A case of lymphomatoid granulomatosis-like lung lesions with abundant infiltrating IgG4-positive plasma cells whose serum IgG4 levels became high following the start of corticosteroid therapy.

A 59-year-old man with a history of prostate hyperplasia was admitted to our hospital for further examination of a lung mass and renal dysfunction. Lung biopsy specimens revealed that inflammatory cells had infiltrated into the blood vessel walls. We initially suspected lymphomatoid granulomatosis, but Epstein Barr virus-encoded small RNA was negative. However, 50% of the infiltrating plasma cells were positive for IgG4. Furthermore, the kidneys and prostate contained abundant IgG4-positive plasma cells. He was diagnosed with IgG4-related sclerosing disease even though serum IgG4 levels were not elevated (45.7 mg/dL). Prednisolone reduced the lung masses and ameliorated renal function, but the serum IgG4 level increased (377 mg/dL). Seronegative IgG4-related sclerosing disease should be considered when patients present with such symptoms and treatment responses, and the secretion of IgG4 might be blocked by its active synthesis.