Flavivirus encephalitis: pathological aspects of mouse and other animal models

Diagnosing arthropod-borne flaviviruses: non-structural protein 1 (NS1) as a biomarker

In recent decades, the presence of flaviviruses of concern for human health in Europe has drastically increased,exacerbated by the effects of climate change - which has allowed the vectors of these viruses to expand into new territories. Co-circulation of West Nile virus (WNV), Usutu virus (USUV), and tick-borne encephalitis virus (TBEV) represents a threat to the European continent, and this is further complicated by the difficulty of obtaining an early and discriminating diagnosis of infection. Moreover, the possibility of introducing non-endemic pathogens, such as Japanese encephalitis virus (JEV), further complicates accurate diagnosis. Current flavivirus diagnosis is based mainly on RT-PCR and detection of virus-specific antibodies. Yet, both techniques suffer from limitations, and the development of new assays that can provide an early, rapid, low-cost, and discriminating diagnosis of viral infection is warranted. In the pursuit of ideal diagnostic assays, flavivirus non-structural protein 1 (NS1) serves as an excellent target for developing diagnostic assays based on both the antigen itself and the antibodies produced against it. This review describes the potential of such NS1-based diagnostic methods, focusing on the application of flaviviruses that co-circulate in Europe.

Lessons Learned from West Nile Virus Infection:Vaccinations in Equines and Their Implications for One Health Approaches

Humans and equines are two dead-end hosts of the mosquito-borne West Nile virus (WNV) with similar susceptibility and pathogenesis. Since the introduction of WNV vaccines into equine populations of the United States of America (USA) in late 2002, there have been only sporadic cases of WNV infection in equines. These cases are generally attributed to unvaccinated and under-vaccinated equines. In contrast, due to the lack of a human WNV vaccine, WNV cases in humans have remained steadily high. An average of 115 deaths have been reported per year in the USA since the first reported case in 1999. Therefore, the characterization of protective immune responses to WNV and the identification of immune correlates of protection in vaccinated equines will provide new fundamental information about the successful development and evaluation of WNV vaccines in humans. This review discusses the comparative epidemiology, transmission, susceptibility to infection and disease, clinical manifestation and pathogenesis, and immune responses of WNV in humans and equines. Furthermore, prophylactic and therapeutic strategies that are currently available and under development are described. In addition, the successful vaccination of equines against WNV and the potential lessons for human vaccine development are discussed.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Comparative study of the pathogenicity of the mosquito origin strain and duck origin strain of Tembusu virus in ducklings and three-week-old mice

•TMUV can infect Kunming mice by i.c., and the pathogenicity of mosquito TMUV is higher than that of duck TMUV. •TMUV can infect SPF ducklings by i.m., and the pathogenicity of duck TMUV is higher than that of mosquito TMUV. •SPF ducklings infected with mosquito TMUV did not produce viremia at 3, 5 and 7 dpi.

Blockade of interferon signaling decreases gut barrier integrity and promotes severe West Nile virus disease

The determinants of severe disease caused by West Nile virus (WNV) and why only ~1% of individuals progress to encephalitis remain poorly understood. Here, we use human and mouse enteroids, and a mouse model of pathogenesis, to explore the capacity of WNV to directly infect gastrointestinal (GI) tract cells and contribute to disease severity. At baseline, WNV poorly infects human and mouse enteroid cultures and enterocytes in mice. However, when STAT1 or type I interferon (IFN) responses are absent, GI tract cells become infected, and this is associated with augmented GI tract and blood-brain barrier (BBB) permeability, accumulation of gut-derived molecules in the brain, and more severe WNV disease. The increased gut permeability requires TNF-α signaling, and is absent in WNV-infected IFN-deficient germ-free mice. To link these findings to human disease, we measured auto-antibodies against type I IFNs in serum from WNV-infected human cohorts. A greater frequency of auto- and neutralizing antibodies against IFN-α2 or IFN-ω is present in patients with severe WNV infection, whereas virtually no asymptomatic WNV-infected subjects have such antibodies (odds ratio 24 [95% confidence interval: 3.0 - 192.5; P = 0.003]). Overall, our experiments establish that blockade of type I IFN signaling extends WNV tropism to enterocytes, which correlates with increased gut and BBB permeability, and more severe 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.

Human brain organoids to explore SARS-CoV-2-induced effects on the central nervous system

Severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) is the causative agent of coronavirus disease 2019 (COVID-19). In less than three years, an estimated 600 million infections with SARS-CoV-2 occurred worldwide, resulting in a pandemic with tremendous impact especially on economic and health sectors. Initially considered a respiratory disease, COVID-19, along with its long-term sequelae (long-COVID) rather is a systemic disease. Neurological symptoms like dementia or encephalopathy were reported early during the pandemic as concomitants of the acute phase and as characteristics of long-COVID. An excessive inflammatory immune response is hypothesized to play a major role in this context. However, direct infection of neural cells may also contribute to the neurological aspects of (long)-COVID-19. To mainly explore such direct effects of SARS-CoV-2 on the central nervous system, human brain organoids provide a useful platform. Infecting these three-dimensional tissue cultures allows the study of viral neurotropism as well as of virus-induced effects on single cells or even the complex cellular network within the organoid. In this review, we summarize the experimental studies that used SARS-CoV-2-infected human brain organoids to unravel the complex nature of (long)-COVID-19-related neurological manifestations.

Clinical and experimental evidence for transplacental vertical transmission of flaviviruses

The Zika virus (ZIKV) epidemic outbreak in Americas in 2016 attracted global attention because of the association of the virus infection with severe birth defects such as microcephaly, mediated through transplacental virus transmission during pregnancy. Less well-known, but also reported is the increasing evidence that prenatal vertical transmission can be caused by other flaviviruses such as dengue virus (DENV). Currently, the mechanism(s) that cause the vertical transmission of flaviviruses is understudied. Here we review the published reports of clinical evidence of intrauterine transmission of ZIKV and other flaviviruses. We also discuss the animal models for flavivirus infection during pregnancy that have been developed to study the mechanisms underlying the transplacental transmission of flaviviruses in order to develop potential countermeasures for its prevention.

Isolation of Exosomes or Extracellular Vesicles from West Nile Virus-Infected N2a Cells, Primary Cortical Neurons, and Brain Tissues

Several flaviviruses compromise the blood-brain barrier integrity, infect the central nervous system, and elicit neuroinvasion to successfully cause neuropathogenesis in the vertebrate host. Therefore, understanding the pathway(s) and mechanism(s) to block the transmission and/or dissemination of flaviviruses and perhaps other neuroinvasive viruses is considered as an important area of research. Moreover, studies that address mechanism(s) of neuroinvasion by flaviviruses are limited. In this chapter, we discuss detailed methods to isolate exosomes or extracellular vesicles (EVs) from mouse and human N2a cells, primary cultures of murine cortical neurons, and mouse brain tissue. Two different methods including differential ultracentrifugation and density gradient exosome (DG-Exo) isolation are described for the preparation of exosomes/EVs from N2a cells and cortical neurons. In addition, we discuss the detailed DG-Exo method for the isolation of exosomes from murine brain tissue. Studies on neuronal exosomes will perhaps enhance our understanding of the mechanism of neuroinvasion by these deadly viruses.

Japanese encephalitis virus induces vasodilation and severe lethality in adult and aged AG129 mice lacking alpha, beta and gamma interferon receptors

Japanese encephalitis virus (JEV) is a single-stranded positive-sense RNA virus belonging to the Flaviviridae family. The JEV is the leading cause of viral encephalitis in children and the elderly which is spread by mosquitoes. JEV infection has been established in different animal models such as mouse, hamster, guinea pig, swine, rat, monkey, rabbit by using the different routes of inoculations. Here, we have shown that the alpha/beta and gamma -receptor deficient AG129 mouse induces fatal encephalitis in both young and aged old mice, when challenged with high titer JEV Indian clinical isolate by both intraperitoneal and intradermal route. The JEV infected AG129 mouse have shown neurological symptoms, JEV-induced pathological features and supported high level viral replication. Additionally, administration of JEV in AG129 mice resulted in the induction of severe peripheral vascular permeability, which is a major hall mark of Dengue infection but not shown in JEV. Taken together, our results demonstrate interferon α/β and γ receptors knock out AG129 mouse does not need adaptation of JEV clinical isolates and could be is a promising JEV challenge mouse model by mimicking the natural intradermal route of administration for rapid screening of novel antivirals and vaccines.

