Adaptation of Saffold virus 2 for high-titer growth in mammalian cells

Saffold viruses (SAFV) are a recently discovered group of human Cardioviruses closely related to Theiler's murine encephalomyelitis viruses (TMEV). Unlike TMEV and encephalomyocarditis virus, each of which is monotypic, SAFV are genetically diverse and include at least eight genotypes. To date, only Saffold virus 3 (SAFV-3) has been grown efficiently in mammalian cells in vitro. Here, we report the successful adaptation of SAFV-2 for efficient growth in HeLa cells after 13 passages in the alpha/beta interferon-deficient human glial cell line U118 MG. Nine amino acid changes were found in the adapted virus, with single mutations in VP2, VP3, and 2B, while 6 mutations arose in VP1. Most capsid mutations were in surface loops. Analysis of SAFV-2 revealed virus growth and cytopathic effect only in human cell lines, with large plaques forming in HeLa cells, with minimal cell association, and without using sialic acid to enter cells. Despite the limited growth of SAFV-2 in rodent cells in vitro, BALB/c mice inoculated with SAFV-2 showed antibody titers of >1:10(6), and fluorescence-activated cell sorting (FACS) analysis revealed only minimal cross-reactivity with SFV-3. Intracerebral inoculation of 6-week-old FVB/n mice produced paralysis and acute neuropathological changes, including meningeal infiltrates, encephalitis, particularly of the limbic system, and spinal cord white matter inflammation.     

The leader protein of cardioviruses inhibits stress granule assembly

Stress granules (SG) are cytoplasmic aggregates of stalled translation preinitiation complexes that form in cells exposed to various environmental stresses. Here, we show that stress granules assemble in cells infected with Theiler's murine encephalomyelitis virus (TMEV) mutants carrying alterations in the leader (L) protein, but not in cells infected with wild-type TMEV. Stress granules also formed in STAT1-deficient cells, suggesting that SG formation was not a consequence of increased type I interferon (IFN) production when cells were infected with the mutant virus. Ectopic expression of the wild-type L protein was sufficient to inhibit stress granule formation induced by sodium arsenite or thapsigargin treatment. In conclusion, TMEV infection induces stress granule assembly, but this process is inhibited by the L protein. Unlike poliovirus-induced stress granules, TMEV-induced stress granules did not contain the nuclear protein Sam68 but contained polypyrimidine tract binding protein (PTB), an internal ribosome entry site (IRES)-interacting protein. Moreover, G3BP was not degraded and was found in SG after TMEV infection, suggesting that SG content could be virus specific. Despite the colocalization of PTB with SG and the known interaction of PTB with viral RNA, in situ hybridization and immunofluorescence assays failed to detect viral RNA trapped in infection-induced SG. Recombinant Theiler's viruses expressing the L protein of Saffold virus 2 (SAFV-2), a closely related human theilovirus, or the L protein of mengovirus, an encephalomyocarditis virus (EMCV) strain, also inhibited infection-induced stress granule assembly, suggesting that stress granule antagonism is a common feature of cardiovirus L proteins.     

Saffold Virus Type 3 (SAFV-3) Persists in HeLa Cells

Saffold virus (SAFV) was identified as a human cardiovirus in 2007. Although several epidemiological studies have been reported, they have failed to provide a clear picture of the relationship between SAFV and human diseases. SAFV genotype 3 has been isolated from the cerebrospinal fluid specimen of patient with aseptic meningitis. This finding is of interest since Theiler’s murine encephalomyelitis virus (TMEV), which is the closely related virus, is known to cause a multiple sclerosis-like syndrome in mice. TMEV persistently infects in mouse macrophage cells in vivo and in vitro, and the viral persistence is essential in TMEV-induced demyelinating disease. The precise mechanism(s) of SAFV infection still remain unclear. In order to clarify the SAFV pathogenicity, in the present study, we studied the possibilities of the in vitro persistent infection of SAFV. The two distinct phenotypes of HeLa cells, HeLa-N and HeLa-R, were identified. In these cells, the type of SAFV-3 infection was clearly different. HeLa-N cells were lyticly infected with SAFV-3 and the host suitable for the efficient growth. On the other hand, HeLa-R cells were persistently infected with SAFV-3. In addition, the SAFV persistence in HeLa-R cells is independent of type I IFN response of host cells although the TMEV persistence in mouse macrophage cells depends on the response. Furthermore, it was suggested that SAFV persistence may be influenced by the expression of receptor(s) for SAFV infection on the host cells. The present findings on SAFV persistence will provide the important information to encourage the research of SAFV pathogenicity.     

