Overview of Neutralization Assays and International Standard for Detecting SARS-CoV-2 Neutralizing Antibody

Optimization of Cellular Transduction by the HIV-Based Pseudovirus Platform with Pan-Coronavirus Spike Proteins

Over the past three years, new SARS-CoV-2 variants have continuously emerged, evolving to a point where an immune response against the original vaccine no longer provided optimal protection against these new strains. During this time, high-throughput neutralization assays based on pseudoviruses have become a valuable tool for assessing the efficacy of new vaccines, screening updated vaccine candidates against emerging variants, and testing the efficacy of new therapeutics such as monoclonal antibodies. Lentiviral vectors derived from HIV-1 are popular for developing pseudo and chimeric viruses due to their ease of use, stability, and long-term transgene expression. However, the HIV-based platform has lower transduction rates for pseudotyping coronavirus spike proteins than other pseudovirus platforms, necessitating more optimized methods. As the SARS-CoV-2 virus evolved, we produced over 18 variants of the spike protein for pseudotyping with an HIV-based vector, optimizing experimental parameters for their production and transduction. In this article, we present key parameters that were assessed to improve such technology, including (a) the timing and method of collection of pseudovirus supernatant; (b) the timing of host cell transduction; (c) cell culture media replenishment after pseudovirus adsorption; and (d) the centrifugation (spinoculation) parameters of the host cell+ pseudovirus mix, towards improved transduction. Additionally, we found that, for some pseudoviruses, the addition of a cationic polymer (polybrene) to the culture medium improved the transduction process. These findings were applicable across variant spike pseudoviruses that include not only SARS-CoV-2 variants, but also SARS, MERS, Alpha Coronavirus (NL-63), and bat-like coronaviruses. In summary, we present improvements in transduction efficiency, which can broaden the dynamic range of the pseudovirus titration and neutralization assays.

The Anti-SARS-CoV-2 S-Protein IgG, Which Is Detected Using the Chemiluminescence Microparticle Immunoassay (CMIA) in Individuals Having Either a History of COVID-19 Vaccination and/or SARS-CoV-2 Infection, Showed a High-Titer Neutralizing Effect

Neutralizing antibodies plays a primary role in protective immunity by preventing severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) from entering the cells. Therefore, characterization of antiviral immunity is important for protection against SARS-CoV-2. In this study, the neutralizing effect of the anti-SARS-CoV-2 S1 protein IgG, which was detected using the chemiluminescence microparticle immunoassay (CMIA)-based SARS-CoV-2 IgG II Quant (Abbott, Waukegan, IL, USA) test in SARS-CoV-2 infected and/or vaccinated individuals, was investigated with a surrogate virus neutralization test (sVNT). In total, 120 Seropositive individuals were included in this study. They were divided into two groups: Vaccinated (n = 60) and Vaccinated + Previously Infected (n = 60). A commercial sVNT, the ACE2-RBD Neutralization Test (Dia.Pro, Milan, Italy), was used to assess the neutralizing effect. The assay is performed in two steps: screening and titration. The screening showed positive results in all seropositive samples. Low titration in 1.7%, medium titration in 5%, and high titration in 93.3% of the Vaccinated group, and medium titration in 1.7% and high titration in 98.3% of the other group, as obtained from the ACE2-RBD titration test. A strong positive and significant correlation was found between the SARS-CoV-2 IgG II Quant test and the ACE2-RBD titration test at the 1/32 titration level for both groups (p < 0.001 for both). This study shows that the SARS-CoV-2 IgG detected using the CMIA method after SARS-CoV-2 infection and/or vaccination has a high neutralizing titration by using the sVNT. In line with these data, knowledge that seropositivity determined by CMIA also indicates a strong neutralizing effect contributes to countrywide planning for protecting the population.

The Protection Level of S-RBD SARS-CoV-2 Immunoglobulin G Antibodies Using the Chemiluminescent Immunoassay Compared to the Surrogate Virus Neutralization Test Method

COVID-19 infection in high-risk populations is fatal and has a poor prognosis, necessitating a test to determine the protectiveness of immune response. Antibody testing is necessary to determine the body's immune response to COVID-19 infection and also vaccination strategies. Among the various methods available, the chemiluminescent immunoassay (CLIA) test is more widely used and accessible to determine antibody levels. This study aimed to determine the protection level of S-RBD SARS-CoV-2 IgG using CLIA compared to the Surrogate Virus Neutralization Test (SVNT). The population of this study comprised all healthcare professionals who experienced S-RBD SARS-CoV-2 IgG antibody level examinations. S-RBD SARS-CoV-2 IgG antibody levels were examined using CLIA and SVNT. The cut-off was determined using a receiver operating characteristic (ROC) curve, and area under the curve (AUC) measurements were evaluated. The result showed a strong positive correlation between S-RBD SARS-CoV-2 IgG CLIA and SVNT, with a value of r = 0.933 and p < 0.001. The value ≥ 37.29 BAU/mL was determined as the cut-off based on SVNT 30% inhibition level with sensitivity, specificity, and positive and negative predictive values of 96.5%, 90.9%, 96.5%, and 90.9%, respectively. A titer of antibodies greater than or equal to 37.29 BAU/mL with CLIA showed the presence of protective antibodies compared to SVNT.

Novel Competitive ELISA Utilizing Trimeric Spike Protein of SARS-CoV-2, Could Identify More Than RBD-RBM Specific Neutralizing Antibodies in Hybrid Sera

Since the initiation of the COVID-19 pandemic, there has been a need for the development of diagnostic methods to determine the factors implicated in mounting an immune response against the virus. The most promising indicator has been suggested to be neutralizing antibodies (nAbs), which mainly block the interaction between the Spike protein (S) of SARS-CoV-2 and the host entry receptor ACE2. In this study, we aimed to develop and optimize conditions of a competitive ELISA to measure serum neutralizing titer, using a recombinant trimeric Spike protein modified to have six additional proline residues (S(6P)-HexaPro) and h-ACE2. The results of our surrogate Virus Neutralizing Assay (sVNA) were compared against the commercial sVNT (cPass, Nanjing GenScript Biotech Co., Nanjing City, China), using serially diluted sera from vaccinees, and a high correlation of ID50-90 titer values was observed between the two assays. Interestingly, when we tested and compared the neutralizing activity of sera from eleven fully vaccinated individuals who subsequently contracted COVID-19 (hybrid sera), we recorded a moderate correlation between the two assays, while higher sera neutralizing titers were measured with sVNA. Our data indicated that the sVNA, as a more biologically relevant model assay that paired the trimeric S(6P) with ACE2, instead of the isolated RBD-ACE2 pairing cPass test, could identify nAbs other than the RBD-RBM specific ones.

