Defining Self-renewal in Human Hematopoietic Stem Cells
Hematopoietic stem cells (HSC) are widely applicable for the treatment of blood disorders such as leukemias and anemias. However, only a fraction of patients can find a suitable HSC donor. As attempts to generate HSC from pluripotent cells or expand functional HSC in culture have met with limited success, it will be necessary to define the identity of the self-renewing HSC and key intrinsic and extrinsic factors utilized by the developing embryo for HSC self-renewal such that the conditions necessary for HSC development can be mimicked in vitro.
In this work, we first defined an in vitro HSPC (hematopoietic stem/progenitor cell) co-culture system on mesenchymal stem cell (MSC) stroma that sustains human multilineage hematopoietic hierarchy. We found that this culture system maintains undifferentiated, transplantable HSPC that retain a relatively stable HSPC surface phenotype and transcriptome over several weeks. However, we also discovered that dysregulation of PBX governed genetic networks may compromise the function of cultured HSC, providing new insights on how to target the compromised HSC transcriptome to improve HSC expansion in culture.
This work also identified a novel surface marker that can be utilized to define the self-renewing HSC during human development. We discovered that the glycophosphatidylinositol-anchored surface protein GPI-80 (Vanin 2), previously implicated in leukocyte adhesion and diapedesis, identifies a functionally distinct subpopulation of human fetal HSPC that possess self-renewal ability. The GPI-80+ HSPC were the only population that maintained proliferative potential and undifferentiated state in MSC stroma co-culture, as well as displayed the ability to engraft in immunodeficient NSG mice. Through defining the identity of the human fetal HSC and characterizing the HSPC transcriptome in the in vivo niche and on mesenchymal stroma, we can determine the factors utilized in HSC self-renewal and identify those that are deficient in culture, which will bring us closer towards generation and expansion of HSC in vitro.