Background: Juvenile idiopathic arthritis (JIA) is an umbrella term of inflammatory joint diseases in children. Oligoarthritis is the most common form in the Western world, representing roughly 60% of all patients. Monocytes and macrophages play an important role in adult arthritides, but their role in oligoarticular JIA is less studied. Polarization highly influences monocytes' and macrophages' effector functions, broadly separated into proinflammatory M1 or anti-inflammatory M2 phenotypes. Here, we set out to investigate the polarization pattern and functional aspects of synovial monocytes in oligoarticular juvenile idiopathic arthritis (JIA). Methods: Paired synovial fluid, blood samples (n = 13), and synovial biopsies (n = 3) were collected from patients with untreated oligoarticular JIA. Monocytes were analyzed for polarization markers by flow cytometry and qPCR. Effector function was analyzed by a phagocytosis assay. Polarization of healthy monocytes was investigated by stimulation with synovial fluid in vitro. Monocyte/macrophage distribution, polarization, and mRNA expression were investigated in biopsies by immunohistochemistry, immunofluorescence, and in situ hybridization. Results: Children with oligoarticular JIA have polarized synovial fluid monocytes of a specific M1(IFNγ)/M2(IL-4)-like pattern. This was evidenced by increased surface expression of CD40 (p < 0.001), CD86 (p < 0.001), and CD206 (p < 0.001), but not CD163, as compared to paired circulating monocytes. Additionally, polarization was extensively explored at the mRNA level and synovial fluid monocytes differentially expressed classical markers of M1(IFNγ)/ M2(IL-4) polarization compared to circulating monocytes. Synovial fluid monocytes were functionally affected, as

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 are highly abundant in synovial fluid of rheumatic inflamed joints. In oligoarticular juvenile idiopathic arthritis (JIA), synovial fluid neutrophils have impaired effector functions and altered phenotype. We hypothesized that these alterations might impact the immunoregulatory interplay between neutrophils and T cells. In this study we analyzed the suppressive effect of neutrophils, isolated from blood and synovial fluid of oligoarticular JIA patients, on CD4+ T cells activated by CD3/CD28 stimulation. JIA blood neutrophils suppressed T cell proliferation but synovial fluid neutrophils from several patients did not. The loss of T cell suppression was replicated in an in vitro transmigration assay, where healthy control neutrophils migrated into synovial fluid through transwell inserts with endothelial cells and synoviocytes. Non-migrated neutrophils suppressed proliferation of activated CD4+ T cells, but migrated neutrophils had no suppressive effect. Neutrophil suppression of T cells was partly dependent on reactive oxygen species (ROS), demonstrated by impaired suppression in presence of catalase. Migrated neutrophils had reduced ROS production compared to non-migrated neutrophils. A proteomic analysis of transwell-migrated neutrophils identified alterations in proteins related to neutrophil ROS production and degranulation, and biological processes involving protein transport, cell-cell contact and inflammation. In conclusion, neutrophils in synovial fluid of children with JIA have impaired capacity to suppress activated T cells, which may be due to reduced oxidative burst and alterations in proteins related to cell-cell contact and inflammation. The lack of T cell suppression by neutrophils in synovial fluid may contribute to local inflammation and autoimmune reactions in the JIA joint.

Background Juvenile idiopathic arthritis (JIA) is an umbrella term of inflammatory joint diseases in children. Oligoarthritis is the most common form in the Western world, representing roughly 60% of all patients. Monocytes and macrophages play an important role in adult arthritides, but their role in oligoarticular JIA is less studied. Polarization highly influences monocytes’ and macrophages’ effector functions, broadly separated into pro-inflammatory M1 or anti-inflammatory M2 phenotypes. Here, we set out to investigate the polarization pattern and functional aspects of synovial monocytes in oligoarticular juvenile idiopathic arthritis (JIA). Methods Paired synovial fluid, blood samples (n = 13) and synovial biopsies (n = 3) were collected from patients with untreated oligoarticular JIA. Monocytes were analyzed for polarization markers by flow cytometry and qPCR. Effector function was analyzed by a phagocytosis assay. Polarization of healthy monocytes was investigated by stimulation with synovial fluid in vitro. Monocyte/macrophage distribution, polarization and mRNA expression were investigated in biopsies by immunohistochemistry, immunofluorescence and in situ hybridization. Results Children with oligoarticular JIA have polarized synovial fluid monocytes of a specific M1(IFNg)/M2(IL-4)-like pattern. This was evidenced by increased surface expression of CD40 (p < 0.001), CD86 (p < 0.001) and CD206 (p < 0.001), but not CD163, as compared to paired circulating monocytes, and extensively explored at the mRNA level were synovial fluid monocytes differentially expressed classical markers of M1(IFNg)/M2(IL-4) polarization. Synovial fluid monocytes were functionally affected, as assessed by reduced capacity to phagocytose (p < 0.01). Synovial fluid induced M2 (CD16 and CD206), but not M1 (CD40 and CD86) markers in healthy monocytes and did not induce cytokine production. Single and co-expression of surface CD40 and CD206, as well as mRNA expression of IL-10 and TNF, was observed in monocytes/macrophages in synovial biopsies. Conclusion Children with untreated oligoarticular JIA have similar and distinct synovial fluid monocyte polarization pattern of mixed pro- and anti-inflammatory features. This pattern was not exclusively a result of the synovial fluid milieu as monocytes/macrophages in the synovial membrane show similar patterns. Our study highlights a distinct polarization pattern in oligoarticular JIA, which could be utilized for future treatment strategies.

Objectives Synovial monocytes in oligoarticular juvenile idiopathic arthritis (oJIA) are polarized, but little is known of how they contribute to disease and attain their pathogenic features. The aim of this study was to investigate the role of monocytes in the pathogenesis of oJIA. Methods The function of synovial monocytes was analysed by several assays believed to reflect key pathogenic events, such as T-cell activation-, efferocytosis- and cytokine production assays through flow cytometry in untreated oJIA patients (n=33). The effect of synovial fluid on healthy monocytes was investigated through mass spectrometry, broad-spectrum phosphorylation assays and functional assays. Additional effects on monocytes were studied through co-cultures with primary fibroblast-like synoviocytes. Results The results demonstrate that synovial monocytes display functional alterations, e.g., increased ability to induce T-cell activation, increased efferocytosis and resistance to cytokine production following activation with LPS. In vitro, synovial fluid induced regulatory features in healthy monocytes through an IL-6/JAK/STAT mechanism. The magnitude of synovial IL-6 driven activation in monocytes was reflected in circulating cytokine levels. An increased ability to induce T-cell activation and markers of antigen presentation could be induced by co-culture with fibroblast-like synoviocytes. Conclusions Synovial monocytes in oJIA are functionally affected, drive chronic inflammation, and promote adaptive immune responses. This phenotype can be replicated in vitro through a combination of synovial fluid (through IL-6/JAK/STAT) and cell-cell interactions. These data support a role of monocytes in the pathogenesis of oJIA and highlight a group of patients more likely to benefit from targeting the IL-6/JAK/STAT axis to restore synovial homeostasis.