Trial of the Impact of Sibeprenlimab on Immunoglobulin A Nephropathy Kidney Tissue

Inclusion Criteria: 1. Participants must be at least 16 years of age or older at the time of signing the informed consent/assent. 2. Source-verified kidney biopsy confirmed diagnosis of IgAN. 3. Participant has estimated glomerular filtration rate (eGFR) \> 45 mL/min/1.73 m2 using serum creatinine (Chronic Kidney Disease-Epidemiology Collaboration \[CKD EPI\] creatinine eGFR 2021 equation for those 18 years and older; Chronic Kidney Disease in Children under age 25 \[CKiD U25\] eGFR equation for those younger than 18 years) Exclusion Criteria: 1. Participants who are breast-feeding and/or who have a positive pregnancy test result prior to receiving sibeprenlimab. 2. Participant has coexisting chronic kidney disease, other than IgAN. 3. Participant has a serum IgG value \<600 mg/dL at screening. 4. Participant is currently receiving or has received within 24 weeks prior to the firstdose of sibeprenlimab, systemic corticosteroids or immunosuppression (note: topical, ophthalmic, rectal, intra-articular, inhaled corticosteroids are allowed). 5. Participant has uncontrolled hypertension (defined as systolic blood pressure \> 140 mmHg or diastolic blood pressure \> 90 mmHg). 6. Participants who would be likely to require prohibited concomitant therapy during the trial.