Double-antigen sandwich ELISA for detection of antibodies to SARS-associated coronavirus in human serum

Development of an effective one-step double-antigen sandwich ELISA based on p72 to detect antibodies against African swine fever virus

African swine fever (ASF), caused by ASF virus (ASFV), is a highly contagious and lethal disease of domestic pigs leading to tremendous economic losses. As there are no vaccines and drugs available. An effective diagnosis to eliminate ASFV-infected pigs is a crucial strategy to prevent and control ASF. To this end, ASFV capsid protein p72 was expressed using Chinese hamster ovary (CHO) cells and subsequently conjugated with horseradish peroxidase (HRP) to develop a one-step double-antigen sandwich enzyme-linked immunosorbent assay (one-step DAgS-ELISA). The performance of this ELISA for detecting ASFV antibodies was evaluated. Overall, a diagnostic sensitivity of 97.96% and specificity of 98.96% was achieved when the cutoff value was set to 0.25. No cross-reaction with healthy pig serum and other swine viruses was observed. The coefficients of variation of the intra-assay and inter-assay were both <10%. Importantly, this ELISA could detect antibodies in standard serum with 12,800-fold dilution, and seroconversion started from the 7th day post-inoculation (dpi), showing excellent analytical sensitivity and great utility. Furthermore, compared to the commercial kit, this ELISA had a good agreement and significantly shorter operation time. Collectively, a novel one-step DAgS-ELISA for detecting antibodies against ASFV is developed, which will be reliable and convenient to monitor ASFV infection.

Trace detection of SARS-CoV-2 N-protein by diamond solution-gate field-effect transistor

In this study, we introduced H-terminated diamond solution-gate field-effect transistor (H-diamond SGFET) to detect trace SARS-CoV-2 N-protein, which plays an important role in replication and transcription of viral RNA. 1-Pyrenebutyric acid-N-hydroxy succinimide ester (Pyr-NHS) was modified on H-diamond surface as linker, on which the specific antibody of SARS-CoV-2 N-protein was catenated. Fourier transform infrared spectrum, scanning electron microscope and energy dispersive spectrum were utilized to demonstrate the modification of H-diamond with Pyr-NHS and antibody. Shifts of I_DS(max) at V_GS = -500 mV in transfer characteristics of H-diamond SGFET was observed to determine N-protein concentration in phosphate buffer solution. Good linear relationship between I_DS(max) and log₁₀(N-protein) was observed from 10^-14 to 10^-5 g/mL with goodness of fit R² = 0.90 and sensitivity of 1.98 μA/Log₁₀ [concentration of N-protein] at V_DS = -500 mV, V_GS = -500 mV. Consequently, this prepared H-diamond SGFET biosensor may provide a new idea for diagnosis of SARS-CoV-2 due to a wide detection range from 10^-14 to 10^-5 g/mL and low limit of detection 10^-14 g/mL.

Fcγ-Receptor-Based Enzyme-Linked Immunosorbent Assays for Sensitive, Specific, and Persistent Detection of Anti-SARS-CoV-2 Nucleocapsid Protein IgG Antibodies in Human Sera

Sensitive and specific serological tests are mandatory for epidemiological studies evaluating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) prevalence as well as coronavirus disease 2019 (COVID-19) morbidity and mortality rates. The accuracy of results is challenged by antibody waning after convalescence and by cross-reactivity induced by previous infections with other pathogens. By employing a patented platform technology based on capturing antigen-antibody complexes with a solid-phase-bound Fcγ receptor (FcγR) and truncated nucleocapsid protein as the antigen, two SARS-CoV-2 IgG enzyme-linked immunosorbent assays (ELISAs), featuring different serum and antigen dilutions, were developed. Validation was performed using a serum panel comprising 213 longitudinal samples from 35 COVID-19 patients and a negative-control panel consisting of 790 pre-COVID-19 samples from different regions of the world. While both assays show similar diagnostic sensitivities in the early convalescent phase, ELISA 2 (featuring a higher serum concentration) enables SARS-CoV-2 IgG antibody detection for a significantly longer time postinfection (≥15 months). Correspondingly, analytical sensitivity referenced to indirect immunofluorescence testing (IIFT) is significantly higher for ELISA 2 in samples with a titer of ≤1:640; for high-titer samples, a prozone effect is observed for ELISA 2. The specificities of both ELISAs were excellent not only for pre-COVID-19 serum samples from Europe, Asia, and South America but also for several challenging African sample panels. The SARS-CoV-2 IgG FcγR ELISAs, methodically combining antigen-antibody binding in solution and isotype-specific detection of immune complexes, are valuable tools for seroprevalence studies requiring the (long-term) detection of anti-SARS-CoV-2 IgG antibodies in populations with a challenging immunological background and/or in which spike-protein-based vaccine programs have been rolled out.

