Apoptotic Cells, Including Macrophages, Are Prominent in Theiler's Virus-Induced Inflammatory, Demyelinating Lesions

Abstract

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are both resistant to infection, possibly due to the absence of coreceptors and increased innate immunity (14-16, 29; unpublished data).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...

Apoptotic and Antiapoptotic Activity of L Protein of Theiler's Murine Encephalomyelitis Virus

“…This form of cell death has been of interest with respect to TMEV because of the marked difference in virulence and persistence manifested by strains of the two subgroups of TMEV: GDVII and other members of the GDVII subgroup are highly virulent, causing a fatal acute disease with no virus persistence, while DA, BeAn, and other members of the TO subgroup cause a chronic, progressive demyelinating disease in which virus routinely persists in the CNS for the life of the mouse. Apoptosis has been reported both in the acute disease caused by DA and GDVII virus and during chronic TMEV-IDD (2,5,20,26). A number of different cell types have been found to undergo apoptosis, including activated macrophages/microglia, T cells, and oligodendrocytes.…”

“…During TMEV-IDD, apoptosis of T cells, microglia/macrophages, and oligodendrocytes has been described (2,5,20,26). In vitro studies have implicated the cardiovirus L protein, which is encoded between the start of the polyprotein and the P1 capsid proteins (Fig.…”

Theiler's Murine Encephalomyelitis Virus Leader Protein Is the Only Nonstructural Protein Tested That Induces Apoptosis When Transfected into Mammalian Cells

“…BeAn virus persists primarily in macrophages in the CNS of infected mice. Schlitt et al (34) found that 74% of TUNEL-positive cells in infected spinal cords (primarily in CNS lesions) were T and B lymphocytes and 8% were macrophages, although virus genomes were detected in Ͻ1% of apoptotic cells, consistent with infection of only a low percentage of macrophages and the fact that TMEV does not infect T or B lymphocytes in culture. Thus, some means other than direct infection was responsible for apoptosis of most CNS macrophages, including TMEV triggering apoptosis through tumor necrosis factor alpha or tumor necrosis factor alpha-related apoptosis-inducing ligand by binding death receptors on activated macrophages in vitro, as reported elsewhere (17).…”

Anticapsid Immunity Level, Not Viral Persistence Level, Correlates with the Progression of Theiler's Virus-Induced Demyelinating Disease in Viral P1-Transgenic Mice

“…However, virus-specific T cells, various proinflammatory chemokines (e.g., monocyte chemoattractant protein 1 and interferon-inducible protein 10), and cytokines (e.g., gamma interferon [IFN-␥] and tumor necrosis factor alpha) in the central nervous system (CNS) are believed to play a critical role (reviewed in references 26 and 44) in this disease process. In addition, the presence of persistent viral infection may lead to demyelinating disease by directly causing host cell lysis (51), which releases sequestrated CNS autoantigens, resulting in the activation of autoreactive T cells (39). The persistence of viral antigens also is believed to perpetuate a massive inflammatory milieu in the CNS by continuously activating virus-specific T cells (25,33).…”