A Rollover Study to Evaluate the Long-Term Safety and Efficacy of Atacicept

Inclusion Criteria: * Must have the ability to understand and sign a written informed consent form, which must be obtained prior to initiation of study assessments * Completed the protocol-defined treatment period on treatment in a parent study of atacicept in patients with IgAN * For Atacicept Drug Holiday Group only: Systolic blood pressure ≤150 mmHg and diastolic blood pressure ≤90mmHg at screening and Day 1 * A participant who was assigned female at birth is eligible if not pregnant (ie, after a confirmed menstrual period, a negative serum pregnancy test at screening and has a negative urine pregnancy test at Day 1), is not breastfeeding (for at least three months prior to screening), and at least one of the following conditions applies: * Is not a woman of childbearing potential (WOCBP) OR * Is a WOCBP who agrees to use a highly effective contraceptive method (ie, has a failure rate of less than 1% per year) at least 7 days prior to enrollment, through 175 days after the last dose of study drug. Exclusion Criteria: * Evidence of rapidly progressive glomerulonephritis (loss of ≥50% of eGFR within 3 months of screening) * For Atacicept Drug Holiday Group only: History of splenectomy * Known hypersensitivity to atacicept or any component of the formulated atacicept * For Atacicept Drug Holiday Group only: Major surgery within 6 weeks prior to screening or planned/expected major surgery during the study period (including the safety follow-up period). Major surgery of…