Toll-like receptor 7 suppresses virus replication in neurons but does not affect viral pathogenesis in a mouse model of Langat virus infection

Resiquimod Induces C-C Motif Chemokine Ligand 2 Via Nuclear Factor-Kappa B in SH-SY5Y Human Neuroblastoma Cells

Toll-like receptor (TLR) 7 plays an important role in recognizing virus-derived nucleic acids. TLR7 signaling in astrocytes and microglia is critical for activating immune responses against neurotrophic viruses. Neurons express TLR7, similar to glial cells; however, the role of neuronal TLR7 has not yet been fully elucidated. This study sought to determine whether resiquimod, the TLR7/8 agonist, induces the expression of inflammatory chemokines in SH-SY5Y human neuroblastoma cells. Immunofluorescence microscopy revealed that TLR7 was constitutively expressed in SH-SY5Y cells. Stimulation with resiquimod induced C-C motif chemokine ligand 2 (CCL2) expression, accompanied by the activation of nuclear factor-kappa B (NF-κB) in SH-SY5Y cells. Resiquimod increased mRNA levels of C-X-C motif chemokine ligand 8 (CXCL8) and CXCL10, while the increase was slight at the protein level. Knockdown of NF-κB p65 eliminated resiquimod-induced CCL2 production. This study provides novel evidence that resiquimod has promising therapeutic potential against central nervous system viral infections through its immunostimulatory effects on neurons.

RIPK3 promotes brain region-specific interferon signaling and restriction of tick-borne flavivirus infection

Innate immune signaling in the central nervous system (CNS) exhibits many remarkable specializations that vary across cell types and CNS regions. In the setting of neuroinvasive flavivirus infection, neurons employ the immunologic kinase receptor-interacting kinase 3 (RIPK3) to promote an antiviral transcriptional program, independently of the traditional function of this enzyme in promoting necroptotic cell death. However, while recent work has established roles for neuronal RIPK3 signaling in controlling mosquito-borne flavivirus infections, including West Nile virus and Zika virus, functions for RIPK3 signaling in the CNS during tick-borne flavivirus infection have not yet been explored. Here, we use a model of Langat virus (LGTV) encephalitis to show that RIPK3 signaling is specifically required in neurons of the cerebellum to control LGTV replication and restrict disease pathogenesis. This effect did not require the necroptotic executioner molecule mixed lineage kinase domain like protein (MLKL), a finding similar to previous observations in models of mosquito-borne flavivirus infection. However, control of LGTV infection required a unique, region-specific dependence on RIPK3 to promote expression of key antiviral interferon-stimulated genes (ISG) in the cerebellum. This RIPK3-mediated potentiation of ISG expression was associated with robust cell-intrinsic restriction of LGTV replication in cerebellar granule cell neurons. These findings further illuminate the complex roles of RIPK3 signaling in the coordination of neuroimmune responses to viral infection, as well as provide new insight into the mechanisms of region-specific innate immune signaling in the CNS.

RIPK3 promotes brain region-specific interferon signaling and restriction of tick-borne flavivirus infection

Innate immune signaling in the central nervous system (CNS) exhibits many remarkable specializations that vary across cell types and CNS regions. In the setting of neuroinvasive flavivirus infection, neurons employ the immunologic kinase receptor-interacting kinase 3 (RIPK3) to promote an antiviral transcriptional program, independently of the traditional function of this enzyme in promoting necroptotic cell death. However, while recent work has established roles for neuronal RIPK3 signaling in controlling mosquito-borne flavivirus infections, including West Nile virus and Zika virus, functions for RIPK3 signaling in the CNS during tick-borne flavivirus infection have not yet been explored. Here, we use a model of Langat virus (LGTV) encephalitis to show that RIPK3 signaling is specifically required in neurons of the cerebellum to control LGTV replication and restrict disease pathogenesis. This effect did not require the necroptotic executioner molecule mixed lineage kinase domain like protein (MLKL), a finding similar to previous observations in models of mosquito-borne flavivirus infection. However, control of LGTV infection required a unique, region-specific dependence on RIPK3 to promote expression of key antiviral interferon-stimulated genes (ISG) in the cerebellum. This RIPK3-mediated potentiation of ISG expression was associated with robust cell-intrinsic restriction of LGTV replication in cerebellar granule cell neurons. These findings further illuminate the complex roles of RIPK3 signaling in the coordination of neuroimmune responses to viral infection, as well as provide new insight into the mechanisms of region-specific innate immune signaling in the CNS.

