Rescue of rabies virus from cloned cDNA and identification of the pathogenicity-related gene: glycoprotein gene is associated with virulence for adult mice

Histopathology and immunohistochemistry findings in kidney and urinary bladder of rabies virus-infected mice

Rabies virus (RABV) is a lethal and neglected zoonosis responsible for over 60,000 deaths annually caused by the neurotropic virus Lyssavirus rabies. Although rabies is well-known for its severe nervous system impairment, little is known regarding the specific alterations caused in extraneural organs. Studies suggest an essential involvement of RABV in the kidneys. However, the extent of the pathological damage caused by RABV in this organ remains to be understood. This study describes the histopathological alterations and RABV antigen expression in the kidneys and urinary bladder. Viral immunostaining was observed, suggesting that RABV can successfully infect these tissues. In addition, the main alterations found in the kidneys were edema in the convoluted tubules and in the glomerulus, interstitial inflammation, atrophy of the glomerular tuft, a decrease in Bowman's capsule and Bowman's space, and the accumulation of glycogen in the tubules, which may indicate the effects of inflammation caused by RABV. Therefore, our results showed the importance of understanding the effects of histopathological alterations induced by RABV and the need for more studies concerning the inflammatory action of the virus during the infection.

Reverse genetic approaches allowing the characterization of the rabies virus street strain belonging to the SEA4 subclade

Rabies virus (RABV) is the causative agent of rabies, a lethal neurological disease in mammals. RABV strains can be classified into fixed strains (laboratory strains) and street strains (field/clinical strains), which have different properties including cell tropism and neuroinvasiveness. RABV Toyohashi strain is a street strain isolated in Japan from an imported case which had been bitten by rabid dog in the Philippines. In order to facilitate molecular studies of RABV, we established a reverse genetics (RG) system for the study of the Toyohashi strain. The recombinant virus was obtained from a cDNA clone of Toyohashi strain and exhibited similar growth efficiency as the original virus in cultured cell lines. Both the original and recombinant strains showed similar pathogenicity with high neuroinvasiveness in mice, and the infected mice developed a long and inconsistent incubation period, which is characteristic of street strains. We also generated a recombinant Toyohashi strain expressing viral phosphoprotein (P protein) fused with the fluorescent protein mCherry, and tracked the intracellular dynamics of the viral P protein using live-cell imaging. The presented reverse genetics system for Toyohashi strain will be a useful tool to explore the fundamental molecular mechanisms of the replication of RABV street strains.

Construction of Vero cell-adapted rabies vaccine strain by five amino acid substitutions in HEP-Flury strain

Rabies virus (RABV) causes fatal neurological disease. Pre-exposure prophylaxis (PrEP) and post-exposure prophylaxis (PEP) using inactivated-virus vaccines are the most effective measures to prevent rabies. In Japan, HEP-Flury, the viral strain, used as a human rabies vaccine, has historically been propagated in primary fibroblast cells derived from chicken embryos. In the present study, to reduce the cost and labor of vaccine production, we sought to adapt the original HEP-Flury (HEP) to Vero cells. HEP was repeatedly passaged in Vero cells to generate ten- (HEP-10V) and thirty-passaged (HEP-30V) strains. Both HEP-10V and HEP-30V grew significantly better than HEP in Vero cells, with virulence and antigenicity similar to HEP. Comparison of the complete genomes with HEP revealed three non-synonymous mutations in HEP-10V and four additional non-synonymous mutations in HEP-30V. Comparison among 18 recombinant HEP strains constructed by reverse genetics and vesicular stomatitis viruses pseudotyped with RABV glycoproteins indicated that the substitution P(L115H) in the phosphoprotein and G(S15R) in the glycoprotein improved viral propagation in HEP-10V, while in HEP-30V, G(V164E), G(L183P), and G(A286V) in the glycoprotein enhanced entry into Vero cells. The obtained recombinant RABV strain, rHEP-PG4 strain, with these five substitutions, is a strong candidate for production of human rabies vaccine.

Evolution of Rabies Virus Isolates: Virulence Signatures and Effects of Modulation by Neutralizing Antibodies

Lyssavirus rabies (RABV) is an RNA virus and, therefore, is subject to mutations due to low RNA polymerase replication fidelity, forming a population structure known as a viral quasispecies, which is the core of RNA viruses' adaptive strategy. Under new microenvironmental conditions, the fittest populations are selected, and the study of this process on the molecular level can help determine molecular signatures related to virulence. Our aim was to survey gene signatures on nucleoprotein and glycoprotein genes that might be involved in virulence modulation during the in vitro evolution of RABV lineages after serial passages in a neuronal cell system with or without the presence of neutralizing antibodies based on replicative fitness, in vivo neurotropism and protein structure and dynamics. The experiments revealed that amino acids at positions 186 and 188 of the glycoprotein are virulence factors of Lyssavirus rabies, and site 186 specifically might allow the attachment to heparan as a secondary cell receptor, while polymorphism at position 333 might allow the selection of escape mutants under suboptimal neutralizing antibodies titers.

Glu333 in rabies virus glycoprotein is involved in virus attenuation through astrocyte infection and interferon responses

The amino acid residue at position 333 of the rabies virus (RABV) glycoprotein (G333) is a major determinant of RABV pathogenicity. Virulent RABV strains possess Arg₃₃₃, whereas the attenuated strain HEP-Flury (HEP) possesses Glu₃₃₃. To investigate the potential attenuation mechanism dependent on a single amino acid at G333, comparative analysis was performed between HEP and HEP³³³R mutant with Arg₃₃₃. We examined their respective tropism for astrocytes and the subsequent immune responses in astrocytes. Virus replication and subsequent interferon (IFN) responses in astrocytes infected with HEP were increased compared with HEP³³³R both in vitro and in vivo. Furthermore, involvement of IFN in the avirulency of HEP was demonstrated in IFN-receptor knockout mice. These results indicate that Glu₃₃₃ contributes to RABV attenuation by determining the ability of the virus to infect astrocytes and stimulate subsequent IFN responses.

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Glu₃₃₃ in G protein is responsible for astrocyte infection with RABV HEP strain

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Arg₃₃₃ mutation in G protein decreases astrocyte tropism of RABV HEP

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RABV HEP evokes higher IFN responses in astrocytes than HEP with Arg₃₃₃ mutation

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Glu₃₃₃-dependent astrocyte infection is involved in the attenuation of RABV HEP

Research Advances on the Interactions between Rabies Virus Structural Proteins and Host Target Cells: Accrued Knowledge from the Application of Reverse Genetics Systems

Rabies is a lethal zoonotic disease caused by lyssaviruses, such as rabies virus (RABV), that results in nearly 100% mortality once clinical symptoms appear. There are no curable drugs available yet. RABV contains five structural proteins that play an important role in viral replication, transcription, infection, and immune escape mechanisms. In the past decade, progress has been made in research on the pathogenicity of RABV, which plays an important role in the creation of new recombinant RABV vaccines by reverse genetic manipulation. Here, we review the latest advances on the interaction between RABV proteins in the infected host and the applied development of rabies vaccines by using a fully operational RABV reverse genetics system. This article provides a background for more in-depth research on the pathogenic mechanism of RABV and the development of therapeutic drugs and new biologics.

