A Study Evaluating the Efficacy and Safety of Mosunetuzumab in Combination With Lenalidomide in Comparison to Rituximab in Combination With Lenalidomide With a US Extension of Mosunetuzumab in Combination With Lenalidomide in Participants With Follicular Lymphoma

Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2

• Histologically documented CD20+ FL (Grades 1-3a)

• Requiring systemic therapy assessed by investigator based on tumor size and/orGroupe d'Etude des Lymphomes Folliculaires (GELF) criteria

• Received at least one prior systemic lymphoma therapy, which included priorimmunotherapy or chemoimmunotherapy

• Availability of a representative tumor specimen and the corresponding pathologyreport at the time of relapse/persistence for confirmation of the diagnosis of FL.Pretreatment sample of at least 1 core-needle, excisional or incisional tumor biopsyis required. Cytological or fine-needle aspiration samples are not acceptable. Freshpretreatment biopsy is preferred. Patients who are unable to undergo biopsyprocedures may be eligible for study enrollment if an archival tumor tissue sample (preferably from the most recent relapse/persistence) as paraffin blocks or at least 15 unstained slides, or in accordance with local regulatory requirements, can besent to the Sponsor.

• Adequate hematologic function (unless due to underlying lymphoma, per theinvestigator)

• Agreement to comply with all local requirements of the lenalidomide riskminimization plan, which includes the global pregnancy prevention program.

• For women of childbearing potential: Agreement to remain abstinent (refrain fromheterosexual intercourse) or use 2 adequate methods of contraception, including atleast 1 method with a failure rate of < 1% per year, for at least 28 days prior toDay 1 of Cycle 1, during the treatment period (including periods of treatmentinterruption), and for at least 28 days after the last dose of lenalidomide, 3months after the final dose of tocilizumab (if applicable), mosunetuzumab, and 12months after final dose of rituximab. Women must refrain from donating eggs duringthis same period.

• For men: Agreement to remain abstinent (refrain from heterosexual intercourse) oruse contraceptive measures and agreement to refrain from donating sperm, as defined:With female partners of childbearing potential or pregnant female partners, men mustremain abstinent or use a condom during the treatment period and for at least 28days after last dose of lenalidomide, 3 months after the final dose of tocilizumab (if applicable), mosunetuzumab and 12 months after the final dose of rituximab. Menmust refrain from donating sperm during this same period.

Exclusion Criteria:

• Grade 3b FL

• Any history of disease transformation and/or diffuse-large B cell lymphoma (DLBCL)

• Documented refractoriness to lenalidomide, defined as no response (partial responseor complete response) or relapse within 6 months of therapy

• Active or history of CNS lymphoma or leptomeningeal infiltration

• Prior standard or investigational anti-cancer therapy as specified: Lenalidomideexposure within 12 months prior to Day 1 of Cycle 1; Chimeric antigen receptor Tcell therapy within 30 days prior to Day 1 of Cycle 1; Radioimmunoconjugate within 12 weeks prior to Day 1 of Cycle 1; Monoclonal antibody or antibody-drug conjugatewithin 4 weeks prior to Cycle 1 Day 1; Treatment with any anti-cancer agent (investigational or otherwise) within 4 weeks or 5 half-lives of the drug, whicheveris shorter, prior to first dose of study treatment

• Clinically significant toxicity (other than alopecia) from prior treatment that hasnot resolved to Grade </= 1 (per National Cancer Institute Common TerminologyCriteria for Adverse Events (NCI CTCAE), Version 5.0) prior to Day 1 of Cycle 1

• Treatment with systemic immunosuppressive medications, including, but not limited toprednisone (> 20 mg), azathioprine, methotrexate, thalidomide, and anti-tumornecrosis factor agents within 2 weeks prior to Day 1 of Cycle 1

• History of solid organ transplantation

• History of severe allergic or anaphylactic reaction to humanized, chimeric or murinemonoclonal antibodies

• Known sensitivity or allergy to murine products

• Known hypersensitivity to biopharmaceuticals produced in Chinese hamster ovary (CHO)cells or any component of the mosunetuzumab, rituximab, tocilizumab, lenalidomide,or thalidomide formulation, including mannitol

• History of erythema multiforme, Grade >/= 3 rash, or blistering following priortreatment with immunomodulatory derivatives

• History of interstitial lung disease, drug-induced pneumonitis, and autoimmunepneumonitis

• Known active bacterial, viral, fungal, or other infection, or any major episode ofinfection requiring treatment with IV antibiotics within 4 weeks of Day 1 of Cycle 1

• Known or suspected chronic active Epstein-Barr virus (EBV) infection

• Known or suspected history of hemophagocytic lymphohistiocytosis

• Clinically significant history of liver disease, including viral or other hepatitis,or cirrhosis

• Active Hepatitis B infection

• Active Hepatitis C infection

• Known history of HIV positive status

• History of progressive multifocal leukoencephalopathy (PML)

• Administration of a live, attenuated vaccine within 4 weeks before first dose ofstudy treatment or anticipation that such a live attenuated vaccine will be requiredduring the study

• Other malignancy that could affect compliance with the protocol or interpretation ofresults

• Active autoimmune disease requiring treatment

• History of autoimmune disease, including, but not limited to: myocarditis,pneumonitis, myasthenia gravis, myositis, autoimmune hepatitis, systemic lupuserythematosus, rheumatoid arthritis, inflammatory bowel disease, vascular thrombosisassociated with antiphospholipid syndrome, Wegener's granulomatosis, Sjögren'ssyndrome, Guillain-Barré syndrome, multiple sclerosis, vasculitis, orglomerulonephritis

• Prior allogeneic stem cell transplantation

• Contraindication to treatment for thromboembolism prophylaxis

• Evidence of any significant, uncontrolled concomitant disease that could affectcompliance with the protocol or interpretation of results, including, but notlimited to, significant cardiovascular disease (e.g., New York Heart AssociationClass III or IV cardiac disease, myocardial infarction within the previous 6 months,unstable arrhythmia, or unstable angina) or significant pulmonary disease (such asobstructive pulmonary disease or history of bronchospasm)

• Major surgical procedure other than for diagnosis within 28 days prior to Day 1 ofCycle 1 Day 1 or anticipation of a major surgical procedure during the course of thestudy

• Pregnant or lactating or intending to become pregnant during the study

• Any serious medical condition or abnormality in clinical laboratory tests that, inthe investigator's judgment, precludes the patient's safe participation in andcompletion of the study, or which could affect compliance with the protocol orinterpretation of results