• Male and female patients including adults (aged at least 18 years to ≤ 60 years) and adolescents (12 -17 years in non-EU countries at screening and 16-17 years in EU countries at screening).

• Diagnosis of idiopathic IC-MPGN as confirmed by kidney biopsy within 12 months prior to screening in adults and within 3 years of screening in adolescents (a biopsy report, review and confirmation by the Investigator is required). If such a biopsy is not available in an adult participant, this must be obtained at screening (performed and assessed locally for adults only).

• Prior to randomization, all participants must have been on a maximally recommended or tolerated dose of renin angiotensin system inhibitors (RASi), e.g an ACEi or ARB for at least 90 days (or as according to local guidelines). The doses of other drugs administered to reduce proteinuria and control the disease including mycophenolic acids (MPAs - mycophenolate mofetil or mycophenolate sodium), corticosteroids, SGLT2 inhibitors and mineralocorticoid receptor antagonists should be stable for at least 90 days prior to randomization

• UPCR ≥ 1.0 g/g (≥ 113 mg/mmol) sampled from the first morning void urine sample at Day -75 and Day -15

• Estimated GFR (using the chronic kidney disease \[CKD\]-EPI formula for adult participants and modified Schwartz formula for adolescents aged 12 to 17 years) or measured GFR ≥ 30 ml/min/1.73m2 at screening and Day -15.

• Mandatory vaccination against Neisseria meningitidis and Streptococcus pneumoniae infection prior to the start of study treatment. If the participant has not been previously vaccinated, or if a booster is required, the vaccine should be given according to local regulations at least 2 weeks prior to the first administration of study treatment. If the study treatment has to start earlier than 2 weeks post vaccination, prophylactic antibiotic treatment should be initiated in accordance with local standard of care.

• If not previously vaccinated, or if a booster is required, vaccination against Haemophilus influenzae infections should be given, if available and according to local regulations, at least 2 weeks prior to the first study treatment administration.