A protein G fragment from the salmonid viral hemorrhagic septicemia rhabdovirus induces cell-to-cell fusion and membrane phosphatidylserine translocation at low pH

Optimised protocols to generate high titre lentiviral vectors using a novel transfection agent enabling extended HEK293T culture following transient transfection and suspension culture

HIV-1 based lentiviral viruses are considered powerful and versatile gene therapy vectors to deliver therapeutic genes to patients with hereditary or acquired diseases. These vectors can efficiently transduce a variety of cell types when dividing or non-dividing to provide permanent delivery and long-term gene expression. Demand for scalable manufacturing protocols able to generate enough high titre vector for widespread use of this technology is increasing and considerable efforts to improve vector production cost-effectively, is ongoing. Current methods for LV production mainly use transient transfection of producer cell lines. Cells can be grown at scale, either in 2D relying on culturing producer cells in multi-tray flask cell culture factories or in roller bottles or can be adapted to grow in 3D suspensions in large batch fermenters. This suits rapid production and testing of new vector constructs pre-clinically for their efficacy, particle titre and safety. In this study, we sought to improve lentiviral titre over time by testing two alternative commercially available transfection reagents Fugene® 6 and Genejuice® with the commonly used polycation, polyethyleneimine. Our aim was to identify less cytotoxic transfection reagents that could be used to generate LV particles at high titre past the often used 72 h period when vector is usually collected before producer cell death is caused due to post transfection cytotoxicity. We show that LV could be produced in extended culture using Genejuice® and even by transfected cells in glass flasks in suspension. Because this delivery agent is less toxic to 293 T producer cells, following optimisation of transfection we found that LV can be harvested for more than 10 days at high titre. Using our protocol, titres of 109 TU/ml and 108 TU/ml were routinely reached via traditional monolayer conditions or suspension cultures, respectively. We propose, this simple change in vector production enables large volumes of high titre vector to be produced, cost effectively for non-clinical in vivo and in vitro applications or for more stringent downstream clinical grade vector purification.

A Cell Membrane Fusion Assay for the Fish Pathogen Spring Viremia of Carp Virus (SVCV)

Fusion of the infected cell membranes is a characteristic effect of a wide number of viral infections. Spring viremia of carp virus (SVCV), a rhabdovirus causing disease in farmed carp, can induce membrane fusion of the infected cells by shifting the pH of the culture medium to slightly acidic. Membrane fusion leads to the formation of clusters of cell nuclei enclosed in a cell membrane, the so-called syncytia, that can be easily visualized by cell staining and light microscope inspection. In the present work, we report a protocol to induce syncytia formation in EPC cells infected with SVCV, where membrane fusion is triggered by a low-pH incubation step. Appearance of syncytia can be observed at 18 hours post infection. The syncytia formation assay described here may serve as an experimental platform to quantitate SVCV, to determine virus infectivity, and a useful tool to study virus entry into the cell as well as to test candidate antiviral compounds that could block the entry of SVCV into cells by inhibiting membrane fusion.

Antiviral Activity of a Turbot (Scophthalmus maximus) NK-Lysin Peptide by Inhibition of Low-pH Virus-Induced Membrane Fusion

Global health is under attack by increasingly-frequent pandemics of viral origin. Antimicrobial peptides are a valuable tool to combat pathogenic microorganisms. Previous studies from our group have shown that the membrane-lytic region of turbot (Scophthalmus maximus) NK-lysine short peptide (Nkl_71–100) exerts an anti-protozoal activity, probably due to membrane rupture. In addition, NK-lysine protein is highly expressed in zebrafish in response to viral infections. In this work several biophysical methods, such as vesicle aggregation, leakage and fluorescence anisotropy, are employed to investigate the interaction of Nkl_71–100 with different glycerophospholipid vesicles. At acidic pH, Nkl_71–100 preferably interacts with phosphatidylserine (PS), disrupts PS membranes, and allows the content leakage from vesicles. Furthermore, Nkl_71–100 exerts strong antiviral activity against spring viremia of carp virus (SVCV) by inhibiting not only the binding of viral particles to host cells, but also the fusion of virus and cell membranes, which requires a low pH context. Such antiviral activity seems to be related to the important role that PS plays in these steps of the replication cycle of SVCV, a feature that is shared by other families of virus-comprising members with health and veterinary relevance. Consequently, Nkl_71–100 is shown as a promising broad-spectrum antiviral candidate.

