Functional modulation of dendritic cells and macrophages by Japanese encephalitis virus through MyD88 adaptor molecule-dependent and -independent pathways

Methotrimeprazine is a neuroprotective antiviral in JEV infection via adaptive ER stress and autophagy

Japanese encephalitis virus (JEV) pathogenesis is driven by a combination of neuronal death and neuroinflammation. We tested 42 FDA-approved drugs that were shown to induce autophagy for antiviral effects. Four drugs were tested in the JE mouse model based on in vitro protective effects on neuronal cell death, inhibition of viral replication, and anti-inflammatory effects. The antipsychotic phenothiazines Methotrimeprazine (MTP) & Trifluoperazine showed a significant survival benefit with reduced virus titers in the brain, prevention of BBB breach, and inhibition of neuroinflammation. Both drugs were potent mTOR-independent autophagy flux inducers. MTP inhibited SERCA channel functioning, and induced an adaptive ER stress response in diverse cell types. Pharmacological rescue of ER stress blocked autophagy and antiviral effect. MTP did not alter translation of viral RNA, but exerted autophagy-dependent antiviral effect by inhibiting JEV replication complexes. Drug-induced autophagy resulted in reduced NLRP3 protein levels, and attenuation of inflammatory cytokine/chemokine release from infected microglial cells. Our study suggests that MTP exerts a combined antiviral and anti-inflammatory effect in JEV infection, and has therapeutic potential for JE treatment.

Secreted NS1 proteins of tick-borne encephalitis virus and West Nile virus block dendritic cell activation and effector functions

The flavivirus non-structural protein 1 (NS1) is secreted from infected cells into the circulation and the serum levels correlate with disease severity. The effect of secreted NS1 (sNS1) on non-infected mammalian immune cells is largely unknown. Here, we expressed recombinant sNS1 proteins of tick-borne encephalitis virus (TBEV) and West Nile virus (WNV) and investigated their effects on dendritic cell (DC) effector functions. Murine bone marrow-derived DCs (BMDCs) showed reduced surface expression of co-stimulatory molecules and decreased release of pro-inflammatory cytokines when treated with sNS1 of TBEV or WNV prior to poly(I:C) stimulation. Transcriptional profiles of BMDCs that were sNS1-exposed prior to poly(I:C) stimulation showed two gene clusters that were downregulated by TBEV or WNV sNS1 and that were associated with innate and adaptive immune responses. Functionally, both sNS1 proteins modulated the capacity for BMDCs to induce specific T-cell responses as indicated by reduced IFN-γ levels in both CD4+ and CD8+ T cells after BMDC co-cultivation. In human monocyte-derived DCs, poly(I:C)-induced upregulation of co-stimulatory molecules and cytokine responses were even more strongly impaired by TBEV sNS1 or WNV sNS1 pretreatment than in the murine system. Our findings indicate that exogenous flaviviral sNS1 proteins interfere with DC-mediated stimulation of T cells, which is crucial for the initiation of cell-mediated adaptive immune responses in human flavivirus infections. Collectively, our data determine soluble flaviviral NS1 as a virulence factor responsible for a dampened immune response to flavivirus infections. IMPORTANCE The effective initiation of protective host immune responses controls the outcome of infection, and dysfunctional T-cell responses have previously been associated with symptomatic human flavivirus infections. We demonstrate that secreted flavivirus NS1 proteins modulate innate immune responses of uninfected bystander cells. In particular, sNS1 markedly reduced the capacity of dendritic cells to stimulate T-cell responses upon activation. Hence, by modulating cellular host responses that are required for effective antigen presentation and initiation of adaptive immunity, sNS1 proteins may contribute to severe outcomes of flavivirus disease.

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.

Mechanism of Immune Evasion in Mosquito-Borne Diseases

In recent decades, mosquito-borne illnesses have emerged as a major health burden in many tropical regions. These diseases, such as malaria, dengue fever, chikungunya, yellow fever, Zika virus infection, Rift Valley fever, Japanese encephalitis, and West Nile virus infection, are transmitted through the bite of infected mosquitoes. These pathogens have been shown to interfere with the host's immune system through adaptive and innate immune mechanisms, as well as the human circulatory system. Crucial immune checkpoints such as antigen presentation, T cell activation, differentiation, and proinflammatory response play a vital role in the host cell's response to pathogenic infection. Furthermore, these immune evasions have the potential to stimulate the human immune system, resulting in other associated non-communicable diseases. This review aims to advance our understanding of mosquito-borne diseases and the immune evasion mechanisms by associated pathogens. Moreover, it highlights the adverse outcomes of mosquito-borne disease.

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.

Recent advances to overcome the burden of Japanese encephalitis: A zoonotic infection with problematic early detection

Japanese encephalitis (JE) is a vector-borne neurotropic disease caused by Japanese encephalitis virus (JEV) associated with high mortality rate distributed from Eastern and Southern Asia to Northern Queensland (Australia). The challenges in early detection and lack of point-of-care biomarkers make it the most important Flavivirus causing encephalitis. There is no specific treatment for the disease, although vaccines are licenced. In this review, we focussed on point-of-care biomarkers as early detection tools and developing the effective therapeutic agents that could halt JE. We have also provided molecular details of JEV, disease progression, and its pathogenesis with recent findings which might bring insights to overcome the disease burden.

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.

Human gut microbiota stimulate defined innate immune responses that vary from phylum to strain

The potential of commensal bacteria to modulate host immunity remains largely uncharacterized, largely due to the vast number of strains that comprise the human gut microbiota. We have developed a screening platform to measure the innate immune responses of myeloid cells to 277 bacterial strains isolated from the gut microbiota of healthy individuals and those with inflammatory bowel diseases. The innate immune responses to gut-derived bacteria are as strong as those toward pathogenic bacteria, and they vary from phylum to strain. Myeloid cells differentially rely upon innate receptors TLR2 or TLR4 to sense taxa, with differential sensing of Bacteroidetes and Proteobacteria that predict in vivo functions. These innate immune responses can be modeled using combinations of up to 8 Toll-like receptor (TLR) agonists. Furthermore, the immunogenicity of strains is stable over time and following fecal microbiota transplantation into new human recipients. Collectively, this high-throughput approach provides an insight into how commensal microorganisms shape innate immune phenotypes.

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.

