The membrane form of the type IIIL-1 receptor accounts for inhibitory function

IL-1 signaling is mediated by the type I IL-1R (IL-1RI), The nonsignaling t ype II receptor has a regulatory function, since it reduces IL-1 effects by scavenging free IL-1 molecules. This regulatory function has been demonstr ated only for the soluble form, released from the membrane receptor by acti on of specific proteases, but is still ill-defined for the membrane recepto r itself, To assess the function of membrane IL-1RII, a modified IL-1RII cD NA was constructed, in which the cleavable domain was replaced with the cor responding uncleavable sequence of the epidermal growth factor receptor. Th e human keratinocyte line HaCaT, which does not express wild-type IL-1RII ( wtIL-1RII), was stably transfected with this modified cDNA (unconventionall y 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-IR ac cessory protein, were responsive to IL-1, Conversely, cells overexpressing wtIL-1RII(811) or uIL-1RII (9D4) showed comparable reduction in responsiven ess to both IL-1 alpha (bound by membrane and soluble receptors) and IL-1 b eta (recognized by the membrane receptor only), suggesting that the membran e form of the IL-1RII is mainly responsible for IL-1 inhibition. In contras t with wtIL-1RII, uIL-1RII did not interact with IL-1R accessory protein, T hus, the membrane form of IL-1RII possesses strong IL-1-inhibitory activity , independent of sequestration of the accessory protein and circumscribed t o its ligand sink function.