a Department of Hepatology, Chancellor’s Building, University of Edinburgh, Edinburgh, Scotland;
http://www.100md.com
《干细胞学杂志》
Potential of Hematopoietic Stem Cell Therapy in Hepatology: A Critical Review
Steven Massona, David J. Harrisonb, John N. Plevrisa, Philip N. Newsomea,bKey Words. Bone marrow ? Cell differentiation ? Cell fusion ? Hematopoietic stem cells Hepatocytes ? Liver regeneration ? Stem cells
Correspondence: Philip Newsome, M.D., Ph.D., Department of Hepatology, Chancellor’s Building, University of Edinburgh, Edinburgh EH16 4SB, U.K. Telephone: 0044-131-242-1712; Fax: 0044-131-242-1633; e-mail: P.Newsome@ed.ac.uk
ABSTRACT
Presently, orthotopic liver transplantation is the major therapeutic option for patients with acute and chronic end-stage liver disease. However, a shortage of suitable donor organs and requirement for immunosuppression restrict its usage, highlighting the need to find suitable alternatives.
A novel and exciting approach could be offered through the current developments in the field of stem cell biology. In the past few years, multiple studies have demonstrated that adult stem cell plasticity is far greater and complex than previously thought, raising expectations that it could lay the foundations for new cellular therapies in regenerative medicine. In this review, the evidence for adult stem cell plasticity will be discussed with respect to hepatology, covering experimental models in animal and human tissues along with a discussion of the factors, putative mechanisms involved, current controversies, and potential clinical implications.
HEMATOPOIETIC STEM CELLS: MULTILINEAGE PLASTICITY
The adult liver has a remarkable regenerative capacity, with mature hepatocytes proliferating rapidly in response to most mild-to-moderate liver injuries and an intrahepatic stem cell compartment contributing in more severe injury. Experimental models of partial hepatectomy or carbon tetrachloride (CCl4) toxicity indicate that regeneration can occur wholly through proliferation of mature hepatocytes. However, in models of more severe injury, such as CCl4 combined with the hepatocarcinogen 2-acetylaminofluorene (2-AAF), which inhibits mature hepatocyte proliferation, additional cellular components contribute. After such injury, large numbers of small oval cells appear, which are able to differentiate into both hepatocytes and ductular cells . Similarly, addition of the DNA alkylating agent retrorsine to CCl4 gives rise to progenitors that express phenotypic characteristics of, but are morphologically distinct from, oval cells . This has led investigators to establish the concept that liver regeneration occurs on three separate levels: hepatocytes, intrahepatic stem cells, and extrahepatic stem cells .
In the first account of hepatic transdifferentiation, lethally irradiated rats underwent cross-sex or cross-strain bone marrow transplantation followed by administration of 2-AAF to suppress hepatocyte proliferation and CCl4 to induce hepatic injury. Examination of the host liver demonstrated hepatic cells that were donor derived, as detected by expression of dipeptidyl peptidase IV enzyme (DPPIV+) in DPPIV– rats or the Y chromosome in female animals . This led to the conclusion that marrow-derived cells could act as progenitors for hepatic cells, albeit in a model of injury in which the replicative capacity of mature host hepatocytes was impaired.
However, liver repopulation by marrow-derived cells has also been seen in the absence of any intentional liver injury. Bone marrow from male donors was infused into irradiated female mice, and the liver tissue was analyzed by fluorescent in situ hybridization (FISH) for the Y chromosome and albumin mRNA, demonstrating significant levels of donor-derived hepatocytes . Further work, again in mice, indicated that a single male HSC transplanted into an irradiated female recipient demonstrated diverse differentiative potential. In addition to hepatic engraftment, epithelial cells from throughout the gastrointestinal tract, bronchus, and skin were donor derived . Notably, in multiorgan engraftment, highest levels and most diffuse clustering of donor cells were seen in alveolar epithelium. The authors postulated that the observed differences in engraftment may relate to the injury induced by irradiation, because lung tissue is known to be more radiosensitive . It remains unclear whether irradiation injury is relevant in the hepatic models, because although radiation-induced liver damage is known to occur , it is usually seen with larger doses than those used in such preparative regimens. Additionally, there was no histological evidence of tissue damage in either study, raising the possibility that a contribution of bone marrow cells may occur even in physiological maintenance or minimal liver injury.