Omicron XBB.1.16-Adapted Vaccine for COVID-19: Interim Immunogenicity and Safety Clinical Trial Results

(1) Background: The global coronavirus disease 2019 vaccination adapts to protect populations from emerging variants. This communication presents interim findings from the new Omicron XBB.1.16-adapted PHH-1V81 protein-based vaccine compared to an XBB.1.5-adapted mRNA vaccine against various acute respiratory syndrome coronavirus 2 (SARS-CoV-2) strains. (2) Methods: In a Phase IIb/III pivotal trial, adults previously vaccinated with a primary scheme and at least one booster dose of an EU-approved mRNA vaccine randomly received either the PHH-1V81 or BNT162b2 XBB.1.5 vaccine booster as a single dose. The primary efficacy endpoint assessed neutralization titers against the Omicron XBB.1.16 variant at day 14. Secondary endpoints evaluated neutralization titers and cellular immunity against different variants. Safety endpoints comprised solicited reactions up to day 7 post-vaccination and serious adverse events until the cut-off date of the interim analysis. Changes in humoral responses were assessed by pseudovirion-based or virus neutralization assays. (3) Results: At the cut-off date, immunogenicity assessments included 599 participants. Both boosters elicited neutralizing antibodies against XBB.1.16, XBB.1.5, and JN.1, with PHH-1V81 inducing a higher response for all variants. The PHH-1V8 booster triggers a superior neutralizing antibody response against XBB variants compared to the mRNA vaccine. A subgroup analysis consistently revealed higher neutralizing antibody responses with PHH-1V81 across age groups, SARS-CoV-2 infection history, and the number of prior vaccination shots. A safety analysis (n = 607) at the day 14 visit revealed favorable safety profiles without any serious vaccine-related adverse events. (4) Conclusions: PHH-1V81 demonstrates superiority on humoral immunogenicity compared to the mRNA vaccine against XBB variants and non-inferiority against JN.1 with a favorable safety profile and lower reactogenicity, confirming its potential as a vaccine candidate.

Incidence of SARS-CoV-2 seropositivity in pediatric healthcare workers prior to widespread vaccination: A 5-month longitudinal cohort study

OBJECTIVES

Determine SARS-CoV-2 IgG antibody incidence over time in unvaccinated pediatric healthcare workers (pHCWs).

DESIGN

A prospective longitudinal cohort of unvaccinated pHCWs measuring the incidence of new infection after initial prevalence was established at 4.1% with seropositive predominance in emergency department (ED)-based pHCWs. Serum samples were collected at follow-up visits to detect new SARS-CoV-2 seropositivity. Univariate analysis was performed to estimate different incidence rates between participant demographics, job, employment location, and community risk factors. Anxiety levels about COVID-19 were collected. SARS-CoV-2 antibody decay postinfection and neutralization antibodies were evaluated. Log-linear Poisson regression models were used to estimate incidence.

RESULTS

Of 642 initially enrolled, 390 pHCWs presented for at least one follow-up serology test after baseline analysis. The incidence of SARS-CoV-2 seropositivity was 8.2%. The seropositive cohort, like the negative one, consisted mainly of females in non-ED settings and nonphysician roles. There were no statistically significant differences in incidence across variables. Seropositive participants dropped antibody titers by 50% at 3 months. Neutralization antibodies correlated to SARS-CoV-2 binding antibodies (r = 0.43, P < 0.0001).

CONCLUSION

The incidence of seropositivity was 8.2%. Although seropositivity was higher among ED staff during the early stages of the pandemic, this difference declined over time, likely due to the universal adoption of personal protective equipment.

Centenarians, semi and supercentenarians, COVID-19 and Spanish flu: a serological assessment to gain insight into the resilience of older centenarians to COVID-19

BACKGROUND

Although it is well known that the older people have been the most susceptible to COVID-19, there are conflicting data on the susceptibility of centenarians. Two epidemiological study have shown that older centenarians (> 101 years old at the time of the 2020 pandemic peak) are more resilient than the remaining centenarians, suggesting that this resilience might be linked to the 1918 Spanish Flu pandemic. To gain insight into this matter, specifically whether the resilience of older centenarians to SARS-CoV-2 infection is linked to the Spanish Flu they had been affected by, we conducted a retrospective serological study. This study examined serum samples from 33 centenarians, encompassing semi- (aged > 104 < 110 years, N = 7) and supercentenarians (aged > 109 years, N = 4), born between 1905 and 1922, against both SARS-CoV-2 and 1918 H1N1 pseudotype virus.

RESULTS

Anamnestic and laboratory data suggest that SARS-CoV-2 infection occurred in 8 centenarians. The infection appeared to have been asymptomatic or mild, and hospitalization was not required, despite 3 out of 8 being between 109 and 110 years old. The levels of anti-spike antibodies in centenarians infected and/or vaccinated were higher, although not significantly, than those produced by a random sample of seventy-year-old individuals used as controls. All centenarians had antibody levels against the 1918 H1N1 virus significantly higher (almost 50 times) than those observed in the quoted group of seventy-year-old subjects, confirming the key role in maintaining immunological memory from a priming that occurred over 100 years ago. Centenarians whose blood was collected prior to the pandemic outbreak demonstrated neutralising antibodies against the 1918 H1N1 virus, but all these subjects tested negative for SARS-CoV-2.

CONCLUSION

This retrospective study shows that older centenarians are quite resilient to COVID-19, as they are capable of producing good levels of neutralising antibodies and experiencing mild or asymptomatic disease. This could be attributed to the 1918 Spanish flu pandemic through mechanisms other than the presence of cross-reactive antibodies between the 1918 H1N1 virus and SARS-CoV-2. Another possibility is that the association is purely temporal, solely correlated with the advanced age of resilient centenarians compared to those born after 1918, since older centenarians are known to have better control of immune-inflammatory responses.

