This study will investigate Umbilical Cord Lining Stem Cells (ULSC) as an investigational medicinal product and its use in patients with polymyositis (PM) or dermatomyositis (DM) to see if a single intravenous (IV) infusion of allogeneic umbilical cord lining stem cells (ULSC) safe, tolerable, and feasible to administer.

Inclusion Criteria:

1. Adult, male or female, age ≥18 years old
2. Diagnosis of definite or probable DM or PM, according to the criteria of Bohan and Peter
3. Patients with PM will either be positive for a myositis-associated antibody or have undergone evaluation to exclude mimics, as deemed appropriate by the Investigator (See Note below).
4. Signs informed consent.

Exclusion Criteria:

1. A diagnosis of inclusion body myositis, juvenile DM or PM, myositis in the context of significant overlap with another systemic autoimmune rheumatologic disease.
2. Non immune myopathies.
3. Cancer associated myositis.
4. Hypersensitivity to study product components. History of hypersensitivity to dimethyl sulfoxide (DMSO).
5. Pregnant or lactating women.
6. Concomitant severe cardiac, pulmonary disease, active infection or other conditions that preclude assessment of safety and efficacy of the study product.
7. Patients with predominant muscle atrophy secondary to uncontrolled or chronic DM or PM, based on clinical, biochemical, and/or radiologic assessment, despite previous optimized treatment.
8. Anticipated need for surgery during the trial period.
9. A history of prevalent noncompliance with medical therapy.
10. Recipient of an organ transplant.
11. Neutropenia (absolute neutrophil count<1,800/mm3 [or <1,000/mm3 in African-American subjects]).
12. Severe impairment in renal function (estimated glomerular filtration rate <30 ml/kg*min).
13. Recent or planned use of vaccination with live attenuated viruses.
14. Active cancer or prior diagnosis of cancer within the past 2 years (patients with basal and squamous cell cancer of skin will not be excluded).
15. Condition that would impair an assessment of muscle strength, including neurological disorders such as Parkinson's disease or severe musculoskeletal condition.
16. Any other condition that, in the judgment of the Investigator or Sponsor, would be a contraindication to enrollment, study product administration, or follow-up.
17. History of Atrial septal defect or ventricular septal defect