RAGE and CCR7 mediate the transmigration of Zika-infected monocytes through the blood-brain barrier

Metabolic response to CNS infection with flaviviruses

Flaviviruses are arthropod-borne RNA viruses found worldwide that, when introduced into the human body, cause diseases, including neuroinfections, that can lead to serious metabolic consequences and even death. Some of the diseases caused by flaviviruses occur continuously in certain regions, while others occur intermittently or sporadically, causing epidemics. Some of the most common flaviviruses are West Nile virus, dengue virus, tick-borne encephalitis virus, Zika virus and Japanese encephalitis virus. Since all the above-mentioned viruses are capable of penetrating the blood-brain barrier through different mechanisms, their actions also affect the central nervous system (CNS). Like other viruses, flaviviruses, after entering the human body, contribute to redox imbalance and, consequently, to oxidative stress, which promotes inflammation in skin cells, in the blood and in CNS. This review focuses on discussing the effects of oxidative stress and inflammation resulting from pathogen invasion on the metabolic antiviral response of the host, and the ability of viruses to evade the consequences of metabolic changes or exploit them for increased replication and further progression of infection, which affects the development of sequelae and difficulties in therapy.

Mechanisms of Neuroinvasion and Neuropathogenesis by Pathologic Flaviviruses

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

Decreased CSTB, RAGE, and Axl Receptor Are Associated with Zika Infection in the Human Placenta

Zika virus (ZIKV) compromises placental integrity, infecting the fetus. However, the mechanisms associated with ZIKV penetration into the placenta leading to fetal infection are unknown. Cystatin B (CSTB), the receptor for advanced glycation end products (RAGE), and tyrosine-protein kinase receptor UFO (AXL) have been implicated in ZIKV infection and inflammation. This work investigates CSTB, RAGE, and AXL receptor expression and activation pathways in ZIKV-infected placental tissues at term. The hypothesis is that there is overexpression of CSTB and increased inflammation affecting RAGE and AXL receptor expression in ZIKV-infected placentas. Pathological analyses of 22 placentas were performed to determine changes caused by ZIKV infection. Quantitative proteomics, immunofluorescence, and western blot were performed to analyze proteins and pathways affected by ZIKV infection in frozen placentas. The pathological analysis confirmed decreased size of capillaries, hyperplasia of Hofbauer cells, disruption in the trophoblast layer, cell agglutination, and ZIKV localization to the trophoblast layer. In addition, there was a significant decrease in CSTB, RAGE, and AXL expression and upregulation of caspase 1, tubulin beta, and heat shock protein 27. Modulation of these proteins and activation of inflammasome and pyroptosis pathways suggest targets for modulation of ZIKV infection in the placenta.

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.

Endothelial Dysfunction, HMGB1, and Dengue: An Enigma to Solve

Dengue is a viral infection caused by dengue virus (DENV), which has a significant impact on public health worldwide. Although most infections are asymptomatic, a series of severe clinical manifestations such as hemorrhage and plasma leakage can occur during the severe presentation of the disease. This suggests that the virus or host immune response may affect the protective function of endothelial barriers, ultimately being considered the most relevant event in severe and fatal dengue pathogenesis. The mechanisms that induce these alterations are diverse. It has been suggested that the high mobility group box 1 protein (HMGB1) may be involved in endothelial dysfunction. This non-histone nuclear protein has different immunomodulatory activities and belongs to the alarmin group. High concentrations of HMGB1 have been detected in patients with several infectious diseases, including dengue, and it could be considered as a biomarker for the early diagnosis of dengue and a predictor of complications of the disease. This review summarizes the main features of dengue infection and describes the known causes associated with endothelial dysfunction, highlighting the involvement and possible relationship between HMGB1 and DENV.

QKI deficiency in macrophages protects mice against JEV infection by regulating cell migration and antiviral response

