Molecular characterization of a nondemyelinating variant of Daniel's strain of Theiler's virus isolated from a persistently infected glioma cell line

Viral mouse models of multiple sclerosis and epilepsy: Marked differences in neuropathogenesis following infection with two naturally occurring variants of Theiler's virus BeAn strain

Following intracerebral inoculation, the BeAn 8386 strain of Theiler's virus causes persistent infection and inflammatory demyelinating encephalomyelitis in the spinal cord of T-cell defective SJL/J mice, which is widely used as a model of multiple sclerosis. In contrast, C57BL/6 (B6) mice clear the virus and develop inflammation and lesions in the hippocampus, associated with acute and chronic seizures, representing a novel model of viral encephalitis-induced epilepsy. Here we characterize the geno- and phenotype of two naturally occurring variants of BeAn (BeAn-1 and BeAn-2) that can be used to further understand the viral and host factors involved in the neuropathogenesis in B6 and SJL/J mice. Next generation sequencing disclosed 15 single nucleotide differences between BeAn-1 and BeAn-2, of which 4 are coding changes and 3 are in the 5'-UTR (5'-untranslated region). The relatively minor variations in the nucleotide sequence of the two BeAn substrains led to marked differences in neurovirulence. In SJL/J mice, inflammatory demyelination in the spinal cord and its clinical consequences were significantly more marked following infection with BeAn-1 than with BeAn-2. Both BeAn substrains caused lymphocyte infiltration and increase of MAC3-positive cells in the hippocampus, but hippocampal damage and seizures were only observed in B6 mice. Seizures occurred in one third of BeAn-2 infected B6 mice, but not in BeAn-1 infected B6 mice. By comparing individual mice by receiver operating characteristic (ROC) curve analysis, the severity of hippocampal neurodegeneration and amount of MAC3-positive microglia/macrophages discriminated seizing from non-seizing B6 mice, whereas T-lymphocyte brain infiltration was not found to be a crucial factor. These data add novel evidence to the view that differential outcome of infection may be not invariably linked to a distinct viral burden but to a finely tuned balance between antiviral immune responses that although essential for host resistance can also contribute to immunopathology.

An elite controller of picornavirus infection targets an epitope that is resistant to immune escape

The emergence of novel viral pathogens can lead to devastating consequences in the infected population. However, on occasion, rare hyper-responsive elite controllers are able to mount a protective primary response to infection and clear the new pathogen. Factors distinguishing elite controllers from other members of the population are not completely understood. We have been using Theiler's murine encephalomyelitis as a model of primary infection in mice and clearance of the virus is limited to one MHC genotype capable of generating a protective response to a single viral peptide VP2_121-130. The genetics of host susceptibility to TMEV, a natural mouse pathogen, has been studied extensively and non-protective CD8 responses to other peptides have been documented, however, little is known why the protective response to infection focuses on the VP2_121-130 peptide. To study this question, we have generated TMEV mutants that encode for mutations within the VP2_121-130 peptide. We find that very few of mutants are able to assemble and infect in vitro. These mutations are not related to virus RNA structure since non-coding mutations do not interfere with assembly. In the rare event when functional VP2_121-130 mutant viruses did emerge, they were attenuated to some level or retained the ability to develop an immune response to the wild-type VP2_121-130 sequence, demonstrating that the virus is incapable of escaping the protective response. These findings advance our understanding of how characteristics of the host immune response and an infectious agent can interact to lead to the appearance of rare super controllers in a population. Furthermore, the immutable nature of the viral antigen highlights the importance of choosing appropriate vaccine antigens and has implications for the development of agents that are able to generate protective CD8 T-cell responses.

Theiler's murine encephalomyelitis virus as a vaccine candidate for immunotherapy