Pathogenesis of Thai duck Tembusu virus in BALB/c mice: Descending infection and neuroinvasive virulence

Duck Tembusu virus (DTMUV) is an emerging flavivirus that causes systemic disease in an avian host. The predominant cluster of DTMUV circulating in Thailand was recently classified as cluster 2.1. The pathogenesis of this virus has been extensively studied in avian hosts but not in mammalian hosts. Six-week-old BALB/c mice were intracerebrally or subcutaneously inoculated with Thai DTMUV to examine clinical signs, pathological changes, viral load and virus distribution. Results demonstrated that the virus caused disease in BALB/c mice by the intracerebral inoculation route. Infected mice demonstrated both systemic and neurological symptoms. Pathological changes and virus distribution were observed in all tested organs. Viral load in the brain was significantly higher than in other organs (p < .05), and the virus caused acute death in BALB/c mice. The virus was disseminated in all parts of the body, but no virus shedding was recorded in saliva and faeces. Findings highlighted the potential of Thai DTMUV to transmit disease in mammalian hosts.

An Advax-Adjuvanted Inactivated Cell-Culture Derived Japanese Encephalitis Vaccine Induces Broadly Neutralising Anti-Flavivirus Antibodies, Robust Cellular Immunity and Provides Single Dose Protection

ccJE+Advax is an inactivated cell culture Japanese encephalitis (JE) vaccine formulated with Advax, a novel polysaccharide adjuvant based on delta inulin. This vaccine has previously shown promise in murine and equine studies and the current study sought to better understand its mechanism of action and assess the feasibility of single dose vaccine protection. Mice immunised with ccJE+Advax had higher serum neutralisation titres than those immunised with ccJE alone or with alum adjuvant. ccJE+Advax induced extraordinarily broad cross-neutralising antibodies against multiple flaviviruses including West Nile virus (WNV), Murray Valley encephalitis virus (MVEV), St Louis encephalitis virus (SLEV) and Dengue virus-1 and -2 (DENV-1 and -2). Notably, the DENV-2 cross-neutralising antibodies from ccJE+Advax immunised mice uniquely had no DENV-2 antibody-dependent infection enhancement (ADIE) activity, in contrast to high ADIE activity seen with DENV-1 cross-reactive antibodies induced by mbJE or ccJE alone or with alum adjuvant. JEV-stimulated splenocytes from ccJE+Advax immunised mice showed increased IL-17 and IFN-γ production, consistent with a mixed Th1 and Th17 response, whereas ccJE-alum was associated with production of mainly Th2 cytokines. In a mouse lethal challenge study against highly virulent JaTH160 JEV strain, ccJE+Advax conferred complete protection in a two-dose schedule with 50 ng of vaccine antigen and near complete protection after a single 200 ng dose of vaccine antigen. There is an ongoing lack of human vaccines against particular flaviviruses, including WNV, SLEV and MVEV. Given its ability to provide single-dose JEV protection and induce broadly neutralising antibodies devoid of ADIE activity, ccJE+Advax vaccine could be useful in situations where rapid protection is desirable, e.g., during a local outbreak or for use in travellers or armies requiring rapid deployment to JEV endemic regions.

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

Background

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

Methods

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

Results

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

Conclusion

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

Enteric Ganglioneuritis, a Common Feature in a Subcutaneous TBEV Murine Infection Model

Tick-borne encephalitis (TBE) is a severe neurologic disease in Europe and Asia. Disease expression ranges from asymptomatic to severe neurological clinical pictures, involving meningitis, encephalitis, meningoencephalitis and potentially fatal outcome. Humans mostly become infected with TBE virus (TBEV) by the bite of an infected tick. Gastrointestinal (GI) symptoms in humans are mainly attributed to the first viremic phase of TBEV infection with unspecific symptoms and/or resulting from severe neurological impairment of the central nervous system (CNS). We used the subcutaneous TBEV-infection of C57BL/6 mice as a model to analyze GI complications of TBE. We observed the acute distension and segmental dilation of the intestinal tract in 10 of 22 subcutaneously infected mice. Histological analysis revealed an intramural enteric ganglioneuritis in the myenteric and submucosal plexus of the small and large intestine. The numbers of infiltrating macrophages and CD3⁺ T lymphocytes correlated with the severity of ganglioneuritis, indicating an immune-mediated pathogenesis due to TBEV-infection of the enteric plexus. Our study demonstrates that the inflammation of enteric intramural ganglia presents to be a common feature in TBEV-infected mice. Accordingly, the results of this mouse model emphasize that GI disease manifestation and consequences for long-term sequelae should not be neglected for TBEV-infections in humans and require further investigation.

Could Chronic Idiopatic Intestinal Pseudo-Obstruction Be Related to Viral Infections?

Chronic idiopathic intestinal pseudo-obstruction (CIIPO) is a disease characterized by symptoms and signs of small bowel obstruction in the absence of displayable mechanical obstruction. Due to the known neuropathic capacity of several viruses, and their localization in the intestine, it has been hypothesized that such viruses could be involved in the pathogenesis of CIIPO. The most frequently involved viruses are John Cunningham virus, Herpesviridae, Flaviviruses, Epstein–Barr virus and Citomegalovirus. Therefore, the present narrative review aims to sum up some new perspectives in the etiology and pathophysiology of CIIPO.

Animal and Human Vaccines against West Nile Virus

West Nile virus (WNV) is a widely distributed enveloped flavivirus transmitted by mosquitoes, which main hosts are birds. The virus sporadically infects equids and humans with serious economic and health consequences, as infected individuals can develop a severe neuroinvasive disease that can even lead to death. Nowadays, no WNV-specific therapy is available and vaccines are only licensed for use in horses but not for humans. While several methodologies for WNV vaccine development have been successfully applied and have contributed to significantly reducing its incidence in horses in the US, none have progressed to phase III clinical trials in humans. This review addresses the status of WNV vaccines for horses, birds, and humans, summarizing and discussing the challenges they face for their clinical advance and their introduction to the market.

In Vitro and In Vivo Models to Study the Zoonotic Mosquito-Borne Usutu Virus

Usutu virus (USUV), a mosquito-borne zoonotic flavivirus discovered in South Africa in 1959, has spread to many European countries over the last 20 years. The virus is currently a major concern for animal health due to its expanding host range and the growing number of avian mass mortality events. Although human infections with USUV are often asymptomatic, they are occasionally accompanied by neurological complications reminiscent of those due to West Nile virus (another flavivirus closely related to USUV). Whilst USUV actually appears less threatening than some other emergent arboviruses, the lessons learned from Chikungunya, Dengue, and Zika viruses during the past few years should not be ignored. Further, it would not be surprising if, with time, USUV disperses further eastwards towards Asia and possibly westwards to the Americas, which may result in more pathogenic USUV strains to humans and/or animals. These observations, inviting the scientific community to be more vigilant about the spread and genetic evolution of USUV, have prompted the use of experimental systems to understand USUV pathogenesis and to boost the development of vaccines and antivirals. This review is the first to provide comprehensive coverage of existing in vitro and in vivo models for USUV infection and to discuss their contribution in advancing data concerning this neurotropic virus. We believe that this paper is a helpful tool for scientists to identify gaps in the knowledge about USUV and to design their future experiments to study the virus.

