Identification of a Critical Ig-Like Domain in IL-18 Receptor and Characterization of a Functional IL-18 Receptor Complex1

Steady state mRNA levels in various human tissues reveal that the proinflammatory cytokine IL-18 is constitutively and ubiq-uitously expressed. However, limited IL-18R -chain (IL-18R) expression in tissues may restrict ligand-acting sites and con-tribute to a specific response for IL-18. To study the IL-18R complex, [125I]IL-18 was studied for binding to the cell surface receptors of IL-18-responsive NK and macrophagic KG-1 cells. After cross-linking, [125I]IL-18 formed three IL-18R complexes with sizes of approximately 93, 160, and 220 kDa. In KG-1 cells, Scatchard analysis revealed the presence of 135 binding sites/cell, with an apparent dissociation constant (Kd) of 250 pM; in NK cells, there were 350 binding sites per cell with an apparent Kd of 146 pM. Each domain of extracellular IL-18R was cloned and individually expressed in Escherichia coli. An mAb specifically recognized the membrane-proximal third domain; this mAb blocked IL-18-induced IFN- production in NK cells. Furthermore, deletion of the membrane-proximal third domain of IL-18R prevented the formation of IL-18R ternary complex with IL-18R -chain. The present studies demonstrate that the biologically active IL-18R complex requires the membrane-proximal third Ig-like domain in IL-18R for the formation of IL-18R ternary complex as well as for signal transduction involved in IL-18-induced IFN- in NK cells. The Journal of Immunology, 2003, 171: 6574–6580