Hydroxycholesterol binds and enhances the anti-viral activities of zebrafish monomeric c-reactive protein isoforms

C-reactive proteins (CRPs) are among the faster acute-phase inflammation-responses proteins encoded by one gene (hcrp) in humans and seven genes (crp1-7) in zebrafish (Danio rerio) with importance in bacterial and viral infections. In this study, we described novel preferential bindings of 25-hydroxycholesterol (25HOCh) to CRP1-7 compared with other lipids and explored the antiviral effects of both 25HOCh and CRP1-7 against spring viremia carp virus (SVCV) infection in zebrafish. Both in silico and in vitro results confirmed the antiviral effect of 25HOCh and CRP1-7 interactions, thereby showing that the crosstalk between them differed among the zebrafish isoforms. The presence of oxidized cholesterols in human atherosclerotic plaques amplifies the importance that similar interactions may occur for vascular and/or neurodegenerative diseases during viral infections. In this context, the zebrafish model offers a genetic tool to further investigate these interactions.

A new coumarin derivative plays a role in rhabdoviral clearance by interfering glycoprotein function during the early stage of viral infection

Coumarin forms an elite class of naturally occurring compounds that possess promising antiviral therapeutic perspectives. In this study, a coumarin derivative 7-[6-(2-methylimidazole) hexyloxy] coumarin (D5) was designed and synthesized to evaluate antiviral activity on a rhabdovirus, spring viraemia of carp virus (SVCV). Our results demonstrated that D5 had a robust antiviral activity with >90% inhibitory rate of SVCV expression in the host cells. And D5 significantly reduced viral-induced apoptosis and recovered virus-activated caspase-3/8/9 activities. Further data determined that SVCV could alter the cytoskeletal structure of EPC cells, characterized by a circumferential ring of microtubules and a disrupted microfilament organization, whereas cytoskeleton structure in D5-treated cells kept the normal morphology. Mechanistically speaking, D5 could interfere with SVCV replication inside or outside of cells through two different approaches. Before the process of virus entry into EPC cells, D5 had an impact on SVCV glycoprotein structure so as to disrupt viral binding to the cell surface or translocation to the cytosol. Another strategy for D5 to against SVCV was that D5 significantly suppressed SVCV-activated autophagy, which was beneficial for the host cells to restrict SVCV viral replication, accompanied by a higher phosphorylation of Akt-mTOR. In summary, our results revealed that D5 was effective in weakening SVCV infection and regulating SVCV-induced undesirable conditions, and this compound provided new therapeutic implications for the treatment of rhabdoviruses.

Neutralization of viral infectivity by zebrafish c-reactive protein isoforms

This work explores the unexpected in vivo and in vitro anti-viral functions of the seven c-reactive protein (crp1-7) genes of zebrafish (Danio rerio). First results showed heterogeneous crp1-7 transcript levels in healthy wild-type zebrafish tissues and organs and how those levels heterogeneously changed not only after bacterial but also after viral infections, including those in adaptive immunity-deficient rag1^-/- mutants. As shown by microarray hybridization and proteomic techniques, crp2/CRP2 and crp5/CRP5 transcripts/proteins were among the most modulated during in vivo viral infection situations including the highest responses in the absence of adaptive immunity. In contrast crp1/CRP1/and crp7/CRP7 very often remained unmodulated. All evidences suggested that zebrafish crp2-6/CRP2-6 may have in vivo anti-viral activities in addition to their well known anti-bacterial and/or physiological functions in mammalians. Confirming those expectations, in vitro neutralization and in vivo protection against spring viremia carp virus (SVCV) infections were demonstrated by crp2-6/CRP2-6 using crp1-7 transfected and/or CRP1-7-enriched supernatant-treated fish cells and crp2-5-injected one-cell stage embryo eggs, respectively. All these findings discovered a crp1-7/CRP1-7 primitive anti-viral functional diversity.These findings may help to study similar functions on the one-gene-coded human CRP, which is widely used as a clinical biomarker for bacterial infections, tissue inflammation and coronary heart diseases.

