Distinct dictation of Japanese encephalitis virus-induced neuroinflammation and lethality via triggering TLR3 and TLR4 signal pathways

NLRP3 activation induces BBB disruption and neutrophil infiltration via CXCR2 signaling in the mouse brain

NLRP3 is an intracellular sensor molecule that affects neutrophil functionality and infiltration in brain disorders such as experimental autoimmune encephalomyelitis (EAE). However, the detailed molecular mechanisms underlying the role of NLRP3 in these processes remain unknown. We found that NLRP3 is crucial for neutrophil infiltration, whereas dispensable for neutrophil priming. Notably, NLRP3 activation in neutrophils induced blood-brain barrier (BBB) disruption and neutrophil infiltration into the brain via CXCL1/2 secretion and subsequent activation of the CXCL1/2-CXCR2 signaling axis. Moreover, CXCL1 and CXCL2 in the inflamed brain directly reduced Claudin-5 expression, which regulates BBB permeability in brain endothelial cells. Furthermore, neutrophil-specific NLRP3 activation aggravated EAE pathogenesis by promoting CXCR2-mediated infiltration of both neutrophils and CD4+ T cells into the central nervous system at disease onset. Thus, the CXCL1/2-CXCR2 axis plays a role in EAE progression. Therefore, this chemokine axis could be a potential therapeutic target for attenuating neuroinflammatory diseases through modulating of neutrophil and CD4+ T cell infiltration and BBB disruption.

Japanese encephalitis virus NS1 inhibits IFN-β production by interacting with DDX3X

Japanese encephalitis virus (JEV) is the causative agent of Japanese encephalitis, which poses great threats to the pig farming industry and human health. To establish infection, JEV has evolved sophisticated strategies to overcome the innate immune responses and finish its life cycle. Previous studies have shown that non-structural protein 1 (NS1) is closely related to its pathogenesis, while its molecular mechanism remains elusive. In this study, host protein ATP-dependent RNA helicase DEAD-box helicase 3 X (DDX3X) was screened to bind with NS1, and both ectopically expressed and virally encoded NS1 further confirmed their interaction. We also proved that the β-roll and wing subdomains of NS1 were responsible for their interaction. In DDX3X-overexpressing cells, the replication of JEV was markedly inhibited, while the viral titers were elevated in DDX3X-silencing cells, indicating that DDX3X might serve as an anti-viral factor during JEV infection. Mechanically, overexpression of DDX3X promotes interferon-beta (IFN-β) transcription, while its transcription was decreased in DDX3X-silencing cells. Consistent with IFN-β, some interferon-stimulated genes (ISGs), including protein kinase R (PKR), myxovirus resistance 1 (MX1), guanylate-binding protein 1 (GBP1), and bone marrow stromal antigen 2 (BST2), were also positively related to the DDX3X expression. Taken together, JEV NS1 blocks IFN-β production by interacting with DDX3X to evade the host's innate immune response and facilitate virus replication. This finding will deepen our understanding of JEV immune-evasion strategies and provide targets for JEV attenuation.IMPORTANCEThis study focused on JEV, a threat to pig farming and human health. The key finding is that NS1 binds to host protein DDX3X via its β-roll and wing subdomains. JEV NS1 evades the host immune response by interacting with DDX3X to restrain type I interferon production. These results deepen our understanding of JEV's immune-evasion strategies and offer potential targets for JEV attenuation.

The Factors Associated with the Blood-Brain Barrier Dysfunction in Tick-Borne Encephalitis

The pathogenesis of the central nervous system (CNS) pathology in tick-borne encephalitis (TBE) remains unclear. We attempted to identify mediators of the blood-brain barrier (BBB) disruption in human TBE in paired serum and cerebrospinal fluid (CSF) samples from 100 TBE patients. CSF albumin quotient (Qalb) was calculated as a measure of BBB impairment. Concentrations of cytokines, cytokine antagonists, adhesion molecules, selectins and matrix metalloproteinases (MMP) were measured with a multiplex bead assay. Single nucleotide polymorphisms (SNP) in genes MIF, TNF, TNFRSF1A, TNFRSF1B, IL-10, TLR3 and TLR4 were studied in patient blood DNA extracts and analyzed for associations with Qalb and/or cytokine concentrations. The multivariate regression models of Qalb were built with the soluble mediators as independent variables. The best models obtained included L-selectin, P-selectin, sVCAM, MMP7, MMP8 (or MMP9) and IL-28A as positive and IL-12p70, IL-15, IL-6Rα/IL-6 ratio and TNF-RII/TNFα ratio as negative correlates of Qalb. The genotype did not associate with Qalb, but polymorphism rs4149570 (in TNFRSF1A) associated with TNFα and rs1800629 (TNF) with MIF concentration. We confirm the association of the TNFα-dependent response, L-selectin and MMP8/MMP9 with BBB disruption and identify its novel correlates (IL-12, IL-15, IL-28A, MMP7). We detect no genotype associations with BBB function in TBE.

Rosmarinic acid-mediated downregulation of RIG-I and p62 in microglia confers resistance to Japanese encephalitis virus-induced inflammation

BACKGROUND

Japanese encephalitis virus (JEV) is a mosquito-borne zoonotic pathogen that causes encephalitis in humans and reproductive failure in pigs. The transmission of JEV between humans and animals poses a significant public health threat and results in substantial economic losses. Excessive inflammation in the central nervous system of JEV-infected patients is a major cause of mortality and disability. Rosmarinic acid (RA), a polyhydroxyphenolic compound isolated from medicinal herbs, has been preliminarily shown to possess anti-inflammatory properties and significantly inhibit JEV-induced neuroinflammation in mice.

RESULTS

This study investigated the antiviral capacity and potential mechanisms of RA in JEV-infected cells. The results demonstrated that RA could inhibit JEV replication in vitro. Furthermore, the expression levels of inflammatory cytokines (including IL-6, IL-1β, CCL-2, and TNF-α), membrane receptors (including RIG-I, TLR3, TLR4, TLR7, and TLR8), NF-κB complex and p62/SQSTM1 were assessed using qPCR, ELISA, and Western blot, respectively. The findings indicated that RA significantly suppressed the expression of IL-6, IL-1α, TNF-α, and CCL-2 in JEV-infected BV-2 cells in a dose-dependent manner. Additionally, RA treatment downregulated the expression levels of RIG-I and p62, while p62 silencing inhibited the upregulation of inflammatory cytokines in JEV-infected BV-2 cells.

CONCLUSION

Our present study highlights the important role of RA-mediated reduction of RIG-I and p62 in microglia, conferring resistance to Japanese encephalitis virus-induced inflammation.

SLC25A12 inhibits Japanese encephalitis virus replication by interacting with the NS1 and enhancing the type I interferon response

Japanese encephalitis virus (JEV) is a mosquito-borne, zoonotic orthoflavivirus causing human encephalitis and reproductive disorders in pigs. Cell-intrinsic antiviral restriction factors are the first line of defense that prevent a virus from establishing a productive infection, while the molecular mechanism of the virus-host interaction is still not fully understood. Our in vitro experiments demonstrated that the Solute Carrier Family 25 Member 12 (SLC25A12) interacted with the JEV nonstructural protein 1 (NS1) and inhibited JEV replication. Furthermore, we showed that knockdown or knockout of SLC25A12 promoted JEV replication, while overexpression of SLC25A12 repressed viral replication. Finally, we demonstrated that SLC25A12 increased IRF7 mRNA levels, which promoted IFN-β expression and subsequently induced antiviral effects. Collectively, our study revealed that SLC25A12 interacted with NS1, inhibiting viral RNA synthesis and transcription and enhancing type I interferon induction for antiviral effects.

The Flavivirus Non-Structural Protein 5 (NS5): Structure, Functions, and Targeting for Development of Vaccines and Therapeutics

Flaviviruses, including dengue (DENV), Zika (ZIKV), West Nile (WNV), Japanese encephalitis (JEV), yellow fever (YFV), and tick-borne encephalitis (TBEV) viruses, pose a significant global emerging threat. With their potential to cause widespread outbreaks and severe health complications, the development of effective vaccines and antiviral therapeutics is imperative. The flaviviral non-structural protein 5 (NS5) is a highly conserved and multifunctional protein that is crucial for viral replication, and the NS5 protein of many flaviviruses has been shown to be a potent inhibitor of interferon (IFN) signalling. In this review, we discuss the functions of NS5, diverse NS5-mediated strategies adopted by flaviviruses to evade the host antiviral response, and how NS5 can be a target for the development of vaccines and antiviral therapeutics.

TLR9 promotes monocytic myeloid-derived suppressor cell induction during JEV infection

Myeloid-derived suppressor cells (MDSCs) are a group of heterologous populations of immature bone marrow cells consisting of progenitor cells of macrophages, dendritic cells and granulocytes. Recent studies have revealed that the accumulation of MDSCs in the mouse spleen plays a pivotal role in suppressing the immune response following JEV infection. However, the mechanisms by which JEV induces MDSCs are poorly understood. Here, it was found that JEV infection induces mitochondrial damage and the release of mitochondrial DNA (mtDNA), which further leads to the activation of TLR9. TLR9 deficiency decreases the M-MDSCs population and their suppressive function both in vitro and in vivo. Moreover, the increase of MHCⅡ expression on antigen-presenting cells and CD28 expression on T cells in TLR9-/- mice was positively correlated with M-MDSCs reduction. Accordingly, the survival rate of TLR9-/- mice dramatically increased after JEV infection. These findings reveal the connections of mitochondrial damage and TLR9 activation to the induction of M-MDSCs during JEV infection.

