Extracardiac progenitor cells repopulate most major cell types in the transplanted human heart

EPCs and pathological angiogenesis: when good cells go bad

Bone-marrow-derived endothelial progenitor cells (EPCs) contribute to angiogenesis-mediated pathological neovascularization, and recent studies have begun to recognize the biological significance of this contribution. This review will discuss the ability of EPCs to contribute to neovascularization in both physiological and pathological conditions. Circulating EPCs were originally identified in 1997 by Asahara as CD34(+) VEGFR2(+) mononuclear cells. These cells differentiated into an endothelial phenotype, expressed endothelial markers, and incorporated into neovessels at sites of ischemia (Asahara et al., 1997). EPCs provide both instructive (release of pro-angiogenic cytokines) and structural (vessel incorporation and stabilization) functions that contribute to the initiation of neo-angiogenesis. EPC populations can be characterized based on surface markers of freshly isolated cells, or they can be described by their in vitro characteristics once placed in culture. However, a major stumbling block to progress in the field has been the lack of consensus among investigators as to the optimal characterization of EPCs. This review intends to address the role of both EPC classes and evaluate how they interact in the setting of pathological angiogenesis. Since the EPCs may be responsible for turning on the "angiogenic switch," strategies have been employed to keep this switch in the "off" position for diseases like cancer, retinopathy, and wet AMD. The expectation is that EPCs will evolve into clinically useful prognostic and predictive tools in cancer and in ocular diseases associated with pathological neovascularization and that targeting this cell type is a key to successful management of patients suffering from diseases associated with pathological neovascularization.

Immunological challenges of cardiac transplantation: the need for better animal models to answer current clinical questions

INTRODUCTION

In the last decade, two advances have shifted attention from cellular rejection to antibody-mediated rejection (AMR) of cardiac transplants. First, more sensitive diagnostic tests for detection of AMR have been developed. Second, improvements in immunosuppression have made severe acute cellular rejection uncommon, but have had less effect on AMR.

DISCUSSION

Antibodies can contribute to graft rejection by activation of complement, by activation of vascular endothelial and smooth muscle cells, and by activation of neutrophils, macrophages or natural killer cells. Because acute rejection is a risk factor for chronic rejection in all types of organ transplants, it is has been proposed that AMR can cause chronic rejection.

CONCLUSION

Small animal models need to be developed to gain further insights into AMR and the role of antibodies in chronic graft arteriopathy. This article reviews the current clinical data and existing mouse models for AMR.

Instructive role of the vascular niche in promoting tumour growth and tissue repair by angiocrine factors

The precise mechanisms whereby anti-angiogenesis therapy blocks tumour growth or causes vascular toxicity are unknown. We propose that endothelial cells establish a vascular niche that promotes tumour growth and tissue repair not only by delivering nutrients and O2 but also through an 'angiocrine' mechanism by producing stem and progenitor cell-active trophogens. Identification of endothelial-derived instructive angiocrine factors will allow direct tumour targeting, while diminishing the unwanted side effects associated with the use of anti-angiogenic agents.

VEGF induces differentiation of functional endothelium from human embryonic stem cells: implications for tissue engineering

OBJECTIVE

Human embryonic stem cells (hESCs) offer a sustainable source of endothelial cells for therapeutic vascularization and tissue engineering, but current techniques for generating these cells remain inefficient. We endeavored to induce and isolate functional endothelial cells from differentiating hESCs.

METHODS AND RESULTS

To enhance endothelial cell differentiation above a baseline of approximately 2% in embryoid body (EB) spontaneous differentiation, 3 alternate culture conditions were compared. Vascular endothelial growth factor (VEGF) treatment of EBs showed the best induction, with markedly increased expression of endothelial cell proteins CD31, VE-Cadherin, and von Willebrand Factor, but not the hematopoietic cell marker CD45. CD31 expression peaked around days 10 to 14. Continuous VEGF treatment resulted in a 4- to 5-fold enrichment of CD31(+) cells but did not increase endothelial proliferation rates, suggesting a primary effect on differentiation. CD31(+) cells purified from differentiating EBs upregulated ICAM-1 and VCAM-1 in response to TNFalpha, confirming their ability to function as endothelial cells. These cells also expressed multiple endothelial genes and formed lumenized vessels when seeded onto porous poly(2-hydroxyethyl methacrylate) scaffolds and implanted in vivo subcutaneously in athymic rats. Collagen gel constructs containing hESC-derived endothelial cells and implanted into infarcted nude rat hearts formed robust networks of patent vessels filled with host blood cells.

CONCLUSIONS

VEGF induces functional endothelial cells from hESCs independent of endothelial cell proliferation. This enrichment method increases endothelial cell yield, enabling applications for revascularization as well as basic studies of human endothelial biology. We demonstrate the ability of hESC-derived endothelial cells to facilitate vascularization of tissue-engineered implants.

Progenitor cells from the explanted heart generate immunocompatible myocardium within the transplanted donor heart

RATIONALE

Chronic rejection, accelerated coronary atherosclerosis, myocardial infarction, and ischemic heart failure determine the unfavorable evolution of the transplanted heart in humans.

OBJECTIVE

Here we tested whether the pathological manifestations of the transplanted heart can be corrected partly by a strategy that implements the use of cardiac progenitor cells from the recipient to repopulate the donor heart with immunocompatible cardiomyocytes and coronary vessels.

METHODS AND RESULTS

A large number of cardiomyocytes and coronary vessels were created in a rather short period of time from the delivery, engraftment, and differentiation of cardiac progenitor cells from the recipient. A proportion of newly formed cardiomyocytes acquired adult characteristics and was integrated structurally and functionally within the transplant. Similarly, the regenerated arteries, arterioles, and capillaries were operative and contributed to the oxygenation of the chimeric myocardium. Attenuation in the extent of acute damage by repopulating cardiomyocytes and vessels decreased significantly the magnitude of myocardial scarring preserving partly the integrity of the donor heart.