Serum AZD7442 (tixagevimab-cilgavimab) concentrations and in vitroIC50 values predict SARS-CoV-2 neutralising antibody titres

Objectives

The evolution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) necessitates rapid methods for assessing monoclonal antibody (mAb) potency against emerging variants. Authentic virus neutralisation assays are considered the gold standard for measuring virus-neutralising antibody (nAb) titres in serum. However, authentic virus-based assays pose inherent practical challenges for measuring nAb titres against emerging SARS-CoV-2 variants (e.g. storing infectious viruses and testing at biosafety level-3 facilities). Here, we demonstrate the utility of pseudovirus neutralisation assay data in conjunction with serum mAb concentrations to robustly predict nAb titres in serum.

Methods

SARS-CoV-2 nAb titres were determined via authentic- and lentiviral pseudovirus-based neutralisation assays using serological data from three AZD7442 (tixagevimab-cilgavimab) studies: PROVENT (NCT04625725), TACKLE (NCT04723394) and a phase 1 dose-ranging study (NCT04507256). AZD7442 serum concentrations were assessed using immunocapture. Serum-based half-maximal inhibitory concentration (IC50) values were derived from pseudovirus nAb titres and serum mAb concentrations, and compared with in vitro IC50 measurements.

Results

nAb titres measured via authentic- and lentiviral pseudovirus-based neutralisation assays were strongly correlated for the ancestral SARS-CoV-2 virus and SARS-CoV-2 Alpha. Serum AZD7442 concentrations and pseudovirus nAb titres were strongly correlated for multiple SARS-CoV-2 variants with all Spearman correlation coefficients ≥ 0.78. Serum-based IC50 values were similar to in vitro IC50 values for AZD7442, for ancestral SARS-CoV-2 and Alpha, Delta, Omicron BA.2 and Omicron BA.4/5 variants.

Conclusions

These data highlight that serum mAb concentrations and pseudovirus in vitro IC50 values can be used to rapidly predict nAb titres in serum for emerging and historical SARS-CoV-2 variants.

Reactivity of anti-SARS-CoV-2 antibodies in Serbian voluntary blood donors

BACKGROUND

The COVID-19 pandemic has major implications on the entire blood supply system worldwide. Seroepidemiological studies are certainly necessary for better understanding the global burden that the COVID-19 pandemic represents.

OBJECTIVES

In this study, we analysed the association between demographic factors, COVID-19 severity, vaccination status and the reactivity of anti-SARS-CoV-2 IgG antibodies in Serbian blood donors.

MATERIALS AND METHODS

In a prospective study, demographic data and data related to previous SARS-CoV-2 infection, COVID-19 severity and vaccination status among whole blood donors were analysed, from February 10 to August 10, 2022, at the Blood Transfusion Institute of Vojvodina, Serbia. The detection and determination of the level of anti-SARS-CoV-2 IgG antibodies were performed using LIAISON® SARS-CoV-2 TrimericS IgG immunoassay.

RESULTS

A total of 1190 blood donors were included, 24.5% were female and 75.5% were male while their average age was 41 years. Anti-SARS-CoV-2 antibody values ranged from 2.40 to 3120 BAU/ml with a mean value of 1354.56 BAU/ml. Statistical analysis showed that COVID-19 severity and vaccination status are linked with reactivity of anti-SARS-CoV-2 antibodies, while gender and age of voluntary blood donors are not related to the values of anti-SARS-CoV-2 antibodies.

CONCLUSION

The values of anti-SARS-CoV-2 antibodies in voluntary blood donors in Serbia are kept relatively high, especially in blood donors who have overcome the severe COVID-19, as well as in donors who have been vaccinated against COVID-19. Further SARS-CoV-2 seroprevalence studies in our country are certainly still necessary so global strategies to fight against COVID-19 would be adequately evaluated.

SARS-CoV-2 Neutralization Assays Used in Clinical Trials: A Narrative Review

Since the emergence of COVID-19, extensive research efforts have been undertaken to accelerate the development of multiple types of vaccines to combat the pandemic. These include inactivated, recombinant subunit, viral vector, and nucleic acid vaccines. In the development of these diverse vaccines, appropriate methods to assess vaccine immunogenicity are essential in both preclinical and clinical studies. Among the biomarkers used in vaccine evaluation, the neutralizing antibody level serves as a pivotal indicator for assessing vaccine efficacy. Neutralizing antibody detection methods can mainly be classified into three types: the conventional virus neutralization test, pseudovirus neutralization test, and surrogate virus neutralization test. Importantly, standardization of these assays is critical for their application to yield results that are comparable across different laboratories. The development and use of international or regional standards would facilitate assay standardization and facilitate comparisons of the immune responses induced by different vaccines. In this comprehensive review, we discuss the principles, advantages, limitations, and application of different SARS-CoV-2 neutralization assays in vaccine clinical trials. This will provide guidance for the development and evaluation of COVID-19 vaccines.

Snapshot of Anti-SARS-CoV-2 IgG Antibodies in COVID-19 Recovered Patients in Guinea

Background: Because the regular vaccine campaign started in Guinea one year after the COVID-19 index case, the profile of naturally acquired immunity following primary SARS-CoV-2 infection needs to be deepened. Methods: Blood samples were collected once from 200 patients (90% of African extraction) who were recovered from COVID-19 for at least ~2.4 months (72 days), and their sera were tested for IgG antibodies to SARS-CoV-2 using an in-house ELISA assay against the Receptor Binding Domain (RBD) of the SARS-CoV-2 spike1 protein (RBD/S1-IH kit). Results: Results revealed that 73% of sera (146/200) were positive for IgG to SARS-CoV-2 with an Optical Density (OD) ranging from 0.13 to 1.19 and a median value of 0.56 (IC95: 0.51-0.61). The median OD value at 3 months (1.040) suddenly decreased thereafter and remained stable around OD 0.5 until 15 months post-infection. The OD median value was slightly higher in males compared to females (0.62 vs. 0.49), but the difference was not statistically significant (p-value: 0.073). In contrast, the OD median value was significantly higher among the 60-100 age group (0.87) compared to other groups, with a noteworthy odds ratio compared to the 0-20 age group (OR: 9.69, p-value: 0.044*). Results from the RBD/S1-IH ELISA kit demonstrated superior concordance with the whole spike1 protein ELISA commercial kit compared to a nucleoprotein ELISA commercial kit. Furthermore, anti-spike1 protein ELISAs (whole spike1 and RBD/S1) revealed higher seropositivity rates. Conclusions: These findings underscore the necessity for additional insights into naturally acquired immunity against COVID-19 and emphasize the relevance of specific ELISA kits for accurate seropositivity rates.

