A Study to Evaluate the Effectiveness and Safety of IONIS-FB-LRx, an Antisense Inhibitor of Complement Factor B, in Adult Participants With Primary IgA Nephropathy

Inclusion Criteria * Females must be non-pregnant and non-lactating, and either surgically sterile or post-menopausal OR use a highly effective method of birth control * Biopsy-proven primary immunoglobulin A (IgA) nephropathy * Hematuria * Proteinuria Exclusion Criteria * Clinically significant abnormalities in medical history (e.g., dementia, stroke, acute coronary syndrome, thrombocytopenia, or major surgery within 3 months of Screening) * Diagnosis of primary or secondary immunodeficiencies of B-lymphocyte function, splenectomy, or history of recurrent meningococcal disease * Active infection 30 days prior to study * Estimated glomerular filtration rate (eGFR) ≤ 40 milliliters per minute per 1.73 square meters (mL/min/1.73m\^2) using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) * Presence of another renal disease including, but not limited to, diabetes and/or diabetic nephropathy, thin basement membrane disease, Alport's disease, IgA Nephritis (Henoch-Schonlein purpura), lupus nephritis, Minimal Change Disease, post-infectious glomerulonephritis or any other cause of proteinuria or secondary IgA nephropathy (including, but not limited to Celiac disease, Crohn's disease, human immunodeficiency virus (HIV), liver cirrhosis) * History of renal transplant or another organ transplant * Treatment with another investigational drug, biological agent, or device within 6 months of screening, or 5 half-lives of investigational agent, whichever is longer * Administration of immunosuppressive/immunomodulatory medication 12 months prior to study drug administration, except for short-term treatments. * Other protocol-specified inclusion/exclusion criteria may apply