Synovial fluid neutrophils from patients with juvenile idiopathic arthritis display a hyperactivated phenotype.

OBJECTIVES Juvenile idiopathic arthritis (JIA) is the most common rheumatic disease in childhood. The predominant subtypes, i.e. oligoarticular (oligo) and polyarticular (poly) JIA, are traditionally considered to be autoimmune diseases with a central role for T cells and autoantibodies. Mounting evidence suggests an important role for neutrophils in JIA pathogenesis. Here, we investigated the phenotypical features of neutrophils present in the blood and inflamed joints of patients. METHODS Synovial fluids and parallel blood samples from JIA patients and blood samples from healthy children were collected. Synovial fluid-treated healthy donor neutrophils and pleural neutrophils from patients with pleural effusion were investigated as controls for synovial fluid exposure and extravasation. Multicolor flow cytometry panels allowed for in-depth phenotypical analysis of neutrophils, focusing on the expression of adhesion molecules, activation and maturation markers and chemoattractant receptors. Multiplex technology was exploited to quantify cytokines in plasma and synovial fluids. RESULTS Synovial fluid neutrophils displayed an activated, hypersegmented phenotype with decreased CD62L expression, upregulation of adhesion molecules CD66b, CD11b and CD15 and downregulation of CXCR1/2. An elevated percentage of CXCR4 positive neutrophils was detected in synovial fluids from patients. Pleural neutrophils showed less pronounced maturation differences. Strikingly, significant percentages of synovial fluid neutrophils showed a profound upregulation of atypical neutrophil markers, including CXCR3, ICAM-1 and HLA-DR. CONCLUSION Our data show that neutrophils in inflamed joints of JIA patients have an activated phenotype. This detailed molecular analysis supports the notion that a complex intertwining between these innate immune cells and adaptive immune events drives JIA.