Each of several strains of fixed rabies virus was found to replicate to high titers in C1300 mouse neuroblastoma (clone NA) cells, without adaptation. Rabies serogroup Lagos bat, Mokola, and Duvenhage viruses also replicated efficiently in NA cells. Kotonkan and Obodhiang viruses replicated efficiently after adaptation, to titers not previously obtained in vitro. Infection in NA cells was frequently more cytopathic than in BHK-21 cells, allowing titration of Kotonkan and Obodhiang viruses by plaque assay. Duvenhage virus caused syncytium formation. Serial propagation of rabies viruses at a high multiplicity of infection in NA cells led to a rapid decline in virus yields; similar "autointerference" has not previously been demonstrated with rabies virus in other cell systems. Rabies virus infection in NA cells exhibited extreme sensitivity to interference by experimentally added defective interfering virions. Although several strains of attenuated rabies virus consistently reverted rapidly to virulence after propagation in NA cells, other strains of attenuated rabies and rabies serogroup viruses acquired increased virulence at a more gradual rate or not at all, suggesting that diverse characters may control virulence. When attenuated Flury HEP rabies virus was serially propagated at a low multiplicity of infection in either NA cells or suckling mouse brain, virulence appeared at a very variable rate, indicating that these systems may selectively enhance replication of randomly occurring virulent virus mutants.
Human peripheral blood mononuclear cells and T-cell lines and clones from individuals immunized with rabies PM vaccine were tested for the ability to recognize antigenic determinants in rabies and rabies-related viruses in an antigen-induced proliferation assay. Some, but not all, of the T cells from these individuals cross-reacted with various laboratory strains of rabies virus with rabies-related viruses such as Duvenhage and Mokola. In addition, these T cells were shown to react with epitopes of either the ribonucleoprotein or the viral glycoprotein. Rabies-specific cytotoxic T-cell responses by a CD4+ T-cell line were evident against antigenic determinants of the ribonucleoprotein and glycoprotein.
Five viruses related to rabies occur in Africa. Two of these, Obodhiang from Sudan and kotonkan from Nigeria, were found in insects and are only distantly related to rabies virus. The other three are antigenically more closely related to rabies. Mokola virus was isolated from shrews in Nigeria, Lagos bat virus from fruit bats in Nigeria, and Duvenhage virus from brain of a man bitten by a bat in South Africa. The public health significance of the rabies-related viruses was emphasized in Zimbabwe where in 1981 a rabies-related virus became epizootic in the dog and cat population. It is postulated that the ancestral origin of rabies virus was Africa where the greatest antigenic diversity occurs and that the ancestor may have been an insect virus. Questions are raised why rabies has not evolved more rapidly in the New World, given the frequency and ease with which antigenic changes can be induced in the laboratory, and how the virus became so extensively established in New World bats.
The inflexibility of existing serological techniques for detection of rabies in surveillance constrains the benefit to be gained from many current control strategies. We analysed 304 serum samples from Tanzanian dogs for the detection of rabies antibodies in a pseudotype assay using lentiviral vectors bearing the CVS-11 envelope glycoprotein. Compared with the widely used gold standard fluorescent antibody virus neutralisation assay, a specificity of 100% and sensitivity of 94.4% with a strong correlation of antibody titres (r = 0.915) were observed with the pseudotype assay. To increase the assay's surveillance specificity in Africa we incorporated the envelope glycoprotein of local viruses, Lagos bat virus, Duvenhage virus or Mokola virus and also cloned the lacZ gene to provide a reporter element. Neutralisation assays using pseudotypes bearing these glycoproteins reveal that they provide a greater sensitivity compared to similar live virus assays and will therefore allow a more accurate determination of the distribution of these highly pathogenic infections and the threat they pose to human health. Importantly, the CVS-11 pseudotypes were highly stable during freeze–thaw cycles and storage at room temperature. These results suggest the proposed pseudotype assay is a suitable option for undertaking lyssavirus serosurveillance in areas most affected by these infections.
