If you choose to join this study, you will take part in 2 stages: a non-randomized stage and a randomized stage. The non-randomized stage of this study is to test the safety of the study drug (in combination with lenalidomide). Non-randomized means that everyone will have the same regimen and not get a random assignment to different intervention groups. You will be enrolled in either the dose-escalation phase or the expansion phase. Whichever study phase you are enrolled in, you may receive mosunetuzumab plus lenalidomide for approximately 12 months. The study drug can be given either intravenously (through a needle in your vein) or subcutaneously (injection under the skin) on Day 1, Day 8, and Day 15 of C1 and Day 1 of every cycle thereafter. A "cycle" is a 21-day period of use of the study drug. Lenalidomide is taken daily, by mouth, at home for 21 days from cycle 2 onwards. The randomized stage of this study is to test the safety of the study drug given through the vein (IV) compared to the study drug as an injection under the skin (SC injection), each in combination with lenalidomide. If you are enrolled in the randomized stage, you will be assigned by chance to either Group A or Group B:

• Group A will receive IV (through the vein) mosunetuzumab in combination with lenalidomide. Lenalidomide is taken daily, by mouth, at home.
• Group B will receive SC (under the skin) mosunetuzumab in combination with lenalidomide.