Mimicking the functional hematopoietic stem cell niche in vitro: recapitulation of marrow physiology by hydrogel-based three-dimensional cultures of mesenchymal stromal cells.

BACKGROUND A culture system that closely recapitulates marrow physiology is essential to study the niche-mediated regulation of hematopoietic stem cell fate at a molecular level. We investigated the key features that play a crucial role in the formation of a functional niche in vitro. DESIGN AND METHODS Hydrogel-based cultures of human placenta-derived mesenchymal stromal cells were established to recapitulate the fibrous three-dimensional architecture of the marrow. Plastic-adherent mesenchymal stromal cells were used as controls. Human bone marrow-derived CD34(+) cells were co-cultured with them. The output hematopoietic cells were characterized by various stem cell-specific phenotypic and functional parameters. RESULTS The hydrogel-cultures harbored a large pool of primitive hematopoietic stem cells with superior phenotypic and functional attributes. Most importantly, like the situation in vivo, a significant fraction of these cells remained quiescent in the face of a robust multi-lineage hematopoiesis. The retention of a high percentage of primitive stem cells by the hydrogel-cultures was attributed to the presence of CXCR4-SDF1α axis and integrin beta1-mediated adhesive interactions. The hydrogel-grown mesenchymal stromal cells expressed high levels of several molecules that are known to support the maintenance of hematopoietic stem cells. Yet another physiologically relevant property exhibited by the hydrogel cultures was the formation of hypoxia-gradient. Destruction of hypoxia-gradient by incubating these cultures in a hypoxia chamber destroyed their specialized niche properties. CONCLUSIONS Our data show that hydrogel-based cultures of mesenchymal stromal cells form a functional in vitro niche by mimicking key features of marrow physiology.