Interferon-gamma induces expression of interleukin-18 binding protein in fibroblast-like synoviocytes.

OBJECTIVE: To investigate expression of the endogenous antagonist of interleukin 18 (IL-18) bioactivity, IL-18 binding protein isoform a (IL-18BPa), in fibroblast-like synoviocytes (FLS). METHODS: Long-term cultured FLS from rheumatoid arthritis (RA), osteoarthritis (OA) and spondylarthropathy patients were analysed for spontaneous and cytokine-induced IL-18BPa expression. Messenger RNA and release of IL-18BPa were assessed by semi-quantitative and quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) as well as immunoblot analysis, respectively. RESULTS: All investigated FLS cultures expressed low amounts of IL-18BPa transcripts. However, there was no detectable release of IL-18BPa from unstimulated synoviocytes. Of the investigated cytokines, only interferon (IFN)-gamma markedly up-regulated IL-18BPa mRNA levels. Induction was accompanied by release of IL-18BPa immunoreactivity from FLS. Conditioned media from IFN-gamma-stimulated FLS cultures reduced IL-12/IL-18-dependent IFN- production by peripheral blood mononuclear cells. CONCLUSION: The present data imply that IFN--activated synoviocytes mediate a negative feedback loop via IL-18BPa, which may limit IL-18 biological activity in arthritis.

Prednisolone induces interleukin-18 expression in mononuclear blood and myeloid progenitor cells.

OBJECTIVE: Interleukin (IL)-18 is involved in host defense mechanisms and inflammatory diseases, among them rheumatoid arthritis (RA). High levels of IL-18 expression in RA joints are contrasted by reduced IL-18 expression in RA peripheral blood mononuclear cells (PBMC). Here, we investigated a putative IL-18 regulating role of corticosteroids. METHODS: IL-18 transcript and protein levels in PBMC from untreated and prednisolone treated RA patients, and from healthy donors were assessed by semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) and immunoblotting. IL-18 regulation was determined in PBMC and U937 cells upon exposure to prednisolone in vitro by RT-PCR and Northern Blot analysis, by ELISA in cell culture supernatants, and in transiently transfected THP-1 cells by IL-18 promoter activity luciferase assays. RESULTS: In RA PBMC, IL-18 transcript levels were dose dependently restored, in parallel with administered prednisolone treatment, to subnormal levels. The corresponding intracellular IL-18 deposits in contrast were depleted. In cultured PBMC and promonocytic cell lines, prednisolone up-regulated IL-18 transcription in parallel with increasing the IL- 18 protein release into cell culture supernatants. CONCLUSION: Prednisolone increases IL-18 expression and release in PBMC and monocytic cell lines.

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-1beta and tumour necrosis factor alpha (TNF-alpha)] and long-term cultured fibroblast-like synoviocytes (FLS) [IL-18, IL-1beta, 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 (FLS(SM)) and RA synovial fluids (FLS(SF)), osteoarthritis (OA) FLS(SM) and FLS(SF) 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-1beta (rHuIL-18/rHuIL-1beta), and TNF-alpha, IL-1beta 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-1beta was observed in RA and OA FLS(SM) as well as spondyloarthropathy FLS(SF). 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-1beta 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-1beta secretion in PBMC from healthy controls, but failed to stimulate TNF-alpha and IL-1beta secretion from PBMC in 11 of 12 RA patients, and all FLS cultures. rHu-IL-1beta, but not rHu-IL-18, induced interstitial collagenase (MMP-1) in FLS. CONCLUSIONS: Persistent high production of proinflammatory cytokines in RA-FLS(SF) may be relevant for chronic progression in RA synovitis. Levels of TNF-alpha and IL-1beta expression are increased in RA-FLS(SF), but are independent of IL-18. The pathological function of enhanced IL-18 expression in RA-FLS(SF) remains to be further elucidated.