Dendritic cells (DC) are bone marrow-derived antigen presenting cells that are crucial for inducing adaptive immune responses and maintaining T cell tolerance. An increasingly precise picture of DC development from bone marrow progenitors is emerging, including the discovery of a DC lineage restricted common DC progenitor (CDP). The molecular mechanisms determining DC commitment of CDP and their further specification into DC subsets remained, however, less clear. This is partly due to the lack of genetic tools and/or culture model systems that would allow the developmental stage-specific analysis of the impact of single factors, like transcription factors and cytokines, on DC development. In this study, I have developed a culture system that reproduces CDP in vitro and addressed the molecular mechanisms behind DC commitment of CDP from multipotent progenitors (MPP). The in vitro amplified CDP recapitulate the function, surface marker and gene expression profile of in vivo CDP, therefore providing a valuable model system for studying DC development. Gene expression analysis by DNA microarrays showed that CDP exhibit a DC-primed transcription profile that includes the upregulation of DC-poietin receptors and DC specific transcription profiles upon differentiation from MPP. Consistent with their capacity to generate both conventional DC (cDC) and plasmacytoid DC (pDC), CDP express pan-DC genes as well as genes specific for either DC subset. Concomitantly with lineage specification, hematopoietic cells cease proliferating and undergo cell cycle arrest. In line with the loss of self-renewal capacity and multilineage potential, CDP differentiation from MPP is accompanied by downregulation of stem/progenitor cell associated gene expression. In addition, this thesis addressed the impact of transforming growth factor-b1 (TGF-b1) on DC differentiation. DNA microarray analysis showed that TGF-b1 pushed the CDP transcription profile towards DC within 24 h, indicating that TGF-b1 accelerates DC differentiation from CDP. This was accompanied with upregulation of genes involved in DC differentiation and/or function and downregulation of proliferation associated genes. Most importantly, TGF-b1 directed DC subset specification towards cDC. TGF-b1 treated CDP acquired a cDC affiliated transcription factor repertoire, with the induction of cDC differentiation instructing transcriptional regulators, including interferon regulatory factor-4 (IRF-4) and several members of the nuclear factor-kB (NF-kB) family. Concurrently, TGF-b1 induced transcription factors that inhibit pDC differentiation, such as inhibitor of DNA binding/differentiation 2 (Id2) and interferon regulatory factor-1 (IRF-1). Taken together, this study describes an in vitro system for studying the consecutive development of MPP towards CDP and their further differentiation into DC subsets. Employing the culture system, the molecular mechanisms instructing DC commitment and subset specification were analysed. For the first time, TGF-b1 was identified as a determining factor that regulates DC subset specification and directs DC differentiation towards cDC
