Myocardial infarction is the leading cause of congestive heart failure and death in developed countries.
Congestive heart failure affects approximately 5 million patients in the United States, with 400 000 new cases per year.
The current pharmacotherapy for congestive heart failure, including neurohormonal inhibition with angiotensin-converting enzyme inhibitors and β-blockers, improves clinical outcomes.
Despite this, other treatment options that include various interventional and surgical therapeutic methods are limited in preventing ventricular remodeling because of their inability to repair or replace damaged myocardium.
Given the high morbidity and mortality rates associated with congestive heart failure, dearth of donor hearts for transplantation, complications associated with immunosuppression, and long-term failure of transplanted organs, novel treatment methods that improve cardiac function and prevent heart failure are in demand.
Although human cardiomyocytes are reported to proliferate and contribute to the increase in muscle mass of the myocardium after myocardial infarction, their capacity for regeneration, mitigation of the adverse effects of ventricular remodeling, and contribution to cardiac function is limited.
Neovascularization in the infarcted myocardium plays an important part in ventricular remodeling.
After myocardial infarction, the newly formed capillary network in the infarcted myocardium cannot adequately keep up with the tissue growth needed for contractile compensation and cannot meet the higher demands of the surviving hypertrophied cardiomyocytes, leading to further expansion of the infarct and fibrosis of the myocardium.
Thus, neovascularization represents a potentially important process by which increasing perfusion to infarcted myocardium may reduce ventricular dilatation and improve cardiac function through the rescue of hibernating myocardium and decreased apoptosis of hypertrophied cardiomyocytes.
Congestive heart failure after an extensive myocardial infarction may occur when compensatory mechanisms are overwhelmed.
Cellular Cardiomyoplasty Approaches
The adult heart appears to contain a subpopulation of cardiomyocytes that are not terminally differentiated and re-enter the cell cycle and undergo nuclear mitotic division soon after myocardial infarction.
Despite this, myocardium does not substantially regenerate after myocardial infarction.
Cellular cardiomyoplasty, which is the replacement or regeneration of cardiomyocytes through cell transplantation, may be attained in one of the following ways:
1) by transplanting stem cells that differentiate into cardiomyocytes or promote angiogenesis;
2) by mobilizing bone marrow resident stem cells to the site of injury with the use of cytokines, such as granulocyte colony-stimulating factor and stem-cell factor; or
3) by administering local treatment with growth factors, such as insulin-like and hepatocyte growth factors, that induce the differentiation of cardiac progenitor cells into cardiomyocytes.
Stem cells, or immature tissue precursor cells, are undifferentiated cells that can proliferate, potentially self-renew, and differentiate into one or more types of specialized cells, including cardiomyocytes.
The genetic and cellular mechanisms that initiate transdifferentiation of stem cells are poorly understood.
Transplanted stem cells also undergo a "homing" process in which they are attracted to the site of injury.
The exact homing mechanism and organ-specific differentiation signals fot stem cells are not clearly understood but may be related to microenvironmental factors that ate favorable to stem-cell growth and function, integrin and other adhesion molecules, homing receptors, ischemia, and increased expression of vascular endothelial growth factor.
Another strategy for cellular cardiomyoplasty involves an indirect approach.
Bone marrow stem cells, which were mobilized by systemic injections of cytokines (such as granulocyte colony-stimulating factor and stem-cell factor) homed to the infarcted myocardium, replicated, differentiated, promoted myocardial repair, and improved cardiac function in a murine model.
A less common strategy for cellular cardiomyoplasty is the use of growth factors, such as insulin-like and hepatocyte growth factors, to attract cardiac progenitor cells, induce the differentiation of cardiac progenitor cells into cardiomyocytes, and promote cardiomyocyte replication.
Insulin-like growth factor also protects against cardiomyocyte death and attenuates left ventricular remodeling in a mouse model.
To be clinically useful, stem-cell proliferation must be rapid and sustained and provide for effective physical and electrical coupling of the new cells.
These processes must occur while the myocardium maintains its usual function of pumping blood and perfusing tissues.
Meanwhile, all these processes depend on the simultaneous formation of nutritive vascular structures.
The ideal donor cell is the subject of intense scientific, ethical, and political debate.
Different donor cells might replace necrotic myocardium and minimize remodeling (Table 1).