Powassan Virus Induces Structural Changes in Human Neuronal Cells In Vitro and Murine Neurons In Vivo

Powassan virus (POWV) is a tick-borne flavivirus (TBFV) that can cause severe encephalitis in humans with a case-fatality rate as high as 11%. Patients who survive severe encephalitic disease can develop long-term neurological sequelae that can be debilitating and life-long. In this study, we have sought to characterize a primary human fetal brain neural stem cell system (hNSC), which can be differentiated into neuron and astrocyte co-cultures, to serve as a translational in vitro system for infection with POWV and a comparative mosquito-borne flavivirus (MBFV), West Nile virus (WNV). We found that both viruses are able to infect both cell types in the co-culture and that WNV elicits a strong inflammatory response characterized by increased cytokines IL-4, IL-6, IL-8, TNF-α and IL-1β and activation of apoptosis pathways. POWV infection resulted in fewer cytokine responses, as well as less detectable apoptosis, while neurons infected with POWV exhibited structural aberrations forming in the dendrites. These anomalies are consistent with previous findings in which tick-borne encephalitis virus (TBEV) infected murine primary neurons formed laminal membrane structures (LMS). Furthermore, these structural aberrations are also recapitulated in brain tissue from infected mice. Our findings indicate that POWV is capable of infecting human primary neurons and astrocytes without causing apparent widespread apoptosis, while forming punctate structures reminiscent with LMS in primary human neurons and in vivo.

Flaviviridae Nonstructural Proteins: The Role in Molecular Mechanisms of Triggering Inflammation

Members of the Flaviviridae family are posing a significant threat to human health worldwide. Many flaviviruses are capable of inducing severe inflammation in humans. Flaviviridae nonstructural proteins, apart from their canonical roles in viral replication, have noncanonical functions strongly affecting antiviral innate immunity. Among these functions, antagonism of type I IFN is the most investigated; meanwhile, more data are accumulated on their role in the other pathways of innate response. This review systematizes the last known data on the role of Flaviviridae nonstructural proteins in molecular mechanisms of triggering inflammation, with an emphasis on their interactions with TLRs and RLRs, interference with NF-κB and cGAS-STING signaling, and activation of inflammasomes.

Flaviviruses: Innate Immunity, Inflammasome Activation, Inflammatory Cell Death, and Cytokines

The innate immune system is the host’s first line of defense against the invasion of pathogens including flavivirus. The programmed cell death controlled by genes plays an irreplaceable role in resisting pathogen invasion and preventing pathogen infection. However, the inflammatory cell death, which can trigger the overflow of a large number of pro-inflammatory cytokines and cell contents, will initiate a severe inflammatory response. In this review, we summarized the current understanding of the innate immune response, inflammatory cell death pathway and cytokine secretion regulation during Dengue virus, West Nile virus, Zika virus, Japanese encephalitis virus and other flavivirus infections. We also discussed the impact of these flavivirus and viral proteins on these biological processes. This not only provides a scientific basis for elucidating the pathogenesis of flavivirus, but also lays the foundation for the development of effective antiviral therapies.

