Characterization of monoclonal antibody against SARS coronavirus nucleocapsid antigen and development of an antigen capture ELISA

Recent advances in immunoassay technologies for the detection of human coronavirus infections

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is the seventh coronavirus (CoV) that has spread in humans and has become a global pandemic since late 2019. Efficient and accurate laboratory diagnostic methods are one of the crucial means to control the development of the current pandemic and to prevent potential future outbreaks. Although real-time reverse transcription-polymerase chain reaction (rRT-PCR) is the preferred laboratory method recommended by the World Health Organization (WHO) for diagnosing and screening SARS-CoV-2 infection, the versatile immunoassays still play an important role for pandemic control. They can be used not only as supplemental tools to identify cases missed by rRT-PCR, but also for first-line screening tests in areas with limited medical resources. Moreover, they are also indispensable tools for retrospective epidemiological surveys and the evaluation of the effectiveness of vaccination. In this review, we summarize the mainstream immunoassay methods for human coronaviruses (HCoVs) and address their benefits, limitations, and applications. Then, technical strategies based on bioinformatics and advanced biosensors were proposed to improve the performance of these methods. Finally, future suggestions and possibilities that can lead to higher sensitivity and specificity are provided for further research.

A Review on Potential Electrochemical Point-of-Care Tests Targeting Pandemic Infectious Disease Detection: COVID-19 as a Reference

Fast and accurate point-of-care testing (POCT) of infectious diseases is crucial for diminishing the pandemic miseries. To fight the pandemic coronavirus disease 2019 (COVID-19), numerous interesting electrochemical point-of-care (POC) tests have been evolved to rapidly identify the causal organism SARS-CoV-2 virus, its nucleic acid and antigens, and antibodies of the patients. Many of those electrochemical biosensors are impressive in terms of miniaturization, mass production, ease of use, and speed of test, and they could be recommended for future applications in pandemic-like circumstances. On the other hand, self-diagnosis, sensitivity, specificity, surface chemistry, electrochemical components, device configuration, portability, small analyzers, and other features of the tests can yet be improved. Therefore, this report reviews the developmental trend of electrochemical POC tests (i.e., test platforms and features) reported for the rapid diagnosis of COVID-19 and correlates any significant advancements with relevant references. POCTs incorporating microfluidic/plastic chips, paper devices, nanomaterial-aided platforms, smartphone integration, self-diagnosis, and epidemiological reporting attributes are also surfed to help with future pandemic preparedness. This review especially screens the low-cost and easily affordable setups so that management of pandemic disease becomes faster and easier. Overall, the review is a wide-ranging package for finding appropriate strategies of electrochemical POCT targeting pandemic infectious disease detection.

Aptamers as promising nanotheranostic tools in the COVID-19 pandemic era

The emergence of SARS‐COV‐2, the causative agent of new coronavirus disease (COVID‐19) has become a pandemic threat. Early and precise detection of the virus is vital for effective diagnosis and treatment. Various testing kits and assays, including nucleic acid detection methods, antigen tests, serological tests, and enzyme‐linked immunosorbent assay (ELISA), have been implemented or are being explored to detect the virus and/or characterize cellular and antibody responses to the infection. However, these approaches have inherent drawbacks such as nonspecificity, high cost, are characterized by long turnaround times for test results, and can be labor‐intensive. Also, the circulating SARS‐COV‐2 variant of concerns, reduced antibody sensitivity and/or neutralization, and possible antibody‐dependent enhancement (ADE) have warranted the search for alternative potent therapeutics. Aptamers, which are single‐stranded oligonucleotides, generated artificially by SELEX (Evolution of Ligands by Exponential Enrichment) may offer the capacity to generate high‐affinity neutralizers and/or bioprobes for monitoring relevant SARS‐COV‐2 and COVID‐19 biomarkers. This article reviews and discusses the prospects of implementing aptamers for rapid point‐of‐care detection and treatment of SARS‐COV‐2. We highlight other SARS‐COV‐2 targets (N protein, spike protein stem‐helix), SELEX augmented with competition assays and in silico technologies for rapid discovery and isolation of theranostic aptamers against COVID‐19 and future pandemics. It further provides an overview on site‐specific bioconjugation approaches, customizable molecular scaffolding strategies, and nanotechnology platforms to engineer these aptamers into ultrapotent blockers, multivalent therapeutics, and vaccines to boost both humoral and cellular immunity against the virus.

This article is categorized under:

Therapeutic Approaches and Drug Discovery > Emerging Technologies

Diagnostic Tools > Biosensing

Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease

Therapeutic Approaches and Drug Discovery > Nanomedicine for Respiratory Disease

A comprehensive review on efficient approaches for combating coronaviruses

Almost 80% of people confronting COVID-19 recover from COVID-19 disease without any particular treatments. They experience heterogeneous symptoms; a wide range of respiratory symptoms, cough, dyspnea, fever, and viral pneumonia. However, some others need urgent intervention and special treatment to get rid of this widespread disease. So far, there isn't any unique drug for the potential treatment of COVID 19. However, some available therapeutic drugs used for other diseases seem beneficial for the COVID-19 treatment. On the other hand, there is a robust global concern for developing an efficient COVID-19 vaccine to control the COVID-19 pandemic sustainably. According to the WHO report, since 8 October 2021, 320 vaccines have been in progress. 194 vaccines are in the pre-clinical development stage that 126 of them are in clinical progression. Here, in this paper, we have comprehensively reviewed the most recent and updated information about coronavirus and its mutations, all the potential therapeutic approaches for treating COVID-19, developed diagnostic systems for COVID- 19 and the available COVID-19 vaccines and their mechanism of action.