Structure and functionalities of the human c-reactive protein compared to the zebrafish multigene family of c-reactive-like proteins

Because of the recent discovery of multiple c-reactive protein (crp)-like genes in zebrafish (Danio rerio) with predicted heterogeneous phospholipid-binding amino acid sequences and heterogeneous transcript expression levels in viral survivors and adaptive-deficient mutants, zebrafish constitute an attractive new model for exploring the evolution of these protein's functions, including their possible participation in fish trained immunity. Circulating human CRP belongs to the short pentraxin family of oligomeric proteins that are characteristic of early acute-phase innate responses and is widely used as a clinical inflammation marker. In contrast to pentameric human CRP (pCRP), zebrafish CRPs are trimeric (tCRP); however monomeric CRP (mCRP) conformations may also be generated when associated with cellular membranes as occurs in humans. Compared to human CRP, zebrafish CRP-like proteins show homologous amino acid sequence stretches that are consistent with, although not yet demonstrated, cysteine-dependent redox switches, calcium-binding spots, phosphocholine-binding pockets, C1q-binding domains, regions interacting with immunoglobulin Fc receptors (FcR), unique mCRP epitopes, mCRP binding peptides to cholesterol-enriched rafts, protease target sites, and/or binding sites to monocyte, macrophage, neutrophils, platelets and/or endothelial cells. Amino acid variations among the zebrafish CRP-like multiprotein family and derived isoforms in these stretches suggest that functional heterogeneity best fits the wide variety of aquatic pathogens. As occurs in humans, phospholipid-tagged tCRP-like multiproteins might also influence local inflammation and induce innate immune responses; however, in addition, different zebrafish tCRP-like proteins and/or isoforms might fine tune new still unknown functions. The information reviewed here could be of value for future studies not only to comparative but also medical immunologists and/or fisheries sectors. This review also introduces some novel speculations for future studies.

TNFα Impairs Rhabdoviral Clearance by Inhibiting the Host Autophagic Antiviral Response

TNFα is a pleiotropic pro-inflammatory cytokine with a key role in the activation of the immune system to fight viral infections. Despite its antiviral role, a few viruses might utilize the host produced TNFα to their benefit. Some recent reports have shown that anti-TNFα therapies could be utilized to treat certain viral infections. However, the underlying mechanisms by which TNFα can favor virus replication have not been identified. Here, a rhabdoviral infection model in zebrafish allowed us to identify the mechanism of action by which Tnfa has a deleterious role for the host to combat certain viral infections. Our results demonstrate that Tnfa signals through its receptor Tnfr2 to enhance viral replication. Mechanistically, Tnfa does not affect viral adhesion and delivery from endosomes to the cytosol. In addition, the host interferon response was also unaffected by Tnfa levels. However, Tnfa blocks the host autophagic response, which is required for viral clearance. This mechanism of action provides new therapeutic targets for the treatment of SVCV-infected fish, and advances our understanding of the previously enigmatic deleterious role of TNFα in certain viral infections.

Tumor necrosis factor alpha (TNFα) is one of the main pro-inflammatory cytokines produced in response to a broad type of infections [1]. Although TNFα has a crucial role in protecting the host organism from pathogens, its deregulation can promote susceptibility to pathogens by impairing pathogen clearance and, ultimately, promoting maintenance of infection and death. In addition, some viruses might utilize the host produced TNFα to their benefit. Thus, anti-TNFα therapies could be utilized to treat certain viral infections. However, the underlying mechanisms by which TNFα can favor certain virus replication have not been identified. Here, we have used a viral infection model in zebrafish to identify the mechanism of action by which TNFα has a deleterious role for the host to combat certain viral infections. Our results demonstrate that Tnfa does not affect viral ability to infect host cells or to antagonize the main host antiviral pathway, namely the interferon pathway. However, Tnfa impairs viral clearance by blocking the host autophagy response, which is usually used by host cells to degrade unnecessary or dysfunctional cellular components, and that we found to be critical to eliminate intracellular viral particles. This mechanism of action provides new therapeutic targets for the treatment of SVCV-infected fish in aquaculture and probably to other viral infection affecting cattle industry and human.