An Integrated Platform for Serological Detection and Vaccination of COVID-19

Coronavirus Disease 2019 (COVID-19), caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), is an ongoing pandemic. Detection and vaccination are essential for disease control, but they are distinct and complex operations that require significant improvements. Here, we developed an integrated detection and vaccination system to greatly simplify these efforts. We constructed recombinant baculoviruses to separately display the nucleocapsid (N) and spike (S) proteins of SARS-CoV-2. Insect cells infected by the recombinant baculoviruses were used to generate a cell-based system to accurately detect patient serum. Notably, although well-recognized by our newly developed detection system in which S-displaying insect cells acted as antigen, anti-S antibodies from many patients were barely detectable by Western blot, evidencing that COVID-19 patients primarily produce conformation-dependent anti-S antibodies. Furthermore, the same baculovirus constructs can display N (N-Bac) or S (S-Bac) on the baculovirus envelope and serve as vector vaccines. Animal experiments show that S-Bac or N-Bac immunization in mice elicited a strong and specific antibody response, and S-Bac in particular stimulated effective neutralizing antibodies without the need for adjuvant. Our integrated system maintains antigen conformation and membrane structure to facilitate serum detection and antibody stimulation. Thus, compared with currently available technologies, our system represents a simplified and efficient platform for better SARS-CoV-2 detection and vaccination.

Usefulness of ELISA Using Total Antibody against Plant-Expressed Recombinant Nucleocapsid Protein of SARS-CoV-2

Here, we aimed to investigate the diagnostic value of a serological assay using the nucleocapsid protein developed for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) detection and evaluated its performance using three commercial enzyme-linked immunosorbent assays (ELISAs), namely, Standard E 2019 novel coronavirus disease (COVID-19) total antibody (Ab) ELISA (SD Biosensor), and EDI novel coronavirus COVID-19 IgG and IgM ELISA. A recombinant nucleocapsid protein (rNP) was expressed from plants and Escherichia coli for the detection of serum total Ab. We prospectively collected 141 serum samples from 32 patients with reverse transcription-PCR (RT-PCR)-confirmed COVID-19 and determined the sensitivity and dynamics of their total Ab response. Specificity was evaluated using 158 prepandemic samples. To validate the assays, we evaluated the performance using two different cutoff values. The sensitivity and specificity for each assay were as follows: 92.91% and 94.30% (plant-rNP), 83.69% and 98.73% (SD Biosensor), 75.89% and 98.10% (E. coli-rNP), 76.47% and 100% (EDI-IgG), and 80.39% and 80% (EDI-IgM). The plant-based rNP showed the highest sensitivity and area under the receiver operating characteristic (ROC) curve (0.980) among all the assays (P < 0.05). The seroconversion rate for total Ab increased sequentially with disease progression, with a sensitivity of 100% after 10 to 12 days of post-symptom onset (PSO) for both rNP-plant-based and SD Biosensor ELISAs. After 2 weeks of PSO, the seroconversion rates were >80% and 100% for EDI-IgM and EDI-IgG ELISA, respectively. Seroconversion occurred earlier with rNP plant-based ELISA (5 days PSO) compared with E. coli-based (7 days PSO) and SD Biosensor (8 days PSO) ELISA. We determined that rNP produced in plants enables the robust detection of SARS-CoV-2 total Abs. The assay can be used for serosurvey and complementary diagnosis of COVID-19. IMPORTANCE At present, the principal diagnostic methods for COVID-19 comprise the identification of viral nucleic acid by genetic approaches, including PCR-based techniques or next-generation sequencing. However, there is an urgent need for validated serological assays which are crucial for the understanding of immune responses against SARS-CoV-2. In this study, a highly sensitive and specific serological antibody assay was developed for the detection of SARS-CoV-2 with an overall accuracy of 93.56% using a recombinant nucleoprotein expressed from plants.

