Key Inclusion Criteria:  Adults ≥ 18 years of age who are willing and able to provide written informed consent Have an ECOG performance status of ≤ 1. Subjects with an ECOG performance status of 2 may be enrolled with Medical Monitor approval. Pathologically confirmed diagnosis of DLBCL, as defined by the 2016 World Health Organization classification including DLBCL NOS high-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangements, Epstein-barr virus (EBV) positive DLBCL, and T cell rich B cell lymphoma (TCRBCL). Subjects with DLBCL transformed from indolent lymphoma (except for Richter's transformation) are eligible. Subjects must have progressive disease following at least two (2) lines of prior systemic therapy for DLBCL; prior treatment must have included an anthracycline and rituximab (or another CD20-targeted agent). Subjects must have failed or be ineligible for ASCT or CAR-T Have at least one bi-dimensionally measurable lesion per Lugano (2014) Willing to provide pre-treatment and on-treatment tumor biopsy tissue. Meet protocol-specified laboratory requirements Life expectancy > 3 months.  Key Exclusion Criteria:  Primary CNS lymphoma or active secondary CNS involvement and/or lymphomatous meningitis Chemotherapy, immunotherapy, major surgery, or investigational agent treatment within 28 days of D0 or 5 half-lives, whichever is shorter Radiotherapy within 14 days of day 0 Autologous stem cell transplant (ASCT) within ˂100 days prior to D0 Chimeric antigen receptor T cell (CAR-T) therapy within ˂28 days prior to D0 Diagnosis of immunodeficiency disorder or history of active autoimmune disease that has required systemic treatment in the past 2 years Uncontrolled significant active infections (controlled Hepatitis B, Hepatitis C, or HIV may be eligible) Prior history of malignancy other than eligible lymphoma sub-types, unless the subject has been free of the disease for ≥ 2 years prior to the start of study treatment