The link between blood cells and the vasculature is rooted in early development. Endothelial cells, lining the interior surface of the entire circulatory system are responsible for the origin of all definitive blood cells. In the adult, such functional interdependency continues. Endothelial cells regulate hematopoietic cell differentiation within the bone marrow niche. They also coordinate homing of inflammatory cells, facilitate immune-surveillance and modulate several functional aspects of hematopoietic cell interaction with tissues under homeostatic and pathologic conditions. The aim of this dissertation is to extend our knowledge of the crosstalk between endothelial cells and hematopoietic cells, especially in regards to macrophages.
First, we investigated the ability of endothelial cells to function as a niche for hematopoietic cell expansion. We showed that endothelial cells derived from various organs of adult mice could support long-term growth of hematopoietic cells. Interestingly, we found that the promotion of hematopoietic cell proliferation was regulated in an organ-specific manner, meaning endothelial cells from different origins supported hematopoietic stem cells at different levels. We further explored the role of AML1, an essential transcription factor in definitive hematopoiesis. We found that AML1 in the endothelium was able to enhance and further support hematopoietic cell proliferation. Finally, we examined the effect of cytokines on hematopoietic cell differentiation and found that IL-3 preferentially promotes myelopoiesis.
Furthermore, we explored the effect of endothelial cells on hematopoietic stem cell differentiation, especially as it relates to macrophages. We demonstrated that endothelial cells provide an instructive niche for M2- macrophage polarization. Intriguingly, endothelial cells induced the formation of macrophage colonies that exhibited an organizational structure with less differentiated cells in the center of the colony and M2-polarized macrophages on the periphery. Also, the initiation and maintenance of the colony was exclusively dependent on direct contact with ECs and the presence of M-CSF. Moreover, we tested the function of these M2- macrophages in vivo and showed that they could promote tumor growth and angiogenesis.
Finally, we studied the association between EC and macrophages in vivo. Using confocal microscopy we found that resident macrophages are in frequent contact with vessels in the mesentery. In fact, the percentage of macrophages associated with vessels is similar to the percentage of pericytes. Through intravital imaging, we discovered that macrophages regulate vascular permeability. Depletion of macrophages from the peritoneum resulted in an elevation vascular permeability. Furthermore, reconstitution of M2 macrophages in vivo was able to rescue the effect. Notably, macrophage motility was also altered upon induction of vascular permeability.
Collectively, this dissertation has increased our understanding of the crosstalk between endothelial and hematopoietic cells. Hopefully, this information will be further expanded by future investigators with the goal to enhance our ability to treat and ameliorate blood-born pathologies.
