Combined peptides of human enterovirus 71 protect against virus infection in mice

Production of antigenically stable enterovirus A71 virus-like particles in Pichia pastoris as a vaccine candidate

Enterovirus A71 (EVA71) causes widespread disease in young children with occasional fatal consequences. In common with other picornaviruses, both empty capsids (ECs) and infectious virions are produced during the viral lifecycle. While initially antigenically indistinguishable from virions, ECs readily convert to an expanded conformation at moderate temperatures. In the closely related poliovirus, these conformational changes result in loss of antigenic sites required to elicit protective immune responses. Whether this is true for EVA71 remains to be determined and is the subject of this investigation.We previously reported the selection of a thermally resistant EVA71 genogroup B2 population using successive rounds of heating and passage. The mutations found in the structural protein-coding region of the selected population conferred increased thermal stability to both virions and naturally produced ECs. Here, we introduced these mutations into a recombinant expression system to produce stabilized virus-like particles (VLPs) in Pichia pastoris.The stabilized VLPs retain the native virion-like antigenic conformation as determined by reactivity with a specific antibody. Structural studies suggest multiple potential mechanisms of antigenic stabilization, however, unlike poliovirus, both native and expanded EVA71 particles elicited antibodies able to directly neutralize virus in vitro. Therefore, anti-EVA71 neutralizing antibodies are elicited by sites which are not canonically associated with the native conformation, but whether antigenic sites specific to the native conformation provide additional protective responses in vivo remains unclear. VLPs are likely to provide cheaper and safer alternatives for vaccine production and these data show that VLP vaccines are comparable with inactivated virus vaccines at inducing neutralising antibodies.

Production of antigenically stable enterovirus A71 virus-like particles in Pichia pastoris as a vaccine candidate

Enterovirus A71 (EVA71) causes widespread disease in young children with occasional fatal consequences. In common with other picornaviruses, both empty capsids (ECs) and infectious virions are produced during the viral lifecycle. While initially antigenically indistinguishable from virions, ECs readily convert to an expanded conformation at moderate temperatures. In the closely related poliovirus, these conformational changes result in loss of antigenic sites required to elicit protective immune responses. Whether this is true for EVA71 remains to be determined and is the subject of this investigation. We previously reported the selection of a thermally resistant EVA71 genogroup B2 population using successive rounds of heating and passage. The mutations found in the structural protein-coding region of the selected population conferred increased thermal stability to both virions and naturally produced ECs. Here, we introduced these mutations into a recombinant expression system to produce stabilised virus-like particles (VLPs) in Pichia pastoris . The stabilised VLPs retain the native virion-like antigenic conformation as determined by reactivity with a specific antibody. Structural studies suggest multiple potential mechanisms of antigenic stabilisation, however, unlike poliovirus, both native and expanded EVA71 particles elicited antibodies able to directly neutralise virus in vitro . Therefore, the anti-EVA71 neutralising antibodies are elicited by sites which are not canonically associated with the native conformation, but whether antigenic sites specific to the native conformation provide additional protective responses in vivo remains unclear. VLPs are likely to provide cheaper and safer alternatives for vaccine production and these data show that VLP vaccines are comparable with inactivated virus vaccines at inducing neutralising antibodies.

Thermal stabilization of enterovirus A 71 and production of antigenically stabilized empty capsids

Enterovirus A71 (EVA71) infection can result in paralysis and may be fatal. In common with other picornaviruses, empty capsids are produced alongside infectious virions during the viral lifecycle. These empty capsids are antigenically indistinguishable from infectious virus, but at moderate temperatures they are converted to an expanded conformation. In the closely related poliovirus, native and expanded antigenic forms of particle have different long-term protective efficacies when used as vaccines. The native form provides long-lived protective immunity, while expanded capsids fail to generate immunological protection. Whether this is true for EVA71 remains to be determined. Here, we selected an antigenically stable EVA71 virus population using successive rounds of heating and passage and characterized the antigenic conversion of both virions and empty capsids. The mutations identified within the heated passaged virus were dispersed across the capsid, including at key sites associated with particle expansion. The data presented here indicate that the mutant sequence may be a useful resource to address the importance of antigenic conformation in EVA71 vaccines.

Development of an Enzyme-Linked Immunosorbent Assay for Detection of the Native Conformation of Enterovirus A71

Enterovirus A71 (EVA71) is a medically important virus that is commonly associated with hand, foot, and mouth disease (HFMD). It is responsible for periodic outbreaks, resulting in significant economic impact and loss of life. Vaccination offers the potential to control future outbreaks, and vaccine development has been increasingly the focus of global research efforts. However, antigenic characterization of vaccine candidates is challenging because there are few tools to characterize the different antigenic forms of the virus. As with other picornaviruses, EVA71 virions exist in two antigenic states, native (NAg) and expanded (HAg). It is likely that the composition of vaccines, in terms of the proportions of NAg and HAg, will be important for vaccine efficacy and batch-to-batch consistency. This paper describes the development of a single-chain fused variable (scFv) domain fragment and the optimization of a sandwich enzyme-linked immunosorbent assay (ELISA) for the specific detection of the NAg conformation of EVA71. NAg specificity of the scFv was demonstrated using purified EVA71, and conversion of NAg to HAg by heating resulted in a loss of binding. We have thus developed an effective tool for characterization of the specific antigenic state of EVA71.

IMPORTANCE EVA71 is a medically important virus that is commonly associated with HFMD, resulting in periodic outbreaks, significant economic impact, and loss of life. Vaccination offers the potential to curtail future outbreaks, and vaccine development has been increasingly the focus of global research efforts. However, antigenic characterization of vaccine candidates is challenging because there are very limited effective tools to characterize the different antigenic forms of EV71. As with other picornaviruses, EVA71 virions exist in two antigenic states, native and expanded. This paper describes the development of an scFv and the optimization of a sandwich ELISA for the specific detection of the native conformation of EVA71 as an effective tool for characterization of the specific antigenic state of EVA71.

Molnupiravir and Its Active Form, EIDD-1931, Show Potent Antiviral Activity against Enterovirus Infections In Vitro and In Vivo

Enterovirus infections can cause hand, foot, and mouth disease (HFDM), aseptic meningitis, encephalitis, myocarditis, and acute flaccid myelitis, leading to death of infants and young children. However, no specific antiviral drug is currently available for the treatment of this type of infection. The Unites States and United Kingdom health authorities recently approved a new antiviral drug, molnupiravir, for the treatment of COVID-19. In this study, we reported that molnupiravir (EIDD-2801) and its active form, EIDD-1931, have broad-spectrum anti-enterovirus potential. Our data showed that EIDD-1931 could significantly reduce the production of EV-A71 progeny virus and the expression of EV-A71 viral protein at non-cytotoxic concentrations. The results of the time-of-addition assay suggest that EIDD-1931 acts at the post-entry step, which is in accordance with its antiviral mechanism. The intraperitoneal administration of EIDD-1931 and EIDD-2801 protected 1-day-old ICR suckling mice from lethal EV-A71 challenge by reducing the viral load in various tissues of the infected mice. The pharmacokinetics analysis indicated that the plasma drug concentration overwhelmed the EC₅₀ for enteroviruses, suggesting the clinical potential of molnupiravir against enteroviruses. Thus, molnupiravir along with its active form, EIDD-1931, may be a promising drug candidate against enterovirus infections.