Other groups have been unable to reproduce these findings. Chimeric animals were generated by transplantation of a single green fluorescent protein (GFP)–marked HSC into sublethally irradiated mice in the absence of specific tissue injury. Single HSCs resulted in significant hematopoietic engraftment, but hepatocytes were produced at a frequency of only approximately 1 in 70,000 cells . Furthermore, GFP+:GFP– parabiotic mice with a common circulatory system were created, surgically enabling evaluation of circulating stem cells and HSC engraftment in a model that does not even require irradiation. Despite successful hematopoietic cross-engraftment, there was no engraftment of non-hematopoietic tissue, leading the authors to conclude that HSCs played no role in the production of nonhematopoietic cells under physiological conditions. A different group used various liver injury models to assess hepatic regeneration after gender-mismatched bone marrow transplantation. No significant contribution was demonstrated .
Nevertheless, several other studies seem to corroborate adult stem cell plasticity, with moderate to severe injury increasing the level of hepatic transdifferentiation, a finding augmented additionally in model systems in which the donor HSCs have a survival advantage . The significance of such a survival advantage is confirmed in a transgenic model based on the protective effect of the antiapoptotic gene, Bcl-2, against Fas-mediated cell death. Bone marrow from mice expressing this transgene, under the control of a liver-specific promoter, was infused into normal mice. Some mice underwent repeated injections with a Fas-agonist antibody to induce liver injury, whereas others did not. Only those that had received antibody injections showed mature hepatocytes expressing Bcl-244, implying that transdifferentiation is inefficient under physiological conditions and that tissue injury such as accumulation of toxic catabolites or apoptotic challenge is required to generate a more robust response. It is interesting to note that a recent study of the contribution of HSCs in a model of liver fibrosis demonstrated higher than previously reported levels of marrow-derived hepatocytes. Transgenic mice expressing GFP were used as a source of bone marrow, and up to 26% of the recipient liver was repopulated by 4 weeks . For the first time, donor bone marrow cells without a survival advantage resulted in robust and efficient regeneration, although there has been some concern raised regarding ambiguous identification and unusual architecture of the reporter cells .
Although the presence and severity of liver injury may be important in regulating the extent of stem cell plasticity and engraftment, the reported variation in the literature is still marked (Table 1). Detailed analysis of the models used demonstrates that different subpopulations of stem cells may have different levels of functional plasticity. Although initial studies mainly used unfractionated bone marrow, subsequent work highlighted the capacity of CD34+ cells for hepatic engraftment . More recently, the SP fraction of marrow cells was demonstrated to contain cells with similar ability , and yet it is rich in CD34– stem cells . Indeed, the authors of these many studies have recognized that supposedly minimal differences in experimental methods may be responsible for the observed discrepancies. Such factors have been discussed in more detail in recent commentaries .
Table 1. Stem cell–derived hepatocytic differentiation in animal models
DO STEM CELL–DERIVED HEPATOCYTES HAVE FUNCTIONAL SIGNIFICANCE?
The characterization of stem cell biology in animal work is clearly important, but clinical application requires convincing evidence that human stem cells also share the properties demonstrated by adult rodent stem cells. Cellular phenotypes of human hepatic stem cells and tissue reactions similar to those seen in animal models have been described in a variety of human acute and chronic liver diseases .
The first reports implying transdifferentiation in human cells used archival biopsies, in which liver specimens from recipients of sex-mismatched bone marrow or liver transplants were analyzed for marrow-derived hepatic cell types. Using immunohistochemistry with FISH staining for the Y chromosome, analysis of liver and bone marrow samples revealed that hepatic cell types had arisen from a marrow-derived population . Such differentiation was also reported in patients who had undergone transfusion of peripheral-blood stem cells for the treatment of hematological malignancy, suggesting that such progenitor cells circulate in the blood . In this study, epithelial cells of the skin and gastrointestinal tract were also seen to derive from cells of donor origin.