Development of new therapeutical/adjuvant molecules by pepscan mapping of autophagy and IFN inducing determinants of rhabdoviral G proteins

Surface glycoproteins of enveloped virus are potent elicitors of both innate and adaptive host immune responses. Therefore, the identification of viral glycoprotein determinants directly implicated in the induction of these responses might be of special interest for designing new therapeutical/adjuvant molecules. In this work we review the contribution of the "pepscan" approach to the screening of viral functions in the sequence of glycoprotein G (gpG) of the fish rhabdovirus of viral hemorrhagic septicemia (VHSV). Among others, by scanning gpG peptides, it has been possible to identify and validate minimal determinants for gpG directly implicated in initiating the fish type I Interferon-associated immune responses as well as in the antiviral autophagy program. Further fine-tunning of the identified peptides in the gpG of VHSV has allowed designing novel adjuvants that decrease DNA vaccine requirements and identify possible innovative antiviral molecules. In addition, these results have also contributed to improve our knowledge on how to stimulate the fish immune system.

Autophagy-inducing peptides from mammalian VSV and fish VHSV rhabdoviral G glycoproteins (G) as models for the development of new therapeutic molecules

It has not been elucidated whether or not autophagy is induced by rhabdoviral G glycoproteins (G) in vertebrate organisms for which rhabdovirus infection is lethal. Our work provides the first evidence that both mammalian (vesicular stomatitis virus, VSV) and fish (viral hemorrhagic septicemia virus, VHSV, and spring viremia carp virus, SVCV) rhabdoviral Gs induce an autophagic antiviral program in vertebrate cell lines. The transcriptomic profiles obtained from zebrafish genetically immunized with either Gsvcv or Gvhsv suggest that autophagy is induced shortly after immunization and therefore, it may be an important component of the strong antiviral immune responses elicited by these viral proteins. Pepscan mapping of autophagy-inducing linear determinants of Gvhsv and Gvsv showed that peptides located in their fusion domains induce autophagy. Altogether these results suggest that strategies aimed at modulating autophagy could be used for the prevention and treatment of rhabdoviral infections such as rabies, which causes thousands of human deaths every year.

Increasing versatility of the DNA vaccines through modification of the subcellular location of plasmid-encoded antigen expression in the in vivo transfected cells

The route of administration of DNA vaccines can play a key role in the magnitude and quality of the immune response triggered after their administration. DNA vaccines containing the gene of the membrane-anchored glycoprotein (gpG) of the fish rhabdoviruses infectious haematopoietic necrosis virus (IHNV) or viral haematopoietic septicaemia virus (VHSV), perhaps the most effective DNA vaccines generated so far, confer maximum protection when injected intramuscularly in contrast to their low efficacy when injected intraperitoneally. In this work, taking as a model the DNA vaccine against VHSV, we focused on developing a more versatile DNA vaccine capable of inducing protective immunity regardless of the administration route used. For that, we designed two alternative constructs to gpG_1-507 (the wild type membrane-anchored gpG of VHSV) encoding either a soluble (gpG_1-462) or a secreted soluble (gpG_LmPle20-462) form of the VHSV-gpG. In vivo immunisation/challenge assays showed that only gpG_LmPle20-462 (the secreted soluble form) conferred protective immunity against VHSV lethal challenge via both intramuscular and intraperitoneal injection, being this the first description of a fish viral DNA vaccine that confers protection when administered intraperitoneally. Moreover, this new DNA vaccine construct also conferred protection when administered in the presence of an oil adjuvant suggesting that DNA vaccines against rhabdoviruses could be included in the formulation of current multicomponent-intaperitoneally injectable fish vaccines formulated with an oil adjuvant. On the other hand, a strong recruitment of membrane immunoglobulin expressing B cells, mainly membrane IgT, as well as t-bet expressing T cells, at early times post-immunisation, was specifically observed in the fish immunised with the secreted soluble form of the VHSV-gpG protein; this may indicate that the subcellular location of plasmid-encoded antigen expression in the in vivo transfected cells could be an important factor in determining the ways in which DNA vaccines prime the immune response.

