Treatment Of Steroid-Refractory Acute Graft-versus-host Disease With Mesenchymal Stromal Cells Versus Best Available Therapy

Inclusion Criteria: * Participant had a previous allogeneic HSCT as indicated for non-malignant (including inborn errors of metabolism, primary immunodeficiencies, haemoglobinopathies, and bone marrow failure syndromes) or haematological malignant disease, irrespective of human leukocyte antigen match * Participant has been clinically diagnosed with Grade II to IV aGvHD at the Screening Visit * Participant has experienced failure of previous first-line aGvHD treatment (ie, SR-aGvHD), defined as: a) aGvHD progression within 3 to 5 days of therapy onset with \>= 2 mg/kg/day of prednisone equivalent or b) failure to improve within 5 to 7 days of treatment initiation with \>= 2 mg/kg/day of prednisone equivalent or c) incomplete response after \> 28 days of immunosuppressive treatment including at least 5 days with \>= 2 mg/kg/day of prednisone equivalent * Participant has an estimated life expectancy \> 28 days at the Screening Visit * Male or female participant who is \>= 12 years of age at the Screening Visit Exclusion Criteria: * Participant has overt relapse or progression or persistence of the underlying disease at the Screening Visit * Participant has received the last HSCT for a solid tumour disease * Participant has GvHD overlap syndrome at the Screening Visit * Participant has received systemic first line treatment for aGvHD other than steroids and a prophylaxis with other than calcineurin inhibitors, mammalian target of rapamycin (mTOR) inhibitors, anti-thymocyte globulin (ATG), mycophenolate mofetil (MMF), methotrexate (MTX), and / or cyclophosphamide before the Screening Visit * Participant has a known pregnancy (as confirmed by a positive pregnancy test at the Screening Visit) and or is breastfeeding at the Screening Visit * Participant has received treatment with any other investigational agent within 30 days or 5 half-lives (whichever is longer) before the Screening Visit (compliance to be confirmed for the period between the Screening Visit and the Baseline Visit at the Baseline Visit).