Pseudovirus-Based Systems for Screening Natural Antiviral Agents: A Comprehensive Review

Since the outbreak of COVID-19, researchers have been working tirelessly to discover effective ways to combat coronavirus infection. The use of computational drug repurposing methods and molecular docking has been instrumental in identifying compounds that have the potential to disrupt the binding between the spike glycoprotein of SARS-CoV-2 and human ACE2 (hACE2). Moreover, the pseudovirus approach has emerged as a robust technique for investigating the mechanism of virus attachment to cellular receptors and for screening targeted small molecule drugs. Pseudoviruses are viral particles containing envelope proteins, which mediate the virus's entry with the same efficiency as that of live viruses but lacking pathogenic genes. Therefore, they represent a safe alternative to screen potential drugs inhibiting viral entry, especially for highly pathogenic enveloped viruses. In this review, we have compiled a list of antiviral plant extracts and natural products that have been extensively studied against enveloped emerging and re-emerging viruses by pseudovirus technology. The review is organized into three parts: (1) construction of pseudoviruses based on different packaging systems and applications; (2) knowledge of emerging and re-emerging viruses; (3) natural products active against pseudovirus-mediated entry. One of the most crucial stages in the life cycle of a virus is its penetration into host cells. Therefore, the discovery of viral entry inhibitors represents a promising therapeutic option in fighting against emerging viruses.

Application of a Biomimetic Nanoparticle-Based Mock Virus to Determine SARS-CoV-2 Neutralizing Antibody Levels in Blood Samples Using a Lateral Flow Assay

The presence of neutralizing antibodies against SARS-CoV-2 in blood, acquired through previous infection or vaccination, is known to prevent the (re)occurrence of outbreaks unless the virus mutates. Therefore, the measurement of neutralizing antibodies constitutes an indispensable tool in assessing an individual's and a population's immunity against SARS-CoV-2. For this reason, we have developed an innovative lateral flow assay (LFA) capable of detecting blood-derived neutralizing antibodies using a biomimetic SARS-CoV-2 mock virus system. Here, functionalized gold nanoparticles (AuNPs) featuring the trimeric spike (S) protein at its surface imitate the virus's structure and are applied to monitor the presence and efficacy of neutralizing antibodies in blood samples. The detection principle relies on the interaction between mock virus and the immobilized angiotensin-converting enzyme 2 (ACE2) receptor, which is inhibited when neutralizing antibodies are present. To further enhance the sensitivity of our competitive assay and identify low titers of neutralizing antibodies, an additional mixing pad is embedded into the device to increase the interaction time between mock virus and neutralizing antibodies. The developed LFA is benchmarked against the WHO International Standard (21/338) and demonstrated reliable quantification of neutralizing antibodies that inhibit ACE2 binding events down to a detection limit of an antibody titer of 59 IU/mL. Additional validation using whole blood and plasma samples showed reproducible results and good comparability to a laboratory-based reference test, thus highlighting its applicability for point-of-care testing.

Quantitative and Standardized Pseudovirus Neutralization Assay for COVID-19

COVID-19 is a global pandemic caused by the highly infectious SARS-CoV-2 virus. Efforts to combat SARS-CoV-2 infection include mass vaccination and development of monoclonal and convalescent plasma therapeutics that require precise measurements of correlative, functional neutralizing antibodies that prevent virus infection. Developing rapid, safe, easy-to-use, and high-quality neutralization assays are essential for the success of the massive effort. Here, we developed a vesicular stomatitis virus-based neutralization assay that was capable of quantifying varying degrees of neutralization in patient serum samples. This assay has two detection readouts, flow cytometry and live cell imaging. The two readout methods produced consistent values of all 50% neutralization titers, further enhancing measurement confidence on the assay. Moreover, the use of available reference standards such as the World Health Organization International Standard (NIBSC code 20/136) enables quantification and standardization of the pseudovirus neutralization assay with neutralizing antibody titers measured in International Units/mL. Quantitative and standardized neutralization assays are critical for reliable efficacy evaluation and comparison of numerous vaccines and therapeutics.

Presence of neutralizing SARS-CoV-2 antibodies in asymptomatic population of N'Djamena, Chad

INTRODUCTION

Neutralizing antibodies (NAbs) are an important specific defence against viral infections, as these antibodies bind to specific receptor(s) and block the viral entry. NAbs assessments are therefore useful in determining individual or herd immunity to SARS-CoV-2. This study aims to deepen the investigation by assessing the positivity rate of neutralizing anti-spike antibodies to understand the real protection of the studied population against SARS-CoV-2.

METHODS

This study involved 260 plasma samples from a larger cohort of 2,700 asymptomatic volunteer donors, enrolled between August and October 2021 in health facilities of N'Djamena. In this study four different kits and techniques including the pseudotype assay have been used and compared with detect the SARS-CoV-2 antibodies. Pseudotyped vesicular stomatitis virus (VSV), was used both the identify and measure the NAbs that to evaluate the performance of two cheaper and easy to use commercial kits, specific for the detection of receptor-binding domain antibodies (anti-RBD) against the SARS-CoV-2 spike protein.

RESULTS

The VSV spike neutralization assay showed that 59.0% (n = 59) samples were positive for NAbs with titers ranging from 1:10 to 1:4800. While 23 out the 41 negative NAbs samples were detected positive using anti-RBD (Abbott) test. Furthermore, a direct and significant strong correlation was found between NAbs and anti-RBD, specifically with Abbott kit. Taken together, the Roche and Abbott methods indicated agreement at the high concentrations of antibodies with the VSV-pseudovirus method. Abbott and Roche indicated a good sensitivity, but the Abbott system test appeared to have better specificity than the Roche test.

