Neutralization-resistant variants of infectious hematopoietic necrosis virus have altered virulence and tissue tropism

Evaluation of a DNA vaccine with self-designed CpG sequences against J genotype IHNV infection in rainbow trout (Oncorhynchus mykiss)

Rainbow trout suffer from infectious hematopoietic necrosis virus (IHNV) outbreaks, which lead to massive mortality and huge economic loss worldwide. The approved commercial vaccine is used for the prevention of IHN in Canada. Given that Chinese domestic J-genotype isolates are different from North American IHNV isolates, the development of an effective DNA vaccine against Chinese J-genotype isolates is urgent. In this study, we developed a DNA vaccine encoding glycoprotein based on our previously isolated IHNV GS21 strain and self-designed CpG sequences were supplemented as molecular adjuvants. The vaccinated rainbow trout were significantly protected against IHNV with approximately a relative percent survival (RPS) of 94.74% compared to the unvaccinated group. Moreover, the specific antibody of IgM and neutralizing antibody (NAb) was significantly provoked after the vaccination. Particularly, the antiviral immune response was rapidly evoked in the early stage of vaccination including the up-regulation of Mx-1, IFN-Ⅰ, and IFN-γ. The IHNV load in vaccinated fish was apparently lower than that in the unvaccinated group. Furthermore, the integration of exogenous genes into the host chromosome and the spread of antimicrobial-resistant genes were not found. These results suggested that our vaccine enhances robust immune responses and evokes considerable protection against IHNV with limited genetically modified risk, which is an effective and promising vaccine candidate for further prevention of IHNV outbreaks.

Immunological Effects of Recombinant Lactobacillus casei Expressing IHNV G Protein and Rainbow Trout (Oncorhynchus mykiss) Chemokine CK6 as an Oral Vaccine

IHNV is a virus that infects salmonids and causes serious economic damage to the salmonid farming industry. There is no specific treatment for the disease caused by this pathogen and the main preventive measure is vaccination, but this is only possible for small groups of individuals. Therefore, it is important to investigate new oral vaccines to prevent IHNV. In this study, the CK6 chemokine protein of rainbow trout and the truncated G protein of IHNV were used to construct a secretory expression recombinant L.casei vaccine for rainbow trout. The results showed that the levels of IgM and IgT antibodies in rainbow trout reached the highest level on the 15th day after the secondary immunization, and the antibodies exhibited high inhibitory activity against viral infection. Furthermore, the expression of relevant cytokines in different tissues was detected and found to be significantly higher in the oral vaccine group than in the control group. It was also found that pPG-612-CK6-G/L.casei 393 could stimulate splenic lymphocyte proliferation and improve mucosal immunity with significant differences between the immunized and control groups. When infected with IHNV, the protection rate of pPG-612-CK6-G/L.casei 393 was 66.67% higher than that of the control group. We found that pPG-612-CK6-G/L.casei 393 expressed and secreted the rainbow trout chemokine CK6 protein and IHNV truncated G protein, retaining the original immunogenicity of rainbow trout while enhancing their survival rate. This indicates that recombinant L.casei provides a theoretical basis and rationale for the development of an oral vaccine against IHNV and has important practical implications for the protection of rainbow trout from IHNV infection.

Infectious hematopoietic necrosis virus truncated G protein effect on survival, immune response, and disease resistance in rainbow trout

