Genetic modification of malignant melanoma cells by transduction of cDNA encoding costimulatory molecules, cytokines or tumor-associated antigens has been shown to induce antitumor immunity. An important step in this scenario is the activation of T cells. CD80 is a pivotal costimulatory molecule for T-cell activation. Another molecule with costimulatory activity is CD70. The purpose of this study was to evaluate the capacity of a combined expression of CD70 and CD80 on melanoma cells to amplify an antitumor response in vitro. Therefore, the CD70- and CD80-negative human malignant melanoma cell line Colo679 was transduced with adeno-associated virus vectors carrying either CD70 or CD80. The resulting cell strains Colo679-CD70, -CD80 and -CD70/CD80 showed strong expression of CD70, CD80 or both, respectively. As expected, the T-cell response to CD70-positive malignant melanoma cells was substantially weaker than to the CD80-positive cells. However, the combined expression of CD70 and CD80 resulted in a T-cell response clearly superior to the single expression of CD80 or CD70 alone. These results provide evidence that CD70 plays an additional role in T-cell activation and should be considered as a molecule of interest in the design of immune gene therapy strategies for the treatment of malignant melanoma.