Inhibition of NADPH oxidase 2 enhances resistance to viral neuroinflammation by facilitating M1-polarization of macrophages at the extraneural tissues

BACKGROUND

Macrophages play a pivotal role in the regulation of Japanese encephalitis (JE), a severe neuroinflammation in the central nervous system (CNS) following infection with JE virus (JEV). Macrophages are known for their heterogeneity, polarizing into M1 or M2 phenotypes in the context of various immunopathological diseases. A comprehensive understanding of macrophage polarization and its relevance to JE progression holds significant promise for advancing JE control and therapeutic strategies.

METHODS

To elucidate the role of NADPH oxidase-derived reactive oxygen species (ROS) in JE progression, we assessed viral load, M1 macrophage accumulation, and cytokine production in WT and NADPH oxidase 2 (NOX2)-deficient mice using murine JE model. Additionally, we employed bone marrow (BM) cell-derived macrophages to delineate ROS-mediated regulation of macrophage polarization by ROS following JEV infection.

RESULTS

NOX2-deficient mice exhibited increased resistance to JE progression rather than heightened susceptibility, driven by the regulation of macrophage polarization. These mice displayed reduced viral loads in peripheral lymphoid tissues and the CNS, along with diminished infiltration of inflammatory cells into the CNS, thereby resulting in attenuated neuroinflammation. Additionally, NOX2-deficient mice exhibited enhanced JEV-specific Th1 CD4 + and CD8 + T cell responses and increased accumulation of M1 macrophages producing IL-12p40 and iNOS in peripheral lymphoid and inflamed extraneural tissues. Mechanistic investigations revealed that NOX2-deficient macrophages displayed a more pronounced differentiation into M1 phenotypes in response to JEV infection, thereby leading to the suppression of viral replication. Importantly, the administration of H2O2 generated by NOX2 was shown to inhibit M1 macrophage polarization. Finally, oral administration of the ROS scavenger, butylated hydroxyanisole (BHA), bolstered resistance to JE progression and reduced viral loads in both extraneural tissues and the CNS, along with facilitated accumulation of M1 macrophages.

CONCLUSION

In light of our results, it is suggested that ROS generated by NOX2 play a role in undermining the control of JEV replication within peripheral extraneural tissues, primarily by suppressing M1 macrophage polarization. Subsequently, this leads to an augmentation in the viral load invading the CNS, thereby facilitating JE progression. Hence, our findings ultimately underscore the significance of ROS-mediated macrophage polarization in the context of JE progression initiated JEV infection.

Molecular Mechanisms Associated with Neurodegeneration of Neurotropic Viral Infection

Viral infections of the central nervous system (CNS) cause variable outcomes from acute to severe neurological sequelae with increased morbidity and mortality. Viral neuroinvasion directly or indirectly induces encephalitis via dysregulation of the immune response and contributes to the alteration of neuronal function and the degeneration of neuronal cells. This review provides an overview of the cellular and molecular mechanisms of virus-induced neurodegeneration. Neurotropic viral infections influence many aspects of neuronal dysfunction, including promoting chronic inflammation, inducing cellular oxidative stress, impairing mitophagy, encountering mitochondrial dynamics, enhancing metabolic rewiring, altering neurotransmitter systems, and inducing misfolded and aggregated pathological proteins associated with neurodegenerative diseases. These pathogenetic mechanisms create a multidimensional injury of the brain that leads to specific neuronal and brain dysfunction. The understanding of the molecular mechanisms underlying the neurophathogenesis associated with neurodegeneration of viral infection may emphasize the strategies for prevention, protection, and treatment of virus infection of the CNS.

Current Advances in Japanese Encephalitis Virus Drug Development

Japanese encephalitis virus (JEV) belongs to the Flaviviridae family and is a representative mosquito-borne flavivirus responsible for acute encephalitis and meningitis in humans. Despite the availability of vaccines, JEV remains a major public health threat with the potential to spread globally. According to the World Health Organization (WHO), there are an estimated 69,000 cases of JE each year, and this figure is probably an underestimate. The majority of JE victims are children in endemic areas, and almost half of the surviving patients have motor or cognitive sequelae. Thus, the absence of a clinically approved drug for the treatment of JE defines an urgent medical need. Recently, several promising and potential drug candidates were reported through drug repurposing studies, high-throughput drug library screening, and de novo design. This review focuses on the historical aspects of JEV, the biology of JEV replication, targets for therapeutic strategies, a target product profile, and drug development initiatives.

Making sense of flavivirus non-strctural protein 1 in innate immune evasion and inducing tissue-specific damage

Flaviviruses include medically important mosquito-borne pathogens, such as Zika virus (ZIKV), Japanese encephalitis virus (JEV), dengue virus (DENV) and West Nile virus (WNV), that cause hundreds of millions of infections each year. Currently, there are no approved effect therapies against mosquito-borne flaviviruses. The flaviviruses encoded nonstructural protein 1 (NS1) is a secreted glycoprotein widely involved in viral replication, immune evasion, and directly causing tissue-specific damage during flaviviruses infection. Upon viral infection of host cell, NS1 can be found in multiple oligomeric forms and include a dimer on the cell surface, and a soluble secreted hexameric lipoparticle. In the recent decade, the detailed crystal structure of several flaviviruses NS1 have been determined and unraveled its broader and deeper functions. Consistent with the potential immune function revealed by its structure, NS1 is involved in the escaping of host signal immune pathway mediated by pattern recognition receptors (PRRs), including RIG-I-like receptors (RLRS) and Toll-like receptors (TLRs). Moreover, the flavivirus NS1 is efficiently secreted by infected cells and circulates in the blood of the host to directly induce specific tissues damage. The NS1 of ZIKV, JEV and WNV changes the permeability of brain microvascular endothelial cell to cause endothelial cell dysfunction and promote virus pathogenesis. DENV NS1 can induce systemic tissues damage in humans through multiple strategies. Mutations of several key amino acids in NS1 can reduce the neurovirulence of the flavivirus. In this article, we provide an overview of the latest research on this fascinating protein in these disparate areas.

Viral inhibitory potential of hyoscyamine in Japanese encephalitis virus-infected embryonated chicken eggs involving multiple signaling pathways

Japanese encephalitis virus (JEV) is the leading cause of viral encephalitis worldwide. The emergence of new genotypes of the virus and a high rate of mutation make it necessary to develop alternative treatment strategies against this deadly pathogen. Although the antiviral properties of Atropa belladonna and some of its active components, such as atropine and scopolamine, have been studied, the effect of another important component, hyoscyamine, against JEV infection has not yet been investigated. In this study, we investigated the antiviral effect of hyoscyamine against JEV and its immunomodulatory activity in embryonated chicken eggs. Pretreatment with hyoscyamine sulphate resulted in a significant decrease in the viral load in both chorioallantoic membrane (CAM) and brain tissues at 48 and 96 hours postinfection. In silico studies showed stable binding and interaction between hyoscyamine and non-structural protein 5 (NS5), suggesting that this could be the basis of its antiviral effect. Embryonated eggs pretreated with hyoscyamine sulphate showed upregulation of Toll-like receptor 3 (TLR3), TLR7, TLR8, interleukin 4 (IL-4), and IL-10 as well as interferons and regulatory factors. Hyoscyamine sulphate was also found to cause significant downregulation of TLR4. The potential use of hyoscyamine for controlling JEV replication and its dissemination to the brain suggest that it may be a promising therapy option against JEV in the future.

Type I/type III IFN and related factors regulate JEV infection and BBB endothelial integrity

BACKGROUND

Japanese encephalitis virus (JEV) remains a predominant cause of Japanese encephalitis (JE) globally. Its infection is usually accompanied by disrupted blood‒brain barrier (BBB) integrity and central nervous system (CNS) inflammation in a poorly understood pathogenesis. Productive JEV infection in brain microvascular endothelial cells (BMECs) is considered the initial event of the virus in penetrating the BBB. Type I/III IFN and related factors have been described as negative regulators in CNS inflammation, whereas their role in JE remains ambiguous.

METHODS

RNA-sequencing profiling (RNA-seq), real-time quantitative PCR, enzyme-linked immunosorbent assay, and Western blotting analysis were performed to analyze the gene and protein expression changes between mock- and JEV-infected hBMECs. Bioinformatic tools were used to cluster altered signaling pathway members during JEV infection. The shRNA-mediated immune factor-knockdown hBMECs and the in vitro transwell BBB model were utilized to explore the interrelation between immune factors, as well as between immune factors and BBB endothelial integrity.

