Replication of rabies virus in organized cultures of Mammalian neural tissues

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.

Different pathogenicity of encephalitic togaviruses in organotypic cultures of spinal cord slices

The pathogenicity of two encephalitic Togaviruses, Sindbis virus (SV), an alphavirus, and West Nile virus (WNV), a flavivirus, was studied in organotypic cultures of fetal mouse spinal cord slices grown in roller tubes. After about 3 weeks in vitro, during which time the cultures became abundantly myelinated, they were infected either by 5 X 10(5) PFU SV or by 5 X 10(6) PFU WNV per culture. The viruses caused different patterns of cytopathogenicity: SV induced severe cytotoxicity in all glia cells and neurons with concomitant demyelination within 48 hr. In contrast, WNV, even 4 days after infection, caused only mild cytopathic effects mainly to neurons and astrocytes and a slight degree of damage to the myelin sheath. A most remarkable finding was the entrapment of WNV particles in the interperiod lines of the myelin sheaths. Treatment of cultures with mouse alpha and beta interferon prior to their infection with either virus protected the cultures from any viral damage. Long-term exposure of non-infected control organotypic cultures of fetal spinal cord slices to mouse interferons had no significant effect on neuronal and glial differentiation, and myelin formation.

Ultrastructural localization of rabies virus antigens in infected trigeminal ganglion of hamsters

The trigeminal ganglion of hamsters infected with CVS strain of rabies virus was investigated by electron microscopy with peroxidase-labeled antibody. Major localization of virus antigens were the following three types of structures; rabies virion, cytoplasmic inclusion, and ribosomal granules consisting of Nissl body. Rabies virions in neurons were mostly found within the cluster of such ribosome-rich regions suggesting a close relationship between the two in the synthesis of virus antigen. The cytoplasmic inclusion appeared to be derived from ribosomal or allied particles with virus-specific antigens. The significance of results obtained is briefly discussed.

Alteration of the in vitro host range of rabies virus after serial chick embryo cell passage using alkaline maintenance medium

HEP Flury strain of rabies virus maintained by 7-day chicken egg passage (parent line) and the same strain serially passaged in primary chick embryo (CE) cells using alkaline maintenance medium (AM line) were inoculated to cells of various species. Growth was negative in primary mouse embryo, L and HeLa cells, and positive in primary hamster kidney and BHK21 cells with both lines. An all-or-none difference between the two lines was observed in primary monkey kidney and Vero cells. The parent line did not multiply in these monkey cells, whereas the AM line grew to high titers. In the case of Vero cells a unique cytopathic effect (CPE) was induced by the AM line. After five consecutive passages in Vero cells, the CPE-inducing agent was identified as rabies virus by a neutralization test. It was infective to intracerebrally inoculated suckling mice but not to adult mice, and its Vero cell-infective titer determined by CPE induction was about 1 log lower than the baby mouse-infective and CE plaque-forming titers. In contrast to the AM line, HEP Flury strain receiving 150 CE cell passages under neutral maintenance medium and three other strains receiving similar CE cell passages all failed to grow in Vero cells.

Enhanced growth and plaquing of rabies virus in static chick embryo cell culture

The 7-day egg passage line of HEP Flury strain of rabies virus was inoculated to primary chick embyro (CE) cells prepared in different ways to compared efficiencies of viral growth and plaquing. Special care to minimize cellular damage due to trypsin at the step of monodispersion and sowing a comparatively large number of cells for monolayer preparation were required for rabies plaquing, whereas such cares were not necessary for plaquing of vesicular stomatitis virus. Plaque number and size were increased by incorporation of a high concentration of thymidine into cell growth medium. Various other means to produce a static state of CE cells were tested, and a maximal plaquing efficiency was obtained when dishes receiving a massive number of dispersed cells in MEM plus 1% calf serum were incubated at 37 C for 1 day without any buffering for monolayer preparation and postinfection incubation was done at 32 C in a CO2-incubator. Bottle cultures of CE cells prepared in a similar manner, when infected with HEP Flury virus, yielded a markedly higher titer of virus that CE cells prepared by our previous standard method.

Maturation of rabies virus by budding from neuronal cell membrane in suckling mouse brain

Two strains of tissue culture-grown rabies virus developed in suckling mouse brain predominantly by the process of virus budding from the neuronal cell membrane.

Further studies on the replication of rabies and rabies-like viruses in organized cultures of mammalian neural tissues

Organized cultures of mammalian spinal and dorsal root ganglions were used for a comparative study of the neurocytopathology caused by rabies and so-called rabies-like viruses. Electron microscopy and titration of infectivity revised earlier data in which no difference could be demonstrated between street and fixed-virus infection. In the present study, fixed virus produced inclusion bodies without apparent virus assembly. Sequential electron microscopy revealed that the main sites of virus assembly were the membranes of the Golgi complex. In contrast, rabies-like viruses freshly isolated from wild rodents produced inclusion bodies all of which were associated with virus replication. Electron microscopic evidence has led us to classify these strains as street virus. Nonneural cell elements from cultivated ganglions were susceptible to fixed virus and the cultures yielded higher titers of infectivity as compared to those of rabies-like viruses. Virus budding was shown to occur at the cell surface as well as at intracytoplasmic membranes.

Experimental infection of dogs and monkeys with two rabies serogroup viruses, Lagos bat and Mokola (IbAn 27377). Gross pathologic and histopathologic changes

Dogs and monkeys were inoculated intracerebrally or intramuscularly with Lagos bat virus or Mokola virus, two viruses serologically and morphologically related to rabies virus. All animals inoculated intracerebrally died, but some animals inoculated intramuscularly survived. One monkey inoculated intramuscularly with Lagos bat virus survived with a hemiparesis. In both dogs and monkeys that died there was a nonsuppurative meningoencephalomyelitis and ganglionitis. Lesions were more extensive than those reported after infection with street rabies virus. Intracytoplasmic inclusions, morphologically indistinguishable from Negri bodies, were seen in the central nervous system of monkeys, but not dogs, that died. With fluorescent-antibody microscopy, granular aggregations of viral antigen were observed in the cytoplasm of many neurons in the central nervous system. In some animals, the retina and ciliary body of the eye had focal inflammatory changes.