There is some concern that Y chromosome–positive cells identified in the female livers in these studies could occur as a result of the transplacental passage of male fetal blood cells during pregnancy , because fetal–maternal microchimerism has previously been documented . However, this cannot be entirely responsible, because these male cells have been seen in at least one human liver from a nulliparous female along with a female with no history of male childbearing . In addition, in the murine models, female recipients were all nulliparous .
Additional analysis of this archival work allows interesting comparisons with the animal models. For example, specimens from human liver allografts revealed varying degrees of injury, with mild biliary obstruction in most and fibrosing cholestatic hepatitis in one. Injury severity correlated directly with hepatic engraftment frequency , yet recipients of bone marrow transplantation also demonstrated significant hepatic engraftment despite the absence of overt hepatic injury, in keeping with the murine model previously discussed . Additionally, histology revealed that in most allografts, hepatocyte engraftment was isolated and scattered, whereas the liver with severe recurrent disease showed more extensive ductular clustering. Again, this is analogous to the animal data suggesting that, as in animals, there may be several modes of human hepatic regeneration with a potential role for adult stem cells in both physiological maintenance and acute injury.
Indeed, this highlights the need for an in vivo system in which to study human stem cells directly (Table 2). Human cord blood cells were infused into sublethally irradiated nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, and at euthanasia, murine livers were analyzed for the presence of human hepatic cells. Despite the absence of intentional liver damage, human-derived hepatocytes were detected in murine liver . Using a similar xenogeneic transplantation model, engrafted human cells were seen to produce human albumin mRNA, as detected by reverse transcription–polymerase chain reaction .
Table 2. Hepatocyte differentiation of hematopoietic cells in human cord blood in immunodeficient mice
The role of liver injury in such models has also been reported. Hepatic injury, induced by one-third partial hepatectomy and 2-AAF after infusion of human cord blood cells, led to the identification of functional hepatocytes . Notably, in another xenogeneic model comparing injured and noninjured mice, only those who had been administered CCl4 were seen to express human albumin . Additionally, this study demonstrated that it was CD34+ HSCs that gave rise to this phenomenon. However, as in murine models, this remains controversial. In another study, human stem cells expressing the receptor for the complement molecule C1q (C1qRp) were infused . This is a human homologue of a mouse stem cell antigen, AA4, recognized by a monoclonal antibody used to define murine multipotent hematopoietic progenitors . Notably, C1qRp is expressed on both CD34+ and CD34– cells, suggesting that this marker may define an even more primitive subpopulation. Perhaps differences in the differentiative potential of the two populations are responsible for the conflicting data regarding the role of injury.
Furthermore, recent studies in the NOD/SCID model demonstrate the role of the stromal cell–derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4) axis in regulating the migration of HSCs to damaged liver . Intriguingly, hepatocyte growth factor, which is upregulated in liver injury and has been shown to augment hepatocytic differentiation of engrafted HSCs , was shown to play a key role in recruiting stem cells via its interaction with SDF-1 . Some of the engrafted human cells differentiated into albumin-producing hepatocyte-like cells. Another group has used the SCID model to examine different mechanistic factors. Consistent downregulation of ?2 microglobluin (?2M), an integral part of the major histocompatibility complex, was observed early after stem cell transplantation in liver tissue in which human albumin expression was upregulated . It was suggested that this switching off of ?2M may be an important mechanism in escaping the host immune system. Although there is clearly much to be established regarding these mechanistic factors, human blood-derived cells can, under strictly defined circumstances perhaps similar to previous animal studies, transdifferentiate into hepatic cell types, and liver injury influences the observed differentiation. As yet, there are no data to demonstrate that human stem cells have therapeutic potential, but this xenogeneic transplantation model will be useful in studying human adult stem cell biology further.
THE ROLE OF CELL FUSION
Despite uncertainty surrounding the mechanisms underlying adult stem cell plasticity, there is much speculation regarding potential clinical implications. Enthusiasm has been heightened by pioneering clinical trials in other disciplines. For example, in cardiology, the delivery of marrow-derived cells into the coronary circulation of human subjects was reported to improve blood flow and cardiac function in ischemic myocardium . In hepatology, the data presented here provide hope that somatic stem cells could eventually be used in tissue replacement protocols for the treatment of inherited and acquired end-stage liver diseases.
