VRP induce IgA Ab production in the DLN in vivo. Groups of BALB/c mice were immunized in the rear footpad with I-Flu alone (1 μg), or I-Flu (1 μg) plus null VRP (1 × 105 IU) at weeks 0 and 4. At the indicated time points, flu-specific IgA Abs were evaluated in DLN homogenates (A), fecal extracts (B), and PP homogenates (C) by ELISA. Values represent mean ± SEM. *, p < 0.05; **, p < 0.02; and ***, p < 0.003 compared with I-Flu alone, as determined by the Mann-Whitney U test.