Is interleukin-18 useful for monitoring rheumatoid arthritis?

Objective: Interleukin-18 (IL-18) is a proinflammatory regulator of immune responses. Its similarities to IL-1 beta and ability to induce tumour necrosis factor-alpha (TNF-alpha) make it potentially important in the pathogenesis of rheumatoid arthritis (RA). Methods: The level of IL-18 was assessed in matched pairs of blood and synovial fluid samples from 90 RA patients ( 47 erosive, 43 non-erosive) by an enzyme-linked immunosorbent assay ( ELISA), and the results compared to 40 healthy controls. Results: In RA patients with erosive joint disease, the IL-18 level was higher than that in non- erosive RA [(median +/- QR) blood: 385 +/- 200 vs. 235 +/- 183 pg/mL, p=0.02; synovial fluid: 392 +/- 392 vs. 224 +/- 324 pg/mL, p=0.05]. IL-18 levels in blood of RA patients were similar and closely related to the local, intra-articular level (r=0.96). The IL-18 level was not related to other markers of inflammation, to the duration of RA, or to the treatment modality. The IL-18 level in RA patients was similar to that of the controls ( 278 +/- 234 vs. 344 +/- 179 pg/mL, not significant). Conclusions: An increased IL-18 level is associated with erosive joint disease, but the measurement of IL-18 does not help to distinguish between RA patients and healthy controls.