Human plasmacytoid dendritic cells at the crossroad of type I interferon-regulated B cell differentiation and antiviral response to tick-borne encephalitis virus

The Tick-borne encephalitis virus (TBEV) causes different disease symptoms varying from asymptomatic infection to severe encephalitis and meningitis suggesting a crucial role of the human host immune system in determining the fate of the infection. There is a need to understand the mechanisms underpinning TBEV-host interactions leading to protective immunity. To this aim, we studied the response of human peripheral blood mononuclear cells (PBMC) to the whole formaldehyde inactivated TBEV (I-TBEV), the drug substance of Encepur, one of the five commercially available vaccine. Immunophenotyping, transcriptome and cytokine profiling of PBMC revealed that I-TBEV generates differentiation of a sub-population of plasmacytoid dendritic cells (pDC) that is specialized in type I interferon (IFN) production. In contrast, likely due to the presence of aluminum hydroxide, Encepur vaccine was a poor pDC stimulus. We demonstrated I-TBEV-induced type I IFN together with Interleukin 6 and BAFF to be critical for B cell differentiation to plasmablasts as measured by immunophenotyping and immunoglobulin production. Robust type I IFN secretion was induced by pDC with the concerted action of both viral E glycoprotein and RNA mirroring previous data on dual stimulation of pDC by both S. aureus and influenza virus protein and nucleic acid that leads to a type I IFN-mediated sustained immune response. E glycoprotein neutralization or high temperature denaturation and inhibition of Toll-like receptor 7 signalling confirmed the importance of preserving the functional integrity of these key viral molecules during the inactivation procedure and manufacturing process to produce a vaccine able to stimulate strong immune responses.

Though vaccination is generally considered effective in reducing tick-borne encephalitis (TBE) incidence, several studies have shown that the antibody response to TBEV vaccination declines with age resulting in more frequent TBE cases among 50+ year-old vaccinees. These observations together with the lack of a specific antiviral drug impose to pinpoint novel host- and pathogen-directed therapies and to improve the control of vaccine efficacy. Thus, we interrogated in vitro human PBMC, whose response to TBEV may provide a picture closer to what occurs in vivo in humans after vaccination or natural infection compared to animal models. The role of E glycoprotein and viral RNA in promoting antiviral and B cell-mediated responses was investigated. Thus, these key viral molecules should be considered, in future, for novel subunit vaccine formulations than the current whole inactivated TBEV-based vaccines, which require laborious manipulation in biosafety level-3 laboratory and animal testing for manufacturing and batch release.

Activation of type I interferon antiviral response in human neural stem cells

BACKGROUND

Neural stem cells (NSCs) residing in the central nervous system play an important role in neurogenesis. Several viruses can infect these neural progenitors and cause severe neurological diseases. The innate immune responses against the neurotropic viruses in these tissue-specific stem cells remain unclear.

METHODS

Human NSCs were transfected with viral RNA mimics or infected with neurotropic virus for detecting the expression of antiviral interferons (IFNs) and downstream IFN-stimulated antiviral genes.

RESULTS

NSCs are able to produce interferon-β (IFN-β) (type I) and λ1 (type III) after transfection with poly(I:C) and that downstream IFN-stimulated antiviral genes, such as ISG56 and MxA, and the viral RNA sensors RIG-I, MDA5, and TLR3, can be expressed in NSCs under poly(I:C) or IFN-β stimulation. In addition, our results show that the pattern recognition receptors RIG-I and MDA5, as well as the endosomal pathogen recognition receptor TLR3, but not TLR7 and TLR8, are involved in the activation of IFN-β transcription in NSCs. Furthermore, NSCs infected with the neurotropic viruses, Zika and Japanese encephalitis viruses, are able to induce RIG-I-mediated IFN-β expression.

CONCLUSION

Human NSCs have the ability to activate IFN signals against neurotropic viral pathogens.

