Two pivotal aspects of the IL-33 system are characterized in this study: First the composition and function of the IL-33 receptor complex on IL-33 responder cells and second the generation of biologically active IL-33 by the IL-33 producer cell. IL-33 binds to the IL-33 receptor -chain (ST2) and as is shown here, this results in the recruitment of interleukin-1 receptor accessory protein (IL-1RAcP) as the co-receptor. The IL-33-dependent interaction of membrane bound and soluble form of IL-1RAcP and IL-33R -chain was demonstrated in co-immunoprecipitation assays. Common notion in the field is...