The major antigenic protein of infectious hematopoietic necrosis virus (IHNV) is the surface glycoprotein G, which contains neutralizing epitopes that induce the production of immune neutralizing antibodies. In this study, the IHNV G gene sequence was truncated according to bioinformatics principles and then recombinantly expressed via an E. coli expression system. We then assessed the specific antibody immunoglobin M (IgM) levels of rainbow trout immunized with recombinant truncated G protein (emulsified with Freund's incomplete adjuvant), and showed that antibody IgM levels of immunized fish were significantly higher than in the control group (p < 0.01). The mRNA expression levels of interferon 1 (IFN1) and interleukin-8 (IL-8) were also up-regulated significantly (p < 0.01) in head kidneys and spleens of rainbow trout immunized with recombinant truncated G protein. Also, after challenge with wild-type IHNV HLJ-09 virus on Day 28, rainbow trout immunized with recombinant truncated G protein showed cumulative survival rates of 60%. These results indicate that the truncated G protein of IHNV expressed by the E. coli prokaryotic expression system can be used as a candidate immunogen for an IHNV subunit vaccine, which lays a theoretical foundation for the study of further potential IHNV subunit vaccines.

Infectious hematopoietic necrosis virus: advances in diagnosis and vaccine development

The aquaculture of salmonid fishes is a multi-billion dollar industry with production over 3 million tons annually. However, infectious hematopoietic necrosis virus (IHNV), which infects and kills salmon and trout, significantly reduces the revenue of the salmon farming industry. Currently, there is no effective treatment for IHNV infected fishes; therefore, early detection and depopulation of the infected fishes remain the most common practices to contain the spread of IHNV. Apart from hygiene practices in aquaculture and isolation of infected fishes, loss of fishes due to IHNV infection can also be significantly reduced through vaccination programs. In the current review, some of the diagnostic methods for IHNV, spanning from clinical diagnosis to cell culture, serological and molecular methods are discussed in detail. In addition, some of the most significant candidate vaccines for IHNV are also extensively discussed, particularly the DNA vaccines.

N-linked glycosylation sites in G protein of infectious hematopoietic necrosis virus (IHNV) affect its virulence and immunogenicity in rainbow trout

Infectious hematopoietic necrosis virus (IHNV) causes infectious hematopoietic necrosis in salmonid fish, resulting in substantial economic losses to the aquaculture industry worldwide. The G protein, which harbors the major antigenic determinants of IHNV, is an envelope glycoprotein that plays an important role in both pathogenicity and immunogenicity of IHNV. Previous studies have demonstrated that changes to viral glycosylation sites may affect replication and immunogenicity, but little is known about the specific contributions of G protein glycosylation to IHNV replication and pathogenicity. In this study, we predicted four N-linked glycosylation sites at position 56, 379, 401, and 438 Asp (N) in G protein, and using a reverse genetics system developed in our laboratory, constructed nine recombinant viruses with single, triple, or quadruple glycosylation site disruptions using alanine substitutions in the following combinations: rIHNV-N56A, rIHNV-N379A, rIHNV-N401A, rIHNV-N438A, rIHNV-N56A-N379A-N401A, rIHNV-N56A-N379A-N438A, rIHNV-N56A-N401A-N438A, rIHNV-N379A-N401A-N438A, and rIHNV-N56A-N379A-N401A-N438A. Our results confirmed that all four asparagines are sites of N-linked glycosylation, and Western blot confirmed that mutation of each predicted N-glycosylation sited impaired glycosylation. Among the nine recombinant IHNVs, replication levels decreased significantly in vitro and in vivo in the triple and quadruple mutants that combined mutation of asparagines 401 and 438, indicating the importance of glycosylation at these sites for efficient replication. Moreover, juvenile rainbow trout mortality after challenge by each of the nine mutants showed that, while eight mutants suffered almost 100% cumulative mortality over 30 days, the mutant with a single alanine substitution at position 438 resulted in cumulative mortality of less than 50% over 30 days. This mutant also elicited specific anti-IHNV IgM production earlier than other mutants, suggesting that glycosylation of asparagine 438 may be important for viral immune escape. In conclusion, our study reveals the effect of G protein glycosylation on the pathogenicity and immunogenicity of IHNV and provides a foundation for developing a live-attenuated vaccine.

