A non-mouse-adapted enterovirus 71 (EV71) strain exhibits neurotropism, causing neurological manifestations in a novel mouse model of EV71 infection

Pathogenicity and landscape of differential gene expression in mice orally infected with clinical coxsackievirus A6 (CA6)

Hand, foot, and mouth disease (HFMD) is caused by more than 20 pathogenic enteroviruses belonging to the Picornaviridae family and Enterovirus genus. Since the introduction of the enterovirus-71 (EV71) vaccine in 2016, the number of HFMD cases caused by EV71 has decreased. However, cases of infections caused by other enteroviruses, such as coxsackievirus A6 (CA6) and coxsackievirus A10, have been increasing accordingly. In this study, we used a clinical isolate of CA6 to establish an intragastric infection mouse model using 7-day-old mice to mimic the natural transmission route, by which we investigated the differential gene expression profiles associated with virus infection and pathogenicity. After intragastric infection, mice exhibited hind limb paralysis symptoms and weight loss, similar to those reported for EV71 infection in mice. The skeletal muscle was identified as the main site of virus replication, with a peak viral load reaching 2.31 × 107 copies/mg at 5 dpi and increased infiltration of inflammatory cells. RNA sequencing analysis identified differentially expressed genes (DEGs) after CA6 infection. DEGs in the blood, muscle, brain, spleen, and thymus were predominantly enriched in immune system responses, including pathways such as Toll-like receptor signaling and PI3K-Akt signaling. Our study has unveiled the genes involved in the host immune response during CA6 infection, thereby enhancing our comprehension of the pathological mechanism of HFMD.IMPORTANCEThis study holds great significance for the field of hand, foot, and mouth disease (HFMD). It not only delves into the disease's etiology, transmission pathways, and severe complications but also establishes a novel mouse model that mimics the natural coxsackievirus A6 infection process, providing a pivotal platform to delve deeper into virus replication and pathogenic mechanisms. Additionally, utilizing RNA-seq technology, it unveils the dynamic gene expression changes during infection, offering valuable leads for identifying novel therapeutic drug targets. This research has the potential to enhance our understanding of HFMD, offering fresh perspectives for disease prevention and treatment and positively impacting children's health worldwide.

Inhibition of lysosome-tethered Ragulator-Rag-3D complex restricts the replication of Enterovirus 71 and Coxsackie A16

Enterovirus 71 (EV71) and Coxsackie A16 (CVA16) are two major causative agents of hand, foot, and mouth disease (HFMD) in young children. However, the mechanisms regulating the replication and pathogenesis of EV71/CVA16 remain incompletely understood. We performed a genome-wide CRISPR-Cas9 knockout screen and identified Ragulator as a mediator of EV71-induced apoptosis and pyroptosis. The Ragulator-Rag complex is required for EV71 and CVA16 replication. Upon infection, the Ragulator-Rag complex recruits viral 3D protein to the lysosomal surface through the interaction between 3D and RagB. Disruption of the lysosome-tethered Ragulator-Rag-3D complex significantly impairs the replication of EV71/CVA16. We discovered a novel EV71 inhibitor, ZHSI-1, which interacts with 3D and significantly reduces the lysosomal tethering of 3D. ZHSI-1 treatment significantly represses replication of EV71/CVA16 as well as virus-induced pyroptosis associated with viral pathogenesis. Importantly, ZHSI-1 treatment effectively protects against EV71 infection in neonatal and young mice. Thus, our study indicates that targeting lysosome-tethered Ragulator-Rag-3D may be an effective therapeutic strategy for HFMD.

Humoral and cellular immunogenicity and efficacy of a coxsackievirus A10 vaccine in mice

Coxsackievirus A10 (CV-A10) has become one of the major pathogens of hand, foot and mouth disease (HFMD), and studies on the vaccine and animal model of CV-A10 are still far from complete. Our study used a mouse-adapted CV-A10 strain, which was lethal for 14-day-old mice, to develop an infected mouse model. Then this model was employed to establish an actively immunized-challenged mouse model to evaluate the efficacy of a formaldehyde-inactivated CV-A10 vaccine, which was prepared from a Vero cell-adapted strain. CV-A10 vaccine at a dose of 0.5 or 2.0 μg was inoculated intraperitoneally in neonatal Kunming mice on the third and ninth day. Then the mice were challenged on day 14. The survival rate of mice immunized with 0.5 or 2.0 μg vaccine were 90% and 100%, respectively, while all Alum-inoculated mice died. Compared to those in the two vaccinated groups, the Alum-inoculated mice showed severe pathological damage, strong viral protein expression and high viral loads. The antisera from vaccinated mice showed high level of neutralizing antibodies against CV-A10. Meanwhile, three potential T cell epitopes located at the carboxyl-terminal regions of the VP1 and VP3 were identified and exhibited CV-A10 serotype-specific. The humoral and cellular immunogenicity analysis showed that immunization with two doses of the vaccine elicited CV-A10 specific neutralizing antibody and T cell response in BALB/c mice. Collectively, these findings indicated that this actively immunized-challenged mouse model will be invaluable in future studies on CV-A10 pathogenesis and evaluation of vaccine candidates.

Ubiquitin E3 ligase SPOP is a host negative regulator of enterovirus 71-encoded 2A protease

IMPORTANCE

EV71 poses a significant health threat to children aged 5 and below. The process of EV71 infection and replication is predominantly influenced by ubiquitination modifications. Our previous findings indicate that EV71 prompts the activation of host deubiquitinating enzymes, thereby impeding the host interferon signaling pathway as a means of evading the immune response. Nevertheless, the precise mechanisms by which the host employs ubiquitination modifications to hinder EV71 infection remain unclear. The present study demonstrated that the nonstructural protein 2Apro, which is encoded by EV71, exhibits ubiquitination and degradation mediated by the host E3 ubiquitin ligase SPOP. In addition, it is the first report, to our knowledge, that SPOP is involved in the host antiviral response.

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.

Research progress on pathogenic and therapeutic mechanisms of Enterovirus A71

In recent years, enterovirus A71 (EV-A71) infection has become a major global public health problem, especially for infants and young children. The results of epidemiological research show that EV-A71 infection can cause acute hand, foot, and mouth disease (HFMD) and complications of the nervous system in severe cases, including aseptic pediatric meningoencephalitis, acute flaccid paralysis, and even death. Many studies have demonstrated that EV-A71 infection may trigger a variety of intercellular and intracellular signaling pathways, which are interconnected to form a network that leads to the innate immune response, immune escape, inflammation, and apoptosis in the host. This article aims to provide an overview of the possible mechanisms underlying infection, signaling pathway activation, the immune response, immune evasion, apoptosis, and the inflammatory response caused by EV-A71 infection and an overview of potential therapeutic strategies against EV-A71 infection to better understand the pathogenesis of EV-A71 and to aid in the development of antiviral drugs and vaccines.

