A Phase 1b, Open Label, Global, Multicenter, Dose Determination, Randomized Dose Expansion Study to Determine the Maximum Tolerated Dose, Assess the Safety and Tolerability, Pharmacokinetics and Preliminary Efficacy of Iberdomide (CC-220) in Combination With R-CHOP-21 and CC-99282 in Combination With R-CHOP-21 for Subjects With Previously Untreated Aggressive B-cell Lymphoma

Inclusion Criteria: * Participants must satisfy the following criteria to be enrolled in the study: 1. Is ≥ 18 years of age at the time of signing the informed consent form (ICF). 2. Participant has histologically confirmed (per local evaluation) diagnosis of de novo, previously untreated, a-BCL according to 2016 WHO classification. 3. Participant has International Prognostic Index (IPI) score 0-5 in Part 1 and IPI 2-5 in Part 2. For the CELMoD and polatuzumab-R-CHP cohort, the subject must also have IPI score 0 to 5 in Part 2A and IPI 2 to 5 in Part 2B. 4. Participants must have measurable disease defined by at least one FDG-avid lesion for FDG-avid subtype and one bi-dimensionally measurable (\> 1.5 cm in longest diameter) disease by computed tomography (CT) or magnetic resonance imaging (MRI), as defined by the Lugano classification (Cheson, 2014). 5. Participant has an Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2. 6. Participants must have the following laboratory values: 1. Absolute neutrophil count (ANC) ≥ 1.5 x 109/L or ≥ 1.0 x 109/L in case of documented bone marrow involvement (\> 50% or tumor cells), without growth factor support for 7 days (14 days if peg-G-CSF) 2. Hemoglobin (Hb) ≥ 8 g/dL 3. Platelets (PLT) ≥ 75 x 109/L or ≥ 50 x 109/L in case of documented bone marrow involvement (\>50% or tumor cells), without transfusion for 7 days 4. Aspartate aminotransferase / serum glutamic oxaloacetic transaminase (AST/SGOT) and alanine aminotransferase / serum glutamate pyruvic transaminase (ALT/SGPT) ≤ 2.5 x upper limit of normal (ULN). In the case of documented liver involvement by lymphoma, ALT/SGPT and AST/SGOT must be ≤ 5.0 x ULN. 5. Serum total bilirubin ≤ 2.0 mg/dL except in cases of Gilbert's syndrome, then ≤ 5.0 mg/dl. Subjects receiving polatuzumab vedotin must have serum total bilirubin \< 1.5 × ULN (26 μmol/L) (corresponding to mild degree as per National Cancer Institute Organ Dysfunction Working Group \[NCI ODWG\] criteria) except in cases of Gilbert's syndrome, then ≤ 3.0 mg/dl (51 μmol/L). 6. Estimated serum creatinine clearance (CrCl) of ≥ 50 mL/min using the modification of diet in renal disease (MDRD) formula. 7. All participants must: 1. Have an understanding that the study drug could have a potential teratogenic risk. 2. Agree to follow all requirements defined in the Pregnancy Prevention Program for CC-220 or CC-99282 Pregnancy Prevention Plan for Participants in Clinical trials. 8. Females of childbearing potential (FCBP) must: a. Have two negative pregnancy tests as verified by the investigator prior to starting study therapy. 9. Male participants must: 1. Practice true abstinence or agree to use a condom during sexual contact with a pregnant female or a female of childbearing potential while participating in the study. Exclusion Criteria: * The presence of any of the following will exclude a participant from enrollment: 1. Any significant medical condition, active infection (including SARS-CoV-2 suspected or confirmed), laboratory abnormality, or psychiatric illness that would prevent the participant from participating in the study. 2. Any condition including the presence of laboratory abnormalities, which places the participant at unacceptable risk if he/she were to participate in the study. 3. Any other subtype of lymphoma. 4. Documented or suspected CNS involvement by lymphoma. 5. Persistent diarrhea or malabsorption ≥ Grade 2 (NCI CTCAE v5.0), despite medical management. 6. Peripheral neuropathy ≥ Grade 2 (NCI CTCAE v5.0). 7. Subjects with a history of progressive multifocal leukoencephalopathy. 8. Chronic systemic immunosuppressive therapy or corticosteroids 9. Impaired cardiac function or clinically significant cardiac disease, including any of the following: a. Left ventricular ejection fraction (LVEF) \< 45% as determined by multigated acquisition (MUGA) scan or echocardiogram (ECHO) 10. Major surgery ≤ 2 weeks prior to starting CC-220 or CC-99282; participant must have recovered from any clinically significant effects of recent surgery. 11. Any condition causing inability to swallow tablets. 12. Known seropositivity for or active viral infection with human immunodeficiency virus (HIV) 13. Known chronic active hepatitis B (hepatitis B virus surface antigen \[HBsAg\] positive and/or hepatitis B core antibody \[anti-HBc\] positive with viral DNA positive) or C (positive serology requiring treatment and/or with evidence of liver damage) infection 14. History of other malignancy, unless being free of the disease for ≥ 3 years; exceptions to the ≥ 3-year time limit include history of the following: 1. Localized nonmelanoma skin cancer 2. Carcinoma in situ of the cervix 3. Carcinoma in situ of the breast 4. Incidental histologic finding of prostate cancer (T1a or T1b as per Tumor Node Metastasis \[TNM\] staging system) or prostate cancer that has been treated with curative intent. 15. Hypersensitivity to the active substance or to murine proteins, or to any of the other excipients of rituximab or polatuzumab vedotin. 16. Known hypersensitivity to any component of CHOP/CHP regimen. 17. Known allergy to thalidomide, pomalidomide, or lenalidomide.