Rabies virus; Lyssavirus; Africa; Pseudotype
Cell-associated ribonucleoprotein (RNP) was isolated from BHK-21 cells infected with several strains of rabies and rabies-related viruses. The RNP-antigen from rabies and related viruses induced the formation of complement-fixing, precipitating, and immunofluorescent antibodies, and proved to be the group-specific antigen common to all rabies viruses. Antigens of the envelope which induce virus-neutralizing antibodies are apparently determinative for the serotype of a virus as evidenced by two-way neutralization tests. A combination of these methods seems to be a useful approach to the serological grouping and typing of rhabdoviruses.
A tissue culture (TC)-fluorescent antibody (FA) technique for the measurement of rabies-neutralizing antibody was found to be reliable and comparable to the standard mouse serum neutralization test. This test was performed with BHK-21 cells infected with the ERA vaccine virus strain on Lab-Tek TC chamber slides. A Flury high egg passage (HEP) rabies virus strain grown on continuous line of African green monkey (Vero) and on BHK-21 cells was investigated to determine its utilization in a TC-FA neutralization procedure. Although both the HEP and ERA viruses infected Vero and BHK-21 cells, the amount of fluorescent antigen observed was most consistent with ERA virus and BHK-21 cells.
Rabies virus isolates from terrestrial animals in six areas of the United States were examined with a panel of monoclonal antibodies to nucleocapsid proteins. Characteristic differences in immunofluorescence reactions permitted the formation of four antigenically distinct reaction groups from the 231 isolates tested. The geographic distribution of these groups corresponded well with separate rabies enzootic areas recognized by surveillance of sylvatic rabies in the United States. Distinctive reaction patterns were also identified for viral proteins from four infected bat species, and identical patterns were found in eight isolated cases of rabies in terrestrial animals. These findings suggest that monoclonal antibodies can be used to study the prevalence, distribution, and transmission of rabies among wildlife species.
We have developed idiotype-anti-idiotype monoclonal antibodies that provide evidence for rabies virus binding to the acetylcholine receptor (AChR). Hybridoma cell lines 7.12 and 7.25 resulted after fusion of NS-1 myeloma cells with spleen cells from a BALB/c mouse immunized with rabies virus strain CVS. Antibody 7.12 reacted with viral glycoprotein and neutralized virus infectivity in vivo. It also neutralized infectivity in vitro when PC12 cells, which express neuronal AChR, but not CER cells or neuroblastoma cells (clone N18), which have no AChR, were used. Antibody 7.25 reacted with nucleocapsid protein. Anti-idiotypic monoclonal antibody B9 was produced from fusion of NS-1 cells with spleen cells from a mouse immunized with 7.12 Fab. In an enzyme-linked immunosorbent assay and immunoprecipitation, B9 reacted with 7.12, polyclonal rabies virus immune dog serum, and purified AChR. The binding of B9 to 7.12 and immune dog serum was inhibited by AChR. B9 also inhibited the binding of 7.12 to rabies virus both in vitro and in vivo. Indirect immunofluorescence revealed that B9 reacted at neuromuscular junctions of mouse tissue. B9 also reacted in indirect immunofluorescence with distinct neurons in mouse and monkey brain tissue as well as with PC12 cells. B9 staining of neuronal elements in brain tissue of rabies virus-infected mice was greatly reduced. Rabies virus inhibited the binding of B9 to PC12 cells. Mice immunized with B9 developed low-titer rabies virus-neutralizing antibody. These mice were protected from lethal intramuscular rabies virus challenge. In contrast, anti-idiotypic antibody raised against nucleocapsid antibody 7.25 did not react with AChR.
Rabies virus is a highly neuronotropic virus and glial cell infection is not prominent in the central nervous system (CNS). Paraffin-embedded tissues from the cerebella of skunks experimentally infected with either a skunk salivary gland isolate of street rabies virus or the challenge virus standard (CVS) strain of fixed rabies virus were examined with immunoperoxidase staining for rabies virus antigen by using an anti-rabies virus nucleocapsid protein monoclonal antibody. A skunk infected with street rabies virus showed prominent infection of Bergmann glia. Although infected Purkinje cells were observed, they usually demonstrated a relatively small amount of antigen in their perikarya. A CVS-infected skunk showed many intensely labeled Purkinje cells and a relatively small number of infected Bergmann glia. These findings indicate that although rabies virus is a highly neuronotropic virus, street rabies virus strains do not always demonstrate strict neuronotropism in the central nervous system.
