Correlations between serum interleukin 6, serum soluble interleukin 6 receptor, and disease activity in systemic juvenile idiopathic arthritis patients treated with or without tocilizumab

Introduction: Interleukin (IL)-6 is a proinflammatory cytokine involved in systemic juvenile idiopathic arthritis (SJIA). Since these patients are often treated with tocilizumab (TCZ), anti-IL-6 receptor (IL-6R) antibody, we investigated correlations between serum IL-6 and soluble IL-6R-levels and disease activity in SJIA patients treated with or without TCZ. Material and methods: 164 serum samples were taken from 42 SJIA patients treated with or without TCZ (69 and 95 samples, respectively). Patients were assigned to three groups according to disease status: 1) systemic (patients with systemic features and/or arthritis), 2) arthritis (patients with arthritis but no systemic features), and 3) inactive (clinically inactive disease). Disease activity was assessed using the Juvenile Arthritis Disease Activity Score-27 (JADAS-27) at the time of blood collection. Results: IL-6 levels were highest in SJIA patients with predominant systemic features, while serum sIL-6R levels were highest in patients with persistent arthritis. Serum IL-6 correlated with JADAS-27 in patients treated with and without TCZ (r = 0.38 and r = 0.65, respectively), whereas serum sIL-6R levels correlated with JADAS-27 in patients treated without (r = 0.30) but not with (r = −0.14) TCZ. The sIL-6R/IL-6 ratio negatively correlated with JADAS-27 in patients treated with and without TCZ (r = –0.49 and r = –0.56, respectively). Conclusions: Serum IL-6 levels correlated more strongly with disease activity parameters than did sIL-6R levels and could be useful for monitoring disease activity in SJIA patients. The sIL-6R/IL-6 ratio might be a promising disease activity marker in both SJIA patients treated with and without TCZ.