CONCLUSIONS

Our data suggest that tissue regeneration by differentiation of recipient cardiac progenitor cells restored a significant portion of the rejected donor myocardium. Ultimately, immunosuppressive therapy may be only partially required improving quality of life and lifespan of patients with cardiac transplantation.

The role of bone-marrow-derived cells in tumor growth, metastasis initiation and progression

Emerging evidence from murine models suggests that tumor-specific endocrine factors systemically stimulate the quiescent bone marrow (BM) compartment, resulting in the expansion, mobilization and recruitment of BM progenitor cells. Discrete subsets of tumor-instigated BM-derived progenitor cells support tumor progression and metastasis by regulating angiogenesis, inflammation and immune suppression. Notably, clinical studies have begun to reveal that increased BM recruitment in tumors is associated with poor prognosis. Thus, the BM-derived tumor microenvironment is an attractive therapeutic target, and drugs targeting the components of the microenvironment are currently in clinical trials. Here, we focus on recent advances and emerging concepts regarding the intriguing role of BM-derived cells in tumor growth, metastasis initiation and progression, and we discuss future directions in the context of novel diagnostic and therapeutic opportunities.

Cell therapy with bone marrow cells for myocardial regeneration

Cell therapy has tremendous potential for the damaged heart, which has limited self-renewing capability. Bone marrow (BM) cells are attractive for cell therapy, as they contain diverse stem and progenitor cell populations that can give rise to various cell types, including cardiomyocytes, endothelial cells, and smooth muscle cells. Studies have shown BM cells to be safe and efficacious in the treatment of myocardial infarction. Possible therapeutic mechanisms mediated by both host and transplanted cells include cardiomyogenesis, neovascularization, and attenuation of adverse remodeling. In this review, different stem and progenitor cells in the bone marrow and their application in cell therapy are reviewed, and evidence for their therapeutic mechanisms is discussed.

Genetic control of wayward pluripotent stem cells and their progeny after transplantation

The proliferative capacity of pluripotent stem cells and their progeny brings a unique aspect to therapeutics, in that once a transplant is initiated the therapist no longer has control of the therapy. In the context of the recent FDA approval of a human ESC trial and report of a neuronal-stem-cell-derived tumor in a human trial, strategies need to be developed to control wayward pluripotent stem cells. Here, we focus on one approach: direct genetic modification of the cells prior to transplantation with genes that can prevent the adverse events and/or eliminate the transplanted cells and their progeny.

Bone marrow-derived endothelial progenitor cells contribute to the angiogenic switch in tumor growth and metastatic progression

Emerging evidence indicates that bone marrow (BM)-derived endothelial progenitor cells (EPCs) contribute to angiogenesis-mediated growth of certain tumors in mice and human. EPCs regulate the angiogenic switch via paracrine secretion of proangiogenic growth factors and by direct luminal incorporation into sprouting nascent vessels. While the contributions of EPCs to neovessel formation in spontaneous and transplanted tumors and to the metastatic transition have been reported to be relatively low, remarkably, specific EPC ablation in vivo has resulted in severe angiogenesis inhibition and impaired primary and metastatic tumor growth. The existence of a BM reservoir of EPCs, and the selective involvement of EPCs in neovascularization, have attracted considerable interest because these cells represent novel target for therapeutic intervention. In addition, EPCs are also being used as pharmacodynamic surrogate markers for monitoring cancer progression, as well as for optimizing efficacy of anti-angiogenic therapies in the clinic. This review will focus primarily on recent advances and emerging concepts in the field of EPC biology and discuss ongoing debates involving the role of EPCs in tumor neovascularization. For detailed information on the in vitro characterization of EPCs contribution to non-tumor pathologies, the reader is directed towards several excellent reviews and publications [F. Bertolini, Y. Shaked, P. Mancuso and R.S. Kerbel, Nat. Rev., Cancer 6 (2006) 835-845. [1]] [J.M. Hill, T. Finkel and A.A. Quyyumi, Vox Sang. 87 Suppl 2 (2004) 31-37. [2]] [A.Y. Khakoo and T. Finkel, Annu. Rev. Med. 56 (2005) 79-101. [3]] [H.G. Kopp, C.A. Ramos and S. Rafii, Curr. Opin. Hematol. 13 (2006) 175-181. [4]; K.K. Hirschi, D.A. Ingram and M.C. Yoder, Arterioscler. Thromb. Vasc. Biol. 28 (2008) 1584-1595. [5]; F. Timmermans, J. Plum, M.C. Yoder, D.A. Ingram, B. Vandekerckhove and J. Case, J. Cell. Mol. Med. 13 (2009) 87-102. [6]] and reviews by Bertolini, Voest and Yoder in this issue.

A heart full of stem cells: the spectrum of myocardial progenitor cells in the postnatal heart

Influencing cellular regeneration processes in the heart has been a long-standing goal in cardiovascular medicine. To some extent, this has been successful in terms of vascular regeneration as well as intercellular connective tissue remodeling processes. Several components of today's routine heart failure medication influence endothelial progenitor cell behavior and support collateral vessel growth in the heart, or have been shown to prevent or reverse fibrosis processes. Cardiomyocyte regeneration, however, has so far escaped therapeutic manipulation strategies. Delivery of exogenous cells of bone marrow origin to the human myocardium may improve heart function, but is not associated with relevant neomyogenesis. However, accumulating evidence indicates that the myocardium contains resident cardiac progenitor cells (CPC) that may be therapeutically useful. This notion indeed represents a paradigm shift but is still controversial. The purpose of this review is to summarize the rapidly expanding current knowledge on CPC, and to assess whether it may be translated into solid therapeutic concepts.