A Determinant for Central Nervous System Persistence Localized in the Capsid of Theiler’s Murine Encephalomyelitis Virus by Using Recombinant Viruses

The demyelinating process in Theiler’s murine encephalomyelitis virus (TMEV) infection in mice requires virus persistence in the central nervous system. Using recombinant TMEV assembled between the virulent GDVII and less virulent BeAn virus cDNAs, we now provide additional evidence supporting the localization of a persistence determinant to the leader P1 (capsid) sequences. Further, recombinant viruses in which BeAn sequences progressively replaced those of GDVII within the capsid starting at the leader NH2 terminus suggest that a conformational determinant requiring homologous sequences in both the VP2 puff and VP1 loop regions, which are in close contact on the virion surface, might underlie persistence.     

Theiler's murine encephalomyelitis virus group includes two distinct genetic subgroups that differ pathologically and biologically.

The intracellular development and RNA composition of Theiler's murine encephalomyelitis virus (TMEV) isolates were determined by electron microscopy, sucrose gradient centrifugation, and RNase T1 fingerprinting. Replication of FA virus, a virulent strain of TMEV, was characterized by the appearance of viral crystalline arrays in the cytoplasm of infected cells. In contrast, cells infected with the less virulent isolates (WW, TO4, BeAn 8386, and Yale) showed no crystalline arrays; instead, virions were found to be arranged between two layers of membranes in the cytoplasm of infected cells. Analysis of the RNAs of TMEV isolates showed that the RNAs were single-stranded molecules having sedimentation coefficients of 35S. RNase T1 fingerprinting of TMEV RNA revealed that striking differences between the virulent and less virulent TMEV isolates existed. Moreover, base composition analysis of RNase T1-resistant oligonucleotides of two TMEV isolates which represented the two subgroups indicated that there were no substantial oligonucleotides common to both subgroups. Based on these findings and the known difference in virulence, we suggest that the TMEV group contains two genetically district subgroups of viruses.     

The predominant virus antigen burden is present in macrophages in Theiler's murine encephalomyelitis virus-induced demyelinating disease.

Theiler's murine encephalomyelitis virus (TMEV) produces a persistent central nervous system infection and chronic, inflammatory demyelinating disease in susceptible mice. TMEV antigen(s) and RNA genome have been detected in astrocytes, oligodendrocytes, and macrophages during persistence. Whether there is a predominant cell type in which TMEV persists has not been resolved. Since TMEV-induced demyelinating lesions are infiltrated with macrophages and a number of other persistent viruses show near-exclusive tropism for these phagocytic cells, we used two-color immunofluorescent staining with conventional and confocal microscopy to colocalize TMEV to cells that stain with monoclonal antibodies (MOMA-2) [unknown antigen], Mac-1 [CD11b], FA-11 [CD66], and 2F8 [scavenger receptor]) to macrophages in BeAn-infected SJL mice. A predominant virus antigen burden within macrophages infiltrating demyelinating lesions was seen. A dichotomy of cells staining for virus antigen(s) was found with infected cells containing either a large or small virus antigen load. Ninety percent of cells with a large virus antigen load were large phagocytes (20 to 50 microns) that were readily detected at low power (5x objective). Cells with smaller amounts of virus antigen(s) turned out to be either these same large phagocytic cells or much smaller cells, approximately equal to 10 microns in diameter. Forty percent of cells with a small virus antigen load were macrophages. The unidentified approximately equal to 10-microns cells that are virus antigen positive and macrophage negative in this study could still be macrophages, or they may be oligodendrocytes. The fact that virus was detected in the cytoplasm and not phagolysosomes of macrophages and the sheer mass of fluorescently stained virus proteins in some macrophages suggest that TMEV persists in these phagocytic cells by active virus replication.     

