The what, why and how hematopoietic stem cells are dominating the regenerative medicine space
We have previously blogged about hematopoietic stem cells (HSCs); they are defined by their ability of self-renewal and capabilities of multi-lineage differentiation by giving rise to all types of blood and immune cells. Development of hematopoietic stem cells takes place during embryogenesis through a complex process that involves multiple anatomical sites. Once HSC precursors have been specified from mesoderm, they give rise to nucleated primitive erythrocytes, followed by hematopoiesis, which generate a pool of HSCs. hematopoietic stem cells maintain the homeostasis in vivo by sustaining the production of blood and immune cells for the life span of the individual.
How easy are hematopoietic stem cells to obtain?
The low frequency of hematopoietic stem cells in the conventional sources such as cord blood (CB), bone marrow (BM) and peripheral blood and the absence of a reliable marker have made their identification and purification a tough task. hematopoietic stem cells can be distinguished from mature blood cells by the presence or absence of lineage-specific cell-surface antigens. Fluorophore-conjugated antibodies and sorting through fluorescence activated cell sorters (FACS) has remarkably improved the hematopoietic stem cells isolation efficiency. The isolation of one of the most thoroughly characterized populations of HSCs relies on the positive expression of the CD34. Human CD34+ cells have been shown to have engraftment potential clinically by successful transplantation therapy.
Any dysfunction of the hematopoietic system can to lead to various hematological disorders. Transplantation of HSCs is now accepted worldwide as treatment for hematological diseases. However, limited number of hematopoietic stem cells primarily residing in their conventional tissue sources is a limiting factor for transplantation. To overcome this shortcoming, various attempts have been made for HSCs expansion and manipulation of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) to generate lineage committed HSC. Liquid culture with cytokines and co-culture with stromal cells are the most primitive methods for HSCs expansion and can only demonstrate moderate expansion of HSC. Supplementation of soluble protein factors to the medium has shown remarkable expansion of CD34+ cells by activating notch signaling. Similarly, over-expression of transcription factors that regulate HSC self-renewal has improved ex vivo HSCs expansion. Recent studies, recently reviewed by Y Nakajima-Takagi et al., 2013 have shown promising out-come of small molecules (SMCs) to serve as the manipulators of fate, state and function of HSCs via targeting of stem cell-specific signaling pathways.
Alternative sources for HSCs
hESCs and iPSCs have shown potential to serve an alternative source for HSCs. Embryoid body (EB) formation and co-culture on stromal feeder cells and primary stromal cell lines derived from aorta-gonad-mesonephros (AGM), fetal liver and fetal BM are two major strategies to generate hematopoietic cells. So far, different protocols have been tried to generate HSCs but unfortunately not much success have been achieved in terms of generation of functional HSCs. Lako’s group has had the most success in the derivation of HSCs from hESCs . Their group co-cultured hESC-derived hematopoietic cells on the stromal feeder cells derived from mouse AGM or fetal liver or with stromal cell lines derived from these tissues and found that hESC can generate CD34+ hematopoietic progenitors cell.
Although studies have shown in-vitro differentiation capabilities of ESCs and iPSCs into myeloid and erythroid cell lineages, more efforts are still required in order to assure the efficacy and safety of expanded HSCs for transplantation and cell-based therapies. It is critical to improve the engraftment rate of hHSCs for taking them form bench to bedside and with these evolving technologies to expand and produce HSCs, the manipulation of HSCs will secure its place as an instrumental tool in transplantation and cell-based therapies.
 Nakajima-Takagi Y, Osawa M, Iwama A. Manipulation of hematopoietic stem cells for regenerative medicine. Anat Rec (Hoboken). 2014 Jan;297(1):111-20. doi: 10.1002/ar.22804.
 Ledran MH, Krassowska A, Armstrong L, Dimmick I, Renstr€om J, Lang R, Yung S, Santibanez-Coref M, Dzierzak E, Stojkovic M, Oostendorp RA, Forrester L, Lako M.. Efficient hemato- poietic differentiation of human embryonic stem cells on stromal cells derived from hematopoietic niches. Cell Stem Cell. 2008 Jul 3;3(1):85-98. doi: 10.1016/j.stem.2008.06.001.