Molecular analysis of the mutational effects of Thai street rabies virus with increased virulence in mice after passages in the BHK cell line

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.

Animal and human RNA viruses: genetic variability and ability to overcome vaccines

RNA viruses, in general, exhibit high mutation rates; this is mainly due to the low fidelity displayed by the RNA-dependent polymerases required for their replication that lack the proofreading machinery to correct misincorporated nucleotides and produce high mutation rates. This lack of replication fidelity, together with the fact that RNA viruses can undergo spontaneous mutations, results in genetic variants displaying different viral morphogenesis, as well as variation on their surface glycoproteins that affect viral antigenicity. This diverse viral population, routinely containing a variety of mutants, is known as a viral 'quasispecies'. The mutability of their virions allows for fast evolution of RNA viruses that develop antiviral resistance and overcome vaccines much more rapidly than DNA viruses. This also translates into the fact that pathogenic RNA viruses, that cause many diseases and deaths in humans, represent the major viral group involved in zoonotic disease transmission, and are responsible for worldwide pandemics.

Genetic and Phenotypic Characterization of a Rabies Virus Strain Isolated from a Dog in Tokyo, Japan in the 1940s

The rabies virus strain Komatsugawa (Koma), which was isolated from a dog in Tokyo in the 1940s before eradication of rabies in Japan in 1957, is known as the only existent Japanese field strain (street strain). Although this strain potentially provides a useful model to study rabies pathogenesis, little is known about its genetic and phenotypic properties. Notably, this strain underwent serial passages in rodents after isolation, indicating the possibility that it may have lost biological characteristics as a street strain. In this study, to evaluate the utility of the Koma strain for studying rabies pathogenesis, we examined the genetic properties and in vitro and in vivo phenotypes. Genome-wide genetic analyses showed that, consistent with previous findings from partial sequence analyses, the Koma strain is closely related to a Russian street strain within the Arctic-related phylogenetic clade. Phenotypic examinations in vitro revealed that the Koma strain and the representative street strains are less neurotropic than the laboratory strains. Examination by using a mouse model demonstrated that the Koma strain and the street strains are more neuroinvasive than the laboratory strains. These findings indicate that the Koma strain retains phenotypes similar to those of street strains, and is therefore useful for studying rabies pathogenesis.

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.

The adaptation of a CTN-1 rabies virus strain to high-titered growth in chick embryo cells for vaccine development

Background

Rabies virus is the causative agent of rabies, a central nervous system disease that is almost invariably fatal. Currently vaccination is the most effective strategy for preventing rabies, and vaccines are most commonly produced from cultured cells. Although the vaccine strains employed in China include CTN, aG, PM and PV, there are no reports of strains that are adapted to primary chick embryo cells for use in human rabies prevention in China.

Results

Rabies virus strain CTN-1 V was adapted to chick embryo cells by serial passage to obtain the CTNCEC25 strain. A virus growth curve demonstrated that the CTNCEC25 strain achieved high titers in chick embryo cells and was nonpathogenic to adult mice by intracerebral inoculation. A comparison of the structural protein genes of the CTNCEC25 strain and the CTN-1 V strain identified eight amino acid changes in the mature M, G and L proteins. The immunogenicity of the CTNCEC25 strain increased with the adaptation process in chick embryo cells and conferred high protective efficacy. The inactivated vaccine induced high antibody responses and provided full protection from an intramuscular challenge in adult mice.

Conclusions

This is the first description of a CTNCEC25 strain that was highly adapted to chick embryo cells, and both its in vitro and in vivo biological properties were characterized. Given the high immunogenicity and good propagation characteristics of the CTNCEC25 strain, it has excellent potential to be a candidate for development into a human rabies vaccine with high safety and quality characteristics for controlling rabies in China.

Involvement of the rabies virus phosphoprotein gene in neuroinvasiveness

Rabies virus (RABV), which is transmitted via a bite wound caused by a rabid animal, infects peripheral nerves and then spreads to the central nervous system (CNS) before causing severe neurological symptoms and death in the infected individual. Despite the importance of this ability of the virus to spread from a peripheral site to the CNS (neuroinvasiveness) in the pathogenesis of rabies, little is known about the mechanism underlying the neuroinvasiveness of RABV. In this study, to obtain insights into the mechanism, we conducted comparative analysis of two fixed RABV strains, Nishigahara and the derivative strain Ni-CE, which cause lethal and asymptomatic infections, respectively, in mice after intramuscular inoculation. Examination of a series of chimeric viruses harboring the respective genes from Nishigahara in the genetic background of Ni-CE revealed that the Nishigahara phosphoprotein (P) gene plays a major role in the neuroinvasiveness by mediating infection of peripheral nerves. The results obtained from both in vivo and in vitro experiments strongly suggested that the Nishigahara P gene, but not the Ni-CE P gene, is important for stable viral replication in muscle cells. Further investigation based on the previous finding that RABV phosphoprotein counteracts the host interferon (IFN) system demonstrated that the Nishigahara P gene, but not the Ni-CE P gene, functions to suppress expression of the beta interferon (IFN-β) gene (Ifn-β) and IFN-stimulated genes in muscle cells. In conclusion, we provide the first data strongly suggesting that RABV phosphoprotein assists viral replication in muscle cells by counteracting the host IFN system and, consequently, enhances infection of peripheral nerves.