Table 1. List of donor cells
||Immunosuppression reguired, ethical debate, short survival, limitted supply
||Lack of immunogenicity, autologous transplantation, high yield, fatigue-resistant, slow-twich fibers
||Arrhytmogenic and lack of gap junction
|Endothelial progenotir cells
||Lack of immunogenicity, autologous transplantation
||Need of expansion because of limited supply
|Embryonic stem cells
||Pluripotent and highly expandable
||Immunosuppression reguired, ethical debate, lack of availability, tumor potential
|Adult mesenchymal stem cells
||Lack of immunogenicity, autologous transplantation, pluripotent and cryopreservable for future use
||Unclear functional and electrophysiologic properties, difficult to isolate and propagate in culture
Transplanted fetal cardiomyocytes could survive, proliferate, and form nascent intercalated disks with host myocardium in murine models.
Transplanting fetal cardiomyocytes into a myocardial scar formed new cardiac tissue and improved cardiac function.
The attenuation of ventricular dilatation, infarct thinning, and cardiac dysfunction were observed as well after embryonic cardiomyocytes were transplanted into rat hearts after myocardial infarction.
Fetal cardiomyocytes may also contribute to the release of cardioprotective factors, such as vascular endothelial growth factor, through a paracrine effect that stimulates nascent blood vessel formation in grafted areas and host ventricle.
Increased microcirculation provides the transplanted cells not only with increased perfusion but may also be a means to remove necrotic debris from myocardial infarction.
However, allogeneic cell transplantation with human fetal cardiomyocytes is limited because of the unresolved ethical debates and the inability to obtain enough cells to repair damaged myocardium.
Skeletal myoblasts function as precursor cells that can undergo mitosis, proliferate, form syncytium, and ultimately form new skeletal myocytes.
Autologous skeletal myoblasts are ideal for transplantation because they are readily available with a skeletal muscle biopsy specimen from the patient, can be returned after in vitro expansion, and do not carry immunologic and ethical concerns as human embryonic stem cells do.
Skeletal myoblasts strongly resist ischemia, allowing for increased survival and engraftment in areas of poor coronary perfusion, which is often seen in patients with coronary artery disease.
Implantation of skeletal myoblasts formed viable myoblast implants and attenuated ventricular dilatation, thereby improving exercise capacity, cardiac function, and left ventricular systolic pressures after myocardial infarction in a rat model.
Similarly, transplanting autologous skeletal myoblasts in an infarcted area minimized left ventricular dysfunction and improved systolic function in the scarred myocardium through colonization of fibrosis by skeletal muscle cells with the expression of myosin heavy chain after myocardial infarction in a sheep model for up to 1 year.
The degree of improvement of cardiac function is related to the number of myoblasts injected.
The potential for intravascular delivery of skeletal myoblasts also makes these cells particularly appealing as the donor cell of choice.
Endothelial Progenitor Cells.
Neovascularization is paramount to the survival of the newly formed cardiomyocytes.
Endothelial progenitor cells are bone marrow residents that can be released into circulation after an acute myocardial infarction and can produce neovascularization in the adult.
Endothelial progenitor cells are ideal donor cells because they allow autologous harvesting, obviating the need for immunosuppression.
The intravenous administration of ex vivo expanded endothelial progenitor cells enhanced neovascularization, reduced left ventricular dilatation, and preserved cardiac function after myocardial infarction in a rat model.
Similarly, intravenously administered granulocyte colony-stimulating factor-mobilized, human bone marrow-derived endothelial progenitor cells that migrated into the infarcted region within 48 hours, transdifferentiated into endothelial cells, induced neovascularization, limited apoptosis of the hypertrophied cardiomyocytes in the peri-infarct area and ventricular remodeling, and improved cardiac function after myocardial infarction in a rat model.
Endothelial progenitor cells might also transdifferentiate into cardiomyocytes and thus participate in myocardial regeneration.
A major obstacle to the clinical application of stem cells is the limited number of cells that can be harvested from the patient.
Methods to potentially expand populations of endothelial progenitor cells ex vivo have been developed.
However, expanding endothelial cells may suppress their homing capacity and limit their effectiveness.
Treatment with statins in patients with coronary artery disease increased the proportion of endothelial progenitor cells in circulation.
