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ObjectiveTo investigate characteristics and risk factors of a novel parenchymal lung disease, increasingly detected in systemic juvenile idiopathic arthritis (sJIA). MethodsIn a multi-center retrospective study, 61 cases were investigated, using physician-reported clinical information and centralized analyses of radiologic, pathologic and genetic data. ResultsLung disease (LD) was associated with distinctive features, including acute erythematous clubbing and a high frequency of anaphylactic reactions to the IL-6 inhibitor, tocilizumab. Serum ferritin elevation and/or significant lymphopenia preceded LD detection. The most prevalent chest CT pattern was septal thickening, involving the periphery of multiple lobes +/- ground glass opacities. Predominant pathology (23/36) was pulmonary alveolar proteinosis and/or endogenous lipoid pneumonia (PAP/ELP), with atypical features, including regional involvement and concomitant vascular changes. Apparent severe delayed drug hypersensitivity occurred in some cases. 5-year survival was 42%. Whole-exome sequencing (20/61) did not identify a novel monogenic defect PAP-related or macrophage activation syndrome (MAS)-related mutations as likely primary cause. Trisomy 21 (T21) increased LD risk, as did young sJIA onset. Refractory sJIA was not required for LD development. Exposure to interleukin (IL)-1 and IL-6 inhibitors (46/61) was associated with multiple LD features. By several indicators, severity of sJIA was comparable in drug-exposed subjects and published sJIA cohorts. MAS at sJIA onset was increased in the drug-exposed, but it was not associated with LD features. ConclusionsA rare, life-threatening LD in sJIA is defined by a constellation of unusual clinical characteristics. The pathology, a PAP/ELP variant, suggests macrophage dysfunction. Inhibitor exposure may promote LD, independent of sJIA severity, in a small subset of treated patients. Treatment/prevention strategies are needed.

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