SHFL inhibits enterovirus A71 infection by triggering degradation of viral 3Dpol protein via the ubiquitin-proteasome pathway

Enterovirus A71 (EV-A71) is a highly contagious virus that poses a major threat to global health, representing the primary etiological agent for hand-foot and mouth disease (HFMD) and neurological complications. It has been established that interferon signaling is critical to establishing a robust antiviral state in host cells, mainly mediated through the antiviral effects of numerous interferon-stimulated genes (ISGs). The host restriction factor SHFL is a novel ISG with broad antiviral activity against various viruses through diverse underlying molecular mechanisms. Although SHFL is widely acknowledged for its broad-spectrum antiviral activity, it remains elusive whether SHFL inhibits EV-A71. In this work, we validated that EV-A71 triggers the upregulation of SHFL both in cell lines and in a mouse model. Knockdown and overexpression of SHFL in EVA71-infected cells suggested that this factor could markedly suppress EV-A71 replication. Our findings further revealed an intriguing mechanism of SHFL that it could interact with the nonstructural proteins 3Dpol of EV-A71 and promoted the degradation of 3Dpol through the ubiquitin-proteasome pathway. Furthermore, the zinc-finger domain and the 36 amino acids (164-199) of SHFL were crucial to the interaction between SHFL and EV-A71 3Dpol . Overall, these findings broadened our understanding of the pivotal roles of SHFL in the interaction between the host and EV-A71.

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.

Ubiquitin-specific protease 24 promotes EV71 infection by restricting K63-linked polyubiquitination of TBK1

TANK-binding kinase 1 (TBK1) is an essential protein kinase for activation of interferon regulatory factor 3 (IRF3) and induction of the type I interferons (IFN-I). Although the biochemical regulation of TBK1 activation has been studied, little is known about how enterovirus 71 (EV71) employs the deubiquitinases (DUBs) to regulate TBK1 activation for viral immune evasion. Here, we found that EV71 infection upregulated the expression of ubiquitin-specific protease 24 (USP24). Further studies revealed that USP24 physically interacted with TBK1, and can reduce K63-linked polyubiquitination of TBK1. Knockdown of USP24 upregulated TBK1 K63-linked polyubiquitination, promoted the phosphorylation and nuclear translocation of IRF3, and in turn improved IFN-I production during EV71 infection. As a consequence, USP24 knockdown dramatically inhibited EV71 infection. This study revealed USP24 as a novel regulator of TBK1 activation, which promotes the understanding of immune evasion mechanisms of EV71 and could provide a potential strategy for treatment of EV71 infection.

Murine models of neonatal susceptibility to a clinical strain of enterovirus A71

Enterovirus A71 (EV-A71) is neurotropic and one of the primary enteric pathogens responsible for severe central nervous system infection in infants and young children. Neonatal mice are ideal models for studying the pathogenesis of infection caused by EV-A71. In this study, we assessed the susceptibility of neonatal BALB/c, C57BL/6, ICR, Kunming, and NIH mice to a clinically isolated EV-A71 strain. One-day-old mice were challenged with a clinical isolate of EV-A71 via intraperitoneal injection, then observed for 13 days for mortality, body-weight changes, and limb paralysis. RT-qPCR was performed to quantify viral RNA in the brain, spinal cord, skeletal muscle, and lungs of BALB/c and C57BL/6 mice. The expression of murine scavenger receptor class B member 2 (mSCARB2) was measured by western blotting. Finally, lesions were assessed by histological examination. We found that neonatal BALB/c and C57BL/6 mice were both susceptible to EV-A71, leading to decreased survival rate, greater body weight loss, and prominent hind-limb paralysis. Tissue viral loads of C57BL/6J mice were markedly higher than those of BALB/c mice, indicating that EV-A71 replicated more efficiently in C57BL/6 mice. Increased expression of mSCARB2 was observed 5 days after infection in C57BL/6 mice, which coincided with the peak in EV-A71 replication. Histological examination indicated that infection caused obvious pathogenic lesions. In conclusion, C57BL/6 are most susceptible to infection caused by EV-A71 and can be used as a model for studying its pathogenesis and test therapeutic options.

Recent advances in enterovirus A71 pathogenesis: a focus on fatal human enterovirus A71 infection

Enterovirus A71 (EV-A71) is one of the major pathogens responsible for hand, foot, and mouth disease (HFMD). Many HFMD outbreaks have been reported throughout the world in the past decades. Compared with other viruses, EV-A71 infection is more frequently associated with severe neurological complications and even death in children. EV-A71 can also infect adults and cause severe complications and death, although such cases are very uncommon. Although fatal cases of EV-A71 infection have been reported, the underlying mechanisms of EV-A71 infection, especially the mode of viral spread into the central nervous system (CNS) and mechanisms of pulmonary edema, which is considered to be the direct cause of death, have not yet been fully clarified, and more studies are needed. Here, we first summarize the pathological findings in various systems of patients with fatal EV-A71 infections, focussing in detail on gross changes, histopathological examination, tissue distribution of viral antigens and nucleic acids, systemic inflammatory cell infiltration, and tissue distribution of viral receptors and their co-localization with viral antigens. We then present our conclusions about viral dissemination, neuropathogenesis, and the mechanism of pulmonary edema in EV-A71 infection, based on pathological findings.

A Mouse-adapted CVA6 Strain Exhibits Neurotropism and Triggers Systemic Manifestations in a Novel Murine Model

CVA6 is one of Enteroviruses causing worldwide epidemics of HFMD with neurological and systemic complications. A suitable animal model is necessary for studying the pathogenesis of CVA6 and evaluating antiviral and vaccine efficacy. In this study, we generated a mouse-adapted CVA6 strain that successfully infected 10-day-old ICR mice via oral route. All infected mice were paralyzed and died within 11 dpi. Analysis of pathological changes and virus loads in fourteen tissues showed that CVA6 triggered systematic damage similar to i.p. inoculation route. Unlike i.p. route, we detected oral and gastrointestinal lesions with the presence of viral antigens. Both specific anti-CVA6 serum and inactivated vaccines successfully generated immune protection in mice. Meanwhile, we also established a successful infection of CVA6 via i.p. and i.m. route in 10-day-old mice. After infection, mice developed remarkably neurological signs and systemic manifestations such as emaciation, polypnea, quadriplegia, depilation and even death. Through i.p. inoculation, pathological examination showed brain and spinal cord damage caused by the virus infection with neuronal reduction, apoptosis, astrocyte activation, and recruitment of neutrophils and monocytes. Following neurological manifestation, the CVA6 infection became systemic, and high viral loads were detected in multiple organs along with morphological changes and inflammation. Moreover, analysis of spleen cells by FACS indicated that CVA6 led to immune system activation, which further contributed to systemic inflammation. Taken together, our novel murine model of CVA6 provides a useful tool for studying the pathogenesis and evaluating antiviral and vaccine efficacy.

