IL-18 binding protein suppresses IL-17-induced osteoclastogenesis and rectifies type 17 helper T cell / regulatory T cell imbalance in rheumatoid arthritis

Patients with rheumatoid arthritis (RA) have increased levels of interleukin-18 (IL-18) and decreased levels of IL-18 binding protein (IL-18BP) in the serum and synovial fluid (SF) compared to those in patients with osteoarthritis (OA) or in healthy controls. In this study, we evaluated the effects of IL-18BP on osteoclastogenesis and T cell differentiation in RA in vitro.

We showed that IL-18BP can rectify the Th17/Treg imbalance and decrease IL-17-induced osteoclastogenesis in PBMCs from patients with RA. Therefore, IL-18BP may have therapeutic potential for RA treatment.

Serum and SF of patients with RA and OA were collected to compare IL-18 and IL-18BP levels by the enzyme-linked immunosorbent assay. Peripheral blood mononuclear cells (PBMCs) and SF mononuclear cells (SFMCs) of RA patients were cultured under type 17 helper T cell (Th17) polarisation conditions with or without IL-18BP. In addition, PBMCs were cultured in the presence of receptor activator of nuclear factor kappa-Β ligand (RANKL) or IL-17A with or without IL-18BP, and tartrate-resistant acid phosphatase (TRAP) staining and real-time quantitative polymerase chain reaction for expression levels of osteoclast-related genes were performed.

IL-18 levels were higher in the serum and SF of patients with RA, whereas IL-18BP was lower in the SF of patients with RA than in the control group. Treatment of patients' PBMCs with IL-18BP decreased the differentiation of CD4+ IL-17A+ and CD4+ RANKL+ T cells, whereas the differentiation of CD4+CD25highFOXP3+ T cell population increased in a dose-dependent manner. These changes in CD4+ T cell differentiation were also observed in the SFMCs of patients with RA. The levels IL-17A and soluble RANKL in the culture medium were significantly decreased by IL-18BP. IL-18BP administration decreased TRAP+ cell counts in a dose-dependent manner on the background of stimulation with RANKL-and IL-17A. In addition, expression levels of TRAP, NFATC1, CTSK, and TNFRSF11A (RANK) genes were lower in the IL-18BP treated cells.