Autologous bone marrow cells secrete angiogenic factors, such as vascular endothelial growth factor and macrophage chemoattractant protein-1, that stimulate the proliferation of endothelial cells and increase collateral perfusion and cardiac function after catheter-based transendocardial injection into ischemic myocardium.
Embryonic Stem Cells.
Human embryonic stem cells are pluripotent cells that can differentiate into all cell types of the body, including cardiomyocytes, but with a much lower efficiency of conversion into cardiomyocytes compared with those of mice.
In a rat model, intramyocardial injection of embryonic stem cells engrafted in the myocardium and improved cardiac function and myocardial contractility after myocardial infarction.
The main drawback with transplanting animal embryonic stem cells into human hearts includes immune rejection due to tissue incompatibility at the HLA level.
The requirement for immunosuppression to prevent destruction of the cellular transplant will probably detract investigators from pursuing clinical trials with animal embryonic stem cells.
Transplantation of human embryonic stem cells is advantageous because of the minimal immunoreactivity, which is due to the reduced expression of immune-related cell-surface proteins.
However, the future application of human embryonic stem cells in clinical trials is limited because of their lack of availability and intense ethical and political issues that are unresolved.
Adult Mesenchymal Stem Cells.
Human adult mesenchymal stem cells are accessible from the bone marrow and peripheral blood, allow autologous transplantation, and are pluripotent cells, which can differentiate into specialized tissues, including cardiomyocytes, endothelial cells, and smooth-muscle cells.
Mesenchymal stem-cell transplantation obviates the need for immunosuppression even when allogenic stem cells are used.
Implanting autologous or allogenic swine mesenchymal stem cells after myocardial infarction sustained engraftment in host myocardium, differentiated into cardiomyocytes, maintained wall thickness, reduced ventricular remodeling, and improved cardiac function.
An improvement in cardiac function and resting blood flow in infarcted myocardium with intramyocardial transplantation of human mesenchymal stem cells and a statistically significantly greater improvement in cardiac function and resting blood flow with cotransplantation of human fetal cardiomyocytes after myocardial infarction in a porcine model also reported.
Mesenchymal stem cells injected with fresh bone marrow into infarcted myocardium induced the overexpression of cardiac tenascin and sympathetic nerve sprouting, resulting in myocardial sympathetic hyperinnervation in swine.
The tenascin gene family of extracellular matrix proteins is implicated in nerve regeneration, cardiac remodeling, vascular remodeling and neointimal proliferation.
This mechanism may explain improved myocardial function after mesenchymal cell transplantation.
However, sympathetic hyperinnervation may lead to life-threatening ventricular tachyarrhythmias.
Route of Delivery
Directly injected in the margin bordering the infarct of the left ventricle of mice bone marrow stem cells migrated into the region bordering the infarction and differentiated into cardiomyocytes and endothelial cells, generating de novo myocardium, improving cardiac function, and leading to neovascularization.
Direct intramyocardial injection may require fewer cells to achieve engraftment compared with intracoronary or intravenous administration.
Although the injection process is simple and can be performed by direct inspection of the potential target zones, this invasive delivery in the form of cardiac surgery is associated with intraoperative and postoperative risks and had a success rate of 40% in 1 study in a mouse model.
A different approach is to implant stem cells through percutaneous catheter-based myocardial injections guided by electromechanical mapping.
Electromechanical mapping can delineate and identify scarred and viable myocardium and allow assessment of the transmural extent of myocardial infarction so that each injection can be precisely targeted to viable areas of hibernating myocardium.
A percutaneous transluminal coronary catheter can be used for intracoronary administration of bone marrow-derived stem cells after myocardial infarction.
This is advantageous over intravenous administration because it can deliver the maximum concentration of cells to the site of infarct and peri-infarct tissue during the first passage.
Intracoronary administration into the infarct artery allows the stem cells to home in and incorporate into the areas bordering the infarct zone in a homogenous manner.
This is in contrast to direct myocardial injection, which may lead to "islands" of cells in the infarcted myocardium, providing a substrate for electrical instability and ventricular tachyarrhythmias.
High-pressure injection of stem cells into the infarct region may facilitate transendothelial passage and migration into infarcted myocardium.
