Infectious cDNA clones of the DA strain of Theiler's murine encephalomyelitis virus

Theiler's Murine Encephalomyelitis Virus Replicates in Primary Neuron Cultures and Impairs Spine Density Formation

In this study, we examined infection with the highly neurovirulent GDVII, the less neurovirulent DA strains, and with a mutant DA, which lacks the L* protein (L*-1) involved in viral persistence and demyelinating disease, to analyze the direct effects of Theiler's murine encephalomyelitis virus (TMEV) replication using primary cultures of mouse brain hippocampal neurons. All viruses replicate in cultured neurons, with GDVII having the highest titers and L*-1 the lowest. Accordingly, all were positive for viral antigen staining 3 days postinfection (dpi), and DA and L*-1 were also positive after 12 dpi. NeuN + immunostaining showed an early and almost complete absence of positive cells in cultures infected with GDVII, an approximately 50% reduction in cultures infected with DA, and fewer changes in L*-1 strains at 3 dpi. Accordingly, staining with chloromethyltetramethylrosamine orange (Mitotracker OrangeTM) as a parameter for cell viability showed similar results. Moreover, at 1 dpi, the strain DA induced higher transcript levels of neuroprotective genes such as IFN-Iβ, IRF7, and IRF8. At 3 dpi, strains GDVII and DA, but not the L*-1 mutant, showed lower PKR expression. In addition, confocal analysis showed that L*-1-infected neurons exhibited a decrease in spine density. Treatment with poly (I:C), which is structurally related to dsRNA and is known to trigger IFN type I synthesis, reduced spine density even more. These results confirmed the use of mouse hippocampal neuron cultures as a model to study neuronal responses after TMEV infection, particularly in the formation of spine density.

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.

The epitope integration site for vaccine antigens determines virus control while maintaining efficacy in an engineered cancer vaccine

Picornaviruses have been developed as potential therapies for gene delivery and vaccination. One drawback to their use is the potential for recombination and viral persistence. Therefore, the engineering strategies used must take into account the possibility for virus escape. We have developed Theiler's murine encephalomyelitis virus (TMEV) as a potential vaccine vector for use in immunotherapy. This study shows that insertion of a vaccine epitope at a unique site within the TMEV leader protein can dramatically increase the type I interferon (IFN) response to infection and promote rapid viral clearance. This live virus vaccine maintains its ability to drive antigen-specific CD8(+) T-cell responses to a model antigen as well as to the weakly immunogenic tumor antigen Her2/neu. Furthermore, the epitope integration site does not affect the efficacy of this vaccine as cancer immunotherapy for treating models of melanoma and breast cancer as demonstrated by delayed tumor outgrowth and increased survival in animals implanted with these tumors. These findings show that an attenuated virus retaining limited ability to replicate nonetheless can effectively mobilize CD8(+) cellular immunity and will be important for the design of picornavirus vectors used as immunotherapy in clinical settings.

Immunization with structural and non-structural proteins of Theiler's murine encephalomyelitis virus alters demyelinating disease

Theiler's murine encephalomyelitis virus (TMEV) causes a demyelinating disease similar to multiple sclerosis in the central nervous system (CNS) of susceptible SJL/J mice. Immune responses to TMEV contribute to viral clearance as well as to demyelination. We constructed recombinant vaccinia viruses (VV) that encode each or all of the capsid proteins (VV(VP1), VV(VP2), VV(VP3), VV(VP4), and VV(all)) or non-structural proteins (VV(P2), VV(P2P3), and VV(3'P3)) of the Daniels strain of TMEV. To determine the role of each of the coding regions of TMEV in vivo, we immunized SJL/J mice with each recombinant VV, with or without subsequent TMEV infection. The groups of mice were compared clinically, immunologically, and histologically. No mice immunized with any recombinant VV without subsequent TMEV infection developed demyelination. However, antibody responses to TMEV were detected in mice immunized with VV(all). In addition, in some mice, VV(P2) immunization induced mild meningitis. VV(VP3) or VV(VP4) immunization of mice prior to TMEV infection ameliorated TMEV-induced pathology or clinical signs of disease. The beneficial effect of VP4 immunization was also seen through DNA immunization with a plasmid encoding VP4 and leader prior to TMEV infection. Therefore, vaccination against not only surface capsid proteins (VV(VP3) and VV(all)) but also non-surface capsid protein (VV(VP4)), and non-structural proteins (VV(P2)) can elicit immune responses to virus or modulate subsequent viral-induced CNS disease.

Saffold virus, a novel human Cardiovirus with unknown pathogenicity

Although cardioviruses have been thought to mainly infect rodents, a novel human cardiovirus, designated Saffold virus (SAFV), was identified in 2007. SAFV is grouped with Theiler-like rat virus and Theiler's murine encephalomyelitis virus (TMEV) in the species Theilovirus of the genus Cardiovirus of the family Picornaviridae. Eight genotypes of SAFV have now been identified. SAFV has been isolated from nasal and stool specimens from infants presenting with respiratory and gastrointestinal symptoms as well as from children with nonpolio acute flaccid paralysis; however, the relationship of SAFV to this symptomatology remains unclear. Of note, the virus has also been isolated from the cerebrospinal fluid specimens of patients with aseptic meningitis. This finding is of interest since TMEV is known to cause a multiple sclerosis-like syndrome in mice. The involvement of SAFV in various diseases (e.g., respiratory illness, gastrointestinal illness, neurological diseases, and type I diabetes) is presently under investigation. In order to clarify the pathogenicity of SAFV, additional epidemiological studies are required. Furthermore, identification of the SAFV cellular receptor will help establish an animal model for SAFV infection and help clarify the pathogenesis of SAFV-related diseases. In addition, investigation of the tissue-specific expression of the receptor may facilitate development of a novel picornavirus vector, which could be a useful tool in gene therapy for humans. The study of viral factors involved in viral pathogenicity using a reverse genetics technique will also be important.

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.