Improving the safety of viral DNA vaccines: development of vectors containing both 5' and 3' homologous regulatory sequences from non-viral origin

Although some DNA vaccines have proved to be very efficient in field trials, their authorisation still remains limited to a few countries. This is in part due to safety issues because most of them contain viral regulatory sequences to driving the expression of the encoded antigen. This is the case of the only DNA vaccine against a fish rhabdovirus (a negative ssRNA virus), authorised in Canada, despite the important economic losses that these viruses cause to aquaculture all over the world. In an attempt to solve this problem and using as a model a non-authorised, but efficient DNA vaccine against the fish rhabdovirus, viral haemorrhagic septicaemia virus (VHSV), we developed a plasmid construction containing regulatory sequences exclusively from fish origin. The result was an "all-fish vector", named pJAC-G, containing 5' and 3' regulatory sequences of β-acting genes from carp and zebrafish, respectively. In vitro and in vivo, pJAC-G drove a successful expression of the VHSV glycoprotein G (G), the only antigen of the virus conferring in vivo protection. Furthermore, and by means of in vitro fusion assays, it was confirmed that G protein expressed from pJAC-G was fully functional. Altogether, these results suggest that DNA vaccines containing host-homologous gene regulatory sequences might be useful for developing safer DNA vaccines, while they also might be useful for basic studies.

Rainbow trout surviving infections of viral haemorrhagic septicemia virus (VHSV) show lasting antibodies to recombinant G protein fragments

Rainbow trout antibodies (Abs) binding to recombinant fragments (frgs) derived from the protein G of the viral haemorrhagic septicemia virus (VHSV)-07.71 strain, could be detected by ELISA (frg-ELISA) in sera from trout surviving laboratory-controlled infections. Abs were detected not only by using sera from trout infected with the homologous VHSV isolate but also with the VHSV-DK-201433 heterologous isolate, which had 13 amino acid changes. Sera from healthy trout and/or from trout surviving infectious haematopoietic necrosis virus (IHNV) infection, were used to calculate cut-off absorbances to differentiate negative from positive sera. Specific anti-VHSV Abs could then be detected by using any of the following frgs: frg11 (56-110), frg15 (65-250), frg16 (252-450) or G21-465. While high correlations were found among the ELISA values obtained with the different frgs, no correlations between any frg-ELISA and complement-dependent 50% plaque neutralization test (PNT) titres could be demonstrated. Between 4 and 10 weeks after VHSV infection, more trout sera were detected as positives by using heterologous frg-ELISA rather than homologous PNT. Furthermore, the percentage of positive sera detected by frg11-ELISA increased with time after infection to reach 100%, while those detected by complement-dependent PNT decreased to 29.4%, thus confirming that the lack of neutralizing Abs does not mean the lack of any anti-VHSV Abs in survivor trout sera. Preliminary results with sera from field samples suggest that further refinements of the frg-ELISA could allow detection of anti-VHSV trout Abs in natural outbreaks caused by different heterologous VHSV isolates. The homologous frg-ELISA method could be useful to follow G immunization attempts during vaccine development and/or to best understand the fish Ab response during VHSV infections. The viral frgs approach might also be used with other fish species and/or viruses.

Pepscan mapping of viral hemorrhagic septicemia virus glycoprotein G major lineal determinants implicated in triggering host cell antiviral responses mediated by type I interferon

Surface glycoproteins of enveloped virus are potent elicitors of type I interferon (IFN)-mediated antiviral responses in a way that may be independent of the well-studied genome-mediated route. However, the viral glycoprotein determinants responsible for initiating the IFN response remain unidentified. In this study, we have used a collection of 60 synthetic 20-mer overlapping peptides (pepscan) spanning the full length of glycoprotein G (gpG) of viral hemorrhagic septicemia virus (VHSV) to investigate what regions of this protein are implicated in triggering the type I IFN-associated immune responses. Briefly, two regions with ability to increase severalfold the basal expression level of the IFN-stimulated mx gene and to restrict the spread of virus among responder cells were mapped to amino acid residues 280 to 310 and 340 to 370 of the gpG protein of VHSV. In addition, the results obtained suggest that an interaction between VHSV gpG and integrins might trigger the host IFN-mediated antiviral response after VHSV infection. Since it is known that type I IFN plays an important role in determining/modulating the protective-antigen-specific immune responses, the identification of viral glycoprotein determinants directly implicated in the type I IFN induction might be of special interest for designing new adjuvants and/or more-efficient and cost-effective viral vaccines as well as for improving our knowledge on how to stimulate the innate immune system.

