Inclusion Criteria:

* Patients must be eligible to receive standard of care CAR T-cell therapy for relapsed or refractory multiple myeloma
* Patients must be ≥18 years of age
* Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2
* Patients must have 20% or more plasma cells in the bone marrow core or aspirate differential within 30 days prior to enrollment.
* Patients must have adequate organ function as defined below:

* Total bilirubin ≤ 1.5 x institutional upper limit of normal
* AST ≤ 3 x institutional upper limit of normal
* ALT ≤ 3 x institutional upper limit of normal
* Creatinine clearance ≥ 40 mL/min for participants with creatinine levels above institutional normal
* The effects of DC/MM fusion vaccine on the developing human fetus are unknown. For this reason, women and men of child-bearing potential must agree to use adequate contraception (hormonal or barrier methods of birth control or abstinence) prior to study enrollment and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while she or her partner are participating in this study, she should inform her treating physician immediately. Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study, for the duration of study participation, and 6 months after completion of treatment.
* Ability to understand and willingness to sign a written informed consent document.

Exclusion Criteria:

* Patients receiving other investigational drugs
* Patients with Plasma Cell Leukemia

* Patients who have known active uncontrolled infections with human immunodeficiency virus (HIV), hepatitis C virus (HCV) or hepatitis B virus (HBV)
* Myocardial infarction within 6 months prior to enrollment or New York Heart Association (NYHA) Class III or IV heart failure, uncontrolled angina, severe uncontrolled ventricular arrhythmias. Prior to study entry, any ECG abnormality at screening will be documented by the investigator as not medically relevant.
* Female patients who are pregnant (positive β-HCG) or breastfeeding.
* Prior organ transplant requiring immunosuppressive therapy.
* Uncontrolled intercurrent illness including, but not limited to: active infection, symptomatic congestive heart failure, unstable angina pectoris, or psychiatric illness/social situations that would limit compliance with study requirements.
* History of intolerance to CAR-T related drugs or GM-CSF.

Inclusion Criteria Prior to Vaccination with DC/MM Fusions:

* Resolution of all CAR T- related grade 3-4 toxicities
* Successful production of at least 2 vaccines with a minimum of 1 x 106 fusion cells
* Absence of disease progression following CAR T-cell therapy
* ECOG performance status ≤ 2
* Patients must have adequate organ function as defined below:

* Total bilirubin ≤ 1.5 x institutional upper limit of normal
* AST ≤ 3 x institutional upper limit of normal
* ALT ≤ 3 x institutional upper limit of normal
* Creatinine within normal limits or Creatinine clearance ≥ 40 mL/min for participants with creatinine levels above institutional normal
* ANC \>1000 in the absence of growth factor support in the prior 7 days
* Platelet count \>50K without the need for transfusion in the prior 7 days
* No myeloma-directed therapy following administration of CAR T-cells