Apoptotic and antiapoptotic activity of L protein of Theiler's murine encephalomyelitis virus

Cellular apoptosis induced by viral genes can play a critical role in determining virulence as well as viral persistence. This form of cell death has been of interest with respect to Theiler's murine encephalomyelitis virus (TMEV) because the GDVII strain and members of the GDVII subgroup are highly neurovirulent, while the DA strain and members of the TO subgroup induce a chronic progressive inflammatory demyelination with persistence of the virus in the central nervous system. The TMEV L protein has been identified as important in the pathogenesis of Theiler's virus-induced demyelinating disease (TMEV-IDD). We now show that DA L is apoptotic following transfection of L expression constructs or following DA virus infection of HeLa cells; the apoptotic activity depends on the presence of the serine/threonine domain of L, especially a serine at amino acid 57. In contrast, GDVII L has little apoptotic activity following transfection of L expression constructs in HeLa cells and is antiapoptotic following GDVII infection of HeLa cells. Of note, both DA and GDVII L cleave caspase-3 in BHK-21 cells, although neither implements the full apoptotic machinery in this cell type as manifested by the induction of terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) staining. The differences in apoptotic activities of DA and GDVII L in varied cell types may play an important role in TMEV subgroup-specific disease phenotypes.

The preparation of an infectious full-length cDNA clone of Saffold virus

The pathogenicity of Saffold virus (SAFV) among humans still remains unclear, although it was identified as a novel human cardiovirus in 2007. In order to encourage the molecular pathogenetic studies of SAFV, we generated an infectious cDNA clone of SAFV type 3 (SAFV-3). The present study demonstrated that the synthesis of the full-length infectious RNA by T7 RNA polymerase was terminated by a homologous sequence motif with the human preproparathyroid hormone (PTH) signal in the SAFV-3 genome. To obtain the infectious RNA using T7 promoter, a variant of T7 RNA polymerase, which fails to recognize the PTH signal, was useful. This study will provide a valuable technical insight into the reverse genetics of SAFV.

The anti-apoptotic protein L(*) of Theiler's murine encephalomyelitis virus (TMEV) contains a mitochondrial targeting signal

L(*) protein of TMEV is out-of-frame with the viral polyprotein from an alternative initiation codon AUG, 13 nucleotides downstream from the authentic polyprotein AUG. Anti-apoptotic activity of L(*) was demonstrated by both 'loss of function' and 'gain of function' experiments. However, the precise mechanism(s) of anti-apoptotic activity of L(*) remains to be clarified. In this study, L(*) was demonstrated to be localized to mitochondria. It was also shown by the GFP fusion protein that N-terminal sequence of L(*) may contain a mitochondrial targeting signal (MTS). Surprisingly, L(*)((5-70))-GFP and L(*)((41-70))-GFP were localized to mitochondria although L(*)((1-70))-GFP was distributed in the cytosol, suggesting L(*) has an MTS between amino acid (AA) positions 41 and 70, and that L(*)((1-4)) inhibits its mitochondrial targeting. Furthermore, L(*)((1-70))-GFP was localized to the mitochondria by co-expression of L(*)((65-156)), indicating that L(*)((65-156)) suppresses the inhibition of mitochondrial targeting by L(*)((1-4)). These results suggest that the intra- or inter-molecular interaction of L(*) regulates its mitochondrial localization. It is also suggested that L(*) may inhibit the intrinsic apoptosis through the localization to mitochondria.

Different strains of Theiler's murine encephalomyelitis virus antagonize different sites in the type I interferon pathway

The DA strain of Theiler's murine encephalomyelitis virus (TMEV), a member of the Cardiovirus genus of the family Picornaviridae, causes persistent infection in susceptible mice, associated with restricted expression of viral proteins, and induces a demyelinating disease of the central nervous system. DA-induced demyelinating disease serves as a model of human multiple sclerosis because of similarities in pathology and because host immune responses contribute to pathogenesis in both disorders. In contrast, the GDVII strain of TMEV causes acute lethal encephalitis with no virus persistence. Cardiovirus L is a multifunctional protein that blocks beta interferon (IFN-beta) gene transcription. We show that both DA L and GDVII L disrupt IFN-beta gene transcription induction by IFN regulatory factor 3 (IRF-3) but do so at different points in the signaling pathway. DA L blocks IFN-beta gene transcription downstream of mitochondrial antiviral signaling protein (MAVS) but upstream of IRF-3 activation, while GDVII L acts downstream of IRF-3 activation. Both DA L and GDVII L block IFN-beta gene transcription in infected mice; however, IFN-beta mRNA is expressed at low levels in the central nervous systems of mice persistently infected with DA. The particular level of IFN-beta mRNA expression set by DA L as well as other factors in the IRF-3 pathway may play a role in virus persistence, inflammation, and the restricted expression of viral proteins during the late stage of demyelinating disease.

Theiler's murine encephalomyelitis virus L* amino acid position 93 is important for virus persistence and virus-induced demyelination

The DA strain and other members of the TO subgroup of Theiler's murine encephalomyelitis virus (TMEV) induce a persistent central nervous system infection associated with an inflammatory white matter demyelinating disease. TO subgroup strains synthesize an 18-kDa protein, L*, out of frame with the polyprotein from an initiation codon 13 nucleotides downstream from the polyprotein's AUG codon. We previously generated a mutant virus from our infectious DA full-length clone that has a change of the L* AUG codon to ACG (with no change in the polyprotein's amino acid sequence). Studies of this mutant virus showed that L* was key to the TO subgroup phenotype because the mutant had a decreased ability to persist and demyelinate. This work was initially called into question because a similar mutant derived from a different full-length DA infectious clone persisted and demyelinated similarly to wild-type DA virus (O. van Eyll and T. Michiels, J. Virol. 74:9071-9077, 2000). We now report that (i) the sequence of the L* coding region differs in the two infectious clones, resulting in a Ser or Leu as the predicted amino acid at position 93 of L* (with no change in the polyprotein's amino acid sequence), (ii) the difference in this amino acid is key to the phenotypic differences between the two mutants, and (iii) the change in amino acid 93 may affect L* phosphorylation. It is of interest that this amino acid only appears critical in determining the virus phenotype when L* is present in a significantly reduced amount (i.e., following translation from an ACG initiating codon).

