A Single Mutation in the VP1 of Enterovirus 71 Is Responsible for Increased Virulence and Neurotropism in Adult Interferon-Deficient Mice

Experimental animal models for development of human enterovirus vaccine

Enterovirus infections induce infectious diseases in young children, such as hand, foot, and mouth disease which is characterized by highly contagious rashes or blisters around the hands, feet, buttocks, and mouth. This predominantly arises from enterovirus A71 or coxsackievirus A16 infections and in severe cases, they can lead to encephalitis, paralysis, pulmonary edema, or even fatality, representing a global health threat. Due to the absence of effective therapeutic strategies for these infections, various experimental animal models are being investigated for the development of vaccines. During the early stages of research on enterovirus infections, non-human primate infections exhibited symptoms like those in humans, leading to their utilization as model animals. However, due to economic and ethical considerations, their current usage is limited. While enterovirus infections do not readily occur in mice, an infection model with mouse-adapted strain in neonatal mice has been employed. Cellular receptors have been identified in human cells, and genetically modified mice expressing these receptors have been used. Most recently, the utilization of Mongolian gerbil model is actively being considered and should be pursued for further animal model development. So, herein, we provide a summarized overview of the current portfolio of available enterovirus infection models, emphasizing their respective advantages and limitations.

Human IVIG treatment in a neurological disease model for Enterovirus A71 infection in 28-day-old AG129 mice

Enterovirus A71 can cause serious neurological disease in young children. Animal models for EV-A71 are needed to evaluate potential antiviral therapies. Existing models have limitations, including lack of lethality or crucial disease signs. Here we report the development of an EV-A71 model in 28-day-old mice. Virus was serially passaged until it produced consistent lethality and rear-limb paralysis. Onset of disease occurred between days 6-9 post-infection, with mortality following weight loss and neurological signs on days 9-14. In addition, a single administration of human intravenous immunoglobulin at doses of 200, 400 and 800 mg/kg at 4h post-infection was evaluated in the model. Protection from weight loss, neurological signs, and mortality (between 50 and 89%) were observed at doses of 400 mg/kg or greater. Based on these results, IVIG was selected for use as a positive control in this acute model, and suggest that IVIG is a potential therapeutic for EV-A71 infections.

Rational attenuation of RNA viruses with zinc finger antiviral protein

Attenuation of a virulent virus is a proven approach for generating vaccines but can be unpredictable. For example, synonymous recoding of viral genomes can attenuate replication but sometimes results in pleiotropic effects that confound rational vaccine design. To enable specific, conditional attenuation of viruses, we examined target RNA features that enable zinc finger antiviral protein (ZAP) function. ZAP recognized CpG dinucleotides and targeted CpG-rich RNAs for depletion, but RNA features such as CpG numbers, spacing and surrounding nucleotide composition that enable specific modulation by ZAP were undefined. Using synonymously mutated HIV-1 genomes, we defined several sequence features that govern ZAP sensitivity and enable stable attenuation. We applied rules derived from experiments with HIV-1 to engineer a mutant enterovirus A71 genome whose attenuation was stable and strictly ZAP-dependent, both in cell culture and in mice. The conditionally attenuated enterovirus A71 mutant elicited neutralizing antibodies that were protective against wild-type enterovirus A71 infection and disease in mice. ZAP sensitivity can thus be readily applied for the rational design of conditionally attenuated viral vaccines.

Efficacy of Coxsackievirus A2 vaccine candidates correlating to humoral immunity in mice challenged with a mouse-adapted strain

BACKGROUND

In recent years, Coxsackievirus A2 (CV-A2) has become one of the main serotypes of enterovirus species A associated with hand, foot and mouth disease (HFMD) in China. It has also caused HFMD epidemics in many countries all over the world. Currently, there are no effective, preventive vaccines against it.

METHODS

A CV-A2 strain was isolated in RD cells and then adapted to grow in Vero cells. This is in compliance with guidelines for cell substrates allowed for human vaccines by the Chinese regulatory authority. Groups of newborn Kunming mice were inoculated on day 3 and day 9 using two formulations of candidate vaccines, empty particles and full particles. They were then challenged on day 14 at a lethal dose with a mouse-adapted strain.

RESULTS

The mice in the control group all died within 14 days post-challenge whereas most of the mice in the candidate vaccine groups survived. It was found that the titers of neutralizing antibodies was dose-dependent in sera of immunized mice. The results also showed that the vaccine candidates stimulated a strong humoral immune response and protected the mice from disease and death. The virus loads in tissues or organs were significantly reduced and pathological changes were either weak or not observed in the immunized groups compared with those in Al(OH)₃ control group. Preliminary mapping of the nucleotide and amino acid residues potentially related to cell tropism of the vaccine strain and virulence of the challenge strain was performed.

CONCLUSION

The results showed that the RD cell-isolated and Vero cell-adapted CV-A2 strain is a promising vaccine candidate. This active immunization-challenge mouse model mimics the vaccination and then exposure to wildtype viruses, compared with passive immunization-challenge model, and is invaluable for efficacy evaluation in studies on multivalent vaccines containing CV-A2 against HFMD.

Isolation and Identification of Type F Bovine Enterovirus from Clinical Cattle with Diarrhoea

Recently, bovine enterovirus (BEV) has caused several respiratory and gastrointestinal diseases outbreaks in cattle. Monitoring the epidemiological and pathogenic characteristics of this virus is crucial to controlling its spread. We isolated a BEV strain with typical cytopathic effects from the faeces of cows with significant diarrhoeal symptoms in China and observed the viral particles within 20–30 nm through transmission electron microscopy. Then, we designated this strain as HB19-1 in this study. The multistep growth curves showed that the virus propagated well in the MDBK cells. Molecular genetic analysis of VP1 indicated that HB19-1 belonged to the BEV-F1 group. Although the challenged ICR mice did not exhibit typical disease symptoms in animal infection assay, we observed significant pathological damage in the lungs, intestines, and muscle tissues. In summary, we isolated a BEV strain HB19-1 causing severe diarrhoea in cattle and proposed reinforcing the epidemiological surveillance of this virus.

A Novel Attenuated Enterovirus A71 Mutant with VP1-V238A,K244R Exhibits Reduced Efficiency of Cell Entry/Exit and Augmented Binding Affinity to Sulfated Glycans

Enterovirus A71 (EV-A71) is one of the major etiological agents of hand, foot, and mouth disease (HFMD), and infection occasionally leads to fatal neurological complications in children. However, only inactivated whole-virus vaccines against EV-A71 are commercially available in Mainland China. Furthermore, the mechanisms underlying the infectivity and pathogenesis of EV-A71 remain to be better understood. By adaptation of an EV-A71 B5 strain in monkey Vero cells in the presence of brilliant black BN (E151), an anti-EV-A71 agent, a double mutant with VP1-V238A,K244R emerged whose infection was enhanced by E151. The growth of the reverse genetics (RG) mutant RG/B5-VP1-V238A,K244R (RG/B5-AR) was promoted by E151 in Vero cells but inhibited in other human and murine cells, while its parental wild type, RG/B5-wt, was strongly prevented by E151 from infection in all tested cells. In the absence of E151, RG/B5-AR exhibited defective cell entry/exit, resulting in reduced viral transmission and growth in vitro. It had augmented binding affinity to sulfated glycans, cells, and tissue/organs, which probably functioned as decoys to restrict viral dissemination and infection. RG/B5-AR was also attenuated, with a 355 times higher 50% lethal dose (LD₅₀) and a shorter timing of virus clearance than those of RG/B5-wt in suckling AG129 mice. However, it remained highly immunogenic in adult AG129 mice and protected their suckling mice from lethal EV-A71 challenges through maternal neutralizing antibodies. Overall, discovery of the attenuated mutant RG/B5-AR contributes to better understanding of virulence determinants of EV-A71 and to further development of novel vaccines against EV-A71.

