New assay improves detection of SARS-CoV-2 antibodies

By Sally Robertson, B.Sc.Jul 20 2020

Researchers at the University of California Santa Cruz have developed a new assay for the detection of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that they say provides significant advantages over the techniques currently used.

Rebecca DuBois and colleagues say the assay provides real-time measurements of both total antibody levels and specific antibody isotopes, with complete quantitative results generated in less than 20 minutes.

They also say the method can be immediately implemented on existing platforms for use in urgent studies such as those monitoring seroconversions in communities or evaluating the effectiveness of potential vaccines.

Furthermore, the researchers say the assay could be developed to provide a novel diagnostic platform for the evaluation of patient antibody levels, for example, or to prioritize patients who should receive plasma from convalescent donors quickly.

A pre-print version of the paper is available in medRxiv*, while the article undergoes peer review.

Rapid, accurate testing is urgently needed

Since the COVID-19 outbreak began in Wuhan, China, late last year, SARS-CoV-2 has infected more than 14.63 million people and caused more than 608,000 deaths.

Rapid and accurate testing methods are essential to understanding the full impact of coronavirus disease 2019 (COVID-19) and to developing approaches that will curb further spread of the virus.

Currently, the primary testing methods involve checking for SARS-CoV-2 viral RNA (nucleic acid testing) and serologic testing, which checks for antibodies generated against the virus.

Nucleic acid testing is the main approach used for the detection of active COVID-19 cases. Still, the limited availability of testing kits and the presence of asymptomatic or mild disease mean that many infections go undiagnosed.

Overview of the BLI-ISA experiment. To begin, a tray of fiber optic biosensors and a 96- or 384-well plate of samples are placed into the Octet BLI instrument (Supplementary Figure 2), and the assay program is run. Throughout the experiment, real-time measurements are recorded as the change in the wavelength of reflected light returning from the biosensor surface. First, biosensors are equilibrated by dipping into wells containing BLI assay buffer. In the antigen loading step, biosensors are dipped into wells containing tagged antigen (e.g. streptavidin SA biosensors dipped into biotinylated antigen). After a wash, antigen-loaded biosensors are placed into diluted plasma, and a Total Antibody Binding signal is measured. After another wash, the antigen-antibody-coated biosensors are dipped into wells containing isotype-specific binding reagents (e.g. colloidal gold-conjugated anti-human IgG), and a Detection signal is measured

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Serologic testing has become an essential approach for gauging the true spread of the virus and for developing countermeasures such as the identification of potential convalescent plasma donors and the discovery of candidate vaccines.

Current serologic tests include the Lateral Flow Immunoassay (LFIA), Enzyme-Linked Immunosorbent Assay (ELISA), Immunofluorescent Assay (IFA), and Chemiluminescent Immunoassay (CLIA).

While the LFIA provides the fastest turnaround and requires minimal training, making it useful for point-of-care testing, its performance has been mixed in terms of sensitivity, say Dubois and team.

The other three tests are generally reliable in terms of sensitivity and specificity and provide quantitative measures that can distinguish between weak and robust antibody responses. However, they are highly complex techniques that take as long as five hours to perform, requiring labor-intensive sample preparation steps such as long incubation times and washing that needs to be performed manually or using fluidics platforms.

“Due to these drawbacks, the development of alternate serological testing methods that are simple, rapid, and quantitative would be advantageous for many applications,” write the researchers.

The team applied a novel, fiber-optics based method

Now, Dubois and the team have described a novel method for measuring plasma antigen-specific antibodies using biolayer interferometry (BLI). This fiber-optics based technology measures the affinity between molecules using reflected white light to generate a real-time signal.

The team used this technology to develop a new application called biolayer interferometry immunosorbent assay (BLI-ISA) that enables rapid and quantitative measurement of plasma antibodies specific to SARS-CoV-2.

The technique uses a simple “dip-and-read” format that provides measurements of both total antibody levels and specific antibody isotypes all in the same assay and enables detection of weak seropositivity.

Furthermore, this rapid assay provides complete, real-time, quantitative data in less than 20 minutes.

The method does not require time-consuming washing of beads or wells, nor does it require enzyme-based signal amplification, which can vary with differing temperature, pH, and reagent manufacturing lots, say Dubois and colleagues.

“As a result, our assay overcomes the lab-to-lab variability that can occur with methods that require extensive washing or enzyme-based signal amplification,” writes the team. “Thus, BLI-ISA provides a solution to standardize other serological testing methods as well as to perform longitudinal studies of biological samples.”

The method could be used on existing platforms and also developed as a diagnostic platform

The researchers say the method could be immediately implemented on existing platforms for use in urgent studies such as serosurveillance to assess seroconversion in communities and studies assessing antibody responses as a result of natural infection or the use of candidate vaccines.

“Finally, we believe that BLI-ISA can be developed as a novel diagnostic platform to evaluate antibodies and other biomolecules in clinical specimens, for example, to evaluate plasma antibody levels to inform patients on vaccinations or to identify and prioritize donors for convalescent plasma therapy donation quickly,” concludes the team.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources