Japanese encephalitis virus disrupts cell-cell junctions and affects the epithelial permeability barrier functions

Viruses and the Brain-A Relationship Prone to Trouble

Neurological disorders, some of which are associated with viral infections, are growing due to the aging and expanding population. Despite strong defenses of the central nervous system, some viruses have evolved ways to breach them, which often result in dire consequences. In this review, we recount the various ways by which different viruses can enter the CNS, and we describe the consequences of such invasions. Consequences may manifest as acute disease, such as encephalitis, meningitis, or result in long-term effects, such as neuromuscular dysfunction, as occurs in poliomyelitis. We discuss evidence for viral involvement in the causation of well-known chronic neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, as well as vascular dementia in the elderly. We also describe the approaches currently available to control a few of the neural viral infections. These include antivirals that are effective against human immunodeficiency virus and herpes simplex virus, as well as vaccines valuable for controlling rabies virus, poliomyelitis virus, and some flavivirus infections. There is an urgent need to better understand, at a molecular level, how viruses contribute to acute and, especially, chronic neurological diseases and to develop more precise and effective vaccines and therapies.

IFN-β production induced by PRRSV is affected by GP3 quantity control and CLND4 interaction

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most harmful pathogens in the swine industry. Our previous studies demonstrated that the small extracellular domain (ECL2) of CLDN4 effectively blocks PRRSV infection. In this study, we explored the in vivo administration of swine ECL2 (sECL2) and found that it blocked HP-PRRSV infection and alleviated histopathological changes in organs. Notably, sECL2 stimulated cytokine production in the lungs. We observed that CLDN4 upregulated the expression of IFN-β at low doses of GP3. While high doses of GP3 inhibited the activity of the IFN-β promotor, regardless of whether CLDN4 was present. GP3 also downregulated IFN-β by decreasing the phosphorylation of TBK1 and IRF3. These findings highlight functional differences in GP3 under quantity control, which account for the variations in IFN-β induction during the early and late stages of infection. Our results indicate that sECL2 is a promising candidate drug for developing treatments to control PRRS.

Poliovirus capsid protein VP3 can penetrate vascular endothelial cells

The poliovirus (PV) enters the central nervous system (CNS) via the bloodstream, suggesting the existence of a mechanism to cross the blood-brain barrier. Here, we report that PV capsid proteins (VP1 and VP3) can penetrate cells, with VP3 being more invasive. Two independent parts of VP3 are responsible for this function. Both peptides can penetrate human umbilical cord vascular endothelial cells, and one peptide of VP3 could also penetrate peripheral blood mononuclear cells. In an in vitro blood-brain barrier model using rat-derived astrocytes, pericytes, and endothelial cells, both peptides were observed to traverse from the blood side to the brain side at 6 h after administration. These results provide insights into the molecular mechanisms underlying PV invasion into the CNS.

Inflammatory & Apoptotic Factor Fluctuations Associated with Japanese Encephalitis Virus Infection in Transgenic IFNAR1-/- Mice

Japanese encephalitis virus (JEV) is an orthoflavivirus that causes Japanese encephalitis, a mosquito-borne viral infection that primarily affects humans and animals. JEV is a major cause of encephalitis in many parts of Asia, particularly in rural and agricultural areas. In this study, we used the IFNAR1-/- mice model to investigate alterations in cytokine and apoptotic factor levels in IFNAR1-/- mice upon JEV infection. A 5-week-adult female C57BL/6 IFN-α/β receptor knockout (IFNAR1-/-) transgenic mice were intramuscularly inoculated with several viral titers and monitored within 10 dpi. The weight changes and survival rates were evaluated during the study period. Gene expression analysis was performed using RT-qPCR, targeting genes related to specific cytokines and apoptotic factors, to identify the inflammatory factors fluctuations associated with JEV strain KBPV-VR-27 infection in IFNAR1-/- mice. The expression of cytokine genes was enhanced in IFNAR1-/- mice infected with JEV KBPV-VR-27. Notably, a significant induction of cytokines, such as IL-13, IL-17α, IFN-β, and IFN-γ, was observed in the brain, while upregulation of IL-6, IFN-β, and IFN-γ was exhibited in the lung. In addition, among the targeted apoptotic factors, only significant induction of Bak was observed in the brain. We also found that the spleen exhibited a higher viral load compared to the brain and lungs. In conclusion, the findings of this study shed light on the varying viral loads across targeted organs, with the brain exhibiting a lower viral load but pronounced expression of targeted pro-inflammatory cytokines in IFNAR1-/- mice.

Zika virus E protein modulates functions of human brain microvascular endothelial cells and astrocytes: implications on blood-brain barrier properties

Neurotropic viruses can cross the otherwise dynamically regulated blood-brain barrier (BBB) and affect the brain cells. Zika virus (ZIKV) is an enveloped neurotropic Flavivirus known to cause severe neurological complications, such as encephalitis and fetal microcephaly. In the present study, we employed human brain microvascular endothelial cells (hBMECs) and astrocytes derived from human progenitors to establish a physiologically relevant BBB model. We used this model to investigate the effects of ZIKV envelope (E) protein on properties of cells comprising the BBB. E protein is the principal viral protein involved in interaction with host cell surface receptors, facilitating the viral entry. Our findings show that the presence of ZIKV E protein leads to activation of both hBMECs and astrocytes. In hBMECs, we observed a decrease in the expression of crucial endothelial junction proteins such as ZO-1, Occludin and VE-Cadherin, which are vital in establishment and maintenance of the BBB. Consequently, the ZIKV E protein induced changes in BBB integrity and permeability. We also found upregulation of genes involved in leukocyte recruitment along with increased proinflammatory chemokines and cytokines upon exposure to E protein. Additionally, the E protein also led to astrogliosis, evident from the elevated expression of GFAP and Vimentin. Both cell types comprising the BBB exhibited inflammatory response upon exposure to E protein which may influence viral access into the central nervous system (CNS) and subsequent infection of other CNS cells. Overall, our study provides valuable insights into the transient changes that occur at the site of BBB upon ZIKV infection.