IMPORTANCE Enterovirus A71 (EV-A71) is highly contagious in children and has been responsible for thousands of deaths in Asia-Pacific region since the 1990s. Unfortunately, the virulence determinants and pathogenesis of EV-A71 are not fully clear. We discovered that a novel EV-A71 mutant, VP1-V238A,K244R, showed growth attenuation with reduced efficiency of cell entry/exit. In the Vero cell line, which has been approved for manufacturing EV-A71 vaccines, the growth defects of the mutant were compensated by a food dye, brilliant black BN. The mutant also showed augmented binding affinity to sulfated glycans and other cellular components, which probably restricted viral infection and dissemination. Therefore, it was virulence attenuated in a mouse model but still retained its immunogenicity. Our findings suggest the mutant as a promising vaccine candidate against EV-A71 infection.

Enterovirus A71 Induces Neurological Diseases and Dynamic Variants in Oral Infection of Human SCARB2-Transgenic Weaned Mice

Enterovirus A71 (EV-A71) and many members of the Picornaviridae family are neurotropic pathogens of global concern. These viruses are primarily transmitted through the fecal-oral route, and thus suitable animal models of oral infection are needed to investigate viral pathogenesis. An animal model of oral infection was developed using transgenic mice expressing human SCARB2 (hSCARB2 Tg), murine-adapted EV-A71/MP4 virus, and EV-A71/MP4 virus with an engineered nanoluciferase gene that allows imaging of viral replication and spread in infected mice. Next-generation sequencing of EV-A71 genomes in the tissues and organs of infected mice was also performed. Oral inoculation of EV-A71/MP4 or nanoluciferase-carrying MP4 virus stably induced neurological symptoms and death in infected 21-day-old weaned mice. In vivo bioluminescence imaging of infected mice and tissue immunostaining of viral antigens indicated that orally inoculated virus can spread to the central nervous system (CNS) and other tissues. Next-generating sequencing further identified diverse mutations in viral genomes that can potentially contribute to viral pathogenesis. This study presents an EV-A71 oral infection murine model that efficiently infects weaned mice and allows tracking of viral spread, features that can facilitate research into viral pathogenesis and neuroinvasion via the natural route of infection. IMPORTANCE Enterovirus A71 (EV-A71), a positive-strand RNA virus of the Picornaviridae, poses a persistent global public health problem. EV-A71 is primarily transmitted through the fecal-oral route, and thus suitable animal models of oral infection are needed to investigate viral pathogenesis. We present an animal model of EV-A71 infection that enables the natural route of oral infection in weaned and nonimmunocompromised 21-day-old hSCARB2 transgenic mice. Our results demonstrate that severe disease and death could be stably induced, and viral invasion of the CNS could be replicated in this model, similar to severe real-world EV-A71 infections. We also developed a nanoluciferase-containing EV-A71 virus that can be used with this animal model to track viral spread after oral infection in real time. Such a model offers several advantages over existing animal models and can facilitate future research into viral spread, tissue tropism, and viral pathogenesis, all pressing issues that remain unaddressed for EV-A71 infections.

Adaptation and Virulence of Enterovirus-A71

Outbreaks of hand, foot, and mouth disease caused by enterovirus-A71 (EV-A71) can result in many deaths, due to central nervous system complications. Outbreaks with many fatalities have occurred sporadically in the Asia-Pacific region and have become a serious public health concern. It is hypothesized that virulent mutations in the EV-A71 genome cause these occasional outbreaks. Analysis of EV-A71 neurovirulence determinants is important, but there are no virulence determinants that are widely accepted among researchers. This is because most studies have been done in artificially infected mouse models and because EV-A71 mutates very quickly to adapt to the artificial host environment. Although EV-A71 uses multiple receptors for infection, it is clear that adaptation-related mutations alter the binding specificity of the receptors and allow the virus to adopt the best entry route for each environment. Such mutations have confused interpretations of virulence in animal models. This article will discuss how environment-adapted mutations in EV-A71 occur, how they affect virulence, and how such mutations can be avoided. We also discuss future perspectives for EV-A71 virulence research.

Viral determinants that drive Enterovirus-A71 fitness and virulence

Hand, Foot and Mouth Disease (HFMD) is usually a self-limiting, mild childhood disease that is caused mainly by Coxsackie virus A16 (CVA16) and Enterovirus A71 (EV-A71), both members of the Picornaviridae family. However, recurring HFMD outbreaks and epidemics due to EV-A71 infection in the Western Pacific region, and the propensity of EV-A71 strains to cause severe neurological complications have made this neurotropic virus a serious public health concern in afflicted countries. High mutation rate leading to viral quasispecies combined with frequent intra- and inter-typic recombination events amongst co-circulating EV-A71 strains have contributed to the great diversity and fast evolution of EV-A71 genomes, making impossible any accurate prediction of the next epidemic strain. Comparative genome sequence analyses and mutagenesis approaches have led to the identification of a number of viral determinants involved in EV-A71 fitness and virulence. These viral determinants include amino acid residues located in the structural proteins of the virus, affecting attachment to the host cell surface, receptor binding, and uncoating events. Critical residues in non-structural proteins have also been identified, including 2C, 3A, 3C proteases and the RNA-dependent RNA polymerase. Finally, mutations altering key secondary structures in the 5’ untranslated region were also found to influence EV-A71 fitness and virulence. While our current understanding of EV-A71 pathogenesis remains fragmented, these studies may help in the rational design of effective treatments and broadly protective vaccine candidates.

Coxsackievirus B4 sewage-isolate induces pancreatitis after oral infection of mice

Numerous serotypes which belong to the genus Enterovirus (EV) show variability in their virulence and clinical manifestations. They are also known to undergo changes caused by mutations and recombination during their circulation in the environment and the population. Various EV serotypes are prevalent in groundwater, wastewater and surface waters. Our previous studies showed that oral infection induces pancreatitis depending on specific conditions, such as gravidity, in an outbred murine model. Our aim in the present study was to further explore the pancreatic histopathology in an outbred mouse model following oral infection with clinical isolates from a patient who had aseptic meningitis and an isolate from a treated-sewage sample recovered from the residential area of the patient. The isolates were identified as coxsackievirus B4 (CVB4) in tissue culture. The CVB4 sewage-isolate induced pancreatitis after oral infection. In contrast, pancreatitis was absent following infection with the clinical isolates. Comparison of polyprotein sequences showed that the treated-sewage strains differed from the patient's isolates by 9 and 11 amino acids. We conclude that the isolates of clinical and environmental origin differed in their pathogenic properties and showed genetic variation.

