Trial of Sibeprenlimab in the Treatment of Immunoglobulin A Nephropathy (IgAN)

Inclusion Criteria: * Subjects who completed Trial 417-201-00007 (at least 20 of the 26 doses over a 2-year duration and the end-of-trial visit) or Trial VIS649-201 (at least 9 of the 12 doses over a 1-year duration and the end-of-trial visit) without safety concerns and who, in the opinion of the investigator, could potentially benefit from treatment with sibeprenlimab for IgAN. * Ability to provide written, informed consent prior to initiation of any trial-specific procedures, and ability, in the opinion of the principal investigator, to comply with all the requirements of the trial. Exclusion Criteria: * Subjects who have not completed participation in Trials 417-201-00007 or VIS649-201. * Subjects with treatment-limiting AEs during Trials 417-201-00007 or VIS649-201 considered related to IMP per investigator judgement. * Noncompliance, due to subject's repeated failure to follow trial procedures during the course of their participation in Trials 417-201-00007 and VIS649-201 (eg, subjects deemed to be noncompliant with the visit schedule, trial assessments, or treatment regimen). The medical monitor should be contacted if the investigator is unsure of a subject's eligibility. * Subjects who have a positive pregnancy test result prior to receiving IMP. * Heterosexually active biological males or subjects of childbearing potential, or their partners, who do not agree to adhere to contraceptive requirements from the time of consent through the end of the subject's participation in the trial and an additional 90 days (biological male subjects) or 30 days (biological female subjects) thereafter. * Biological male subjects who do not agree to avoid donation of sperm from the time of consent through the end of the subject's participation in the trial and an additional 90 days thereafter. * Subject has nephrotic syndrome, defined for this purpose as 24-hour urine protein \> 3.5 g with concurrent hypoalbuminemia (serum albumin \< 2.5 g/dL), hyperlipidemia (total cholesterol \> 350 mg/dL), and edema. Subjects with isolated nephrotic range proteinuria (\> 3.5 g/day) will be eligible. * Subject has a history or current evidence of a serious and/or unstable cardiovascular, respiratory, gastrointestinal, hematologic, autoimmune, rheumatologic, blood dyscrasias or other medical disorder, including psychiatric disorders, cirrhosis, or ongoing malignancy. History of minor skin cancers (not including melanoma) or surgically treated, limited cervical carcinomas (ie, carcinoma in situ) are not exclusionary. * History of a previous hypersensitivity or severe allergic reaction with generalized urticaria, angioedema or anaphylaxis to any of the ingredients of the sibeprenlimab SC injection formulation. * Subject has a body mass index \< 16 kg/m\^2. * Subject has received an organ transplant (ie, solid or a bone marrow or hematologic stem cell transplantation). * Subject is currently receiving, or has received within 16 weeks prior to enrollment, systemic immunosuppressive therapy (ie, steroids such as targeted release budesonide, complement inhibitors, mycophenolate, rituximab). Note: topical, ophthalmic, rectal, intra-articular, inhaled corticosteroids, and short courses of oral/intravenous steroids (\<= 14 days) are allowed.