Safety and Efficacy of ULSC on Disease Severity and Steroid Tapering in Participants With Dermatomyositis/ Polymyositis (DM/PM), Also Known as Idiopathic Inflammatory Myopathy (IIM)

Inclusion Criteria:

1. Participants will be ≥18 years old.

2. Diagnosis of idiopathic inflammatory myositis (IIM) based on 2017 EULAR/ACRClassification Criteria for adult IIM, corresponding to a score of ≥ 5.5 (≥ 6.7 withmuscle biopsy).

3. Active disease as defined by any one of the following test results:

4. Elevated Creatine Kinase (CK) or Aldolase (more than 1.5 x the upper limit ofnormal) at screening, OR

5. MRI positive for active, muscle inflammation within 12 weeks prior toscreening, OR

6. EMG read as active myositis within 12 weeks prior to screening, OR

7. muscle biopsy obtained within 12 weeks of the screening showing activeinflammatory disease.

8. Muscle weakness or active cutaneous manifestations of dermatomyositis assessed atScreening and documented with either of the following scores:

9. Bilateral MMT-8 score of ≤142/150, OR

10. CDASI Total Activity score of ≥ 7.

11. Participants must be receiving standard of care treatment with one or moreimmunosuppressants or at least 5 mg prednisone (or corticosteroid equivalent).

12. Immunosuppressive doses should be stable for at least 12 weeks prior toenrollment. Participants must remain on stable immunosuppressive therapy forthe duration of the trial unless discontinuation is warranted due to toxicityor another clinical reason.

13. Hydroxychloroquine (HCQ) doses should be stable for at least 12 weeks prior toenrollment.

14. Steroid doses should be stable for at least 4 weeks prior to enrollment. Atscreening and enrollment, maximum dose allowed is 25 mg/day prednisone (orcorticosteroid equivalent).

15. Allowed immunosuppressants include: methotrexate, azathioprine, mycophenolate,cyclosporine, IVIG, and others to be evaluated at the discretion of theinvestigator.

16. Participants will either be:

17. positive for a myositis-associated antibody, OR

18. will have undergone evaluation to exclude mimics, as deemed appropriate by theInvestigator. Note: The minimum workup to be performed on all prospective participants to excludemimics is as standardly followed, which may include:

• Medical history and physical exam to determine the clinical course andprogression of symptoms, the distribution of weakness, and the absence offeatures of myelopathy, neuropathy, neuromuscular disease, myotonic dystrophy,and congenital myopathy;

• Elevated Creatine Kinase (CK) or Aldolase levels in blood;

• Electromyography (EMG) and/or MRI of clinically affected 99+proximal musclegroup;

• Myositis-specific and myositis-associated autoantibodies in blood;

• Muscle biopsy with characteristic features of IIM and excluding features ofmuscular dystrophy, metabolic myopathies, drug-induce myopathy, inclusion bodymyopathy, and necrotizing myopathy;

• Blood tests for exclusion of HIV, Hepatitis B (HBV), and Hepatitis C (HCV). [Screening tests are done for HIV ELISA, HBs Ag, HBc Ab, and HCV Ab with reflexfor HCV RNA (PCR) for exclusion criteria.]

7. Adequate pulmonary function, defined as saturated oxygen (SpO2 ≥ 94%) on room air.

8. Left ventricular ejection fraction (LVEF) ≥ 30% as assessed by echocardiogram (ECHO)or multigated acquisition (MUGA) scan performed within 8 weeks prior to Screening.

9. Participants must have the ability to comply with the requirements of the study.

10. All participants of reproductive age/capacity will be required to use adequatecontraception, defined as at least one form of a highly effective contraceptive (i.e., condoms, hormonal birth control, IUD), with any partners during the studyperiod and for at least three months beyond the study period, for safety.

11. Participant will have the ability to understand and provide written informedconsent.

Exclusion Criteria:

The presence of any of the following criteria excludes a participant from study enrollment:

1. A diagnosis of inclusion body myositis, juvenile DM or PM, myositis in the contextof significant autoimmune rheumatologic disease.

2. Diagnosis of IIM as part of an overlap syndrome (except overlap with Sjogren'ssyndrome).

3. Initiation of Rituxan (rituximab) treatment within 12 weeks of randomization. Ifparticipant is already on Rituxan, they must remain on a stable dose throughout thetrial.

4. Use of other biologic or investigational drug within 6 half-lives for the agent.

5. Diagnosis of myositis-associated interstitial lung disease or cardiac involvementsufficient to limit participation in the trial in the discretion of the PI.

6. End-stage IIM with irreversible muscle involvement seen on biopsy.

7. Patients with predominant muscle atrophy secondary to uncontrolled or chronic DM orPM, based on clinical, biochemical, and/or radiologic assessment, despite previousoptimized treatment.

8. Non-immune myopathies.

9. Cancer associated with myositis.

10. Hypersensitivity to study product components including history of hypersensitivityto dimethyl sulfoxide (DMSO).

11. Pregnant or lactating participants.

12. Concomitant severe cardiac, pulmonary disease, active infection, or other conditionsthat preclude assessment of safety and efficacy of the study product.

13. Anticipated need for surgery during the trial period.

14. A history of prevalent noncompliance with medical therapy.

15. Recipient of an organ transplant.

16. Neutropenia [absolute neutrophil count <1,800/mm^3 (or <1,000/mm^3 inAfrican-American participants)].

17. Severe impairment in renal function (estimated glomerular filtration rate <30ml/kg*min).

18. Recent or planned use of vaccination with live attenuated viruses.

19. Active cancer or prior diagnosis of cancer within the past 2 years, exceptnon-melanoma skin cancer and carcinoma in situ of cervix Potential participantsdiagnosed with IIM <3 years before screening will have mandatory evaluation forcancer.

20. Participants with current or prior hepatitis B infection that could be at risk forreactivation. [For eligibility, screening test results must be HBsAg (DNA) negativeand anti-HBc (core antibody total) negative).]

21. Participant with HIV or active Hepatitis C. [For eligibility, the followingscreening tests must have negative or non-reactive results: HIV ELISA, HCV Ab withreflex for HCV RNA (PCR); if HCV Ab test is positive, exclude if HCV RNA (PCR) ispositive.]

22. Condition that would impair an assessment of muscle strength, including neurologicaldisorders such as Parkinson's disease or severe musculoskeletal condition.

23. Any other condition that, in the judgment of the Investigator or Sponsor, would be acontraindication to enrollment, study product administration, or follow-up.