The Myeloid-Related Proteins 8 and 14 Complex, a Novel Ligand of Toll-Like Receptor 4, and Interleukin-1β Form a Positive Feedback Mechanism in Systemic-Onset Juvenile Idiopathic Arthritis

Objective. Fever of unknown origin is a diagnostic challenge in children, especially for differentiation of systemic-onset juvenile idiopathic arthritis (systemic-onset JIA) and infectious diseases. We undertook this study to analyze the relevance of myeloid-related proteins (MRPs) 8 and 14, endogenous activators of Toll-like receptor 4, in diagnosis and pathogenesis of systemic-onset JIA. Methods. Serum concentrations of MRP-8/MRP-14 were analyzed in 60 patients with systemic-onset JIA, 85 patients with systemic infections, 40 patients with acute lymphoblastic leukemia, 5 patients with acute myeloblastic leukemia, 18 patients with neonatal-onset multisystem inflammatory disease (NOMID), and 50 healthy controls. In addition, we investigated the link between interieukin-1 beta (IL-1 beta) and MRP-8/MRP-14 in systemic-onset JIA. Results. Serum MRP-8/MRP-14 concentrations were significantly (P < 0.001) elevated in patients with active systemic-onset JIA (mean +/- 95% confidence interval 14,920 +/- 4,030 ng/ml) compared with those in healthy controls (340 70 ng/ml), patients with systemic infections (2,640 +/- 720 ng/ml), patients with acute lymphoblastic leukemia (650 280 ng/ml), patients with acute myeloblastic leukemia (840 +/- 940 ng/ml), and patients with NOMID (2,830 +/- 580 ng/ml). In contrast to C-reactive protein levels, MRP-8/MRP-14 concentrations distinguished systemic-onset JIA from infections, with a specificity of 95%. MRP-14 in serum of patients with systemic-onset JIA was a strong inducer of IL-1 beta expression in phagocytes. Conclusion. The analysis of MRP-8/MRP-14 in serum is an excellent tool for the diagnosis of systemic-onset JIA, allowing early differentiation between patients with systemic-onset JIA and those with other inflammatory diseases. MRP-8/MRP-14 and IL-1 beta represent a novel positive feedback mechanism activating phagocytes via 2 major signaling pathways of innate immunity during the pathogenesis of systemic-onset JIA.