FIG. 2.

Systemic (serum IgG) and secreted (fecal IgA) CfaB-specific antibody responses elicited in BALB/c mice subjected to different immunization regimens. Serum IgG (A) and fecal IgA (B) anti-CfaB responses were measured in ELISA plates coated with heat-denatured CfaB protein purified from a CFA/I⁺ ETEC strain. Sera were collected from the following immunization groups (at least five animals per group): pRECFA (▴), mice immunized with two i.m. doses of pRECFA (100 μg/dose); HG3 (▪), mice immunized with two p.o. doses of IPTG-induced S. enterica serovar Typhimurium HG3 (10¹⁰ CFU/dose); pRE4 + HG3 (+), mice immunized with two i.m. doses of the empty vector pRE4 (100 μg/dose) followed by two p.o. doses of the HG3 strain; pRECFA + HG3 (⧫), the complete primer-booster immunization regimen, with mice immunized with two i.m. doses of pRECFA followed by two p.o. doses of the HG3 strain; HG3 + pRECFA (*), inverse primer-booster immunization regimen consisting of two p.o. doses of the HG3 strain followed by two i.m. pRECFA doses. Data corresponding to three determinations are presented as means ± standard errors (SE) of the titers (serum IgG) or of absorbance values at 492 nm of the reactions with diluted fecal extracts (fecal IgA). All values were deducted from the background values obtained with nonimmune sera. All bars marked with an asterisk show statistically significantly different values (P < 0.05) compared to the results of the pRECFA (serum IgG)- or HG3 (fecal IgA)-vaccinated mouse group.