Differential usage of carbohydrate co-receptors influences cellular tropism of Theiler's murine encephalomyelitis virus infection of the central nervous system

Theiler's murine encephalomyelitis viruses (TMEV) are ubiquitous pathogens of mice, producing either rapidly fatal encephalitis (high-neurovirulence strains) or persistent central nervous system infection and inflammatory demyelination (low-neurovirulence strains). Although a protein entry receptor has not yet been identified, carbohydrate co-receptors that effect docking and concentration of the virus on the cell surface are known for both TMEV neurovirulence groups. Low-neurovirulence TMEV use α2,3-linked N-acetylneuramic acid (sialic acid) on an N-linked glycoprotein, whereas high-neurovirulence TMEV use the proteoglycan heparan sulfate (HS) as a co-receptor. While the binding of low-neurovirulence TMEV to sialic acid can be inhibited completely, only a third of the binding of high-neurovirulence TMEV to HS is inhibitable, suggesting that high-neurovirulence strains use another co-receptor or bind directly to the putative protein entry receptor. Four amino acids on the surface (VP2 puff B) of low-neurovirulence strains make contact with sialic acid through non-covalent hydrogen bonds. Since these virus residues are conserved in all TMEV strains, the capsid conformation of this region is probably responsible for sialic acid binding. A persistence determinant that maps within the virus coat using recombinant TMEV is also conformational in nature. Low-neurovirulence virus variants that do not bind to sialic acid fail to persist in the central nervous system of mice, indicating a role for sialic acid binding in TMEV persistence. Analysis of high-neurovirulence variants that do not bind HS demonstrates that HS co-receptor usage influences neuronal tropism in brain, whereas, the HS co-receptor use is not required for the infection of spinal cord anterior horn cells associated with poliomyelitis.     

Role of sialyloligosaccharide binding in Theiler's virus persistence.

Theiler's murine encephalomyelitis viruses (TMEVs) belong to the Picornaviridae family and are divided into two groups, typified by strain GDVII virus and members of the TO (Theiler's original) group. The highly virulent GDVII group causes acute encephalitis in mice, while the TO group is less virulent and causes a chronic demyelinating disease which is associated with viral persistence in mice. This persistent central nervous system infection with demyelination resembles multiple sclerosis (MS) in humans and has thus become an important model for studying MS. It has been shown that some of the determinants associated with viral persistence are located on the capsid proteins of the TO group. Structural comparisons of two persistent strains (BeAn and DA) and a highly virulent strain (GDVII) showed that the most significant structural variations between these two groups of viruses are located on the sites that may influence virus binding to cellular receptors. Most animal viruses attach to specific cellular receptors that, in part, determine host range and tissue tropism. In this study, atomic models of TMEV chimeras were built with the known structures of GDVII, BeAn, and DA viruses. Comparisons among the known GDVII, BeAn, and DA structures as well as the predicted models for the TMEV chimeras suggested that a gap on the capsid surface next to the putative receptor binding site, composed of residues from VP1 and VP2, may be important in determining viral persistence by influencing virus attachment to cellular receptors, such as sialyloligosaccharides. Our results showed that sialyllactose, the first three sugar molecules of common oligosaccharides on the surface of mammalian cells, inhibits virus binding to the host cell and infection with the persistent BeAn virus but not the nonpersistent GDVII and chimera 39 viruses.     

Theiler's murine encephalomyelitis virus--characterization of newly isolated viruses from Japanese mouse colonies

The hemagglutinating-inhibition (HI) test was used to detect antibodies for Theiler's murine encephalomyelitis virus (TMEV), and the virus was isolated from sero-positive mice derived from colonies in Japan. HI antibody was detected in conventional mice (38.7%; 137/354) at titers ranging from 1:8 to 1:512, but it not in SPF mice (0/90). To isolate the virus, weanling mice inoculated intracerebrally with samples obtained from sero-positive mice were sacrificed and 10% brain homogenates were subcultured. New isolates designated as YOC and AB strains were obtained, and their physicochemical and biological properties were characterized. The results indicated that the new isolates were similar to Theiler's original (TO) strain according to the following observations of persistent paralysis of the hind limbs, resistance to ether treatment, a particles size of 10 approximately 50 nm in diameter, stability at pH 3, a density of 1.35 g/cm3 and three major and one minor viral proteins, (VPO; 38 Kd, VP 1; 33 Kd, VP2; 32Kd, VP3; 25 Kd). Immunoblotting analysis also showed that VP 2 of YOC and encephalomyocarditis virus of the Cardiovirus group, reacted strongly with the antisera against the viruses as well as with the GDVII strain. These results suggest that TMEV infection does exist in conventional mouse colonies in Japan, and that these viruses resemble the TO strain of TMEV.     

