IL-18-binding protein (IL-18BP) is a secreted glycoprotein in the IL-1 family of ligands, however, it bears no amino acid sequence homology to the membrane-associated IL-18 and IL-1 receptor proteins. IL-18BP functions as a natural anti-inflammatory and immunosuppressive molecule neutralizing the effects of high IL-18 levels during inflammation (1). It is constitutively expressed and secreted in mononuclear cells and can be enhanced by IFN gamma, which suggests IL-18 activity is modulated by a negative feedback mechanism via IL-18BP. There are four known variants of IL-18BP (a, b, c & d). The isoform IL-18BPd, also known as MC54L, lacks a complete Ig domain and the ability to bind or neutralize IL-18 (2). IL-18BPd has been shown to be produced as a secreted full-length molecule that binds cell surface glycosaminoglycans in the extracellular matrix and also as a furin-cleaved form that contains only the IL-18 binding domain (3). It is related to a secreted viral protein, vIL-18BP, and may contribute to the lack of inflammatory responses characteristic of molluscum contagiosum virus lesions (4). 

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