Adult versus Embryonic Stem Cells: It's Still a Tie
Authors: Markus Grompe
Journal: Molecular Therapy (2002)

The field of gene therapy has always had a special interest in stem cell biology because the ex vivo approach to gene transfer involved the genetic manipulation of cells followed by retransplantation [1Dzierzak E.A. Papayannopoulou T. Mulligan R.C. Lineage-specific expression of a human β-globin gene in murine bone marrow transplant recipients reconstituted with retrovirus-transduced stem cells.Nature. 1988; 331: 35-41Crossref PubMed Scopus (229) Google Scholar]. Much of the early work in this area was specifically directed at genetic manipulation of hematopoietic stem cells (HSCs) and the first clinical success in our field has come from this approach [2Cavazzana-Calvo M. Gene therapy of human severe combined immunodeficiency (SCID)-X1 disease.Science. 2000; 288: 669-672Crossref PubMed Scopus (2232) Google Scholar]. Thus, for many investigators in gene therapy, research required knowledge in transplantation biology, as well as an understanding of the gene transfer technology itself. For many years the HSC appeared to be the only stem cell of relevance for cell therapy applications. Sessions or scientific meetings on "Stem Cell Biology" were exclusively involved with hematopoiesis. This has changed dramatically recently. Multiple new aspects of stem cell biology in the adult have emerged, including the developmental plasticity of HSCs [3Gussoni E. Dystrophin expression in the mdx mouse restored by stem cell transplantation.Nature. 1999; 401: 390-394Crossref PubMed Scopus (1642) Google Scholar, 4Petersen B.E. Bone marrow as a potential source of hepatic oval cells.Science. 1999; 284: 1168-1170Crossref PubMed Scopus (2185) Google Scholar] and the discovery of other stem cells amenable to in vitro manipulation and transplantation. These include the mesenchymal stem cell [5Prockop D.J. Marrow stromal cells as stem cells for nonhematopoietic tissues.Science. 1997; 276: 71-74Crossref PubMed Scopus (4130) Google Scholar], neural stem cells [6Lendahl U. Zimmerman L.B. 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In addition, the development of human embryonic stem cells increased the interest in their use for cell therapy [11Thomson J.A. Embryonic stem cell lines derived from human blastocysts.Science. 1998; 282: 1145-1147Crossref PubMed Scopus (12274) Google Scholar]. Because the creation of embryonic stem (ES) cell lines involves the procurement and destruction of human embryos, this approach to cell therapy has elicited an intense response outside the scientific community. Some groups wish to see research on human embryonic stem cells banned. In their lobbying, they have argued that "embryonic stem cells are not needed" because adult bone marrow stem cells have multi-lineage plasticity and, hence, could be used for cell therapy of any tissue. Furthermore, proponents of a ban emphasized that ES cell-derived cells had never been used in a therapeutic proof-of-principle experiment, whereas many preclinical successes are known for adult cell therapy (for example, bone marrow transplantation). Two recent publications are highly relevant to this debate, but no clear winner has yet emerged. Both papers represent landmark achievements in the stem cell biology field and are very important to gene therapists. The first paper, from the group of Catherine Verfaillie at the University of Minnesota by Jiang, Jahagirdar, and coworkers [12Jiang Y. Pluripotency of mesenchymal stem cells derived from adult marrow.Nature. 2002; 418: 41-49Crossref PubMed Scopus (5161) Google Scholar], describes the multipotentiality of cells, termed MAPCs (multipotent adult progenitor cells) derived from adult bone marrow. The same group had previously described how MAPCs could be generated from mouse, rat, and human marrow, expanded for multiple passages, and differentiated into multiple cell types in vitro [13Reyes M. Purification and ex vivo expansion of postnatal human marrow mesodermal progenitor cells.Blood. 2001; 98: 2615-2625Crossref PubMed Scopus (1116) Google Scholar, 14Schwartz R.E. Multipotent adult progenitor cells from bone marrow differentiate into functional hepatocyte-like cells.J. Clin. Invest. 