Tick-borne encephalitis in Europe and Russia: Review of pathogenesis, clinical features, therapy, and vaccines

Tick-borne encephalitis (TBE) is an illness caused by tick-borne encephalitis virus (TBEV) infection which is often limited to a febrile illness, but may lead to very aggressive downstream neurological manifestations. The disease is prevalent in forested areas of Europe and northeastern Asia, and is typically caused by infection involving one of three TBEV subtypes, namely the European (TBEV-Eu), the Siberian (TBEV-Sib), or the Far Eastern (TBEV-FE) subtypes. In addition to the three main TBEV subtypes, two other subtypes; i.e., the Baikalian (TBEV-Bkl) and the Himalayan subtype (TBEV-Him), have been described recently. In Europe, TBEV-Eu infection usually results in only mild TBE associated with a mortality rate of <2%. TBEV-Sib infection also results in a generally mild TBE associated with a non-paralytic febrile form of encephalitis, although there is a tendency towards persistent TBE caused by chronic viral infection. TBE-FE infection is considered to induce the most severe forms of TBE. Importantly though, viral subtype is not the sole determinant of TBE severity; both mild and severe cases of TBE are in fact associated with infection by any of the subtypes. In keeping with this observation, the overall TBE mortality rate in Russia is ∼2%, in spite of the fact that TBEV-Sib and TBEV-FE subtypes appear to be inducers of more severe TBE than TBEV-Eu. On the other hand, TBEV-Sib and TBEV-FE subtype infections in Russia are associated with essentially unique forms of TBE rarely seen elsewhere if at all, such as the hemorrhagic and chronic (progressive) forms of the disease. For post-exposure prophylaxis and TBE treatment in Russia and Kazakhstan, a specific anti-TBEV immunoglobulin is currently used with well-documented efficacy, but the use of specific TBEV immunoglobulins has been discontinued in Europe due to concerns regarding antibody-enhanced disease in naïve individuals. Therefore, new treatments are essential. This review summarizes available data on the pathogenesis and clinical features of TBE, plus different vaccine preparations available in Europe and Russia. In addition, new treatment possibilities, including small molecule drugs and experimental immunotherapies are reviewed. The authors caution that their descriptions of approved or experimental therapies should not be considered to be recommendations for patient care.

Tick-Borne Flaviviruses and the Type I Interferon Response

Flaviviruses are globally distributed pathogens causing millions of human infections every year. Flaviviruses are arthropod-borne viruses and are mainly transmitted by either ticks or mosquitoes. Mosquito-borne flaviviruses and their interactions with the innate immune response have been well-studied and reviewed extensively, thus this review will discuss tick-borne flaviviruses and their interactions with the host innate immune response.

Bovine herpesvirus type 5 replication and induction of apoptosis in vitro and in the trigeminal ganglion of experimentally-infected cattle

Bovine herpesvirus (BoHV) types 1 and 5 are neuroinvasive. Cases of BoHV-1-induced encephalitis are not as frequent as those caused by BoHV-5. In this study, the capability of BoHV-5 to induce apoptosis in cell cultures and in the trigeminal ganglion during acute infection of experimentally-infected cattle was analyzed. Apoptotic changes in cell cultures agree with the ability of the viral strains to replicate in each cell line. Marked differences were observed between the in vitro induction of apoptosis by BoHV-1Cooper and BoHV-5 97/613 strains. Apoptotic neurons were clearly evident in the trigeminal ganglion of BoHV-1-infected calves. For BoHV-5 a fewer number of positive neurons was observed. There is an association between the magnitude of bovine herpesviruses replication and the induction of apoptosis in trigeminal ganglion. These findings suggest that the induction of apoptosis and the innate immune response orchestrate the final outcome of alpha herpesviruses infection of the bovine nervous system.