Strains from both Theiler's virus subgroups encode a determinant for demyelination.

The GDVII strain and other members of the GDVII subgroup of Theiler's murine encephalomyelitis viruses (TMEV) cause an acute lethal neuronal infection in mice, whereas the DA strain and other members of the TO subgroup of TMEV cause a chronic demyelinating disease associated with a persistent virus infection. We used GDVII/DA chimeric infectious cDNAs to produce intratypic recombinant viruses in order to clarify reasons for the TMEV subgroup-specific difference in demyelinating activity. We found that both the GDVII and DA strains contain a genetic determinant(s) for demyelinating activity. No demyelination occurs following GDVII strain inoculation because this strain produces an early neuronal disease that kills mice before white matter disease and persistent infection can occur.     

Genetic analysis of a Theiler-like virus isolated from rats

Although cardioviruses related to Theiler's murine encephalomyelitis virus (TMEV) appear to be common in mice and rats, few TMEV isolates have been obtained from rat colonies. In 1991, a cardiovirus isolate designated NGS910 was obtained from sentinel rats exposed to cage bedding previously used by adult rats that were TMEV seropositive, but had never manifested clinical signs of disease. To determine to which group and subgroup of cardiovirus this virus belongs, the sequence of the viral genome was determined. The NGS910 genome consisted of 8,021 nucleotides and the 5'-nontranslated region had a predicted secondary structure that is similar to members of the TMEV group of cardioviruses. The Leader-P3D open reading frame (L ORF) of NGS910 had strong homology with L ORFs of other TMEVs (72% identity), but lower homology with EMCV cardioviruses (55 to 56%). Phylogenetic analyses on the basis of aligned nucleotide sequences of the L ORF (6,924 b) and the internal L* ORF (471 b) supported this classification of NGS910 as a TMEV strain. However, within the TMEV group, NGS910 wassufficiently divergent from other isolates that it could not be regarded as simply a mutant strain of a known TMEV. As genetic distances between NGS910 and other TMEVs were greater than those between Mengo virus of EMCV and other EMCVs, we propose to designate the NGS910 isolate as a rat Theiler-like virus.     

Predominant binding of Theiler's viruses to a 34-kilodalton receptor protein on susceptible cell lines.

Western immunoblots of BHK-21 cell lysates probed with the highly virulent GDVII and the less virulent BeAn strains of Theiler's murine encephalomyelitis virus (TMEV) revealed predominant binding to a 34-kDa membrane protein and much lower levels of binding to 100- and 18-kDa membrane proteins. Complete inhibition of virus binding to both the 34- and 18-kDa membrane species by excess unlabeled TMEV demonstrated specificity of binding. Virus binding was also blocked by wheat germ agglutinin, which specifically binds to sialic acid residues and blocks TMEV binding to whole BHK-21 cells. Radiolabeled TMEV also bound to 100-, 34-, and 18-kDa membrane proteins expressed on other TMEV permissive cell lines but not on the nonpermissive cell lines tested. These data suggest that a 34-kDa cellular protein may be the primary determinant of susceptibility to TMEV infection by mediating the binding of GDVII and BeAn viruses to susceptible cells.     

Differences in avidity and epitope recognition of CD8(+) T cells infiltrating the central nervous systems of SJL/J mice infected with BeAn and DA strains of Theiler's murine encephalomyelitis virus

Theiler's murine encephalomyelitis virus (TMEV) infection induces immune-mediated demyelinating disease in susceptible mouse strains and serves as a relevant infectious model for human multiple sclerosis. To investigate the pathogenic mechanisms, two strains of TMEV (DA and BeAn), capable of inducing chronic demyelination in the central nervous system (CNS), have primarily been used. Here, we have compared the T-cell responses induced after infection with DA and BeAn strains in highly susceptible SJL/J mice. CD4(+) T-cell responses to known epitopes induced by these two strains were virtually identical. However, the CD8(+) T-cell response induced following DA infection in susceptible SJL/J mice was unable to recognize two of three H-2K(s)-restricted epitope regions of BeAn, due to single-amino-acid substitutions. Interestingly, T cells specific for the H-2K(s)-restricted epitope (VP1(11-20)) recognized by both strains showed a drastic increase in frequency as well as avidity after infection with DA virus. These results strongly suggest that the level and avidity of virus-specific CD8(+) T cells infiltrating the CNS could be drastically different after infection with these two strains of TMEV and may differentially influence the pathogenic and/or protective outcome.     