Development and Evaluation of Quantitative Immunoglobulin G Enzyme-Linked Immunosorbent Assay for the Diagnosis of Coronavirus Disease 2019 Using Truncated Recombinant Nucleocapsid Protein as Assay Antigen

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of coronavirus disease 2019 (COVID-19). Real-time RT-PCR is the most commonly used method for COVID-19 diagnosis. However, serological assays are urgently needed as complementary tools to RT-PCR. Hachim et al. 2020 and Burbelo et al. 2020 demonstrated that anti-nucleocapsid(N) SARS-CoV-2 antibodies are higher and appear earlier than the spike antibodies. Additionally, cross-reactive antibodies against N protein are more prevalent than those against spike protein. We developed a less cross-reactive immunoglobulin G (IgG) indirect ELISA by using a truncated recombinant SARS-CoV-2 N protein as assay antigen. A highly conserved region of coronaviruses N protein was deleted and the protein was prepared using an E. coli protein expression system. A total of 177 samples collected from COVID-19 suspected cases and 155 negative control sera collected during the pre-COVID-19 period were applied to evaluate the assay's performance, with the plaque reduction neutralization test and the commercial SARS-CoV-2 spike protein IgG ELISA as gold standards. The SARS-CoV-2 N truncated protein-based ELISA showed similar sensitivity (91.1% vs. 91.9%) and specificity (93.8% vs. 93.8%) between the PRNT and spike IgG ELISA, as well as also higher specificity compared to the full-length N protein (93.8% vs. 89.9%). Our ELISA can be used for the diagnosis and surveillance of COVID-19.

Nanotechnology-Based Approaches for the Detection of SARS-CoV-2

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as a pandemic has been validated as an extreme clinical calamity and has affected several socio-economic activities globally. Proven transmission of this virus occurs through airborne droplets from an infected person. The recent upsurge in the number of infected individuals has already exceeded the number of intensive care beds available to patients. These extraordinary circumstances have elicited the need for the development of diagnostic tools for the detection of the virus and, hence, prevent the spread of the disease. Early diagnosis and effective immediate treatment can reduce and prevent an increase in the number of cases. Conventional methods of detection such as quantitative real-time polymerase chain reaction and chest computed tomography scans have been used extensively for diagnostic purposes. However, these present several challenges, including prolonged assay requirements, labor-intensive testing, low sensitivity, and unavailability of these resources in remote locations. Such challenges urgently require fast, sensitive, and accurate diagnostic techniques for the timely detection and treatment of coronavirus disease 2019 (COVID-19) infections. Point-of-care biosensors that include paper- and chip-based diagnostic systems are rapid, cost-effective, and user friendly. In this article nanotechnology-based potential biosensors for SARS-CoV-2 diagnosis are discussed with particular emphasis on a lateral flow assay, a surface-enhanced Raman scattering-based biosensor, a localized surface plasmon resonance-based biosensor, Förster resonance energy transfer, an electrochemical biosensor, and artificial intelligence-based biosensors. Several biomolecules, such as nucleic acids, antibodies/enzymes, or aptamers, can serve as potential detection molecules on an appropriate platform, such as graphene oxide, nanoparticles, or quantum dots. An effective biosensor can be developed by using appropriate combinations of nanomaterials and technologies.

Developments in biosensors for CoV detection and future trends

This review summarizes the state of art of biosensor technology for Coronavirus (CoV) detection, the current challenges and the future perspectives. Three categories of affinity-based biosensors (ABBs) have been developed, depending on their transduction mechanism, namely electrochemical, optical and piezoelectric biosensors. The biorecognition elements include antibodies and DNA, which undergo important non-covalent binding interactions, with the formation of antigen-antibody and ssDNA/oligonucleotide-complementary strand complexes in immuno- and DNA-sensors, respectively. The analytical performances, the advantages and drawbacks of each type of biosensor are highlighted, discussed, and compared to traditional methods. It is hoped that this review will encourage scientists and academics to design and develop new biosensing platforms for point-of-care (POC) diagnostics to manage the coronavirus disease 2019 (COVID-19) pandemic, providing interesting reference for future studies.

Aptamers, the bivalent agents as probes and therapies for coronavirus infections: A systematic review

The recently known coronavirus, SARS-CoV-2, has turn into the greatest global health challenge, affecting a large number of societies. The lack of specific treatment and gold-standard diagnostic system has made the situation more complicated. Efforts have led to production of several diagnostic kits that are associated with limitations such as inadequate sensitivity and accuracy. Aptamers as multipotent biological probes could be promising candidates to design sensitive and specific biosensors. Although few studies have introduced specific aptamer types of coronavirus, they may help us select the best approach to obtain specific aptamers for this virus. On the other hand, some of already-introduced aptamers have shown the inhibitory effects on coronavirus that could be applied as therapeutics. The present study has provided a systematic overview on use of aptamer-based biosensors and drugs to diagnose and treat coronavirus.