Viruses and Type 1 Diabetes: From Enteroviruses to the Virome

For over a century, viruses have left a long trail of evidence implicating them as frequent suspects in the development of type 1 diabetes. Through vigorous interrogation of viral infections in individuals with islet autoimmunity and type 1 diabetes using serological and molecular virus detection methods, as well as mechanistic studies of virus-infected human pancreatic β-cells, the prime suspects have been narrowed down to predominantly human enteroviruses. Here, we provide a comprehensive overview of evidence supporting the hypothesised role of enteroviruses in the development of islet autoimmunity and type 1 diabetes. We also discuss concerns over the historical focus and investigation bias toward enteroviruses and summarise current unbiased efforts aimed at characterising the complete population of viruses (the “virome”) contributing early in life to the development of islet autoimmunity and type 1 diabetes. Finally, we review the range of vaccine and antiviral drug candidates currently being evaluated in clinical trials for the prevention and potential treatment of type 1 diabetes.

Mechanisms of viral persistence in the brain and therapeutic approaches

There is growing recognition of the diversity of viruses that can infect the cells of the central nervous system (CNS). While the majority of CNS infections are successfully cleared by the immune response, some viral infections persist in the CNS. As opposed to resolved infections, persistent viruses can contribute to ongoing tissue damage and neuroinflammatory processes. In this manuscript, we provide an overview of the current understanding of factors that lead to viral persistence in the CNS including how viruses enter the brain, how these pathogens evade antiviral immune system responses, and how viruses survive and transmit within the CNS. Further, as the CNS may serve as a unique viral reservoir, we examine the ways in which persistent viruses in the CNS are being targeted therapeutically.

Efficacy of a coxsackievirus A6 vaccine candidate in an actively immunized mouse model

Coxsackievirus A6 (CV-A6) has been emerging as a major pathogen of hand, foot and mouth disease (HFMD). Study on the pathogenesis of CV-A6 infection and development of vaccines is hindered by a lack of appropriate animal models. Here, we report an actively immunized-challenged mouse model to evaluate the efficacy of a Vero-cell-based, inactivated CV-A6 vaccine candidate. The neonatal Kunming mice were inoculated with a purified, formaldehyde-inactivated CV-A6 vaccine on days 3 and 9, followed by challenging on day 14 with a naturally selected virulent strain at a lethal dose. Within 14 days postchallenge, all mice in the immunized groups survived, while 100% of the Alum-only inoculated mice died. Neutralizing antibodies (NtAbs) were detected in the serum of immunized suckling mice, and the NtAb levels correlated with the survival rate of the challenged mice. The virus loads in organs were reduced, and pathological changes and viral protein expression were weak in the immunized mice compared with those in Alum-only inoculated control mice. Elevated levels of interleukin-4, 6, interferon γ and tumour necrosis factor α were also observed in Alum-only control mice compared with immunized mice. Importantly, the virulent CV-A6 challenge strain was selected quickly and conveniently from a RD cell virus stock characterized with the natural multi-genotypes. The virulent determinants were mapped to V124M and I242 V at VP1. Together, our results indicated that this actively immunized mouse model is invaluable for future studies to develop multivalent vaccines containing the major component of CV-A6 against HFMD.

Recent advances in the understanding of enterovirus A71 infection: a focus on neuropathogenesis

Introduction: Hand, foot, and mouth disease caused by enterovirus A71 (EV-A71) is more frequently associated with neurological complications and deaths compared to other enteroviruses.Areas covered: The authors discuss current understanding of the neuropathogenesis of EV-A71 based on various clinical, human, and animal model studies. The authors discuss the important advancements in virus entry, virus dissemination, and neuroinvasion. The authors highlight the role of host immune system, host genetic factors, viral quasispecies, and heparan sulfate in EV-A71 neuropathogenesis.Expert opinion: Comparison of EV-A71 with EV-D68 and PV shows similarity in primary target sites and dissemination to the central nervous system. More research is needed to understand cellular tropisms, persistence of EV-A71, and other possible invasion routes. EV-A71 infection has varied clinical manifestations which may be attributed to multiple receptors usage. Future development of antivirals and vaccines should target neurotropic enteroviruses. Repurposing drug and immunomodulators used in combination could reduce the severity of EV-A71 infection. Only a few drugs have been tested in clinical trials, and in the absence of antiviral and vaccines (except China), active virus surveillance, good hand hygiene, and physical distancing should be advocated. A better understanding of EV-A71 neuropathogenesis is critical for antiviral and multivalent vaccines development.

TAR DNA-Binding Protein 43 is Cleaved by the Protease 3C of Enterovirus A71

Enterovirus A71 (EV-A71) is one of the etiological pathogens leading to hand, foot, and mouth disease (HFMD), which can cause severe neurological complications. The neuropathogenesis of EV-A71 infection is not well understood. The mislocalization and aggregation of TAR DNA-binding protein 43 (TDP-43) is the pathological hallmark of amyotrophic lateral sclerosis (ALS). However, whether TDP-43 was impacted by EV-A71 infection is unknown. This study demonstrated that TDP-43 was cleaved during EV-A71 infection. The cleavage of TDP-43 requires EV-A71 replication rather than the activated caspases due to viral infection. TDP-43 is cleaved by viral protease 3C between the residues 331Q and 332S, while mutated TDP-43 (Q331A) was not cleaved. In addition, mutated 3C which lacks the protease activity failed to induce TDP-43 cleavage. We also found that TDP-43 was translocated from the nucleus to the cytoplasm, and the mislocalization of TDP-43 was induced by viral protease 2A rather than 3C. Taken together, we demonstrated that TDP-43 was cleaved by viral protease and translocated to the cytoplasm during EV-A71 infection, implicating the possible involvement of TDP-43 in the pathogenesis of EV-A71infection.

Pan-Enterovirus Amplicon-Based High-Throughput Sequencing Detects the Complete Capsid of a EVA71 Genotype C1 Variant via Wastewater-Based Epidemiology in Arizona

We describe the complete capsid of a genotype C1-like Enterovirus A71 variant recovered from wastewater in a neighborhood in the greater Tempe, Arizona area (Southwest United States) in May 2020 using a pan-enterovirus amplicon-based high-throughput sequencing strategy. The variant seems to have been circulating for over two years, but its sequence has not been documented in that period. As the SARS-CoV-2 pandemic has resulted in changes in health-seeking behavior and overwhelmed pathogen diagnostics, our findings highlight the importance of wastewater-based epidemiology (WBE ) as an early warning system for virus surveillance.

Comparison of coxsackievirus A12 genome isolated from patients with different symptoms in Yunnan, Southwest China

Aim: Coxsackievirus A12 (CVA12) mainly causes hand–foot–mouth disease (HFMD) or herpangina (HA). Thus, the genomic characteristics of CVA12 isolates from Southwest China, especially the discrepancy between CVA12-HFMD and CVA12-HA were analyzed. Patients & methods: The full-length genome sequences of CVA12-HFMD and CVA12-HA were obtained and phylogenetic, nucleotide mutation, amino acid substitution and recombinant analyses were performed. Results: All CVA12 were closest to the Netherlands (2013) and have possibly recombined in the capsid coding region (P1) with other HEV-A. In the coding region, 54 base mutation result in 11 nonsynonymous mutations and four of them were identical mutations between CVA12-HFMD and CVA12-HA. Conclusion: Whether the presence of four consistent nonsynonymous mutation sites affect the virulence of the CVA12 deserves further study.

