The pathogenesis of rabies and other lyssaviral infections: recent studies

Association between RABV G Proteins Transported from the Perinuclear Space to the Cell Surface Membrane and N-Glycosylation of the Sequon Asn(204)

In this study, G proteins of the rabies virus (RABV) Kyoto strain were detected in the cytoplasm but not distributed at the cell membrane of mouse neuroblastoma (MNA) cells. G proteins of CVS-26 were detected in both the cell membrane and perinuclear space of MNA cells. We found that N-glycosylation of street RABV G protein by the insertion of the sequon Asn(204) induced the transfer of RABV G proteins to the cell surface membrane. Fixed RABV budding from the plasma membrane has been found to depend not only on G protein but also on other structural proteins such as M protein. However, the differing N-glycosylation of G protein could be associated with the distinct budding and antigenic features of RABV in street and fixed viruses. Our study of the association of N-glycan of G protein at Asn(204) with the transport of RABV G protein to the cell surface membrane contributes to the understanding of the evolution of fixed virus from street virus, which in turn would help for determine the mechanism underlying RABV budding and enhanced host immune responses.

Review of human rabies prophylaxis and treatment

Rabies is a devastating encephalitis caused by RNA viruses that use mammals as reservoirs. In the United States, most naturally acquired human cases have come from bats. The use of appropriate preexposure and postexposure prophylaxis can be nearly 100% effective. If prophylaxis is not used, or is implemented incorrectly, the patient may develop clinical rabies, which is almost universally fatal. All health care practitioners should be familiar with the appropriate evaluation of patients presenting with a possible rabies exposure and ensure that expeditious and appropriate prophylaxis is provided to help prevent the development of this lethal disease.

Infectivity of rabies virus-exposed macrophages

Rabies virus distributes widely in infected mice, including lymphoid tissues and spleen macrophages. The infection characteristics in murine macrophages and the infectivity of virus-exposed macrophages were examined upon inoculation in mice. In vitro, Mf4/4 spleen macrophages supported mild virus production (10(4)-fold less than neuroblastoma), with formation of typical virions. Bone marrow-derived macrophages (BMM) were most efficient to capture virus, but new virus production was not detected. Virus-induced cell death was significantly stronger in BMM, which might have eliminated BMM with productive infection. Still, viral RNA remained detectable in the remaining BMM for at least 4 weeks. Injection of in vitro-infected Mf4/4 in the nose or brain proved efficient to propagate infection in mice, even when cells were pre-incubated with neutralizing antibodies. Surprisingly, injection of ex-vivo-infected BMM in the brain also led to lethal infection in 8 out of 12 mice. Injection of infected Mf4/4 in the muscle mostly favoured a protective antibody response. Despite that macrophages are less fit to support virus production, they can still act as a source of infectious virus upon transfer in mice. This may be relevant for screening donor organs/cells, for which RT-PCR should be preferred over the traditional antigen or virus isolation assays.

Canine rabies ecology in southern Africa

Rabies is a widespread disease in African domestic dogs and certain wild canine populations. Canine rabies became established in Africa during the 20th century, coinciding with ecologic changes that favored its emergence in canids. I present a conceptual and terminologic framework for understanding rabies ecology in African canids. The framework is underpinned by 2 distinct concepts: maintenance and persistence. Maintenance encompasses the notion of indefinite transmission of infection within a local population and depends on an average transmission ratio > or =1. Maintenance in all local populations is inherently unstable, and the disease frequently becomes extinct. Persistence, the notion of long-term continuity, depends on the presence of rabies in > or =1 local population within the canine metapopulation at any time. The implications for understanding rabies ecology and control are reviewed, as are previous studies on rabies ecology in African canids.