2002; 109: 1291-1302Crossref PubMed Scopus (1110) Google Scholar]. They now demonstrate the multipotentiality of MAPCs in transplantation experiments in vivo. They performed two major categories of experiments. First, extensively expanded (> 50 cell doublings) and clonally marked MAPCs were injected into host blastocysts in a procedure analogous to that used previously for ES cells. Clonally marked MAPCs generated high-level chimeras (up to 45%), which survived into postnatal life (up to 20 weeks) and were healthy. Some animals achieved high-level donor chimerism even when only one MAPC was injected. MAPCs contributed to virtually all tissues, including the brain, retina, lung, skeletal muscle, myocardium, liver, intestine, kidney, skin, spleen, and blood. Second, MAPCs were injected into adult NOD-SCID mice. Similar to the results obtained with injection of blastocysts, the MAPCs again contributed substantially to multiple tissues, although only 1 × 106 cells were transplanted and although no selective advantage for the donor cells was provided. Chimerism ranged from 1 to 10% in the hematopoietic tissues (blood, marrow, and spleen) but also in some solid epithelial organs (lung, liver, and intestine). In contrast to the blastocyst injections, no engraftment was seen in skeletal, myocardium, skin, kidney, or brain. When selection was applied in the form of whole-body irradiation of the host, the degree of tissue replacement increased, but the spectrum of engrafted tissues remained the same. Together, these experiments demonstrate remarkable properties of MAPCs. In the blastocyst injections, they behaved very similarly to ES cells, although the average level of chimerism was lower than that typically obtained with ES cells and although no germline transmission has yet been described. The transplantation experiments in adult animals show results superior to those reported for transplantation of ES cells or HSCs. Undifferentiated ES cells form teratomas upon transplantation into adult mice and HSCs contribute at high levels only to hematopoietic lineages and only minimally to epithelial tissues. The second paper, from the laboratory of Ron McKay by Kim, Auerbach, and coworkers [15Kim J.H. Dopamine neurons derived from embryonic stem cells function in an animal model of parkinson's disease.Nature. 2002; 418: 50-56Crossref PubMed Scopus (1407) Google Scholar], provides the long-awaited proof of concept for the therapeutic efficiency of ES-derived cells for Parkinson's disease. The authors developed a complex protocol to differentiate multipotent ES cells to dopaminergic mid-brain neurons in vitro. The protocol involves gene transfer of the nuclear receptor related-1 (Nurr1) transcription factor into ES cells followed by five sequential tissue culture stages. The cells derived by this process were extensively characterized in terms of their pattern of gene expression, as well as their physiologic behavior. In vitro, the ES cell-derived dopamine producing cells had all the characteristics expected for dopaminergic neurons. To further characterize these cells and to show their therapeutic potential, they were transplanted into an animal model of Parkinson's disease. First, the morphological and electrophysiological properties of endogenous and ES cell-derived neurons were compared in brain slices. The ES-derived neurons behaved the same as endogenous cells. In addition, there was clear improvement in the abnormal behavior of the mice with Parkinson's disease. As a matter of fact, the defect in the cell-transplanted animals was overcorrected, because they demonstrated abnormal rotation in the opposite direction of untreated controls. Very importantly, no tumors or other mass effects were observed as a result of the transplantation of the ES cell-derived neurons. The work described by Kim, Auerbach, et al. [15Kim J.H. Dopamine neurons derived from embryonic stem cells function in an animal model of parkinson's disease.Nature. 2002; 418: 50-56Crossref PubMed Scopus (1407) Google Scholar] therefore provides strong encouragement for the concept that ES cells could be converted to therapeutically useful and safe cells for transplantation in vitro. Indeed, the McKay laboratory also provided evidence that murine ES cells could be converted to insulin secreting cells with potential utility for the treatment of diabetes [16Lumelsky N. Differentiation of embryonic stem cells to insulin-secreting structures similar to pancreatic islets.Science. 