CONCLUSION

Our findings indicated a high presence of NAbs against SARS-CoV-2 spike protein among asymptomatic individuals in N'Djamena. This could be one of the reasons for the low severity of Covid-19 observed in this area, given the key role of NAbs in blocking SARS-CoV-2 infection.

Naked-Eye Readable Microarray for Rapid Profiling of Antibodies against Multiple SARS-CoV-2 Variants

Novel high-throughput protein detection technologies are critically needed for population-based large-scale SARS-CoV-2 antibody detection as well as for monitoring quality and duration of immunity against virus variants. Current protein microarray techniques rely heavily on labeled transduction methods that require sophisticated instruments and complex operations, limiting their clinical potential, particularly for point-of-care (POC) applications. Here, we developed a label-free and naked-eye readable microarray (NRM) based on a thickness-sensing plasmon ruler, enabling antibody profiling within 30 min. The NRM chips provide 100% accuracy for neutralizing antibody detection by efficiently screening antigen types and experimental conditions and allow for the profiling of antibodies against multiple SARS-CoV-2 variants in clinical samples. We further established a flexible "barcode" NRM assay with a simple tape-based operation, enabling an effective smartphone-based readout and analysis. These results demonstrate new strategies for high-throughput protein detection and highlight the potential of novel protein microarray techniques for realistic clinical applications.

Development of an automated, high-throughput SARS-CoV-2 neutralization assay based on a pseudotyped virus using a vesicular stomatitis virus (VSV) vector

ABSTRACTThe global outbreak of COVID-19 has caused a severe threat to human health; therefore, simple, high-throughput neutralization assays are desirable for developing vaccines and drugs against COVID-19. In this study, a high-titre SARS-CoV-2 pseudovirus was successfully packaged by truncating the C-terminus of the SARS-CoV-2 spike protein by 21 amino acids and infecting 293 T cells that had been stably transfected with the angiotensin-converting enzyme 2 (ACE2) receptor and furin (named AF cells), to establish a simple, high-throughput, and automated 384-well plate neutralization assay. The method was optimized for cell amount, virus inoculation, incubation time, and detection time. The automated assay showed good sensitivity, accuracy, reproducibility, Z' factor, and a good correlation with the live virus neutralization assay. The high-throughput approach would make it available for the SARS-CoV-2 neutralization test in large-scale clinical trials and seroepidemiological surveys which would aid the accelerated vaccine development and evaluation.

Development of a Multispecies Double-Antigen Sandwich ELISA Using N and RBD Proteins to Detect Antibodies against SARS-CoV-2

SARS-CoV-2 infects humans and a broad spectrum of animal species, such as pets, zoo animals, and nondomestic animals. Monitoring infection in animals is important in terms of the risk of interspecies transmission and the emergence of new viral variants. Economical, fast, efficient, and sensitive diagnostic tests are needed to analyze animal infection. Double-antigen sandwich ELISA has the advantage of being multispecies and can be used for detecting infections caused by pathogens that infect several animal hosts. This study aimed to develop a double-antigen sandwich ELISA using two SARS-CoV-2 proteins, N and RBD. We compared its performance, when using these proteins separately, with an indirect ELISA and with a surrogate virus neutralization test. Positive and negative controls from a cat population (n = 31) were evaluated to compare all of the tests. After confirming that double-antigen sandwich ELISA with both RBD and N proteins had the best performance (AUC= 88%), the cutoff was adjusted using positive and negative samples from cats, humans (n = 32) and guinea pigs (n = 3). The use of samples from tigers (n = 2) and rats (n = 51) showed good agreement with the results previously obtained using the microneutralization test. Additionally, a cohort of samples from dogs with unknown infection status was evaluated. These results show that using two SARS-CoV-2 proteins in the double-antigen sandwich ELISA increases its performance and turns it into a valuable assay with which to monitor previous infection caused by SARS-CoV-2 in different animal species.

The breadth of the neutralizing antibody response to original SARS-CoV-2 infection is linked to the presence of Long COVID symptoms

The associations between longitudinal dynamics and the breadth of SARS-CoV-2 neutralizing antibody (nAb) response with various Long COVID phenotypes before vaccination are not known. The capacity of antibodies to cross-neutralize a variety of viral variants may be associated with ongoing pathology and persistent symptoms. We measured longitudinal neutralizing and cross-neutralizing antibody responses to pre- and post-SARS-CoV-2 Omicron variants in participants infected early in the COVID-19 pandemic, before widespread rollout of SARS-CoV-2 vaccines. Cross-sectional regression models adjusted for clinical covariates and longitudinal mixed-effects models were used to determine the impact of the breadth and rate of decay of neutralizing responses on the development of Long COVID symptoms, as well as Long COVID phenotypes. We identified several novel relationships between SARS-CoV-2 antibody neutralization and the presence of Long COVID symptoms. Specifically, we show that, although nAb responses to the original, infecting strain of SARS-CoV-2 were not associated with Long COVID in cross-sectional analyses, cross-neutralization ID50 levels to the Omicron BA.5 variant approximately 4 months following acute infection was independently and significantly associated with greater odds of Long COVID and with persistent gastrointestinal and neurological symptoms. Longitudinal modeling demonstrated significant associations in the overall levels and rates of decay of neutralization capacity with Long COVID phenotypes. A higher proportion of participants had antibodies capable of neutralizing Omicron BA.5 compared with BA.1 or XBB.1.5 variants. Our findings suggest that relationships between various immune responses and Long COVID are likely complex but may involve the breadth of antibody neutralization responses.

Cell-mediated and Neutralizing Antibody Responses to the SARS-CoV-2 Omicron BA.4/BA.5-adapted Bivalent Vaccine Booster in Kidney and Liver Transplant Recipients

Background

The immunogenicity elicited by the Omicron BA.4/BA.5-adapted bivalent booster vaccine after solid organ transplantation (SOT) has not been characterized.

