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


CONCLUSIONS
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.
Department of Medicine III, Okayama University Medical School, Japan.
M
M
Yamamura
M
M
Kawashima
M
M
Taniai
H
H
Yamauchi
T
T
Tanimoto
M
M
Kurimoto
Y
Y
Morita
Y
Y
Ohmoto
H
H
Makino
D000917:Antibody Formation / Q000187:drug effects; D001172:Arthritis, Rheumatoid / Q000378:metabolism*; D006801:Humans; D007371:Interferon-gamma / Q000096:biosynthesis / Q000502:physiology*; D018664:Interleukin-12 / Q000494:pharmacology; D020382:Interleukin-18 / Q000097:blood / Q000378:metabolism*; D053727:Interleukin-18 Receptor alpha Subunit; D007596:Joints / Q000378:metabolism*; D010003:Osteoarthritis / Q000378:metabolism; D018123:Receptors, Interleukin / Q000037:antagonists & inhibitors; D053724:Receptors, Interleukin-18; D013582:Synovial Fluid / Q000737:chemistry; D013583:Synovial Membrane / Q000166:cytology; D013601:T-Lymphocytes / Q000276:immunology / Q000378:metabolism
C508572:IL18R1 protein, human; D020382:Interleukin-18; D053727:Interleukin-18 Receptor alpha Subunit; D018123:Receptors, Interleukin; D053724:Receptors, Interleukin-18; D018664:Interleukin-12; D007371:Interferon-gamma