Objective:
To examine the levels of interleukin-18 (il-18) bioactivity within the rheumatoid arthritis (ra) joint, and the differential effects of il-12 and il-18 on interferon-gamma (ifngamma) production by t cell infiltrates.
Methods:
Expression of il-18 protein and messenger rna (mrna) was determined by enzyme-linked immunosorbent assay and reverse transcriptase-polymerase chain reaction, respectively. the biologic activity of il-18 was detected on the basis of ifngamma secretion from il-18-responding human myelomonocytic kg-1 cells. to determine the extent of inhibitory activity on binding of il-18 to its receptor, a [125i]-il-18 binding inhibition assay was performed, using a chinese hamster ovary cell line transfected with a murine il-18 receptor.
Results:
The amount of il-18 protein detected in both the serum and synovial fluid of ra patients was markedly larger than that detected in the serum and synovial fluid ofosteoarthritis (oa) patients, and serum il-18 levels correlated with the levels of serum c-reactive protein. ifngamma production 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 largely diminished in the presence of anti-il-18 antibody. in contrast, the activity of il-18 binding inhibition in the serum and synovial fluid of ra patients was not significantly elevated compared with that in oa patients. ra synovial tissues showed increased expression of il-18 mrna and increased il-18 protein synthesis compared with that in oa tissues. purified cd14+ macrophages, 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 ifngamma production by ra synovial tissue cells, in contrast to il-12, which was directly stimulatory. however, il-12-induced ifngamma production was synergistically enhanced by il-18, and yet was >50% reduced by neutralization of endogenous il-18 with anti-il-18 antibody.
Conclusion:
These results indicate that il-18, produced predominantly by tissue macrophages, primarily potentiates il-12-induced ifngamma production by t cell infiltrates in ra synovium. detection of significant il-18 bioactivity in the joints, despite the presence of il-18 binding inhibitors, supports an integral role of this cytokine in perpetuating the ifngamma-dominant t cell cytokine response in ra.