The cooperative role of CD4+ helper T (Th) cells has been reported for CD8+ cytotoxic T (Tc) cells in tumor eradication. However, its molecular mechanisms have not been well elucidated. We have recently demonstrated that CD4+ Th cells can acquire major histocompatibility complex/peptide I (pMHC I) complexes and costimulatory molecules by dendritic cell (DC) activation, and further stimulate naïve CD8+ T cell proliferation and activation. In this study, we used CD4+ Th1 and CD8+ Tc1 cells derived from ovalbumin (OVA)-specific T cell receptor (TCR) transgenic OT II and OT I mice to study CD4+ Th1 cell's help effects on active CD8+ Tc1 cells and the molecular mechanisms involved in CD8+ Tc1-cell immunotherapy of OVA-expressing EG7 tumors. Our data showed that CD4+ Th1 cells with acquired pMHC I by OVA-pulsed DC (DCOVA) stimulation are capable of prolonging survival and reducing apoptosis formation of active CD8+ Tc1 cells in vitro, and promoting CD8+ Tc1 cell tumor localization and memory responses in vivo by 3-folds. A combined adoptive T-cell therapy of CD8+ Tc1 with CD4+ Th1 cells resulted in regression of well-established EG7 tumors (5 mm in diameter) in all 10/10 mice. The CD4+ Th1's help effect is mediated via the helper cytokine IL-2 specifically targeted to CD8+ Tc1 cells in vivo by acquired pMHC I complexes. Taken together, these results will have important implications for designing adoptive T-cell immunotherapy protocols in treatment of solid tumors.