Interleukin-18 Binding Protein (IL-18BP): A Long Journey From Discovery to Clinical Application

Abstract

IL-18 binding protein (IL-18BP) was originally discovered in 1999 while attempting to identify an IL-18 receptor ligand binding chain (also known as IL-18Rα) by subjecting concentrated human urine to an IL-18 ligand affinity column. The IL-18 ligand chromatography purified molecule was analyzed by protein microsequencing. The result revealed a novel 40 amino acid polypeptide. To isolate the complete open reading frame (ORF), various human and mouse cDNA libraries were screened using cDNA probe derived from the novel IL-18 affinity column bound molecule. The identified entire ORF gene was thought to be an IL-18Rα gene. However, IL-18BP has been proven to be a unique soluble antagonist that shares homology with a variety of viral proteins that are distinct from the IL-18Rα and IL-18Rβ chains. The IL-18BP cDNA was used to generate recombinant IL-18BP (rIL-18BP), which was indispensable for characterizing the role of IL-18BP in vitro and in vivo. Mammalian cell lines were used to produce rIL-18BP due to its glycosylation-dependent activity of IL-18BP (approximately 20 kDa). Various forms of rIL-18BP, intact, C-terminal his-tag, and Fc fusion proteins were produced for in vitro and in vivo experiments. Data showed potent neutralization of IL-18 activity, which seems promising for clinical application in immune diseases involving IL-18. However, it was a long journey from discovery to clinical use although there have been various clinical trials since IL-18BP was discovered in 1999. This review primarily covers the discovery of IL-18BP along with how basic research influences the clinical development of IL-18BP.

Keywords: Autoimmune diseases; Basic research; Clinical trials; Discovery of IL-18BP; rIL-18BP.

Figure 1. Schematic drawing of IL-18 affi-column to isolate IL-18BP from concentrated human urine. (A) Concentrated human urine was passed through IL-18 affi-column then eluted bound molecules. (B) The eluted fraction was visualized by silver staining. (C) Edman degradation protein microsequencing revealed a novel 40 amino acid sequence. (D) The screening cDNA libraries discover full ORF of four human and two mouse IL-18BP isoforms.

Figure 2. Amino acid alignment of four human and two mouse IL-18BP isoforms. (A) Human IL-18BPa and IL-18BPc possess a complete Ig domain with distinct C-terminal sequence. Human IL-18BP b and d possess incomplete Ig domain resulting in losing activity. Both mouse IL-18BPa (former mouse IL-18BPd) and c have complete Ig domain. Mouse IL-18BPa and IL-18BPc correspond to human IL-18BPa and IL-18BPc, respectively. The biological activity of 6 IL-18BP isoforms was exhibited on the right. (B) The amino acid sequence of 6 IL-18BP isoforms was aligned.