Interleukin 18 binding protein (IL-18?BP) is a secreted glycoprotein, which functions as an IL-18 antagonist by binding to IL-18 and blocking its biological activity. IL?18?BP bears no amino acid sequence homology to the membrane-associated IL-18 and IL-1 receptor proteins. The gene for human IL-18?BP has been localized to chromosome 11q13. It encodes for at least four isoforms by alternative splicing. The IL-18?BP isoforms a?and?c each contain one immunoglobulin (Ig)-like C2-type domain while isoforms?b and?d lack a complete Ig domain. The complete Ig domain has been shown to be essential to the binding and neutralizing properties of the binding proteins. Two isoforms of mouse IL18?BP (c?and?d) containing the complete Ig domain have also been isolated and shown to neutralize IL-18 bioactivity. Human and mouse IL-18?BPs share approximately 61% amino acid sequence identity. Several poxviruses also encode proteins with sequence similarity to the human and mouse IL-18?BP. Viral IL-18?BPs have been shown to bind and inhibit IL-18 responses and may be involved in modulating host immune responses. The expression of IL-18?BP is markedly up-regulated by IFN-gamma, suggesting that IL-18 activity is modulated by a negative feedback mechanism mediated by IL-18?BP.