Different subcellular localization of Theiler's murine encephalomyelitis virus leader proteins of GDVII and DA strains in BHK-21 cells

The highly virulent GDVII strain of Theiler's murine encephalomyelitis virus causes acute and fatal encephalomyelitis, whereas the DA strain causes mild encephalomyelitis followed by a chronic inflammatory demyelinating disease with virus persistence. The differences in the amino acid sequences of the leader protein (L) of the DA and GDVII strains are greater than those for any other viral protein. We examined the subcellular distribution of DA L and GDVII L tagged with the FLAG epitope in BHK-21 cells. Wild-type GDVII L was localized predominantly in the cytoplasm, whereas wild-type DA L showed a nucleocytoplasmic distribution. A series of the L mutant experiments demonstrated that the zinc finger domain, acidic domain, and C-terminal region of L were necessary for the nuclear accumulation of DA L. A GDVII L mutant with a deletion of the serine/threonine (S/T)-rich domain showed a nucleocytoplasmic distribution, in contrast to the predominant cytoplasmic distribution of wild-type GDVII L. A chimeric DA/GDVII L, D/G, which encodes the N region of DA L including the zinc finger domain and acidic domain, followed by the GDVII L sequence including the S/T-rich domain, was distributed exclusively throughout the cytoplasm but not in the nucleus, as observed with wild-type GDVII L. Another chimeric L, G/D (which is the converse of the D/G construct), accumulated in the nucleus as well as the cytoplasm, as was observed for wild-type DA L. The findings suggest that the differential distribution of DA L and GDVII L is determined primarily by the S/T-rich domain. The S/T-rich domain may be important for the viral activity through the regulation of the subcellular distribution of L.

A subgenomic segment of Theiler's murine encephalomyelitis virus RNA causes demyelination

The DA strain of Theiler's murine encephalomyelitis virus (TMEV) causes a persistent central nervous system (CNS) infection of mice with a restricted virus gene expression and induces an inflammatory demyelinating disease that is thought to be immune mediated and a model of multiple sclerosis (MS). The relative contribution of virus vis-à-vis the immune system in the pathogenesis of DA-induced white matter disease remains unclear, as is also true in MS. To clarify the pathogenesis of DA-induced demyelination, we used Cre/loxP technology to generate a transgenic mouse that has tamoxifen (Tm)-inducible expression of a subgenomic segment of DA RNA in oligodendrocytes and Schwann cells. Tm-treated young transgenic mice developed progressive weakness leading to death, with abnormalities of oligodendrocytes and Schwann cells and demyelination, but without inflammation, demonstrating that DA virus can play a direct pathogenic role in demyelination. Tm treatment of mice at a later age resulted in milder disease, with evidence of peripheral nerve remyelination and focal fur depigmentation; surviving weak mice had persistent expression of the recombined transgene in the CNS, suggesting that the DA subgenomic segment can cause cellular dysfunction but not death, possibly similar to the situation seen during DA virus persistence. These studies demonstrate that DA RNA or a DA protein(s) is toxic to myelin-synthesizing cells. This Cre/loxP transgenic system allows for spatially and temporally controlled expression of the viral transgene and is valuable for clarifying nonimmune (and immune) mechanisms of demyelination induced by TMEV as well as other viruses.

Expression of L* protein of Theiler's murine encephalomyelitis virus in the chronic phase of infection

The DA strain and other members of the TO subgroup of Theiler's murine encephalomyelitis virus synthesize the L* protein from an alternative initiation codon. L* is considered to play a key role in viral persistence and demyelination in susceptible strains of mice, although this hypothesis is still controversial. By using a mutant virus that expresses FLAG epitope-tagged L*, it was demonstrated previously that L* is expressed exclusively in neurons in vivo in the acute phase of infection in the central nervous system (CNS). However, in the mutant virus, the C-H-C-C zinc-binding motif in the leader protein (L) was disrupted by the insertion of the FLAG epitope, resulting in clearance of the virus from the CNS. Therefore, a further two mutant viruses were newly generated, expressing FLAG epitope-tagged L* in which the C-H-C-C zinc-binding motif within L is spared. Both mutant viruses caused persistence and demyelination successfully in spinal cords and enabled us to identify L* immunohistochemically in the demyelinating lesions.

Theiler's murine encephalomyelitis virus leader protein amino acid residue 57 regulates subgroup-specific virus growth on BHK-21 cells

Strains of Theiler's murine encephalomyelitis virus (TMEV) are divided into two subgroups, TO and GDVII. TMEV strains show subgroup-specific virus growth and cell tropism and induce subgroup-specific diseases. Using site-directed mutagenesis, we demonstrated that the amino acid at position 57 of the leader protein (L(57)), which is located at the most N-terminal part of the polyprotein, regulates subgroup-specific virus growth on BHK-21 cells. Further study suggested that L(57) may regulate viral RNA encapsidation, although it does not affect the synthesis of viral proteins or the assembly of viral intermediates.

Infectious RNA isolated from the spinal cords of mice chronically infected with Theiler's murine encephalomyelitis virus

The DA strain of Theiler's murine encephalomyelitis virus (TMEV) persistently infects cells of the spinal cord during the chronic phase of infection. Although in situ hybridization and reverse transcription-PCR have demonstrated the presence of viral RNA in the spinal cord, it has not been determined whether this RNA is infectious and, if so, how many PFU equivalents of virus the RNA can yield. In this study, we demonstrated that TMEV RNA isolated from the spinal cords of chronically infected mice is infectious and that there is at least 30-fold more infectious RNA than infectious virus in the spinal cords of these mice.

Leader (L) and L* proteins of Theiler's murine encephalomyelitis virus (TMEV) and their regulation of the virus' biological activities

Theiler's murine encephalomyelitis virus (TMEV) is divided into two subgroups on the basis of their different biological activities. GDVII subgroup strains produce fatal poliomyelitis in mice without virus persistence or demyelination. In contrast, TO subgroup strains induce demyelinating disease with virus persistence in the spinal cords of weanling mice. Two proteins, whose open reading frames are located in the N-terminus of the polyprotein, recently have been reported to be important for TMEV biological activities. One is leader (L) protein and is processed from the most N-terminus of the polyprotein; its function is still unknown. Although the homology of capsid proteins between DA (a representative strain of TO subgroup) and GDVII strains is over 94% at the amino acid level, that of L shows only 85%. Therefore, L is thought to be a key protein for the subgroup-specific biological activities of TMEV. Various studies have demonstrated that L plays important roles in the escape of virus from host immune defenses in the early stage of infection. The second protein is a 17–18 kDa protein, L*, which is synthesized out-of-frame with the polyprotein. Only TO subgroup strains produce L* since GDVII subgroup strains have an ACG rather than AUG at the initiation site and therefore do not synthesize L*. 'Loss and gain of function' experiments demonstrate that L* is essential for virus growth in macrophages, a target cell for TMEV persistence. L* also has been demonstrated to be necessary for TMEV persistence and demyelination. Further analysis of L and L* will help elucidate the pathomechanism(s) of TMEV-induced demyelinating disease.