Japanese Encephalitis Virus Infected Human Monocyte-Derived Dendritic Cells Activate a Transcriptional Network Leading to an Antiviral Inflammatory Response

A comprehensive understanding of the human immune response to virus infection is imperative for developing effective therapies, antivirals, and vaccines. Dendritic cells (DCs) are among the first cells to encounter the virus and are also key antigen-presenting cells that link the innate and adaptive immune system. In this study, we focus on the human immune response to the mosquito-borne Japanese encephalitis virus (JEV), which is the leading cause of virus-induced encephalitis in south-east Asia and has the potential to become a global pathogen. We describe the gene regulatory circuit of JEV infection in human monocyte-derived DCs (moDCs) along with its functional validation. We observe that JEV can productively infect human moDCs leading to robust transcriptional activation of the interferon and NF-κB-mediated antiviral and inflammatory pathways. This is accompanied with DC maturation and release of pro-inflammatory cytokines and chemokines TNFα, IL-6, IL-8, IL-12, MCP-1. and RANTES. JEV-infected moDCs activated T-regulatory cells (Tregs) in allogenic mixed lymphocyte reactions (MLR) as seen by upregulated FOXP3 mRNA expression, suggestive of a host response to reduce virus-induced immunopathology. The virus also downregulated transcripts involved in Peroxisome Proliferator Activated Receptor (PPAR) signalling and fatty acid metabolism pathways suggesting that changes in cellular metabolism play a crucial role in driving the DC maturation and antiviral responses. Collectively, our data describe and corroborate the human DC transcriptional network that is engaged upon JEV sensing.

STAT2-dependent restriction of Zika virus by human macrophages but not dendritic cells

Zika virus (ZIKV) is an emerging mosquito-borne flavivirus that poses significant threats to global public health. Macrophages and dendritic cells are both key sentinel cells in the host immune response and play critical roles in the pathogenesis of flavivirus infections. Recent studies showed that ZIKV could productively infect monocyte-derived dendritic cells (moDCs), but the role of macrophages in ZIKV infection remains incompletely understood. In this study, we first compared ZIKV infection in monocyte-derived macrophages (MDMs) and moDCs derived from the same donors. We demonstrated that while both MDMs and moDCs were susceptible to epidemic (Puerto Rico) and pre-epidemic (Uganda) strains of ZIKV, virus replication was largely restricted in MDMs but not in moDCs. ZIKV induced significant apoptosis in moDCs but not MDMs. The restricted virus replication in MDMs was not due to inefficient virus entry but was related to post-entry events in the viral replication cycle. In stark contrast with moDCs, ZIKV failed to inhibit STAT1 and STAT2 phosphorylation in MDMs. This resulted in the lack of efficient antagonism of the host type I interferon-mediated antiviral responses. Importantly, depletion of STAT2 but not STAT1 in MDMs significantly rescued the replication of ZIKV and the prototype flavivirus yellow fever virus. Overall, our findings revealed a differential interplay between macrophages and dendritic cells with ZIKV. While dendritic cells may be exploited by ZIKV to facilitate virus replication, macrophages restricted ZIKV infection.

Innate Immune Sensing of Viruses and Its Consequences for the Central Nervous System

Viral infections remain a global public health concern and cause a severe societal and economic burden. At the organismal level, the innate immune system is essential for the detection of viruses and constitutes the first line of defense. Viral components are sensed by host pattern recognition receptors (PRRs). PRRs can be further classified based on their localization into Toll-like receptors (TLRs), C-type lectin receptors (CLR), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), NOD-like receptors (NLRs) and cytosolic DNA sensors (CDS). TLR and RLR signaling results in production of type I interferons (IFNα and -β) and pro-inflammatory cytokines in a cell-specific manner, whereas NLR signaling leads to the production of interleukin-1 family proteins. On the other hand, CLRs are capable of sensing glycans present in viral pathogens, which can induce phagocytic, endocytic, antimicrobial, and pro- inflammatory responses. Peripheral immune sensing of viruses and the ensuing cytokine response can significantly affect the central nervous system (CNS). But viruses can also directly enter the CNS via a multitude of routes, such as the nasal epithelium, along nerve fibers connecting to the periphery and as cargo of infiltrating infected cells passing through the blood brain barrier, triggering innate immune sensing and cytokine responses directly in the CNS. Here, we review mechanisms of viral immune sensing and currently recognized consequences for the CNS of innate immune responses to viruses.

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.

Recent advances in understanding Japanese encephalitis

Japanese encephalitis (JE) is a clinical manifestation of the brain inflammation caused by JE virus (JEV). This virus imparts permanent neurological damage, thus imposing a heavy burden on public health and society. Neuro-inflammation is the hallmark of JEV infection. The prolonged pro-inflammatory response is due primarily to microglial activation, which eventually leads to severe encephalitis. A continual effort is going on in the scientific community toward an understanding of cellular and molecular factors that are involved in JEV neuro-invasion and inflammatory processes. This review not only gives a comprehensive update on the recent advances on understanding virus structure and mechanisms of pathogenesis but also briefly discusses crucial unresolved issues. We also highlight challenging areas of research that might open new avenues for controlling virus-induced neuro-inflammation.

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.

Review of Emerging Japanese Encephalitis Virus: New Aspects and Concepts about Entry into the Brain and Inter-Cellular Spreading

Japanese encephalitis virus (JEV) is an emerging flavivirus of the Asia-Pacific region. More than two billion people live in endemic or epidemic areas and are at risk of infection. Recently, the first autochthonous human case was recorded in Africa, and infected birds have been found in Europe. JEV may spread even further to other continents. The first section of this review covers established and new information about the epidemiology of JEV. The subsequent sections focus on the impact of JEV on humans, including the natural course and immunity. Furthermore, new concepts are discussed about JEV’s entry into the brain. Finally, interactions of JEV and host cells are covered, as well as how JEV may spread in the body through latently infected immune cells and cell-to-cell transmission of virions or via other infectious material, including JEV genomic RNA.

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.

Invasion of a murine in vitro blood-brain barrier co-culture model by dengue virus serotypes 1 to 4

The blood-brain barrier (BBB) is a physical barrier that restricts the passage of cells and molecules as well as pathogens into the central nervous system (CNS). Some viruses enter the CNS by disrupting the BBB, while others can reach the CNS without altering the integrity of the BBB. Even though dengue virus (DENV) is not a distinctive neurotropic virus, the virus is considered to be one of the leading causes of neurological manifestations. In this study, we found that DENV is able to compromise the integrity of a murine in vitro blood-brain barrier (BBB) model, resulting in hyperpermeability, as shown by a significant increase in sucrose and albumin permeability. Infection of brain endothelial cells (ECs) was facilitated by the presence of glycans, in particular, mannose and N-acetyl glucosamine residues, on cell surfaces and viral envelope proteins, and the requirement for glycan moieties for cell infection was serotype-specific. Direct viral disruption of brain ECs was observed, leading to a significant decrease in tight-junction protein expression and peripheral localization, which contributed to the changes in BBB permeability. In conclusion, the hyperpermeability and breaching mechanism of BBB by DENV are primarily due to direct consequences of viral infection of ECs, as shown in this in vitro study.

