Genetic determinants of the demyelinating disease caused by Theiler's virus

Viral mouse models used to study multiple sclerosis: past and present

Multiple sclerosis (MS) is a common inflammatory demyelinating disease of the central nervous system. Although the etiology of MS is unknown, genetics and environmental factors, such as infections, play a role. Viral infections of mice have been used as model systems to study this demyelinating disease of humans. Three viruses that have long been studied in this capacity are Theiler’s murine encephalomyelitis virus, mouse hepatitis virus, and Semliki Forest virus. This review describes the viruses themselves, the infection process, the disease caused by infection and its accompanying pathology, and the model systems and their usefulness in studying MS.

Circulation of 3 lineages of a novel Saffold cardiovirus in humans

Saffold virus may be the first human cardiovirus species.

Cardioviruses cause serious disease, mainly in rodents, including diabetes, myocarditis, encephalomyelitis, and multiple sclerosis–like disseminated encephalomyelitis. Recently, a human virus isolate obtained 25 years ago, termed Saffold virus, was sequenced and classified as a cardiovirus. We conducted systematic molecular screening for Saffold-like viruses in 844 fecal samples from patients with gastroenteritis from Germany and Brazil, across all age groups. Six cardioviruses were identified in patients <6 years of age. Viral loads were 283,305–5,044,412,175 copies/g of stool. Co-infections occurred in 4 of 6 children. No evidence for outbreak-like epidemic patterns was found. Phylogenetic analysis identified 3 distinct genetic lineages. Viral protein 1 amino acids were 67.9%–77.7% identical and had a distance of at least 39.4% from known cardioviruses. Because closely related strains were found on 2 continents, global distribution in humans is suspected. Saffold-like viruses may be the first human cardiovirus species to be identified.

Analysis of cellular mutants resistant to Theiler's virus infection: differential infection of L929 cells by persistent and neurovirulent strains

Theiler's murine encephalomyelitis virus (TMEV) is a natural pathogen of the mouse and belongs to the Picornaviridae family. TMEV strains are divided into two subgroups on the basis of their pathogenicity. The first group contains two neurovirulent strains, FA and GDVII, which cause a rapid fatal encephalitis. The second group includes persistent strains, like DA and BeAn, which produce a biphasic neurological disease in susceptible mice. Persistence of these viruses in the white matter of the spinal cord leads to chronic inflammatory demyelination. L929 cells, which are susceptible to TMEV infection, were subjected to physicochemical mutagenesis. Cellular clones that became resistant to TMEV infection were selected by viral infection. Three such mutants resistant to strain GDVII were characterized to determine the step of the virus cycle that was inhibited. The mutation present in one of these mutant cell lines inhibited, by more than 1,000-fold, the entry of strain GDVII but hardly decreased infection by strain DA. In the two other cellular mutants, replication of the viral genome was slowed down. Interestingly, one of these mutant cell lines resisted infection by both the persistent and neurovirulent strains while the second cell line resisted infection by strain GDVII but remained susceptible to the persistent virus. These results show that although they have 95% identity at the amino acid sequence level, neurovirulent and persistent viruses use partly distinct pathways for both entry into cells and genome replication.

Adaptation of Theiler's virus to L929 cells: mutations in the putative receptor binding site on the capsid map to neutralization sites and modulate viral persistence

Persistent strains of Theiler's virus, a murine picornavirus, produce a life-long infection of the central nervous system of the mouse and induce a chronic demyelinating disease. Strain DA1, a molecular clone of such a persistent strain, produces a prominent cytopathic effect in BHK-21 cells but is less efficient at infecting L929 cells. We cloned the cDNA of a derivative of virus DA1, adapted to promote a rapid cytopathic effect in L929 cells. Adaptation of the new variant (named KJ6) to L929 cells correlated with an enhanced viral entry rather than with an increased replication rate of the genome. Mutations responsible for L929 cells adaptation occurred in amino acids exposed at the surface of the capsid, in the CD loop of VP1 (100-102) and in the EF loop of VP2 (162-171-173), suggesting that these residues could be involved in receptor recognition. These two clusters of amino acids are precisely known to be part of neutralization epitopes. They also differentiate persistent from neurovirulent strains of Theiler's virus. Adaptation of the virus to L929 cells was accompanied by attenuation of its virulence for the mouse. Taken together, these data suggest a close relationship between receptor binding, virus neutralization, and virus phenotype.

