This clinical trial is focused on studying a condition known as steroid-refractory acute graft-versus-host disease (SR aGvHD), which can occur after a patient receives a stem cell transplant from a donor. This condition happens when the donor’s immune cells attack the recipient’s body. The trial will compare the effectiveness and safety of a new treatment using mesenchymal stromal cells, also known by the code name MC0518, against the best available therapies currently used for this condition. Mesenchymal stromal cells are a type of cell therapy derived from bone marrow that is cultured outside the body before being infused into the patient.

Participants in the study will receive either the new treatment or one of the existing therapies, which may include medications like infliximab, anti-T lymphocyte immunoglobulin (from rabbits), methoxsalen, ruxolitinib, or etanercept. These treatments are administered in different ways, such as through intravenous infusion or oral intake, depending on the specific medication. The purpose of the study is to evaluate how well these treatments work in improving the condition by assessing the overall response rate at specific intervals, such as 28 days after starting the treatment.

The trial will follow participants over a period of time to monitor their response to the treatment, any side effects, and overall health outcomes. The study aims to provide valuable information on the best treatment options for children and adolescents with SR aGvHD, potentially improving their quality of life and long-term health. Participants will be closely monitored by healthcare professionals throughout the study to ensure their safety and well-being.