In situ genetic analysis of cellular chimerism

Copy number variants are a recently discovered source of large-scale genomic diversity present in all individuals. We capitalize on these inherent genomic differences, focusing on deletion polymorphisms, to develop informative fluorescence in situ hybridization probes with the ability to unequivocally distinguish between donor and recipient cells in situ. These probes are accurate, specific, highly polymorphic and, notably, can be used to assign genetic identity in situ in a completely gender-independent fashion. We anticipate that these polymorphic deletion probes will be useful in further understanding the dynamics of cellular chimerism after transplantation, including the details of chronic organ rejection, post-transplant lymphoproliferative disorder and graft-versus-host disease, and in optimizing future tissue engineering and pluripotent stem cell therapies.

Host-derived smooth muscle cells accumulate in cardiac allografts: role of inflammation and monocyte chemoattractant protein 1

Transplant arteriosclerosis is characterized by inflammation and intimal thickening caused by accumulation of smooth muscle cells (SMCs) both from donor and recipient. We assessed the relationship between clinical factors and the presence of host-derived SMCs in 124 myocardial biopsies from 26 consecutive patients who received hearts from opposite-sex donors. Clinical and demographic information was obtained from the patients' medical records. Host-derived SMCs accounted for 3.35±2.3% of cells in arterioles (range, 0.08–12.51%). As shown by linear regression analysis, an increased number of SMCs was associated with rejection grade (mean, 1.41±1.03, p = 0.034) and the number of leukocytes (19.1±12.7 per 20 high-power fields, p = 0.01). The accumulation of host-derived SMCs was associated with an increased number of leukocytes in the allografts. In vitro, monocyte chemoattractant protein 1 (MCP-1) released from leukocytes was crucial for SMC migration. After heart allotransplantion, mice treated with MCP-1-specific antibodies had significantly fewer host-derived SMCs in the grafts than mice treated with isotypic antibody controls. We conclude that the number of host-derived SMCs in human cardiac allografts is associated with the rejection grade and that MCP-1 may play pivotal role in recruiting host-derived SMCs into cardiac allografts.

Endothelial cells in allograft rejection

In organ transplantation, blood borne cells and macromolecules (e.g., antibodies) of the host immune system are brought into direct contact with the endothelial cell lining of graft vessels. In this location, graft endothelial cells play several roles in allograft rejection, including the initiation of rejection responses by presentation of alloantigen to circulating T cells; the development of inflammation and thrombosis; and as targets of injury and agents of repair.

Endothelial cells in allograft rejection

In organ transplantation, blood borne cells and macromolecules (e.g., antibodies) of the host immune system are brought into direct contact with the endothelial cell lining of graft vessels. In this location, graft endothelial cells play several roles in allograft rejection, including the initiation of rejection responses by presentation of alloantigen to circulating T cells; the development of inflammation and thrombosis; and as targets of injury and agents of repair.

Cell therapy for heart disease: great expectations, as yet unmet

Regenerative medicine is often touted as an achievement of the new millennium, but many approaches to improve health by stimulating the organism's own capacity for healing have existed for a long time. Some components of today's regenerative medicine, however, are indeed fundamentally new developments, and one of those is the concept of increasing the number of contractile cells in the heart to cure heart failure, either by stimulating intrinsic regeneration processes or by transplanting exogenous cells. The aim of this paper is to review the current status of some key aspects of cell therapy and obstacles to clinical translation.

Pravastatin improves function in hibernating myocardium by mobilizing CD133+ and cKit+ bone marrow progenitor cells and promoting myocytes to reenter the growth phase of the cardiac cell cycle

3-hydroxy-3-methyl glutaryl coenzyme A reductase inhibitors have been reported to increase circulating bone marrow progenitor cells and variably improve global function in heart failure. The potential role of improved perfusion versus direct effects of statins on cardiac myocytes has not been established. We chronically instrumented swine with a left anterior descending artery (LAD) stenosis to produce chronic hibernating myocardium with regional contractile dysfunction in the absence of heart failure. Hemodynamics, function, perfusion, and histopathology were assessed in pigs treated for 5 weeks with pravastatin (n=12) versus untreated controls (n=10). Regional LAD wall thickening was depressed under baseline conditions (LAD 3.7+/-0.3 versus 6.6+/-0.3 in remote regions, P<0.01). It remained unchanged in untreated animals but increased from 3.8+/-0.6 to 5.2+/-0.5 mm after pravastatin (P<0.01). There was no increase in myocardial perfusion at rest or during vasodilation. Pravastatin mobilized circulating CD133(+)/cKit(+) bone marrow progenitor cells and increased myocardial tissue levels (LAD CD133(+) cells from 140+/-33 to 884+/-167 cells/10(6) myocyte nuclei and cKit(+) cells from 223+/-49 to 953+/-123 cells/10(6) myocyte nuclei). Pravastatin increased myocytes in mitosis (phospho-histone-H3; 9+/-5 to 43+/-7 nuclei/10(6) myocyte nuclei, P<0.05) and the growth phase of the cell cycle (Ki67; 410+/-82 to 1261+/-235 nuclei/10(6) myocyte nuclei, P<0.05) in diseased but not normal hearts. As a result, pravastatin increased LAD myocyte nuclear density from 830+/-41 to 1027+/-55 nuclei/mm(2) (P<0.05). These data indicate that, in the absence of impaired endothelial function and heart failure, dysfunctional hibernating myocardium improves after pravastatin. This effect is independent of myocardial perfusion and related to mobilization of CD133(+)/cKit(+) bone marrow progenitor cells which stimulate myocyte proliferation resulting in quantitative increases in myocyte nuclear density.