RESULTS

RNA-Seq data of JEV-infected hBMECs identified 417, 1256, and 2748 differentially expressed genes (DEGs) at 12, 36, and 72 h post-infection (hpi), respectively. The altered genes clustered into distinct pathways in gene ontology (GO) terms and KEGG pathway enrichment analysis, including host antiviral immune defense and endothelial cell leakage. Further investigation revealed that pattern-recognition receptors (PRRs, including TLR3, RIG-I, and MDA5) sensed JEV and initiated IRF/IFN signaling. IFNs triggered the expression of interferon-induced proteins with tetratricopeptide repeats (IFITs) via the JAK/STAT pathway. Distinct PRRs exert different functions in barrier homeostasis, while treatment with IFN (IFN-β and IFN-λ1) in hBMECs stabilizes the endothelial barrier by alleviating exogenous destruction. Despite the complex interrelationship, IFITs are considered nonessential in the IFN-mediated maintenance of hBMEC barrier integrity.

CONCLUSIONS

This research provided the first comprehensive description of the molecular mechanisms of host‒pathogen interplay in hBMECs responding to JEV invasion, in which type I/III IFN and related factors strongly correlated with regulating the hBMEC barrier and restricting JEV infection. This might help with developing an attractive therapeutic strategy in JE.

Mice as an Animal Model for Japanese Encephalitis Virus Research: Mouse Susceptibility, Infection Route, and Viral Pathogenesis

Japanese encephalitis virus (JEV), a zoonotic flavivirus, is principally transmitted by hematophagous mosquitoes, continually between susceptible animals and incidentally from those animals to humans. For almost a century since its discovery, JEV was geographically confined to the Asia-Pacific region with recurrent sizable outbreaks involving wildlife, livestock, and people. However, over the past decade, it has been detected for the first time in Europe (Italy) and Africa (Angola) but has yet to cause any recognizable outbreaks in humans. JEV infection leads to a broad spectrum of clinical outcomes, ranging from asymptomatic conditions to self-limiting febrile illnesses to life-threatening neurological complications, particularly Japanese encephalitis (JE). No clinically proven antiviral drugs are available to treat the development and progression of JE. There are, however, several live and killed vaccines that have been commercialized to prevent the infection and transmission of JEV, yet this virus remains the main cause of acute encephalitis syndrome with high morbidity and mortality among children in the endemic regions. Therefore, significant research efforts have been directed toward understanding the neuropathogenesis of JE to facilitate the development of effective treatments for the disease. Thus far, multiple laboratory animal models have been established for the study of JEV infection. In this review, we focus on mice, the most extensively used animal model for JEV research, and summarize the major findings on mouse susceptibility, infection route, and viral pathogenesis reported in the past and present, and discuss some unanswered key questions for future studies.

Antiviral effect of melatonin on Japanese encephalitis virus infection involves inhibition of neuronal apoptosis and neuroinflammation in SH-SY5Y cells

Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, causes high mortality rates in humans and it is the most clinically important and common cause of viral encephalitis in Asia. To date, there is no specific treatment for JEV infection. Melatonin, a neurotropic hormone, is reported to be effective in combating various bacterial and viral infections. However, the effects of melatonin on JEV infection have not yet been studied. The investigation tested the antiviral effects of melatonin against JEV infection and elucidated the possible molecular mechanisms of inhibition. Melatonin inhibited the viral production in JEV-infected SH-SY5Y cells in a time- and dose-dependent manner. Time-of-addition assays demonstrated a potent inhibitory effect of melatonin at the post-entry stage of viral replication. Molecular docking analysis revealed that melatonin negatively affected viral replication by interfering with physiological function and/or enzymatic activity of both JEV nonstructural 3 (NS3) and NS5 protein, suggesting a possible underlying mechanism of JEV replication inhibition. Moreover, treatment with melatonin reduced neuronal apoptosis and inhibited neuroinflammation induced by JEV infection. The present findings reveal a new property of melatonin as a potential molecule for the further development of anti-JEV agents and treatment of JEV infection.

SARS-CoV-2 Spike protein induces TLR4-mediated long-term cognitive dysfunction recapitulating post-COVID-19 syndrome in mice

Cognitive dysfunction is often reported in patients with post-coronavirus disease 2019 (COVID-19) syndrome, but its underlying mechanisms are not completely understood. Evidence suggests that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Spike protein or its fragments are released from cells during infection, reaching different tissues, including the CNS, irrespective of the presence of the viral RNA. Here, we demonstrate that brain infusion of Spike protein in mice has a late impact on cognitive function, recapitulating post-COVID-19 syndrome. We also show that neuroinflammation and hippocampal microgliosis mediate Spike-induced memory dysfunction via complement-dependent engulfment of synapses. Genetic or pharmacological blockage of Toll-like receptor 4 (TLR4) signaling protects animals against synapse elimination and memory dysfunction induced by Spike brain infusion. Accordingly, in a cohort of 86 patients who recovered from mild COVID-19, the genotype GG TLR4-2604G>A (rs10759931) is associated with poor cognitive outcome. These results identify TLR4 as a key target to investigate the long-term cognitive dysfunction after COVID-19 infection in humans and rodents.

Exaggerated levels of some specific TLRs, cytokines and chemokines in Japanese encephalitis infected BV2 and neuro 2A cell lines associated with worst outcome

Japanese encephalitis (JE) disease, a viral brain fever is caused by Japanese encephalitis virus (JEV). Despite the availability of effective vaccines against this deadly infection, JE is the leading cause of epidemic viral encephalitis in children in South-east Asia. There is no treatment available for the JE disease which might be due to incomplete understanding of the pathogenesis of JE virus. The JEV infections lead to permanent neurological deficits even in those who survive from the infection. Activated microglia may play a potentially detrimental role by eliciting the expression of pro-inflammatory cytokines such as interleukin (IL)-1β, IL-6, and tumor necrosis factor-α (TNF-α) influencing the surrounding brain tissue. Microglial activation, proinflammatory cytokine release and leukocytes trafficking are associated following JEV infection in central nervous system (CNS). How the pattern recognition receptors sense the viral nucleic acid and how the microglial and neuronal cells behaves following JEV infection is still unelucidated. There is scarcity of data on the expression levels of toll like receptors (TLRs), cytokines and chemokines in JEV infection in invitro model. To explore the molecular mechanisms of JEV infection of microglial cells and neuronal cells, we studied the expression profile of TLRs, cytokines and chemokines in JEV infected microglial cell line BV2 and Neuronal cell line Neuro 2A. For the present study, we developed the mouse model of encephalitis by intracerebral (IC) injection of JE virus for virus propagation, disease progression and damage study. Our results demonstrate the exaggerated release of some specific TLRs, cytokines and chemokines in invitro cell culture of microglial and Neuro 2A cell line, which are associated with bad outcome in invivo study.

Mechanisms of Neuroinvasion and Neuropathogenesis by Pathologic Flaviviruses

Flaviviruses are present on every continent and cause significant morbidity and mortality. In many instances, severe cases of infection with flaviviruses involve the invasion of and damage to the central nervous system (CNS). Currently, there are several mechanisms by which it has been hypothesized flaviviruses reach the brain, including the disruption of the blood-brain barrier (BBB) which acts as a first line of defense by blocking the entry of many pathogens into the brain, passing through the BBB without disruption, as well as travelling into the CNS through axonal transport from peripheral nerves. After flaviviruses have entered the CNS, they cause different neurological symptoms, leading to years of neurological sequelae or even death. Similar to neuroinvasion, there are several identified mechanisms of neuropathology, including direct cell lysis, blockage of the cell cycle, indication of apoptosis, as well as immune induced pathologies. In this review, we aim to summarize the current knowledge in the field of mechanisms of both neuroinvasion and neuropathogenesis during infection with a variety of flaviviruses and examine the potential contributions and timing of each discussed pathway.

Japanese Encephalitis Virus-Infected Cells

RNA virus infections have been a leading cause of pandemics. Aided by global warming and increased connectivity, their threat is likely to increase over time. The flaviviruses are one such RNA virus family, and its prototypes such as the Japanese encephalitis virus (JEV), Dengue virus, Zika virus, West Nile virus, etc., pose a significant health burden on several endemic countries. All viruses start off their life cycle with an infected cell, wherein a series of events are set in motion as the virus and host battle for autonomy. With their remarkable capacity to hijack cellular systems and, subvert/escape defence pathways, viruses are able to establish infection and disseminate in the body, causing disease. Using this strategy, JEV replicates and spreads through several cell types such as epithelial cells, fibroblasts, monocytes and macrophages, and ultimately breaches the blood-brain barrier to infect neurons and microglia. The neurotropic nature of JEV, its high burden on the paediatric population, and its lack of any specific antivirals/treatment strategies emphasise the need for biomedical research-driven solutions. Here, we highlight the latest research developments on Japanese encephalitis virus-infected cells and discuss how these can aid in the development of future therapies.

