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/or Groupe d'Etude des Lymphomes Folliculaires (GELF) criteria
* Received at least one prior systemic lymphoma therapy, which included prior immunotherapy or chemoimmunotherapy
* Availability of a representative tumor specimen and the corresponding pathology report 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 biopsy is required. Cytological or fine-needle aspiration samples are not acceptable. Fresh pretreatment biopsy is preferred. Patients who are unable to undergo biopsy procedures 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 be sent to the Sponsor.
* Adequate hematologic function (unless due to underlying lymphoma, per the investigator)
* Agreement to comply with all local requirements of the lenalidomide risk minimization plan, which includes the global pregnancy prevention program.
* For women of childbearing potential: Agreement to remain abstinent (refrain from heterosexual intercourse) or use 2 adequate methods of contraception, including at least 1 method with a failure rate of \< 1% per year, for at least 28 days prior to Day 1 of Cycle 1, during the treatment period (including periods of treatment interruption), and for at least 28 days after the last dose of lenalidomide, 3 months after the final dose of tocilizumab (if applicable), mosunetuzumab, and 12 months after final dose of rituximab. Women must refrain from donating eggs during this same period.
* For men: Agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures and agreement to refrain from donating sperm, as defined: With female partners of childbearing potential or pregnant female partners, men must remain abstinent or use a condom during the treatment period and for at least 28 days 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. Men must 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 response or 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: Lenalidomide exposure within 12 months prior to Day 1 of Cycle 1; Chimeric antigen receptor T cell 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 conjugate within 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, whichever is shorter, prior to first dose of study treatment
* Clinically significant toxicity (other than alopecia) from prior treatment that has not resolved to Grade \</= 1 (per National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE), Version 5.0) prior to Day 1 of Cycle 1
* Treatment with systemic immunosuppressive medications, including, but not limited to prednisone (\> 20 mg), azathioprine, methotrexate, thalidomide, and anti-tumor necrosis 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 murine monoclonal 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 prior treatment with immunomodulatory derivatives
* History of interstitial lung disease, drug-induced pneumonitis, and autoimmune pneumonitis
* Known active bacterial, viral, fungal, or other infection, or any major episode of infection 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 of study treatment or anticipation that such a live attenuated vaccine will be required during the study
* Other malignancy that could affect compliance with the protocol or interpretation of results
* Active autoimmune disease requiring treatment
* History of autoimmune disease, including, but not limited to: myocarditis, pneumonitis, myasthenia gravis, myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, vascular thrombosis associated with antiphospholipid syndrome, Wegener's granulomatosis, Sjögren's syndrome, Guillain-Barré syndrome, multiple sclerosis, vasculitis, or glomerulonephritis
* Prior allogeneic stem cell transplantation
* Contraindication to treatment for thromboembolism prophylaxis
* Evidence of any significant, uncontrolled concomitant disease that could affect compliance with the protocol or interpretation of results, including, but not limited to, significant cardiovascular disease (e.g., New York Heart Association Class III or IV cardiac disease, myocardial infarction within the previous 6 months, unstable arrhythmia, or unstable angina) or significant pulmonary disease (such as obstructive pulmonary disease or history of bronchospasm)
* Major surgical procedure other than for diagnosis within 28 days prior to Day 1 of Cycle 1 Day 1 or anticipation of a major surgical procedure during the course of the study
* Pregnant or lactating or intending to become pregnant during the study
* Any serious medical condition or abnormality in clinical laboratory tests that, in the investigator's judgment, precludes the patient's safe participation in and completion of the study, or which could affect compliance with the protocol or interpretation of results
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