© The American Society of Gene & Cell Therapy
commentary
See page 1368
From Mesoderm to Mesodermatology: Bone Marrow Mesenchymal Cells Heal Skin Wounds Marketa Tolarova1 and Jakub Tolar2 doi:10.1038/mt.2015.84
R
ecessive dystrophic epidermolysis bullosa (RDEB) is caused by loss-offunction mutations in the COL7A1 gene that result in reduced levels or complete absence of type VII collagen (C7). The disease is characterized by progressively worsening blistering, inflammation, pain, scarring, and contractures. C7 is an extracellular matrix protein that homopolymerizes into anchoring fibrils (AFs) and plays a critical role in adherence of skin layers at the dermal-epidermal junction (DEJ) in cutaneous and mucosal membranes. Multiple cell types can express C7—keratinocytes, fibroblasts (FBs), mesenchymal stromal/stem cells (MSCs, Figure 1), perhaps even hematopoietic cells—but are the cell types functionally and qualitatively similar or equal? In several methodical and incisive studies, injections of allogeneic FBs or MSCs have been used for wound healing in individuals with RDEB,1–6 and the study by Kühl and colleagues reported in this issue adds to our knowledge regarding the use of MSCs as medications for wound healing in RDEB.7 Kühl et al.7 found that human MSCs express C7 in vitro at approximately the same level as FBs. When injected into the skin of a murine RDEB model (these RDEB “hypomorphic” mice express C7 at approximately 10% of wild-type levels), human MSCs deposited human C7 at the DEJ in a dose-dependent manner. Although xenografted MSCs did not proliferate, they persisted for about 1 month; human C7 decreased proportionately but was detectable up to 3 months after injections. Resistance to injury (measured by putting adhesive tape on shaved skin 2nd Faculty of Medicine, Charles University, Prague, Czech Republic; 2Stem Cell Institute and Division of Blood and Marrow Transplantation, Department of Pediatrics, University of Minnesota, Minneapolis, Minnesota, USA Correspondence: Jakub Tolar, 420 Delaware Street SE, MMC 366, Minneapolis, Minnesota 55455, USA. E-mail: [email protected]
and ripping it off) was improved, and rudimentary AFs (C7 homotrimers) increased in number along the DEJ. There were no significant local side effects and no systemic immune reactivity to human C7. In fact, whereas wounds generated in wild-type mice did not benefit from MSC injections, the naturally occurring wounds in RDEB mice showed faster wound closure after MSC local treatment than when injected with vehicle alone. Crucially, this productive tissue repair reaction may not have been solely a consequence of C7 expression but may also have been due to a synergy with the immune-modulatory and reparative functions of MSCs. This work demonstrates the possibility of an unlikely symbiosis between two essentially unrelated medical fields: hematology and dermatology. MSCs, routinely derived from hematopoietic organs such as bone marrow or umbilical cord blood,8 are shown here to merge with the goals of dermatologists
to alleviate blistering in this prototypical genodermatosis. They are clearly connected in development; hematopoietic cells, connective tissue stromal cells, and dermis are all derived from mesoderm, one of the primary germ layers of the embryo. Therefore, thinking in the new conceptual space of mesoderm derivatives and their impact on skin (mesodermatology) might facilitate a thoughtful examination of MSCs and other bone marrow–derived cells in skin disorders. This functional link between bone marrow and skin has a long history. In 1876 Julius Cohnheim argued for a bold new way of understanding tissue boundaries and hypothesized that cells mediating post-traumatic fibrosis in skin and other organs might originate in blood and bone marrow.9 In 1966 Alexander Friedenstein, in the first clear functional indication of existence of mesenchymal cells in bone marrow, demonstrated the donor origin of bone tissue after bone marrow transplantation in mice.10,11 The idea of nonhematopoietic progenitor cells present in bone marrow aspirate emerged—a cell type adherent to plastic and easily propagated in vitro, capable of differentiation into bone, fat, and cartilage—and equated with the term mesenchymal stem cell. Instead of anchoring the concept, however, a much
1
Molecular Therapy vol. 23 no. 8 august 2015
Figure 1
Mesenchymal stromal/stem cell. Colored scanning electron micrograph. Copyright © 2015 Photo Researchers, Inc. All Rights Reserved. Credit: Steve Gschmeissner/Science Source (registered trademark of Photo Researchers, Inc.).