Bivalent DNA vaccine induces significant immune responses against infectious hematopoietic necrosis virus and infectious pancreatic necrosis virus in rainbow trout

Infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV) are important pathogens of salmon and trout. An active bivalent DNA vaccine was constructed with the glycoprotein gene of Chinese IHNV isolate Sn1203 and VP2-VP3 gene of Chinese IPNV isolate ChRtm213. Rainbow trout (5 g) were vaccinated by intramuscular injection with 1.0 µg of the bivalent DNA vaccine and then challenged with an intraperitoneal injection of IHNV, IPNV, or both, at 30 and 60 days post-vaccination (d.p.v.). High protection rates against IHNV were observed, with 6% and 10% cumulative mortality, respectively, compared with 90-94% in the mock-vaccinated groups. IPNV loads (531-fold and 135-fold, respectively) were significantly reduced in the anterior kidneys of the vaccinated trout. Significant protection against co-infection with IHNV and IPNV was observed, with cumulative mortality rates of 6.67% and 3.33%, respectively, compared with 50.0% and 43.3%, respectively, in the mock-vaccinated groups. No detectable infective IHNV or IPNV was recovered from vaccinated trout co-infected with IHNV and IPNV. The bivalent DNA vaccine increased the expression of Mx-1 and IFN-γ at 4, 7, and 15 d.p.v, and IgM at 21 d.p.v., and induced high titres (≥160) of IHNV and IPNV neutralizing antibodies at 30 and 60 d.p.v.

A effective DNA vaccine against diverse genotype J infectious hematopoietic necrosis virus strains prevalent in China

Infectious hematopoietic necrosis virus (IHNV) is the most important pathogen threatening the aquaculture of salmonid fish in China. In this study, a DNA vaccine, designated pIHNch-G, was constructed with the glycoprotein (G) gene of a Chinese IHNV isolate SD-12 (also called Sn1203) of genotype J. The minimal dose of vaccine required, the expression of the Mx-1 gene in the muscle (vaccine delivery site) and anterior kidney, and the titers of the neutralizing antibodies produced were used to evaluate the vaccine efficacy. To assess the potential utility of the vaccine in controlling IHNV throughout China, the cross protective efficacy of the vaccine was determined by challenging fish with a broad range of IHNV strains from different geographic locations in China. A single 100ng dose of the vaccine conferred almost full protection to rainbow trout fry (3g) against waterborne or intraperitoneal injection challenge with IHNV strain SD-12 as early as 4days post-vaccination (d.p.v.), and significant protection was still observed at 180d.p.v. Intragenogroup challenges showed that the DNA vaccine provided similar protection to the fish against all the Chinese IHNV isolates tested, suggesting that the vaccine can be widely used in China. Mx-1 gene expression was significantly upregulated in the muscle tissue (vaccine delivery site) and anterior kidney in the vaccinated rainbow trout at both 4 and 7d.p.v. Similar levels of neutralizing antibodies were determined with each of the Chinese IHNV strains at 60 and 180d.p.v. This DNA vaccine should play an important role in the control of IHN in China.

Molecular Evolution and Phylogeography of Co-circulating IHNV and VHSV in Italy

Infectious haematopoietic necrosis virus (IHNV) and viral haemorrhagic septicaemia virus (VHSV) are the most important viral pathogens impacting rainbow trout farming. These viruses are persistent in Italy, where they are responsible for severe disease outbreaks (epizootics) that affect the profitability of the trout industry. Despite the importance of IHNV and VHSV, little is known about their evolution at a local scale, although this is likely to be important for virus eradication and control. To address this issue we performed a detailed molecular evolutionary and epidemiological analysis of IHNV and VHSV in trout farms from northern Italy. Full-length glycoprotein gene sequences of a selection of VHSV (n = 108) and IHNV (n = 89) strains were obtained. This revealed that Italian VHSV strains belong to sublineages Ia1 and Ia2 of genotype Ia and are distributed into 7 genetic clusters. In contrast, all Italian IHNV isolates fell within genogroup E, for which only a single genetic cluster was identified. More striking was that IHNV has evolved more rapidly than VHSV (mean rates of 11 and 7.3 × 10⁻⁴ nucleotide substitutions per site, per year, respectively), indicating that these viruses exhibit fundamentally different evolutionary dynamics. The time to the most recent common ancestor of both IHNV and VHSV was consistent with the first reports of these pathogens in Italy. By combining sequence data with epidemiological information it was possible to identify different patterns of virus spread among trout farms, in which adjacent facilities can be infected by either genetically similar or different viruses, and farms located in different water catchments can be infected by identical strains. Overall, these findings highlight the importance of combining molecular and epidemiological information to identify the determinants of IHN and VHS spread, and to provide data that is central to future surveillance strategies and possibly control.

