New blood test can measure the amount of SARS-CoV-2 neutralizing antibodies present in the bloodstream


The Lausanne University Hospital (CHUV) and EPFL teamed up to develop a new test that’s sensitive enough to measure the amount of SARS-CoV-2 neutralizing antibodies present in the bloodstream.

The scientists’ discovery, published in the prestigious Science Translational Medicine, opens promising new avenues for tracking immunity acquired by infection or vaccination. With this test, experts can measure the level of protection against variants of the virus and monitor their prevalence over time.

Blood tests detect the presence of antibodies against an infectious agent, such as SARS-CoV-2, in a patient’s bloodstream. Some antibodies simply indicate whether the individual has been previously exposed to either the virus or a vaccine, while others – known as neutralizing antibodies – provide immunity against infection or re-infection.

In the case of SARS-CoV-2, neutralizing antibodies work by interfering with the virus’ spike protein, which is the key that the virus uses to enter respiratory system cells by binding to the ACE2 receptors on the cells’ surface.

The research was carried out by the teams at the CHUV’s Service of Immunology and Allergy, which are led by Prof. Giuseppe Pantaleo and Dr. Craig Fenwick, and by EPFL’s Laboratory of Virology and Genetics, headed by Prof. Didier Trono and Dr. Priscilla Turelli.

The new antibody test is a highly sensitive and extremely accurate way of measuring how well a sample of blood serum can prevent the spike protein in its trimeric form – as found on the surface of the SARS-CoV-2 virus – from binding to ACE2 receptors. It completes the diagnostic arsenal in development at EPFL, which also includes the microchip device presented two months ago.

Because the new method requires a simple blood test, it can be deployed easily on a large scale. The test results show whether a patient has developed immunity against one or more variants of SARS-CoV-2. The research team was able to develop the test rapidly thanks to the core facilities set up and supported over many years by the Swiss Vaccine Research Institute.

The recent worldwide outbreak of severe acute respiratory syndrome related coronavirus 2 (SARS-CoV-2)(Wu et al., 2020) has led to unprecedented pressure on national healthcare systems(Fauci et al., 2020). The World Health Organization (WHO) advised the international community to perform extensive diagnostic tests to reduce the spreading of the virus and decrease the number of unreported cases (i.e.,asymptomatic or mild cases)(Li et al., 2020, Zhao et al., 2020). This strongly motivates the researchers to develop reliable testing tools to make SARS-CoV-2 diagnostics easier, cheaper and more accessible(Dincer et al., 2019, Choi, 2020, Morales-Narváez and Dincer, 2020, Zhu et al., 2020, Santiago, 2020, Seo et al., 2020, Bhalla et al., 2020).

While quantitative reverse transcription polymerase chain reaction (qRT-PCR) is the most reliable method to detect the genome of SARS-CoV-2 at the early stage of the infection(Corman et al., 2020, Chu et al., 2020, Moitra et al., 2020), serological tests for viral antibodies are equally important as they can identify false negative qRT-PCR responses because the virus concentration tends to become low at the late stage of the infection(LaMarca et al., 2020). In addition, sampling for antibodies are easier because antibodies are more stable than RNAs.

Even though it takes several days to develop sufficient amount of antibody in blood plasma or serum once a patient is infected by SARS-CoV-2, serological analysis is crucial for the identification of asymptomatic infections to further control the spread of the virus(Paiva et al., 2020, Du et al., 2020, Cui and Zhou, 2020, Day, 2020). Finally, the antibody tests can help to track how effectively the patient’s immune system is fighting the infection and are potentially helpful for plasma transfusion therapies(Krammer and Simon, 2020, Winter and Hegde, 2020, Long et al., 2020, Roback and Guarner, 2020, Duan et al., 2020, Amanat et al., 2020, LaMarca et al., 2020, Lee et al., 2020).

Several serological tests have received the Emergency Use Authorization (EUA) from the U.S. Food and Drug Administration (FDA)(U.S. Food and Drug Administration, 2020). Among them, enzyme linked immunosorbent assays (ELISA), chemiluminescent immunoassays, and neutralization assays(Muruato et al., 2020) are reliable but necessitate of trained operators and require hours or even days to perform the analysis(John Hopkins Center for Health Security, 2020). On the other hand, rapid diagnostic tests such as lateral flow assays, are easier to use and provide the results quickly (i.e, 10–30min), but they offer only qualitative information and their accuracy is not always sufficient(Carter et al., 2020, Udugama et al., 2020, John Hopkins Center for Health Security, 2020). A comprehensive survey of the existing serological tests published by FDA is summarized in TableS1 in the Supplementary Material.

More recently, new diagnostic sensors(Xu et al., 2020) have been developed to support the standard SARS-CoV-2 diagnostic techniques by detecting the viral RNA by using CRISPR (clustered regularly interspaced short palindromic repeats) based assays(Huang et al., 2020) and plasmonics(Qiu et al., 2020), the viral surface proteins by field-effect transistors(Seo et al., 2020), membrane-engineered mammalian cells(Mavrikou et al., 2020), and toroidal plasmonic devices(Ahmadivand et al., 2020). In addition, new lateral flow assays based on immunochromatographic strips have also been established to identify the antibodies produced in blood in response to the viral infection with qualitative outputs (i.e.,positive or negative)(Zeng et al., 2020).

Motivated by finding a reliable, rapid, and cost-effective alternative to existing serological methodologies for antibody detection(Dincer et al., 2019), we develop an opto-microfluidic biosensor platform to quantify the concentration of anti-SARS-CoV-2 spike protein antibodies in diluted human plasma by correlating the wavelength shift of the localized surface plasmon resonance (LSPR) peak of gold nanostructures in the microfluidic device upon binding interactions with the SARS-CoV-2 spike protein. The LSPR detection principle is based on the local refractive index changes around the metal nanostructures due to the biomolecule binding events (i.e.,antigen–antibody binding).

This leads to a red shift of the LSPR peak of the noble metal nanostructures, which is directly proportional to the target antibody concentration(Willets and VanDuyne, 2007, Mayer and Hafner, 2011). Another advantage of LSPR-based sensing is that the short decay length of the electromagnetic field in localized surface plasmons greatly reduces interfering effects from the bulk solution, which is desirable when analyzing complex samples such as blood plasma or serum containing fibrinogen, globulins, etc.(Szunerits and Boukherroub, 2012).

Our optofluidic platform consists of a gold nanospike covered glass substrate, fabricated by gold electrodeposition (ED), integrated in a microfluidic chip coupled with a reflection probe to detect the presence of antibodies against the SARS-CoV-2 spike protein within ∼30min in a diluted human plasma (1:1000), with the limit of detection (LOD) of ∼0.08ng/mL (∼0.5pM), which falls under the clinical relevant concentration range (ng/mL–μg/mL in diluted plasma samples)(Brown et al., 2018, Humphrey and Batty, 1974, Long et al., 2020).

Our work successfully demonstrates, for the first time, an opto-microfluidic chip to detect antibodies specific to the SARS-CoV-2 spike protein in real human plasma with high sensitivity and selectivity, without labeling agents, which can be expanded as a potential point-of-care antibody testing platform for real sample analysis.

reference link :

More information: Craig Fenwick et al, A high-throughput cell- and virus-free assay shows reduced neutralization of SARS-CoV-2 variants by COVID-19 convalescent plasma, Science Translational Medicine (2021). DOI: 10.1126/scitranslmed.abi8452



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