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

The leader polypeptide of Theiler's murine encephalomyelitis virus is required for the assembly of virions in mouse L cells

Deletion of the entire leader polypeptide of the GDVII strain of Theiler's murine encephalomyelitis virus (TMEV) results in the production of an attenuated virus that grows in baby hamster kidney (BHK) cells but cannot grow at all in mouse L-929 cells. This study examined the reasons for the failure of dl-L, the GDVII variant that lacks the leader polypeptide, to grow in mouse cells. At low multiplicities of infection, it was difficult to detect any viral proteins in mouse cells. However, levels of positive- and negative-strand RNA molecules were only moderately reduced in these infections. Viral RNA showed no major defect in translatability, as the mutant viral RNA was nearly as efficient as that of the wild-type (WT) virus in directing protein synthesis in vitro in assays using extracts prepared from mouse L cells. Viral protein synthesis was detected in dl-L-infected mouse cells as multiplicities of infection were increased and approached the levels observed in WT infections. Despite this, there was a total lack of virus production in high-multiplicity infections, and this was found to correlate with the failure of viral proteins and early virion precursors to assemble into virions in mouse cells. Thus, the inability of dl-L to grow in mouse cells reflects complex effects on various stages of the virus infection but is primarily a defect in virus assembly.

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.

Viruses and apoptosis

Successful viral replication requires not only the efficient production and spread of progeny, but also evasion of host defense mechanisms that limit replication by killing infected cells. In addition to inducing immune and inflammatory responses, infection by most viruses triggers apoptosis or programmed cell death of the infected cell. This cell response often results as a compulsory or unavoidable by-product of the action of critical viral replicative functions. In addition, some viruses seem to use apoptosis as a mechanism of cell killing and virus spread. In both cases, successful replication relies on the ability of certain viral products to block or delay apoptosis until sufficient progeny have been produced. Such proteins target a variety of strategic points in the apoptotic pathway. In this review we summarize the great amount of recent information on viruses and apoptosis and offer insights into how this knowledge may be used for future research and novel therapies.

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.

A Theiler’s Virus Alternatively Initiated Protein Inhibits the Generation of H-2K-Restricted Virus-Specific Cytotoxicity

In susceptible mouse strains, the wild-type Daniel’s (wt-DA) strain of Theiler’s murine encephalomyelitis virus induces a persistent central nervous system (CNS) infection with chronic demyelination. The virus is cleared from resistant mice with no resulting demyelination. We characterized the role of the DA L* protein in late demyelination and persistent infection. The DA genome has two alternative reading frames, encoding the virus polyprotein and L*, respectively. The mutant virus DAL*-1 fails to synthesize L* and does not persist in the CNS of wt-DA-susceptible SJL/J or B10.S mice. Since class I-restricted cytotoxicity has been shown to determine resistance to virus persistence and demyelination in this model, virus-specific cytotoxicity in the CNS of DA-resistant (B6 or B10) and -susceptible (SJL/J and B10.S) mice during the acute stage of DA and DAL*-1 infection was characterized. Following intracerebral inoculation with DAL*-1, virus-specific Db- and Kb-restricted CTLs were demonstrated in the CNS of resistant B10 mice, whereas only Db-restricted CTL were found in wt-DA-inoculated mice. CTLs specific to wt-DA or DAL*-1 recognized class I-presented peptides from either of the viruses. Of particular interest, Ks-restricted virus-specific cytotoxicity-restricted CTLs were identified in the CNS of susceptible SJL/J (H-2s) and B10.S (H-2s) mice inoculated with DAL*-1. In contrast, no virus-specific CTLs were identified in the CNS of SJL/J and B10.S mice inoculated with wt-DA. We propose that L* inhibits the generation of H-2K-restricted virus-specific cytotoxicity in the CNS, permitting a persistent infection in susceptible strains, with subsequent inflammatory demyelination in the CNS similar to that in human multiple sclerosis.