Methods

We assessed cell-mediated and neutralizing IgG antibody responses against the BA.4/BA.5 spike receptor-binding domain at baseline and 2 wk after the administration of an mRNA-based bivalent (ancestral strain and BA.4/BA.5 subvariants) vaccine among 30 SOT recipients who had received ≥3 monovalent vaccine doses. Previous coronavirus disease 2019 history was present in 46.7% of them. We also recruited a control group of 19 nontransplant healthy individuals. Cell-mediated immunity was measured by fluorescent ELISpot assay for interferon (IFN)-γ secretion, whereas the neutralizing IgG antibody response against the BA.4/BA.5 spike receptor-binding domain was quantified with a competitive ELISA.

Results

The median number of BA.4/BA.5 spike-specific IFN-γ-producing spot-forming units (SFUs) increased from baseline to 2 wk postbooster (83.8 versus 133.0 SFUs/106 peripheral blood mononuclear cells; P = 0.0017). Seropositivity rate also increased (46.7%-83.3%; P = 0.001), as well as serum neutralizing activity (4.2%-78.3%; P < 0.0001). Patients with no prior coronavirus disease 2019 history experienced higher improvements in cell-mediated and neutralizing responses after booster vaccination. There was no correlation between BA.4/BA.5 spike-specific IFN-γ-producing SFUs and neutralizing activity. Nontransplant controls showed more robust postbooster cell-mediated immunity than SOT recipients (591.1 versus 133.0 IFN-γ-producing SFUs/106 peripheral blood mononuclear cells; P < 0.0001), although no differences were observed for antibody responses in terms of postbooster seropositivity rates or neutralizing activity.

Conclusions

Booster with the BA.4/BA.5-adapted bivalent vaccine generated strong subvariant-specific responses among SOT recipients. Booster-induced cell-mediated immunity, however, remained lower than in immunocompetent individuals.

Pseudotyping Improves the Yield of Functional SARS-CoV-2 Virus-like Particles (VLPs) as Tools for Vaccine and Therapeutic Development

Virus-like particles (VLPs) have been proposed as an attractive tool in SARS-CoV-2 vaccine development, both as (1) a vaccine candidate with high immunogenicity and low reactogenicity and (2) a substitute for live virus in functional and neutralization assays. Though multiple SARS-CoV-2 VLP designs have already been explored in Sf9 insect cells, a key parameter ensuring VLPs are a viable platform is the VLP spike yield (i.e., spike protein content in VLP), which has largely been unreported. In this study, we show that the common strategy of producing SARS-CoV-2 VLPs by expressing spike protein in combination with the native coronavirus membrane and/or envelope protein forms VLPs, but at a critically low spike yield (~0.04-0.08 mg/L). In contrast, fusing the spike ectodomain to the influenza HA transmembrane domain and cytoplasmic tail and co-expressing M1 increased VLP spike yield to ~0.4 mg/L. More importantly, this increased yield translated to a greater VLP spike antigen density (~96 spike monomers/VLP) that more closely resembles that of native SARS-CoV-2 virus (~72-144 Spike monomers/virion). Pseudotyping further allowed for production of functional alpha (B.1.1.7), beta (B.1.351), delta (B.1.617.2), and omicron (B.1.1.529) SARS-CoV-2 VLPs that bound to the target ACE2 receptor. Finally, we demonstrated the utility of pseudotyped VLPs to test neutralizing antibody activity using a simple, acellular ELISA-based assay performed at biosafety level 1 (BSL-1). Taken together, this study highlights the advantage of pseudotyping over native SARS-CoV-2 VLP designs in achieving higher VLP spike yield and demonstrates the usefulness of pseudotyped VLPs as a surrogate for live virus in vaccine and therapeutic development against SARS-CoV-2 variants.

Correlation between pseudotyped virus and authentic virus neutralisation assays, a systematic review and meta-analysis of the literature

Background

The virus neutralization assay is a principal method to assess the efficacy of antibodies in blocking viral entry. Due to biosafety handling requirements of viruses classified as hazard group 3 or 4, pseudotyped viruses can be used as a safer alternative. However, it is often queried how well the results derived from pseudotyped viruses correlate with authentic virus. This systematic review and meta-analysis was designed to comprehensively evaluate the correlation between the two assays.

Methods

Using PubMed and Google Scholar, reports that incorporated neutralisation assays with both pseudotyped virus, authentic virus, and the application of a mathematical formula to assess the relationship between the results, were selected for review. Our searches identified 67 reports, of which 22 underwent a three-level meta-analysis.

Results

The three-level meta-analysis revealed a high level of correlation between pseudotyped viruses and authentic viruses when used in an neutralisation assay. Reports that were not included in the meta-analysis also showed a high degree of correlation, with the exception of lentiviral-based pseudotyped Ebola viruses.

Conclusion

Pseudotyped viruses identified in this report can be used as a surrogate for authentic virus, though care must be taken in considering which pseudotype core to use when generating new uncharacterised pseudotyped viruses.

Development of monoclonal antibody-based blocking ELISA for detecting SARS-CoV-2 exposure in animals

The global pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) poses a significant threat to public health. Besides humans, SARS-CoV-2 can infect several animal species. Highly sensitive and specific diagnostic reagents and assays are urgently needed for rapid detection and implementation of strategies for prevention and control of the infection in animals. In this study, we initially developed a panel of monoclonal antibodies (mAbs) against SARS-CoV-2 nucleocapsid protein. To detect SARS-CoV-2 antibodies in a broad spectrum of animal species, an mAb-based blocking enzyme-linked immunosorbent assay (bELISA) was developed. Test validation using a set of animal serum samples with known infection status obtained an optimal percentage of inhibition cut-off value of 17.6% with diagnostic sensitivity of 97.8% and diagnostic specificity of 98.9%. The assay demonstrates high repeatability as determined by a low coefficient of variation (7.23%, 4.89%, and 3.16%) between-runs, within-run, and within-plate, respectively. Testing of samples collected over time from experimentally infected cats showed that the bELISA was able to detect seroconversion as early as 7 days post-infection. Subsequently, the bELISA was applied for testing pet animals with coronavirus disease 2019 (COVID-19)-like symptoms and specific antibody responses were detected in two dogs. The panel of mAbs generated in this study provides a valuable tool for SARS-CoV-2 diagnostics and research. The mAb-based bELISA provides a serological test in aid of COVID-19 surveillance in animals. IMPORTANCE Antibody tests are commonly used as a diagnostic tool for detecting host immune response following infection. Serology (antibody) tests complement nucleic acid assays by providing a history of virus exposure, no matter symptoms developed from infection or the infection was asymptomatic. Serology tests for COVID-19 are in high demand, especially when the vaccines become available. They are important to determine the prevalence of the viral infection in a population and identify individuals who have been infected or vaccinated. ELISA is a simple and practically reliable serological test, which allows high-throughput implementation in surveillance studies. Several COVID-19 ELISA kits are available. However, they are mostly designed for human samples and species-specific secondary antibody is required for indirect ELISA format. This paper describes the development of an all species applicable monoclonal antibody (mAb)-based blocking ELISA to facilitate the detection and surveillance of COVID-19 in animals.

