Correspondence of the Functional Epitopes of Poxvirus and Human Interleukin-18-Binding Proteins

Abstract

Molluscum contagiosum virus, a human poxvirus that causes persistent small benign skin tumors, encodes a variety of putative immune defense proteins. Three such proteins, MC51L, MC53L, and MC54L, have 20 to 35% amino acid sequence identities with human interleukin-18 (hIL-18)-binding protein (hIL-18BP), a naturally occurring antagonist of the proinflammatory cytokine IL-18. We previously demonstrated that seven amino acids within the immunoglobulin-like domain of hIL-18BP were important for high-affinity binding to hIL-18. Model building indicated that MC54L, which has been shown to bind hIL-18, contains five of the seven amino acids at corresponding positions in its immunoglobulin-like domain, the exceptions being the conservative substitution of isoleucine for a leucine and the nonconservative substitution of valine for a phenylalanine. We found that individual alanine substitutions for these six identical or highly conserved amino acids of MC54L caused changes in affinity and binding free energy for hIL-18 that were quantitatively similar to those produced by mutagenesis of hIL-18BP. Furthermore, when the nonconserved valine of MC54L was mutated to phenylalanine, making it more like hIL-18BP, its affinity for hIL-18 increased more than 10-fold. In addition, the carboxyl-terminal half of MC54L, which has no similarity with hIL-18BP, was dispensable for hIL-18 binding. Thus, despite their relatively low overall sequence identity, MC54L and hIL-18BP have similar hIL-18 binding sites and functional epitopes. On the other hand, MC51L and MC53L have nonconservative substitutions of three to six of the seven critical amino acids of hIL-18BP and neither protein bound hIL-18, suggesting that they may interact with unidentified ligands.

Structural basis for antagonism of human interleukin 18 by poxvirus interleukin 18-binding protein

“…Lys-53 also forms a hydrogen bond and a salt bridge, respectively, with Glu-69 and Glu-77 of ectvIL-18BP, effectively lodging hIL-18 Lys-53 into a remarkably secure position. These intimate interactions form a network of stabilizing forces at binding site A, fully explaining some previous mutagenesis studies on various IL-18BPs and hIL-18 (14)(15)(16)(17). These studies identified Tyr-53 and Phe-67 of ectvIL-18BP and Lys-53 of hIL-18 as residues that contribute most significantly to complex formation between these 2 molecules.…”

M‐CSF induces the expression of a membrane‐bound form of IL‐18 in a subset of human monocytes differentiating in vitro toward macrophages

“…IL-18-induced cellular activation is counteracted by the IL-18 binding protein (IL-18BP), a natural occurring antagonist that binds IL-18 with very high affinity [34]. Interestingly, a similar binding has been reported for viral IL-18BP homologs that are encoded by poxviruses such as Molluscum contagiosum virus (MCV) and represent a mechanism of escape from the immune response [35,36]. Thus, similar to IL-1β, the balance between IL-18 and its antagonist dictates the role of the cytokine in both health and disease.…”

A novel isoform of pro-interleukin-18 expressed in ovarian tumors is resistant to caspase-1 and -4 processing

“…Thus, it is likely that the alternate splicing, leading to the generation of D3pro-IL-18, may represent a regulatory mechanism that inhibits the production of biologically active IL-18 in some ovarian tumors. In this context, it should be outlined that similar to the IL-1b/IL-1R system, the IL-18/IL-18R system also has several natural inhibitors which may negatively regulate the IL-18 biological activity, protecting the organism from unappropriate IL-18-mediated damage (Novick et al, 1999;Kim et al, 2000;Young et al, 2000;Xiang and Moss, 2001;Bufler et al, 2002). Thus, a soluble IL-18 binding protein (BP), which prevent binding of mature IL-18 to its receptor, and inhibits IL-18 biological activity has been identified Figure 6 D3pro-IL-18 binds to caspase-1.…”

“…1 mg of rabbit Ig was run in a control lane Caspase-1 and -4-resistant variant of pro-IL-18 A Gaggero et al (Novick et al, 1999). Four isoforms of this IL-18 BP, displaying differential ability to bind and inhibit IL-18 activity exists in humans (Kim et al, 2000), and a poxvirus-encoded IL-18 BP analog displaying similar inhibitory activity has been described (Xiang and Moss, 2001). In addition, a IL-1b homologue (IL-1F7b) has been reported to interact with IL-18 BP and cooperate in blocking IL-18 activity (Bufler et al, 2002).…”