To examine the mechanism of regulation of interleukin-18 (IL-18) bioactivity by IL-18 binding protein (IL-18BP) induction.
Levels of IL-18 and IL-18BPa in synovial fluid samples from patients with osteoarthritis (OA) or rheumatoid arthritis (RA) were determined by enzyme-linked immunosorbent assays (ELISAs), followed by calculation of free IL-18. IL-18 and IL-18BPa synthesis in RA synovial fibroblasts that had been treated with proinflammatory and antiinflammatory cytokines were assessed by quantitative real-time polymerase chain reaction and ELISA, respectively, followed by IL-18 bioactivity determination using KG-1 cells. Chemical signaling inhibitors were used for determination of the signal transduction pathways involved in IL-18BPa/IL-18 regulation. Tumor necrosis factor alpha (TNFalpha)-induced caspase 1 activity was determined by a colorimetric assay.
IL-18BPa was lower in RA synovial fluid than in OA synovial fluid (P < 0.05; n = 8), and free IL-18 was higher in RA synovial fluid than in OA synovial fluid. TNFalpha induced RA synovial fibroblast IL-18BPa and IL-18 in a time-dependent manner (P < 0.05). Evaluation of signaling pathways suggested that TNFalpha induced IL-18 production through the ERK-1/2, protein kinase Cdelta (PKCdelta), and Src pathways, whereas IL-18BPa synthesis was mediated through the NFkappaB, PKC, Src, and JNK pathways. Furthermore, addition of exogenous IL-18BPa-Fc reduced the RA synovial fibroblast phosphorylation of ERK-1/2 induced by TNFalpha.
These results suggest that IL-18BPa reduces IL-18 bioactivity induced by TNFalpha, by regulating the ERK-1/2 pathway in RA synovial fibroblasts. Targeting IL-18 bioactivity by induction or addition of IL-18BPa may provide another therapeutic option in the management of RA.