Mouse Scarb2 Modulates EV-A71 Pathogenicity in Neonatal Mice

Enterovirus A71 (EV-A71) is a human pathogen that causes hand, foot, and mouth disease, which can progress to severe neurological disease. EV-A71 infects humans via the human scavenger receptor B2 (hSCARB2). It can also infect neonatal mice experimentally. Wild-type (WT) EV-A71 strains replicate primarily in the muscle of neonatal mice; however, susceptibility lasts only for a week after birth. Mouse-adapted (MA) strains, which can be obtained by serial passages in neonatal mice, are capable of infecting both muscle and neurons of the central nervous system. It is not clear how the host range and tropism of EV-A71 are regulated and why neonatal mice lose their susceptibility during development. We hypothesized that EV-A71 infection in neonatal mice is mediated by mouse Scarb2 (mScarb2) protein. Rhabdomyosarcoma (RD) cells expressing mScarb2 were prepared. Both WT and MA strains infected mScarb2-expressing cells, but the infection efficiency of the WT strain was much lower than that of the MA strain. Infection by WT and MA strains in vivo was abolished completely in Scarb2^-/- mice. Scarb2^+/- mice, in which Scarb2 expression was approximately half of that in Scarb2^+/+ mice, showed a milder pathology than Scarb2^+/+ mice after infection with the WT strain. The Scarb2 expression level in muscle decreased with aging, which was consistent with the reduced susceptibility of aged mice to infection. These results indicated that EV-A71 infection is mediated by mScarb2 and that the severity of the disease, the spread of virus, and the susceptibility period are modulated by mScarb2 expression. IMPORTANCE EV-A71 infects humans naturally but can also infect neonatal mice. The tissue tropism and severity of EV-A71 disease are determined by several factors, among which the virus receptor is thought to be important. We show that EV-A71 can infect neonatal mice using mScarb2. However, the infection efficiency of WT strains via mScarb2 is so low that an elevated virus-receptor interaction associated with mouse adaptation mutation and decrease in mScarb2 expression level during development modulate the severity of the disease, the spread of virus, and the susceptibility period in the artificial neonatal mice model.

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.

Enterovirus Replication and Dissemination Are Differentially Controlled by Type I and III Interferons in the Gastrointestinal Tract

Echovirus infections are associated with a broad spectrum of illness, particularly in neonates, and are primarily transmitted through the fecal-oral route. Little is known regarding how echoviruses infect the gastrointestinal tract and how the intestinal epithelium controls echoviral replication.

Prototypes virus of hand, foot and mouth disease infections and severe cases in Gansu, China: a spatial and temporal analysis

Background

Little research has been conducted on the spatio-temporal relationship between the severe cases and the enteroviruses infections of hand, foot and mouth disease (HFMD). This study aimed to investigate epidemic features and spatial clusters of HFMD incidence rates and assess the relationship between Enterovirus 71 (EV71) and Coxsackievirus A16 (CoxA16) and severe cases of HMFD in Gansu province, China.

Methods

Weekly county-specific data on HFMD between 1st January and 31st December 2018 were collected from the China Infectious Disease Information System (CIDIS), including enterovirus type (EV71 and CoxA16), severe and non-severe cases in Gansu province, China. Temporal risk [frequency index (α), duration index (β) and intensity index (γ)] and spatial cluster analysis were used to assess epidemic features and identify high-risk areas for HFMD. Time-series cross-correlation function and regression model were used to explore the relationship between the ratios of two types of viruses (i.e. EV71/Cox16) (EC) and severe cases index (i.e. severe cases/non-severe cases) (SI) of HFMD.

Results

Some counties in Dingxi City, Gansu were identified as a hot spot for the temporal risk indices. Time-series cross-correlation analysis showed that SI was significantly associated with EC (r = 0.417, P < 0.05) over a 4-week time lag. The regression analysis showed that SI was positively associated with EC (β = 0.04, 95% confidence interval (CI) 0.02–0.06).

Conclusion

The spatial patterns of HFMD incidence were associated with enteroviruses in Gansu. The research suggested that the EC could be considered a potential early warning sign for predicting severe cases of HFMD in Gansu province.

Pharmacological perturbation of CXCL1 signaling alleviates neuropathogenesis in a model of HEVA71 infection

Hand, foot and mouth disease (HFMD) caused by Human Enterovirus A71 (HEVA71) infection is typically a benign infection. However, in minority of cases, children can develop severe neuropathology that culminate in fatality. Approximately 36.9% of HEVA71-related hospitalizations develop neurological complications, of which 10.5% are fatal. Yet, the mechanism by which HEVA71 induces these neurological deficits remain unclear. Here, we show that HEVA71-infected astrocytes release CXCL1 which supports viral replication in neurons by activating the CXCR2 receptor-associated ERK1/2 signaling pathway. Elevated CXCL1 levels correlates with disease severity in a HEVA71-infected mice model. In humans infected with HEVA71, high CXCL1 levels are only present in patients presenting neurological complications. CXCL1 release is specifically triggered by VP4 synthesis in HEVA71-infected astrocytes, which then acts via its receptor CXCR2 to enhance viral replication in neurons. Perturbing CXCL1 signaling or VP4 myristylation strongly attenuates viral replication. Treatment with AZD5069, a CXCL1-specific competitor, improves survival and lessens disease severity in infected animals. Collectively, these results highlight the CXCL1-CXCR2 signaling pathway as a potential target against HFMD neuropathogenesis.

Characterization of Pathogenesis and Inflammatory Responses to Experimental Parechovirus Encephalitis

Human parechovirus type 3 (PeV-A3) infection has been recognized as an emerging etiologic factor causing severe nerve disease or sepsis in infants and young children. But the neuropathogenic mechanisms of PeV-A3 remain unknown. To understand the pathogenesis of PeV-A3 infection in the neuronal system, PeV-A3-mediated cytopathic effects were analyzed in human glioblastoma cells and neuroblastoma cells. PeV-A3 induced interferons and inflammatory cytokine expression in these neuronal cells. The pronounced cytopathic effects accompanied with activation of death signaling pathways of apoptosis, autophagy, and pyroptosis were detected. A new experimental disease model of parechovirus encephalitis was established. In the disease model, intracranial inoculation with PeV-A3 in C57BL/6 neonatal mice showed body weight loss, hindlimb paralysis, and approximately 20% mortality. PeV-A3 infection in the hippocampus and cortex regions of the neonatal mouse brain was revealed. Mechanistic assay supported the in vitro results, indicating detection of PeV-A3 replication, inflammatory cytokine expression, and death signaling transduction in mouse brain tissues. These in vitro and in vivo studies revealed that the activation of death signaling and inflammation responses is involved in PeV-A3-mediated neurological disorders. The present results might account for some of the PeV-A3-associated clinical manifestations.