Hazara orthonairovirus nucleoprotein facilitates viral cell-to-cell spread by modulating tight junction protein, claudin-1

Background

Tight junctions act as a barrier that prevents invasion of pathogens through epithelial cells. This study aims to elucidate the correlation between tight junctions and nairoviruses using Hazara orthonairovirus (HAZV) as a surrogate model for Crimean-Congo hemorrhagic fever virus.

Methods

mRNA, total protein, and cell surface protein levels of tight junction proteins were examined by quantitative real-time reverse transcription polymerase chain reaction, immunoblot and flow cytometry, respectively. HAZV growth was measured by plaque assay. Immunofluorescence assay was used to examine viral cell-to-cell spread. The interaction between HAZV nucleoprotein and claudin-1 was analyzed by immunoprecipitation.

Results

HAZV infection induced mRNA of several tight junction proteins, especially claudin-1. HAZV infection also induced cell surface expression of claudin-1 protein. Claudin-1 overexpression inhibited the growth of HAZV by blocking its cell-to-cell spread. In contrast, HAZV nucleoprotein completely inhibited HAZV-induced cell surface expression of claudin-1, and this inhibition required interaction between HAZV nucleoprotein and claudin-1.

Conclusion

HAZV nucleoprotein was shown to bind to claudin-1 to negatively regulate its cell surface expression, and so can promote cell-to-cell spread of HAZV. This is the first presentation of a possible mechanism behind how nairoviruses counteract tight junction barrier function.

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.

Zika virus replication on endothelial cells and invasion into the central nervous system by inhibiting interferon β translation

The blood-brain barrier (BBB) is one of the tightest physical barriers to prevent pathogens from invading the central nervous system (CNS). However, the mechanism by which Zika virus (ZIKV) crossing the BBB remains unresolved. We found ZIKV induced high morbidity and mortality in newborn mice, accompanied by inflammatory injury on CNS. ZIKV was found to replicate primarily in the cortex and hippocampus in neonatal mouse brains. An in vitro model revealed that ZIKV had no impact on hBMECs permeability but led to endothelial activation, as shown by the enhancement of adhesion molecules expression and F-actin redistribution. ZIKV replication in hBMECs might be associated with the suppression of IFN-β translation via inhibiting RPS6 phosphorylation. On the other hand, ZIKV infection induced IFN-stimulated genes (ISGs), activated the mitogen-activated protein kinase (MAPK) signaling pathway, and promoted chemokine secretion. This study provides an understanding of virus replication and transmigration across the BBB during ZIKV infection.

A Journey to the Central Nervous System: Routes of Flaviviral Neuroinvasion in Human Disease

Many arboviruses, including viruses of the Flavivirus genera, are known to cause severe neurological disease in humans, often with long-lasting, debilitating sequalae in surviving patients. These emerging pathogens impact millions of people worldwide, yet still relatively little is known about the exact mechanisms by which they gain access to the human central nervous system. This review focusses on potential haematogenous and transneural routes of neuroinvasion employed by flaviviruses and identifies numerous gaps in knowledge, especially regarding lesser-studied interfaces of possible invasion such as the blood-cerebrospinal fluid barrier, and novel routes such as the gut-brain axis. The complex balance of pro-inflammatory and antiviral immune responses to viral neuroinvasion and pathology is also discussed, especially in the context of the hypothesised Trojan horse mechanism of neuroinvasion. A greater understanding of the routes and mechanisms of arboviral neuroinvasion, and how they differ between viruses, will aid in predictive assessments of the neuroinvasive potential of new and emerging arboviruses, and may provide opportunity for attenuation, development of novel intervention strategies and rational vaccine design for highly neurovirulent arboviruses.

Signaling events evoked by domain III of envelop glycoprotein of tick-borne encephalitis virus and West Nile virus in human brain microvascular endothelial cells

Tick-borne encephalitis virus and West Nile virus can cross the blood–brain barrier via hematogenous route. The attachment of a virion to the cells of a neurovascular unit, which is mediated by domain III of glycoprotein E, initiates a series of events that may aid viral entry. Thus, we sought to uncover the post-attachment biological events elicited in brain microvascular endothelial cells by domain III. RNA sequencing of cells treated with DIII of TBEV and WNV showed significant alteration in the expression of 309 and 1076 genes, respectively. Pathway analysis revealed activation of the TAM receptor pathway. Several genes that regulate tight-junction integrity were also activated, including pro-inflammatory cytokines and chemokines, cell-adhesion molecules, claudins, and matrix metalloprotease (mainly ADAM17). Results also indicate activation of a pro-apoptotic pathway. TLR2 was upregulated in both cases, but MyD88 was not. In the case of TBEV DIII, a MyD88 independent pathway was activated. Furthermore, both cases showed dramatic dysregulation of IFN and IFN-induced genes. Results strongly suggest that the virus contact to the cell surface emanates a series of events namely viral attachment and diffusion, breakdown of tight junctions, induction of virus uptake, apoptosis, reorganization of the extracellular-matrix, and activation of the innate immune system.

