Recent advances in research on human enterovirus 71

Genetic variation and evolutionary characteristics of Echovirus 11: new variant within genotype D5 associated with neonatal death found in China

Echovirus 11 (E11) has gained attention owing to its association with severe neonatal infections. From 2018 to 2023, a surge in severe neonatal cases and fatalities linked to a novel variant of genotype D5 was documented in China, France, and Italy. However, the prevention and control of E11 variants have been hampered by limited background data on the virus circulation and genetic variance. Therefore, the present study investigated the circulating dynamics of E11 and the genetic variation and molecular evolution of genotype D5 through the collection of strains from the national acute flaccid paralysis (AFP) and hand, foot, and mouth disease (HFMD) surveillance system in China during 2000-2022 and genetic sequences published in the GenBank database. The results of this study revealed a prevalent dynamic of E11 circulation, with D5 being the predominant genotype worldwide. Further phylogenetic analysis of genotype D5 indicated that it could be subdivided into three important geographic clusters (D5-CHN1: 2014-2019, D5-CHN2: 2016-2022, and D5-EUR: 2022-2023). Additionally, variant-specific (144) amino acid mutation sites and positive-selection pressure sites (132, 262) were identified in the VP1 region. Cluster-specific recombination patterns were also identified, with CVB5, E6, and CVB4 as the major recombinant viruses. These findings provide a preliminary landscape of E11 circulation worldwide and basic scientific data for further study of the pathogenicity of E11 variants.

Antiviral effects of ferric ammonium citrate

Iron is an essential nutrient for cell survival and is crucial for DNA replication, mitochondrial function and erythropoiesis. However, the immunological role of iron in viral infections has not been well defined. Here we found the iron salt ferric ammonium citrate (FAC) inhibited Influenza A virus, HIV virus, Zika virus, and Enterovirus 71 (EV71) infections. Of note, both iron ion and citrate ion were required for the antiviral capability of FAC, as other iron salts and citrates did not exhibit viral inhibition. Mechanistically, FAC inhibited viral infection through inducing viral fusion and blocking endosomal viral release. These were further evidenced by the fact that FAC induced liposome aggregation and intracellular vesicle fusion, which was associated with a unique iron-dependent cell death. Our results demonstrate a novel antiviral function of FAC and suggest a therapeutic potential for iron in the control of viral infections.

Interleukin-18 protects mice from Enterovirus 71 infection

Previous study has demonstrated that the NLRP3 inflammasome is essential for protecting murine host against Enterovirus 71 (EV71) infection. However, the underlying mechanism remained unknown. Here we discovered that the pleiotropic cytokine interleukin-18 (IL-18), an NLRP3 inflammasome-dependent effector protein, exhibits a protective capability against EV71 challenge. Deficiency of IL-18 in mice exacerbated EV71 infection, which was reflected by increased viral replication, elevated production of interferons (IFN-β, IFN-γ), proinflammatory cytokines (TNF-α, IL-6) and chemokine CCL2,as well as decreased survival of experimental animals. Conversely, administration of recombinant IL-18 considerably restrained EV71 infection in IL-18 deficient mice. Thus, our results revealed a protective role for IL-18 against EV71 challenge, and indicated a novel therapeutic application for IL-18 in EV71 associated hand, foot, and mouth disease (HFMD).

The role of Misshapen NCK-related kinase (MINK), a novel Ste20 family kinase, in the IRES-mediated protein translation of human enterovirus 71

Human Enterovirus 71 (EV71) commonly causes Hand, Foot and Mouth Disease in young children, and occasional occurrences of neurological complications can be fatal. In this study, a high-throughput cell-based screening on the serine/threonine kinase siRNA library was performed to identify potential antiviral agents against EV71 replication. Among the hits, Misshapen/NIKs-related kinase (MINK) was selected for detailed analysis due to its strong inhibitory profile and novelty. In the investigation of the stage at which MINK is involved in EV71 replication, virus RNA transfection in MINK siRNA-treated cells continued to cause virus inhibition despite bypassing the normal entry pathway, suggesting its involvement at the post-entry stage. We have also shown that viral RNA and protein expression level was significantly reduced upon MINK silencing, suggesting its involvement in viral protein synthesis which feeds into viral RNA replication process. Through proteomic analysis and infection inhibition assay, we found that the activation of MINK was triggered by early replication events, instead of the binding and entry of the virus. Proteomic analysis on the activation profile of p38 Mitogen-activated Protein Kinase (MAPK) indicated that the phosphorylation of p38 MAPK was stimulated by EV71 infection upon MINK activation. Luciferase reporter assay further revealed that the translation efficiency of the EV71 internal ribosomal entry site (IRES) was reduced after blocking the MINK/p38 MAPK pathway. Further investigation on the effect of MINK silencing on heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) localisation demonstrated that cytoplasmic relocalisation of hnRNP A1 upon EV71 infection may be facilitated via the MINK/p38 MAPK pathway which then positively regulates the translation of viral RNA transcripts. These novel findings hence suggest that MINK plays a functional role in the IRES-mediated translation of EV71 viral RNA and may provide a potential target for the development of specific antiviral strategies against EV71 infection.

