Fig. 1.

Characterization of RBD and RBD-Fc, mouse immunization with recombinant RBD proteins. (A) SDS-PAGE of RBD-Fc (2 μg, ~100 kDa without DTT and ~50 kDa with DTT) are consistent with their theoretically calculated MWs. (B) SDS-PAGE of RBD (with 6 × His tag, 2 μg) in the presence or absence of DTT. The apparent molecular weight (MW) of RBD (heterogeneity ranging from 25 to 38 kDa) due to glycosylation was verified by deglycosylation. (C) ELISA measurement of binding of the recombinant RBD and RBD-Fc to hACE2-mFc (mouse Fc, Sino Biological). 200 ng RBD or RBD-Fc was coated on plate with incubation of serially diluted hACE2-mFc. Binding was detected by using HRP conjugated anti-mouse Fc antibody. Experiments were performed in duplicate and the error bars denote ± SD, n = 2. (D) Mouse immunization and sera sampling schedule. Four groups of BALB/c mice (n = 5) received same doses of the RBD vaccine or the control DPBS on day 0 and boost again on day 14. Sera were collected on day 0 (pre-vaccination), day 13 and 27 (post-vaccination).