Neuroimmunology of central nervous system viral infections: the cells, molecules and mechanisms involved

Chemokine-mediated cell migration into the central nervous system in progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) has been associated with different forms of immune compromise. This study analyzes the chemokine signals and attracted immune cells in cerebrospinal fluid (CSF) during PML to define immune cell subpopulations relevant for the PML immune response. In addition to chemokines that indicate a general state of inflammation, like CCL5 and CXCL10, the CSF of PML patients specifically contains CCL2 and CCL4. Single-cell transcriptomics of CSF cells suggests an enrichment of distinct CD4+ and CD8+ T cells expressing chemokine receptors CCR2, CCR5, and CXCR3, in addition to ITGA4 and the genetic PML risk genes STXBP2 and LY9. This suggests that specific immune cell subpopulations migrate into the central nervous system to mitigate PML, and their absence might coincide with PML development. Monitoring them might hold clues for PML risk, and boosting their recruitment or function before therapeutic immune reconstitution might improve its risk-benefit ratio.

New insight into DAVF pathology—Clues from meningeal immunity

In recent years, with the current access in techniques, studies have significantly advanced the knowledge on meningeal immunity, revealing that the central nervous system (CNS) border acts as an immune landscape. The latest concept of meningeal immune system is a tertiary structure, which is a comprehensive overview of the meningeal immune system from macro to micro. We comprehensively reviewed recent advances in meningeal immunity, particularly the new understanding of the dural sinus and meningeal lymphatics. Moreover, based on the clues from the meningeal immunity, new insights were proposed into the dural arteriovenous fistula (DAVF) pathology, aiming to provide novel ideas for DAVF understanding.

Genes conferring immunity against viral infections

Immunity against viral infection is different from that for other infections. The virus utilizes the host’s immune mechanism for its survival and infection. Innate immune reaction is initially activated by conserved pathogen-associated molecular patterns, pattern recognition receptors, retinoic acid-inducible gene I, MDA5, LGP2, and toll-like receptors (TLRs) such as TLR3 and TLR7. Some important molecules responsible for viral infection include cytokines and chemokines. The current chapter includes the basic mechanism of action of immune-response genes and a list of important genes and their protein IDs. Chemokines include CCL19 and CCL21, and reactivate antiviral CCR7, monocyte chemoattractant protein 1, a CC chemokine, and interleukin 8, an interferon-stimulated gene.

New advances in CNS immunity against viral infection

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Advances in CNS immunity and anatomy bridge the CNS and the peripheral immune system.

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Region-specific antiviral responses alter BBB permeability during viral invasion.

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CNS barriers have anatomical specializations with tailored defenses against pathogens.

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Immunocytotherapies for persistent CNS infections can promote non-cytopathic viral clearance.

The central nervous system (CNS) is an immunologically specialized organ where restrictive barrier structures protect the parenchyma from inflammation and infection. This protection is important in preventing damage to non-renewable resident cell populations, such as neurons, responsible for functions ranging from executive to autonomic. Despite these barriers, the CNS can be infected through several entry portals, giving rise to meningitis and encephalitis. Following infection, resident cells recruit peripherally derived immune cells to sites of viral infection. In this review, we discuss recent advances in immune recruitment and entry at barrier structures as well as current immunotherapeutic strategies for the treatment of persistent viral infections.

Interferon regulated gene (IRG) expression-signature in a mouse model of chikungunya virus neurovirulence

Interferon regulated genes (IRGs) are critical in controlling virus infections. Here, we analyzed the expression profile of IRGs in the brain tissue in a mouse model of chikungunya virus (CHIKV) neurovirulence. Neurovirulence is one of the newer complications identified in disease caused by re-emerging strains of CHIKV, an alphavirus with positive-strand RNA in the Togaviridae family. In microarray analysis, we identified significant upregulation of 269 genes, out of which a predominant percentage (76%) was IRGs. The highly modulated IRGs included Ifit1, Ifi44, Ddx60, Usp18, Stat1, Rtp4, Mnda, Gbp3, Gbp4, Gbp7, Oasl2, Oas1g, Ly6a, Igtp, and Gbp10, along with many others exhibiting lesser changes in expression levels. We found that these IRG mRNA transcripts are modulated in parallel across CHIKV-infected mouse brain tissues, human neuronal cell line IMR-32 and hepatic cell line Huh-7. The genes identified to be highly modulated both in mouse brain and human neuronal cells were Ifit1, Ifi44, Ddx60, Usp18, and Mnda. In Huh-7 cells, however, only two IRGs (Gbp4 and Gbp7) showed a similar level of upregulation. Concordant modulation of IRGs in both mice and human cells indicates that they might play important roles in regulating CHIKV replication in the central nervous system (CNS). The induction of several IRGs in CNS during infection underscores the robustness of IRG-mediated innate immune response in CHIKV restriction. Further studies on these IRGs would help in evolving possibilities for their targeting in host-directed therapeutic interventions against CHIKV.

