Differential stability and fusion activity of Lyssavirus glycoprotein trimers

Single amino acid change at position 255 in rabies virus glycoprotein decreases viral pathogenicity

Previous studies have indicated that the amino acid at position 333 in the glycoprotein (G) is closely related to rabies virus (RABV) pathogenicity. However, whether there are other amino acid residues in G that relate to pathogenicity remain unclear. The aim of this study is to find new amino acid residues in G that could strongly reduce RABV pathogenicity. The present study found that the pathogenicity of a virulent strain was strongly attenuated when the amino acid glycine (Gly) replaced the aspartic acid (Asp) at position 255 in G (D255G) as intracranial (i.c.) infection with this D255G mutant virus did not cause death in adult mice. The indexes of neurotropism of the D255G mutant strain and the parent GD-SH-01 are 0.72 and 10.0, respectively, which indicate that the D255G mutation decreased the neurotropism of RABV. In addition, the D255G mutation significantly decreased RABV replication in the mouse brain. Furthermore, the D255G mutation enhanced the immune response in mice, which contributed to the clearance of RABV after infection. The Asp255 → Gly255 mutation was genetically stable in vitro and in vivo. In this study, we describe a new referenced amino acid site in G that relates to the pathogenicity of RABV.

Structure of the low pH conformation of Chandipura virus G reveals important features in the evolution of the vesiculovirus glycoprotein

Chandipura virus (CHAV), a member of the vesiculovirus genus, is an emerging human pathogen. As for other rhabdoviruses, CHAV entry into susceptible cells is mediated by its single envelope glycoprotein G which is both involved in receptor recognition and fusion of viral and cellular membranes. Here, we have characterized the fusion properties of CHAV-G. As for vesicular stomatitis virus (VSV, the prototype of the genus) G, fusion is triggered at low pH below 6.5. We have also analyzed the biochemical properties of a soluble form of CHAV-G ectodomain (CHAV-Gth, generated by thermolysin limited-proteolysis of recombinant VSV particles in which the G gene was replaced by that of CHAV). The overall behavior of CHAV-Gth is similar to that previously reported for VSV-Gth. Particularly, CHAV-Gth pre-fusion trimer is not stable in solution and low-pH-induced membrane association of CHAV-Gth is reversible. Furthermore, CHAV-Gth was crystallized in its low pH post-fusion conformation and its structure was determined at 3.6Å resolution. An overall comparison of this structure with the previously reported VSV-Gth post-fusion conformation, shows a high structural similarity as expected from the comparison of primary structure. Among the three domains of G, the pleckstrin homology domain (PHD) appears to be the most divergent and the largest differences are confined to the secondary structure of the major antigenic site of rhabdoviruses. Finally, local differences indicate that CHAV has evolved alternate structural solutions in hinge regions between PH and fusion domains but also distinct pH sensitive switches. Globally the comparison between the post fusion conformation of CHAV and VSV-G highlights several features essential for the protein's function. It also reveals the remarkable plasticity of G in terms of local structures.

Characterization of pH-sensitive molecular switches that trigger the structural transition of vesicular stomatitis virus glycoprotein from the postfusion state toward the prefusion state

Vesicular stomatitis virus (VSV; the prototype rhabdovirus) fusion is triggered at low pH and mediated by glycoprotein G, which undergoes a low-pH-induced structural transition. A unique feature of rhabdovirus G is that its conformational change is reversible. This allows G to recover its native prefusion state at the viral surface after its transport through the acidic Golgi compartments. The crystal structures of G pre- and postfusion states have been elucidated, leading to the identification of several acidic amino acid residues, clustered in the postfusion trimer, as potential pH-sensitive switches controlling the transition back toward the prefusion state. We mutated these residues and produced a panel of single and double mutants whose fusion properties, conformational change characteristics, and ability to pseudotype a virus lacking the glycoprotein gene were assayed. Some of these mutations were also introduced in the genome of recombinant viruses which were further characterized. We show that D268, located in the segment consisting of residues 264 to 273, which refolds into postfusion helix F during G structural transition, is the major pH sensor while D274, D395, and D393 have additional contributions. Furthermore, a single passage of recombinant virus bearing the mutation D268L (which was demonstrated to stabilize the G postfusion state) resulted in a pseudorevertant with a compensatory second mutation, L271P. This revealed that the propensity of the segment of residues 264 to 273 to refold into helix F has to be finely tuned since either an increase (mutation D268L alone) or a decrease (mutation L271P alone) of this propensity is detrimental to the virus.

