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 often involves opening one of the major body cavities (abdomen or chest) and/or use of general anesthesia. Types of surgery that have the highest risk include heart or lung, liver, abdomen, or major operations on the bones and joints (eg, hip replacement) * Clinically significant history of alcohol or drug abuse in the 1 year prior to Day 1 as per Investigator opinion * Unwillingness or lack of capacity to follow all study procedures * For Atacicept Drug Holiday Group only: Treatment with other investigational agents within the last 4 weeks or 5 half-lives, whichever is longer, prior to screening * Evidence of nephrotic syndrome (serum albumin \<30g/L in association with UPCR \>3.5 mg/mg) within 6 months of screening * Currently on chronic dialysis, or expected to initiate dialysis within 12 weeks of screening * Renal or other organ transplantation prior to, or expected during, the study, with the exception of corneal transplants * Clinically significant or predefined abnormalities per central laboratory tests at screening, meeting any of the criteria: Clinical evidence of immunosuppression and/or hypogammaglobulinemia as determined by the Investigator. * For Atacicept Drug Holiday Group only: Aspartate aminotransferase, alanine aminotransferase or alkaline phosphatase level \>2.5 × upper limit of normal (ULN) or total bilirubin \>1.5 x ULN. If the participant has a known history of Gilberts (history of isolated increase in total bilirubin without increase in liver transaminases), contact the Medical Monitor for further discussion. * For Atacicept Drug Holiday Group only: Administration of live and live-attenuated vaccinations within 30 days prior to enrollment. * For Atacicept Drug Holiday Group only: History or current diagnosis of any demyelinating disease such as, but not restricted to, multiple sclerosis (MS) or optic neuritis (ON) * Active clinically significant viral, bacterial or fungal infection, or any major episode of infection requiring hospitalization or treatment with parenteral anti-infectives within 4 weeks prior to enrollment, or completion of oral anti-infectives within 2 weeks prior to enrollment, ora history of recurrent infections (ie, 3 or more of the same type of infection in a 12-month rolling period). Vaginal candidiasis, onychomycosis and genital or oral herpes simplex virus considered by the Investigator to be sufficiently controlled are not exclusionary. * Atacicept Drug Holiday Group only: If the participant is undergoing current treatment for latent tuberculosis infection (LTBI), they must have received at least 4 continuous weeks of an appropriate LTBI treatment prior to screening without evidence of re-exposure to be eligible for this study. If on LTBI treatment at the Screening visit, the participant will be expected to complete an appropriate LTBI treatment regimen to remain in the trial Prohibited medications: * Use of systemic corticosteroids (including oral budesonide) or immunosuppressive medications (eg, MMF, azathioprine, cyclophosphamide, hydroxychloroquine) for the treatment of IgAN within 2 months prior to Screening * For glucocorticosteroids (GCS), "Systemic" is defined as oral, rectal or injectable (intravenous or intramuscular) routes of administration, Other routes of administration are allowed, including intra-articular, inhaled, topical, ophthalmic, optic and intranasal * Use of B-cell-directed biologic therapies including belimumab, rituximab, ocrelizumab within 12 months of screening * Use of other biologics (eg, anti-TNF, abatacept, anti-IL-6) and investigational biologics for the treatment of IgAN within 6 months of screening