced by isopropyl-beta-d-thiogalactopyranoside (IPTG) were obtained. Dot blot results showed that the eight scFv antibodies could recognize SVCV. The soluble scFv antibodies showed a molecular weight 29 kD by Western blot. All scFv antibodies could recognize SVCV proteins specifically without cross-reaction with other virus proteins by ELISA. Indirect immunofluorescence results showed that all of these scFv antibodies reacted positively with virus in the SVCV-infected cells. These scFv antibodies will be useful tools to establish immunological detection methods for SVCV.

Direct detection of unamplified spring viraemia of carp virus RNA using unmodified gold nanoparticles

Spring viraemia of carp (SVC) is a viral disease that mainly affects carp Cyprinus carpio and other cyprinid fish, causing severe economic losses. Rapid detection and identification of spring viraemia of carp virus (SVCV) is crucial for effective disease management. Recent advances in nanoscience are having a significant impact on many scientific fields, especially biodiagnostics, where a number of nanoparticle-based assays have been introduced for biomolecular detection. Single- and double-stranded oligonucleotides can be adsorbed on gold nanoparticles (AuNPs) in colloidal solution under certain conditions. We exploited this phenomenon to develop a specific hybridization assay for direct detection of SVCV-RNA without prior amplification. The result of the hybridization process could be detected visually within 1 min when the colour of the reaction mixture changed from red to blue (positive reaction) or remains red (negative). The lower detection limit of the assay was estimated to be 10-3 TCID50 ml-1 SVCV-RNA, and it has the feasibility to detect the target virus-RNA in clinical specimens without previous amplification. In order to obtain an indication of the assay's performance on clinical samples we compared the optimized assay with nested RT-PCR in detection of SVCV-RNA in infected fish samples. The concordance of the 2 methods was defined as 100% when compared to nested RT-PCR positive and negative samples. The SVC-AuNPs assay requires only 15 min, eliminates the need for thermal cycling or detection instruments and is a specific and rapid tool for detection of SVCV-RNA directly from clinical samples.

First detection of pike fry-like rhabdovirus in barbel and spring viraemia of carp virus in sturgeon and pike in aquaculture in the Czech Republic

Rapid antigen detection enzyme-linked immunosorbent assay (ELISA) testing of cell cultures with organ homogenate from fish, collected from farms with a predominance of common carp or in natural aquaculture in the Czech Republic between 1995 and 2008, identified piscine vesiculovirus in 27 of 178 samples. Using reverse transcription semi-nested PCR, targeting a 550 nucleotide region of the glycoprotein (G) gene, piscine vesiculovirus was confirmed in 23 of the 27 organ samples diagnosed by ELISA as infected. PCR products were amplified and sequenced from 18 isolates from common carp Cyprinus carpio (family Cyprinidae), 2 isolates from northern pike Esox lucius (family Esocidae), and 1 isolate each from Siberian sturgeon Acipenser baerii (family Acipenseridae), common barbel Barbus barbus (family Cyprinidae), and koi carp Cyprinus carpio koi (family Cyprinidae). The sequences (based on 401 nucleotides) clustered into 2 genogroups. The majority of isolates (n = 22), including those from sturgeon and pike, grouped with the spring viraemia of carp virus (SVCV) Genogroup I and Subgroup Id. The 22 isolates could be further subdivided into 2 groups: Id1 (n = 20) and Id2 (n = 2). A marker (a non-conservative nucleotide substitution) for the Id1 SVCV group was identified. It was specifically found in all sequences of Id1 isolates when testing SVCV originating from different countries. The remaining isolate from barbel, was classified in the pike fry-like rhabdovirus Genogroup IV. This is the first confirmation of natural SVCV infection in sturgeon and pike, and pike fry-like rhabdovirus infection in barbel. In the case of the pike fry-like rhabdovirus, this is also its first identification in the Czech Republic. According to the presence/absence of evident clinical signs of rhabdoviral disease in the 3 infected hosts, only the sturgeon seemed to be susceptible to the monitored rhabdovirus.