IMPORTANCE

Vesicular stomatitis virus enters cells via endocytosis. Endosome acidification induces a structural transition of its unique glycoprotein (G), which mediates fusion between viral and endosomal membranes. G conformational change is reversible upon increases in pH. This allows G to recover its native prefusion state at the viral surface after its transport through the acidic Golgi compartments. We mutated five acidic residues, proposed to be pH-sensitive switches controlling the structural transition back toward the prefusion state. Our results indicate that residue D268 is the major pH sensor, while other acidic residues have additional contributions, and reveal that the propensity of the segment consisting of residues 264 to 273 to adopt a helical conformation is finely regulated. This segment might be a good target for antiviral compounds.

Vaccines for lyssaviruses other than rabies

Several new lyssaviruses have emerged in the past decade and it is likely that more remain to be discovered. There are six recognized genotypes of lyssavirus other than the rabies virus (genotype 1). All but one of these has been associated with human cases, with the resulting disease clinically similar to rabies. Rabies vaccines provide a means of pre- and postexposure prophylaxis against rabies and some of the other genotypes, but not all. Those that are crossprotected fall into phylogroup 1 of the genus, and those not protected in phylogroup 2. The crossprotection of phylogroup 1 viruses by rabies vaccines and the development of new, broader range or specific vaccines for phylogroup 2 viruses are reviewed.

An electrochemiluminescence assay for analysis of rabies virus glycoprotein content in rabies vaccines

Vaccine potency testing is necessary to evaluate the immunogenicity of inactivated rabies virus (RABV) vaccine preparations before human or veterinary application. Currently, the NIH test is recommended by the WHO expert committee to evaluate RABV vaccine potency. However, numerous disadvantages are inherent concerning cost, number of animals and biosafety requirements. As such, several in vitro methods have been proposed for the evaluation of vaccines based on RABV glycoprotein (G) quality and quantity, which is expected to correlate with vaccine potency. In this study an antigen-capture electrochemiluminescent (ECL) assay was developed utilizing anti-RABV G monoclonal antibodies (MAb) to quantify RABV G. One MAb 2-21-14 was specific for a conformational epitope so that only immunogenic, natively folded G was captured in the assay. MAb 2-21-14 or a second MAb (62-80-6) that binds a linear epitope was used for detection of RABV G. Vaccine efficacy was also assessed in vivo using pre-exposure vaccination of mice. Purified native RABV G induced a RABV neutralizing antibody (rVNA) response with a geometric mean titer of 4.2IU/ml and protected 100% of immunized mice against RABV challenge, while an experimental vaccine with a lower quality and quantity of G induced a rVNA titer<0.05IU/ml and protected <50% of immunized mice. These preliminary results support the hypothesis that in vivo immunogenicity may be predicted from the in vitro measurement of RABV G using an ECL assay. Based upon these results, the ECL assay may have utility in replacement of the NIH test.

Molecular characterization of China human rabies vaccine strains

To understand the molecular characteristics of China human rabies vaccine strains, we report the full-length genome of the aG strain and present a comprehensive analysis of this strain and almost all available lyssavirus genomes (58 strains) from GenBank (as of Jan 6, 2011). It is generally considered that the G protein plays a predominant role in determining the pathogenicity of the virus, to this end we predicted the tertiary structure of the G protein of aG strain, CTN181 strain and wild type strain HN10 based on the crystal structure of Vesicular stomatitis virus (VSV) G. The predicted RABV G structure has a similar topology to VSV G and the ectodomain can be divided into 4 distinct domains DI - DIV. By mapping the characterized mutations to this structure between China vaccine strains and their close street strains, we speculate that the G303(P-H) mutations of CTN181 and HN10 causing DII 3D change may be associated with the II attenuated virulence in both strains. Specifically, the two signature mutations (G165P and G231P) in the aG strain are within ßsheets, suggesting that both sites are of structural importance.

Characterization of a rabies virus isolate from a ferret badger (Melogale moschata) with unique molecular differences in glycoprotein antigenic site III

Rabies virus was isolated from the brain of a Chinese ferret badger (Melogale moschata) and identified as having an R333Q substitution within its glycoprotein antigenic site III. Additionally, compared with vaccine strains and other rabies virus isolates from dogs and ferret badgers in China, the isolate had five other amino acid substitutions in its glycoprotein: P(-17)L in the signal peptide, R88H, L225M, and D422E in the ectoplasmic region, and G478E in the cytoplasmic region. This isolate possessed high virulence in suckling, weanling and adult mice. These data indicate that this is a unique rabies virus with a molecular signature that differentiates it from other strains circulating in terrestrial mammals in China. We propose that rabies virus circulates in some ferret badgers in an independent epidemiological cycle unique to China following spillover from domestic dogs or other hosts infected with rabies.

