A Possible Role for Wnt3a in Enhancing StemRegenin1-Mediated Expansion of Hematopoietic Stem Cells

Abstract 4130

Insufficient engraftment after umbilical cord blood stem cell transplantation (UCB-SCT) may be improved by the administration of ex vivo expanded UCB-derived hematopoietic stem cells (HSC). Although culturing HSC with cytokines such as SCF, Flt3L and TPO results in robust proliferation, it is accompanied with extensive differentiation and loss of self renewal capacity. Inhibition of the Aryl hydrocarbon Receptor (AhR) leads to potent expansion of hematopoietic progenitor cells (HPC) ^{(Boitano et al. Science 2010)}. Reportedly, Wnt3a inhibits differentiation in different types of stem cells including hematopoietic stem cells^{(Reya et al. Nature 2003, Ten Berge et al. Nature Cell Biology 2011)}. Here, we evaluated possible additive or synergistic effects of combining the AhR antagonist StemRegenin1 (SR1) with recombinant purified Wnt3a to expand hematopoietic stem cells ex vivo.

UCB-derived CD34-selected cells were cultured in serum-free Glycostem Basal Growth Medium (GBGM) supplemented with the early-acting growth factors SCF, Flt3L and TPO (SFT medium) with or without the addition of SR1 and Wnt3a. Cell number, viability and subset composition within the CD34⁺ cells were measured using flowcytometry. The multilineage differentiation potential and self renewal capacity of expanded CD34⁺HPC were evaluated in stroma-supported long-term culture-initiating cell (LTC-IC) assays.

Culturing CD34⁺ cells in SFT medium resulted in a mean 10.2-fold increase in CD34⁺ cells after 1 week of culture (n=3). Addition of SR1 to the SFT-medium resulted in a 16-fold increase of CD34+ input cells within 7 days, while on the other hand, addition of Wnt3a alone to the SFT-medium resulted in a 7-fold increase in CD34+ cells. However, combining SR1 and Wnt3a in the SFT medium resulted in a 20-fold expansion of CD34⁺ cells compared to input. The early additive effect of Wnt3a on SFT+SR1-induced expansion of CD34⁺ cells, however, disappeared upon prolonged culture up to 2–3 weeks.

Approximately 3–10% of UCB-derived CD34⁺ cells could be characterized as phenotypic HSC, as was defined by CD34⁺CD38^lowCD45RA^lowCD90⁺ cells. After culture, we sorted different CD34⁺-populations to evaluate their functional capacity. Evaluation of LTC-IC frequencies yielded a frequency of 1/23 LTC-IC in the phenotypic HSC-subset (CD34⁺CD38^lowCD45RA^lowCD90⁺) after 28 days of culture in SFT medium with SR1. However, no LTC-IC appeared to be present in the multipotent progenitor subset (MPP, CD34⁺CD38^lowCD45RA^lowCD90^low). These results indicate that phenotypic HSC maintain their functional LTC-IC capacity after expansion culture.

Collectively, our results confirm that SR1 expands HSC with preservation of self-renewal capacity and ability to differentiate into various hematopoietic lineages. In addition, we show that Wnt3a initially enhances SFT+SR1-driven expansion of CD34⁺ HPC, but reduces the increase in number of CD34⁺ cells at later stages of culture. These data may suggest that the period of expansion needed for clinical application may be shortened by combining SR1 and Wnt3a.