The Production of Antibody by Invading B Cells Is Required for the Clearance of Rabies Virus from the Central Nervous System

Background

The pathogenesis of rabies is associated with the inability to deliver immune effectors across the blood-brain barrier and to clear virulent rabies virus from CNS tissues. However, the mechanisms that facilitate immune effector entry into CNS tissues are induced by infection with attenuated rabies virus.

Methodology/Principal Findings

Infection of normal mice with attenuated rabies virus but not immunization with killed virus can promote the clearance of pathogenic rabies virus from the CNS. T cell activity in B cell–deficient mice can control the replication of attenuated virus in the CNS, but viral mRNA persists. Low levels of passively administered rabies virus–neutralizing antibody reach infected cells in the cerebellum of B cell–deficient mice but are not sufficient to mediate virus clearance. Production of rabies virus-specific antibody by B cells invading CNS tissues is required for this process, and a substantial proportion of the B cells that accumulate in the CNS of mice infected with attenuated rabies virus produce virus-specific antibodies.

Conclusions/Significance

The mechanisms required for immune effectors to enter rabies virus-infected tissues are induced by infection with attenuated rabies virus but not by infection with pathogenic rabies viruses or immunization with killed virus. T cell activities can inhibit rabies virus replication, but the production of rabies virus–specific antibodies by infiltrating B cells, as opposed to the leakage of circulating antibody across the BBB, is critical to elimination of the virus. These findings suggest that a pathogenic rabies virus infection may be treatable after the virus has reached the CNS tissues, providing that the appropriate immune effectors can be targeted to the infected tissues.     

Gamma Interferon (IFN-γ) Receptor Restricts Systemic Dengue Virus Replication and Prevents Paralysis in IFN-α/β Receptor-Deficient Mice

We previously reported that mice lacking alpha/beta and gamma interferon receptors (IFN-α/βR and -γR) uniformly exhibit paralysis following infection with the dengue virus (DENV) clinical isolate PL046, while only a subset of mice lacking the IFN-γR alone and virtually no mice lacking the IFN-α/βR alone develop paralysis. Here, using a mouse-passaged variant of PL046, strain S221, we show that in the absence of the IFN-α/βR, signaling through the IFN-γR confers approximately 140-fold greater resistance against systemic vascular leakage-associated dengue disease and virtually complete protection from dengue-induced paralysis. Viral replication in the spleen was assessed by immunohistochemistry and flow cytometry, which revealed a reduction in the number of infected cells due to IFN-γR signaling by 2 days after infection, coincident with elevated levels of IFN-γ in the spleen and serum. By 4 days after infection, IFN-γR signaling was found to restrict DENV replication systemically. Clearance of DENV, on the other hand, occurred in the absence of IFN-γR, except in the central nervous system (CNS) (brain and spinal cord), where clearance relied on IFN-γ from CD8⁺ T cells. These results demonstrate the roles of IFN-γR signaling in protection from initial systemic and subsequent CNS disease following DENV infection and demonstrate the importance of CD8⁺ T cells in preventing DENV-induced CNS disease.     

Interferon-dependent immunity is essential for resistance to primary dengue virus infection in mice, whereas T- and B-cell-dependent immunity are less critical

Dengue virus (DEN) causes dengue fever and dengue hemorrhagic fever/dengue shock syndrome, which are major public health problems worldwide. The immune factors that control DEN infection or contribute to severe disease are neither well understood nor easy to examine in humans. In this study, we used wild-type and congenic mice lacking various components of the immune system to study the immune mechanisms in the response to DEN infection. Our results demonstrate that alpha/beta interferon (IFN-α/β) and IFN-γ receptors have critical, nonoverlapping functions in resolving primary DEN infection. Furthermore, we show that IFN-α/β receptor-mediated action limits initial DEN replication in extraneural sites and controls subsequent viral spread into the central nervous system (CNS). In contrast, IFN-γ receptor-mediated responses seem to act at later stages of DEN disease by restricting viral replication in the periphery and eliminating virus from the CNS. Mice deficient in B, CD4⁺ T, or CD8⁺ T cells had no increased susceptibility to DEN; however, RAG mice (deficient in both B and T cells) were partially susceptible to DEN infection. In summary, (i) IFN-α/β is critical for early immune responses to DEN infection, (ii) IFN-γ-mediated immune responses are crucial for both early and late clearance of DEN infection in mice, and (iii) the IFN system plays a more important role than T- and B-cell-dependent immunity in resistance to primary DEN infection in mice.     