Flavivirus NS1 Triggers Tissue-Specific Disassembly of Intercellular Junctions Leading to Barrier Dysfunction and Vascular Leak in a GSK-3β-Dependent Manner

The flavivirus nonstructural protein 1 (NS1) is secreted from infected cells and contributes to endothelial barrier dysfunction and vascular leak in a tissue-dependent manner. This phenomenon occurs in part via disruption of the endothelial glycocalyx layer (EGL) lining the endothelium. Additionally, we and others have shown that soluble DENV NS1 induces disassembly of intercellular junctions (IJCs), a group of cellular proteins critical for maintaining endothelial homeostasis and regulating vascular permeability; however, the specific mechanisms by which NS1 mediates IJC disruption remain unclear. Here, we investigated the relative contribution of five flavivirus NS1 proteins, from dengue (DENV), Zika (ZIKV), West Nile (WNV), Japanese encephalitis (JEV), and yellow fever (YFV) viruses, to the expression and localization of the intercellular junction proteins β-catenin and VE-cadherin in endothelial cells from human umbilical vein and brain tissues. We found that flavivirus NS1 induced the mislocalization of β-catenin and VE-cadherin in a tissue-dependent manner, reflecting flavivirus disease tropism. Mechanistically, we observed that NS1 treatment of cells triggered internalization of VE-cadherin, likely via clathrin-mediated endocytosis, and phosphorylation of β-catenin, part of a canonical IJC remodeling pathway during breakdown of endothelial barriers that activates glycogen synthase kinase-3β (GSK-3β). Supporting this model, we found that a chemical inhibitor of GSK-3β reduced both NS1-induced permeability of human umbilical vein and brain microvascular endothelial cell monolayers in vitro and vascular leakage in a mouse dorsal intradermal model. These findings provide insight into the molecular mechanisms regulating NS1-mediated endothelial dysfunction and identify GSK-3β as a potential therapeutic target for treatment of vascular leakage during severe dengue disease.

Intrauterine Viral Infections: Impact of Inflammation on Fetal Neurodevelopment

Intrauterine viral infections during pregnancy by pathogens such as Zika virus, Cytomegalovirus, Rubella and Herpes Simplex virus can lead to prenatal as well as postnatal neurodevelopmental disorders. Although maternal viral infections are common during pregnancy, viruses rarely penetrate the trophoblast. When they do cross, viruses can cause adverse congenital health conditions for the fetus. In this context, maternal inflammatory responses to these neurotropic pathogens play a significant role in negatively affecting neurodevelopment. For instance, intrauterine inflammation poses an increased risk of neurodevelopmental disorders such as microcephaly, schizophrenia, autism spectrum disorder, cerebral palsy and epilepsy. Severe inflammatory responses have been linked to stillbirths, preterm births, abortions and microcephaly. In this review, we discuss the mechanistic basis of how immune system shapes the landscape of the brain and how different neurotropic viral pathogens evoke inflammatory responses. Finally, we list the consequences of neuroinflammation on fetal brain development and discuss directions for future research and intervention strategies.

Pro-inflammatory and anti-inflamatory cytokine genes polymorphisms and susceptibility to Japanese encephalitis disease in the North Indian population

BACKGROUND

Japanese encephalitis virus (JEV) is the major cause of viral encephalitis in many regions of Asia. Cytokines, including pro-inflammatory and anti-inflammatory are key regulators playing a detrimental role in the host response to JE infection, pathogenesis and disease outcome. Evidently, the host's cytokine response is genetically determined, representing the complexity of interindividual differences regarding immune response to viral infection. The current study assesses the association of single nucleotide polymorphisms of classical interleukin IL-1β and IL-10 with JEV susceptibility and disease severity in north Indian population.

METHODS

We performed a case-control study using 85 JE patients and 85 healthy controls. Polymorphisms in the IL-1β (-511 C/T) and IL-10 (-1082 A/G) genes were genotyped using PCR-RFLP. All continuous variables were expressed as mean ± standard deviation, and categorical variables were expressed in percentage.

RESULTS

The mRNA level of IL-1β and IL-10 were found significantly increased in JE patients. In severe JE patients, IL-1β mRNA level was significantly higher with heterozygous (C/T) and homozygous (C/C) genotype compared to wild (T/T) genotype and mRNA level of IL-10 was higher in heterozygous genotype (A/G) compared to wild genotype (A/A). The C/T and C/C genotypes of IL-1β were significantly associated with higher risk of JE infection (p < 0.05, OR = 7.25 and 4.40) whereas, the A/G genotype of IL-10 was associated with a reduced risk of JEV infection (p < 0.05, OR = 0.30). The C allele of IL-1β was associated with fever and neck stiffness (p < 0.05) and CT genotype was associated with disease severity and worse outcomes in JE patients. Along with this, IL-10 polymorphism was found associated with fever, and AG genotype was found to be associated with worse disease outcomes such as neurological sequelae (p < 0.05).

CONCLUSION

Mutant allele and genotype at IL-1β (-511 C/T) and IL-10 (-1082 A/G) gene polymorphism show increased expression of IL-1β and IL-10 in JE patients which contribute to disease severity as well as adverse outcomes of disease. Overall this is the first report from northern India, which shows the association of IL-1β and IL-10 polymorphisms with JEV infection.

Comparative analysis of neuroinvasion by Japanese encephalitis virulent and vaccine viral strains in an in vitro model of human blood-brain barrier

Japanese encephalitis virus (JEV) is the major cause of viral encephalitis in South East Asia. It has been suggested that, as a consequence of the inflammatory process during JEV infection, there is disruption of the blood-brain barrier (BBB) tight junctions that in turn allows the virus access to the central nervous system (CNS). However, what happens at early times of JEV contact with the BBB is poorly understood. In the present work, we evaluated the ability of both a virulent and a vaccine strain of JEV (JEV RP9 and SA14-14-2, respectively) to cross an in vitro human BBB model. Using this system, we demonstrated that both JEV RP9 and SA14-14-2 are able to cross the BBB without disrupting it at early times post viral addition. Furthermore, we find that almost 10 times more RP9 infectious particles than SA14-14 cross the model BBB, indicating this BBB model discriminates between the virulent RP9 and the vaccine SA14-14-2 strains of JEV. Beyond contributing to the understanding of early events in JEV neuroinvasion, we demonstrate this in vitro BBB model can be used as a system to study the viral determinants of JEV neuroinvasiveness and the molecular mechanisms by which this flavivirus crosses the BBB during early times of neuroinvasion.

