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.