MESENCHYMAL STROMAL CELLS (MSC) have shown promising albeit not always consistent therapeutic effects in the treatment of severe steroid-resistant acute Graf versus Host Disease. Remarkably, in all reported clinical studies the toxicity of Mesenchymal stromal cells administration has been found consistently negligible. The investigators believe that Umbilical Cord (UC) derived Mesenchymal stromal cells may represent a stronger immunosuppressive tool for such clinical emergency and no data suggest any change in the safety profile of these cells. For this reason, and in the best interest of the patient, the investigators plan to test the safety and activity of Umbilical Cord Mesenchymal stromal cells when given sequentially to another partially effective treatment of steroid resistant acute graf versus host disease such as Pentostatin.