Cardiogenic differentiation and transdifferentiation of progenitor cells

In recent years, cell transplantation has drawn tremendous interest as a novel approach to preserving or even restoring contractile function to infarcted hearts. A typical human infarct involves the loss of approximately 1 billion cardiomyocytes, and, therefore, many investigators have sought to identify endogenous or exogenous stem cells with the capacity to differentiate into committed cardiomyocytes and repopulate lost myocardium. As a result of these efforts, dozens of stem cell types have been reported to have cardiac potential. These include pluripotent embryonic stem cells, as well various adult stem cells resident in compartments including bone marrow, peripheral tissues, and the heart itself. Some of these cardiogenic progenitors have been reported to contribute replacement muscle through endogenous reparative processes or via cell transplantation in preclinical cardiac injury models. However, considerable disagreement exists regarding the efficiency and even the reality of cardiac differentiation by many of these stem cell types, making these issues a continuing source of controversy in the field. In this review, we consider approaches to cell fate mapping and establishing the cardiac phenotype, as well as the present state of the evidence for the cardiogenic and regenerative potential of the major candidate stem cell types.

Sustained persistence of transplanted proangiogenic cells contributes to neovascularization and cardiac function after ischemia

Circulating blood-derived vasculogenic cells improve neovascularization of ischemic tissue by a broad repertoire of potential therapeutic actions. Whereas initial studies documented that the cells incorporate and differentiate to cardiovascular cells, other studies suggested that short-time paracrine mechanisms mediate the beneficial effects. The question remains to what extent a physical incorporation is contributing to the beneficial effects of cell therapy. By using the inducible suicide gene thymidine kinase to deplete transplanted cells, we determined the contribution of physical incorporation in 3 animal models. After acute myocardial infarction, depletion of cells 14 days after infusion resulted in a reduction of capillary density and a substantial deterioration of heart function. Likewise, neovascularization of Matrigel plugs and ischemic limbs was significantly suppressed when infused cells were depleted 7 days after infusion. Induction of cell death in the previously transplanted cells reduced perfusion and led to vascular leakage as evidenced by Evans blue extravasation. These results indicate that physical incorporation and persistence of cells contribute to cell-mediated improvement of neovascularization and cardiac function. Long-term paracrine activities and/or cell intrinsic mechanisms may have contributed to the maintenance of functional improvement.

The role of chemokines in transplant graft arterial disease

Despite the development of effective immunosuppressive therapy, transplant graft arterial disease (GAD) remains the major limitation to long-term graft survival. Multiple immune and nonimmune risk factors contribute to this vasculopathic intimal hyperplastic process. Thus, initial interplay between host inflammatory cells and donor endothelial cells triggers alloimmune responses, whereas alloantigen-independent factors such as prolonged ischemia, surgical manipulation, ischemia-reperfusion injury, and hyperlipidemia enhance the antigen-dependent events. Intrinsic to all stages of this process are chemokines, a family of 8- to 10-kDa proteins mediating directional migration of immune cells to sites of inflammation and injury. Beyond their role in immune-cell chemotaxis, chemokines also contribute to cellular activation, vascular remodeling, and angiogenesis. Expression of chemokines and their cognate receptors in allografts correlates with acute organ rejection, as well as GAD. Moreover, chemokine or chemokine receptor blockade prolongs graft survival and attenuates GAD in experimental models. Further studies will likely confirm a substantial utility for antichemokine therapy in human organ transplantation.

Potential strategies for myocardial regeneration in pediatric patients

Owing to the heart’s limited ability for self-repair, heart failure is a leading cause of death among all patient populations. Thus, a cell-based regenerative strategy for cardiac repair would be highly attractive. A variety of cell sources have been identified as candidates for myocardial repair, including skeletal myoblasts, various bone marrow stem cells, resident cardiac progenitors and embryonic stem cells. However, nearly all studies geared towards myocardial regeneration, both in animal models and in clinical trials, have focused on adult ischemic disease with regional muscle injury. Pediatric patients suffer from more diverse forms of heart disease, including congenital and acquired cardiomyopathies with global muscle dysfunction, as well as disorders of cardiac development, for example, left ventricular hypoplasia, atrial or ventricular septal defects. In this article, a broad range of cell-based therapies are discussed, emphasizing the rapidly evolving science surrounding these strategies and the outstanding questions before application to pediatric patients. It is probable that many of the cell types and delivery strategies capable of repairing adult myocardial diseases will require additional investigations to take advantage of the unique opportunities and challenges of pediatric patients.

Cardiac allograft vasculopathy: recent developments

Cardiac allograft vasculopathy (CAV) continues to limit the long-term success of cardiac transplantation. Recent insights have underscored the fact that innate and adaptive immune responses are involved in the pathogenesis of CAV. Vascular lesions are the result of cumulative endothelial injuries induced both by alloimmune responses and by nonspecific insults (including ischemia-reperfusion injury, viral infections, and metabolic disorders) in the context of impaired repair mechanisms. Intravascular ultrasound is the most sensitive method for detection of CAV, and progressive intimal thickening in the first posttransplant year identifies patients at high risk for future cardiovascular events. Encouraging results with regard to the detection of CAV by noninvasive methods should be an incentive to apply routine noninvasive imaging during mid- to long-term follow-up. Improved immunosuppressive drugs, including mycophenolate mofetil and proliferation signal inhibitors, as well as statins (in part via immunomodulation), have beneficial effects on CAV progression, although there is still a need to confirm the impact of vasodilators in improving outcome after heart transplantation. Coronary revascularization for CAV is only palliative, with no long-term survival benefit. Three main strategies for CAV prevention are currently under investigation: inhibition of growth factors and cytokines, cell therapy, and tolerance induction. However, because individual responses to an allograft change over time, assays to monitor the recipient's immune response and individualized methods for therapeutic immune modulation are clearly needed.

