FIG 2.

RBD DNA vaccination induces strong humoral responses in mice. (A) C57BL/6 mice (n = 5) were immunized three times (via intramuscular electroporation) with 10 or 25 μg of either pARBD-NQ (denoted A) or pBRBD-NQ (denoted B). Animals immunized with pVAX-1 are referred to as “control.” (B and C) After the third immunization, total serum anti-RBD IgG responses were measured by ELISA. (C) Fold change in OD₄₅₀ values were compared at a 1/4,500 dilution. (D) An ELISA was used to determine the isotypes of vaccine-induced RBD-specific antibodies; post-third-immunization sera were subjected to a similar analysis. (E) Spleens from immunized animals were isolated 10 days after the third immunization. Pooled splenocytes were added to RBD- or IgG-coated ELISpot plates, and the numbers of antigen-specific ASCs were determined. The bars (B and D) and horizontal lines (C and E) indicate the medians of the groups. OD₄₅₀, optical density at 450 nm. *, P ≤ 0.05; **, P ≤ 0.01.