A Study to Evaluate Mosunetuzumab in Participants With Systemic Lupus Erythematosus With or Without Active Lupus Nephritis

Inclusion Criteria: * Diagnosis of SLE for ≥ 6 months as assessed using the 2019 European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) Classification Criteria at screening Exclusion Criteria: * Pregnant or breastfeeding, or intention of becoming pregnant during the study or within the time frame in which contraception is required * Treatment with investigational therapy within 30 days or 5 drug-elimination half-lives (whichever is longer) prior to initiation of study treatment and during the study * Major surgery requiring hospitalization during the 4 weeks prior to screening or during screening, or any planned surgery or procedure requiring hospitalization during the 12 weeks following study drug administration * Alcohol or substance abuse within the 12 months prior to screening * Active infection of any kind, excluding fungal infection of the nail beds * Any major episode of infection as defined by the protocol * History of serious recurrent or chronic infection * History of progressive multifocal leukoencephalopathy (PML) * Tuberculosis (TB) infection * History of cancer, including solid tumors, hematological malignancies, and carcinoma in situ, within the past 5 years * Active overlap syndrome with mixed connective tissue disease or systemic sclerosis within the 12 months prior to screening or during screening * Catastrophic or severe antiphospholipid syndrome within the 12 months prior to screening or during screening. Antiphospholipid syndrome adequately controlled by anticoagulant therapy for at least 2 months prior to screening is acceptable * High risk for clinically significant bleeding or any condition requiring plasmapheresis, IV immunoglobulin, or acute blood product transfusions * Active severe or unstable lupus-associated neuropsychiatric disease or where, in the opinion of the investigator, it is likely to require treatment with protocol-disallowed therapies. Examples of neuropsychiatric SLE manifestations include, but are not limited to the following: meningitis, retinitis, cerebral vasculitis, myelopathy, demyelination syndromes, acute confusional state, psychosis, acute stroke or stroke syndrome, cranial neuropathy, status epilepticus or seizures, cerebellar ataxia, and mononeuritis multiplex * History of any non-SLE disease treated with oral, intravenous, or intramuscular corticosteroids for more than 14 days in total during the one year prior to Day 1 * History of treatment with any T cell-engaging bispecific antibodies or CAR-T therapy within the past 2 years * Receipt of any live or attenuated vaccine in the 28 days prior to or during screening