Rabies Infection: An Overview of Lyssavirus-Host Protein Interactions

Viruses are obligatory intracellular parasites that use cell proteins to take the control of the cell functions in order to accomplish their life cycle. Studying the viral-host interactions would increase our knowledge of the viral biology and mechanisms of pathogenesis. Studies on pathogenesis mechanisms of lyssaviruses, which are the causative agents of rabies, have revealed some important host protein partners for viral proteins, especially for most studied species, i.e. RABV. In this review article, the key physical lyssavirus-host protein interactions, their contributions to rabies infection, and their exploitation are discussed to improve the knowledge about rabies pathogenesis.

Molecular characterisation of rabies virus detected in livestock animals in the southern part of Egypt during 2018 and 2019

Brain samples were collected from 33 animals of different species, including buffalo, cattle, dog, donkey, fox and wolf, that had been suspected to be infected by rabies virus (RABV) in different geographical regions of Aswan and Luxor governorates in Egypt. The samples were submitted for histopathological examination and the presence of the nucleic acid and antigens of RABV was tested by RT-PCR and indirect fluorescent antibody technique (IFAT), respectively. Sixteen samples were found positive by all the three examinations. Three samples were selected for further study from animals in which the highest virus loads were detected. The partial sequence of the RABV N gene was determined and analysed from the samples of a buffalo, a cow and a donkey. The viruses in the samples were found to share 95-98% and 95-97% nucleotide and amino acid sequence identities, respectively. In comparison to reference sequences, a few amino acid substitutions occurred in the N protein antigenic sites I and IV in the immunodominant epitopes of the viruses detected in the cow and the donkey but not in the one from the buffalo. The phylogenetic analysis revealed that the RABVs sequenced from the samples belonged to genotype 1, Africa-4 clade, and formed two distinct sub-clades within the Egyptian clade. These findings indicate the circulation of RABV among livestock animals in the southern part of Egypt and raise public health concerns. The amino acid changes detected in this work may contribute to the antigenic diversification of RABVs.

The Viral Hemorrhagic Septicemia Virus (VHSV) Markers of Virulence in Rainbow Trout (Oncorhynchus mykiss)

Viral hemorrhagic septicemia virus (VHSV) is a highly contagious virus leading to high mortality in a large panel of freshwater and marine fish species. VHSV isolates originating from marine fish show low pathogenicity in rainbow trout. The analysis of several nearly complete genome sequences from marine and freshwater isolates displaying varying levels of virulence in rainbow trout suggested that only a limited number of amino acid residues might be involved in regulating the level of virulence. Based on a recent analysis of 55 VHSV strains, which were entirely sequenced and phenotyped in vivo in rainbow trout, several amino acid changes putatively involved in virulence were identified. In the present study, these amino acid changes were introduced, alone or in combination, in a highly-virulent VHSV 23–75 genome backbone by reverse genetics. A total of 35 recombinant VHSV variants were recovered and characterized for virulence in trout by bath immersion. Results confirmed the important role of the NV protein (R116S) and highlighted a major contribution of the nucleoprotein N (K46G and A241E) in regulating virulence. Single amino acid changes in these two proteins drastically affect virus pathogenicity in rainbow trout. This is particularly intriguing for the N variant (K46G) which is unable to establish an active infection in the fins of infected trout, the main portal of entry of VHSV in this species, allowing further spread in its host. In addition, salmonid cell lines were selected to assess the kinetics of replication and cytopathic effect of recombinant VHSV and discriminate virulent and avirulent variants. In conclusion, three major virulence markers were identified in the NV and N proteins. These markers explain almost all phenotypes (92.7%) observed in trout for the 55 VHSV strains analyzed in the present study and herein used for the backward validation of virulence markers. The identification of VHSV specific virulence markers in this species is of importance both to predict the in vivo phenotype of viral isolates with targeted diagnostic tests and to improve prophylactic methods such as the development of safer live-attenuated vaccines.

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.

Amino Acid Mutation in Position 349 of Glycoprotein Affect the Pathogenicity of Rabies Virus

Rabies, caused by rabies virus (RABV), is a zoonotic disease infecting mammals including humans. Studies have confirmed that glycoprotein (G) is most related to RABV pathogenicity. In the present study, to discover more amino acid sites related to viral pathogenicity, artificial mutants have been constructed in G of virulent strain GD-SH-01 backbone. Results showed that pathogenicity of GD-SH-01 significantly decreased when Gly₃₄₉ was replaced by Glu₃₄₉ through in vivo assays. Gly₃₄₉→Glu₃₄₉ of G did not significantly influence viral growth and spread in NA cells. Gly₃₄₉→Glu₃₄₉ of G increased the immunogenicity of GD-SH-01 in periphery and induced more expression of interferon alpha (IFN-α) in the brain in mice. It was observed that Gly₃₄₉→Glu₃₄₉ of G led to enhanced blood–brain barrier (BBB) permeability at day 5 postinfection. All together, these data revealed that Gly₃₄₉→Glu₃₄₉ of G mutation decreased RABV pathogenicity through enhanced immune response and increased BBB permeability. This study provides a new referenced site G349 that could attenuate pathogenicity of RABV.

Phosphoprotein Gene of Wild-Type Rabies Virus Plays a Role in Limiting Viral Pathogenicity and Lowering the Enhancement of BBB Permeability

Enhancement of blood–brain barrier (BBB) permeability is necessary for clearing virus in the central nervous system (CNS). It has been reported that only laboratory-attenuated rabies virus (RABV) induces inflammatory response to lead BBB transient breakdown rather than wild-type (wt) strains. As a component of ribonucleoprotein (RNP), phosphoprotein (P) of RABV plays a key role in viral replication and pathogenicity. To our knowledge, the function of RABV P gene during RABV invasion was unclear so far. In order to determine the role of RABV P gene during RABV infection, we evaluated the BBB permeability in vivo after infection with wt RABV strain (GD-SH-01), a lab-attenuated RABV strain (HEP-Flury), and a chimeric RABV strain (rHEP-SH-P) whose P gene cloned from GD-SH-01 was expressed in the genomic backbone of HEP-Flury. We found that rHEP-SH-P caused less enhancement of BBB permeability and was less pathogenic to adult mice than GD-SH-01 and HEP-Flury. In an effort to investigate the mechanism, we found that the replication of rHEP-SH-P has been limited due to the suppressed P protein expression and induced less response to maintain BBB integrity. Our data indicated that the P gene of wt RABV was a potential determinant in hampering viral replication in vivo, which kept BBB integrity. These findings provided an important foundation for understanding the viral invasion and development of novel vaccine.

