A Study to Evaluate the Safety and Efficacy of Mosunetuzumab (BTCT4465A) in Combination With Polatuzumab Vedotin in B-Cell Non-Hodgkin Lymphoma

Key Inclusion Criteria: * ECOG PS of 0, 1, or 2 * Histologically confirmed FL, DLBCL, or MCL * Must have received at least one prior systemic treatment regimen containing an anti-CD20-directed therapy for DLBCL or FL * For MCL, participants must have received at least two prior systemic treatment regiments, which include 1) anti-CD20-directed therapy, 2) BTK inhibitor, and 3) anthracycline or bendamustine * Relapsed to prior regimen(s) after having a documented history of response (complete response \[CR\], CR unconfirmed \[CRu\], or partial response \[PR\]) of \>/= 6 months in duration from completion of regimen(s); or, refractory to any prior regimen, defined as no response to the prior therapy, or progression within 6 months of completion of the last dose of therapy * Measurable disease, defined as at least one bi-dimensionally measurable nodal lesion, defined as \> 1.5 cm in its longest dimension, or at least one bi-dimensionally measurable extranodal lesion, defined as \> 1.0 cm in its longest dimension * Adequate hematologic, renal, and hepatic function Key Exclusion Criteria: * Prior treatment with mosunetuzumab or other CD20-directed bispecific antibodies * Prior treatment with polatuzumab vedotin * Current \> Grade 1 peripheral neuropathy * Prior use of any monoclonal antibody, radioimmunoconjugate or antibody-drug conjugate (ADC) within 4 weeks before first dose of study treatment * Treatment with any chemotherapeutic agent, or treatment with any other 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 * Treatment with radiotherapy within 2 weeks prior to the first dose of study treatment * Autologous stem-cell transplantation (SCT) within 100 days prior to first study treatment administration * Prior treatment with chimeric antigen receptor T-cell (CAR-T) therapy within 30 days before first study treatment administration * Prior allogeneic SCT * Prior solid organ transplantation * Known or suspected history of hemophagocytic lymphohistiocytosis (HLH) * Patients with history of confirmed progressive multifocal leukoencephalopathy (PML) * Current or past history of central nervous system (CNS) lymphoma or CNS disease * History of autoimmune disease