A lentiviral expression system demonstrates that L* protein of Theiler's murine encephalomyelitis virus (TMEV) is essential for virus growth in a murine macrophage-like cell line

The DA subgroup strains of Theiler's murine encephalomyelitis virus (TMEV) synthesize L* protein, which is translated out of frame with the polyprotein from an alternative AUG, 13 nucleotides downstream from the authentic polyprotein AUG. By a 'loss of function' experiment using a mutant virus, DAL*-1, in which the L* AUG is mutated to an ACG, L* protein is shown to play an important role in virus persistence, TMEV-induced demyelination, and virus growth in macrophages. In the present study, we established an L* protein-expressed macrophage-like cell line and confirmed the importance of L* protein in virus growth in this cell line.

Epitope-tagged L* protein of Theiler's murine encephalomyelitis virus is expressed in the central nervous system in the acute phase of infection

TO subgroup strains of Theiler's murine encephalomyelitis virus (TMEV) synthesize L* protein from an alternative initiation codon. We first demonstrated L* expression in the central nervous system (CNS) of TMEV-infected mice during the acute phase of infection by immunoprecipitation and immunoblotting with anti-L* antibody. In addition, we generated mutant viruses which synthesize FLAG or 3xFLAG epitope-tagged L* protein. With a mutant virus expressing 3xFLAG epitope-tagged L*, designated DA/3xFLAGL*, we investigated L* in the CNS in the acute phase of infection. DA/3xFLAGL* did not change the virus tropism in comparison with wild-type virus, and L* was clearly identified in the CNS in both susceptible and resistant strains of mice. Double immunolabeling studies showed that L* is colocalized with TMEV polyprotein and exclusively expressed in neurons.

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.

Mutation in loop I of VP1 of Theiler's virus delays viral RNA release into cells and enhances antibody-mediated neutralization: a mechanism for the failure of persistence by the mutant virus

The DA strain of Theiler's murine encephalomyelitis viruses (TMEV) causes a central nervous system (CNS) demyelinating disease with viral persistence despite the presence of high serum anti-TMEV antibody titers. The DA virus mutant, T81D, was created to have a mutation at position 81 in loop I of VP1, close to the putative virus receptor binding site. T81D showed slower replication in vitro and in vivo. T81D-infected mice developed anti-TMEV antibody responses with no virus persistence. We tested whether the differences between the viruses were due to alteration in virus-cell interactions, or in the resistance to neutralization by anti-TMEV antibody. Using radiolabeled viruses, we found no difference in binding to permissive cell lines between the mutant and wild-type viruses. In a semipermissive cell line, DA virus bound more efficiently than T81D. During the uncoating step, both viruses decapsidated without the production of stable intermediates and 80% of viruses were eluted or decapsidated after 45 minutes. At the final step of uncoating, however, T81D showed a slower rate of RNA release than DA virus into cells using a photoinactivation assay. Anti-TMEV monoclonal and polyclonal antibodies neutralized T81D virus more efficiently than DA virus in suspension. Further, these anti-TMEV antibodies were able to neutralize viruses that had already attached to cells but not internalized (postadsorption neutralization [PAN]). However, DA virus showed significant resistance to PAN after cells were incubated at 37 degrees C compared with T81D-infected cells. The development of resistance to PAN appeared to correlate with the rate of RNA release from virions into cells. In T81D virus infection, the slow RNA release and high susceptibility to neutralization by antibodies would result in a failure to establish virus persistence in vivo. Conversely, rapid RNA release and resistance to neutralization could favor virus persistence in DA virus infection.

Nucleotide sequence and construction of an infectious cDNA clone of an EMCV strain isolated from aborted swine fetus

A full-length cDNA clone of an Encephalomyocarditis virus (EMCV) strain (2887A) isolated from aborted swine fetus was constructed and sequenced. Sequence comparison showed more than 99% nucleotide and amino acid sequence identity with two other EMCV strains, EMCV-PV21 and -R. However, the 2887A genomic sequence showed only about 84% nucleotide identity and 96% amino acid identity with EMCV-B, -D and -PV2 variants. RNA synthesized by in vitro transcription of this cDNA clone was infectious upon transfection of BHK21 cells, as shown by cytopathic effects and identification by neutralization test, and by propagation of the virus released into the culture media. The transcript RNA led to the production of infectious particles despite the presence of two nongenomic nucleotide residues at the 5' end, the short poly(C) tract (C(10)TCTC(3)TC(10)), the short poly(A) tail (7A), and the presence of six nongenomic nucleotides at the 3' end. The rescued virus was also found to be highly pathogenic for mice by intra-peritoneal inoculation producing a fatal disease indistinguishable from that of wild-type virus. An important finding concerning the molecular basis of infectivity was that the in vitro synthesized EMCV RNA transcript is infectious, although it contains a very short poly(A). The availability of the infectious cDNA clone of the reproductive failure strain of EMCV should prove to be useful for studying the molecular basis of the pathogenicity of EMCV in pig.

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.

Influence of the Theiler's virus L* protein on macrophage infection, viral persistence, and neurovirulence

The genome of picornaviruses contains a large open reading frame (ORF) translated as a precursor polypeptide that is processed to yield all the proteins necessary for the viral life cycle. In persistent but not in neurovirulent strains of Theiler's virus, an overlapping ORF encodes an additional 18-kDa protein called L*. We confirmed previous work showing that the L* ORF of persistent strains facilitates the infection of macrophage cell lines, and we present evidence that this effect is due to the L* protein itself rather than to competition for the translation of the two overlapping ORFs. The introduction of an AUG codon to restore the L* ORF of the neurovirulent GDVII strain also enhanced the infection of macrophages, in spite of the divergent evolution of this protein. The presence or the absence of the L* AUG initiation codon had only a weak influence on the neurovirulence of the GDVII strain and on the persistence of the DA1 strain. The results obtained with DA1 in vivo contrast with the results reported previously for DAFL3, another molecular clone of the same virus strain, where the AUG-to-ACG mutation of the L* initiation codon totally blocked viral persistence (G. D. Ghadge, L. Ma, S. Sato, J. Kim, and R. P. Roos, J. Virol. 72:8605-8612, 1998). Thus, a factor that is critical for the persistence of a given clone of Theiler's virus is dispensable for the persistence of a closely related clone, indicating that different adjustments in the expression of persistence determinants occur in related viral strains.

