Selective tropism of a neurotropic coronavirus for ependymal cells, neurons, and meningeal cells.

The ability of a neurotropic virus, mouse hepatitis virus type 3 (MHV3), to invade the central nervous system (CNS) and to recognize cells selectively within the brain was investigated in vivo and in vitro. In vivo, MHV3 induced in C3H mice a genetically controlled infection of meningeal cells, ependymal cells, and neurons. In vitro, purified MHV3 bound to the surface of isolated ependymal cells and cultured cortical neurons but not to oligodendrocytes or cultured astrocytes. MHV3 replicated within cultured cortical neurons and neuroblastoma cells (NIE 115); infected cultured neurons nonetheless survived and matured normally for a 7-day period postinfection. On the other hand, MHV3 had a low affinity for cortical glial cells or glioma cells (C6 line), both of which appear to be morphologically unaltered by viral infection. Finally, MHV3 infected and disrupted cultured meningeal cells. This suggests that differences in the affinity of cells for MHV3 are determinants of the selective vulnerability of cellular subpopulations within the CNS. In vivo, a higher titer of virus was needed for CNS penetration in the genetically resistant (A/Jx) mice than in the susceptible (C57/BL6) mouse strain. However, in spite of viral invasion, no neuropathological lesions developed. In vitro viral binding to adult ependymal cells of susceptible and resistant strains of mice was identical. Genetic resistance to MHV3-CNS infection appeared to be mediated both by a peripheral mechanism limiting viral penetration into the CNS and by intra-CNS mechanisms, presumably at a stage after viral attachment to target cells.     

Activated MEK suppresses activation of PKR and enables efficient replication and in vivo oncolysis by Deltagamma(1)34.5 mutants of herpes simplex virus 1

Herpes simplex virus mutants lacking the gamma(1)34.5 gene are not destructive to normal tissues but are potent cytolytic agents in human tumor cells in which the activation of double-stranded RNA-dependent protein kinase (PKR) is suppressed. Thus, replication of a Deltagamma(1)34.5 mutant (R3616) in 12 genetically defined cancer cell lines correlates with suppression of PKR but not with the genotype of RAS. Extensive analyses of two cell lines transduced with either dominant negative MEK (dnMEK) or constitutively active MEK (caMEK) indicated that in R3616 mutant-infected cells dnMEK enabled PKR activation and decreased virus yields, whereas caMEK suppressed PKR and enabled better viral replication and cell destruction in transduced cells in vitro or in mouse xenografts. The results indicate that activated MEK mediates the suppression of PKR and that the status of MEK predicts the ability of Deltagamma(1)34.5 mutant viruses to replicate in and destroy tumor cells.     

Differential response of human cells to deletions and stop codons in the gamma(1)34.5 gene of herpes simplex virus.

Earlier studies have shown that herpes simplex virus mutants lacking the gamma(1)34.5 gene are totally avirulent on intracerebral inoculation of the virus into mice and induce premature shutoff of protein synthesis in human neuroblastoma (SK-N-SH) cells but not in Vero cells. We report the following. (i) Whereas deletion mutant R3616, lacking 1,000 bp of the gamma(1)34.5 gene, caused premature shutoff of protein synthesis in both SK-N-SH and human foreskin fibroblasts (HFF), mutants R4009 and R930 (mutant F), carrying stop codons in all six frames, 27 and 210 codons from the initiation codon of the gamma(1)34.5 genes, respectively, induced shutoff of protein synthesis in SK-N-SH cells but not in HFF. The differences in behavior between the R3616 deletion and R4009 stop codon mutants cannot be attributed to differences in the rate of induction of premature shutoff of protein synthesis and the multiplicity of infection. HFF do not produce detectable truncated gamma(1)34.5 protein or truncated mRNA. (ii) Some clonal lines of SK-N-SH cells carrying a gamma(1)34.5 gene driven by a metallothionein promoter express the gamma(1)34.5 gene constitutively and do not require induction by cadmium to complement the gamma(1)34.5- virus. One clonal cell line complements the gamma(1)34.5- virus only after induction by cadmium. These results are consistent with previous conclusions that the phenotype of premature shutoff of protein synthesis is associated with absence of the gamma(1)34.5 protein and indicate that the amounts of gamma(1)34.5 protein necessary to complement the gamma(1)34.5- viruses are small. We conclude that human cells differ in the manner in which they respond to the presence of stop codons. Shutoff of protein synthesis in HFF infected with the stop codon mutants could have been precluded by small amounts of gamma(1)34.5 protein produced by splicing out of an intron containing the stop codon, downstream initiation of translation, or tRNA suppression of the stop codon.     