The induction of sterilizing T-cell responses to tumors is a major goal in the development of T-cell vaccines for treating cancer. Although specific components of anti-viral CD8+ immunity are well characterized, we still lack the ability to mimic viral CD8+ T-cell responses in therapeutic settings for treating cancers. Infection with the picornavirus Theiler's murine encephalomyelitis virus (TMEV) induces a strong sterilizing CD8+ T-cell response. In the absence of sterilizing immunity, the virus causes a persistent infection. We capitalized on the ability of TMEV to induce strong cellular immunity even under conditions of immune deficiency by modifying the virus to evaluate its potential as a T-cell vaccine. The introduction of defined CD8+ T-cell epitopes into the leader sequence of the TMEV genome generates an attenuated vaccine strain that can efficiently drive CD8+ T-cell responses to the targeted antigen. This virus activates T-cells in a manner that is capable of inducing targeted tissue damage and glucose dysregulation in an adoptive T-cell transfer model of diabetes mellitus. As a therapeutic vaccine for the treatment of established melanoma, epitope-modified TMEV can induce strong cytotoxic T-cell responses and promote infiltration of the T-cells into established tumors, ultimately leading to a delay in tumor growth and improved survival of vaccinated animals. We propose that epitope-modified TMEV is an excellent candidate for further development as a human T-cell vaccine for use in immunotherapy.

Identification of a novel neuropathogenic Theiler's murine encephalomyelitis virus

Theiler's murine encephalitis viruses (TMEV) are divided into two subgroups based on their neurovirulence. Persistent strains resemble Theiler's original viruses (referred to as the TO subgroup), which largely induce a subclinical polioencephalomyelitis during the acute phase of the disease and can persist in the spinal cord of susceptible animals, inducing a chronic demyelinating disease. In contrast, members of the neurovirulent subgroup cause an acute encephalitis characterized by the rapid onset of paralysis and death within days following intracranial inoculation. We report herein the characterization of a novel neurovirulent strain of TMEV, identified using pyrosequencing technology and referred to as NIHE. Complete coverage of the NIHE viral genome was obtained, and it shares <90% nucleotide sequence identity to known TMEV strains irrespective of subgroup, with the greatest sequence variability being observed in genes encoding the leader and capsid proteins. The histopathological analysis of infected brain and spinal cord demonstrate inflammatory lesions and neuronal necrosis during acute infection with no evidence of viral persistence or chronic disease. Intriguingly, genetic analysis indicates the putative expression of the L protein, considered a hallmark of strains within the persistent subgroup. Thus, the identification and characterization of a novel neurovirulent TMEV strain sharing features previously associated with both subgroups will lead to a deeper understanding of the evolution of TMEV strains and new insights into the determinants of neurovirulence.

TGF-beta1 suppresses T cell infiltration and VP2 puff B mutation enhances apoptosis in acute polioencephalitis induced by Theiler's virus

GDVII and DA strains of Theiler's murine encephalomyelitis virus (TMEV) differ in VP2 puff B. One week after GDVII virus infection, SJL/J mice had large numbers of TUNEL+ apoptotic cells with a relative lack of T cell infiltration in the brain. DA viruses with mutation in puff B induced higher levels of apoptosis than wild-type DA virus, but levels of inflammation in brains were similar between DA and DA virus mutants. The difference in inflammation among TMEVs could be due to TGF-beta1 expression that was seen only in GDVII virus infection and negatively correlated with CD3+ T cell infiltration.

Direct comparison of demyelinating disease induced by the Daniel's strain and BeAn strain of Theiler's murine encephalomyelitis virus

We compared CNS disease following intracerebral injection of SJL mice with Daniel's (DA) and BeAn 8386 (BeAn) strains of Theiler's murine encephalomyelitis virus (TMEV). In tissue culture, DA was more virulent then BeAn. There was a higher incidence of demyelination in the spinal cords of SJL/J mice infected with DA as compared to BeAn. However, the extent of demyelination was similar between virus strains when comparing those mice that developed demyelination. Even though BeAn infection resulted in lower incidence of demyelination in the spinal cord, these mice showed significant brain disease similar to that observed with DA. There was approximately 100 times more virus specific RNA in the CNS of DA infected mice as compared to BeAn infected mice. This was reflected by more virus antigen positive cells (macrophages/microglia and oligodendrocytes) in the spinal cord white matter of DA infected mice as compared to BeAn. There was no difference in the brain infiltrating immune cells of DA or BeAn infected mice. However, BeAn infected mice showed higher titers of TMEV specific antibody. Functional deficits as measured by Rotarod were more severe in DA infected versus BeAn infected mice. These findings indicate that the diseases induced by DA or BeAn are distinct.

