Rapid detection of SARS-CoV-2 antibodies using flow cytometry

Several classes of antibodies against the receptor binding domain (RBD) of the SARS-CoV-2 spike protein have been identified in COVID-19 patients, and are largely responsible for the protective immunity induced by the current COVID-19 vaccines. These antibodies have potent neutralizing activity, and three isotypes (IgG, IgM, and IgA) of anti-SARS-CoV-2 spike RBD antibodies have been identified in patients following natural infection with SARS-CoV-2 or immunization with the COVID-19 vaccine.

In this free on-demand SelectScience^® webinar, join Dr. Julie Lovchik, Scientist at Sartorius, as she introduces a newly developed flow cytometry kit, which provides a rapid, robust, and sensitive serological assay for the measurement of IgG, IgM, and IgA anti-SARS-CoV-2 spike RBD antibodies.

Read on for highlights from the live Q&A session, or watch the webinar on-demand, at a time that suits you.

How many samples can be tested using this assay?

JL: We have several formats of the kit available. You can get it either for 1x or 5x 96-well plates, or enough for 1x or 5x 384-well plates. The number of samples, of course, would depend on how many dilution batches and how many multiples of the samples you would run, but those are the available kits.

What concentration ranges can be detected?

JL: The kit allows the user to run the assay qualitatively or quantitatively. Generally, you can detect a range in the low picogram to high nanogram concentrations. For qualitative analysis, which might be useful for initial screening studies or on a large number of samples, the assay can detect as low as 30 to 60 pg/mL. You may recall from the presentation I just showed you, that this is far below the negative threshold cut-off values.

For quantitative analysis, the lower level of quantification (LLOQ), is higher and slightly between the antibody isotypes. For example, the linear range for the accurate quantification of IgG antibodies is approximately 50 ng/mL down to 300 pg/mL when using the antibody standard included in the kit, and preparing the 1 to 3 dilutions.

Can this kit be used with other flow cytometry machines?

JL: The assay was developed for use with the iQue® platform, including the compensation adjustments and the predefined analysis templates that auto-generate the concentration-response curves. Additionally, the low sample volume needed is very unique to the iQue high-throughput system. Particularly, this assay requires only 10 µL of pre-diluted serum or plasma, and then after the final wash, it’s only 10 µL of buffer that's added before the acquisition, thanks to the low volume sample required for the iQue platform. Other systems may require substantially more of your sample.

What SARS-CoV-2 variant is the source of the RBD?

JL: The RBD in this assay is the original wild type from the original wild type strain of SARS-CoV-2, which is just to remind you, the same SARS-CoV-2 strain that's utilized in the current vaccines.

Is the cytometer part of the iQue^® kit?

JL: No, the cytometer would be a separate purchase, and just for your information, we have multiple kits that can be used on this cytometer for various functions.

Can the assay be run on another cytometer other than the iQue^®?

JL: It really was developed for use with the iQue platform. All the compensation and everything is already set, so that would be difficult to translate to another machine. Also, the reagent quantity, the fact that it's only 10 µL in the final wash for acquisition, is very unusual for a lot of other machines.

Is there an option to purchase the kit without the cytometer?

JL: Yes, you can purchase the kit individually. It's not part of the machine.

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