The coronavirus mouse hepatitis virus (MHV) induces a minimal type I interferon (IFN) response in several cell types in vitro despite the fact that the type I IFN response is important in protecting the mouse from infection in vivo. When infected with MHV, mice deficient in IFN-associated receptor expression (IFNAR ؊/؊ ) became moribund by 48 h postinfection. MHV also replicated to higher titers and exhibited a more broad tissue tropism in these mice, which lack a type I IFN response. Interestingly, MHV induced IFN-␤ in the brains and livers, two main targets of MHV replication, of infected wild-type mice. MHV infection of primary cell cultures indicates that hepatocytes are not responsible for the IFN-␤ production in the liver during MHV infection. Furthermore, macrophages and microglia, but not neurons or astrocytes, are responsible for IFN-␤ production in the brain. To determine the pathway by which MHV is recognized in macrophages, IFN-␤ mRNA expression was quantified following MHV infection of a panel of primary bone marrow-derived macrophages generated from mice lacking different pattern recognition receptors (PRRs). Interestingly, MDA5, a PRR thought to recognize primarily picornaviruses, was required for recognition of MHV. Thus, MHV induces type I IFN in macrophages and microglia in the brains of infected animals and is recognized by an MDA5-dependent pathway in macrophages. These findings suggest that secretion of IFN-␤ by macrophages and microglia plays a role in protecting the host from MHV infection of the central nervous system.The type I interferon (IFN) response, consisting of IFN-␣/␤, represents one of the first lines of defense against viral infection. During viral infection of a cell, pathogen-associated molecular patterns, such as double-stranded RNA (dsRNA), are exposed and recognized by pattern recognition receptors (PRRs). These PRRs include Toll-like receptors (TLRs), such as TLR3, that reside on the cell membrane or in endosomal compartments and the cytoplasmic receptors RIG-I (retinoic acid-inducible gene I) and MDA5 (melanoma differentiationassociated protein 5) (8). Recognition by PRRs leads to the activation of IRF-3 (IFN regulatory factor 3), which dimerizes and translocates to the nucleus. In the nucleus it associates with the promoter region of the IFN-␤ gene along with NF-B, AP-1, and CBP/p300, driving IFN-␤ transcription. Once IFN-␤ is produced, it is secreted from infected cells and is able to bind to the IFN-associated receptor (IFNAR) on neighboring cells, amplifying the production of IFN-␤ and inducing the expression of IFN-␣ and many IFN-stimulated genes (37, 41). This series of events generates an antiviral milieu that helps to limit viral spread. Therefore, it is not surprising that many viruses have developed mechanisms to subvert or alter the type I IFN response, making it more difficult for the host to combat viral infection (8,20).Mouse hepatitis virus (MHV), a group 2 coronavirus (CoV), is a positive-strand RNA virus. Depending on the strain, MHV can infect both the ...

Murine coronavirus (mouse hepatitis virus, MHV) is a collection of strains that induce disease in several organ systems of mice. Infection with neurotropic strains JHM and A59 causes acute encephalitis, and in survivors, chronic demyelination, the latter of which serves as an animal model for multiple sclerosis. The MHV receptor is a carcinoembryonic antigen-related cell adhesion molecule, CEACAM1a; paradoxically, CEACAM1a is poorly expressed in the central nervous system (CNS), leading to speculation of an additional receptor. Comparison of highly neurovirulent JHM isolates with less virulent variants and the weakly neurovirulent A59 strain, combined with the use of reverse genetics, has allowed mapping of pathogenic properties to individual viral genes. The spike protein, responsible for viral entry, is a major determinant of tropism and virulence. Other viral proteins, both structural and nonstructural, also contribute to pathogenesis in the CNS. Studies of host responses to MHV indicate that both innate and adaptive responses are crucial to antiviral defense. Type I interferon is essential to prevent very early mortality after infection. CD8 T cells, with the help of CD4 T cells, are crucial for viral clearance during acute disease and persist in the CNS during chronic disease. B cells are necessary to prevent reactivation of virus in the CNS following clearance of acute infection. Despite advances in understanding of coronavirus pathogenesis, questions remain regarding the mechanisms of viral entry and spread in cell types expressing low levels of receptor, as well as the unique interplay between virus and the host immune system during acute and chronic disease.