1283
© The American Society of Gene & Cell Therapy
commentary more complex picture emerged as more data were uncovered on the heterogeneity, clonality, self-renewal, and functional diversity of these cells in both fresh and cultured forms. Almost in contrast to the complexity of the basic biology of MSCs, the relative ease with which they can be obtained, cultured, and prepared under good manufacturing practices for human use has led to a remarkable number of clinical trials. MSCs have been used clinically for two decades, and MSC infusions have shown clinical benefit in some applications, such as connective tissue injury or graft-versus-host disease (the main immune complication of allogeneic hematopoietic cell transplantation). For other indications, however, their value is less clear. Nevertheless, through application of MSCs in different injuries and disease states, several concepts have emerged: (i) MSCs are multifunctional; (ii) the capacity of MSCs to contribute to the musculoskeletal system and to modulate immune and inflammatory responses can be separated; (iii) the magnitude and quality of donor MSC engagement depends on recipient factors; (iv) MSCs interact with their cellular partners directly, by cell-to-cell contact, or indirectly, by short-range or longrange signals (free protein mediators or exosomes12); and (v) the changeability of MSCs, their conceptual resistance to traditional hierarchical thinking, and their versatility in adopting various cell fates such as mesenchymal-to-epithelial transition, may be an advantage in clinical applications. It is possible that MSCs are a mobile link via blood circulation and a modular link among stromal components of various organs. They can provide homeostasis and tissue maintenance in health, can mediate productive repair in injury states, and can also drive pathology, such as in the microenvironment of some cancers. Kühl et al. show here that skin injection of MSCs can improve wound healing in a murine model of RDEB.7 One of the key insights of this work is that the MSCs are multitalented: in addition to secreting C7, they are able to mobilize myofibroblasts, ameliorate skin inflammation, and increase skin stability. Their
1284
study shows the practical, clinically relevant advantages of increased density of MSCs in the wound, thereby paving the way for translation of MSC biology into clinical practice for RDEB individuals with moderate severity of disease, where local therapy of individual wounds can change their quality of life. For those with the severe form of RDEB, hematologists and dermatologists have worked together to understand the contribution of bone marrow cells to wound healing in RDEB preclinical models,13–16 and then to use bone marrow or umbilical cord blood for hematopoietic cell transplantation to change generalized severe RDEB from a fatal to a treatable, controllable, and eventually curable disease.17,18 The next challenge is to separate cells used as clinical-grade MSCs (fundamentally changed by the artificial environment of cell culture) from their physiological counterparts, the resident MSCs (such as pericytes and other components of tissue niches),19 where MSC identity is defined by local microenvironment and their fate is directed by extracellular matrix and neighboring cells. If we can combine the current clinical insights on the use of MSCs with new tools to explore them in their physiological circumstances, we will unveil not only their functional diversity but possible ways to manipulate them, including niche-directed therapies. As both skin and bone marrow stromal cells and their niches have been studied extensively, we should be able to integrate this knowledge into a mesodermatological concept of MSCs. This, in turn, should lead to its practical translation into an array of bone marrow–derived therapies to enhance regeneration and healing in RDEB, other genodermatoses, and other extracellular matrix disorders. ACKNOWLEDGMENTS
J.T. is supported in part by National Institutes of Health grants R01 AR063070 and R01 AR059947, Department of Defense grant W81XWH, the EB Research Partnership, the EB Medical Research Foundation, DEBRA, and the Sohana Research Fund. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The authors thank Nancy Griggs Morgan for assistance with manuscript preparation and editing.
