Protective memory response against tumor challenge in mice receiving S typhimurium-mediated DNA oral immunization. (A) Tumor-take curves. After immunization with SL7207 (pCMVβ; squares) or SL7207 (triangles), mice received a first subcutaneous challenge with F1A11 tumor cells (solid symbols); after 40 days, tumor-free mice received a second contralateral challenge (open symbols). Tumor growth was inspected every other day by palpation. Each group included 15 mice. (B) Tumor size (in square millimeters) progression in immunized mice. Statistically significant differences are indicated: P < .01 (**).