Presence of Virus Neutralizing Antibodies in Cerebral Spinal Fluid Correlates with Non-Lethal Rabies in Dogs

Background

Rabies is traditionally considered a uniformly fatal disease after onset of clinical manifestations. However, increasing evidence indicates that non-lethal infection as well as recovery from flaccid paralysis and encephalitis occurs in laboratory animals as well as humans.

Methodology/Principal Findings

Non-lethal rabies infection in dogs experimentally infected with wild type dog rabies virus (RABV, wt DRV-Mexico) correlates with the presence of high level of virus neutralizing antibodies (VNA) in the cerebral spinal fluid (CSF) and mild immune cell accumulation in the central nervous system (CNS). By contrast, dogs that succumbed to rabies showed only little or no VNA in the serum or in the CSF and severe inflammation in the CNS. Dogs vaccinated with a rabies vaccine showed no clinical signs of rabies and survived challenge with a lethal dose of wild-type DRV. VNA was detected in the serum, but not in the CSF of immunized dogs. Thus the presence of VNA is critical for inhibiting virus spread within the CNS and eventually clearing the virus from the CNS.

Conclusions/Significance

Non-lethal infection with wt RABV correlates with the presence of VNA in the CNS. Therefore production of VNA within the CNS or invasion of VNA from the periphery into the CNS via compromised blood-brain barrier is important for clearing the virus infection from CNS, thereby preventing an otherwise lethal rabies virus infection.     

Regional differences in blood-brain barrier permeability changes and inflammation in the apathogenic clearance of virus from the central nervous system

The loss of blood-brain barrier (BBB) integrity in CNS inflammatory responses triggered by infection and autoimmunity has generally been associated with the development of neurological signs. In the present study, we demonstrate that the clearance of the attenuated rabies virus CVS-F3 from the CNS is an exception; increased BBB permeability and CNS inflammation occurs in the absence of neurological sequelae. We speculate that regionalization of the CNS inflammatory response contributes to its lack of pathogenicity. Despite virus replication and the expression of several chemokines and IL-6 in both regions being similar, the up-regulation of MIP-1beta, TNF-alpha, IFN-gamma, and ICAM-1 and the loss of BBB integrity was more extensive in the cerebellum than in the cerebral cortex. The accumulation of CD4- and CD19-positive cells was higher in the cerebellum than the cerebral cortex. Elevated CD19 levels were paralleled by kappa-L chain expression levels. The timing of BBB permeability changes, kappa-L chain expression in CNS tissues, and Ab production in the periphery suggest that the in situ production of virus-neutralizing Ab may be more important in virus clearance than the infiltration of circulating Ab. The data indicate that, with the possible exception of CD8 T cells, the effectors of rabies virus clearance are more commonly targeted to the cerebellum. This is likely the result of differences in the capacity of the tissues of the cerebellum and cerebral cortex to mediate the events required for BBB permeability changes and cell invasion during virus infection.     