Molecular Mechanism and Role of Japanese Encephalitis Virus Infection in Central Nervous System-Mediated Diseases

The Japanese encephalitis virus (JEV) is the most common cause of neurodegenerative disease in Southeast Asia and the Western Pacific region; approximately 1.15 billion people are at risk, and thousands suffer from permanent neurological disorders across Asian countries, with 10-15 thousand people dying each year. JEV crosses the blood-brain barrier (BBB) and forms a complex with receptors on the surface of neurons. GRP78, Src, TLR7, caveolin-1, and dopamine receptor D2 are involved in JEV binding and entry into the neurons, and these receptors also play a role in carcinogenic activity in cells. JEV binds to GRP78, a member of the HSP70 overexpressed on malignant cells to enter neurons, indicating a higher chance of JEV infection in cancer patients. However, JEV enters human brain microvascular endothelial cells via an endocytic pathway mediated by caveolae and the ezrin protein and also targets dopamine-rich areas for infection of the midbrain via altering dopamine levels. In addition, JEV complexed with CLEC5A receptor of macrophage cells is involved in the breakdown of the BBB and central nervous system (CNS) inflammation. CLEC5A-mediated infection is also responsible for the influx of cytokines into the CNS. In this review, we discuss the neuronal and macrophage surface receptors involved in neuronal death.

Neuroinflammation in neurodegeneration via microbial infections

Recent epidemiological studies show a noticeable correlation between chronic microbial infections and neurological disorders. However, the underlying mechanisms are still not clear due to the biological complexity of multicellular and multiorgan interactions upon microbial infections. In this review, we show the infection leading to neurodegeneration mediated by multiorgan interconnections and neuroinflammation. Firstly, we highlight three inter-organ communications as possible routes from infection sites to the brain: nose-brain axis, lung-brain axis, and gut-brain axis. Next, we described the biological crosstalk between microglia and astrocytes upon pathogenic infection. Finally, our study indicates how neuroinflammation is a critical player in pathogen-mediated neurodegeneration. Taken together, we envision that antibiotics targeting neuro-pathogens could be a potential therapeutic strategy for neurodegeneration.

Nucleotide-Binding Oligomerization Domain 1 (NOD1) Positively Regulates Neuroinflammation during Japanese Encephalitis Virus Infection

Japanese encephalitis virus (JEV) is a neurotropic flavivirus that invades the central nervous system and causes neuroinflammation and extensive neuronal cell death. Nucleotide-binding oligomerization domain 1 (NOD1) is a type of pattern recognition receptor that plays a regulatory role in both bacterial and nonbacterial infections. However, the role of NOD1 in JEV-induced neuroinflammation remains undisclosed. In this study, we evaluated the effect of NOD1 activation on the progression of JEV-induced neuroinflammation using a human astrocytic cell line and NOD1 knockout mice. The results showed that JEV infection upregulated the mRNA and protein expression of NOD1, ultimately leading to an enhanced neuroinflammatory response in vivo and in vitro. Inhibition of NOD1 in cultured cells or mice significantly abrogated the inflammatory response triggered by JEV infection. Moreover, compared to the wild-type mice, the NOD1 knockout mice showed resistance to JEV infection. Mechanistically, the NOD1-mediated neuroinflammatory response was found to be associated with increased expression or activation/phosphorylation of downstream receptor-interacting protein 2 (RIPK2), mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), Jun N-terminal protein kinase (JNK), and NF-κB signaling molecules. Thus, NOD1 targeting could be a therapeutic approach to treat Japanese encephalitis. IMPORTANCE Neuroinflammation is the main pathological manifestation of Japanese encephalitis (JE) and the most important factor leading to morbidity and death in humans and animals infected by JEV. An in-depth understanding of the basic mechanisms of neuroinflammation will contribute to research on JE treatment. This study proved that JEV infection can activate the NOD1-RIPK2 signal cascade to induce neuroinflammation through the proven downstream MAPK, ERK, JNK, and NF-κB signal pathway. Thus, our study unveiled NOD1 as a potential target for therapeutic intervention for JE.

Association of single nucleotide polymorphisms in the CD209, MMP9, TNFA and IFNG genes with susceptibility to Japanese encephalitis in children from North India

Japanese encephalitis (JE), an acute encephalitis syndrome disease caused by infection with JE virus (JEV), is an important mosquito borne disease in developing countries. The clinical outcomes of JEV infection show inter individual differences. Only in a minor percent of the infected subjects, the disease progresses into acute encephalitis syndrome. Single nucleotide polymorphisms in the host immune response related genes are known to affect susceptibility to JE. In the present study, 238 JE cases and 405 healthy controls (HCs) without any known history of encephalitis were investigated for SNPs in the CD209 MX1, TLR3, MMP9, TNFA and IFNG genes which are important in the immune response against JEV by PCR based methods. The results revealed higher frequencies of heterozygous genotypes of CD209 rs4804803, MMP9 rs17576, TNFA rs1800629 and IFNG rs2430561 in JE cases compared to HCs. These SNPs were associated with JE in an over-dominant genetic model (Odds ratio with 95% CI 1.51 (1.09-2.10) for CD209 rs4804803, 1.52 (1.09-2.11) for MMP9 rs17576, and 1.55 (1.12-2.15) for IFNG rs2430561). The association of G/A genotype of TNFA rs1800629 with JE was confirmed in a larger sample size. The results suggest the association of CD209 rs4804803, MMP9 rs17576, IFNG rs2430561 and TNFA rs1800629 polymorphisms with susceptibility to JE.

Toll-like Receptors in Viral Encephalitis

Viral encephalitis is a rare but serious syndrome. In addition to DNA-encoded herpes viruses, such as herpes simplex virus and varicella zoster virus, RNA-encoded viruses from the families of Flaviviridae, Rhabdoviridae and Paramyxoviridae are important neurotropic viruses. Whereas in the periphery, the role of Toll-like receptors (TLR) during immune stimulation is well understood, TLR functions within the CNS are less clear. On one hand, TLRs can affect the physiology of neurons during neuronal progenitor cell differentiation and neurite outgrowth, whereas under conditions of infection, the complex interplay between TLR stimulated neurons, astrocytes and microglia is just on the verge of being understood. In this review, we summarize the current knowledge about which TLRs are expressed by cell subsets of the CNS. Furthermore, we specifically highlight functional implications of TLR stimulation in neurons, astrocytes and microglia. After briefly illuminating some examples of viral evasion strategies from TLR signaling, we report on the current knowledge of primary immunodeficiencies in TLR signaling and their consequences for viral encephalitis. Finally, we provide an outlook with examples of TLR agonist mediated intervention strategies and potentiation of vaccine responses against neurotropic virus infections.

Proteomic landscape of Japanese encephalitis virus-infected fibroblasts

Advances in proteomics have enabled a comprehensive understanding of host-pathogen interactions. Here we have characterized Japanese encephalitis virus (JEV) infection-driven changes in the mouse embryonic fibroblast (MEF) proteome. Through tandem mass tagging (TMT)-based mass spectrometry, we describe changes in 7.85 % of the identified proteome due to JEV infection. Pathway enrichment analysis showed that proteins involved in innate immune sensing, interferon responses and inflammation were the major upregulated group, along with the immunoproteasome and poly ADP-ribosylation proteins. Functional validation of several upregulated anti-viral innate immune proteins, including an active cGAS-STING axis, was performed. Through siRNA depletion, we describe a crucial role of the DNA sensor cGAS in restricting JEV replication. Further, many interferon-stimulated genes (ISGs) were observed to be induced in infected cells. We also observed activation of TLR2 and inhibition of TLR2 signalling using TLR1/2 inhibitor CU-CPT22-blocked production of inflammatory cytokines IL6 and TNF-α from virus-infected N9 microglial cells. The major proteins that were downregulated by infection were involved in cell adhesion (collagens), transport (solute carrier and ATP-binding cassette transporters), sterol and lipid biosynthesis. Several collagens were found to be transcriptionally downregulated in infected MEFs and mouse brain. Collectively, our data provide a bird's-eye view into how fibroblast protein composition is rewired following JEV infection.

Pathobiology of Japanese encephalitis virus infection

Japanese encephalitis virus (JEV) is a flavivirus, spread by the bite of carrier Culex mosquitoes. The subsequent disease caused is Japanese encephalitis (JE), which is the leading global cause of virus-induced encephalitis. The disease is predominant in the entire Asia-Pacific region with the potential of global spread. JEV is highly neuroinvasive with symptoms ranging from mild fever to severe encephalitis and death. One-third of JE infections are fatal, and half of the survivors develop permanent neurological sequelae. Disease prognosis is determined by a series of complex and intertwined signaling events dictated both by the virus and the host. All flaviviruses, including JEV replicate in close association with ER derived membranes by channelizing the protein and lipid components of the ER. This leads to activation of acute stress responses in the infected cell-oxidative stress, ER stress, and autophagy. The host innate immune and inflammatory responses also enter the fray, the components of which are inextricably linked to the cellular stress responses. These are especially crucial in the periphery for dendritic cell maturation and establishment of adaptive immunity. The pathogenesis of JEV is a combination of direct virus induced neuronal cell death and an uncontrolled neuroinflammatory response. Here we provide a comprehensive review of the JEV life cycle and how the cellular stress responses dictate the pathobiology and resulting immune response. We also deliberate on how modulation of these stress pathways could be a potential strategy to develop therapeutic interventions, and define the persisting challenges.