Depletion of FoxP3+ Tregs improves control of larval Echinococcus multilocularis infection by promoting co-stimulation and Th1/17 immunity

Introduction

The growth potential of the tumor‐like Echinococcus multilocularis metacestode (causing alveolar echinococcosis, AE) is directly linked to the nature/function of the periparasitic host immune‐mediated processes. Previous studies had shown that regulatory T cells (Tregs) become gradually up‐regulated in the course of both chronic human and murine AE. Thus we now tackled the role of FoxP3⁺ Tregs and FoxP3⁺‐Treg‐regulated immune response in contributing to the control of this helminthic infection.

Methods

The infection outcome in E. multilocularis‐infected DEREG mice was measured upon determining parasite load (wet weight of parasitic metacestode tissue). Flow cytometry and qRT‐PCR were used to assess Treg, Th17‐, Th1‐, Th2‐type immune responses and antigen presenting cell activation.

Results

We showed that E. multilocularis‐infected DEREG‐mice treated with DT (as compared to infected control DEREG‐mice without DT application) exhibited a significantly lower parasite load, associated with a persisting capacity of co‐stimulation, and an increased Th1/Th17‐polarization.

Conclusions

FoxP3⁺ Tregs appear as one of the key players in immune regulatory processes favoring (i) metacestode survival by inhibiting the maturation potential of co‐stimulatory activity and (ii) T cell exhaustion (suppressing Th1/Th17‐type immune responses). We showed as well that prospectively, targeting FoxP3⁺ Tregs could be an option to develop an immunotherapy against AE.

Interferon (IFN)-induced protein 35 (IFI35) negatively regulates IFN-β-phosphorylated STAT1-RIG-I-CXCL10/CCL5 axis in U373MG astrocytoma cells treated with polyinosinic-polycytidylic acid

Interferon (IFN)-stimulated genes (ISGs) exert multiple functions in immune system. IFN-induced protein 35 (IFI35) is a member of ISGs, and has been suggested to regulate innate immune reaction. However, the physiological functions and pathological roles of IFI35 in the central nervous system are not characterized well. In the present study, we found that the expression of IFI35 was induced by a Toll-like receptor 3 (TLR3) ligand polyinosinic-polycytidylic acid (poly IC) in U373MG human astrocytoma cells. Knockdown of IFI35 using RNA interference resulted in increased expression of IFN-β, phosphorylated STAT1 (P-STAT1), retinoic acid-inducible gene-I (RIG-I), CXCL10 and CCL5 induced by poly IC. Poly IC-induced expression of CXCL10 and CCL5 was decreased by knockdown of RIG-I. These results suggest that IFI35 may negatively regulate the TLR3-IFN-β-P-STAT1-RIG-I-CXCL10/CCL5 axis in U373MG cells, and IFI35 may play a role at least partially in the regulation of innate immune reactions in astrocytes.

Interferon gamma modulation of disease manifestation and the local antibody response to alphavirus encephalomyelitis

Infection of mice with Sindbis virus (SINV) produces encephalomyelitis and provides a model for examination of the central nervous system (CNS) immune response to alphavirus infection. Clearance of infectious virus is accomplished through a cooperative effort between SINV-specific antibody and IFN-γ, but the regulatory interactions are poorly understood. To determine the effects of IFN-γ on clinical disease and the antiviral immune response, C57BL/6 mice lacking IFN-γ (Ifng-/-) or IFN-γ receptor (Ifngr1-/-) were studied in comparison to WT mice. Maximum production of Ifng mRNA and IFN-γ protein in the CNS of WT and Ifngr1-/- mice occurred 5-7 days after infection, with higher levels of IFN-γ in Ifngr1-/- mice. Onset of clinical disease was earlier in mice with impaired IFN-γ signalling, although Ifngr1-/- mice recovered more rapidly. Ifng-/- and Ifngr1-/- mice maintained body weight better than WT mice, associated with better food intake and lower brain levels of inflammatory cytokines. Clearance of infectious virus from the spinal cords was slower, and CNS, but not serum, levels of SINV-specific IgM, IgG2a and IgG2b were lower in Ifngr1-/- and Ifng-/- mice compared to WT mice. Decreased CNS antiviral antibody was associated with lower expression of mRNAs for B-cell attracting chemokines CXCL9, CXCL10 and CXCL13 and fewer B cells in the CNS. Therefore, IFN-γ signalling increases levels of CNS pro-inflammatory cytokines, leading to clinical disease, but synergistically clears virus with SINV-specific antibody at least in part by increasing chemokine production important for infiltration of antibody-secreting B cells into the CNS.