Advances in Viral Diagnostic Technologies for Combating COVID-19 and Future Pandemics

The emergence of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) threatens the health of the global population and challenges our preparedness for pandemic threats. Previous outbreaks of coronaviruses and other viruses have suggested the importance of diagnostic technologies in fighting viral outbreaks. Nucleic acid detection techniques are the gold standard for detecting SARS-CoV-2. Viral antigen tests and serological tests that detect host antibodies have also been developed for studying the epidemiology of COVID-19 and estimating the population that may have immunity to SARS-CoV-2. Nevertheless, the availability, cost, and performance of existing viral diagnostic technologies limit their practicality, and novel approaches are required for improving our readiness for global pandemics. Here, we review the principles and limitations of major viral diagnostic technologies and highlight recent advances of molecular assays for COVID-19. In addition, we discuss emerging technologies, such as clustered regularly interspaced short palindromic repeats (CRISPR) systems, high-throughput sequencing, and single-cell and single-molecule analysis, for improving our ability to understand, trace, and contain viral outbreaks. The prospects of viral diagnostic technologies for combating future pandemic threats are presented.

Analysis of preferred codon usage in the coronavirus N genes and their implications for genome evolution and vaccine design

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The nucleotide variations among the N genes of 13 different coronaviruses (CoVs) were interpreted.

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Overall, 18 amino acids observed with varying preferred codons.

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The effective number of codon values ranged from 40.43 to 53.85, revealing a slight codon bias.

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A highly significant correlation between GC3s and ENc values was observed in porcine epidemic diarrhea CoV, followed by Middle East respiratory syndrome CoV.

The nucleocapsid (N) protein of a coronavirus plays a crucial role in virus assembly and in its RNA transcription. It is important to characterize a virus at the nucleotide level to discover the virus’s genomic sequence variations and similarities relative to other viruses that could have an impact on the functions of its genes and proteins. This entails a comprehensive and comparative analysis of the viral genomes of interest for preferred nucleotides, codon bias, nucleotide changes at the 3^rd position (NT3s), synonymous codon usage and relative synonymous codon usage. In this study, the variations in the N proteins among 13 different coronaviruses (CoVs) were analysed at the nucleotide and amino acid levels in an attempt to reveal how these viruses adapt to their hosts relative to their preferred codon usage in the N genes. The results revealed that, overall, eighteen amino acids had different preferred codons and eight of these were over-biased. The N genes had a higher AT% over GC% and the values of their effective number of codons ranged from 40.43 to 53.85, indicating a slight codon bias. Neutrality plots and correlation analyses showed a very high level of GC3s/GC correlation in porcine epidemic diarrhea CoV (pedCoV), followed by Middle East respiratory syndrome-CoV (MERS CoV), porcine delta CoV (dCoV), bat CoV (bCoV) and feline CoV (fCoV) with r values 0.81, 0.68, -0.47, 0.98 and 0.58, respectively. These data implied a high rate of evolution of the CoV genomes and a strong influence of mutation on evolutionary selection in the CoV N genes. This type of genetic analysis would be useful for evaluating a virus’s host adaptation, evolution and is thus of value to vaccine design strategies.

CD8+ T Cells Responding to the Middle East Respiratory Syndrome Coronavirus Nucleocapsid Protein Delivered by Vaccinia Virus MVA in Mice

Middle East respiratory syndrome coronavirus (MERS-CoV), a novel infectious agent causing severe respiratory disease and death in humans, was first described in 2012. Antibodies directed against the MERS-CoV spike (S) protein are thought to play a major role in controlling MERS-CoV infection and in mediating vaccine-induced protective immunity. In contrast, relatively little is known about the role of T cell responses and the antigenic targets of MERS-CoV that are recognized by CD8+ T cells. In this study, the highly conserved MERS-CoV nucleocapsid (N) protein served as a target immunogen to elicit MERS-CoV-specific cellular immune responses. Modified Vaccinia virus Ankara (MVA), a safety-tested strain of vaccinia virus for preclinical and clinical vaccine research, was used for generating MVA-MERS-N expressing recombinant N protein. Overlapping peptides spanning the whole MERS-CoV N polypeptide were used to identify major histocompatibility complex class I/II-restricted T cell responses in BALB/c mice immunized with MVA-MERS-N. We have identified a H2-d restricted decamer peptide epitope in the MERS-N protein with CD8+ T cell antigenicity. The identification of this epitope, and the availability of the MVA-MERS-N candidate vaccine, will help to evaluate MERS-N-specific immune responses and the potential immune correlates of vaccine-mediated protection in the appropriate murine models of MERS-CoV infection.

Development of Monoclonal Antibody and Diagnostic Test for Middle East Respiratory Syndrome Coronavirus Using Cell-Free Synthesized Nucleocapsid Antigen

Protein nativity is one of the most critical factors for the quality of antigens used as immunogens and the reactivities of the resultant antibodies. The preparation and purification of native viral antigens in conventional cell-based protein expression systems are often accompanied by technical hardships. These challenges are attributable mainly to protein aggregation and insolubility during expression and purification, as well as to very low expression levels associated with the toxicity of some viral proteins. Here, we describe a novel approach for the production of monoclonal antibodies (mAbs) against nucleocapsid protein (NP) of the Middle East respiratory syndrome coronavirus (MERS-CoV). Using a wheat germ cell-free protein synthesis system, we successfully prepared large amounts of MERS-CoV NP antigen in a state that was highly soluble and intact for immunization. Following mouse immunization and hybridoma generation, we selected seven hybridoma clones that produced mAbs with exclusive reactivity against MERS-CoV NP. Epitope mapping and subsequent bioinformatic analysis revealed that these mAbs recognized epitopes located within relatively highly conserved regions of the MERS-CoV amino-acid sequence. Consistently, the mAbs exhibited no obvious cross-reactivity with NPs derived from other related viruses, including SARS coronavirus. After determining the optimal combinations of these mAbs, we developed an enzyme-linked immunosorbent assay and a rapid immunochromatographic antigen detection test that can be reliably used for laboratory diagnosis of MERS-CoV. Thus, this study provides strong evidence that the wheat germ cell-free system is useful for the production of diagnostic mAbs against emerging pathogens.