Pathogenesis of rabies

Rabies is a central nervous system (CNS) disease that is almost invariably fatal. The causative agent is rabies virus (RV), a negative-stranded RNA virus of the rhabdovirus family. RV pathogenesis, like that of other viruses, is a multigenic trait. Recent findings indicate that in addition to the RV G protein viral elements that regulate gene expression, especially expression of the L gene, are also likely to play a role in RV pathogenesis. In vivo, RV infects almost exclusively neurons, and neuroinvasiveness is the major defining characteristic of a classical RV infection. A key factor in the neuroinvasion of RV is transsynaptic neuronal spread. While the ability of RV to spread from the post-synaptic site to the pre-synaptic site is mediated by the RV G protein, the RV P protein might be an important determinant of retrograde transport of the virus within axons. Although the mechanism(s) by which an RV infection cause(s) a lethal neurological disease are still not well understood, the most significant factor underlying the lethal outcome of an RV infection appears to be the neuronal dysfunction due to drastically inhibited synthesis of proteins required in maintaining neuronal functions.

The application of reverse genetics technology in the study of rabies virus (RV) pathogenesis and for the development of novel RV vaccines

Rabies is a central nervous system (CNS) disease that is almost invariably fatal. Neurotropism, neuroinvasiveness, and transsynaptic spread are the main features that determine the pathogenesis of rabies. Recent advances in rabies virus (RV) research, which made direct genetic manipulations of the RV genome possible, greatly improved the understanding of the role of different viral and host cell factors in the pathogenesis of rabies. Here the authors discuss molecular mechanisms associated with rabies RV infection and its spread to the CNS.

Viral diseases of ferrets

Distemper and rabies vaccination are highly recommended because of the almost invariable fatal outcome of these conditions. Vaccination should constitute an important part of a ferret's preventative medicine program. With the current and anticipated development and licensing of new vaccines, practitioners are invited to gain awareness of the latest vaccine information. Establishment of a practice vaccination protocol with regards to the site of administration of rabies and distemper vaccines is paramount to document any future abnormal tissue reactions. Influenza is the most common zoonotic disease that is seen in ferrets. Although it generally is benign in most ferrets, veterinarians must take this condition seriously. The characteristic continuous antigenic variation of this virus may lead to more virulent strains; the recent emergence of avian influenza virus outbreaks; and the increased susceptibility of elderly, young, and immunosuppressed individuals.

Luciferase imaging of a neurotropic viral infection in intact animals

The identification of viral determinants of virulence and host determinants of susceptibility to virus-induced disease is essential for understanding the pathogenesis of infection. Obtaining this information requires infecting large numbers of animals to assay amounts of virus in a variety of organs and to observe the onset and progression of disease. As an alternative approach, we have used a murine model of viral encephalitis and an in vivo imaging system that can detect light generated by luciferase to monitor over time the extent and location of virus replication in intact, living mice. Sindbis virus causes encephalomyelitis in mice, and the outcome of infection is determined both by the strain of virus used for infection and by the strain of mouse infected. The mode of entry into the nervous system is not known. Virulent and avirulent strains of Sindbis virus were engineered to express firefly luciferase, and the Xenogen IVIS system was used to monitor the location and extent of virus replication in susceptible and resistant mice. The amount of light generated directly reflected the amount of infectious virus in the brain. This system could distinguish virulent and avirulent strains of virus and susceptible and resistant strains of mice and suggested that virus entry into the nervous system could occur by retrograde axonal transport either from neurons innervating the initial site of replication or from the olfactory epithelium after viremic spread.

A novel expression cassette of lyssavirus shows that the distantly related Mokola virus can rescue a defective rabies virus genome

By comparing three expression vectors for the rabies virus (Rv) minigenome, we show that the characteristic of the Rv RNA is important for efficient rescue despite its not being crucial for replication. Moreover, we show that the coexpression of the viral proteins from helper Rv and Mokola virus could rescue the Rv minigenome while Rv-related European bat lyssavirus 1 could not, suggesting that the signals controlling transcription and replication are conserved in the distantly related Rv and Mokola virus.