2001; 292: 1389-1394Crossref PubMed Scopus (1306) Google Scholar]. In light of these new discoveries, it is timely to compare the respective benefits and disadvantages of adult-derived MAPCs and embryo-derived ES cells (Table 1). First, I will highlight the similarities. Both cell types can be expanded from a single clone for many passages in vitro, and virtually unlimited numbers of cells can be generated. Both cell types are multi-potential in vitro and in vivo and, hence, might serve as sources for the production of a wide variety of transplantable cell types. However, there are also some important differences that have clear implications for the potential therapeutic use of these cells. MAPC cultures are established by plating CD45-negative cells onto plastic and harvesting clones that emerge over time. The cells are then plated at low density (400–800 cells/cm2) and serially passaged again at low density. Maintenance of the stem cell phenotype is critically dependent on this low density, and for this reason, it takes a long time to obtain substantial numbers of cells suitable for transplantation. The advantage of deriving MAPCs de novo from the prospective patient is that they would be immunologically matched, thus avoiding the need for immune suppression. Of course, established MAPCs from allogeneic donors could also be used, similar to ES cells. In that case, the problem with the slow expansion of these cells would be solved. In contrast to MAPCs, ES cells are grown at very high density and can be passaged at ratios as high as 1:10. They are therefore easier (and presumably cheaper) to grow than MAPCs. On the other hand, ES cells are derived from a limited pool of donor embryos and not from the patient to be treated by cell therapy. For this reason, ES-derived differentiated cells will have some degree of immunological mismatch, and immune suppression would likely be needed in patients transplanted with such cells. The immunological problems associated with the use of ES cells could be overcome by transfer of patient nuclei (therapeutic cloning) [17Rideout 3rd, W.M. Hochedlinger K. Kyba M. Daley G.Q. Jaenisch R. Correction of a genetic defect by nuclear transplantation and combined cell and gene therapy.Cell. 2002; 109: 17-27Abstract Full Text Full Text PDF PubMed Scopus (497) Google Scholar], but this would make the use of ES cells at least as complicated as establishing MAPCs. Another difference between the two cell types is their potential to form tumors. Here, MAPCs seem to have the advantage. Undifferentiated ES cells form teratomas when transplanted into adult animals. In contrast, MAPCs not only do not form tumors but also appear to differentiate spontaneously into multiple useful cell types when transplanted in vivo. Because of the clearly established tumorigenicity of ES cells, very high standards for the purification of the desired differentiated cell types will be needed. The ES cell-derived neurons described by Kim, Auerbach, et al. did not form tumors, but the follow-up time in their experiments was limited, and each differentiation scheme may have different risks for the formation of tumors.TABLE 1Comparison of MAPCs and ES cellsES cellsMAPCsAdvantagesExpandable in vitroExpandable in vitroMultipotential in vitroMultipotential in vitroMultipotential in vivo (blastocyst injection)Multipotential in vivo (blastocyst injection)Germline transmission possibleMultipotential in vivo (direct transplantation in the adult)Proof of principle for therapeutic use existsNot tumorigenicEasy to grow at high densityAdult-derived, ethics not controversialHuman ES cells availableCan be immunologically matched to patient Human MAPCs availableDisadvantagesTumor potential when transplanted in adultTake time to establishNot matched to host immunologicallyGrow at low densityEmbryo-derived, ethically contentiousTherapeutic use unprovenGermline transmission not yet demonstrated Open table in a new tab One disadvantage of the MAPCs at this time is that the system is very young and has not yet been tested in many different laboratories. In addition, it has not yet been shown that they can correct a disease phenotype. However, the transplantation properties of the cells reported by Jiang, Jahagirdar, et al. [12Jiang Y. Pluripotency of mesenchymal stem cells derived from adult marrow.Nature. 2002; 418: 41-49Crossref PubMed Scopus (5161) Google Scholar] predict that the cells will be useful for the treatment of hematopoietic diseases, as well as liver disorders. Despite the substantial progress represented by the two Nature papers, many questions and hurdles remain before the discoveries can be brought to the patient. Clearly, transplantable cells from both ES cells and MAPCs have much potential for human therapy. However, not enough is known at this time about either of these approaches to decide which technology may be superior in the clinic. Transplantation experiments have only been carried out in rodent animal models. Despite the promising properties of MAPCs it is far from obvious that they are the adult equals of ES cells. To ensure the highest chance for the development of reliable sources of cells for transplantation, continued research in both systems is needed. However, if MAPCs really do represent a good source for therapeutic cells, the ethical considerations may eventually tip the balance in their favor.