Immune responses to non-tumor antigens in the central nervous system

The central nervous system (CNS), once viewed as an immune-privileged site protected by the blood-brain barrier (BBB), is now known to be a dynamic immunological environment through which immune cells migrate to prevent and respond to events such as localized infection. During these responses, endogenous glial cells, including astrocytes and microglia, become highly reactive and may secrete inflammatory mediators that regulate BBB permeability and recruit additional circulating immune cells. Here, we discuss the various roles played by astrocytes, microglia, and infiltrating immune cells during host immunity to non-tumor antigens in the CNS, focusing first on bacterial and viral infections, and then turning to responses directed against self-antigens in the setting of CNS autoimmunity.

Protein kinase C activation mediates interferon-β-induced neuronal excitability changes in neocortical pyramidal neurons

BACKGROUND

Cytokines are key players in the interactions of the immune and nervous systems. Recently, we showed that such interplay is mediated by type I interferons (IFNs), which elevate the excitability of neocortical pyramidal neurons. A line of indirect evidence suggested that modulation of multiple ion channels underlies the effect. However, which currents are principally involved and how the IFN signaling cascade is linked to the respective ion channels remains elusive.

METHODS

We tested several single and combined ionic current modulations using an in silico model of a neocortical layer 5 neuron. Subsequently we investigated resulting predictions by whole-cell patch-clamp recordings in layer 5 neurons of ex vivo neocortical rat brain slices pharmacologically reproducing or prohibiting neuronal IFN effects.

RESULTS

The amount and type of modulation necessary to replicate IFN effects in silico suggested protein kinase C (PKC) activation as link between the type I IFN signaling and ion channel modulations. In line with this, PKC activation with 4β-phorbol 12-myristate 13-acetate (4β-PMA) or Bryostatin1 augmented the excitability of neocortical layer 5 neurons comparable to IFN-β in our ex vivo recordings. In detail, both PKC activators attenuated the rheobase and increased the input-output gain as well as the input resistance, thereby augmenting the neuronal excitability. Similar to IFN-β they also left the threshold of action potential generation unaffected. In further support of PKC mediating type I IFN effects, IFN-β, 4β-PMA and Bryostatin1 reduced the amplitude of post-train after-hyperpolarizations in a similar manner. In conjunction with this finding, IFN-β reduced M-currents, which contribute to after-hyperpolarizations and are modulated by PKC. Finally, blocking PKC activation with GF109203X at the catalytic site or calphostin C at the regulatory site prevented the main excitatory effects of IFN-β.

CONCLUSION

Multiple ion channel modulations underlie the neuromodulatory effect of type I IFNs. PKC activation is both sufficient and necessary for mediating the effect, and links the IFN signaling cascade to the intrinsic ion channels. Therefore, we regard PKC activation as unitary mechanism for the neuromodulatory potential of type I IFNs in neocortical neurons.

HHV-7 in adults with drug-resistant epilepsy: a pathological role in hippocampal sclerosis?

BACKGROUND

Human herpesvirus-7 (HHV-7) is a β-herpesvirus associated with febrile seizures. No association between HHV-7 and epilepsy has been confirmed.

OBJECTIVES

The aim of this study was to investigate the presence of HHV-7 protein (KR4) in brain tissue from patients with drug-resistant epilepsy and to determine whether inflammatory molecules are activated in the presence of HHV-7 infection.

STUDY DESIGN

We used immunohistochemistry (IHC) to detect HHV-7 protein KR4 in samples from 305 patients with drug-resistant epilepsy. Liquid nitrogen-preserved hippocampal sclerosis (HS) samples from 63 of these patients were available, and we used nested polymerase chain reaction (PCR) to detect HHV-7 DNA. Inflammatory molecules including tumor necrosis factor-alpha (TNF-α), transforming growth factor-beta (TGF-β), interleukin-1 (IL-1) and interleukin-6 (IL-6) were identified by real-time PCR (rt-PCR) and IHC.

RESULTS AND CONCLUSIONS

The study sample included 201 male subjects. The mean age was 23.9, SD 6.2 years (range 15-45). HS was the pathology in 69 samples (23%). The HHV-7 protein was detected in 27 (9%) of the 305 samples and in none of the 42 controls. The factors associated with HHV-7 infection were HS (11/69), glial scar (8/58), arachnoid cyst (2/21), focal cortical dysplasia (2/31) and vascular malformation (4/52). HHV-7 antigen was distributed mainly in the cytoplasm of astrocyte and oligodendrocyte in HS samples. HHV-7 DNA was detected in 20 of the 63 nitrogen preserved HS samples. The expression of TGF-β was up-regulated in samples that were positive for the HHV-7 protein and was mainly detected in neurons. This finding suggests a possible association between HHV-7 positivity, activation of TGF-β and drug-resistant epilepsy, especially HS, but these data need to be replicated.

