Steroid resistant acute Graft versus Host Disease (GvHD) is a very serious complicationassociated with allogeneic haematopoietic stem cell transplantation. There is no established consensus on the therapy for this disease and the prognosis for these patients is poor. Early reports of MSC therapy for acute GvHD involving small numbers of patients were promising, however, results from large scale multi-centre phase III trials have questioned the therapeutic efficacy of MSC therapy for GvHD. Our hypothesis is that MSC require licensing for therapeutic efficacy in GvHD. This project addresses a number of very important and clinically relevant questions; Why are IFN-gamma licensed MSC more efficacious in the treatment of acute GvHD in pre-clinical models? What are the differences between licensed and unlicensed MSC that facilitates the enhanced efficacy? The final element that this project will address is whether or not correlates of MSC therapeutic efficacy can be identified and translated into the clinic. This research is both internationally competitive and innovative in that no correlates have been identified for MSC therapeutic efficacy and the provision of such information would transform the field of MSC therapy for the treatment of GvHD.