• Subprotocol A and B: Diagnosis of SLE according to 2019 European League Against Rheumatism and the American College of Rheumatology (ACR) classification criteria.

• Subprotocol A and B: Participant must be positive for at least one of the following autoantibodies at screening (performed by central laboratory) or through documented history:

‣ Antinuclear antibodies (ANA) ≥ 1:80

⁃ Anti-double stranded deoxyribonucleic acid (anti-dsDNA) antibodies elevated to above normal range (ie, positive results)

⁃ Anti-Smith antibodies elevated to above normal (ie, positive results).

• Subprotocol A and B (with LN): Active, biopsy-proven, proliferative LN demonstrating class III or class IV with or without co-existing features of Class V LN (or pure Class V LN for Subprotocol B only) according to 2018 International Society of Nephrology/Renal Pathology Society (ISN/RPS) criteria. The local biopsy report will be used.

• Subprotocol A and B (with LN): Inadequate response, loss of response or intolerance to at least 1 therapy (Subprotocol A) or 2 immunosuppressive therapies (Subprotocol B with LN) at the maximally tolerated doses as recommended by the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines (KDIGO, 2024). Inadequate response is defined as: UPCR ≥ 1.0 mg/mg.

• Subprotocol B (SLE without nephritis): Systemic Lupus Erythematosus Disease Activity Index 2K ≥ 6.

• Subprotocol B (SLE without nephritis): Refractory SLE participants with inadequate response to multiple therapies (excluding hydroxychloroquine or corticosteroids) and have failed either a biologic agent or cyclophosphamide.

• Subprotocol A and B: If receiving any of the following medications, participants must be on these doses prior to day 1:

‣ Prednisone dose ≤ 20 mg/day (or its equivalent in other corticosteroid forms) and at a stable dose for 5 days

⁃ Hydroxychloroquine dose ≤ 400 mg/day and at a stable dose for 4 weeks. Other equivalent antimalarials (chloroquine, quinacrine) are also accepted at a stable dose for 4 weeks.

⁃ MMF dose ≤ 3 g/day or MPA dose ≤ 2160 mg/day and at a stable dose for 2 weeks.

⁃ AZA dose ≤ 2 mg/kg/day and at a stable dose for 2 weeks.

• Subprotocol C: Diagnosis of RA according to the 2010 ACR/ European Alliance of Associations for Rheumatology (EULAR) classification criteria.

• Subprotocol C: Active disease defined as DAS28-CRP \> 3.2 with at least 1 swollen joint at screening.

• Subprotocol C: Refractory disease defined as:

• Moderate to severe active disease despite having received treatment with:

‣ at least 1 conventional synthetic disease-modifying antirheumatic drug (csDMARD), AND

⁃ at least 2 biologic disease-modifying antirheumatic drugs (bDMARDs) of different mechanisms of action OR 1 bDMARD and at least 1 targeted synthetic disease-modifying antirheumatic drugs (tsDMARD).

• Inadequate response or intolerance to csDMARDs, bDMARDs, and tsDMARDs should be defined as:

‣ Participant having active disease despite a minimum of 12 weeks of treatment with a csDMARD, bDMARD, or tsDMARD.

⁃ Intolerance to treatment as defined by participant having experienced an adverse effect from treatment with a csDMARD, bDMARD, or tsDMARD.