Type I IFN signaling limits hemorrhage-like disease after infection with Japanese encephalitis virus through modulating a prerequisite infection of CD11b+Ly-6C+ monocytes

Background

The crucial role of type I interferon (IFN-I, IFN-α/β) is well known to control central nervous system (CNS) neuroinflammation caused by neurotrophic flaviviruses such as Japanese encephalitis virus (JEV) and West Nile virus. However, an in-depth analysis of IFN-I signal-dependent cellular factors that govern CNS-restricted tropism in JEV infection in vivo remains to be elucidated.

Methods

Viral dissemination, tissue tropism, and cytokine production were examined in IFN-I signal-competent and -incompetent mice after JEV inoculation in tissues distal from the CNS such as the footpad. Bone marrow (BM) chimeric models were used for defining hematopoietic and tissue-resident cells in viral dissemination and tissue tropism.

Results

The paradoxical and interesting finding was that IFN-I signaling was essentially required for CNS neuroinflammation following JEV inoculation in distal footpad tissue. IFN-I signal-competent mice died after a prolonged neurological illness, but IFN-I signal-incompetent mice all succumbed without neurological signs. Rather, IFN-I signal-incompetent mice developed hemorrhage-like disease as evidenced by thrombocytopenia, functional injury of the liver and kidney, increased vascular leakage, and excessive cytokine production. This hemorrhage-like disease was closely associated with quick viral dissemination and impaired IFN-I innate responses before invasion of JEV into the CNS. Using bone marrow (BM) chimeric models, we found that intrinsic IFN-I signaling in tissue-resident cells in peripheral organs played a major role in inducing the hemorrhage-like disease because IFN-I signal-incompetent recipients of BM cells from IFN-I signal-competent mice showed enhanced viral dissemination, uncontrolled cytokine production, and increased vascular leakage. IFN-I signal-deficient hepatocytes and enterocytes were permissive to JEV replication with impaired induction of antiviral IFN-stimulated genes, and neuron cells derived from both IFN-I signal-competent and -incompetent mice were vulnerable to JEV replication. Finally, circulating CD11b⁺Ly-6C⁺ monocytes infiltrated into the distal tissues inoculated by JEV participated in quick viral dissemination to peripheral organs of IFN-I signal-incompetent mice at an early stage.

Conclusion

An IFN-I signal-dependent model is proposed to demonstrate how CD11b⁺Ly-6C⁺ monocytes are involved in restricting the tissue tropism of JEV to the CNS.

The increased intrathecal expression of the monocyte-attracting chemokines CCL7 and CXCL12 in tick-borne encephalitis

Tick-borne encephalitis (TBE) is a relatively severe and clinically variable central nervous system (CNS) disease with a significant contribution of a secondary immunopathology. Monocytes/macrophages play an important role in the CNS inflammation, but their pathogenetic role and migration mechanisms in flavivirus encephalitis in humans are not well known. We have retrospectively analyzed blood and cerebrospinal fluid (CSF) monocyte counts in 240 patients with TBE presenting as meningitis (n = 110), meningoencephalitis (n = 114), or meningoencephalomyelitis (n = 16), searching for associations with other laboratory parameters, clinical presentation, and severity. We have measured concentrations of selected monocytes-attracting chemokines (CCL7, CXCL12, CCL20) in serum and CSF of the prospectively recruited patients with TBE (n = 15), with non-TBE aseptic meningitis (n = 6) and in non-infected controls (n = 8). The data were analyzed with non-parametric tests, p < 0.05 considered significant. Monocyte CSF count correlated with other CSF inflammatory parameters, but not with the peripheral monocytosis, consistent with an active recruitment into CNS. The monocyte count did not correlate with a clinical presentation. The median CSF concentration of CCL7 and CXCL12 was increased in TBE, and that of CCL7 was higher in TBE than in non-TBE meningitis. The comparison of serum and CSF concentrations pointed to the intrathecal synthesis of CCL7 and CXCL12, but with no evident concentration gradients toward CSF. In conclusion, the monocytes are recruited into the intrathecal compartment in concert with other leukocyte populations in TBE. CCL7 and CXCL12 have been found upregulated intrathecally but are not likely to be the main monocyte chemoattractants.

Peripheral Injection of Tim-3 Antibody Attenuates VSV Encephalitis by Enhancing MHC-I Presentation

Viral encephalitis is the most common cause of encephalitis. It is responsible for high morbidity rates, permanent neurological sequelae, and even high mortality rates. The host immune response plays a critical role in preventing or clearing invading pathogens, especially when effective antiviral treatment is lacking. However, due to blockade of the blood-brain barrier, it remains unclear how peripheral immune cells contribute to the fight against intracerebral viruses. Here, we report that peripheral injection of an antibody against human Tim-3, an immune checkpoint inhibitor widely expressed on immune cells, markedly attenuated vesicular stomatitis virus (VSV) encephalitis, marked by decreased mortality and improved neuroethology in mice. Peripheral injection of Tim-3 antibody enhanced the recruitment of immune cells to the brain, increased the expression of major histocompatibility complex-I (MHC-I) on macrophages, and as a result, promoted the activation of VSV-specific CD8⁺ T cells. Depletion of macrophages abolished the peripheral injection-mediated protection against VSV encephalitis. Notably, for the first time, we found a novel post-translational modification of MHC-I by Tim-3, wherein, by enhancing the expression of MARCH9, Tim-3 promoted the proteasome-dependent degradation of MHC-I via K48-linked ubiquitination in macrophages. These results provide insights into the immune response against intracranial infections; thus, manipulating the peripheral immune cells with Tim-3 antibody to fight viruses in the brain may have potential applications for combating viral encephalitis.

Pre-treatment with Scopolamine Naturally Suppresses Japanese Encephalitis Viral Load in Embryonated Chick Through Regulation of Multiple Signaling Pathways

Suitable recognition of invasive microorganisms is a crucial factor for evoking a strong immune response that can combat the pathogen. Toll-like receptors (TLRs) play a pivotal role in the induction of this innate immune response through stimulation of interferons (IFNs) that control viral replication in the host via distinct signaling pathways. Though the antiviral property of Atropa belladonna has been established, yet the role of one of its active components scopolamine in modulating various factors of the innate immune branch has not yet been investigated until date. Thus, the present study was conducted to assess the antiviral effects of scopolamine and its immunomodulatory role against Japanese encephalitis virus (JEV) infections in embryonated chick. Pre-treatment with scopolamine hydrobromide showed a significant decrease in the viral loads of chorioallantoic membrane (CAM) and brain tissues. Molecular docking analysis revealed that scopolamine hydrobromide binds to the active site of non-structural protein 5 (NS5) that has enzymatic activities required for replication of JEV, making it a highly promising chemical compound against the virus. The binding contributions of different amino acid residues at or near the active site suggest a potential binding of this compound. Pre-treatment with the scopolamine hydrobromide showed significant upregulation of different TLRs like TLR3, TLR7, and TLR8, interleukins like IL-4, and IL-10, as well as IFNs and their regulatory factors. However, virus-infected tissues (direct infection group) exhibited higher TLR4 expression as compared to scopolamine hydrobromide pre-treated, virus-infected tissues (medicine pre-treated group). These results indicate that scopolamine hydrobromide contributes much to launch antiviral effects by remoulding the TLR and IFN signaling pathways that are involved in sensing and initiating the much-needed anti-JEV responses.

Toll-like receptor 4 mediates blood-brain barrier permeability and disease in C3H mice during Venezuelan equine encephalitis virus infection

Venezuelan equine encephalitis virus (VEEV) is an encephalitic alphavirus that can cause debilitating, acute febrile illness and potentially result in encephalitis. Currently, there are no FDA-licensed vaccines or specific therapeutics for VEEV. Previous studies have demonstrated that VEEV infection results in increased blood-brain barrier (BBB) permeability that is mediated by matrix metalloproteinases (MMPs). Furthermore, after subarachnoid hemorrhage in mice, MMP-9 is upregulated in the brain and mediates BBB permeability in a toll-like receptor 4 (TLR4)-dependent manner. Here, we demonstrate that disease in C3H mice during VEEV TC-83 infection is dependent on TLR4 because intranasal infection of C3H/HeN (TLR4^WT) mice with VEEV TC-83 resulted in mortality as opposed to survival of TLR4-defective C3H/HeJ (TLR4^mut) mice. In addition, BBB permeability was induced to a lesser extent in TLR4^mut mice compared with TLR4^WT mice during VEEV TC-83 infection as determined by sodium fluorescein and fluorescently-conjugated dextran extravasation. Moreover, MMP-9, MMP-2, ICAM-1, CCL2 and IFN-γ were all induced to significantly lower levels in the brains of infected TLR4^mut mice compared with infected TLR4^WT mice despite the absence of significantly different viral titers or immune cell populations in the brains of infected TLR4^WT and TLR4^mut mice. These data demonstrate the critical role of TLR4 in mediating BBB permeability and disease in C3H mice during VEEV TC-83 infection, which suggests that TLR4 is a potential target for the development of therapeutics for VEEV.

