Correlation of serum interleukin-6 levels with joint involvement and thrombocytosis in systemic juvenile rheumatoid arthritis.

We measured interleukin-6 (IL-6) levels in 70 serum samples obtained from 25 patients with systemic-onset juvenile rheumatoid arthritis (JRA), using the hybridoma cell line B9. Patients with systemic-onset JRA had significantly elevated serum IL-6 levels during active disease (mean +/- SD 92.1 +/- 75.1 hybridoma growth factor units/ml; P less than 0.00001 versus healthy age-matched controls), but not during remission. Serum IL-6 levels correlated with the extent and severity of joint involvement (P less than 0.001) and with platelet counts (P less than 0.05). Our data suggest that IL-6 plays a significant role in the pathogenesis of systemic-onset JRA.