Phenotypic Consequence of Rearranging the N Gene of RABV HEP-Flury

Nucleoprotein (N) is a key element in rabies virus (RABV) replication. To further investigate the effect of N on RABV, we manipulated an infectious cDNA clone of the RABV HEP-Flury to rearrange the N gene from its wild-type position of 1 (N-P-M-G-L) to 2 (P-N-M-G-L), 3 (P-M-N-G-L), or 4 (P-M-G-N-L), using an approach that left the viral nucleotide sequence unaltered. Subsequently, viable viruses were recovered from each of the rearranged cDNA and examined for their gene expression levels, growth kinetics in cell culture, pathogenicity in suckling mice and protection in mice. The results showed that gene rearrangement decreased N mRNA transcription and vRNA replication. As a result, all viruses with rearranged genomes showed worse replication than that of rHEP-Flury in NA cells at a MOI of 0.01, but equivalent or slightly better replication levels at a MOI of 3. Consequently, the lethality in suckling mice infected with N4 was clearly attenuated compared with rHEP-Flury. However, the protection to mice was not enhanced. This study not only gives us insight into the understanding of the phenotype of RABV N gene rearrangement, but also helps with rabies vaccine candidate construction.

Segmentation of the rabies virus genome

We established a system for the recovery of a segmented recombinant rabies virus, the virus genome RNA of which was divided into two parts: segment 1 encoding the nucleoprotein, phosphoprotein, matrix protein, and glycoprotein genes, and segment 2 encoding the large RNA-dependent RNA polymerase gene. The morphology of the segmented recombinant rabies virus was bullet-like in shape with a length of approximately 130 nm, which is shorter than the 200-nm long non-segmented recombinant rabies virus. The segmented recombinant rabies virus was maintained for at least 18 passages. The virus multiplication rate of the segmented recombinant rabies virus was lower than that of the non-segmented recombinant rabies virus during the passages, and the relative amounts of virus genome RNAs for segment 1 and segment 2 differed in the supernatant of the segmented recombinant rabies virus infected cells. These results suggest that the segmented recombinant rabies virus packages either segment 1 or segment 2 into each virus particle. Thus, co-infection with segmented recombinant rabies virus particles packaging segment 1 or segment 2 may be necessary for the production of progeny virus.

Near-infrared fluorescent protein iRFP720 is optimal for in vivo fluorescence imaging of rabies virus infection

In vivo imaging is a noninvasive method that enables real-time monitoring of viral infection dynamics in a small animal, which allows a better understanding of viral pathogenesis. In vivo bioluminescence imaging of virus infection is widely used but, despite its advantage over bioluminescence that no substrate administration is required, fluorescence imaging is not used because of severe autofluorescence. Recently, several far-red and near-infrared (NIR) fluorescent proteins (FPs) have been developed and shown to be useful for whole-body fluorescence imaging. Here, we report comparative testing of far-red and NIR FPs in the imaging of rabies virus (RABV) infection. Using the highly neuroinvasive 1088 strain, we generated recombinant RABV that expressed FPs such as Katushka2S, E2-Crimson, iRFP670 or iRFP720. After intracerebral inoculation to nude mice, the 1088 strain expressing iRFP720, the most red-shifted FP, was detected the earliest with the highest signal-to-noise ratio using a filter set for >700 nm, in which the background signal level was very low. Furthermore, we could also track viral dissemination from the spinal cord to the brain in nude mice after intramuscular inoculation of iRFP720-expressing 1088 into the hind limb. Hence, we conclude that the NIR FP iRFP720 used with a filter set for >700 nm is useful for in vivo fluorescence imaging not only for RABV infection but also for other virus infections. Our findings will also be useful for developing dual-optical imaging of virus-host interaction dynamics using bioluminescence reporter mice for inflammation imaging.

Molecular Function Analysis of Rabies Virus RNA Polymerase L Protein by Using an L Gene-Deficient Virus

While the RNA-dependent RNA polymerase L protein of rabies virus (RABV), a member of the genus Lyssavirus of the family Rhabdoviridae, has potential to be a therapeutic target for rabies, the molecular functions of this protein have remained largely unknown. In this study, to obtain a novel experimental tool for molecular function analysis of the RABV L protein, we established by using a reverse genetics approach an L gene-deficient RABV (Nishi-ΔL/Nluc), which infects, propagates, and correspondingly produces NanoLuc luciferase in cultured neuroblastoma cells transfected to express the L protein. trans-Complementation with wild-type L protein, but not that with a functionally defective L protein mutant, efficiently supported luciferase production by Nishi-ΔL/Nluc, confirming its potential for function analysis of the L protein. Based on the findings obtained from comprehensive genetic analyses of L genes from various RABV and other lyssavirus species, we examined the functional importance of a highly conserved L protein region at positions 1914 to 1933 by a trans-complementation assay with Nishi-ΔL/Nluc and a series of L protein mutants. The results revealed that the amino acid sequence at positions 1929 to 1933 (NPYNE) is functionally important, and this was supported by other findings that this sequence is critical for binding of the L protein with its essential cofactor, P protein, and thus also for L protein's RNA polymerase activity. Our findings provide useful information for the development of an anti-RABV drug targeting the L-P protein interaction.IMPORTANCE To the best of our knowledge, this is the first report on the establishment of an L gene-deficient, reporter gene-expressing virus in all species of the order Mononegavirales, also highlighting its applicability to a trans-complementation assay, which is useful for molecular function analyses of their L proteins. Moreover, this study revealed for the first time that the NPYNE sequence at positions 1929 to 1933 in the RABV L protein is important for L protein's interaction with the P protein, consistent with and extending the results of a previous study showing that the P protein-binding domain in the L protein is located in its C-terminal region, at positions 1562 to 2127. This study indicates that the NPYNE sequence is a promising target for the development of an inhibitor of viral RNA synthesis, which has high potential as a therapeutic drug for rabies.

