Interleukin 18 induces the sequential activation of natural killer cells and cytotoxic T lymphocytes to protect syngeneic mice from transplantation with Meth A sarcoma.

We have recently reported that interleukin 18 (IL-18) pretreatment induces immunologically mediated antitumor effects in BALB/c mice injected i.p. with syngeneic Meth A sarcoma. In this study, mice were pretreated with IL-18 before Meth A transplantation, and immunocompetency in pretreated or untreated tumor-bearing mice (TBM) 3, 9, and 15 days after transplantation was compared with that of normal mice. On day 3, pretreated TBM mitogen-stimulated spleen cells produced significantly decreased levels of IL-2 and IFN-gamma during 24-h culture. In contrast, IL-10 and granulocyte macrophage colony-stimulating factor productions were significantly enhanced in pretreated TBM cultures, and natural killer (NK) cell activity was also significantly augmented. Splenomegaly was also observed in the pretreated TBM on day 3, and the proliferating cells were identified as asialo GM1+ cells by flow cytometry. Cytotoxic activity of pretreated TBM spleen cells after a 5-day mixed lymphocyte-tumor cell culture did not differ from that of untreated TBM and normal mice on day 3 but was significantly enhanced on days 9 and 15 compared with that observed in normal mice and untreated TBM. Concurrently, the production of IL-2 and of IL-10 recovered and decreased, respectively, and NK activity dropped to normal levels. The effects of IL-18 on cytokine production and NK activity observed on day 3 treated TBM were also reproduced in normal mice. In conclusion, IL-18 seems to enhance the generation of NK activity early after tumor transplantation and simultaneously induces an increase and a decrease in the production of IL-10 and IL-2, respectively. As NK activity subsides to normal levels and IL-10 synthesis decreases, IL-2 synthesis is restored, and cytolytic cell activity is significantly enhanced. These results provide new insight into the immunologically mediated antitumor effects of IL-18.