A practical method for evaluating the in vivo efficacy of EVA-71 vaccine using a hSCARB2 knock-in mouse model

Hand-foot-and-mouth disease is a contagious disease common among children under 5 years old worldwide. It is caused by strains of enterovirus, especially EV-A71, which can lead to severe disease. Vaccines are the only way to fight this disease. Accordingly, it is necessary to establish an efficient and accurate methodology to evaluate vaccine efficacy in vivo. Here, we established a practical method using a hSCARB2 knock-in mouse model, which was susceptible to EV-A71 infection at 5-6 weeks of age, to directly determine the efficacy of vaccines. Unlike traditional approaches, one-week-old hSCARB2 mice were immunized twice with a licensed vaccine, with an interval of a week. The titre of antibodies was measured after 1 week. Mice at 4 weeks of age were challenged with EV-A71 intraperitoneally and intracranially, respectively. The unimmunized hSCARB2 mice displayed systemic clinical symptoms and succumbed to the disease at a rate of approximately 50%. High viral loads were detected in the lungs, brain, and muscles, accompanied by clear pathological changes. The expression of IL-1β, IL-13, IL-17, and TNF-α was significantly upregulated. By contrast, the immunized group was practically normal and indistinguishable from the control mice. These results indicate that the hSCARB2 knock-in mouse is susceptible to infection in adulthood, and the in vivo efficacy of EV-A71 vaccine could be directly evaluated in this mouse model. The method developed here may be used in the development of new vaccines against HFMD or quality control of licensed vaccines.

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.

Muscle Tissue Damage and Recovery After EV71 Infection Correspond to Dynamic Macrophage Phenotypes

Enterovirus 71 (EV71) is a positive single-stranded RNA virus from the enterovirus genus of the Picornaviridae family. Most young children infected with EV71 develop mild symptoms of hand, foot and mouth disease, but some develop severe symptoms with neurological involvement. Limb paralysis from EV71 infection is presumed to arise mainly from dysfunction of motor neurons in the spinal cord. However, EV71 also targets and damages skeletal muscle, which may also contribute to the debilitating symptoms. In this study, we have delineated the impacts of EV71 infection on skeletal muscle using a mouse model. Mouse pups infected with EV71 developed limb paralysis, starting at day 3 post-infection and peaking at day 5-7 post-infection. At later times, mice recovered gradually but not completely. Notably, severe disease was associated with high levels of myositis accompanied by muscle calcification and persistent motor end plate abnormalities. Interestingly, macrophages exhibited a dynamic change in phenotype, with inflammatory macrophages (CD45⁺CD11b⁺Ly6C^hi) appearing in the early stage of infection and anti-inflammatory/restorative macrophages (CD45⁺CD11b⁺Ly6C^low/-) appearing in the late stage. The presence of inflammatory macrophages was associated with severe inflammation, while the restorative macrophages were associated with recovery. Altogether, we have demonstrated that EV71 infection causes myositis, muscle calcification and structural defects in motor end plates. Subsequent muscle regeneration is associated with a dynamic change in macrophage phenotype.

A hSCARB2-transgenic mouse model for Coxsackievirus A16 pathogenesis

Background

Coxsackievirus A16 (CA16) is one of the neurotropic pathogen that has been associated with severe neurological forms of hand, foot, and mouth disease (HFMD), but its pathogenesis is not yet clear. The limited host range of CA16 make the establishment of a suitable animal model that can recapitulate the neurological pathology observed in human HFMD more difficult. Because the human scavenger receptor class B, member 2 (hSCARB2) is a cellular receptor for CA16, we used transgenic mice bearing human SCARB2 and nasally infected them with CA16 to study the pathogenicity of the virus.

Methods

Coxsackievirus A16 was administered by intranasal instillation to groups of hSCARB2 transgenic mice and clinical signs were observed. Sampled at different time-points to document and characterize the mode of viral dissemination, pathological change and immune response of CA16 infection.

Results

Weight loss and virus replication in lung and brain were observed in hSCARB2 mice infected with CA16, indicating that these animals could model the neural infection process. Viral antigens were observed in the alveolar epithelia and brainstem cells. The typical histopathology was interstitial pneumonia with infiltration of significant lymphocytes into the alveolar interstitial in lung and diffuse punctate hemorrhages in the capillaries of the brainstem. In addition, we detected the expression levels of inflammatory cytokines and detected high levels of interleukin IL-1β, IL-6, IL-18, and IFN-γ in nasal mucosa, lungs and brain tissues.

Conclusions

The hSCARB2-transgenic mice can be productively infected with CA16 via respiratory route and exhibited a clear tropism to lung and brain tissues, which can serve as a model to investigate the pathogenesis of CA16 associated respiratory and neurological disease.

Enterovirus-A71 exploits peripherin and Rac1 to invade the central nervous system

Enterovirus-A71 (EV-A71) has been associated with severe neurological forms of hand, foot, and mouth disease (HFMD). EV-A71 infects motor neurons at neuromuscular junctions (NMJs) to invade the central nervous system (CNS). Here, we investigate the role of peripherin (PRPH) during EV-A71 infection, a type III intermediate neurofilament involved in neurodegenerative conditions. In mice infected with EV-A71, PRPH co-localizes with viral particles in the muscles at NMJs and in the spinal cord. In motor neuron-like and neuroblastoma cell lines, surface-expressed PRPH facilitates viral entry, while intracellular PRPH influences viral genome replication through interactions with structural and non-structural viral components. Importantly, PRPH does not play a role during infection with coxsackievirus A16, another causative agent of HFMD rarely associated with neurological complications, suggesting that EV-A71 ability to exploit PRPH represents a unique attribute for successful CNS invasion. Finally, we show that EV-A71 also exploits some of the many PRPH-interacting partners. Of these, small GTP-binding protein Rac1 represents a potential druggable host target to limit neuroinvasion of EV-A71.

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.

Pathogenesis Study of Enterovirus 71 Using a Novel Human SCARB2 Knock-In Mouse Model

Enterovirus 71 (EV71) can cause a severe hand-foot-mouth disease in children. However, the precise mechanism of EV71-associated disease, particularly the neuropathogenesis and pulmonary disorder, is still not fully understood because no suitable animal models are available. The human scavenger receptor class B, member 2 (hSCARB2), is a cellular receptor for EV71. Here, we generated a novel knock-in (KI) mouse model using the CRISPR/Cas9 system to insert the hSCARB2 gene into the mouse Rosa26 locus to study the pathogenesis of EV71. The hSCARB2 KI mice infected with clinical isolates of EV71 showed neurological symptoms, such as ataxia, paralysis, and death. Viral replication was detected in mainly astrocytes and a limited number of neurons and microglia, accompanied by gliosis. Vascular leakage and alveoli filled with erythrocytes were detected, suggesting that edema and hemorrhage, which are observed in human patients, also occurred in EV71-infected KI mice. In addition, proinflammatory cytokines and chemokines were significantly increased in the serum of infected KI mice. These pathological features of the KI mice after infection resembled those of EV71 encephalomyelitis in humans. Therefore, our KI mouse model is suitable to study the pathogenesis of EV71 and is of great significance for development of antiviral drugs and vaccines to treat or prevent EV71 infection.IMPORTANCE Enterovirus 71 (EV71) is associated with severe hand-foot-mouth disease. Recently, outbreaks of EV71 infection with high mortality have been reported in the Asia-Pacific region, posing a great challenge for global public health. To date, the precise mechanism of EV71-induced disease, particularly the neuropathogenesis and respiratory disorders, is still not fully understood because no suitable animal models are available. Human scavenger receptor class B, member 2 (hSCARB2), has been identified as a cellular receptor for EV71. Here, we introduce a novel CRISPR/Cas9-mediated hSCARB2 knock-in (KI) mouse model for the study of EV71 pathogenesis, which is of great significance for the development of antiviral drugs and vaccines.

