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448 Part V: Therapeutic Principles Chapter 30: Regenerative Medicine: Multipotential Cell Therapy for Tissue Repair 449

Despite the much-anticipated potential of hESCs to differentiate of sickle cell anemia, hematopoietic stem cells derived from
and replace malfunctioning cells in the body, progress toward clinical gene-corrected murine iPSCs have established preclinical proof-
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use has been hindered by the possibility of teratoma formation or the of-concept for combined gene correction and stem cell engineering.
immune rejection of the allogeneic transplanted cells and production Furthermore, insights from murine embryogenesis were applied to in
issues. vitro induction of mesoderm and ESC differentiation to blood cells via
coculture with feeder cells or generation of embryoid bodies. These
seemingly straightforward concepts, however, have proven challenging
INDUCED PLURIPOTENT STEM CELLS to mimic in human ESCs and iPSCs. Despite many attempts, current
Generation of iPSCs has connected several previous observations into a technology appears to lead only to low hematopoietic chimerism after
coherent outline. For example, the ability of transcription factor MyoD transplantation of hematopoietic stem cells derived from pluripotent
to change fibroblasts to myoblasts and of transcription factor Anten- human cells. 30,31 An alternative to generation of transplantable human
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napedia to change development of antennae into legs in Drosophilla, hematopoietic stem cells is direct conversion of fibroblasts to hemato-
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uncovered potential of a differentiated cell to assume an alternative cell poietic stem cells without the iPSC intermediate. This is done by using
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fate as a result of defined, externally provided signals. forced expression of OCT4 and differentiation of human pluripotent
The understanding of induced pluripotency has become more progenitor cells by forced expression of GATA-1, ETV2, and TAL-1 into
refined as additional reprogramming factors are identified, the critical hemoendothelial cells. 33
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role of epigenetic regulation is uncovered, and with the dynamics of The replacement of hematopoiesis by marrow transplantation is
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iPSC generation (from initially random event to deterministic process) the prototype of regenerative medicine. While the initial experimen-
more fully developed. 19 tation with marrow transfers on both sides of the Atlantic was almost
The reprogramming technology applied to human cells iPSCs immediately recognized as a pioneering effort in hematology, it was
allows for modeling various, typically genetic, disorders. 20,21 Further- only later understood as a turning point in the larger field of regenera-
more, organoid cultures derived from the patients themselves allow for tive medicine. The critical evidence was the ability of a relatively small
high throughput drug testing that would be impossible without the sup- number of donor cells to repopulate the host and reconstitute its full
ply of differentiated cells from patient-specific iPSCs. lymphohematopoietic system. Although initially applied to leukemia
The first preclinical example of iPSC technology conceptually and lymphoma therapy in an effort to replace the malignant lympho-
applied to human disease has been amelioration of the sickle cell ane- hematopoiesis with a healthy wild-type system, it later became clear
mia phenotype in a murine model. At the time of this writing, the first that the immune elimination of the tumor (graft-versus-leukemia,
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iPSC-based clinical trial opened in Japan for individuals with exudative graft-versus-lymphoma) is the dominant mechanism behind successful
age-related macular degeneration. 23 therapy in many cases.
New knowledge derived from the rapidly expanding iPSC field has This remarkable regenerative capacity of hematopoietic stem cells
also reenergized the technology of direct reprogramming, whereby one established marrow, and later cord blood, transplantation as the blue-
differentiated cellular phenotype (such as a dermal fibroblast) can be print for other stem cell therapies.
induced to convert into another somatic cell (such as a neuron) without
the intermediate iPSC stage. In contrast to the expandable iPSC-based
generation of differentiated cells, the process of direct reprogramming MESENCHYMAL STROMAL CELLS
makes it more challenging to produce the large numbers of cells needed Originally defined by how they were identified—they adhered to the
for therapeutic intervention. surface of a culture dish —marrow-derived mesenchymal stromal/
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An example of this strategy has been in vivo trans-differentiation stem cells (MSCs) were then identified as key support cells in the cellu-
of exocrine pancreatic cells or biliary epithelial cells into insulin- lar niche. Evidence from different sources (e.g., marrow, umbilical cord
producing endocrine cells in rodent models. 24,25 A conceptually different blood, and adipose tissue) suggests that MSCs have different functions
concept to solve the same clinical challenge has been blastocyst com- in various organs (e.g., as pericytes in adventitia of blood vessels, or as
plementation whereby rat iPSCs were injected into blastocysts that had supporting cells in marrow periosteal and endovascular hematopoietic
been derived from mice deficient in pancreatic organogenesis, which niches). 35–37 In addition to this developmental heterogeneity, cultured
resulted in the development of a functional rat pancreas in mice. In MSCs display various levels of “stemness,” and the cellular products
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addition to reducing the cell numbers needed to create a physiologically used in therapeutic applications may be more a cell culture artifact
meaningful effect, the efficacy of both strategies may be enhanced by than a counterpart to physiologic functionally integrated MSCs. This
targeting them into a permissive cellular niche. is not necessarily a disadvantage, as the culture process enables both
amplification of cell numbers and defined release criteria for clinical
use.
HEMATOPOIETIC STEM CELLS The most striking application of MSCs in medicine to date relies not
The earliest advances with clinical potential will most likely arise from on the regenerative capacity of MSCs alone but on the immunosuppres-
understanding reprogramming in hematopoietic stem cells. Not only sive potential of MSC cultures in the setting of severe graft-versus-host
was hematopoietic cell transplantation the first stem cell therapy, devel- disease (GVHD), 38–40 a serious complication of allogeneic hematopoi-
oped close to half a century ago, but reports of using defined factors to etic cell transplantation (HCT). The treatment options for individuals
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turn committed blood progenitor cells into transplantable hematopoi- with glucocorticoid-resistant severe GVHD have been inadequate, and
etic cells 27,28 suggest that robust generation of clinical-grade, patient-specific mortality in this subgroup remains high. In a paradigm-changing study,
autologous grafts for transplantation is possible. it was demonstrated that culture-expanded MSCs can ameliorate severe
Equally important has been combination of pluripotentiality of GVHD. 38,42 The induction and maintenance of MSC-driven regulation
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ESCs and iPSCs with their commitment to specific lineages, such as of immune and inflammatory reactions has made it possible to assess
hemogenic differentiation program. Derivation of hematopoietic stem their role in autoimmune and inflammatory disorders such as Crohn
cells from murine ESC and their genetic correction has been used in disease, arthritis, diabetes, organ rejection, and bridge therapy before
murine severe combined immune deficiency ; similarly in a model solid-organ transplantation. 44–46
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