RNase L Mediated Protection from Virus Induced Demyelination

IFN-α/β plays a critical role in limiting viral spread, restricting viral tropism and protecting mice from neurotropic coronavirus infection. However, the IFN-α/β dependent mechanisms underlying innate anti-viral functions within the CNS are poorly understood. The role of RNase L in viral encephalomyelitis was explored based on its functions in inhibiting translation, inducing apoptosis, and propagating the IFN-α/β pathway through RNA degradation intermediates. Infection of RNase L deficient (RL^−/−) mice with a sub-lethal, demyelinating mouse hepatitis virus variant revealed that the majority of mice succumbed to infection by day 12 p.i. However, RNase L deficiency did not affect overall control of infectious virus, or diminish IFN-α/β expression in the CNS. Furthermore, increased morbidity and mortality could not be attributed to altered proinflammatory signals or composition of cells infiltrating the CNS. The unique phenotype of infected RL^−/− mice was rather manifested in earlier onset and increased severity of demyelination and axonal damage in brain stem and spinal cord without evidence for enhanced neuronal infection. Increased tissue damage coincided with sustained brain stem infection, foci of microglia infection in grey matter, and increased apoptotic cells. These data demonstrate a novel protective role for RNase L in viral induced CNS encephalomyelitis, which is not reflected in overall viral control or propagation of IFN-α/β mediated signals. Protective function is rather associated with cell type specific and regional restriction of viral replication in grey matter and ameliorated neurodegeneration and demyelination.     

Memory CD4+ T-cell-mediated protection from lethal coronavirus encephalomyelitis

The antiviral role of CD4⁺ T cells in virus-induced pathologies of the central nervous system (CNS) has not been explored extensively. Control of neurotropic mouse hepatitis virus (JHMV) requires the collaboration of CD4⁺ and CD8⁺ T cells, with CD8⁺ T cells providing direct perforin and gamma interferon (IFN-γ)-mediated antiviral activity. To distinguish bystander from direct antiviral contributions of CD4⁺ T cells in virus clearance and pathology, memory CD4⁺ T cells purified from wild type (wt), perforin-deficient (PKO), and IFN-γ-deficient (GKO) immune donors were transferred to immunodeficient SCID mice prior to CNS challenge. All three donor CD4⁺ T-cell populations controlled CNS virus replication at 8 days postinfection, indicating IFN-γ- and perforin-independent antiviral function. Recipients of GKO CD4⁺ T cells succumbed more rapidly to fatal disease than untreated control infected mice. In contrast, wt and PKO donor CD4⁺ T cells cleared infectious virus to undetectable levels and protected from fatal disease. Recipients of all CD4⁺ T-cell populations exhibited demyelination. However, it was more severe in wt CD4⁺ T-cell recipients. These data support a role of CD4⁺ T cells in virus clearance and demyelination. Despite substantial IFN-γ-independent antiviral activity, IFN-γ was crucial in providing protection from death. IFN-γ reduced neutrophil accumulation and directed macrophages to white matter but did not ameliorate myelin loss.     

Differential Host Immune Responses after Infection with Wild-Type or Lab-Attenuated Rabies Viruses in Dogs

Rabies virus (RABV) induces encephalomyelitis in humans and animals. One of the major problems with rabies is that the infected individuals most often do not develop virus neutralizing antibodies (VNA). In this study we have investigated the host immune response to RABV infection in dogs, using a live-attenuated (TriGAS) or a wild-type (wt) (DRV-NG11) RABV isolated from a rabid dog.

Methodology/Principal Findings

The experimental infection of dogs with TriGAS induced high levels of VNA in the serum, whereas wt RABV infection did not. Dogs infected with TriGAS developed antibodies against the virus including its glycoprotein, whereas dogs infected with DRV-NG11 only developed rabies antibodies that are presumably specific for the nucleoprotein, (N) and not the glycoprotein (G). We show that infection with TriGAS induces early activation of B cells in the draining lymph nodes and persistent activation of DCs and B cells in the blood. On the other hand, infection with DRV-NG11 fails to induce the activation of DCs and B cells and further reduces CD4 T cell production. Further, we show that intrathecal (IT) immunization of TriGAS not only induced high levels of VNA in the serum but also in the CSF while intramuscular (IM) immunization of TriGAS induced VNA only in the serum. In addition, high levels of total protein and WBC were detected in the CSF of IT immunized dogs, indicating the transient enhancement of blood-brain barrier (BBB) permeability, which is relevant to the passage of immune effectors from periphery into the CNS.

Conclusions/Significance

IM infection of dogs with TriGAS induced the production of serum VNA whereas, IT immunization of TriGAS in dogs induces high levels of VNA in the periphery as well as in the CSF and transiently enhances BBB permeability. In contrast, infection with wt DRV-NG11 resulted in the production of RABV-reactive antibodies but VNA and antibodies specific for G were absent. As a consequence, all of the dogs infected with wt DRV-NG11 succumbed to rabies. Thus the failure to activate protective immunity is one of the important features of RABV pathogenesis in dogs.     

