Innate immune response induced by Theiler's murine encephalomyelitis virus infection.

Although the causative agents of human multiple sclerosis (MS) are not known, it is suspected that a viral infection may be associated with the initiation of the disease. Several viral disease models in mice have been studied to understand the pathogenesis of demeylination. In particular, Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) has been extensively studied as a relevant model. Various cytokines and chemokines are produced upon viral infection by different cell types, including antigen-presenting cells (APCs) such as macrophages; dendritic cells (DCs); and glial cells, such as astrocytes, microglia, and oligodendrocytes. The upregulation of the corresponding molecules are also found in MS and are likely to play an important role in the protection and/or pathogenesis of chronic inflammatory demyelinating disease. In this review, the type of cells and molecules, gene-activation mechanisms as well as their potential roles in protection and pathogenesis will be discussed.

Type I interferon signals control Theiler's virus infection site, cellular infiltration and T cell stimulation in the CNS.

Theiler's murine encephalomyelitis virus (TMEV) establishes a persistent infection in the central nervous system (CNS). To examine the role of type I interferon (IFN-I)-mediated signals in TMEV infection, mice lacking a subunit of the type I IFN receptor (IFN-IR KO mice) were utilized. In contrast to wild type mice, IFN-IR KO mice developed rapid fatal encephalitis accompanied with greater viral load and infiltration of immune cells to the CNS. The proportion of virus-specific CD4(+) and CD8(+) T cell responses in the CNS was significantly lower in IFN-IR KO mice during the early stage of infection. Levels of IFN-γ and IL-17 produced by isolated primed CD4(+) T cells in response to DCs from TMEV-infected IFN-IR KO mice were also lower than those stimulated by DCs from TMEV-infected wild type control mice. The less efficient stimulation of virus-specific T cells by virus-infected antigen-presenting cells is attributable in part to the low level expression of activation markers on TMEV-infected cells from IFN-IR KO mice. However, due to high levels of cellular infiltration and viral loads in the CNS, the overall numbers of virus-specific T cells are higher in IFN-IR KO mice during the later stage of viral infection. These results suggest that IFN-I-mediated signals play important roles in controlling cellular infiltration to the CNS and shaping local T cell immune responses.

Differential virus replication, cytokine production, and antigen-presenting function by microglia from susceptible and resistant mice infected with Theiler's virus.

Infection with Theiler's murine encephalomyelitis virus (TMEV) in the central nervous system (CNS) causes an immune system-mediated demyelinating disease similar to human multiple sclerosis in susceptible but not resistant strains of mice. To understand the underlying mechanisms of differential susceptibility, we analyzed viral replication, cytokine production, and costimulatory molecule expression levels in microglia and macrophages in the CNS of virus-infected resistant C57BL/6 (B6) and susceptible SJL/J (SJL) mice. Our results indicated that message levels of TMEV, tumor necrosis factor alpha, beta interferon, and interleukin-6 were consistently higher in microglia from virus-infected SJL mice than in those from B6 mice. However, the levels of costimulatory molecule expression, as well as the ability to stimulate allogeneic T cells, were significantly lower in TMEV-infected SJL mice than in B6 mice. In addition, microglia from uninfected naïve mice displayed differential viral replication, T-cell stimulation, and cytokine production, similar to those of microglia from infected mice. These results strongly suggest that different levels of intrinsic susceptibility to TMEV infection, cytokine production, and T-cell activation ability by microglia contribute to the levels of viral persistence and antiviral T-cell responses in the CNS, which are critical for the differential susceptibility to TMEV-induced demyelinating disease between SJL and B6 mice.

Proapoptotic Bid is required for pulmonary fibrosis.

The molecular mechanisms of pulmonary fibrosis are poorly understood. Previous reports indicate that activation of TGF-beta1 is essential for the development of pulmonary fibrosis. Here, we report that the proapoptotic Bcl-2 family member Bid is required for the development of pulmonary fibrosis after the intratracheal instillation of bleomycin. Mice lacking Bid exhibited significantly less pulmonary fibrosis in response to bleomycin compared with WT mice. The attenuation in pulmonary fibrosis was observed despite similar levels of inflammation, lung injury, and active TGF-beta1 in bronchoalveolar lavage fluid 5 days after the administration of bleomycin in mice lacking Bid and in WT controls. Bleomycin induced similar levels cell death in vitro in alveolar epithelial cells isolated from WT and bid(-/-) mice. By contrast, alveolar epithelial cells from bid(-/-) mice were resistant to TGF-beta1-induced cell death. These results indicate that Bcl-2 family members are critical regulators for the development of pulmonary fibrosis downstream of TGF-beta1 activation.

Gender bias in Theiler's virus-induced demyelinating disease correlates with the level of antiviral immune responses.

Multiple sclerosis is an immune-mediated disease of the CNS and shows a sex-biased distribution in which 60-75% of all cases are female. A mouse model of multiple sclerosis, Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease, also displays a gender bias. However, in the C57L/J strain of mice, males are susceptible to disease whereas females are completely resistant. In this study we determined the gender differences in the TMEV-specific immune response, which may be responsible for the gender bias in clinical disease. Our data clearly demonstrate that female C57L/J mice induce significantly higher levels of TMEV-specific neutralizing Ab as well as a stronger peripheral T cell response throughout the course of viral infection. In contrast, male mice have a higher level of TMEV-specific CD4(+) and CD8(+) T cell infiltration into the CNS as well as viral persistence. These results suggest that a higher level of the initial antiviral immune response in female mice may be able to effectively clear virus from the periphery and CNS and therefore prevent further disease manifestations. Male mice in contrast do not mount as effective an immune response, thereby allowing for eventual viral persistence in the CNS and continuous T cell expansion leading to clinical symptoms.

Induction of chemokines in human astrocytes by picornavirus infection requires activation of both AP-1 and NF-kappa B.

Infection with different picornaviruses can cause meningitis/encephalitis in humans and experimental animals. To investigate the mechanisms of such inflammatory diseases, potential chemokine gene activation in human astrocytes was investigated following infection with Theiler's murine encephalomyelitis virus (TMEV), coxsackievirus B3 (CVB3), or coxsackievirus B4 (CVB4). We report that all these viruses are potent inducers for the expression of interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1) genes in primary human astrocytes, as well as in an established astrocyte cell line (U-373MG). Further studies indicated that both activator protein-1 (AP-1) and NF-kappaB transcription factors are required in the activation of chemokine genes in human astrocytes infected with various picornaviruses. Interestingly, the pattern of activated chemokine genes in human astrocytes is quite restricted compared to that in mouse astrocytes infected with the same viruses, suggesting species differences in gene activation. This may result in potential differences in the pathogenic outcome in each species.