Japanese Encephalitis Virus Infects the Thalamus Early Followed by Sensory-Associated Cortex and Other Parts of the Central and Peripheral Nervous Systems

Japanese encephalitis (JE) is a known CNS viral infection that often involves the thalamus early. To investigate the possible role of sensory peripheral nervous system (PNS) in early neuroinvasion, we developed a left hindlimb footpad-inoculation mouse model to recapitulate human infection by a mosquito bite. A 1-5 days postinfection (dpi) study, demonstrated focal viral antigens/RNA in contralateral thalamic neurons at 3 dpi in 50% of the animals. From 4 to 5 dpi, gradual increase in viral antigens/RNA was observed in bilateral thalami, somatosensory, and piriform cortices, and then the entire CNS. Infection of neuronal bodies and adjacent nerves in dorsal root ganglia (DRGs), trigeminal ganglia, and autonomic ganglia (intestine, etc.) was also observed from 5 dpi. Infection of explant organotypic whole brain slice cultures demonstrated no viral predilection for the thalamus, while DRG and intestinal ganglia organotypic cultures confirmed sensory and autonomic ganglia susceptibility to infection, respectively. Early thalamus and sensory-associated cortex involvement suggest an important role for sensory pathways in neuroinvasion. Our results suggest that JE virus neuronotropism is much more extensive than previously known, and that the sensory PNS and autonomic system are susceptible to infection.

Japanese encephalitis virus-induced neuropathology in mouse model infected through the conjunctival route

Background & objectives:

Mouse is a preferred animal model for studying pathogenesis of Japanese encephalitis virus (JEV) infections, and different routes of inoculation have been tried. Some neurotropic viruses can reach the brain following infection through ocular route. This study was undertaken to establish JEV-induced clinical disease in mouse model through conjunctival route and document the neuropathological effects.

Methods:

Ten two-week old Swiss albino mice were inoculated with 5 μl Vero cell cultured virus containing 10^4.7 TCID₅₀ JEV through conjunctival route. Clinical signs of mice were observed twice daily. After necropsy examination, different organs including eyes and olfactory bulbs were collected for histopathological examination, quantification of viral copy number and antigen by real-time TaqMan assay and immunohistochemistry, respectively.

Results:

Infected mice showed characteristic clinical signs of JE by 4 days post-infection (dpi). Histopathological lesions in brain included perivascular cuffing by mononuclear cells, focal gliosis, necrosis of neurons and neuronophagia and astrocytosis in the cerebrum, cerebellum and the brainstem. JEV viral load was highest in the brain followed by intestine, heart, liver, spleen, lung and kidney. JEV antigen was detected in the bipolar and ganglion cells of the retina and in the mitral cells and periglomerular cells of olfactory bulb and other parts of the brain.

Interpretation & conclusions:

JEV infection in mice through conjunctival route produced characteristic clinical signs of the disease and neuropathological lesions. Demonstration of JEV antigen in association with neuropathological lesions in the central nervous system and neuronal cells of the eye showed that conjunctival route could be an effective alternate route for virus invasion into the brain. These findings have biosafety implications for researchers, veterinary practitioners and pig farmers.

New Insights into the Susceptibility of Immunocompetent Mice to Usutu Virus

Usutu virus (USUV) is a mosquito-borne flavivirus that shares many similarities with the closely related West Nile virus (WNV) in terms of ecology and clinical manifestations. Initially distributed in Africa, USUV emerged in Italy in 1996 and managed to co-circulate with WNV in many European countries in a similar mosquito-bird life cycle. The rapid geographic spread of USUV, the seasonal mass mortalities it causes in the European avifauna, and the increasing number of infections with neurological disease both in healthy and immunocompromised humans has stimulated interest in infection studies to delineate USUV pathogenesis. Here, we assessed the pathogenicity of two USUV isolates from a recent Belgian outbreak in immunocompetent mice. The intradermal injection of USUV gave rise to disorientation and paraplegia and was associated with neuronal death in the brain and spinal cord in a single mouse. Intranasal inoculation of USUV could also establish the infection; viral RNA was detected in the brain 15 days post-infection. Overall, this pilot study probes the suitability of this murine model for the study of USUV neuroinvasiveness and the possibility of direct transmission in mammals.

Comparative Pathology of West Nile Virus in Humans and Non-Human Animals

West Nile virus (WNV) continues to be a major cause of human arboviral neuroinvasive disease. Susceptible non-human vertebrates are particularly diverse, ranging from commonly affected birds and horses to less commonly affected species such as alligators. This review summarizes the pathology caused by West Nile virus during natural infections of humans and non-human animals. While the most well-known findings in human infection involve the central nervous system, WNV can also cause significant lesions in the heart, kidneys and eyes. Time has also revealed chronic neurologic sequelae related to prior human WNV infection. Similarly, neurologic disease is a prominent manifestation of WNV infection in most non-human non-host animals. However, in some avian species, which serve as the vertebrate host for WNV maintenance in nature, severe systemic disease can occur, with neurologic, cardiac, intestinal and renal injury leading to death. The pathology seen in experimental animal models of West Nile virus infection and knowledge gains on viral pathogenesis derived from these animal models are also briefly discussed. A gap in the current literature exists regarding the relationship between the neurotropic nature of WNV in vertebrates, virus propagation and transmission in nature. This and other knowledge gaps, and future directions for research into WNV pathology, are addressed.

Oral Antibiotic Treatment of Mice Exacerbates the Disease Severity of Multiple Flavivirus Infections

Although the outcome of flavivirus infection can vary from asymptomatic to lethal, environmental factors modulating disease severity are poorly defined. Here, we observed increased susceptibility of mice to severe West Nile (WNV), Dengue, and Zika virus infections after treatment with oral antibiotics (Abx) that depleted the gut microbiota. Abx treatment impaired the development of optimal T cell responses, with decreased levels of WNV-specific CD8⁺ T cells associated with increased infection and immunopathology. Abx treatments that resulted in enhanced WNV susceptibility generated changes in the overall structure of the gut bacterial community and in the abundance of specific bacterial taxa. As little as 3 days of treatment with ampicillin was sufficient to alter host immunity and WNV outcome. Our results identify oral Abx therapy as a potential environmental determinant of systemic viral disease, and they raise the possibility that perturbation of the gut microbiota may have deleterious consequences for subsequent flavivirus infections.

Twenty Years of Progress Toward West Nile Virus Vaccine Development

Although West Nile virus (WNV) has been a prominent mosquito-transmitted infection in North America for twenty years, no human vaccine has been licensed. With a cumulative number of 24,714 neurological disease cases and 2314 deaths in the U.S. since 1999, plus a large outbreak in Europe in 2018 involving over 2000 human cases in 15 countries, a vaccine is essential to prevent continued morbidity, mortality, and economic burden. Currently, four veterinary vaccines are licensed, and six vaccines have progressed into clinical trials in humans. All four veterinary vaccines require multiple primary doses and annual boosters, but for a human vaccine to be protective and cost effective in the most vulnerable older age population, it is ideal that the vaccine be strongly immunogenic with only a single dose and without subsequent annual boosters. Of six human vaccine candidates, the two live, attenuated vaccines were the only ones that elicited strong immunity after a single dose. As none of these candidates have yet progressed beyond phase II clinical trials, development of new candidate vaccines and improvement of vaccination strategies remains an important area of research.

Upregulated expression of the antioxidant sestrin 2 identified by transcriptomic analysis of Japanese encephalitis virus-infected SH-SY5Y neuroblastoma cells

Japanese encephalitis virus (JEV) exerts a profound burden of viral encephalitis. We have investigated the differentially expressed transcripts in the neuronal transcriptome during JEV infection by RNA sequencing (RNA-Seq) of virus-infected SH-SY5Y human neuroblastoma cells. Gene ontology analysis revealed significant enrichment from two main pathways: endoplasmic reticulum (ER)-nucleus signaling (P value: 5.75E-18; false discovery rate [FDR] 3.11E-15) and the ER unfolded protein response (P value: 7.58E-18; FDR 3.11E-15). qPCR validation showed significant upregulation and differential expression (P < 0.01) of ER stress-signaling transcripts (SESN2, TRIB3, DDIT3, DDIT4, XBP1, and ATF4) at 24 h post-infection for both low (LN) and high (HN) neurovirulence JEV strains. Immunoblot analysis following JEV infection of SH-SY5Y cells showed an increase in levels of SESN2 protein following JEV infection. Similarly, Zika virus (MR766) infection of SH-SY5Y showed a titer-dependent increase in ER stress-signaling transcripts; however, this was absent or diminished for DDIT4 and ATF4, respectively, suggestive of differences in the induction of stress-response transcripts between flaviviruses. Interestingly, SLC7A11 and SLC3A2 mRNA were also both deregulated in JEV-infected SH-SY5Y cells and encode the two constituent subunits of the plasma membrane xCT amino acid antiporter that relieves oxidative stress by export of glutamate and import of cystine. Infection of SH-SY5Y and HEK293T cells by the JEV HN strain Sw/Mie/40/2004 lead to significant upregulation of the SLC7A11 mRNA to levels comparable to DDIT3. Our findings suggest upregulation of antioxidants including SESN2 and, also, the xCT antiporter occurs to counteract the oxidative stress elicited by JEV infection.