L* protein of Theiler's murine encephalomyelitis virus is required for virus growth in a murine macrophage-like cell line

We sought to confirm the importance of L* protein for growth of Theiler's murine encephalomyelitis virus (TMEV) in a macrophage-like cell line, J774-1. The protein is out of frame with the polyprotein and synthesized in DA but not GDVII subgroup strains of TMEV. A recombinant virus, DANCL*/GD, which substitutes the DA 5' noncoding and L* coding regions for the corresponding regions of GDVII and synthesizes L* protein, grew with little restriction in J774-1 cells. In contrast, another recombinant virus, DANCL*-1/GD, which has an ACG rather than an AUG as the starting codon of L* protein at nucleotide 1079, resulting in no synthesis of L* protein, did not grow well. No significant difference between the rates of adsorption to J774-1 cells of these viruses was observed. RNase protection assay demonstrated that DANCL*/GD viral RNA significantly increased, whereas only a minimal increase was observed for DANCL*-1/GD. The present study suggests that L* protein is required for virus growth in macrophages.

Theiler's murine encephalomyelitis virus (TMEV) subgroup strain-specific infection in neural and non-neural cell lines

GDVII subgroup strains of Theiler's murine encephalomyelitis virus (TMEV) are highly virulent and produce acute polioencephalomyelitis in mice. Neither viral persistence nor demyelination is demonstrated in the few surviving mice. In contrast, DA subgroup strains are less virulent and establish a persistent central nervous system infection which results in demyelinating disease. We previously reported a subgroup-specific infection in a macrophage-like cell line, J774-1 cells; i.e., GDVII strain does not replicate in J774-1 cells, whereas the DA strain actively replicates in these cells. In addition, this subgroup-specific virus growth is shown to be related to the presence of L* protein, a 17 kDa protein translated out-of-frame of the viral polyprotein from an AUG located 13 nucleotides downstream from the polyprotein's AUG. The present paper demonstrated that this subgroup-specific infection is observed in murine monocyte/macrophage lineage cell lines, but not in other murine cell lines including neural cells. An RNase protection assay also suggested that L* protein-related virus growth is regulated at the step of viral RNA replication. As macrophages are reported to be the major cell harboring virus during the chronic demyelinating stage, the activity of L* protein with respect to virus growth in macrophages may be a key factor in clarifying the mechanism(s) of TMEV persistence, which is probably a trigger to spinal cord demyelination.

Alternative translation initiation of Theiler's murine encephalomyelitis virus

DA strain and other members of the TO subgroup of Theiler's murine encephalomyelitis virus (TMEV) produce a chronic demyelinating disease in which the virus persists but has a restricted expression. We previously reported that TO subgroup strains, in addition to synthesizing the picornaviral polyprotein, use an alternative initiation codon just downstream from the polyprotein's AUG to translate an 18-kDa protein called L* that is out of frame with the polyprotein (H. H. Chen et al., Nat. Med. 1:927-931, 1995; W. P. Kong and R. P. Roos, J. Virol. 65:3395-3399, 1991). L* is critically important for virus persistence and the induction of the demyelinating disease (Chen et al., 1995; G. D. Ghadge et al. J. Virol. 72:8605-8612, 1998). We have proposed that variations in the amount of translation initiation from the L* AUG versus the polyprotein AUG may occur in different cell types and therefore affect the degree of expression of viral capsid proteins. We now demonstrate that ribosomal translation initiation at the polyprotein's initiation codon affects initiation at the L* AUG, suggesting that ribosomes land at the polyprotein's initiation codon before scanning downstream and initiating at the L* AUG. We also find that the viral 5' untranslated region affects utilization of the L* AUG. Surprisingly, mutant DA cDNAs were found to be infectious despite the presence of mutations of the polyprotein initiation codon or placement of a stop codon upstream of the L* AUG in the polyprotein's reading frame. Sequencing studies showed that these viruses had a second site mutation, converting the reading frame of L* into the polyprotein's reading frame; the results suggest that translation of the polyprotein during infection of these mutant viruses can be initiated at the L* AUG. These data are important in our understanding of translation initiation of TMEV and other RNAs that contain an internal ribosome entry site.

Expression of lymphotoxin gene inserted into Theiler's murine encephalomyelitis virus

Theiler's murine encephalomyelitis virus (TMEV) belongs to the Picornaviridae genus. DA subgroup strains of this virus induce early, non-fatal polioencephalomyelitis followed by demyelination in the spinal cord, with virus persistence. We investigated the use of DA strain as a vector for the introduction of a foreign gene into the central nervous system. Human lymphotoxin (LT) gene was inserted in the L region, the most upstream part of the polyprotein coding region of DA genome. Expression of LT was demonstrated by an immunoblot and an enzyme-linked immunosorbent assay on BHK-21 cells that were infected with the recombinant virus. In addition, the expressed LT showed cytotoxicity against L-929 cells.

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.

DNA vaccination against Theiler's murine encephalomyelitis virus leads to alterations in demyelinating disease

Although the etiology of multiple sclerosis (MS) is not known, several factors play a role in this disease: genetic contributions, immunologic elements, and environmental factors. Viruses and virus infections have been associated with the initiation and/or enhancement of exacerbations in MS. Theiler's murine encephalomyelitis virus (TMEV) infection of mice is one of the animal models used to mimic MS. In other animal model systems, DNA vaccination has been used to protect animals against a variety of virus infections. To explore the utility of DNA vaccination, we have constructed eukaryotic expression vectors encoding the TMEV capsid proteins VP1, VP2, and VP3. SJL/J mice were vaccinated intramuscularly once, twice, or three times with the different capsid protein cDNAs. This was followed by intracerebral TMEV infection to determine the effects of DNA vaccination on the course of TMEV-induced central nervous system (CNS) demyelinating disease. We found that vaccination of mice three times with cDNA encoding VP2 led to partial protection of mice from CNS demyelinating disease as determined by a decrease in clinical symptoms and histopathology. Vaccination of mice with cDNA encoding VP3 also led to a decrease in clinical symptoms. In contrast, mice vaccinated with cDNA encoding VP1 experienced a more severe disease with an earlier onset of clinical signs and enhanced histopathology compared with control mice. There was no correlation between anti-TMEV antibody titers and disease course. These results indicate that DNA immunization can modify chronic virus-induced demyelinating disease and may eventually lead to potential treatments for illnesses such as MS.