Development and characterization of neutralizing monoclonal antibodies against the S1 subunit protein of QX-like avian infectious bronchitis virus strain Sczy3

Infectious bronchitis (IB) is a highly contagious disease in chickens caused by infectious bronchitis virus (IBV). The present study was carried out with the aim to develop anti-spike 1 (S1) subunit monoclonal antibodies (MAbs) that could react with IBV strains of different genotypes. The high antigenicity region of S1 gene of an QX-like IBV strain Sczy3 was amplified and ligated into the prokaryotic expression vector pET-32a(+), and the recombinant His-S1 fusion proteins were expressed and purified. The purified whole viral antigen of Sczy3 strain was used to immunize BALB/c mice to produce hybridoma-secreting anti-IBV MAbs. Eleven anti-IBV MAbs were generated, and two MAbs 1C8 and 2C10 were positive in indirect ELISA against both His-S1 protein and the purified whole viral antigen. These two MAbs showed positive reaction with IBV in Western blot, and the isotype were both IgM. These two MAbs react specifically with IBV but not with Newcastle disease virus (NDV) or avian influenza virus (AIV) subtype H9 or H5, and could cross-react with other 10 IBV strains in five different genotypes. End-point neutralizing assay performed in chicken embro kidney (CEK) cells revealed that the neutralization titer of 1C8 and 2C10 against Sczy3 reached 1:2.82 and 1:4.70, respectively. The anti-S1 MAbs produced in the present work may be valuable in developing an antigen-capture ELISA test for antigen detection or a competitive ELISA test for antibody detection or therapeutic medicine for IB in poultry.

Aptasensors for viral diagnostics

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We discuss progress in aptamer-based detection of viruses.

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We consider the use of aptasensors for point-of-care diagnostics of viruses.

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Aptamers have distinct advantages over antibodies for virus recognition.

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There is strong demand for multiplexed diagnostic measurement of pathogens.

Novel viral diagnostic tools need to be affordable, fast, accurate and easy to use with sensitivity and specificity equivalent or superior to current standards. At present, viral diagnostics are based on direct detection of viral components or indirect detection by measuring antibodies generated in response to viral infection. While sensitivity of detection and quantification are still important challenges, we expect major advances from new assay formats and synthetic binding molecules, such as aptamers. Compared to traditional antibody-based detection, aptamers could provide faster adaptation to continuously evolving virus strains and higher discriminating capacity between specific virus serotypes. Aptamers are very stable and easily modifiable, so are ideal molecules for detection and chemical sensing applications. Here, we review the use of aptasensors for detection of viral pathogens and consider the feasibility of aptasensors to become standard devices for point-of-care diagnostics of viruses.

Single-Stranded DNA Aptamers against Pathogens and Toxins: Identification and Biosensing Applications

Molecular recognition elements (MREs) can be short sequences of single-stranded DNA, RNA, small peptides, or antibody fragments. They can bind to user-defined targets with high affinity and specificity. There has been an increasing interest in the identification and application of nucleic acid molecular recognition elements, commonly known as aptamers, since they were first described in 1990 by the Gold and Szostak laboratories. A large number of target specific nucleic acids MREs and their applications are currently in the literature. This review first describes the general methodologies used in identifying single-stranded DNA (ssDNA) aptamers. It then summarizes advancements in the identification and biosensing application of ssDNA aptamers specific for bacteria, viruses, their associated molecules, and selected chemical toxins. Lastly, an overview of the basic principles of ssDNA aptamer-based biosensors is discussed.

Immunoreactivity characterisation of the three structural regions of the human coronavirus OC43 nucleocapsid protein by Western blot: implications for the diagnosis of coronavirus infection

Previous studies have reported that a prokaryotic-expressed recombinant nucleocapsid protein (NP) is a suitable reagent for the epidemiological screening of coronavirus infection. In this study, soluble recombinant human coronavirus OC43 (HCoV-OC43) NP was produced to examine the antigenicity of the HCoV-OC43 NP of betacoronavirus. Using the purified recombinant NP as an antigen, a polyclonal antibody from rabbit serum with specificity for HCoV-OC43 NP was generated; this antibody reacts specifically with HCoV-OC43 NP and does not cross-react with other human CoV NPs (including those of SARS-CoV and HCoV-229E) by Western blot. Sera from 26 young adults, 17 middle-aged and elderly patients with respiratory infection, and 15 cord blood samples were also tested. Strong reactivity to the NPs of HCoV-OC43 was observed in 96%, 82%, and 93% of the serum samples from the young adults, respiratory patients, and cord blood samples, respectively. To identify the immunoreactivities of the three structural regions of the NP that are recognised by the rabbit polyclonal antibody and human serum, the antigenicities of three protein fragments, including the N-terminal domain (aa 1-173), the central-linker region (aa 174-300), and the C-terminal domain (aa 301-448), were evaluated by Western blot. The rabbit polyclonal antibody demonstrated greater immunoreactivity to the central-linker region and the C-terminal domain than to the N-terminal domain. Three different patterns for the immunoreactivities of the three structural regions of HCoV-OC43 NP were observed in human serum, suggesting variability in the immune responses that occur during HCoV-OC43 infection in humans. The central-linker region of the NP appeared to be the most highly immunoreactive region for all three patterns observed. The goal of this study was to offer insight into the design of diagnostic tools for HCoV infection.

