Objective Systemic juvenile idiopathic arthritis (JIA) is associated with a recently recognized, albeit poorly defined and characterized, lung disease (LD). The objective of this study was to describe the clinical characteristics, risk factors, and histopathologic and immunologic features of this novel inflammatory LD associated with systemic JIA (designated SJIA‐LD). Methods Clinical data collected since 2010 were abstracted from the medical records of patients with systemic JIA from the Cincinnati Children's Hospital Medical Center. Epidemiologic, cellular, biochemical, genomic, and transcriptional profiling analyses were performed. Results Eighteen patients with SJIA‐LD were identified. Radiographic findings included diffuse ground‐glass opacities, subpleural reticulation, interlobular septal thickening, and lymphadenopathy. Pathologic findings included patchy, but extensive, lymphoplasmacytic infiltrates and mixed features of pulmonary alveolar proteinosis (PAP) and endogenous lipoid pneumonia. Compared to systemic JIA patients without LD, those with SJIA‐LD were younger at the diagnosis of systemic JIA (odds ratio [OR] 6.5, P = 0.007), more often had prior episodes of macrophage activation syndrome (MAS) (OR 14.5, P < 0.001), had a greater frequency of adverse reactions to biologic therapy (OR 13.6, P < 0.001), and had higher serum levels of interleukin‐18 (IL‐18) (median 27,612 pg/ml versus 5,413 pg/ml; P = 0.047). Patients with SJIA‐LD lacked genetic, serologic, or functional evidence of granulocyte–macrophage colony‐stimulating factor pathway dysfunction, a feature that is typical of familial or autoimmune PAP. Moreover, bronchoalveolar lavage (BAL) fluid from patients with SJIA‐LD rarely demonstrated proteinaceous material and had less lipid‐laden macrophages than that seen in patients with primary PAP (mean 10.5% in patients with SJIA‐LD versus 66.1% in patients with primary PAP; P < 0.001). BAL fluid from patients with SJIA‐LD contained elevated levels of IL‐18 and the interferon‐γ–induced chemokines CXCL9 and CXCL10. Transcriptional profiling of the lung tissue from patients with SJIA‐LD identified up‐regulated type II interferon and T cell activation networks. This signature was also present in SJIA‐LD human lung tissue sections that lacked substantial histopathologic findings, suggesting that this activation signature may precede and drive the lung pathology in SJIA‐LD. Conclusion Pulmonary disease is increasingly detected in children with systemic JIA, particularly in association with MAS. This entity has distinct clinical and immunologic features and represents an uncharacterized inflammatory LD.

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Objectives Systemic juvenile idiopathic arthritis (sJIA) is a childhood arthritis with features of autoinflammation and high risk of macrophage activation syndrome (MAS). IL-18 has been shown to have key roles in sJIA and MAS. We aimed to examine IL-18 levels in sJIA in relation to disease activity and history of MAS and other disease biomarkers namely S100 proteins and CXCL9. Methods Total IL-18, CXCL9 and S100 proteins levels were determined in 40 sJIA patients, and IL-18 levels were compared between patients with regards to disease activity, history of MAS, and other biomarkers. Results Total IL-18 levels were significantly higher in patients with active sJIA (median 16 499 pg/ml; interquartile range (IQR) 4816–61 839), and remained persistently elevated even in the majority of patients with inactive disease (1164 pg/ml; IQR 587–3444). Patients with history of MAS had significantly higher IL-18 levels (13 380 pg/ml; IQR 4212–62 628) as compared with those without MAS history (956.5 pg/ml; IQR 276.3–4262.5). Total IL-18 performed well with area under the curve of 0.8145 and 0.84 in predicting disease activity and history of MAS, respectively. We observed moderate correlation between IL-18 and CXCL9 (R = 0.56), S100A8/A9 (R = 0.47) and S100A12 (R = 0.46). The correlation was stronger for ferritin (R = 0.74) and overall for those with active disease. Conclusion Total IL-18 levels were elevated in the majority of sJIA patients regardless of clinical features, but were higher in patients with active disease and history of MAS. Change in IL-18 may reflect increased disease activity or development of MAS.