A Single Mutation in the VP1 Gene of Enterovirus 71 Enhances Viral Binding to Heparan Sulfate and Impairs Viral Pathogenicity in Mice

Enterovirus 71 (EV71) is the major causative pathogen of human hand, foot, and mouth disease (hHFMD) and has evolved to use various cellular receptors for infection. However, the relationship between receptor preference and EV71 virulence has not been fully revealed. By using reverse genetics, we identified that a single E98K mutation in VP1 is responsible for rapid viral replication in vitro. The E98K mutation enhanced binding of EV71-GZCII to cells in a heparan sulfate (HS)-dependent manner, and it attenuated the virulence of EV71-GZCII in BALB/c mice, indicating that the HS-binding property is negatively associated with viral virulence. HS is widely expressed in vascular endothelial cells in different mouse tissues, and weak colocalization of HS with scavenger receptor B2 (SCARB2) was detected. The cGZCII-98K virus bound more efficiently to mouse tissue homogenates, and the cGZCII-98K virus titers in mouse tissues and blood were much lower than the cGZCII virus titers. Together, these findings suggest that the enhanced adsorption of the cGZCII-98K virus, which likely occurs through HS, is unable to support the efficient replication of EV71 in vivo. Our study confirmed the role of HS-binding sites in EV71 infection and highlighted the importance of the HS receptor in EV71 pathogenesis.

Characterization of Plaque Variants and the Involvement of Quasi-Species in a Population of EV-A71

Viral plaque morphologies in human cell lines are markers for growth capability and they have been used to assess the viral fitness and selection of attenuated mutants for live-attenuated vaccine development. In this study, we investigate whether the naturally occurring plaque size variation reflects the virulence of the variants of EV-A71. Variants of two different plaque sizes (big and small) from EV-A71 sub-genotype B4 strain 41 were characterized. The plaque variants displayed different in vitro growth kinetics compared to the parental wild type. The plaque variants showed specific mutations being present in each variant strain. The big plaque variants showed four mutations I97L, N104S, S246P and N282D in the VP1 while the small plaque variants showed I97T, N237T and T292A in the VP1. No other mutations were detected in the whole genome of the two variants. The variants showed stable homogenous small plaques and big plaques, respectively, when re-infected in rhabdomyosarcoma (RD) and Vero cells. The parental strain showed faster growth kinetics and had higher viral RNA copy number than both the big and small plaque variants. Homology modelling shows that both plaque variants have differences in the structure of the VP1 protein due to the presence of unique spontaneous mutations found in each plaque variant This study suggests that the EV-A71 sub-genotype B4 strain 41 has at least two variants with different plaque morphologies. These differences were likely due to the presence of spontaneous mutations that are unique to each of the plaque variants. The ability to maintain the respective plaque morphology upon passaging indicates the presence of quasi-species in the parental population.

Acarbose, as a potential drug, effectively blocked the dynamic metastasis of EV71 from the intestine to the whole body

Enterovirus 71 (EV71) is one of the main pathogens causing hand-foot-and-mouth disease (HFMD). The nose and mouth are usually the main infection entries of EV71 virus. However, its dynamic transport pathway from mouth to the whole body remains unknown. The reveal of this physiological mechanism in vivo will help to understand its transport direction, find its key proliferation nodes, and develop new preventive strategies. We trained a new strain of GFP-EV71 virus to be susceptible to mice brain by intracranial injection of mice. The adapted virus was oral-administrated to suckling mice. Then, the dynamic distributions of the virus in vivo were detected by living image system and fluorescence quantitation polymerase chain reaction (qPCR). We figured out the dynamic pathway of EV71 transport in vivo from intestine to peripheral tissue, then to the other organs. Small intestine was identified as a gateway for EV71 infection in vivo. Ileum was proved to be the main part of proliferation and transport of EV71 in small intestine of mice. EV71 was verified to enter small intestinal villus of mice through the infection of small intestinal epithelial cell. Acarbose displayed a good preventive effect on EV71 infection both in vivo and in vitro. Acarbose possibly decreased the intestinal infection of EV71 by blocking the receptor-binding sites on the surface of EV71 virion or by inhibiting various glycolic receptors on the cell surface. Thus, acarbose and its analogue may be the potential medicines to prevent EV71 infection.

Intra-host emergence of an enterovirus A71 variant with enhanced PSGL1 usage and neurovirulence

Enterovirus A71 (EV-A71) is one of the main causative agents of hand-foot-and-mouth disease and is occasionally associated with severe neurological complications. EV-A71 pathophysiology is poorly understood due to the lack of small animal models that robustly support viral replication in relevant organs/tissues. Here, we show that adult severe combined immune-deficient (SCID) mice can serve as an EV-A71 infection model to study neurotropic determinants and viral tropism. Mice inoculated intraperitoneally with an EV-A71 clinical isolate had an initial infection of the lung compartment, followed by neuroinvasion and infection of (motor)neurons, resulting in slowly progressing paralysis of the limbs. We identified a substitution (V135I) in the capsid protein VP2 as a key requirement for neurotropism. This substitution was also present in a mouse-adapted variant, obtained by passaging the clinical isolate in the brain of one-day-old mice, and induced exclusive neuropathology and rapid paralysis, confirming its role in neurotropism. Finally, we showed that this residue enhances the capacity of EV-A71 to use mouse PSGL1 for viral entry. Our data reveal that EV-A71 initially disseminates to the lung and identify viral and host determinants that define the neurotropic character of EV-A71, pointing to a hitherto understudied role of PSGL1 in EV-A71 tropism and neuropathology.

Electrostatic interactions at the five-fold axis alter heparin-binding phenotype and drive enterovirus A71 virulence in mice

Enterovirus A71 (EV-A71) causes hand, foot and mouth disease epidemics with neurological complications and fatalities. However, the neuropathogenesis of EV-A71 remains poorly understood. In mice, adaptation and virulence determinants have been mapped to mutations at VP2-149, VP1-145 and VP1-244. We investigate how these amino acids alter heparin-binding phenotype and shapes EV-A71 virulence in one-day old mice. We constructed six viruses with varying residues at VP1-98, VP1-145 (which are both heparin-binding determinants) and VP2-149 (based on the wild type 149K/98E/145Q, termed KEQ) to generate KKQ, KKE, KEE, IEE and IEQ variants. We demonstrated that the weak heparin-binder IEE was highly lethal in mice. The initially strong heparin-binding IEQ variant acquired an additional mutation VP1-K244E, which confers weak heparin-binding phenotype resulting in elevated viremia and increased virus antigens in mice brain, with subsequent high virulence. IEE and IEQ-244E variants inoculated into mice disseminated efficiently and displayed high viremia. Increasing polymerase fidelity and impairing recombination of IEQ attenuated the virulence, suggesting the importance of population diversity in EV-A71 pathogenesis in vivo. Combining in silico docking and deep sequencing approaches, we inferred that virus population diversity is shaped by electrostatic interactions at the five-fold axis of the virus surface. Electrostatic surface charges facilitate virus adaptation by generating poor heparin-binding variants for better in vivo dissemination in mice, likely due to reduced adsorption to heparin-rich peripheral tissues, which ultimately results in increased neurovirulence. The dynamic switching between heparin-binding and weak heparin-binding phenotype in vivo explained the neurovirulence of EV-A71.