Use of adult stem cells overcomes many of the moral and ethical barriers of ES cell manipulation, and if somatic cells genuinely can switch lineage barriers, then HSCs are an ideal source. There is already considerable experience in their handling, and they are relatively accessible. Additionally, their administration may induce immunological tolerance, because they may potentially induce hematopoietic microchimerism, thus obviating the need for immunosuppression. Rather than relying on cadaveric organs from deceased donors who are often immunologically disparate, HSCs offer ready availability of liver-repopulating stem cells or progenitors obtained from living donors.
Despite such enthusiasm, at present there remains significant uncertainty as to what such cells would accomplish in the clinical setting, and there are many issues to be addressed before translation into clinical practice. First, a better understanding of the mechanisms that modulate the role of HSCs in physiological maintenance and liver injury is required. Specifically, more work is required to establish whether HSCs may play a therapeutic role in chronic liver disease, which is a more clinically relevant target. Second, the mechanism of genomic plasticity needs to be further defined. If transdifferentiation truly is responsible, then there could be wide-ranging utility for a range of acquired liver diseases, whereas if fusion is responsible, then this could be exploited to deliver corrective genes for hepatic metabolic disorders, as long as genetic stability in the reprogrammed cells could be assured. However, given the great disparity in levels of transdifferentiation reported and controversy regarding the mechanism responsible, these issues seem far from resolved and bring into question therapeutic strategies based on this idea. Additionally, much work is needed to assess practical issues; if HSCs are capable of engrafting directly as hepatocytes, then direct intraorgan injection may facilitate a more robust response and obviate the need for irradiation to deplete the recipient’s bone marrow. Alternatively, if marrow engraftment is a prerequisite, then peripheral administration with marrow preparation and manipulation of the hepatic microenvironment to maximally attract HSCs will likely be required.
In conclusion, the new findings in adult stem cell biology are transforming our understanding of tissue repair with promising hopes of regenerative hepatology. However, perhaps we should remain cautious at present. Adult stem cell plasticity does occur, but it is a rare event even under selective pressure, and it remains to be seen whether this will be clinically significant in the human context.
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Steven Massona, David J. Harrisonb, John N. Plevrisa, Philip N. Newsomea,bKey Words. Bone marrow ? Cell differentiation ? Cell fusion ? Hematopoietic stem cells Hepatocytes ? Liver regeneration ? Stem cells
Correspondence: Philip Newsome, M.D., Ph.D., Department of Hepatology, Chancellor’s Building, University of Edinburgh, Edinburgh EH16 4SB, U.K. Telephone: 0044-131-242-1712; Fax: 0044-131-242-1633; e-mail: P.Newsome@ed.ac.uk
ABSTRACT
Presently, orthotopic liver transplantation is the major therapeutic option for patients with acute and chronic end-stage liver disease. However, a shortage of suitable donor organs and requirement for immunosuppression restrict its usage, highlighting the need to find suitable alternatives.
A novel and exciting approach could be offered through the current developments in the field of stem cell biology. In the past few years, multiple studies have demonstrated that adult stem cell plasticity is far greater and complex than previously thought, raising expectations that it could lay the foundations for new cellular therapies in regenerative medicine. In this review, the evidence for adult stem cell plasticity will be discussed with respect to hepatology, covering experimental models in animal and human tissues along with a discussion of the factors, putative mechanisms involved, current controversies, and potential clinical implications.
HEMATOPOIETIC STEM CELLS: MULTILINEAGE PLASTICITY
The adult liver has a remarkable regenerative capacity, with mature hepatocytes proliferating rapidly in response to most mild-to-moderate liver injuries and an intrahepatic stem cell compartment contributing in more severe injury. Experimental models of partial hepatectomy or carbon tetrachloride (CCl4) toxicity indicate that regeneration can occur wholly through proliferation of mature hepatocytes. However, in models of more severe injury, such as CCl4 combined with the hepatocarcinogen 2-acetylaminofluorene (2-AAF), which inhibits mature hepatocyte proliferation, additional cellular components contribute. After such injury, large numbers of small oval cells appear, which are able to differentiate into both hepatocytes and ductular cells . Similarly, addition of the DNA alkylating agent retrorsine to CCl4 gives rise to progenitors that express phenotypic characteristics of, but are morphologically distinct from, oval cells . This has led investigators to establish the concept that liver regeneration occurs on three separate levels: hepatocytes, intrahepatic stem cells, and extrahepatic stem cells .