The next-generation coronavirus diagnostic techniques with particular emphasis on the SARS-CoV-2

The potential zoonotic coronaviruses (SARS-CoV, MERS-CoV, and SARS-CoV-2) are of global health concerns. Early diagnosis is the milestone in their mitigation, control, and eradication. Many diagnostic techniques are showing great success and have many advantages, such as the rapid turnover of the results, high accuracy, and high specificity and sensitivity. However, some of these techniques have several pitfalls if samples were not collected, processed, and transported in the standard ways and if these techniques were not practiced with extreme caution and precision. This may lead to false-negative/positive results. This may affect the downstream management of the affected cases. These techniques require regular fine-tuning, upgrading, and optimization. The continuous evolution of new strains and viruses belong to the coronaviruses is hampering the success of many classical techniques. There are urgent needs for next generations of coronaviruses diagnostic assays that overcome these pitfalls. This new generation of diagnostic tests should be able to do simultaneous, multiplex, and high-throughput detection of various coronavirus in one reaction. Furthermore, the development of novel assays and techniques that enable the in situ detection of the virus on the environmental samples, especially air, water, and surfaces, should be given considerable attention in the future. These approaches will have a substantial positive impact on the mitigation and eradication of coronaviruses, including the current SARS-CoV-2 pandemic.

Improved diagnosis of SARS-CoV-2 by using nucleoprotein and spike protein fragment 2 in quantitative dual ELISA tests

The novel coronavirus, severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), is the causative agent of the 2020 worldwide coronavirus pandemic. Antibody testing is useful for diagnosing historic infections of a disease in a population. These tests are also a helpful epidemiological tool for predicting how the virus spreads in a community, relating antibody levels to immunity and for assessing herd immunity. In the present study, SARS-CoV-2 viral proteins were recombinantly produced and used to analyse serum from individuals previously exposed, or not, to SARS-CoV-2. The nucleocapsid (Npro) and spike subunit 2 (S2Frag) proteins were identified as highly immunogenic, although responses to the former were generally greater. These two proteins were used to develop two quantitative enzyme-linked immunosorbent assays (ELISAs) that when used in combination resulted in a highly reliable diagnostic test. Npro and S2Frag-ELISAs could detect at least 10% more true positive coronavirus disease-2019 (COVID-19) cases than the commercially available ARCHITECT test (Abbott). Moreover, our quantitative ELISAs also show that specific antibodies to SARS-CoV-2 proteins tend to wane rapidly even in patients who had developed severe disease. As antibody tests complement COVID-19 diagnosis and determine population-level surveillance during this pandemic, the alternative diagnostic we present in this study could play a role in controlling the spread of the virus.

SARS-CoV, MERS-CoV and SARS-CoV-2: A Diagnostic Challenge

The highly pathogenic MERS-CoV, SARS-CoV and SARS-CoV-2 cause acute respiratory syndrome and are often fatal. These new viruses pose major problems to global health in general and primarily to infection control and public health services. Accurate and selective assessment of MERS-CoV, SARS-CoV and SARS-CoV-2 would assist in the effective diagnosis of infected individual, offer clinical guidance and aid in assessing clinical outcomes. In this mini-review, we review the literature on various aspects, including the history and diversity of SARS-CoV-2, SARS-CoV and MERS-CoV, their detection methods in effective clinical diagnosis, clinical assessment of COVID-19, safety guidelines recommended by World Health Organization and legal regulations. This review article also deals with existing challenges and difficulties in the clinical diagnosis of SARS-CoV-2. Developing alternative diagnostic platforms by spotting the shortcomings of the existing point-of-care diagnostic devices would be useful in preventing future outbreaks.

