Genetic determinants of Sindbis virus neuroinvasiveness.

After peripheral inoculation of mice, Sindbis virus replicates in a variety of tissues, leading to viremia. In some cases, the virus can enter the central nervous system (CNS) and cause lethal encephalitis. The outcome of infection is age and virus strain dependent. Recently, two pairs of Sindbis virus variants differing in neurovirulence and neuroinvasiveness were derived by limited serial passaging in mouse brain. Two early passage isolates (SVA and SVB) were neurotropic but did not cause lethal encephalitis. SVB, but not SVA, was neuroinvasive. A second independent pair of isolates (SVN and SVNI), which had undergone more extensive mouse brain passaging, were highly neurotropic and caused lethal encephalitis. Only SVNI could reach the brain after peripheral inoculation. From these isolates, virion RNAs were obtained and used to construct full-length cDNA clones from which infectious RNA transcripts could be recovered. The strains recovered from these clones were shown to retain the appropriate phenotypes in weanling mice. Construction and analysis of recombinant viruses were used to define the genetic loci determining neuroinvasion. For SVB, neuroinvasiveness was determined by a single residue in the E2 glycoprotein (Gln-55). For SVNI, neuroinvasive loci were identified in both the 5' noncoding region (position 8) and the E2 glycoprotein (Met-190). Either of these changes on the SVN background was sufficient to confer a neuroinvasive phenotype, although these recombinants were less virulent. To completely mimic the SVNI phenotype, three SVNI-specific substitutions on the SVN background were required: G at position 8, E2 Met-190, and Lys-260, which by itself had no effect on neuroinvasion. These genetically defined strains should be useful for dissecting the molecular mechanisms leading to Sindbis virus invasion of the CNS.

Loss of active neuroinvasiveness in attenuated strains of West Nile virus: pathogenicity in immunocompetent and SCID mice.

The neuropathogenicity of West Nile virus (WNV) and two derived attenuated strains WN25 and WN25A, was studied in young adult ICR mice and in severe combined immunodeficient (SCID) mice. Similarity in serology and RNA fingerprints were found between WNV and WN25. The viral envelope proteins of the attenuates differed from WNV in their slower mobility in SDS-PAGE due probably to the presence of N-linked glycan. The three strains were lethal to ICR mice by intracerebral (IC) inoculation, but when inoculated intraperitoneally (IP), WNV caused viremia, invaded the CNS and was lethal, whereas the attenuates showed no viremia or invasion of the CNS. The attenuates elicited antibodies to comparable levels as WNV in IP-infected mice, conferring upon them immunity to IC challenge with the wild type. In IP-inoculated SCID mice the three strains exhibited similar high viremiae that lasted until death of the animals. All strains invaded the CNS and proliferated in the mouse brain to similar high titers, but differed largely in the time of invasion: WNV invaded the CNS of SCID mice (and two other mouse strains) much earlier than the attenuates, which showed large intervals in their time of invasion into individual mouse brains within the group. The data presented for SCID mice indicate that WN25 and WN25A have truly lost the neuroinvasive property, and that this property materialized by a prescribed, active process specific for WNV.

A novel variant of Sindbis virus is both neurovirulent and neuroinvasive in adult mice.

A strain of Sindbis virus (SV), recently isolated from mosquitoes in Israel, was used as a source for variants which differ in neuroinvasiveness and virulence that were generated by serial passage of SV in suckling and weanling mouse brain. At the 15th passage a neurovirulent variant was observed and designated SVN (neurovirulent). After 7 more passages in weanling mouse brains, another variant was observed and designated SVNI (neuroinvasive) and both were isolated and purified. All strains caused similar viremia after intraperitoneal (I.P.) injection of weanling mice, but whereas SV was neuroinvasive but nonvirulent, SVN was neurovirulent but noninvasive and SVNI was both virulent and invasive. SVNI is the first SV variant which is both neurovirulent and neuroinvasive in weanling mice. Co-injection I.P. of SV + SVN resulted in presence of SV alone in the mouse brain; co-injection of SVNI + SVN resulted in full-titered replication of both strains in the brain. We assume that this is achieved through a breach of the blood brain barrier effected by SVNI replication and used by SVN for co-invasion. SV probably invades the brain by a different mechanism. I.P. infection with SVNI of inbred BALB/c mice gave rise to clinical signs only in a few mice even though substantial viremia was demonstrated.

Spinal cord slices with attached dorsal root ganglia: a culture model for the study of pathogenicity of encephalitic viruses.

We describe here a culture system for long-term growth of organotypic slices of spinal cord, with attached dorsal root ganglia (DRG) derived from 13-14 day mouse fetuses. This is a unique in vitro tool in which both central and peripheral nervous tissue grow and differentiate in culture to become heavily myelinated. During cultivation the slices and the ganglia become flattened so as to allow microscopical and immunocytochemical staining. When both central and peripheral myelin had been formed (usually around the third week of cultivation), cultures were infected with 5 x 10(6) PFU of West Nile Virus (WNV). Progeny virions appeared first in about 10% of the neurons and were subsequently observed between lamellae in the central myelin sheath of several axons. Such viral arrangement in relation to myelin membranes, might provide a novel concept for a possible mechanism underlying slow viral infection.

West Nile virions aligned along myelin lamellae in organotypic spinal cord cultures.

Organotypic spinal cord cultures infected with West Nile Virus (WNV) exhibited a remarkable arrangement of virions among lamellae of the myelin sheath. Virions were first observed in neurons and only at day 4 after infection appeared within the myelin lamellae. Virions were observed only in the central myelin, aligned along the interperiod lines and therefore attached to the outer side of the oligodendrocyte membrane. In spite of this peculiar location of the virions, their presence was not associated with severe damage to the axon or to the myelin sheath. The causes of the formation of this viral pattern, its morphological features, and the role which it might have in viral infection in vivo are considered.

