Familial pulmonary alveolar proteinosis and type I interferonopathy by mutation of STAT2 (TIMS2).

Mutations that enhance type I interferon (IFN-I) activity cause monogenic autoinflammatory disorders termed type I interferonopathies. Along with the typical neurologic and rheumatologic manifestations, severe pulmonary disease is increasingly recognized yet poorly understood. We studied three siblings presenting with early-onset, life-threatening pulmonary alveolar proteinosis (PAP) and autoinflammatory stigmata. Genetic analysis uncovered a novel homozygous variant (R223Q) in STAT2, a key mediator of IFN-I signaling, which also facilitates feedback inhibition via USP18. R223Q STAT2 preserved signal transduction and viral control in vitro. However, cells homozygous for the R223Q variant failed to terminate IFN-I responses, owing to impaired localization of USP18. Unlike in classical forms of PAP, GM-CSF signaling remained intact. Instead, persistent IFN-I signaling antagonized monocyte migration toward chemokines essential for lung trafficking. Informed by these findings, the youngest sibling received JAK inhibitor and anti-IFN-I receptor therapy with marked clinical improvement. Collectively, type I interferonopathy by mutation of STAT2 (TIMS2) compromises monocyte chemotaxis and underlies a novel mechanism of PAP.