Development of an enzyme-linked immunosorbent spot assay to measure serum-neutralizing antibodies against coxsackievirus B3

Immuno-Colorimetric Neutralization Test: A Surrogate for Widely Used Plaque Reduction Neutralization Tests in Public Health Virology

Since their first documentation in 1952, plaque reduction neutralization tests (PRNTs) have become the choice of test for the measurement of neutralizing antibodies against a particular virus. However, PRNTs can be performed only against viruses that cause cytopathic effects (CPE). PRNTs also require skilled personnel and can be time-consuming depending on the time required for the virus to cause CPE. Hence, their application limits large-scale studies or epidemiological and laboratory investigations. Since 1978, many surrogate PRNTs or immunocolorimetric assay (ICA)-based focus reduction neutralization tests (FRNT) have been developed. In this article, ICAs and their utility in FRNTs for the characterization of neutralizing antibodies, homologous or heterologous cross-neutralization, and laboratory diagnosis of viruses of public health importance have been discussed. Additionally, possible advancements and automations have been described that may help in the development and validation of novel surrogate tests for emerging viruses.

Development of a rapid neutralization testing system for Rhinovirus C15 based on the enzyme-linked immunospot assay

Human Rhinoviruses (RVs) are dominant pathogens causing a wide range of respiratory tract diseases, posing a huge threat to public health worldwide. Viruses belonging to the RV-C species are more likely to cause severe illnesses and are strongly associated with asthma onset or exacerbations than RV-A or RV-B. Rapid and sensitive detection of neutralizing antibodies (NAbs) against RV-C can promote the development of vaccines and antiviral drugs and help in the diagnosis of viral infection. In this study, a rapid neutralization testing system for RV-C15, based on an enzyme-linked immunospot assay (Nt-ELISPOT) was developed. A monoclonal antibody (MAb), named 9F9, with high binding efficacy for RV-C15 conjugated to horseradish peroxidase (HRP), was used to detect RV-C15-infected cells at a concentration of 2 μg/ml. The optimal infectious dose of RV-C15 was set at 1 × 10⁴ TCID₅₀/well and the cells were fixed with 0.5% formaldehyde diluted in PBS after incubation for 20 h. Compared with the traditional cytopathic effect (CPE)-based neutralization assay (Nt-CPE), Nt-ELISPOT significantly shortened the detection period and showed good consistency with the detection of neutralizing titers of both sera and NAbs. Using Nt-ELISPOT, three anti-RV-C15 NAbs were obtained with IC₅₀ values of 0.16, 0.27, and 11.8 μg/ml, respectively. Moreover, 64 human serum samples collected from a wide range of age groups were tested for NAb against RV-C15 by Nt-ELISPOT. The total seroprevalence was 48.4% (31/64) and the positive rate was lowest in the group under 6 years old. Thus, the Nt-ELISPOT established in this study can be used as a high-throughput and rapid neutralization assay for the screening of NAbs and for seroepidemiological investigation against RV-C15.

Establishment of a rapid ELISPOT assay for influenza virus titration and neutralizing antibody detection

Seasonal influenza is an acute respiratory infection causing around 500 000 global deaths annually. There is an unmet medical need to develop more effective antiviral drugs and vaccines against influenza infection. A rapid, accurate, high-throughput titration assay for influenza virus particles or neutralizing antibodies would be extremely useful in these research fields. However, commonly used methods such as tissue culture infective dose and plaque-forming units (PFU) for virus particle quantification, and the plaque reduction neutralization test (PRNT) for antibody determination are time-consuming, laborious, and have limited accuracy. In this study, we developed an efficient assay based on the enzyme-linked immunospot (ELISPOT) technique for the influenza virus and neutralizing antibody titration. Two broad-spectrum antibodies recognizing the nucleoproteins of influenza A and B viruses were used in the assay to broadly and highly sensitively detect influenza virus-infected cells at 16 hours postinfection. An optimized cell culture medium with no tosyl phenylalanyl chloromethyl ketone trypsin and high dose oseltamivir acid was used to improve quantitation accuracy. This ELISPOT assay displayed a good correlation (R2  = 0.9851) with the PFU assay when used to titrate 30 influenza virus isolates. The assay was also applied to measure influenza-neutralizing antibodies in 40 human sera samples, showing a good correlation (R2  = 0.9965) with the PRNT assay. This ELISPOT titration assay is a rapid, accurate, high-throughput assay for quantification of influenza virus and neutralizing antibodies, and provides a powerful tool for research into and development of drugs and vaccines against influenza.