Elucidating the pivotal role of convalescent plasma therapy in critically ill COVID-19 patients: A review

World Health Organization (WHO) declared coronavirus disease (COVID-19) a pandemic in March 2020. Currently almost every country in the world has reported cases with moderate to high mortality rates. The European Union (EU), the United States of America (USA) and the United Kingdom (UK) are the severely affected countries. Nevertheless, the WHO is very much concern about countries with weak health systems. The clinical characteristics of COVID-19 varies extensively, ranging from asymptomatic infections to severe as well as critical pneumonia with high mortality rates in the elderly and patients with co-morbid medical illness. Convalescent Plasma Therapy (CPT) has been successfully used in treating various viral disease outbreaks such as 1918 influenza pneumonia pandemic, poliomyelitis, measles, mumps, Machupo virus, Junin virus, Lassa virus, Ebola etc. High-titer specific antibodies maybe capable of binding to Coronavirus- 19 (CoV-19) and neutralize the viral particles, inhibit entry to uninfected cells, and trigger potent effector mechanisms such as complement activation as well as phagocytosis. Therefore, in most countries with very weak health systems with no Intensive Care Units (ICUs) or trained ICU physicians, early initiation of CPT for severely COVID- 19 patients may be rewarding. Therefore, solidarity control trials on CPT for COVID- 19 patients involving large number of patients are urgently needed.

Nucleic Acid and Immunological Diagnostics for SARS-CoV-2: Processes, Platforms and Pitfalls

Accurate diagnosis at an early stage of infection is essential for the successful management of any contagious disease. The coronavirus disease 2019 (COVID-19), caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) virus is a pandemic that has affected 214 countries affecting more than 37.4 million people causing 1.07 million deaths as of the second week of October 2020. The primary diagnosis of the infection is done either by the molecular technique of RT-qPCR by detecting portions of the RNA of the viral genome or through immunodiagnostic tests by detecting the viral proteins or the antibodies produced by the host. As the demand for the test increased rapidly many naive manufacturers entered the market with novel kits and more and more laboratories also entered the diagnostic arena making the test result more error-prone. There are serious debates globally and regionally on the sensitivity and specificity of these tests and about the overall accuracy and reliability of the tests for decision making on control strategies. The significance of the test is also complexed by the presence of asymptomatic carriers, re-occurrence of infection in cured patients as well as by the varied incubation periods of the infection and shifting of the viral location in the host tissues. In this paper, we review the techniques available for SARS-CoV-2 diagnosis and probable factors that can reduce the sensitivity and specificity of the different test methods currently in vogue. We also provide a checklist of factors to be considered to avoid fallacious practices to reduce false positive and false negative results by the clinical laboratories.

A multi-target lateral flow immunoassay enabling the specific and sensitive detection of total antibodies to SARS COV-2

A rapid test for detecting total immunoglobulins directed towards the nucleocapsid protein (N) of severe acute syndrome coronavirus 2 (SARS CoV-2) was developed, based on a multi-target lateral flow immunoassay comprising two test lines. Both test lines bound to several classes of immunoglobulins (G, M, and A). Specific anti-SARS immunoglobulins were revealed by a colorimetric probe formed by N and gold nanoparticles. Targeting the total antibodies response to infection enabled achieving 100% diagnostic specificity (95.75-100, C.I. 95%, n = 85 healthy and with other infections individuals) and 94.6% sensitivity (84.9-98.9, C.I. 95%, n = 62 SARS CoV-2 infected subjects) as early as 7 days post confirmation of positivity. Agreeing results with a reference serological ELISA were achieved, except for the earlier detection capability of the rapid test. Follow up of the three seroconverting patients endorsed the hypothesis of the random rise of the different immunoglobulins and strengthened the 'total antibodies' approach for the trustworthy detection of serological response to SARS CoV-2 infection.