CCR2 Inhibition Reduces Neurotoxic Microglia Activation Phenotype After Japanese Encephalitis Viral Infection

Controlling the proinflammatory response of microglia by targeting chemokines (C-C motif) receptor 2 (CCR2) could be an important therapeutic approach for Japanese encephalitis virus (JEV) infection. Here, through JEV infection to BV2 microglia and young BALB/c mice, we investigated that CCR2 is highly upregulated after JEV infection and plays a key role in determining microglia activation phenotype and associated with neurotoxic proinflammatory mediators of TNF-α and IFNγ. In addition, we found JEV infection to BV2 microglia causes an increase in microglial proliferation and cell body area at day 1 and day 3. Using the agonist molecule of CCR2 inhibition; RS102895, significantly reduces microglia reactive phenotype and nitric oxide production. Further, to define the role of CCR2 in functional responses of microglia and their activation phenotype, we performed in vitro cell scratch functional assay and ImageJ analysis. When compared with control, microglia cells showed a significant increase in elongated or rod-like activated phenotype in JEV-infected cells at 24 h post-infection and CCR2 inhibition significantly reduced the elongated activation phenotype induced by JEV infection, suggesting that CCR2 acts as a critical regulator for microglia activation phenotype after JEV infection. We found that JEV-infected mice treated with RS102895 had less microglia activation and reduced mRNA expression of CCR2 and proinflammatory mediators such as IFN-γ in cortical tissue. Collectively, our data indicate that CCR2 drives reactive phenotype of microglia and its inhibition reduces microglia activation and neurotoxic proinflammatory mediators after JEV infection.

Degradation of MicroRNA miR-466d-3p by Japanese Encephalitis Virus NS3 Facilitates Viral Replication and Interleukin-1β Expression

Japanese encephalitis virus (JEV) infection alters microRNA (miRNA) expression in the central nervous system (CNS). However, the mechanism contributing to miRNA regulation in the CNS is not known. We discovered global degradation of mature miRNA in mouse brains and neuroblastoma (NA) cells after JEV infection. Integrative analysis of miRNAs and mRNAs suggested that several significantly downregulated miRNAs and their targeted mRNAs were clustered into an inflammation pathway. Transfection with miRNA 466d-3p (miR-466d-3p) decreased interleukin-1β (IL-1β) expression and inhibited JEV replication in NA cells. However, miR-466d-3p expression increased after JEV infection in the presence of cycloheximide, indicating that viral protein expression reduced miR-466d-3p expression. We generated all the JEV coding proteins and demonstrated NS3 helicase protein to be a potent miRNA suppressor. The NS3 proteins of Zika virus, West Nile virus, and dengue virus serotype 1 (DENV-1) and DENV-2 also decreased miR-466d-3p expression. Results from helicase-blocking assays and in vitro unwinding assays demonstrated that NS3 could unwind pre-miR-466d and induce miRNA dysfunction. Computational models and an RNA immunoprecipitation assay revealed arginine-rich domains of NS3 to be crucial for pre-miRNA binding and degradation of host miRNAs. Importantly, site-directed mutagenesis of conserved residues in NS3 revealed that R226G and R202W reduced the binding affinity and degradation of pre-miR-466d. These results expand the function of flavivirus helicases beyond unwinding duplex RNA to degrade pre-miRNAs. Hence, we revealed a new mechanism for NS3 in regulating miRNA pathways and promoting neuroinflammation.IMPORTANCE Host miRNAs have been reported to regulate JEV-induced inflammation in the CNS. We found that JEV infection could reduce expression of host miRNA. The helicase region of the NS3 protein bound specifically to miRNA precursors and could lead to incorrect unwinding of miRNA precursors, thereby reducing the expression of mature miRNAs. This observation led to two major findings. First, our results suggested that JEV NS3 protein induced miR-466d-3p degradation, which promoted IL-1β expression and JEV replication. Second, arginine molecules on NS3 were the main miRNA-binding sites, because we demonstrated that miRNA degradation was abolished if arginines at R226 and R202 were mutated. Our study provides new insights into the molecular mechanism of JEV and reveals several amino acid sites that could be mutated for a JEV vaccine.

Transcriptome Analysis Reveals the Neuro-Immune Interactions in Duck Tembusu Virus-Infected Brain

The duck Tembusu virus (DTMUV) is a mosquito-borne flavivirus. It causes severe symptoms of egg-drop, as well as neurological symptoms and brain damage in ducks. However, the specific molecular mechanisms of DTMUV-induced neurovirulence and host responses in the brain remain obscure. To better understand the host-pathogen and neuro-immune interactions of DTMUV infection, we conducted high-throughput RNA-sequencing to reveal the transcriptome profiles of DTMUV-infected duck brain. Totals of 117, 212, and 150 differentially expressed genes (DEGs) were identified at 12, 24, and 48 h post infection (hpi). Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses uncovered genes and pathways related to the nervous system and immune responses in duck brain. Neuro-related genes, including WNT3A, GATA3, and CHRNA6, were found to be significantly downregulated. RIG-I-like receptors (DHX58, IFIH1) and Toll-like receptors (TLR2 and TLR3) were activated, inducing the expression of 22 interferon stimulated genes (ISGs) and antigen-processing and -presenting genes (TAP1 and TAP2) in the brain. Our research provides comprehensive information for the molecular mechanisms of neuro-immune and host-pathogen interactions of DTMUV.

RIPK3 Promotes JEV Replication in Neurons via Downregulation of IFI44L

Japanese encephalitis virus (JEV), the leading cause of viral encephalitis in Asia, is neurovirulent and neuroinvasive. Neurons are the main target of JEV infection and propagation. Receptor interacting serine/threonine-protein kinase 3 (RIPK3) has been reported to contribute to neuroinflammation and neuronal death in many central nervous system diseases. In this study, we found that the progression of JE was alleviated in RIPK3-knockout (RIPK3^–/–) mice in both peripheral and intracerebral infection. RIPK3-knockdown (RIPK3-RNAi) neuro2a cells showed higher cell viability during JEV infection. Moreover, the JEV load was significantly decreased in RIPK3^–/– mouse-derived primary neurons and RIPK3-RNAi neuro2a cells compared with wild-type neurons, but this was not observed in microglia. Furthermore, RNA sequencing of brain tissues showed that the level of the interferon (IFN)-induced protein 44-like gene (IFI44L) was significantly increased in JEV-infected RIPK3^–/– mouse brains, RIPK3^–/– neurons, and RIPK3-RNAi-neuro2a cells. Then, it was demonstrated that the propagation of JEV was inhibited in IFI44L-overexpressing neuro2a cells and enhanced in IFI44L and RIPK3 double knockdown neuro2a cells. Taken together, our results showed that the increased expression of RIPK3 following JEV infection played complicated roles. On the one hand, RIPK3 participated in neuroinflammation and neuronal death during JEV infection. On the other hand, RIPK3 inhibited the expression of IFI44L to some extent, leading to the propagation of JEV in neurons, which might be a strategy for JEV to evade the cellular innate immune response.

Double-faced implication of CD4+ Foxp3+ regulatory T cells expanded by acute dengue infection via TLR2/MyD88 pathway

Dengue infection causes dengue fever (DF) and dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). CD4⁺ Foxp3⁺ Tregs are expanded in patients during dengue infection, and appear to be associated with clinical severity. However, molecular pathways involved in Treg proliferation and the reason for their insufficient control of severe diseases are poorly understood. Here, dengue infection induced the proliferation of functional CD4⁺ Foxp3⁺ Tregs via TLR2/MyD88 pathway. Surface TLR2 on Tregs was responsible for their proliferation, and dengue-expanded Tregs subverted in vivo differentiation of effector CD8⁺ T cells. An additional interesting finding was that dengue-infected hosts displayed changed levels of susceptibility to other diseases in TLR2-dependent manner. This change included enhanced susceptibility to tumors and bacterial infection, but highly enhanced resistance to viral infection. Further, the transfer of dengue-proliferated Tregs protected the recipients from dengue-induced DHF/DSS and LPS-induced sepsis. In contrast, dengue-infected hosts were more susceptible to sepsis, an effect attributable to early TLR2-dependent production of proinflammatory cytokines. These facts may explain the reason why in some patients, dengue-proliferated Tregs is insufficient to control DF and DHF/DSS. Also, our observations lead to new insights into Treg responses activated by dengue infection in a TLR2-dependent manner, which could differentially act on subsequent exposure to other disease-producing situations.