Regulation of eIF2alpha phosphorylation by different functions that act during discrete phases in the herpes simplex virus type 1 life cycle

Multiple herpes simplex virus type 1 functions control translation by regulating phosphorylation of the initiation factor eIF2 on its alpha subunit. Both of the two known regulators, the gamma(1)34.5 and Us11 gene products, are produced late in the viral life cycle, although the gamma(1)34.5 gene is expressed prior to the gamma(2) Us11 gene, as gamma(2) genes require viral DNA replication for their expression while gamma(1) genes do not. The gamma(1)34.5 protein, through a GADD34-related domain, binds a cellular phosphatase (PP1alpha), maintaining pools of active, unphosphorylated eIF2. Infection of a variety of cultured cells with a gamma(1)34.5 mutant virus results in the accumulation of phosphorylated eIF2alpha and the inhibition of translation prior to the completion of the viral lytic program. Ectopic, immediate-early Us11 expression prevents eIF2alpha phosphorylation and the inhibition of translation observed in cells infected with a gamma(1)34.5 mutant by inhibiting activation of the cellular kinase PKR and the subsequent phosphorylation of eIF2alpha; however, a requirement for the Us11 protein, produced in its natural context as a gamma(2) polypeptide, remains to be demonstrated. To determine if Us11 regulates late translation, we generated two Us11 null viruses. In cells infected with a Us11 mutant, elevated levels of activated PKR and phosphorylated eIF2alpha were detected, viral translation rates were reduced 6- to 7-fold, and viral replication was reduced 13-fold compared to replication in cells infected with either wild-type virus or a virus in which the Us11 mutation was repaired. This establishes that the Us11 protein is critical for proper late translation rates. Moreover, it demonstrates that the shutoff of protein synthesis observed in cells infected with a gamma(1)34.5 mutant virus, previously ascribed solely to the gamma(1)34.5 mutation, actually results from the combined loss of gamma(1)34.5 and Us11 functions, as the gamma(2) Us11 mRNA is not translated in cells infected with a gamma(1)34.5 mutant.     

Perforin and gamma interferon-mediated control of coronavirus central nervous system infection by CD8 T cells in the absence of CD4 T cells

Infection of the central nervous system (CNS) with the neurotropic JHM strain of mouse hepatitis virus produces acute and chronic demyelination. The contributions of perforin-mediated cytolysis and gamma interferon (IFN-gamma) secretion by CD8(+) T cells to the control of infection and the induction of demyelination were examined by adoptive transfer into infected SCID recipients. Untreated SCID mice exhibited uncontrolled virus replication in all CNS cell types but had little or no demyelination. Memory CD8(+) T cells from syngeneic wild-type (wt), perforin-deficient, or IFN-gamma-deficient (GKO) donors all trafficked into the infected CNS in the absence of CD4(+) T cells and localized to similar areas. Although CD8(+) T cells from all three donors suppressed virus replication in the CNS, GKO CD8(+) T cells expressed the least antiviral activity. A distinct viral antigen distribution in specific CNS cell types revealed different mechanisms of viral control. While wt CD8(+) T cells inhibited virus replication in all CNS cell types, cytolytic activity in the absence of IFN-gamma suppressed the infection of astrocytes, but not oligodendroglia. In contrast, cells that secreted IFN-gamma but lacked cytolytic activity inhibited replication in oligodendroglia, but not astrocytes. Demyelination was most severe following viral control by wt CD8(+) T cells but was independent of macrophage infiltration. These data demonstrate the effective control of virus replication by CD8(+) T cells in the absence of CD4(+) T cells and support the necessity for the expression of distinct effector mechanisms in the control of viral replication in distinct CNS glial cell types.     

Critical role for protein tyrosine phosphatase SHP-1 in controlling infection of central nervous system glia and demyelination by Theiler's murine encephalomyelitis virus