Classical swine fever virus infection suppresses claudin-1 expression to facilitate its replication in PK-15 cells

Classical swine fever (CSF) is one of the most epidemic viral diseases in swine industry. The causative pathogen is CSF virus (CSFV), a small enveloped RNA virus of Flaviviridae family. Claudin-1 was reported to be involved in the infections of a number of viruses, including many from Flaviviridae family, but no studies have investigated the role of porcine claudin-1 during CSFV infection in PK-15 cells. In this study, on the one hand, we demonstrated that CSFV infection reduced the claudin-1 expression at both mRNA and protein levels; on the other hand, CSFV infection was enhanced after claudin-1 knockdown, but inhibited by claudin-1 overexpression in a dose-dependent manner. Furthermore, negative correlation was demonstrated between the claudin-1 expression and CSFV titer. In conclusion, claudin-1 might be a barrier for CSFV infection in PK-15 cells, while CSFV bypasses the barrier through lysosome mediated degradation of claudin-1, which could be repressed by bafilomycin A1. Although the elaborate mechanisms how claudin-1 plays its roles in CSFV infection require further investigations, this study may advance our understanding of the molecular host-pathogen interaction mechanisms underlying CSFV infection and suggests enhancement of porcine claudin-1 as a potential preventive or therapeutic strategy for CSF control.

Porcine Reproductive and Respiratory Syndrome Virus Structural Protein GP3 Regulates Claudin 4 To Facilitate the Early Stages of Infection

Claudins (CLDN) are a family of proteins that represent the most important components of tight junctions, where they establish the paracellular barrier that controls the flow of molecules in the intercellular space between epithelial cells. Several types of viruses make full use of CLDN to facilitate entry into cells. Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogens in the swine industry. In this study, we found that CLDN4 functions as an anti-PRRSV factor by blocking its absorption during the early stages of infection. The small extracellular loop (ECL2) of CLDN4 restricted the viral particles outside cells by binding to GP3. A novel function of GP3-mediated regulation of CLDN4 transcription was suggested. CLDN4 can be decreased through downregulating the level of CLDN4 transcription by ubiquitinating the transcription factor, SP1. The mechanism by which highly pathogenic PRRSV infects the epithelium was proposed. Importantly, ECL2 was found to block PRRSV absorption and infection and neutralize the virus. A more in-depth understanding of PRRSV infection is described, and novel therapeutic antiviral strategies are discussed.IMPORTANCE In the present study, the role of CLDN4 in PRRSV infection was studied. The results showed that CLDN4 blocked absorption into cells and restricted extracellular viral particles via the interaction between the CLDN4 small extracellular loop, ECL2, and the viral surface protein GP3. GP3 was found to downregulate CLDN4 through ubiquitination of the transcription factor SP1 to facilitate viral entry. The mechanism by which highly pathogenic PRRSV infects the epithelium is suggested. A novel function of GP3 in regulating gene transcription was discovered. Moreover, ECL2 could block PRRSV absorption and infection, as well as neutralizing the virus in the supernatant, which may lead to the development of novel therapeutic antiviral strategies.

Titanium dioxide nanoparticles exaggerate respiratory syncytial virus-induced airway epithelial barrier dysfunction

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in children worldwide. While most develop a mild, self-limiting illness, some develop severe acute lower respiratory infection and persistent airway disease. Exposure to ambient particulate matter has been linked to asthma, bronchitis, and viral infection in multiple epidemiological studies. We hypothesized that coexposure to nanoparticles worsens RSV-induced airway epithelial barrier dysfunction. Bronchial epithelial cells were incubated with titanium dioxide nanoparticles (TiO2-NP) or a combination of TiO2-NP and RSV. Structure and function of epithelial cell barrier were analyzed. Viral titer and the role of reactive oxygen species (ROS) generation were evaluated. In vivo, mice were intranasally incubated with TiO2-NP, RSV, or a combination. Lungs and bronchoalveolar lavage (BAL) fluid were harvested for analysis of airway inflammation and apical junctional complex (AJC) disruption. RSV-induced AJC disruption was amplified by TiO2-NP. Nanoparticle exposure increased viral infection in epithelial cells. TiO2-NP induced generation of ROS, and pretreatment with antioxidant, N-acetylcysteine, reversed said barrier dysfunction. In vivo, RSV-induced injury and AJC disruption were augmented in the lungs of mice given TiO2-NP. Airway inflammation was exacerbated, as evidenced by increased white blood cell infiltration into the BAL, along with exaggeration of peribronchial inflammation and AJC disruption. These data demonstrate that TiO2-NP exposure exacerbates RSV-induced AJC dysfunction and increases inflammation by mechanisms involving generation of ROS. Further studies are required to determine whether NP exposure plays a role in the health disparities of asthma and other lung diseases, and why some children experience more severe airway disease with RSV infection.

Vascular underpinning of COVID-19

COVID-19 management guidelines have largely attributed critically ill patients who develop acute respiratory distress syndrome, to a systemic overproduction of pro-inflammatory cytokines. Cardiovascular dysfunction may also represent a primary phenomenon, with increasing data suggesting that severe COVID-19 reflects a confluence of vascular dysfunction, thrombosis and dysregulated inflammation. Here, we first consolidate the information on localized microvascular inflammation and disordered cytokine release, triggering vessel permeability and prothrombotic conditions that play a central role in perpetuating the pathogenic COVID-19 cascade. Secondly, we seek to clarify the gateways which SARS-CoV-2, the causative COVID-19 virus, uses to enter host vascular cells. Post-mortem examinations of patients' tissues have confirmed direct viral endothelial infection within several organs. While there have been advances in single-cell RNA sequencing, endothelial cells across various vascular beds express low or undetectable levels of those touted SARS-CoV-2 entry factors. Emerging studies postulate alternative pathways and the apicobasal distribution of host cell surface factors could influence endothelial SARS-CoV-2 entry and replication. Finally, we provide experimental considerations such as endothelial polarity, cellular heterogeneity in organoids and shear stress dynamics in designing cellular models to facilitate research on viral-induced endothelial dysfunctions. Understanding the vascular underpinning of COVID-19 pathogenesis is crucial to managing outcomes and mortality.