Sigma viruses from three species of Drosophila form a major new clade in the rhabdovirus phylogeny

The sigma virus (DMelSV), which is a natural pathogen of Drosophila melanogaster, is the only Drosophila-specific rhabdovirus that has been described. We have discovered two new rhabdoviruses, D. obscura and D. affinis, which we have named DObsSV and DAffSV, respectively. We sequenced the complete genomes of DObsSV and DMelSV, and the L gene from DAffSV. Combining these data with sequences from a wide range of other rhabdoviruses, we found that the three sigma viruses form a distinct clade which is a sister group to the Dimarhabdovirus supergroup, and the high levels of divergence between these viruses suggest that they deserve to be recognized as a new genus. Furthermore, our analysis produced the most robustly supported phylogeny of the Rhabdoviridae to date, allowing us to reconstruct the major transitions that have occurred during the evolution of the family. Our data suggest that the bias towards research into plants and vertebrates means that much of the diversity of rhabdoviruses has been missed, and rhabdoviruses may be common pathogens of insects.

Molecular analysis of spring viraemia of carp virus in China: a fatal aquatic viral disease that might spread in East Asian

Spring viraemia of carp (SVC) is a fatal viral disease for cyprinid fish, which is caused by spring viraemia of carp virus (SVCV). To date, no SVC outbreak has been reported in China. Between 1998 and 2002, outbreaks of SVC were reported in ornamental and wild fish in Europe and America, imported from multiple sources including China. Based on phylogenetic analysis, the viral strain isolated from America was shown to be originated from Asia. These outbreaks not only resulted in huge economic losses, but also raise an interesting question as to whether SVCV really exists in China and if so, is it responsible for SVC outbreaks? From 2002 to 2006, we screened 6700 samples from ornamental fish farms using the cell culture method of the Office International des Epizooties (OIE), and further verified the presence of SVCV by ELISA and real-time quantitative RT-PCR. Two infected samples were found and the complete genome of SVCV was sequenced from one of the isolates, termed SVCV-C1. Several unique hallmarks of SVCV-C1 were identified, including six amino acid (KSLANA) insertion in the viral RNA-dependent RNA polymerase (L) protein and ten nucleotide insertion in the region between glycoprotein (G) and L genes in European SVCV strains. Phylogenetic tree analysis of the full-length G protein of selected SVCV isolates from the United Kingdom and United States revealed that G proteins could be classified into Ia and Id sub genogroups. The Ia sub genogroup can be further divided into newly defined sub genogroups Ia-A and Ia-B. The isolates derived from the United States and China including the SVCV-C1 belongs to in the Ia-A sub genogroup. The SVCV-C1 G protein shares more than 99% homology with the G proteins of the SVCV strains from England and the United States, making it difficult to compare their pathogenicity. Comparison of the predicted three-dimensional structure based on the published G protein sequences from five SVCV strains revealed that the main differences were in the loops of the pleckstrin homology domains. Since SVCV is highly pathogenic, we speculate that SVC may therefore pose a serious threat to farmed cyprinid fish in China.

DNA vaccine protects ornamental koi (Cyprinus carpio koi) against North American spring viremia of carp virus

The emergence of spring viremia of carp virus (SVCV) in the United States constitutes a potentially serious alien pathogen threat to susceptible fish stocks in North America. A DNA vaccine with an SVCV glycoprotein (G) gene from a North American isolate was constructed. In order to test the vaccine a challenge model utilizing a specific pathogen-free domestic koi stock and a cold water stress treatment was also developed. We have conducted four trial studies demonstrating that the pSGnc DNA vaccine provided protection in vaccinated fish against challenge at low, moderate, and high virus doses of the homologous virus. The protection was significant (p < 0.05) as compared to fish receiving a mock vaccine construct containing a luciferase reporter gene and to non-vaccinated controls in fish ranging in age from 3 to 14 months. In all trials, the SVCV-G DNA immunized fish were challenged 28-days post-vaccination (546 degree-days) and experienced low mortalities varying from 10 to 50% with relative percent survivals ranging from 50 to 88%. The non-vaccinated controls and mock construct vaccinated fish encountered high cumulative percent mortalities ranging from 70 to 100%. This is the first report of a SVCV DNA vaccine being tested successfully in koi. These experiments prove that the SVCV DNA (pSGnc) vaccine can elicit specific reproducible protection and validates its potential use as a prophylactic vaccine in koi and other vulnerable North American fish stocks.