RIG-I knockdown impedes neurogenesis in a murine model of Japanese encephalitis

Retinoic acid inducible gene I (RIG-I) is a well established pattern recognition receptor (PRR) in neurons infected with Japanese encephalitis virus (JEV) as reported previously from our laboratory. Japanese encephalitis (JE) virus infection in brain has been shown to decrease the proliferation of neural stem/progenitor cells (NSPCs) which has its implications in neurological sequelae in JE survivors. We have found that ablation of RIG-I both in vivo and in vitro models results in significant decrease in NSPC proliferation post JEV infection. We hypothesize that knockdown of RIG-I diminishes the expression of antiviral molecules resulting in an increase in viral replication, which in turn results in enhancement of the expression of cell cycle inhibitors, hence affecting the proliferation of NSPCs.

Roles of TLR3 and RIG-I in mediating the inflammatory response in mouse microglia following Japanese encephalitis virus infection

Japanese encephalitis virus (JEV) infection can cause central nervous system disease with irreversible neurological damage in humans and animals. Evidence suggests that overactivation of microglia leads to greatly increased neuronal damage during JEV infection. However, the mechanism by which JEV induces the activation of microglia remains unclear. Toll-like receptor 3 (TLR3) and retinoic acid-inducible gene I (RIG-I) can recognize double-stranded RNA, and their downstream signaling results in production of proinflammatory mediators. In this study, we investigated the roles of TLR3 and RIG-I in the inflammatory response caused by JEV infection in the mouse microglial cell line. JEV infection induced the expression of TLR3 and RIG-I and the activation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38MAPK). Knockdown of TLR3 and RIG-I attenuated activation of ERK, p38MAPK, activator protein 1 (AP-1), and nuclear factor κB (NF-κB). Secretion of TNF-α, IL-6, and CCL-2, which was induced by JEV, was reduced by TLR3 and RIG-I knockdown and inhibitors of phosphorylated ERK and p38MAPK. Furthermore, viral proliferation was increased following knockdown of TLR3 and RIG-I. Our findings suggest that the signaling pathways of TLR3 and RIG-I play important roles in the JEV-induced inflammatory response of microglia.

Rift valley Fever virus encephalitis is associated with an ineffective systemic immune response and activated T cell infiltration into the CNS in an immunocompetent mouse model

Background

Rift Valley fever virus (RVFV) causes outbreaks of severe disease in livestock and humans throughout Africa and the Arabian Peninsula. In people, RVFV generally causes a self-limiting febrile illness but in a subset of individuals, it progresses to more serious disease. One manifestation is a delayed-onset encephalitis that can be fatal or leave the afflicted with long-term neurologic sequelae. In order to design targeted interventions, the basic pathogenesis of RVFV encephalitis must be better understood.

Methodology/Principal Findings

To characterize the host immune responses and viral kinetics associated with fatal and nonfatal infections, mice were infected with an attenuated RVFV lacking NSs (ΔNSs) that causes lethal disease only when administered intranasally (IN). Following IN infection, C57BL/6 mice developed severe neurologic disease and succumbed 7–9 days post-infection. In contrast, inoculation of ΔNSs virus subcutaneously in the footpad (FP) resulted in a subclinical infection characterized by a robust immune response with rapid antibody production and strong T cell responses. IN-inoculated mice had delayed antibody responses and failed to clear virus from the periphery. Severe neurological signs and obtundation characterized end stage-disease in IN-inoculated mice, and within the CNS, the development of peak virus RNA loads coincided with strong proinflammatory responses and infiltration of activated T cells. Interestingly, depletion of T cells did not significantly alter survival, suggesting that neurologic disease is not a by-product of an aberrant immune response.

Conclusions/Significance

Comparison of fatal (IN-inoculated) and nonfatal (FP-inoculated) ΔNSs RVFV infections in the mouse model highlighted the role of the host immune response in controlling viral replication and therefore determining clinical outcome. There was no evidence to suggest that neurologic disease is immune-mediated in RVFV infection. These results provide important insights for the future design of vaccines and therapeutic options.

Rift Valley fever virus (RVFV) is a mosquito-borne virus that causes severe disease in people and livestock throughout Africa and the Arabian Peninsula. Human disease is usually self-limiting, but a small proportion of individuals develop fatal encephalitis. The role of the host immune response in determining disease outcome is largely unknown. In order to compare the quality and character of immune responses in nonfatal and fatal cases, we used an attenuated RVFV to inoculate mice by two routes. Subcutaneous inoculation resulted in a subclinical systemic infection that was rapidly cleared due to a robust adaptive response. In contrast, intranasal inoculation stimulated weaker immune responses that failed to control virus replication and culminated in uniformly fatal encephalitis. With many encephalitic viruses, the onset of disease is mediated by changes in blood brain barrier permeability and often, subsequent injury to the CNS by an uncontrolled immune response. However, our results suggest that development of RVFV disease does not depend on either mechanism, but rather results from direct virus-mediated damage in the CNS. Future therapeutic drug design should take into account all possible routes of virus exposure as well as the role of therapies that boost the adaptive response to better combat disease.