Pathogenicity and Immunogenicity of Recombinant Rabies Viruses Expressing the Lagos Bat Virus Matrix and Glycoprotein: Perspectives for a Pan-Lyssavirus Vaccine

Lagos bat virus (LBV) is a phylogroup II lyssavirus exclusively found in Africa. Previous studies indicated that this virus is lethal to mice after intracranial and intramuscular inoculation. The antigenic composition of LBV differs substantially from that of rabies virus (RABV) and current rabies vaccines do not provide cross protection against phylogroup II lyssaviruses. To investigate the potential role of the LBV matrix protein (M) and glycoprotein (G) in pathogenesis, reverse genetics technology was used to construct recombinant viruses. The genes encoding the glycoprotein, or the matrix and glycoprotein of the attenuated RABV strain SPBN, were replaced with those of LBV resulting in SPBN-LBVG and SPBN-LBVM-LBVG, respectively. To evaluate the immunogenicity of the LBV G, the recombinant RABV SPBNGAS-LBVG-GAS was constructed with the LBV G inserted between two mutated RABV G genes (termed GAS). All the recombinant viruses were lethal to mice after intracranial (i.c.) inoculation although the pathogenicity of SPBNGAS-LBVG-GAS was lower compared to the other recombinant viruses. Following intramuscular (i.m.) inoculation, only SPBN-LBVM-LBVG was lethal to mice, indicating that both the M and G of LBV play a role in the pathogenesis. Most interestingly, serum collected from mice that were inoculated i.m. with SPBNGAS-LBVG-GAS neutralized phylogroup I and II lyssaviruses including RABV, Duvenhage virus (DUVV), LBV, and Mokola virus (MOKV), indicating that this recombinant virus has potential to be developed as a pan-lyssavirus vaccine.

Investigation of rabies virus glycoprotein carboxyl terminus as an in vitro predictive tool of neurovirulence. A 3R approach

In the field of live viral vaccines production, there is an unmet need for in vitro tests complying a 3R approach (Refine, Replace and Reduce the use of animal experimentation) to replace the post-licensing safety tests currently assayed in animals. Here, we performed a pilot study evaluating whether virulence of rabies virus, RABV, can be forecast by an in vitro test of neurite outgrowth. The rationale to use neurite outgrowth as a read-out for this test is based on the salient property of the cytoplasmic domain of the G-protein (Cyto-G) of virulent RABV strains - not of attenuated RABV strains - to stimulate neurite outgrowth in vitro. We observed that neurite elongation triggered by the Cyto-Gs encoded by different RABV field isolates correlate with the distinct virulence scores obtained in a mouse model of experimental rabies. Our results cast the idea that it could be feasible to predict RABV virulence by testing the in vitro property of a RABV strain to promote neurite outgrowth without the use of animal experimentation.

Development of infectious clones of a wild-type Korean rabies virus and evaluation of their pathogenic potential

Most reverse genetic (RG) systems for rabies viruses (RVs) have been constructed on the genome background of laboratory-adapted strains. In this study, we developed an RG system using a Korean wild type (KGH) strain to investigate the pathogenic potential of different strains. We developed a RG system with the KGH strain for the first time. Following the complete genome sequencing of the KGH strain, pKGH infectious clones were constructed using the CMV/T7 promoter, and HamRz and HdvRz were introduced to allow self-cleavage of the synthesized RNA. We successfully recovered the rescued virus by constructing chimeric RVs in which we replaced a part of the construct with the partial gene from the fixed RC-HL strain. The rescued viruses formed clearer and countable plaques in an immunostaining plaque assay, with a distinct plaque morphology. Furthermore, compared with the chimeric RVs, the pKGH/RCinsΔ4 strain containing the KGH strain G protein exhibited a decreased efficiency of cell-to-cell spreading in BHK-21 cells and significantly reduced (100-1000 fold) replication kinetics. However, pKGH/RCinsΔ4 strain-infected mice revealed 100% morbidity at 11days post-infection, whereas other chimeric RV strains showed no mortality. Our RG system is a useful tool for studying differences in the cell-to-cell spreading efficiency and replication with respect to the different internalization patterns of street and fixed laboratory-adapted viruses.

The Rabies Virus L Protein Catalyzes mRNA Capping with GDP Polyribonucleotidyltransferase Activity

The large (L) protein of rabies virus (RABV) plays multiple enzymatic roles in viral RNA synthesis and processing. However, none of its putative enzymatic activities have been directly demonstrated in vitro. In this study, we expressed and purified a recombinant form of the RABV L protein and verified its guanosine 5′-triphosphatase and GDP polyribonucleotidyltransferase (PRNTase) activities, which are essential for viral mRNA cap formation by the unconventional mechanism. The RABV L protein capped 5′-triphosphorylated but not 5′-diphosphorylated RABV mRNA-start sequences, 5′-AACA(C/U), with GDP to generate the 5′-terminal cap structure G(5′)ppp(5′)A. The 5′-AAC sequence in the substrate RNAs was found to be strictly essential for RNA capping with the RABV L protein. Furthermore, site-directed mutagenesis showed that some conserved amino acid residues (G1112, T1170, W1201, H1241, R1242, F1285, and Q1286) in the PRNTase motifs A to E of the RABV L protein are required for cap formation. These findings suggest that the putative PRNTase domain in the RABV L protein catalyzes the rhabdovirus-specific capping reaction involving covalent catalysis of the pRNA transfer to GDP, thus offering this domain as a target for developing anti-viral agents.

Role of the glycoprotein G in lyssavirus pathogenicity

Rabies is a fatal zoonotic disease caused by the highly neurotropic members of the Lyssavirus genus (Rhabdoviridae family). These viruses contain an RNA genome that encodes information for five viral proteins: the nucleoprotein (N), the glycoprotein (G), the phosphoprotein (P), matrix (M) and an RNA-dependent RNA polymerase (L). The glycoprotein is the major contributor of viral pathogenicity. However, nucleotide changes in specific regions of the G-protein influence the ability of the rabies virus to cause death in experimental animals but also the ability to move within the neuronal network. In addition to the glycoprotein, other regions of the viral genome may also contribute to pathogenicity, underlining the multigenic nature of the lyssavirus.

Genetically modified rabies virus ERA strain is safe and induces long-lasting protective immune response in dogs after oral vaccination

Oral immunization in free-roaming dogs is one of the most practical approaches to prevent rabies for developing countries. The safe, efficient and long-lasting protective oral rabies vaccine for dogs is highly sought. In this study, rabies virus (RABV) Evelyn-Rokitnicki-Abelseth (ERA) strain wild-type (rERA) and a genetically modified type (rERAG333E) containing a mutation from arginine to glutamic acid at residue 333 of glycoprotein (G333E) were generated by reverse genetic. The recombinant virus rERAG333E retained growth properties of similar to the parent strain rERA in BHK-21 cell culture. The G333E mutation showed genetic stability during passage into neuroblastoma cells and in the brains of suckling mice and was significantly reduced the virulence of rERA in mice. rERAG333E was immunogenic in dogs by intramuscular inoculation. Mice orally vaccinated with rERAG333E induced strong and one year longer virus neutralizing antibodies (VNA) to RABV, and were completely protected from challenge with lethal street virus at 12months after immunization. Dogs received oral vaccination with rERAG333E induced strong protective RABV VNA response, which lasted for over 3years, and moderate saliva RABV-specific IgA. Moreover, sizeable booster responses to RABV VNA were induced by a second oral dose 1year after the first dose. These results demonstrated that the genetically modified ERA vaccine strain has the potential to serve as a safe and efficient oral live vaccine against rabies in dogs.

