Functional role played by the glycosylphosphatidylinositol anchor glycan of CD48 in interleukin-18-induced interferon-gamma production.

Interleukin (IL)-18 induces T cells and natural killer cells to produce not only interferon-gamma but also other cytokines by binding to the IL-18 receptor (IL-18R) alpha and beta subunits. However, little is known about how IL-18, IL-18Ralpha, and IL-18Rbeta form a high-affinity complex on the cell surface and transduce the signal. We found that IL-18 and IL-18Ralpha bind to glycosylphosphatidylinositol (GPI) glycan via the third mannose 6-phosphate diester and the second beta-GlcNAc-deleted mannose 6-phosphate of GPI glycan, respectively. To determine which GPI-anchored glycoprotein is involved in the complex of IL-18 and IL-18Ralpha, IL-18Ralpha of IL-18-stimulated KG-1 cells was immunoprecipitated together with CD48 by anti-IL-18Ralpha antibody. More than 90% of CD48 was detected as beta-GlcNAc-deleted GPI-anchored glycoprotein, and soluble recombinant human CD48 without GPI glycan bound to IL-18Ralpha, indicating that CD48 is associated with IL-18Ralpha via both the peptide portion and the GPI glycan. To investigate whether the carbohydrate recognition of IL-18 is involved in physiological activities, KG-1 cells were digested with phosphatidylinositol-specific phospholipase C before IL-18 stimulation. Phosphatidylinositol-specific phospholipase C treatment inhibited the phosphorylation of tyrosine kinases and the following IL-18-dependent interferon-gamma production. These observations suggest that the complex formation of IL-18.IL-18Ralpha. CD48 via both the peptide portion and GPI glycan triggers the binding to IL-18Rbeta, and the IL-18.IL-18Ralpha.CD48.IL-18Rbeta complex induces cellular signaling.