The molecular mechanisms associated with rabies virus (RV) virulence are not fully understood. In this study, the RV Flury low-egg-passage (LEP) and high-egg-passage (HEP) strains were used as models to explore the attenuation mechanism of RV. The results of our studies confirmed that the R333Q mutation in the glycoprotein (GR333Q) is crucial for the attenuation of Flury RV in mice. The R333Q mutation is stably maintained in the HEP genome background but not in the LEP genome background during replication in mouse brain tissue or cell culture. Further investigation using chimeric viruses revealed that the polymerase L gene determines the genetic stability of the GR333Q mutation during replication. Moreover, a recombinant RV containing the LEP G protein with the R333Q mutation and the HEP L gene showed significant attenuation, genetic stability, enhancement of apoptosis, and immunogenicity. These results indicate that attenuation of the RV Flury strain results from the coevolution of G and L elements and provide important information for the generation of safer and more effective modified live rabies vaccine.
Ocular pathology in the first European case of human bat-borne rabies is described. The patient was a 30-year-old bat scientist who seven weeks after bat bite developed neurological symptoms and died 23 days later. Rabies virus antigens were detected in brain smears. After extensive virological studies the virus turned out to be a rabies-related virus, closely resembling the Duvenhage virus isolated from bats in South Africa in 1980. By light microscopy focal chronic inflammatory infiltration of the ciliary body and of the choroid was found. PAS-positive exudate was seen in the subretinal and in the outer plexiform layers of the retina, and retinal veins showed endothelial damage and perivascular inflammation. Many of the retinal ganglion cells were destroyed. The presence of rabies-related viral antigen in the retinal ganglion cells was shown by positive cytoplasmic immunofluorescence, though electron microscopy failed to identify definite viral structures in the retina. By immunohistochemistry glial fibrillary acidic protein was observed in the Müller's cells, which are normally negative for this antigen but express it as a reactive change when the retina is damaged. Synaptophysin, a constituent of presynaptic vesicles of normal retinal neurons, was not detected in the retina.
Using four neutralizing monoclonal antibodies which presumably bind to the same antigenic site on the CVS glycoprotein (antigenic site III as defined by cross-neutralization tests), we isolated 58 mutants of the CVS strain of rabies virus. These mutants were highly resistant to the selecting antibodies and grew efficiently in cell cultures. We classified them into five groups on the basis of the pattern of resistance to the four antibodies. We determined pathogenicities of the mutants for adult mice by intracerebral inoculation. Group 2 mutants were nonpathogenic or had attenuated pathogenicity. On the contrary, mutants from the other groups were pathogenic, causing paralysis and death as does CVS. We determined the nucleotide alterations of representative mutants from each group by using the dideoxy method of RNA sequencing. In the glycoproteins of eight nonpathogenic or attenuated mutants, we identified an amino acid substitution at position 333. Arginine 333 was replaced by either glutamine or glycine. In the glycoprotein of eight pathogenic mutants, we identified an amino acid substitution at lysine 330, asparagine 336, or isoleucine 338. Thus, although all substitutions affected neutralization and were located close to each other in the glycoprotein sequence, only substitutions at position 333 affected pathogenicity.