Does enterovirus 71 urge for effective vaccine control strategies? Challenges and current opinion

Enterovirus 71 (EV71) is an infectious virus affecting all age groups of people around the world. It is one of the major aetiologic agents for HFMD (hand, foot and mouth disease) identified globally. It has led to many outbreaks and epidemics in Asian countries. Infection caused by this virus that can lead to serious psychological problems, heart diseases and respiratory issues in children younger than 10 years of age. Many studies are being carried out on the pathogenesis of the virus, but little is known. The host immune response and other molecular responses against the virus are also not clearly determined. This review deals with the interaction between the host and the EV71 virus. We discuss how the virus makes use of its proteins to affect the host's immunity and how the viral proteins help their replication. Additionally, we describe other useful resources that enable the virus to evade the host's immune responses. The knowledge of the viral structure and its interactions with host cells has led to the discovery of various drug targets for the treatment of the virus. Additionally, this review focusses on the antiviral drugs and vaccines developed by targeting various viral surface molecules during their infectious period. Furthermore, it is asserted that the improvement of prevailing vaccines will be the simplest method to manage EV71 infection swiftly. Therefore, we summarise numerous vaccines candidate for the EV71, such as the use of an inactivated complete virus, recombinant VP1 protein, artificial peptides, VLPs (viral-like particles) and live attenuated vaccines for combating the viral outbreaks promptly.

Enterovirus A71 Vaccines

Enterovirus A71 (EV-A71) is a major causative agent of hand, foot, and mouth disease (HFMD) and herpangina. Moreover, EV-A71 infection can lead to neurological complications and death. Vaccination is the most efficient way to control virus infection. There are currently three inactivated, whole EV-A71 vaccines licensed by the China NMPA (National Medical Products Administration). Several other types of vaccines, such as virus-like particles and recombinant VP1 (capsid protein), are also under development. In this review, we discuss recent advances in the development of EV-A71 vaccines.

Immunization with a fusion protein vaccine candidate generated from truncated peptides of human enterovirus 71 protects mice from lethal enterovirus 71 infections

Background

Prophylactic vaccines are critical in preventing hand, foot, and mouth disease (HFMD) primarily caused by human enterovirus 71 (EV71) infection. Children aged less than 5 years are especially susceptible to EV71 infections. In addition to the development of vaccines containing the inactivated virus, those containing virus-like particles (VLPs) with repeated antigens also constitute an effective preventive strategy for EV71 infections, with safety and productivity advantages. We previously developed a fusion protein composed with truncated peptides of the EV71 capsid protein, which assembled into spherical particles. This study aimed to assess the immunoprotective effects of this fusion protein as a vaccine candidate in a mouse model of EV71 infection.

Methods

To evaluate the protective effect of fusion protein vaccine candidate, neonatal mice born by immunized female mice, as well as normal neonatal mice immunized twice were infected with EV71 virus. Whereafter, the survival rates, clinical scores and viral loads were measured.

Results

The high dosage and booster immunization helped induce specific serum antibodies with high neutralization titers, which were transferred to neonatal mice, thereby facilitating effective resistance towards EV71 infection. An active immune response was also observed in neonatal mice which generated following immunization.

Conclusions

The present results suggest that this fusion protein is a suitable vaccine candidate in treating EV71 infections.

Plant-Produced Anti-Enterovirus 71 (EV71) Monoclonal Antibody Efficiently Protects Mice Against EV71 Infection

Enterovirus 71 (EV71) is the main causative agent of severe hand-foot-mouth disease. EV71 affects countries mainly in the Asia-Pacific region, which makes it unattractive for pharmaceutical companies to develop drugs or vaccine to combat EV71 infection. However, development of these drugs and vaccines is vital to protect younger generations. This study aims to develop a specific monoclonal antibody (mAb) to EV71 using a plant platform, which is a cost-effective and scalable production technology. A previous report showed that D5, a murine anti-EV71 mAb, binds to VP1 protein of EV71, potently neutralizes EV71 in vitro, and effectively protects mice against EV71 infection. Herein, plant-produced chimeric D5 (cD5) mAb, variable regions of murine D5 antibody linked with constant regions of human IgG1, was transiently expressed in Nicotiana benthamiana using geminiviral vectors. The antibody was expressed at high levels within six days of infiltration. Plant-produced cD5 retained its in vitro high-affinity binding and neutralizing activity against EV71. Furthermore, a single dose (10 µg/g body weight) of plant-produced cD5 mAb offered 100% protection against infection in mice after a lethal EV71 challenge. Therefore, our results showed that plant-produced anti-EV71 mAb is an effective, safe, and affordable therapeutic option against EV71 infection.

Antivirals and vaccines for Enterovirus A71

Enterovirus A71 (EV-A71) is an important emerging virus posing a threat to children under five years old. EV-A71 infection in infants or young children can cause hand-foot-and-mouth disease, herpangina, or severe neurological complications. However, there are still no effective antivirals for treatment of these infections. In this review, we summarize the antiviral compounds developed to date based on various targets of the EV-A71 life cycle. Moreover, development of a vaccine would be the most effective approach to prevent EV-A71 infection. Therefore, we also summarize the development and clinical progress of various candidate EV-A71 vaccines, including inactivated whole virus, recombinant VP1 protein, synthetic peptides, viral-like particles, and live attenuated vaccines.

An inactivated enterovirus 71 vaccine is safe and immunogenic in healthy adults: A phase I, double blind, randomized, placebo-controlled, study of two dosages

BACKGROUND

Hand, foot and mouth disease (HFMD), especially that caused by enterovirus 71 (EV71) infection, is a public health concern in the Asia-Pacific region. We report a phase I clinical trial of an EV71 candidate vaccine (INV21) based on a binary ethylenimine inactivated B2 sub-genotype formulated with aluminum hydroxide.

METHODS

In this double-blind, placebo-controlled, randomized, dose escalation study adult volunteers received two vaccinations 28 days apart of low or high dose formulations of the candidate vaccine and were then monitored for safety and reactogenicity for four weeks after each dose, and for their immune responses up to 28 weeks.

