['We evaluated the antitumor effects of IL-21 gene transfer into mouse RenCa renal cell carcinoma cells, so that cells could spontaneously secrete IL-21. We also investigated the mechanisms underlying this antitumor effect.', 'The IL-21 gene was introduced into RenCa cells by the liposome mediated method using Lipofectamine. The in vivo antitumor effect of IL-21 secreting RenCa cells was assessed by subcutaneous injection into syngeneic BALB/c mice. Mechanisms underlying the antitumor effects were investigated in syngeneic mice in which CD8 T, CD4 T or natural killer cells had been depleted using the corresponding antibody. The cytotoxic activity of splenocytes in mice injected with IL-21 secreting RenCa cells was determined using the CytoTox 96 nonradioactive cytotoxicity assay. Immunohistochemical examinations were performed to investigate infiltrating cells around tumor sites in vivo. Tumor vaccine study was also performed.', 'IL-21 secreting RenCa cells were almost all rejected following subcutaneous injection into syngeneic mice. The antitumor effect of IL-21 secreting RenCa cells remained in mice in which CD4 T cells had been depleted but it was totally abrogated in mice depleted of CD8 T cells or natural killer cells. Cytotoxic activities of splenocytes were higher in IL-21 secreting RenCa cell rejected mice than in parental RenCa mice. Immunohistochemical study also supported the involvement of CD8 T cells and natural killer cells in the antitumor effect of IL-21 secreting RenCa cells. Moreover, mitomycin C treated IL-21 secreting RenCa cells inhibited the growth of parental RenCa at distant site.'] Does interleukin-21 activate cytotoxic T lymphocytes and natural killer cells to generate antitumor response in mouse renal cell carcinoma?