ELEVATED SERUM LEVELS OF INTERLEUKIN‐1 RECEPTOR ANTAGONIST IN POLYMYOSITIS/DERMATOMYOSITIS. A Biologic Marker of Disease Activity with a Possible Role in the Lack of Acute‐Phase Protein Response

Objective . To determine whether the lack of acute‐phase protein (APP) response in numerous patients with polymyositis/dermatomyositis (PM/DM) is related to an imbalance between cytokines and cytokine inhibitors. Methods . Levels of C‐reactive protein (CRP), interleukin‐1β (IL‐1β), tumor necrosis factor α (TNFα), IL‐6, IL‐1 receptor antagonist (IL‐1ra), TNF soluble receptor 55 kd (sTNFR 55 kd), and sTNFR 75 kd were tested in the serum of 15 patients with PM/DM, 14 patients with spondylarthropathies (SPA), and 12 healthy blood donors. Serum IL‐1β, TNFα, IL‐6, IL‐1ra, sTNFR were measured by specific immunoassays. Results . Serum levels of CRP were lower in PM/DM patients than in SPA patients. Normal or slightly elevated CRP values were found in 10 of the 15 PM/DM patients, 7 of whom had active myositis. Serum IL‐6 levels were significantly higher in SPA patients than in PM/DM patients, whereas serum IL‐1ra and sTNFR levels were significantly higher in PM/DM than in SPA patients. IL‐1ra levels were particularly elevated in patients with active myositis and decreased in response to treatment. Conclusion . These differences in cytokine levels, particularly IL‐1ra, between PM/DM and SPA patients are indicative of distinct pathogenic mechanisms. High levels of IL‐1ra may account for the weak APP response in some PM/DM patients. Our results suggest that measurement of IL‐1ra, together with clinical examination, may provide useful information for the followup of PM/DM patients.