IFN-gamma-dependent and -independent mechanisms in adverse effects caused by concomitant administration of IL-18 and IL-12.

IL-18 is a new type of inflammatory cytokine similar to but distinct from IL-12 and IL-1beta. One intriguing property of IL-18 is synergism with IL-12 in many respects. In this study we examined the in vivo synergistic effects of IL-18/IL-12 in mice and found lethal toxicity accompanying an elevated IFN-gamma level in the serum. Since treatment with IL-18 alone did not have any apparent toxicity, and treatment with IL-12 alone showed only limited toxicity in our system, the synergy between the two cytokines was all the more remarkable. The major symptoms of the toxicity were weight loss, diarrhea, hemorrhagic colitis, splenomegaly, fatty liver, and atrophic thymus, most of which are similarly found in endotoxin-induced septic shock. However, in contrast to septic shock, TNF-alpha was not induced. The involvement of IFN-gamma in the toxicity was further studied in detail. Treatment of athymic nude mice with anti-asialo-GM1 did not reduce the toxicity, whereas anti-IFN-gamma treatment of wild-type mice alleviated it. When IFN-gamma-deficient mice were treated with IL-18/IL-12, the majority of them showed mortality and toxicity with severe pulmonary edema. These results indicate that IL-18/IL-12 treatment induces severe adverse effects through not only IFN-gamma-dependent mechanisms but also IFN-gamma-independent processes.