DC-SIGN promotes Japanese encephalitis virus transmission from dendritic cells to T cells via virological synapses

Skin-resident dendritic cells (DCs) likely encounter incoming viruses in the first place, and their migration to lymph nodes following virus capture may promote viral replication. However, the molecular mechanisms underlying these processes remain unclear. In the present study, we found that compared to cell-free viruses, DC-bound viruses showed enhanced capture of JEV by T cells. Additionally, JEV infection was increased by co-culturing DCs and T cells. Blocking the C-type lectin receptor DC-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) with neutralizing antibodies or antagonists blocked JEV transmission to T cells. Live-cell imaging revealed that DCs captured and transferred JEV viral particles to T cells via virological synapses formed at DC-T cell junctions. These findings indicate that DC-SIGN plays an important role in JEV transmission from DCs to T cells and provide insight into how JEV exploits the migratory and antigen-presenting capabilities of DCs to gain access to lymph nodes for dissemination and persistence in the host.

Regulation of inflammation in Japanese encephalitis

Background

Uncontrolled inflammatory response of the central nervous system is a hallmark of severe Japanese encephalitis (JE). Although inflammation is necessary to mount an efficient immune response against virus infections, exacerbated inflammatory response is often detrimental. In this context, cells of the monocytic lineage appear to be important forces driving JE pathogenesis.

Main body

Brain-infiltrating monocytes, macrophages and microglia play a major role in central nervous system (CNS) inflammation during JE. Moreover, the role of inflammatory monocytes in viral neuroinvasion during JE and mechanisms of cell entry into the CNS remains unclear. The identification of cellular and molecular actors in JE inflammatory responses may help to understand the mechanisms behind excessive inflammation and to develop therapeutics to treat JE patients. This review addresses the current knowledge about mechanisms of virus neuroinvasion, neuroinflammation and therapeutics critical for JE outcome.

Conclusion

Understanding the regulation of inflammation in JE is challenging. Elucidation of the remaining open questions will help to the development of therapeutic approaches avoiding detrimental inflammatory responses in JE.

Exacerbation of Japanese Encephalitis by CD11chi Dendritic Cell Ablation Is Associated with an Imbalance in Regulatory Foxp3+ and IL-17+CD4+ Th17 Cells and in Ly-6Chi and Ly-6Clo Monocytes

Japanese encephalitis (JE) is neuroinflammation characterized by uncontrolled infiltration of peripheral leukocytes into the central nervous system (CNS). We previously demonstrated exacerbation of JE following CD11c^hi dendritic cell (DC) ablation in CD11c-DTR transgenic mice. Moreover, CD11c^hi DC ablation led to abnormal differentiation of CD11b⁺Ly-6C^hi monocytes and enhanced permeability of the blood-brain barrier (BBB), resulting in promoting the progression of JE. Here, we examined changes in lymphoid and myeloid-derived leukocyte subpopulations associated with pro- and anti-inflammation during JE progression. The analyses of this study focused on regulatory CD4⁺Foxp3⁺ regulatory T cells (Tregs), IL-17⁺CD4⁺ Th17 cells, and CD11b⁺Ly-6C^hi and Ly-6C^lo monocytes. CD11c^hi DC ablation resulted in the accumulation of IL-17⁺CD4⁺ Th17 cells in the CNS, thereby leading to lower ratio of Tregs to Th17 cells. This result was corroborated by the higher expression levels of IL-17 and RORγT in CD4⁺ T cells from the brains of CD11c^hi DC-ablated mice. In addition, CD11c^hi DC-ablated mice showed higher frequency and total number of inflammatory CD11b⁺Ly-6C^hi monocytes, whereas CD11b⁺Ly-6C^lo monocytes were detected with lower frequency and total number in CD11c^hi DC-ablated mice. Furthermore, CD11c^hi DC ablation altered the phenotype and function of CD11b⁺Ly-6C^lo monocytes, resulting in lower levels of activation marker and anti-inflammatory cytokine (IL-10 and TGF-β) expression. Collectively, these results indicate that CD11c^hi DC ablation caused an imbalance in CD4⁺ Th17/Treg cells and CD11b⁺Ly-6C^hi/Ly-6C^lo monocytes in the lymphoid tissue and CNS during JE progression. This imbalanced orchestration of pro- and anti-inflammatory leukocytes following CD11c^hi DC ablation may contribute to the exacerbation of JE.

TLR7 Deficiency Leads to TLR8 Compensative Regulation of Immune Response against JEV in Mice

Japanese encephalitis virus (JEV) is a highly fatal pathogen to human beings. Toll-like receptor 7 (TLR7) plays a role as the first host defense against most single-stranded RNA flaviviruses. This study aims to investigate the role of TLR7 in inducing adaptive immune response in mice against JEV. In vitro and in vivo studies were conducted to examine the expression of toll-like receptors (TLRs) in mice. After JEV infection, physical parameters of mice (survival rate and body weight) were evaluated, and organs or cells were collected for further analysis. The expression of TLR7 was increased significantly as compare to other TLR molecules post-JEV infection. The expression of CD80, CD86, and CD273 on bone marrow-derived dendritic cells was increased significantly in TLR7^−/− mice. Furthermore, viral load was also increased significantly in TLR7^−/− mice as compare to C57BL/6 mice. But there was no significant difference among survival rate and body weight in TLR7^−/− mice as compare to C57BL/6. Interestingly, we also found that TLR8 was upregulated in TLR7^−/− mice. The study concluded that TLR8 was upregulated in TLR7-deficient mice, and it might play a compensatory role in the immune response in TLR7^−/− mice.

Mesenchymal stem cells alleviate Japanese encephalitis virus-induced neuroinflammation and mortality

Background

Japanese encephalitis virus (JEV) is the leading cause of viral encephalitis in Asia. Japanese encephalitis (JE) caused by JEV is characterized by extensive inflammatory cytokine secretion, microglia activation, blood-brain barrier (BBB) breakdown, and neuronal death, all of which contribute to the vicious cycle of inflammatory damage. There are currently no effective treatments for JE. Mesenchymal stem cells (MSCs) have been demonstrated to have a therapeutic effect on many central nervous system (CNS) diseases by regulating inflammation and other mechanisms.

Methods

In vivo, 8- to 10-week-old mice were infected intraperitoneally with JEV and syngeneic bone marrow MSCs were administered through the caudal vein at 1 and 3 days post-infection. The mortality, body weight, and behavior were monitored daily. Brains from each group were harvested at the indicated times for hematoxylin and eosin staining, immunohistochemical observation, flow cytometric analysis, TUNEL staining, Western blot, quantitative real-time polymerase chain reaction, and BBB permeability assays. In vitro, co-culture and mixed culture experiments of MSCs with either microglia or neurons were performed, and then the activation state of microglia and survival rate of neurons were tested 48 h post-infection.

