SPR Assay.

SPR was used to detect the affinity between antibody and RBD by BIAcore 8K system, all of which was carried out at room temperature. Proteins were exchanged into 20 mM HEPES, pH 7.4, 150 mM NaCl, 0.05% (v/v) Tween 20 before using. Protein A biosensor chips (GE Healthcare) were used to capture antibody and RBD protein with serial two-fold dilution and five concentration gradients flowed through the chip surface from low to high concentration. At the end of each cycle, the sensor was regenerated with Gly-HCl (pH 1.7). We used 1:1 (Langmuir) binding fit model to calculate the affinity between antibody and RBD on BIAevaluation software.