Japanese encephalitis (JE) is a major reason to cause viral encephalitis, with 50% patients suffering from severe neuro-inflammation and permanent neural injury. Effective anti-viral treatment is urgently needed. Here, we found RNA binding protein quaking (QKI) was involved in the progression of JE by regulating migration and anti-viral response of macrophages. After JE virus (JEV) infection, QKI-deficient mice had lower viral loads in the brain and fewer neurological symptoms. In comparison with control mice, proinflammatory cytokines in the brain of QKI-deficient animals revealed distinct patterns, with lower levels of IL-6 (interleukin-6) and IFN-β (interferon-β) at the early stage but higher levels at the end of JE. Then we found infiltration of CCR2 positive ((C-C motif) receptor 2) peripheral macrophages and CCR2 expression on macrophages were inhibited in QKI-deficient mice, while the expression of CCR2 ligands was not changed. Bioinformatical analysis showed that a QRE (quaking response element) located on 3'UTR (untranslated region) of Ccr2. We further verified that QKI was able to interact with Ccr2 mRNA and regulate its degradation in vitro. Additionally, since the IFN-β production was increased in QKI-ablation mice after JEV infection, the anti-viral response was analyzed. Results in QKI-silenced N9 cells showed that the expression of RIG-I (retinoic acid-inducible gene-I) and TBK1 (TANK binding kinase 1) was increased, thus further inducing IRF3 (interferon regulatory factor 3) phosphorylation and interferon activation. Overall, these results revealed QKI mediated the anti-viral process via interfering migration of macrophages to CNS (central nervous system) and enhancing RIG-I/IRF3/IFN-β pathway to restrict virus dissemination.

Meningeal lymphatic vessels mediate neurotropic viral drainage from the central nervous system

Recent studies have demonstrated that brain meningeal lymphatic vessels (MLVs) act as a drainage path directly into the cervical lymph nodes (CLNs) for macromolecules contained in the cerebrospinal fluid (CSF). However, the role of MLVs during CNS viral infection remains unexplored. Here, we found that infection with several neurotropic viruses in mice promotes MLV expansion but also causes impaired MLV-mediated drainage of macromolecules. Notably, MLVs could drain virus from the CNS to CLNs. Surgical ligation of the lymph vessels or photodynamic ablation of dorsal MLVs increased neurological damage and mortality of virus-infected mice. By contrast, pretreatment with vascular endothelial growth factor C promoted expansion of functional MLVs and alleviated the effects of viral infection. Together, these data indicate that functional MLVs facilitate virus clearance, and MLVs represent a critical path for virus spreading from the CNS to the CLNs. MLV-based therapeutic strategies may thus be useful for alleviating infection-induced neurological damage.

Concomitant pyroptotic and apoptotic cell death triggered in macrophages infected by Zika virus

Zika virus (ZIKV) is a positive-sense RNA flavivirus and can cause serious neurological disorders including microcephaly in infected fetuses. As a mosquito-borne arbovirus, it enters the bloodstream and replicates in various organs. During pregnancy, it can be transmitted from the blood of the viremic mother to the fetus by crossing the placental barrier. Monocytes and macrophages are considered the earliest blood cell types to be infected by ZIKV. As a first line defense, these cells are crucial components in innate immunity and host responses and may impact viral pathogenesis in humans. Previous studies have shown that ZIKV infection can activate inflammasomes and induce proinflammatory cytokines in monocytes. In this report, we showed that ZIKV could infect and induce cell death in human and murine macrophages. In addition to the presence of cleaved caspase-3, indicating that apoptosis was involved, we identified the cleaved caspase-1 and gasdermin D (GSDMD) as well as increased secretion of IL-1β and IL-18. This suggests that the inflammasome was activated and that may lead to pyroptosis in infected macrophages. The pyroptosis was NLRP3-dependent and could be suppressed in the macrophages treated with shRNA to target and knockdown caspase-1. It was also be inhibited by an inhibitor for caspase-1, indicating that the pyroptosis was triggered via a canonical approach. Our findings in this study demonstrate a concomitant occurrence of apoptosis and pyroptosis in ZIKV-infected macrophages, with two mechanisms involved in the cell death, which may have potentially significant impacts on viral pathogenesis in humans.

The RAGE/DIAPH1 Signaling Axis & Implications for the Pathogenesis of Diabetic Complications

Increasing evidence links the RAGE (receptor for advanced glycation end products)/DIAPH1 (Diaphanous 1) signaling axis to the pathogenesis of diabetic complications. RAGE is a multi-ligand receptor and through these ligand-receptor interactions, extensive maladaptive effects are exerted on cell types and tissues targeted for dysfunction in hyperglycemia observed in both type 1 and type 2 diabetes. Recent evidence indicates that RAGE ligands, acting as damage-associated molecular patterns molecules, or DAMPs, through RAGE may impact interferon signaling pathways, specifically through upregulation of IRF7 (interferon regulatory factor 7), thereby heralding and evoking pro-inflammatory effects on vulnerable tissues. Although successful targeting of RAGE in the clinical milieu has, to date, not been met with success, recent approaches to target RAGE intracellular signaling may hold promise to fill this critical gap. This review focuses on recent examples of highlights and updates to the pathobiology of RAGE and DIAPH1 in diabetic complications.