Stem Cells and Transplant Arteriosclerosis

Stem cells can differentiate into a variety of cells to replace dead cells or to repair damaged tissues. Recent evidence indicates that stem cells are involved in the pathogenesis of transplant arteriosclerosis, an alloimmune initiated vascular stenosis that often results in transplant organ failure. Although the pathogenesis of transplant arteriosclerosis is not yet fully understood, recent developments in stem cell research have suggested novel mechanisms of vascular remodeling in allografts. For example, stem cells derived from the recipient may repair damaged endothelial cells of arteries in transplant organs. Further evidence suggests that stem cells or endothelial progenitor cells may be released from both bone marrow and non–bone marrow tissues. Vascular stem cells appear to replenish cells that died in donor vessels. Concomitantly, stem/progenitor cells may also accumulate in the intima, where they differentiate into smooth muscle cells. However, several issues concerning the contribution of stem cells to the pathogenesis of transplant arteriosclerosis are controversial, eg, whether bone marrow–derived stem cells can differentiate into smooth muscle cells that form neointimal lesions of the vessel wall. This review summarizes recent research on the role of stem cells in transplant arteriosclerosis, discusses the mechanisms of stem cell homing and differentiation into mature endothelial and smooth muscle cells, and highlights the controversial issues in the field.

Progenitor cells and vascular disease

Vascular progenitor cells have been the focus of much attention in recent years; both from the point of view of their pathophysiological roles and their potential as therapeutic agents. However, there is as yet no definitive description of either endothelial or vascular smooth muscle progenitor cells. Cells with the ability to differentiate into mature endothelial and vascular smooth muscle reportedly reside within a number of different tissues, including bone marrow, spleen, cardiac muscle, skeletal muscle and adipose tissue. Within these niches, vascular progenitor cells remain quiescent, until mobilized in response to injury or disease. Once mobilized, these progenitor cells enter the circulation and migrate to sites of damage, where they contribute to the remodelling process. It is generally perceived that endothelial progenitors are reparative, acting to restore vascular homeostasis, while smooth muscle progenitors contribute to pathological changes. Indeed, the number of circulating endothelial progenitor cells inversely correlates with exposure to cardiovascular risk factors and numbers of animal models and human studies have demonstrated therapeutic roles for endothelial progenitor cells, which can be enhanced by manipulating them to overexpress vasculo-protective genes. It remains to be determined whether smooth muscle progenitor cells, which are less well studied than their endothelial counterparts, can likewise be manipulated to achieve therapeutic benefit. This review outlines our current understanding of endothelial and smooth muscle progenitor cell biology, their roles in vascular disease and their potential as therapeutic agents.

Cardiomyocyte death and renewal in the normal and diseased heart

During post-natal maturation of the mammalian heart, proliferation of cardiomyocytes essentially ceases as cardiomyocytes withdraw from the cell cycle and develop blocks at the G0/G1 and G2/M transition phases of the cell cycle. As a result, the response of the myocardium to acute stress is limited to various forms of cardiomyocyte injury, which can be modified by preconditioning and reperfusion, whereas the response to chronic stress is dominated by cardiomyocyte hypertrophy and myocardial remodeling. Acute myocardial ischemia leads to injury and death of cardiomyocytes and nonmyocytic stromal cells by oncosis and apoptosis, and possibly by a hybrid form of cell death involving both pathways in the same ischemic cardiomyocytes. There is increasing evidence for a slow, ongoing turnover of cardiomyocytes in the normal heart involving death of cardiomyocytes and generation of new cardiomyocytes. This process appears to be accelerated and quantitatively increased as part of myocardial remodeling. Cardiomyocyte loss involves apoptosis, autophagy, and oncosis, which can occur simultaneously and involve different individual cardiomyocytes in the same heart undergoing remodeling. Mitotic figures in myocytic cells probably represent maturing progeny of stem cells in most cases. Mitosis of mature cardiomyocytes that have reentered the cell cycle appears to be a rare event. Thus, cardiomyocyte renewal likely is mediated primarily by endogenous cardiac stem cells and possibly by blood-born stem cells, but this biological phenomenon is limited in capacity. As a consequence, persistent stress leads to ongoing remodeling in which cardiomyocyte death exceeds cardiomyocyte renewal, resulting in progressive heart failure. Intense investigation currently is focused on cell-based therapies aimed at retarding cardiomyocyte death and promoting myocardial repair and possibly regeneration. Alteration of pathological remodeling holds promise for prevention and treatment of heart failure, which is currently a major cause of morbidity and mortality and a major public health problem. However, a deeper understanding of the fundamental biological processes is needed in order to make lasting advances in clinical therapeutics in the field.

Contribution of recipient-derived cells in allograft neointima formation and the response to stent implantation

Allograft coronary disease is the dominant cause of increased risk of death after cardiac transplantation. While the percutaneous insertion of stents is the most efficacious revascularization strategy for allograft coronary disease there is a high incidence of stent renarrowing. We developed a novel rabbit model of sex-mismatched allograft vascular disease as well as the response to stent implantation. In situ hybridization for the Y-chromosome was employed to detect male cells in the neointima of stented allograft, and the population of recipient derived neointimal cells was measured by quantitative polymerase chain reaction and characterized by immunohistochemistry. To demonstrate the participation of circulatory derived cells in stent neointima formation we infused ex vivo labeled peripheral blood mononuclear cells into native rabbit carotid arteries immediately after stenting. Fourteen days after stenting the neointima area was 58% greater in the stented vs. non-stented allograft segments (p = 0.02). Male cells were detected in the neointima of stented female-to-male allografts. Recipient-derived cells constituted 72.1±5.7% and 81.5±4.2% of neointimal cell population in the non-stented and stented segments, respectively and the corresponding proliferation rates were only 2.7±0.5% and 2.3±0.2%. Some of the recipient-derived neointimal cells were of endothelial lineage. The ex vivo tagged cells constituted 9.0±0.4% of the cells per high power field in the stent neointima 14 days after stenting. These experiments provide important quantitative data regarding the degree to which host-derived blood-borne cells contribute to neointima formation in allograft vasculopathy and the early response to stent implantation.