Lentiviral Knockdown of Transcription Factor STAT1 in Peromyscus leucopus to Assess Its Role in the Restriction of Tick-borne Flaviviruses

Cellular infection with tick-borne flaviviruses (TBFVs) results in activation of the interferon (IFN) signaling pathway and subsequent upregulation of numerous genes termed IFN stimulated genes (ISGs) ( Schoggins et al., 2011 ). Many ISGs function to prevent virus pathogenesis by acting in a broad or specific manner through protein-protein interactions (Duggal and Emerman, 2012). The potency of the IFN signaling response determines the outcome of TBFV infection (Best, 2017; Carletti et al., 2017 ). Interestingly, data from our lab show that TBFV replication is significantly restricted in cells of the reservoir species Peromyscus leucopus thereby suggesting a potent antiviral response ( Izuogu et al., 2017 ). We assessed the relative contribution of IFN signaling to resistance in P. leucopus by knocking down a major transcription factor in the IFN response pathway. Signal transducer and activator of transcription 1 (STAT1) was specifically targeted in P. leucopus cells by shRNA technology. We further tested the impact of gene knockdown on the ability of cells to respond to IFN and restrict virus replication; the results indicate that when STAT1 expression is altered, P. leucopus cells have a decreased response to IFN stimulation and are significantly more susceptible to TBFV replication.

Adaptive Immune Responses to Zika Virus Are Important for Controlling Virus Infection and Preventing Infection in Brain and Testes

The recent association between Zika virus (ZIKV) and neurologic complications, including Guillain-Barré syndrome in adults and CNS abnormalities in fetuses, highlights the importance in understanding the immunological mechanisms controlling this emerging infection. Studies have indicated that ZIKV evades the human type I IFN response, suggesting a role for the adaptive immune response in resolving infection. However, the inability of ZIKV to antagonize the mouse IFN response renders the virus highly susceptible to circulating IFN in murine models. Thus, as we show in this article, although wild-type C57BL/6 mice mount cell-mediated and humoral adaptive immune responses to ZIKV, these responses were not required to prevent disease. However, when the type I IFN response of mice was suppressed, then the adaptive immune responses became critical. For example, when type I IFN signaling was blocked by Abs in Rag1^-/- mice, the mice showed dramatic weight loss and ZIKV infection in the brain and testes. This phenotype was not observed in Ig-treated Rag1^-/- mice or wild-type mice treated with anti-type I IFNR alone. Furthermore, we found that the CD8⁺ T cell responses of pregnant mice to ZIKV infection were diminished compared with nonpregnant mice. It is possible that diminished cell-mediated immunity during pregnancy could increase virus spread to the fetus. These results demonstrate an important role for the adaptive immune response in the control of ZIKV infection and imply that vaccination may prevent ZIKV-related disease, particularly when the type I IFN response is suppressed as it is in humans.

Encephalitic Arboviruses: Emergence, Clinical Presentation, and Neuropathogenesis

Arboviruses are arthropod-borne viruses that exhibit worldwide distribution, contributing to systemic and neurologic infections in a variety of geographical locations. Arboviruses are transmitted to vertebral hosts during blood feedings by mosquitoes, ticks, biting flies, mites, and nits. While the majority of arboviral infections do not lead to neuroinvasive forms of disease, they are among the most severe infectious risks to the health of the human central nervous system. The neurologic diseases caused by arboviruses include meningitis, encephalitis, myelitis, encephalomyelitis, neuritis, and myositis in which virus- and immune-mediated injury may lead to severe, persisting neurologic deficits or death. Here we will review the major families of emerging arboviruses that cause neurologic infections, their neuropathogenesis and host neuroimmunologic responses, and current strategies for treatment and prevention of neurologic infections they cause.

Type I Interferon response in olfactory bulb, the site of tick-borne flavivirus accumulation, is primarily regulated by IPS-1

BACKGROUND

Although type I interferons (IFNs)-key effectors of antiviral innate immunity are known to be induced via different pattern recognition receptors (PRRs), the cellular source and the relative contribution of different PRRs in host protection against viral infection is often unclear. IPS-1 is a downstream adaptor for retinoid-inducible gene I (RIG-I)-like receptor signaling. In this study, we investigate the relative contribution of IPS-1 in the innate immune response in the different brain regions during infection with tick-borne encephalitis virus (TBEV), a flavivirus that causes a variety of severe symptoms like hemorrhagic fevers, encephalitis, and meningitis in the human host.

