Early appearance of abnormally migrating serum IgM in control and cyclophosphamide-treated NZB/NZW mice.

An augmented incidence of neoplasms was observed in autoimmune NZB/NZW mice receiving longterm immunosuppressive therapy with cyclophosphamide. This observation afforded a unique opportunity to test for paraproteins in 58 serial samples of serum from 39 mice dying with tumors or with renal disease/vasculitis. In 6 animals, abnormally migrating monoclonal IgM precipitin lines appeared at 40--56 weeks of age and persisted until death. In 3 animals, paraproteins appeared 32 to 43 weeks before reticuloendothelial neoplasms were observed. Three mice with abnormally migrating IgM died with renal disease/vasculitis. Abnormal IgM did not correlate with histologic changes of Sjögren's syndrome, lymphadenopathy, or activity of autoimmune disease. In 1 instance, an IgM paraprotein appeared and persisted in a mouse receiving prolonged treatment with a high dose of cyclophosphamide.