Characterization of complete genome sequence of the spring viremia of carp virus isolated from common carp (Cyprinus carpio) in China

The complete genome of spring viraemia of carp virus (SVCV) strain A-1 isolated from cultured common carp (Cyprinus carpio) in China was sequenced and characterized. Reverse transcription-polymerase chain reaction (RT-PCR) derived clones were constructed and the DNA was sequenced. It showed that the entire genome of SVCV A-1 consists of 11,100 nucleotide base pairs, the predicted size of the viral RNA of rhabdoviruses. However, the additional insertions in bp 4633-4676 and bp 4684-4724 of SVCV A-1 were different from the other two published SVCV complete genomes. Five open reading frames (ORFs) of SVCV A-1 were identified and further confirmed by RT-PCR and DNA sequencing of their respective RT-PCR products. The 5 structural proteins encoded by the viral RNA were ordered 3'-N-P-M-G-L-5'. This is the first report of a complete genome sequence of SVCV isolated from cultured carp in China. Phylogenetic analysis indicates that SVCV A-1 is closely related to the members of the genus Vesiculovirus, family Rhabdoviridae.

Comparison of multiple genes of spring viremia of carp viruses isolated in the United States

Five spring viremia of carp viruses (SVCV), Rhabdovirus carpio, were isolated in the United States (US) between 2002 and 2004. Single tube reverse transcription-polymerase chain reaction (RT-PCR) was used to generate overlapping cDNA fragments from the US isolates of SVCV. Multiple pairs of specific primers were designed to amplify a portion of the phosphoprotein gene, the matrix gene, and the glycoprotein gene of SVCV genogroup Id (corresponding to nucleotides 2174-4942 of GenBank accession NC_002803). Sequences were proofread and aligned to generate a consensus sequence for each isolate. Phylogenetic analysis of the 2705 nucleotide consensus sequence revealed that all five US isolates belong to SVCV genogroup Ia, Asian origin isolates, and a PCR primer binding site unique to SVCV genogroup Ia was identified.

DNA vaccination can protect Cyprinus Carpio against spring viraemia of carp virus

Several DNA constructs containing the spring viraemia of carp virus (SVCV) glycoprotein (G) gene were investigated for their ability to induce protection against SVCV following injection into myofibres. The constructs were pooled into four groups and co-injected with a plasmid encoding murine granulocyte-macrophage colony-stimulating factor. Group 1 contained one full-length and two truncated G constructs under the control of the cytomegalovirus (CMV) promoter. Group 2 contained full-length constructs with the CMV promoter, the simian virus 40 promoter and a muscle-specific promoter. Group 3 contained constructs in which the G-gene was fused with a second gene in order to improve secretion of the G-protein or to enhance destruction of transfected myocytes by T cells. Group 4 contained constructs with the CMV-Intron A promoter in plasmids with or without CpG motifs. A small-scale trial in goldfish showed that antibody responses in at least half the fish were induced by three injections of plasmids from Groups 1 and 3 whereas T-cell like responses with stimulation indices of above 3 were induced in at least half the fish by Groups 2 and 4. A single-dose of each plasmid mix was then used to protect carp in a large-scale trial. Following challenge with a heterologous strain of SVCV that killed 64% of fish, the strongest protection was observed in carp that received the full length G-gene expressed by two plasmids driven by the CMV-Intron A promoter (Group 4), with a relative percentage survival of 48% (p=0.00008).

Fish rhabdoviruses: molecular epidemiology and evolution

Rhabdoviruses may cause serious diseases in wild and farmed fish. Within the Rhabdoviridae six genera have been established: Ephemerovirus, Cytorhabdovirus, Nucleorhabdovirus, Lyssavirus, Vesiculovirus, and Novirhabdovirus. Viruses that infect fish are official or tentative members of the genera Vesiculovirus and Novirhabdovirus, or are listed as unassigned rhabdoviruses. In this report, we summarize and discuss published and our own unpublished data on the molecular epidemiology and phylogeography of fish rhabdoviruses including intrapopulational differences and subgrouping of fish rhabdoviruses, in particular the species spring viremia of carp virus (SVCV), infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV).

