On the origin of B cells. Elucidating the Role of Tyrosine Kinase Receptors in B Cell Development.

B cells are essential cells that are part of the adaptive immune system with the capacity to produce antibodies against exogenous antigens. These cells are produced through a process called hematopoiesis, tightly regulated by extrinsic and intrinsic factors. Among these, tyrosine kinase receptors and their cytokines have been shown to have an important role in adult lymphopoiesis regulation. However, little is known about their involvement in the ontogeny of the blood system, and specifically, the B lineage that takes place in the embryo. In this thesis, I have aimed to deepen our understanding of the regulation and differentiation of hematopoietic cells in the embryo, both in mouse and human. In the first paper, we have identified a new distinct B cell progenitor in mouse that is uniquely present in a narrow window of fetal liver lymphopoiesis and that possesses B/myeloid potential and gene identity. We have also shown that the tyrosine kinase receptor, CSF1R, marks and regulates this progenitor and that upon loss of CSF1R signaling, embryonic B lymphopoiesis was specifically impaired. These CSF1R+ ProB progenitors are relevant for infant and childhood leukemia in which CSF1R has recently been found to be rearranged.In the second paper, we have demonstrated that the first CD19+ B cell emerging in the human embryo expresses IL7R and seems to derive from a newly identified IL7R+ progenitor. Using single cell gene expression analysis, we have shown that this IL7R+ progenitor shifts from a myeloid to a lympho-myeloid gene signature throughout development. Importantly, we have developed an in vitro modeling system that allowed us to recapitulate this transition and hierarchy using human pluripotent stem cells and used this system to study one of the most commonly found fusion genes, ETV6-RUNX1, in childhood B cell leukemia (B-ALL). Our investigation suggests that the IL7R+ progenitor can be a relevant target cell for in utero B-ALL, initiating pre-leukemic mutation.In the third paper, we have developed a new mouse model that allows a time or stage specific deletion of the tyrosine kinase receptor, FLT3. Previous loss of function studies, using mice deficient either for the receptor or for its ligand, FLT3L, have demonstrated that FLT3 signaling is required for lymphoid and also myeloid development. However, they have not been able to address the specific cellular stage of FLT3 requirement in the hematopoietic differentiation since the receptor was constitutively lost in the entire blood hierarchy and from the first HSC in the embryo. We showed here that FLT3 is required for adult hematopoiesis, and also after lymphoid commitment, while being dispensable after commitment to the B lineage.Overall, the papers included in this thesis highlight the need for studying embryonic hematopoiesis specifically, since regulatory requirements can change during ontogeny. It also shows the importance of developing new tools in order to study the onset of infant and childhood blood disorders that are thought to derive from mutations occurring already in utero