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We have developed a coculture system that establishes DLK[superscript +] fetal hepatic progenitors as the authentic supportive cells for expansion of hematopoietic stem (HSCs) and progenitor cells. In 1-week cultures supplemented with serum and supportive cytokines, both cocultured DLK[superscript +] fetal hepatic progenitors and their conditioned medium supported rapid expansion of hematopoietic progenitors and a small increase in HSC numbers. In 2- and 3-week cultures DLK[superscript +] cells, but not their conditioned medium, continuously and significantly (>20-fold) expanded both hematopoietic stem and progenitor cells. Physical contact between HSCs and DLK[superscript +] cells was crucial to maintaining this long-term expansion. Similar HSC expansion (approximately sevenfold) was achieved in cocultures using a serum-free, low cytokine- containing medium. In contrast, DLK[superscript -] cells are incapable of expanding hematopoietic cells, demonstrating that hepatic progenitors are the principle supportive cells for HSC expansion in the fetal liver., National Institutes of Health (U.S.) (Grant P01 HL 32262), National Institutes of Health (U.S.) (Grant DK067356)

A recent Science paper reported a purine derivative that expands human cord blood hematopoietic stem cells in culture ( Boitano et al., 2010) by antagonizing the aryl hydrocarbon receptor. Major problems need to be overcome before ex vivo HSC expansion can be used clinically.

This article reviews the regulation of production of red blood cells at several levels: (1) the ability of erythropoietin and adhesion to a fibronectin matrix to stimulate the rapid production of red cells by inducing terminal proliferation and differentiation of committed erythroid CFU-E progenitors; (2) the regulated expansion of the pool of earlier BFU-E erythroid progenitors by glucocorticoids and other factors that occurs during chronic anemia or inflammation; and (3) the expansion of thehematopoietic cell pool to produce more progenitors of all hematopoietic lineages., National Institutes of Health (U.S.) (grant P01 HL 32262), National Institutes of Health (U.S.) (grant DK06735), Amgen Inc., Swedish Research Council, Maja och Hjalmar Leanders Stiftelse, National Institutes of Health (U.S.). Ruth L. Kirschstein National Research Service Award, Sweden-America Foundation

Previously we showed that the ~2% of fetal liver cells reactive with an anti-CD3ε monoclonal antibody support ex vivo expansion of both fetal liver and bone marrow hematopoietic stem cells (HSCs); these cells express two proteins important for HSC ex vivo expansion, IGF2, and angiopoietin-like 3. Here we show that these cells do not express any CD3 protein and are not T cells; rather, we purified these HSC-supportive stromal cells based on the surface phenotype of SCF+DLK+. Competitive repopulating experiments show that SCF+DLK+ cells support the maintenance of HSCs in ex vivo culture. These are the principal fetal liver cells that express not only angiopoietin-like 3 and IGF2, but also SCF and thrombopoietin, two other growth factors important for HSC expansion. They are also the principal fetal liver cells that express CXCL12, a factor required for HSC homing, and also α-fetoprotein (AFP), indicating that they are fetal hepatic stem or progenitor cells. Immunocytochemistry shows that >93% of the SCF+ cells express DLK and Angptl3, and a portion of SCF+ cells also expresses CXCL12. Thus SCF+DLK+ cells are a highly homogenous population that express a complete set of factors for HSC expansion and are likely the primary stromal cells that support HSC expansion in the fetal liver., National Institutes of Health (U.S.) (NIH) Grant DK067356), National Institutes of Health (U.S.) (NIH National Research Service Award Fellowship)