Serum interleukin 18 and interleukin 18 binding protein in rheumatoid arthritis.

OBJECTIVE To measure serum interleukin 18 (IL18) and IL18 binding protein (IL18BP) levels in patients with inflammatory arthropathies, and to identify associations with disease status and the response to treatment. METHODS Serum samples were obtained before and after methotrexate treatment from patients with rheumatoid arthritis (RA) and psoriatic arthritis (PsA) attending an early arthritis clinic. IL18 and IL18BP were measured by enzyme linked immunosorbent assay (ELISA). RESULTS Sixty patients with RA and 13 with PsA were evaluated. Serum IL18 levels were significantly higher in RA than in PsA (p<0.001). After six months' treatment with methotrexate, IL18 levels were reduced, but the differences were not significant (p=0.052). In cross sectional analyses, no correlations between IL18 levels and measures of disease activity or structural damage in RA were found. In longitudinal analyses, no correlations between IL18 levels and the response to treatment or the degree of progressive joint damage were found. Similarly, IL18BP levels were raised in RA, and were not associated with measures of the clinical status or the response to treatment. CONCLUSION Raised serum levels of IL18 are consistent with a pathophysiological role in RA. However, in this study measurement of circulating IL18 and IL18BP did not correlate significantly with clinical measures of disease activity or the response to treatment in patients with early RA.