REFERENCES
1. Wong, T, Gammon, L, Liu, L, Mellerio, JE, DoppingHepenstal, PJ, Pacy, J et al. (2008). Potential of fibroblast cell therapy for recessive dystrophic epidermolysis bullosa. J Invest Dermatol 128: 2179–2189. 2. Petrof, G, Martinez-Queipo, M, Mellerio, JE, Kemp, P and McGrath, JA (2013). Fibroblast cell therapy enhances initial healing in recessive dystrophic epidermolysis bullosa wounds: results of a randomized, vehicle-controlled trial. Br J Dermatol 169: 1025–1033. 3. Venugopal, SS, Yan, W, Frew, JW, Cohn, HI, Rhodes, LM, Tran, K et al. (2013). A phase II randomized vehicle-controlled trial of intradermal allogeneic fibroblasts for recessive dystrophic epidermolysis bullosa. J Am Acad Dermatol 69: 898–908. 4. Conget, P, Rodriguez, F, Kramer, S, Allers, C, Simon, V, Palisson, F et al. (2010). Replenishment of type VII collagen and re-epithelialization of chronically ulcerated skin after intradermal administration of allogeneic mesenchymal stromal cells in two patients with recessive dystrophic epidermolysis bullosa. Cytotherapy 12: 429–431. 5. Kern, JS, Loeckermann, S, Fritsch, A, Hausser, I, Roth, W, Magin, TM et al. (2009). Mechanisms of fibroblast cell therapy for dystrophic epidermolysis bullosa: high stability of collagen VII favors long-term skin integrity. Mol Ther 17: 1605–1615. 6. Petrof, G, Lwin, SM, Martinez-Queipo, M, AbdulWahab, A, Tso, S, Mellerio, JE et al. (2015). Potential of systemic allogeneic mesenchymal stromal cell therapy for children with recessive dystrophic epidermolysis bullosa. J Invest Dermatol; e-pub ahead of print 23 April 2015. 7. Kühl, T, Mezger, M, Hausser, I, Handgretinger, R, Bruckner-Tuderman, L and Nyström, A (2015). High local concentrations of intradermal MSCs restore skin integrity and facilitate wound healing in dystrophic eppidermolysis bullosa. Mol Ther 23: 1368–1379 8. Nevala-Plagemann, C, Lee, C and Tolar, J (2015). Placenta-based therapies for the treatment of epidermolysis bullosa. Cytotherapy 17: 786–795. 9. Cohnheim, J (1867). Ueber Entzündung und Eiterung. Virch Arch Path Anat 40: 1–79. 10. Friedenstein, AJ, Piatetzky, S, II and Petrakova, KV (1966). Osteogenesis in transplants of bone marrow cells. J Embryol Exp Morphol 16: 381–390. 11. Friedenstein, AJ, Petrakova, KV, Kurolesova, AI and Frolova, GP (1968). Heterotopic transplants of bone marrow. Analysis of precursor cells for osteogenic and hematopoietic tissues. Transplantation 6: 230–247. 12. Lai, RC, Yeo, RW and Lim, SK (2015). Mesenchymal stem cell exosomes. Semin Cell Dev Biol. 40: 82–88. 13. Chino, T, Tamai, K, Yamazaki, T, Otsuru, S, Kikuchi, Y, Nimura, K et al. (2008). Bone marrow cell transfer into fetal circulation can ameliorate genetic skin diseases by providing fibroblasts to the skin and inducing immune tolerance. Am J Pathol 173: 803–814. 14. Tolar, J, Ishida-Yamamoto, A, Riddle, M, McElmurry, RT, Osborn, M, Xia, L et al. (2009). Amelioration of epidermolysis bullosa by transfer of wild-type bone marrow cells. Blood 113: 1167–1174. 15. Tamai, K, Yamazaki, T, Chino, T, Ishii, M, Otsuru, S, Kikuchi, Y et al. (2011). PDGFRalpha-positive cells in bone marrow are mobilized by high mobility group box 1 (HMGB1) to regenerate injured epithelia. Proc Natl Acad Sci USA 108: 6609–6614. 16. Iinuma, S, Aikawa, E, Tamai, K, Fujita, R, Kikuchi, Y, Chino, T et al. (2015). Transplanted bone marrow– derived circulating PDGFRα+ cells restore type VII collagen in recessive dystrophic epidermolysis bullosa mouse skin graft. J Immunol 194: 1996–2003. 17. Wagner, JE, Ishida-Yamamoto, A, McGrath, JA, Hordinsky, M, Keene, DR, Woodley, DT et al. (2010). Bone marrow transplantation for recessive dystrophic epidermolysis bullosa. N Engl J Med 363: 629–639. 18. Tolar, J and Wagner, JE (2015). A biologic Velcro patch. N Engl J Med 372: 382–384. 19. Schofield, R (1978). The relationship between the spleen colony-forming cell and the haemopoietic stem cell. Blood Cells 4: 7–25.
