The potential for the human heart to regenerate constitutes a revolutionary paradigm shift in our understanding of myocardial pathophysiology; besides cellular hypertrophy, necrosis, apoptosis, and fibrosis, it appears that under the right conditions we can enable another myocardial response to injury – regeneration. While rate of cell turnover in the human heart is debated, the existence of the phenomenon is a) clearly established, and b) clearly inadequate to recoup the myocytes lost during myocardial infarction. One possible exception might be found in the syndrome of Anomalous Left Coronary Artery from the Pulmonary Artery (ALCAPA, or Bland-White-Garland syndrome); these infants have evidence of myocardial infarction, but if surgically corrected early may recover normal left ventricular function [15]. While the mechanism of this phenomenon is unknown, it seems likely that the regenerative potential of an infant’s myocardium might be substantially greater than that of adults’.

As a potential therapeutic intervention in adults, stem cell transplantation may be viewed as an attempt to augment the inadequate endogenous regenerative capacity of the heart, and to prevent the maladaptive sequelae. Mechanisms by which transplanted stem cells enhance cardiac regeneration include the formation of new cardiac myocytes, new blood vessels, modulation of inflammation, and attenuation of remodeling and fibrosis [16].

Skeletal myoblasts are stem cells located under the basal lamina in skeletal muscle; they were postulated to participate in repair following injury, and were among the first cells to be considered for myocardial regenerative therapy [17]. They can be obtained as an autologous product, have high scalability in culture, and appear to be resistant to ischemia. In animal models skeletal myoblasts appeared promising [18, 19]. Early phase clinical studies did encounter a safety concern and it was speculated that ventricular tachyarrhythmias might arise from failure of the graft electromechanically to couple with endogenous cardiomyocytes [20, 21].

The adult bone marrow is home to a vast array of support cells and multipotent precursor lineage stem cells. Bone marrow derived stem cells (BMDSC) are the earliest and most investigated in all of stem cell research. In adult humans, BMDSC can be collected by iliac crest aspiration of marrow with expansion in culture, or obtained by cell sorting from peripheral blood after cytokine mobilization. BMDSC exhibit plasticity in vitro permitting differentiation into many cell types including those of cardiogenic lineage, such as contracting cardiomyocytes, coronary arterioles and capillaries [7]. BMDSCs appear to constitute many different cell types, and of these, preclinical studies [22] and clinical trials have examined several for their potential to promote favourable healing after myocardial infarction. Freshly prepared mononuclear cell fraction of BMDSCs have been the subject of the largest experience to date, but more specific fractions from the bone marrow include precursors of hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs) and endothelial progenitor cells (EPCs), which are found in varying quantities within the mononuclear population [23].

HSCs are negative for lineage markers (lin⁻) but express hematopoietic marker CD45 with Sca-1+, CD34+, CD133+ and c-kit+ (CD117) [24]. The transdifferentiation of HSC into cardiomyocytes and endothelial cells in the heart has been demonstrated in animal models. Experiments with the administration of c-kit⁺/lin⁻ HSC in infarcted rat myocardium demonstrated significant cardiogenesis in conjunction with improved myocardial perfusion, angiogenesis and collateral vessel formation [25], translating into improved myocardial function, whether by transdifferentiation or another mechanism [26]. EPCs have the ability to differentiate into endothelial cells, and exist both in the bone marrow and in the peripheral circulation. EPCs share a common precursor with the HSCs and are CD34+ but are negative for CD45 [27]. Kawamoto et al. demonstrated engraftment, neovascularization and improved cardiac function with both intravenous and intramyocardial delivery of EPCs [28, 29].

MSC constitute a small percentage of the bone marrow; they express Stro-1, CD90, CD106, CD13 but no classic hematopoietic or endothelial cell markers [30, 31], and exhibit the capacity to differentiate into cells of several tissue types, including osteoblasts, chondroblasts and adipocytes. Our lab and others have demonstrated that bone marrow-derived MSCs transplanted into infarcted myocardium result in functional improvement with reduction of infarct scar area [32–34]. MSC also possess immunomodulatory characteristics, inhibiting immune responses during allogeneic transplantation in humans [35]. The use of allograft MSCs in animal models and in human subjects has followed; graft rejection has not been observed, and this approach reduced scar size and improved cardiac function in ischemic cardiomyopathy (ICM) [16, 36, 37].