Dominant CD4-dependent RNA-dependent RNA polymerase-specific T-cell responses in children acutely infected with human enterovirus 71 and healthy adult controls

Human enterovirus 71 (EV71) is one of the major causes of hand, foot and mouth disease (HFMD), which leads to significant mortality in infected children. A prophylactic vaccine is urgently needed. However, little is known about the protective T-cell immunity in individuals infected with the EV71 virus. In this study, we performed a comprehensive ex vivo interferon-γ ELISPOT analysis in 31 children infected with EV71 as well as in 40 healthy adult controls of the CD4(+) and CD8(+) T-cell responses to overlapping peptides spanning the VP1 structural protein and RNA-dependent RNA polymerase (RdRp) non-structural protein. EV71-specific CD4 T-cell responses were detected in most of the acute patients and were mostly CD4-dependent RdRp-specific responses. CD8-dependent VP1 and RdRp-specific responses were also detected in a small proportion of recently infected children. There was no significant association between the strength of the T-cell responses and disease severity observed during the acute EV71 infection phase. Interestingly, an RdRp-specific, but no VP1-specific, CD4-dependent T-cell response was detected in 30% of the adult controls, and no T-cell responses were detected in healthy children. In addition, 24 individual peptides containing potential T-cell epitope regions were identified. The data suggest that CD4-dependent RdRp-specific T-cell responses may play an important role in protective immunity, and the epitopes identified in this study should provide valuable information for future therapeutic and prophylactic vaccine design as well as basic research.

Evaluation of the stability of enterovirus 71 virus-like particle

Enterovirus 71 (EV71) is responsible for the outbreaks of hand-foot-and-mouth disease that caused significant mortality in children, but no vaccine is available yet. EV71 virus-like particle (VLP) is the empty capsid consisting of viral structural proteins but can elicit potent immune responses, rendering VLP a promising EV71 vaccine candidate. To evaluate whether VLP remains stable after long-term storage, which is crucial for advancing the VLP vaccine to the clinical setting, we evaluated the effects of NaCl concentration, buffers and temperatures on the VLP stability. We first validated the use of dynamic light scattering (DLS) for measuring the hydrodynamic diameter (≈30-35 nm) of VLP, which was close to the VLP diameter (≈25-27 nm) as measured by transmission electron microscopy (TEM). Using these techniques, we found that EV71 VLP remained stable for 5 months in sodium phosphate (NaPi) buffers with various NaCl concentrations. EV71 VLP also remained morphologically stable in NaPi, citrate and TE(+) buffers for 5 months, yet the enzyme-linked immunosorbent assay (ELISA) revealed that the VLP stored in citrate and TE(+) buffers partially lost the immunogenicity after 5 months. In contrast, the VLP stored in the NaPi buffer at 4°C remained stable macroscopically and microscopically for 5 months, as judged from the DLS, TEM and ELISA. The VLP stored at 25°C and 37°C also retained stability for 1 month, which would obviate the need of a cold chain during the shipping. These data altogether proved the stability of EV71 VLP and suggested that the VLP is amenable to bioprocessing and storage.

The Pathogenesis and Prevention of Encephalitis due to Human Enterovirus 71

Human enterovirus 71 (HEV71) has emerged as a major cause of viral encephalitis in Southeast Asia, with increased epidemic activity observed since 1997. This is reflected in a large increase in scientific publications relating directly to HEV71. New research is elucidating details of the viral life cycle, confirming similarities between HEV71 and other enteroviruses. Scavenger receptor B2 (SCARB2) is a receptor for HEV71, although other receptors are likely to be identified. Currently, the only strategies to prevent HEV71-associated disease are early diagnosis and aggressive supportive management of identified cases. As more information emerges regarding the molecular processes of HEV71 infection, further advances may lead to the development of effective antiviral treatments and ultimately a vaccine-protection strategy. The protective efficacies of several inactivated HEV71 vaccines have been confirmed in animal models, suggesting that an effective vaccine may become available in the next decade.