RESULTS

Of 36 adults enrolled, 35 completed the study as planned. Either no or mild adverse events were observed, mainly injection site pain and tiredness. Seroconversion was 100% after two vaccinations. High geometric mean neutralizing antibody titers (GMT) were observed 14 days post first dose, peaking 14 days post second dose (at Day 42) in both high and low dose groups; GMTs on days 14, 28, 42, and 56 were 128, 81, 323, 203 and 144, 100, 451, 351 in low- and high-dose groups, respectively. Titers for both doses declined gradually to Day 196 but remained higher than baseline and the placebo groups, which had low GMTs throughout the duration of the study. Cross-neutralizing antibody activity against heterologous sub-genotypes was demonstrated.

CONCLUSION

These data show that the EV71 candidate vaccine is safe and immunogenic in adults and supports further clinical development as a potential pediatric vaccine by initiating a dose-escalation study for determining the dose-dependent safety and immunogenicity of the vaccine in young naïve children.

Induction of immune responses by a novel recombinant fusion protein of enterovirus A71 in BALB/c mice

Fusion protein technology is used in biotechnology and medical developments. In this study, recombinant fusion proteins from enterovirus A71 (EV-A71) subgenotype B5, Thailand were designed based two surface proteins (VP1 and VP2) and an internal protein (VP4), and named "VP0" (consisting of VP4-VP2) and "EV71" (consisting of VP4-VP2-VP1), respectively. The recombinant fusion proteins VP0 and EV71 were expressed in insect cells and successfully produced and secreted into the media. Both recombinant fusion proteins were shown to have immunogenic properties in BALB/c mice when formulated with Freund's complete/incomplete adjuvant (FA). Interestingly, EV71 formulated with FA- induced a level of IgG antibodies level similar to that induced by the recombinant protein VP1 formulated with FA (the positive control). Our results showed that VP1 alone is better at eliciting a strong cell-mediated immune response. Nontheless, EV71 formulated with FA was capable of inducing lymphocyte proliferation and increasing the cytokine-related mRNA expression levels of interferon-γ (IFN-γ), interleukin-2 (IL-2), and IL-10 in mice after immunization. Additionally, the number of CD4⁺ and CD8⁺ T lymphocyte cells after stimulation with purified EV71 in splenic cell culture showed highly specific CD4⁺ and CD8⁺ T-cell production. We suggest that EV71, which consists of VP4-VP2-VP1, could be used as the foundation for developing a novel recombinant fusion protein-based vaccine for EV-A71.

Induction of a high-titered antibody response using HIV gag-EV71 VP1-based virus-like particles with the capacity to protect newborn mice challenged with a lethal dose of enterovirus 71

Enterovirus 71 (EV71) is the most frequently detected causative agent in hand, foot, and mouth disease (HFMD) and is a serious threat to public health in the Asia-Pacific region. Many EV71 vaccines are under development worldwide, and although both inactivated virus vaccines and virus-like particles (VLPs) are considered to be effective, the main focus has been on inactivated EV71vaccines. There have been very few studies on EV71 VLPs. In this study, using a strategy based on HIV gag VLPs, we constructed a gag-VP1 fusion gene to generate a recombinant baculovirus expressing the EV71 structural protein VP1 together with gag, which was then used to infect TN5 cells to form VLPs. The VLPs were characterized using transmission electron microscopy, electrophoresis and staining with Coomassie blue, and Western blotting. Mice immunized with gag-VP1 VLPs showed strong humoral and cellular immune responses. Finally, immunization of female mice with gag-VP1 VLPs provided effective protection of their newborn offspring against challenge with a lethal dose EV71. These results demonstrate a successful approach for producing EV71 VP1 VLPs based on the ability of HIV gag to self-assemble, thus providing a good foundation for producing high-titered anti-EV71 antibody by immunization with VLP-based gag EV71 VP1 protein.

Flos Farfarae Inhibits Enterovirus 71-Induced Cell Injury by Preventing Viral Replication and Structural Protein Expression

Enterovirus 71 (EV71) infection can cause airway symptoms, brainstem encephalitis, neurogenic shock, and neurogenic pulmonary edema with high morbidity and mortality. There is no proven therapeutic modality. Flos Farfarae is the dried flower bud of Tussilago farfara L. that has been used to manage airway illnesses for thousands of years. It has neuro-protective activity and has been used to manage neuro-inflammatory diseases. However, it is unknown whether Flos Farfarae has activity against EV71-induced neuropathy. The current study used both human foreskin fibroblast (CCFS-1/KMC) and human rhabdomyosarcoma (RD) cell lines to test the hypothesis that a hot water extract of Flos Farfarae could effectively inhibit EV71 infection. The authenticity of Flos Farfarae was confirmed by HPLC-UV fingerprint. Through plaque reduction assays and flow cytometry, Flos Farfarae was found to inhibit EV71 infection ([Formula: see text]). Inhibition of viral replication and protein expression were further confirmed by reverse transcription polymerase chain reaction (RT-PCR) and quantitative RT-PCR (qRT-PCR), and western blot, respectively. The estimated IC[Formula: see text]s were 106.3[Formula: see text][Formula: see text]g/mL in CCFS-1/KMC, and 15.0[Formula: see text][Formula: see text]g/mL in RD cells. Therefore, Flos Farfarae could be beneficial to inhibit EV71 infection by preventing viral replication and structural protein expression.

Antiviral effects of Reduning injection against Enterovirus 71 and possible mechanisms of action

The present study was designed to evaluate the protective effects of Reduning injection against Enterovirus 71 (EV71) in Vero cells and in mice. The Vero cells were infected with 100 and 50 TCID50 (50% tissue culture infective dose) of EV71, respectively. The inhibition of Reduning injection on cytopathic effect (CPE) was detected. Meanwhile, a mouse model produced by intraperitoneal EV71-infection (10(6) TCID50), was used to investigate the protective effects of Reduning injection. The total survival rate, living time, daily survival rate, weight ratio, and score for symptoms were examined. The viral loads in Vero cells and muscle tissues were detected using real-time PCR. Finally, the content of cytokines was analyzed by ELISA. In the Vero cells, 2.5 mg crude drug·mL(-1) of Reduning injection inhibited CPE induced by EV71 infection. In the mice, 1.3 g crude drug·kg(-1) of Reduning injection rescued death triggered by infection, in comparison with model group. Moreover, the survival rate, weight ratio, and clinical scores were also improved. The viral RNA copies in the Vero cells and the mice muscle tissues were reduced. Besides, the steep EV71-induced accumulations of TNF-α and MCP-1 were decreased by Reduning injection. In conclusion, Reduning injection showed promising protective effects against EV71 in Vero cells and in mice.

Enterovirus 71: a whole virion inactivated enterovirus 71 vaccine

INTRODUCTION

Enterovirus A71 (EV71) is the predominant causative agent of hand, foot, and mouth disease (HFMD), which is often associated with severe cases and even deaths. EV71-associated epidemics have emerged as a serious threat to public health, particularly in the Asia-Pacific region.