Efficacy of Coxsackievirus A5 Vaccine Candidates in an Actively Immunized Mouse Model

Coxsackievirus A5 (CV-A5) has recently emerged as a main hand, foot, and mouth disease (HFMD) pathogen. Following a large-scale vaccination campaign against enterovirus 71 (EV-71) in China, the number of HFMD-associated cases with EV-71 was reduced, especially severe and fatal cases. However, the total number of HFMD cases remains high, as HFMD is also caused by other enterovirus serotypes. A multivalent HFMD vaccine containing 4 or 6 antigens of enterovirus serotypes is urgently needed. A formaldehyde-inactivated CV-A5 vaccine derived from Vero cells was used to inoculate newborn Kunming mice on days 3 and 10. The mice were challenged on day 14 with a mouse-adapted CV-A5 strain at a dose that was lethal for 14-day-old suckling mice. Within 14 days postchallenge, groups of mice immunized with three formulations, empty particles (EPs), full particles (FPs), and a mixture of the EP and FP vaccine candidates, all survived, while 100% of the mock-immunized mice died. Neutralizing antibodies (NtAbs) were detected in the sera of immunized mice, and the NtAb levels were 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 or not observed in the immunized mice compared with those in alum-inoculated control mice. Another interesting finding was the identification of CV-A5 dense particles (DPs), facilitating morphogenesis study. These results demonstrated that the Vero cell-adapted CV-A5 strain is a promising vaccine candidate and could be used as a multivalent HFMD vaccine component in the future.IMPORTANCE The vaccine candidate strain CV-A5 was produced with a high infectivity titer and a high viral particle yield. Three particle forms, empty particles (EPs), full particles (FPs), and dense particles (DPs), were obtained and characterized after purification. The immunogenicities of EP, FP, and the EP and FP mixture were evaluated in mice. Mouse-adapted CV-A5 was generated as a challenge strain to infect 14-day-old mice. An active immunization challenge mouse model was established to evaluate the efficacy of the inactivated vaccine candidate. This animal model mimics vaccination, similar to immune responses of the vaccinated. The animal model also tests protective efficacy in response to the vaccine against the disease. This work is important for the preparation of multivalent vaccines against HFMD caused by different emerging strains.

TLR7 Is Critical for Anti-Viral Humoral Immunity to EV71 Infection in the Spinal Cord

Enterovirus 71 (EV71) is a positive single-stranded RNA (ssRNA) virus from the enterovirus genus of Picornaviridae family and causes diseases ranged from the mild disease of hand, foot and mouth disease (HFMD) to the severe disease of neurological involvement in young children. TLR7 is an intracellular pattern recognition receptor (PRR) recognizing viral ssRNA. In this study, we investigated the role of TLR7 in EV71 infection in mouse pups (10-12 days old) and found that wild-type (WT) and TLR7 knock-out (TLR7KO) mice infected with EV71 showed similar limb paralysis at the onset and peak of the disease, comparable loss of motor neurons, and similar levels of antiviral molecules in the spinal cord. These results suggest that TLR7 is not the absolute PRR for EV71 in the spinal cord. Interestingly, TLR7KO mice infected with EV71 exhibited significantly delayed recovery from limb paralysis compared with WT mice. TLR7KO mice infected with EV71 showed significantly decreased levels of IgM and IgG2, important antibodies for antiviral humoral immunity. Furthermore, TLR7KO mice infected with EV71 showed a decrease of germinal center B cells in the spleen compared with WT mice. Altogether, our study suggests that TLR7 plays a critical role in anti-viral humoral immunity rather than in being a PRR in the spinal cord during EV71 infection in young mice.

Enterovirus 71 induces autophagy in mice via mTOR inhibition and ERK pathway activation

AIMS

Enterovirus 71 (EV71) is one of the main viruses that cause hand-foot-mouth disease; however, its pathogenic mechanism remains unclear. This study characterized the relationship between EV71 infection and autophagy in vivo and explored the molecular mechanism underlying EV71-induced autophagy.

MATERIALS AND METHODS

A mouse model of EV71 infection was prepared by intraperitoneally injecting one-day-old BALB/c suckling mice with 30 μL/g of EV71 virus stock solution for 3 days. The behavior, fur condition, weight, and mice mortality were monitored, and disease scores were calculated. The pathological damage to the brain, lung, and muscle tissues after the viral infection was assessed by hematoxylin and eosin staining. Western blot and immunofluorescence analyses were used to detect the expression levels of viral protein 1, Beclin-1, microtubule-associated protein light chain 3B, mammalian target of rapamycin (mTOR), phosphorylated (p)-mTOR, extracellular signal-regulated protein kinase (ERK) 1/2, and p-ERK.

KEY FINDINGS

EV71 infection can trigger autophagy in the brains, lungs, and muscles of infected mice. The autophagy response triggered by EV71 is achieved by the simultaneous mTOR inhibition and the ERK pathway activation. Blocking the mTOR pathway may aggravate autophagy, whereas ERK inhibition alleviates autophagy but cannot completely prevent it.

SIGNIFICANCE

EV71 infection can induce autophagy in mice, involving mTOR and ERK signaling pathways. These two signaling pathways are independent and do not interfere with each other.

Cathelicidin antimicrobial peptides suppress EV71 infection via regulating antiviral response and inhibiting viral binding

Cathelicidin antimicrobial peptides (human LL-37 and mouse CRAMP) are mainly virucidal to enveloped virus. However, the effects and relative mechanisms of LL-37 and CRAMP on non-enveloped virus are elusive. We herein found that CRAMP expression was significantly up-regulated post non-enveloped Enterovirus 71 (EV71) infection in different tissues of newborn ICR mice, while EV71 replication gradually declined post CRAMP up-regulation, indicating the antiviral potential of cathelicidin against EV71. In vitro antiviral assay showed that LL-37 and CRAMP markedly reduced cytopathic effects (CPE), intracellular viral RNA copy numbers, viral VP1 protein levels, and extracellular virons in U251 cells post EV71 infection, indicating that LL-37 and CRAMP significantly inhibited EV71 replication. Mechanism of action assay showed that LL-37 and CRAMP were not virucidal to EV71, but markedly regulated antiviral immune response in U251 cells. Co-incubation of LL-37 or CRAMP with U251 cells markedly increased the basal interferon-β (IFN-β) expression and interferon regulatory transcription factor 3 (IRF3) phosphorylation, modestly enhanced IFN-β production and IRF3 phosphorylation upon EV71 infection, and significantly reduced interleukin-6 (IL-6) production and p38 mitogen-activated protein kinase (MAPK) activation post EV71 infection. Additionally, LL-37 and CRAMP directly inhibited viral binding to U251 cells. Collectively, LL-37 and CRAMP markedly inhibited EV71 replication via regulating antiviral response and inhibiting viral binding, providing potent candidates for peptide drug development against EV71 infection.