Results

MSC treatment reduced JEV-induced mortality and improved the recovery from JE in our mouse model. The inflammatory response, microglia activation, neuronal damage, BBB destruction, and viral load (VL) were significantly decreased in the MSC-treated group. In co-culture experiments, MSCs reprogrammed M1-to-M2 switching in microglia and improved neuron survival. Additionally, the VL was decreased in Neuro2a cells in the presence of MSCs accompanied by increased expression of interferon-α/β.

Conclusion

MSC treatment alleviated JEV-induced inflammation and mortality in mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0486-5) contains supplementary material, which is available to authorized users.

Tick-borne pathogens induce differential expression of genes promoting cell survival and host resistance in Ixodes ricinus cells

Background

There has been an emergence and expansion of tick-borne diseases in Europe, Asia and North America in recent years, including Lyme disease, tick-borne encephalitis and human anaplasmosis. The primary vectors implicated are hard ticks of the genus Ixodes. Although much is known about the host response to these bacterial and viral pathogens, there is limited knowledge of the cellular responses to infection within the tick vector. The bacterium Anaplasma phagocytophilum is able to bypass apoptotic processes in ticks, enabling infection to proceed. However, the tick cellular responses to infection with the flaviviruses tick-borne encephalitis virus (TBEV) and louping ill virus (LIV), which cause tick-borne encephalitis and louping ill respectively, are less clear.

Results

Infection and transcriptional analysis of the Ixodes ricinus tick cell line IRE/CTVM20 with the viruses LIV and TBEV, and the bacterium A. phagocytophilum, identified activation of common and distinct cellular pathways. In particular, commonly-upregulated genes included those that modulate apoptotic pathways, putative anti-pathogen genes, and genes that influence the tick innate immune response, including selective activation of toll genes.

Conclusion

These data provide an insight into potential key genes involved in the tick cellular response to viral or bacterial infection, which may promote cell survival and host resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2011-1) contains supplementary material, which is available to authorized users.

The host cell response to tick-borne encephalitis virus

Tick-borne encephalitis virus is the most prevalent autochthonous arbovirus in Europe and an important travel-associated virus. Complications of the infection could lead to lethal encephalitis in susceptible individuals. However, despite its clinical relevance and expanding geographical distribution, most of our knowledge on its pathogenesis is inferred from studies on other flaviviruses. Molecular details of the host cell response to infection are scarce leading to a poor understanding of the antiviral pathways and viral countermeasures that are critical to determine the outcome of the infection. In this work the relevant literature is reviewed and the key elements of tick-borne encephalitis virus infection of human cells are identified, which requires further investigation.

Regulation of Viral Replication, Apoptosis and Pro-Inflammatory Responses by 17-AAG during Chikungunya Virus Infection in Macrophages

Chikungunya virus (CHIKV) infection has re-emerged as a major public health concern due to its recent worldwide epidemics and lack of control measures. Although CHIKV is known to infect macrophages, regulation of CHIKV replication, apoptosis and immune responses towards macrophages are not well understood. Accordingly, the Raw264.7 cells, a mouse macrophage cell line, were infected with CHIKV and viral replication as well as new viral progeny release was assessed by flow cytometry and plaque assay, respectively. Moreover, host immune modulation and apoptosis were studied through flow cytometry, Western blot and ELISA. Our current findings suggest that expression of CHIKV proteins were maximum at 8 hpi and the release of new viral progenies were remarkably increased around 12 hpi. The induction of Annexin V binding, cleaved caspase-3, cleaved caspase-9 and cleaved caspase-8 in CHIKV infected macrophages suggests activation of apoptosis through both intrinsic and extrinsic pathways. The pro-inflammatory mediators (TNF and IL-6) MHC-I/II and B7.2 (CD86) were also up-regulated during infection over time. Further, 17-AAG, a potential HSP90 inhibitor, was found to regulate CHIKV infection, apoptosis and pro-inflammatory cytokine/chemokine productions of host macrophages significantly. Hence, the present findings might bring new insight into the therapeutic implication in CHIKV disease biology.

Ablation of CD11c(hi) dendritic cells exacerbates Japanese encephalitis by regulating blood-brain barrier permeability and altering tight junction/adhesion molecules

Japanese encephalitis (JE), characterized by extensive neuroinflammation following infection with neurotropic JE virus (JEV), is becoming a leading cause of viral encephalitis due to rapid changes in climate and demography. The blood-brain barrier (BBB) plays an important role in restricting neuroinvasion of peripheral leukocytes and virus, thereby regulating the progression of viral encephalitis. In this study, we explored the role of CD11c(hi) dendritic cells (DCs) in regulating BBB integrity and JE progression using a conditional depletion model of CD11c(hi) DCs. Transient ablation of CD11c(hi) DCs resulted in markedly increased susceptibility to JE progression along with highly increased neuro-invasion of JEV. In addition, exacerbated JE progression in CD11c(hi) DC-ablated hosts was closely associated with increased expression of proinflammatory cytokines (IFN-β, IL-6, and TNF-α) and CC chemokines (CCL2, CCL3, CXCL2) in the brain. Moreover, our results revealed that the exacerbation of JE progression in CD11c(hi) DC-ablated hosts was correlated with enhanced BBB permeability and reduced expression of tight junction and adhesion molecules (claudin-5, ZO-1, occluding, JAMs). Ultimately, our data conclude that the ablation of CD11c(hi) DCs provided a subsidiary impact on BBB integrity and the expression of tight junction/adhesion molecules, thereby leading to exacerbated JE progression. These findings provide insight into the secondary role of CD11c(hi) DCs in JE progression through regulation of BBB integrity and the expression of tight junction/adhesion molecules.

Product review on the JE vaccine IXIARO

Japanese encephalitis virus, as the most important vaccine-preventable cause of viral encephalitis in Asia, is estimated to cause over 68,000 clinical cases yearly. In endemic areas, most Japanese encephalitis infections occur in children younger than 10 y and clinical manifestation of this disease is critical, because there is no effective treatment available. As JEV infections are regarded as one of the most serious viral causes of encephalitis and mass immunization programmes are generally recommended for residents in endemic areas, a safe and effective JEV vaccine was needed to protect them as well as others at risk. Due to the safety concerns with the mouse brain derived vaccine, second generation vaccines against JE produced in cell culture like Vero cells were developed. IXIARO® is a purified, inactivated aluminum-adjuvanted JE vaccine, based on the SA14-14-2 virus strain, and is available in North America, Europe, Canada, Switzerland, Singapore, Hong Kong and Israel as well as in Australia & New Zealand (as JESPECT®).The safety, tolerability and immunogenicity profile of IXIARO® is well established through a number of clinical studies comparing IXIARO® with placebo as well as mouse brain derived vaccine. Recent data show that the global incidence of JE remains substantial, especially young children in endemic areas are most susceptible. As vaccination is the most feasible, reliable and cost effective tool for JE control, IXIARO® with confirmed excellent safety profile is highly recommendable, in particular for vaccination of children at risk. The European Commission as well as the FDA approved the extension of indication of IXIARO® to the pediatric segment (2 months of age and older) based on these data.