We previously characterized the expression and function of the protein tyrosine phosphatase SHP-1 in the glia of the central nervous system (CNS). In the present study, we describe the role of SHP-1 in virus infection of glia and virus-induced demyelination in the CNS. For in vivo studies, SHP-1-deficient mice and their normal littermates received an intracerebral inoculation of an attenuated strain of Theiler's murine encephalomyelitis virus (TMEV). At various times after infection, virus replication, TMEV antigen expression, and demyelination were monitored. It was found that the CNS of SHP-1-deficient mice uniquely displayed demyelination and contained substantially higher levels of virus than did that of normal littermate mice. Many infected astrocytes and oligodendrocytes were detected in both brains and spinal cords of SHP-1-deficient but not normal littermate mice, showing that the virus replicated and spread at a much higher rate in the glia of SHP-1-deficient animals. To ascertain whether the lack of SHP-1 in the glia was primarily responsible for these differences, glial samples from these mice were cultured in vitro and infected with TMEV. As in vivo, infected astrocytes and oligodendrocytes of SHP-1-deficient mice were much more numerous and produced more virus than did those of normal littermate mice. These findings indicate that SHP-1 is a critical factor in controlling virus replication in the CNS glia and virus-induced demyelination.     

Full resistance of herpes simplex virus type 1-infected primary human cells to alpha interferon requires both the Us11 and gamma(1)34.5 gene products

The gamma(1)34.5 gene product is important for the resistance of herpes simplex virus type 1 (HSV-1) to interferon. However, since the inhibition of protein synthesis observed in cells infected with a gamma(1)34.5 mutant virus results from the combined loss of the gamma(1)34.5 gene product and the failure to translate the late Us11 mRNA, we sought to characterize the relative interferon sensitivity of mutants unable to produce either the Us11 or the gamma(1)34.5 polypeptide. We now demonstrate that primary human cells infected with a Us11 mutant virus are hypersensitive to alpha interferon, arresting translation upon entry into the late phase of the viral life cycle. Furthermore, immediate-early expression of Us11 by a gamma(1)34.5 deletion mutant is sufficient to render translation resistant to alpha interferon. Finally, we establish that the Us11 gene product is required for wild-type levels of replication in alpha interferon-treated cells and, along with the gamma(1)34.5 gene, is an HSV-1-encoded interferon resistance determinant.     

Replication of herpes simplex virus 1 depends on the gamma 134.5 functions that facilitate virus response to interferon and egress in the different stages of productive infection

The ability of the gamma(1)34.5 protein to suppress the PKR response plays a crucial role in herpes simplex virus pathogenesis. In this process, the gamma(1)34.5 protein associates with protein phosphatase 1 to form a large complex that dephosphorylates eIF-2alpha and thereby prevents translation shutoff mediated by PKR. Accordingly, gamma(1)34.5 null mutants are virulent in PKR-knockout mice but not in wild-type mice. However, gamma(1)34.5 deletion mutants, with an extragenic compensatory mutation, inhibit PKR activity but remain avirulent, suggesting that the gamma(1)34.5 protein has additional functions. Here, we show that a substitution of the gamma(1)34.5 gene with the NS1 gene from influenza A virus renders viral resistance to interferon involving PKR. The virus replicates as efficiently as wild-type virus in SK-N-SH and CV-1 cells. However, in mouse 3T6 cells, the virus expressing the NS1 protein grows at an intermediate level between the wild-type virus and the gamma(1)34.5 deletion mutant. This decrease in growth, compared to that of the wild-type virus, is due not to an inhibition of viral protein synthesis but rather to a block in virus release or egress. Virus particles are predominantly present in the nucleus and cytoplasm. Notably, deletions in the amino terminus of the gamma(1)34.5 protein lead to a significant decrease in virus growth in mouse 3T6 cells, which is independent of eIF-2alpha dephosphorylation. In correlation, a series of deletions in the amino-terminal domain impair nuclear as well as cytoplasmic egress. These results indicate that efficient viral replication depends on the gamma(1)34.5 functions required to prevent the PKR response and to facilitate virus egress in the different stages during virus infection.     

The immune system preferentially clears Theiler's virus from the gray matter of the central nervous system.

Infection of susceptible strains of mice with Daniel's (DA) strains of Theiler's murine encephalomyelitis virus (DAV) results in virus persistence in the central nervous system (CNS) white matter and chronic demyelination similar to that observed in multiple sclerosis. We investigated whether persistence is due to the immune system more efficiently clearing DAV from gray than from white matter of the CNS. Severe combined immunodeficient (SCID) and immunocompetent C.B-17 mice were infected with DAV to determine the kinetics, temporal distribution, and tropism of the virus in CNS. In early disease (6 h to 7 days postinfection), DAV replicated with similar kinetics in the brains and spinal cords of SCID and immunocompetent mice and in gray and white matter. DAV RNA was localized within 48 h in CNS cells of all phenotypes, including neurons, oligodendrocytes, astrocytes, and macrophages/microglia. In late disease (13 to 17 days postinfection), SCID mice became moribund and permitted higher DAV replication in both gray and white matter. In contrast, immunocompetent mice cleared virus from the gray matter but showed replication in the white matter of their brains and spinal cords. Reconstitution of SCID mice with nonimmune splenocytes or anti-DAV antibodies after establishment of infection demonstrated that both cellular and humoral immune responses decreased virus from the gray matter; however, the cellular responses were more effective. SCID mice reconstituted with splenocytes depleted of CD4+ or CD8+ T lymphocytes cleared virus from the gray matter but allowed replication in the white matter. These studies demonstrate that both neurons and glia are infected early following DAV infection but that virus persistence in the white matter is due to preferential clearance of virus from the gray matter by the immune system.     