Analysis of the complete nucleotide sequence of the picornavirus Theiler's murine encephalomyelitis virus indicates that it is closely related to cardioviruses

Theiler's murine encephalomyelitis viruses (TMEV) are naturally occurring enteric pathogens of mice which constitute a separate serological group within the picornavirus family. Persistent TMEV infection in mice provides a relevant experimental animal model for the human demyelinating disease multiple sclerosis. To provide information about the TMEV classification, genome organization, and protein processing map, we determined the complete nucleotide sequence of the TMEV genome and deduced the amino acid sequence of the polyprotein coding region. The RNA genome, which is typical of the picornavirus family, is 8,098 nucleotides long. The 5' untranslated region is 1,064 nucleotides long (making it the longest in the picornavirus family after the aphthoviruses) and lacks a poly(C) tract. Computer-generated comparison of the 5' and 3' noncoding regions and polyprotein revealed the highest level of nucleotide and predicted amino acid identity between the TMEV and the cardioviruses encephalomyocarditis virus (EMCV) and Mengo virus. The TMEV polyprotein, which appears to be processed like EMCV since the amino acids flanking the putative proteolytic cleavage sites have been conserved, begins with a short leader peptide followed by 11 other gene products in the standard L-4-3-4 picornavirus arrangement. Because of these similarities, we propose that the TMEV be grouped with the cardioviruses. However, since TMEV and EMCV have different biophysical properties and show no cross-neutralization, they most likely belong in a separate cardiovirus subgroup.     

Infection with Theiler's murine encephalomyelitis virus directly induces proinflammatory cytokines in primary astrocytes via NF-kappaB activation: potential role for the initiation of demyelinating disease

Theiler's virus infection in the central nervous system (CNS) induces a demyelinating disease very similar to human multiple sclerosis. We have assessed cytokine gene activation upon Theiler's murine encephalomyelitis virus (TMEV) infection and potential mechanisms in order to delineate the early events in viral infection that lead to immune-mediated demyelinating disease. Infection of SJL/J primary astrocyte cultures induces selective proinflammatory cytokine genes (interleukin-12p40 [IL-12p40], IL-1, IL-6, tumor necrosis factor alpha, and beta interferon [IFN-beta]) important in the innate immune response to infection. We find that TMEV-induced cytokine gene expression is mediated by the NF-kappaB pathway based on the early nuclear NF-kappaB translocation and suppression of cytokine activation in the presence of specific inhibitors of the NF-kappaB pathway. Further studies show this to be partly independent of dsRNA-dependent protein kinase (PKR) and IFN-alpha/beta pathways. Altogether, these results demonstrate that infection of astrocytes and other CNS-resident cells by TMEV provides the early NF-kappaB-mediated signals that directly activate various proinflammatory cytokine genes involved in the initiation and amplification of inflammatory responses in the CNS known to be critical for the development of immune-mediated demyelination.     

Transgenic expression of Theiler's murine encephalomyelitis virus genes in H-2(b) mice inhibits resistance to virus-induced demyelination

