Synovial fluid neutrophils in oligoarticular juvenile idiopathic arthritis have an altered phenotype and impaired effector functions

Abstract

Background Neutrophils are the most prevalent immune cells in the synovial fluid in inflamed joints of children with oligoarticular juvenile idiopathic arthritis (JIA). Despite this, little is known about neutrophil function at the site of inflammation in JIA and how local neutrophils contribute to disease pathogenesis. This study aimed to characterize the phenotype and function of synovial fluid neutrophils in oligoarticular JIA. Methods Neutrophils obtained from paired blood and synovial fluid from patients with active oligoarticular JIA were investigated phenotypically (n = 17) and functionally (phagocytosis and oxidative burst, n = 13) by flow cytometry. In a subset of patients (n = 6), blood samples were also obtained during inactive disease at a follow-up visit. The presence of CD206-expressing neutrophils was investigated in synovial biopsies from four patients by immunofluorescence. Results Neutrophils in synovial fluid had an activated phenotype, characterized by increased CD66b and CD11b levels, and most neutrophils had a CD16hi CD62Llowaged phenotype. A large proportion of the synovial fluid neutrophils expressed CD206, a mannose receptor not commonly expressed by neutrophils but by monocytes, macrophages, and dendritic cells. CD206-expressing neutrophils were also found in synovial tissue biopsies. The synovial fluid neutrophil phenotype was not dependent on transmigration alone. Functionally, synovial fluid neutrophils had reduced phagocytic capacity and a trend towards impaired oxidative burst compared to blood neutrophils. In addition, the effector functions of the synovial fluid neutrophils correlated negatively with the proportion of CD206+ neutrophils. Conclusions Neutrophils in the inflamed joint in oligoarticular JIA were altered, both regarding phenotype and function. Neutrophils in the synovial fluid were activated, had an aged phenotype, had gained monocyte-like features, and had impaired phagocytic capacity. The impairment in phagocytosis and oxidative burst was associated with the phenotype shift. We speculate that these neutrophil alterations might play a role in the sustained joint inflammation seen in JIA.

Neutrophils Lose the Capacity to Suppress T Cell Proliferation Upon Migration Towards Inflamed Joints in Juvenile Idiopathic Arthritis

“…In line with this, our transwell-migrated non-suppressive neutrophils had impaired production of ROS and lower levels of NOX2 compared to non-migrated neutrophils. We have previously seen that oligoarticular JIA synovial fluid neutrophils tend to have impaired oxidative burst compared to neutrophils in the circulation ( 2 ), and low ROS production might be part of the explanation to why synovial neutrophils from several of the patients in this study are unable to suppress T cell proliferation. However, as ROS scavenging by catalase could only partly neutralize neutrophil-mediated inhibition of T cell proliferation, most likely other mechanisms for T cell suppression are also involved.…”

“…We have previously shown that neutrophils in JIA synovial fluid trend towards reduced oxidative burst compared to neutrophils in the blood ( 2 ), and we therefore sought to investigate if migration in our system would affect neutrophil ROS production. When exposed to PMA, neutrophils which had migrated towards synovial fluid had impaired ROS production compared to neutrophils incubated in the same synovial fluid ( Figure 2D ).…”

“…Children with oligoarticular JIA present with persistent, unexplained and often asymmetric arthritis in one to four joints before the age of sixteen, often associated with anti-nuclear autoantibodies ( 13 ). We and others have shown that neutrophils in synovial fluid from children with oligoarticular JIA are activated and express atypical neutrophil surface markers ( 2 , 5 ). JIA synovial fluid neutrophils also have impaired phagocytosis and trend towards impaired oxidative burst compared to circulating neutrophils, related to the phenotype shift ( 2 ).…”

“…We and others have shown that neutrophils in synovial fluid from children with oligoarticular JIA are activated and express atypical neutrophil surface markers ( 2 , 5 ). JIA synovial fluid neutrophils also have impaired phagocytosis and trend towards impaired oxidative burst compared to circulating neutrophils, related to the phenotype shift ( 2 ).…”

Juvenile idiopathic arthritis: from aetiopathogenesis to therapeutic approaches

“…Association with HLA class II and the presence of ANA suggests that an adaptive immune response is predominant in the pathogenesis of oligoarticular JIA. However, activated neutrophils with altered phenotype and dysfunction, and impaired synovial monocytes and macrophages with reduced capacity to phagocytose have recently been identified in synovial fluid of patients with oligoarticular JIA [ 28 , 29 ]. Together with high levels of monocyte-derived cytokines this emphasises the importance of the innate immune system in oligoarticular JIA pathogenesis [ 28 , 29 ].…”