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

Abstract

Steady state mRNA levels in various human tissues reveal that the proinflammatory cytokine IL-18 is constitutively and ubiquitously expressed. However, limited IL-18R α-chain (IL-18Rα) expression in tissues may restrict ligand-acting sites and contribute 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-18Rα was cloned and individually expressed in Escherichia coli. An mAb specifically recognized the membrane-proximal third domain; this mAb blocked IL-18-induced IFN-γ production in NK cells. Furthermore, deletion of the membrane-proximal third domain of IL-18Rα prevented the formation of IL-18R ternary complex with IL-18R β-chain. The present studies demonstrate that the biologically active IL-18R complex requires the membrane-proximal third Ig-like domain in IL-18Rα for the formation of IL-18R ternary complex as well as for signal transduction involved in IL-18-induced IFN-γ in NK cells.

Interleukin-1 as a Key Factor in the Development of Inflammatory Diseases

“…The IL-18 receptor has two chains, including IL-18R_ α and IL-18R_ β. The IL-18 receptor binding site to the α chain of the receptor is similar to the IL-1 receptor type I (18,19). The intracellular chain of the IL-18 receptor contains a Toll domain which phylogenetically link IL-18 to the Toll-like receptors.…”

Clinical and Biological Effects of Recombinant Human Interleukin-18 Administered by Intravenous Infusion to Patients with Advanced Cancer

“…Such concentrations would be expected to engage both high-affinity (K d , f300-500 pmol/L) and low-affinity (K d , f30-50 nmol/L) IL-18 receptors expressed on the cell surface (7,16,17). In vitro, IL-18 concentrations of 1 to 1,000 ng/mL have been found to activate lymphocytes (18 -20).…”

Recombinant Fc-IL-18BPc Isoform Inhibits IL-18-Induced Cytokine Production

“…(13,22) However, IL-18BP is a distinct protein from the IL-1 and IL-18 receptor families and locates in chromosome 11q13 at the inverted position of the nuclear mitotic apparatus protein-1. (13) The human IL-18BP genomic DNA encodes at least four different isoforms derived from alternative splicing isolated from several human cDNA libraries.…”

“…IL-18BP is a distinct gene composed of a single immunoglobulin domain. (13,22) Interestingly, several poxviruses encode IL-18BP orthologues as virulence factors that contribute to immune escape by suppressing antiviral immune responses of the host. (23)(24)(25)(26) These viral IL-18BP orthologues are evidence that IL-18BP has a pivotal role in viral infection as a mediator of cytotoxic immune responses.…”