Beneficial and Detrimental Effects of Regulatory T Cells in Neurotropic Virus Infections

Neurotropic viruses infect the central nervous system (CNS) and cause acute or chronic neurologic disabilities. Regulatory T cells (Treg) play a critical role for immune homeostasis, but may inhibit pathogen-specific immunity in infectious disorders. The present review summarizes the current knowledge about Treg in human CNS infections and their animal models. Besides dampening pathogen-induced immunopathology, Treg have the ability to facilitate protective responses by supporting effector T cell trafficking to the infection site and the development of resident memory T cells. Moreover, Treg can reduce virus replication by inducing apoptosis of infected macrophages and attenuate neurotoxic astrogliosis and pro-inflammatory microglial responses. By contrast, detrimental effects of Treg are caused by suppression of antiviral immunity, allowing for virus persistence and latency. Opposing disease outcomes following Treg manipulation in different models might be attributed to differences in technique and timing of intervention, infection route, genetic background, and the host’s age. In addition, mouse models of virus-induced demyelination revealed that Treg are able to reduce autoimmunity and immune-mediated CNS damage in a disease phase-dependent manner. Understanding the unique properties of Treg and their complex interplay with effector cells represents a prerequisite for the development of new therapeutic approaches in neurotropic virus infections.

DEF Cell-Derived Exosomal miR-148a-5p Promotes DTMUV Replication by Negative Regulating TLR3 Expression

Duck tembusu virus (DTMUV) is a single-stranded, positive-polarity RNA flavivirus that has caused considerable economic losses in China in recent years. Innate immunity represents the first line of defense against invading pathogens and serves as an important role in resisting viral infections. In this study, we found that the infection of ducks by DTMUV triggers Toll-like receptors (TLRs) and (RIG-I)-like receptors (RLRs) signaling pathways and inducing abundant of pro-inflammatory factors and type I interferons (IFNs), in which melanoma differentiation-associated gene 5 (MDA5) and Toll-like receptor 3 (TLR3) play important immunity roles, they can inhibit the replication process of DTMUV via inducing type I IFNs. Moreover, we demonstrated that type I IFNs can inhibit the DTMUV replication process in a time- and dose-dependent manner. Exosomes are small membrane vesicles that have important roles in intercellular communication. MicroRNAs (miRNAs) are small non-coding RNAs that can modulate gene expression and are common substances in exosomes. In our experiment, we successfully isolated DEF cells derived exosome for the first time and explored its function. Firstly, we found the expression of miR-148a-5p is significantly decreased following DTMUV infect. Then we found miR-148a-5p can target TLR3 and down-regulate the expression of TLR3, serving as a negative factor in innate immunity. Unfortunately, we cannot find miRNAs with different expression changes that can target MDA5. Lastly, our experimental results showed that TLR3 was one of the causes of miR-148a-5p reduction, suggesting that the high level of TLR3 after DTMUV infect can both trigger innate immunity and suppress miR-148a-5p to resist DTMUV.

Japanese encephalitis - the prospects for new treatments

Japanese encephalitis is a mosquito-borne disease that occurs in Asia and is caused by Japanese encephalitis virus (JEV), a member of the genus Flavivirus. Although many flaviviruses can cause encephalitis, JEV causes particularly severe neurological manifestations. The virus causes loss of more disability-adjusted life years than any other arthropod-borne virus owing to the frequent neurological sequelae of the condition. Despite substantial advances in our understanding of Japanese encephalitis from in vitro studies and animal models, studies of pathogenesis and treatment in humans are lagging behind. Few mechanistic studies have been conducted in humans, and only four clinical trials of therapies for Japanese encephalitis have taken place in the past 10 years despite an estimated incidence of 69,000 cases per year. Previous trials for Japanese encephalitis might have been too small to detect important benefits of potential treatments. Many potential treatment targets exist for Japanese encephalitis, and pathogenesis and virological studies have uncovered mechanisms by which these drugs could work. In this Review, we summarize the epidemiology, clinical features, prevention and treatment of Japanese encephalitis and focus on potential new therapeutic strategies, based on repurposing existing compounds that are already suitable for human use and could be trialled without delay. We use our newly improved understanding of Japanese encephalitis pathogenesis to posit potential treatments and outline some of the many challenges that remain in tackling the disease in humans.

Role of NS1 and TLR3 in Pathogenesis and Immunity of WNV

West Nile Virus (WNV) is a mosquito-transmitted flavivirus which causes encephalitis especially in elderly and immunocompromised individuals. Previous studies have suggested the protective role of the Toll-like receptor 3 (TLR3) pathway against WNV entry into the brain, while the WNV non-structural protein 1 (NS1) interferes with the TLR3 signaling pathway, besides being a component of viral genome replication machinery. In this study, we investigated whether immunization with NS1 could protect against WNV neuroinvasion in the context of TLR3 deficiency. We immunized mice with either an intact or deleted TLR3 system (TLR3KO) with WNV envelope glycoprotein (gE) protein, NS1, or a combination of gE and NS1. Immunization with gE or gE/NS1, but not with NS1 alone, induced WNV neutralizing antibodies and protected against WNV brain invasion and inflammation. The presence of intact TLR3 signaling had no apparent effect on WNV brain invasion. However, mock-immunized TLR3KO mice had higher inflammatory cell invasion upon WNV brain infection than NS1-immunized TLR3KO mice and wild type mice. Thus, immunization against NS1 may reduce brain inflammation in a context of TLR3 signaling deficiency.

Indispensable Role of CX3CR1+ Dendritic Cells in Regulation of Virus-Induced Neuroinflammation Through Rapid Development of Antiviral Immunity in Peripheral Lymphoid Tissues

A coordinated host immune response mediated via chemokine network plays a crucial role in boosting defense mechanisms against pathogenic infections. The speed of Ag presentation and delivery by CD11c⁺ dendritic cells (DCs) to cognate T cells in lymphoid tissues may decide the pathological severity of the infection. Here, we investigated the role of CX₃CR1 in the neuroinflammation induced by infection with Japanese encephalitis virus (JEV), a neurotrophic virus. Interestingly, CX₃CR1 deficiency strongly enhanced susceptibility to JEV only after peripheral inoculation via footpad. By contrast, both CX₃CR1^+/+ and CX₃CR1^-/- mice showed comparable susceptibility to JEV following inoculation via intranasal and intraperitoneal routes. CX₃CR1^-/- mice exhibited lethal neuroinflammation after JEV inoculation via footpad route, showing high mortality, morbidity, pro-inflammatory cytokine expression, and uncontrolled CNS-infiltration of peripheral leukocytes including Ly-6C^hi monocytes and Ly-6G^hi granulocytes. Furthermore, the absence of CX₃CR1⁺CD11c⁺ DCs appeared to enhance susceptibility of CX₃CR1^-/- mice to JE after peripheral JEV inoculation. CX₃CR1 ablation impaired the migration of CX₃CR1⁺CD11c⁺ DCs from JEV-inoculated sites to draining lymph nodes (dLNs), resulting in decreased NK cell activation and JEV-specific CD4⁺/CD8⁺ T-cell responses. However, CX₃CR1-competent mice showed rapid temporal expression of viral Ags in dLNs. Subsequently, JEV was rapidly cleared, with concomitant generation of antiviral NK cell activation and T-cell responses mediated by rapid migration of JEV Ag⁺CX₃CR1⁺CD11c⁺ DCs. Using biallelic functional CX₃CR1 expression system, the functional expression of CX₃CR1 on CD11c^hi DCs appeared to be essentially required for inducing rapid and effective responses of NK cell activation and Ag-specific CD4⁺ T cells in dLNs. Strikingly, adoptive transfer of CX₃CR1⁺CD11c⁺ DCs was found to completely restore the resistance of CX₃CR1^-/- recipients to JEV, as corroborated by the rapid delivery of JEV Ags in dLNs and attenuation of neuroinflammation in the CNS. Collectively, these results indicate that CX₃CR1⁺CD11c⁺ DCs play an important role in generating rapid and effective responses of antiviral NK cell activation and Ag-specific T cells after peripheral inoculation with the virus, thereby resulting in conferring resistance to viral infection by reducing the peripheral viral burden.

PD1+CCR2+CD8+ T Cells Infiltrate the Central Nervous System during Acute Japanese Encephalitis Virus Infection

Japanese encephalitis (JE) is a viral encephalitis disease caused by Japanese encephalitis virus (JEV) infection. Uncontrolled inflammatory responses in the central nervous system (CNS) are a hallmark of severe JE. Although the CCR2-CCL2 axis is important for monocytes trafficking during JEV infection, little is known about its role in CNS trafficking of CD8⁺ T cells. Here, we characterized a mouse model of JEV infection, induced via intravenous injection (i.v.) and delineated the chemokines and infiltrating peripheral immune cells in the brains of infected mice. The CNS expression of chemokines, Ccl2, Ccl3, and Ccl5, and their receptors, Ccr2 or Ccr5, was significantly up-regulated after JEV infection and was associated with the degree of JE pathogenesis. Moreover, JEV infection resulted in the migration of a large number of CD8⁺ T cells into the CNS. In the brains of JEV-infected mice, infiltrating CD8⁺ T cells expressed CCR2 and CCR5 and were found to comprise mainly effector T cells (CD44⁺CD62L^-). JEV infection dramatically enhanced the expression of programmed death 1 (PD-1) on infiltrating CD8⁺ T cells in the brain, as compared to that on peripheral CD8⁺ T cells in the spleen. This effect was more pronounced on infiltrating CCR2⁺CD8⁺ T cells than on CCR2^-CD8⁺ T cells. In conclusion, we identified a new subset of CD8⁺ T cells (PD1⁺CCR2⁺CD8⁺ T cells) present in the CNS of mice during acute JEV infection. These CD8⁺ T cells might play a role in JE pathogenesis.