Because coronary flow impairment and myocardial cell necrosis are possible, the quantity of cells and duration of infusion must be carefully determined.
Intravenous administration of stem cells is an attractive and practical mode of delivery because it does not require cardiac surgery or catheterization.
If stem cells have an effective cellular homing mechanism to localize in the infarcted myocardium, intravenous administration of stem cells may be possible.
Microenvironmental factors, expression of matrix and adhesion molecules by injured tissue, homing receptors, and various factors relating to migration are believed to be involved in the homing process of stem cells.
However, homing of stem cells to other organs could limit the percentage of cells reaching the infarct region after intravenous administration.
Many important fundamental questions about stem cell transplantation still remain unanswered.
The optimal donor cell and the optimal number of stem cells to be transplanted have not been determined.
There is a threshold of the number of stem cells needed to generate adequate heart muscle to contribute to cardiac function.
Adult stem cells are limited in supply in each patient and therefore are difficult to isolate and purify.
The stem cells from the patient's body must be isolated and expanded in culture to obtain a sufficient amount for stem-cell transplantation.
Manipulation in vitro of stem cells by providing optimal culture condition, which include various cytokines and growth factors to augment organ-specific engraftment, may be needed to facilitate in vivo incorporation.
Whether the most benefit from transplantation is derived early after acute myocardial infarction during extreme local inflammation, later during the ventricular remodeling phase, or late at the end stage of ischemic cardiomyopathy is uncertain.
The inflammatory process is strongest in the first days after acute myocardial infarction and may be responsible for the negative results after immediate cell transplantation.
Transplantation after 2 weeks after infarct scar formation may reduce the benefits of cell transplantation.
Therefore, transplanting stem cells between 7 and 14 days after acute myocardial infarction seems reasonable.
The ideal strategy of delivery (intramyocardial injection, intracoronary administration, or systemic delivery) has not been clarified.
The long-term viability and function of stem cells are also uncertain.
The strategy of coadministering stem cells with angioblasts may have synergistic effects by augmenting perfusion to both the chronically ischemic native myocardium and the newly administered stem cells.
Data on stem-cell transplantation in myocardial infarction are limited because the large proportion of the work has been done in laboratories and animal models.
While these results are interesting and perhaps safe and promising, the data are very preliminary, with the clinical studies being conducted in an uncontrolled manner with few patients.
Whether stem-cell transplantation will fulfill the potential in replacing damaged myocardium in patients and prevent congestive heart failure, and therefore apply to the large population of postinfarction patients, must be evaluated by rigorous large-scale clinical trials.
Safety is always a major concern in dealing with clinical trials involving stem cells.
Implanted stem cells may differentiate into fibroblasts rather than myocytes.
This may enhance scar formation, further depressing myocardial function and creating a substrate for life-threatening arrhythmias.
There may also be life-threatening consequences if stem cells incompletely integrate into the myocardium and adversely affect electrical conduction and syncytial contraction of the heart.
Tumor formation associated with embryonic stem cells, such as teratomas, may also occur.
Late-onset toxicity may occur from using whole populations of bone marrow mononuclear cells, which contain different organ-specific stem cells.
These nonessential cells may incorporate into regenerating myocardium, resulting in the generation of noncardiac tissues.
Stem-cell transplantation in acute myocardial infarction is still in its infancy.
The potential for stem cells to acutely regenerate contracting myocardium and improve immediate and long-term prognosis after acute myocardial infarction faces some formidable challenges.
Whether stem-cell transplantation offers a sustained clinical benefit by reversing ventricular remodelling in myocardial infarction is unknown, given that too few patients have undergone stem-cell transplantation to derive any meaningful efficacy and safety data.
Preliminary data from animal models suggest implanting stem cells can regenerate that infarcted myocardium.
Scepticism exists with this treatment method, especially given the initial excitement of angiogenesis studies that did not live up to expectations and the disappointment in gene therapy trials.
More work needs to be done to clarify the biology of stem cells and address any unanswered questions for clinical use.
Currently, the effectiveness of stem-cell transplantation alone is difficult to interpret because the clinical studies have been done in conjunction with percutaneous or surgical revascularization.
Thus, larger double-blinded, controlled studies with therapeutic end points are imperative to clarify the role of stem-cell transplantation for myocardial regeneration.