Theiler's Murine Encephalomyelitis Virus Induces Apoptosis in Gamma Interferon-Activated M1 Differentiated Myelomonocytic Cells through a Mechanism Involving Tumor Necrosis Factor Alpha (TNF-α) and TNF-α-Related Apoptosis-Inducing Ligand

Infection of susceptible mice with the low-neurovirulence Theiler’s murine encephalomyelitis virus strain BeAn results in an inflammatory demyelinating disease similar to multiple sclerosis. While the majority of virus antigen is detected in central nervous system macrophages (Mfs), few infiltrating Mfs are infected. We used the myelomonocytic precursor M1 cell line to study BeAn virus-Mf interactions in vitro to elucidate mechanisms for restricted virus expression. We have shown that restricted BeAn infection of M1 cells differentiated in vitro (M1-D) results in apoptosis. In this study, BeAn infection of gamma interferon (IFNg)-activated M1-D cells also resulted in apoptosis but with no evidence of virus replication or protein expression. RNase protection assays of M1-D cellular RNA revealed up-regulation of Fas and the p55 chain of the tumor necrosis factor alpha (TNF-a) receptor transcripts with IFN-g activation. BeAn infection of activated cells resulted in increased caspase 8 mRNA transcripts and the appearance of TNF-a-related apoptosis-inducing ligand (TRAIL) 4 h postinfection. Both unactivated and activated M1-D cells expressed TRAIL receptors (R1 and R2), but only activated cells were killed by soluble TRAIL. Activated cells were also susceptible to soluble FasL- and TNF-a-induced apoptosis. The data suggest that IFN-g-activated M1-D cell death receptors become susceptible to their ligands and that the cells respond to BeAn virus infection by producing the ligands TNF-a and TRAIL to kill the susceptible cells. Unactivated cells are not susceptible to FasL or TRAIL and require virus replication to initiate apoptosis. Therefore, two mechanisms of apoptosis induction can be triggered by BeAn infection: an intrinsic pathway requiring virus replication and an extrinsic pathway signaling through the death receptors. Theiler’s murine encephalomyelitis virus (TMEV) is a member of the family Picornaviridae, genus Cardiovirus, and a natural enteric pathogen of mice. TMEV has been divided into two groups based on neurovirulence following intracerebral inoculation of genetically susceptible mice. Infection with BeAn, a low-neurovirulence strain, results in a chronic, demyelinating disease of the central nervous system (CNS). Persistence of BeAn virus leads to activation of major histocompatibility complex class II-restricted CD4 1 Th1 lymphocytes directed at virus epitopes (11, 13, 14, 33, 34) and immunopathologic damage of myelin. This animal model has been used as an experimental analogue of multiple sclerosis. During TMEV persistence, the major virus antigen burden resides in CNS macrophages (Mfs) (12, 29, 37); however, only a relatively small percentage of Mfs infiltrating demyelinating lesions contain detectable virus antigen (29), and an infected Mf produces only 1 to 5 PFU (12). To investigate virus-Mf interactions that might lead to restricted virus replication, we have used the myelomonocytic precursor cell line, M1. Recently, it was reported that the M1 cell line was susceptible to virus infection only after differentiation into Mf-like cells