A Review of Intra- and Extracellular Antigen Delivery Systems for Virus Vaccines of Finfish

Vaccine efficacy in aquaculture has for a long time depended on evaluating relative percent survival and antibody responses after vaccination. However, current advances in vaccine immunology show that the route in which antigens are delivered into cells is deterministic of the type of adaptive immune response evoked by vaccination. Antigens delivered by the intracellular route induce MHC-I restricted CD8+ responses while antigens presented through the extracellular route activate MHC-II restricted CD4+ responses implying that the route of antigen delivery is a conduit to induction of B- or T-cell immune responses. In finfish, different antigen delivery systems have been explored that include live, DNA, inactivated whole virus, fusion protein, virus-like particles, and subunit vaccines although mechanisms linking these delivery systems to protective immunity have not been studied in detail. Hence, in this review we provide a synopsis of different strategies used to administer viral antigens via the intra- or extracellular compartments. Further, we highlight the differences in immune responses induced by antigens processed by the endogenous route compared to exogenously processed antigens. Overall, we anticipate that the synopsis put together in this review will shed insights into limitations and successes of the current vaccination strategies used in finfish vaccinology.

Antibody-mediated neutralization of African swine fever virus: myths and facts

Almost all viruses can be neutralized by antibodies. However, there is some controversy about antibody-mediated neutralization of African swine fever virus (ASFV) with sera from convalescent pigs and about the protective relevance of antibodies in experimentally vaccinated pigs. At present, there is no vaccine available for this highly lethal and economically relevant virus and all classical attempts to generate a vaccine have been unsuccessful. This failure has been attributed, in part, to what many authors describe as the absence of neutralizing antibodies. The findings of some studies clearly contradict the paradigm of the impossibility to neutralize ASFV by means of monoclonal or polyclonal antibodies. This review discusses scientific evidence of these types of antibodies in convalescent and experimentally immunized animals, the nature of their specificity, the neutralization-mediated mechanisms demonstrated, and the potential relevance of antibodies in protection.

The reverse genetics applied to fish RNA viruses

Aquaculture has expanded rapidly to become a major economic and food-producing sector worldwide these last 30 years. In parallel, viral diseases have emerged and rapidly spread from farm to farm causing enormous economic losses. The most problematic viruses encountered in the field are mainly, but not exclusively, RNA viruses belonging to the Novirhabdovirus, Aquabirnavirus, Alphavirus and Betanodavirus genera. The recent establishment of reverse genetics systems to recover infectious fish RNA viruses entirely from cDNA has made possible to genetically manipulate the viral genome. These systems have provided powerful tools to study all aspects of the virus biology and virus-host interactions but also gave the opportunity to use these viruses as live vaccines or as gene vectors. This review provides an overview on the recent breakthroughs achieved by using these reverse genetics systems in terms of viral protein function, virulence and host-specificity factor, vaccine development and vector design.