Development of a mouse monoclonal antibody cocktail for post-exposure rabies prophylaxis in humans

As the demand for rabies post-exposure prophylaxis (PEP) treatments has increased exponentially in recent years, the limited supply of human and equine rabies immunoglobulin (HRIG and ERIG) has failed to provide the required passive immune component in PEP in countries where canine rabies is endemic. Replacement of HRIG and ERIG with a potentially cheaper and efficacious alternative biological for treatment of rabies in humans, therefore, remains a high priority. In this study, we set out to assess a mouse monoclonal antibody (MoMAb) cocktail with the ultimate goal to develop a product at the lowest possible cost that can be used in developing countries as a replacement for RIG in PEP. Five MoMAbs, E559.9.14, 1112-1, 62-71-3, M727-5-1, and M777-16-3, were selected from available panels based on stringent criteria, such as biological activity, neutralizing potency, binding specificity, spectrum of neutralization of lyssaviruses, and history of each hybridoma. Four of these MoMAbs recognize epitopes in antigenic site II and one recognizes an epitope in antigenic site III on the rabies virus (RABV) glycoprotein, as determined by nucleotide sequence analysis of the glycoprotein gene of unique MoMAb neutralization-escape mutants. The MoMAbs were produced under Good Laboratory Practice (GLP) conditions. Unique combinations (cocktails) were prepared, using different concentrations of the MoMAbs that were capable of targeting non-overlapping epitopes of antigenic sites II and III. Blind in vitro efficacy studies showed the MoMab cocktails neutralized a broad spectrum of lyssaviruses except for lyssaviruses belonging to phylogroups II and III. In vivo, MoMAb cocktails resulted in protection as a component of PEP that was comparable to HRIG. In conclusion, all three novel combinations of MoMAbs were shown to have equal efficacy to HRIG and therefore could be considered a potentially less expensive alternative biological agent for use in PEP and prevention of rabies in humans.

Human mortality from endemic canine rabies is estimated to be 55,000 deaths per year in Africa and Asia, yet rabies remains a neglected disease throughout most of these countries. More than 99% of human rabies cases are caused by infections resulting from a dog-bite injury. In the vast majority of human exposures to rabies, patients require post-exposure prophylaxis (PEP), which includes both passive (rabies immunoglobulin, RIG) and active immunization (rabies vaccine). The number of victims requiring PEP has increased exponentially in recent years, and human and equine RIG (HRIG and ERIG) were not sufficiently available in countries where canine rabies is endemic. Rabies virus-neutralizing monoclonal antibodies (MAbs) of mouse (Mo) origin have been identified as promising alternatives to HRIG and ERIG. We have developed and assessed both in vitro and in vivo unique mouse monoclonal antibody (MoMAb) cocktails, which are highly efficacious. Three novel combinations were shown to have an equal or superior efficacy to HRIG and therefore could be considered a potentially less expensive alternative for passive prophylactic use to prevent the development of rabies in humans, particularly where needed most in developing countries.

Rabies virus glycoprotein expression in Drosophila S2 cells. I: design of expression/selection vectors, subpopulations selection and influence of sodium butyrate and culture medium on protein expression

The cDNA encoding the rabies virus glycoprotein (RVGP) gene was cloned in expression plasmids under the control of the inductive metallothionein promoter. They were designed in order to bear or not a secretion signal (i) and a cDNA coding for the selection hygromycin. These vectors were transfected into S2 cells, cell populations selected and subpopulations were then obtained by reselection with hygromycin. Cell cultures were examined for kinetics of cell growth, detection of RVGP mRNA and expression of RVGP. All cell populations were shown to express the RVGP mRNA upon induction. S2MtRVGPHy cell population, transfected with one vector that contains RGPV gene and selection gene, was shown to express higher amounts of RVGP as evaluated by flow cytometry ( approximately 52%) and ELISA (0.64 microg/10(7)cells at day 7). Subpopulation selection allowed a higher RVGP expression, specially for the S2MtRVGPHy(+) (5.5 microg/10(7)cells at day 7). NaBu treatment leading to lower cell growth and higher RVGP expression allowed an even higher RVGP synthesis by S2MtRVGPHy(+) (8.4 microg/10(7)cells at day 7). SF900II medium leading to a higher S2MtRVGPHy(+)cell growth allowed a higher final RVGP synthesis in this cell culture. RVGP synthesis may be optimized by the expression/selection vectors design, cell subpopulations selection, chromatin exposure and culture medium employed.