Potential chemotherapeutic targets for Japanese encephalitis: current status of antiviral drug development and future challenges

INTRODUCTION

Japanese encephalitis (JE) remains a public health threat in Asia. Although several vaccines have been licensed, ∼ 67,900 cases of the disease are estimated to occur annually, probably because the vaccine coverage is low. Therefore, effective antiviral drugs are required to control JE. However, no licensed anti-JE drugs are available, despite extensive efforts to develop them.

AREAS COVERED

We provide a general overview of JE and JE virus, including its transmission cycle, distribution, structure, replication machinery, immune evasion mechanisms and vaccines. The current situation in antiviral drug development is then reviewed and future perspectives are discussed.

EXPERT OPINION

Although the development of effective anti-JE drugs is an urgent issue, only supportive care is currently available. Recent progress in our understanding of the viral replication machinery and immune evasion strategies has identified new targets for anti-JE drug development. To date, most candidate drugs have only been evaluated in single-drug formulations, and efficient drug delivery to the CNS has virtually not been considered. However, an effective anti-JE treatment is expected to be achieved with multiple-drug formulations and a targeted drug delivery system in the near future.

The pathogenesis of 3 neurotropic flaviviruses in a mouse model depends on the route of neuroinvasion after viremia

Neurotropic flavivirus infection of humans results in viremia subsequently; in some cases, it causes meningitis encephalomyelitis, although the pathways from viremia to central nervous system (CNS) invasion are uncertain. Here, we intravenously infected BALB/c mice with 3 neurotropic flaviviruses, then examined the clinical manifestations and histopathologic changes. The Sofjin strain of tick-borne encephalitis virus-infected mice exhibited dose-dependent survival. The animals showed distention of the small intestine caused by peripheral neuritis because of infection of the myenteric plexus. Histopathologically, the strongly neurotropic Sofjin strain invaded the CNS of viremic mice via the autonomic nerves running from the plexus. The JaTH-160 strain of Japanese encephalitis virus was isolated from the lymph nodes during the preclinical phase of viral encephalitis. Therefore, this strain might infect the CNS via a hematogenous pathway, including through lymphoid tissues. The NY99-6922 strain of the West Nile virus caused clinical signs suggestive of intestinal, lymphoid, and/or neurologic involvement; the infected mice had prolonged viremia, suggesting that NY99-6922 may mainly use the hematogenous pathway; however, there was also histopathologic evidence of involvement of the autonomic nervous system pathway. In conclusion, the three neurotropic flaviviruses showed different pathogenesis, which were dependent upon overlapping but distinct pathways to CNS invasion after viremia.

Viral Infection of the Central Nervous System and Neuroinflammation Precede Blood-Brain Barrier Disruption during Japanese Encephalitis Virus Infection

Japanese encephalitis is an acute zoonotic, mosquito-borne disease caused by Japanese encephalitis virus (JEV). Japanese encephalitis is characterized by extensive inflammation in the central nervous system (CNS) and disruption of the blood-brain barrier (BBB). However, the pathogenic mechanisms contributing to the BBB disruption are not known. Here, using a mouse model of intravenous JEV infection, we show that virus titers increased exponentially in the brain from 2 to 5 days postinfection. This was accompanied by an early, dramatic increase in the level of inflammatory cytokines and chemokines in the brain. Enhancement of BBB permeability, however, was not observed until day 4, suggesting that viral entry and the onset of inflammation in the CNS occurred prior to BBB damage. In vitro studies revealed that direct infection with JEV could not induce changes in the permeability of brain microvascular endothelial cell monolayers. However, brain extracts derived from symptomatic JEV-infected mice, but not from mock-infected mice, induced significant permeability of the endothelial monolayer. Consistent with a role for inflammatory mediators in BBB disruption, the administration of gamma interferon-neutralizing antibody ameliorated the enhancement of BBB permeability in JEV-infected mice. Taken together, our data suggest that JEV enters the CNS, propagates in neurons, and induces the production of inflammatory cytokines and chemokines, which result in the disruption of the BBB.

IMPORTANCE

Japanese encephalitis (JE) is the leading cause of viral encephalitis in Asia, resulting in 70,000 cases each year, in which approximately 20 to 30% of cases are fatal, and a high proportion of patients survive with serious neurological and psychiatric sequelae. Pathologically, JEV infection causes an acute encephalopathy accompanied by BBB dysfunction; however, the mechanism is not clear. Thus, understanding the mechanisms of BBB disruption in JEV infection is important. Our data demonstrate that JEV gains entry into the CNS prior to BBB disruption. Furthermore, it is not JEV infection per se, but the inflammatory cytokines/chemokines induced by JEV infection that inhibit the expression of TJ proteins and ultimately result in the enhancement of BBB permeability. Neutralization of gamma interferon (IFN-γ) ameliorated the enhancement of BBB permeability in JEV-infected mice, suggesting that IFN-γ could be a potential therapeutic target. This study would lead to identification of potential therapeutic avenues for the treatment of JEV infection.

miR-146a suppresses cellular immune response during Japanese encephalitis virus JaOArS982 strain infection in human microglial cells

Background

Japanese encephalitis virus (JEV) is the causative agent of Japanese encephalitis which is more prevalent in South and Southeast Asia. JEV is a neurotropic virus which infiltrates into the brain through vascular endothelial cells. JEV infects neurons and microglial cells which causes neuronal damage and inflammation. However, JEV also evades the cellular immune response to survive in host cells. Viruses are known to modulate the expression of microRNAs, which in turn modulate cellular immune response by targeting expression of antiviral genes. The aim of this study is to understand the anti-inflammatory role of miR-146a during JEV infection, which facilitates immune evasion.

Methods

Human brain microglial cells (CHME3) were infected by JEV: JaOArS982 and P20778 strain, and expression of miR-146a were analyzed. Overexpression and knockdown studies of miR-146a were done to see the effect on NF-κB pathway and antiviral Jak-STAT pathway. Regulatory role of miR-146a on expression of interferon-stimulated genes was determined by real-time PCR and luciferase assays.

Results

JEV infection elevated the expression of miR-146a in JaOArS982 strain which caused downregulation of TRAF6, IRAK1, IRAK2, and STAT1 genes. Exogenous overexpression of miR-146a led to suppression of NF-κB activation and abrogation of Jak-STAT pathway upon JEV infection which led to downregulation of interferon-stimulated genes (IFIT-1 and IFIT-2) and facilitated viral replication. JEV infection initially upregulated cytokine production and activated STAT1 activity but STAT1 levels reduced at later time point, which led to the downregulation of interferon-stimulated genes.

