Inclusion Criteria:

* Diagnosis of multiple myeloma with measurable disease by IMWG criteria.
* Eligible for standard of care BCMA CAR-T cell therapy.
* Life expectancy ≥ 12 weeks per assessment from the enrolling physician.
* At least 18 years of age.
* ECOG performance status ≤ 2
* Adequate organ function as defined below:

* Total bilirubin ≤ 1.5 x IULN
* AST(SGOT)/ALT(SGPT) ≤ 3.0 x IULN
* Creatinine clearance \> 30 mL/min by Cockcroft-Gault
* The effects of NT-I7 on the developing human fetus are unknown. For this reason, women of childbearing potential and men must agree to use adequate contraception prior to study entry until 90 days after completion of NT-I7 therapy/placebo (corresponding to Day 125 post CAR-T). Should a woman become pregnant or suspect she is pregnant while participating in this study or should a man suspect he has fathered a child, s/he must inform her treating physician immediately.
* Ability to understand and willingness to sign an IRB approved written informed consent document. Legally authorized representatives may sign and give informed consent on behalf of study participants.

Exclusion Criteria:

* Received prior BCMA-directed therapy.
* Prior or concurrent malignancy whose natural history has the potential to interfere with the safety or efficacy assessment of the investigational regimen. Patients with prior or concurrent malignancy that does NOT meet that definition are eligible for this trial.
* Currently receiving or have received any other investigational agents within 14 days prior to CAR-T infusion.
* A history of allergic reactions attributed to compounds of similar chemical or biologic composition to NT-I7or other agents used in the study.
* Uncontrolled intercurrent illness including but not limited to: ongoing or active infection (bacterial, fungal, viral, or tuberculosis, including known hepatitis A, B, or C, or HIV (testing not required)), symptomatic congestive heart failure, unstable angina pectoris, or cardiac arrhythmia (except well-controlled atrial fibrillation). Patients with a known history or current symptoms of cardiac disease, or history of treatment with cardiotoxic agents, should have a clinical risk assessment of cardiac function using the New York Heart Association Function Classification; to be eligible for this trial, patients should be a class 2B or better.
* Pregnant and/or breastfeeding. Women of childbearing potential must have a negative serum pregnancy test within 14 days prior to starting CAR-T therapy.
* Receipt of live, attenuated vaccine within 30 days prior to first day of treatment.
* Had an allogeneic tissue/solid organ transplant or allogeneic stem cell transplant.
* Not able to receive subcutaneous therapy.
* Prior history of T cell malignancy.
* Prior history of congenital immunodeficiency syndrome.
* Prior history of autoimmune disease with significant disease activity in the past 2 years, including but not limited to systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, Sézary syndrome, vasculitis or glomerulonephritis, Bell's palsy, Guillain-Barré syndrome, or multiple sclerosis.
* Prior history of plasma cell leukemia, systemic amyloidosis, POEMS syndrome, or multiple myeloma with CNS involvement.
* Planning to start maintenance therapy prior to Day 100 post-CAR-T therapy.
* A history of clinically significant pulmonary disorders, such as severe asthma, severe COPD, restrictive lung disease, symptomatic pulmonary embolism within 3 months prior to study enrollment, or active or prior interstitial lung disease/pneumonitis.