Evaluation of creatinine clearance 9. Vital Signs (including ambulatory O2 sat) per Section 11 5.11.4 Survival Follow-Up Participants will be followed in clinic or by phone every 6 months for 5 years from the last dose of treatment. 5.12 Agent Administration 5.12.1 Nivolumab Patients will be treated with Nivolumab 240 mg on days 1 and 15 of a 28-day cycle. Nivolumab will be given every two weeks (±3 days). Patients may be dosed no less than 12 days from the previous dose of drug. Nivolumab is to be administered as a 60-minute IV infusion (+/- 10 minutes), using a volumetric pump with a 0.2/1.2 micron in-line filter at the protocol-specified dose. The drug can be diluted with 0.9% normal saline for delivery but the total drug concentration of the solution cannot be below 1.0 mg/mL. It is not to be administered as an IV push or bolus injection. Vital signs (BP, HR, Temp, RR, O2 sat) will be taken within 15 minutes prior to start of nivolumab infusion and within 15 minutes following completion of the infusion. 5.12.2 DC/MM Fusion Vaccine Patients will receive the DC/myeloma fusion vaccine/GM-CSF (vaccine) on day 1 of cycles 1, 2, and 3. Vaccine will be administered by subcutaneous injection with 100 ug GM-CSF given subcutaneously at the vaccine site on day of vaccination and daily for a total of 4 days of each cycle. The vaccine will be administered at least 1 hour following DF/HCC Protocol #: 18-280 Protocol Version Date: 10.17.2020 19 Nivolumab infusion. On the first day of GM-CSF administration, 100 ug of GM-CSF subcutaneously will be administered subcutaneously. The patient will be trained to inject the remaining three GM-CSF injections (100ug dose once a day at the vaccination site) for self-administration subcutaneously at home. 5.13 General Concomitant Medication and Supportive Care Guidelines Patients started on systemic anti-neoplastic chemotherapy or immune suppression while on treatment will be taken off the study drug. The administration of live vaccinations while on treatment is prohibited. Short courses of corticosteroids for adverse events may be permitted after discussion with the Study Chair, Jacalyn Rosenblatt, MD. 5.14 Criteria for Taking a Participant Off Treatment Duration of therapy will depend on individual response, evidence of disease progression and tolerance. In the absence of treatment delays due to adverse event(s), treatment may continue for up to 6 cycles of nivolumab or until one of the following criteria applies:  Disease progression by IMWG criteria, after the administration of study treatment  Intercurrent illness that prevents further administration of treatment  Unacceptable adverse event(s)  Participant demonstrates an inability or unwillingness to comply with the oral medication regimen and/or documentation requirements  Participant decides to withdraw from the protocol therapy  General or specific changes in the participant's condition render the participant unacceptable for further treatment in the judgment of the treating investigator Participants will be removed from the protocol therapy when any of these criteria apply. The reason for removal from protocol therapy, and the date the participant was removed, must be documented in the case report form (CRF).