It is now clear that CD4+ T cells play a crucial role in the generation of CD8+ T effector and memory T-cell immune responses. In this study, we enhanced the CD4+ T-cell immune responses in mice by constructing a DNA vaccine encoding an invariant (Ii) chain in which the class II-associated Ii peptide (CLIP) region is replaced with a CD4+ T-helper epitope, PADRE (Ii-PADRE) (invariant Pan HLA-DR reactive epitope). C57BL/6 mice vaccinated with DNA encoding Ii-PADRE showed significantly greater PADRE-specific CD4+ T-cell immune responses than mice vaccinated with DNA encoding the Ii chain alone (Ii DNA). More important, administration of DNA encoding human papillomavirus (HPV) E6 or E7 antigen with DNA encoding Ii-PADRE led to significantly stronger E6- or E7-specific CD8+ T-cell immune responses and more potent protective and therapeutic anti-tumor effects against an E6/E7-expressing tumor model in mice than administration of E6 or E7 DNA with Ii DNA. Overall, our data indicate that administration of DNA vaccines with Ii-PADRE DNA represents an effective approach to enhancing the generation of CD4+ T cells and eliciting stronger antigen-specific CD8+ T-cell immune responses. Therefore, this strategy may be expected to have significant potential for clinical translation.