Leflunomide Plus Low Dose Corticosteroid in Immunoglobulin A (IgA) Nephropathy With Renal Insufficiency

Inclusion Criteria: * 14 to 65 years old participants, No restrictions on gender or race * Diagnosis of primary IgA nephropathy * Renal biopsy within 6 months before screening. Renal pathology shows diffuse IgA deposition in the Mesangial area and dense deposition in the Mesangial area under electron microscope. glomeruli more than 8 * persistent proteinuria ≥ 1 g/24 hr (or urine protein/creatinine ratio ≥ 1.0 mg/g), eGFR at 25-50 ml/min/1.73 m2 (calculated by Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula) * signed written consent; patients under 18 years old need to have legal guardians to sign informed consent at the same time Exclusion Criteria: * Secondary IgA nephropathy (such as lupus nephritis, Henoch-Schönlein purpura, hepatitis B associated glomerulonephritis, hepatitis C associated glomerulonephritis, liver cirrhosis and other autoimmune diseases) * eGFR \< 25 ml/min/1.73m2 or eGFR \> 50 ml/min/1.73m2 (calculated by CKD-EPI formula) * Special types of IgA nephropathy need to be excluded, such as crescent IgA glomerulonephritis (defined as the presence of crescents in over 50% of the glomeruli), or minimal lesions with IgA deposition * Acute kidney injury within 3 months before screening * Received immunosuppressive therapy within 3 months before screening * Patients who had acute or chronic infections that need treatment, and the researchers judged that they were not suitable for inclusion in the study * Pregnancy, lactation or unreliable birth control * Dialysis treatment before screening * Allergic or taboo to planned drugs (such as leflunomide, glucocorticoids, etc.) * Severe acute or chronic diseases that the researchers believe may bring an excessive risk to the subjects * A history of malignant tumors within 5 years, with the exception of carcinoma in situ and papillary thyroid carcinoma which have been adequately treated * Participated in other clinical trials and / or used other research drugs within 4 weeks prior to screening