Mesenchymal stroma/stem cells: Haematologists’ friend or foe?

Summary Mesenchymal stromal cells (MSCs) are non‐haematopoietic cells found in fetal and adult organs, that play important roles in tissue repair, inflammation and immune modulation. MSCs residing in the bone marrow interact closely with haematopoietic cells and comprise an important component of the microenvironment supporting haematopoiesis, in both health and disease states. Since their identification in 1970, basic scientific and preclinical research efforts have shed light on the role of MSCs in the regulation of haematopoiesis and evoked interest in their clinical application in haematopoietic stem cell transplantation (HSCT) and malignant haematology. Over the last two decades, these research efforts have led to numerous clinical trials, which have established the safety of MSC therapy; however, the optimal mode of administration and the benefit remain inconclusive. In this paper, we will review the clinical experience with use of MSCs in HSCT for enhancement of engraftment, prevention and treatment of graft‐versus‐host disease and haemorrhagic cystitis. Then, we will discuss the contradictory evidence regarding tumour‐promoting versus tumour‐suppressing effects of MSCs in haematological malignancies, which may have relevance for future clinical applications.