IL6 and systemic-onset juvenile idiopathic arthritis: On a serum/plasma biomarker level Still’s disease is further hallmarked by elevated levels of bioactive, free IL-18 (not complexed by IL-18 binding protein (IL-18BP)) [4]; as well as S100 proteins [6], sCD163 [7], IL-6, IL-8, IL-17, and TNF-α [8].