In vitro and in silico Models to Study Mosquito-Borne Flavivirus Neuropathogenesis, Prevention, and Treatment

Mosquito-borne flaviviruses can cause disease in the nervous system, resulting in a significant burden of morbidity and mortality. Disease models are necessary to understand neuropathogenesis and identify potential therapeutics and vaccines. Non-human primates have been used extensively but present major challenges. Advances have also been made toward the development of humanized mouse models, but these models still do not fully represent human pathophysiology. Recent developments in stem cell technology and cell culture techniques have allowed the development of more physiologically relevant human cell-based models. In silico modeling has also allowed researchers to identify and predict transmission patterns and discover potential vaccine and therapeutic candidates. This review summarizes the research on in vitro and in silico models used to study three mosquito-borne flaviviruses that cause neurological disease in humans: West Nile, Dengue, and Zika. We also propose a roadmap for 21st century research on mosquito-borne flavivirus neuropathogenesis, prevention, and treatment.

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.

West Nile Virus Capsid Protein Interacts With Biologically Relevant Host Lipid Systems

West Nile and dengue viruses are closely related flaviviruses, originating mosquito-borne viral infections for which there are no effective and specific treatments. Their capsid proteins sequence and structure are particularly similar, forming highly superimposable α-helical homodimers. Measuring protein-ligand interactions at the single-molecule level yields detailed information of biological and biomedical relevance. In this work, such an approach was successfully applied on the characterization of the West Nile virus capsid protein interaction with host lipid systems, namely intracellular lipid droplets (an essential step for dengue virus replication) and blood plasma lipoproteins. Dynamic light scattering measurements show that West Nile virus capsid protein binds very low-density lipoproteins, but not low-density lipoproteins, and this interaction is dependent of potassium ions. Zeta potential experiments show that the interaction with lipid droplets is also dependent of potassium ions as well as surface proteins. The forces involved on the binding of the capsid protein with lipid droplets and lipoproteins were determined using atomic force microscopy-based force spectroscopy, proving that these interactions are K⁺-dependent rather than a general dependence of ionic strength. The capsid protein interaction with host lipid systems may be targeted in future therapeutic strategies against different flaviviruses. The biophysical and nanotechnology approaches employed in this study may be applied to characterize the interactions of other important proteins from different viruses, in order to understand their life cycles, as well as to find new strategies to inhibit them.

ITRAQ-Based Quantitative Proteomics Reveals the Proteome Profiles of Primary Duck Embryo Fibroblast Cells Infected with Duck Tembusu Virus

Outbreaks of duck Tembusu virus (DTMUV) have caused substantial economic losses in the major duck-producing regions of China since 2010. To improve our understanding of the host cellular responses to virus infection and the pathogenesis of DTMUV infection, we applied isobaric tags for relative and absolute quantification (iTRAQ) labeling coupled with multidimensional liquid chromatography-tandem mass spectrometry to detect the protein changes in duck embryo fibroblast cells (DEFs) infected and mock-infected with DTMUV. In total, 434 cellular proteins were differentially expressed, among which 116, 76, and 339 proteins were differentially expressed in the DTMUV-infected DEFs at 12, 24, and 42 hours postinfection, respectively. The Gene Ontology analysis indicated that the biological processes of the differentially expressed proteins were primarily related to cellular processes, metabolic processes, biological regulation, response to stimulus, and cellular organismal processes and that the molecular functions in which the differentially expressed proteins were mainly involved were binding and catalytic activity. Some selected proteins that were found to be differentially expressed in DTMUV-infected DEFs were further confirmed by real-time PCR. The results of this study provide valuable insight into DTMUV-host interactions. This could lead to a better understanding of DTMUV infection mechanisms.

Intestinal Dysmotility Syndromes following Systemic Infection by Flaviviruses

Although chronic gastrointestinal dysmotility syndromes are a common worldwide health problem, underlying causes for these disorders are poorly understood. We show that flavivirus infection of enteric neurons leads to acute neuronal injury and cell death, inflammation, bowel dilation, and slowing of intestinal transit in mice. Flavivirus-primed CD8⁺ T cells promote these phenotypes, as their absence diminished enteric neuron injury and intestinal transit delays, and their adoptive transfer reestablished dysmotility after flavivirus infection. Remarkably, mice surviving acute flavivirus infection developed chronic gastrointestinal dysmotility that was exacerbated by immunization with an unrelated alphavirus vaccine or exposure to a non-infectious inflammatory stimulus. This model of chronic post-infectious gastrointestinal dysmotility in mice suggests that viral infections with tropism for enteric neurons and the ensuing immune response might contribute to the development of bowel motility disorders in humans. These results suggest an opportunity for unique approaches to diagnosis and therapy of gastrointestinal dysmotility syndromes.

Endothelins in inflammatory neurological diseases

Endothelins were discovered more than thirty years ago as potent vasoactive compounds. Beyond their well-documented cardiovascular properties, however, the contributions of the endothelin pathway have been demonstrated in several neuroinflammatory processes and the peptides have been reported as clinically relevant biomarkers in neurodegenerative diseases. Several studies report that endothelin-1 significantly contributes to the progression of neuroinflammatory processes, particularly during infections in the central nervous system (CNS), and is associated with a loss of endothelial integrity at the blood brain barrier level. Because of the paucity of clinical trials with endothelin-1 antagonists in several infectious and non-infectious neuroinflammatory diseases, it remains an open question whether the 21 amino acid peptide is a mediator/modulator rather than a biomarker of the progression of neurodegeneration. This review focuses on the potential roles of endothelins in the pathology of neuroinflammatory processes, including infectious diseases of viral, bacterial or parasitic origin in which the synthesis of endothelins or its pharmacology have been investigated from the cell to the bedside in several cases, as well as in non-infectious inflammatory processes such as neurodegenerative disorders like Alzheimers Disease or central nervous system vasculitis.

Neuronal Degeneration in Mice Induced by an Epidemic Strain of Saint Louis Encephalitis Virus Isolated in Argentina

Saint Louis encephalitis virus (SLEV) is a neglected flavivirus that causes severe neurological disorders. The epidemic strain of SLEV, CbaAr-4005, isolated during an outbreak in Córdoba city (Argentina), causes meningitis and encephalitis associated with neurological symptoms in a murine experimental model. Here, we identified the affected brain areas and the damage triggered by this neurotropic arbovirus. We performed a detailed analysis of brain neurodegeneration associated with CbaAr-4005 SLEV infection in mice. The motor cortex, corpus striatum and cerebellum were the most affected structures. Neurodegeneration was also found in the olfactory bulb, thalamus, hypothalamus, hippocampus, and hindbrain. SLEV infection triggered brain cell apoptosis as well as somatodendritic and terminal degeneration. In addition, we observed massive excitotoxic-like degeneration in many cortical structures. Apoptosis was also detected in the neuroblastoma cell line N2a cultured with SLEV. The results evidenced that SLEV CbaAr-4005 infection induced severe degenerative alterations within the central nervous system of infected mice, providing new information about the targets of this flavivirus infection.

