Dendritic cells (DC) represent key regulators of the immune system, yet their development from hemopoietic precursors is poorly defined. In this study, we describe an in vitro system for amplification of a Flt3+CD11b+ progenitor from mouse bone marrow with specific cytokines. Such progenitor cells develop into both CD11b+ and CD11b− DC, and CD8α+ and CD8α− DC in vivo. Furthermore, with GM-CSF, these progenitors synchronously differentiated into fully functional DC in vitro. This two-step culture system yields homogeneous populations of Flt3+CD11b+ progenitor cells in high numbers and allows monitoring the consecutive steps of DC development in vitro under well-defined conditions. We used phenotypic and functional markers and transcriptional profiling by DNA microarrays to study the Flt3+CD11b+ progenitor and differentiated DC. We report here on an extensive analysis of the surface Ag expression of Flt3+CD11b+ progenitor cells and relate that to surface Ag expression of hemopoietic stem cells. Flt3+CD11b+ progenitors studied exhibit a broad overlap of surface Ags with stem cells and express several stem cell Ags such as Flt3, IL-6R, c-kit/SCF receptor, and CD93/AA4.1, CD133/AC133, and CD49f/integrin α6. Thus, Flt3+CD11b+ progenitors express several stem cell surface Ags and develop into both CD11b+ and CD11b− DC, and CD8α+ and CD8α− DC in vivo, and thus into both of the main conventional DC subtypes.

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Receptor and non-receptor tyrosine kinases constitute a large family of proteins that play a pivotal role in hematopoiesis. Here we conducted a comprehensive survey of tyrosine kinase gene expression in primary erythroid progenitor cells from bone marrow by employing a PCR-based strategy that targets the conserved kinase encoding region. We demonstrate that erythroid progenitor cells express several receptor and nonreceptor tyrosine kinases, like c-kit, Jak1, Ryk, FAK, Syk, Arg, Csk and members of the insulin receptor family. Speci®c changes in the expression pro®le of tyrosine kinases were observed following di erentiation induction. We also report on the identi®cation of a new ligand dependent modulator of erythropoiesis, ®broblast growth factor receptor-4 (FGFR-4). FGFR-4 is e ectively expressed in erythroid progenitors and downregulated when cells di erentiate. Furthermore, the FGFR-4 ligand, basic ®broblast growth factor (bFGF), enhanced erythroid cell proliferation induced by SCF or insulin, and thus modulated both erythroid proliferation and di erentiation in vitro.