1 Stem cells: characterization and measurement

The process of blood formation is sustained throughout an individual's life by a small population of haemopoietic stem cells (HSCs). The HSC compartment represents a hierarchy of HSC subsets with decreasing proliferative ability. This heterogeneity is reflected in the varying time periods that HSCs may contribute to the initiation and maintenance of donor-type haemopoietic multilineage chimerism in vivo. The phenotype of HSC is incompletely defined rendering morphological or flow cytometric quantitation unreliable. Functional HSC assays, both in vitro (CAFC, LTC-IC) and in vivo (repopulation of NOD/SCID mice) may be superior to phenotypic analysis; however, such assays have not been truly validated in a human transplant setting.

The quiescence and proliferation of HSCs is highly regulated by the stroma in haemopoietic organs. Many of the cytokines that have been cloned in recent years are actually elaborated and presented by the haemopoietic organ stroma and are supposed to serve as local regulators in order to gain specificity and avoid pleitropic and thus undesired side effects. Most probably, additional stroma-derived factors will be characterized as suggested by the observation that HSCs produce more progeny in stroma-contact than in its absence or in stroma-conditioned medium, irrespectively of the exogenous cytokines included.

Stem cells are considered to possess the ability to self-renew and are therefore attractive vehicles for gene therapy. The same assumed characteristic fuels attempts to amplify their numbers ex vivo, and is expected to enable more rapid haemopoietic recovery of conditioned recipients as well as enlarge HSC grafts of insufficient size before actual transplantation.