Quantitative and sensitive detection of the SARS-CoV spike protein using bispecific monoclonal antibody-based enzyme-linked immunoassay

The severe acute respiratory syndrome coronavirus (SARS-CoV) spike protein is known to mediate receptor interaction and immune recognition and thus it is considered as a major target for vaccine design. The spike protein plays an important role in virus entry, virus receptor interactions, and virus tropism. Sensitive diagnosis of SARS is essential for the control of the disease in humans. Recombinant SARS-CoV S1 antigen was produced and purified for the development of monoclonal and bi-specific monoclonal antibodies. The hybridomas secreting anti-S1 antibodies, F26G18 and P136.8D12, were fused respectively with the YP4 hybridoma to generate quadromas. The sandwich ELISA was formed by using F26G18 as a coating antibody and biotinylated F26G18 as a detection antibody with a detection limit of 0.037 μg/ml (p < 0.02). The same detection limit was found with P136.8D12 as a coating antibody and biotinylated F26G18 as a detection antibody. The sensitivity was improved (detection limit of 0.019 μg/ml), however, when using bi-specific monoclonal antibody (F157) as the detection antibody. In conclusion, the method described in this study allows sensitive detection of a recombinant SARS spike protein by sandwich ELISA with bi-specific monoclonal antibody and could be used for the diagnosis of patients suspected with SARS.

Novel system for detecting SARS coronavirus nucleocapsid protein using an ssDNA aptamer

The outbreak of severe acute respiratory syndrome (SARS) in 2002 affected thousands of people and an efficient diagnostic system is needed for accurate detection of SARS coronavirus (SARS CoV) to prevent or limit future outbreaks. Of the several SARS CoV structural proteins, the nucleocapsid protein has been shown to be a good diagnostic marker. In this study, an ssDNA aptamer that specifically binds to SARS CoV nucleocapsid protein was isolated from a DNA library containing 45-nuceotide random sequences in the middle of an 88mer single-stranded DNA. After twelve cycles of systematic evolution of ligands by exponential enrichment (SELEX) procedure, 15 ssDNA aptamers were identified. Enzyme-linked immunosorbent assay (ELISA) analysis was then used to identify the aptamer with the highest binding affinity to the SARS CoV nucleocapsid protein. Using this approach, an ssDNA aptamer that binds to the nucleocapsid protein with a K(d) of 4.93±0.30nM was identified. Western blot analysis further demonstrated that this ssDNA aptamer could be used to efficiently detect the SARS CoV nucleocapsid protein when compared with a nucleocapsid antibody. Therefore, we believe that the selected ssDNA aptamer may be a good alternative detection probe for the rapid and sensitive detection of SARS.

Production of specific antibodies against SARS-coronavirus nucleocapsid protein without cross reactivity with human coronaviruses 229E and OC43

Severe acute respiratory syndrome (SARS) is a life-threatening disease for which accurate diagnosis is essential. Although many tools have been developed for the diagnosis of SARS, false-positive reactions in negative sera may occur because of cross-reactivity with other coronaviruses. We have raised polyclonal and monoclonal antibodies (Abs) using a recombinant form of the SARS virus nucleocapsid protein. Cross-reactivity of these anti-SARS Abs against human coronavirus (HCoV) 229E and HCoV OC43 were determined by Western blotting. The Abs produced reacted with recombinant SARS virus nucleocapsid protein, but not with HCoV 229E or HCoV OC43.

The Nucleocapsid Protein of the SARS Coronavirus: Structure, Function and Therapeutic Potential

As in other coronaviruses, the nucleocapsid protein is one of the core components of the SARS coronavirus (CoV). It oligomerizes to form a closed capsule, inside which the genomic RNA is securely stored thus providing the SARS-CoV genome with its first line of defense from the harsh conditions of the host environment and aiding in replication and propagation of the virus. In addition to this function, several reports have suggested that the SARS-CoV nucleocapsid protein modulates various host cellular processes, so as to make the internal milieu of the host more conducive for survival of the virus. This article will analyze and discuss the available literature regarding these different properties of the nucleocapsid protein. Towards the end of the article, we will also discuss some recent reports regarding the possible clinically relevant use of the nucleocapsid protein, as a candidate diagnostic tool and vaccine against SARS-CoV infection.

RNA aptamer-based sensitive detection of SARS coronavirus nucleocapsid protein

Severe acute respiratory syndrome coronavirus (SARS-CoV) is the etiological agent of a newly emerged disease SARS. The SARS-CoV nucleocapsid (N) protein is one of the most abundant structural proteins and serves as a diagnostic marker for accurate and sensitive detection of the virus. Using a SELEX (systematic evolution of ligand by exponential enrichment) procedure and recombinant N protein, we selected a high-affinity RNA aptamer capable of binding to N protein with a dissociation constant of 1.65 nM. Electrophoretic mobility shift assays and RNA competition experiments showed that the selected aptamer recognized selectively the C-terminal region of N protein with high specificity. Using a chemiluminescence immunosorbent assay and a nanoarray aptamer chip with the selected aptamer as an antigen-capturing agent, we could sensitively detect N protein at a concentration as low as 2 pg/ml. These aptamer-antibody hybrid immunoassays may be useful for rapid, sensitive detection of SARS-CoV N protein.