Continuous engraftment and differentiation of male recipient Y-chromosome-positive cardiomyocytes in donor female human heart transplants

BACKGROUND

The mechanisms of stem cell engraftment and differentiation in transplanted organs are still unknown. The aim of our study was to assess the time course of extracardiac progenitor cell colonization of cardiac allografts using the human sex-mismatched heart transplant model. The possible mechanisms by which stem cells acquire a cardiac phenotypic lineage were also investigated.

METHODS

Thirty-four endomyocardial biopsies were obtained from 17 sex-mismatched orthotopic heart transplant patients (mean age, 43.50 +/- 23.95 years). Cells of recipient origin were identified by fluorescence in situ hybridization for combined XY-chromosomes.

RESULTS

The mean incremental number of cardiomyocytes of recipient origin per month was 0.064 +/- 0.04, suggesting ongoing engraftment and transdifferentiation in the absence of cell fusion. Regression analysis showed a positive correlation between the Y-chromosome-positive cardiomyocytes and the rejection score (r(2) = 0.99; 95% confidence interval -0.14 + 0.02; p = 0.006) suggesting that colonization was more pronounced in cases of more severe cardiac injury. At multivariable analysis, time since transplantation was the only independent predictor of the proportion of XY-chromosome-positive cardiac cell engraftment (beta = 0.025, p < 0.0001, 95% confidence interval, 0.012-0.038).

CONCLUSION

The phenotypic transformation in this human chronic heart injury model is the result of transdifferentiation of male stem cells (atrial or circulating cells) into new cardiomyocytes. Immunologic injury predicts recipient cardiomyocyte engraftment and may be one of the mobilizing stimuli.

Stem cells in the heart: what's the buzz all about? Part 2: Arrhythmic risks and clinical studies

New approaches for cardiac repair have been enabled by the discovery that the heart contains its own reservoir of stem cells. In Part 1 of this review, we discussed various cardiac stem cell populations, reviewed our own work on cardiosphere-derived cells from human hearts, and outlined large animal preclinical models testing the regenerative potential of cardiac stem cells. Here we continue with a discussion on other adult stem cell sources with clinical potential. We summarize the critical safety issues associated with stem cell therapy and present the possible proarrhythmic and antiarrhythmic effects of stem cell transplantation. We discuss the outcomes of clinical stem cell trials and identify the technical, ethical, and practical issues facing the clinical application of cardiac stem cells.

Recipient-derived neoangiogenesis of arterioles and lymphatics in quilty lesions of cardiac allografts

BACKGROUND

The contribution of extracardiac cells to tissue turnover in heart allografts has recently been demonstrated. Complex subendocardial infiltrates, known as Quilty lesions, are frequently observed in cardiac allografts. The origin of the different cellular components of Quilty lesions is not known.

METHODS

Different constituents of these lymphonodular infiltrates were analyzed with regard to donor or recipient derivation. Laser-assisted microdissection with subsequent short tandem repeat polymerase chain reaction (PCR)-based "genetic fingerprinting" was employed. Combined immunofluorescence and fluorescence in situ hybridization for sex chromosomes was performed for confirmation in cases of gender-mismatched transplantation. Expression of angiogenic factors (FGF-2, PDGF-alpha, PDGF-alpha-receptor, and VEGF-alpha) was analyzed by quantitative real-time reverse-transcription PCR and immunohistochemistry.

RESULTS

The inflammatory, nonvascular component of Quilty lesions was completely recipient-derived. Blood vessels were of mixed origin. Different compartments of blood vessels displayed different rates of recipient derivation (endothelium up to 50%, smooth muscle cells up to 15%). Lymphatic vessels were mainly recipient-derived. Of the angiogenic molecules, VEGF-alpha expression was significantly increased in the adjacent myocardium, compared to controls and the Quilty lesions themselves.

CONCLUSIONS

The inflammatory compartment of Quilty lesions is of recipient origin and shows chimeric neoangiogenesis of blood and lymphatic vessels. VEGF-alpha produced in the adjacent myocardium appears to stimulate the chimeric neoangiogenesis.

Stem cell-derived cardiomyocytes after bone marrow and heart transplantation

Cardiomyocytes are a stable cell population with only limited potential for renewal after injury. Tissue regeneration may be due to infiltration of stem cells, which differentiate into cardiomyocytes. We have analysed the influx of stem cells in the heart of patients who received either a gender-mismatched BMT (male donor to female recipient) or a gender-mismatched cardiac transplant (HTX; female donor to male recipient). The proportion of infiltrating cells was determined by Y-chromosome in situ hybridization combined with immunohistochemical cell characterization. In BM transplanted patients and in cardiac allotransplant recipients, cardiomyocytes of apparent BM origin were detected. The proportions were similar in both groups and amounted up to 1% of all cardiomyocytes. The number of stem cell-derived cardiomyocytes did not alter significantly in time, but were relatively high in cases where large numbers of BM-derived Y-chromosome-positive infiltrating inflammatory cells were present. The number of Y-chromosome-positive endothelial cells was small and present only in small blood vessels. The number of BM-derived cardiomyocytes in both BMT and HTX is not significantly different between the two types of transplantation and is at most 1%.

