ABSTRACT: Various approaches employing cytokines and cytokine gene-modified tumor cells have been explored to induce antitumor responses, yet their widespread application has been limited due to efficacy concerns and adverse effects. In this study, interleukin-7 was engineered for expression both as a natural secretory form (sIL-7) and as a membrane-bound form fused with the B7.1 type I transmembrane protein (mbIL-7/B7) on CT26 colon cancer cells. Analysis of the resulting cell clones demonstrated that ectopically expressed sIL-7 and mbIL-7/B7 both retained similar capacities to induce the expansion and activation of CD8(+) T cells and to enhance antitumor responses in vitro. While the sIL-7 or mbIL-7/B7 clones showed similar growth in culture, the mbIL-7/B7 clone exhibited lower tumorigenicity in mice compared to the sIL-7 clone or wild-type CT26 cells. Specifically, the mbIL-7/B7 clone failed to form tumors in approximately 60% of the mice injected with it. Moreover, 80% of mice that rejected the mbIL-7/B7 clone developed long-term systemic immunity against CT26 cells. Analysis of immune cells within the tumor masses revealed significant increases in CD4(+) T cells, CD8(+) T cells, and dendritic cells in tumors formed by the mbIL-7/B7 clone compared to those formed by the sIL-7 clone. These findings suggest that the membrane-bound form of IL-7 with B7.1 is more effective than the secretory form in establishing antitumor immunity within the tumor microenvironment. Our strategy of expressing the mbIL-7/B7 chimera holds promise as a novel approach for tumor therapy, particularly in cases requiring IL-7 supplementation.