Dynamic changes of throat swabs RNA and serum antibodies for SARS-CoV-2 and their diagnostic performances in patients with COVID-19

Dynamic changes of RNA and antibodies in SARS-CoV-2 infected patients remain largely unknown, and influence factors for antibody production have not been fully clarified. In this study, consecutive throat swabs specimens (n = 1875) from 187 patients were collected to analyse the dynamic changes of RNA. Moreover, 162 serial serum samples from 31 patients were tested for seroconversion of IgM and IgG. Meanwhile, IgM and IgG were also detected in 409 COVID-19 patients and 389 controls. Additionally, the logistic regression analysis was executed to identify the possible influence factors for antibody production. The median positive conversion time for RNA was day 7 (IQR, 3-11), and the positive rate was highest in day 1-5 (74.59 %) and then gradually decreased. The median time of seroconversion for IgM and IgG were both day 12 (IQR, 10-15). The sensitivity and specificity for IgM (or IgG) was 87.04% and 96.92%, respectively. Multivariate logistic regression indicated that reduced lymphocytes and short positive conversion time for SARS-CoV-2 RNA were independent factors for negative results of IgM and IgG. In conclusion, RNA and antibodies should be combined for COVID-19 diagnosis, and delayed seroconversion was influenced by the decreased lymphocytes and short positive conversion time for RNA.

Principles of computer-controlled linear motion applied to an open-source affordable liquid handler for automated micropipetting

OTTO is an open-source automated liquid handler that can be fabricated at a cost of $1,500 using off-the-shelf and 3D-printable parts as an alternative to commercial devices. Open-source approaches have been applied to build syringe pumps, centrifuges, and other laboratory equipment. These devices are affordable but generally rely on a single motor to perform simple operations and thus do not fully utilize the potential of the Maker Movement. Open-source linear actuators and microcontrollers enable the fabrication of more complex laboratory instruments that rely on 3D positioning and accurate dispensing of fluids, such as automated liquid handlers. These instruments can be built rapidly and affordably, thereby providing access to highly reproducible sample preparation for common biological assays such as qPCR. We applied the design principles of speed and accuracy, unattended automation, and open-source components to build an automated liquid handler that controls micropipetting of liquids in 3D space at speeds and positional resolutions required for qPCR. In benchmarking studies, OTTO showed accuracy and sample preparation times comparable to manual qPCR. The ability to control linear motion and liquid dispensing using affordable off-the-shelf and 3D-printable parts can facilitate the adoption of open-source automated liquid handlers for qPCR, bioplotting, and other bioinstrumentation applications.

Tackling COVID-19 infection through complement-targeted immunotherapy

The complement system is an ancient part of innate immunity sensing highly pathogenic coronaviruses by mannan‐binding lectin (MBL) resulting in lectin pathway activation and subsequent generation of the anaphylatoxins (ATs) C3a and C5a as important effector molecules. Complement deposition on endothelial cells and high blood C5a serum levels have been reported in COVID‐19 patients with severe illness, suggesting vigorous complement activation leading to systemic thrombotic microangiopathy (TMA). Complement regulator gene variants prevalent in African‐Americans have been associated with a higher risk for severe TMA and multi‐organ injury. Strikingly, severe acute respiratory syndrome Coronavirus 2 (SARS‐CoV‐2)‐infected African‐Americans suffer from high mortality. These findings allow us to apply our knowledge from other complement‐mediated diseases to COVID‐19 infection to better understand severe disease pathogenesis. Here, we discuss the multiple aspects of complement activation, regulation, crosstalk with other parts of the immune system, and the options to target complement in COVID‐19 patients to halt disease progression and death.

Evaluation of Nucleocapsid and Spike Protein-Based Enzyme-Linked Immunosorbent Assays for Detecting Antibodies against SARS-CoV-2

At present, PCR-based nucleic acid detection cannot meet the demands for coronavirus infectious disease (COVID-19) diagnosis. Two hundred fourteen confirmed COVID-19 patients who were hospitalized in the General Hospital of Central Theater Command of the People’s Liberation Army between 18 January and 26 February 2020 were recruited. Two enzyme-linked immunosorbent assay (ELISA) kits based on recombinant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein (rN) and spike protein (rS) were used for detecting IgM and IgG antibodies, and their diagnostic feasibility was evaluated.