In the first account of hepatic transdifferentiation, lethally irradiated rats underwent cross-sex or cross-strain bone marrow transplantation followed by administration of 2-AAF to suppress hepatocyte proliferation and CCl4 to induce hepatic injury. Examination of the host liver demonstrated hepatic cells that were donor derived, as detected by expression of dipeptidyl peptidase IV enzyme (DPPIV+) in DPPIV– rats or the Y chromosome in female animals . This led to the conclusion that marrow-derived cells could act as progenitors for hepatic cells, albeit in a model of injury in which the replicative capacity of mature host hepatocytes was impaired.
However, liver repopulation by marrow-derived cells has also been seen in the absence of any intentional liver injury. Bone marrow from male donors was infused into irradiated female mice, and the liver tissue was analyzed by fluorescent in situ hybridization (FISH) for the Y chromosome and albumin mRNA, demonstrating significant levels of donor-derived hepatocytes . Further work, again in mice, indicated that a single male HSC transplanted into an irradiated female recipient demonstrated diverse differentiative potential. In addition to hepatic engraftment, epithelial cells from throughout the gastrointestinal tract, bronchus, and skin were donor derived . Notably, in multiorgan engraftment, highest levels and most diffuse clustering of donor cells were seen in alveolar epithelium. The authors postulated that the observed differences in engraftment may relate to the injury induced by irradiation, because lung tissue is known to be more radiosensitive . It remains unclear whether irradiation injury is relevant in the hepatic models, because although radiation-induced liver damage is known to occur , it is usually seen with larger doses than those used in such preparative regimens. Additionally, there was no histological evidence of tissue damage in either study, raising the possibility that a contribution of bone marrow cells may occur even in physiological maintenance or minimal liver injury.
Other groups have been unable to reproduce these findings. Chimeric animals were generated by transplantation of a single green fluorescent protein (GFP)–marked HSC into sublethally irradiated mice in the absence of specific tissue injury. Single HSCs resulted in significant hematopoietic engraftment, but hepatocytes were produced at a frequency of only approximately 1 in 70,000 cells . Furthermore, GFP+:GFP– parabiotic mice with a common circulatory system were created, surgically enabling evaluation of circulating stem cells and HSC engraftment in a model that does not even require irradiation. Despite successful hematopoietic cross-engraftment, there was no engraftment of non-hematopoietic tissue, leading the authors to conclude that HSCs played no role in the production of nonhematopoietic cells under physiological conditions. A different group used various liver injury models to assess hepatic regeneration after gender-mismatched bone marrow transplantation. No significant contribution was demonstrated .
Nevertheless, several other studies seem to corroborate adult stem cell plasticity, with moderate to severe injury increasing the level of hepatic transdifferentiation, a finding augmented additionally in model systems in which the donor HSCs have a survival advantage . The significance of such a survival advantage is confirmed in a transgenic model based on the protective effect of the antiapoptotic gene, Bcl-2, against Fas-mediated cell death. Bone marrow from mice expressing this transgene, under the control of a liver-specific promoter, was infused into normal mice. Some mice underwent repeated injections with a Fas-agonist antibody to induce liver injury, whereas others did not. Only those that had received antibody injections showed mature hepatocytes expressing Bcl-244, implying that transdifferentiation is inefficient under physiological conditions and that tissue injury such as accumulation of toxic catabolites or apoptotic challenge is required to generate a more robust response. It is interesting to note that a recent study of the contribution of HSCs in a model of liver fibrosis demonstrated higher than previously reported levels of marrow-derived hepatocytes. Transgenic mice expressing GFP were used as a source of bone marrow, and up to 26% of the recipient liver was repopulated by 4 weeks . For the first time, donor bone marrow cells without a survival advantage resulted in robust and efficient regeneration, although there has been some concern raised regarding ambiguous identification and unusual architecture of the reporter cells .
