To evaluate the safety and efficacy of Tai Ai(Recombinant Human B Lymphocyte Stimulator Receptor-Antibody Fusion Protein for Injection) in the treatment of IgA nephropathy.

Both RC18 and Recombinant Human B Lymphocyte Stimulator Receptor-Antibody Fusion Protein for Injection are other names of Tai Ai.

After a 35-day screen period, subjects are randomly allocated into 3 groups receiving subcutaneous injection of Tai Ai 160mg, Tai Ai 240mg, and placebo once a week individually. The treatment lasts 24 weeks. Subjects, the sponsor, investigators are blinded in the whole process of the trial.

Inclusion Criteria:

1. Signing the informed consent;
2. Biopsy confirmed diagnosis of IgA nephropathy;
3. Male or female, between 18 and 70 years age;
4. During screening, 24-hour urine protein excretion ≥0.75 g/24h at Visit 1 and/or Visit 2 and at Visit 3;
5. Estimated glomerular filtration rate (eGFR) (CKD-EPI ) >35 ml/min per 1.73m^2;
6. Have received the Angiotension converting enzyme Inhibitors(ACEI)/Angiotensin receptor blocker(ARB) standard treatment for 12 weeks prior to randomization, and have stabled the dosage (within the maximum tolerated dosage) for 4 weeks prior to randomization.

Exclusion Criteria:

1. Abnormal laboratory tests;
2. Any secondary IgA nephropathy caused by Henoch-Schönlein purpura, ankylosing spondylitis, systemic lupus erythematosus, sjogren syndrome, viral hepatitis, liver cirrhosis, rheumatoid arthritis, mixed connective tissue disease, polyarteritis nodosa, erythema nodosum, psoriasis, ulcerative colitis, crohn's disease, tumor, AIDS ,etc.;
3. Any nephropathy with special pathologic or clinical types, such as nephrotic syndrome, crescentic glomerulonephritis(with >50% of biopsied glomeruli), minimal change disease with IgA deposition; and IgA nephropathy requiring corticosteroids treatment.
4. Suffering from cardiovascular and cerebrovascular events (myocardial infarction, unstable angina, ventricular arrhythmia, New York heart association grade III-IV heart failure, stroke, etc.) within the last 12 weeks;
5. Treating with systemic corticosteroids drug(excluding topical or nasal steroids) within 3 months prior to randomizing;
6. Treating with systemic immunosuppressor within 3 months prior to randomizing: cyclophosphamide, azathioprine, mycophenolate mofetil, leflunomide, tacrolimus, cyclosporine, rituximab, tripterygium wilfordii, etc.;
7. Requiring hospitalization or intravenous antibiotics treatment due to active infection within 3 months prior to randomizing;
8. Active tuberculosis or latent carrier without treatment;
9. Herpes zoster infected patients or patients with positive HIV antibody or positive HCV antibody;
10. Active hepatitis or severe liver disease, and HBV infection (According to the HBV screening test, ① the HBsAg-positive; ②HBsAg-negative and HBcAb-positive, the HBV-DNA should be tested to determine the situation: the HBV-DNA positive subjects should be excluded, while the HBV-DNA negative subjects can participated in.)
11. With malignant tumors;
12. Pregnancy ，lactation, or patients with childbearing plans during the trial;
13. Nephrotoxic drugs is unavoidable during the study period;
14. Allergy to human-derived biologics;
15. Receiving any other investigating drug 4 weeks or 5 times half-life of the experimental drug (whichever is longer) prior to randomization;
16. Not suitable for the study judged by investigator.