Preventive effects of folic acid on Zika virus-associated poor pregnancy outcomes in immunocompromised mice

Zika virus (ZIKV) infection may lead to congenital microcephaly and pregnancy loss in pregnant women. In the context of pregnancy, folic acid (FA) supplementation may reduce the risk of abnormal pregnancy outcomes. Intriguingly, FA may have a beneficial effect on the adverse pregnancy outcomes associated with ZIKV infection. Here, we show that FA inhibits ZIKV replication in human umbilical vein endothelial cells (HUVECs) and a cell culture model of blood-placental barrier (BPB). The inhibitory effect of FA against ZIKV infection is associated with FRα-AMPK signaling. Furthermore, treatment with FA reduces pathological features in the placenta, number of fetal resorptions, and stillbirths in two mouse models of in utero ZIKV transmission. Mice with FA treatment showed lower viral burden and better prognostic profiles in the placenta including reduced inflammatory response, and enhanced integrity of BPB. Overall, our findings suggest the preventive role of FA supplementation in ZIKV-associated abnormal pregnancy and warrant nutritional surveillance to evaluate maternal FA status in areas with active ZIKV transmission.

Zinc Chelation Specifically Inhibits Early Stages of Dengue Virus Replication by Activation of NF-κB and Induction of Antiviral Response in Epithelial Cells

Zinc is an essential micronutrient which regulates diverse physiological functions and has been shown to play a crucial role in viral infections. Zinc has a necessary role in the replication of many viruses, however, antiviral action of zinc has also been demonstrated in in vitro infection models most likely through induction of host antiviral responses. Therefore, depending on the host machinery that the virus employs at different stages of infection, zinc may either facilitate, or inhibit virus infection. In this study, we show that zinc plays divergent roles in rotavirus and dengue virus infections in epithelial cells. Dengue virus infection did not perturb the epithelial barrier functions despite the release of virus from the basolateral surface whereas rotavirus infection led to disruption of epithelial junctions. In rotavirus infection, zinc supplementation post-infection did not block barrier disruption suggesting that zinc does not affect rotavirus life-cycle or protects epithelial barriers post-infection suggesting the involvement of cellular pathways in the beneficial effect of zinc supplementation in enteric infections. Zinc depletion by N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN) inhibited dengue virus and Japanese encephalitis virus (JEV) infection but had no effect on rotavirus. Time-of-addition experiments suggested that zinc chelation affected both early and late stages of dengue virus infectious cycle and zinc chelation abrogated dengue virus RNA replication. We show that transient zinc chelation induces ER stress and antiviral response by activating NF-kappaB leading to induction of interferon signaling. These results suggest that modulation of zinc homeostasis during virus infection could be a component of host antiviral response and altering zinc homeostasis may act as a potent antiviral strategy against flaviviruses.

Role of adherens junctions and apical-basal polarity of neural stem/progenitor cells in the pathogenesis of neurodevelopmental disorders: a novel perspective on congenital Zika syndrome

Radial glial cells (RGCs) are the neural stem/progenitor cells (NSPCs) that give rise to most of neurons and glial cells that constitute the adult central nervous system. A hallmark of RGCs is their polarization along the apical-basal axis. They extend a long basal process that contacts the pial surface and a short apical process to the ventricular surface. Adherens junctions (AJs) are organized as belt-like structures at the most-apical lateral plasma membrane of the apical processes. These junctional complexes anchor RGCs to each other and allow the recruitment of cytoplasmic proteins that act as apical-basal determinants. It has been proposed that disruption of AJs underlies the onset of different neurodevelopmental disorders. In fact, studies performed in different animal models indicate that loss of function of AJs-related proteins in NSPCs can disrupt cell polarity, imbalance proliferation and/or differentiation rates and increase cell death, which, in turn, lead to disruption of the cytoarchitecture of the ventricular zone, protrusion of non-polarized cells into the ventricles, cortical thinning, and ventriculomegaly/hydrocephalus, among other neuropathological findings. Recent Zika virus (ZIKV) outbreaks and the high comorbidity of ZIKV infection with congenital neurodevelopmental defects have led to the World Health Organization to declare a public emergency of international concern. Thus, noteworthy advances have been made in clinical and experimental ZIKV research. This review summarizes the current knowledge regarding the function of AJs in normal and pathological corticogenesis and focuses on the neuropathological and cellular mechanisms involved in congenital ZIKV syndrome, highlighting the potential role of cell-to-cell junctions between NSPCs in the etiopathogenesis of such syndrome.

Pathways Exploited by Flaviviruses to Counteract the Blood-Brain Barrier and Invade the Central Nervous System

Human infection by different flaviviruses may cause severe neurologic syndromes, through pathogenic mechanisms that are still largely unknown. Japanese encephalitis virus (JEV), West Nile virus (WNV), Zika virus (ZIKV), yellow fever virus (YFV), dengue virus (DENV), and tick-borne encephalitis virus (TBEV) are believed to reach the central nervous system by a hematogenous route, upon crossing the blood-brain barrier. Although the disruption of BBB during flavivirus infection has been largely evidenced in experimental models, the relevance of BBB breakdown for virus entering the brain was not completely elucidated. In vitro models of BBB had demonstrated that these viruses replicated in brain microvascular endothelial cells (BMECs), which induced downregulation of tight junction proteins and increased the permeability of the barrier. Other reports demonstrated that infection of BMECs allowed the basolateral release of infectious particles, without a remarkable cytopathic effect, what might be sufficient for virus invasion. Virus replication and activation of other cells associated to the BBB, mostly astrocytes and microglia, were also reported to affect the endothelial barrier permeability. This event might occur simultaneously or after BMECs infection, being a secondary effect leading to BBB disruption. Importantly, activation of BMECs, astrocytes, and microglia by flaviviruses was associated to the expression and secretion of inflammatory mediators, which are believed to recruit leukocytes to the CNS. The leukocyte infiltrate could further mediate viral invasion through a Trojan horse mechanism and might contribute to BBB breakdown and to neurological alterations. This review discussed the previous studies regarding in vitro and in vivo models of JEV, WNV, ZIKV, YFV, DENV, and TBEV infection and addressed the pathways for BBB overcome and invasion of the CNS described for each virus infection, aiming to increment the knowledge and stimulate further discussion about the role of BBB in the neuropathogenesis of flavivirus infection.