Although the presence and severity of liver injury may be important in regulating the extent of stem cell plasticity and engraftment, the reported variation in the literature is still marked (Table 1). Detailed analysis of the models used demonstrates that different subpopulations of stem cells may have different levels of functional plasticity. Although initial studies mainly used unfractionated bone marrow, subsequent work highlighted the capacity of CD34+ cells for hepatic engraftment . More recently, the SP fraction of marrow cells was demonstrated to contain cells with similar ability , and yet it is rich in CD34– stem cells . Indeed, the authors of these many studies have recognized that supposedly minimal differences in experimental methods may be responsible for the observed discrepancies. Such factors have been discussed in more detail in recent commentaries .
Table 1. Stem cell–derived hepatocytic differentiation in animal models
DO STEM CELL–DERIVED HEPATOCYTES HAVE FUNCTIONAL SIGNIFICANCE?
The characterization of stem cell biology in animal work is clearly important, but clinical application requires convincing evidence that human stem cells also share the properties demonstrated by adult rodent stem cells. Cellular phenotypes of human hepatic stem cells and tissue reactions similar to those seen in animal models have been described in a variety of human acute and chronic liver diseases .
The first reports implying transdifferentiation in human cells used archival biopsies, in which liver specimens from recipients of sex-mismatched bone marrow or liver transplants were analyzed for marrow-derived hepatic cell types. Using immunohistochemistry with FISH staining for the Y chromosome, analysis of liver and bone marrow samples revealed that hepatic cell types had arisen from a marrow-derived population . Such differentiation was also reported in patients who had undergone transfusion of peripheral-blood stem cells for the treatment of hematological malignancy, suggesting that such progenitor cells circulate in the blood . In this study, epithelial cells of the skin and gastrointestinal tract were also seen to derive from cells of donor origin.
There is some concern that Y chromosome–positive cells identified in the female livers in these studies could occur as a result of the transplacental passage of male fetal blood cells during pregnancy , because fetal–maternal microchimerism has previously been documented . However, this cannot be entirely responsible, because these male cells have been seen in at least one human liver from a nulliparous female along with a female with no history of male childbearing . In addition, in the murine models, female recipients were all nulliparous .
Additional analysis of this archival work allows interesting comparisons with the animal models. For example, specimens from human liver allografts revealed varying degrees of injury, with mild biliary obstruction in most and fibrosing cholestatic hepatitis in one. Injury severity correlated directly with hepatic engraftment frequency , yet recipients of bone marrow transplantation also demonstrated significant hepatic engraftment despite the absence of overt hepatic injury, in keeping with the murine model previously discussed . Additionally, histology revealed that in most allografts, hepatocyte engraftment was isolated and scattered, whereas the liver with severe recurrent disease showed more extensive ductular clustering. Again, this is analogous to the animal data suggesting that, as in animals, there may be several modes of human hepatic regeneration with a potential role for adult stem cells in both physiological maintenance and acute injury.
Indeed, this highlights the need for an in vivo system in which to study human stem cells directly (Table 2). Human cord blood cells were infused into sublethally irradiated nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, and at euthanasia, murine livers were analyzed for the presence of human hepatic cells. Despite the absence of intentional liver damage, human-derived hepatocytes were detected in murine liver . Using a similar xenogeneic transplantation model, engrafted human cells were seen to produce human albumin mRNA, as detected by reverse transcription–polymerase chain reaction .
Table 2. Hepatocyte differentiation of hematopoietic cells in human cord blood in immunodeficient mice
The role of liver injury in such models has also been reported. Hepatic injury, induced by one-third partial hepatectomy and 2-AAF after infusion of human cord blood cells, led to the identification of functional hepatocytes . Notably, in another xenogeneic model comparing injured and noninjured mice, only those who had been administered CCl4 were seen to express human albumin . Additionally, this study demonstrated that it was CD34+ HSCs that gave rise to this phenomenon. However, as in murine models, this remains controversial. In another study, human stem cells expressing the receptor for the complement molecule C1q (C1qRp) were infused . This is a human homologue of a mouse stem cell antigen, AA4, recognized by a monoclonal antibody used to define murine multipotent hematopoietic progenitors . Notably, C1qRp is expressed on both CD34+ and CD34– cells, suggesting that this marker may define an even more primitive subpopulation. Perhaps differences in the differentiative potential of the two populations are responsible for the conflicting data regarding the role of injury.
