• Patient with histologically proven previously untreated CD20+ follicular lymphoma grade 1, 2, or 3a (including patient watched during up to 10 years after initial diagnosis) as assessed by the investigators according to the World Health Organization (WHO) 2016 classification12, or classical follicular lymphoma according to the WHO 2022 classification13. Diagnostic tissue must be available for central pathology review, exploratory endpoints and secondary data use. (Patients with absolute lymphocyte count \> 20 G/L must be discussed with the Sponsor before screening/inclusion).

• FLIPI 2-5.

• All Ann Arbor stages (including stage I if FLIPI ≥ 2).

• Must need treatment as evidenced by at least one of the following criteria:

‣ Bulky disease defined as:

⁃ a nodal or extranodal mass/lesion \> 7 cm in its largest diameter or,

⁃ involvement of at least 3 nodal or extranodal sites (each with a diameter greater than \> 3 cm)

⁃ Presence of at least one of the following B symptoms:

⁃ fever (\> 38°C) of unclear etiology

⁃ night sweats

⁃ weight loss greater than 10% within the prior 6 months

⁃ Symptomatic splenomegaly

⁃ Any compressive syndrome (for example, but not restricted to- ureteral, orbital, gastrointestinal)

⁃ Any one of the following cytopenias due to lymphoma:

⁃ hemoglobin \< 10g/dL (6.25 mmol/L)

⁃ platelets \<100 x 109/L, or

⁃ absolute neutrophil count (ANC) \< 1.5 x 109/L

⁃ Pleural or peritoneal serous effusion (irrespective of cell content)

⁃ β2microglobulin \> ULN or lactate dehydrogenase (LDH) \> ULN

• At least one bi-dimensionally measurable nodal lesion, defined as \> 1.5 cm in its longest dimension, or at least one bi-dimensionally measurable extra nodal lesion, defined as \> 1.0 cm in its longest dimension (and 18F-2-fluoro-2-deoxy-D-glucose (FDG)-avid lesion).

• Patient who understood and voluntarily signed and dated an informed consent prior to any study-specific assessments/procedures.

• Must be ≥ 18 years at the time of signing the informed consent form (ICF).

• Eastern Cooperative Oncology Group (ECOG) performance status 0 to 2.

• Estimated minimum life expectancy of 3 months.

⁃ Adequate hematological function within 28 days prior to signing informed consent, including:

∙ Absolute neutrophil count (ANC) ≥ 1 x 109/L

‣ Platelet count ≥ 75 x 109/L, or ≥ 30 x 109/L if bone marrow infiltration or splenomegaly

‣ Hemoglobin ≥ 8.0 g/dL (5 mmol/L) unless related to bone marrow infiltration or splenomegaly. Transfusion is allowed before starting treatment (no required window)

⁃ Normal laboratory values:

∙ Measured or estimated creatinine clearance ≥ 40 mL/min calculated by institutional standard method (MDRD or Cockcroft-Gault)

‣ aspartate aminotransferase (AST) or alanine aminotransférase (ALT) ≤ 2.5 x the upper limit of normal (ULN), except in patients with documented liver or pancreatic involvement by lymphoma ≤ 5 x ULN

‣ Serum total bilirubin ≤ 1.5 x ULN (or ≤ 3 x ULN for patients with Gilbert syndrome), except in patients with documented liver or pancreatic involvement by lymphoma ≤ 3 x ULN

⁃ left ventricular ejection fraction (LVEF) within normal range (i.e. \> 50% as evaluated by Transthoracic Echocardiography or \> 45% as evaluated by isotopic method (MUGA scan)).

⁃ Patients should be able to receive adequate prophylaxis and/or therapy for thromboembolic events (aspirin, low molecular weight heparin or direct oral anticoagulants).

⁃ Patients with a curative anticoagulation therapy can be enrolled. A patient with deep vein thrombosis due to compressive syndrome is eligible if a curative anticoagulation therapy has been started at least 1 week before initiating study treatment: low molecular weight heparin possible at treatment onset, then direct oral anticoagulants according to local practices.

⁃ Must be able to adhere to the study visit schedule and other protocol requirements.

⁃ Negative SARS-CoV-2 test within 7 days prior to randomization. Rapid antigen test is also acceptable. If a patient has a positive SARS-CoV-2 test before randomization, another test should be done and be negative within 7 days before initiation of treatment.

⁃ Negative HIV test before randomization, with the following exception:

⁃ Patients with a positive HIV test before randomization are eligible provided they are stable on antiretroviral therapy for at least 4 weeks, have a CD4 count ≥ 200/ µL, have an undetectable viral load, and have not had a history of opportunistic infection attributable to AIDS within the last 12 months.

⁃ For women of childbearing potential (WOCBP) (refer to section 14.7): must have a negative result for pregnancy test (highly sensitive serum) at screening and within 7 days before initiation of study treatment, and agree to abstain from breastfeeding during study participation, and for at least 28 days after the final dose of lenalidomide (if applicable), 3 months after the final dose of mosunetuzumab and tocilizumab (if applicable), 6 months after the final dose of chemotherapies (if applicable), 12 months after the final dose of rituximab (if applicable), and 18 months after the final dose of obinutuzumab (if applicable).

⁃ For men (refer to section 14.7): Agreement to remain abstinent (refrain from heterosexual intercourse) or use a condom, and agreement to refrain from donating sperm, as defined below: with a female partner of childbearing potential or pregnant female partner, men must remain abstinent or use a condom during the treatment period (including periods of treatment interruption), and for at least 28 days after the final dose of lenalidomide (if applicable), 2 months after the final dose of tocilizumab (if applicable), 6 months after the final dose of chemotherapies (if applicable), 12 months after the final dose of rituximab (if applicable), and 3 months after the final dose of obinutuzumab (if applicable).

⁃ Patient covered by any social security system (France).

⁃ Patient who understands and speaks one of the country official languages, unless local regulation authorizes independent translators.