Integrated expression profiles of mRNA and miRNA in polarized primary murine microglia

Neuroinflammation contributes to many neurologic disorders including Alzheimer’s disease, multiple sclerosis, and stroke. Microglia is brain resident myeloid cells and have emerged as a key driver of the neuroinflammatory responses. MicroRNAs (miRNAs) provide a novel layer of gene regulation and play a critical role in regulating the inflammatory response of peripheral macrophages. However, little is known about the miRNA in inflammatory activation of microglia. To elucidate the role that miRNAs have on microglial phenotypes under classical (M1) or alternative (M2) activation under lipopolysaccharide (‘M1’-skewing) and interleukin-4 (‘M2a’-skewing) stimulation conditions, we performed microarray expression profiling and bioinformatics analysis of both mRNA and miRNA using primary cultured murine microglia. miR-689, miR-124, and miR-155 were the most strongly associated miRNAs predicted to mediate pro-inflammatory pathways and M1-like activation phenotype. miR-155, the most strongly up-regulated miRNA, regulates the signal transducer and activator of transcription 3 signaling pathway enabling the late phase response to M1-skewing stimulation. Reduced expression in miR-689 and miR-124 are associated with dis-inhibition of many canonical inflammatory pathways. miR-124, miR-711, miR-145 are the strongly associated miRNAs predicted to mediate anti-inflammatory pathways and M2-like activation phenotype. Reductions in miR-711 and miR-124 may regulate inflammatory signaling pathways and peroxisome proliferator-activated receptor-gamma pathway. miR-145 potentially regulate peripheral monocyte/macrophage differentiation and faciliate the M2-skewing phenotype. Overall, through combined miRNA and mRNA expression profiling and bioinformatics analysis we have identified six miRNAs and their putative roles in M1 and M2-skewing of microglial activation through different signaling pathways.

Induction of cytopathogenicity in human glioblastoma cells by chikungunya virus

Chikungunya virus (CHIKV), an arthritogenic old-world alphavirus, has been implicated in the central nervous system (CNS) infection in infants and elderly patients. Astrocytes are the major immune cells of the brain parenchyma that mediate inflammation. In the present study we found that a local isolate of CHIKV infect and activate U-87 MG cells, a glioblastoma cell line of human astrocyte origin. The infection kinetics were similar in infected U-87 MG cells and the human embryo kidney (HEK293) cells as indicated by immunofluorescence and plaque assays, 24h post-infection (p.i.). In infected U-87 MG cells, apoptosis was detectable from 48h p.i. evidenced by DNA fragmentation, PARP cleavage, loss of mitochondrial membrane potential, nuclear condensation and visible cytopathic effects in a dose and time-dependent manner. XBP1 mRNA splicing and eIF2α phosphorylation studies indicated the occurrence of endoplasmic reticulum stress in infected cells. In U-87 MG cells stably expressing a green fluorescent protein-tagged light chain-3 (GFP-LC3) protein, CHIKV infection showed increased autophagy response. The infection led to an enhanced expression of the mRNA transcripts of the pro-inflammatory cytokines IL-1β, TNF-α, IL-6 and CXCL9 within 24h p.i. Significant up-regulation of the proteins of RIG-I like receptor (RLR) pathway, such as RIG-I and TRAF-6, was observed indicating the activation of the cytoplasmic-cellular innate immune response. The overall results show that the U-87 MG cell line is a potential in vitro model for in depth study of these molecular pathways in response to CHIKV infection. The responses in these cells of CNS origin, which are inherently defective in Type I interferon response, could be analogous to that occurring in infants and very old patients who also have a compromised interferon-response. The results also point to the intriguing possibility of using this virus for studies to develop oncolytic virus therapy approaches against glioblastoma, a highly aggressive malignancy.

Resistance to oncolytic myxoma virus therapy in nf1(-/-)/trp53(-/-) syngeneic mouse glioma models is independent of anti-viral type-I interferon

Despite promising preclinical studies, oncolytic viral therapy for malignant gliomas has resulted in variable, but underwhelming results in clinical evaluations. Of concern are the low levels of tumour infection and viral replication within the tumour. This discrepancy between the laboratory and the clinic could result from the disparity of xenograft versus syngeneic models in determining in vivo viral infection, replication and treatment efficacy. Here we describe a panel of primary mouse glioma lines derived from Nf1 (+/-) Trp53 (+/-) mice in the C57Bl/6J background for use in the preclinical testing of the oncolytic virus Myxoma (MYXV). These lines show a range of susceptibility to MYXV replication in vitro, but all succumb to viral-mediated cell death. Two of these lines orthotopically grafted produced aggressive gliomas. Intracranial injection of MYXV failed to result in sustained viral replication or treatment efficacy, with minimal tumour infection that was completely resolved by 7 days post-infection. We hypothesized that the stromal production of Type-I interferons (IFNα/β) could explain the resistance seen in these models; however, we found that neither the cell lines in vitro nor the tumours in vivo produce any IFNα/β in response to MYXV infection. To confirm IFNα/β did not play a role in this resistance, we ablated the ability of tumours to respond to IFNα/β via IRF9 knockdown, and generated identical results. Our studies demonstrate that these syngeneic cell lines are relevant preclinical models for testing experimental glioma treatments, and show that IFNα/β is not responsible for the MYXV treatment resistance seen in syngeneic glioma models.