Internalization and fusion mechanism of vesicular stomatitis virus and related rhabdoviruses

Members of the Rhabdoviridae infect a wide variety of animals and plants, and are the causative agents of many important diseases. Rhabdoviruses enter host cells following internalization into endosomes, with the glycoprotein (G protein) mediating both receptor binding to host cells and fusion with the cellular membrane. The recently solved crystal structure of vesicular stomatitis virus G has allowed considerable insight into the mechanism of rhabdovirus entry, in particular the low pH-dependent conformational changes that lead to fusion activation. Rhabdovirus entry shows several distinct features compared with other enveloped viruses; first, the entry process appears to consist of two distinct fusion events, initial fusion into vesicles within endosomes followed by back-fusion into the cytosol; second, the conformational changes in the G protein that lead to fusion activation are reversible; and third, the G protein is structurally distinct from other viral fusion proteins and is not proteolytically cleaved. The internalization and fusion mechanisms of rhabdoviruses are discussed in this article, with a focus on viral systems where the G protein has been studied extensively: vesicular stomatitis virus and rabies virus, as well as viral hemorrhagic septicemia virus.

Dual antiviral activity of human alpha-defensin-1 against viral haemorrhagic septicaemia rhabdovirus (VHSV): inactivation of virus particles and induction of a type I interferon-related response

It is well known that human alpha-defensin-1, also designated as human neutrophil peptide 1 (HNP1), is a potent inhibitor towards several enveloped virus infecting mammals. In this report, we analyzed the mechanism of the antiviral action of this antimicrobial peptide (AMP) on viral haemorrhagic septicaemia virus (VHSV), a salmonid rhabdovirus. Against VHSV, synthetic HNP1 possesses two antiviral activities. The inactivation of VHSV particles probably through interfering with VHSV-G protein-dependent fusion and the inhibition of VHSV replication in target cells by up-regulating genes related to the type I interferon (IFN) response, such as Mx. Neither induction of IFN-stimulated genes (ISGs) by HNP1 nor their antiviral activity against fish rhabovirus has been previously reported. Therefore, we can conclude that besides to acting as direct effector, HNP1 acts across species and can elicit one of the strongest antiviral responses mediated by innate immune system. Since the application of vaccine-based immunization strategies is very limited, the used of chemicals is restricted because of their potential harmful impact on the environment and no antimicrobial peptides from fish that exhibit both antiviral and immunoenhancing capabilities have been described so far, HNP1 could be a model molecule for the development of antiviral agents for fish. In addition, these results further confirm that molecules that mediate the innate resistance of animals to virus may prove useful as templates for new antivirals in both human and animal health.

Viral membrane fusion: is glycoprotein G of rhabdoviruses a representative of a new class of viral fusion proteins?

Enveloped viruses always gain entry into the cytoplasm by fusion of their lipid envelope with a cell membrane. Some enveloped viruses fuse directly with the host cell plasma membrane after virus binding to the cell receptor. Other enveloped viruses enter the cells by the endocytic pathway, and fusion depends on the acidification of the endosomal compartment. In both cases, virus-induced membrane fusion is triggered by conformational changes in viral envelope glycoproteins. Two different classes of viral fusion proteins have been described on the basis of their molecular architecture. Several structural data permitted the elucidation of the mechanisms of membrane fusion mediated by class I and class II fusion proteins. In this article, we review a number of results obtained by our laboratory and by others that suggest that the mechanisms involved in rhabdovirus fusion are different from those used by the two well-studied classes of viral glycoproteins. We focus our discussion on the electrostatic nature of virus binding and interaction with membranes, especially through phosphatidylserine, and on the reversibility of the conformational changes of the rhabdovirus glycoprotein involved in fusion. Taken together, these data suggest the existence of a third class of fusion proteins and support the idea that new insights should emerge from studies of membrane fusion mediated by the G protein of rhabdoviruses. In particular, the elucidation of the three-dimensional structure of the G protein or even of the fusion peptide at different pH's might provide valuable information for understanding the fusion mechanism of this new class of fusion proteins.