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.

Sodium dodecylsulphate induces a breach in the blood-brain barrier and enables a West Nile virus variant to penetrate into mouse brain.

A novel and convenient assay was used to determine the effect of recombinant Interleukin-2 (IL-2) on the function of the blood-brain barrier (BBB). The assay is based on a variant of the West Nile virus, WN-25, which had lost its neuroinvasiveness but not its neurovirulence. WN-25, when injected intravenously, can cause the death of mice only if the function of the BBB is impaired. Sodium dodecylsulphate (SDS), a component in IL-2 excipient, was found to cause a short term breach in the BBB, enabling the penetration of viruses into the brain. Minimal amounts (30 ng/mouse) can induce a breach of about 10 min, which allows 0.1% of the injected virus to enter the brain. These findings demonstrate the possible use of SDS as a mean for intentional introduction of drugs into the brain, however they also call attention to the danger of using detergents as additives for drugs given intravenously.

Determination of human IgG and IgM class antibodies to West Nile virus by enzyme linked immunosorbent assay (ELISA).

An enzyme-linked immunosorbent assay (ELISA) was developed and used for the detection of IgG and IgM antibodies to West Nile virus in human sera. Thirteen paired sera of clinical cases and 24 control sera taken randomly from a blood bank repository were tested. The sera were reacted in microtiter plates coated with PEG-treated WNV antigen. IgG or IgM antibodies were quantitated by the use of alkaline-phosphatase-conjugated anti-human IgG or IgM antibodies. Of the 24 randomly collected serum samples, 7 were positive in the IgG-ELISA test. One positive by the IgM-ELISA was found to contain rheumatoid factor. In 12 of 13 paired sera of clinical cases, IgM as well as IgG antibodies were detected in the second serum sample taken about 3 wk after the onset of clinical signs. The IgM positive sera were screened for rheumatoid factor (RF) on IgG-coated plates. None of them contained RF. Antibody titers obtained by ELISA showed a good correlation with titers obtained by hemagglutination inhibition, complement fixation, and neutralization tests. The ELISA tests for detection of IgM and IgG antibodies to WNV therefore can replace the other serological methods for epidemiological surveillance and diagnostic purposes.

Ultrastructural and virological aspects of Langat virus-induced SSPE in suckling hamsters.

The morphology of subacute sclerosing panencephalitis (SSPE) lesions in hamsters following i.p. inoculation of Langat virus was studied by light microscopy and electron microscopy. The lesions' temporal relationship to virus presence and antibody production was studied by immunofluorescence, virus isolation from brains and brain cell cultures, and by antibody assay of serum and spinal fluid. All 10-day-old hamsters infected with Langat virus developed SSPE lesions, most prominently in the cerebellum, without any overt signs. The lesions appeared 10 days after infection, progressed in severity until Day 21 and persisted unaltered at least 3 months. They consisted of neuronal degeneration, calcification, and intracellular lipid accumulation. Virus was isolated from Days 3 to 15 and disappeared on Day 21. Demonstration by cerebellar immunofluorescence of viral antigen was observed 10 days after inoculation. Antibody appeared in the serum on Day 6, rose to very high titres by Day 21, and thereafter declined. Cell-associated virus was not demonstrable in negative brain-cell cultures. These findings suggest that SSPE in hamsters, associated with Langat virus infection, is biologically different from measles SSPE. Purkinje-cell lipid accumulation and mineralization sparing mitochondria were manifestations of the response of neurons to cell injury at the threshold of irreversibility.

Assay of togavirus haemagglutination-inhibition antibodies by the micro method: loss of information and its rectification.

Haemagglutination inhibition (HI) activity was assayed by the micro and macro methods in immune sera prepared against four togaviruses. HI titres were always 4 to 8 times lower by the micro method, and coincided with 4 to 8 times lower haemagglutinin titres in micro method assays. Because of this phenomenon, positive sera with HI macro method titres lower than 1: 80 will be false negative for HI by the micro method when tests begin at a 1: 10 serum dilution. This was confirmed for a number of human sera tested for West Nile virus HI activity. As a consequence of the difference between titres given by the two assay methods, results of seroepidemiological studies may be distorted. Use of a system that combines both the macro and micro methods could rectify this distortion.

Some properties of togavirus hemagglutinin studied with the aid of kaolin-adsorbed virus.

Acidification (at pH 5.75) of Semliki Forest virus and West Nile virus suspensions completely eliminated their hemagglutinating activity within several minutes, but did not affect their infectivity or change their ability to absorb homologous hemagglutination-inhibition antibodies. In order to assay antibody absorption it was necessary to remove all of the immune complex from the reaction mixture, because the immune complex inhibited additional hemagglutinin. Removal of the immune complex can be accomplished by the use of kaolin-absorbed virus. This procedure is simple and dependable and has been carried out with viruses from several groups--Toga-, Myxo-, Paramyxo- and Picornaviridae.

Prevalence of antibodies to arboviruses in various animals in Israel.

A serological survey of the prevalence of arbovirus antibodies in various mammals and birds was made in Israel during the years 1959-60, employing the haemagglutination-inhibition technique and using group-A and group-B antigens. High proportions of the animals of several species were found to be positive to group-B arboviruses. Most of these animals showed higher titres against the West Nile virus than against that of turkey meningoencephalitis, but in some cases there were higher titres against the latter virus. Group-A positive sera were also encountered, but in a smaller proportion of the tested animals. Most of the group-A positive sera were also group-B positive, and very few were group-A positive only.