Interleukin-18 (IL-18) was originally described in 1989 as interferon-gamma inducing factor (IGIF). IL-18 is related to the IL-1 family and is structurally related to IL-1β (Okamura H et al. (1995) Nature; 378:88-91). IL-18 is primarily produced by macrophages and T cells as a precursor protein (pro-IL-18) and secreted as an active protein following cleavage by caspase-1 (Dinarello C A et al (1999) J Allergy Clin Immunol; 103:11-24). In normal physiology IL-18, in synergy with IL-12, is associated with induction of cell-mediated immunity following infection with microbial products such as lipopolysaccharide (LPS) (Sareneva T et al (2000) J Immunol; 165(4):1933-8). After stimulation with IL-18, natural killer (NK) cells and T cells release the cytokine interferon gamma (INF-γ) which plays an important role in activating macrophages and other cells. IL-18 has also various functions in addition to an ability to induce interferon gamma. These biological properties include activation of NF-κB, Fas ligand expression, the induction of both CC and CXC chemokines, and increased production of competent human immunodeficiency virus. Due to the ability of IL-18 to induce INF-γ production in T cells and macrophages, it plays an important role in Th1-type immune responses and participates in both innate and acquired immunity.
IL-18 binds with high affinity and signals through the IL-18 receptor (IL-18R), a heteromeric complex of alpha and beta chains encoded by the genes IL18R1 and IL18 RAP, respectively (Torigoe K et al (1997) J Biol Chem; 272(41):25737-42). The bioactivity of IL-18 is negatively regulated by the IL-18 binding protein (IL18BP), a naturally occurring and highly specific inhibitor. This soluble protein forms a complex with free IL-18 preventing its interaction with the IL-18 receptor, thus neutralizing and inhibiting its biological activity (Dinarello Calif. (2000) Ann Rheum Dis; 59 Suppl 1:i17-20). IL-18BP is a constitutively secreted protein with high affinity binding to IL-18. Alternate mRNA splicing variants of IL-18BP result in four isoforms. The prominent ‘a’ isoform is present in the serum of healthy humans at 20-fold molar excess compared with IL-18 (Dinarello and Kaplanski (2005) Expert Rev Clin Immunol, 1(4), 619-632).
Apart from its physiological role, IL-18 has been shown to mediate a variety of autoimmune and inflammatory diseases. It has been demonstrated that IL-18 expression is up-regulated in several autoimmune diseases, such as Crohn's disease, psoriasis, rheumatoid arthritis, multiple sclerosis and cardiovascular diseases (Braddock et al. (2004) Expert Opin Biol Ther; 4(6):847-860). IL-18 is also up-regulated in certain inflammatory diseases such as chronic obstructive pulmonary disease (COPD) (Imaoka et al. (2008) Eur Respir; J31:287-297), idiopathic pulmonary fibrosis (IPF) (Kitasato et al. (2004) Am J Resp Cell Mol Biol; 31:619-625), macrophage activation syndrome (MAS) (Dinarello and Kaplanski (2005) Expert Rev Clin Immunol; 1(4): 619-632), adult onset Still's disease (AOSD) (Arlet J B et al. (2006) Ann Rheum Dis 65(12):1596-601) and systemic juvenile idiopathic arthritis (SJIA) (Akashi et al. (1994) Br J Haematol; 87(2):243-50).
Recent studies have shown that high amounts of IL-18 and INF-γ in both inherited and acquired forms of hemophagocytic lymphohistiocytosis (HLH). HLH is characterized by activated lymphocytes and histiocytes secreting high amounts of inflammatory cytokines (Janka G E et al (2007) Eur J Paediatr; 166:95-109) and by impaired function of NK cells and cytotoxic T-cells. Most important, it has been shown that both forms are characterized by dis-regulation of IL-18 (Mazodier et al (2005) Immunobiology 106(10):3483-89).
Developing therapeutic molecules for targets such as IL-18 which are regulated by natural inhibitors can be very challenging. The presence of an increased amount of systemic or local total IL-18 does not always reflect the level of biologically active protein (IL-18 free of IL-18BP).
A therapeutic compound binding both free IL-18 and IL-18 in complex to IL-18BP, although potentially capable of neutralizing the activity of IL-18, would be required in higher dose than one which can selectively bind only biologically active free IL-18. Therapeutic compounds which need to be administered in high dose may lead to more pronounced side effects or become immunogenic. High dosages also translate into high production costs.
Therapeutic compounds competing for IL-18 when bound to IL18BP may disturb the delicate balances of free/active IL-18 and IL-18BP bound/inactive IL-18 existing in patients stricken with diseases/disorders characterized by IL-18 dis-regulation.
Finally, investigating diseases characterized by dis-regulation of IL-18 free, in the absence of a diagnostic tool capable of detecting and/or measuring IL-18 free from IL-18BP as component of the total IL-18 would be very difficult.