We transduced a human gastric carcinoma cell line, HR, with the interleukin 2 (IL-2) gene. Stable HR transfectants secreted nanogram quantities of biologically active IL-2 and had significantly increased expression of IL-2 mRNA relative to that in parental cells. Expression of intracellular IL-2 protein was not quantitatively different in the parental and IL-2 gene-transduced cells, although the former did not secrete IL-2. Surface expression of IL-2 receptors was comparable in the parental and transduced cells at the mRNA or protein levels. Nevertheless, in vitro proliferation of IL-2 gene-transduced HR cells was significantly more rapid than that of parental cells. Both parental and IL-2 gene-transduced HR cells were equally sensitive to lysis by IL-2-activated natural killer (A-NK) cells, as measured in 4 hr 51Cr-release assasys or to apoptosis induced by NK or A-NK cells, assessed in 1 hr 3H-TdR-release assays. In 24 hr MTT assays, however, the IL-2 gene-transduced cells were significantly more sensitive to these effector cells than were parental cells. Upon intrasplenic injection of 5 x 10(6) parental or transduced HR cells into nude mice, liver metastases developed. Metastases of parental HR cells killed the animals in 24 days. In contrast, metastases of the IL-2 gene-transduced HR cells became necrotic by day 14 and were found to be surrounded by murine NK cells and macrophages. Survival of nude mice injected with IL-2 gene-transduced HR cells was significantly prolonged (>50 days) relative to that of mice injected with parental HR. Thus, IL-2 gene-transduced HR cells produced sufficient amounts of functional IL-2 in vivo to be able to recruit to the tumor site and support functions of endogenous cytotoxic immune effector cells, which were responsible for regression of hepatic metastases and significant improvement of survival in these mice.