Enterovirus A71 (EV-A71) is the primary cause of hand, foot and mouth disease, and it can also infect the central nervous system and cause fatal outbreaks in young children. EV-A71 pathogenesis remains elusive. In this study, we demonstrated that EV-A71 variants with strong affinity to heparan sulfate (heparin) have a growth advantage in cell culture, but are disadvantaged in vivo. When inoculated into one-day old mice, strong heparin-binding virus variants are more likely to be adsorbed to peripheral tissues, resulting in impaired ability to disseminate, and are cleared from the bloodstream rapidly. The lower viremia level resulted in no neuroinvasion. In contrast, weak heparin-binding variants show greater levels of viremia, dissemination and subsequent neurovirulence in mice. We also provide evidence that the EV-A71 heparin-binding pattern is mediated by electrostatic surface charges on the virus capsid surface. In mice, EV-A71 undergoes adaptive mutation to acquire greater negative surface charges, thus generating new virulent variants with weak heparin-binding ability which allows greater viral spread. Our study underlines the importance of electrostatic surface charges in shaping EV-A71 virulence.

In Vitro and In Vivo Inhibition of the Infectivity of Human Enterovirus 71 by a Sulfonated Food Azo Dye, Brilliant Black BN

Hand, foot, and mouth disease (HFMD), a highly contagious disease in children, is caused by human enteroviruses, including enterovirus 71 (EV71), coxsackievirus A16 (CVA16), and coxsackievirus A6 (CVA6). Although HFMD is usually mild and self-limiting, EV71 infection occasionally leads to fatal neurological disorders. Currently, no commercial antiviral drugs for HFMD treatment are available. Here, numerous sulfonated azo dyes, widely used as food additives, were identified as having potent antiviral activities against human enteroviruses. Among them, brilliant black BN (E151) was able to inhibit all EV71, CVA16, and CVA6 strains tested. In rhabdomyosarcoma cells, the 50% inhibitory concentrations of the dye E151 for various strains of EV71 ranged from 2.39 μM to 28.12 μM, whereas its 50% cytotoxic concentration was 1,870 μM. Food azo dyes, including E151, interacted with the vertex of the 5-fold axis of EV71 and prevented viral entry. Their efficacy in viral inhibition was regulated by amino acids at VP1-98, VP1-145, and/or VP1-246. Dye E151 not only prevented EV71 attachment but also eluted attached viruses in a concentration-dependent manner. Moreover, E151 inhibited the interaction between EV71 and its cellular uncoating factor cyclophilin A. In vivo studies demonstrated that E151 at a dose of 200 mg/kg of body weight/day given on the initial 4 days of challenge protected AG129 mice challenged with 10× the 50% lethal dose of wild-type EV71 isolates. Taken together, these data highlight E151 as a promising antiviral agent against EV71 infection.IMPORTANCE Human enterovirus 71 (EV71) is one of the causative agents of hand, foot, and mouth disease in children and is responsible for thousands of deaths in the past 20 years. Food azo dyes have been widely used since the nineteenth century; however, their biological effects on humans and microbes residing in humans are poorly understood. Here, we discovered that one of these dyes, brilliant black BN (E151), was particularly effective in inhibiting the infectivity of EV71 in both cell culture and mouse model studies. Mechanistic studies demonstrated that these sulfonated dyes mainly competed with EV71 attachment factors for viral binding to block viral attachment/entry to host cells. As no commercial antiviral drugs against EV71 are currently available, our findings open an avenue to exploit the development of permitted food dye E151 as a potential anti-EV71 agent.

Genome Analysis of Coxsackievirus A4 Isolates From Hand, Foot, and Mouth Disease Cases in Shandong, China

Coxsackievirus A4 (CVA4) is one of the most prevalent pathogens associated with hand, foot and mouth disease (HFMD), an acute febrile illness in children, and is also associated with acute localized exanthema, myocarditis, hepatitis and pancreatitis. Despite this, limited CVA4 genome sequences are currently available. Herein, complete genome sequences from CVA4 strains (n = 21), isolated from patients with HFMD in Shandong province, China between 2014 and 2016, were determined and phylogenetically characterized. Phylogenetic analysis of the VP1 gene from a larger CVA4 collection (n = 175) showed that CVA4 has evolved into four separable genotypes: A, B, C, and D; and genotype D could be further classified in to two sub-genotypes: D1 and D2. Each of the 21 newly described genomes derived from isolates that segregated with sub-genotype D2. The CVA4 genomes displayed significant intra-genotypic genetic diversity with frequent synonymous substitutions occurring at the third codon positions, particularly within the P2 region. However, VP1 was relatively stable and therefore represents a potential target for molecular diagnostics assays and also for the rational design of vaccine epitopes. The substitution rate of VP1 was estimated to be 5.12 × 10^-3 substitutions/site/year, indicative of ongoing CVA4 evolution. Mutations at amino acid residue 169 in VP1 gene may be responsible for differing virulence of CVA4 strains. Bayesian skyline plot analysis showed that the population size of CVA4 has experienced several dynamic fluctuations since 1948. In summary, we describe the phylogenetic and molecular characterization of 21 complete genomes from CVA4 isolates which greatly enriches the known genomic diversity of CVA4 and underscores the need for further surveillance of CVA4 in China.

A decade of sustained selection pressure on two surface sites of the VP1 protein of Enterovirus A71 suggests that immune evasion may be an indirect driver for virulence

Enterovirus A71 (EV-A71) is an emerging pathogen in the Enterovirus A species group. EV-A71 causes hand, foot and mouth disease (HFMD), with virulent variants exhibiting polio-like acute flaccid paralysis and other central nervous system manifestations. We analysed all enterovirus A71 complete genomes with collection dates from 2008 to mid-2018. All sub-genotypes exhibit a strong molecular clock with omega (dN/dS) suggesting strong purifying selection. In sub-genotypes B5 and C4, positive selection can be detected at two surface sites on the VP1 protein, also detected in positive selection studies performed prior to 2008. Toggling of a limited repertoire of amino acids at these positively selected residues over the last decade suggests that EV-A71 may be undergoing a sustained frequency-dependent selection process for immune evasion, raising issues for vaccine development. These same sites have also been previously implicated in virus-host binding and strain-associated severity of HFMD, suggesting that immune evasion may be an indirect driver for virulence (154 words).

Enterovirus A71 Infection Activates Human Immune Responses and Induces Pathological Changes in Humanized Mice

Since the discovery of enterovirus A71 (EV-A71) half a century ago, it has been recognized as the cause of large-scale outbreaks of hand-foot-and-mouth disease worldwide, particularly in the Asia-Pacific region, causing great concern for public health and economic burdens. Detailed mechanisms on the modulation of immune responses after EV-A71 infection have not been fully known, and the lack of appropriate models hinders the development of promising vaccines and drugs. In the present study, NOD-scid IL2Rγ^-/- (NSG) mice with a human immune system (humanized mice) at the age of 4 weeks were found to be susceptible to a human isolate of EV-A71 infection. After infection, humanized mice displayed limb weakness, which is similar to the clinical features found in some of the EV-A71-infected patients. Histopathological examination indicated the presence of vacuolation, gliosis, or meningomyelitis in brain stem and spinal cord, which were accompanied by high viral loads detected in these organs. The numbers of activated human CD4⁺ and CD8⁺ T cells were upregulated after EV-A71 infection, and EV-A71-specific human T cell responses were found. Furthermore, the secretion of several proinflammatory cytokines, such as human gamma interferon (IFN-γ), interleukin-8 (IL-8), and IL-17A, was elevated in the EV-A71-infected humanized mice. Taken together, our results suggested that the humanized mouse model permits insights into the human immune responses and the pathogenesis of EV-A71 infection, which may provide a platform for the evaluation of anti-EV-A71 drug candidates in the future.IMPORTANCE Despite causing self-limited hand-food-and-mouth disease in younger children, EV-A71 is consistently associated with severe forms of neurological complications and pulmonary edema. Nevertheless, only limited vaccines and drugs have been developed over the years, which is possibly due to a lack of models that can more accurately recapitulate human specificity, since human is the only natural host for wild-type EV-A71 infection. Our humanized mouse model not only mimics histological symptoms in patients but also allows us to investigate the function of the human immune system during infection. It was found that human T cell responses were activated, accompanied by an increase in the production of proinflammatory cytokines in EV-A71-infected humanized mice, which might contribute to the exacerbation of disease pathogenesis. Collectively, this model allows us to delineate the modulation of human immune responses during EV-A71 infection and may provide a platform to evaluate anti-EV-A71 drug candidates in the future.