Rapid Neutralization Testing System for Zika Virus Based on an Enzyme-Linked Immunospot Assay

Zika virus (ZIKV) is a mosquito-borne flavivirus that has been associated with neuropathology in fetuses and adults, imposing a serious health concern. Therefore, the development of a vaccine is a global health priority. Notably, neutralization tests have a significant value for vaccine development and virus diagnosis. The cytopathic effect (CPE)-based neutralization test (Nt-CPE) is a common neutralization method for ZIKV. However, this method has some drawbacks, such as being time-consuming and labor-intensive and having low-throughput, which precludes its application in the detection of large numbers of specimens. To improve this problem, we developed a neutralization test based on an enzyme-linked immunospot assay (Nt-ELISPOT) for ZIKV and performed the assay in a 96-well format. A monoclonal antibody (mAb), 11C11, with high affinity and reactivity to ZIKV was used to detect ZIKV-infected cells. To optimize this method, the infectious dose of ZIKV was set at a multiplicity of infection (MOI) of 0.0625, and a detection experiment was performed after incubating for 24 h. As a result, under these conditions, the Nt-ELISPOT had good consistency with the traditional Nt-CPE to measure neutralizing titers of sera and neutralizing antibodies. Additionally, three neutralizing antibodies against ZIKV were screened by this method. Overall, we successfully developed an efficient neutralization test for ZIKV that is high-throughput and rapid. This Nt-ELISPOT can potentially be applied to detecting neutralizing titers of large numbers of specimens in vaccine evaluation and neutralizing antibody screening for ZIKV.

Synthesis and anti-CVB3 activity of 4-amino acid derivative substituted pyrimidine nucleoside analogues

Seven novel 4-amino acid derivative substituted pyrimidine nucleoside analogues were designed, synthesized, and tested for their anti-CVB3 activity. Initial biological studies indicated that among these 4-amino acid derivative substituted pyrimidine nucleoside analogues, 4-N-(2'-amino-glutaric acid-1'-methylester)-1-(2'- deoxy-2'-β-fluoro-4'-azido)-furanosyl-cytosine 2 exhibited the most potent anti-CVB activity (IC₅₀ = 9.3 μM). The cytotoxicity of these compounds has also been assessed. The toxicity of compound 2 was similar to that of ribavirin.

Development of an efficient neutralization assay for Coxsackievirus A10

Coxsackievirus A10 (CVA10) recently has become one of the major pathogens of hand, foot, and mouth disease (HFMD) in children worldwide, but no cure or vaccine against CVA10 is available yet. Serological evaluation of herd immunity to CVA10 will promote the development of vaccine. The traditional neutralization assay based on inhibition of cytopathic effect (Nt-CPE) is a common method for measuring neutralizing antibody titer against CVA10, which is time-consuming and labor-intensive. In this study, an efficient neutralization test based on a monoclonal antibody (mAb) 3D1 against CVA10, called Elispot-based neutralization test (Nt-Elispot), was developed. In the Nt-Elispot, the mAb 3D1 labeled with horseradish peroxidase (HRP) was used to detect the CVA10-infected RD cells at a 1:4000 dilution and the optimal infectious dose of CVA10 was set at 10⁵ TCID₅₀/well when combined with a fixed incubation time of 14 h. Compared with the Nt-CPE, the Nt-Elispot method effectively shortened the detection period and presented a good correlativity with it. Using the Nt-Elispot, a total of 123 sera from healthy children were tested for neutralizing antibody against CVA10, demonstrating that the overall seroprevalence was 49.3% (54/123) and the geometric mean titer (GMT) had been calculated as 574.2. Furthermore, 2 anti-CVA10 neutralizing mAbs were obtained by screening via the Nt-Elispot. Overall, the established Nt-Elispot could be used as an efficient and high-throughput method for evaluating immunity to CVA10 and screening the neutralizing antibodies.

Serological survey of neutralizing antibodies to eight major enteroviruses among healthy population

Human enteroviruses (EVs) are the most common causative agents infecting human, causing many harmful diseases, such as hand, foot, and mouth disease (HFMD), herpangina (HA), myocarditis, encephalitis, and aseptic meningitis. EV-related diseases pose a serious worldwide threat to public health. To gain comprehensive insight into the seroepidemiology of major prevalent EVs in humans, we firstly performed a serological survey for neutralizing antibodies (nAbs) against Enterovirus A71 (EV-A71), Coxsackie virus A16 (CV-A16), Coxsackie virus A6 (CV-A6), Coxsackie virus A10 (CV-A10), Coxsackie virus B3 (CV-B3), Coxsackie virus B5 (CV-B5), Echovirus 25 (ECHO25), and Echovirus 30 (ECHO30) among the healthy population in Xiamen City in 2016, using micro-neutralization assay. A total of 515 subjects aged 5 months to 83 years were recruited by stratified random sampling. Most major human EVs are widely circulated in Xiamen City and usually infect infants and children. The overall seroprevalence of these eight EVs were ranged from 14.4% to 42.7%, and most of them increased with age and subsequently reached a plateau. The co-existence of nAbs against various EVs are common among people ≥ 7 years of age, due to the alternate infections or co-infections with different serotypes of EVs, while most children were negative for nAb against EVs, especially those < 1 year of age. This is the first report detailing the seroepidemiology of eight prevalent EVs in the same population, which provides scientific data supporting further studies on the improvement of EV-related disease prevention and control.