Protein 2A is not required for Theiler's virus replication

Nonpolar mutations were introduced into all 12 regions of the genome of Theiler's murine encephalomyelitis virus. In agreement with data previously reported for other picornaviruses, mutations in regions 2B, 2C, 3A, 3B, 3C, and 3D totally abrogated viral RNA replication. Viruses with deletions in each of the capsid proteins retained RNA replication proficiency, although they were unable to propagate from cell to cell. As reported previously, mutations in the leader protein did not impair RNA replication or virus production in BHK-21 cells. Surprisingly, region 2A also appeared to be dispensable for the replication process. Indeed, up to 77 of the 133 amino acids of 2A could be deleted without significantly affecting RNA replication. 2A mutant viruses had only a slow cytopathic effect for BHK-21 cells and were totally avirulent for mice. As was the case for mutants lacking the leader protein, viruses with deletions in 2A propagated in BHK-21 cells, but their propagation was highly restricted in L929 cells.

Virus-induced immunopathology

Several viruses cause damage to the tissue by immunopathological mechanisms. This chapter presents the principal examples of immunopathogenesis caused by the viruses, accompanied by speculations about its management. The most common mechanism of lesion development in virus induced immunopathology involves T cells. Usually, it seems that when CD8+ T cells act as the controlling cell type, lesions are acute and the outcome is decided quickly. The classic example is provided by LCM in mice. The newest candidate may turn out to be SNV infection in humans. Lesions orchestrated primarily by CD4+ T cells can be either acute or long-lasting. Curiously, in the LCMV example, if CD8+ T cells are removed from the scene, immunopathological responses may still occur and these involve CD4+ T cells. Such responses are far more chronic and of lower grade than those mediated by CD8+ T lymphocytes. One possible sequel to chronic inflammatory responses to viruses is that autoreactive inflammatory reactions are initiated and an autoimmune disease occurs. The adage that an ounce of prevention is worth a pound of cure is certainly true in the field of viral pathogenesis. Preventing viral infection or manipulating immune processes during the initial phases of infection is far more successful than attempting to counteract pathological events once underway.

Rapid cell variation can determine the establishment of a persistent viral infection

Evidence for a mechanism of initiation of viral persistence in which the cell, and not the virus, plays a critical role has been obtained using the important animal pathogen foot-and-mouth disease virus (FMDV). We have developed a virulence assay consisting of quantification of the ability of virus to kill cells and of cells to divide in the presence of virus and to initiate a carrier state. Cells were cured of FMDV at early times following a cytolytic infection of BHK-21 monolayers with FMDV. When cured cells were subjected to the virulence assay they showed an increased ability to survive a second infection by FMDV but not by other RNA viruses. This altered phenotype was maintained as a stable genetic trait. When the virus present in such early surviving cells was used to infect BHK-21 cells, it proved to be as virulent as the initial cytolytic FMDV and, furthermore, its ability to kill BHK-21 cells increased upon replication in the surviving cells. Both the level of genetic heterogeneity and the rate of evolution of FMDV were similar to those previously documented during acute and persistent FMDV infections. The results suggest that, in contrast to most other viral systems, the critical element in the establishment of a persistent infection of BHK-21 cells with FMDV is the ability of the host cells to vary genetically and phenotypically, which promotes selection of cells with increased resistance to virus. The possible relevance of this mechanism to viral persistence in vivo is discussed.

Analysis of the molecular basis of neuropathogenesis of RNA viruses in experimental animals: relevance for human disease?