Cells obtained from the heart with the Lin⁻/c-kit⁺ phenotype appear to have the potential to differentiate into functional cardiac myocytes, smooth muscle cells, and endothelial cells in vitro; these CSCs have been applied in animal models and in human subjects with the expectation of forming differentiated cardiac myocytes in vivo. CSC harvested from the adult heart (for example, from the right atrial appendage normally incised during many open heart surgeries) also exhibit other markers including Sca-1⁺ cells [38], side population (SP) cells [39], and ISL-1+ cells. Sca1-positive cells constitute 0.3 % of the myocyte compartment, and appear to have the potential for expressing cardiac transcription factors and transdifferentiating into cardiac myocytes in vivo [40]. Similarly, cellular and animal studies of Sca-1+/CD31cells [41], and SP cells [42] demonstrate cardiomyogenic potential. Together with Islet-1+ and c-kit⁺ CSCs , these and perhaps other cardiac stem cells exist in niches [43] from which proliferation, migration and regeneration may occur.

Reconstitution of the cardiac stem cell niche has been proposed to underlie the potential efficacy of “cardiospheres,” biologically engineered cultured multicellular structures containing a core of c-kit⁺ with supporting cells [44, 45], and cells derived from cultured cardiospheres have also advanced through the translational pathway [46].

Pluripotent stem cells are those which may generate all three germ layers (ectoderm, mesoderm, and endoderm) and which self-renew indefinitely; these include embryonic stem cells (ESC) obtained from the developing embryo. ESC also have the potential for cardiac differentiation [47], but development of these cell types for therapy has been limited compared to other cells. Despite mechanistically feasible findings, both ethical issues and the potential of tumorgenicity [48] have led investigators to other approaches towards pluripotency, including nuclear reprogramming of differentiated adult cells [49, 50]. Like ESC, induced pluripotent stem cells (iPSC) have been shown in small animals to improve heart function after infarction [51].

Human umbilical cord blood is a rich source of both HSC [52] and MSC [53] and these cells maintain proliferative potential which is neither embryonic nor adult, but probably in between [54]. Cord blood-derived cells have been used clinically for the treatment of hematologic diseases [55]. Transdifferentiation into cardiac myocytes and other lineages has been demonstrated [56], and transplantation into infarcted myocardium resulted in neoangiogenesis, reduced infarct size and improved ventricular function in animal models [57–59]. The availability of large quantities of cord blood may favour therapeutic development [60].

Paradoxically, as clinicians and epidemiologists bemoan the increasing trend towards obesity in industrialized societies, human adipose tissue also contains cells with multipotent potential [61]. Studies in vitro studies demonstrate ADSC can be persuaded to differentiate into cardiac myocyte-like cells [62, 63]. Showing similar characteristics as MSCs [64, 65], ADSC have been shown to regenerate damaged myocardium in animal studies [66], leading to translational studies with adipose tissue derived products in human subjects.

Most studies of cell therapy for myocardial disease have focused on that resulting from ischemic damage. Whether in the acute phase of myocardial infarction, or in the later stages of chronic ICM, this closely related set of disorders is well-understood after decades of clinical and preclinical investigation, and is particularly well-represented by small and large animal models [67]. Nonetheless, important non-ischemic heart diseases also have been subjected to translational investigation for stem cell therapies.

In animal models, inherited non-ischemic cardiomyopathy [68–70], anthracycline chemotherapy-associated cardiomyopathy [71, 72], post-myocarditis cardiomyopathy [73, 74], and pacing tachycardia-induced cardiomyopathy [75] have been studied as potential targets for cell therapies. While clinical investigation and trials in these non-ischemic conditions remain fewer than those for acute and chronic ischemic myocardial diseases, translation into early phase human investigation for non-ischemic cardiomyopathies has begun.

The hope that stem cells could replace damaged heart tissue [19, 22] by transdifferentiation into functioning cardiac myocytes and vascular cells (akin to re-populating a bare patch of lawn with grass seed) has proved a misleading oversimplification. Transdifferentiation may occur as a low-frequency event, but is unlikely to explain the magnitude and temporal course of myocardial regeneration; instead, the modulation and amplifaction of endogenous processes by the administered cell product appears to represent a major mechanism of action [76–78]. In a porcine myocardial infarct model, the administration of bone marrow derived MSCs resulted in 20-fold increase in endogenous c-kit+ CSCs; this principle was further supported by the finding that co-culture with MSCs enriched cardiopoetic cells obtained from heart biopsies [33]. Based on this finding of cell-cell interaction between niche-modulating cells and cardiotypic cells, co-administration of MSCs and CSCs in porcine chronic ICM was twice as effective as either cell type alone [79].