Molecular characterization of the virulent infectious hematopoietic necrosis virus (IHNV) strain 220-90

Background

Infectious hematopoietic necrosis virus (IHNV) is the type species of the genus Novirhabdovirus, within the family Rhabdoviridae, infecting several species of wild and hatchery reared salmonids. Similar to other rhabdoviruses, IHNV has a linear single-stranded, negative-sense RNA genome of approximately 11,000 nucleotides. The IHNV genome encodes six genes; the nucleocapsid, phosphoprotein, matrix protein, glycoprotein, non-virion protein and polymerase protein genes, respectively. This study describes molecular characterization of the virulent IHNV strain 220-90, belonging to the M genogroup, and its phylogenetic relationships with available sequences of IHNV isolates worldwide.

Results

The complete genomic sequence of IHNV strain 220-90 was determined from the DNA of six overlapping clones obtained by RT-PCR amplification of genomic RNA. The complete genome sequence of 220-90 comprises 11,133 nucleotides (GenBank GQ413939) with the gene order of 3'-N-P-M-G-NV-L-5'. These genes are separated by conserved gene junctions, with di-nucleotide gene spacers. An additional uracil nucleotide was found at the end of the 5'-trailer region, which was not reported before in other IHNV strains. The first 15 of the 16 nucleotides at the 3'- and 5'-termini of the genome are complementary, and the first 4 nucleotides at 3'-ends of the IHNV are identical to other novirhadoviruses. Sequence homology and phylogenetic analysis of the glycoprotein genes show that 220-90 strain is 97% identical to most of the IHNV strains. Comparison of the virulent 220-90 genomic sequences with less virulent WRAC isolate shows more than 300 nucleotides changes in the genome, which doesn't allow one to speculate putative residues involved in the virulence of IHNV.

Conclusion

We have molecularly characterized one of the well studied IHNV isolates, 220-90 of genogroup M, which is virulent for rainbow trout, and compared phylogenetic relationship with North American and other strains. Determination of the complete nucleotide sequence is essential for future studies on pathogenesis of IHNV using a reverse genetics approach and developing efficient control strategies.

Molecular characterization of the Great Lakes viral hemorrhagic septicemia virus (VHSV) isolate from USA

Background

Viral hemorrhagic septicemia virus (VHSV) is a highly contagious viral disease of fresh and saltwater fish worldwide. VHSV caused several large scale fish kills in the Great Lakes area and has been found in 28 different host species. The emergence of VHS in the Great Lakes began with the isolation of VHSV from a diseased muskellunge (Esox masquinongy) caught from Lake St. Clair in 2003. VHSV is a member of the genus Novirhabdovirus, within the family Rhabdoviridae. It has a linear single-stranded, negative-sense RNA genome of approximately 11 kbp, with six genes. VHSV replicates in the cytoplasm and produces six monocistronic mRNAs. The gene order of VHSV is 3'-N-P-M-G-NV-L-5'. This study describes molecular characterization of the Great Lakes VHSV strain (MI03GL), and its phylogenetic relationships with selected European and North American isolates.

Results

The complete genomic sequences of VHSV-MI03GL strain was determined from cloned cDNA of six overlapping fragments, obtained by RT-PCR amplification of genomic RNA. The complete genome sequence of MI03GL comprises 11,184 nucleotides (GenBank GQ385941) with the gene order of 3'-N-P-M-G-NV-L-5'. These genes are separated by conserved gene junctions, with di-nucleotide gene spacers. The first 4 nucleotides at the termini of the VHSV genome are complementary and identical to other novirhadoviruses genomic termini. Sequence homology and phylogenetic analysis show that the Great Lakes virus is closely related to the Japanese strains JF00Ehi1 (96%) and KRRV9822 (95%). Among other novirhabdoviruses, VHSV shares highest sequence homology (62%) with snakehead rhabdovirus.

Conclusion

Phylogenetic tree obtained by comparing 48 glycoprotein gene sequences of different VHSV strains demonstrate that the Great Lakes VHSV is closely related to the North American and Japanese genotype IVa, but forms a distinct genotype IVb, which is clearly different from the three European genotypes. Molecular characterization of the Great Lakes isolate will be helpful in studying the pathogenesis of VHSV using a reverse genetics approach and developing efficient control strategies.