A protein critical for a Theiler's virus-induced immune system-mediated demyelinating disease has a cell type-specific antiapoptotic effect and a key role in virus persistence

TO subgroup strains of Theiler's murine encephalomyelitis virus (TMEV) induce a persistent central nervous system infection and demyelinating disease in mice. This disease serves as an experimental model of multiple sclerosis (MS) because the two diseases have similar inflammatory white matter pathologies and because the immune system appears to mediate demyelination in both processes. We previously reported (H. H. Chen, W. P. Wong, L. Zhang, P. L. Ward, and R. P. Roos, Nat. Med. 1:927-931, 1995) that TO subgroup strains use an alternative initiation codon (in addition to the AUG used to synthesize the picornavirus polyprotein from one long open reading frame) to translate L*, a novel protein that is out of frame with the polyprotein and which plays a key role in the demyelinating disease. We now demonstrate that L* has antiapoptotic activity in macrophage cells and is critical for virus persistence. The antiapoptotic action of L* as well as the differential translation of L* and virion capsid proteins may foster virus persistence in macrophages and interfere with virus clearance. The regulation of apoptotic activity in inflammatory cells may be important in the pathogenesis of TMEV-induced demyelinating disease as well as MS.

Replacement of loop II of VP1 of the DA strain with loop II of the GDVII strain of Theiler's murine encephalomyelitis virus alters neurovirulence, viral persistence, and demyelination

Theiler's murine encephalomyelitis viruses, which are murine picornaviruses, can cause central nervous system inflammatory disease. To study the role of loop II in capsid protein VP1, two mutant viruses of strain DA in which DA loop II amino acids were replaced with strain GDVII amino acids were constructed. Infection of mice with the two mutant viruses led to dramatically different patterns of disease.

L* protein of the DA strain of Theiler's murine encephalomyelitis virus is important for virus growth in a murine macrophage-like cell line

Strain GDVII and other members of the GDVII subgroup of Theiler's murine encephalomyelitis virus (TMEV) are highly virulent and cause acute polioencephalomyelitis in mice. Neither viral persistence nor demyelination is demonstrated in the few surviving mice. On the other hand, strain DA and other members of the TO subgroup of TMEV are less virulent and establish a persistent infection in the spinal cord, which results in a demyelinating disease. We previously reported that GDVII does not actively replicate in a murine macrophage-like cell line, J774-1, whereas DA strain productively infects these cells (M. Obuchi, Y. Ohara, T. Takegami, T. Murayama, H. Takada, and H. Iizuka, J. Virol. 71:729-733, 1997). In the present study, we used recombinant viruses between these strains of the two subgroups to demonstrate that the DA L coding region of DA strain is important for virus growth in J774-1 cells. Additional experiments with a mutant virus indicate that L* protein, which is synthesized out of frame with the polyprotein from an additional alternative initiation codon in the L coding region of TO subgroup strains, is a key determinant responsible for the cell-type-specific restriction of virus growth. L* protein may play a critical role in the DA-induced restricted demyelinating infection by allowing growth in macrophages, a major site for virus persistence.

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

Wild-type Daniel's strain of Theiler's virus (wt-DA) induces a chronic demyelination in susceptible mice which is similar to multiple sclerosis. A variant of wt-DA (designated DA-P12) generated during the 12th passage of persistent infection of a G26-20 glioma cell line failed to persist and induce demyelination in SJL/J mice. To identify the determinants responsible for this change in phenotype, we sequenced the capsid coding sequence (nucleotides [nt] 2991 to 3994) and found three mutations in VP1: residues 99 (Gly to Ser), 100 (Gly to Asp), and 103 (Asn to Lys). To study the role of these mutations in neurovirulence and demyelination, we prepared a recombinant virus, DAP-1C-2A/DA, with replacement of wt-DA nt 2991 to 3994 with the corresponding region of DA-P12, and viruses with individual point mutations at VP1 residues 99(Ser), 100(Asp), and 103(Lys). DAP-1C-2A/DA and viruses with a mutation at VP1 residue 99 or 100 (but not 103) completely attenuated the ability of wt-DA to induce demyelination. Failure to induce demyelination was not due to a general failure in growth, since DA-P12 and other mutant viruses lysed L-2 cells in vitro as effectively as wt-DA. The change in disease phenotype was independent of the specific B- or T-cell immune recognition because a decrease in the neurovirulence of mutant viruses was observed in neonatal mice and immune-deficient RAG1 -/- mice. This difference in neurovirulence is not the complete explanation for the failure of DA-P12 to demyelinate, since virus with a mutation at residue 103(Lys) had decreased neurovirulence but did induce demyelination. Therefore, point mutation at VP1 residue 99 or 100 altered the ability of wt-DA to demyelinate, perhaps related to a disruption in interaction between virus and receptor on certain neural cells.

Central nervous system cytokine mRNA expression following Theiler's murine encephalomyelitis virus infection

DA strain of Theiler's murine encephalomyelitis virus (TMEV) produces a biphasic disease with an initial self-limited acute gray matter polioencephalomyelitis in all strains of mice followed by, in the case of certain susceptible strains of mice, a chronic inflammatory demyelination of the spinal cord with a persistent virus infection. A pathogenic role for T-helper 1 (Th1) cells during the demyelinating phase of disease has been proposed. We characterized the cytokine mRNA expression in the brain and spinal cord of susceptible and resistant strains of mice during the early encephalomyelitic disease and the late demyelination, using a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) assay. At the time of the encephalomyelitis, both resistant and susceptible mice expressed proinflammatory cytokine mRNAs followed by T-cell derived mRNAs; susceptible mice expressed more IL-12 p40 mRNA than resistant mice. During this early disease, there was no significant difference in Th1 cytokine mRNA expression in the brain and spinal cord among the four strains and relatively little Th2 type cytokine upregulation above levels seen in mock-infected controls. During the late demyelinating disease, susceptible but not resistant mice had evidence of viral genome and a continuous expression of Th1 type cytokine mRNAs. The expression of Th2 cytokine mRNAs varied among the different strains and did not correlate with susceptibility or resistance. The results indicate the complexity of cytokine mRNA expression following TMEV infection and the dependence of the expression on disease pathology, the time following infection and the genetics of the host.