Temporal changes in stem cells in the circulation and myocardium of mice with Coxsackie virus B3-induced myocarditis

Our goal was to investigate temporal changes in stem cell in the circulation and myocardium of mice with Coxsackie virus B3-induced myocarditis. Groups of mice were administered Eagle's minimal essential medium or virus solution. The animals were further divided into six subgroups based on the following time points post-inoculation: 1, 3, 7, 14, 21, and 28 days. Ten animals were studied in each subgroup. Circulating blood mononuclear cells were collected from the heart and analyzed using flow cytometry. Myocardial inflammation, stem cell expression, and cell proliferation were detected by histology and immunofluorescence. H&E staining revealed neutrophil infiltration and bleeding by day 3 post-infection. Myeloperoxidase and reactive oxygen species levels peaked by day 3 and were followed by myocyte loss and collagen deposition. Circulating mesenchymal stem cells also peaked by day 3. In contrast, hematopoietic stem cells remained sustained increase within day 14. Immunohistochemical microscopy also showed a marked increase in cardiac stem cells by day 14. The kinetics of this increase was consistent with a rise in proliferating cells expressing nuclear and cytoplasmic proteins that are typical of cardiomyocyte or vascular endothelial cells. These results demonstrate the rapid kinetics of progenitor cells during viral myocarditis and suggest that the optimal time to administer cell therapy to induce heart repair is within 2 weeks after viral infection.

Pressure-induced cardiac overload induces upregulation of endothelial and myocardial progenitor cells

AIM

The regulation of angiogenesis in the hypertrophied overloaded heart is incompletely understood. Bone-marrow-derived progenitor cells have been shown to contribute to endothelial homeostasis, repair, and new blood vessel formation. We therefore studied the effects of pressure overload on angiogenesis and progenitor cells.

METHODS AND RESULTS

Pressure overload induced by transaortic constriction (TAC, C57/Bl6 mice, 360 microm for 35 days) increased left ventricular (LV) systolic pressure, the ratio of heart weight to tibia length, cardiomyocyte diameters, and cardiac apoptosis and fibrosis compared to sham-operated mice. In the TAC group, the number of cycling Ki67 pos cells increased from none to 0.1 +/- 0.02% in cardiomyocytes and from 0.17 +/- 0.02% to 0.65 +/- 0.1% in non-cardiomyocytes, P < 0.001. stem cell antigen 1(pos)/vascular endothelial growth factor receptor 2 pos endothelial progenitor cells (EPC) increased to 210 +/- 25% in the blood and to 196 +/- 21% in the bone marrow (P < 0.01). TAC upregulated cultured spleen-derived DiLDL pos/lectin pos EPC to 221 +/- 37%, P < 0.001. Cardiac hypertrophy and upregulation of EPC secondary to cardiac pressure overload were associated with increased extra-cardiac neoangiogenesis (54 +/- 12% increase, P < 0.05). In endothelial nitric oxide synthase double knockout mice, the upregulation of EPC by TAC was abolished. Maladaptive myocardial remodelling in TAC mice was characterized by a reduction of CD31 pos cells. In mice transplanted with green fluorescent protein pos bone marrow, TAC markedly increased myocardial bone marrow-derived CD31 pos cells from 2.37 +/- 0.4% to 7.76 +/- 1.5% and MEF2 pos cells from 1.8 +/- 0.4/mm2 to 20.5 +/- 5.3/mm2, P < 0.05.

CONCLUSION

Pressure-induced myocardial hypertrophy leads to upregulation of systemic EPCs, increased extra-cardiac angiogenesis, and upregulation of intra-myocardial bone marrow-derived endothelial and myocyte precursor cells. The data show that afterload-dependent regulation of bone marrow-derived progenitor cells contributes to angiogenesis in myocardial hypertrophy.

Controlled release of stromal cell-derived factor-1 alpha in situ increases c-kit+ cell homing to the infarcted heart

Stromal-derived factor 1alpha (SDF-1alpha) is a key stem cell homing factor that is crucial for mobilization of stem cells from bone marrow to peripheral blood and subsequent engraftment to the tissue of diseased organs. It has been reported that SDF-1alpha is transiently over-expressed in ischemic myocardium. Therefore, there may be a limited time window after acute myocardial infarction (AMI) during which stem cells are recruited to injured myocardium for repair. This study aimed at investigating whether controlled release of SDF-1alpha via a novel conjugated poly(ethylene glycol) (PEG) (PEGylated) fibrin patch at the infarct site would increase the rate of stem cell recruitment and offer potential therapeutic benefits. Recombinant mouse SDF-1alpha was covalently bound to the PEGylated fibrinogen as evidenced by immunoprecipitation and western blotting. The PEGylated fibrinogen, bound with recombinant mouse SDF-1alpha, was mixed with thrombin to form the PEGylated fibrin patch. The release kinetics of SDF-1alpha were detected in vitro using enzyme-linked immunosorbent assay. Using a mouse AMI model produced by a ligature on the left anterior descending coronary artery, a PEGylated fibrin patch bound with SDF-1alpha (100 ng) was placed on the surface of the infarct area of the left ventricle. Infarct size, left ventricular (LV) function, and the percentage of sca-1(+)/c-kit(+) cells within the infarct area were measured at days 7, 14, and 28 after AMI. In vitro results showed that SDF-1alpha was successfully bound to the PEGylated fibrin patch and can be released from the patch constantly for up to 10 days. Two weeks after infarction, the myocardial recruitment of c-kit(+) cells was significantly higher in the group treated with the SDF-1alpha PEGylated fibrin patch (n = 9) than in the AMI control group (n = 10) (p < 0.05; 11.20 +/- 1.71% vs. 4.22 +/- 0.96%, respectively). At day 28 post-AMI, unlike the control group, the group with the SDF-1alpha-releasing patch maintained stable release of SDF-1alpha concurrent with additional stem cell homing. Moreover, LV function was significantly better than in the control group. These data demonstrate that the PEGylated fibrin patch based SDF-1alpha delivery can improve the rate of c-kit(+) cell homing and improve LV function in hearts with postinfarction LV remodeling.

