This study is for adults with a type of blood cancer called Diffuse Large B-Cell Lymphoma (DLBCL) that hasn't improved with other treatments. The study tests two drugs, **DPX-Survivac** and **pembrolizumab**, with or without another drug called **cyclophosphamide**. About 102 people will join the study and be randomly divided into two groups. One group takes all three drugs, while the other group takes only DPX-Survivac and pembrolizumab.

Participants will receive DPX-Survivac shots and pembrolizumab through an IV (a way to get medicine directly into your veins). Some will also take cyclophosphamide pills. The study will have cycles of treatment, with breaks in between.

• **Duration:** The study involves several cycles over time, with regular visits.
• **Eligibility:** Participants must have tried other treatments and meet specific health criteria.
• **Risks and Benefits:** As with any study, there are potential risks and benefits which will be explained before joining.