Molecular determinants for virulence in coxsackievirus B1 infection

Studies demonstrated that a strain derived from an infectious clone of coxsackievirus B1 (CVB1N) (N. Iizuka, H. Yonekawa, and A. Nomoto, J. Virol. 65:4867-4873, 1991) was 3 to 4 log10 less virulent than the myotropic Tucson strain of CVB1 (CVB1T) following intraperitoneal inoculation of newborn mice. Replacement of nucleotides (nt) 69 to 804 from the 5' untranslated region (5' UTR) and 1A coding region of CVB1N or nt 117 to 161 from the 5' UTR with the corresponding part from CVB1T restored greater than 90% of the virulence. Sequencing of the 5' UTR of CVB1T demonstrated areas with a greater similarity to particular echoviruses than to CVB1N, suggesting that recombination events might have occurred, perhaps influencing the virulence phenotype.

Nucleotide sequence of classical swine fever virus strain Alfort/187 and transcription of infectious RNA from stably cloned full-length cDNA

The complete nucleotide sequence of the genome of classical swine fever virus (CSFV) strain Alfort/187 was determined from three cDNA libraries constructed by cloning of DNA fragments obtained from independent sets of reverse transcription and PCR. The cDNA fragments were then assembled and inserted downstream of a T7 promoter in a P15A-derived plasmid vector to obtain the full-length cDNA clone pA187-1. The first nucleotide of the CSFV genome was positioned at the transcription start site of the T7 promoter. Cleavage at an SrfI restriction site introduced at the exact 3' end of the cloned viral cDNA allowed the in vitro synthesis of full-length viral RNA by runoff transcription. This RNA proved to be infectious after transfection into porcine kidney cells. Infectivity was not increased after capping of the synthetic RNA. Virus recovered from transfected cells was titrated in porcine kidney cells by endpoint dilution using indirect immunofluorescence and a CSFV-specific monoclonal antibody. RNA transcripts generated from plasmid DNA isolated from bacteria which had been cultured and cloned 10 times remained infectious, indicating that the full-length clone is stable in bacterial cells. A silent point mutation introduced at position 11842 of the genome was retained in the recombinant virus recovered from transfected cells. An infectious chimeric construct was obtained by replacing a 696-bp fragment in pA187-1 with the corresponding cDNA fragment from the CSFV strain CAP. The stably cloned full-length CSFV cDNA allows site-specific mutagenesis of the viral genome and thus will be useful for detailed molecular characterization of the virus as well as for studies of viral pathogenesis.

The leader polypeptide of Theiler's virus is essential for neurovirulence but not for virus growth in BHK cells

A leader polypeptide of unknown function is encoded by cardioviruses, such as Theiler's murine encephalomyelitis virus. Although the deletion of this polypeptide has little effect on the growth of parental GDVII virus in baby hamster kidney (BHK) cells, the mutant virus is completely attenuated and fails to kill mice receiving intracerebral inoculations of high doses of the virus.

A picornaviral protein synthesized out of frame with the polyprotein plays a key role in a virus-induced immune-mediated demyelinating disease

The DA strain and other members of the TO subgroup of Theiler's murine encephalomyelitis virus (TMEV) induce a chronic demyelinating disease with a restricted virus expression. This disease serves as an experimental model of multiple sclerosis; in both diseases the immune system contributes to a similar demyelinating pathology. Like all picornaviruses, TMEV encodes a polyprotein translated from one long open reading frame. The polyprotein is then processed into structural and non-structural viral proteins. Here, we demonstrate that the DA strain of TMEV has an additional alternative open reading frame that encodes a protein called L* that is present in infected cells. Virus with a mutation of L* has a dramatically decreased demyelinating activity, indicating that L* plays a critical role in TO subgroup-induced demyelinating disease. L* is associated with membranes, suggesting that L* may interact with the immune system and thereby mediate the viral-induced demyelinating disease.

Theiler's virus as a vector for foreign gene delivery

DA strain and other strains of the TO subgroup of Theiler's murine encephalomyelitis viruses are members of the Cardiovirus genus of picornaviruses and produce a persistent demyelinating disease in mice. A recent study from our laboratory (W.-P. Kong, G. D. Ghadge, and R. P. Roos, Proc. Natl. Acad. Sci. USA 91:1796-1800, 1994) demonstrated that the leader, which is encoded at the N terminus of the Theiler's murine encephalomyelitis virus polyprotein, can be partially replaced by foreign sequences as well as completely deleted, with no loss of infectivity in BHK-21 cells. In this study, we have inserted up to 724 nucleotides into the leader coding region of an infectious DA clone. Recombinant viruses were produced, and the inserts were shown to be stable for at least three passages in BHK-21 cells.

Construction of rubella virus genome-length cDNA clones and synthesis of infectious RNA transcripts

Plasmids containing a complete cDNA copy of the rubella virus (RUB) genomic RNA were constructed. Transfection into cell culture of genome-length RNA transcribed in vitro from one of these cDNA clones, Robo102, resulted in the production of virus which preserved the genetic and phenotypic characteristics of the parental virus from which the cDNA clone was derived. Prior to construction of the RUB genome-length cDNA clones, the 5'-terminal sequence of the RUB genomic RNA was determined to be 5'CAAUGG...3' following the cap structure. Analysis of the specific infectivity of RUB genomic RNA isolated from virions revealed that in Vero cells, the specific infectivity of RUB genomic RNA is roughly equivalent to that of Sindbis virus genomic RNA. In RUB virion RNA preparations, the subgenomic RNA was detected. It was demonstrated that subgenomic RNA was packaged into RUB virions; however, the presence of the subgenomic RNA was not essential for infectivity of the genomic RNA.