AREAS COVERED

We searched PubMed using the terms 'enterovirus 71', 'hand, foot, and mouth disease', and 'vaccine', with no date or language restrictions for all publications before April 27, 2016. Among various vaccine candidates, the alum-adjuvant inactivated EV71 vaccines are most promising. Three alum-adjuvant inactivated EV71 vaccines developed by mainland China showed high efficacy, good immunogenicity persistence and acceptable safety profiles in clinical trials. Recently, two of these EV71 vaccines have been approved for marketing in China and the other one is undergoing the review process of licensure. In this manuscript, we summarized previous study results as well as discussed the regulatory affairs and post-market surveillances issues. Expert commentary: The marketing of EV71 vaccines is a milestone in the controlling of HFMD. International clinical trials are needed to further assess the efficacy and cross-immunogenicity. Establishing a sensitive pathogen monitoring system would be essential to monitor the variation of genotypes and control HFMD epidemics.

A safe and sensitive enterovirus A71 infection model based on human SCARB2 knock-in mice

Enterovirus A71 infection has become a severe threat for global public health. Vaccines for controlling and preventing Enterovirus A71 epidemics are highly demanded, however, vaccine evaluation has been hindered by the lack of suitable Enterovirus A71 infection animal models. Here we established an hSCARB2 knockin mouse model for real-time monitoring of enterovirus A71 infection in vivo. This model was sensitive to the infection of both replication-competent virus rEV71(FY)-EGFP and single round pseudotype virus pEV71(FY)-Luc. The intensity of bioluminescence correlated well with viral loads in infected tissues (R=0.86, P<0.01). Pathological changes recapitulated human infectious and clinical features of enterovirus A71, including both general characteristics of "hand foot and mouth" and the severe symptoms in the CNS. A formalin-inactivated enterovirus A71 vaccine can elicit antibodies in R26-hSCARB2 mice, which play effective roles in protecting knockin mice against enterovirus A71 infection as indicated by bioluminescence. Therefore, this work provides a safe, sensitive and visualizing model for exploring mechanisms of enterovirus A71 infection and examining human enterovirus A71 vaccines and antiviral therapies.

Determinants of EV71 immunogenicity and protection against lethal challenge in a mouse model

Circulating enterovirus 71 (EV71)-associated hand, foot, and mouth disease (HFMD) is a major public health problem in the Asian-Pacific region. An EV71 vaccine for HFMD prevention is currently being developed. However, viral determinants that could influence the vaccine's efficacy have not been well characterized. In this study, we isolated and characterized several EV71 strains that are currently circulating in northern and southern China. We determined that VP1 variation is a major determinant of EV71 immunogenicity. A single amino acid variation in VP1 can lead to significant differences in the breadth and potency of immune responses against primary EV71 isolates as well as the sensitivity of EV71 to heterologous neutralizing antibody responses. We also identified EV71 strains that could induce potent immunogenic and cross-neutralizing antibody responses against diverse EV71 strains. Furthermore, these neutralizing antibodies could protect neonatal mice from lethal dose challenge with various circulating EV71 viruses. Our study provides useful information for EV71 vaccine development and evaluation.

EV71 virus-like particles produced by co-expression of capsid proteins in yeast cells elicit humoral protective response against EV71 lethal challenge

BACKGROUND

Enterovirus 71 (EV71) is the most common causative pathogens of hand, foot and mouth disease (HFMD) associated with severe neurological complications. There is a great need to develop prophylactic vaccine against EV71 infection.

RESULTS

EV71 virus-like particle (VLP) was produced in yeast expression system by the co-expression of four EV71 structural proteins VP1-VP4. Immunization with the recombinant VLPs elicited potent anti-EV71 antibody responses in adult mice and anti-VLP sera were able to neutralize EV71 virus in vitro. Neonatal mice model demonstrated VLP immunization conferred protection to suckling mice against the lethal viral challenge.

CONCLUSIONS

Co-expression of four EV71 structural proteins VP1-VP4 in yeast expression systems is an effective method to produce EV71 VLPs. VLP-based vaccine shows great potential to prevent EV71 infection.

Hexon-modified recombinant E1-deleted adenoviral vectors as bivalent vaccine carriers for Coxsackievirus A16 and Enterovirus 71

Hand, foot and mouth disease (HFMD) is a major public health concern in Asia; more efficient vaccines against HFMD are urgently required. Adenoviral (Ad) capsids have been used widely for the presentation of foreign antigens to induce specific immune responses in the host. Here, we describe a novel bivalent vaccine for HFMD based on the hexon-modified, E1-deleted chimpanzee adenovirus serotype 68 (AdC68). The novel vaccine candidate was generated by incorporating the neutralising epitope of Coxsackievirus A16 (CA16), PEP71, into hypervariable region 1 (HVR1), and a shortened neutralising epitope of Enterovirus 71 (EV71), sSP70, into HVR2 of the AdC68 hexon. In order to enhance the immunogenicity of EV71, VP1 of EV71 was cloned into the E1-region of the AdC68 vectors. The results demonstrated that these two epitopes were well presented on the virion surface and had high affinity towards specific antibodies, and VP1 of EV71 was also significantly expressed. In pre-clinical mouse models, the hexon-modified AdC68 elicited neutralising antibodies against both CA16 and EV71, which conferred protection to suckling mice against a lethal challenge of CA16 and EV71. In summary, this study demonstrates that the hexon-modified AdC68 may represent a promising bivalent vaccine carrier against EV71 and CA16 and an epitope-display platform for other pathogens.

Characterization of the enterovirus 71 P1 polyprotein expressed in Pichia pastor as a candidate vaccine

Human enterovirus 71 (EV71) plays an important role in hand, foot, and mouth disease (HFMD), which recently caused the death of hundreds of children in the Asia-Pacific region. However, there are no specific treatments available for EV71 infections; thus, a safe and effective vaccine is needed urgently. In this study, we developed an effective and economical method for producing EV71 polyprotein (P1 protein) in Pichia pastoris. Furthermore, we evaluated the potential of P1 protein as a candidate vaccine against EV71 virus. The data revealed that P1 protein induced persistent high cross-neutralization antibodies for different EV71 subtypes, and elicited significant splenocyte proliferation. The high levels of interleukin-10(IL-10) and interferon-gamma (IFN-γ) showed that P1 protein induced Th1 and Th2 immune responses. Interestingly, vaccinating female mice with the P1 protein conferred cross-protection against different EV71 subtypes to their neonatal offspring.Compared with heat-inactivated EV71, the P1 protein elicited improved humoral and cellular immune responses and showed good cross-protection with different EV71 subtypes. Therefore, the EV71-P1 protein produced by P. pastoris is a promising candidate vaccine against EV71.

