Objectives. To investigate the expression of and monokine induction by inte
rleukin 18 (IL-18; also called interferon-gamma inducing factor, IGIF), in
peripheral blood mononuclear cells (PBMC) and cultured synoviocytes from rh
eumatoid arthritis (RA) patients.
Methods. We carried out IL-18 Western blotting and semi-quantitative revers
e 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, I
L-6, interferon gamma (INF-gamma) and [granulocyte-macrophage colony stimul
ating factor (GM-CSF)] from RA patients and controls. FLS were isolated fro
m RA synovial membranes (FLSSM) and RA synovial fluids (FLSSF), osteoarthri
tis (OA) FLSSM and FLSSF: from spondyloarthropathy patients. FLS were chara
cterized by fluorescence-activated cell sorting of the FLS. PBMC and FLS fr
om 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 M
MP-1 were measured by ELISA in supernatants.
Results. Constitutive expression of IL-18 mRNA was significantly reduced wh
ereas 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 a
s well as spondyloarthropathy FLSSF. In contrast, high constitutive express
ion 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 eq
ually 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 pa
tients, and all FLS cultures. rHu-IL-1 beta, but not rHu-IL-18, induced int
erstitial 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 TN
F-alpha and IL-1 beta expression are increased in RA-FLSSF, but are indepen
dent of IL-18. The pathological function of enhanced IL-18 expression in RA
-FLSSF remains to be further elucidated.