Abstract By studying BALB/c mice deficient in immune components, we show that the protective immunity to rat ErbB2+ tumors rests on the Ab response elicited by the electroporation of a DNA vaccine encoding the extracellular and transmembrane domains of rat ErbB2. In vivo, the adoptive transfer of vaccine-elicited anti-rat ErbB2 Abs protected against a challenge of rat ErbB2+ carcinoma cells (TUBO cells). In vitro, such Abs inhibited TUBO cell growth by impairing cell cycle progression and inducing apoptosis. To correlate intrinsic mechanisms of Ab action with their tumor-inhibitory potential, first we showed that TUBO cells constitutively express phosphorylated transgenic ErbB2/autochthonous ErbB3 heterodimers and exhibit a basal level of Akt phosphorylation, suggesting a constitutive activation of the PI3K/Akt pathway. Treatment with anti-ErbB2 Abs caused a drastic reduction in the basal level of Akt phosphorylation in the absence of an impairment of PI3K enzymatic activity. Notably, the same Ab treatment induced an increase in PTEN phosphatase activity that correlated with a reduced PTEN phosphorylation. In conclusion, vaccine-induced anti-ErbB2 Abs directly affected the transformed phenotype of rat ErbB2+ tumors by impairing ErbB2-mediated PI3K/Akt signaling.