DPX-Survivac and Checkpoint Inhibitor in DLBCL

This is a Phase 2, non-randomized, open-label, uncontrolled, efficacy and safety trial. Participants will receive 2 priming injections (0.5ml) of DPX-Survivac 3 weeks apart on Study Days 7 and 28. In addition, up to 6 maintenance injections (0.1ml) over the course of the study occurring on Study Days 84, 140, 196, 252, 308, and 364. All injections will be given under the skin of the upper thigh. Participants will receive metronomic oral cyclophosphamide (50mg BID; 7 days on / 7 days off) for study period. Pembrolizumab 200mg will be administered intravenously every 3 weeks, commencing on study day 7, to a total of 18 infusions. If a participant is removed from the trial prior to the completion of at least 4 doses of Pembrolizumab and 3 injections of DPX-Survivac, that particiapnt may be replaced to determine the efficacy of treatment in a minimum of 16 participants. DPX-Survivac injection sites will be evaluated throughout the study and if evidence of significant reaction, an Injection site reaction biopsy will be sought. During the course of the study, blood will be drawn to evaluate immune cells and the effect that DPX Survivac will have on the participants immune system. During all treatment cycles a physical exam and questions about the participants general health will be performed. Participants will undergo "re-staging" to assess the status of their disease at approximately study day 70 (if there is evidence of Grade 2 or greater injection site reaction or ulceration evident on study day 49) or routinely at approximately study day 91, and again at end of study or study withdrawal for all participants. A follow-up tumour biopsy will be requested between study day 77-83 for participants with any grade 2 or greater Injection site reaction or ulceration on SD49 or between SD98 and SD104 if no evidence of injection site reaction or ulceration. Upon completion of study, participants will be monitored every 2 months for 1 year.