Differential chemokine responses in the murine brain following lyssavirus infection

The hallmark of lyssavirus infection is lethal encephalomyelitis. Previous studies have reported distinct lyssavirus isolate-related differences in severity of cellular recruitment into the encephalon in a murine model of infection following peripheral inoculation with rabies virus (RABV) and European bat lyssavirus (EBLV)-1 and -2. In order to understand the role of chemokines in this process, comparative studies of the chemokine pattern, distribution and production in response to infection with these lyssaviruses were undertaken. Expression of CCL2, CCL5 and CXCL10 was observed throughout the murine brain with a distinct caudal bias in distribution, similar to both inflammatory changes and virus antigen distribution. CCL2 immunolabelling was localized to neuronal and astroglial populations. CCL5 immunolabelling was only detected in the astroglia, while CXCL10 labelling, although present in the astroglia, was more prominent in neurons. Isolate-dependent differences in the amount of chemokine immunolabelling in specific brain regions and chemokine production by neurons in vitro were observed, with a greater expression of CCL5 in vivo and CXCL10 production in vitro after EBLV infection. Additionally, strong positive associations between chemokine immunolabelling and perivascular cuffing and, to a lesser extent, virus antigen score were also observed. These differences in chemokine expression may explain the variation in severity of encephalitic changes observed in animals infected with different lyssavirus isolates.

NK cells impede glioblastoma virotherapy through NKp30 and NKp46 natural cytotoxicity receptors

The role of the immune response to oncolytic Herpes simplex viral (oHSV) therapy for glioblastoma is controversial because it might enhance or inhibit efficacy. We found that within hours of oHSV infection of glioblastomas in mice, activated natural killer (NK) cells are recruited to the site of infection. This response substantially diminished the efficacy of glioblastoma virotherapy. oHSV-activated NK cells coordinated macrophage and microglia activation within tumors. In vitro, human NK cells preferentially lysed oHSV-infected human glioblastoma cell lines. This enhanced killing depended on the NK cell natural cytotoxicity receptors (NCRs) NKp30 and NKp46, whose ligands are upregulated in oHSV-infected glioblastoma cells. We found that HSV titers and oHSV efficacy are increased in Ncr1(-/-) mice and a Ncr1(-/-) NK cell adoptive transfer model of glioma, respectively. These results demonstrate that glioblastoma virotherapy is limited partially by an antiviral NK cell response involving specific NCRs, uncovering new potential targets to enhance cancer virotherapy.

Clearance of virus infection from the CNS

Viruses that cause encephalomyelitis infect neurons and recovery from infection requires noncytolytic clearance of virus from the nervous system to avoid damaging these irreplaceable cells. Several murine model systems of virus infection have been used to identify clearance mechanisms. Quantitative analysis of Sindbis virus clearance over 6 months shows three phases: day 5-7, clearance of infectious virus, but continued presence of viral RNA; day 8-60, decreasing levels of viral RNA; day 60-180, maintenance of viral RNA at low levels. Antiviral antibody and interferon-γ have major roles in clearance with a likely role for IgM as well as IgG antibody. Long-term residence of virus-specific immune cells in the nervous system is necessary to prevent virus reactivation.

Hippocampal protection in mice with an attenuated inflammatory monocyte response to acute CNS picornavirus infection

Neuronal injury during acute viral infection of the brain is associated with the development of persistent cognitive deficits and seizures in humans. In C57BL/6 mice acutely infected with the Theiler's murine encephalomyelitis virus, hippocampal CA1 neurons are injured by a rapid innate immune response, resulting in profound memory deficits. In contrast, infected SJL and B6xSJL F1 hybrid mice exhibit essentially complete hippocampal and memory preservation. Analysis of brain-infiltrating leukocytes revealed that SJL mice mount a sharply attenuated inflammatory monocyte response as compared to B6 mice. Bone marrow transplantation experiments isolated the attenuation to the SJL immune system. Adoptive transfer of B6 inflammatory monocytes into acutely infected B6xSJL hosts converted these mice to a hippocampal damage phenotype and induced a cognitive deficit marked by failure to recognize a novel object. These findings show that inflammatory monocytes are the critical cellular mediator of hippocampal injury during acute picornavirus infection of the brain.

