Figure 4: Effect of preimmunity to the 17D strain of YFV on immunogenicity of the dengue tetravalent DNA vaccine. Groups of six mice were preimmunized by two inoculations, with a two-week interval, with 5.6 106 (closed circle) or 5.6 104 (open circle) PFU of YFV or inoculated with PBS (closed triangle). Two and six weeks after the second inoculation for preimmunization, these mice were vaccinated with 40 g of the dengue tetravalent DNA vaccine (10 g for each type). Pooled sera were examined for neutralizing antibody titers (upper panel) and individual sera were examined for ELISA antibody levels (lower panel) against DENV14 and YFV (indicated above the upper panel). The positive control for the DENV14 ELISA was obtained from mice only vaccinated with the tetravalent vaccine, while that for the YFV ELISA was obtained from mice preimmunized with 5.6 106 PFU of YFV and then vaccinated with the tetravalent vaccine. Since this experiment was performed in parallel to the experiment shown in Figure 3, time courses obtained with the control mice (closed triangles) are the same as those shown in Figure 3. Signifi cant differences in mean ELISA values from non-preimmunized mice were indicated by * (P<0.05), ** (P<0.01) and *** (P<0.001) for mice preimmunized with a 1/2 dose of JEVAX and + (P<0.05), ++ (P<0.01) and +++ (P<0.001) for mice preimmunized with a 1/10 dose of JEVAX.