At present, PCR-based nucleic acid detection cannot meet the demands for coronavirus infectious disease (COVID-19) diagnosis. Two hundred fourteen confirmed COVID-19 patients who were hospitalized in the General Hospital of Central Theater Command of the People’s Liberation Army between 18 January and 26 February 2020 were recruited. Two enzyme-linked immunosorbent assay (ELISA) kits based on recombinant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein (rN) and spike protein (rS) were used for detecting IgM and IgG antibodies, and their diagnostic feasibility was evaluated. Among the 214 patients, 146 (68.2%) and 150 (70.1%) were successfully diagnosed with the rN-based IgM and IgG ELISAs, respectively; 165 (77.1%) and 159 (74.3%) were successfully diagnosed with the rS-based IgM and IgG ELISAs, respectively. The positive rates of the rN-based and rS-based ELISAs for antibody (IgM and/or IgG) detection were 80.4% and 82.2%, respectively. The sensitivity of the rS-based ELISA for IgM detection was significantly higher than that of the rN-based ELISA. We observed an increase in the positive rate for IgM and IgG with an increasing number of days post-disease onset (d.p.o.), but the positive rate of IgM dropped after 35 d.p.o. The positive rate of rN-based and rS-based IgM and IgG ELISAs was less than 60% during the early stage of the illness, 0 to 10 d.p.o., and that of IgM and IgG was obviously increased after 10 d.p.o. ELISA has a high sensitivity, especially for the detection of serum samples from patients after 10 d.p.o., so it could be an important supplementary method for COVID-19 diagnosis.

A preliminary study on serological assay for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 238 admitted hospital patients

In this study, we aimed to evaluate the diagnostic value of serological assay for SARS-CoV-2. A newly-developed ELISA assay for IgM and IgG antibodies against N protein of SARS-CoV-2 was used to screen the serums of 238 admitted hospital patients between February 6 and February 14, 2020 with confirmed or suspected SARS-CoV-2. SARS-CoV-2 RNA was detected on pharyngeal swab specimens using real time RT-PCR. 194 (81.5%) of the serums were detected to be antibody (IgM and/or IgG) positive, significantly higher than the positive rate of viral RNA (64.3%). There was no difference in the positive rate of antibodies between the confirmed patients (83.0%, 127/153) and the suspected patients (78.8%, 67/85), whose nucleic acid tests were negative. The antibody positive rates were very low in the first five days after initial onset of symptoms, and then rapidly increased as the disease progressed. After 10 days, the antibody positive rates jumped from below 50% to over 80%. However, the positive rates of viral RNA maintained above 60% in the first 11 days after initial onset of symptoms, and then rapidly decreased. Overall, the suspected patients were most likely infected by SARS-CoV-2. Before the 11th day after initial onset of symptoms, nucleic acid test is key for confirmation of viral infection. The combination of serological assay can greatly improve the diagnostic efficacy. After the 11th day post-disease onset, the diagnosis for viral infection should be majorly dependent on serological assay.

Humoral Immune Responses in COVID-19 Patients: A Window on the State of the Art

The novel SARS-CoV-2 is a recently emerging virus causing a human pandemic. A great variety of symptoms associated with COVID-19 disease, ranging from mild to severe symptoms, eventually leading to death. Specific SARS-CoV-2 RT-PCR is the standard method to screen symptomatic people; however, asymptomatic subjects and subjects with undetectable viral load escape from the screening, contributing to viral spread. Currently, the lock down imposed by many governments is an important measure to contain the spread, as there is no specific antiviral therapy or a vaccine and the main treatments are supportive. Therefore, there is urgent need to characterize the virus and the viral-mediated responses, in order to develop specific diagnostic and therapeutic tools to prevent viral transmission and efficiently cure COVID-19 patients. Here, we review the current studies on two viral mediated-responses, specifically the cytokine storm occurring in a subset of patients and the antibody response triggered by the infection. Further studies are needed to explore both the dynamics and the mechanisms of the humoral immune response in COVID-19 patients, in order to guide future vaccine design and antibody-based therapies for the management of the disease.

