Modeling hematopoietic stem cell dynamics in bioengineered niches

Bone marrow transplantation is a well-established medical procedure for the treatment of various hematologic diseases. However, the relatively low number of hematopoietic stem cells (HSCs) that can be harvested, especially from umbilical cord blood, limits even broader applicability of the procedure. A better understanding of the biology of HSCs, and in particular how these rare cells are regulated by microenvironmental niches in the bone marrow, could ultimately enable robust in vitro expansion of HSCs. In this thesis, several novel strategies were developed to study and manipulate HSC biology in vitro, through the regulation of key niche factors, cellular metabolism, and the complex multicellular crosstalk in a niche-mimicking context. First, we made use of gastruloids, an embryonic stem cell-based model of early mouse development, to mimic key aspects of embryonic hematopoiesis in vitro. Simultaneously with the establishment of a vascular network, we detected by immunophenotyping the emergence of primitive blood progenitor cells during the late developmental stages of gastruloids. These embryonic blood progenitors were spatially localized close to a vascular-like plexus in the anterior portion of the gastruloid. Colony-forming assays demonstrated an interesting differentiation potential of these cells. These data demonstrate the potential of gastruloids as an easily accessible in vitro model to study blood development in an embryo-like context. Second, a bioengineering approach was used to study HSC fate choices upon exposure to niche-specific ligands in a well-defined artificial environment. A combination of time-lapse microscopy and single-cell multigene expression analysis was used to define differentiation and cell-cycle states of mouse and human HSCs. Strikingly, selected artificial niches reduced proliferation and maintained the long-term multilineage potential of HSCs in vitro. These artificial niches hold significant potential for the study of mechanisms involved in HSC fate regulation. Third, the influence of the metabolism on fate choices of adult HSCs was studied, specifically focusing on fatty acid β-oxidation. Malonyl-CoA was used to reversibly block fatty acid β-oxidation in mouse HSCs. In vitro treatment of HSCs with malonyl-CoA promoted their expansion and increased lymphoid reconstitution upon in vivo transplantation. This study sheds new light on the role of the metabolic environment in HSC regulation. Surprisingly, exposure to only a single, readily available metabolite was found to be sufficient to strongly influence HSC behavior. Fourth, a miniaturized multicellular culture system was developed to mimic the hallmarks of the dynamics of HSCs in their native bone marrow. Primary human mesenchymal stem/progenitor cells and endothelial cells were aggregated in a high-throughput manner in biomimetic hydrogel microwells to form self-organizing bone marrow organoids. Immunostaining demonstrated the formation of branched vascular networks, rendering the organoids permissive for the homing of human hematopoietic stem and progenitor cells. These results suggest that bone marrow organoids may be a suitable platform for the modeling of physiological and clinically-relevant stem cell dynamics in vitro. Altogether, this thesis presents several innovative approaches for modeling key features of native stem cell microenvironments that will contribute to a better understanding of the biology of HSCs and their regulatory niche.