Conformation- and fusion-defective mutations in the hypothetical phospholipid-binding and fusion peptides of viral hemorrhagic septicemia salmonid rhabdovirus protein G

Fourteen single and two double point mutants in the highly conserved region (positions 56 to 159) of the G gene of viral hemorrhagic septicaemia virus (VHSV), a salmonid rhabdovirus, were selected and obtained in plasmids by site-directed mutagenesis. Fish cell monolayers transfected with the mutant plasmids were then assayed for protein G (pG) expression, conformation-dependent monoclonal antibody (MAb) reactivity, and cell-cell fusion. Some mutations located in the phospholipid-binding p2 peptide (positions 82 to 110; mutants P86A, A96E, G98A, and R107A) abolished both MAb recognition and fusion activity, while others (P79A, L85S, and R103A) abolished MAb recognition but retained fusion at similar or lower pHs compared to those for the wild type. Phospholipid-binding assays of p2-derived synthetic peptides suggested that phosphatidylserine binding was not affected by the mutations studied. On the other hand, three (P79A, L85S, and T135E) of the four mutants retaining fusion activity mapped around two locations showing amino acid variation in 22 VHSV isolates and in neutralizing MAb-resistant mutants described previously. Mutations located in the hypothetical fusion peptide (positions 142 to 159; mutants F147K, P148K, and W154K) abolished both MAb recognition and fusion activity. The existence of mutants with altered conformation and defective fusion in both p2 and fusion peptides provides further evidence in favor of the participation of these and adjacent regions in some of the steps of the VHSV fusion processes, as suggested by previous studies. In addition, because the studied region induced strong immunological responses in trout, some of the mutants described here might be used to design attenuated VHSV vaccines.

Characterisation of the syncytia formed by VHS salmonid rhabdovirus G gene transfected cells

Protein G expression and cell-to-cell fusion of cells transfected with the G gene of viral haemorrhagic septicaemia virus (VHSV) has been characterised. The presence of protein G in the membrane of transfected cells was confirmed by staining with Abs (Abs) and FACS. The subsequent formation of syncytia by membrane fusion of transfected cells required transfection with a wild type G gene and a low pH step. Mice Abs made against the protein G regions involved in fusion and neutralising monoclonal Abs (MAbs) as well as MAbs against some linear epitopes inhibited syncytia formation, thus confirming that syncytia formation was G-dependent. Similarly, Abs from trout immunised with purified VHSV or protein G inhibited syncytia formation whereas Abs from non-immunised or non-infected animals did not. Abs from mice or trout with the highest neutralisation titres also showed the highest percentage of inhibition of syncytia. While the main utility of these observations might be to further the understanding of the complex trout antibody response against VHSV and in the follow up of VHSV immunisation attempts, they may also have some future diagnostic potential for countries were work with VHSV is not allowed.

Reversible inhibition of spreading of in vitro infection and imbalance of viral protein accumulation at low pH in viral hemorrhagic septicemia rhabdovirus, a salmonid rhabdovirus

The inhibition of viral hemorrhagic septicemia rhabdovirus (VHSV) in vitro infection by pHs of <7 (low pH) has been previously reported. Nevertheless, the details of the mechanism underlying this effect remain obscure. We present evidence showing that low-pH inhibition occurs during a viral postadsorption step. Thus, while VHSV bound, replicated within single cells, and presented its G protein on the membranes of infected cells at both low and physiological pHs, both cell-to-cell spreading of infection (as estimated by the appearance of foci of infected cells) and fusion (as estimated by a syncytium assay) were inhibited by this low pH. The decreased VHSV titers and the inhibition of both cell-to-cell spreading of infection and fusion could be reversed by adjusting the pH to 7.5 at any time during infection. This effect should be taken into account to avoid false negatives in the diagnosis of VHSV by cell culture. On the other hand, the cell-to-cell spreading of infection at pH 7.5 could be stopped at any time by reducing the pH to 6.5. Since at low pH there were changes in the protein G conformation and smaller and imbalanced amounts of N with respect to M1, M2, and G viral proteins, alterations of the assembly and/or budding of VHSV are most probably involved in the absence of newly released infective virions.