Recent Progress on Functional Genomics Research of Enterovirus 71

Enterovirus 71 (EV71) is one of the main pathogens that causes hand-foot-and-mouth disease (HFMD). HFMD caused by EV71 infection is mostly self-limited; however, some infections can cause severe neurological diseases, such as aseptic meningitis, brain stem encephalitis, and even death. There are still no effective clinical drugs used for the prevention and treatment of HFMD. Studying EV71 protein function is essential for elucidating the EV71 replication process and developing anti-EV71 drugs and vaccines. In this review, we summarized the recent progress in the studies of EV71 non-coding regions (5' UTR and 3' UTR) and all structural and nonstructural proteins, especially the key motifs involving in viral infection, replication, and immune regulation. This review will promote our understanding of EV71 virus replication and pathogenesis, and will facilitate the development of novel drugs or vaccines to treat EV71.

Severity of enterovirus A71 infection in a human SCARB2 knock-in mouse model is dependent on infectious strain and route

Enterovirus A71 (EV-A71) is a major etiological agent of human hand, foot and mouth disease, and it can cause severe neurological complications. Although several genotypes of EV-A71 strains are prevalent in different regions of the world, the genotype C4 has circulated in mainland China for more than 20 years. The pathogenicity of different EV-A71 clinical isolates varies and needs to be explored. In this study, hSCARB2 knock-in mice (N = 181) with a wide range of ages were tested for their susceptibility to two EV-A71 strains with the subgenotypes C4 and C2, and two infection routes (intracranial and venous) were compared. The clinical manifestations and pathology and their relationship to the measured viral loads in different tissues were monitored. We observed that 3 weeks is a crucial age, as mice younger than 3-week-old that were infected became extremely ill. However, mice older than 3 weeks displayed diverse clinical symptoms. Significant differences were observed in the pathogenicity of the two strains with respect to clinical signs, disease incidence, survival rate, and body weight change. We concluded that hSCARB2 knock-in mice are a sensitive model for investigating the clinical outcomes resulting from infection by different EV-A71 strains. The intracranial infection model appears to be suitable for evaluating EV-A71 neurovirulence, whereas the venous infection model is appropriate for studying the pathogenicity of EV-A71.

Molecular epidemiology analysis of enterovirus 71 strains isolated in Dak Lak, Vietnam, 2011-2016

Human enterovirus 71 (EV71) is the major etiologic agent of hand, foot, and mouth disease (HFMD). EV71 outbreaks have been reported in Dak Lak in recent years, however, the genotypes/subgenotypes information and phylogeny of circulating EV71 strains are limited. The objectives of this study were to determine the genotypes/subgenotypes and investigate the phylogeny of EV71 isolates in Dak Lak over a 6-year period. Viruses were isolated from clinical samples from patients with HFMD. In total, 43 EV71 isolates circulated in Dak Lak during 2011-2016 were used for the phylogenetic analysis using complete VP1 gene. The phylogenetic analysis of the VP1 gene revealed that two major genotypes, B and C, were found. Among the 43 EV71 strains, 29 belonged to subgenotype C4, 2 belonged to subgenotype C5, and 12 belonged to subgenotype B5. Of these, the subgenotype C4 was predominant in 2011-2013 and this was later replaced by the subgenotype B5 in 2014. The subgenotype B5 was dominant between 2014 and 2015, and then C4 recirculated in 2016. Our study also indicated that the subgenotypes C4 and B5 emerged into Dak Lak were closely related to variants causing epidemics of HFMD in the southern and central region of Vietnam and Thailand. Sequence analysis showed that nine amino acid mutations were detected in the VP1 region. Our results identified two significant amino acid substitutions (D31N and E145G/Q) associated with enhancing EV71 virulence.

Amplification and next generation sequencing of near full-length human enteroviruses for identification and characterisation from clinical samples

More than 100 different enterovirus (EV) genotypes infect humans and contribute to substantial morbidity. However, current methods for characterisation of full-length genomes are based on Sanger sequencing of short genomic regions, which are labour-intensive and do not enable comprehensive characterisation of viral populations. Here, we describe a simple and sensitive protocol for the amplification and sequencing of near full-length genomes of human EV species using next generation sequencing. EV genomes were amplified from 89% of samples tested, with C_t values ranging between 15.7 and 39.3. These samples included 7 EV-A genotypes (CVA2, 5–7, 10, 16 and EV71), 19 EV-B genotypes (CVA9, CVB1-6, ECHO3, 4, 6, 7, 9, 11, 16, 18, 25, 29, 30, and EV69), 3 EV-C genotypes (CVA19 and PV2, 3) and 1 EV-D genotype (EV70). We characterised 70 EVs from 58 clinical stool samples and eight reference strains, with a minimum of 100X depth. We found evidence of co-infection in four clinical specimens, each containing two distinct EV genotypes (CVB3/ECHO7, CVB3/ECHO18 and ECHO9/30). Characterisation of the complete genome provided conclusive genotyping of EVs, which can be applied to investigate the intra-host virus evolution of EVs, and allows further identification and investigation of EV outbreaks.

Changes in the EV-A71 Genome through Recombination and Spontaneous Mutations: Impact on Virulence

Enterovirus 71 (EV-A71) is a major etiological agent of hand, foot and mouth disease (HFMD) that mainly affects young children less than five years old. The onset of severe HFMD is due to neurological complications bringing about acute flaccid paralysis and pulmonary oedema. In this review, we address how genetic events such as recombination and spontaneous mutations could change the genomic organization of EV-A71, leading to an impact on viral virulence. An understanding of the recombination mechanism of the poliovirus and non-polio enteroviruses will provide further evidence of the emergence of novel strains responsible for fatal HFMD outbreaks. We aim to see if the virulence of EV-A71 is contributed solely by the presence of fatal strains or is due to the co-operation of quasispecies within a viral population. The phenomenon of quasispecies within the poliovirus is discussed to reflect viral fitness, virulence and its implications for EV-A71. Ultimately, this review gives an insight into the evolution patterns of EV-A71 by looking into its recombination history and how spontaneous mutations would affect its virulence.

