Clay Smith

Summary. We have developed an approach for identifying primitive mobilized peripheral blood cells (PBSC) that express high levels of aldehyde dehydrogenase (ALDH). PBSC were stained with a fluorescent ALDH substrate, termed BODIPY™‐aminoacetaldehyde (BAAA), and then analysed using flow cytometry. A population of cells with a low side scatter (SSC) and a high level of BAAA staining, termed the SSC^loALDH^br population, was readily discriminated and comprised a mean of 3 ± 5% of leukapheresis samples. A mean of 73 ± 11% of the SSC^loALDH^br population expressed CD34 and 56 ± 25% of all the mobilized CD34⁺ cells resided within the SSC^loALDH^br population. The SSC^loALDH^br population was largely depleted of cells with mature phenotypes and enriched for cells with immature phenotypes. Sorted SSC^loALDH^br and SSC^loALDH^br CD34⁺ PBSC were enriched for progenitors with the ability to (1) generate colony‐forming units (CFU) and long‐term culture (LTC)‐derived CFU, (2) expand in primary and secondary LTC, and (3) generate multiple cell lineages. In 21 cancer patients who had undergone autologous PBSC transplantation, the number of infused SSC^loALDH^br cells/kg highly correlated with the time to neutrophil and platelet engraftment (P < 0·015 and P < 0·003 respectively). In summary, peripheral blood SSC^loALDH^br cells have the phenotypic and functional properties of primitive haematopoietic cells and their number correlates with engraftment following autologous transplantation.

Background: The highly orchestrated process of blood cell development and homeostasis is termed “hematopoiesis.” Understanding the biology of hematopoietic stem cells as well as hematopoiesis is important to developing improved treatments for hematologic malignancies, congenital disorders, chemotherapy-related cytopenias, and blood and marrow transplants. Methods: The author reviews the current state of the art regarding hematopoietic stem cells and hematopoiesis. Results: Several new concepts, including stem cell plasticity, suggest the possibility that stem cells may have the ability to differentiate into other tissues in addition to blood cells. Conclusions: While much is known about hematopoietic stem cells and hematopoiesis, much remains to be clarified about the environmental and genetic processes that govern the growth and development of the blood system. In addition, careful studies remain to be conducted to determine whether hematopoietic stem cells can differentiate into extra-hematopoietic tissues.

A novel Hoechst 33342 dye efflux assay was recently developed that identifies a population of hematopoietic cells termed side population (SP) cells. In the bone marrow of multiple species, including mice and primates, the SP is composed primarily of CD34⁻cells, yet has many of the functional properties of hematopoietic stem cells (HSCs). This report characterizes SP cells from human umbilical cord blood (UCB). The SP in unfractionated UCB was enriched for CD34⁺ cells but also contained a large population of CD34⁻ cells, many of which were mature lymphocytes. SP cells isolated from UCB that had been depleted of lineage-committed cells (Lin⁻ UCB) contained CD34⁺ and CD34⁻ cells in approximately equivalent proportions. Similar to previous descriptions of human HSCs, the CD34⁺Lin⁻ SP cells were CD38^dimHLA-DR^dimThy-1^dimCD45RA⁻CD71⁻and were enriched for myelo-erythroid precursors. In contrast, the CD34⁻Lin⁻ SP cells were CD38⁻HLA-DR⁻Thy-1⁻CD71⁻and failed to generate myelo-erythroid progeny in vitro. The majority of these cells were CD7⁺CD11b⁺CD45RA⁺, as might be expected of early lymphoid cells, but did not express other lymphoid markers. The CD7⁺CD34⁻Lin⁻ UCB SP cells did not proliferate in simple suspension cultures but did differentiate into natural killer cells when cultured on stroma with various cytokines. In conclusion, the human Lin⁻ UCB SP contains both CD34⁺ multipotential stem cells and a novel CD7⁺CD34⁻Lin⁻ lymphoid progenitor. This observation adds to the growing body of evidence that CD34⁻ progenitors exist in humans.