Host Molecules Regulating Neural Invasion of Zika Virus and Drug Repurposing Strategy

Zika virus (ZIKV) is a mosquito-borne, single-stranded RNA virus belonging to the genus Flavivirus. Although ZIKV infection is usually known to exhibit mild clinical symptoms, intrauterine ZIKV infections have been associated with severe neurological manifestations, including microcephaly and Guillain Barre syndrome (GBS). Therefore, it is imperative to understand the mechanisms of ZIKV entry into the central nervous system (CNS) and its effect on brain cells. Several routes of neuro-invasion have been identified, among which blood–brain barrier (BBB) disruption is the commonest mode of access. The molecular receptors involved in viral entry remain unknown; with various proposed molecular ZIKV-host interactions including potential non-receptor mediated cellular entry. As ZIKV invade neuronal cells, they trigger neurotoxic mechanisms via cell-autonomous and non-cell autonomous pathways, resulting in neurogenesis dysfunction, viral replication, and cell death, all of which eventually lead to microcephaly. Together, our understanding of the biological mechanisms of ZIKV exposure would aid in the development of anti-ZIKV therapies targeting host cellular and/or viral components to combat ZIKV infection and its neurological manifestations. In this present work, we review the current understanding of ZIKV entry mechanisms into the CNS and its implications on the brain. We also highlight the status of the drug repurposing approach for the development of potential antiviral drugs against ZIKV.

Intracellular translocation of HMGB1 is important for Zika virus replication in Huh7 cells

Neonatal microcephaly and adult Guillain-Barré syndrome are severe complications of Zika virus (ZIKV) infection. The robustly induced inflammatory cytokine expressions in ZIKV-infected patients may constitute a hallmark for severe disease. In the present study, the potential role of high mobility group box 1 protein (HMGB1) in ZIKV infection was investigated. HMGB1 protein expression was determined by the enzyme-linked immunosorbent assay (ELISA) and immunoblot assay. HMGB1's role in ZIKV infection was also explored using treatment with dexamethasone, an immunomodulatory drug, and HMGB1-knockdown (shHMGB1) Huh7 cells. Results showed that the Huh7 cells were highly susceptible to ZIKV infection. The infection was found to induce HMGB1 nuclear-to-cytoplasmic translocation, resulting in a > 99% increase in the cytosolic HMGB1 expression at 72-h post-infection (h.p.i). The extracellular HMGB1 level was elevated in a time- and multiplicity of infection (MOI)-dependent manner. Treatment of the ZIKV-infected cells with dexamethasone (150 µM) reduced HMGB1 extracellular release in a dose-dependent manner, with a maximum reduction of 71 ± 5.84% (P < 0.01). The treatment also reduced virus titers by over 83 ± 0.50% (P < 0.01). The antiviral effects, however, were not observed in the dexamethasone-treated shHMGB1 cells. These results suggest that translocation of HMGB1 occurred during ZIKV infection and inhibition of the translocation by dexamethasone coincided with a reduction in ZIKV replication. These findings highlight the potential of targeting the localization of HMGB1 in affecting ZIKV infection.

High-mobility group box 1 protein promotes dengue virus replication by interacting with untranslated regions of viral genome

Dengue virus (DENV) is most prevalent arthropod-borne human pathogen belongs to Flaviviridae family causes thousands of deaths annually. HMGB1 is highly conserved, ubiquitously expressed, non-histone nuclear protein which plays important role in diseases like metabolic disorders, cancer, and viral infections. However, the importance of HMGB1 in DENV infection is understudied. In this study, we observed that DENV-2 induces cytoplasmic translocation and secretion of HMGB1. Interestingly, inhibition of HMGB1 secretion by ethyl pyruvate (EP) enhanced viral propagation while silencing of HMGB1 resulted in abrogated viral replication in DENV-2 infected A549 cells. RNA-Electrophoretic mobility shift assay and immunoprecipitation showed that HMGB1 interacts with 5'-3' UTRs of DENV-2 genome. This interaction further stimulates production of proinflammatory cytokines like TNF-α, IL-6 and IL-1β which have been implicated in pathogenesis of severe DENV disease. Together, our finding suggests that DENV-2 modulates HMGB1 translocation and HMGB1-DENV-2 UTRs RNA interaction further induces proinflammatory cytokines production in A549 cells. This study discloses HMGB1 as an important host factor contributing to disease pathogenesis and hence can be targeted as an alternative approach for antiviral development against DENV virus infection.

