Structural Insights Into The Interaction Of IL-18 With Its Receptors

Interleukin 18(IL-18) is one agonistic member in the IL-1 family that plays important roles in both innate and adaptive immune responses. IL-18 binds its ligand-binding receptor IL-18Rα and co-receptor IL-18R? to form a signaling ternary complex, which initiates downstream NF-κB and MAPK pathways. Overproduction of IL-18 in human is related to several autoimmune diseases and chronic inflammatory disorders.To understand the mechanism how IL-18 recognizes its receptors and assembles the signaling complex, human IL-18 and the ectodomains of IL-18Rαand IL-18R? were expressed in E.coli and insect cells respectively and were purified via affinity chromatography and gel filtration chromatography. The binary complex of IL-18 with IL-18Rαand the ternary complex of IL-18 with IL-18Rα and IL-18R? were reconstituted in vitro and the crystallization conditions were screened. Crystals of IL-18-IL-18Rαbinary complex were successfully obtained and the structure was solved at a resolution of 2.8 ? by X-ray diffraction method. Analysis of the structure determined the interacting residues between IL-18 and IL-18Rα. Via site-specific mutagenesis andNF-κB luciferase reporter system, nine critical residues on IL-18 for the interaction with IL-18Rαwere revealed. It also suggested that surface charge complementarity determines the specific recognition of IL-18 by IL-18Rα. Structural comparison revealed the molecular basis for IL-18 binding protein and monoclonal neutralizing antibody to inhibit the IL-18 activity.The first complex structure of IL-18 with its receptor was reported in this work which clarified the structural basis for the specific recognition of IL-18 by IL-18Rα. Afterwards a Japanese group reported the complex structure of IL-18-IL-18Rα-IL-18R? ternary complex. Based on these results as well as the complex structures of IL-1? and IL-33 with their receptors, a general ligand-receptor assembly andactivation model in the IL-1 family was proposed.