Indirect immunofluorescence confirmed the antigenic relationship between kotankan and Obodhiang viruses and Mokola virus that had originally been shown by complement fixation test. This relationship suggests inclusion of these two arthropod isolates in the rabies subgroup of the Rhabdoviridae family. Cross-reactivity with Mokola virus was also demonstrated by direct immunofluorescence but was easily eliminated when conjugates were diluted. No crossreactivities were found by neutralization tests or by surface immunofluorescence. Other than these immunological ties to the rabies serogroup, other biological characteristics of kotonkan and Obodhiang viruses were distinct. Maximum yield of infectivity of kotonkan and Obodhiang in cell culture was at 30 C, antigen usually filled the cytoplasm of infected cells diffusely, and syncytia were formed before severe cytonecrosis. By electron microscopy, virus particles and their nucleocapsids appeared cone shaped (mean lengths: kotonkan, 182 nm; Obodhiang, 170 nm). Viral morphogenesis took place on plasma membranes of cells in culture, mouse brain neurons, and inflammatory cells (macrophages) in brain lesions. All of these characteristics of the two viruses, and the known association of kotonkan virus with an acute, febrile illness of cattle in Nigeria, suggest a biological relationship with bovine ephemeral fever virus. The latter is known to exist in the same geographic area but exhibits no serological cross-reaction with either kotonkan or Obodhiang virus. The question of whether these two viruses deserve placement in an expanded rabies subgroup (at the cost of a less precise definition of the subgroup) or in a separate subgroup (which would include bovine ephemeral fever virus) of the Rhabdoviridae family will only be answered by further physicochemical characterization and comparison.
A reverse genetics approach which allows the generation of infectious defective rabies virus (RV) particles entirely from plasmid-encoded genomes and proteins (K.-K. Conzelmann and M. Schnell, J. Virol. 68:713-719, 1994) was used to investigate the ability of a heterologous lyssavirus glycoprotein (G) and chimeric G constructs to function in the formation of infectious RV-like particles. Virions containing a chloramphenicol acetyltransferase (CAT) reporter gene (SDI-CAT) were generated in cells simultaneously expressing the genomic RNA analog, the RV N, P, M, and L proteins, and engineered G constructs from transfected plasmids. The infectivity of particles was determined by a CAT assay after passage to helper virus-infected cells. The heterologous G protein from Eth-16 virus (Mokola virus, lyssavirus serotype 3) as well as a construct in which the ectodomain of RV G was fused to the cytoplasmic and transmembrane domains of the Eth-16 virus G rescued infectious SDI-CAT particles. In contrast, a chimeric protein composed of the amino-terminal half of the Eth-16 virus G and the carboxy-terminal half of RV G failed to produce infectious particles. Site-directed mutagenesis was used to convert the antigenic site III of RV G to the corresponding sequence of Eth-16 G. This chimeric protein rescued infectious SDI-CAT particles as efficiently as RV G. Virions containing the chimeric protein were specifically neutralized by an anti-Eth-16 virus serum and escaped neutralization by a monoclonal antibody directed against RV antigenic site III. The results show that entire structural domains as well as short surface epitopes of lyssavirus G proteins may be exchanged without affecting the structure required to mediate infection of cells.
A gene encoding the nucleoprotein (N) of rabies virus was inserted into the genome of the baculovirus Autographa californica nuclear polyhedrosis virus. Recombinant gene expression was controlled by the strong polyhedrin gene promoter. Insect cells (Spodoptera frugiperda) infected by a baculovirus recombinant containing the rabies virus N gene produced abundant amounts of a novel 55-kilodalton protein of a size comparable to that of the rabies virus N protein, as demonstrated by polyacrylamide gel electrophoresis. This new gene product possessed the antigenic and immunogenic properties of native viral N protein, as shown by the ability of the new protein to react in immunoprecipitation and immunofluorescence assays with antirabies antibodies, to serve as a substitute for infectious rabies virus in adsorbing suspensions for diagnostic tests, and to induce high-titered antiserum. The baculovirus expression system provides a safe, convenient, and inexpensive source of rabies virus N protein for the production of both antiserum and adsorbing suspensions for use in rabies diagnoses.