The Second-Site Mutation in the Herpes Simplex Virus Recombinants Lacking the γ134.5 Genes Precludes Shutoff of Protein Synthesis by Blocking the Phosphorylation of eIF-2α

In cells infected with the herpes simplex virus 1 (HSV-1) recombinant R3616 lacking both copies of the γ₁34.5 gene, the double-stranded protein kinase R (PKR) is activated, eIF-2α is phosphorylated, and protein synthesis is shut off. Although PKR is also activated in cells infected with the wild-type virus, the product of the γ₁34.5 gene, infected-cell protein 34.5 (ICP34.5), binds protein phosphatase 1α and redirects it to dephosphorylate eIF-2α, thus enabling sustained protein synthesis. Serial passage in human cells of a mutant lacking the γ₁34.5 gene yields second-site, compensatory mutants lacking various domains of the α47 gene situated next to the U_S11 gene (I. Mohr and Y. Gluzman, EMBO J. 15:4759–4766, 1996). We report the construction of two recombinant viruses: R5103, lacking the γ₁34.5, U_S8, -9, -10, and -11, and α47 (U_S12) genes; and R5104, derived from R5103 and carrying a chimeric DNA fragment containing the U_S10 gene and the promoter of the α47 gene fused to the coding domain of the U_S11 gene. R5104 exhibited a protein synthesis profile similar to that of wild-type virus, whereas protein synthesis was shut off in cells infected with R5103 virus. Studies on the wild-type parent and mutant viruses showed the following: (i) PKR was activated in cells infected with parent or mutant virus but not in mock-infected cells, consistent with earlier studies; (ii) lysates of R3616, R5103, and R5104 virus-infected cells lacked the phosphatase activity specific for eIF-2α characteristic of wild-type virus-infected cells; and (iii) lysates of R3616 and R5103, which lacked the second-site compensatory mutation, contained an activity which phosphorylated eIF-2α in vitro, whereas lysates of mock-infected cells or cells infected with HSV-1(F) or R5104 did not phosphorylate eIF-2α. We conclude that in contrast to wild-type virus-infected cells, which preclude the shutoff of protein synthesis by causing rapid dephosphorylation of eIF-2α, in cells infected with γ₁34.5⁻ virus carrying the compensatory mutation, eIF-2α is not phosphorylated. The activity made apparent by the second-site mutation may represent a more ancient mechanism evolved to preclude the shutoff of protein synthesis.     

Identification of the infected target cell type in spongiform myeloencephalopathy induced by the neurotropic Cas-Br-E murine leukemia virus.

The Cas-Br-E murine leukemia virus (MuLV) induces a progressive hindlimb paralysis accompanied by a spongiform myeloencephalopathy in susceptible mice. In order to better understand the pathological process leading to these neurodegenerative lesions, we have investigated the nature of the cell type(s) infected by the virus during the course of the disease in CFW/D and SWR/J mice. For this purpose, we used in situ hybridization with virus-specific probes in combination with cell-type-specific histochemical (lectin) and immunological markers as well as morphological assessment. In the early stage of infection, endothelial cells represented the main cell type expressing viral RNA in the central nervous system (CNS). With disease progression and the appearance of lesions, microglial cells became the major cell type infected, accounting for up to 65% of the total infected cell population in diseased areas. Morphologically, these cells appeared activated and were frequently found in clusters. Infection and activation of microglial cells were almost exclusively restricted to diseased regions of the CNS. Neurons in diseased regions were not discernibly infected with virus at either early or late times of disease progression. Similarly, the proportion of infected astrocytes was typically < 1%. Although some endothelial cells and oligodendrocytes were infected by the virus, their infection was not limited to diseased CNS regions. These results are consistent with a model of indirect motor neuron degeneration, subsequent to the infection of nonneuronal CNS cells and especially of microglial cells. Infected microglial cells may play a role in the disease process by releasing not only virions or viral env-gene-encoded gp70 proteins but also other factors which may be directly or indirectly toxic to neurons. Parallels between microglial cell infection by MuLV and by lentiviruses, and specifically by human immunodeficiency virus, are discussed.     