Protective Vaccine-Induced CD4+ T Cell-Independent B Cell Responses against Rabies Infection

A major goal in rabies virus (RV) research is to develop a single-dose postexposure prophylaxis (PEP) that would simplify vaccination protocols, reduce costs associated with rabies prevention in humans, and save lives. Live replication-deficient RV-based vaccines are emerging as promising single-dose vaccines to replace currently licensed inactivated RV-based vaccines. Nonetheless, little is known about how effective B cells develop in response to live RV-based vaccination. Understanding this fundamental property of rabies immunology may help in developing a single-dose RV vaccine. Typically, vaccines induce B cells secreting high-affinity, class-switched antibodies during germinal center (GC) reactions; however, there is a lag time between vaccination and the generation of GC B cells. In this report, we show that RV-specific antibodies are detected in mice immunized with live but not inactivated RV-based vaccines before B cells displaying a GC B cell phenotype (B220⁺GL7^hiCD95^hi) are formed, indicating a potential role for T cell-independent and early extrafollicular T cell-dependent antibody responses in the protection against RV infection. Using two mouse models of CD4⁺ T cell deficiency, we show that B cells secreting virus-neutralizing antibodies (VNAs) are induced via T cell-independent mechanisms within 4 days postimmunization with a replication-deficient RV-based vaccine. Importantly, mice that are completely devoid of T cells (B6.129P2-Tcrβ^tm1Mom Tcrδ^tm1Mom/J) show protection against pathogenic challenge shortly after immunization with a live replication-deficient RV-based vaccine. We show that vaccines that can exploit early pathways of B cell activation and development may hold the key for the development of a single-dose RV vaccine wherein the rapid induction of VNA is critical.     

Rabies virus ocular disease: T-cell-dependent protection is under the control of signaling by the p55 tumor necrosis factor alpha receptor, p55TNFR

Following brain infection, the Challenge Virus Standard strain of rabies virus infects the retina. Rabies virus ocular infection induces the infiltration of neutrophils and predominantly T cells into the eye. The role of tumor necrosis factor alpha (TNF-alpha)-lymphotoxin signaling in the control of rabies virus ocular infection and inflammatory cell infiltration was assessed using mice lacking the p55 TNF-alpha receptor (p55TNFR(-/-) mice). The incidence of ocular disease and the intensity of retinal infection were greater in p55TNFR(-/-) mice than in C57BL/6 mice: the aggravation correlated with less neutrophil and T-cell infiltration. This indicates that cellular infiltration is under the control of the p55 TNF-alpha receptor and suggests that inflammatory cells may protect the eye against rabies virus ocular infection. The role of T cells following rabies virus ocular disease was assessed by comparison of rabies virus infection in nude mice with their normal counterparts. Indeed, the incidence and severity of the rabies virus ocular disease were higher in athymic nude mice than in BALB/c mice, indicating that T lymphocytes are protective during rabies virus ocular infection. Moreover, few T cells and neutrophils underwent apoptosis in rabies virus-infected retina. Altogether, these data suggest that T lymphocytes and neutrophils are able to enter the eye, escape the immune privilege status, and limit rabies virus ocular disease. In conclusion, rabies virus-mediated eye disease provides a new model for studying mechanisms regulating immune privilege during viral infection.     

GITR Intrinsically Sustains Early Type 1 and Late Follicular Helper CD4 T Cell Accumulation to Control a Chronic Viral Infection

