Antibody blocking of RBD and ACE2 interaction

The Fc-tagged RBD proteins (4 μg/ml) were captured on the protein A biosensor for 300 s. Then, the sensors were blocked by a control Fc protein (150 μg/ml) for 200 s to occupy the free protein A on the sensor. The serially diluted antibodies (0.041–30 nM) were then incubated with the sensors for 200 s to allow antibody and RBD binding. After 10 s of baseline in kinetics buffer, the sensors were dipped in to the ACE2 solution (10 μg/ml) for 200 s to record the response signal. For analysis of the IC₅₀ of the blocking, the ACE2 response values were normalized to the starting points. The blocking percentages at each concentrations were calculated as: (normalized ACE2 response of isotype antibody- normalized ACE2 response of tested antibody)/ normalized ACE2 response of isotype antibody *100. The dose-blocking curves were plotted and the blocking IC₅₀ values were calculated by nonlinear fit in the GraphPad prism 8 Software.