Development of a Cell-Based SARS-CoV-2 Pseudovirus Neutralization Assay Using Imaging and Flow Cytometry Analysis

COVID-19 is an ongoing, global pandemic caused by the novel, highly infectious SARS-CoV-2 virus. Efforts to mitigate the effects of SARS-CoV-2, such as mass vaccination and development of monoclonal therapeutics, require precise measurements of correlative, functional neutralizing antibodies that block virus infection. The development of rapid, safe, and easy-to-use neutralization assays is essential for faster diagnosis and treatment. Here, we developed a vesicular stomatitis virus (VSV)-based neutralization assay with two readout methods, imaging and flow cytometry, that were capable of quantifying varying degrees of neutralization in patient serum samples. We tested two different spike-pseudoviruses and conducted a time-course assay at multiple multiplicities of infection (MOIs) to optimize the assay workflow. The results of this assay correlate with the results of previously developed serology and surrogate neutralization assays. The two pseudovirus readout methods produced similar values of 50% neutralization titer values. Harvest-free in situ readouts for live-cell imaging and high-throughput analysis results for flow cytometry can provide unique capabilities for fast evaluation of neutralization, which is critical for the mitigation of future pandemics.

An Overview of the Conventional and Novel Methods Employed for SARS-CoV-2 Neutralizing Antibody Measurement

SARS-CoV-2 is the etiological agent of the coronavirus disease-19 (COVID-19) and is responsible for the pandemic that started in 2020. The virus enters the host cell through the interaction of its spike glycoprotein with the angiotensin converting enzyme-2 (ACE2) on the host cell's surface. Antibodies present an important role during the infection and pathogenesis due to many reasons, including the neutralization of viruses by binding to different spike epitopes. Therefore, measuring the neutralizing antibody titers in the whole population is important for COVID-19's epidemiology. Different methods are described in the literature, and some have been used to validate the main vaccines used worldwide. In this review, we discuss the main methods used to quantify neutralizing antibody titers, their advantages and limitations, as well as new approaches to determineACE2/spike blockage by antibodies.

Do quantitative levels of antispike-IgG antibodies aid in predicting protection from SARS-CoV-2 infection? Results from a longitudinal study in a police cohort

In a COVID-19 sero-surveillance cohort study with predominantly healthy and vaccinated individuals, the objectives were (i) to investigate longitudinally the factors associated with the quantitative dynamics of antispike (anti-S1) IgG antibody levels, (ii) to evaluate whether the levels were associated with protection from SARS-CoV-2 infection, and (iii) to assess whether the association was different in the pre-Omicron compared with the Omicron period. The QuantiVac Euroimmun ELISA test was used to quantify anti-S1 IgG levels. The entire study period (16 months), the 11-month pre-Omicron period and the cross-sectional analysis before the Omicron surge included 3219, 2310, and 895 reactive serum samples from 949, 919, and 895 individuals, respectively. Mixed-effect linear, mixed-effect time-to-event, and logistic regression models were used to achieve the objectives. Age and time since infection or vaccination were the only factors associated with a decline of anti-S1 IgG levels. Higher antibody levels were significantly associated with protection from SARS-CoV-2 infection (0.89, 95% confidence interval [CI] 0.82-0.97), and the association was higher during the time period when Omicron was predominantly circulating compared with the ones when Alpha and Delta variants were predominant (adjusted hazard ratio for interaction 0.66, 95% CI 0.53-0.84). In a prediction model, it was estimated that >8000 BAU/mL anti-S1 IgG was required to reduce the risk of infection with Omicron variants by approximately 20%-30% for 90 days. Though, such high levels were only found in 1.9% of the samples before the Omicron surge, and they were not durable for 3 months. Anti-S1 IgG antibody levels are statistically associated with protection from SARS-CoV-2 infection. However, the prediction impact of the antibody level findings on infection protection is limited.

An update on lateral flow immunoassay for the rapid detection of SARS-CoV-2 antibodies

Over the last three years, after the outbreak of the COVID-19 pandemic, an unprecedented number of novel diagnostic tests have been developed. Assays to evaluate the immune response to SARS-CoV-2 have been widely considered as part of the control strategy. The lateral flow immunoassay (LFIA), to detect both IgM and IgG against SARS-CoV-2, has been widely studied as a point-of-care (POC) test. Compared to laboratory tests, LFIAs are faster, cheaper and user-friendly, thus available also in areas with low economic resources. Soon after the onset of the pandemic, numerous kits for rapid antibody detection were put on the market with an emergency use authorization. However, since then, scientists have tried to better define the accuracy of these tests and their usefulness in different contexts. In fact, while during the first phase of the pandemic LFIAs for antibody detection were auxiliary to molecular tests for the diagnosis of COVID-19, successively these tests became a tool of seroprevalence surveillance to address infection control policies. When in 2021 a massive vaccination campaign was implemented worldwide, the interest in LFIA reemerged due to the need to establish the extent and the longevity of immunization in the vaccinated population and to establish priorities to guide health policies in low-income countries with limited access to vaccines. Here, we summarize the accuracy, the advantages and limits of LFIAs as POC tests for antibody detection, highlighting the efforts that have been made to improve this technology over the last few years.

The Breadth of the Neutralizing Antibody Response to Original SARS-CoV-2 Infection is Linked to the Presence of Long COVID Symptoms

Background

The associations between longitudinal dynamics and the breadth of SARS-CoV-2 neutralizing antibody response with various Long COVID (LC) phenotypes prior to vaccination are not known. The capacity of antibodies to cross neutralize a variety of viral variants may be associated with ongoing pathology and persistent symptoms.