Deleterious effect of Usutu virus on human neural cells

In the last decade, the number of emerging Flaviviruses described worldwide has increased considerably. Among them Zika virus (ZIKV) and Usutu virus (USUV) are African mosquito-borne viruses that recently emerged. Recently, ZIKV has been intensely studied due to major outbreaks associated with neonatal death and birth defects, as well as neurological symptoms. USUV pathogenesis remains largely unexplored, despite significant human and veterinary associated disorders. Circulation of USUV in Africa was documented more than 50 years ago, and it emerged in Europe two decades ago, causing massive bird mortality. More recently, USUV has been described to be associated with neurological disorders in humans such as encephalitis and meningoencephalitis, highlighting USUV as a potential health threat. The aim of this study was to evaluate the ability of USUV to infect neuronal cells. Our results indicate that USUV efficiently infects neurons, astrocytes, microglia and IPSc-derived human neuronal stem cells. When compared to ZIKV, USUV led to a higher infection rate, viral production, as well as stronger cell death and anti-viral response. Our results highlight the need to better characterize the physiopathology related to USUV infection in order to anticipate the potential threat of USUV emergence.

Usutu virus (USUV) is an African mosquito-borne virus closely related to West Nile virus and belongs to the Japanese encephalitis virus serogroup in the Flavivirus genus. Recently several neurological disorders such as encephalitis, meningitis and meningoencephalitis were associated with USUV-infection in immunocompromised and immunocompetent patients. The goal of our work was to study the ability of USUV to infect neuronal cells and to characterize the effects of USUV infection in these cells. We have shown that USUV can infect efficiently several neuronal cells (mature neurons, astrocytes, microglia, IPSc-derived human neuronal stem cells (NSCs)). Interestingly, USUV replicates in human astrocytes more efficiently than another mosquito-borne flavivirus, Zika virus, reduces cell proliferation and induces strong anti-viral response. Moreover, USUV induces caspase-dependent apoptosis in NSCs. Our results suggest that USUV infection may lead to encephalitis and/or meningoencephalitis via neuronal toxicity and inflammatory response.

Phenotypic characterisation of cell populations in the brains of horses experimentally infected with West Nile virus

BACKGROUND

West Nile virus (WNV), a mosquito borne member of the Flaviviridae, is one of the most commonly diagnosed agents of viral encephalitis in horses and people worldwide.

OBJECTIVES

A cassette of markers for formalin-fixed paraffin-embedded tissue and an archive of tissues from experimental infections in the horse were used to investigate the equine neuroimmune response to WNV meningoencephalomyelitis to phenotype the early response to WNV infection in the horse.

STUDY DESIGN

Quantitative analysis using archived tissue from experimentally infected horses.

METHODS

The thalamus and hindbrain from 2 groups of 6 horses were compared and consisted of a culture positive tissues from WNV experimentally horses, in the other, normal horses. Formalin-fixed paraffin-embedded tissue from the thalamus and hindbrain were immunolabeled for microglia, astrocytes, B cells, macrophages/neutrophils, CD3⁺ T cells. Fresh frozen tissues were immunolabeled for CD4⁺ and CD8⁺ T lymphocyte cell markers. Cell counts were obtained using a computer software program. Differences, after meeting assumptions of abnormality, were computed using a general linear model with a Tukey test (P<0.05) for pairwise comparisons.

RESULTS

In WNV-challenged horses, Iba-1⁺ microglia, CD3⁺ T lymphocyte and MAC387⁺ macrophage staining were significantly increased. The T cell response for the WNV-challenged horses was mixed, composed of CD4⁺ and CD8⁺ T lymphocytes. A limited astrocyte response was also observed in WNV-challenged horses, and MAC387⁺ and B cells were the least abundant cell populations.

MAIN LIMITATIONS

The results of this study were limited by a single collection time post-infection. Furthermore, a comprehensive analysis of cellular phenotypes is needed for naturally infected horses. Unfortunately, in clinical horses, there is high variability of sampling in terms of days post-infection and tissue handling.

CONCLUSIONS

The data show that WNV-challenged horses recruit a mixed T cell population at the onset of neurologic disease.

Overview on the Current Status of Zika Virus Pathogenesis and Animal Related Research

There is growing evidence that Zika virus (ZIKV) infection is linked with activation of Guillan-Barré syndrome (GBS) in adults infected with the virus and microcephaly in infants following maternal infection. With the recent outpour in publications by numerous research labs, the association between microcephaly in newborns and ZIKV has become very apparent in which large numbers of viral particles were found in the central nervous tissue of an electively aborted microcephalic ZIKV-infected fetus. However, the underlying related mechanisms remain poorly understood. Thus, development of ZIKV-infected animal models are urgently required. The need to develop drugs and vaccines of high efficacy along with efficient diagnostic tools for ZIKV treatment and management raised the demand for a very selective animal model for exploring ZIKV pathogenesis and related mechanisms. In this review, we describe recent advances in animal models developed for studying ZIKV pathogenesis and evaluating potential interventions against human infection, including during pregnancy. The current research directions and the scientific challenges ahead in developing effective vaccines and therapeutics are also discussed.

The host microRNA miR-301a blocks the IRF1-mediated neuronal innate immune response to Japanese encephalitis virus infection

Effective recognition of viral components and the subsequent stimulation of the production of type I interferons (IFNs) is crucial for the induction of host antiviral immunity. The failure of the host to efficiently produce type I IFNs in response to infection by the Japanese encephalitis virus (JEV) is linked with an increased probability for the disease to become lethal. JEV is a neurotropic virus of the Flaviviridae family that causes encephalitis in humans. JEV infection is regulated by several host factors, including microRNAs, which are conserved noncoding RNAs that participate in various physiological and pathological processes. We showed that the JEV-induced expression of miR-301a led to inhibition of the production of type I IFN by reducing the abundances of the transcription factor IFN regulatory factor 1 (IRF1) and the signaling protein suppressor of cytokine signaling 5 (SOCS5). Mechanistically, induction of miR-301a expression during JEV infection required the transcription factor nuclear factor κB. In mouse neurons, neutralization of miR-301a restored the host innate immune response by enabling IFN-β production, thereby restricting viral propagation. Inhibition of miR-301a in mouse brain rescued the production of IRF1 and SOCS5, increased the generation of IFN-β, and reduced the extent of JEV replication, thus improving mouse survival. Thus, our study suggests that the JEV-induced expression of miR-301a assists viral pathogenesis by suppressing IFN production, which might be targeted by antiviral therapies.

CD8 T cells protect adult naive mice from JEV-induced morbidity via lytic function

Following Japanese encephalitis virus (JEV) infection neutralizing antibodies are shown to provide protection in a significant proportion of cases, but not all, suggesting additional components of immune system might also contribute to elicit protective immune response. Here we have characterized the role of T cells in offering protection in adult mice infected with JEV. Mice lacking α/β–T cells (TCRβ–null) are highly susceptible and die over 10–18 day period as compared to the wild-type (WT) mice which are resistant. This is associated with high viral load, higher mRNA levels of proinflammatory cytokines and breach in the blood-brain-barrier (BBB). Infected WT mice do not show a breach in BBB; however, in contrast to TCRβ-null, they show the presence of T cells in the brain. Using adoptive transfer of cells with specific genetic deficiencies we see that neither the presence of CD4 T cells nor cytokines such as IL-4, IL-10 or interferon-gamma have any significant role in offering protection from primary infection. In contrast, we show that CD8 T cell deficiency is more critical as absence of CD8 T cells alone increases mortality in mice infected with JEV. Further, transfer of T cells from beige mice with defects in granular lytic function into TCRβ-null mice shows poor protection implicating granule-mediated target cell lysis as an essential component for survival. In addition, for the first time we report that γ/δ-T cells also make significant contribution to confer protection from JEV infection. Our data show that effector CD8 T cells play a protective role during primary infection possibly by preventing the breach in BBB and neuronal damage.

Japanese encephalitis virus (JEV) commonly infects human beings in developing countries including those in Southeast Asia. While the majority of the infected people suffer from mild illness, a minority suffers from encephalitis which may lead to death. The virus is transmitted by mosquito bites and elimination of mosquitoes is not a practical answer to prevent the disease, therefore, prevention by vaccination is a desired goal. While various vaccines are clinically tried and some are marketed further improvement in vaccines is still possible. In a complex disease like JE many components of the immune system contribute to variable extent in protection. We show here that one subset of T cells called CD8 cells which are capable of killing infected cells are very critical for providing protection against JEV infection in mice. In the absence of T cells we also observed that virus reaches the brain early, unlike in the presence of T cells, and this possibly results in high virus load in the brain leading to worsening of the condition and death. Thus, our data help in identifying the role of CD8 T cells in protection from lethal JEV infection and the information may be useful for modifying and/or developing vaccine for prevention of JEV-mediated disease.