The neurovirulent GDVII strain of Theiler's virus can replicate in glial cells.

The distribution, spread, neuropathology, tropism, and persistence of the neurovirulent GDVII strain of Theiler's virus in the central nervous system (CNS) was investigated in mice susceptible and resistant to chronic demyelinating infection with TO strains. Following intracerebral inoculation, the virus spread rapidly to specific areas of the CNS. There were, however, specific structures in which infection was consistently undetectable. Virus spread both between adjacent cell bodies and along neuronal pathways. The distribution of the infection was dependent on the site of inoculation. The majority of viral RNA-positive cells were neurons. Many astrocytes were also positive. Infection of both of these cell types was lytic. In contrast, viral RNA-positive oligodendrocytes were rare and were observed only in well-established areas of infection. The majority of oligodendrocytes in these areas were viral RNA negative and were often the major cell type remaining; however, occasional destruction of these cells was observed. No differences in any of the above parameters were observed between CBA and BALB/c mice, susceptible and resistant, respectively, to chronic CNS demyelinating infection with TO strains of Theiler's virus. By using Southern blot hybridization to detect reverse-transcribed PCR-amplified viral RNA sequences, no virus persistence could be detected in the CNS of immunized mice surviving infection with GDVII. In conclusion, the GDVII strain of Theiler's murine encephalomyelitis virus cannot persist in the CNS, but this is not consequent upon an inability to infect glial cells, including oligodendrocytes.     

Signals that dictate nuclear,nucleolar, and cytoplasmic shuttling of the gamma(1)34.5 protein of herpes simplex virus type 1

The gamma(1)34.5 protein of herpes simplex virus type 1 (HSV-1) is required for viral neurovirulence in vivo. In infected cells, this viral protein prevents the shutoff of protein synthesis mediated by double-stranded-RNA-dependent protein kinase PKR. This is accomplished by recruiting protein phosphatase 1 to dephosphorylate the alpha subunit of translation initiation factor eIF-2 (eIF-2 alpha). Moreover, the gamma(1)34.5 protein is implicated in viral egress and interacts with proliferating cell nuclear antigen. In this report, we show that the gamma(1)34.5 protein encoded by HSV-1(F) is distributed in the nucleus, nucleolus, and cytoplasm in transfected or superinfected cells. Deletion analysis revealed that the Arg-rich cluster from amino acids 1 to 16 in the gamma(1)34.5 protein functions as a nucleolar localization signal. The region from amino acids 208 to 236, containing a bipartite basic amino acid cluster, is able to mediate nuclear localization. R(215)A and R(216)A substitutions in the bipartite motif disrupt this activity. Intriguingly, leptomycin B, an inhibitor of nuclear export, blocks the cytoplasmic accumulation of the gamma(1)34.5 protein. L(134)A and L(136)A substitutions in the leucine-rich motif completely excluded the gamma(1)34.5 protein from the cytoplasm. These results suggest that the gamma(1)34.5 protein continuously shuttles between the nucleus, nucleolus, and cytoplasm, which may be a requirement for the different activities of the gamma(1)34.5 protein in virus-infected cells.     

Attenuation of herpes simplex virus neurovirulence with picornavirus cis-acting genetic elements

Viral pathogenesis depends on a suitable milieu in target host cells permitting viral gene expression, propagation, and spread. In many instances, viral genomes can be manipulated to select for propagation in certain tissues or cell types. This has been achieved for the neurotropic poliovirus (PV) by exchange of the internal ribosomal entry site (IRES), which is responsible for translation of the uncapped plus-strand RNA genome. The IRES of human rhinovirus type 2 (HRV2) confers neuron-specific replication deficits to PV but has no effect on viral propagation in malignant glioma cells. We report here that placing the critical gamma(1)34.5 virulence genes of herpes simplex virus type 1 (HSV) under translation control of the HRV2 IRES results in neuroattenuation in mice. In contrast, IRES insertion permits HSV propagation in malignant glioma cell lines that do not support replication of HSV recombinants carrying gamma(1)34.5 deletions. Our observations indicate that the conditions for alternative translation initiation at the HRV2 IRES in malignant glioma cells differ from those in normal central nervous system (CNS) cells. Picornavirus regulatory sequences mediating cell type-specific gene expression in the CNS can be utilized to target cancerous cells at the level of translation regulation outside their natural context.     

