Leflunomide for Maintenance of Remission in IgG4 Related Disease

Inclusion Criteria: 1. Age between 18 and 80 years. 2. Diagnosis of IgG4-RD according to either Consensus statement on the pathology of IgG4-related disease (for those who have undergone biopsies) or 2011 Comprehensive diagnostic criteria for IgG4-related disease. Both of the two criteria for diagnosis are specified below. (1)Consensus statement on the pathology of IgG4-related disease 1. Histopathologic features consisting of dense lymphoplasmacytic infiltrate, fibrosis(usually storiform in character)and/or obliterative phlebitis within involved organs. 2. Either an elevated IgG4+/IgG+cell ratio of \>40% within the affected organs or elevated IgG4-bearing plasma cells per high-power field is necessary. The cut-off number of IgG4-bearing plasma cells per high-power field is different depending upon the types of affected organs and specimens(through surgery or needle puncture biopsy). (2)2011 Comprehensive diagnostic criteria for IgG4-related disease 1. Clinical examination showing characteristic diffuse/localized swelling or masses in single or multiple organs. 2. Hematological examination shows elevated serum IgG4 concentrations(135 mg/dl). 3. Histopathologic examination shows marked lymphocyte and plasmacyte infiltration and fibrosis or Infiltration of IgG4+ plasma cells(ratio of IgG4+/IgG+ cells \> 40% and \>10 IgG4+ plasma cells/HPF). Definite: a + b + c,Probable: a + c,Possible: a + b 4. Excluded from malignant tumors of each organ (e.g. cancer, lymphoma) and similar diseases (e.g. Sjögren's syndrome, primary sclerosing cholangitis, Castleman's disease, secondary retroperitoneal fibrosis, Wegener's granulomatosis, sarcoidosis, Churg-Strauss syndrome) by additional histopathological examination. 5. Even when patients cannot be diagnosed using the Comprehensive diagnostic criteria, they may be diagnosed using organ-specific diagnostic criteria for IgG4-RD, such as Diagnostic criteria for IgG4-Mikulicz's disease. Exclusion Criteria: 1. Subjects having received steroids or immunosuppressants in recent 3 months will be excluded. 2. Subjects who were hypersensitive to leflunomide will be excluded. 3. ALT and/or AST is more than two folds of the upper limit of relevant reference value at baseline. 4. WBC is less than 3×10\*9/L at baseline. 5. Female patients who are pregnant or breastfeeding. 6. Known significant concurrent medical disease.