Protection of flounder against hirame rhabdovirus (HIRRV) with a DNA vaccine containing the glycoprotein gene

Hirame rhabdovirus (HIRRV) is an important virus of cultured flounder (Paralichthys olivaceus). We tested the protective immunogenicity of DNA-based vaccines against this virus. Genes encoding the nucleocapsid protein (N) and the C-terminal half of the glycoprotein (G) were amplified by RT-PCR and separately cloned into the eukaryotic expression vector pcDNA 3.1. The G protein expressed by transfected cells was detected by western blot analysis. PCR analyses demonstrated the presence of injected plasmids in fish muscle tissue at 14 days post injection. Immunocytochemistry of muscle tissue injected with the plasmid DNA showed expression of the target protein in myofibrils and sarcoplasm. Flounder fry with an average weight of 3 g were injected with 5 microg of plasmid DNA and challenged at 21 days after immunization. Fish injected with vector DNA or PBS showed >95% cumulative mortality by 16 days after inoculation with the virus. In contrast, fish injected with plasmids containing the N gene, G gene, or N + G gene mixture showed 70, 5, and 2.5% cumulative mortality, respectively. These results show that the G gene is effective for the induction of protective immunity against HIRRV infection in injected fish.

Complete nucleotide sequence of the hirame rhabdovirus, a pathogen of marine fish

Reverse transcription-polymerase chain reaction (RT-PCR) derived clones were constructed for the hirame rhabdovirus (HIRRV) CA-9703 strain from Korea, and the DNA was sequenced. The 3'-end of genomic RNA was cloned by poly(A)-tailing of the genomic RNA before reverse transcription, and the 5'-end of the genome was cloned by poly(G)- or poly(C)-tailing of the first strand, followed by PCR. The remainder of the genomic DNA was cloned by reverse transcription-polymerase chain reaction using primers that were based on the published rhabdovirus sequences. The complete genome of HIRRV CA-9703 strain comprises 11,034 nucleotides and encodes six genes in the order of: 3'-leader, N, P, M, G, NV, L, and 5'-trailer. These genes are separated by conserved sequences or gene junctions, with one-nucleotide gene spacers. The first 16 of the 19 nucleotides at the ends of the HIRRV genome are complementary, and the first four nucleotides at the 3'-ends of the HIRRV, infectious hematopoietic necrosis virus (IHNV), viral hemorrhagic septicemia virus (VHSV), and snakehead rhabdovirus (SHRV) genomes are identical. The HIRRV proteins share the highest amino acid sequence homology (ranging from 72% to 92%) with the proteins of IHNV, of all the known fish rhabdoviruses, and the highest sequence homology with respect to the L protein was shared among HIRRV, IHNV, VHSV, and SHRV. Although there were differences in the degrees of relatedness, phylogenetic trees that were derived from multiple sequence alignments of the rhabdovirus proteins showed similar patterns of relationship among these viruses, in which fish Novirhabdoviruses formed a separate clade from spring viremia of carp virus (SVCV), unassigned fish rhabdovirus that was closer to mammalian rhabdoviruses.

Development of a DNA vaccine against hirame rhabdovirus and analysis of the expression of immune-related genes after vaccination

Intramuscular injection of Japanese flounder, Paralichthys olivaceus (average weight approximately 2 g) with 1 and 10 microg of a plasmid DNA vaccine encoding the hirame rhabdovirus (HIRRV) glycoprotein gene (pCMV-HRVg) was found to provide strong protection against HIRRV. We also conducted a real-time PCR analysis to quantify immune-related genes, e.g. MHC class Ialpha, IIalpha, IIbeta, TCR-alpha, beta1, beta2 and delta, to characterize the immune response at 1 and 7 days after DNA vaccination. In general, the copy numbers were at least 2-fold higher than those of the non-vaccinated fish. Interestingly, the gene expression of TCR beta1 and beta2 increased 1 day post-DNA vaccination, after which their copy numbers returned to levels similar to those before vaccination. These results suggest that the immune system of Japanese flounder was activated immediately after DNA immunization.