Low-affinity nerve-growth factor receptor (P75NTR) can serve as a receptor for rabies virus

A random-primed cDNA expression library constructed from the mRNA of neuroblastoma cells (NG108) was used to clone a specific rabies virus (RV) receptor. A soluble form of the RV glycoprotein (Gs) was utilized as a ligand to detect positive cells. We identified the murine low-affinity nerve-growth factor receptor, p75NTR. BSR cells stably expressing p75NTR were able to bind Gs and G-expressing lepidopteran cells. The ability of the RV glycoprotein to bind p75NTR was dependent on the presence of a lysine and arginine in positions 330 and 333 respectively of antigenic site III, which is known to control virus penetration into motor and sensory neurons of adult mice. P75NTR-expressing BSR cells were permissive for a non-adapted fox RV isolate (street virus) and nerve growth factor (NGF) decreased this infection. In infected cells, p75NTR associates with the RV glycoprotein and could be precipitated with anti-G monoclonal antibodies. Therefore, p75NTR is a receptor for street RV.

Rabies virus quasispecies: implications for pathogenesis

Passage of the mouse-adapted rabies virus strain CVS-24 (where CVS is challenge virus standard) in BHK cells results in the rapid selection of a dominant variant designated CVS-B2c that differs genotypically and phenotypically from the dominant variant CVS-N2c present in mouse-brain- or neuroblastoma-cell-passaged CVS-24. The glycoprotein of CVS-B2c has 10 amino acid substitutions compared with that of CVS-N2c. Because CVS-B2c can be reproducibly selected in BHK cells, it is likely to be a conserved minor subpopulation of CVS-24. CVS-N2c is more neurotropic in vitro and in vivo than CVS-B2c, which replicates more readily in nonneuronal cells in vitro and in vivo. These characteristics appear to be relevant to the pathogenicity of the two variants. CVS-N2c is more pathogenic for adult mice than CVS-B2c. In contrast, CVS-B2c is more pathogenic for neonatal mice. These differences in pathogenicity are reflected in the selection pattern when mixtures of CVS-N2c and CVS-B2c were used to infect neonatal and adult mice. Although CVS-N2c was highly selected in adult mice, no selection for either variant was seen in neonates, suggesting that certain aspects of development, such as maturation of the nervous and immune systems, may contribute to the selection process. We speculate that the existence of different variants within a rabies virus strain may facilitate the virus in overcoming barriers to its spread, both within the host and between species.

Neuronal cell surface molecules mediate specific binding to rabies virus glycoprotein expressed by a recombinant baculovirus on the surfaces of lepidopteran cells

The existence of specific rabies virus (RV) glycoprotein (G) binding sites on the surfaces of neuroblastoma cells is demonstrated. Spodoptera frugiperda (Sf21) cells expressing G of the RV strain CVS (Gcvs-Sf21 cells) bind specifically to neuroblastoma cells of different species but not to any other cell type (fibroblast, myoblast, epithelial, or glioma). Attachment to mouse neuroblastoma NG108-15 cells is abolished by previous treatment of Gcvs-Sf2 cells with anti-G antibody. Substitutions for lysine at position 330 and for arginine at position 333 in RV G greatly reduce interaction between Gcvs-Sf21 cells and NG108-15 cells. These data are consistent with in vivo results: an avirulent RV mutant bearing the same double mutation is not able to infect sensory neurons or motoneurons (P. Coulon, J.-P. Ternaux, A. Flamand, and C. Tuffereau, J. Virol. 72:273-278, 1998) after intramuscular inoculation into a mouse. Furthermore, infection of NG108-15 cells by RV but not by vesicular stomatitis virus leads to a reduction of the number of binding sites at the neuronal-cell surface. Our data strongly suggest that these specific attachment sites on neuroblastoma cells represent a neuronal receptor(s) used by RV to infect certain types of neurons in vivo.