Fig. 1.

Key mediators of the IL-1 family. A productive IL-1 signaling (green frame) requires the binding of the IL-1 agonists IL-1α or IL-1β to the transmembrane IL-1RI, following the approximation of IL-1RAP and the formation of the signal-transducing heterotrimeric complex. Binding of IL-1Ra to IL-1RI prevents proper formation of the signaling complex and blocks signal transduction. IL-1RII competes with IL-1RI for the binding of the IL-1 agonists but is not able to transduce a signal because of a truncated cytosolic domain. Finally, sIL-1RI is able to bind both agonists and antagonist and therefore plays a double regulatory role, and sIL-1RII is able to bind free agonists and sequester them from proper binding, either on its own or with greater affinities while connected to sIL-1RAP (red frame)