Novel Approaches to the Prevention and Treatment of Rabies

Rabies is a highly lethal disease caused by the neurotropic rabies virus (RABV), and it remains an important public health problem globally. Effective vaccines have been developed for pre- and post-exposure prophylaxis (PEP). PEP is only effective if it is initiated promptly after recognizing exposure. Once neurological symptoms develop, however, it is widely accepted that there is no effective treatment available. Recent studies indicate that the presence of RABV-specific immunity (i.e. Virus neutralizing antibodies, VNA) and the transient enhancement of the BBB permeability are absolutely required for effective virus clearance from the CNS. In principle, it has been shown in mice using various live-attenuated RABVs or recombinant RABVs expressing three copies of the G or expressing chemokine/cytokines, which can induce high levels of VNA in the serum and also capable of transiently enhancing the BBB permeability that it is possible to clear the virus from CNS. Also, it has been demonstrated that, intravenous administration of VNA together with MCP-1 (shown to transiently open up BBB) can clear RABV from the CNS in both immunocompetent and immunocompromised mice, as late as 5 days after lethal challenge. Novel therapeutic approaches aimed at allowing the peripheral VNA to cross the BBB by administration of the VNA in combination with biological or chemical agents that can transiently open up the BBB would be useful to establish an effective therapy for rabies in humans. In this review, we focus on the some of the approaches that can be used to meet the challenges in the field of rabies treatment.

A recombinant rabies virus expressing a phosphoprotein-eGFP fusion is rescued and applied to the rapid virus neutralization antibody assay

Rabies remains a worldwide concern, and dogs are a major vector for rabies virus (RABV) transmission. Vaccination is used in China to control the spread of rabies in dogs, a practice which necessitates effective, efficient, and high-throughput methods to confirm vaccination. The current rapid fluorescent focus inhibition test (RFFIT) method to measure virus-neutralizing antibody titers in the serum involves multiple steps, and more efficient methods are needed to match the increasing demand for this type of monitoring. In this study, based on the parental rRC-HL strain, a recombinant RABV rRV-eGFP expressing enhanced green fluorescent protein (eGFP) fused with RABV P protein was generated by a reverse genetic technique. The rRV-eGFP grew stably and successfully expressed P-eGFP fusion in Neuro-2A (NA) host cells. Furthermore, the P protein was shown to co-localize with eGFP in rRV-eGFP-infected NA cells. Since eGFP is easily detected in infected cells under a fluorescence microscope, rRV-eGFP could be used to establish a more rapid virus-neutralizing antibody titers assay based on RFFIT, designated as the RFFIT-eGFP method. From 69 canine serum samples, the RFFIT-eGFP method was shown to be as specific and as sensitive as the RFFIT method, suggesting that it might represent a faster tool than conventional RFFIT for measuring RABV virus-neutralizing antibody titers in canine sera without sacrificing accuracy.

Using rabies virus vaccine strain SRV9 as viral vector to express exogenous gene

Rabies virus (RABV) can cause a fatal neurological disease in human and animals, and vaccines were generally applied for the immunoprophylaxis of rabies. Here, a recombinant viral vector carrying the exogenous gene expression component between phosphoprotein (P) and matrix protein (M) genes of RABV was constructed based on the vaccine strain SRV9 used in China. To develop a reverse genetic system, the full-length cDNA plasmids of SRV9 were constructed using the eukaryotic expression vector pCI or pcDNA3.1(+). However, recovery efficiency based on the pcDNA3.1 vector was significantly higher than that of the pCI vector. The exogenous gene expression component PE-PS-BsiWI-PmeI or PS-BsiWI-PmeI-PE was introduced in different locations between the P and M genes of SRV9. When the enhanced green fluorescent protein (eGFP) was used as a reporter gene, both locations could rescue recombinant RABV (rRABV) expressing eGFP with high efficiency. Characterization of rRABV expressing eGFP in vitro revealed that its growth was similar to that of the parental virus. Animal experiments showed that rRABV expressing eGFP could replicate and express eGFP in the brains of suckling mice. Furthermore, rRABV of SRV9 was nonpathogenic for 3-week-old mice and could be cleared from the central nervous system at 5 days post-inoculation. Our results showed that the recombinant SRV9 virus could be used as a useful viral vector for exogenous gene expression.

Sequencing and molecular characterization of CTNCEC25, a China fixed rabies virus vaccine strain CTN-1 adapted to primary chicken embryo cells

Background

Rabies virus is the main etiologic agent of the widespread neurological disease rabies. Recently, the China rabies virus vaccine strain CTN-1 adapted to chicken embryo cells, which has been designated as CTNCEC25, was obtained and demonstrated to have high immunogenicity. However, the full genome sequence of CTNCEC25 and its phylogenetic relationship with other rabies virus street and vaccine strains have not been characterized.

Results

The complete genome of CTNCEC25 was sequenced and analyzed. The length of CTNCEC25 genome is 11,924 nucleotides (nt), comprising a 3′ leader sequence of 59 nt, nucleoprotein (N) gene of 1,425 nt, phosphoprotein (P) gene of 989 nt, matrix protein (M) gene of 803 nt, glycoprotein (G) gene of 2,067 nt, RNA-dependent RNA polymerase gene (L) of 6,474 nt and a 5′ trailer region of 71 nt. A comparison of the entire genomes of CTN-1 and CTNCEC25 identified 16 nt substitutions and 1 deletion, resulting in 8 amino acid (aa) changes in the five structural proteins with one in L (aa 1602), two in M (aa 99 and 191) and six in mature G (aa 147, 333, 389, 421 and 485). The percentage homology of the CTNCEC25 genomic sequence with other fully sequenced rabies virus strains ranged from 81.4% to 99.9%. Phylogenetic analysis indicated that CTNCEC25 was more closely related with those recently isolated China street strains than other vaccine strains. Virus growth analysis showed that CTNCEC25 achieved high rate of propagation in cultured cells.

