The development and kinetics of functional antibody-dependent cell-mediated cytotoxicity (ADCC) to SARS-CoV-2 spike protein

Highlights: • Inducible EGFP and Luciferase dual reporters target cell line. • Antibody-Dependent Cell-mediated Cytotoxicity (ADCC) to SARS-CoV-2 spike protein. • Cell lysis mediated primarily via the NK FcγRIIIa receptor (CD16). • Functional SARS-CoV-2 spike (S) protein-based ADCC antibody assay. Since the COVID-19 pandemic, functional non-neutralizing antibody responses to SARS-CoV-2, including antibody-dependent cell-mediated cytotoxicity (ADCC), are poorly understood. We developed an ADCC assay utilizing a stably transfected, dual-reporter target cell line with inducible expression of a SARS-CoV-2 spike protein on the cell surface. Using this assay, we analyzed 61 convalescent serum samples from adults with PCR-confirmed COVID-19 and 15 samples from healthy uninfected controls. We found that 56 of 61 convalescent serum samples induced ADCC killing of SARS-CoV-2 S target cells, whereas none of the 15 healthy controls had detectable ADCC. We then found a modest decline in ADCC titer over a median 3-month follow-up in 21 patients who had serial samples available for analysis. We confirmed that the antibody-dependent target cell lysis was mediated primarily via the NK FcγRIIIa receptor (CD16). This ADCC assay had high sensitivity and specificity for detecting serologic immune responses to SARS-CoV-2.