Quantitative assays reveal cell fusion at minimal levels of SARS-CoV-2 spike protein and fusion from without

Cell entry of the pandemic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is mediated by its spike protein S. As a main antigenic determinant, S protein is in focus of various therapeutic strategies. Besides particle-cell fusion, S mediates fusion between infected and uninfected cells resulting in syncytia formation. Here, we present sensitive assay systems with a high dynamic range and high signal-to-noise ratios covering not only particle-cell and cell-cell fusion but also fusion from without (FFWO). In FFWO, S-containing viral particles induce syncytia independently of de novo synthesis of S. Neutralizing antibodies, as well as sera from convalescent patients, inhibited particle-cell fusion with high efficiency. Cell-cell fusion, in contrast, was only moderately inhibited despite requiring levels of S protein below the detection limit of flow cytometry and Western blot. The data indicate that syncytia formation as pathological consequence during coronavirus disease 2019 (COVID-19) can proceed at low levels of S protein and may not be effectively prevented by antibodies.