Conclusion

Upregulation of miR-146a by JEV JaOArS982 strain leads to suppression of NF-κB activity and disruption of antiviral Jak-STAT signaling which helps the virus to evade the cellular immune response. This effect of JEV infection on miR-146a expression was found to be strain specific.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0249-0) contains supplementary material, which is available to authorized users.

Flaviviruses, an expanding threat in public health: focus on dengue, West Nile, and Japanese encephalitis virus

The flaviviruses dengue, West Nile, and Japanese encephalitis represent three major mosquito-borne viruses worldwide. These pathogens impact the lives of millions of individuals and potentially could affect non-endemic areas already colonized by mosquito vectors. Unintentional transport of infected vectors (Aedes and Culex spp.), traveling within endemic areas, rapid adaptation of the insects into new geographic locations, climate change, and lack of medical surveillance have greatly contributed to the increase in flaviviral infections worldwide. The mechanisms by which flaviviruses alter the immune and the central nervous system have only recently been examined despite the alarming number of infections, related deaths, and increasing global distribution. In this review, we will discuss the expansion of the geographic areas affected by flaviviruses, the potential threats to previously unaffected countries, the mechanisms of pathogenesis, and the potential therapeutic interventions to limit the devastating consequences of these viruses.

Potential biomarkers for the clinical prognosis of severe dengue

Currently, several assays can confirm acute dengue infection at the point-of-care. However, none of these assays can predict the severity of the disease symptoms. A prognosis test that predicts the likelihood of a dengue patient to develop a severe form of the disease could permit more efficient patient triage and treatment. We hypothesise that mRNA expression of apoptosis and innate immune response-related genes will be differentially regulated during the early stages of dengue and might predict the clinical outcome. Aiming to identify biomarkers for dengue prognosis, we extracted mRNA from the peripheral blood mononuclear cells of mild and severe dengue patients during the febrile stage of the disease to measure the expression levels of selected genes by quantitative polymerase chain reaction. The selected candidate biomarkers were previously identified by our group as differentially expressed in microarray studies. We verified that the mRNA coding for CFD, MAGED1, PSMB9, PRDX4 and FCGR3B were differentially expressed between patients who developed clinical symptoms associated with the mild type of dengue and patients who showed clinical symptoms associated with severe dengue. We suggest that this gene expression panel could putatively serve as biomarkers for the clinical prognosis of dengue haemorrhagic fever.

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

Japanese encephalitis (JE) is major emerging neurologic disease caused by JE virus. To date, the impact of TLR molecules on JE progression has not been addressed. Here, we determined whether each TLR modulates JE, using several TLR-deficient mouse strains (TLR2, TLR3, TLR4, TLR7, TLR9). Surprisingly, among the tested TLR-deficient mice there were contrasting results in TLR3(-/-) and TLR4(-/-) mice, i.e. TLR3(-/-) mice were highly susceptible to JE, whereas TLR4(-/-) mice showed enhanced resistance to JE. TLR3 ablation induced severe CNS inflammation characterized by early infiltration of inflammatory CD11b(+)Ly-6Chigh monocytes along with profoundly increased viral burden, proinflammatory cytokine/chemokine expression as well as BBB permeability. In contrast, TLR4(-/-) mice showed mild CNS inflammation manifested by reduced viral burden, leukocyte infiltration and proinflammatory cytokine expression. Interestingly, TLR4 ablation provided potent in vivo systemic type I IFN innate response, as well as ex vivo type I IFN production associated with strong induction of antiviral PRRs (RIG-I, MDA5), transcription factors (IRF-3, IRF-7), and IFN-dependent (PKR, Oas1, Mx) and independent ISGs (ISG49, ISG54, ISG56) by alternative activation of IRF3 and NF-κB in myeloid-derived DCs and macrophages, as compared to TLR3(-/-) myeloid-derived cells which were more permissive to viral replication through impaired type I IFN innate response. TLR4 ablation also appeared to mount an enhanced type I IFN innate and humoral, CD4(+) and CD8(+) T cell responses, which were mediated by altered immune cell populations (increased number of plasmacytoid DCs and NK cells, reduced CD11b(+)Ly-6C(high) monocytes) and CD4(+)Foxp3(+) Treg number in lymphoid tissue. Thus, potent type I IFN innate and adaptive immune responses in the absence of TLR4 were closely coupled with reduced JE lethality. Collectively, these results suggest that a balanced triggering of TLR signal array by viral components during JE progression could be responsible for determining disease outcome through regulating negative and positive factors.

Japanese encephalitis virus expands regulatory T cells by increasing the expression of PD-L1 on dendritic cells

The mechanisms underlying Japanese encephalitis virus (JEV) pathogenesis need to be thoroughly explored to delineate therapeutic approaches. It is believed that JEV manipulates the innate and adaptive compartments of the host's immune system to evade immune response and cross the blood-brain barrier. The present study was thus designed to investigate the functional modulation of DCs after exposure to JEV and to assess the consequences on CD4(+) T-lymphocyte functions. Human monocyte-derived DCs were either infected with 1 MOI of live virus, UV-inactivated virus, or were mock-infected. Replication-competent JEV induced a significant increase in the expression of maturation markers 48 h postinfection, along with that of programmed cell death 1 ligand 1 (PD-L1; also called B7-H1 and CD274). JEV-infected DCs expanded the Treg cells in allogenic mixed lymphocyte reactions. The expansion of Treg cells by JEV-infected DCs was significantly reduced upon blocking PD-L1 using an antagonist. In addition, JEV-infected DCs significantly altered the proliferation and reduced the polarization of Th cells toward the Th1-cell phenotype. The results, for the first time, suggest that JEV evades the host's immune system by modulating the crosstalk between DCs and T lymphocytes via the PD-L1 axis.

The fine line between protection and pathology in neurotropic flavivirus and alphavirus infections

ABSTRACT: 

Flavivirus and alphavirus are two families of medically important arboviruses known to cause devastating neurologic disease. Exciting knowledge regarding epidemiology, disease and host immune responses are constantly unraveling. In this review, we aim to piece existing knowledge of neurotropic flavi- and alpha-viruses into a general, coherent picture of host–pathogen interactions. Special interest lies in the protective and pathologic host immunity to flavi- and alpha-viral infections, with a strong focus on West Nile virus, Japanese Encephalitis virus and Venezuelan equine encephalitis virus as representatives of their family.