Cardiomyocytes derived from human embryonic stem cells in pro-survival factors enhance function of infarcted rat hearts

Cardiomyocytes derived from human embryonic stem (hES) cells potentially offer large numbers of cells to facilitate repair of the infarcted heart. However, this approach has been limited by inefficient differentiation of hES cells into cardiomyocytes, insufficient purity of cardiomyocyte preparations and poor survival of hES cell-derived myocytes after transplantation. Seeking to overcome these challenges, we generated highly purified human cardiomyocytes using a readily scalable system for directed differentiation that relies on activin A and BMP4. We then identified a cocktail of pro-survival factors that limits cardiomyocyte death after transplantation. These techniques enabled consistent formation of myocardial grafts in the infarcted rat heart. The engrafted human myocardium attenuated ventricular dilation and preserved regional and global contractile function after myocardial infarction compared with controls receiving noncardiac hES cell derivatives or vehicle. The ability of hES cell-derived cardiomyocytes to partially remuscularize myocardial infarcts and attenuate heart failure encourages their study under conditions that closely match human disease.

Bone marrow-derived endothelial progenitor cells are a major determinant of nascent tumor neovascularization

Tumors build vessels by cooption of pre-existing vasculature and de novo recruitment of bone marrow (BM)-derived endothelial progenitor cells (EPCs). However, the contribution and the functional role of EPCs in tumor neoangiogenesis are controversial. Therefore, by using genetically marked BM progenitor cells, we demonstrate the precise spatial and temporal contribution of EPCs to the neovascularization of three transplanted and one spontaneous breast tumor in vivo using high-resolution microscopy and flow cytometry. We show that early tumors recruit BM-derived EPCs that differentiate into mature BM-derived endothelial cells (ECs) and luminally incorporate into a subset of sprouting tumor neovessels. Notably, in later tumors, these BM-derived vessels are diluted with non-BM-derived vessels from the periphery, which accounts for purported differences in previously published reports. Furthermore, we show that specific ablation of BM-derived EPCs with alpha-particle-emitting anti-VE-cadherin antibody markedly impaired tumor growth associated with reduced vascularization. Our results demonstrate that BM-derived EPCs are critical components of the earliest phases of tumor neoangiogenesis.

Immunosuppression-related fibroproliferative polyps of the tongue

Polypoid tongue lesions arising after bone marrow transplantation have been described. Their etiopathogenesis has been unclear, as has their relationship to similar lesions arising in other settings of chronic immunodeficiency. We identified 12 polypoid lesions (from 8 immunosuppressed patients aged 6 months to 13 years) among all tongue lesions biopsied over the course of 13 years at our institution. Clinical history, histologic and ultrastructural features, special stains (Gram, Grocott methenamine silver, acid-fast bacilli, CD34, actin, desmin, human herpesvirus-8), in situ hybridization for Epstein-Barr virus, and cytogenetic features were studied. Immunocompromise was from bone marrow transplantation for severe combined immunodeficiency (n = 1) and acute lymphoblastic leukemia (n = 3), hypogammaglobulinemia (n = 2), 22q11 deletion syndrome (n = 1), and asthma therapy (n = 1). Histologic examination revealed fibrous stromal cores with squamous epithelial covering and various degrees of ulceration and accompanying inflammation and granulation tissue. In 2 patients lesions were multiple in number. Fibroblasts were variably positive for smooth muscle actin and desmin and negative for CD34. Special stains, immunohistochemistry, in situ hybridization, and ultrastructural examination identified no organisms except occasional surface bacteria. The tongue lesion from 1 patient with Down's syndrome showed t(2;9)(p11;q34)+21 (translocation not seen in peripheral blood). Another patient had constitutional del 22q11. All transplant patients had Philadelphia chromosome-positive acute lymphoblastic leukemia (ALL) (translocations involving 9q34 and 22q11). Patients with congenital immunosuppression had polyps arise at significantly younger ages than did patients with acquired immunosuppression. Immunosuppression-related lingual polyps are a fibroproliferative process occurring in patients with bone marrow transplantation and other immune-deficient conditions. Our findings indicate that these polyps are driven by both immunosuppression and chromosomal rearrangement.

Vascular Remodeling in Transplant Vasculopathy

As therapeutic strategies to prevent acute rejection progressively improve, transplant vasculopathy (TV) constitutes the single most important limitation for long-term functioning of solid organ allografts. In TV, allograft arteries characteristically develop severe, diffuse intimal hyperplastic lesions that eventually compromise luminal flow and cause ischemic graft failure. Traditional immunosuppressive strategies that check acute allograft rejection do not prevent TV; indeed 50% of transplant recipients will have significant disease within five years of organ transplantation, and 90% will have significant TV a decade after their surgery. TV can involve the entire length of the transplanted arterial bed, including penetrating intraorgan arterioles. Indeed, the luminal narrowing of such penetrating vessels may be the most functionally significant because arterioles represent the major contributors to tissue vascular resistance. Because of the diffuseness of TV involvement in the allograft vascular bed, the only currently definitive therapy requires re-transplantation. Nevertheless, as we better understand the pathogenesis and critical mediators of these lesions, pharmacological advances can be anticipated. Other articles in this thematic review series focus on the specifics of the inciting injury, the cytokines and chemokines that drive TV development, and the nature of the recruited cells in TV lesions, as well as the pathogenic similarities between TV and other vascular lesions such as atherosclerosis. This review focuses on the mechanisms of vascular wall remodeling in TV, including the intimal accumulation of smooth muscle-like cells and associated extracellular matrix, medial smooth muscle cell degeneration, and adventitial fibrosis. A brief overview highlights the aneurysmal changes that can accrue when vessel wall inflammation has a cytokine profile distinct from the typical proinflammatory interferon-gamma-dominated milieu.