Evolution of cell recognition by viruses: a source of biological novelty with medical implications

The picture beginning to form from genome analyses of viruses, unicellular organisms, and multicellular organisms is that viruses have shared functional modules with cells. A process of coevolution has probably involved exchanges of genetic information between cells and viruses for long evolutionary periods. From this point of view present-day viruses show flexibility in receptor usage and a capacity to alter through mutation their receptor recognition specificity. It is possible that for the complex DNA viruses, due to a likely limited tolerance to generalized high mutation rates, modifications in receptor specificity will be less frequent than for RNA viruses, albeit with similar biological consequences once they occur. It is found that different receptors, or allelic forms of one receptor, may be used with different efficiency and receptor affinities are probably modified by mutation and selection. Receptor abundance and its affinity for a virus may modulate not only the efficiency of infection, but also the capacity of the virus to diffuse toward other sites of the organism. The chapter concludes that receptors may be shared by different, unrelated viruses and that one virus may use several receptors and may expand its receptor specificity in ways that, at present, are largely unpredictable.

Experimental Models of Virus-Induced Demyelination

This chapter reviews two of the most widely studied animal models of virus-induced demyelinating disease. These are Theiler's murine encephalomyelitis virus and murine hepatitis virus. Both viruses produce acute inflammatory encephalitis that is followed by chronic central-nervous-system (CNS) demyelinating disease. The clinical and pathologic correlates of virus-induced demyelination are largely immune mediated. Furthermore, several pathologic mechanisms have been proposed to explain the development of myelin damage and neurologic deficits, and each of the proposed mechanisms may play a role in disease progression depending on the genetic constitution of the infected animal. The induction of demyelinating disease by virus may be directly relevant to human MS. Several viruses are known to cause demyelination in humans and viral infection is an epidemiologic factor that is consistently associated with clinical exacerbation of MS. It is suggested that viral infection may be a cause of MS, although no specific virus has been identified as a causative agent.

Prolonged gray matter disease without demyelination caused by Theiler's murine encephalomyelitis virus with a mutation in VP2 puff B

Theiler's murine encephalomyelitis virus (TMEV) is divided into two subgroups based on neurovirulence. During the acute phase, DA virus infects cells in the gray matter of the central nervous system (CNS). Throughout the chronic phase, DA virus infects glial cells in the white matter, causing demyelinating disease. Although GDVII virus also infects neurons in the gray matter, infected mice developed a severe polioencephalomyelitis, and no virus is detected in the white matter or other areas in the CNS in rare survivors. Several sequence differences between the two viruses are located in VP2 puff B and VP1 loop II, which are located near each other, close to the proposed receptor binding site. We constructed a DA virus mutant, DApBL2M, which has the VP1 loop II of GDVII virus and a mutation at position 171 in VP2 puff B. While DApBL2M virus replicated less efficiently than DA virus during the acute phase, DApBL2M-induced acute polioencephalitis was comparable to that in DA virus infection. Interestingly, during the chronic phase, DApBL2M caused prolonged gray matter disease in the brain without white matter involvement in the spinal cord. This is opposite what is observed during wild-type DA virus infection. Our study is the first to demonstrate that conformational differences via interaction of VP2 puff B and VP1 loop II between GDVII and DA viruses can play an important role in making the transition of infection from the gray matter in the brain to the spinal cord white matter during TMEV infection.

The CD4-mediated immune response is critical in determining the outcome of infection using Theiler's viruses with VP1 capsid protein point mutations

Daniel's strain of Theiler's virus (DA) induces a chronic demyelinating disease in the central nervous system (CNS) of susceptible SJL mice, which serves as an excellent model of multiple sclerosis. We previously demonstrated that point mutations near a putative virus receptor-binding site [VP1 99 (Gly to Ser) or 100 (Gly to Asp)] totally attenuate the ability of DA to persist and induce demyelination in SJL mice. The current studies demonstrate that class II-restricted CD4(+) T cells play a major role in clearing VP1 mutant DA viruses from the CNS to prevent demyelination. Infection of SJL CD4((-/-)) mice with DA-VP1-99(Ser) or DA-VP1-100(Asp) resulted in virus persistence and prominent demyelination in the spinal cord. In contrast, infection of SJL CD8((-/-)) mice with DA-VP1-99(Ser) or DA-VP1-100 did not result in virus persistence or demyelination. In addition, no virus-specific cytotoxicity was observed in CNS-infiltrating lymphocytes following infection of SJL mice with VP1 mutant viruses. The mutant DA-VP1-99(Ser) and DA-VP1(100) viruses were in fact neurovirulent when compared to the wild-type DA virus, as they induced an overwhelming encephalitis and early lethality (2 to 4 days postinfection) in mice deficient in the IFN-alpha/beta receptor. Therefore, the nondemyelinating phenotype observed with DA-VP1-99(Ser) and DA-VP1-100(Asp) viruses is dependent in part on the CD4-mediated host immune response.

