Trial of Mosunetuzumab (BTCT4465A) as Consolidation Therapy in Participants With Diffuse Large B-Cell Lymphoma Following First-Line Immunochemotherapy and as Monotherapy or in Combination With Polatuzumab Vedotin in Elderly/Unfit Participants With Previously Untreated Diffuse Large B-Cell Lymphoma
Trial of Mosunetuzumab (BTCT4465A) as Consolidation Therapy in Participants With Diffuse Large B-Cell Lymphoma Following First-Line Immunochemotherapy and as Monotherapy or in Combination With Polatuzumab Vedotin in Elderly/Unfit Participants With Previously Untreated Diffuse Large B-Cell Lymphoma
This study will evaluate the safety, pharmacokinetics, and preliminary efficacy of
 mosunetuzumab following first-line diffuse large B-cell lymphoma (DLBCL) immunochemotherapy
 in participants with a best response of stable disease or partial response, or in
 elderly/unfit participants with previously untreated DLBCL, or subcutaneous mosunetuzumab in
 combination with polatuzumab vedotin IV in elderly/unfit participants with previously
 untreated DLBCL.
Inclusion Criteria for All Cohorts

 -  At least one bi-dimensionally measurable nodal lesion, defined as > 1.5 cm in its
 longest dimension, or one bi-dimensionally measurable extranodal lesion, defined as >
 1.0 cm in its longest diameter

 -  Adequate hematologic function

 -  Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2

 Inclusion Criteria Specific to Cohort A

 Participants in Cohort A must also meet the following criteria for study entry:

 -  Histologically confirmed DLBCL according to World Health Organization (WHO) 2016
 expected to express the cluster of differentiation-20 (CD20) antigen

 -  One prior therapy with any systemic anthracycline-based chemoimmunotherapy containing
 regimen for previously untreated DLBCL

 -  Best response of SD or PR to prior systemic chemoimmunotherapy at the end of induction
 treatment in accordance with the Lugano 2014 criteria

 Inclusion Criteria Specific to Cohorts B and C

 Participants in Cohorts B and C must also meet the following criteria for study entry:

 -  Previously untreated, histologically confirmed, DLBCL according to WHO 2016
 classification

 -  Age >/= 80 years, or

 -  Age 60-79 years and considered ineligible for chemoimmuotherapy (R-CHOP) with at least
 one of the following: Impairment in at least one activity of daily living as defined
 in the protocol; impairment in at least one instrumental activity of daily living as
 defined in the protocol; impairment in cardiac function, renal function, liver
 function, or other comorbidities such that the participant is unfit for full-dose
 immunochemotherapy, such as rituximab plus cyclophosphamide, doxorubicin, vincristine,
 and prednisone (R-CHOP)

 Exclusion Criteria for All Cohorts

 Participants who meet any of the following criteria will be excluded from study entry:

 -  Transformed lymphoma

 -  CNS lymphoma

 -  Prior treatment with mosunetuzumab

 -  Prior stem cell transplant (autologous and allogeneic)

 -  History of confirmed progressive multifocal leukoencephalopathy (PML)

 -  Known or suspected chronic active Epstein Barr virus (CAEBV), hepatitis B, hepatitis C
 (HCV), or Human Immunodeficiency Virus (HIV)

 -  History of hemophagocytic lymphohistiocytosis (HLH)

 -  Prior solid organ transplantation

 -  Current or past history of central nervous system (CNS) disease, such as stroke,
 epilepsy, CNS vasculitis, or neurodegenerative disease

 -  Clinically significant history of liver disease

 -  Prior treatment with radiotherapy within 2 weeks prior to Cycle 1, Day 1 (C1D1)

 -  Significant cardiovascular disease

 Exclusion Criteria Specific to Cohort A

 Participants in Cohort A who meet the following criteria will be excluded from study entry:

 - Prior treatment with chemotherapy, immunotherapy, or biologic therapy 4 weeks prior to
 C1D1

 Exclusion Criterion Specific to Cohorts B and C

 Participants in Cohorts B and C who meet the following criterion will be excluded from
 study entry:

 - Prior treatment for DLBCL with chemotherapy, immunotherapy, and biologic therapy

 Exclusion Criteria Specific to Cohort C

 Participants in Cohort C who meet the following criteria will be excluded from study entry:

 - Current Grade >1 peripheral neuropathy by clinical examination or demyelinating form of
 Charcot-Marie-Tooth disease