Inclusion Criteria:

1. Is willing and able to provide written informed consent/assent and to comply with all protocol procedures and assessments required for the study.
2. Is age ≥12 years and \>40kg at informed consent/assent.
3. Has had an initial allogeneic HSCT for any indication using any graft source, donor source, conditioning regimen intensity or prophylaxis.
4. Has evidence of myeloid engraftment
5. Has a clinical diagnosis of aGVHD Grades III-IV or Grade II with LGI involvement based on Mount Sinai Acute GVHD International Consortium (MAGIC) grading criteria.
6. Began systemic corticosteroid treatment for aGVHD ≤72 hours prior to the start of study drug dosing AND must receive 2 mg/kg/day methylprednisolone or equivalent on Day 1.

Exclusion Criteria:

1. Evidence of morphological relapsed, progressive, persistent, or untreated malignancy, with the exception of nonmelanoma skin cancer and in situ ductal carcinoma of the breast.
2. An unplanned donor lymphocyte infusion for persistent or recurrent malignancy after HSCT.
3. Evidence of persistent molecular disease requiring treatment that was not specified prior to HSCT.
4. Evidence of cGVHD or overlap syndrome
5. Use of immunosuppressants other than corticosteroids for the treatment of aGVHD.
6. Use of any systemic corticosteroids of \>0.5 mg/kg/day methylprednisolone or equivalent for any indication other than aGVHD within 7 days before the onset of aGVHD.