Differential cell line susceptibility to the emerging Zika virus: implications for disease pathogenesis, non-vector-borne human transmission and animal reservoirs

Zika virus (ZIKV) is unique among human-pathogenic flaviviruses by its association with congenital anomalies and trans-placental and sexual human-to-human transmission. Although the pathogenesis of ZIKV-associated neurological complications has been reported in recent studies, key questions on the pathogenesis of the other clinical manifestations, non-vector-borne transmission and potential animal reservoirs of ZIKV remain unanswered. We systematically characterized the differential cell line susceptibility of 18 human and 15 nonhuman cell lines to two ZIKV isolates (human and primate) and dengue virus type 2 (DENV-2). Productive ZIKV replication (⩾2 log increase in viral load, ZIKV nonstructural protein-1 (NS1) protein expression and cytopathic effects (CPE)) was found in the placental (JEG-3), neuronal (SF268), muscle (RD), retinal (ARPE19), pulmonary (Hep-2 and HFL), colonic (Caco-2),and hepatic (Huh-7) cell lines. These findings helped to explain the trans-placental transmission and other clinical manifestations of ZIKV. Notably, the prostatic (LNCaP), testicular (833KE) and renal (HEK) cell lines showed increased ZIKV load and/or NS1 protein expression without inducing CPE, suggesting their potential roles in sexual transmission with persistent viral replication at these anatomical sites. Comparatively, none of the placental and genital tract cell lines allowed efficient DENV-2 replication. Among the nonhuman cell lines, nonhuman primate (Vero and LLC-MK2), pig (PK-15), rabbit (RK-13), hamster (BHK21) and chicken (DF-1) cell lines supported productive ZIKV replication. These animal species may be important reservoirs and/or potential animal models for ZIKV. The findings in our study help to explain the viral shedding pattern, transmission and pathogenesis of the rapidly disseminating ZIKV, and are useful for optimizing laboratory diagnostics and studies on the pathogenesis and counter-measures of ZIKV.

Utilisation of ISA Reverse Genetics and Large-Scale Random Codon Re-Encoding to Produce Attenuated Strains of Tick-Borne Encephalitis Virus within Days

Large-scale codon re-encoding is a new method of attenuating RNA viruses. However, the use of infectious clones to generate attenuated viruses has inherent technical problems. We previously developed a bacterium-free reverse genetics protocol, designated ISA, and now combined it with large-scale random codon-re-encoding method to produce attenuated tick-borne encephalitis virus (TBEV), a pathogenic flavivirus which causes febrile illness and encephalitis in humans. We produced wild-type (WT) and two re-encoded TBEVs, containing 273 or 273+284 synonymous mutations in the NS5 and NS5+NS3 coding regions respectively. Both re-encoded viruses were attenuated when compared with WT virus using a laboratory mouse model and the relative level of attenuation increased with the degree of re-encoding. Moreover, all infected animals produced neutralizing antibodies. This novel, rapid and efficient approach to engineering attenuated viruses could potentially expedite the development of safe and effective new-generation live attenuated vaccines.

Comparison of the live-attenuated Japanese encephalitis vaccine SA14-14-2 strain with its pre-attenuated virulent parent SA14 strain: similarities and differences in vitro and in vivo

Japanese encephalitis virus (JEV) is the main cause of acute viral encephalitis, primarily affecting children and young adults in the Asia-Pacific region. JEV is a vaccine-preventable pathogen, with four types of JE vaccine licensed in different regions of the world. To date, the most common JEV strain used in vaccine development and production is SA14-14-2, an attenuated strain derived from its wild-type parental strain SA14. In this study, we directly compared the phenotypic and genotypic characteristics of SA14 and SA14-14-2 to determine the biological and genetic properties associated with their differential virulence. In susceptible BHK-21 cells, SA14-14-2 grew slightly more slowly and formed smaller plaques than SA14, but unlike SA14, it showed almost no expression of the viral protein NS1', the product of a conserved predicted RNA pseudoknot-mediated ribosomal frameshift. In weanling ICR mice, SA14-14-2 was highly attenuated in terms of both neuroinvasiveness and neurovirulence, with its median lethal doses invariably over five logs higher than those of SA14 when inoculated intramuscularly and intracerebrally. Interestingly, the neurovirulence of SA14-14-2 was dependent on mouse age, with the 1- to 7-day-old mice being highly susceptible and the 14- to 21-day-old mice becoming resistant to intracerebral inoculation. At the genome level, SA14-14-2 differed from SA14 by 57 nucleotides, including one silent G-to-A substitution at position 3599 within the predicted RNA pseudoknot for NS1' synthesis; of the 57 differences, 25 resulted in amino acid substitutions. Our data pave the way for the development of new genetically modified JE vaccines.

CCR5 ameliorates Japanese encephalitis via dictating the equilibrium of regulatory CD4(+)Foxp3(+) T and IL-17(+)CD4(+) Th17 cells

Background

CCR5 is a CC chemokine receptor involved in the migration of effector leukocytes including macrophages, NK, and T cells into inflamed tissues. Also, the role of CCR5 in CD4⁺Foxp3⁺ regulatory T cell (Treg) homing has recently begun to grab attention. Japanese encephalitis (JE) is defined as severe neuroinflammation of the central nervous system (CNS) following infection with mosquito-borne flavivirus JE virus. However, the potential contribution of CCR5 to JE progression via mediating CD4⁺Foxp3⁺ Treg homing has not been investigated.

Methods

Infected wild-type (Ccr5^+/+) and CCR5-deficient (Ccr5^−/−) mice were examined daily for mortality and clinical signs, and neuroinflammation in the CNS was evaluated by infiltration of inflammatory leukocytes and cytokine expression. In addition, viral burden, NK- and JEV-specific T cell responses were analyzed. Adoptive transfer of CCR5⁺CD4⁺Foxp3⁺ Tregs was used to evaluate the role of Tregs in JE progression.

Results

CCR5 ablation exacerbated JE without altering viral burden in the extraneural and CNS tissues, as manifested by increased CNS infiltration of Ly-6C^hi monocytes and Ly-6G^hi granulocytes. Compared to Ccr5^+/+ mice, Ccr5^−/− mice unexpectedly showed increased responses of IFN-γ⁺NK and CD8⁺ T cells in the spleen, but not CD4⁺ T cells. More interestingly, CCR5-ablation resulted in a skewed response to IL-17⁺CD4⁺ Th17 cells and correspondingly reduced CD4⁺Foxp3⁺ Tregs in the spleen and brain, which was closely associated with exacerbated JE. Our results also revealed that adoptive transfer of sorted CCR5⁺CD4⁺Foxp3⁺ Tregs into Ccr5^−/− mice could ameliorate JE progression without apparently altering the viral burden and CNS infiltration of IL-17⁺CD4⁺ Th17 cells, myeloid-derived Ly-6C^hi monocytes and Ly-6G^hi granulocytes. Instead, adoptive transfer of CCR5⁺CD4⁺Foxp3⁺ Tregs into Ccr5^−/− mice resulted in increased expression of anti-inflammatory cytokines (IL-10 and TGF-β) in the spleen and brain, and transferred CCR5⁺ Tregs were found to produce IL-10.

Conclusions

CCR5 regulates JE progression via governing timely and appropriate CNS infiltration of CD4⁺Foxp3⁺ Tregs, thereby facilitating host survival. Therefore, this critical and extended role of CCR5 in JE raises possible safety concerns regarding the use of CCR5 antagonists in human immunodeficiency virus (HIV)-infected individuals who inhabit regions in which both HIV and flaviviruses, such as JEV and West Nile virus, are endemic.