Detection of severe acute respiratory syndrome (SARS) coronavirus nucleocapsid protein in human serum using a localized surface plasmon coupled fluorescence fiber-optic biosensor

In order to enhance the sensitivity of conventional immunoassay technology for the detection of SARS coronavirus (SARS-CoV) nucleocapsid protein (N protein), we developed a localized surface plasmon coupled fluorescence (LSPCF) fiber-optic biosensor that combines sandwich immunoassay with the LSP technique. Experimentally, a linear relationship between the fluorescence signal and the concentration of recombinant SARS-CoV N (GST-N) protein in buffer solution could be observed from 0.1 pg/mL to 1 ng/mL. In addition, the concentration of GST-N protein in diluted serum across a similar range could also be measured. The correlation coefficients (linear scale) for these two measurements were 0.9469 and 0.9624, respectively. In comparison with conventional enzyme linked immunosorbent assay (ELISA), the detection limit of the LSPCF fiber-optic biosensor for the GST-N protein was improved at least 10⁴-fold using the same monoclonal antibodies. Therefore, the LSPCF fiber-optic biosensor shows an ability to detect very low concentration (∼1 pg/mL) of SARS-CoV N protein in serum. The biosensor should help with the early diagnosis of SARS infection.

Molecular targets for diagnostics and therapeutics of severe acute respiratory syndrome (SARS-CoV)

PURPOSE

The large number of deaths in a short period of time due to the spread of severe acute respiratory syndrome (SARS) infection led to the unparalleled collaborative efforts world wide to determine and characterize the new coronavirus (SARS-CoV). The full genome sequence was determined within weeks of the first outbreak by the Canadian group with international collaboration. As per the World Health Organization (WHO), the continual lack of a rapid laboratory test to aid the early diagnosis of suspected cases of SARS makes this area a priority for future research. To prevent deaths in the future, early diagnosis and therapy of this infectious disease is of paramount importance.

METHODS

This review describes the specific molecular targets for diagnostics and therapeutics of viral infection.

RESULTS

The three major diagnostic methods available for SARS includes viral RNA detection by reverse transcription polymerase chain reaction (RT-PCR), virus induced antibodies by immunofluorescence assay (IFA) or by enzyme linked immunosorbant assay (ELISA) of nucleocapsid protein (NP). The spike glycoprotein of SARS-CoV is the major inducer of neutralizing antibodies. The receptor binding domain (RBD) in the S1 region of the spike glycoprotein contains multiple conformational epitopes that induces highly potent neutralizing antibodies. The genetically engineered attenuated form of the virus or viral vector vaccine encoding for the SARS-CoV spike glycoprotein has been shown to elicit protective immunity in vaccinated animals.

CONCLUSION

NP is the preferred target for routine detection of SARS-CoV infection by ELISA which is an economical method compared to other methods. The RBD of the spike glycoprotein is both a functional domain for cell receptor binding and also a major neutralizing determinant of SARS-CoV. The progress in evaluating a therapeutic or vaccine would depend on the avail ability of clinically relevant animal model.

Determination and application of immunodominant regions of SARS coronavirus spike and nucleocapsid proteins recognized by sera from different animal species

Knowledge of immunodominant regions in major viral antigens is important for rational design of effective vaccines and diagnostic tests. Although there have been many reports of such work done for SARS–CoV, these were mainly focused on the immune responses of humans and mice. In this study, we aim to search for and compare immunodominant regions of the spike (S) and nucleocapsid (N) proteins which are recognized by sera from different animal species, including mouse, rat, rabbit, civet, pig and horse. Twelve overlapping recombinant protein fragments were produced in Escherichia coli, six each for the S and N proteins, which covered the entire coding region of the two proteins. Using a membrane-strip based Western blot approach, the reactivity of each antigen fragment against a panel of animal sera was determined. Immunodominant regions containing linear epitopes, which reacted with sera from all the species tested, were identified for both proteins. The S3 fragment (aa 402–622) and the N4 fragment (aa 220–336) were the most immunodominant among the six S and N fragments, respectively. Antibodies raised against the S3 fragment were able to block the binding of a panel of S-specific monoclonal antibodies (mAb) to SARS–CoV in ELISA, further demonstrating the immunodominance of this region. Based on these findings, one-step competition ELISAs were established which were able to detect SARS–CoV antibodies from human and at least seven different animal species. Considering that a large number of animal species are known to be susceptible to SARS–CoV, these assays will be a useful tool to trace the origin and transmission of SARS–CoV and to minimise the risk of animal-to-human transmission.