Furthermore, recent studies in the NOD/SCID model demonstrate the role of the stromal cell–derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4) axis in regulating the migration of HSCs to damaged liver . Intriguingly, hepatocyte growth factor, which is upregulated in liver injury and has been shown to augment hepatocytic differentiation of engrafted HSCs , was shown to play a key role in recruiting stem cells via its interaction with SDF-1 . Some of the engrafted human cells differentiated into albumin-producing hepatocyte-like cells. Another group has used the SCID model to examine different mechanistic factors. Consistent downregulation of ?2 microglobluin (?2M), an integral part of the major histocompatibility complex, was observed early after stem cell transplantation in liver tissue in which human albumin expression was upregulated . It was suggested that this switching off of ?2M may be an important mechanism in escaping the host immune system. Although there is clearly much to be established regarding these mechanistic factors, human blood-derived cells can, under strictly defined circumstances perhaps similar to previous animal studies, transdifferentiate into hepatic cell types, and liver injury influences the observed differentiation. As yet, there are no data to demonstrate that human stem cells have therapeutic potential, but this xenogeneic transplantation model will be useful in studying human adult stem cell biology further.
THE ROLE OF CELL FUSION
Despite uncertainty surrounding the mechanisms underlying adult stem cell plasticity, there is much speculation regarding potential clinical implications. Enthusiasm has been heightened by pioneering clinical trials in other disciplines. For example, in cardiology, the delivery of marrow-derived cells into the coronary circulation of human subjects was reported to improve blood flow and cardiac function in ischemic myocardium . In hepatology, the data presented here provide hope that somatic stem cells could eventually be used in tissue replacement protocols for the treatment of inherited and acquired end-stage liver diseases.
Use of adult stem cells overcomes many of the moral and ethical barriers of ES cell manipulation, and if somatic cells genuinely can switch lineage barriers, then HSCs are an ideal source. There is already considerable experience in their handling, and they are relatively accessible. Additionally, their administration may induce immunological tolerance, because they may potentially induce hematopoietic microchimerism, thus obviating the need for immunosuppression. Rather than relying on cadaveric organs from deceased donors who are often immunologically disparate, HSCs offer ready availability of liver-repopulating stem cells or progenitors obtained from living donors.
Despite such enthusiasm, at present there remains significant uncertainty as to what such cells would accomplish in the clinical setting, and there are many issues to be addressed before translation into clinical practice. First, a better understanding of the mechanisms that modulate the role of HSCs in physiological maintenance and liver injury is required. Specifically, more work is required to establish whether HSCs may play a therapeutic role in chronic liver disease, which is a more clinically relevant target. Second, the mechanism of genomic plasticity needs to be further defined. If transdifferentiation truly is responsible, then there could be wide-ranging utility for a range of acquired liver diseases, whereas if fusion is responsible, then this could be exploited to deliver corrective genes for hepatic metabolic disorders, as long as genetic stability in the reprogrammed cells could be assured. However, given the great disparity in levels of transdifferentiation reported and controversy regarding the mechanism responsible, these issues seem far from resolved and bring into question therapeutic strategies based on this idea. Additionally, much work is needed to assess practical issues; if HSCs are capable of engrafting directly as hepatocytes, then direct intraorgan injection may facilitate a more robust response and obviate the need for irradiation to deplete the recipient’s bone marrow. Alternatively, if marrow engraftment is a prerequisite, then peripheral administration with marrow preparation and manipulation of the hepatic microenvironment to maximally attract HSCs will likely be required.
In conclusion, the new findings in adult stem cell biology are transforming our understanding of tissue repair with promising hopes of regenerative hepatology. However, perhaps we should remain cautious at present. Adult stem cell plasticity does occur, but it is a rare event even under selective pressure, and it remains to be seen whether this will be clinically significant in the human context.
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