Broad protection with an inactivated vaccine against primary-isolated lethal enterovirus 71 infection in newborn mice

Background

Circulating enterovirus 71 (EV-A71)-associated hand, foot, and mouth disease is on the rise in the Asian-Pacific region. Although animal models have been developed using mouse-adapted EV-A71 strains, mouse models using primary EV-A71 isolates are scarce. Lethal animal models with circulating EV-A71 infection would contribute to studies of pathogenesis as well as vaccine development and evaluation.

Results

In this study, we established a lethal mouse model using primary EV-A71 isolates from patients infected with serotypes that are currently circulating in humans. We also characterized the dose-dependent virulence and pathologic changes of circulating EV-A71 in this mouse model. Most importantly, we have established this mouse model as a suitable system for EV-A71 vaccine evaluation. An inactivated EV-A71 vaccine candidate offered complete protection from death induced by various circulating EV-A71 viruses to neonatal mice that were born to immunized female mice. The sera of the immunized dams and their pups showed higher neutralization titers against multiple circulating EV-A71 viruses.

Conclusions

Thus, our newly established animal model using primary EV-A71 isolates is helpful for future studies on viral pathogenesis and vaccine and drug development.

Enhanced enterovirus 71 virus-like particle yield from a new baculovirus design

Enterovirus 71 (EV71) is responsible for the outbreaks of hand‐foot‐and‐mouth disease in the Asia‐Pacific region. To produce the virus‐like particle (VLP) vaccine, we previously constructed recombinant baculoviruses to co‐express EV71 P1 polypeptide and 3CD protease using the Bac‐to‐Bac^® vector system. The recombinant baculoviruses resulted in P1 cleavage by 3CD and subsequent VLP assembly in infected insect cells, but caused either low VLP yield or excessive VLP degradation. To tackle the problems, here we explored various expression cassette designs and flashBAC GOLD™ vector system which was deficient in v‐cath and chiA genes. We found that the recombinant baculovirus constructed using the flashBAC GOLD™ system was insufficient to improve the EV71 VLP yield. Nonetheless, BacF‐P1‐C3CD, a recombinant baculovirus constructed using the flashBAC GOLD^TM system to express P1 under the polh promoter and 3CD under the CMV promoter, dramatically improved the VLP yield while alleviating the VLP degradation. Infection of High Five^TM cells with BacF‐P1‐C3CD enhanced the total and extracellular VLP yield to ≈268 and ≈171 mg/L, respectively, which enabled the release of abundant VLP into the supernatant and simplified the downstream purification. Intramuscular immunization of mice with 5 μg purified VLP induced cross‐protective humoral responses and conferred protection against lethal virus challenge. Given the significantly improved extracellular VLP yield (≈171 mg/L) and the potent immunogenicity conferred by 5 μg VLP, one liter High Five^TM culture produced ≈12,000 doses of purified vaccine, thus rendering the EV71 VLP vaccine economically viable and able to compete with inactivated virus vaccines. Biotechnol. Bioeng. 2015;112: 2005–2015. © 2015 Wiley Periodicals, Inc.

Therapeutic and prevention strategies against human enterovirus 71 infection

Human enterovirus 71 (HEV71) is the cause of hand, foot and mouth disease and associated neurological complications in children under five years of age. There has been an increase in HEV71 epidemic activity throughout the Asia-Pacific region in the past decade, and it is predicted to replace poliovirus as the extant neurotropic enterovirus of highest global public health significance. To date there is no effective antiviral treatment and no vaccine is available to prevent HEV71 infection. The increase in prevalence, virulence and geographic spread of HEV71 infection over the past decade provides increasing incentive for the development of new therapeutic and prevention strategies against this emerging viral infection. The current review focuses on the potential, advantages and disadvantages of these strategies. Since the explosion of outbreaks leading to large epidemics in China, research in natural therapeutic products has identified several groups of compounds with anti-HEV71 activities. Concurrently, the search for effective synthetic antivirals has produced promising results. Other therapeutic strategies including immunotherapy and the use of oligonucleotides have also been explored. A sound prevention strategy is crucial in order to control the spread of HEV71. To this end the ultimate goal is the rapid development, regulatory approval and widespread implementation of a safe and effective vaccine. The various forms of HEV71 vaccine designs are highlighted in this review. Given the rapid progress of research in this area, eradication of the virus is likely to be achieved.

Establishment of a panel of in-house polyclonal antibodies for the diagnosis of enterovirus infections

The aim of this study was to establish a reliable method of virus detection for the diagnosis of critical enterovirus infections such as acute infective encephalitis, encephalomyelitis and myocarditis. Because histopathological and immunohistochemical analyses of paraffin-embedded tissues play an important role in recognizing infectious agents in tissue samples, six in-house polyclonal antibodies raised against three representative enteroviruses using an indirect immunofluorescence assay and immunohistochemistry were examined. This panel of polyclonal antibodies recognized three serotypes of enterovirus. Two of the polyclonal antibodies were raised against denatured virus particles from enterovirus A71, one was raised against the recombinant VP1 protein of coxsackievirus B3, and the other for poliovirus type 1 were raised against denatured virus particles, the recombinant VP1 protein and peptide 2C. Western blot analysis revealed that each of these antibodies recognized the corresponding viral antigen and none cross-reacted with non-enteroviruses within the family Picornaviridae. However, all cross-reacted to some extent with the antigens derived from other serotypes of enterovirus. Indirect immunofluorescence assay and immunohistochemistry revealed that the virus capsid and non-structural proteins were localized in the cytoplasm of affected culture cells, and skeletal muscles and neurons in neonatal mice experimentally-infected with human enterovirus. The antibodies also recognized antigens derived from recent clinical isolates of enterovirus A71, coxsackievirus B3 and poliovirus. In addition, immunohistochemistry revealed that representative antibodies tested showed the same recognition pattern according to each serotype. Thus, the panel of in-house anti-enterovirus polyclonal antibodies described herein will be an important tool for the screening and pathological diagnosis for enterovirus infections, and may be useful for the classification of different enterovirus serotypes, including coxsackieviruses A and B, echoviruses, enterovirus A71 and poliovirus.

Update on the development of enterovirus 71 vaccines

INTRODUCTION

Enterovirus 71 (EV71) is an etiological agent that causes severe neurological complications in children. EV71 outbreaks have occurred throughout the Asia-Pacific region, posing a severe global public health threat; however, no specific therapeutic strategy exists for treating EV71-infected children.

