Safety and Efficacy Study of Adult Human Mesenchymal Stem Cells to Treat Acute Graft Versus Host Disease (GVHD)

Protocol 260 - Participants will be randomized with equal probability to the treatment arms (2 million cells/kilogram (kg) of Prochymal® or 8 million cells/kg of Prochymal®) using a stratified block design. The stratification factor is acute GVHD grade. For the purpose of stratification, the GVHD grades are II and III-IV. Treatment with investigational agent will be administered on study Days 1 and 4. Participants will be followed for safety and efficacy until Day 28 after initiation of treatment with the investigational agent, or until withdrawal or death, whichever occurs first. Protocol 261- This study is designed as a long-term safety follow-up of participants who take part in the preceding clinical study of Prochymal® (Protocol 260) for the treatment of acute GVHD. Participants will be enrolled in Study 261 upon completion of the preceding Study 260.