www.moleculartherapy.org vol. 23 no. 8 august 2015
commentary
See page 1368
From Mesoderm to Mesodermatology: Bone Marrow Mesenchymal Cells Heal Skin Wounds Marketa Tolarova1 and Jakub Tolar2 doi:10.1038/mt.2015.84
R
ecessive dystrophic epidermolysis bullosa (RDEB) is caused by loss-offunction mutations in the COL7A1 gene that result in reduced levels or complete absence of type VII collagen (C7). The disease is characterized by progressively worsening blistering, inflammation, pain, scarring, and contractures. C7 is an extracellular matrix protein that homopolymerizes into anchoring fibrils (AFs) and plays a critical role in adherence of skin layers at the dermal-epidermal junction (DEJ) in cutaneous and mucosal membranes. Multiple cell types can express C7—keratinocytes, fibroblasts (FBs), mesenchymal stromal/stem cells (MSCs, Figure 1), perhaps even hematopoietic cells—but are the cell types functionally and qualitatively similar or equal? In several methodical and incisive studies, injections of allogeneic FBs or MSCs have been used for wound healing in individuals with RDEB,1–6 and the study by Kühl and colleagues reported in this issue adds to our knowledge regarding the use of MSCs as medications for wound healing in RDEB.7 Kühl et al.7 found that human MSCs express C7 in vitro at approximately the same level as FBs. When injected into the skin of a murine RDEB model (these RDEB “hypomorphic” mice express C7 at approximately 10% of wild-type levels), human MSCs deposited human C7 at the DEJ in a dose-dependent manner. Although xenografted MSCs did not proliferate, they persisted for about 1 month; human C7 decreased proportionately but was detectable up to 3 months after injections. Resistance to injury (measured by putting adhesive tape on shaved skin 2nd Faculty of Medicine, Charles University, Prague, Czech Republic; 2Stem Cell Institute and Division of Blood and Marrow Transplantation, Department of Pediatrics, University of Minnesota, Minneapolis, Minnesota, USA Correspondence: Jakub Tolar, 420 Delaware Street SE, MMC 366, Minneapolis, Minnesota 55455, USA. E-mail: [email protected]
and ripping it off) was improved, and rudimentary AFs (C7 homotrimers) increased in number along the DEJ. There were no significant local side effects and no systemic immune reactivity to human C7. In fact, whereas wounds generated in wild-type mice did not benefit from MSC injections, the naturally occurring wounds in RDEB mice showed faster wound closure after MSC local treatment than when injected with vehicle alone. Crucially, this productive tissue repair reaction may not have been solely a consequence of C7 expression but may also have been due to a synergy with the immune-modulatory and reparative functions of MSCs. This work demonstrates the possibility of an unlikely symbiosis between two essentially unrelated medical fields: hematology and dermatology. MSCs, routinely derived from hematopoietic organs such as bone marrow or umbilical cord blood,8 are shown here to merge with the goals of dermatologists
to alleviate blistering in this prototypical genodermatosis. They are clearly connected in development; hematopoietic cells, connective tissue stromal cells, and dermis are all derived from mesoderm, one of the primary germ layers of the embryo. Therefore, thinking in the new conceptual space of mesoderm derivatives and their impact on skin (mesodermatology) might facilitate a thoughtful examination of MSCs and other bone marrow–derived cells in skin disorders. This functional link between bone marrow and skin has a long history. In 1876 Julius Cohnheim argued for a bold new way of understanding tissue boundaries and hypothesized that cells mediating post-traumatic fibrosis in skin and other organs might originate in blood and bone marrow.9 In 1966 Alexander Friedenstein, in the first clear functional indication of existence of mesenchymal cells in bone marrow, demonstrated the donor origin of bone tissue after bone marrow transplantation in mice.10,11 The idea of nonhematopoietic progenitor cells present in bone marrow aspirate emerged—a cell type adherent to plastic and easily propagated in vitro, capable of differentiation into bone, fat, and cartilage—and equated with the term mesenchymal stem cell. Instead of anchoring the concept, however, a much
1
Molecular Therapy vol. 23 no. 8 august 2015
Figure 1
Mesenchymal stromal/stem cell. Colored scanning electron micrograph. Copyright © 2015 Photo Researchers, Inc. All Rights Reserved. Credit: Steve Gschmeissner/Science Source (registered trademark of Photo Researchers, Inc.).