Conclusions

In this study, the complete genome of CTNCEC25 was sequenced and characterized. Our results showed that CTNCEC25 was more closely related to wild street strains circulating in China than other vaccine strains. Sequence analysis showed that the G protein ectodomain amino acid sequence identity between CTNCEC25 and other rabies virus strains was at least 90% identical. Furthermore, CTNCEC25 achieved high virus titers in cultured cells. Given that CTNCEC25 has high immunogenicity and induced strong protective immune response in animals, these results collectively demonstrated that CTNCEC25 is an ideal vaccine strain candidate for producing human vaccine with high quality and safety in China.

Gene order rearrangement of the M gene in the rabies virus leads to slower replication

The matrix protein (M) is one of only five genes in the RV genome and is an important multifunctional protein. Besides to allow for the release of newly replicated virions pairing with G, the M protein also functions in virus replication, pathogenicity, and host cell apoptosis. The goal of present study is to generate recombinant viruses with M gene rearranged, thus laying the foundation for further exploring what will happen when the gene for M is relocated on the RV single-strand RNA. We used rHEP-Flury, an attenuated virus that remains virulent for less than 3 days in sucking mice, to reshuffle the M gene, using an approach that leaves the other viral nucleotide sequence intact. Two viruses with translocated M genes (N1M2 and N1M4) were recovered from each of the rearranged cDNAs, whose gene order is 3'-N-M-P-G-L-5' and 3'-N-P-G-M-L-5' respectively. The growth dynamics of these viruses showed slower replication than the wild-type virus in multiple-step growth curves, but they can grow to a comparable titer in tests of single-step growth curves. Further experimentation with these rearranged viruses will provide insights into the relationships between genome structure and virus phenotypes.

Development of a chimeric strain of porcine reproductive and respiratory syndrome virus with an infectious clone and a Korean dominant field strain

The K418 chimeric virus of porcine reproductive and respiratory syndrome virus (PRRSV) was engineered by replacing the genomic region containing structure protein genes of an infectious clone of PRRSV, FL12, with the same region obtained from a Korean dominant field strain, LMY. The K418 reached 10⁶ TCID₅₀/ml of viral titer with similar growth kinetics to those of parental strains and had a cross-reactive neutralizing antibody response to field serum from the entire country. The chimeric clone pK418 can be used as a practical tool for further studying the molecular characteristics of PRRSV proteins through genetic manipulation. Furthermore, successful construction of the K418 will allow for the development of customized vaccine candidates against PRRSV, which has evolved rapidly in Korea.

A recombinant rabies virus encoding two copies of the glycoprotein gene confers protection in dogs against a virulent challenge

The rabies virus (RABV) glycoprotein (G) is the principal antigen responsible for the induction of virus neutralizing antibodies (VNA) and is the major modality of protective immunity in animals. A recombinant RABV HEP-Flury strain was generated by reverse genetics to encode two copies of the G-gene (referred to as HEP-dG). The biological properties of HEP-dG were compared to those of the parental virus (HEP-Flury strain). The HEP-dG recombinant virus grew 100 times more efficiently in BHK-21 cell than the parental virus, yet the virulence of the dG recombinant virus in suckling mice was lower than the parental virus. The HEP-dG virus can improve the expression of G-gene mRNA and the G protein and produce more offspring viruses in cells. The amount of G protein revealed a positive relationship with immunogenicity in mice and dogs. The inactivated HEP-dG recombinant virus induced higher levels of VNA and conferred better protection against virulent RABV in mice and dogs than the inactivated parental virus and a commercial vaccine. The protective antibody persisted for at least 12 months. These data demonstrate that the HEP-dG is stable, induces a strong VNA response and confers protective immunity more effectively than the RABV HEP-Flury strain. HEP-dG could be a potential candidate in the development of novel inactivated rabies vaccines.

Comparison of complete genome sequences of dog rabies viruses isolated from China and Mexico reveals key amino acid changes that may be associated with virus replication and virulence

Rabies is a global problem, but its impact and prevalence vary across different regions. In some areas, such as parts of Africa and Asia, the virus is prevalent in the domestic dog population, leading to epidemic waves and large numbers of human fatalities. In other regions, such as the Americas, the virus predominates in wildlife and bat populations, with sporadic spillover into domestic animals. In this work, we attempted to investigate whether these distinct environments led to selective pressures that result in measurable changes within the genome at the amino acid level. To this end, we collected and sequenced the full genome of two isolates from divergent environments. The first isolate (DRV-AH08) was from China, where the virus is present in the dog population and the country is experiencing a serious epidemic. The second isolate (DRV-Mexico) was taken from Mexico, where the virus is present in both wildlife and domestic dog populations, but at low levels as a consequence of an effective vaccination program. We then combined and compared these with other full genome sequences to identify distinct amino acid changes that might be associated with environment. Phylogenetic analysis identified strain DRV-AH08 as belonging to the China-I lineage, which has emerged to become the dominant lineage in the current epidemic. The Mexico strain was placed in the D11 Mexico lineage, associated with the West USA-Mexico border clade. Amino acid sequence analysis identified only 17 amino acid differences in the N, G and L proteins. These differences may be associated with virus replication and virulence-for example, the short incubation period observed in the current epidemic in China.

Development of a reverse genetics system for the aG strain of rabies virus in China

The aG rabies virus strain has been attenuated through multiple passages in cells and is now used as a vaccine strain in China. We attempted to develop a reverse genetics system using the aG strain. Recombinant full-length genomic cDNA was flanked by a hammerhead ribozyme and the hepatitis delta virus ribozyme. Three helper plasmids encoding the nucleoprotein, the phosphoprotein, and the large protein were produced and introduced together with a plasmid containing the full-length aG viral genome into BHK-21 cells by transfection. Recombinant virus was successfully recovered from the cloned cDNA under the control of a CMV promoter driven by RNA polymerase II. The recombinant virus was confirmed by RT-PCR, and the titer of the recombinant virus was 6.2 log LD50.

Characterization of Recombinant Rabies Virus Carrying Double Glycoprotein Genes

A recombinant rabies virus carrying double glycoprotein (G) genes, R(NPMGGL) strain, was generated by a reverse genetics system utilizing cloned cDNA of the RC-HL strain, and the biological properties of the virus were compared to those of the recombinant RC-HL (rRC-HL) strain. The extents of virus growth in cultured cells and virulence for adult mice of the R(NPMGGL) strain were almost same as those of the rRC-HL strain, while G protein content of the purified R(NPMGGL) virion and G protein expression level in R(NPMGGL)-infected cells were 1.5-fold higher than those of the rRC-HL strain. As a result of serial passages of the R(NPMGGL) strain in cultured cells, the expression level of G protein in cultured cells infected with the passaged R(NPMGGL) strain was maintained and virus titers rose with adaptation to the cultured cells. Furthermore, analysis of neutralization titers in mice immunized with UVinactivated virus suggested that the R(NPMGGL) strain had higher immunogenicity than that of the rRC-HL strain. The results suggest that the R(NPMGGL) strain carrying double G genes might be a useful candidate for development of a new inactivated rabies vaccine.