Viral CNS infections: role of glial pattern recognition receptors in neuroinflammation

Viruses are the major causative agents of central nervous system (CNS) infection worldwide. RNA and DNA viruses trigger broad activation of glial cells including microglia and astrocytes, eliciting the release of an array of mediators that can promote innate and adaptive immune responses. Such responses can limit viral replication and dissemination leading to infection resolution. However, a defining feature of viral CNS infection is the rapid onset of severe neuroinflammation and overzealous glial responses are associated with significant neurological damage or even death. The mechanisms by which microglia and astrocytes perceive neurotropic RNA and DNA viruses are only now becoming apparent with the discovery of a variety of cell surface and cytosolic molecules that serve as sensors for viral components. In this review we discuss the role played by members of the Toll-like family of pattern recognition receptors (PRRs) in the inflammatory responses of glial cells to the principle causative agents of viral encephalitis. Importantly, we also describe the evidence for the involvement of a number of newly described intracellular PRRs, including retinoic acid-inducible gene I and DNA-dependent activator of IFN regulatory factors, that are thought to function as intracellular sensors of RNA and DNA viruses, respectively. Finally, we explore the possibility that cross-talk exists between these disparate viral sensors and their signaling pathways, and describe how glial cytosolic and cell surface/endosomal PRRs could act in a cooperative manner to promote the fulminant inflammation associated with acute neurotropic viral infection.

Encephalitis due to emerging viruses: CNS innate immunity and potential therapeutic targets

The emerging viruses represent a group of pathogens that are intimately connected to a diverse range of animal vectors. The recent escalation of air travel climate change and urbanization has meant humans will have increased risk of contacting these pathogens resulting in serious CNS infections. Many RNA viruses enter the CNS by evading the BBB due to axonal transport from the periphery. The systemic adaptive and CNS innate immune systems express pattern recognition receptors PRR (TLRs, RiG-1 and MDA-5) that detect viral nucleic acids and initiate host antiviral response. However, several emerging viruses (West Nile Fever, Influenza A, Enterovirus 71 Ebola) are recognized and internalized by host cell receptors (TLR, MMR, DC-SIGN, CD162 and Scavenger receptor B) and escape immuno surveillance by the host systemic and innate immune systems. Many RNA viruses express viral proteins WNF (E protein), Influenza A (NS1), EV71 (protein 3C), Rabies (Glycoprotein), Ebola proteins (VP24 and VP 35) that inhibit the host cell anti-virus Interferon type I response promoting virus replication and encephalitis. The therapeutic use of RNA interference methodologies to silence gene expression of viral peptides and treat emerging virus infection of the CNS is discussed.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Tregs and infections: on the potential value of modifying their function

CD4(+) T cells, which express a master transcription factor, Foxp3, have been recognized as bona fide Tregs. These cells are essential to maintain immune homeostasis in healthy as well as infected mice and humans. Extensive investigations in the last decade have provided ways to manipulate the Foxp3(+) Treg response therapeutically so the role of such cells in microbe-induced inflammatory reactions can be evaluated. This review focuses on our current understanding of the mechanisms required for the generation and sustenance of Tregs in vivo and the potential value of modulating Tregs to control microbe-induced immunopathological responses.

RIG-I mediates innate immune response in mouse neurons following Japanese encephalitis virus infection

Background

Neuroinflammation associated with Japanese encephalitis (JE) is mainly due to the activation of glial cells with subsequent release of proinflammatory mediators from them. The recognition of viral RNA, in part, by the pattern recognition receptor retinoic acid-inducible gene I (RIG-I) has been indicated to have a role in such processes. Even though neurons are also known to express this receptor, its role after JE virus (JEV) infections is yet to be elucidated.

Methodology/Principal Findings

Upon infecting murine neuroblastoma cells and primary cortical neurons with JEV the expression profile of key proinflammatory cyto/chemokines were analyzed by qRT-PCR and bead array, both before and after ablation of RIG-I. Immunoblotting was performed to evaluate the levels of key molecules downstream to RIG-I leading to production of proinflammatory mediators. Changes in the intracellular viral antigen expression were confirmed by intracellular staining and immunoblotting. JEV infection induced neuronal expression of IL-6, IL-12p70, MCP-1, IP-10 and TNF-α in a time-dependent manner, which showed significant reduction upon RIG-I ablation. Molecules downstream to RIG-I showed significant changes upon JEV-infection, that were modulated following RIG-I ablation. Ablation of RIG-I in neurons also increased their susceptibility to JEV.

Conclusions/Significance

In this study we propose that neurons are one of the potential sources of proinflammatory cyto/chemokines in JEV-infected brain that are produced via RIG-I dependent pathways. Ablation of RIG-I in neurons leads to increased viral load and reduced release of the cyto/chemokines.