Claudin-1 inhibits human parainfluenza virus type 2 dissemination

Tight junctions enable epithelial cells to form physical barriers that act as an innate immune defense against respiratory infection. However, the involvement of tight junction molecules in paramyxovirus infections, which include various respiratory pathogens, has not been examined in detail. Human parainfluenza virus type 2 (hPIV2) infects airway epithelial cells and causes respiratory illness. In the present study, we found that hPIV2 infection of cultured cells induces expression of claudin-1 (CLDN1), an essential component of tight junctions. This induction seemed to be intrinsically restricted by V, an accessory protein that modulates various host responses, to enable efficient virus propagation. By generating CLDN1 over-expressing and knockout cell lines, we showed that CLDN1 is involved in the restriction of hPIV2 spread via cell-to-cell contact. Taken together, we identified CLDN1 an inhibitory factor for hPIV2 dissemination, and that its V protein acts to counter this.

Dengue virus enters and exits epithelial cells through both apical and basolateral surfaces and perturbs the apical junctional complex

Dengue is the most relevant mosquito-borne viral disease in the world. It has been estimated that 390 million infections of dengue occur each year. Dengue virus (DENV) infection can be asymptomatic or can produce a self-limited febrile illness called dengue fever (DF) or a severe form of the infection called severe dengue. In some viruses, the entry and egress from cells, occur in a specific domain of polarized endothelial and epithelial cells. In this study, we investigated whether the entry and release of DENV was polarized in epithelial cells, and evaluated the effect of DENV infection on cellular junctions of epithelial cells. We used MDCK epithelial cells, which serve as an excellent model to study a functional barrier due to the presence of an apical junctional complex (AJC), and showed that entry and release of DENV from the cells, is bipolar. Additionally, we performed paracellular flux, diffusion of membrane lipid, immunofluorescence and immunoblotting assays to evaluate the integrity of the AJC during DENV infection. We observed that at later stages of infection, DENV altered the barrier function causing a decrease in the transepithelial electrical resistance and the degradation and delocalization of TJ and AJ proteins. The present study contributes to understand how DENV traverse epithelia in order to cause a productive infection, and provides insights into the mechanism of DENV pathogenesis.

Flavivirus Receptors: Diversity, Identity, and Cell Entry

Flaviviruses are emerging and re-emerging arthropod-borne pathogens responsible for significant mortality and morbidity worldwide. The genus comprises more than seventy small, positive-sense, single-stranded RNA viruses, which are responsible for a spectrum of human and animal diseases ranging in symptoms from mild, influenza-like infection to fatal encephalitis and haemorrhagic fever. Despite genomic and structural similarities across the genus, infections by different flaviviruses result in disparate clinical presentations. This review focusses on two haemorrhagic flaviviruses, dengue virus and yellow fever virus, and two neurotropic flaviviruses, Japanese encephalitis virus and Zika virus. We review current knowledge on host-pathogen interactions, virus entry strategies and tropism.

Heparan sulfate is an important mediator of Ebola virus infection in polarized epithelial cells

BACKGROUND

Currently, no FDA-approved vaccines or treatments are available for Ebola virus disease (EVD), and therapy remains largely supportive. Ebola virus (EBOV) has broad tissue tropism and can infect a variety of cells including epithelial cells. Epithelial cells differ from most other cell types by their polarized phenotype and barrier function. In polarized cells, the apical and basolateral membrane domains are demarcated by tight junctions, and specialized sorting machinery, which results in a difference in composition between the two membrane domains. These specialized sorting functions can have important consequences for viral infections. Differential localization of a viral receptor can restrict virus entry to a particular membrane while polarized sorting can lead to a vectorial virus release. The present study investigated the impact of cell polarity on EBOV infection.

METHODS

Characteristics of EBOV infection in polarized cells were evaluated in the polarized Caco-2 model grown on semipermeable transwells. Transepithelial resistance (TEER), which is a function of tight junctions, was used to assess epithelial cell polarization. EBOV infection was assessed with immunofluorescence microscopy and qPCR. Statistical significance was calculated using one-way ANOVA and significance was set at p < 0.05.

RESULTS

Our data indicate that EBOV preferentially infects cells from the basolateral route, and this preference may be influenced by the resistance across the Caco-2 monolayer. Infection occurs without changes in cellular permeability. Further, our data show that basolateral infection bias may be dependent on polarized distribution of heparan sulfate, a known viral attachment factor. Treatment with iota-carrageenan, or heparin lyase, which interrupts viral interaction with cellular heparan sulfate, significantly reduced cell susceptibility to basolateral infection, likely by inhibiting virus attachment.

CONCLUSIONS

Our results show cell polarity has an impact on EBOV infection. EBOV preferentially infects polarized cells through the basolateral route. Access to heparan sulfate is an important factor during basolateral infection and blocking interaction of cellular heparan sulfate with virus leads to significant inhibition of basolateral infection in the polarized Caco-2 cell model.

Exploitation of microRNAs by Japanese Encephalitis virus in human microglial cells

JEV infection in CNS leads to the JE neuroinflammation. Children and old age individual have been reported to be more prone to JEV infection. MicroRNAs are endogenous, small non-coding RNAs, which regulate the gene expression. These are ∼22 nucleotide long, conserved RNA sequence that binds at the 3'UTR of a target mRNA and regulate the post-transcriptional gene expression. The role of microRNAs has been reported in several diseases like cancer, viral infection, neuro-degeneration, diabetes etc. In the present study, the human microglial cells were infected with JEV (JaOArS982). The control and infected samples were subject to microarray profiling for microRNA expression. The microarray profile yielded differentially expressed microRNAs from JEV infected samples. The microRNA gene targets, gene ontology, annotations, and pathways were identified through various bioinformatics tools. Additionally, the pathways were mostly found common to "ubiquitin mediated proteolysis," "cytokine signaling," "maintenance of barrier function/cell junctions," JAK/STAT pathway" "Toll-like receptor signaling," "Wnt-signaling," "adhesion molecules," "apoptosis," "endocytosis," "vesicle mediated transport" etc.