Suspension cultures of BHK-21 cells maintained at 32 to 33 C were infected with the Flury LEP strain of rabies virus. By using a cell concentration of 2.0 × 106 to 2.5 × 106 cells per ml infected at a multiplicity of 0.05, high titers of extracellular virus were reached in 96 to 120 h, and potent inactivated vaccines were prepared from culture fluids harvested between 96 to 168 h. The addition of 1% bovine serum to the maintenance medium resulted in an increase in virus yields and vaccine potency. Estimation of the number of infected cells by immunofluorescent procedures proved a rapid and reliable guide to the virus content of suspension cultures.
Chicken embryo cells infected with the HEP Flury strain of rabies virus adapted to tissue culture produced a hemadsorption (HAD) phenomenon by using goose erthyrocytes. The optimal conditions for HAD included the incubation of cell cultures at 37C for 3 days after virus inoculation, the use of a 0.4% suspension of goose erythrocytes in phosphate buffer adjusted at pH 6.2, and adsorption of erythrocytes at 4C. This phenomenon was inhibited with anti-rabies serum. Virus titer obtained with the HAD technique was almost the same as with the fluorescent antibody technique or the intracerebral inoculation of suckling mice. Results of the neutralization test by using the HAD technique could be easily determined 3 days after inoculation of chicken embryo cells with the mixture of 100 mean tissue culture infective doses of virus and diluted serum. The neutralizing antibody titers coincided with those obtained in mice.
An outbreak of bovine rabies occurred on a ranch when cattle were bitten by vampire bats. Microscopic lesions showed a nonsuppurative encephalitis with intracytoplasmic inclusion bodies. Immunohistochemistry demonstrated viral antigen in the brain, and monoclonal antibodies identified a serotype 1 (vampire strain) of the rabies virus.
The predominant role of Eptesicus serotinus in the epizootic of bat rabies in Europe was further outlined by the first isolation of the rabies virus from this species in France. The distribution of the virus was studied in naturally infected E. serotinus bats at the time of death and suggested that the papillae of the tongue and the respiratory mucosa may play a role in virus production and excretion. The analysis of 501 French rabies virus isolates from various animal species by antinucleocapsid monoclonal antibodies indicated that transmission of the disease from bats to terrestrial animals is unlikely. The antigenic profile of two isolates from French bats corresponded to that of European bat lyssavirus type 1 (EBL1). Comparisons of 12 different isolates from bats with antinucleocapsid and antiglycoprotein monoclonal antibodies and by direct sequencing of the polymerase chain reaction amplification product of the N gene indicated that EBL1, EBL2, Duvenhage virus (serotype 4 of lyssavirus), and the European fox rabies virus (serotype 1) are phylogenetically distant. They formed four tight genetic clusters named genotypes. EBL1 was shown to be antigenically and genetically more closely related to Duvenhage virus than to EBL2. We propose that EBL1 and EBL2 constitute two distinct genotypes which further serologic characterization will probably classify as new serotypes. We also report a simple method for the rapid characterization of EBL based on the digestion of the polymerase chain reaction product of the N gene by three restriction endonucleases.
Although the cell-to-cell spread of many viruses in vitro is inhibited by antibody, the effect of antibody on such spread of rabies viruses is uncertain. Thus, we examined the effects of anti-rabies virus immune sera and monoclonal antibodies (MAbs) on the in vitro spread of pathogenic rabies viruses in neuronal and nonneuronal cells. Both anti-rabies virus immune sera and neutralizing antiglycoprotein MAbs inhibited the cell-to-cell spread of street rabies virus, challenge virus standard, and ERA rabies viruses in cultures of neuroblastoma cells and of nonneuronal BHK-21 and chicken embryo-related cells. Furthermore, the cell-to-cell spread of virus was inhibited by greater than or equal to 75% with less than 1 IU/ml of human antirabies immunoglobulin. Nonneutralizing antinucleocapsid MAbs did not inhibit viral spread. After the immune serum was removed from the monolayers, virus spread rapidly to uninfected cells. Thus, antibody controlled the cell-to-cell spread of the virus but did not eliminate it from the cultures. Because antibody was more effective in inhibiting viral spread in fibroblast and epithelioid cells than in neuroblastoma cells infected at a high multiplicity of infection, we suggest that the inhibition of viral cell-to-cell spread by antibody in vivo would more likely occur at an initial site of exposure and before nerves are infected.