Virulence variation among epidemic and non-epidemic strains of Saint Louis encephalitis virus circulating in Argentina

Saint Louis encephalitis virus caused an outbreak of febrile illness and encephalitis cases in Córdoba, Argentina, in 2005. During this outbreak, the strain CbaAr-4005 was isolated from Culex quinquefasciatus mosquitoes. We hypothesised that this epidemic variant would be more virulent in a mouse model than two other non-epidemic strains (78V-6507 and CorAn-9275) isolated under different epidemiological conditions. To test this hypothesis, we performed a biological characterisation in a murine model, including mortality, morbidity and infection percentages and lethal infection indices using the three strains. Mice were separated into age groups (7, 10 and 21-day-old mice) and analysed after infection. The strain CbaAr-4005 was the most infective and lethal of the three variants, whereas the other two strains exhibited a decreasing mortality percentage with increasing animal age. The strain CbaAr-4005 produced the highest morbidity percentages and no significant differences among age groups were observed. The epidemic strain caused signs of illness in all inoculated animals and showed narrower ranges from the onset of symptoms than the other strains. CbaAr-4005 was the most virulent for Swiss albino mice. Our results highlight the importance of performing biological characterisations of arbovirus strains likely to be responsible for emerging or reemerging human diseases.

Blockage of indoleamine 2,3-dioxygenase regulates Japanese encephalitis via enhancement of type I/II IFN innate and adaptive T-cell responses

Background

Japanese encephalitis (JE), a leading cause of viral encephalitis, is characterized by extensive neuroinflammation following infection with neurotropic JE virus (JEV). Indoleamine 2,3-dioxygenase (IDO) has been identified as an enzyme associated with immunoregulatory function. Although the regulatory role of IDO in viral replication has been postulated, the in vivo role of IDO activity has not been fully addressed in neurotropic virus-caused encephalitis.

Methods

Mice in which IDO activity was inhibited by genetic ablation or using a specific inhibitor were examined for mortality and clinical signs after infection. Neuroinflammation was evaluated by central nervous system (CNS) infiltration of leukocytes and cytokine expression. IDO expression, viral burden, JEV-specific T-cell, and type I/II interferon (IFN-I/II) innate responses were also analyzed.

Results

Elevated expression of IDO activity in myeloid and neuron cells of the lymphoid and CNS tissues was closely associated with clinical signs of JE. Furthermore, inhibition of IDO activity enhanced resistance to JE, reduced the viral burden in lymphoid and CNS tissues, and resulted in early and increased CNS infiltration by Ly-6C^hi monocytes, NK, CD4⁺, and CD8⁺ T-cells. JE amelioration in IDO-ablated mice was also associated with enhanced NK and JEV-specific T-cell responses. More interestingly, IDO ablation induced rapid enhancement of type I IFN (IFN-I) innate responses in CD11c⁺ dendritic cells (DCs), including conventional and plasmacytoid DCs, following JEV infection. This enhanced IFN-I innate response in IDO-ablated CD11c⁺ DCs was coupled with strong induction of PRRs (RIG-I, MDA5), transcription factors (IRF7, STAT1), and antiviral ISG genes (Mx1, Mx2, ISG49, ISG54, ISG56). IDO ablation also enhanced the IFN-I innate response in neuron cells, which may delay the spread of virus in the CNS. Finally, we identified that IDO ablation in myeloid cells derived from hematopoietic stem cells (HSCs) dominantly contributed to JE amelioration and that HSC-derived leukocytes played a key role in the enhanced IFN-I innate responses in the IDO-ablated environment.

Conclusions

Inhibition of IDO activity ameliorated JE via enhancement of antiviral IFN-I/II innate and adaptive T-cell responses and increased CNS infiltration of peripheral leukocytes. Therefore, our data provide valuable insight into the use of IDO inhibition by specific inhibitors as a promising tool for therapeutic and prophylactic strategies against viral encephalitis caused by neurotropic viruses.

Duck Tembusu Virus Exhibits Pathogenicity to Kunming Mice by Intracerebral Inoculation

In this study, Kunming mice were used as the animal models to study the pathogenicity of TMUV. Three groups of 3-week-old female Kunming mice (n = 15 mice per group) were infected with the SDSG strain of TMUV in 50 μL allantoic fluid (10^4.8 ELD₅₀/0.2 ml) respectively by the intracerebral (i.c.), subcutaneous (s.c.) and intranasal (i.n.) routes. The control group (n = 15 mice) was inoculated with 50 μL sterile phosphate-buffered saline. Clinical signs, gross, and microscopic lesions, viral loads in different tissues, and serum antibody titers were examined and recorded. Kunming mice infected intracerebrally showed typical clinical symptoms, including severe hindlimb paralysis, weight loss and death. Only dead mice presented severe intestinal mucosal edema. No gross lesions were observed in mice sequentially euthanized. However, microscopic lesions in the brain, spleen, liver, kidney, and lung were very typical including varying degrees of viral encephalitis, lymphocytes depletion, liver cell necrosis and nephritis, etc. Viral loads in different tissues were detected by the SYBR Green I real-time PCR assay. Viral loads in the brain, liver, and spleen were first detected and maintained a longer time, which indicated that these organs may be the target organs of TMUV. The level of viral loads was consistent with the severity of clinical signs and microscopic lesions in different tissues. The neutralizing antibody began to seroconvert at 8 dpi. Clinical signs, microscopic lesions, viral loads and serum neutralizing antibodies weren’t observed in other groups. In summary, TMUV can cause systemic infections and death in Kunming mice by i.c., which provides some experimental basis for further study of the significance of TMUV in public health.

MicroRNA transcriptome profiling of mice brains infected with Japanese encephalitis virus by RNA sequencing

Japanese encephalitis (JE) is a mosquito borne viral disease, caused by Japanese encephalitis virus (JEV) infection producing severe neuroinflammation in the central nervous system (CNS) with the associated disruption of the blood brain barrier. MicroRNAs (miRNAs) are a family of 21-24 nt small non-coding RNAs that play important post-transcriptional regulatory roles in gene expression and have critical roles in virus pathogenesis. We examined the potential roles of miRNAs in JEV-infected suckling mice brains and found that JEV infection changed miRNA expression profiles when the suckling mice began showing nervous symptoms. A total of 1062 known and 71 novel miRNAs were detected in JEV-infected group, accompanied with 1088 known and 75 novel miRNAs in mock controls. Among these miRNAs, one novel and 25 known miRNAs were significantly differentially expressed, including 18 up-regulated and 8 down-regulated miRNAs which were further confirmed by real-time PCR. Gene ontology (GO) and signaling pathway analysis of the predicted target mRNAs of the modulated miRNAs showed that they are correlated with the regulation of apoptosis, neuron differentiation, antiviral immunity and infiltration of mouse brain, and the validated targets of 12 differentially expressed miRNAs were enriched for the regulation of cell programmed death, proliferation, transcription, muscle organ development, erythrocyte differentiation, gene expression, plasma membrane and protein domain specific binding. KEGG analysis further reveals that the validated target genes were involved in the Pathways in cancer, Neurotrophin signaling pathway, Toll like receptor signaling pathway, Endometrial cancer and Jak-STAT signaling pathway. We constructed the interaction networks of miRNAs and their target genes according to GO terms and KEGG pathways and the expression levels of several target genes were examined. Our data provides a valuable basis for further studies on the regulatory roles of miRNAs in JE pathogenesis.

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.

Attenuation of tick-borne encephalitis virus using large-scale random codon re-encoding

Large-scale codon re-encoding (i.e. introduction of a large number of synonymous mutations) is a novel method of generating attenuated viruses. Here, it was applied to the pathogenic flavivirus, tick-borne encephalitis virus (TBEV) which causes febrile illness and encephalitis in humans in forested regions of Europe and Asia. Using an infectious clone of the Oshima 5–10 strain ("wild-type virus"), a cassette of 1.4kb located in the NS5 coding region, was modified by randomly introducing 273 synonymous mutations ("re-encoded virus"). Whilst the in cellulo replicative fitness of the re-encoded virus was only slightly reduced, the re-encoded virus displayed an attenuated phenotype in a laboratory mouse model of non-lethal encephalitis. Following intra-peritoneal inoculation of either 2.10⁵ or 2.10⁶ TCID50 of virus, the frequency of viraemia, neurovirulence (measured using weight loss and appearance of symptoms) and neuroinvasiveness (detection of virus in the brain) were significantly decreased when compared with the wild-type virus. Mice infected by wild-type or re-encoded viruses produced comparable amounts of neutralising antibodies and results of challenge experiments demonstrated that mice previously infected with the re-encoded virus were protected against subsequent infection by the wild-type virus. This constitutes evidence that a mammalian species can be protected against infection by a virulent wild-type positive-stranded RNA virus following immunisation with a derived randomly re-encoded strain. Our results demonstrate that random codon re-encoding is potentially a simple and effective method of generating live-attenuated vaccine candidates against pathogenic flaviviruses.