Zika Virus Requires the Expression of Claudin-7 for Optimal Replication in Human Endothelial Cells

Zika virus (ZIKV) infection has been associated with a series of neurological pathologies. In patients with ZIKV-induced neurological disorders, the virus is detectable in the central nervous system. Thus, ZIKV is capable of neuroinvasion, presumably through infection of the endothelial cells that constitute the blood-brain barrier (BBB). We demonstrate that susceptibility of BBB endothelial cells to ZIKV infection is modulated by the expression of tight-junction protein claudin-7 (CLDN7). Downregulation of CLDN7 reduced viral RNA yield, viral protein production, and release of infectious viral particles in several endothelial cell types, but not in epithelial cells, indicating that CLDN7 implication in viral infection is cell-type specific. The proviral activity of CLDN7 in endothelial cells is ZIKV-specific since related flaviviruses were not affected by CLDN7 downregulation. Together, our data suggest that CLDN7 facilitates ZIKV infection in endothelial cells at a post-internalization stage and prior to RNA production. Our work contributes to a better understanding of the mechanisms exploited by ZIKV to efficiently infect and replicate in endothelial cells and thus of its ability to cross the BBB.

Brain Microvascular Endothelial Cell-Derived HMGB1 Facilitates Monocyte Adhesion and Transmigration to Promote JEV Neuroinvasion

Infection with Japanese encephalitis virus (JEV) induces high morbidity and mortality, including potentially permanent neurological sequelae. However, the mechanisms by which viruses cross the blood-brain barrier (BBB) and invade into the central nervous system (CNS) remain unclear. Here, we show that extracellular HMGB1 facilitates immune cell transmigration. Furthermore, the migration of immune cells into the CNS dramatically increases during JEV infection which may enhance viral clearance, but paradoxically expedite the onset of Japanese encephalitis (JE). In this study, brain microvascular endothelial cells (BMECs) were utilized for the detection of HMGB1 release, and leucocyte, adhesion, and the integrity of the BBB in vitro. Genetically modified JEV-expressing EGFP (EGFP-JEV) and the BBB model were established to trace JEV-infected immune cell transmigration, which mimics the process of viral neuroinfection. We find that JEV causes HMGB1 release from BMECs while increasing adhesion molecules. Recombinant HMGB1 enhances leukocyte-endothelium adhesion, facilitating JEV-infected monocyte transmigration across endothelia. Thus, JEV successfully utilizes infected monocytes to spread into the brain, expanding inside of the brain, and leading to the acceleration of JE onset, which was facilitated by HMGB1. HMGB1-promoted monocyte transmigration may represent the mechanism of JEV neuroinvasion, revealing potential therapeutic targets.

Journey to a Receptor for Advanced Glycation End Products Connection in Severe Acute Respiratory Syndrome Coronavirus 2 Infection: With Stops Along the Way in the Lung, Heart, Blood Vessels, and Adipose Tissue

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected millions of people worldwide and the pandemic has yet to wane. Despite its associated significant morbidity and mortality, there are no definitive cures and no fully preventative measures to combat SARS-CoV-2. Hence, the urgency to identify the pathobiological mechanisms underlying increased risk for and the severity of SARS-CoV-2 infection is mounting. One contributing factor, the accumulation of damage-associated molecular pattern molecules, is a leading trigger for the activation of nuclear factor-kB and the IRF (interferon regulatory factors), such as IRF7. Activation of these pathways, particularly in the lung and other organs, such as the heart, contributes to a burst of cytokine release, which predisposes to significant tissue damage, loss of function, and mortality. The receptor for advanced glycation end products (RAGE) binds damage-associated molecular patterns is expressed in the lung and heart, and in priming organs, such as the blood vessels (in diabetes) and adipose tissue (in obesity), and transduces the pathological signals emitted by damage-associated molecular patterns. It is proposed that damage-associated molecular pattern-RAGE enrichment in these priming tissues, and in the lungs and heart during active infection, contributes to the widespread tissue damage induced by SARS-CoV-2. Accordingly, the RAGE axis might play seminal roles in and be a target for therapeutic intervention in SARS-CoV-2 infection.

Evidence That Zika Virus Is Transmitted by Breastfeeding to Newborn A129 (Ifnar1 Knock-Out) Mice and Is Able to Infect and Cross a Tight Monolayer of Human Intestinal Epithelial Cells