CD4 T cells are critical for control of persistent infections; however, the key signals that regulate CD4 T help during chronic infection remain incompletely defined. While several studies have addressed the role of inhibitory receptors and soluble factors such as PD-1 and IL-10, significantly less work has addressed the role of T cell co-stimulatory molecules during chronic viral infection. Here we show that during a persistent infection with lymphocytic choriomeningitis virus (LCMV) clone 13, mice lacking the glucocorticoid-induced tumor necrosis factor receptor related protein (GITR) exhibit defective CD8 T cell accumulation, increased T cell exhaustion and impaired viral control. Differences in CD8 T cells and viral control between GITR^+/+ and GITR^-/- mice were lost when CD4 T cells were depleted. Moreover, mixed bone marrow chimeric mice, as well as transfer of LCMV epitope-specific CD4 or CD8 T cells, demonstrated that these effects of GITR are largely CD4 T cell-intrinsic. GITR is dispensable for initial CD4 T cell proliferation and differentiation, but supports the post-priming accumulation of IFNγ⁺IL-2⁺ Th1 cells, facilitating CD8 T cell expansion and early viral control. GITR-dependent phosphorylation of the p65 subunit of NF-κB as well as phosphorylation of the downstream mTORC1 target, S6 ribosomal protein, were detected at day three post-infection (p.i.), and defects in CD4 T cell accumulation in GITR-deficient T cells were apparent starting at day five p.i. Consistently, we pinpoint IL-2-dependent CD4 T cell help for CD8 T cells to between days four and eight p.i. GITR also increases the ratio of T follicular helper to T follicular regulatory cells and thereby enhances LCMV-specific IgG production. Together, these findings identify a CD4 T cell-intrinsic role for GITR in sustaining early CD8 and late humoral responses to collectively promote control of chronic LCMV clone 13 infection.     

Intracerebral administration of recombinant rabies virus expressing GM-CSF prevents the development of rabies after infection with street virus

Recently it was found that prior immunization with recombinant rabies virus (RABV) expressing granulocyte-macrophage colony-stimulating factor (GM-CSF) (LBNSE-GM-CSF) resulted in high innate/adaptive immune responses and protection against challenge with virulent RABV (Wen et al., JVI, 2011). In this study, the ability of LBNSE-GM-CSF to prevent animals from developing rabies was investigated in mice after infection with lethal doses of street RABV. It was found that intracerebral administration of LBNSE-GM-CSF protected more mice from developing rabies than sham-treated mice as late as day 5 after infection with street RABV. Intracerebral administration of LBNSE-GM-CSF resulted in significantly higher levels of chemokine/cytokine expression and more infiltration of inflammatory and immune cells into the central nervous system (CNS) than sham-administration or administration with UV-inactivated LBNSE-GM-CSF. Enhancement of blood-brain barrier (BBB) permeability and increases in virus neutralizing antibodies (VNA) were also observed in mice treated with LBNSE-GM-CSF. On the other hand, intracerebral administration with UV-inactivated LBNSE-GM-CSF did not increase protection despite the fact that VNA were induced in the periphery. However, intracerebral administration with chemoattractant protein-1 (MCP-1, also termed CCL2) increased significantly the protective efficacy of UV-inactivated LBNSE-GM-CSF. Together these studies confirm that direct administration of LBNSE-GM-CSF can enhance the innate and adaptive immunity as well as the BBB permeability, thus allowing infiltration of inflammatory cells and other immune effectors enter into the CNS to clear the virus and prevent the development of rabies.     

IL-21R Signaling Suppresses IL-17+ Gamma Delta T Cell Responses and Production of IL-17 Related Cytokines in the Lung at Steady State and After Influenza A Virus Infection

Influenza A virus (IAV) infection of the respiratory tract elicits a robust immune response, which is required for efficient virus clearance but at the same time can contribute to lung damage and enhanced morbidity. IL-21 is a member of the type I cytokine family and has many different immune-modulatory functions during acute and chronic virus infections, although its role in IAV infection has not been fully evaluated. In this report we evaluated the contributions of IL-21/IL-21 receptor (IL-21R) signaling to host defense in a mouse model of primary IAV infection using IL-21R knock out (KO) mice. We found that lack of IL-21R signaling had no significant impact on virus clearance, adaptive T cell responses, or myeloid cell accumulations in the respiratory tract. However, a subset of inflammatory cytokines were elevated in the bronchoalveolar lavage fluid of IL-21R KO mice, including IL-17. Although there was only a small increase in Th17 cells in the lungs of IL-21R KO mice, we observed a dramatic increase in gamma delta (γδ) T cells capable of producing IL-17 both after IAV infection and at steady state in the respiratory tract. Finally, we found that IL-21R signaling suppressed the accumulation of IL-17⁺ γδ T cells in the respiratory tract intrinsically. Thus, our study reveals a previously unrecognized role of IL-21R signaling in regulating IL-17 production by γδ T cells.     