V(beta)8(+) T cells protect from demyelinating disease in a viral model of multiple sclerosis

Previous studies illustrated the influence of T cell subsets on susceptibility or resistance to demyelination in the Theiler's murine encephalomyelitis virus (TMEV) model of multiple sclerosis. Genetic segregation analysis showed a correlation with disease phenotype in this model with particular V(beta) genes. In this study we investigated the contribution of specific V(beta) TCR to the pathogenesis of virus-induced demyelinating disease. Spectratype analysis of cells infiltrating the CNS early in infection demonstrated an over-representation of V(beta)8(+) T cells in mice expressing a susceptible H-2 haplotype. We infected transgenic mice expressing the V(beta)8.2 TCR directed against a non-TMEV antigen and found an increase in demyelinating disease in mice of either susceptible or resistant background compared with littermate controls. In addition, depletion studies with an anti-V(beta)8-specific antibody in both susceptible (B10.Q) and resistant (C57BL/6) mice resulted in increased demyelination. TCR analysis of VP2-specific cytotoxic T cell clones from mice with a resistant genotype identified only the V(beta)8.1 TCR, suggesting that limited T cell diversity is critical to TMEV clearance. Together, these results support a protective role for V(beta)8(+) T cells in virus-induced demyelinating disease.

Distinct attenuation phenotypes caused by mutations in the translational starting window of Theiler's murine encephalomyelitis virus

Upon initiation of translation of picornavirus RNA, the ribosome is believed to bind the internal ribosome entry site of the template and then to form a productive complex with a downstream RNA segment, the starting window. The presence or absence of an AUG triplet within the starting window of the RNA of Theiler's murine encephalomyelitis virus (a picornavirus) is known to modulate its neurovirulence. In this study, mutants of this virus in which the starting windows, lying upstream of the viral polyprotein reading frame, had AUGs with different nonoptimal contexts were engineered. Upon intracerebral inoculation of mice, the mutants proved to be partially attenuated, as judged by a significant increase in the dose causing paralysis in 50% of the animals (PD50). Mutants with similar PD50s might differ from one another by eliciting either a severe, fatal tetraplegy or only mild, recoverable neurologic lesions. Some of the mutants triggered a chronic inflammatory reaction in the white matter of the spinal cord in the absence of detectable viral RNA or antigen. Thus, point mutations changing the context of an AUG within the starting window outside the polyprotein reading frame may differently affect the morbidity and mortality caused by a viral infection and may result in distinct attenuation phenotypes.

Theiler's viruses with mutations in loop I of VP1 lead to altered tropism and pathogenesis

Theiler's murine encephalomyelitis viruses are picornaviruses that can infect the central nervous system. The DA strain produces an acute polioencephalomyelitis followed by a chronic demyelinating disease in its natural host, the mouse. The ability of DA virus to induce a demyelinating disease renders this virus infection a model for human demyelinating diseases such as multiple sclerosis. Here we describe the generation and characterization of DA virus mutants that contain specific mutations in the viral capsid protein VP1 at sites believed to be important contact regions for the cellular receptor(s). A mutant virus with a threonine-to-aspartate (T81D) substitution in VP1 loop I adjacent to the putative virus receptor binding site exhibited a large-plaque phenotype but had a slower replication cycle in vitro. When this mutant virus was injected into susceptible mice, an altered tropism was seen during the acute stage of the disease and the chronic demyelinating disease was not produced. A virus with a threonine-to-valine substitution (T81V) did not cause any changes in the pattern or extent of disease seen in mice, whereas a virus with a tryptophan substitution at this position (T81W) produced a similar acute disease but was attenuated for the development of the chronic disease. A change in amino acids in a hydrophobic patch located in the wall of the pit, VP1 position 91, to a hydrophilic threonine (V91T) resulted in a profound attenuation of the acute and chronic disease without persistence of virus. This report illustrates the importance of the loop I of VP1 and a site in the wall of the pit in pathogenesis and that amino acid substitutions at these sites result in altered virus-host interactions.