• Age ≥ 18 years

• Pathologically confirmed diffuse large B cell lymphoma, transformed indolent lymphoma, primary mediastinal B cell lymphoma, high grade B cell lymphoma, follicular lymphoma grade 3B

• Subjects must have histologically confirmed CD20+ lymphoma and documented in the most recent representative pathology report.

• Presence of CD19 is not required to be confirmed (except if patients have received anti-CD19 therapy in the past). Patients treated with prior anti-CD19 therapy must have confirmation of CD19 expression in a biopsy done after progression on the last CD19 directed therapy.

• At least 2 prior lines of systemic therapy including CART or ASCT (up to 4 prior lines of therapy allowed). Note that bridging therapy prior to ASCT or CART will be counted as a separate line of therapy)

• At least one prior line of systemic therapy for patients ineligible for ASCT/CART or patients unwilling to undergo CAR-T/ASCT for logistic or other reasons (up to 4 prior lines of therapy allowed)

• Have radiologically measurable lymphadenopathy or extranodal involvement.

• Eastern Cooperative Oncology Group performance status (PS) ≤ 2 (ECOG \>2 can be enrolled if PS compromised from lymphoma e.g. spinal cord compression and expected to improve rapidly with therapy)

• Must have adequate organ and marrow status Hemoglobin ≥8 g/dL (red blood cell transfusions are allowed)

‣ Absolute neutrophil count (ANC) ≥1,000/mm\^3 or ≥500/mm\^3 if due to disease involvement in the bone marrow (G-CSF use is allowed).

⁃ Platelet count ≥75,000 cells/mm\^3 or ≥50,000/mm\^3 if due to disease involvement in the bone marrow (platelets transfusions are allowed)

⁃ Estimated Creatinine Clearance (CrCl) ≥40 mL/min (Cockcroft-Gault formula or other institutional standard methods)

⁃ Serum aspartate transaminase (AST) or alanine transaminase (ALT) ≤2.5 x upper limit of normal (ULN)

⁃ Direct bilirubin ≤ 2 x ULN (≤3 if due to Gilbert's syndrome or liver involvement by the lymphoma)

• Negative HIV test at screening, with the following exception: Individuals with a positive HIV test at screening are eligible provided, prior to enrollment, they are stable on antiretroviral therapy for at least 1 year, have a CD4 count ≥ 200/μL, and have an undetectable viral load.

• Signed Informed Consent Form(s)

• Ability to comply with all the study-related procedures, in the investigator's judgement

• Subject is willing to take aspirin prophylaxis (subjects with low or intermediate risk for thromboembolism) or prophylactic anticoagulant (if high risk for a thromboembolic event)

• Contraception requirements

‣ Patient is willing to adhere to pregnancy risk minimization plan associated with lenalidomide treatment.

⁃ For all females of child-bearing potential; a negative serum pregnancy test (beta-hCG) at the Screening Visit and a negative serum or urine pregnancy test at baseline prior to the first dose of study drug.

⁃ Female subjects of childbearing potential must practice at least 2 protocol-specified methods of birth control, that are effective from 30 days prior to treatment through at least 12 months after the last dose of study drug.

⁃ Female subjects of non-childbearing potential do not need to use birth control.

⁃ Female who is not pregnant, breastfeeding, donating eggs (ova, oocytes), or considering

⁃ becoming pregnant during the study or for 12 months after the last dose of study drug.

⁃ If male, and subject is sexually active with female partner(s) of childbearing potential, he must agree, from 30 days prior to treatment initiation through 12 months after the last dose of study drug, to practice the protocol-specified contraception.

⁃ Male who is not considering fathering a child or donating sperm during the study or for 12 months after the last dose of study drug.

• COVID19 eligibility criteria: Patient should have no known active severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. If a patient has signs/symptoms suggestive of SARS-CoV-2 infection or have had recent known exposure to someone with SARS-CoV-2 infection, the subject must have a negative molecular (e.g., PCR) test, or 2 negative antigen test results at least 24 hours apart, to rule out SARS-CoV-2 infection. Note: SARS-CoV-2 diagnostic tests should be applied following local requirements/recommendations.

• Subjects who do not meet SARS-CoV-2 infection eligibility criteria must be screen failed and may only rescreen after they meet the following SARS-CoV-2 infection viral clearance criteria:

‣ No signs/symptoms suggestive of active SARS-CoV-2 infection

⁃ Negative molecular (e.g., PCR) result or 2 negative antigen test results at least 24 hours apart