P060 Il-38 in arthritis maturation and degradation of this new IL-1 family anti-inflammatory cytokine

Introduction Interleukin 38 (IL-38) is an IL-1 family cytokine with potential anti-inflammatory activity. Recently we showed that over-expression of full length IL-38 reduces inflammation in arthritic mice by 20%–30%, reduces macrophage infiltration and the expression of cytokines implicated in the biology of Th17 cells.1 However, IL-38 does not have a classical signal peptide, and it is unclear how it is released for example from monocytes/macrophages. It needs post-translational processing (N-terminal truncation) to acquire full biological activity, but the required proteases are unknown. Objectives The aim is to study the maturation of IL-38 in the inflammatory process. Methods THP1 macrophagic cells were transduced with a lentivirus encoding full length IL-38 to over-express this cytokine. Secretion and maturation (truncation) of endogenous or over-expressed IL-38 has been studied by western blot. The potential proteases implicated in IL-38 N-terminal truncation have been analysed in silico (Prosper webserver) and by biochemical tests using recombinant proteases and their inhibitors. Results In the synovial fluid of RA patients, we observed 2 truncated forms of IL-38. To determine if apoptosis is a mechanism responsible for the maturation of IL-38, we tested different stress conditions on THP1 macrophages that overexpress IL-38. In conditioned media, three truncated forms of IL-38 were revealed by Western Blot under conditions of apoptosis, exacerbated oxidative stress, or necrosis. However, none of these conditioned media appears to have a high anti-inflammatory effect on macrophages. Our results rather suggest that cell death by apoptosis or necrosis results in IL-38 degradation and loss of biological effect. In silico, several potential cleavage sites were found for calpain, MMP-2, MMP-9 and cathepsin G. Incubation of recombinant full length IL-38 with MMP-2 or cathepsin G leads to significant degradation of this cytokine. On apoptotic THP1 cells overexpressing IL-38, an inhibitor of MMPs and TACE (TAPI) also reduces the release and degradation of IL-38. Conclusions The degradation and loss of the anti-inflammatory effect of IL-38 appears to occur during cell death in the extracellular space, and could involve MMPs. The processes involved in the maturation of IL-38 need to be further explored and could open new therapeutic opportunities for the treatment of arthritis. Reference . Boutet MA, et al. IL-38 overexpression induces anti-inflammatory effects in mice arthritis models and in human macrophages in vitro. Ann Rheum Dis2017;76:1304–1312. Disclosure of interest M. Harel: None declared, T. Garraud: None declared, B. Le Goff: None declared, F. Blanchard Grant/research support from: Inserm, the Arthritis Foundation, the French Society of Rheumatology, Pfizer and the Region Pays de la Loire (Bioregate)