Study of Mosunetuzumab Plus Lenalidomide Compared to Anti-CD20 Anti-body + Chemotherapy in Follicular Lymphoma FLIPI2-5

This study is a phase III, randomized, open-label, international, multicenter, interventional trial, designed to compare the efficacy and safety of mosunetuzumab in combination with lenalidomide versus anti-CD20 monoclonal antibody (mAb) plus chemotherapy in patients with previously untreated Follicular Lymphoma International Prognostic Index (FLIPI) 2-5 follicular lymphoma This study is composed of a screening period (up to 6 weeks before randomization, i.e., 42 days), a treatment period (30 months i.e., 125w), a safety follow-up period (90 days i.e., 3 months), and a survival follow-up period (up to 7 years after the last randomized patient). The enrollment will last approximately 34 months. The total duration of the study will be therefore approximately 10 years. Once a patient provides written consent, they may enter the screening phase, with a duration up to 6 weeks prior to randomization and initiation of treatment. Upon completion of the required assessments in the screening phase, and fulfillment of the eligibility criteria, patients will be randomized. Investigators will be requested to indicate their treatment choice among permitted immuno-chemotherapy regimens just before randomization. The treatment period for each patient starts with the first intake. The patients will receive protocol-specified treatments until: * inability to achieve a response at the end of induction phase (at M12 evaluation for experimental arm, and at M6 evaluation for control arms), * relapse or progression of the disease, * withdrawal of consent, * or unacceptable toxicity In the experimental arm, patients will be treated for 1 cycle of 3 weeks for mosunetuzumab and then 11 cycles of 4 weeks for mosunetuzumab and lenalidomide (47 weeks, around 11 months) during the induction phase, and for a maximum of 9 additional cycles of 8 weeks during the maintenance phase (72 weeks, around 17 months), up to around 125 weeks (30 months). Patients should start the maintenance phase 7 to 8 weeks after the start of last induction cycle (C12). In the control arm, patients will be treated for 8 or 6 cycles of 3 or 4 weeks for anti-CD20 mAb +cyclophosphamide-doxorubicine-vincristine-prednisone (CHOP) or anti-CD20 mAb + Bendamustine, respectively, depending on the assigned arm (24 weeks, around 5 months) during the induction phase, and for a maximum of 12 additional cycles of 8 weeks during the maintenance phase (96 weeks, around 22 months), up to around 125 weeks (30 months). Patients should start the maintenance phase, 6 to 7 or 7 to 8 weeks after the start of last induction cycle (C8 or C6). The option to cross-over from the control arm to the experimental arm is not allowed. All randomized patients will be followed for progression-free survival and overall survival using the same schedule. Patients will be followed up from End of treatment evaluation every 3 months during the first two years, then every 6 months during the next 3 years, then yearly until the end of study. The end of study will occur when all randomized patients have been followed-up for survival for at least 7 years (or discontinued study early).