Theiler's murine encephalomyelitis virus (TMEV) persists in the mouse central nervous system principally in macrophages, and infected macrophages in culture undergo apoptosis. We have detected abundant apoptotic cells in perivascular cuffs and inflammatory, demyelinating lesions of SJL mice chronically infected with TMEV. T cells comprised 74% of apoptotic cells, while 8% were macrophages, 0.6% were astrocytes, and ϳ17% remained unidentified. In situ hybridization revealed viral RNA in ϳ1% of apoptotic cells.Theiler's murine encephalomyelitis virus (TMEV), a member of the Cardiovirus genus in the family Picornaviridae, produces a central nervous system (CNS) infection in mice leading to a chronic, inflammatory demyelinating pathology. TMEV preferentially replicates in CNS macrophages-microglia (hereafter, macrophages) during persistence and to a lesser extent in oligodendrocytes and astrocytes (11,21,26). A central role of macrophages in TMEV persistence was demonstrated by depletion of peripheral macrophages with mannosylated liposomes (26). One model of persistence involves macrophageto-macrophage spread limited by a block(s) in virus replication as well as host antiviral immune responses but with dissemination to other cells, e.g., astrocytes and oligodendrocytes, where infection is cytolytic.TMEV infection of macrophages in culture is dependent on the state of macrophage differentiation and activation (14,29). Myelomonocytic precursors and activated macrophages are both resistant to infection, possibly due to the absence of coreceptors and increased innate immunity (14-16, 29; unpublished data). This limited window of susceptibility might account in part for the infection of only a small percentage of macrophages in the mouse spinal cord (11). Once infected, macrophages (both in mice and in cell culture) restrict TMEV replication at the level of viral assembly (11,14,18,23).TMEV-infected macrophages in culture undergo apoptosis (14,23,24,29), either through an intrinsic pathway in nonactivated macrophages that requires virus replication and results in caspase activation or through an extrinsic pathway in gamma interferon-activated macrophages that involves signaling through receptors for tumor necrosis factor alpha and tumor necrosis factor alpha-related apoptosis-inducing ligand (16). However, the only report of apoptosis in TMEV infection of mice indicated that terminal deoxynucleotidyltransferase (TdT)-mediated UTP nick end labeling (TUNEL)-positive cells were seldom observed during viral persistence in the CNS (34), which suggests that apoptosis may be of little importance. Here, we report the detection of numerous apoptotic cells, primarily in the white matter in inflammatory demyelinating lesions in spinal cord sections of SJL mice chronically infected with BeAn virus.Quantitation of apoptotic cells in spinal cords of mice with demyelinating disease. Cells in the spinal cords of 12 SJL mice inoculated intracerebrally with 10 6 PFU of BeAn virus and undergoing demyelination were assessed for apoptosis b...