A study to find how well atacicept works and how safe it is in participants with autoimmune kidney disease.

The purpose of this study is to evaluate the safety and efficacy of Atacicept in multiple autoimmune glomerular diseasing including:

• IgAN (IgA Nephropathy)
• pMN (Primary Membranous Nephropathy)
• MCD/FSGS (Minimal Change Disease/Focal Segmental Glomerulosclerosis)

Inclusion Criteria:

• Weight of at least 40 kg
• On a stable prescribed standard of care (SoC) treatment regimen according to local guidelines and the specific requirements for each disease
• Systolic blood pressure ≤160 mmHg and diastolic blood pressure ≤90 mmHg at Screening.

Diagnosis of IgAN, IgAVN, pMN, MCD, FSGS, or primary nephrotic syndrome

For patients enrolling in IgAN cohorts (eligibility varies by cohort):

• Age ≥ 18 years
• Biopsy proven IgAN, IgAVN, or recurrent IgAN in kidney transplant
• UPCR ≥ 0.5 g/g or UPE ≥ 0.5 g/day on 24h urine
• eGFR≥ 20 mL/min/1.73m2

For patients enrolling in pediatric IgAN Cohorts (eligibility varies by cohort):

• Age ≥ 2 years and < 18 years
• Biopsy proven IgAN or IgAVN
• On stable prescribed regimen of RAASi (or SoC) for at least 8 weeks
• UPCR ≥ 1.0 g/g or UPE ≥ 1.0 g/d on 24h urine
• eGFR≥ 30 mL/min/1.73m2

For patients enrolling in pMN cohorts (eligibility varies by cohort):

• Age ≥ 18 years
• Biopsy-proven pMN
• Anti PLA2R antibodies ≥ 25 RU/mL
• UPCR ≥ 1.5 g/g or UPE ≥ 1.5 g/d on 24h urine
• At low risk for spontaneous remission (based on severity or duration of disease)

For patients enrolling in Nephrotic Syndrome cohorts (MCD, FSGS, or pediatric idiopathic nephrotic syndrome):

• Age ≥ 10 years
• eGFR ≥30 mL/min/1.73m2
• Adults with biopsy diagnosis of primary MCD or FSGS (adults) or children with challenging clinical course with steroids (frequenlty relapsing, steroid-dependent, or steroid-resistant)
• UPCR ≥ 1.0 g/g or UPE ≥ 1.0 g/d on 24h urine
• Evidence of anti-nephrin antibodies

Key Exclusion Criteria:

• Evidence of rapidly progressive glomerulonephritis (loss of ≥50% of eGFR) within 12 weeks prior to and at Screening)
• Active viral or bacterial infections
• Existing conditions or clinically significant laboratory abnormalities that may interfere with participation in this study
• Administration of live and live-attenuated vaccinations within 30 days prior to enrollment
• Immunosuppressant medications within 4 weeks prior to dosing with atacicept (except transplant maintenance immunosuppression).
• Known hypersensitivity to atacicept or any component of the formulated atacicept
• Additional criteria apply to each cohort/disease.