Transplantation of Enterovirus 71 Virion Protein Particle Vaccine Protects Against Enterovirus 71 Infection in a Neonatal Mouse Model

Background

Enterovirus 71 (EV71) is the pathogen most likely to cause HFMD in young children (1–5 years old). A small number of virion protein (VP) vaccine candidates are considered as the protective molecules in EV71 models. This study aimed to observe comprehensive immunogenicity for a promising EV71 vaccine depending on VP1 in neonatal mouse EV71 models.

Material/Methods

VP1 was isolated from patients and associated peptides were synthesized. EV71 particles were inactivated and mixed with Freund’s complete adjuvant to prepare peptide vaccines. An EV71 vaccine was administered to establish the mouse model and the mice were infected with EV71. Hematoxylin and eosin staining was used to examine inflammatory response in EV71-infected neonatal mice. A semi-quantitative reverse transcription-polymerase chain reaction assay was performed to evaluate the levels of EV71 virus in skeletal muscle, small intestines, and brain tissues.

Results

Three peptides were selected from 20 VP1 peptides due to their exhibition of the highest immunogenicity. The peptide injection improved inflammation and decreased EV71 particle levels in muscle, small intestines, and brain tissues. The injection also decreased lesions in the small intestines of EV71-infected mice and protected brain tissues from the EV71 infection.

Conclusions

The present study confirmed the immuno-protective effects of VP1 vaccine transplantation in mice infected with EV71 virus. Our results provide valuable information that can be used in further studies investigating the specific mechanism of the anti-EV71 vaccine.

Sodium Copper Chlorophyllin Is Highly Effective against Enterovirus (EV) A71 Infection by Blocking Its Entry into the Host Cell

Human enteroviruses (HEVs) pose an ongoing threat to global public health. Particularly, enterovirus-A71 (EV-A71), the main pathogen causing hand-foot-and-mouth disease (HFMD), has caused ongoing outbreaks globally in recent years associated with severe neurological manifestations and several deaths. Currently, no effective antivirals are available for the prevention or treatment of EV-A71 infection. In this study, we found that sodium copper chlorophyllin (CHL), a health food additive and an over-the-counter anticancer medicine or treatment to reduce the odor of urine or feces, exhibited potent inhibitory activity against infection by divergent EV-A71 and coxsackievirus-A16 (CV-A16) isolates at a low micromolar concentration with excellent safety. The antiviral activity of each was confirmed by colorimetric viral infection and qRT-PCR assays. A series of mechanistic studies showed that CHL did not target the host cell but blocked the entry of EV-A71 and CV-A16 into the host cell at the postattachment stage. In the mouse model, CHL could significantly reduce the viral titer in the lungs and muscles. Since CHL has been used in clinics for many years with excellent safety, it has the potential to be further developed into a prophylactic or therapeutic to prevent or treat HFMD caused by EV-A71 or CV-A16 infection.

Molecular Pathogenicity of Enteroviruses Causing Neurological Disease

Enteroviruses are single-stranded positive-sense RNA viruses that primarily cause self-limiting gastrointestinal or respiratory illness. In some cases, these viruses can invade the central nervous system, causing life-threatening neurological diseases including encephalitis, meningitis and acute flaccid paralysis (AFP). As we near the global eradication of poliovirus, formerly the major cause of AFP, the number of AFP cases have not diminished implying a non-poliovirus etiology. As the number of enteroviruses linked with neurological disease is expanding, of which many had previously little clinical significance, these viruses are becoming increasingly important to public health. Our current understanding of these non-polio enteroviruses is limited, especially with regards to their neurovirulence. Elucidating the molecular pathogenesis of these viruses is paramount for the development of effective therapeutic strategies. This review summarizes the clinical diseases associated with neurotropic enteroviruses and discusses recent advances in the understanding of viral invasion of the central nervous system, cell tropism and molecular pathogenesis as it correlates with host responses.

Development of an Enterovirus 71 Vaccine Efficacy Test Using Human Scavenger Receptor B2 Transgenic Mice

Enterovirus 71 (EV71) is a causative agent of hand-foot-mouth disease, and it sometimes causes severe neurological disease. Development of effective vaccines and animal models to evaluate vaccine candidates are needed. However, the animal models currently used for vaccine efficacy testing, monkeys and neonatal mice, have economic, ethical, and practical drawbacks. In addition, EV71 strains prepared for lethal challenge often develop decreased virulence during propagation in cell culture. To overcome these problems, we used a mouse model expressing human scavenger receptor B2 (hSCARB2) that showed lifelong susceptibility to EV71. We selected virulent EV71 strains belonging to the subgenogroups B4, B5, C1, C2, and C4 and propagated them using a culture method for EV71 without an apparent reduction in virulence. Here, we describe a novel EV71 vaccine efficacy test based on these hSCARB2 transgenic (Tg) mice and these virulent viruses. Adult Tg mice were immunized subcutaneously with formalin-inactivated EV71. The vaccine elicited sufficient levels of neutralizing antibodies in the immunized mice. The mice were subjected to lethal challenge with virulent viruses via intravenous injection. Survival, clinical signs, and body weight changes were observed for 2 weeks. Most immunized mice survived without clinical signs or histopathological lesions. The viral replication in immunized mice was much lower than that in nonimmunized mice. Mice immunized with the EV71 vaccine were only partially protected against lethal challenge with coxsackievirus A16. These results indicate that this new model is useful for in vivo EV71 vaccine efficacy testing.IMPORTANCE The development of new vaccines for EV71 relies on the availability of small animal models suitable for in vivo efficacy testing. Monkeys and neonatal mice have been used, but the use of these animals has several drawbacks, including high costs, limited susceptibility, and poor experimental reproducibility. In addition, the related ethical issues are considerable. The new efficacy test based on hSCARB2 Tg mice and virulent EV71 strains propagated in genetically modified cell lines presented here can overcome these disadvantages and is expected to accelerate the development of new EV71 vaccines.

An infectious clone of enterovirus 71(EV71) that is capable of infecting neonatal immune competent mice without adaptive mutations

Enterovirus 71 (EV71) is a major pathogen that causes hand, foot and mouth disease (HFMD), which is a life threatening disease in certain children. The pathogenesis of EV71-caused HFMD is poorly defined due to the lack of simple and robust animal models with severe phenotypes that recapitulate symptoms observed in humans. Here, we generated the infectious clone of a clinical isolate from a severe HFMD patient. Virus rescued from the cDNA clone was infectious in cell lines. When administrated intraperitoneally to neonatal ICR, BALB/c and C57 immune competent mice at a dosage of1.4 × 10⁴ pfu per mouse, the virus caused weight loss, paralysis and death in the infected mice after 4–5 days of infection. In the infected mice, detectable viral replication was detected in various tissues such as heart, liver, brain, lung, kidney, small intestine, leg skeletal muscle and medulla oblongata. The histology of the infected mice included massive myolysis, glomerular atrophy, villous blunting in small intestine, widened alveolar septum, diminished alveolar spaces and lymphocytes infiltration into the lung. By using the UV-inactivated virus as a control, we elucidated that the virus first amplified in the leg skeletal muscle tissue and the muscle tissue served as a primary viral replication site. In summary, we generated a stable EV71 infectious clone that is capable of infecting neonatal immune competent mice without adaptive mutations and provide a simple, valuable animal model for the studies of EV71pathogenesis and therapy.

