The membrane form of the type II IL-1 receptor accounts for inhibitory function.

IL-1 signaling is mediated by the type I IL-1R (IL-1RI). The nonsignaling type II receptor has a regulatory function, since it reduces IL-1 effects by scavenging free IL-1 molecules. This regulatory function has been demonstrated only for the soluble form, released from the membrane receptor by action of specific proteases, but is still ill-defined for the membrane receptor itself. To assess the function of membrane IL-1RII, a modified IL-1RII cDNA was constructed, in which the cleavable domain was replaced with the corresponding uncleavable sequence of the epidermal growth factor receptor. The human keratinocyte line HaCaT, which does not express wild-type IL-1RII (wtIL-1RII), was stably transfected with this modified cDNA (unconventionally cleavable IL-1RII (uIL-1RII)). Cells transfected with uIL-1RII expressed the membrane form of IL-1RII, but were unable to produce the 60-kDa soluble receptor. Upon analysis of IL-1 responsiveness, parental HaCaT and vector-transfected cells (E27), expressing IL-1RI and the accessory chain IL-1R accessory protein, were responsive to IL-1. Conversely, cells overexpressing wtIL-1RII (811) or uIL-1RII (9D4) showed comparable reduction in responsiveness to both IL-1alpha (bound by membrane and soluble receptors) and IL-1beta (recognized by the membrane receptor only), suggesting that the membrane form of the IL-1RII is mainly responsible for IL-1 inhibition. In contrast with wtIL-1RII, uIL-1RII did not interact with IL-1R accessory protein. Thus, the membrane form of IL-1RII possesses strong IL-1-inhibitory activity, independent of sequestration of the accessory protein and circumscribed to its ligand sink function.