Project Summary/Abstract Interleukin-37 (IL-37), a member of the IL-1 family, has been a neglected cytokine mostly because there is no mouse homologue for IL-37. However, we expressed human IL-37 in mice, which revealed that IL-37 broadly suppresses innate inflammation and acquire immunity and that recombinant human IL-37 accomplishes the same in wild type mice. Similar to IL-1α and IL-33, IL-37 is a dual function cytokine in that IL-37 translocates to the nucleus but also binds its cell surface receptor complex. The present application focuses on the mechanism for these properties. The translocation of IL-37 to the nucleus depends on caspase-1 cleavage followed by the carboxyl domain binding to chromatin. To determine to what extent nuclear IL-37 contributes to suppression of innate and acquire immunity, we have generated a new strain of mice that lacks the ability for IL-37 to translocate to the nucleus by to mutating the caspase-1 recognition site on IL-37 without altering the activity of caspase-1 in the same cell. In order to establish an independent role for nuclear IL-37, the cell surface receptors for IL-37 (IL-18 Receptor α−chain) will be prevented using a blocking antibody in short-term models. In both short and long-term models, we will use mice deficient in IL-1R8, the IL-37 co-receptor. The second Aim of this proposal directly addresses the binding of IL-37 to the IL-18 Binding Protein (IL-18BP) and investigates whether administration of IL-18BP reduces the protection afforded by endogenous IL-37. We have generated a transgenic mouse expressing human IL-18BP to determine whether administration of recombinant human IL-37 is less effective in mice expressing human IL-18BP. Since recombinant human IL-37 possesses several properties to suppress innate and acquired immunity, the translational component of the proposal is the development of IL-37 as a therapeutic. As with several members of the IL-1 family, the N- and C-terminal for optimal biologic activity is unknown. We have produced a recombinant form of IL-37 fused to the Fc domain of human IgG1 (IL-37Fc fusion protein) and demonstrated its efficacy. We will now determine the N- and C-termini of IL-37 for an optimal form of recombinant IL-37 to limit of innate inflammation. Once this has been established, an improved IL-37Fc fusion protein will also be produced and tested in preclinical models. The overall goal of these studies is to advance the biology and clinical significance of IL-37 as well as to exploit its anti- inflammatory properties as a therapeutic.