Enterovirus and Encephalitis

Enterovirus-induced infection of the central nervous system (CNS) results in acute inflammation of the brain (encephalitis) and constitutes a significant global burden to human health. These viruses are thought to be highly cytolytic, therefore normal brain function could be greatly compromised following enteroviral infection of the CNS. A further layer of complexity is added by evidence showing that some enteroviruses may establish a persistent infection within the CNS and eventually lead to pathogenesis of certain neurodegenerative disorders. Interestingly, enterovirus encephalitis is particularly common among young children, suggesting a potential causal link between the development of the neuroimmune system and enteroviral neuroinvasion. Although the CNS involvement in enterovirus infections is a relatively rare complication, it represents a serious underlying cause of mortality. Here we review a selection of enteroviruses that infect the CNS and discuss recent advances in the characterization of these enteroviruses with regard to their routes of CNS infection, tropism, virulence, and immune responses.

A neonatal murine model of coxsackievirus A4 infection for evaluation of vaccines and antiviral drugs

Coxsackievirus A4 (CVA4) infection can cause hand, foot and mouth disease (HFMD), an epidemic illness affecting neonatal and paediatric cohorts, which can develop to severe neurological disease with high mortality. In this study, we established the first ICR mouse model of CVA4 infection for the evaluation of inactivated vaccines and antiviral drug screening. The CVA4 YT226R strain was selected to infect the neonatal mice and three infectious factors were optimized to establish the infection model. The 3-day-old neonatal mice exhibited clinical symptoms such as hind limb paralysis and death. The severe inflammatory reactions were closely related to the abnormal expression of the acute phase response proinflammatory cytokine IL-6 and an imbalance in the IFN-γ/IL-4 ratio. Importantly, the inactivated CVA4 whole-virus vaccine induced humoral immune responses in adult females and the maternal antibodies afforded mice complete protection against lethal dose challenges of homologous or heterologous CVA4 strains. Both IFN-α2a and antiserum inhibited the replication of CVA4 and increased the survival rates of neonatal mice during the early stages of infection. This neonatal murine model of CVA4 infection will be useful for the development of prophylactic and therapeutic vaccines and for screening of antiviral drugs targeting CVA4 to decrease morbidity and mortality.

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.

Pathogenesis of hand-foot-mouth disease caused by enterovirus 71

Hand-foot-mouth disease (HFMD) is a global infectious disease. The infected population is mainly infants and young children. Enterovirus 71 (EV71) is the main pathogen. In addition to HFMD, EV71 infection can also affect the nervous system and other organs, resulting in aseptic meningitis, brainstem encephalitis, and poliomyelitis-like paralysis, causing serious harm to children's health. At present, the pathogenesis of HFMD caused by EV71 is still unclear, and there is no effective treatment. In this paper, we discuss the factors influencing EV71 infection from the aspects of virus gene recombination and spontaneous mutation, host genes, and receptor sites, review the pathogenesis of HFMD caused by EV71 based on the study findings from animal infection models, and explore the main problems in the study of pathogenesis of this condition, in order to provide reference for the prevention and treatment of HFMD and for the development of new drugs or effective vaccines for EV71 infection.

Interplays between Enterovirus A71 and the innate immune system

Enterovirus A71 (EV-A71) is a growing threat to public health, particularly in the Asia-Pacific region. EV-A71 infection is most prevalent in infants and children and causes a wide spectrum of clinical complications, including hand-foot-and-mouth disease (HFMD), pulmonary and neurological disorders. The pathogenesis of EV-A71 infection is poorly understood at present. It is likely that viral factors and host immunity, and their interplay, affect the pathogenesis and outcome of EV-A71 infection. The mammalian innate immune system forms the first layer of defense against viral infections and triggers activation of adaptive immunity leading to full protection. In this review, we discuss recent advances in our understanding of the interaction between EV-A71 and the innate immune system. We discuss the role of pattern-recognition receptors (PRRs), including Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), and inflammasomes, in the detection of EV-A71 infection and induction of antiviral immunity. As a counteraction, EV-A71 viral proteins target multiple innate immune pathways to facilitate viral replication in host cells. These novel insights at the virus-host interphase may support the future development of vaccines and therapeutics against EV-A71 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.

Hypoxia and therapeutic treatment of EV-A71 with an immune modulator TLR7 agonist in a new immunocompetent mouse model

Background

Enterovirus 71 (EV71 or EV-A71) was first identified in California about half a century ago. In recent years, outbreaks of EV-A71 were prevalent worldwide, including Taiwan, Malaysia, Singapore, Japan, and China. Between 2008 and 2011, China alone reported 1894 deaths associated with EV-A71 infection. In mild cases, EV-A71 can cause herpangina and hand-foot-and-mouth disease (HFMD). However, in severe cases, it could cause neurological disorders, including meningitis and encephalitis. Cardiopulmonary failure is common among hospitalized children with EV-A71 infection. No effective FDA-approved therapeutics against EV-A71 are clinically available.

Methods

We report the establishment of an immunocompetent wild type strain 129 (wt-129) mouse model, which can be cross-species infected with human EV-A71 clinical isolates via an intraperitoneal route.

Results

One intriguing disease phenotype of this new model is the development of characteristic “White-Jade” patches in the muscle, which lost sporadically the normal pink color of uninfected muscle. Viral VP1 protein and massive leukocyte infiltration were detected in muscles with or without white-jades. We demonstrated further that hypoxia is a general phenomenon associated with white-jades in both immunocompetent and immunodeficient mouse models. Therefore, hypoxia appears to be a feature intrinsic to EV-A71 infection, irrespective of its host’s immunogenetic background. To date, no effective treatment for EV-A71 is available. Here, using this new wt-129 mouse model, we showed that timely treatment with compound R837 (a TLR7 immune modulator) via oral or intraperitoneal routes, rescued the hypoxia, limb paralysis, and death at a high therapeutic efficacy.

Conclusions

In this new immunocompetent mouse 129 model, we observed an unexpected white-jade phenotype and its associated hypoxia. The successful treatment with TLR7 immune modulators via an oral route, provide us a new research direction for EV-A71 basic science and translational research. It remains an open issue whether R837 or its related compounds, will be a promising drug candidate in clinical trials in EV-A71 endemic or epidemic areas in the future.

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.