The membrane-proximal region of vesicular stomatitis virus glycoprotein G ectodomain is critical for fusion and virus infectivity

The glycoprotein (G) of vesicular stomatitis virus (VSV) is responsible for binding of virus to cells and for mediating virus entry following endocytosis by inducing fusion of the viral envelope with the endosomal membrane. The fusion peptide of G is internal (residues 116 to 137) and exhibits characteristics similar to those of other internal fusion peptides, but recent studies have implicated the region adjacent to the transmembrane domain as also being important for G-mediated membrane fusion. Sequence alignment of the membrane-proximal region of G from several different vesiculoviruses revealed that this domain is highly conserved, suggesting that it is important for G function. Mutational analysis was used to show that this region is not essential for G protein oligomerization, transport to the cell surface, or incorporation into virus particles but that it is essential for acid-induced membrane fusion activity and for virus infectivity. Deletion of the 13 membrane-proximal amino acids (N449 to W461) dramatically reduced cell-cell fusion activity and reduced virus infectivity approximately 100-fold, but mutation of conserved aromatic residues (W457, F458, and W461) either singly or together had only modest effects on cell-cell fusion activity; recombinant virus encoding these mutants replicated as efficiently as wild-type (WT) VSV. Insertion of heterologous sequences in the juxtamembrane region completely abolished membrane fusion activity and virus infectivity, as did deletion of residues F440 to N449. The insertion mutants showed some changes in pH-dependent conformational changes and in virus binding, which could partially explain the defects in membrane fusion activity, but all the other mutants were similar to WT G with respect to conformational changes and virus binding. These data support the hypothesis that the membrane-proximal domain contributes to G-mediated membrane fusion activity, yet the conserved aromatic residues are not essential for membrane fusion or virus infectivity.

A novel fish rhabdovirus from sweden is closely related to the Finnish rhabdovirus 903/87

A novel rhabdovirus, preliminary designated as the Sea trout rhabdovirus 28/97 (STRV 28/97), was isolated from sea trout (Salmo trutta trutta) in Sweden in 1996. The fish showed central nervous symptoms, and at the autopsy petechial bleedings in the mesenteric fat were visible. STRV 28/97 was shown to be serologically related to the vesiculotype rhabdovirus 903/87 isolated from brown trout (Salmo trutta lacustris) in Finland [1,3]. The sequences for the nucleocapsid protein, phosphoprotein, matrix protein, glycoprotein and beginning of the polymerase protein of STRV 28/97 were determined. At the amino acid level the genes were over 97% similar to virus 903/87. The nucleocapsid proteins, glycoproteins and beginning of the polymerase protein of STRV 28/97 and virus 903/87 were clustered with the vesiculoviruses and the phosphoproteins close to the vesiculoviruses in protein parsimony analysis. The matrix proteins formed a distinct clade in protein parsimony analysis.

Determination of the complete genomic sequence and analysis of the gene products of the virus of Spring Viremia of Carp, a fish rhabdovirus

The complete genome of spring viremia of carp virus (SVCV) was cloned and the sequence of 11019 nucleotides was determined. It contains five open reading frames (ORF's) encoding for the nucleoprotein N; phosphoprotein P; matrix protein M; glycoprotein G; and the viral RNA dependent RNA polymerase L. Genes are organised in the order typical for rhabdoviruses: 3'-N-P-M-G-L-5'. The short leader and trailer regions of SVCV exhibit inverse complementarity and are similar to the respective 3' and 5' ends of the genome of vesicular stomatitis virus. To verify the predicted open reading frames proteins were expressed in bacteria and analysed with a polyclonal anti-SVCV serum. Furthermore, monospecific antisera against the distinct viral proteins were generated. Comparison of genome and protein confirm the assignment of SVCV to the genus Vesiculovirus.

Heterologous exchanges of the glycoprotein and the matrix protein in a Novirhabdovirus

Infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV) are two salmonid rhabdoviruses replicating at low temperatures (14 to 20 degrees C). Both viruses belong to the Novirhabdovirus genus, but they are only distantly related and do not cross antigenically. By using a recently developed reverse-genetic system based on IHNV (S. Biacchesi et al., J. Virol. 74:11247-11253, 2000), we investigated the ability to exchange IHNV glycoprotein G with that of VHSV. Thus, the IHNV genome was modified so that the VHSV G gene replaced the complete IHNV G gene. A recombinant virus expressing VHSV G instead of IHNV G, rIHNV-Gvhsv, was generated and was shown to replicate as well as the wild-type rIHNV in cell culture. This study was extended by exchanging IHNV G with that of a fish vesiculovirus able to replicate at high temperatures (up to 28 degrees C), the spring viremia of carp virus (SVCV). rIHNV-Gsvcv was successfully recovered; however, its growth was restricted to 14 to 20 degrees C. These results show the nonspecific sequence requirement for the insertion of heterologous glycoproteins into IHNV virions and also demonstrate that an IHNV protein other than the G protein is responsible for the low-temperature restriction on growth. To determine to what extent the matrix (M) protein interacts with G, a series of chimeric pIHNV constructs in which all or part of the M gene was replaced with the VHSV counterpart was engineered and used to recover the respective recombinant viruses. Despite the very low percentage (38%) of amino acid identity between the IHNV and VHSV matrix proteins, viable chimeric IHNVs, harboring either the matrix protein or both the glycoprotein and the matrix protein from VHSV, were recovered and propagated. Altogether, these data show the extreme flexibility of IHNV to accommodate heterologous structural proteins.

Molecular characterisation of the nucleocapsid protein gene, glycoprotein gene and gene junctions of rhabdovirus 903/87, a novel fish pathogenic rhabdovirus

The sequences of the nucleocapsid and glycoprotein genes and the gene junctions of the fish pathogenic rhabdovirus 903/87 were determined from cDNA and PCR clones. The mRNA of the nucleocapsid is most likely 1492 nucleotides long and encodes a protein of 426 amino acids, whereas the mRNA of the glycoprotein is likely to be 1682 nucleotides long and the protein 517 amino acids. When the nucleocapsid and glycoprotein genes of virus 903/87 were compared at amino acid level with other rhabdoviruses they showed the highest homology with the Vesiculovirus genus. By sequencing the junctions between the N, P, M, G and L genes it was determined that transcription start and stop codons were conserved between virus 903/87 and the vesiculoviruses. Virus 903/87 has no open reading frame coding for a non-virion gene between the glycoprotein and the polymerase gene. Phylogenetic studies based on rhabdovirus nucleocapsid and glycoprotein genes suggested that virus 903/87 is related to viruses in the Vesiculovirus genus.

Viruses of lower vertebrates

Viruses of lower vertebrates recently became a field of interest to the public due to increasing epizootics and economic losses of poikilothermic animals. These were reported worldwide from both wildlife and collections of aquatic poikilothermic animals. Several RNA and DNA viruses infecting fish, amphibians and reptiles have been studied intensively during the last 20 years. Many of these viruses induce diseases resulting in important economic losses of lower vertebrates, especially in fish aquaculture. In addition, some of the DNA viruses seem to be emerging pathogens involved in the worldwide decline in wildlife. Irido-, herpes- and polyomavirus infections may be involved in the reduction in the numbers of endangered amphibian and reptile species. In this context the knowledge of several important RNA viruses such as orthomyxo-, paramyxo-, rhabdo-, retro-, corona-, calici-, toga-, picorna-, noda-, reo- and birnaviruses, and DNA viruses such as parvo-, irido-, herpes-, adeno-, polyoma- and poxviruses, is described in this review.

Protection of rainbow trout against infectious hematopoietic necrosis virus four days after specific or semi-specific DNA vaccination

A DNA vaccine against a fish rhabdovirus, infectious hematopoietic necrosis virus (IHNV), was shown to provide significant protection as soon as 4 d after intramuscular vaccination in 2 g rainbow trout (Oncorhynchus mykiss) held at 15 degrees C. Nearly complete protection was also observed at later time points (7, 14, and 28 d) using a standardized waterborne challenge model. In a test of the specificity of this early protection, immunization of rainbow trout with a DNA vaccine against another fish rhabdovirus, viral hemorrhagic septicemia virus, provided a significant level of cross-protection against IHNV challenge for a transient period of time, whereas a rabies virus DNA vaccine was not protective. This indication of distinct early and late protective mechanisms was not dependent on DNA vaccine doses from 0.1 to 2.5 microg.

