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).

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.

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.

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.