AREAS COVERED

Five manufacturers have produced inactivated EV71 whole virus vaccines in mainland China, Taiwan, and Singapore, which have completed Phase III (mainland China) and Phase I (Taiwan and Singapore) clinical trials. Various EV71 vaccine candidates are being researched in animal models, including live-attenuated virus vaccine, recombinant VP1 vaccine, VP1-based DNA vaccine, synthetic peptide vaccine and virus-like particle vaccine. In this review, the present situation is summarized, and feasible improvements to the EV71 vaccine are explored.

EXPERT OPINION

Although inactivated EV71 vaccines are safe, efficient and elicit strong immune responses to protect adults, children and infants against infection, the quality control of production is critical.

Characterization of the enterovirus 71 VP1 protein as a vaccine candidate

Enterovirus 71 (EV71) is an important agent responsible for hand-foot-and-mouth disease (HFMD), which can cause severe neurological complications and death in children. However, there is no specific treatment for EV71 infection, and a safe and effective vaccine is needed urgently. In this study, an effective and economical method for the production of EV71-VP1 protein was developed, and the VP1 protein was evaluated in humoral and cellular immune responses as an EV71 vaccine. The results revealed that the VP1 protein induced high titers of cross-neutralizing antibodies for different EV71 subtypes, and elicited significant splenocyte proliferation. The high levels of IFN-r and IL-10 showed the VP1 protein induced a mixed Th1 and Th2 immune response. Vaccinated female mice could confer protection in their neonatal offspring. Compared with the inactivated EV71, the VP1 protein elicited similar humoral and cellular responses, but the engineered protein is safer, less expensive and can be produced more efficiently. Therefore, EV71-VP1 protein can induce effective immunologic protection against EV71 and is an ideal candidate against EV71 infection.

Deferoxamine compensates for decreases in B cell counts and reduces mortality in enterovirus 71-infected mice

Enterovirus 71 is one of the major causative agents of hand, foot and mouth disease in children under six years of age. No vaccine or antiviral therapy is currently available. In this work, we found that the number of B cells was reduced in enterovirus 71-infected mice. Deferoxamine, a marine microbial natural product, compensated for the decreased levels of B cells caused by enterovirus 71 infection. The neutralizing antibody titer was also improved after deferoxamine treatment. Furthermore, deferoxamine relieved symptoms and reduced mortality and muscle damage caused by enterovirus 71 infection. This work suggested that deferoxamine has the potential for further development as a B cell-immunomodulator against enterovirus 71.

Recombinant tandem multi-linear neutralizing epitopes of human enterovirus 71 elicited protective immunity in mice

BACKGROUND

Human Enterovirus 71 (EV71) has emerged as the leading cause of viral encephalitis in children, especially in the Asia-Pacific regions. EV71 vaccine development is of high priority at present, and neutralization antibodies have been documented to play critical roles during in vitro and in vivo protection against EV71 infection.

RESULTS

In this study, a novel strategy to produce EV71 vaccine candidate based on recombinant multiple tandem linear neutralizing epitopes (mTLNE) was proposed. The three well identified EV71 linear neutralizing epitopes in capsid proteins, VP1-SP55, VP1-SP70 and VP2-SP28, were sequentially linked by a Gly-Ser linker ((G4S)3), and expressed in E.coli in fusion with the Trx and His tag at either terminal. The recombinant protein mTLNE was soluble and could be purified by standard affinity chromatography. Following three dosage of immunization in adult mice, EV71-specific IgG and neutralization antibodies were readily induced by recombinant mTLNE. IgG subtyping demonstrated that lgG1 antibodies dominated the mTLNE-induced humoral immune response. Especially, cytokine profiling in spleen cells from the mTLNE-immunized mice revealed high production of IL-4 and IL-6. Finally, in vivo challenge experiments showed that passive transfer with anti-mTLNE sera conferred full protection against lethal EV71 challenge in neonatal mice.

CONCLUSION

Our results demonstrated that this rational designed recombinant mTLNE might have the potential to be further developed as an EV71 vaccine in the future.

Animal models of enterovirus 71 infection: applications and limitations

Human enterovirus 71 (EV71) has emerged as a neuroinvasive virus that is responsible for several outbreaks in the Asia-Pacific region over the past 15 years. Appropriate animal models are needed to understand EV71 neuropathogenesis better and to facilitate the development of effective vaccines and drugs. Non-human primate models have been used to characterize and evaluate the neurovirulence of EV71 after the early outbreaks in late 1990s. However, these models were not suitable for assessing the neurovirulence level of the virus and were associated with ethical and economic difficulties in terms of broad application. Several strategies have been applied to develop mouse models of EV71 infection, including strategies that employ virus adaption and immunodeficient hosts. Although these mouse models do not closely mimic human disease, they have been applied to determine the pathogenesis of and treatment and prevention of the disease. EV71 receptor-transgenic mouse models have recently been developed and have significantly advanced our understanding of the biological features of the virus and the host-parasite interactions. Overall, each of these models has advantages and disadvantages, and these models are differentially suited for studies of EV71 pathogenesis and/or the pre-clinical testing of antiviral drugs and vaccines. In this paper, we review the characteristics, applications and limitation of these EV71 animal models, including non-human primate and mouse models.

Development of enterovirus 71 vaccines: from the lab bench to Phase III clinical trials

The widespread epidemics of enterovirus 71 (EV71) seriously affected the Western Pacific Region. Young children, especially those younger than 3 years are the most susceptible population to the EV71-associated diseases. Several Asian countries have begun to focus on the research and development of EV71 vaccines. Five inactivated whole-virus EV71 candidate vaccines (three were manufactured in mainland China based on a C4 genotype strain, one in Taiwan based on a B4 genotype strain and one in Singapore based on a B2 genotype strain) have been assessed in clinical trials. Three candidate vaccines developed in mainland China have already completed Phase III clinical trials recently. The tested EV71 vaccine could provide good efficacy, satisfactory safety, and high immunogenicity. Thus, inactivated EV71 vaccines are expected to become the first available vaccines against EV71 in the near future.

Immunization of N terminus of enterovirus 71 VP4 elicits cross-protective antibody responses

BACKGROUND

Enterovirus 71 (EV71) is major cause of hand, foot and mouth disease. Large epidemics of EV71 infection have been recently reported in the Asian-Pacific region. Currently, no vaccine is available to prevent EV71 infection.

RESULTS

The peptide (VP4N20) consisting of the first 20 amino acids at the N-terminal of VP4 of EV71 genotype C4 were fused to hepatitis B core (HBcAg) protein. Expression of fusion proteins in E. coli resulted in the formation of chimeric virus-like particles (VLPs). Mice immunized with the chimeric VLPs elicited anti-VP4N20 antibody response. In vitro microneutralization experiments showed that anti-chimeric VLPs sera were able to neutralize not only EV71 of genotype C4 but also EV71 of genotype A. Neonatal mice model confirmed the neutralizing ability of anti-chimeric VLPs sera. Eiptope mapping led to the identification of a "core sequence" responsible for antibody recognition within the peptide.