Tick-borne flaviviruses antagonize both IRF-1 and type I IFN signaling to inhibit dendritic cell function

Tick-borne encephalitis virus (TBEV), a member of the Flaviviridae family, is a leading cause of viral encephalitis in Europe and Asia. Dendritic cells (DCs), as early cellular targets of infection, provide an opportunity for flaviviruses to inhibit innate and adaptive immune responses. Flaviviruses modulate DC function, but the mechanisms underpinning this are not defined. We examined the maturation phenotype and function of murine bone marrow-derived DCs infected with Langat virus (LGTV), a naturally attenuated member of the TBEV serogroup. LGTV infection failed to induce DC maturation or a cytokine response. Treatment with LPS or LPS/IFN-γ, strong inducers of inflammatory cytokines, resulted in enhanced TNF-α and IL-6 production, but suppressed IL-12 production in infected DCs compared with uninfected "bystander" cells or mock-infected controls. LGTV-mediated antagonism of type I IFN (IFN-I) signaling contributed to inhibition of IL-12p40 mRNA expression at late time points after stimulation. However, early suppression was still observed in DCs lacking the IFN-I receptor (Ifnar(-/-)), suggesting that additional mechanisms of antagonism exist. The early IFN-independent inhibition of IL-12p40 was nearly abolished in DCs deficient in IFN regulatory factor-1 (IRF-1), a key transcription factor required for IL-12 production. LGTV infection did not affect Irf-1 mRNA expression, but rather diminished IRF-1 protein levels and nuclear localization. The effect on IRF-1 was also observed in DCs infected with the highly virulent Sofjin strain of TBEV. Thus, antagonism of IRF-1 is a novel mechanism that synergizes with the noted ability of flaviviruses to suppress IFN-α/β receptor-dependent signaling, resulting in the orchestrated evasion of host innate immunity.

Japanese encephalitis virus infection modulates the expression of suppressors of cytokine signaling (SOCS) in macrophages: implications for the hosts' innate immune response

Viruses have evolved various mechanisms to subvert the host's immune system and one of them is preventing the infected cells from sending out chemotactic signals to activate the adaptive immune response. Japanese encephalitis virus (JEV) is a neuropathologic flavivirus that is responsible for significant number of child mortalities in various parts of South-East Asia. In this study we show that JEV modulates suppressors of cytokine signaling (SOCS)1 and 3 expression in macrophages to bring about changes in the JAK-STAT signaling cascade, so as to inhibit proinflammatory cyto/chemokine release. Using real time PCR, immunoblotting and immunofluorescent staining, we show that the expression of type 1 interferons and intracellular expression of viral genes are also affected over time. Also, following the initial activation of SOCS1 and 3, there is production of interferon-inducible anti-viral proteins in the cells which may be responsible for inhibiting viral replication. However, even at later time points, viral genes were still detected from the macrophages, albeit at lesser quantities, than earlier time points, indicative of intracellular persistence of the virus in a latent form. On knocking down SOCS1 and SOCS3 we found a significant decrease in viral gene expression at an early time point, indicating the dysregulation of the signaling cascade leading to increased production of interferon-inducible anti-viral proteins. Taken together, our study provides an insight into the role of JEV infection in modulating the JAK-STAT pathway with the help of SOCS leading to the generation of an antiviral innate immune response.

Japanese Encephalitis Virus Generated Neurovirulence, Antigenicity, and Host Immune Responses

In response to a JE virus attack, infected body cells start secretion of different cytokines and activate innate immune response. Virus starts neuronal invasion by entering into nerve cells and inflecting the central nervous system. It avoids exposure of body’s natural immunity and generates neurotrophic effects. Virus causes acute susceptibility to CNS and establishes encephalitis syndrome that results in very high fatality in children. In survivors, JEV inhibits the growth and proliferation of NCPs and imposes permanent neuronal disorders like cognitive, motor, and behavioral impairments. However, body cells start TCR mediated interactions, to recognize viral antigens with class I MHC complex on specific target cells, and operate mass killing of virus infected cells by increased CTL activity. Thus, both cell mediated and antibody interactions plays a central role in protection against JEV. In the present review article virus generated neurovirulence, antigenicity, and host immune responses are described in detail. More emphasis is given on diagnosis, clinical care, and active immunization with well-designed potential antiflavivirus vaccines. Further, for achieving an elite success against JEV, global eradication strategies are to be needed for making vaccination program more responsible and effective in endemic areas.

Characterization of immune responses induced by inactivated, live attenuated and DNA vaccines against Japanese encephalitis virus in mice

Vaccination is the most effective countermeasure for protecting individuals from Japanese encephalitis virus (JEV) infection. There are two types of JEV vaccines currently used in China: the Vero cell-derived inactivated vaccine and the live attenuated vaccine. In this study, we characterized the immune response and protective efficacy induced in mice by the inactivated vaccine, live attenuated vaccine and the DNA vaccine candidate pCAG-JME, which expresses JEV prM-E proteins. We found that the live attenuated vaccine conferred 100% protection and resulted in the generation of high levels of specific anti-JEV antibodies and cytokines. The pCAG-JME vaccine induced protective immunity as well as the live attenuated vaccine. Unexpectedly, immunization with the inactivated vaccine only induced a limited immune response and partial protection, which may be due to the decreased activity of dendritic cells and the expansion of CD4+CD25+Foxp3+ regulatory T cells observed in these mice. Altogether, our results suggest that the live attenuated vaccine is more effective in providing protection against JEV infection than the inactivated vaccine and that pCAG-JME will be a potential JEV vaccine candidate.

Histone deacetylase inhibition by Japanese encephalitis virus in monocyte/macrophages: a novel viral immune evasion strategy

Japanese encephalitis virus (JEV) is a common cause of encephalitis in humans who are dead-end hosts producing negligible viremia. The virus reaches the brain and causes massive inflammation. Our study seeks to understand the virus-host interaction using the murine monocyte/macrophage cell line RAW264.7, an antigen presenting cell involved in eliciting an innate immune response. We have discovered several interesting phenomena occurring in JEV-infected RAW264.7 cells which diverge from established observations. JEV remains inside RAW264.7 and appears to have little negative effect on cell viability. Expression studies of major histocompatibility complexes (MHC) and co-stimulatory molecules show inhibition of antigen presentation. There is enhanced immune suppression creating an anti-viral milieu. Expression of pro-inflammatory cytokines and chemokines is suppressed along with increased expression of anti-inflammatory molecules. Histone deacetylases (HDACs) have known inflammatory properties. In our study, through modulation of HDACs JEV seems to induce a crucial anti-inflammatory and anti-viral role in host macrophages.

Immune evasion strategies of flaviviruses

Flavivirus is a genus of the family Flaviviridae. It includes West Nile virus (WNV), dengue virus (DENV), yellow fever virus (YFV), Japanese encephalitis virus (JEV), tick-borne encephalitis virus (TBEV), and several other viruses which lead to extensive morbidity and mortality in humans. To establish infection and replication in the hosts, flaviviruses have evolved a variety of strategies to modulate the host's immune responses. In this review, the strategies employed by flaviviruses to evade the innate and adaptive immunity of host are summarized based on current studies, with a major focus on the inhibition of interferon, complement, natural killer (NK) cell, B cell, and T cell responses. This review aims to provide an overview of the current understanding for the mechanisms used by flaviviruses to escape the host's immune response, which will facilitate the future studies on flavivirus pathogenesis and the development of anti-flavivirus therapeutics.

