FIG. 3.

Factors affecting serum anti-CfaB IgG responses elicited in mice subjected to the DNA priming-Salmonella boosting immunization regimen. (A) Nature of the bacterial carrier given to mice primed with pRECFA. Mice were boosted p.o. with live (HG3) or ethanol-inactivated (HG3 k) S. enterica serovar Typhimurium HG3 or with live (ETEC) or ethanol-inactivated (ETEC k) ETEC 258909-3. As a control, the mean CfaB-specific serum IgG titer of mice immunized with two doses of pRECFA is indicated (pRECFA). (B) Age-dependent CfaB-specific antibody responses elicited in mice who were first immunized with pRECFA at the age of 1 week (young), 6 weeks (adult), or 52 weeks (old). Mice were immunized with two i.m. doses (100 μg/dose) of pRECFA (black bars) or with the complete primer-booster regimen (white bars). (C) Interval between the last priming dose with pRECFA and the first oral booster with the CfaB-expressing HG3 strain. CfaB-specific antibody levels detected for mouse groups subjected to the primer-booster regimens with intervals of 2, 4, 8, 16, and 52 weeks between the priming and boosting doses are indicated by white bars. As controls, anti-CfaB antibody titers detected in sera of mice who were vaccinated only with two doses of pRECFA and then bled at the same periods as those subjected to the complete primer-booster regimen are indicated by black bars. (D) Amount of DNA required to prime CfaB-specific antibody responses in mice receiving oral boosters with the HG3 strain. The amount of DNA inoculated in each dose is indicated (100, 50, 10, or 1 μg). Data corresponding to one representative determination are presented as means ± SE of the titers. All bars marked with an asterisk show statistically significantly different values (P < 0.05) compared to the results obtained with pRECFA-vaccinated mouse groups that were subjected to the same testing conditions.