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A method to enhance the expansion of human hematopoietic stem cells ex vivo

(De Felice L et al, Cancer Res 2005;65:1505)
Summary by Teleca I. Brooks and Ru Chen

Hematopoietic stem cells (HSC) give rise to the different types of blood and immune cells. They are found in the umbilical cord blood, peripheral blood taken from patients ready to donate stem cells for transplantation, and the bone marrow. HSCs are considered to be one of the most studied stem cells.  HSCs have found a place in the clinical setting as they are used to treat patients with cancers and other blood disorders.  Based on the clinical importance of HSC, it would be ideal to develop efficient methods to expand the HSCs ex vivo. In this case, a small amount of cells would be taken from the donor, as compared to the current method to take large numbers of stem cells.

The present study tested a strategy to increase the number of HSCs. For expansion, the HSCs will need to make many cells of its kind by a process termed self-renewal, which is defined as the ability of to generate identical cells.

An agent that inhibits the actions of an enzyme called histone deacetylase has been used to assist in the expansion process. Histone deacetylases remove groups surround the genetic material of the cells to restrict their need to commit into specialized cells. The investigators added the enzyme to other factors previously shown to expand the HSCs. The interesting finding is that the expanded cells retained functions expected of HSCs.

SCIENTIFIC

The expansion of hematopoietic stem cells (HSCs) is relevant for cell therapies. However, due to the low frequency of HSCs, there is a need to expand these cells by in vitro methods. However, the ideal senario is to have the cells retain their ‘stemness’ during ex vivo amplification for cell therapies. Previous research suggests that cocktails of cytokines could mediate ex vivo expansion. However, this method did not show efficiency in retaining the `stemness’, as indicated by short-term repopulating capacity.

This study investigated the effect of valproic acid (VPA), a specific histone deacetylase inhibitor, on ex vivo culture of HSC. Cord blood (CB), mobilized peripheral blood (MPB) and bone marrow (BM) served as sources of HSC. After 7 days of culture, the majority of cells were immature and hyperacetylated when the cultures contained VPA and cytokines. Similar observation was not seen in cultures with only cytokines. By day 21, the cells that were cultured in the presence of cytokines and VPA were CD34+/CD90+/CD38-/ Ac133+. In addition, a higher percentage of cells remained in the G0/G1 phase in the presence of VPA while cells treated with cytokines alone were in the G2/S phase of the cell cycle. Cell culture assays confirm the aforementioned data indicating that cytokines and VPA combination preserve a higher replating potential of the expanded cells. VPA treatment showed an increase in histone H4 acetylation level on HOXB4, a key transcription factor related to HSC self-renewal. Together, the studies suggest that VPA can retain the expansion of HSC with no significant evidence of cell differentiation. The outcome require `fine-tuning’ since cells with lineage-specific markers (CD13, CD15, CD235a and CD61) were detectable in the VPA-containing cultures.

Comment:  The major weakness of this research is the lack of animal studies to verify the stem cell properties of the expanded cells.