Background: Diffuse large B-cell lymphoma (DLBCL) escapes host immune responses via inhibition of the B2M and CD58 pathways and upregulation of PD-1 ligands. However, treatment results with PD-1 blockade have been disappointing. One potential mechanism is the recruitment of intra-tumoral regulatory T-cells (Tregs) which suppress anti-tumor immunity and inhibit NK cell cytotoxicity. Targeting regulatory T cells with the CCR4 antibody mogamulizumab (KW-0761) represents a molecularly informed strategy to overcome this resistance. Mogamulizumab has been safely administered in combination with pembrolizumab (MK-3475) in patients with solid malignancies and may promote CD8 T-cell dependent effector and NK cell-dependent cytotoxicity in lymphomas. Methods: This is a multi-center NIH-ETCTN phase Ib/randomized phase II study. The phase I will evaluate the safety and tolerability of mogamulizumab in combination with pembrolizumab in patients with R/R DLBCL and determine the recommended phase II dose (RP2D). A traditional 3+3 design with a starting dose of pembrolizumab 200mg IV on day 1 of a 21-day cycle and mogamulizumab 1mg/kg IV on days 1, 8, 15 and then 1.5 mg/kg IV every 21 days. The phase II will evaluate the efficacy of the combination by PFS (primary endpoint) ORR and CR (secondary endpoints). This will be a randomized 1:1 study with allowed crossover, comparing the combination to single agent pembrolizumab Correlative studies will evaluate the association of tumor infiltrating CD8 and NK cells with response to treatment, somatic mutations in B2M and CD58 and with MHC-I expression on DLBCL cells. Functional characterization of circulating immune cells in the peripheral blood and measurement of pro and anti-inflammatory cytokines will be used to assess the levels, activation status and effector function of Tregs and circulating T cells. Inclusion criteria include measurable disease, ≥2 prior lines of therapy including or ineligible for autologous stem cell transplant, ECOG ≤ 2 and normal organ function. Prior or planned allogeneic stem cell transplant, as well as prior treatment with an anti PD-1/PD-L1/CTLA4 antibody, preexisting autoimmune disease or CNS involvement by lymphoma are exclusion criteria. The study aims to enroll up to 12 patients on the phase I and up to 58 patients on the phase II and can be opened at any ETCTN participating site. To date it has been opened at two sites and is accruing the first patients for the phase I portion. Clinical trial information: NCT03309878.