A small non-interface surface epitope in human IL18 mediates the dynamics and self-assembly of IL18-IL18BP heterodimers

Interleukin 18 (IL18) is a pro-inflammatory cytokine that modulates innate and adaptive immune responses. IL18 activity is tightly controlled by the constitutively secreted IL18 binding protein (IL18BP). PDB structures of human IL18 showed that a short stretch of amino acids between 68 and 81 adopted a disordered conformation in all IL18-IL18BP complexes while adopting a 3(10) helical structure in other IL18 structures, including the receptor complexes. The C74 of human IL18, which was reported to form a novel intermolecular disulfide bond in the human tetrameric assembly, is also located in this short epitope. These observations reflected the importance of this short surface epitope for the structure and dynamics of the IL18-IL18BP heterodimers. We have analyzed all known IL18-IL18BP complexes in the PDB by all-atom MD simulations. The analysis also included two computed complex models adopting a helical structure for the surface epitope. Heterodimer simulations showed a stabilizing impact of the small surface region at the helical form by reducing flexibility of the complex backbone. Analysis of the symmetry-related human IL18-IL18BP tetramer showed that the unfolding of this small surface region also contributed to the IL18-IL18BP stability through a completely exposed C74 sidechain to form an intermolecular disulfide bond in the self-assembled human IL18-IL18BP dimer. Our findings showed how the conformation of the short IL18 epitope between amino acids 68 and 81 would affect IL18 activity by mediating the intermolecular interactions of IL18.