Fig. 1.

RBD-specific neutralizing Ab was induced in mice. (A) A vaccinia virus which contains the full length of the spike gene was used to immunize mice. Titers of the RBD-specific Ab were tested using ELISA in sera collected 1 week after the second immunization. ‘A’, ‘B’ and ‘C’: three individual immunized mice, ‘N’: mouse without immunization. (B) Neutralizing activity was measured by syncytia inhibition assay. Immunized mouse sera collected 1 week after the second booster immunization was screened by syncytia inhibition assay. ‘No ACE2’: HEK293T cells without ACE2 plasmid transfection. ‘+’: positive control in which HEK293T cells were transfected with ACE2 plasmid and no antiserum added. Neutralizing effects of normal mouse sera (N) and immunized mouse sera (A, B and C) were measured in groups of ACE2 transfected HEK293T cells. Mouse whose serum bears highest neutralizing activity was selected for raising mAbs.