Programming Endothelial Cells to Support Hematopoiesis In Vivo and In Vitro

Abstract

Hematopoietic stem and progenitor cells (HSPCs) reside in specialized microenvironments known as the HSPC niche. Vascular endothelial cells are the most abundant and one of the most significant components of the HSPC niche, providing molecular factors and physical interactions that support the maintenance of the resident HSPCs. However, the detailed cell fate specification of these niche endothelial cells has not been fully elucidated. In this thesis, I present the identification of transcription factor candidates that govern the genetic programs that contribute to the HSPC niche supportive fate of endothelial cells. These transcription factors were identified from differential gene expression analyses of endothelial cell populations isolated from the sinusoidal vasculature in the hematopoietic tissues and in the non-hematopoietic tissues. In an in vivo zebrafish animal model, I overexpressed transcription factor candidates tfec and mafbb using an adult liver sinusoidal endothelial cell-specific enhancer and generated a synthetic extramedullary hematopoietic niche in the adult liver sinusoidal vasculature. These reprogrammed liver sinusoidal endothelial cells express hematopoietic stem and progenitor cell supportive genes and attract the homing of primary HSPCs to the liver in adult zebrafish. Furthermore, in an in vitro human induced pluripotent stem cell (iPSC) derived endothelial cell model, I engineered human iPSC lines with inducible overexpression of transcription factor candidates TFEC and MAFB. These iPSC-derived endothelial cells express cell surface receptors and cytokines that support HSPCs. Upon co-culture with these engineered endothelial cells, primary human HSPCs show superior stem cell colony formation capacities in vitro and improved engraftment potential when transplanted into immunocompromised mouse models in vivo. Our findings revealed a transcription factor code that specifies the HSPC niche supportive fate of endothelial cells and distinguishes endothelial cells in the hematopoietic niche vs. non-niche organs. These results may translate into novel therapies targeting the sinusoidal endothelial cell niches to better stimulate and support HSPCs in vivo and in vitro.