FLOW CYTOMETRY MULTIPLEXED METHOD FOR THE DETECTION OF NEUTRALIZING HUMAN ANTIBODIES TO THE NATIVE SARS-CoV-2 SPIKE PROTEIN

Horndler, Lydia; Delgado, Pilar; Balabanov, Ivaylo; Cornish, Georgina; Llamas, Miguel Angel; Serrano-Villar, Sergio; Fresno, Manuel; Santen, Hisse Martien van; Alarcon, Balbino

A correct identification of seropositive individuals for the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) infection is of paramount relevance to assess the degree of protection of a human population to present and future outbreaks of the COVID-19 pandemic. We describe here a sensitive and quantitative flow cytometry method using the cytometer-friendly non-adherent Jurkat T cell line that stably expresses the full-length native spike S protein of SARS-CoV-2 and a truncated form of the human EGFR that serves a normalizing role. S protein and huEGFRt coding sequences are separated by a T2A self-cleaving sequence, allowing to accurately quantify the presence of anti-S immunoglobulins by calculating a ratio of the mean fluorescence intensities obtained by double- staining with the sera and a monoclonal antibody specific for EGFR. We show that the method allows to detect immune individuals regardless of the result of other serological tests or even repeated PCR monitoring. It can also be employed to detect neutralizing activity in the sera of individuals. Finally, the method can be used in a multiplexed format to simultaneously measure all anti-S human immunoglobulin isotypes in blood and mucosal fluids including total saliva.