IL-18 receptor β-induced changes in the presentation of IL-18 binding sites affect ligand binding and signal transduction

IL-18 is a pleiotropic proinflammatory cytokine that is involved in induction of inflammatory mediators, regulation of the cytotoxic activity of NK cells and T cells, and differentiation and activation of both Th1 and Th2 cells. IL-18 signals through its specific cell surface receptor IL-18R, which comprises two subunits: IL-18Ralpha and IL-18Rbeta. IL-18Ralpha alone has a weak affinity for IL-18 binding, while the IL-18Ralpha/beta complex has a high affinity. By using several anti-IL48 mAbs and IL-18 binding protein, we have, examined whether these site-specific inhibitors could block the binding of IL-18 to IL-18Ralpha and to the IL-18Ralpha/beta complex. Here we show that IL-18 binding to IL-18Ralpha was inhibited by a neutralizing mAb, 125-2H, while binding of IL-18 to the alpha/beta receptor complex was not. This suggests that IL-18Rbeta-induced conformational changes may occur in IL-18Ra upon dimerization, leading to changes in the presentation of IL-18 binding sites. Epitope mapping of 125-2H using human-mouse IL-18 chimeras identified a region in IL-18 that was required for 125-2H recognition. This region, as examined by IL-18R binding and functional analysis, appeared to be critical for triggering signal transduction through the heterodimeric receptor.