RNA viruses with segmented genomes were the first model used for molecular analysis of viral neuropathogenesis, since they could be analysed genetically by reassortment. Four viruses with non-segmented genomes have been used as models of neurovirulence and demyelinating disease: JHM coronavirus, Theiler's virus, Sindbis virus and Semliki Forest virus (SFV). Virus gene expression in the central nervous system of infected animals has been measured by in situ hybridization and immunocytochemistry. Cell tropism has been analysed by neural cell culture. Infectious clones have been constructed for Theiler's virus, Sindbis virus and SFV, and these allow analysis of the sequences involved in the determination of neuropathogenesis, through the construction of chimeric viruses and site-specific mutagenesis. Measles and rubella viruses have been studied in animal systems because of their importance for human disease. The importance of two recently discovered mechanisms of neuropathogenesis, antibody-induced modulation of virus multiplication, and persistence of virus in the absence of multiplication, remains to be assessed.

Identification of a region of the poliovirus genome involved in persistent infection of HEp-2 cells

Poliovirus mutants were selected during the persistent infection of human neuroblastoma cells. These viruses could establish secondary persistent infections in HEp-2 nonneural cells. We report the identification of a region of the genome of a persistent virus (S11) that was sufficient to confer to a recombinant virus the phenotype that causes persistent infection in HEp-2 cells. This region, between nucleotides 1148 and 3481, contained 11 missense mutations mapping exclusively in the genes of capsid proteins VP1 and VP2. Because recombinant viruses carrying only one of these two mutated genes were not able to cause persistent infection, it seems very probable that two or more mutations in these genes are required for expression of the phenotype that causes persistent infection.

Theiler's virus infection of beta 2-microglobulin-deficient mice

Theiler's virus, a murine picornavirus, persists in the central nervous systems of susceptible mice and induces a chronic demyelinating disease. Susceptibility or resistance to this disease is controlled in part by the H2-D locus of the major histocompatibility complex (MHC). For this reason, it has been proposed that CD8+ class I-restricted cytotoxic T cells play a main role in the pathogenesis of this viral infection. We recently reported the existence of anti-virus CD8+ cytotoxic T cells in the course of Theiler's virus infection. In the present study, we examined the role of these effector cells in mice in which the beta 2-microglobulin gene had been disrupted. These mice fail to express class I MHC molecules and therefore lack CD8+ T cells. The mice are derived from a C57BL/6 x 129/Ola cross and are H-2b, a haplotype associated with resistance to Theiler's virus infection. beta 2-Microglobulin-deficient mice (beta 2m-/-mice) failed to clear the virus, developed demyelination, and, interestingly, did not succumb to early infection. These results demonstrate that CD8+ T cells are required to clear Theiler's virus infection. In contrast with a current hypothesis, they also demonstrate that CD8+ T cells are not major mediators of the demyelinating disease.

Pathogenesis of virus-induced demyelination

Demyelination is a component of several viral diseases of humans. The best known of these are subacute sclerosing panencephalitis (SSPE) and progressive multifocal leukoencephalopathy (PML). There are a number of naturally occurring virus infections of animals that involve demyelination and many of these serve as instructive models for human demyelinating diseases. In addition to the naturally occurring diseases, many viruses have been shown to be capable of producing demyelination in experimental situations. In discussing virus-associated demyelinating disease, the chapter reviews the architecture and functional organization of the CNS and considers what is known of the interaction of viruses with CNS cells. It also discusses the immunology of the CNS that differs in several important aspects from that of the rest of the body. Experimental models of viral-induced demyelination have also been considered. Viruses capable of producing demyelinating disease have no common taxonomic features; they include both DNA and RNA viruses, enveloped and nonenveloped viruses. The chapter attempts to summarize the important factors influencing viral demyelination, their common features, and possible mechanisms.

The interaction of two groups of murine genes determines the persistence of Theiler's virus in the central nervous system

Theiler's murine encephalomyelitis virus is responsible for a chronic inflammatory demyelinating disease of the central nervous system of the mouse. The disease is associated with persistent viral infection of the spinal cord. Some strains of mice are susceptible to viral infection, and other strains are resistant. The effect of the genetic background of the host on viral persistence has not been thoroughly investigated. We studied the amount of viral RNA in the spinal cords of 17 inbred strains of mice and their F1 crosses with the SJL/J strain and observed a large degree of variability among strains. The pattern of viral persistence among mouse strains could be explained by the interaction of two loci. One locus is localized in the H-2D region of the major histocompatibility complex, whereas the other locus is outside this complex and is not linked to the Tcrb locus on chromosome 6.