We investigated the role of the immune system in protecting against virus-induced demyelination by generating lines of transgenic B10 (H-2(b)) congenic mice expressing three independent contiguous coding regions of the Theiler's murine encephalomyelitis virus (TMEV) under the control of a class I major histocompatibility complex (MHC) promoter. TMEV infection of normally resistant B10 mice results in virus clearance and development of inflammatory demyelination in the spinal cord. Transgenic expression of the viral capsid genes resulted in inactivation of virus-specific CD8(+) T lymphocytes (class I MHC immune function) directed against the relevant peptides, but it did not affect production of virus capsid-specific antibodies or lymphocyte proliferation to the virus antigen (class II MHC immune functions). Following intracerebral infection with TMEV, all three lines of mice survived the acute encephalitis but transgenic mice expressing VP1 (or the cluster of virus capsid proteins [VP4, VP2, and VP3] mapping to the left of VP1 in the TMEV genome) developed virus persistence and subsequent demyelination in spinal cord white matter. Transgenic mice expressing noncapsid proteins mapping to the right of VP1 (2A, 2B, 2C, 3A, 3B, 3C, and 3D) cleared the virus and did not develop demyelination. These results are consistent with the hypothesis that virus capsid gene products of TMEV stimulate class I-restricted CD8(+) T-cell immune responses, which are important for virus clearance and for protection against myelin destruction. Presented within the context of self-antigens, inactivation of these cells by ubiquitous expression of relevant virus capsid peptides partially inhibited resistance to virus-induced demyelination.     

Differential Influence of Interleukin-12 in the Pathogenesis of Autoimmune and Virus-Induced Central Nervous System Demyelination

Experimental allergic encephalomyelitis (EAE) and Theiler’s murine encephalomyelitis virus (TMEV) disease are two demyelinating diseases of the central nervous system (CNS) that serve as animal models for multiple sclerosis. Th1 cells are thought to play a role in the pathogenesis of CNS demyelination in both these diseases. We show here the differential influence of interleukin 12, a critical cytokine for the development of Th1 cells in EAE and TMEV disease.     

High Numbers of Viral RNA Copies in the Central Nervous System of Mice during Persistent Infection with Theiler's Virus

The low-neurovirulence Theiler's murine encephalomyelitis viruses (TMEV), such as BeAn virus, cause a persistent infection of the central nervous system (CNS) in susceptible mouse strains that results in inflammatory demyelination. The ability of TMEV to persist in the mouse CNS has traditionally been demonstrated by recovering infectious virus from the spinal cord. Results of infectivity assays led to the notion that TMEV persists at low levels. In the present study, we analyzed the copy number of TMEV genomes, plus- to minus-strand ratios, and full-length species in the spinal cords of infected mice and infected tissue culture cells by using Northern hybridization. Considering the low levels of infectious virus in the spinal cord, a surprisingly large number of viral genomes (mean of 3.0 x 10(9)) was detected in persistently infected mice. In the transition from the acute (approximately postinfection [p.i.] day 7) to the persistent (beginning on p.i. day 28) phase of infection, viral RNA copy numbers steadily increased, indicating that TMEV persistence involves active viral RNA replication. Further, BeAn viral genomes were full-length in size; i.e., no subgenomic species were detected and the ratio of BeAn virus plus- to minus-strand RNA indicated that viral RNA replication is unperturbed in the mouse spinal cord. Analysis of cultured macrophages and oligodendrocytes suggests that either of these cell types can potentially synthesize high numbers of viral RNA copies if infected in the spinal cord and therefore account for the heavy viral load. A scheme is presented for the direct isolation of both cell types directly from infected spinal cords for further viral analyses.     

Direct activation of innate and antigen-presenting functions of microglia following infection with Theiler's virus

Microglia are resident central nervous system (CNS) macrophages. Theiler's murine encephalomyelitis virus (TMEV) infection of SJL/J mice causes persistent infection of CNS microglia, leading to the development of a chronic-progressive CD4(+) T-cell-mediated autoimmune demyelinating disease. We asked if TMEV infection of microglia activates their innate immune functions and/or activates their ability to serve as antigen-presenting cells for activation of T-cell responses to virus and endogenous myelin epitopes. The results indicate that microglia lines can be persistently infected with TMEV and that infection significantly upregulates the expression of cytokines involved in innate immunity (tumor necrosis factor alpha, interleukin-6 [IL-6], IL-18, and, most importantly, type I interferons) along with upregulation of major histocompatibility complex class II, IL-12, and various costimulatory molecules (B7-1, B7-2, CD40, and ICAM-1). Most significantly, TMEV-infected microglia were able to efficiently process and present both endogenous virus epitopes and exogenous myelin epitopes to inflammatory CD4(+) Th1 cells. Thus, TMEV infection of microglia activates these cells to initiate an innate immune response which may lead to the activation of naive and memory virus- and myelin-specific adaptive immune responses within the CNS.