Genetic susceptibility to West Nile virus infection in Camargue horses

West Nile virus (WNV) is a mosquito-borne zoonotic neurotropic virus capable to cause lethal meningoencephalitis (WNE) in infected hosts such as birds, horses, and humans. Due to their sensitivity, horses serve as sentinel species in areas at risk. We studied a population of Camargue horses living in Southern France in two zones with endemic WNV circulation where WNV outbreaks were recorded in 2000 and 2003-4. Two sets of microsatellite markers located in MHC and Ly49 genomic regions were genotyped as well as multiple SNPs in ten immunity-related candidate gene regions. Associations between genetic polymorphisms and resistance/susceptibility to WNE were tested. While single marker associations were weak, compound two-gene genotypes of SNPs located within the MAVS, NCR2 and IL-10 genes and microsatellites HMS082 and CZM013 were associated with susceptibility to WNE. Combinations of microsatellite markers CZM009, ABGe17402 and ABGe9019 were associated with simple seroconversion without clinical signs of WNE (resistance). In addition, a distribution of polymorphic markers between WNV-IgG seropositive horses and a control group of WNV-IgG seronegative horses was tested. One SNP in the OAS1 gene (NC_009151.3:g.21961328A>G) was significantly associated with the seropositive phenotype (p_corr = 0.023; OR = 40.5 CI (4.28; 383.26); RR = 8.18 CI (1.27; 52.89) in the Camargue breed. In compound genotypes, SNP markers for SLC11A1, MAVS, OAS1, TLR4, ADAM17 and NCR2 genes and ten microsatellites showed non-random distribution between seropositive and seronegative groups of horses. Further analysis of associated markers could contribute to our understanding of anti-WNV defense mechanisms in horses.

Toll-like receptors in the pathogenesis of neuroinflammation

Toll-like receptors (TLRs) are discovered as crucial pattern recognition receptors (PRRs) involved in the recognition of pathogen-associated molecular patterns (PAMPs). Later studies showed their involvement in the recognition of various damage/danger-associated molecular patterns (DAMPs) generated by host itself. Thus, TLRs are capable of recognizing wide-array of patterns/molecules derived from pathogens and host as well and initiating a proinflammatory immune response through the activation of NF-κB and other transcription factors causing synthesis of proinflammatory molecules. The process of neuroinflammation is seen under both sterile and infectious inflammatory diseases of the central nervous system (CNS) and may lead to the development of neurodegeneration. The present article is designed to highlight the importance of TLRs in the pathogenesis of neuroinflammation under diverse conditions. TLRs are expressed by various immune cells present in CNS along with neurons. However out of thirteen TLRs described in mammals, some are present and active in these cells, while some are absent and are described in detail in main text. The role of various immune cells present in the brain and their role in the pathogenesis of neuroinflammation depending on the type of TLR expressed is described. Thereafter the role of TLRs in bacterial meningitis, viral encephalitis, stroke, Alzheimer's disease (AD), Parkinson's disease (PD), and autoimmune disease including multiple sclerosis (MS) is described. The article is designed for both neuroscientists needing information regarding TLRs in neuroinflammation and TLR biologists or immunologists interested in neuroinflammation.

Differential Expression Levels of Inflammatory Chemokines and TLRs in Patients Suffering from Mild and Severe Japanese Encephalitis

Japanese encephalitis (JE) is a vector-borne viral disease with clinical manifestations ranging from asymptomatic to severe neurological symptoms and even leading to death. The exact pathophysiology for diverse clinical spectrum of the disease is complex and has not yet been defined. Studies have postulated that during JE infection, inflammatory cytokines and chemokines are produced after the initial recognition of viral antigens through the engagement of toll-like receptors (TLR) pathways. However, there is paucity of knowledge on the expression levels of chemokines and TLRs among mild and severely affected JE patients. Hence, to better understand disease pathogenesis, we examined the mRNA expression of chemokines, CCL2 and CCL5, and their respective receptors CCR2 and CCR5 along with TLRs viz. TLR3, TLR7, TLR8, and TLR9 in context of mild and severely Japanese encephalitis virus (JEV)-infected (n = 19) and healthy (n = 19) individuals. Our study showed significant downregulation of CCL2, CCL5, CCR2, CCR5, and TLR3 by log 0.87, 1.02, 0.82, 0.68, and 0.37-fold respectively, among mild cases compared with controls. Significant difference of gene expression among mild and severe JE cases for CCL2 (p < 0.001), CCL5 (p < 0.01), and TLR7 (p < 0.05) was observed. In conclusion, our results proposes that chemokines viz. CCL2 and CCL5 along with TLR7 may be associated with degree of pathogenesis of JE and could be putative therapeutic targets for preventing severe inflammation during viral encephalitis.

Vaccination and immunization strategies to design Aedes aegypti salivary protein based subunit vaccine tackling Flavivirus infection

Flavivirus causes arthropod-borne severe diseases that sometimes lead to the death. The Flavivirus species including Dengue virus, Zika virus and yellow fever virus are transmitted by the bite of Aedes mosquitoes. All these viral species target the people living in their respective endemic zone causing a high mortality rate. Recent studies show that immune factors present in the Ae. aegypti saliva is the hidden culprit promoting blood meal collection, suppressing host immune molecules and promoting disease establishment. This study was designed to develop a subunit vaccine using Aedes mosquito salivary proteins targeting the aforementioned Flaviviruses. Subunit vaccine was designed very precisely by combining the immunogenic B-cell epitope with CTL and HTL epitopes and also suitable adjuvant and linkers. Immunogenicity, allergenicity and physiochemical characterization were also performed for scientific validation. Molecular docking and molecular dynamics simulations studies were carried out to confirm the stable affinity between the vaccine protein (3D) and TLR3 receptor. At last, in silico cloning was executed to get the subunit vaccine restriction clone into pET28a vectro to express it in microbial expression system. Additionally, this study warrants the experimental evaluation for the validation purposes.

Toll-Like Receptors and RIG-I-Like Receptors Play Important Roles in Resisting Flavivirus

Flaviviridae family is a class of single-stranded RNA virus, which is fatal to human and animals and mainly prevalent in subtropic and tropic countries. Even though people and animals are barraged with flavivirus infection every year, we have not invented either vaccines or antiviral for most flavivirus infections yet. Innate immunity is the first line of defense in resisting pathogen invasion, serving an important role in a resisting virus. Toll-like receptors (TLRs) and retinoic acid-inducible gene I- (RIG-I-) like receptors (RLRs) are crucial pattern recognition receptors (PRRs) that play essential roles in recognizing and clearing pathogens, including resisting flavivirus. In the present review, we provide a significant reference for further research on the function of innate immunity in resisting flavivirus.

Dual TLR2/9 Recognition of Herpes Simplex Virus Infection Is Required for Recruitment and Activation of Monocytes and NK Cells and Restriction of Viral Dissemination to the Central Nervous System

The importance of TLR2 and TLR9 in the recognition of infection with herpes simplex virus (HSV) and HSV-caused diseases has been described, but some discrepancies remain concerning the benefits of these responses. Moreover, the impact of TLR2/9 on innate and adaptive immune responses within relevant mucosal tissues has not been elucidated using natural mucosal infection model of HSV. Here, we demonstrate that dual TLR2/9 recognition is essential to provide resistance against mucosal infection with HSV via an intravaginal route. Dual TLR2/9 ablation resulted in the highly enhanced mortality with exacerbated symptoms of encephalitis compared with TLR2 or TLR9 deficiency alone, coinciding with highly increased viral load in central nervous system tissues. TLR2 appeared to play a minor role in providing resistance against mucosal infection with HSV, since TLR2-ablated mice showed higher survival rate compared with TLR9-ablated mice. Also, the high mortality in dual TLR2/9-ablated mice was closely associated with the reduction in early monocyte and NK cell infiltration in the vaginal tract (VT), which was likely to correlate with low expression of cytokines and CCR2 ligands (CCL2 and CCL7). More interestingly, our data revealed that dual TLR2/9 recognition of HSV infection plays an important role in the functional maturation of TNF-α and iNOS-producing dendritic cells (Tip-DCs) from monocytes as well as NK cell activation in VT. TLR2/9-dependent maturation of Tip-DCs from monocytes appeared to specifically present cognate Ag, which effectively provided functional effector CD4⁺ and CD8⁺ T cells specific for HSV Ag in VT and its draining lymph nodes. TLR2/9 expressed in monocytes was likely to directly facilitate Tip-DC-like features after HSV infection. Also, dual TLR2/9 recognition of HSV infection directly activated NK cells without the aid of dendritic cells through activation of p38 MAPK pathway. Taken together, these results indicate that dual TLR2/9 recognition plays a critical role in providing resistance against mucosal infection with HSV, which may involve a direct regulation of Tip-DCs and NK cells in VT. Therefore, our data provide a more detailed understanding of TLR2/9 role in conferring antiviral immunity within relevant mucosal tissues after mucosal infection with HSV.