SUMMARY/ABSTRACT The Mount Sinai BMT CTN Consortium (MSBCC) combines three large blood and marrow transplant (BMT) centers (Mount Sinai, Vanderbilt, and the Mayo Clinic; >950 total annual HCTs including >250 allogeneic HCTs) and >100 immune effector cell therapies. The MSBCC has demonstrated a strong track record of BMT CTN productivity and possesses significant strengths to accomplish BMT CTN strategic goals. In terms of scientific leadership, Dr. Levine will be Steering Committee Chair starting in 2024. The MSBCC has provided extremely strong accrual to BMT CTN clinical trials, with a proven ability to enroll large numbers of under-represented minorities. It possesses extensive expertise in transplant for non- malignant blood diseases (especially sickle cell disease). Graft versus host disease (GVHD) is a high priority for the BMT CTN and the MSBCC benefits from the successful collaborations of the Mount Sinai Acute GVHD International Consortium (MAGIC) which now houses a large natural history database (>6000 patients) that has enabled a highly innovative, biomarker-based approach to this major complication of allogeneic BMT. As shown in our preliminary data, steroid-refractory (SR) acute (a) GVHD remains an unmet medical need despite the fact that it has become less common and less lethal. Nearly half of patients with SR-aGVHD do not respond to ruxolitinib and these patients have a dismal 100 day survival rate of only 20%. Remestemcel-L, a bone marrow derived mesenchymal stromal cell (MSC) product has shown significant efficacy with virtually no toxicity in treating SR-aGVHD in children and adults. Although an earlier MSC product, Prochymal, did not prove effective for adults with SR-aGVHD, its lack of potency was a major contributor to its poor efficacy. The improved product, Ryoncil, passes a potency assay before release; this improved, quality controlled product is now available to be tested in a clinical trial. Our proposal shows that third line treatment with this new, high potency product increases day 100 survival of patients with SR-aGVHD to greater than 60%. We thus propose a clinical trial to improve day 100 survival in patients with acute GVHD resistant to both steroids and a second line of treatment. To accomplish this goal we propose to treat patients in need of a third line of treatment with high potency remestemcel-L (Ryoncil). Our proposal is supported by the manufacturer, Mesoblast. Our specific aims are: (1) To participate vigorously in BMT CTN clinical trials and committees and (2) To treat resistant GVHD with high-potency remestemcel-L to improve survival. RELEVANCE: The Blood and Marrow Transplant Clinical Trials Network conducts clinical trials to improve outcomes for patients facing life-threatening diseases. The Mount Sinai BMT CTN Consortium will advance the mission of the BMT CTN by enrolling patients onto BMT CTN clinical trials and providing scientific leadership to the BMT CTN. The proposed clinical trial will miti...