1283
© The American Society of Gene & Cell Therapy
commentary more complex picture emerged as more data were uncovered on the heterogeneity, clonality, self-renewal, and functional diversity of these cells in both fresh and cultured forms. Almost in contrast to the complexity of the basic biology of MSCs, the relative ease with which they can be obtained, cultured, and prepared under good manufacturing practices for human use has led to a remarkable number of clinical trials. MSCs have been used clinically for two decades, and MSC infusions have shown clinical benefit in some applications, such as connective tissue injury or graft-versus-host disease (the main immune complication of allogeneic hematopoietic cell transplantation). For other indications, however, their value is less clear. Nevertheless, through application of MSCs in different injuries and disease states, several concepts have emerged: (i) MSCs are multifunctional; (ii) the capacity of MSCs to contribute to the musculoskeletal system and to modulate immune and inflammatory responses can be separated; (iii) the magnitude and quality of donor MSC engagement depends on recipient factors; (iv) MSCs interact with their cellular partners directly, by cell-to-cell contact, or indirectly, by short-range or longrange signals (free protein mediators or exosomes12); and (v) the changeability of MSCs, their conceptual resistance to traditional hierarchical thinking, and their versatility in adopting various cell fates such as mesenchymal-to-epithelial transition, may be an advantage in clinical applications. It is possible that MSCs are a mobile link via blood circulation and a modular link among stromal components of various organs. They can provide homeostasis and tissue maintenance in health, can mediate productive repair in injury states, and can also drive pathology, such as in the microenvironment of some cancers. Kühl et al. show here that skin injection of MSCs can improve wound healing in a murine model of RDEB.7 One of the key insights of this work is that the MSCs are multitalented: in addition to secreting C7, they are able to mobilize myofibroblasts, ameliorate skin inflammation, and increase skin stability. Their
1284
study shows the practical, clinically relevant advantages of increased density of MSCs in the wound, thereby paving the way for translation of MSC biology into clinical practice for RDEB individuals with moderate severity of disease, where local therapy of individual wounds can change their quality of life. For those with the severe form of RDEB, hematologists and dermatologists have worked together to understand the contribution of bone marrow cells to wound healing in RDEB preclinical models,13–16 and then to use bone marrow or umbilical cord blood for hematopoietic cell transplantation to change generalized severe RDEB from a fatal to a treatable, controllable, and eventually curable disease.17,18 The next challenge is to separate cells used as clinical-grade MSCs (fundamentally changed by the artificial environment of cell culture) from their physiological counterparts, the resident MSCs (such as pericytes and other components of tissue niches),19 where MSC identity is defined by local microenvironment and their fate is directed by extracellular matrix and neighboring cells. If we can combine the current clinical insights on the use of MSCs with new tools to explore them in their physiological circumstances, we will unveil not only their functional diversity but possible ways to manipulate them, including niche-directed therapies. As both skin and bone marrow stromal cells and their niches have been studied extensively, we should be able to integrate this knowledge into a mesodermatological concept of MSCs. This, in turn, should lead to its practical translation into an array of bone marrow–derived therapies to enhance regeneration and healing in RDEB, other genodermatoses, and other extracellular matrix disorders. ACKNOWLEDGMENTS
J.T. is supported in part by National Institutes of Health grants R01 AR063070 and R01 AR059947, Department of Defense grant W81XWH, the EB Research Partnership, the EB Medical Research Foundation, DEBRA, and the Sohana Research Fund. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The authors thank Nancy Griggs Morgan for assistance with manuscript preparation and editing.