CONCLUSIONS

Immunization of chimeric VLPs is able to elicit antibodies displaying a broad neutralizing activity against different genotypes of EV71 in vitro. The "core sequence" of EV71-VP4 is highly conserved across EV71 genotypes. The chimeric VLPs have a great potential to be a novel vaccine candidate with a broad cross-protection against different EV71 genotypes.

Preclinical evaluation of the immunogenicity and safety of an inactivated enterovirus 71 candidate vaccine

Human enterovirus 71 (EV71) is a significant cause of morbidity and mortality from Hand, Foot and Mouth Disease (HFMD) and neurological complications, particularly in young children in the Asia-Pacific region. There are no vaccines or antiviral therapies currently available for prevention or treatment of HFMD caused by EV71. Therefore, the development of therapeutic and preventive strategies against HFMD is of growing importance. We report the immunogenic and safety profile of inactivated, purified EV71 preparations formulated with aluminum hydroxide adjuvant in preclinical studies in mice and rabbits. In mice, the candidate vaccine formulations elicited high neutralizing antibody responses. A toxicology study of the vaccine formulations planned for human use performed in rabbits showed no vaccine-related pathological changes and all animals remained healthy. Based on these preclinical studies, Phase 1 clinical testing of the EV71 inactivated vaccine was initiated.

Patterns of polymorphism and divergence in the VP1 gene of enterovirus 71 circulating in the Asia-Pacific region between 1994 and 2013

Enterovirus 71 has been implicated in several outbreaks of hand, foot and mouth disease in the Asia-Pacific region. The present study aimed to achieve comprehensive evolutionary dynamic aspects of EV71 during 1994-2013, based on phylogenetic analyses of the VP1 sequences. The results indicated that 4 genotypes, namely C4, C1, C2 and B4 are the predominant strains, especially in Southeast Asian countries. No common ancestor was shared in different countries. Fourteen sites of substitutions were detected in the VP1 gene sequences; including the most common sites related to neutralization at position V249I [47.1% (189/401)] and A289T [42.6% (171/401)]. However, the sites Q22H and Q22R associated with increased virulence were recognized only in 13.7% (55/401) and 18% (72/401), respectively. None of the above mutations seemed to become fixed because the ratio of Ka/Ks was greater than 1.0. Mutations K43E, A58T, S184T, and T240S could possibly change the spatial structure. Two mutations, G145E and T240S, could obviously affect the hydrophobicity of VP1 and thus alter the EV71 immunoreactivity. In conclusion, the VP1 gene of EV71 strains circulating in the Asia-Pacific region during 1994-2013, showed polymorphisms and divergence with very slow evolution rate, which may be one of the reasons for periodic outbreaks in this area.

Progress on the research and development of inactivated EV71 whole-virus vaccines

The prevalence of diseases caused by EV71 infection has become a serious public health problem in the Western Pacific region. Due to a lack of effective treatment options, controlling EV71 epidemics has mainly focused on the research and development (R&D) of EV71 vaccines. Thus far, five organizations have completed pre-clinical studies focused on the development of inactivated EV71 whole-virus vaccines, including vaccine strain screening, process optimization, safety and immunogenicity evaluation, and are in different stages of clinical trials. Among these organizations, three companies in Mainland China [Beijing Vigoo Biological Co., Ltd. (Vigoo), Sinovac Biotech Ltd. (Sinovac) and Institute of Medical Biology, Chinese Academy of Medical Science (CAMS)] have recently completed Phase III trials for the vaccines they developed. In addition, the other two vaccines, developed by National Health Research Institutes (NHRI) of Taiwan and Inviragen Pte., Ltd (Inviragen), of Singapore, have also completed Phase I clinical trials. Published clinical trial results indicate that the inactivated EV71 vaccines have good safety and immunogenicity in the target population (infants) and confer a relatively high rate of protection against EV71 infection-related diseases. The results of clinical trials suggest a promising future for the clinical use of EV71 vaccines. Here, we review and highlight the recent progress on the R&D of inactivated EV71 whole-virus vaccines.

Recombinant baculovirus associated with bilosomes as an oral vaccine candidate against HEV71 infection in mice

BACKGROUND

Human enterovirus 71 (HEV71) is one of the major pathogen responsible for hand, foot and mouth disease (HFMD). Currently no effective vaccine or antiviral drugs are available. Like poliovirus, EV71 is transmitted mainly by the feco-oral route. To date the majority of the studied EV71 vaccine candidates are administered parenterally. Injectable vaccines induce good systemic immunity but mucosal responses are often unsatisfactory, whereas mucosal vaccines provide both systemic and mucosal immunity. Therefore, oral immunization appears to be an attractive alternative to parenteral immunization.

METHODOLOGY/PRINCIPAL FINDINGS

In this report, we studied the efficacy of an orally administered vaccine candidate developed using recombinant baculovirus displaying VP1 (Bac-VP1) in a murine model. Gastrointestinal delivery of Bac-VP1 significantly induced VP1-specific humoral (IgG) and mucosal (IgA) immune responses. Further, we studied the efficacy of the Bac-VP1 associated with bilosomes and observed that the Bac-VP1 associated with bilosomes elicited significantly higher immune responses compared to bilosomes non-associated with Bac-VP1. However, mice immunized subcutaneously with live Bac-VP1 had significantly enhanced VP1 specific serum IgG levels and higher neutralizing antibody titers compared with mice orally immunized with live Bac-VP1 alone or associated with bilosomes.

CONCLUSION

Bilosomes have been shown to possess inherent adjuvant properties when associated with antigen. Therefore Bac-VP1 with bilosomes could be a promising oral vaccine candidate against EV71 infections. Thus, Bac-VP1 loaded bilosomes may provide a needle free, painless approach for immunization against EV71, thereby increasing patient compliance and consequently increasing vaccination coverage.

Antiviral drug discovery for the treatment of enterovirus 71 infections

Enterovirus 71 (EV71) is a small, positive-sense, single-stranded RNA virus in the genus Enterovirus, family Picornavirus. It causes hand, foot and mouth disease in infants and children, which in a small percentage of cases progresses to central nervous system infection, ranging from aseptic meningitis to fatal encephalitis. Sporadic cases of EV71 infection occur throughout the world, but large epidemics have occurred recently in Southeast Asia and China. There are currently no approved vaccines or antiviral therapies for the prevention or treatment of EV71 infection. This paper reviews efforts to develop antiviral therapies against EV71.