Rabies virus glycoprotein expression in Drosophila S2 cells. I. Functional recombinant protein in stable co-transfected cell line

Recombinant rabies virus glycoprotein (rRVGP) was expressed in Drosophila melanogaster Schneider 2 (S2) cells. The cDNA encoding the entire RVGP gene was cloned in an expression plasmid under the control of the constitutive actin promoter (Ac), which was co-transfected into S2 cells together with a hygromycin selection plasmid. Selected S2 cell populations (S2AcRVGP) had a decreased ability to grow and consume substrates, when compared to the non-transfected cells (S2). They were shown, by PCR, to express the RVGP gene and mRNA and, by immunoblotting, to synthesize the rRVGP in its expected molecular mass of 65 kDa. ELISA kinetic studies showed the rRVGP expression in cell lysates and supernatants attaining concentrations of 300 microg/L. By flow cytometry analysis, about 30% of the cells in the co-transfected populations were shown to express the rRVGP. Cell populations selected by limiting dilution expressed higher rRVGP yields. Mice immunized with rRVGP were shown to synthesize antibodies against rabies virus and be protected against experimental infection with rabies virus. The data presented here show that S2 cells can be suitable hosts for the rRVGP expression, allowing its synthesis in a high degree of physical and biological integrity.

Emergence of lyssaviruses in the Old World: the case of Africa

Rabies has a long history of occurrence throughout Africa, spanning hundreds of years. At least four distinct Lyssavirus species persist throughout the continent, among carnivores, bats and other mammals. Rabies virus is the most cosmopolitan member, with primary reservoirs within dogs and mongoose, but other wildlife vectors are important in viral maintenance, such as jackals. Besides a prominent toll on humans and domestic animals, the disease has an underappreciated role in conservation biology, especially for such highly endangered fauna as African wild dogs and Ethiopian wolves. Both Duvenhage and Lagos bat viruses are adapted to bats, but their epidemiology, together with Mokola virus, is poorly understood. Significantly, less than ideal cross-reactivity with modern biologicals used for veterinary and public health interventions is a major cause for concern among these emerging viral agents.

DNA vaccine for rabies: relevance of the trans-membrane domain of the glycoprotein in generating an antibody response

Various studies have demonstrated the potential of immunization with DNA vaccines encoding the rabies virus glycoprotein (RV-G) to elicit humoral responses. In the present study, we have designed four constructs using a VR1020 vector, wherein the RV-G ectodomain has been cloned without the signal sequence (SS) and the trans-membrane domain (TD) (rGVR), without the SS but with the TD (rGVRt), with the SS but without the TD (rGVRs) and with the SS and the TD (rGVRst), under the control of a cytomegalovirus (CMV) promoter, and downstream of the tissue plasminogen activator (TPA) signal sequence. In addition, RV-G has been expressed as a His6 tag fusion protein, both in Escherichia coli as well as in baculovirus expression systems. Using a prime-boost strategy, BALB/cJ mice administered with the rGVRt construct either in saline (intramuscularly) or adsorbed onto gold microcarriers (delivered intradermally by gene gun) generated the highest rabies virus neutralizing antibody (RVNA) titers. Inclusion of the SS, in addition to the TD (rGVRst), led to a significant decrease in RVNA titers, compared to the rGVRt construct. The DNA vaccine construct lacking both the SS and the TD domain and the vaccine having only the SS generated lower antibody responses, compared to the rGVRt construct. After priming with DNA vaccine, boosting with both E. coli- as well as baculovirus-expressed rRV-G led to an increase in the RVNA titers. The present results demonstrate that a DNA vaccine encoding the full-length sequence of the ectodomain plus TD of the mature native RV-G is capable of expressing an 'ideal' immunogen to produce RVNA titers.

Inhibition of cell fusion in Junin virus-infected cells by sera from Argentine hemorrhagic fever patients

BACKGROUND

Junin virus (JV), a member of the Arenaviridae family, is the etiological agent of Argentine hemorrhagic fever (AHF). A low pH-pulse, induces fusion of Vero cells infected with JV to form syncytia, whose production can be inhibited by neutralizing antibodies against the JV major glycoprotein.

