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Functional hematopoietic stem cells from human ES cells – significant progress toward?


This title is a citation from the last paper [1]about making functional hematopoiesis (hematopoietic stem cells) from human embryonic stem (ES) cells, which appeared in July’s issue of Cell Stem Cell [2]journal. I put ? mark at the end for a reason that i’ll discuss here.

Paper is very good and compared to others from the same topic, definitely advanced. But when i read a comment I asked myself some global questions, such us: Do we really need to produce hematopoietic stem cells (HSC) from ES for clinical needs? And what is the progress in this area in the last decade? I’d like to discuss here about it with people who are really interested or working in this field.

Let’s start from an overview of the problem. The first question: why do we need to make HSCs from ES cells? [3] The answer is that it could be a therapeutic alternative to bone marrow transplant (BMT) in hematological clinic. To become an alternative method, it should have definite advantages compared with others. So, yes we really need more HSC in BMT clinic, because it’s frequently not enough in many available grafts (such as bone marrow or cord blood samples or mobilized apheresis products) for successful engraftment and to rescue patients. So the first point is the number of functional HSC in one transplant – we need expansion! The second point is immunological compatibility – transplant should be a match! Third point is availability of the transplant.

Based on these advantages many scientific groups around the world are trying very hard [4] to improve protocols to make it work. The main problem was to get good engraftment of HSC derived from human ES cells. Seems like cells were not functional in the first attempts to transplant into immunocompromised mice. Only recently in 2004-2006 Mickie Bhatia [5] and Dan Kaufman [6] groups got engraftment of human ES-derived HSC close to 1% only after intra-bone BMT [7].

Ok, now let’s come back to the most recent work of Maria Ledran,
published [1] in Cell Stem Cell. This group has significantly improved the protocol of HSC derivation. The most important achievements of this work are the following:
* authors successfully mimic HSC niche conditions in vitro, using stromal cells. So it seems like for normal function HSC required natural microenvironment;
* study highlighted role of transforming growths factor beta in efficient generation of functional HSC from ES cells even without stromal cells; thus shows that not only adhesion signaling is important but also humoral factors, released by stromal cells withing the niche;
* improvement of cell culture conditions allowed engraftment efficiency of 2-16% (that’s much higher than was reported before); HSC were functional – and gave long-term multilineage reconstitution and were able to self-renew.

Colony assay, demonstrates hematopoietic progenitor capacity of human ES cells in vitro:

(Credit: Maria H. Ledran, et al. 2008; modified)

Ok, now we have functional HSC from human ES cells, without genetic manipulations with up to 16% of engraftment level (we jumped from 1% hooraay!).
I’d like to notice that this study was done on NOG mice (immunocompomised mice with very low innate immunity and absence of NK cells) and unfortunately didn’t compare efficiency of human somatic HSC engraftment in the same model. But if we look at previous study of Bhatia group it shows that engraftment of ES-derived HSC was worse than that somatic HSC from cord blood [5], and even more – they look different on molecular level.

To give a green light to derivation of HSC from ES for clinical needs we have to compare all advantages and disadvantages of this method with the qualities of current techniques. So authors pointed out that potentially new source of HSC could has enough number of functional stem cells, less immunogenic or even autologous and easily available. I’d challenge all of these advantages and indicate what kind of bad things can we expect from bringing this technology in clinical future:

* we can get a lot of HSC, but longer cell culture time and crazy expansion (compared with expansion of somatic HSC) level will increase risk of mutation acquisition and leukemia development after transplant;
* HSC derived from allogeneic human ES will be immunogenic (maybe a little less) after transplant anyway;
* we can create autologous cancer-free HSC product from ES cells if we will be able to make somatic cell nuclear transfer or iPS techniques work for patients, in this case I don’t think high price of procedure allows for it to move to wide clinical use;
* we should enrich or even sort out HSC contaminated by undifferentiated ES cells or other unwanted derivates (such as endothelial cells which could be immunogenic) to make sure there will be no risk of tumors growth after transplant, which will make procedure more complicated and costly;
* human ES-derived HSC are functionally different from somatic HSC by unknown reasons and have more potential risk of complications after transplantation ( lung capillary emboli for instance [5]).

About a hundred protocols were published about expansion of somatic (adult) HSC (cord or mobilized blood) for clinical needs. Some of them are currently on clinical trials and really work. To overcome compatibility problem, on the one hand we have banks of matched grafts and on the other hand we know or can develop some technologies of separating malignant cells in bone marrow from healthy HSC, so adult transplant could be autologous.

Overall i can not get the points of huge money and time investment in derivation of HSC from ES cells for clinic. Expansion of somatic HSC almost there and I honestly think that we will get only more problems with ES-derivates (even autologous). Of course i don’t think it’s a waste of time, because it gives us a lot of scientific information about hematopoiesis and development. It’s very applicable in basic science or maybe for drug screening. But do we really need it in terms of clinical perspectives? What do you think?

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read more about this work:
Growing blood in a dish; using stem cells to screen for breast cancer [8] (by the Niche)
The Origins of Blood: Induction of Hematopoietic Stem Cells from Different Sources [9] (Cell Stem Cell comment)