Antiviral activity of punicalagin toward human enterovirus 71 in vitro and in vivo

Human enterovirus 71 is one of the major causative agents of hand, foot and mouth disease in children and has caused mortalities in large-scale outbreaks in the Asia-Pacific region in recent years. No vaccine or antiviral therapy is available currently in the clinic. In this work, we investigated the antiviral effect of punicalagin on enterovirus 71 both in vitro and in vivo. The results showed that punicalagin reduced the viral cytopathic effect on rhabdomyosarcoma cells with an IC₅₀) value of 15 μg/ml. Moreover, punicalagin treatment of mice challenged with a lethal dose of enterovirus 71 resulted in a reduction of mortality and relieved clinical symptoms by inhibiting viral replication. Our work suggested that punicalagin have the potential for further development as antiviral agents against enterovirus 71.

Adoptive transfer of macrophages from adult mice reduces mortality in mice infected with human enterovirus 71

Human enterovirus 71 (EV71) causes hand, foot and mouth disease in children under 6 years of age, and the neurological complications of this virus can lead to death. Until now, no vaccines or drugs have been available for the clinical control of this epidemic. Macrophages can engulf pathogens and mediate a series of host immune responses that play a role in the defence against infectious diseases. Using immunohistochemistry, we observed the localizations of virus in muscle tissues of EV71-infected mice. The macrophages isolated from the adult mice could kill the virus gradually in vitro, as shown using quantitative real-time PCR (qRT-PCR) and virus titration. Co-localisation of lysosomes and virus within macrophages suggested that the lysosomes were possibly responsible for the phagocytosis of EV71. Activation of the macrophages in the peritoneal cavity of mice four days pre-infection reduced the mortality of mice upon lethal EV71 infection. The adoptive transfer of macrophages from adult mice inhibited virus replication in the muscle tissues of infected mice, and this was followed by a relief of symptoms and a significant reduction of mortality, which suggested that the adoptive transfer of macrophages from adult humans represents a potential strategy to treat EV71-infected patients.

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.

Enterovirus 71: pathogenesis, control and models of disease

Enterovirus 71 (EV71) is one of the major agents responsible for hand, foot and mouth disease. The increasing incidence of hand, foot and mouth disease outbreaks, epidemics due to EV71 infection in South East Asia and the propensity of EV71 strains to cause severe neurological complications in young children underscore the need to further our knowledge and understanding of the mechanisms involved in EV71 pathogenesis; such knowledge could then be translated to the identification of biomarkers of disease severity, and the development of effective therapeutics and vaccines. This article reviews the current knowledge of EV71 pathogenesis, control measures and models of infection.

Production of EV71 vaccine candidates

Enterovirus 71 (EV71) is now recognized as an emerging neurotropic virus in Asia and with Coxsackie virus (CV) it is the other major causative agent of hand-foot-mouth diseases (HFMD). Effective medications and/or prophylactic vaccines against HFMD are urgently needed. From a scientific (the feasibility of bioprocess, immunological responses and potency in animal challenge model) and business development (cost of goods) points of view, we in this review address and discuss the pros and cons of different EV71 vaccine candidates that have been produced and evaluated in animal models. Epitope-based synthetic peptide vaccine candidates containing residues 211–225 of VP1 formulated with Freund’s adjuvant (CFA/IFA) elicited low EV71 virus neutralizing antibody responses, but were protective in the suckling mouse challenge model. Among recombinant EV71 subunits (rVP1, rVP2 and rVP3) expressed in E. coli, purified and formulated with CFA/IFA, only VP1 elicited mouse antibody responses with measurable EV71-specific virus neutralization titers. Immunization of mice with either a DNA plasmid containing VP1 gene or VP1 expressed in Salmonella typhimurium also generated neutralizing antibody responses and protected animals against a live EV71 challenge. Recombinant EV71 virus-like particles (rVLP) produced from baculovirus formulated either with CFA/IFA or alum elicited good virus neutralization titers in both mice and non-human primates, and were found to be protective in the suckling mouse EV71 challenge model. Synthetic peptides or recombinant EV71 subunit vaccines (rVP1 and rVLP) formulated in alum were found to be poorly immunogenic in rabbits. Only formalin-inactivated (FI) EV71 virions formulated in alum elicited cross-neutralizing antibodies against different EV71 genotypes in mice, rabbits and non-human primates but induced weak neutralizing responses against CAV16. From a regulatory, economic and market acceptability standpoint, FI-EV71 virion vaccines are the most promising candidates and are currently being evaluated in human clinical trials. We further describe and analyze some new bioprocesses technologies that have great potential applications in EV71 vaccine development. This review also demonstrates the opportunities and challenges that the Asian vaccine industry faces today.

Immunological evaluation and comparison of different EV71 vaccine candidates

Enterovirus 71 (EV71) and coxsackievirus A16 (CVA16) are major causative agents of hand, foot, and mouth diseases (HFMDs), and EV71 is now recognized as an emerging neurotropic virus in Asia. Effective medications and/or prophylactic vaccines against HFMD are not available. The current results from mouse immunogenicity studies using in-house standardized RD cell virus neutralization assays indicate that (1) VP1 peptide (residues 211–225) formulated with Freund's adjuvant (CFA/IFA) elicited low virus neutralizing antibody response (1/32 titer); (2) recombinant virus-like particles produced from baculovirus formulated with CFA/IFA could elicit good virus neutralization titer (1/160); (3) individual recombinant EV71 antigens (VP1, VP2, and VP3) formulated with CFA/IFA, only VP1 elicited antibody response with 1/128 virus neutralization titer; and (4) the formalin-inactivated EV71 formulated in alum elicited antibodies that cross-neutralized different EV71 genotypes (1/640), but failed to neutralize CVA16. In contrast, rabbits antisera could cross-neutralize strongly against different genotypes of EV71 but weakly against CVA16, with average titers 1/6400 and 1/32, respectively. The VP1 amino acid sequence dissimilarity between CVA16 and EV71 could partially explain why mouse antibodies failed to cross-neutralize CVA16. Therefore, the best formulation for producing cost-effective HFMD vaccine is a combination of formalin-inactivated EV71 and CAV16 virions.

Expression and purification of enterovirus type 71 polyprotein P1 using Pichia pastoris system

Enterovirus type 71(EV71) causes severe hand-foot-and-mouth disease (HFMD) resulting in hundreds of deaths of children every year; However, currently, there is no effective treatment for EV71. In this study, the EV71 poly-protein (EV71-P1 protein) gene was processed and cloned into the eukaryotic expression vector pPIC9k and then expressed in Pichia pastoris strain GS115. The EV71 P1 protein with a molecular weight of 100 kD was produced and secreted into the medium. The soluble EV71 P1 protein was purified by column chromatography with a recovery efficiency of 70%. The result of the immunological analysis showed that the EV71 P1 protein had excellent immunogenicity and could stimulate the production of EV71-VP1 IgG antibody in injected rabbits. We suggest that EV71-P1 protein is an ideal candidate for an EV71 vaccine to prevent EV71 infection.