The present study was designed to investigate the effect of a series of cytokines on the release of the type II IL-1 decoy receptor, which represents a unique pathway of negative regulation of the IL-1 system. After 20 min, IL-1, IL-2, IL-4, IL-6, IL-10, IL-12, IL-13, IFN-gamma, granulocyte-CSF, macrophage-CSF, and TGF-beta had little or no effect on IL-1 binding by human polymorphonuclear cells. In contrast granulocyte-macrophage-CSF and, to a greater extent, TNF markedly reduced IL-1 binding. The action of TNF was rapid, reaching 50% of its maximum (80%) at 2 min, and plateauing at 20 min with decrease in receptor number and no significant change in affinity. Loss of surface receptor was associated to rapid release of a 45-kDa IL-1-binding molecule identified as the decoy RII. TNF-induced release of the decoy RII was independent of protein synthesis and reactive oxygen intermediates. Monocytes showed a similar response to TNF, except for the size of the released molecule (approximately 60 kDa). TNF induced rapid release of its own receptors. In contrast IL-1beta affected neither its own receptors nor the TNF-R. TNF and, more efficiently, PMA caused release of the decoy RII in fibroblasts transfected with the full-length decoy RII or with a cytoplasmatic deletion mutant. TNF-induced decoy RII release represents an unidirectional pathway of communication in the interplay between the IL-1 and TNF system.