Immunization and bleeding scheme. Mice were divided into six groups, ranging from 3 to 10 mice per group: CpG (n = 3), CpG/PBS (n = 3) and PBS (n = 4) served as the negative controls for DNA (n = 10), DNA/VLP (n = 10) and VLP (n = 10) groups, respectively. Immunization dose and route of administration for each group is as follows: CpG (16 μg CpG/100 μl/mouse/i.m. injection), PBS (100 μl endotoxin-free PBS/mouse/i.p. injection), DNA (50 μg of pVR1012-EBOVGPΔMLD + pMLV-GagPol at a 2:1 ratio + 16 μg CpG/100 μl/mouse/i.m. injection), VLP (33 μg retroVLP/100 μl/mouse/i.p. injection). The DNA, VLP, and CpG were formulated with endotoxin-free PBS. Immunization and blood collection schedules are shown. Mice were killed (†) at week 39 and 40. The symbols of syringes and blood drops represent immunization and blood collection, respectively.