Sequence analysis-based characterization and identification of neurovirulence-associated variants of 36 EV71 strains from China

Enterovirus 71 (EV71) is the main pathogen of hand-foot-mouth disease (HFMD) and causes several neurological complications. As new strains of EV71 are constantly discovered, it is important to understand the genomic characteristics of the viruses and the mechanism of virulence. Herein, we isolated five strains of EV71 from HFMD patients with or without neurovirulence and sequenced their whole genomes. We then performed whole genome sequence analysis of totally 36 EV71 strains. The phylogenetic analysis of the VP1 region revealed all five isolated strains are clustered into C4a of C4 subgenotype. In addition, by comparing the complete genome sequences of 36 strains, 253 variable amino acid positions were found, 14 of which were identified to be associated with neurovirulence (P < 0.05). Moreover, a similar pattern of amino acid variants combination was identified in four strains without neurovirulence, indicating this type of variant pattern might be associated with avirulence. The strains with neurovirulence appeared to be distinguished from those without neurovirulence by the variants in VP1 and P2 regions, implying VP1 and P2 are the important regions associated with neurovirulence. Indeed, 3-D modeling of VP1 and P2 regions of non-neurovirulent and neurovirulent strains revealed that the different variants resulted in different protein structures and amino acid composition of ligand binding site, which might account for their difference in neurovirulence. In summary, our study reveals 14 variable amino acid positions of VP1, P2 and P3 regions are related to the virulence and that mutations in the capsid proteins of EV71 might contribute to neurovirulence.

An emerging and expanding clade accounts for the persistent outbreak of Coxsackievirus A6-associated hand, foot, and mouth disease in China since 2013

Enterovirus (EV)-A71 and Coxsackievirus (CV)-A16 have historically been the major pathogens of hand, foot, and mouth disease (HMFD) in China; however, CV-A6， which had previously received little attention, became the predominant pathogen in 2013, and has remained one of the common pathogens since then. In this work, we conducted a molecular epidemiology study of CV-A6-associated HFMD in Xiamen from 2009 to 2015. The data showed CV-A6 pandemics had a certain periodicity rather than occurring randomly. Evolution analysis based on near-complete VP1 nucleotide sequences showed subgenotype D5 lineage 4 strains account for the persistent outbreak of CV-A6-associated HFMD in China since 2013. Alignment analysis revealed eight candidate amino acid substitutions in VP1, which may provide useful information for the research of CV-A6 virulence enhancement. This study contributed to elucidating the circulation patterns and genetic characteristics of CV-A6 in China; however, further surveillance and intervention in CV-A6 epidemics is recommended.

Nonstructural protein 2A modulates replication and virulence of enterovirus 71

Enterovirus 71 (EV71) can cause hand, foot, and mouth disease in children, and severe infections can induce neurological complications and even death. However, the pathogenesis of EV71 remains unknown. The 2A proteinase (2A^pro) of EV71 plays an important role in segmenting the precursor polyprotein during viral replication, inhibiting host protein synthesis, and evading innate immunity. This study was to determine the function of EV71 2A^pro in replication and virulence. A chimeric strain (SDLY 107-2A-1) was recombined by replacing 2A^pro of a severe strain (SDLY107) with that of a mild strain (SDLY1) based on an infectious cDNA clone. The replication kinetics of the chimeric strain in vitro and in vivo were determined by qRT-PCR, which showed that the chimeric strain replicated slower and generated less viral RNA than the severe strain. The pathological change and viral load of chimeric strain infected mice were intermediate between severe strain infected mice and mild strain infected mice. Cellular cytotoxicity assays revealed that 2A^pro was associated with the neurotoxicity of EV71. Histopathological and immunohistochemical assays detected tissue pathological damage in the lungs, muscles, brain, and intestinal tissues. Together, these results suggest that 2A^pro modulates replication and virulence of EV71. This provides a theoretical basis for virulence determination of EV71.

A highly conserved amino acid in VP1 regulates maturation of enterovirus 71

Enterovirus 71 (EV71) is the major causative agent of hand, foot and mouth disease (HFMD) in children, causing severe clinical outcomes and even death. Here, we report an important role of the highly conserved alanine residue at position 107 in the capsid protein VP1 (VP1A107) in the efficient replication of EV71. Substitutional mutations of VP1A107 significantly diminish viral growth kinetics without significant effect on viral entry, expression of viral genes and viral production. The results of mechanistic studies reveal that VP1A107 regulates the efficient cleavage of the VP0 precursor during EV71 assembly, which is required, in the next round of infection, for the transformation of the mature virion (160S) into an intermediate or A-particle (135S), a key step of virus uncoating. Furthermore, the results of molecular dynamic simulations and hydrogen-bond networks analysis of VP1A107 suggest that flexibility of the VP1 BC loop or the region surrounding the VP1107 residue directly correlates with viral infectivity. It is possible that sufficient flexibility of the region surrounding the VP1107 residue favors VP0 conformational change that is required for the efficient cleavage of VP0 as well as subsequent viral uncoating and viral replication. Taken together, our data reveal the structural role of the highly conserved VP1A107 in regulating EV71 maturation. Characterization of this novel determinant of EV71 virulence would promote the study on pathogenesis of Enteroviruses.

Impact of Coxsackievirus A6 emergence on hand, foot, and mouth disease epidemic in Osaka City, Japan

Hand, foot, and mouth disease (HFMD) is an acute febrile illness characterized by fever; sore throat; and vesicular eruptions on the hands, feet, and oral mucosa. Until 2010, HFMD was predominantly associated with enterovirus (EV) A71 and coxsackievirus (CV) A16 in Japan. In 2011, CV-A6 emerged as a primary causative agent, causing the largest HFMD epidemic in Japan since 1981. Since then, CV-A6 has caused large HFMD epidemics every 2 years. The phylogenetic analysis of complete Viral Protein 1 (VP1) sequences revealed that most CV-A6 strains detected from 2011 to 2015 in Osaka City were classified into a different clade compared with CV-A6 strains detected from 1999 until 2009. The majority of CV-A6 strains detected in 2011 and most CV-A6 strains detected from 2013 to 2015 were mainly divided into two distinct genetic groups. Each epidemic strain carried unique amino acid substitutions in the presumed DE, EF, and GH loops of the VP1 protein that is exposed on the surface of the virion. There is a possibility that the appearance of substitutions on the surface of the virion and an accumulation of a susceptible population are significant factors in recent HFMD epidemics.

Characterization of significant molecular determinants of virulence of Enterovirus 71 sub-genotype B4 in Rhabdomyosarcoma cells

One of the leading causes of the hand, foot and mouth disease (HFMD) is Enterovirus 71 (EV-A71), displaying symptoms such as fever and ulcers in children but some strains can produce cardiopulmonary oedema which leads to death. There is no FDA-approved vaccine for prevention of severe HFMD. The molecular determinants of virulence for EV-A71 are unclear. It could be a single or a combination of amino acids that determines virulence in different EV-A71 genotype/sub-genotypes. Several EV-A71 strains bearing single nucleotide (nt) mutations were constructed and the contribution of each mutation to virulence was evaluated. The nt(s) that contributed to significant reduction in virulence in vitro were selected and each mutation was introduced separately into the genome to construct the multiply mutated EV-A71 strain (MMS) which carried six substitutions of nt(s) at the 5'-NTR (U700C), VP1-145 (E to G), VP1-98E, VP1-244K and G64R in the vaccine seed strain that had a partial deletion within the 5'-NTR region (nt. 475-485) of Δ11bp. In comparison to the wild type strain, the MMS showed low virulence as it produced very low RNA copy number, plaque count, VP1 and had 10⁵-fold higher TCID₅₀, indicative of a promising LAV candidate that should be further evaluated in vivo.