METHODS

IPS-1 knockout mice were infected with TBEV/Langat virus (LGTV), and viral burden in the peripheral and the central nervous systems, type I IFN induction, brain infiltrating cells, and inflammatory response was analyzed.

RESULTS

We show that IPS-1 is indispensable for controlling TBEV and LGTV infections in the peripheral and central nervous system. Our data indicate that IPS-1 regulates neuropathogenicity in mice. IFN response is differentially regulated in distinct regions of the central nervous system (CNS) influencing viral tropism, as LGTV replication was mainly restricted to olfactory bulb in wild-type (WT) mice. In contrast to the other brain regions, IFN upregulation in the olfactory bulb was dependent on IPS-1 signaling. IPS-1 regulates basal levels of antiviral interferon-stimulated genes (ISGs) like viperin and IRF-1 which contributes to the establishment of early viral replication which inhibits STAT1 activation. This diminishes the antiviral response even in the presence of high IFN-β levels. Consequently, the absence of IPS-1 causes uncontrolled virus replication, in turn resulting in apoptosis, activation of microglia and astrocytes, elevated proinflammatory response, and recruitment of inflammatory cells into the CNS.

CONCLUSIONS

We show that LGTV replication is restricted to the olfactory bulb and that IPS-1 is a very important player in the olfactory bulb in shaping the innate immune response by inhibiting early viral replication and viral spread throughout the central nervous system. In the absence of IPS-1, higher viral replication leads to the evasion of antiviral response by inhibiting interferon signaling. Our data suggest that the local microenvironment of distinct brain regions is critical to determine virus permissiveness.

The microRNAs Let7c and miR21 are recognized by neuronal Toll-like receptor 7 to restrict dendritic growth of neurons

Inflammatory responses are known to play critical roles in the regulation of neurodevelopment and neurodegeneration. Although microglial cells are recognized as professional immune cells in brains, recent evidence suggests that neurons also express important receptors and regulators of innate immunity, including Toll-like receptor 7 (TLR7), which is a receptor for single-stranded RNAs (ssRNAs). Here, we report that neuronal TLR7 recognizes endogenous ligands such as the miRNAs Let7c and miR21 and plays a negative role in controlling neuronal growth in a cell-autonomous manner. We show here that hippocampal CA1 neurons in Tlr7(-/Y) mice had more complex dendritic arbors compared with those of wild-type littermates at postnatal (P) day 7, but not at P21. This observation strengthens a role of TLR7 in restricting neuronal growth during development. In cultured neurons, transient knockdown of Tlr7 promoted axonal and dendritic growth, supporting the cell-autonomous effect of TLR7 on neuronal growth. We observed perceptible levels of Let7c and miR21 in the exosomes of the neuronal cultures as well as in developing brains. Treatment with Let7c and miR21 restricted dendritic growth of wild-type neurons but not Tlr7(-/-) neurons. Our study suggests that neuronal TLR7 is activated by endogenous ligands and thus regulates neuronal morphology. Neuronal innate immune responses may influence neurodevelopment and neurodegeneration through the regulation of neuronal morphology.

Investigations on CXCL13 in anti-N-methyl-D-aspartate receptor encephalitis: a potential biomarker of treatment response

IMPORTANCE

Anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis is a severe but treatable autoimmune encephalitis affecting mainly young adults and children. The lack of suitable biomarkers of disease activity makes treatment decisions and identification of relapses challenging.

OBJECTIVE

To determine the levels of the B-cell-attracting C-X-C motif chemokine 13 (CXCL13) in serum samples and cerebrospinal fluid (CSF) of patients with anti-NMDAR encephalitis and whether they can be used as biomarkers of treatment response and outcome.

DESIGN, SETTINGS, AND PARTICIPANTS

Retrospective cohort study of 167 patients consecutively diagnosed as having anti-NMDAR encephalitis between May 1, 2008, and January 31, 2013. Concentration of CXCL13 was determined with enzyme-linked immunosorbent assay in all available patients' samples (272 CSF and 55 serum samples). Samples from 25 patients with noninflammatory neurological disorders and 9 with neuroborreliosis served as controls. Expression of CXCL13 in the brain biopsy of a patient with anti-NMDAR encephalitis was determined by immunohistochemistry.