REFERENCES
1. Wong, T, Gammon, L, Liu, L, Mellerio, JE, DoppingHepenstal, PJ, Pacy, J et al. (2008). Potential of fibroblast cell therapy for recessive dystrophic epidermolysis bullosa. J Invest Dermatol 128: 2179–2189. 2. Petrof, G, Martinez-Queipo, M, Mellerio, JE, Kemp, P and McGrath, JA (2013). Fibroblast cell therapy enhances initial healing in recessive dystrophic epidermolysis bullosa wounds: results of a randomized, vehicle-controlled trial. Br J Dermatol 169: 1025–1033. 3. Venugopal, SS, Yan, W, Frew, JW, Cohn, HI, Rhodes, LM, Tran, K et al. (2013). A phase II randomized vehicle-controlled trial of intradermal allogeneic fibroblasts for recessive dystrophic epidermolysis bullosa. J Am Acad Dermatol 69: 898–908. 4. Conget, P, Rodriguez, F, Kramer, S, Allers, C, Simon, V, Palisson, F et al. (2010). Replenishment of type VII collagen and re-epithelialization of chronically ulcerated skin after intradermal administration of allogeneic mesenchymal stromal cells in two patients with recessive dystrophic epidermolysis bullosa. Cytotherapy 12: 429–431. 5. Kern, JS, Loeckermann, S, Fritsch, A, Hausser, I, Roth, W, Magin, TM et al. (2009). Mechanisms of fibroblast cell therapy for dystrophic epidermolysis bullosa: high stability of collagen VII favors long-term skin integrity. Mol Ther 17: 1605–1615. 6. Petrof, G, Lwin, SM, Martinez-Queipo, M, AbdulWahab, A, Tso, S, Mellerio, JE et al. (2015). Potential of systemic allogeneic mesenchymal stromal cell therapy for children with recessive dystrophic epidermolysis bullosa. J Invest Dermatol; e-pub ahead of print 23 April 2015. 7. Kühl, T, Mezger, M, Hausser, I, Handgretinger, R, Bruckner-Tuderman, L and Nyström, A (2015). High local concentrations of intradermal MSCs restore skin integrity and facilitate wound healing in dystrophic eppidermolysis bullosa. Mol Ther 23: 1368–1379 8. Nevala-Plagemann, C, Lee, C and Tolar, J (2015). Placenta-based therapies for the treatment of epidermolysis bullosa. Cytotherapy 17: 786–795. 9. Cohnheim, J (1867). Ueber Entzündung und Eiterung. Virch Arch Path Anat 40: 1–79. 10. Friedenstein, AJ, Piatetzky, S, II and Petrakova, KV (1966). Osteogenesis in transplants of bone marrow cells. J Embryol Exp Morphol 16: 381–390. 11. Friedenstein, AJ, Petrakova, KV, Kurolesova, AI and Frolova, GP (1968). Heterotopic transplants of bone marrow. Analysis of precursor cells for osteogenic and hematopoietic tissues. Transplantation 6: 230–247. 12. Lai, RC, Yeo, RW and Lim, SK (2015). Mesenchymal stem cell exosomes. Semin Cell Dev Biol. 40: 82–88. 13. Chino, T, Tamai, K, Yamazaki, T, Otsuru, S, Kikuchi, Y, Nimura, K et al. (2008). Bone marrow cell transfer into fetal circulation can ameliorate genetic skin diseases by providing fibroblasts to the skin and inducing immune tolerance. Am J Pathol 173: 803–814. 14. Tolar, J, Ishida-Yamamoto, A, Riddle, M, McElmurry, RT, Osborn, M, Xia, L et al. (2009). Amelioration of epidermolysis bullosa by transfer of wild-type bone marrow cells. Blood 113: 1167–1174. 15. Tamai, K, Yamazaki, T, Chino, T, Ishii, M, Otsuru, S, Kikuchi, Y et al. (2011). PDGFRalpha-positive cells in bone marrow are mobilized by high mobility group box 1 (HMGB1) to regenerate injured epithelia. Proc Natl Acad Sci USA 108: 6609–6614. 16. Iinuma, S, Aikawa, E, Tamai, K, Fujita, R, Kikuchi, Y, Chino, T et al. (2015). Transplanted bone marrow– derived circulating PDGFRα+ cells restore type VII collagen in recessive dystrophic epidermolysis bullosa mouse skin graft. J Immunol 194: 1996–2003. 17. Wagner, JE, Ishida-Yamamoto, A, McGrath, JA, Hordinsky, M, Keene, DR, Woodley, DT et al. (2010). Bone marrow transplantation for recessive dystrophic epidermolysis bullosa. N Engl J Med 363: 629–639. 18. Tolar, J and Wagner, JE (2015). A biologic Velcro patch. N Engl J Med 372: 382–384. 19. Schofield, R (1978). The relationship between the spleen colony-forming cell and the haemopoietic stem cell. Blood Cells 4: 7–25.
www.moleculartherapy.org vol. 23 no. 8 august 2015