A novel inactivated enterovirus 71 vaccine can elicit cross-protective immunity against coxsackievirus A16 in mice

Hand, foot, and mouth disease (HFMD) is a highly contagious disease that mainly affects infants and children. Enterovirus 71 (EV71) and coxsackievirus A16 (CA16) are the major pathogens of HFMD. Two EV71 vaccines were recently licensed in China and the administration of the EV71 vaccines is believed to significantly reduce the number of HFMD-related severe or fatal cases. However, a monovalent EV71 vaccine cannot cross-protect against CA16 infection, this may result in that it cannot effectively control the overall HFMD epidemic. In this study, a chimeric EV71, whose VP1/210-225 epitope was replaced by that of CA16, was constructed using a reverse genetics technique to produce a candidate EV71/CA16 bivalent vaccine strain. The chimeric EV71 was infectious and showed similar growth characteristics as its parental strain. The replacement of the VP1/210-225 epitope did not significantly affect the antigenicity and immunogenicity of EV71. More importantly, the chimeric EV71 could induce protective immunity against both EV71 and CA16, and protect neonatal mice against either EV71 or CA16 lethal infections, the chimeric EV71 constructed in this study was shown to be a feasible and promising candidate bivalent vaccine against both EV71 and CA16. The construction of a chimeric enterovirus also provides an alternative platform for broad-spectrum HFMD vaccines development.

A neonatal mouse model for the evaluation of antibodies and vaccines against coxsackievirus A6

Coxsackievirus A6 (CA6) can induce atypical hand, foot, and mouth disease, which is characterized by severe rash, onychomadesis and a higher rate of infection in adults. Increasing epidemiological data indicated that outbreaks of CA6-associated hand, foot, and mouth disease have markedly increased worldwide in recent years. However, the current body of knowledge on the infection, pathogenic mechanism, and immunogenicity of CA6 is still very limited. In this study, we established the first neonatal mouse model for the evaluation of antibodies and vaccines against CA6. The CA6 strain CA6/141 could infect a one-day-old BALB/c mouse through intraperitoneal and intracerebral routes. The infected mice developed clinical symptoms, such as inactivity, wasting, hind-limb paralysis and even death. Pathological examination indicated that CA6 showed special tropism to skeletal muscles and skin, but not to nervous system or cardiac muscles. Infections with CA6 could induce vesicles in the dermis without a rash in mice, and the CA6 antigen was mainly localized in hair follicles. The strong tropism of CA6 to the skin may be related to its severe clinical features in infants. This mouse model was further applied to evaluate the efficacy of a therapeutic antibody and an experimental vaccine against CA6. A potential mAb 1D5 could fully protect mice from a lethal CA6 infection and also showed good therapeutic effects in the CA6-infected mice. In addition, an inactivated CA6 vaccine was evaluated through maternal immunization and showed 100% protection of neonatal mice from lethal CA6 challenge. Collectively, these results indicate that this infection model will be a useful tool in future studies on vaccines and antiviral reagents against CA6.

Development of an HSV-1 neutralization test with a glycoprotein D specific antibody for measurement of neutralizing antibody titer in human sera

Background

Investigating the neutralizing antibody (NAb) titer against HSV-1 is essential for monitoring the immune protection against HSV-1 in susceptible populations, which would facilitate the development of vaccines against herpes infection and improvement of HSV-1 based oncolytic virotherapy.

Results

In this study, we have developed a neutralization test based on the enzyme-linked immunospot assay (ELISPOT-NT) to determine the neutralizing antibody titer against HSV-1 in human serum samples. This optimized assay employed a monoclonal antibody specifically recognizing glycoprotein D to detect the HSV-1 infected cells. With this test, the neutralizing antibody titer against HSV-1 could be determined within one day by automated interpretation of the counts of cell spots. We observed good correlation in the results obtained from ELISPOT-NT and plaque reduction neutralization test (PRNT) by testing 22 human serum samples representing different titers. Moreover, 269 human serum samples collected from a wide range of age groups were tested, the average neutralizing antibody titer (log₂NT₅₀) was 8.3 ± 2.8 and the prevalence of NAbs was 83.6 % in this cohort, it also revealed that the average neutralizing antibody titer in different groups increased with the age, and no significant difference in neutralizing antibody titers was observed between males and females.

Conclusions

These results prove that this novel assay would serve as an accurate and simple assay for the assessment of the neutralizing antibody titers against HSV-1 in large cohorts.

A rapid and quantitative assay for measuring neutralizing antibodies of Coxsackievirus B3

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We have established a robust single round infection system of CVB3.

