The many faces of Merlin: IgG4-associated pulmonary-renal disease.

Pulmonary-renal syndrome is a common and serious disorder with a broad differential diagnosis. We describe a case of a middle-aged man presenting with interstitial pulmonary disease and severe renal impairment caused by a hypocomplementemic immune-complex-mediated interstitial nephritis. Serum levels of IgG4 were elevated, and renal biopsy specimens revealed the presence of interstitial IgG4(+) plasma cells. There was a rapid improvement of both pulmonary and renal abnormalities after the initiation of corticosteroids. To our knowledge, this report is the first to show interstitial pulmonary disease in association with interstitial kidney disease as the predominant and presenting symptoms of IgG4-related disease.