Expression of interleukin-18 and its monokine-directed function in rheumatoid arthritis

Objectives. To investigate the expression of and monokine induction by interleukin 18 (IL-18; also called interferon-gamma inducing factor, IGIF), in peripheral blood mononuclear cells (PBMC) and cultured synoviocytes from rheumatoid arthritis (RA) patients. Methods. We carried out IL-18 Western blotting and semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) of cytokines in PBMC [IL-18, IL-1 beta and tumour necrosis factor alpha (TNF-alpha)] and long-term cultured fibroblast-like synoviocytes (FLS) [IL-18, IL-1 beta, TNF-alpha, IL-6, interferon gamma (INF-gamma) and [granulocyte-macrophage colony stimulating factor (GM-CSF)] from RA patients and controls. FLS were isolated from RA synovial membranes (FLSSM) and RA synovial fluids (FLSSF), osteoarthritis (OA) FLSSM and FLSSF: from spondyloarthropathy patients. FLS were characterized by fluorescence-activated cell sorting of the FLS. PBMC and FLS from RA patients and control subjects were stimulated with recombinant human IL-18 and IL-1 beta (rHuIL-18/rHuIL-1 beta), and TNF-alpha, IL-1 beta and MMP-1 were measured by ELISA in supernatants. Results. Constitutive expression of IL-18 mRNA was significantly reduced whereas that of TNF-alpha was enhanced in RA PBMC. Persistent low expression of IL-18, TNF-alpha, GM-CSF and IL-1 beta was observed in RA and OA FLSSM as well as spondyloarthropathy FLSSF. In contrast, high constitutive expression of IL-18 in FLS (CD90/Thy-1- and CD54-positive, CD14- and CD86-negative), accompanied by persistent high levels of TNF-alpha, GM-CSF and IL-1 beta expression, was restricted to synovial fluid-derived FLS obtained from RA patients. IFN-gamma was not detectable in any culture, but IL-6 mRNA was equally expressed in all FLS cultures. rHuIL-18 was effective in stimulating TNF-alpha and IL-1 beta secretion in PBMC from healthy controls, but failed to stimulate TNF-alpha and IL-1 beta secretion from PBMC in 11 of 12 RA patients, and all FLS cultures. rHu-IL-1 beta, but not rHu-IL-18, induced interstitial collagenase (MMP-1) in FLS. Conclusions. Persistent high production of proinflammatory cytokines in RA-FLSSF may be relevant for chronic progression in RA synovitis. Levels of TNF-alpha and IL-1 beta expression are increased in RA-FLSSF, but are independent of IL-18. The pathological function of enhanced IL-18 expression in RA-FLSSF remains to be further elucidated.