Induction and Role of Indoleamine 2,3 Dioxygenase in Mouse Models of Influenza A Virus Infection

Influenza infection stimulates protective host immune responses but paradoxically enhances lung indoleamine 2,3 dioxygenase (IDO) activity, an enzyme that suppresses helper/effector T cells and activates Foxp3-lineage regulatory CD4 T cells (Tregs). Influenza A/PR/8/34 (PR8) infection stimulated rapid elevation of IDO activity in lungs and lung-draining mediastinal lymph nodes (msLNs). Mice lacking intact IDO1 genes (IDO1-KO mice) exhibited significantly lower morbidity after sub-lethal PR8 infection, and genetic or pharmacologic IDO ablation led to much faster recovery after virus clearance. More robust influenza-specific effector CD8 T cell responses manifested in lungs of PR8-infected IDO1-KO mice, though virus clearance rates were unaffected by IDO ablation. Similar outcomes manifested in mice infected with a less virulent influenza A strain (X31). IDO induction in X31-infected lungs was dependent on IFN type II (IFNγ) signaling and was restricted to non-hematopoietic cells, while redundant IFN type 1 or type II signaling induced IDO exclusively in hematopoietic cells from msLNs. Memory T cells generated in X31-primed IDO1-KO mice protected mice from subsequent challenge with lethal doses of PR8 (100×LD₅₀). However recall T cell responses were less robust in lung interstitial tissues, and classic dominance of TCR Vβ8.3 chain usage amongst memory CD8⁺ T cells specific for influenza nucleoprotein (NP₃₆₆) did not manifest in IDO1-KO mice. Thus, influenza induced IDO activity in lungs enhanced morbidity, slowed recovery, restrained effector T cell responses in lungs and shaped memory T cell repertoire generation, but did not attenuate virus clearance during primary influenza A infection.     

The Role of Interferon in Influenza Virus Tissue Tropism

We have studied the pathogenesis of influenza virus infection in mice that are unable to respond to type I or II interferons due to a targeted disruption of the STAT1 gene. STAT1−/− animals are 100-fold more sensitive to lethal infection with influenza A/WSN/33 virus than are their wild-type (WT) counterparts. Virus replicated only in the lungs of WT animals following intranasal (i.n.) virus inoculation, while STAT1−/− mice developed a fulminant systemic influenza virus infection following either i.n. or intraperitoneal inoculation. We investigated the mechanism underlying this altered virus tropism by comparing levels of virus replication in fibroblast cell lines and murine embryonic fibroblasts derived from WT mice, STAT−/− mice, and mice lacking gamma interferon (IFNγ−/− mice) or the IFN-α receptor (IFNαR−/− mice). Influenza A/WSN/33 virus replicates to high titers in STAT1−/− or IFNαR−/− fibroblasts, while cells derived from WT or IFNγ−/− animals are resistant to influenza virus infection. Immunofluorescence studies using WT fibroblast cell lines demonstrated that only a small subpopulation of WT cells can be infected and that in the few infected WT cells, virus replication is aborted at an early, nuclear phase. In all organs examined except the lung, influenza A WSN/33 virus infection is apparently prevented by an intact type I interferon response. Our results demonstrate that type I interferon plays an important role in determining the pathogenicity and tissue restriction of influenza A/WSN/33 virus in vivo and in vitro.     