Salmonid viral haemorrhagic septicaemia virus: fusion-related enhancement of virus infectivity by peptides derived from viral glycoprotein G or a combinatorial library

To search for enhancers and/or inhibitors of viral haemorrhagic septicaemia virus (VHSV, a salmonid rhabdovirus) infectivity, a total of 51 peptides from a pepscan of viral envelope protein G, a recombinant peptide from protein G (frg11) and 80 peptide mixtures from an alpha-helix-favoured combinatorial library were screened. However, contrary to what occurs in many other enveloped viruses, only peptides enhancing rather than inhibiting VHSV infectivity were found. Because some of the enhancer pepscan G peptides and frg11 were derived from phospholipid-binding or fusion-related regions identified previously, it was suggested that enhancement of virus infectivity might be related to virus-cell fusion. Furthermore, enhancement was significant only when the viral peptides were pre-incubated with VHSV at the optimal low pH of fusion, before being adjusted to physiological pH and assayed for infectivity. Enhancement of VHSV infectivity caused by the pre-incubation of VHSV with peptide p5 (SAAEASAKATAEATAKG), one of the individual enhancer peptides defined from the screening of the combinatorial library, was independent of the pre-incubation pH. However, it was also related to fusion because the binding of p5 to protein G induced VHSV to bypass the endosome pathway of infection and reduced the low-pH threshold of fusion, thus suggesting an alternative virus entry pathway for p5-VHSV complexes. Further investigations into VHSV enhancer peptides might shed some light on the mechanisms of VHSV fusion.

Antibody response to a fragment of the protein G of VHS rhabdovirus in immunised trout

A fragment (called frg#11, amino acids, aa 56-110) of the protein G (pG) of viral haemorrhagic septicaemia virus (VHSV) was designed after previous results showed it to be recognised by approximately 40% of the trout immunised to VHSV [Dis. Aquat. Organ. 34 (1999) 167]. frg#11 was then cloned, expressed, purified and used to study the production of antibodies to its epitopes in trout immunised to VHSV. Anti-frg#11 trout antibodies could be detected in serum from individual trout surviving VHSV exposure, immunised by injection with purified VHSV or DNA-immunised with its pG gene whereas it was not detected in non-infected and non-immunised trout. The trout serum antibodies which reacted more strongly by ELISA using solid-phase frg#11 (continuous or linear epitopes on the sequence of the pG) had the lowest VHSV-neutralising activity (epitopes which are pG conformation-dependent). Because antibodies recognising continuous as well as conformation-dependent epitopes of the pG seem to be involved in protective trout immunological responses to VHSV, the estimation of anti-frg#11 antibodies could help to the dissection of the complex trout antibody response to VHSV infections. In addition, these preliminary results suggest that the determination of anti-frg#11 antibodies might also be used to complement in vitro viral neutralising assays which seem to be restricted to pG conformation-dependent epitopes.

Membrane recognition by vesicular stomatitis virus involves enthalpy-driven protein-lipid interactions

Vesicular stomatitis virus (VSV) infection depends on the fusion of viral and cellular membranes, which is mediated by virus spike glycoprotein G at the acidic environment of the endosomal compartment. VSV G protein does not contain a hydrophobic amino acid sequence similar to the fusion peptides found among other viral glycoproteins, suggesting that membrane recognition occurs through an alternative mechanism. Here we studied the interaction between VSV G protein and liposomes of different phospholipid composition by force spectroscopy, isothermal titration calorimetry (ITC), and fluorescence spectroscopy. Force spectroscopy experiments revealed the requirement for negatively charged phospholipids for VSV binding to membranes, suggesting that this interaction is electrostatic in nature. In addition, ITC experiments showed that VSV binding to liposomes is an enthalpically driven process. Fluorescence data also showed the lack of VSV interaction with the vesicles as well as inhibition of VSV-induced membrane fusion at high ionic strength. Intrinsic fluorescence measurements showed that the extent of G protein conformational changes depends on the presence of phosphatidylserine (PS) on the target membrane. Although the increase in PS content did not change the binding profile, the rate of the fusion reaction was remarkably increased when the PS content was increased from 25 to 75%. On the basis of these data, we suggest that G protein binding to the target membrane essentially depends on electrostatic interactions, probably between positive charges on the protein surface and negatively charged phospholipids in the cellular membrane. In addition, the fusion is exothermic, indicating no entropic constraints to this process.