A double antibody sandwich enzyme-linked immunosorbent assay for detection of soft-shelled turtle iridovirus antigens

A double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) for detection of the soft-shelled turtle iridovirus (STIV) was developed using a specific monoclonal antibody (mAb) against STIV and anti-STIV rabbit serum. Using DAS-ELISA, the detection limit of STIV was found to be 10³ PFU/ml. The positive rate of 15 STIV samples was 100%, while the positive rate of 100 other aquatic virus samples was 0%. These data show that DAS-ELISA is highly specific and sensitive for the detection of STIV. In clinical tests, 128 samples isolated from pond-reared turtles were subjected to DAS-ELISA and PCR. The overall agreement between the results obtained by DAS-ELISA and PCR was 98.4%. The results indicate that the DAS-ELISA method could be used for diagnosing diseases caused by STIV.

Multispecies detection of antibodies to influenza A viruses by a double-antigen sandwich ELISA

A double-antigen sandwich ELISA was developed for the detection of antibodies to influenza A viruses. A recombinant nucleoprotein (rNP) of influenza A virus was used as a capture antigen and an HRP-conjugate for detecting the antibodies. A total of 125 serum samples from birds of different species including chickens, geese, open-billed storks, Khaki Campbell ducks, lesser whistling ducks, and pigeons with known antibodies were tested by ELISA. The sensitivity and the specificity of ELISA were found to be 98% and 97.3%, respectively. The assay was able to detect the presence of influenza A antibodies as early as the fourth day post-inoculation in ducks infected experimentally with influenza A (H5N1) virus. Excellent agreement (97.6%) was obtained between this sandwich ELISA and the hemagglutination inhibition (HI) tests (kappa=0.95). The double-antigen sandwich ELISA correlated well with a commercial avian influenza (AI) multispecies ELISA and was slightly more sensitive than the AI multispecies ELISA. These findings indicate that the double-antigen sandwich ELISA based on rNP may offer an effective screening method for serodiagnosis of influenza A virus. The double-antigen sandwich ELISA also enables the detection of antibodies to influenza A viruses in different species without the need for species-specific secondary antibodies.

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.

Indirect Enzyme-Linked Immunosorbent Assay based on the nucleocapsid protein of SARS-like coronaviruses

The nucleocapsid protein (N) is a major structural protein of coronaviruses. The N protein of bat SARS-like coronavirus (SL-CoV) has a high similarity with that of SARS-CoV. In this study, the SL-CoV N protein was expressed in Escherichia coli, purified and used as antigen. An Indirect Enzyme-Linked Immunosorbent Assay (indirect ELISA) was developed for detection of SARS- or SL-CoV infections in bat populations. The detection of 573 bat sera with this indirect ELISA demonstrated that SL-CoVs consistently circulate in Rhinilophus species, further supporting the proposal that bats are natural reservoirs of SL-CoVs. This method uses 1–2 μl of serum sample and can be used for preliminary screening of infections by SARS- or SL-CoV with a small amount of serum sample.

A line immunoassay utilizing recombinant nucleocapsid proteins for detection of antibodies to human coronaviruses

Most coronaviruses infecting humans cause mild diseases, whereas severe acute respiratory syndrome (SARS)-associated coronavirus is an extremely dangerous pathogen. Here, we report the development of a serologic assay for detection of antibodies to human coronaviruses (HCoVs) based on recombinant nucleocapsid (N) proteins of all known pathogenic strains (229E, NL63, OC43, HKU1, SARS). The novel immunoassay is highly useful for epidemiologic surveys, where use of nucleic acid diagnostics often is limited. Purified recombinant antigens were immobilized on nitrocellulose membranes and applied in a line immunoassay, which allows rapid detection of antibodies to 5 different HCoVs in a single experiment. For assay evaluation, serum samples from persons infected with 229E or OC43 (acute/convalescent), recovered SARS patients and healthy donors were analyzed. Screening for nucleocapsid (N)-specific immunoglobulin G (IgG) in convalescent sera reached 100% sensitivity. With this new technique, we found that recently identified NL63 and HKU1 contribute significantly to the overall spectrum of coronavirus infections. Possibly, cross-reactive antibody responses were observed using 229E and OC43 serum pairs. However, the potential of this assay could clearly be demonstrated employing SARS-positive serum samples, where nonspecific binding to nucleocapsids of other HCoVs was not observed. This coronavirus strain-specific line immunoassay represents a powerful tool for serologic diagnostics.

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.