DNA vaccines encoding viral glycoproteins induce nonspecific immunity and Mx protein synthesis in fish

Protective immunity by vaccination with plasmid DNA encoding a viral glycoprotein (G) has long been assumed to result from the induction of a specific immune response. We report here that the initial protection may be due to the induction of alpha/beta interferon, with long-term protection due to a specific response to the encoded viral G. DNA vaccines encoding the Gs of three serologically unrelated fish rhabdoviruses were used to vaccinate rainbow trout against a lethal challenge with infectious hematopoietic necrosis virus (IHNV). All three vaccines, each encoding the G gene of either IHNV (IHNV-G), snakehead rhabdovirus (SHRV) (SHRV-G), or spring viremia of carp virus (SVCV) (SVCV-G), elicited protective immunity against IHNV. Vaccinated fish were challenged at 30 or 70 days postvaccination with lethal doses of IHNV. At 30 days postvaccination, only 5% of fish that had received any of the G vaccines died, whereas more than 50% of the control fish succumbed to virus challenge. When fish were vaccinated and challenged at 70 days postvaccination, only 12% of the IHNV-G-vaccinated fish died compared to 68% for the SHRV-G- and 76% for the SVCV-G-vaccinated fish. Assays for trout Mx protein, an indicator of alpha/beta interferon induction, showed that only fish vaccinated with a G-containing plasmid produced high levels of Mx protein in the kidneys and liver. Interestingly, at day 7 after virus challenge, all of the fish vaccinated with the IHNV-G plasmid were negative for Mx, but the SHRV-G- and SVCV-G-vaccinated fish still showed detectable levels of Mx. These results suggest that DNA vaccines in fish induce an early, nonspecific antiviral protection mediated by an alpha/beta interferon and, later, a specific immune response.

Molecular characterization of the glycoproteins from two warm water rhabdoviruses: snakehead rhabdovirus (SHRV) and rhabdovirus of penaeid shrimp (RPS)/spring viremia of carp virus (SVCV)

We have determined the complete coding sequences for the glycoprotein (G) genes from two rhabdoviruses that infect warm water aquatic animals, the snakehead rhabdovirus (SHRV) and rhabdovirus of penaeid shrimp (RPS). Surprisingly, the G nucleotide sequence from RPS, a virus which has been isolated from diseased shrimp in Hawaii on numerous occasions, was over 99% identical to the G nucleotide sequence from spring viremia of carp virus (SVCV), a fish virus from Europe and Asia. This is the first report of SVCV isolation outside of Europe and Asia, and it is also the first report of SVCV infecting a non-vertebrate species. The G gene from SHRV was most closely related to the G genes from the three Novirhabdoviruses, viral hemorrhagic septicemia virus (VHSV), infectious hematopoietic necrosis virus (IHNV), and hirame rhabdovirus (HIRRV), with 47, 37, and 36% amino acid identity, respectively. In addition, a phylogenetic analysis using the amino acid sequence from rhabdovirus G genes indicated that SHRV should be classified within the Novirhabdovirus genus. Finally, the SHRV-G gene was successfully expressed in mammalian cells under the control of the cytomegalovirus (CMV) promoter, establishing that it can potentially be used in the production of pseudotyped retroviruses designed to infect fish.

Fish rhabdovirus cell entry is mediated by fibronectin

Three monoclonal antibodies (MAbs) generated against rainbow trout gonad cells (RTG-2) have been selected for their ability to protect cells from the viral hemorrhagic septicemia virus (VHSV) infection, a salmonid rhabdovirus. Protection from infection was restricted to the salmonid-derived cell lines indicating species specificity of the blocking MAbs. Surprisingly, the blocking activity of these MAbs was also effective against other nonantigenically related fish rhabdoviruses. Indirect immunofluorescence and immunoelectron microscopy observations demonstrated that the three MAbs were all directed against an abundant cell plasma membrane component, and immunoprecipitation studies indicated that the target consisted of a heterodimeric complex with molecular masses of 200 and 44 kDa. Biochemical data provided the following evidence that fibronectin is part of this complex and that it could represent the main receptor for fish rhabdoviruses. (i) An antiserum generated against the 200-kDa protein reacted against the recombinant rainbow trout fibronectin expressed in Escherichia coli. (ii) The purified rainbow trout fibronectin was able to bind specifically to VHSV. To our knowledge, this is the first identification of a cellular component acting as a primary receptor for a virus replicating in lower vertebrates and, more interestingly, for viruses belonging to the Rhabdoviridae family.