MAIN OUTCOMES AND MEASURES

Percentage of patients with anti-NMDAR encephalitis and elevated CXCL13 in CSF.

RESULTS

Compared with control individuals, 70% of patients with early-stage anti-NMDAR encephalitis had increased CXCL13 in CSF (>7 pg/mL; P < .001) but none in serum samples (>1047 pg/mL; P > .99). High concentration of CSF CXCL13 was associated with the presence of prodromal fever or headache (P = .01), limited response to therapy (P = .003), clinical relapses (P = .03), and intrathecal NMDAR-antibody synthesis (P < .001). Among patients with monophasic disease assessed 2 to 6 months after starting treatment, 10 of 15 with limited treatment response vs 0 of 13 with favorable response had increased CSF CXCL13 (specificity, 100%; 95% CI, 75-100 and sensitivity, 67%; 95% CI, 38-88; P = .02). Six of 12 patients had elevated CSF CXCL13 at relapse including 3 with previously normal levels. In brain, abundant mononuclear cells in perivascular infiltrates and scattered intraparenchymal microglia expressed CXCL13.

CONCLUSIONS AND RELEVANCE

Seventy percent of patients with early-stage anti-NMDAR encephalitis had increased CSF CXCL13 concentration that correlated with intrathecal NMDAR-antibody synthesis. Prolonged or secondary elevation of CXCL13 was associated with limited response to treatment and relapses. CXCL13 is a potentially useful biomarker of treatment response and outcome in anti-NMDAR encephalitis.

A tick-borne encephalitis model in infant rats infected with langat virus

Tick-borne encephalitis virus (TBEV) is the causative agent of human TBE, a severe infection that can cause long-lasting neurologic sequelae. Langat virus (LGTV), which is closely related to TBEV, has a low virulence for human hosts and has been used as a live vaccine against TBEV. Tick-borne encephalitis by natural infection of LGTV in humans has not been described, but one of 18,500 LGTV vaccinees developed encephalitis. The pathogenetic mechanisms of TBEV are poorly understood and, currently, no effective therapy is available. We developed an infant rat model of TBE using LGTV as infective agent. Infant Wistar rats were inoculated intracisternally with 10 focus-forming units of LGTV and assessed for clinical disease and neuropathologic findings at Days 2, 4, 7, and 9 after infection. Infection with LGTV led to gait disturbance, hypokinesia, and reduced weight gain or weight loss. Cerebrospinal fluid concentrations of RANTES, interferon-γ, interferon-β, interleukin-6, and monocyte chemotactic protein-1 were increased in infected animals. The brains of animals with LGTV encephalitis exhibited characteristic perivascular inflammatory cuffs and glial nodules; immunohistochemistry documented the presence of LGTV in the thalamus, hippocampus, midbrain, frontal pole, and cerebellum. Thus, LGTV meningoencephalitis in infant rats mimics important clinical and histopathologic features of human TBE. This new model provides a tool to investigate disease mechanisms and to evaluate new therapeutic strategies against encephalitogenic flaviviruses.

Type I interferon protects mice from fatal neurotropic infection with Langat virus by systemic and local antiviral responses

Vector-borne flaviviruses, such as tick-borne encephalitis virus (TBEV), West Nile virus, and dengue virus, cause millions of infections in humans. TBEV causes a broad range of pathological symptoms, ranging from meningitis to severe encephalitis or even hemorrhagic fever, with high mortality. Despite the availability of an effective vaccine, the incidence of TBEV infections is increasing. Not much is known about the role of the innate immune system in the control of TBEV infections. Here, we show that the type I interferon (IFN) system is essential for protection against TBEV and Langat virus (LGTV) in mice. In the absence of a functional IFN system, mice rapidly develop neurological symptoms and succumb to LGTV and TBEV infections. Type I IFN system deficiency results in severe neuroinflammation in LGTV-infected mice, characterized by breakdown of the blood-brain barrier and infiltration of macrophages into the central nervous system (CNS). Using mice with tissue-specific IFN receptor deletions, we show that coordinated activation of the type I IFN system in peripheral tissues as well as in the CNS is indispensable for viral control and protection against virus induced inflammation and fatal encephalitis.