Detecting molecular adaptation at individual codons in the glycoprotein gene of the geographically diversified infectious hematopoietic necrosis virus, a fish rhabdovirus

Salmonid fishes, the principal hosts of the infectious hematopoietic necrosis virus (IHNV), are a candidate species for aquaculture in many countries. IHNV causes an acute disease resulting in severe economic loss in salmonid fish farming. Previous phylogenetic analyses revealed the existence of multiple genogroups of this virus throughout the geographical range of its host. Here, we report the importance of natural selection in shaping the evolution of certain codons at the surface glycoprotein (G-protein) gene of this virus. Maximum likelihood (ML)-based codon substitution analyses revealed that approximately 2.8% of the codons for the entire G-protein are shown to have higher nonsynonymous substitution per nonsynonymous site (dn) than the synonymous substitutions per synonymous site (ds) (dn/ds=omega>4.335). Thus, the data suggest that positive selection (omega>1) is the major driving force in the evolution of certain codons. However, majority of these positively selected sites cannot be mapped to the regions of antigenic determinants of IHNV. Based on the reports of previous studies, epitopes with positively selected sites are immunodominant and viruses can escape from immune responses by producing antigenic variation at positively selected sites, therefore, vaccines directed against these neutralizing epitopes of IHNV that consist of no positively selected sites will be more effective. Some of the positively selected sites showed radical change in amino acids with respect to their charge and polarity; however, it is unclear how these changes affect the fitness of the virus.

Virulence Comparisons of Infectious Hematopoietic Necrosis Virus U and M Genogroups in Sockeye Salmon and Rainbow Trout

Infectious hematopoietic necrosis virus (IHNV) is an aquatic rhabdovirus that infects salmonids in the Pacific Northwest of the United States, Europe, and Asia. Isolates of IHNV have been phylogenetically classified into three major viral genogroups, designated U, M, and L. To characterize virulence of IHNV in the context of these three viral genogroups, seven strains of IHNV (three U genogroup strains, three M strains, and one L strain) were compared for their pathogenicity in juvenile sockeye salmon Oncorhynchus nerka, kokanee (lacustrine sockeye salmon), and rainbow trout O. mykiss. Fish were waterborne-exposed to the different viral strains, and virulence was assessed by comparing mortality curves and final cumulative percent mortality (CPM) in both species of fish at 10°C and 15°C. In sockeye salmon and kokanee, the U genogroup virus types were extremely virulent, causing average CPMs of 69-100%, while the M genogroup virus types caused very little or no mortality (CPM = 0-4%). The endangered Redfish Lake sockeye salmon stock exhibited extreme differences in susceptibility to the U and M genogroups. Conversely, in two stocks of rainbow trout, the M genogroup virus types were more virulent, inducing average CPMs of 25-85%, while the U genogroup viruses caused lower mortality (CPM = 5-41%). In both fish species, the single L genogroup strain caused low to intermediate mortality (CPM = 13-53%). Viral glycoprotein sequence comparisons of the seven challenge strains revealed three amino acid sites (247, 256, and 270) that consistently differed between the U and M genogroups, possibly contributing to pathogenicity differences.

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.