Cryptic etiopathological conditions of equine nervous system with special emphasis on viral diseases

The importance of horse (Equus caballus) to equine practitioners and researchers cannot be ignored. An unevenly distributed population of equids harbors numerous diseases, which can affect horses of any age and breed. Among these, the affections of nervous system are potent reason for death and euthanasia in equids. Many episodes associated with the emergence of equine encephalitic conditions have also pose a threat to human population as well, which signifies their pathogenic zoonotic potential. Intensification of most of the arboviruses is associated with sophisticated interaction between vectors and hosts, which supports their transmission. The alphaviruses, bunyaviruses, and flaviviruses are the major implicated groups of viruses involved with equines/humans epizootic/epidemic. In recent years, many outbreaks of deadly zoonotic diseases such as Nipah virus, Hendra virus, and Japanese encephalitis in many parts of the globe addresses their alarming significance. The equine encephalitic viruses differ in their global distribution, transmission and main vector species involved, as discussed in this article. The current review summarizes the status, pathogenesis, pathology, and impact of equine neuro-invasive conditions of viral origin. A greater understanding of these aspects might be able to provide development of advances in neuro-protective strategies in equine population.

Zika-Virus-Encoded NS2A Disrupts Mammalian Cortical Neurogenesis by Degrading Adherens Junction Proteins

Zika virus (ZIKV) directly infects neural progenitors and impairs their proliferation. How ZIKV interacts with the host molecular machinery to impact neurogenesis in vivo is not well understood. Here, by systematically introducing individual proteins encoded by ZIKV into the embryonic mouse cortex, we show that expression of ZIKV-NS2A, but not Dengue virus (DENV)-NS2A, leads to reduced proliferation and premature differentiation of radial glial cells and aberrant positioning of newborn neurons. Mechanistically, in vitro mapping of protein-interactomes and biochemical analysis suggest interactions between ZIKA-NS2A and multiple adherens junction complex (AJ) components. Functionally, ZIKV-NS2A, but not DENV-NS2A, destabilizes the AJ complex, resulting in impaired AJ formation and aberrant radial glial fiber scaffolding in the embryonic mouse cortex. Similarly, ZIKA-NS2A, but not DENV-NS2A, reduces radial glial cell proliferation and causes AJ deficits in human forebrain organoids. Together, our results reveal pathogenic mechanisms underlying ZIKV infection in the developing mammalian brain.

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.

Claudins in viral infection: from entry to spread

Tight junctions are critically important for many physiological functions, including the maintenance of cell polarity, regulation of paracellular permeability, and involvement in signal transduction pathways to regulate integral cellular processes. Furthermore, tight junctions enable epithelial cells to form physical barriers, which act as an innate immune mechanism that can impede viral infection. Viruses, in turn, have evolved mechanisms to exploit tight junction proteins to gain access to cells or spread through tissues in an infected host. Claudin family proteins are integral components of tight junctions and are thought to play crucial roles in regulating their permeability. Claudins have been implicated in the infection process of several medically important human pathogens, including hepatitis C virus, dengue virus, West Nile virus, and human immunodeficiency virus, among others. In this review, we summarize the role of claudins in viral infections and discuss their potential as novel antiviral targets. A better understanding of claudins during viral infection may provide insight into physiological roles of claudins and uncover novel therapeutic antiviral strategies.

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.

In Vitro Infection with Dengue Virus Induces Changes in the Structure and Function of the Mouse Brain Endothelium

Background

The neurological manifestations of dengue disease are occurring with greater frequency, and currently, no information is available regarding the reasons for this phenomenon. Some viruses infect and/or alter the function of endothelial organs, which results in changes in cellular function, including permeability of the blood-brain barrier (BBB), which allows the entry of infected cells or free viral particles into the nervous system.

Methods

In the present study, we standardized two in vitro models, a polarized monolayer of mouse brain endothelial cells (MBECs) and an organized co-culture containing MBECs and astrocytes. Using these cell models, we assessed whether DENV-4 or the neuro-adapted dengue virus (D4MB-6) variant infects cells or induces changes in the structure or function of the endothelial barrier.

Results

The results showed that MBECs, but not astrocytes, were susceptible to infection with both viruses, although the percentage of infected cells was higher when the neuro-adapted virus variant was used. In both culture systems, DENV infection changed the localization of the tight junction proteins Zonula occludens (ZO-1) and Claudin-1 (Cln1), and this process was associated with a decrease in transendothelial resistance, an increase in macromolecule permeability and an increase in the paracellular passing of free virus particles. MBEC infection led to transcriptional up-regulation of adhesion molecules (VCAM-1 and PECAM) and immune mediators (MCP-1 and TNF- α) that are associated with immune cell transmigration, mainly in D4MB-6-infected cells.

Conclusion

These results indicate that DENV infection in MBECs altered the structure and function of the BBB and activated the endothelium, affecting its transcellular and paracellular permeability and favoring the passage of viruses and the transmigration of immune cells. This phenomenon can be harnessed for neurotropic and neurovirulent strains to infect and induce alterations in the CNS.

