A Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of Subcutaneously Administered Mosunetuzumab to Participants With Systemic Lupus Erythematosus

Inclusion Criteria:

• Diagnosis of SLE according to the 2019 European League Against Rheumatism/AmericanCollege of Rheumatology Classification Criteria at least 12 weeks or more prior toscreening

• Presence of one or more of the following SLE autoantibodies documented within the 12months prior to screening or during screening: positive ANA (greater than or equalto 1:160); anti dsDNA above the upper limit of normal (ULN); anti-Sm above the ULN

• Active SLE disease, as demonstrated by a SLEDAI-2K total score of greater than orequal to 4 at screening

• Current receipt of one or more of the following classes of standard therapies forthe treatment of SLE at stable doses: oral corticosteroids (OCSs), antimalarialagents, conventional immunosuppressants

• For women of childbearing potential: agreement to remain abstinent (refrain fromheterosexual intercourse) or use contraception as defined by the protocol

• For men on mycophenolate mofetil (MMF): With a female partner of childbearingpotential, men who are not surgically sterile must remain abstinent (refrain fromheterosexual intercourse) or use contraception as defined by the protocol

Exclusion Criteria:

• Pregnant or breastfeeding, or intending to become pregnant during the study orwithin 3 months after the final dose of mosunetuzumab and 3 months after the finaldose of tocilizumab

• Active severe or unstable lupus-associated neuropsychiatric disease that is likelyto require treatment with protocol-prohibited therapies

• Active overlap syndrome with mixed connective tissue disease or systemic sclerosiswithin 12 months prior to screening or during screening

• Catastrophic or severe antiphospholipid syndrome within 12 months prior to screeningor during screening

• Presence of significant lupus-associated renal disease and/or renal impairment thatis likely to require treatment with protocol-prohibited therapies

• Peripheral CD19+ B-cell count < 25 cells/uL

• Receipt of an investigational therapy within 30 days or 5 drug-eliminationhalf-lives (whichever is longer) prior to initiation of study treatment and duringthe study

• Receipt of any of the following excluded therapies: any anti-CD19 or anti-CD20therapy such as blinatumomab, obinutuzumab, rituxumab, ocrelizumab, or ofatumumabless than 12 months prior to screening or during screening; inhibitors of JAK,Bruton tyrosine kinase, or tyrosine kinase 2, including baricitinib, tofacitinib,upadacitinib, filgotinib, ibrutinib, or fenebrutinib, or any investigational agentwithin 30 days prior to screening or during screening; tacrolimus, ciclosporin, orvoclosporin within 30 day prior to screening or during screening; cyclophosphamideor a biologic therapy such as but not limited to belimumab, ustekinumab,anifrolumab, secukinumab, or atacicept during 2 months prior to screening or duringscreening; any live or attenuated vaccine during 28 days prior to screening orduring screening

• High risk for any clinically significant bleeding or any condition requiringplasmapheresis, IV immunoglobulin, or acute blood product transfusions

• Significant or uncontrolled medical disease that would preclude participation

• HIV infection, acute or chronic hepatitis B virus (HBV), acute or chronic hepatitisC (HCV) infection, tuberculosis (TB) infection, known or suspected chronic activeEpstein-Barr virus (EBV) infection, or cytomegalovirus (CMV) infection

• Active infection of any kind, excluding fungal infection of the nail beds

• Any major episode of infection that fulfills any of the following criteria: requireshospitalization during 8 weeks prior to screening or during screening; requirestreatment with IV antibiotics (or anti-infective medications) during 8 weeks priorto screening or during screening; requires treatment with oral antibiotics (oranti-infective medications) during 2 weeks prior to screening or during screening

• History of serious recurrent or chronic infection

• History of progressive multifocal leukoencephalopathy (PML)

• History of cancer, including solid tumors, hematological malignancies, and carcinomain situ, within the past 5 years

• Major surgery requiring hospitalization during 4 weeks prior to screening or duringscreening or any planned surgery or procedure requiring hospitalization during 12weeks following study drug administration

• Current alcohol or drug abuse or history of alcohol or drug abuse within 12 monthsprior to screening or during screening

• Intolerance or contraindication to study therapies including history of severeallergic or anaphylactic reactions to monoclonal antibodies or knownhypersensitivity to any component of mosunetuzumab injection

• Positive serum human chorionic gonadotropin measures at screening

• Any of the following laboratory parameters: aspartate transaminase (AST) or alaninetransaminase (ALT) > 2.5 x upper limit of normal (ULN); total bilirubin > 1.5 x ULN;absolute neutrophil count (ANC) < 2.0 x 10^9/L (< 2000/mm^3); platelet count < 100 x 10^9/L (100,000 mm^3); hemoglobin < 100 g/L (10 g/dL); estimated glomerularfiltration rate (eGFR) < 30 ml/min/1.73 m^2 calculated according to the ChronicKidney Disease Epidemiology Collaboration equation; positive serum human chorionicgonadotropin measured at screening