Interleukin-18 (IL-18) is a pro-inflammatory cytokine that belongs to the interleukin-1 (IL-1) superfamily. It plays an important role in host defense against microbes but also contributes to pathogenesis of several inflammatory diseases, including rheumatoid arthritis, septic shock and Crohn's disease. IL-18 signaling is initiated by its cell-surface binding to the receptor (IL-18R) alpha subunit, followed by the recruitment of the receptor beta subunit to form a ternary signaling complex. IL-18 activities are regulated in vivo by a naturally occurring antagonist, the IL-18 binding protein (IL-18BP) through a negative feedback mechanism. Poxviruses, including the smallpox (variola) virus, also express functional IL-18BP homologues to evade IL-18-mediated host immune responses. IL-18, IL-18R and IL-18BP are therefore attractive targets for developing therapeutics agonist inflammatory or infectious diseases where down- or up-modulating IL-18 activities is indicated. However, there is a lack of thorough understanding of how IL-18 activates its receptor and how IL-18BP inhibits IL-18. We propose to determine the crystal structures of various protein complexes of IL-18 with IL-18R or IL-18BP and perform functional studies based on the structural information. In addition, we will perform structure-based design of IL-18 variants that may either serve as a more effective cytokine capable of evading the neutralization of poxvirus IL-18BPs, or as a receptor antagonist capable of blocking IL-18 activities.
Aim 1. To determine the molecular mechanism by which IL-18BPs neutralize IL-18.
Aim 2. To determine the molecular mechanism by which IL-18 specifically recognizes 1 subunit of IL-18R.
Aim 3. To determine the molecular mechanism by which IL-18 triggers the hetero-dimerization of IL-18R1 and 2 subunits. The objective of this application is the detailed characterization of a collection of IL-18 complexes by a combination of biophysical and biochemical methods, including x-ray crystallography, Surface Plasmon Resonance (SPR) and IL-18 bioassay. Accomplishing this objective is an important first step for achieving our long-term goal of understanding how IL-18 activates its receptors to initiate cell signaling and how this activity is regulated by IL-18BP.This research obtains/assumes extra dimensions. Since the targeted proteins are of significant medical relevance, our studies will provide a platform for designing selective inhibitors that may ultimately be developed into new therapeutics against a number of human diseases. Public Health Relevance: Our contribution here is expected to provide detailed molecular recognition of IL-18 by IL-18BP and IL-18R. This contribution is significant because it will fill the gap of our current knowledge on IL-18 activation pathway, and will provide important clues on how to modulate IL-18 activity. It may benefit efforts in developing treatments against some autoimmune and inflammatory diseases, in developing immunotherapies against other infectious diseases and cancer and in combating bioterrorism.
Our contribution here is expected to provide detailed molecular recognition of IL-18 by IL-18BP and IL-18R. This contribution is significant because it will fill the gap of our current knowledge on IL-18 activation pathway, and will provide important clues on how to modulate IL-18 activity. It may benefit efforts in developing treatments against some autoimmune and inflammatory diseases, in developing immunotherapies against other infectious diseases and cancer and in combating bioterrorism.
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