Inclusion Criteria:

          -  Patient must have a confirmed diagnosis of:

               -  De novo DLBCL including the clinical subtypes of primary mediastinal, -
                  cell/histiocyte-rich large B-cell lymphoma and intravascular DLBCL OR

               -  De novo transformed DLBCL from follicular lymphoma (FL) OR

               -  Any high grade CD20+ B-cell lymphoma per World Health Organization (WHO) 2016 OR

               -  CD20+ aggressive B-cell lymphoma unclassifiable.

          -  Patient must be deemed an appropriate candidate for R-CHOP therapy.

          -  Patients must be naive to prior therapy for the study diagnosis.

          -  Patient must have advanced stage III/IV early stage disease where provider determines
             single modality therapy with chemo-immunotherapy is most appropriate (i.e radiation
             deferred).

          -  Patient must have measurable disease (defined as >= 1.5 cm in diameter) with
             correlated fludeoxyglucose F-18 (FDG)-avidity on positron emission tomography (PET)
             scan with Deauville score of 4 or 5 at time of diagnosis.

          -  Patient must have Eastern Cooperative Oncology Group (ECOG) performance status =< 2.

          -  Absolute neutrophil count (ANC) >= 1000/mm^3, independent of growth factor support or
             >= 500/mm^3 in cases of ongoing bone marrow involvement (in either case, these must be
             independent of transfusion support) documented =< 28 days prior to registration.

          -  Platelets >= 100,000/mm^3, or >= 50,000 in cases of ongoing bone marrow involvement
             (In either case, these must be independent of transfusion support) documented =< 28
             days prior to registration.

          -  Total bilirubin =< 1.5 x upper limit of normal (ULN) documented =< 28 days prior to
             registration.

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase [SGOT])
             /alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase [SPGT]) =< 3 x
             ULN documented =< 28 days prior to registration.

          -  Creatinine clearance >= 25 mL/min documented =< 28 days prior to registration.

          -  Females of child-bearing potential (FOCBP) and men who are sexually active with FOCBP
             must agree to follow instructions for method(s) of contraception for the duration of
             treatment and the designated post-treatment period.

               -  NOTE: A FOCBP is any woman (regardless of sexual orientation, having undergone a
                  tubal ligation, or remaining celibate by choice) who meets the following
                  criteria:

                    -  Has not undergone a hysterectomy or bilateral oophorectomy.

                    -  Has had menses at any time in the preceding 12 consecutive months (and
                       therefore has not been naturally postmenopausal for > 12 months).

          -  Females of childbearing potential (FOCBP) must have a negative urine or serum
             pregnancy test within 7 days prior to registration on study.

          -  Patients must have the ability to understand and willingness to sign a written
             informed consent prior to registration on study.

        Exclusion Criteria:

          -  Patients who have received prior therapy intended to treat the study diagnosis are not
             eligible.

          -  Patients who have received prior anti-PD-1/L1 therapy for any indication are not
             eligible.

          -  Patients who require urgent cytoreductive therapy (e.g. surgery or radiation) are not
             eligible.

          -  Subjects with known immunodeficiency, known autoimmune disease, or concurrent use of
             immunomodulatory agents are not eligible.

          -  Patients who have a condition requiring systemic treatment with corticosteroids (> 10
             mg daily prednisone equivalents) or other immunosuppressive medications =< 14 days
             prior to registration are not eligible.

               -  NOTE: Inhaled steroids and adrenal replacement steroid doses >10 mg daily
                  prednisone equivalents are permitted in the absence of active autoimmune disease.
                  A brief (less than 3 weeks) course of corticosteroids for prophylaxis (eg,
                  contrast dye allergy) or for treatment of non-autoimmune conditions (eg,
                  delayed-type hypersensitivity reaction caused by a contact allergen) is
                  permitted.

          -  Patients with known central nervous system (CNS) involvement are not eligible.

          -  Patients with an active malignancy requiring therapy such as radiation, chemotherapy,
             or immunotherapy are not eligible.

               -  NOTE: Exceptions to this are as follows: localized non-melanotic skin cancer and
                  any cancer that in the judgment of the investigator has been treated with
                  curative intent and will not interfere with the study treatment plan and response
                  assessment. Prostate and breast cancer patients undergoing hormone therapy with
                  no currently active disease are eligible.

          -  Patients with known human immunodeficiency virus (HIV) are not eligible.

          -  Patients with clinically active hepatitis A, B, or C infections are not eligible.

               -  NOTE: Patients with a history of hepatitis may be eligible if they have a normal
                  titer. Such cases should be approved by the study principal investigator (PI).

          -  Patients who have any life-threatening illness, medical condition, or organ system
             dysfunction which, in the investigator?s opinion, could compromise the subject?s
             safety or put the study outcomes at risk are not eligible.

          -  Pregnant or nursing females are not eligible.

          -  Patients with uncontrolled intercurrent illness including, but not limited to, any of
             the following are not eligible:

               -  Ongoing or active systemic infection.

               -  Symptomatic congestive heart failure.

               -  Myocardial infarction within 6 months prior to registration.

               -  Unstable angina pectoris.

               -  Uncontrolled or symptomatic cardiac arrhythmias.

               -  Any class 3 (moderate) or class 4 (severe) cardiac disease as defined by the New
                  York Heart Association Functional classification.

               -  Psychiatric illness/social situations that would limit compliance with study
                  requirements.