Interferon-inducible Mx proteins in fish

Mx proteins are members of a family of interferon-inducible genes expressed when cells are treated with double-stranded RNA or virus infection. These proteins are important components of the antiviral response and form the first line of the body's defense against virus infections. The exact mechanism of action for these proteins has not been discovered, but mice missing the Mx genes are extremely sensitive to influenza virus infection. Mammals have between two and three Mx genes whose functions may vary with regard to the inhibition of a specific virus, cellular localization, and activity. The cDNA of three rainbow trout Mx proteins has been cloned and a comparison of their sequences with that of avian and mammalian species reveals striking conservation of domains. They all maintain the tripartite ATP/GTP binding domain and the dynamin family signature in the amino terminal half of the protein. In the carboxyl terminal half of the Mx proteins are the localization signals and the leucine zipper motifs which account for the trimerization of Mx in the cell. Like the rat and human Mx proteins, the different trout Mx proteins exhibit distinctly different immunohistochemical staining patterns in cells transfected with plasmids expressing RBTMx1, RBTMx2, or RBTMx3. To date, the antiviral function of the trout Mx proteins has not been satisfactorily established.

Neutralization susceptibility of African swine fever virus is dependent on the phospholipid composition of viral particles

In this study we have investigated the generation of African swine fever (ASF) virus variants resistant to neutralizing antibodies after cell culture propagation. All highly passaged ASF viruses analyzed were resistant to neutralization by antisera from convalescent pigs or antibodies generated against individual viral proteins which neutralized low-passage viruses. A molecular analysis of neutralizable and nonneutralizable virus isolates by sequencing of the genes encoding for neutralizing proteins revealed that the absence of neutralization of high-passage viruses is not due to antigenic variability of critical epitopes. A comparative analysis of phospholipid composition of viral membranes between low- and high-passage viruses revealed differences in the relative amount of phosphatidylinositol in these two groups of viruses, independent of the cells in which the viruses were grown. Further purification of low- and high-passage viruses by Percoll sedimentation showed differences in the phospholipid composition identical to those found with the partially purified viruses and confirmed the susceptibility of these viruses to neutralization. The incorporation of phosphatidylinositol into membranes of high-passage viruses rendered a similar neutralization susceptibility to low-passage viruses, in which this is a major phospholipid. In contrast, other phospholipids did not interfere with high-passage virus neutralization, suggesting that phosphatidylinositol is essential for a correct epitope presentation to neutralizing antibodies. Additionally, the removal of phosphatidylinositol form a low-passage virus by a specific lipase transformed this virus from neutralizable to nonneutralizable. These data constitute clear evidence of the importance of the lipid composition of the viral membranes for the protein recognition by antibodies and may account in part for the past difficulties in reproducibly demonstrating ASF virus-neutralizing antibodies by using high-passage viruses.

Molecular epizootiology and evolution of the glycoprotein and non-virion protein genes of infectious hematopoietic necrosis virus, a fish rhabdovirus

Infectious hematopoietic necrosis virus (IHNV) causes a highly lethal, economically important disease of salmon and trout. The virus is enzootic throughout western North America, and has been spread to Asia and Europe. The nucleotide sequences of the glycoprotein (G) and non-virion (NV) genes of 12 diverse IHNV isolates were determined in order to examine the molecular epizootiology of IHN, the primary structure and conservation of NV, and the evolution of the virus. The G and NV genes and their encoded proteins were highly conserved, with a maximum pairwise nucleotide divergence of 3.6 and 4.4%, and amino acid divergence of 3.7 and 6.2%, respectively. Conservation of NV protein sequence (111 amino acids in length) confirms that the protein is functional and plays an important role in virus replication. The phylogenetic relationship of viruses was found to correlate with the geographic origin of virus isolates rather than with host species or time of isolation. These data are consistent with stable maintenance of virus in enzootic foci. Two main IHNV genetic lineages were identified; one in the Columbia River Basin (Oregon, Washington and Idaho), the other in the Sacramento River Basin (California). The first major IHNV outbreak in chinook salmon in 1973 in the Columbia River was genetically linked to importation of virus-infected fish eggs from the Sacramento River where outbreaks in chinook salmon are common. However, the introduced virus apparently did not persist, subsequent virus outbreaks in Columbia River chinook salmon being associated with Columbia River genetic lineages. In general, virus monoclonal antibody reactivity profiles and phylogenetic relationships correlated well.