Ex vivo expansion of hematopoietic stem cells: mission accomplished?

A small number of hematopoietic stem cells (HSCs) with self-renewal and multi-lineage repopulation capacity maintain hematopoiesis during the lifetime of an individual. Moreover, HSCs and their potential exist in excess as one individual can share its HSCs with another leading to creation of a genetically identical hematopoietic system. For over half a century this property of HSCs has been utilised by successful allogeneic clinical HSC transplantation for treatment of patients with inherited or acquired genetic and neoplastic diseases of the hematopoietic and immune system. There are now more than twenty thousand allogeneic HSC transplants per year worldwide [1]. However, although more than 17.5 million potential HSC donors are registered and additional 500,000 cord bloods are stored for potential allogeneic HSC transplantation [2], timely availability of appropriately human leukocyte antigen (HLA)-compatible HSCs with sufficient quality for patients still poses a problem in the field. Even if a donor is available, toxicity of the procedure could be reduced by increasing HSC numbers in transplants. One way to solve these issues would be by generation of quality-controlled, off the shelf HSC products via in vitro HSC expansion, a "holy grail" procedure many have been hunting for. Here, we discuss accumulating knowledge on signalling pathways involved in HSC maintenance as well as recent achievements to apply the findings to ex vivo HSC expansion for clinical use. Although the specific issue concerns only highly specialised medicine today, newly generated knowledge will be critical for the whole field of stem cell transplantation and regenerative medicine in the future.