Functional Role Played by the Glycosyiphosphatidylinositol Anchor Glycan of CD48 in Interleukin-18-induced Interferon-γ Production.

Interleukin (IL)-18 induces T cells and natural killer cells to produce not only interferon-γ but also other cytokines by binding to the IL-18 receptor (IL-18R) α and β subunits. However, little is known about how IL18, IL-18Rα, and IL-18Rβ form a high-affinity complex on the cell surface and transduce the signal. We found that IL-18 and IL-18Rα bind to glycosylphosphatidylinositol (GPI) glycan via the third mannose 6-phosphate diester and the second β-GlcNAc-deleted mannose 6-phosphate of GPI glycan, respectively. To determine which GPIanchored glycoprotein is involved in the complex of IL-18 and IL-18Rα, IL-18Rα of IL-18-stimulated KG-1 cells was immunoprecipitated together with CD48 by anti-IL-18Rα antibody. More than 90% of CD48 was detected as β-GlcNAc-deleted GPI-anchored glycoprotein, and soluble recombinant human CD48 without GPI glycan bound to IL-18Rα, indicating that CD48 is associated with IL-18Rα 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-γ production. These observations suggest that the complex formation of IL-18·IL-18Rα·CD48 via both the peptide portion and GPI glycan triggers the binding to IL-18Rβ, and the IL-18-IL-18Rα-CD48·IL-18Rβ complex induces cellular signaling. [ABSTRACT FROM AUTHOR]