IMPORTANCE

The type I interferon (IFN) system is important to control viral infections; however, the interactions between tick-borne encephalitis virus (TBEV) and the type I IFN system are poorly characterized. TBEV causes severe infections in humans that are characterized by fever and debilitating encephalitis, which can progress to chronic illness or death. No treatment options are available. An improved understanding of antiviral innate immune responses is pivotal for the development of effective therapeutics. We show that type I IFN, an effector molecule of the innate immune system, is responsible for the extended survival of TBEV and Langat virus (LGTV), an attenuated member of the TBE serogroup. IFN production and signaling appeared to be essential in two different phases during infection. The first phase is in the periphery, by reducing systemic LGTV replication and spreading into the central nervous system (CNS). In the second phase, the local IFN response in the CNS prevents virus-induced inflammation and the development of encephalitis.

TLR7 is a key regulator of innate immunity against Japanese encephalitis virus infection

Toll-like receptor 7 (TLR7) known to recognize guanidine-rich ssRNA has been shown to mount vital host defense mechanism against many viruses including flaviviruses. Signal transduction through TLR7 has been shown to produce type-1 interferon and proinflammatory mediators, thereby initiating essential innate immune response against ssRNA viruses in hosts. Systemic and brain specific TLR7 knock-down mice (TLR7(KD)) were generated using vivo-morpholinos. These mice were then subcutaneously challenged with lethal dose of JEV (GP78 strain) and were subsequently analyzed for survival. Significant difference in susceptibility to JEV between wild-type and systemic TLR7(KD) mice was observed whereas, no difference in susceptibility to JEV infection was seen in brain-specific TLR7(KD) mice. Significant decreases in IFN-α and antiviral proteins were also observed in both TLR7(KD) mice along with increased viral loads in their brain. Owing to increased viral load, increases in levels of various proinflammatory cyto/chemokines, increased microglial activation and infiltration of peripheral immune cells in brain of TLR7(KD) mice were also observed. Immunocytochemistry and RNA co-immunoprecipitation performed with JEV-infected N2a or HT22 cells indicated endosomal localization and confirmed interaction between JEV ssRNA with TLR7. Treatment of mice with imiquimod, a TLR7 agonist, prior to JEV infection resulted in their increased survival. Overall, our results suggest that the TLR7 response following JEV infection promotes type-1 interferon production and generation of antiviral state which might contribute to protective effect in systemic infection.

Role of pattern recognition receptors in flavivirus infections

The flaviviral encephalitis has now become a major health concern in global scale. The efficient detection of viral infection and induction of the innate antiviral response by host's innate immune system are crucial to determine the outcome of infection. The intracellular pattern recognition receptors TLRs, RLRs, NLRs and CLRs play a central role in detection and initiation of robust antiviral response against flaviviral infection. Both cytoplasmic RLRs, RIG-I and MDA5 have been shown to be implicated in sensing flaviviral genomic RNA. Similarly among TLRs mainly TLR3 and TLR7 are known to respond in flaviviral infections as they are known to sense dsRNA and ssRNA moiety as their natural cognate ligand. Several studies have also shown the roles of NLRs and CLRs in mounting an innate antiviral response against flavivirus but, it is yet to be completely understood. Until now only few reports have implicated NLRs and CLRs in induction of antiviral and proinflammatory state following flaviviral infection. The current review therefore aims to comprehensively analyze past as well as current understanding on the role of PRRs in flaviviral infections.