The CVS strain of fixed rabies virus causes acute, fatal encephalomyelitis in young adult ICR mice. Variant RV194-2, which was selected from CVS virus in cell culture with a neutralizing antiglycoprotein monoclonal antibody, has a single amino acid change in the glycoprotein. The infections caused by CVS virus and RV194-2 virus were compared in mice for 14 days postinoculation of 5 x 10(7) PFU into the right masseter muscle. All CVS virus-infected mice died (mean time to death, 7.9 days), compared with a mortality rate of 8.5% for RV194-2 virus-infected mice. RV194-2 virus spread to the ipsilateral trigeminal ganglion during the first 2 days postinoculation, and both viruses spread to the ipsilateral motor nucleus of the trigeminal nerve in the pons. Both viruses spread centrifugally and caused infection of bilateral trigeminal ganglia on day 3. The viruses spread throughout the central nervous system (CNS) at similar rates, but CVS virus infected many more neurons than did RV194-2 virus. Rabies virus antigen was observed in only occasional CNS neurons after day 6 of RV194-2 virus infection. By this time, CVS virus had caused severe widespread infection. In this model, virulence depends on improved efficiency of viral spread between CNS neurons rather than the rate of spread or topographical distribution of the infection.
Mice lethally infected with street rabies virus failed to develop cytotoxic T cells specific for rabies virus-infected target cells, whereas high levels of cell-mediated cytotoxicity (CMC) were generated after nonfatal infection with the attenuated high egg passage (HEP) or ERA rabies virus strains. Furthermore concurrent infection with street, but not with HEP, rabies virus suppresses development of a primary (but not a secondary) CMC response specific for influenza virus. No cross- reactivity is found between effector T-cell populations from mice immunized with HEP or with influenza virus. It thus appears that street rabies virus, which is not known to replicate in the cells of immune system, induces some general defect in the primary CMC lymphocyte response, though restimulation of memory T-cell populations is unimpaired and there is no defect in antibody formation. Development of fatal rabies may reflect the operation of this selective immunosuppressive mechanism.
A total of 112 rabies virus-infected skunk brain samples from naturally occurring cases (64 from Missouri, 48 from Kentucky) were code labeled and grouped into two morphologic categories according to the appearance and size of the discrete particles observed by immunofluorescent-antibody staining. The reactivity of the blind-labeled samples was then determined using a panel of 23 antinucleocapsid monoclonal antibodies to test whether morphologic appearance was associated with antigenicity. Two categories were defined and designated; they were morphologic type I (MO-I) and morphologic type II (MO-II). MO-I viruses produced granular-type fluorescing aggregates with no evidence of inclusions compatible in appearance with Negri bodies. MO-II viruses produced at least one and usually many large, round, or oval fluorescing inclusion bodies. No viruses categorized as MO-I reacted with antinucleocapsid monoclonal antibodies 103-7, 239-10, and 120-2; all MO-II viruses reacted positively to these three antibodies, with one exception involving antinucleocapsid monoclonal antibody 239-10. The results indicate a strong association between the reactivity of these antibodies and the morphologic appearance of the aggregates of antigen.
Exposure of both mammalian and reptilian cells in tissue culture to different strains of fixed rabies virus resulted in a carrier type of infection. No cytopathic effect was observed in either type of culture; infected cultures could be maintained by cell transfer for unlimited numbers of passages. A consistent pattern of cyclically rising and falling levels of viral infection was observed by fluorescent-antibody staining techniques and by titration of released infectious virus. Resistance to super-infection by vesicular stomatis virus and the production of an interferon-like substance by infected cells indicated that the maintenance of a carrier type of infection may be interferon-mediated. The degree of susceptibility of rabies-infected cells to immunolysis by antirabies antibody in the presence of complement was found to be correlated with the amount of virus maturation occurring by budding through the cell membrane and not with the presence of immunofluorescent antigen in the cytoplasm of infected cells.
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.