Enterovirus 71 targets the cardiopulmonary system in a robust oral infection mouse model

Severe infection with the re-emerging enterovirus 71 (EV71 or EV-A71) can cause cardiopulmonary failure. However, in patients' heart and lung, viral protein has not been detected. In mouse models, heart disease has not been reported. EV71-infected brainstem is generally believed to be responsible for the cardiopulmonary collapse. One major limitation in EV71 research is the lack of an efficient oral infection system using non-mouse-adapted clinical isolates. In a robust oral infection NOD/SCID mouse model, we detected EV71 protein at multiple organs, including heart and lung, in 100% of moribund mice with limb paralysis. Infiltrating leukocytes were always detected in heart and muscle, and VP1-positive M2 macrophages were abundant in the lung. Functional dissection on the pathogenesis mechanism revealed severe apoptosis, inflammatory cytokines, and abnormal electrocardiogram (EKG) in orally infected hearts. Therefore, cardiopulmonary disease could be one plausible cause of death in this mouse model. Inoculation of EV71 through an oral route resulted in viral infection in the intestine, viremia, and EV71 appeared to spread to peripheral tissues via blood circulation. Infectious virus was no longer detected in the blood on day 5 post-infection by the plaque formation assay. We demonstrated that both EV71 clinical isolate and cloned virus can target the cardiopulmonary system via a natural infection-like oral route.

A Novel Murine Model Expressing a Chimeric mSCARB2/hSCARB2 Receptor Is Highly Susceptible to Oral Infection with Clinical Isolates of Enterovirus 71

Enterovirus 71 (EV71) infection is generally associated with hand-foot-and-mouth disease (HFMD) and may cause severe neurological disorders and even death. An effective murine oral infection model for studying the pathogenesis of various clinical EV71 isolates is lacking. We developed a transgenic (Tg) mouse that expresses an EV71 receptor, that is, human scavenger receptor class B member 2 (hSCARB2), in a pattern highly similar to that of endogenous murine SCARB2 (mSCARB2) protein. A FLAG-tagged SCARB2 cDNA fragment composed of exons 3 to 12 was inserted into a murine Scarb2 gene-containing bacterial artificial chromosome (BAC) clone, and the resulting transgene was used for establishment of chimeric receptor-expressing Tg mice. Tg mice intragastrically (i.g.) infected with clinical isolates of EV71 showed neurological symptoms, such as ataxia and paralysis, and fatality. There was an age-dependent decrease in susceptibility to viral infection. Pathological characteristics of the infected Tg mice resembled those of encephalomyelitis in human patients. Viral infection was accompanied by microglial activation. Clodronate treatment of the brain slices from Tg mice enhanced viral replication, while lipopolysaccharide treatment significantly inhibited it, suggesting an antiviral role for microglia during EV71 infection. Taken together, this Tg mouse provides a model that closely mimics natural infection for studying EV71 pathogenesis and for evaluating the efficacy of vaccines or other antiviral drugs.IMPORTANCE The availability of a murine model of EV71 infection is beneficial for the understanding of pathogenic mechanisms and the development and assessment of vaccines and antiviral drugs. However, the lack of a murine oral infection model thwarted the study of pathogenesis induced by clinically relevant EV71 strains that are transmitted via the oral-oral or oral-fecal route. Our Tg mice could be intragastrically infected with clinically relevant EV71 strains in an efficient way and developed neurological symptoms and pathological changes strikingly resembling those of human infection. Moreover, these mice showed an age-dependent change in susceptibility that is similar to the human case. This Tg mouse, when combined with the use of other genetically modified mice, potentially contributes to studying the relationship between developmental changes in immunity and susceptibility to virus.

Replication and cytokine profiles of different subgenotypes of enterovirus 71 isolated from Thai patients in peripheral blood mononuclear cells

Enterovirus 71 (EV71) and coxsackievirus A16 (CA16) are common causative agents of mild and self-limiting symptoms of childhood hand, foot, and mouth disease (HFMD). However, some EV71-infected HFMD patients can develop severe neurological and/or fatal cardiopulmonary complications. In Thailand, HFMD associated with the EV71 subgenotypes C4a and B5 were reported to be associated with diverse outcomes. However, variations in enterovirus subgenotypes and virulence factors have not been fully elucidated; this study elucidated these variations in peripheral blood mononuclear cells (PBMCs) exposed to different subgenotypes of isolated enteroviruses for 24 and 48 h. Following infection, viral titers were determined by plaque assay. Infected cells and intracellular cytokines were quantified using flow cytometry, and multiplex assay was used to examine cytokine release. All isolated subgenotypes showed replication capability in PBMCs; specifically, the replication titer of EV71 C4a tended to be higher than titers of EV71 B5 and CA16. Additionally, the infectivity of EV71 B5 was higher in monocytes than in lymphocytes. Compared with EV71 B5, EV71 C4a and CA16 had greater ability to induce intra- and extracellular cytokine responses. These findings provide new insights into variations in cellular immune responses to different EV71 subgenotypes isolated from Thai patients, which should be considered for the development of vaccines and therapeutic agents.

Development of live attenuated Enterovirus 71 vaccine strains that confer protection against lethal challenge in mice

Besides causing mild hand, foot and mouth infections, Enterovirus A71 (EV-A71) is associated with neurological complications and fatality. With concerns about rising EV-A71 virulence, there is an urgency for more effective vaccines. The live attenuated vaccine (LAV) is a more valuable vaccine as it can elicit both humoral and cellular immune responses. A miRNA-based vaccine strain (pIY) carrying let-7a and miR-124a target genes in the EV-A71 genome which has a partial deletion in the 5′NTR (∆11 bp) and G64R mutation (3D^p°^l) was designed. The viral RNA copy number and viral titers of the pIY strain were significantly lower in SHSY-5Y cells that expressed both let-7a and miR-124a. Inhibition of the cognate miRNAs expressed in RD and SHSY-5Y cells demonstrated de-repression of viral mRNA translation. A previously constructed multiply mutated strain, MMS and the pIY vaccine strain were assessed in their ability to protect 4-week old mice from hind limb paralysis. The MMS showed higher amounts of IFN-γ ex vivo than the pIY vaccine strain. There was absence of EV-A71 antigen in the skeletal muscles and spinal cord micrographs of mice vaccinated with the MMS and pIY strains. The MMS and pIY strains are promising LAV candidates developed against severe EV-A71 infections.

Type III interferon signaling restricts enterovirus 71 infection of goblet cells

Recent worldwide outbreaks of enterovirus 71 (EV71) have caused major epidemics of hand, foot, and mouth disease with severe neurological complications, including acute flaccid paralysis. EV71 is transmitted by the enteral route, but little is known about the mechanisms it uses to cross the human gastrointestinal tract. Using primary human intestinal epithelial monolayers, we show that EV71 infects the epithelium from the apical surface, where it preferentially infects goblet cells. We found that EV71 infection did not alter epithelial barrier function but did reduce the expression of goblet cell-derived mucins, suggesting that it alters goblet cell function. We also show that the intestinal epithelium responds to EV71 infection through the selective induction of type III interferons (IFNs), which restrict EV71 replication. Collectively, these findings define the early events associated with EV71 infections of the human intestinal epithelium and show that host IFN signaling controls replication in an IFN-specific manner.