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RNA transcripts derived from recombinant chimeras between the highly virulent GDVII virus and the less virulent BeAn virus were constructed to study the molecular pathogenesis of Theiler's murine encephalomyelitis virus infection. The presence of the BeAn 5' noncoding sequences in chimera 2 (BeAn 5' noncoding sequences joined with the GDVII nucleotides encoding the polyprotein and present in the 3' end) resulted in dramatic attenuation of GDVII neurovirulence and development of poliomyelitis in mice. This reduced neurovirulence was associated with slower virus growth and lower peak titers in the brain and spinal cord than with parental GDVII virus replication. On the other hand, the sites of replication following chimera 2 infection were the same as those seen in GDVII-infected mice; the distribution of virus antigen and histopathological changes indicated that chimera 2 replicates in neurons in the brain, e.g., in the neocortex, hippocampus, caudate putamen, and brain stem, as well as in anterior-horn cells in the spinal cord. Chimera 2 was efficiently cleared from the mouse central nervous system by day 30 postinfection, in marked contrast to the persistence of the BeAn parent in the central nervous system. This suggests that elements in the BeAn sequences that encode the polyprotein or are present in the 3' noncoding region are necessary for viral persistence. It is of interest that chimera 2-infected mice developed localized inflammatory, demyelinating lesions which were detected at day 28 postinfection but these lesions did not become larger with time. Thus, virus persistence appears to be required for maintenance and progression of immune-mediated demyelination. If the demyelinating lesions become sufficiently large, clinical signs and disease may develop.

Intracerebral inoculation of Theiler's murine encephalomyelitis virus (TMEV) into susceptible mouse strains produces a chronic demyelinating disease in which mononuclear cell-rich infiltrates in the central nervous system (CNS) are prominent. Current evidence strongly supports an immune-mediated basis for myelin breakdown, with an effector role proposed for TMEV-specific, major histocompatibility complex (MHC) class II-restricted delayed-type hypersensitivity (DTH) responses in which lymphokine-activated macrophages mediate bystander demyelination. The present study examined the possibility that concomitant or later-appearing neuroantigen-specific autoimmune T cell responses, such as those demonstrated in chronic-relapsing experimental allergic encephalomyelitis (R-EAE), may contribute to the demyelinating process following TMEV infection. T cell responses against intact, purified major myelin proteins (myelin basic protein (MBP) and proteolipid protein (PLP], and against altered myelin constituents were readily demonstrable in SJL/J mice with R-EAE, but were not detectable in SJL/J mice with TMEV-induced demyelinating disease. TMEV-infected mice also did not display T cell responses against the peptide fragments of MBP(91-104) and PLP(139-151) recently shown to be encephalitogenic in SJL/J mice. In addition, induction of neuroantigen-specific tolerance to a heterogeneous mixture of CNS antigens, via the i.v. injection of syngeneic SJL/J splenocytes covalently coupled with mouse spinal cord homogenate, resulted in significant suppression of clinical and histologic signs of R-EAE and the accompanying MBP- and PLP-specific DTH responses. In contrast, neuroantigen-specific tolerance failed to alter the development of clinical and histologic signs of TMEV-induced demyelinating disease or the accompanying virus-specific DTH and humoral immune responses. These findings demonstrate that TMEV-induced demyelinating disease can occur in the apparent absence of neuroantigen-specific autoimmune responses. The relationship of the present results to the immunopathology of multiple sclerosis is discussed.

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Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease is a relevant mouse model of multiple sclerosis. Demyelination is linked to persistent TMEV infection of the central nervous system and characterized by perivascular inflammatory mononuclear infiltrates and primary demyelination. Our previous results have shown that susceptibility correlates with the temporal development of chronic virus-specific delayed-type hypersensitivity (DTH) responses and suggest that inflammatory processes mediated by T cells specific for an immunodominant determinant on virus capsid protein 2 (VP2(74-86)) play a major immunopathologic role in SJL/J mice. In this study we have further defined the T cell-dependent nature and specificity of the demyelinating process in susceptible SJL/J mice by showing that thymectomized irradiated bone marrow-restored mice fail to develop chronic demyelination and that i.v. adoptive transfer of polyclonal TMEV-specific T cells before intracerebral infection with a suboptimal dose of the BeAn strain of TMEV led to increased incidence and accelerated onset of clinical disease. The data also show that demyelination is dependent on the activity of virus-specific CD4+ T cells because in vivo depletion with anti-CD4, but not anti-CD8, mAb led to significantly diminished incidence and severity of demyelination concomitant with a decrease in TMEV-specific DTH reactivity. In addition, the adoptive transfer of a TMEV-specific, DTH-mediating CD4+ I-A(s)-restricted Th1 line (sTV1) specific for the immunodominant VP2(74-86) epitope also led to increased incidence and accelerated onset of clinical disease only in TMEV-infected recipients. Collectively, the results of this and the companion paper demonstrate the highly significant immunopathologic contribution of CD4+ T cell responses specific for an immunodominant viral epitope to the chronic central nervous system demyelination observed in TMEV-infected SJL/J mice.