Mean F4-specific serum IgA, IgG, and IgM titers following two i.d. immunizations (black arrows) with pcDNA1/faeG19, pWRGFaeG, pWRGFaeGopt, pWRGFaeGopt plus the LT vectors or pWRG7079 (controls), an oral F4 protein boost (all pigs, gray arrows) and an F4+ ETEC challenge (double arrow). Shown are significant differences (P < 0.05) of the pWRGFaeGopt group with the pcDNA1/faeG19 group (1); the control and the pcDNA1/faeG19 group (2); the control, pcDNA1/faeG19 and the pWRGFaeG group (3); all other groups (4) and significant differences of the pWRGFaeGopt+LT group with the pcDNA1/faeG19 group (5); the control and the pcDNA1/faeG19 group (6); the control, pcDNA1/faeG19 and the pWRGFaeG group (7) and all other groups (8). Significant differences between time points are mentioned in the text.