Brain dendritic cells: biology and pathology

Dendritic cells (DC) are the professional antigen-presenting cells of the immune system. In their quiescent and mature form, the presentation of self-antigens by DC leads to tolerance; whereas, antigen presentation by mature DC, after stimulation by pathogen-associated molecular patterns, leads to the onset of antigen-specific immunity. DC have been found in many of the major organs in mammals (e.g. skin, heart, lungs, intestines and spleen); while the brain has long been considered devoid of DC in the absence of neuroinflammation. Consequently, microglia, the resident immune cell of the brain, have been charged with many functional attributes commonly ascribed to DC. Recent evidence has challenged the notion that DC are either absent or minimal players in brain immune surveillance. This review will discuss the recent literature examining DC involvement within both the young and aged steady-state brain. We will also examine DC contributions during various forms of neuroinflammation resulting from neurodegenerative autoimmune disease, injury, and CNS infections. This review also touches upon DC trafficking between the central nervous system and peripheral immune compartments during viral infections, the new molecular technologies that could be employed to enhance our current understanding of brain DC ontogeny, and some potential therapeutic uses of DC within the CNS.

Impaired cross-presentation of CD8α+ CD11c+ dendritic cells by Japanese encephalitis virus in a TLR2/MyD88 signal pathway-dependent manner

Cross-presentation is the pathway by which exogenous antigens are routed for presentation by MHC class I molecules leading to activation of antiviral CD8(+) T-cell responses. However, there is little information describing the modulation of cross-presentation and the impact of pathogen-derived signals associated with Japanese encephalitis virus (JEV), which is one of the most common causes of encephalitis in humans. In this study, we demonstrate that JEV infection could suppress in vivo cross-presentation of soluble and cell-associated antigens, thereby generating weak CD8(+) T-cell responses to exogenous antigens, as evaluated by CFSE dilution of adoptively transferred CD8(+) T cells and in vivo CTL killing activity. Furthermore, CD8α(+) CD11c(+) dendritic cells (DCs), which are known to be far more efficient at cross-presenting soluble antigens, played a specific role in contributing to JEV-mediated inhibition of the cross-presentation of exogenous antigens through interference with effective antigen uptake. Finally, this study provides evidence that TLR2-MyD88 and p38 MAPK signal pathway might be involved in JEV-mediated inhibition of cross-presentation of soluble and cell-associated antigens. These observations suggest that the modulation of cross-presentation of exogenous antigens through TLR signaling has important implications for antiviral immune responses against JEV infection and the development of effective vaccination strategies.

Pathogenic and vaccine strains of Japanese encephalitis virus elicit different levels of human macrophage effector functions

In India, Japanese encephalitis virus (JEV) remains one of the major causative agents of pediatric encephalitis. Macrophages support various neurotropic viruses and influence the immune response. However, the functional status of human macrophages during JEV infection remains unidentified. In this study, we examined the cytokine response and co-stimulatory marker levels in primary human monocyte derived macrophages (MDMs) infected with JE057434 (neurovirulent, primary clinical isolate) or SA14-14-2 (non-neurovirulent, live-attenuated vaccine) JEV strains. We also examined the differential susceptibility of these JEV strains to antiviral effects of interferon and nitric oxide. The results indicate that both JEV strains are capable of inducing various cytokines (type-I IFN, TNFα, IL6 and IL8) and co-stimulatory molecules (CD86 and CD80) in MDMs. However, they varied in replication potential and corresponding interferon sensitivity. SA14-14-2 was highly susceptible to interferon and nitric oxide when compared to JE057434. Thus, reduction in infectious virion production and increased sensitivity of SA14-14-2 towards interferon in MDMs could potentially play a role in limiting viral spread to additional target tissues.

The Roles and Perspectives of Toll-Like Receptors and CD4(+) Helper T Cell Subsets in Acute Viral Encephalitis

Acute viral encephalitis caused by neurotrophic viruses, such as mosquito-borne flaviviruses, is an emerging and re-emerging disease that represents an immense global health problem. Considerable progression has been made in understanding the pathogenesis of acute viral encephalitis, but the immune-pathological processes occurring during the progression of encephalitis and the roles played by various molecules and cellular components of the innate and adaptive systems still remain undefined. Recent findings reveal the significant contribution of Toll-like receptors (TLRs) and regulatory CD4⁺ T cells in the outcomes of infectious diseases caused by neurotrophic viruses. In this review, we discuss the ample evidence focused on the roles of TLRs and CD4⁺ helper T cell subsets on the progression of acute viral encephalitis. Finally, we draw attention to the importance of these molecules and cellular components in defining the pathogenesis of acute viral encephalitis, thereby providing new therapeutic avenues for this disease.

Mice with different susceptibility to Japanese encephalitis virus infection show selective neutralizing antibody response and myeloid cell infectivity

Background

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that causes public health problems in Asian countries. Only a limited number of JEV-infected individuals show symptoms and develop severe encephalitis, indicating host-dependent susceptibilities.

Methodology/Principal Findings

C3H/HeN and DBA/2 mice, which exhibit different mortalities when infected by intraperitoneal inoculation with JEV, were used as experimental models to compare viral pathogenesis and host responses. One hundred infectious virus particles killed 95% of C3H/HeN mice whereas only 40% of DBA/2 mice died. JEV RNA was detected with similar low levels in peripheral lymphoid organs and in the sera of both mouse strains. High levels of viral and cytokine RNA were observed simultaneously in the brains of C3H/HeN and DBA/2 mice starting on days 6 and 9 post-infection, respectively. The kinetics of the cytokines in sera correlated with the viral replication in the brain. Significantly earlier and higher titers of neutralizing antibodies were detected in the DBA/2 strain. Primary embryonic fibroblasts, bone marrow-derived dendritic cells and macrophages from the two mouse strains were cultured. Fibroblasts displayed similar JEV replication abilities, whereas DBA/2-derived myeloid antigen-presenting cells had lower viral infectivity and production compared to the C3H/HeN–derived cells.

Conclusions/Significance

Mice with different susceptibilities to JEV neuroinvasion did not show changes in viral tropism and host innate immune responses prior to viral entry into the central nervous system. However, early and high neutralizing antibody responses may be crucial for preventing viral neuroinvasion and host fatality. In addition, low permissiveness of myeloid dendritic cells and macrophages to JEV infection in vitro may be elements associated with late and decreased mouse neuroinvasion.