Several cell-based assays are now being used to determine the correlates of protection in groups and individuals with symptomatic COVID-19 immunity and vaccine reactions. The cell-based assays have demonstrated high levels of concordance with living as well as pseudotyped virions of the novel coronavirus.
For a return to normalcy and the resumption of economic activity across the globe, national borders will need to be reopened and businesses will have to start functioning as before. However, for this to happen, safe and effective vaccines, as well as other treatment modalities, will have to be developed and implemented as soon as possible.
Hence, scientists need to be able to appraise the factors that determine protective immunity against the SARS-CoV-2 virus, also known as the severe acute respiratory syndrome 2. This is because the SARS-CoV-2 is known to be a major causative agent of COVID-19.
Building Immunity against Viral Infections
Scientists and researchers, therefore, believe that the immune responses that lead to protective immunity against this virus will also protect against COVID-19. Cell-based assays are used largely to determine the plausibility of this theory, and to what extent it might be true.
To be useful, these cell-based assays need to be both standardized and reproducible. Such standardized antibody assays can then be used by multiple research organizations across the globe to establish and analyze the correlates of risk and protection in the immune response against the various strains of the coronavirus.
Myriad neutralization assays are already being used by various companies and laboratories to find an effective vaccine for the COVID-19 virus. One of the most popular research methods is to use the plasma samples obtained from convalescent patients to compare the different COVID-19 neutralization platforms. Usually, these plasma samples are obtained from patients who had mild to moderate COVID-19 symptoms, before making a full recovery.
A group of Seattle-based researchers from the Vaccine and Infectious Diseases Division of the Fred Hutch Cancer Research Center is currently leading one of the most promising studies being conducted in this field. Dr. Anton M. Sholukh, Ph.D. is currently leading the study, having spent over two decades learning vaccine design and analyzing the role of different antibody isotopes in developing immune protection against viral infections.
The Role of Assays and Antibodies
To test the reliability of cell-based assays for antibody monitoring, researchers compared live recombinant assays containing the SARS-CoV-2 virus and a surrogate ELISA-based test. Some pseudovirus-based assays were also tested, with three different cell lines and two pseudovirus-based packaging techniques being identified during the process. The aforementioned packaging techniques are characterized by the use of the vesicular stomatitis virus as well as the lentivirus.
The correlation between the concentration of immunoglobulin antibodies and the speed of neutralization was also explored by researchers during the testing process. All the sera samples used during these tests were obtained during a localized outbreak of COVID-19 in a region in the western USA.
The influence of proteolytic cleavage on the process of virus neutralization by serum antibodies was one of the many factors explored with the help of cell-based assays during these tests and clinical trials. Multiple cell lines were used for the assays during these studies, which made it easier to choose an assay that would provide physiologically relevant outcomes to the tests.
Neutralization Readouts in Cell-Based Assays
In cell-based assays, readouts of 50 to 80 percent neutralization titers typically indicate a high correlation with one another. Correlation with the concentration of IgG antibodies against spike glycoprotein and the SARS-CoV-2 receptor binding is also quite high. A positive correlation with the results of the ELISA-based virus neutralization experiment further cemented the findings of this test, despite the correlation being somewhat more modest in this case.
Cell-based assays can, therefore, be used to quickly distinguish between samples having low and high neutralizing potency as well as to obtain preliminary qualitative results during the testing process. When the instruments for performing a cell-based assay are not available, or when such an assay is not reasonably feasible, surrogate ELISA-based assays can be used as a temporary alternative to achieve some of the aforementioned goals.
Usually, biotinylated soluble ACE2 receptors are used to perform such surrogate assays. They compete with serum antibodies for a receptor-binding domain or a binding spot on immobilized spike protein. A soluble, receptor-binding domain can also be used for this purpose.
The research conducted so far seems to demonstrate a high level of concordance between cell-based assays with pseudotyped virions and live ones, making them reliable for antibody monitoring. Cell-based assays can also be used to monitor patients receiving different types of treatment for COVID-19 symptoms, to correlate the lymphocyte counts with inhibitory activity, as an indicator of drug efficacy. However, as countries around the world work towards formulating an effective COVID-19 vaccine, further studies are needed to determine the efficacy of this method and many others.