Severe acute respiratory syndrome coronavirus as an agent of emerging and reemerging infection

Before the emergence of severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) in 2003, only 12 other animal or human coronaviruses were known. The discovery of this virus was soon followed by the discovery of the civet and bat SARS-CoV and the human coronaviruses NL63 and HKU1. Surveillance of coronaviruses in many animal species has increased the number on the list of coronaviruses to at least 36. The explosive nature of the first SARS epidemic, the high mortality, its transient reemergence a year later, and economic disruptions led to a rush on research of the epidemiological, clinical, pathological, immunological, virological, and other basic scientific aspects of the virus and the disease. This research resulted in over 4,000 publications, only some of the most representative works of which could be reviewed in this article. The marked increase in the understanding of the virus and the disease within such a short time has allowed the development of diagnostic tests, animal models, antivirals, vaccines, and epidemiological and infection control measures, which could prove to be useful in randomized control trials if SARS should return. The findings that horseshoe bats are the natural reservoir for SARS-CoV-like virus and that civets are the amplification host highlight the importance of wildlife and biosecurity in farms and wet markets, which can serve as the source and amplification centers for emerging infections.

Sensitive and specific enzyme-linked immunosorbent assay using chemiluminescence for detection of severe acute respiratory syndrome viral infection

Here we report the development of a more-sensitive immunoassay for severe acute respiratory syndrome (SARS) based on an enzyme-linked immunosorbent assay using chemiluminescence (CLEIA) to detect the viral nucleocapsid (N) antigen in nasopharyngeal aspirate (NPA) from patients infected with SARS coronavirus (CoV). The CLEIA was established with an optical combination of monoclonal antibodies (MAbs) against SARS CoV N protein prepared from mice immunized with recombinant N protein without cultivating the virus. The capture and detecting MAbs of the CLEIA reacted to the carboxyl-terminal and amino-terminal peptides of the N protein, respectively. The CLEIA was capable of detecting recombinant N protein at 1.56 pg/ml and viral N protein in SARS CoV cell culture lysates at 0.087 of 50% tissue culture infective doses/ml. The CLEIA showed no cross-reactivities to recombinant N proteins of common human CoV (229E, OC43, and NL63) or lysates of cells infected with 229E and OC43. In addition, an evaluation with 18 SARS-positive NPA samples, all confirmed SARS positive by quantitative PCR and antibodies to SARS CoV, revealed that all (18/18) were found positive by the CLEIA; thus, the sensitivity of detection was 100%. When we tested 20 SARS-negative NPA samples, the CLEIA was shown to have high specificity (100%). The sensitivity of our novel SARS CLEIA was significantly higher than the previous EIA and comparable to the other methods using reverse transcription-PCR.

The SARS-CoV nucleocapsid protein: a protein with multifarious activities

Ever since the discovery of SARS-CoV in the year 2003, numerous researchers around the world have been working relentlessly to understand the biology of this virus. As in other coronaviruses, nucleocapsid (N) is one of the most crucial structural components of the SARS-CoV. Hence major attention has been focused on characterization of this protein. Independent studies conducted by several laboratories have elucidated significant insight into the primary function of this protein, which is to encapsidate the viral genome. In addition, many reports also suggest that this protein interferes with different cellular pathways, thus implying it to be a key regulatory component of the virus too. In the first part of this review, we will discuss these different properties of the N-protein in a consolidated manner. Further, this protein has also been proposed to be an efficient diagnostic tool and a candidate vaccine against the SARS-CoV. Hence, towards the end of this article, we will discuss some recent progress regarding the possible clinically relevant use of the N-protein.

Recombinant viral proteins for use in diagnostic ELISAs to detect virus infection

ELISAs provide a valuable tool in the detection and diagnosis of virus infection. The ability to produce recombinant viral proteins will ensure that future ELISAs are safe, specific and rapid. This latter point being the most crucial advantage in that even if a virus cannot be cultured, provided gene sequence is available, it is possible to rapidly respond to emerging viruses and new viral strains of existing pathogens. Indeed, ELISAs based on peptides (corresponding to epitopes) also hold great promise, as in this case no cloning or expression of a recombinant protein is required. Both recombinant protein and peptide based systems lend themselves to large scale production and purification. These approaches can also be used to distinguish recombinant vaccines from parental or wild type viruses.

Recombinant truncated nucleocapsid protein as antigen in a novel immunoglobulin M capture enzyme-linked immunosorbent assay for diagnosis of severe acute respiratory syndrome coronavirus infection

We report the development of an immunoglobulin M (IgM) antibody capture enzyme-linked immunosorbent assay (MAC-ELISA) for severe acute respiratory syndrome coronavirus (SARS-CoV) by using recombinant truncated SARS-CoV nucleocapsid protein as the antigen. The newly developed MAC-ELISA had a specificity and sensitivity of 100% as evaluated by using sera from healthy volunteers and patients with laboratory-confirmed SARS. Using serial serum samples collected from SARS patients, the times to seroconversion were determined by IgM antibody detection after SARS-CoV infection. The median time to seroconversion detection was 8 days (range, 5 to 17 days) after disease onset, and the seroconversion rates after the onset of illness were 33% by the first week, 97% by the second week, and 100% by the third week. Compared with the results of our previous report on the detection of IgG, the median seroconversion time by IgM detection was 3 days earlier and the seroconversion rate by the second week after the illness for IgM was significantly higher than by IgG assay. Our results indicating that the IgM response appears earlier than IgG after SARS-CoV infection in consistent with those for other pathogens. Our newly developed MAC-ELISA system offers a new alternative for the confirmation of SARS-CoV infection.

Analysis of proteins that interact with nucleocapsid protein of SARS-CoV using 15-mer phage-displayed library

Analysis of proteins that interact with N protein of SARS-CoV using 15-mer phage-displayed library will help to explore the virus pathogenesis and to develop new drugs and vaccines against SARS. In this study, we cloned, expressed and purified N protein of SARS-CoV. This 46-kD N protein was verified by SDS-PAGE and Western-blot. Then, the peptides binding-specific to N protein were identified using 15-mer phage-displayed library. Surprisingly, all of the 89 clones from monoclonal ELISA were positive (S/N>2.1) and the result was further confirmed experimentally once again. Six N protein-binding peptides, designated separately as SNA1, SNA2, SNA4, SNA5, SNA9 and SNG11, were selected for sequencing. Sequence analysis suggested that SNA5 shared approximatively 100% sequence identity to SNA4, SNA2, SNA9 and SNA1. In addition, the binding specificity of the 15-mer peptides with the SARS-CoV N protein was further demonstrated by blocking ELISA using the synthetical 15-mer peptide according to the deduced amino acid sequence of SNA5. Also, the deduced amino sequence of SNA5 was compared with proteins in translated database using the tblastx program, and the results showed that the proteins with the highest homology were Ubiquinol-cytochrome c reductase iron-sulfur subunits (UCRI or UQCR), otherwise known as the Rieske iron-sulfur proteins (RISP). Notablely, in the [2Fe-2S] redox centre of UCRI, there were 6 residues [GGW(Y)F(Y)CP] compatible to the residues (position 2→7, GGWFCP7) of the NH2-terminal of the 15-mer peptide, which indicated higher binding specificity between the N protein of SARS-CoV and the redox centre of UCRI to some extent. Here, the possible molecular mechanisms of SARS-CoV N protein in the pathogenesis of SARS are discussed.

Microarray-in-a-tube for detection of multiple viruses

Background: The detection of multiple viruses is important for pathogenic diagnosis and disease control. Microarray detection is a good method, but requires complex procedures for multiple virus detection.

Methods: We developed a novel PCR assay, the microarray-in-a-tube system, which integrates multiple PCR processes and DNA microarrays for multiple virus detection. A 5 × 5 oligonucleotide microarray for detecting 4 respiratory tract viruses (severe acute respiratory syndrome–associated coronavirus, influenza A virus, influenza B virus, and enterovirus) with inner controls was arranged on the inner surface of a specially designed Eppendorf cap with a flat, optically transparent window.

Results: We were able to perform all detection processes in the encapsulated system without opening the cap. The 4 viruses were successfully amplified by one-step reverse transcription–PCR in the encapsulated tube. After the PCR process, the microarray-in-a-tube was inverted, and the fluorescence-labeled PCR products were directly hybridized on the microarray. Hybridization signals were obtained with an ordinary fluorescent microscope. The sensitivity of the system for virus detection reached 10² copies/μL. With the help of inner controls, the system provided reliable results without false negatives and false positives.

Conclusions: The microarray-in-a-tube system is a rapid, labor-saving tool for multiple virus detection with several advantages, such as convenience, prevention of cross-contamination of the PCR products, and potential for multiple-gene detection.

Identification of a B-cell antigenic epitope at the N-terminus of SARS-CoV M protein and characterization of monoclonal antibody against the protein

To identify the potential B-cell antigenic epitopes within the N-terminus of SARS-CoV (SARS-associated coronavirus, SARS-CoV) M protein and characterize monoclonal antibody (MAb) against the protein as well as its recognizing region, we expressed and purified a portion of SARS-CoV M protein (amino acid 1–43) in Escherichia coli (E. coli). By using Western blot and enzyme-linked immunosorbent assay (ELISA), we showed that the purified recombinant M protein could be recognized by four SARS-CoV-positive human sera even when those sera were 12,800-fold diluted. Furthermore, we characterized one representative IgG2 MAb, 3H9, which exhibited a strong immunoreaction to both recombinant M protein and native viral protein of SARS-CoV. We found a B-cell antigenic epitope located between amino acid 1–15 and defined the MAb recognizing region within amino acid 16–28 of M. These findings not only suggest that both recombinant M protein and its specific MAbs may be used as the diagnostic reagents for SARS, but also provide a potential target site for the design of an epitope-based vaccine against SARS.

Naturally occurring anti-Escherichia coli protein antibodies in the sera of healthy humans cause analytical interference in a recombinant nucleocapsid protein-based enzyme-linked immunosorbent assay for serodiagnosis of severe acute respiratory syndrome

We reported the analytical interference of anti-Escherichia coli protein (EP) antibodies in human sera and residual EP in a recombinant nucleocapsid protein-based enzyme-linked immunosorbent assay as a possible source of false positives in severe acute respiratory syndrome serodiagnosis. The rate of false positives was significantly reduced by adding mouse anti-EP antiserum in the blocking step.

Yeast expression and characterization of SARS-CoV N protein

The severe acute respiratory syndrome human coronavirus (SARS-CoV) nucleocapsid protein (N protein) is its most antigenic structural protein. The N protein gene has been cloned into a yeast expression vector pPIC9, transformed into Pichia pastoris strain GS115 and induced for expression by methanol. SDS-PAGE and Western blot showed that the N protein was expressed at a level of 3 mg/ml of culture medium. Characterization by mass spectrometry, circular dichroism and fluorescence luminescence assays showed that the expressed N protein displayed a β-sheet secondary structure in solution and it is stable in the pH range between 5.0 and 8.0. The P. pastoris-expressed N protein is believed to more closely resemble native SARS N protein than the bacterially expressed N protein.