Valine/isoleucine variants drive selective pressure in the VP1 sequence of EV-A71 enteroviruses

BACKGROUND

In 2011-2012, Northern Vietnam experienced its first large scale hand foot and mouth disease (HFMD) epidemic. In 2011, a major HFMD epidemic was also reported in South Vietnam with fatal cases. This 2011-2012 outbreak was the first one to occur in North Vietnam providing grounds to study the etiology, origin and dynamic of the disease. We report here the analysis of the VP1 gene of strains isolated throughout North Vietnam during the 2011-2012 outbreak and before.

METHODS

The VP1 gene of 106 EV-A71 isolates from North Vietnam and 2 from Central Vietnam were sequenced. Sequence alignments were analyzed at the nucleic acid and protein level. Gene polymorphism was also analyzed. A Factorial Correspondence Analysis was performed to correlate amino acid mutations with clinical parameters.

RESULTS

The sequences were distributed into four phylogenetic clusters. Three clusters corresponded to the subgenogroup C4 and the last one corresponded to the subgenogroup C5. Each cluster displayed different polymorphism characteristics. Proteins were highly conserved but three sites bearing only Isoleucine (I) or Valine (V) were characterized. The isoleucine/valine variability matched the clusters. Spatiotemporal analysis of the I/V variants showed that all variants which emerged in 2011 and then in 2012 were not the same but were all present in the region prior to the 2011-2012 outbreak. Some correlation was found between certain I/V variants and ethnicity and severity.

CONCLUSIONS

The 2011-2012 outbreak was not caused by an exogenous strain coming from South Vietnam or elsewhere but by strains already present and circulating at low level in North Vietnam. However, what triggered the outbreak remains unclear. A selective pressure is applied on I/V variants which matches the genetic clusters. I/V variants were shown on other viruses to correlate with pathogenicity. This should be investigated in EV-A71. I/V variants are an easy and efficient way to survey and identify circulating EV-A71 strains.

Impact of genetic changes, pathogenicity and antigenicity on Enterovirus- A71 vaccine development

Enterovirus-A71 (EV-A71) is an etiological agent of the hand, foot and mouth disease (HFMD). EV-A71 infection produces high fever and ulcers in children. Some EV-A71 strains produce severe infections leading to pulmonary edema and death. Although the protective efficacy of the inactivated vaccine (IV) was ≥90% against mild HFMD, there was approximately 80% protection against severe HFMD. The monovalent EV-A71 IV elicits humoral immunity but lacks long-term immunogenicity. Spontaneous mutations of the EV-A71 genome could lead to antigenicity changes and the virus may not be neutralized by antibodies elicited by the IV. A better alternative would be the live attenuated vaccine (LAV) that elicits cellular and humoral immunity. The LAV induces excellent antigenicity and chances of reversion is reduced by presence of multiple mutations which could reduce pathogenicity. Besides CV-A16, outbreaks have been caused by CV-A6 and CV-A10, hence the development of bivalent and trivalent vaccines is required.

In Vivo Imaging with Bioluminescent Enterovirus 71 Allows for Real-Time Visualization of Tissue Tropism and Viral Spread

Hand, foot, and mouth disease (HFMD) is a reemerging illness caused by a variety of enteroviruses. The main causative agents are enterovirus 71 (EV71), coxsackievirus A16 (CVA16), and, most recently, coxsackievirus A6 (CVA6). Enterovirus infections can vary from asymptomatic infections to those with a mild fever and blisters on infected individuals' hands, feet, and throats to infections with severe neurological complications. Viral persistence for weeks postinfection (wpi) has also been documented by the demonstration of virus in children's stools. However, little is known about disease progression, viral spread, and tissue tropism of these viruses. These types of studies are limited because many recently developed mouse models mimic the severe neurological complications that occur in a small percentage of enterovirus infections. In the present study, we documented real-time EV71 infection in two different mouse strains by the use of in vivo imaging. Infection of BALB/c mice with a bioluminescent mouse-adapted EV71 construct (mEV71-NLuc) resulted in a lack of clinical signs of disease but in relatively high viral replication, as visualized by luminescence, for 2 wpi. In contrast, mEV71-NLuc infection of AG129 mice (alpha/beta and gamma interferon receptor deficient) showed rapid spread and long-term persistence of the virus in the brain. Interestingly, AG129 mice that survived infection maintained luminescence in the brain for up to 8 wpi. The results we present here will allow future studies on EV71 antiviral drug susceptibility, vaccine efficacy, transmissibility, and pathogenesis. IMPORTANCE We report here that a stable full-length enterovirus 71 (EV71) reporter construct was used to visualize real-time viral spread in AG129 and BALB/c mice. To our knowledge, this is the first report of in vivo imaging of infection with any member of the Picornaviridae family. The nanoluciferase (NLuc) gene, one of the smallest luciferase genes currently available, was shown to be stable in the EV71 genome for eight passages on rhabdomyosarcoma cells. Real-time visualization of EV71 infection in mice identified areas of tropism that would have been missed by traditional methods, including full characterization of EV71 replication in BALB/c mice. Additionally, the bioluminescent construct allowed for increased speed and sensitivity of cell culture assays and will allow future studies involving various degrees of enterovirus infection in mice, not just severe infections. Our data suggest that interferon plays an important role in controlling EV71 infection in the central nervous system of mice.

Identification of a novel bovine enterovirus possessing highly divergent amino acid sequences in capsid protein

BACKGROUND

Bovine enterovirus (BEV) belongs to the species Enterovirus E or F, genus Enterovirus and family Picornaviridae. Although numerous studies have identified BEVs in the feces of cattle with diarrhea, the pathogenicity of BEVs remains unclear. Previously, we reported the detection of novel kobu-like virus in calf feces, by metagenomics analysis. In the present study, we identified a novel BEV in diarrheal feces collected for that survey. Complete genome sequences were determined by deep sequencing in feces. Secondary RNA structure analysis of the 5' untranslated region (UTR), phylogenetic tree construction and pairwise identity analysis were conducted.

RESULTS

The complete genome sequences of BEV were genetically distant from other EVs and the VP1 coding region contained novel and unique amino acid sequences. We named this strain as BEV AN12/Bos taurus/JPN/2014 (referred to as BEV-AN12). According to genome analysis, the genome length of this virus is 7414 nucleotides excluding the poly (A) tail and its genome consists of a 5'UTR, open reading frame encoding a single polyprotein, and 3'UTR. The results of secondary RNA structure analysis showed that in the 5'UTR, BEV-AN12 had an additional clover leaf structure and small stem loop structure, similarly to other BEVs. In pairwise identity analysis, BEV-AN12 showed high amino acid (aa) identities to Enterovirus F in the polyprotein, P2 and P3 regions (aa identity ≥82.4%). Therefore, BEV-AN12 is closely related to Enterovirus F. However, aa sequences in the capsid protein regions, particularly the VP1 encoding region, showed significantly low aa identity to other viruses in genus Enterovirus (VP1 aa identity ≤58.6%). In addition, BEV-AN12 branched separately from Enterovirus E and F in phylogenetic trees based on the aa sequences of P1 and VP1, although it clustered with Enterovirus F in trees based on sequences in the P2 and P3 genome region.

CONCLUSIONS

We identified novel BEV possessing highly divergent aa sequences in the VP1 coding region in Japan. According to species definition, we proposed naming this strain as "Enterovirus K", which is a novel species within genus Enterovirus. Further genomic studies are needed to understand the pathogenicity of BEVs.