Patients suffering from diffuse large B-cell lymphoma (DLBCL) who relapse within 12 months of chemotherapy usually undergo salvage therapies, followed by autologous transplant with a low success rate. These treatments for relapse have significant toxicities and may not be tolerated well by the patients. These patients need an effective means of identifying relapse at an early time point to be treated effectively. Detection of circulating tumor DNA (ctDNA) has been reported to be a sensitive and more specific method to detect relapse at an early stage compared to PET/ CT scans. Purpose of this trial is to monitor patients who have undergone successful chemotherapy for the presence of ctDNA. Patients who test positive for ctDNA would be treated with Nivolumab for a period of 2 years to avoid complete relapse.