The arbovirus Tick-borne encephalitis virus (TBEV; genus Flavivirus) is transmitted by ticks of the Ixodes genus. TBEV causes febrile illness and encephalitis in humans in forested regions of Europe and Asia. The incidence of TBE is increasing across Central and Eastern European countries despite the availability of several licensed inactivated vaccines and appropriate vaccination programmes. Large-scale codon re-encoding, a recently developed attenuation method that modifies viral RNA nucleotide composition of large coding regions without alteration of the encoded proteins, has been successfully applied to a variety of RNA viruses. In contrast with previous empirical methods of generating live attenuated vaccines, large-scale codon re-encoding facilitates rapid generation of vaccine candidates using reverse genetics methods, by direct control of the attenuation phenotype. Additional benefits include reduced costs and induction of long-term immunity. Here, we have applied the large-scale codon re-encoding method to the TBEV to demonstrate the principle of developing a live attenuated virus vaccine which protects mice against subsequent infection with the wild type virulent virus. This study therefore illustrates that codon re-encoding is potentially an easily derived and effective method of producing live attenuated vaccine candidates against positive-stranded RNA viruses.

Neuroinvasion of the highly pathogenic influenza virus H7N1 is caused by disruption of the blood brain barrier in an avian model

Influenza A virus (IAV) causes central nervous system (CNS) lesions in avian and mammalian species, including humans. However, the mechanism used by IAV to invade the brain has not been determined. In the current work, we used chickens infected with a highly pathogenic avian influenza (HPAI) virus as a model to elucidate the mechanism of entry of IAV into the brain. The permeability of the BBB was evaluated in fifteen-day-old H7N1-infected and non-infected chickens using three different methods: (i) detecting Evans blue (EB) extravasation into the brain, (ii) determining the leakage of the serum protein immunoglobulin Y (IgY) into the brain and (iii) assessing the stability of the tight-junction (TJ) proteins zonula occludens-1 and claudin-1 in the chicken brain at 6, 12, 18, 24, 36 and 48 hours post-inoculation (hpi). The onset of the induced viremia was evaluated by quantitative real time RT-PCR (RT-qPCR) at the same time points. Viral RNA was detected from 18 hpi onward in blood samples, whereas IAV antigen was detected at 24 hpi in brain tissue samples. EB and IgY extravasation and loss of integrity of the TJs associated with the presence of viral antigen was first observed at 36 and 48 hpi in the telencephalic pallium and cerebellum. Our data suggest that the mechanism of entry of the H7N1 HPAI into the brain includes infection of the endothelial cells at early stages (24 hpi) with subsequent disruption of the TJs of the BBB and leakage of virus and serum proteins into the adjacent neuroparenchyma.

Pathology of fatal lineage 1 and 2 West Nile virus infections in horses in South Africa

Since 2007, West Nile virus (WNV) has been reported in South African horses, causing severe neurological signs. All cases were of lineage 2, except for one case that clustered with lineage 1 viruses. In the present study, gross and microscopic lesions of six South African lineage 2-infected horses and the one lineage 1 case are described. Diagnoses were confirmed by real-time reverse-transcriptase polymerase chain reaction (RT-PCR) of central nervous system (CNS) tissue and one by RT-PCR of a brain virus isolate. The CNS of all cases was negative by RT-PCR or immunohistochemistry (IHC) for African horse sickness (AHS), equine encephalosis virus, equine herpes viruses 1 and 4, other zoonotic flaviviruses, alphaviruses, and shunivirus, and either by immunofluorescence or IHC for rabies. Gross visceral lesions were nonspecific but often mimicked those of AHS. The CNS histopathology of WNV lineage 2 cases resembled the nonsuppurative polioencephalomyelitis reported in the Northern Hemisphere lineage 1 and recent Hungarian lineage 2 cases. Occasional meningitis, focal spinal ventral horn poliomalacia, dorsal and lateral horn poliomyelitis, leucomyelitis, asymmetrical ventral motor spinal neuritis and frequent olfactory region involvement were also seen. Lineage 2 cases displayed marked variations in CNS lesion severity, type and distribution, and suggested various viral entry routes into the CNS, based on findings in experimental mice and hamsters. Lineage 1 lesions were comparable to the milder lineage 2 cases. West Nile virus IHC on CNS sections with marked lesions from all cases elicited only two antigen-positive cells in the olfactory cortex of one case. The presence in the CNS of T-lymphocytes, B-lymphocytes, plasma cells and macrophage-monocytes was confirmed by cluster of differentiation (CD) 3, CD20, multiple myeloma oncogene 1 (MUM1) and macrophage (MAC) 387 IHC.

Kyasanur Forest disease virus infection in mice is associated with higher morbidity and mortality than infection with the closely related Alkhurma hemorrhagic fever virus

BACKGROUND

Kyasanur Forest disease virus (KFDV) and Alkhurma hemorrhagic fever virus (AHFV) are closely related members of the Flavivirus genus and are important causes of human disease in India and the Arabian Peninsula, respectively. Despite high genetic similarity, the viruses have distinctly different host ranges and ecologies. Human cases of KFDV or AHFV develop a spectrum of disease syndromes ranging from liver pathology to neurologic disease. Case reports suggest KFDV is more commonly associated with hepatic and gastrointestinal manifestations whereas AHFV is more commonly associated with neurologic disease.

METHODOLOGY/PRINCIPAL FINDINGS

Inoculation of three immunocompetent laboratory mouse strains revealed that KFDV was consistently more lethal than AHFV. In subsequent studies utilizing C57BL/6J mice, we demonstrated that KFDV infection was associated with higher viral loads and significantly higher mortality. KFDV-infected mice rapidly developed more severe disease than AHFV-infected mice, as evidenced by significant abnormalities on clinical chemistry panels and more severe pathology in the brain and gastrointestinal tract.

CONCLUSIONS/SIGNIFICANCE

Infections of C57BL/6J mice with KFDV or AHFV resulted in clinical disease syndromes that closely approximate the diseases seen in human cases. Despite high genetic similarity, there were clear differences in survival, viral kinetics, clinical chemistry data and histology. These results suggest that distinct mouse models for AHFV and KFDV are necessary in order to gain a better understanding of the unique pathogenesis of each virus, as well as to provide platforms for testing promising vaccines and therapeutics.

Protective role of TNF-α, IL-10 and IL-2 in mice infected with the Oshima strain of Tick-borne encephalitis virus

Tick-borne encephalitis virus (TBEV) causes acute central nervous system disease. Here, we investigated the roles of the TNF-α, IL-10 and other cytokines in appropriate KO mice following infection with Oshima and Sofjin strains of TBEV. Following infection with the Oshima strain, mortality rates were significantly increased in TNF-α KO and IL-10 KO mice compared with wild type (WT) mice. These results suggested that TNF-α and IL-10 play protective roles against fatal infection due to Oshima strain infection. However, viral loads and proinflammatory cytokine levels in the brain of TNF-α KO andIL-10 KO mice were not significantly different compared with those of WT mice. On the other hand, all WT, TNF-α KO and IL-10 KO mice died following infection with Sofjin strain. Interestingly, Sofjin-infected mice did not exhibit an up-regulated mRNA level of IL-2 in the spleen in all groups of mice, whereas Oshima-infected mice showed significantly increased level of IL-2 compared with mock-infected mice. From these results, we suggest that TNF-α, IL-10 and IL-2 are key factors for disease remission from fatal encephalitis due to infection with Oshima strain of TBEV.