Chimeric Theiler's virus with altered tropism for the central nervous system

Theiler's virus is a neurotropic murine picornavirus which, depending on the strain, causes either an acute encephalitis or a persistent demyelinating disease. Following intracranial inoculation, the demyelinating strains infect sequentially the grey matter of the brain, the grey matter of the spinal cord, and finally the white matter of the spinal cord, where they persist and cause chronic demyelination. The neurovirulent strains cause a generally fatal encephalitis with lytic infection of neurons. The study of chimeric Theiler's viruses, obtained by recombining the genomes of demyelinating and neurovirulent strains, has shown that the viral capsid contains determinants for persistence and demyelination. In this article we describe the recombinant virus R5, in which the capsid protein VP1 and a small portion of protein 2A come from the neurovirulent GDVII strain and the rest of the genome comes from the persistent DA strain. The capsid of virus R5 also contains one mutation at amino acid 34 of VP3 (Asn-->His). Virus R5 does not persist in the central nervous system (CNS) of immunocompetent SJL/J or BALB/c mice. However, it replicates efficiently and persists in the CNS of BALB/c nu/nu mice, showing that its growth in the CNS is not impaired. In BALB/c nu/nu mice, whereas virus DA causes mortality with large amounts of viral antigens in the white matter of the spinal cord, virus R5 does not kill the animals, persists in the neurons of the grey matter of the brain, and never reaches the white matter of the spinal cord. This phenotype is due to the chimerism of the capsid and/or to the mutation in VP3. These results indicate that the capsid plays an important role in the characteristic migration of Theiler's virus within the CNS.

A single amino acid change determines persistence of a chimeric Theiler's virus

The DA strain of Theiler's virus persists in the central nervous system of mice and causes chronic inflammation and demyelination. On the other hand, the GDVII strain causes an acute encephalitis and does not persist in surviving animals. Series of recombinants between infectious cDNA clones of the genomes of DA and GDVII viruses have been constructed. The analysis of the phenotypes of the recombinant viruses has shown that determinants of persistence and demyelination are present in the capsid proteins of DA virus. Chimeric viruses constructed by the different research groups gave consistent results, with one exception. Chimeras GD1B-2A/DAFL3 and GD1B-2C/DAFL3, which contain part of capsid protein VP2, capsid proteins VP3 and VP1, and different portions of P2 of GDVII in a DA background, were able to persist and cause demyelination. Chimera R4, whose genetic map is identical to that of GD1B-2A/DAFL3, was not. After exchanging the viral chimeras between laboratories and verifying each other's observations, new chimeras were generated in order to explain this difference. Here we report that the discrepancy can be attributed to a single amino acid difference in the sequence of the capsid protein VP2 of the two parental DA strains. DAFL3 (University of Chicago) and the chimeras derived from it, GD1B-2A/DAFL3 and GD1B-2C/DAFL3, contain a Lys at position 141, while TMDA (Institut Pasteur) and R4, the chimera derived from it, contain an Asn in that position. This amino acid is located at the tip of the EF loop, on the rim of the depression spanning the twofold axis of the capsid. These results show that a single amino acid change can confer the ability to persist and demyelinate to a chimeric Theiler's virus, and they pinpoint a region of the viral capsid that is important for this phenotype.

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.

Involvement of cardiovirus leader in host cell-restricted virus expression

Theiler murine encephalomyelitis virus designates a number of picornavirus strains that are classified into two subgroups on the basis of their different biological activities. DA strain and other members of the TO subgroup produce a chronic demyelinating disease in which the virus persists and manifests a restricted expression. Mutagenesis studies of the DA strain leader (L) coding region, which is located at the 5' end of the polyprotein coding region, demonstrate that L is completely dispensable for infection of some cells; in addition, nucleotides can be inserted into the L coding region with no loss in infectivity, indicating that Theiler murine encephalomyelitis virus may be used as a vector for delivering foreign sequences. In other cells, L is critical for plaque formation and efficient viral multiplication. These findings raise the possibility that L may play a role in the DA-induced demyelinating restricted infection. The functions of L, and even its presence within the genome, vary among picornaviruses, reflecting the various requirements for viral growth among different host cells.

Importance of amino acid 101 within capsid protein VP1 for modulation of Theiler's virus-induced disease

We constructed a Theiler's virus mutant designated DA3304, in which the amino acid at position 101 of VP1 was changed from a threonine to an alanine. Because of this single amino acid change, DA3304 could still produce a biphasic central nervous system disease similar to that produced by the wild-type DA virus. However, DA3304 was significantly attenuated in both the acute and the chronic phases and induced smaller demyelinating lesions than the wild-type DA virus. The data are most compatible with the attenuated phenotype in DA3304 being due to the change of binding efficiency between the virus and receptor resulting from the physical alteration at the mutation site.

Chimeric cDNA studies of Theiler's murine encephalomyelitis virus neurovirulence

Strain GDVII and other members of the GDVII subgroup of Theiler's murine encephalomyelitis virus are highly neurovirulent and rapidly fatal, while strain DA and other members of the TO subgroup produce a chronic, demyelinating disease. GDVII/DA chimeric cDNA studies suggest that a major neurovirulence determinant is within the GDVII 1B through 1D capsid protein coding region, although the additional presence of upstream GDVII sequences, including the 5' untranslated region, contributes to full neurovirulence. Our studies indicate that there are limitations in precisely delineating neurovirulence determinants with chimeric cDNAs between evolutionarily diverged viruses, such as GDVII and DA.

Role of maturation cleavage in infectivity of picornaviruses: activation of an infectosome

Maturation of picornaviruses involves assembly of a "provirion," which undergoes an autocatalytic cleavage of VP0 to VP2 plus VP4. RNA transcripts from a cDNA clone of human rhinovirus 14 mutated at asparagine 68, one of the residues in the maturation cleavage site, generated normal yields of 150S particles which were noninfectious in the plaque assay because they were unable to initiate a second cycle of infection. These cleavage-defective provirions were otherwise indistinguishable from mature virions in sedimentation coefficient, binding affinity to monoclonal antibodies against neutralization sites IA, II, and III, attachment to HeLa cell receptors, and rate of cell-mediated conformational changes to form 125S A-particles and 80S empty capsids. These results suggest that maturation cleavage is required for the function of a previously undescribed intermediate which transfers packaged RNA across the membrane and into the cytosol. For this hypothetical intermediate, we propose the name infectosome. Since the native virus has a particle/PFU ratio of about 800, such an intermediate will be difficult to find. Mutations at serine 10 in VP2 reduced maturation cleavage to a rate sufficiently slow to show that the infectivity of virus particles increased with the degree of cleavage of VP0 to VP4 and VP2. This article describes the first characterization of a pure form of a picornaviral provirion, and hence the first direct evidence that provirions of picornaviruses lack infectivity.