T cell-, interleukin-12-, and gamma interferon-driven viral clearance in measles virus-infected brain tissue

Genetic studies with immunocompetent mice show the importance of both T cells and gamma interferon (IFN-γ) for survival of a measles virus (MV) challenge; however, the direct role of T cells and IFN-γ within the MV-infected brain has not been addressed. Organotypic brain explants represent a successful ex vivo system to define central nervous system (CNS)-specific mechanisms of leukocyte migration, activation, and MV clearance. Within the heterogeneous, brain-derived, primed leukocyte population which reduced MV RNA levels in brain explants by 60%, CD3 T cells are the active antiviral cells, as purified CD3-positive cells are highly antiviral and CD3-negative leukocytes are unable to reduce the viral load. Neutralization of CCL5 and CXCL10 decreases leukocyte migration to areas of infection by 70%. However, despite chemokines directing the migration of T cells to infected neurons, chemokine neutralization revealed that migration is not required for viral clearance, suggesting a cytokine-mediated antiviral mechanism. In accordance with our hypothesis, the ability of leukocytes to clear the virus is abrogated when explants are treated with anti-IFN-γ neutralizing antibodies. IFN-γ applied to infected slices in the absence of primed leukocytes reduces the viral load by more than 80%; therefore, in brain tissue, IFN-γ is both necessary and sufficient to clear MV. Secretion of IFN-γ is stimulated by interleukin-12 (IL-12) in the brain, as neutralization of IL-12 results in loss of antiviral activity and stimulation of leukocytes with IL-12/IL-18 enhances their immune effector function of viral clearance. MV-primed leukocytes can reduce both West Nile and mouse hepatitis viral RNAs, indicating that cytokine-mediated viral clearance occurs in an antigen-independent manner. The IFN-γ signal is transduced within the brain explant by the Jak/STAT signaling pathway, as inhibition of Jak kinases results in a loss of antiviral activity driven by either brain-derived leukocytes or recombinant IFN-γ. These results reveal that primed T cells directly act to clear MV infection of the brain by using a noncytolytic IL-12- and IFN-γ-dependent mechanism in the CNS and that this mechanism relies upon Jak/STAT signaling.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Neurons under viral attack: victims or warriors?

When the central nervous system (CNS) is under viral attack, defensive antiviral responses must necessarily arise from the CNS itself to rapidly and efficiently curb infections with minimal collateral damage to the sensitive, specialized and non-regenerating neural tissue. This presents a unique challenge because an intact blood-brain barrier (BBB) and lack of proper lymphatic drainage keeps the CNS virtually outside the radar of circulating immune cells that are at constant vigilance for antigens in peripheral tissues. Limited antigen presentation skills of CNS cells in comparison to peripheral tissues is because of a total lack of dendritic cells and feeble expression of major histocompatibility complex (MHC) proteins in neurons and glia. However, research over the past two decades has identified immune effector mechanisms intrinsic to the CNS for immediate tackling, attenuating and clearing of viral infections, with assistance pouring in from peripheral circulation in the form of neutralizing antibodies and cytotoxic T cells at a later stage. Specialized CNS cells, microglia and astrocytes, were regarded as sole sentinels of the brain for containing a viral onslaught but neurons held little recognition as a potential candidate for protecting itself from the proliferation and pathogenesis of neurotropic viruses. Accumulating evidence however indicates that extracellular insult causes neurons to express immune factors characteristic of lymphoid tissues. This article aims to comprehensively analyze current research on this conditional alteration in the protein expression repertoire of neurons and the role it plays in CNS innate immune response to counter viral infections.

MicroRNA regulation of IFN-beta protein expression: rapid and sensitive modulation of the innate immune response

IFN-beta production is an inaugural event in the innate immune response to viral infections, with relatively small fold changes in IFN-beta expression resulting in the activation of important antiviral signaling cascades. In our rapid SIV/macaque model of HIV encephalitis, the virus enters the CNS within 4 d of infection, accompanied by a marked IFN-beta response that wanes as SIV replication is controlled. The centrality of IFN-beta to the innate antiviral response in the CNS combines with the potential inflammatory damage associated with long-term activation of this pathway to suggest that IFN-beta may be subject to regulatory fine-tuning in addition to well-established transcriptional and message stability mechanisms of regulation. In this paper, we present for the first time evidence that microRNAs (miRNAs), including miR-26a, -34a, -145, and let-7b, may directly regulate IFN-beta in human and macaque cells. In primary primate macrophages, the main cell type implicated in HIV and SIV infection in the CNS, specific miRNAs reduce, whereas miRNA inhibitors enhance, IFN-beta protein production. The potential biologic significance of this regulation is supported by evidence of an apparent negative feedback loop, with increased expression of three IFN-beta-regulating miRNAs by primate macrophages exposed to recombinant IFN-beta or stimulated to produce IFN-beta. Thus, miRNAs may contribute significantly to the regulation of IFN-beta in innate immune responses.