The interferon-inducible gene viperin restricts West Nile virus pathogenesis

Type I interferon (IFN) signaling coordinates an early antiviral program in infected and uninfected cells by inducing IFN-stimulated genes (ISGs) that modulate viral entry, replication, and assembly. However, the specific antiviral functions in vivo of most ISGs remain unknown. Here, we examined the contribution of the ISG viperin to the control of West Nile virus (WNV) in genetically deficient cells and mice. While modest increases in levels of WNV replication were observed for primary viperin(-/-) macrophages and dendritic cells, no appreciable differences were detected in deficient embryonic cortical neurons or fibroblasts. In comparison, viperin(-/-) adult mice infected with WNV via the subcutaneous or intracranial route showed increased lethality and/or enhanced viral replication in central nervous system (CNS) tissues. In the CNS, viperin expression was induced in both WNV-infected and adjacent uninfected cells, including activated leukocytes at the site of infection. Our experiments suggest that viperin restricts the infection of WNV in a tissue- and cell-type-specific manner and may be an important ISG for controlling viral infections that cause CNS disease.     

Dephosphorylation of eIF-2alpha mediated by the gamma(1)34.5 protein of herpes simplex virus type 1 is required for viral response to interferon but is not sufficient for efficient viral replication

The gamma(1)34.5 protein of herpes simplex virus type 1 (HSV-1) functions to block the shutoff of protein synthesis involving double-stranded RNA-dependent protein kinase (PKR). In this process, the gamma(1)34.5 protein recruits cellular protein phosphatase 1 (PP1) to form a high-molecular-weight complex that dephosphorylates eIF-2alpha. Here we show that the gamma(1)34.5 protein is capable of mediating eIF-2alpha dephosphorylation without any other viral proteins. While deletion of amino acids 1 to 52 from the gamma(1)34.5 protein has no effect on eIF-2alpha dephosphorylation, further truncations up to amino acid 146 dramatically reduce the activity of the gamma(1)34.5 protein. An additional truncation up to amino acid 188 is deleterious, indicating that the carboxyl-terminal domain alone is not functional. Like wild-type HSV-1, the gamma(1)34.5 mutant with a truncation of amino acids 1 to 52 is resistant to interferon, and resistance to interferon is coupled to eIF-2alpha dephosphorylation. Intriguingly, this mutant exhibits a similar growth defect seen for the gamma(1)34.5 null mutant in infected cells. Restoration of the wild-type gamma(1)34.5 gene in the recombinant completely reverses the phenotype. These results indicate that eIF-2alpha dephosphorylation mediated by the gamma(1)34.5 protein is required for HSV response to interferon but is not sufficient for viral replication. Additional functions or activities of the gamma(1)34.5 protein contribute to efficient viral infection.     

Type I interferons are essential in controlling neurotropic coronavirus infection irrespective of functional CD8 T cells

Neurotropic coronavirus infection induces expression of both beta interferon (IFN-beta) RNA and protein in the infected rodent central nervous system (CNS). However, the relative contributions of type I IFN (IFN-I) to direct, cell-type-specific virus control or CD8 T-cell-mediated effectors in the CNS are unclear. IFN-I receptor-deficient (IFNAR(-/-)) mice infected with a sublethal and demyelinating neurotropic virus variant and those infected with a nonpathogenic neurotropic virus variant both succumbed to infection within 9 days. Compared to wild-type (wt) mice, replication was prominently increased in all glial cell types and spread to neurons, demonstrating expanded cell tropism. Furthermore, increased pathogenesis was associated with significantly enhanced accumulation of neutrophils, tumor necrosis factor alpha, interleukin-6, chemokine (C-C motif) ligand 2, and IFN-gamma within the CNS. The absence of IFN-I signaling did not impair induction or recruitment of virus-specific CD8 T cells, the primary adaptive mediators of virus clearance in wt mice. Despite similar IFN-gamma-mediated major histocompatibility complex class II upregulation on microglia in infected IFNAR(-/-) mice, class I expression was reduced compared to that on microglia in wt mice, suggesting a synergistic role of IFN-I and IFN-gamma in optimizing class I antigen presentation. These data demonstrate a critical direct antiviral role of IFN-I in controlling virus dissemination within the CNS, even in the presence of potent cellular immune responses. By limiting early viral replication and tropism, IFN-I controls the balance of viral replication and immune control in favor of CD8 T-cell-mediated protective functions.