OBJECTIVES

To characterize the existence of an antifusogenic activity present in sera obtained from natural infections of AHF over a 20-year period and to study both the fusogenic activity of one pathogenic and two attenuated strains of JV in Vero cells, at different pH. The study sample consisted of sera obtained from two provinces in the Argentine Republic. Vero cells grown in monolayers, were infected with different strains of JV and a 2 h pulse, at different pH, was performed. Syncytium production was evaluated 12 h later, after staining with Giemsa. Neutralization tests against the attenuated strain XJCl3 were carried out and the antifusogenic activity of immunosera was studied by incubating serum with JV-infected Vero cells. Also the fusion activity in Vero cells infected with three JV strains was assayed.

RESULTS AND CONCLUSIONS

A pathogenic strain XJ exhibited the highest fusogenic activity at pH 5. Syncytium formation was prevented by patients' sera obtained from different geographical locations, independently of time of infection. However, when Vero cells were infected with XJ, a significant reduction of syncytium production was observed, though the level of inhibition was lower than that detected in other JV strains-infected cells. These results could be explained by the existence of a conserved domain on JV proteins and also antigenic heterogeneity among strains.

Stable trimerization of recombinant rabies virus glycoprotein ectodomain is required for interaction with the p75NTR receptor

Native rabies virus glycoprotein (RVGvir) is a trimeric, membrane-anchored protein that has been shown to interact with the p75NTR neurotrophin receptor. In order to determine if the RVG trimeric oligomerization state is required for its binding with p75NTR, different soluble recombinant molecules containing the entire RVG ectodomain (RVGect) were expressed alone or fused at its C terminus to the trimerization domain of the bacteriophage T4 fibritin, termed 'foldon'. The oligomerization status of recombinant RVG was investigated using sedimentation in sucrose gradient and p75NTR binding assays. It was found that, in the absence of the fibritin foldon, recombinant RVGect forms unstable trimers that dissociate into monomers in a concentration-dependent manner. C-terminal fusion with the foldon induces stable RVG trimerization, which is concentration-independent. Furthermore, the fibritin foldon maintains the native antigenic structure of the carboxy part of RVGect. Cell binding experiments showed that RVG trimerization is required for efficient interaction with p75NTR. However, the exact mode of trimerization appears unimportant, as trimeric recombinant RVGect (fused to the fibritin foldon) and RVGvir both recognize p75NTR with similar nanomolar affinities, as shown by surface plasmon resonance experiments. Altogether, these results show that the C-terminal fusion of the RVG ectodomain with the fibritin foldon is a powerful way to obtain a recombinant trimeric native-like structure of the p75NTR binding domain of RVG.

Evidence that rabies virus forms different kinds of fusion machines with different pH thresholds for fusion

Fusion of rabies virus with membranes is triggered at a low pH and is mediated by a viral glycoprotein (G). Fusion of rabies virus with liposomes was monitored by using a lipid mixing assay based on fluorescence resonance energy transfer. Fusion was detected below pH 6.4, and its extent increased with H(+) concentrations to be maximal around pH 6.15. The origin of the partial fusion activity of rabies virus under suboptimal pH conditions (i.e., between pH 6.15 and 6.4) was investigated. We demonstrate unambiguously that fusion at a suboptimal pH is distinct from the phenomenon of low-pH-induced inactivation and that it is not due to heterogeneity of the virus population. We also show that viruses that do not fuse under suboptimal pH conditions are indeed bound to the target liposomes and that the fusion complexes they have formed are blocked at an early stage of the fusion pathway. Our conclusion is that along the fusion reaction, different kinds of fusion machines with different pH thresholds for fusion can be formed. Possible explanations of this difference of pH sensitivity are discussed.

Region at amino acids 164 to 303 of the rabies virus glycoprotein plays an important role in pathogenicity for adult mice

The authors have previously reported that the glycoprotein of the pathogenic Nishigahara strain of rabies virus is required to lethality for adult mice. A cluster region of amino acid substitutions exists at the positions 164 to 303 on the glycoprotein between avirulent and virulent strains. In this study, the authors generated a chimeric strain having the region at the positions 164 to 303 of the glycoprotein derived from the pathogenic Nishigahara strain in the genetic background of the avirulent RC-HL strain. The chimeric R(G 164-303) strain restores the lethality for adult mice. This result clearly shows that the region at the position 164 to 303 of glycoprotein plays an important role in the lethality for adult mice. Moreover, the authors observed that the lethality for adult mice correlated well with the viral growth in a brain but not with the pH-dependent fusion activity in vitro.