Coronavirus infection of the murine central nervous system (CNS) provides a model for studies of viral encephalitis and demyelinating disease. Mouse hepatitis virus (MHV) neurotropism varies by strain: MHV-A59 causes mild encephalomyelitis and demyelination, while the highly neurovirulent strain JHM.SD (MHV-4) causes fatal encephalitis with extensive neuronal spread of virus. In addition, while neurons are the predominant CNS cell type infected in vivo, the canonical receptor for MHV, the carcinoembryonic antigen family member CEACAM1a, has been demonstrated only on endothelial cells and microglia. In order to investigate whether CEACAM1a is also expressed in other cell types, ceacam1a mRNA expression was quantified in murine tissues and primary cells. As expected, among CNS cell types, microglia expressed the highest levels of ceacam1a, but lower levels were also detected in oligodendrocytes, astrocytes, and neurons. Given the low levels of neuronal expression of ceacam1a, primary neurons from wild-type and ceacam1a knockout mice were inoculated with MHV to determine the extent to which CEACAM1a-independent infection might contribute to CNS infection. While both A59 and JHM.SD infected small numbers of ceacam1a knockout neurons, only JHM.SD spread efficiently to adjacent cells in the absence of CEACAM1a. Quantification of mRNA for the ceacam1a-related genes ceacam2 and psg16 (bCEA), which encode proposed alternative MHV receptors, revealed low ceacam2 expression in microglia and oligodendrocytes and psg16 expression exclusively in neurons; however, only CEACAM2 mediated infection in human 293T cells. Therefore, neither CEACAM2 nor PSG16 is likely to be an MHV receptor on neurons, and the mechanism for CEACAM1a-independent neuronal spread of JHM.SD remains unknown.Murine coronavirus (mouse hepatitis virus [MHV]) is a member of the Coronaviridae family of large, enveloped RNA viruses. Central nervous system (CNS) infection with neurotropic strains of MHV provides a model for studying acute virus-induced neurological disease, with or without chronic demyelination. These neurotropic strains differ widely in terms of tropism, spread, host response, and disease outcome, making them useful for identifying viral and host determinants of neurovirulence (57). Two strains commonly used to study coronavirus-induced CNS disease are the highly neurovirulent strain JHM.SD (formerly called MHV-4) and the more neuroattenuated and hepatotropic strain A59 (4, 26). Following intracranial (i.c.) or intranasal (i.n.) inoculation, JHM.SD causes severe and uniformly lethal encephalitis, whereas A59 induces a less severe encephalomyelitis followed by chronic demyelination (57). The extreme neurovirulence of JHM.SD maps largely to the spike glycoprotein, as a recombinant A59 virus expressing the JHM.SD spike (rA59/S JHM.SD ) showed increased virulence and viral dissemination throughout the brain compared to parental A59 (41, 42). Viral genes other than the spike gene also contribute to neurovirulence (7,23).MHV binds to a target cell vi...

Immunotherapy of established solid tumors is rarely achieved, and the mechanisms leading to success remain to be elucidated. We previously showed that extended control of advanced-stage autochthonous brain tumors is achieved following adoptive transfer of naive C57BL/6 splenocytes into sublethally irradiated line SV11 mice expressing the SV40 T Ag (T Ag) oncoprotein, and was associated with in vivo priming of CD8+ T cells (TCD8) specific for the dominant epitope IV (T Ag residues 404–411). Using donor lymphocytes derived from mice that are tolerant to epitope IV or a newly characterized transgenic mouse line expressing an epitope IV-specific TCR, we show that epitope IV-specific TCD8 are a necessary component of the donor pool and that purified naive epitope IV-specific TCD8 are sufficient to promote complete and rapid regression of established tumors. While transfer of naive TCR-IV cells alone induced some initial tumor regression, increased survival of tumor-bearing mice required prior conditioning of the host with a sublethal dose of gamma irradiation and was associated with complete tumor eradication. Regression of established tumors was associated with rapid accumulation of TCR-IV T cells within the brain following initial priming against the endogenous T Ag in the peripheral lymphoid organs. Additionally, persistence of functional TCR-IV cells in both the brain and peripheral lymphoid organs was associated with long-term tumor-free survival. Finally, we show that production of IFN-γ, but not perforin or TNF-α, by the donor lymphocytes is critical for control of autochthonous brain tumors.