Improved Recovery of Rabies Virus from Cloned cDNA Using a Vaccinia Virus-Free Reverse Genetics System

To improve efficiency of recovery of rabies virus from cloned cDNA, we established a BHK cell clone that stably expresses T7 RNA polymerase, which we named BHK/T7-9. We also constructed new helper plasmids for expression of nucleoprotein and RNA polymerase of the RC-HL strain using the pTM1 plasmid vector, which makes the T7 RNA polymerase-transcripts from the plasmid cap-independent for translation. After co-transfection of these helper plasmids and the previously constructed full-length genome plasmid of the RC-HL strain to BHK/T7-9 cells, recombinant rabies virus was efficiently recovered from the cloned cDNA.

Multigenic Relation to the Attenuation of Rabies Virus

Rabies virus Nishigahara strain causes lethal infection in adult mice after intracerebral inoculation. On the other hand, the RC-HL strain, derived from the Nishigahara strain, does not cause lethal infection in adult mice. We previously demonstrated that a chimeric virus, R(G), with the open reading frame of the G gene (G-ORF) from the Nishigahara strain in the background of the RC-HL genome, is virulent. Reversely, in order to demonstrate that the G gene of the RC-HL strain is related to the attenuated phenotype, we established a reverse genetics system of the Nishigahara strain and generated a chimeric virus, Ni(G), with the G-ORF from RC-HL in the background of the Nishigahara genome. Contrary to our prediction, Ni(G) killed adult mice after intracerebral inoculation with neuropathic symptoms like those of Nishigahara strain infection. Therefore, the G-ORF of the RC-HL strain is not the sole determinant of the attenuated phenotype. In additional investigation, we examined other genes, including N, P, M and L genes, and generated chimeric viruses exhaustively. We found that chimeric viruses with a single gene from the RC-HL were not attenuated and that chimeric viruses with the G-ORF and at least one other ORF from the RC-HL were attenuated. In conclusion, attenuation from the Nishigahara to RC-HL strain is multigenic.

Rabies virus glycoprotein is an important determinant for the induction of innate immune responses and the pathogenic mechanisms

Our previous studies have suggested that street and fixed rabies viruses (RABVs) induce diseases in the mouse model via different mechanisms. In the present study, attempts were made to determine if it is the glycoprotein (G) that is responsible for the observed differences in the pathogenic mechanisms. To this end, an infectious clone from fixed virus B2c was established and used as a backbone for exchange of the G from street viruses. The rate of viral replication, expression of viral proteins, and the induction of innate immune responses were compared in cells or in mice infected with each of the viruses. Furthermore, the infiltration of inflammatory cells into the CNS and the enhancement of blood-brain barrier (BBB) permeability were also compared. It was found that fixed viruses induced stronger innate immune responses (expression of chemokines, infiltration of inflammatory cells, and enhancement of BBB permeability) than street RABV or recombinant viruses expressing the G from street RABVs. Fixed viruses induce disease via an immune-mediated pathogenic mechanism while street viruses or recombinant viruses expressing the G from street RABVs induce diseases via a mechanism other than immune-mediated pathogenesis. Therefore, RABV G is an important determinant for the induction of innate immune responses and consequently the pathogenic mechanisms.

Addition of a single N-glycan to street rabies virus glycoprotein enhances virus production

Most street rabies virus G proteins have two N-glycosylation sites, i.e. Asn37 and Asn319, whereas additional sites are found in fixed (laboratory adapted) viruses. In this study, we performed a pseudotyped virus assay using G-deficient rabies virus and demonstrated that single-N-glycan additions to the G protein of street rabies virus strain 1088, which are found in adapted strains, enhanced virus production in neural and non-neural cell lines, while additions to Asn194 or Asn247 enhanced production greatly. Moreover, we found that N-glycan additions at Asn194 or Asn247 facilitated the production of cell-associated virus. In contrast, deletion of the sequon at Asn37 reduced viral production, while a deletion at Asn319 resulted in extensive loss of production. Furthermore, G proteins lacking an N-glycan at Asn319 failed to fold into their correct structure and lost their fusion activity, indicating that Asn319 N-glycosylation is important for the functional expression of street virus G proteins.

Complete genome sequence of a rabies virus isolate from cattle in guangxi, southern china

A street rabies virus (RV) isolate, GXHXN, was obtained from brain tissue of rabid cattle in the Guangxi Zhuang Autonomous Region of China in 2009. GXHXN is the first isolate from cattle in China with its entire genome sequenced and is closely related to BJ2011E from horse in Beijing, WH11 from donkey in the Hubei Province, and isolates from dogs in the Guangxi and Fujian Provinces, with homologies of 97.6% to 99.6%. It is more distantly related to isolates from domestic cat, pig, Chinese ferret badger, and vaccine strains, with homologies of 83.1% to 88.0%.

Relationship between within-host fitness and virulence in the vesicular stomatitis virus: correlation with partial decoupling

Given the parasitic nature of viruses, it is sometimes assumed that rates of viral replication and dissemination within hosts (within-host fitness) correlate with virulence. However, there is currently little empirical evidence supporting this principle. To test this, we quantified the fitness and virulence of 21 single- or double-nucleotide mutants of the vesicular stomatitis virus in baby hamster kidney cells (BHK-21). We found that, overall, these two traits correlated positively, but significant outliers were identified. Particularly, a single mutation in the conserved C terminus of the N nucleocapsid (U1323A) had a strongly deleterious fitness effect but did not alter or even slightly increased virulence. We also found a double mutant of the M matrix protein and G glycoprotein (U2617G/A3802G mutant) with high fitness yet low virulence. We further characterized these mutants in primary cultures from mouse brain cells and in vivo and found that their relative fitness values were similar to those observed in BHK-21 cells. The mutations had weak effects on the virus-induced death rate of total brain cells, although they specifically reduced neuron death rates. Furthermore, increased apoptosis levels were detected in neurons infected with the U2617G/A3802G mutant, consistent with its known inability to block interferon secretion. In vivo, this mutant had reduced virulence and, despite its low brain titer, it retained a relatively high fitness value owing to its ability to suppress competitor viruses. Overall, our results are in broad agreement with the notion that viral fitness and virulence should be positively correlated but show that certain mutations can break this association and that the fitness-virulence relationship can depend on complex virus-host and virus-virus interactions.