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A new quantitative assay for measuring NtAb of CVB3 in clinical samples.

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Seroprevalence of CVB3 in pre-school children is lower than that in adults.

Coxsackievirus B3 (CVB3) infection has been found to account for an increasing proportion cases of hand, foot and mouth disease (HFMD) in recent epidemiology studies. CVB3 is a single stranded, non-enveloped RNA virus and the infection can cause prominent health threat to pre-school children. Here, by taking approaches of reverse genetics, we established a single-round infection system for CVB3. The pseudovirus was produced by sequential transfection of CVB3 capsid expresser plasmid and CVB3 replicon RNA bearing firefly luciferase as a reporter. The CVB3 pseudovirus system was used for quantifying neutralizing antibody (NtAb) levels of 720 human serum samples and showed superior specificity and sensitivity comparing traditional cytopathic effect (CPE) assay. Furthermore, we compared the seroprevalence of CVB3 NtAbs in pre-school children and healthy adults, and found that only 11.94% of pre-school children were NtAbs positive which suggested that most children were naive to CVB3 infection; while there is much higher positive rate in adults (60%) indicating that most adults have experienced CVB3 infection during childhood. This rapid and quantitative assay greatly facilitates evaluating the level of NtAbs against CVB3 in populations and will help to advance CVB3 vaccine development.

Development of a coxsackievirus A16 neutralization test based on the enzyme-linked immunospot assay

Coxsackievirus A16 (CA16) is one of the major pathogens responsible for hand, foot and mouth disease (HFMD). The assessment of the humoral immunity response is indispensable in the development of vaccines against enteroviruses. The neutralization test based on the inhibition of cytopathic effects (Nt-CPE) is a common method for measuring neutralizing antibodies against CA16. However, an efficient neutralization test needs to be developed for seroepidemiological surveys and clinical trials of CA16 vaccines because Nt-CPE is time-consuming and labor-intensive. In this study, a high-throughput neutralization test for CA16 based on the enzyme-linked immunospot assay (Nt-ELISPOT) was developed. The monoclonal antibody 7D10, which reacted with the viral protein VP1, was used to detect the cells infected with CA16. The neutralizing titers of sera were proven to be unchanged over an infectious dose range from 10 to 10,000TCID50 per well. The Nt-ELISPOT results correlated well with the Nt-CPE results (R(2) = 0.9250), and the detection period was shortened from five days to approximately 30h. Overall, the Nt-ELISPOT is a reliable and efficient method for measuring neutralizing antibodies against CA16.

Novel N-benzenesulfonyl sophocarpinol derivatives as coxsackie B virus inhibitors

Novel N-benzenesulfonyl sophocarpinic acid/ester and sophocarpinol derivatives were synthesized and evaluated for their antienteroviral activities against coxsackievirus type B3 (CVB3) from sophocarpine (1), a natural medicine isolated from Chinese herb. Structure-activity relationship (SAR) analysis revealed that the double bond and its geometrical configuration and position at the C-11 attachment did not greatly affect the potency. Among these derivatives, sophocarpinol 24d exerted the promising activities against not only CVB3 but also CVB1, CVB2, CVB5, and CVB6 with IC50 ranging from 0.62 to 3.63 μM (SI from 46 to 275), indicating a broad-spectrum antienteroviral characteristic. The SAR results provided the powerful information for further strategic optimization and development of a novel scaffold of broad-spectrum antiviral candidates against enteroviruses.

In vivo time-related evaluation of a therapeutic neutralization monoclonal antibody against lethal enterovirus 71 infection in a mouse model

Enterovirus 71 (EV71) is a neurotropic virus capable of inducing severe neurological symptoms and death. No direct targeting antivirals are useful in the treatment of severe EV71 infection. Because of low toxicity and good specificity, monoclonal antibodies (MAb) are a potential candidate for the treatment of viral infections. Therefore, we developed an EV71-specific conformational MAb with high in vitro cross-neutralization activity to heterologous EV71 subgenotypes. The in vivo treatment experiment at different days post-infection indicated that a single treatment of MAb CT11F9 within day 3 post-infection fully protected mice from morbidity and mortality (0% PBS vs. 100% at 10 µg/g per body weight ***P<0.0001). Immunohistochemical and histological analysis confirmed that CT11F9 significantly prohibited EV71 VP1 expression in various tissues and prevented EV71-induced myonecrosis. Moreover, thrice-treatment at day 4, 5, 6 post-infection was associated with an increased survival rate (18.2% single vs. 50% thrice at 20 µg/g per body weight), and the mice recovered from limb paralysis. Competitive ELISA also confirmed that CT11F9-recognized epitopes were immunodominant in humans. In conclusion, MAb CT11F9 is an ideal candidate to be humanized and used in severe EV71 infection.