Pivotal role of antibody and subsidiary contribution of CD8+ T cells to recovery from infection in a murine model of Japanese encephalitis

The immunological correlates for recovery from primary Japanese encephalitis virus (JEV) infection in humans and experimental animals remain poorly defined. To investigate the relative importance of the adaptive immune responses, we have established a mouse model for Japanese encephalitis in which a low-dose virus inoculum was administered into the footpads of adult C57BL/6 mice. In this model, ~60% of the mice developed a fatal encephalitis and a virus burden in the central nervous system (CNS). Using mice lacking B cells (μMT(-/-) mice) and immune B cell transfer to wild-type mice, we show a critically important role for humoral immunity in preventing virus spread to the CNS. T cell help played an essential part in the maintenance of an effective antibody response necessary to combat the infection, since mice lacking major histocompatibility complex class II showed truncated IgM and blunted IgG responses and uniformly high lethality. JEV infection resulted in extensive CD8(+) T cell activation, judged by upregulation of surface markers CD69 and CD25 and cytokine production after stimulation with a JEV NS4B protein-derived H-2D(b)-binding peptide and trafficking of virus-immune CD8(+) T cells into the CNS. However, no significant effect of CD8(+) T cells on the survival phenotype was found, which was corroborated in knockout mice lacking key effector molecules (Fas receptor, perforin, or granzymes) of cytolytic pathways triggered by T lymphocytes. Accordingly, CD8(+) T cells are mostly dispensable for recovery from infection with JEV. This finding highlights the conflicting role that CD8(+) T cells play in the pathogenesis of JEV and closely related encephalitic flaviviruses such as West Nile virus.     

IL-1β Signaling Promotes CNS-Intrinsic Immune Control of West Nile Virus Infection

West Nile virus (WNV) is an emerging flavivirus capable of infecting the central nervous system (CNS) and mediating neuronal cell death and tissue destruction. The processes that promote inflammation and encephalitis within the CNS are important for control of WNV disease but, how inflammatory signaling pathways operate to control CNS infection is not defined. Here, we identify IL-1β signaling and the NLRP3 inflammasome as key host restriction factors involved in viral control and CNS disease associated with WNV infection. Individuals presenting with acute WNV infection displayed elevated levels of IL-1β in their plasma over the course of infection, suggesting a role for IL-1β in WNV immunity. Indeed, we found that in a mouse model of infection, WNV induced the acute production of IL-1β in vivo, and that animals lacking the IL-1 receptor or components involved in inflammasome signaling complex exhibited increased susceptibility to WNV pathogenesis. This outcome associated with increased accumulation of virus within the CNS but not peripheral tissues and was further associated with altered kinetics and magnitude of inflammation, reduced quality of the effector CD8⁺ T cell response and reduced anti-viral activity within the CNS. Importantly, we found that WNV infection triggers production of IL-1β from cortical neurons. Furthermore, we found that IL-1β signaling synergizes with type I IFN to suppress WNV replication in neurons, thus implicating antiviral activity of IL-1β within neurons and control of virus replication within the CNS. Our studies thus define the NLRP3 inflammasome pathway and IL-1β signaling as key features controlling WNV infection and immunity in the CNS, and reveal a novel role for IL-1β in antiviral action that restricts virus replication in neurons.     

Invariant NKT Cells Regulate the CD8 T Cell Response during Theiler's Virus Infection

Invariant NKT cells are innate lymphocytes with a broad tissue distribution. Here we demonstrate that iNKT cells reside in the central nervous system (CNS) in the absence of inflammation. Their presence in the CNS dramatically augments following inoculation of C57Bl/6 mice with the neurotropic Theiler''s murine encephalomyelitis virus (TMEV). At the peak of inflammation the cellular infiltrate comprises 45 000 iNKT cells for 1 250 CD8 T cells specific for the immunodominant TMEV epitope. To study the interaction between these two T cell subsets, we infected both iNKT cell deficient Jα18^-/- mice and iNKT cell enriched Vα14 transgenic mice with TMEV. The CD8 T cell response readily cleared TMEV infection in the iNKT cell deficient mice. However, in the iNKT cell enriched mice TMEV infection persisted and was associated with significant mortality. This was caused by the inhibition of the CD8 T cell response in the cervical lymph nodes and spleen after T cell priming. Taken together we demonstrate that iNKT cells reside in the CNS in the absence of inflammation and that their enrichment is associated with the inhibition of the anti-viral CD8 T cell response and an augmented mortality during acute encephalomyelitis.