Junctional proteins of the blood-brain barrier: New insights into function and dysfunction

The blood-brain barrier (BBB) is a highly complex and dynamic barrier. It is formed by an interdependent network of brain capillary endothelial cells, endowed with barrier properties, and perivascular cells (astrocytes and pericytes) responsible for inducing and maintaining those properties. One of the primary properties of the BBB is a strict regulation of paracellular permeability due to the presence of junctional complexes (tight, adherens and gap junctions) between the endothelial cells. Alterations in junction assembly and function significantly affect BBB properties, particularly barrier permeability. However, such alterations are also involved in remodeling the brain endothelial cell surface and regulating brain endothelial cell phenotype. This review summarizes the characteristics of brain endothelial tight, adherens and gap junctions and highlights structural and functional alterations in junctional proteins that may contribute to BBB dysfunction.

Influenza virus damages the alveolar barrier by disrupting epithelial cell tight junctions

A major cause of respiratory failure during influenza A virus (IAV) infection is damage to the epithelial-endothelial barrier of the pulmonary alveolus. Damage to this barrier results in flooding of the alveolar lumen with proteinaceous oedema fluid, erythrocytes and inflammatory cells. To date, the exact roles of pulmonary epithelial and endothelial cells in this process remain unclear.Here, we used an in vitro co-culture model to understand how IAV damages the pulmonary epithelial-endothelial barrier. Human epithelial cells were seeded on the upper half of a transwell membrane while human endothelial cells were seeded on the lower half. These cells were then grown in co-culture and IAV was added to the upper chamber.We showed that the addition of IAV (H1N1 and H5N1 subtypes) resulted in significant barrier damage. Interestingly, we found that, while endothelial cells mounted a pro-inflammatory/pro-coagulant response to a viral infection in the adjacent epithelial cells, damage to the alveolar epithelial-endothelial barrier occurred independently of endothelial cells. Rather, barrier damage was associated with disruption of tight junctions amongst epithelial cells, and specifically with loss of tight junction protein claudin-4.Taken together, these data suggest that maintaining epithelial cell integrity is key in reducing pulmonary oedema during IAV infection.

Disruption of in vitro endothelial barrier integrity by Japanese encephalitis virus-Infected astrocytes

Blood-brain barrier (BBB) characteristics are induced and maintained by crosstalk between brain microvascular endothelial cells and neighboring cells. Using in vitro cell models, we previously found that a bystander effect was a cause for Japanese encephalitis-associated endothelial barrier disruption. Brain astrocytes, which neighbor BBB endothelial cells, play roles in the maintenance of BBB integrity. By extending the scope of relevant studies, a potential mechanism has been shown that the activation of neighboring astrocytes could be a cause of disruption of endothelial barrier integrity during the course of Japanese encephalitis viral (JEV) infection. JEV-infected astrocytes were found to release biologically active molecules that activated ubiquitin proteasome, degraded zonula occludens-1 (ZO-1) and claudin-5, and disrupted endothelial barrier integrity in cultured brain microvascular endothelial cells. JEV infection caused astrocytes to release vascular endothelial growth factor (VEGF), interleukin-6 (IL-6), and matrix metalloproteinases (MMP-2/MMP-9). Our data demonstrated that VEGF and IL-6 released by JEV-infected astrocytes were critical for the proteasomal degradation of ZO-1 and the accompanying disruption of endothelial barrier integrity through the activation of Janus kinase-2 (Jak2)/signal transducer and activator of transcription-3 (STAT3) signaling as well as the induction of ubiquitin-protein ligase E3 component, n-recognin-1 (Ubr 1) in endothelial cells. MMP-induced endothelial barrier disruption was accompanied by MMP-mediated proteolytic degradation of claudin-5 and ubiquitin proteasome-mediated degradation of ZO-1 via extracellular VEGF release. Collectively, these data suggest that JEV infection could activate astrocytes and cause release of VEGF, IL-6, and MMP-2/MMP-9, thereby contributing, in a concerted action, to the induction of Japanese encephalitis-associated BBB breakdown. GLIA 2015;63:1915-1932.

Claudins and pathogenesis of viral infection

Since their discovery, tremendous progress has been made in our understanding of the roles of claudins in tight junction physiology. In addition, interactions between claudins and other cellular proteins have highlighted their novel roles in cell physiology. Moreover, the importance of claudins is becoming apparent in the pathophysiology of several diseases, including viral infections. Notable is the discovery of CLDN1 as an essential host factor for hepatitis C virus (HCV) entry, which led to detailed characterization of CLDN1 and its association with tetraspanin CD81 for the initiation of HCV infection. CLDN1 has also been shown to facilitate dengue virus entry. Furthermore, owing to the roles of claudins in forming anatomical barriers, several viruses have been shown to alter claudin expression at the tight junction. This review summarizes the role of claudins in viral infection, with particular emphasis on HCV.

Flaviviruses are neurotropic, but how do they invade the CNS?

Flaviruses (FV) are RNA viruses carried by mosquitoes. Neurological signs including acute encephalitis, meningitis and acute flaccid paralysis develop in a small percentage of infected individuals; long term sequlae are, Parkinsonism, dystonias and cognitive changes. FV neuroinfection is neurotropic involving subcortical nuclei (substantia nigra and thalamus) anterior horn neurons and neocortex. Glycosylation of the FV E envelope protein is one determinant of neuroinvasion, increasing both axonal and trans-epithelial transportation. Neutralizing antibodies against the E and NS proteins prevents FV uptake into several cell types, including axons. CD8+ T cells are vital for clearance of WNF infected cells from the CNS, whereas TLR-3 and TLR-7 mediated anti-virus response through increased serum inflammatory cytokines to disrupt the BBB providing infected leucocytes and free virus access to the CNS (so called Trojan horse) Cellular virus attachment factors, promoting FV cell entry are widely distributed and include DC-SIGN (that detects complex carbohydrate molecules); Glycosamino glycans (GAG), Heparan sulphate(HSPG) Semaphorin 7A, Low Density Lipid receptors (LDLR); these are not FV specific virus entry receptors. The FV also crosses epithelial and endothelial barriers by disrupting Tight Junction complexes to increase BBB permeability. This review describes the multiple pathways responsible for the neuroinvasive properties of the Flaviviruses.