The Road to Maturity - Lineage Commitment in early Hematopoiesis

The road to maturity – how do hematopoietic stem cells (HSC) differentiate into mature blood cells? The pathways of lineage commitment during normal hematopoiesis are of great significance in order to understand the underlying events that lead to leukemia, and to the design of proper treatments for prevention and remission of the disease. The route of hematopoiesis can be thought of as a hierarchical tree, with the rare HSCs at the top, transitioning down along the pathways as different progenitors. These progenitors continue on their way to become mature blood cells. The roads of blood cell production have been extensively studied in the adult mouse model, whereas less is known about the differentiation of cells during fetal mapping. It is not even known if blood cell commitment follows the same route in adult as in fetal life. The fetal map may be even more important to study since some pre-leukemic events are of prenatal origin. This thesis focuses on the role of different cytokines in lineage commitment, as well as on identifying the first lymphoid committed progenitor in the early fetus. The c-fms like tyrosine kinase 3 receptor (Flt3) is known to be important for lymphopoiesis. However, Flt3 is often found mutated in acute myeloid leukemia (AML), and then associated with poor prognosis. Despite the role in AML, no role for Flt3 or its ligand has been found in myelopoiesis. But more distinct stages of early myeloid progenitors can now be identified, and the role of Flt3 in myelopoiesis could be investigated in detail. We found that early myeloid progenitors express high levels of Flt3, and in mice deficient in Flt3 signaling myeloid progenitors are reduced. Taken together the data clearly show a role of Flt3 signaling in early myelopoiesis, which has implications for understanding the role of Flt3 mutations in AML. In the next study the key cytokines in B lymphopoiesis were investigated. The role of Flt3 signaling together with interleukin 7 (IL7) and a cytokine called Thymic stromal lymphopoietin (TSLP) was studied. TSLP has been suggested to have a key role in IL7 independent B cell development, although direct evidence has been lacking. By using different knockout mice the role of the three signaling pathways was investigated side by side in fetal and adult mice. Mice deficient in all three signaling pathways lacked B cells, as did mice deficient in IL7 and Flt3 signaling. The conclusion is that the IL7 and Flt3 signaling pathways are the main factors driving both fetal and adult B lymphopoiesis. In fetal hematopoiesis, as mentioned previously, the road to maturity is not well understood. We aimed at identifying the first lymphoid commitment step in the early embryo. A population in the fetal liver at 11.5 days post coitus (dpc), expressing the markers Flt3 and IL7 receptor alfa (IL7Rα), was purified. It was shown to have combined lymphoid and granulocyte/macrophage potential but no megakaryocyte or erythroid potential at the single cell level. This population could represent the first lympho-myeloid restricted cells in ontogeny, and further evidence suggests that it might be the first progenitor that seeds the thymus. By using a reporter mouse for recombination activating gene 1, Rag1, (an early sign of lymphoid commitment), a lympho-myeloid restricted population, expressing IL7Rα and Rag1-GFP, could be traced back to 9.5 dpc. Fetal liver colonization has been shown to begin at 9.5 dpc and definitive adult HSCs appear first one day later at 10.5 dpc. This study identifies a lymphomyeloid restricted progenitor in the early embryo, notably arising prior to the establishment of definitive HSCs, and suggests that lymphoid commitment might take place outside the fetal liver niche