Inclusion Criteria:

1. Documentation of IgAN diagnosis established on kidney biopsy obtained any time prior to or during the Screening Period for participants with eGFR ≥ 30 mL/min/1.73 m^2.
2. For participants in the AdKD cohorts, eGFR 20 to 29 mL/min/1.73 m2 a kidney biopsy is required within 6 months prior to Screening or during the Screening Period.
3. UPCR ≥ 0.75 g/g or UP ≥1 g/day calculated from the mean of two 24-hour urine during the Screening Period.
4. Estimated GFR ≥ 30 mL/min/1.73 m2 at Screening.
5. Stable and maximum allowed or tolerated RASI (ACEI and/or ARB) dose for ≥ 3 months prior to Screening with no planned change during Screening through Week 106.
6. Participants who are receiving SGLT2I, DEARA, MRA or ERA must be on a stable and maximum allowed or tolerated dose for ≥ 3 months prior to Screening with no planned change in dose through Week 106.

Exclusion Criteria:

1. Diagnosis of rapid progressive glomerulonephritis as measured by eGFR loss ≥ 50% over a period of 3 months prior to Screening.
2. Secondary IgAN (eg, due to systemic lupus erythematosus (SLE), cirrhosis, or celiac disease; IgAV-N may be eligible).
3. Concomitant clinically significant renal disease other than IgAN.
4. Prior use of immunosuppressive treatment within 3 months of screening.
5. Uncontrolled diabetes mellitus with glycosylated hemoglobin (HbA1c) > 8.5%.
6. Henoch-Schonlein purpura (IgAV) requiring systemic immunosuppressive therapy within 12 months of Screening.
7. History of kidney transplant or planned kidney transplant during the Treatment Period.
8. Splenectomy or functional asplenia.
9. History of Neisseria meningitidis infection.
10. Active systemic bacterial, viral, or fungal infection within 14 days prior to randomization.