DPX-Survivac and Pembrolizumab With and Without Intermittent Low-Dose Cyclophosphamide, in Subjects With Relapsed/Refractory Diffuse Large B-Cell Lymphoma

This is a Phase 2b, randomized, open label study to assess the safety and efficacy of DPX-Survivac and pembrolizumab, with and without low-dose cyclophosphamide (CPA) in subjects with relapsed or refractory DLBCL. The study will enroll up to 102 subjects. Eligible subjects will be randomized (1:1) to receive: * Arm 1: DPX-Survivac, pembrolizumab and intermittent, low-dose CPA; or, * Arm 2: DPX-Survivac and pembrolizumab All subjects will receive two 0.5 mL doses of DPX-Survivac 3 weeks apart on day 7 (D7) and D28 followed by up to twelve 0.1 mL doses of DPX-Survivac, 8 weeks apart (Q8W). All subjects will receive pembrolizumab intravenously (IV) at a flat dose of 200 mg starting at D7 and on day 1 of each 3-week cycle thereafter (i.e., D28, D49, D70 etc.) (Q3W). For subjects randomized to Arm 1, intermittent oral CPA at a dose of 50 mg twice a day (BID) is administered from D0 to D6 (7 days) followed by 7 days off. This 14-day cycle of "7 days on and 7 days off" will be repeated until the end of study treatment.