Serial passage of a street rabies virus in mouse neuroblastoma cells resulted in attenuation: potential role of the additional N-glycosylation of a viral glycoprotein in the reduced pathogenicity of street rabies virus

Street rabies viruses are field isolates known to be highly neurotropic. However, the viral elements related to their pathogenicity have yet to be identified at the nucleotide or amino acid level. Here, through 30 passages in mouse neuroblastoma NA cells, we have established an attenuated variant of street rabies virus strain 1088, originating from a rabid woodchuck followed by 2 passages in the brains of suckling mice. The variant, 1088-N30, was well adapted to NA cells and highly attenuated in adult mice after intramuscular (i.m.) but not intracerebral (i.c.) inoculations. 1088-N30 had seven nucleotide substitutions, and the R196S mutation of the G protein led to an additional N-glycosylation. Street viruses usually possess one or two N-glycosylation sites on the G protein, 1088 has two, while an additional N-glycosylation site is observed in laboratory-adapted strains. We also established a cloned variant 1088-N4#14 by limiting dilution. Apart from the R196S mutation, 1088-N4#14 possessed only one amino acid substitution in the P protein, which is found in several field isolates. 1088-N4#14 also efficiently replicated in NA cells and was attenuated in adult mice after i.m. inoculations, although it was more pathogenic than 1088-N30. The spread of 1088-N30 in the brain was highly restricted after i.m. inoculations, although the pattern of 1088-N4#14's spread was intermediate between that of the parental 1088 and 1088-N30. Meanwhile, both variants strongly induced humoral immune responses in mice compared to 1088. Our results indicate that the additional N-glycosylation is likely related to the reduced pathogenicity. Taken together, we propose that the number of N-glycosylation sites in the G protein is one of the determinants of the pathogenicity of street rabies viruses.

[Rhabdoviruses]

The family Rhabdoviridae has a non-segmented single stranded negative-sense RNA and its genome ranges in size from approximately 11 kb to almost 16 kb. It is one of the most ecologically diverse families of RNA viruses with members infecting a wide range of organisms. The five structural protein genes are arranged in the same linear order (3'-N-P-M-G-L-5') and may be interspersed with one more additional accessory gene. For many years, a full of knowledge of the rhabdoviridae has been established on extensive studies of two kinds of prototype viruses; vesicular stomatitis virus (VSV) and rabies virus (RABV). Among them, the genus Lyssavirus includes RABV and rabies-related viruses naturally infect mammals and chiropterans via bite-exposure by rabid animals and finally cause fatal encephalitis. In this review, we describe the sketch of the various virological features of the Rhabdoviridae, especially focusing on VSV and RABV.

Rabies virus as a research tool and viral vaccine vector

Until recently, single-stranded negative sense RNA viruses (ssNSVs) were one of only a few important human viral pathogens, which could not be created from cDNA. The inability to manipulate their genomes hindered their detailed genetic analysis. A key paper from Conzelmann's laboratory in 1994 changed this with the publication of a method to recover rabies virus (RABV) from cDNA. This discovery not only dramatically changed the broader field of ssNSV biology but also opened a whole new avenue for studying RABV pathogenicity, developing novel RABV vaccines as well a new generation of RABV-based vaccine vectors, and creating research tools important in neuroscience such as neuronal tracing.

Development of recombinant rabies viruses vectors with Gaussia luciferase reporter based on Chinese vaccine strain CTN181

The recombinant rabies virus (RV) vectors encoding the secreted gene marker Gaussia luciferase (Gluc) were generated based on Chinese vaccine strain CTN181. Vectors included replication competent CTN-Gluc, CTN/G_Q333R-Gluc, in which the amino acid in position 333 of glycoprotein was mutated from glutamine (Q) to arginine (R), and replication constrained CTNΔG-Gluc, in which the glycoprotein encoding gene (G) was deleted. The growth of recombinant RVs in transfected cells was confirmed through biochemical assays of Gluc activities. Gluc expression in recombinant CTNΔG-Gluc virus was highest while that in CTN/G_Q333R-Gluc virus was lowest. The optimal time to harvest recombinant RVs was determined and the function of pathogenic and nonpathogenic rabies glycoprotein in virus recovery was examined. The addition of glycoprotein was slightly beneficial for virus recovery and the titer of rescued virus was lowered even when the amino acid in G333 position of glycoprotein was mutated from nonpathogenic Gln to pathogenic Arg. Conclusions: Viral vectors based on a human rabies vaccine strain CTN181 were successful. Gluc was useful as an in vitro gene marker for monitoring the growth of recombinant RVs iteratively in cell culture.

Restricted growth of U-type infectious haematopoietic necrosis virus (IHNV) in rainbow trout cells may be linked to casein kinase II activity

Previously, we demonstrated that a representative M genogroup type strain of infectious haematopoietic necrosis virus (IHNV) from rainbow trout grows well in rainbow trout-derived RTG-2 cells, but a U genogroup type strain from sockeye salmon has restricted growth, associated with reduced genome replication and mRNA transcription. Here, we analysed further the mechanisms for this growth restriction of U-type IHNV in RTG-2 cells, using strategies that assessed differences in viral genes, host immune regulation and phosphorylation. To determine whether the viral glycoprotein (G) or non-virion (NV) protein was responsible for the growth restriction, four recombinant IHNV viruses were generated in which the G gene of an infectious IHNV clone was replaced by the G gene of U- or M-type IHNV and the NV gene was replaced by NV of U- or M-type IHNV. There was no significant difference in the growth of these recombinants in RTG-2 cells, indicating that G and NV proteins are not major factors responsible for the differential growth of the U- and M-type strains. Poly I:C pretreatment of RTG-2 cells suppressed the growth of both U- and M-type IHNV, although the M virus continued to replicate at a reduced level. Both viruses induced type 1 interferon (IFN1) and the IFN1 stimulated gene Mx1, but the expression levels in M-infected cells were significantly higher than in U-infected cells and an inhibitor of the IFN1-inducible protein kinase PKR, 2-aminopurine (2-AP), did not affect the growth of U- or M-type IHNV in RTG-2 cells. These data did not indicate a role for the IFN1 system in the restricted growth of U-type IHNV in RTG-2 cells. Prediction of kinase-specific phosphorylation sites in the viral phosphoprotein (P) using the NetPhosK program revealed differences between U- and M-type P genes at five phosphorylation sites. Pretreatment of RTG-2 cells with a PKC inhibitor or a p38MAPK inhibitor did not affect the growth of the U- and M-type viruses. However, 100 μm of the casein kinase II (CKII) inhibitor, 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB), reduced the titre of the U type 8.3-fold at 24 h post-infection. In contrast, 100 μm of the CKII inhibitor reduced the titre of the M type only 1.3-fold at 48 h post-infection. Our data suggest that the different growth of U- and M-type IHNV in RTG-2 cells may be linked to a differential requirement for cellular protein kinases such as CKII for their growth.