Methods

We measured longitudinal neutralizing and cross-neutralizing antibody responses to pre- and post-SARS-CoV-2 Omicron variants in participants infected during the early waves of the COVID-19 pandemic, prior to wide-spread rollout of SARS-CoV-2 vaccines. Cross sectional regression models adjusted for various clinical covariates and longitudinal mixed effects models were used to determine the impact of the breadth and rate of decay of neutralizing responses on the development of Long COVID symptoms in general, as well as LC phenotypes.

Results

We identified several novel relationships between SARS-CoV-2 antibody neutralization and the presence of LC symptoms. Specifically, we show that, although neutralizing antibody responses to the original, infecting strain of SARS-CoV-2 were not associated with LC in cross-sectional analyses, cross-neutralization ID50 levels to the Omicron BA.5 variant approximately 4 months following acute infection was independently and significantly associated with greater odds of LC and with persistent gastrointestinal and neurological symptoms. Longitudinal modeling demonstrated significant associations in the overall levels and rates of decay of neutralization capacity with LC phenotypes. A higher proportion of participants had antibodies capable of neutralizing Omicron BA.5 compared with BA.1 or XBB.1.5 variants.

Conclusions

Our findings suggest that relationships between various immune responses and LC are likely complex but may involve the breadth of antibody neutralization responses.

Development of monoclonal antibody-based blocking ELISA for detecting SARS-CoV-2 exposure in animals

The global pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) poses a significant threat to public health. Besides humans, SARS-CoV-2 can infect several animal species. Highly sensitive and specific diagnostic reagents and assays are urgently needed for rapid detection and implementation of strategies for prevention and control of the infection in animals. In this study, we initially developed a panel of monoclonal antibodies (mAbs) against SARS-CoV-2 nucleocapsid (N) protein. To detect SARS-CoV-2 antibodies in a broad spectrum of animal species, a mAb-based bELISA was developed. Test validation using a set of animal serum samples with known infection status obtained an optimal percentage of inhibition (PI) cut-off value of 17.6% with diagnostic sensitivity of 97.8% and diagnostic specificity of 98.9%. The assay demonstrates high repeatability as determined by a low coefficient of variation (7.23%, 6.95%, and 5.15%) between-runs, within-run, and within-plate, respectively. Testing of samples collected over time from experimentally infected cats showed that the bELISA was able to detect seroconversion as early as 7 days post-infection. Subsequently, the bELISA was applied for testing pet animals with COVID-19-like symptoms and specific antibody responses were detected in two dogs. The panel of mAbs generated in this study provides a valuable tool for SARS-CoV-2 diagnostics and research. The mAb-based bELISA provides a serological test in aid of COVID-19 surveillance in animals.

IMPORTANCE

Antibody tests are commonly used as a diagnostic tool for detecting host immune response following infection. Serology (antibody) tests complement nucleic acid assays by providing a history of virus exposure, no matter symptoms developed from infection or the infection was asymptomatic. Serology tests for COVID-19 are in high demand, especially when the vaccines become available. They are important to determine the prevalence of the viral infection in a population and identify individuals who have been infected or vaccinated. ELISA is a simple and practically reliable serological test, which allows high-throughput implementation in surveillance studies. Several COVID-19 ELISA kits are available. However, they are mostly designed for human samples and species-specific secondary antibody is required for indirect ELISA format. This paper describes the development of an all species applicable monoclonal antibody (mAb)-based blocking ELISA to facilitate the detection and surveillance of COVID-19 in animals.

SARS-CoV-2-Neutralizing Antibody Response and Correlation of Two Serological Assays with Microneutralization

SARS-CoV-2 has caused a huge pandemic affecting millions of people and resulting innumerous deaths. A better understanding of the correlation between binding antibodies and neutralizing antibodies is necessary to address protective immunity post-infection or vaccination. Here, we investigate the humoral immune response and the seroprevalence of neutralizing antibodies following vaccination with adenovirus-based vector in 177 serum samples. A Microneutralization (MN) assay was used as a reference method to assess whether neutralizing antibody titers correlated with a positive signal in two commercially available serological tests:a rapid lateral flow immune-chromatographic assay (LFIA) and an enzyme-linked Fluorescence Assay (ELFA). Neutralizing antibodies were detected in most serum samples (84%). COVID-19 convalescent individuals showed high antibody titers and significant neutralizing activity. Spearman correlation coefficients between the serological and neutralization results ranged from 0.8 to 0.9, suggesting a moderate to strong correlation between commercial immunoassays test results (LFIA and ELFA) and virus neutralization.

Neutralizing Antibodies against SARS-CoV-2 Beta and Omicron Variants Inhibition Comparison after BNT162b2 mRNA Booster Doses with a New PETIA sVNT Assay

BACKGROUND

Monitoring antibody response following SARS-CoV-2 vaccination is strategic, and neutralizing antibodies represent the gold standard. The neutralizing response to Beta and Omicron VOCs was evaluated versus the gold standard by a new commercial automated assay.

METHODS

Serum samples from 100 healthcare workers from the Fondazione Policlinico Universitario Campus Biomedico and the Pescara Hospital were collected. IgG levels were determined by chemiluminescent immunoassay (Abbott Laboratories, Wiesbaden, Germany) and serum neutralization assay as the gold standard. Moreover, a new commercial immunoassay, the PETIA test Nab (SGM, Rome, Italy), was used for neutralization evaluation. Statistical analysis was performed with R software, version 3.6.0.

RESULTS

Anti-SARS-CoV-2 IgG titers decayed during the first ninety days after the vaccine second dose. The following booster dose significantly (p < 0.001) increased IgG levels. A correlation between IgG expression and neutralizing activity modulation was found with a significant increase after the second and the third booster dose (p < 0.05. Compared to the Beta variant of the virus, the Omicron VOC was associated with a significantly larger quantity of IgG antibodies needed to achieve the same degree of neutralization. The best Nab test cutoff for high neutralization titer (≥1:80) was set for both Beta and Omicron variants.

CONCLUSION

This study correlates vaccine-induced IgG expression and neutralizing activity using a new PETIA assay, suggesting its usefulness for SARS-CoV2 infection management.