Zika virus (ZIKV) belongs to the Flavivirus genus in the Flaviviridae family. Mainly transmitted via mosquito bites (Aedes aegypti, Aedes albopictus), ZIKV has been classified in the large category of arthropod-borne viruses, or arboviruses. However, during the past two outbreaks in French Polynesia (2013–2014) and Latin America (2015–2016), several cases of ZIKV human-to-human transmission were reported, either vertically via transplacental route but also horizontally after sexual intercourse. Interestingly, high viral burdens were detected in the colostrum and breast milk of infected women and mother-to-child transmission of ZIKV during breastfeeding was recently highlighted. In a previous study, we highlighted the implication of the mammary epithelium (blood–milk barrier) in ZIKV infectious particles excretion in breast milk. However, mechanisms of their further transmissibility to the newborn via oral route through contaminated breast milk remain unknown. In this study, we provide the first experimental proof-of-concept of the existence of the breastfeeding as a route for mother-to-child transmission of ZIKV and characterized the neonatal oral transmission in a well-established mouse model of ZIKV infection. From a mechanistical point-of-view, we demonstrated for the first time that ZIKV was able to infect and cross an in vitro model of tight human intestinal epithelium without altering its barrier integrity, permitting us to consider the gut as an entry site for ZIKV after oral exposure. By combining in vitro and in vivo experiments, this study strengthens the plausibility of mother-to-child transmission of ZIKV during breastfeeding and helps to better characterize underlying mechanisms, such as the crossing of the newborn intestinal epithelium by ZIKV. As a consequence, these data could serve as a basis for a reflection about the implementation of measures to prevent ZIKV transmission, while keeping in mind breastfeeding-associated benefits.

Zika Virus Infection Promotes Local Inflammation, Cell Adhesion Molecule Upregulation, and Leukocyte Recruitment at the Blood-Brain Barrier

The blood-brain barrier (BBB) largely prevents toxins and pathogens from accessing the brain. Some viruses have the ability to cross this barrier and replicate in the central nervous system (CNS). Zika virus (ZIKV) was responsible in 2015 to 2016 for a major epidemic in South America and was associated in some cases with neurological impairments. Here, we characterized some of the mechanisms behind its neuroinvasion using an innovative in vitro human BBB model. ZIKV efficiently replicated, was released on the BBB parenchyma side, and triggered subtle modulation of BBB integrity as well as an upregulation of inflammatory and cell adhesion molecules (CAMs), which in turn favored leukocyte recruitment. Finally, we showed that ZIKV-infected mouse models displayed similar CAM upregulation and that soluble CAMs were increased in plasma samples from ZIKV-infected patients. Our observations suggest a complex interplay between ZIKV and the BBB, which may trigger local inflammation, leukocyte recruitment, and possible cerebral vasculature impairment.IMPORTANCE Zika virus (ZIKV) can be associated with neurological impairment in children and adults. To reach the central nervous system, viruses have to cross the blood-brain barrier (BBB), a multicellular system allowing a tight separation between the bloodstream and the brain. Here, we show that ZIKV infects cells of the BBB and triggers a subtle change in its permeability. Moreover, ZIKV infection leads to the production of inflammatory molecules known to modulate BBB integrity and participate in immune cell attraction. The virus also led to the upregulation of cellular adhesion molecules (CAMs), which in turn favored immune cell binding to the BBB and potentially increased infiltration into the brain. These results were also observed in a mouse model of ZIKV infection. Furthermore, plasma samples from ZIKV-infected patients displayed an increase in CAMs, suggesting that this mechanism could be involved in neuroinflammation triggered by ZIKV.

Flavivirus Infection Associated with Cerebrovascular Events

Arthropod-borne viruses (arboviruses) of the genus Flavivirus are distributed globally and cause significant human disease and mortality annually. Flavivirus infections present a spectrum of clinical manifestations, ranging from asymptomatic to severe manifestations, including hemorrhage, encephalitis and death. Herein, we describe 3 case reports of cerebrovascular involvement in patients infected by dengue and Zika viruses in Sao Jose do Rio Preto, São Paulo State, Brazil, a hyperendemic area for arbovirus circulation, including dengue, yellow fever, chikungunya and Saint Louis encephalitis viruses. Our findings highlight the potential threat that unusual clinical manifestations may pose to arbovirus disease management and recovery.

Immune cell trafficking across the blood-brain barrier in the absence and presence of neuroinflammation

To maintain the homeostatic environment required for proper function of CNS neurons the endothelial cells of CNS microvessels tightly regulate the movement of ions and molecules between the blood and the CNS. The unique properties of these blood vascular endothelial cells are termed blood-brain barrier (BBB) and extend to regulating immune cell trafficking into the immune privileged CNS during health and disease. In general, extravasation of circulating immune cells is a multi-step process regulated by the sequential interaction of adhesion and signalling molecules between the endothelial cells and the immune cells. Accounting for the unique barrier properties of CNS microvessels, immune cell migration across the BBB is distinct and characterized by several adaptations. Here we describe the mechanisms that regulate immune cell trafficking across the BBB during immune surveillance and neuroinflammation, with a focus on the current state-of-the-art in vitro and in vivo imaging observations.