Interferon-gamma-inducing activity of interleukin-18 in the joint with rheumatoid arthritis

Objective. To examine the levels of interleukin-18 (IL-18) bioactivity with in the rheumatoid arthritis (RA) joint, and the differential effects of IL- 12 and IL-18 on interferon-gamma (IFN gamma) production by T cell infiltrat es. Methods. Expression of IL-18 protein and messenger RNA (mRNA) was determine d by enzyme-linked immunosorbent assay and reverse transcriptase-polymerase chain reaction, respectively. The biologic activity of IL-18 was detected on the basis of IFN gamma secretion from IL-18-responding human myelomonocy tic KG-1 cells. To determine the extent of inhibitory activity on binding o f IL-18 to its receptor, a [I-125]-IL18 binding inhibition assay was perfor med, using a Chinese hamster ovary cell line transfected with a murine IL-1 8 receptor. Results. The amount of IL-18 protein detected in both the serum and synovia l fluid of RA patients was markedly larger than that detected in the serum and synovial fluid of osteoarthritis (OA) patients, and serum IL-18 levels correlated with the levels of serum C-reactive protein. IFN gamma productio n by KG-1 cells was more strongly stimulated in synovial fluid samples from RA patients than in samples from OA patients, and this activity was largel y diminished in the presence of anti-IL-18 antibody. In contrast, the activ ity of IL-18 binding inhibition in the serum and synovial fluid of RA patie nts was not significantly elevated compared with that in OA patients. RA sy novial tissues showed increased expression of IL-18 mRNA and increased IL-1 8 protein synthesis compared with that in OA tissues. Purified CD14+ macrop hages, but not activated fibroblast cell lines, from RA synovium were able to release mature IL-18, although both cell types expressed its transcripts . IL-18 alone showed a negligible effect on IFN gamma production by RA syno vial tissue cells, in contrast to IL-12, which was directly stimulatory. Ho wever, IL-12-induced IFN gamma production was synergistically enhanced by I L-18, and yet was >50% reduced by neutralization of endogenous IL-18 with a nti-IL-18 antibody. Conclusion. These results indicate that IL-18, produced predominantly by ti ssue macrophages, primarily potentiates IL-12-induced IFN gamma production by T cell infiltrates in RA synovium. Detection of significant IL-18 bioact ivity in the joints, despite the presence of IL-18 binding inhibitors, supp orts an integral role of this cytokine in perpetuating the IFN gamma -domin ant T cell cytokine response in RA.