Formalin-inactivated vaccine provokes cross-protective immunity in a mouse model of human enterovirus 71 infection

Human enterovirus 71 (HEV71) has emerged as a major cause of epidemics of hand, foot and mouth disease associated with severe neurological sequelae in the Asia-Pacific region. In this study, a passive protection mouse model was used to evaluate the protective efficacy of formalin-inactivated HEV71 vaccines derived from a Chinese C4 genotype strain. Pregnant mice were immunised using a prime/boost strategy and ≥50U of vaccine protected five-day-old pups from lethal challenge with a mouse-adapted (B3 genotype) strain of HEV71. Immunised mice developed a neutralising antibody response to both the immunising C4 strain and to the mouse-adapted strain. Mice born to immunised dams showed significantly less myositis and reduced viral loads in tissues.

Antibody dependent enhancement infection of enterovirus 71 in vitro and in vivo

BACKGROUND

Human enterovirus 71 (EV71) has emerged as a significant cause of acute encephalitis and deaths in young children. The clinical manifestations caused by EV71 varied from mild hand, foot and mouth disease to severe neurological complications and deaths, but its pathogenesis remains elusive. Antibody dependent enhancement (ADE) infection has been reported in various viruses and has been shown to contribute to disease severity.

RESULTS

In this study, the presence of sub-neutralizing antibody was demonstrated to enhance EV71 infection in THP-1 cells and increase the mortality of EV71 infection in a suckling mouse model. Further, a secondary infection model was established to characterize the correlation between ADE and disease severity, and primary asymptomatic EV71 infection was shown to increase the mortality of the secondary EV71 infection in suckling mice.

CONCLUSIONS

Together, these in vitro and in vivo experiments strongly supported the hypothesis of ADE infection of EV71. The present findings indicate ADE might contribute to the pathogenesis of severe EV71 infection, and raise practical issues of vaccine development and antibody-based therapy.

Development and evaluation of a reverse transcription-loop-mediated isothermal amplification assay for rapid detection of enterovirus 71

Human enterovirus 71 (EV71) is the major etiological agent of hand, foot, and mouth disease (HFMD), which is a common infectious disease in young children and infants. EV71 can cause various clinical manifestations and has been associated with severe neurological complications; it has resulted in fatalities during recent outbreaks in Asian-Pacific regions since 1997. The early and rapid detection is critical for prevention and control of EV71 infection, since no vaccine or antiviral drugs are currently available. In this study, a simple and sensitive reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay was developed for rapid detection of EV71. The detection limit of the RT-LAMP assay was approximately 0.01 PFU per reaction mixture, and no cross-reactive amplification with other enteroviruses was observed. The assay was evaluated further with 40 clinical specimens and exhibited 92.9% sensitivity and 100% specificity. This RT-LAMP assay may become a useful alternative in clinical diagnosis of EV71, especially in resource-limited hospitals or rural clinics of China and other countries in the Asian-Pacific region.

Inferring nonneutral evolution from contrasting patterns of polymorphisms and divergences in different protein coding regions of enterovirus 71 circulating in Taiwan during 1998-2003

Background

Enterovirus (EV) 71 is one of the common causative agents for hand, foot, and, mouth disease (HFMD). In recent years, the virus caused several outbreaks with high numbers of deaths and severe neurological complications. Despite the importance of these epidemics, several aspects of the evolutionary and epidemiological dynamics, including viral nucleotide variations within and between different outbreaks, rates of change in immune-related structural regions vs. non-structural regions, and forces driving the evolution of EV71, are still not clear.

Results

We sequenced four genomic segments, i.e., the 5' untranslated region (UTR), VP1, 2A, and 3C, of 395 EV71 viral strains collected from 1998 to 2003 in Taiwan. The phylogenies derived from different genomic segments revealed different relationships, indicating frequent sequence recombinations as previously noted. In addition to simple recombinations, exchanges of the P1 domain between different species/genotypes of human enterovirus species (HEV)-A were repeatedly observed. Contrasting patterns of polymorphisms and divergences were found between structural (VP1) and non-structural segments (2A and 3C), i.e., the former was less polymorphic within an outbreak but more divergent between different HEV-A species than the latter two. Our computer simulation demonstrated a significant excess of amino acid replacements in the VP1 region implying its possible role in adaptive evolution. Between different epidemic seasons, we observed high viral diversity in the epidemic peaks followed by severe reductions in diversity. Viruses sampled in successive epidemic seasons were not sister to each other, indicating that the annual outbreaks of EV71 were due to genetically distinct lineages.

Conclusions

Based on observations of accelerated amino acid changes and frequent exchanges of the P1 domain, we propose that positive selection and subsequent frequent domain shuffling are two important mechanisms for generating new genotypes of HEV-A. Our viral dynamics analysis suggested that the importation of EV71 from surrounding areas likely contributes to local EV71 outbreaks.