Cytokines which may be involved in this effect include vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), insulin growth factor 1 (IGF-1), thymosin β4 (TB4), and stromal cell-derived factors 1 (SDF-1) [80], and it is likely that these cytokines are expressed with some temporal and spatial specificity including autocrine feedback, to modulate a local stem cell niche, influencing endogenous stem cells for proliferation, differentiation, cardiac remodelling and repair [81, 82]. Other paracrine effects may alter the metabolism [83] and recruitable contractility [84] of viable cardiac myocytes, and fusion of host cells with transplanted stem cells may occur, with the resulting cell exhibiting some of the characteristics of each contributor [85].

Cell therapy studies for acute MI have included more than 1000 patients, mostly receiving BMDSC via intracoronary infusion. Results are mixed. For example, two studies can be compared. In a well-conducted, trial enrolling 101 patients with a first, anterior MI and receiving reperfusion by PCI, Lunde et al. randomized patients to receive bone marrow mononuclear cells (median cell number 68 × 10⁶) by intracoronary infusion with stop-flow balloon occlusion. Control patients were not subjected to sham/placebo interventions. At 6 month cardiac MRI, no benefit was detected from cell infusion [100].

In contrast, the REPAIR-AMI trial randomized 204 patients with an acute MI to receive a ~200 × 106 bone marrow mononuclear cells or placebo 3–7 days after initial coronary intervention, again by stop flow intracoronary infusion. In this case, both LV ejection fraction changes and clinical endpoints seemed to be improved by cell therapy [101].

Among the many potential confounders for these trials, a few might be particularly significant. Important variability exists even among patients meeting the most tightly controlled inclusion criteria; differences in the severity and extent of coronary disease, pre-infarction angina, time to treatment, and others will influence the extent of myocardial injury, the efficacy of its salvage, the rate of recovery of myocardial contractility after reperfusion, and the net effect on global LV function. Ejection fraction, though undoubtedly a powerful marker of future clinical risk, is determined by the estimation of end-diastolic LV volume and end-systolic volume; it may vary substantially both on a beat to beat basis and as loading conditions change. Meta-analyses have been attempted and suggest that bone marrow cell infusion after acute MI may result in improved LVEF, reduced left ventricular end systolic volume (LVESV), and reduce infarct size [102]. Trends suggesting greater benefit in patients with more severely reduced EF are seen in these and other [103] trials of cell therapy for acute MI, including studies based on intravenous administration [104]. However, only large trials, such as the ongoing BAMI (The Effect of Intracoronary Reinfusion of BM-MNC on All Cause Mortality in Acute Myocardial Infarction) trial can approach balancing all these variables.

In comparison with trials for acute MI, those enrolling patients with ICM include a broader range of cell types and delivery strategies. There is also significant heterogeneity among the outcome measures studied and the techniques used to characterize changes after cell therapy. Cardiac MRI is particularly useful in this regard, allowing quantification of both global and regional myocardial function, and measurement of the mass of viable myocardium and scar. While MRI remains the most complete technique, CT now approaches MRI in many of these measures. Many patients with ICM have implanted pacemakers and defibrillators, which create artifacts partially obscuring the heart with either MRI or CT. SPECT, PET, and echocardiography have been used as well.

Clinical assessments included in some trials have included the highly subjective (NYHA functional class) and more quantitative (MVO2 max) measures. The potential for placebo effect is particularly great when highly motivated patients with serious conditions undergo procedures with novel therapies [105], reinforcing the value of randomized blinded placebo controlled trials in this arena.

While the myocardial disease related to coronary ischemia has been the subject of much more preclinical, translational, and clinical study, other forms of cardiomyopathy are important and have motivated investigation as well. Idiopathic dilated cardiomyopathy (DCM), and specific entities such as familial cardiomyopathies or chemotherapy-associated cardiomyopathy are beginning to be studied with the hope that cell therapy may regenerate or repair damaged myocardium.

Arguero et al. conducted the first clinical trial including bone marrow-derived cells delivery by surgical transepicardial injection, including in five patients with DCM [123]. Similarly, Arom et al. administered peripherally circulating cells by thoracoscopic injection in 20 patients with ischemic cardiomyopathy and 21 patients with DCM [124]. Both studies reported improved LVEF and functional status.

Fischer-Rasokat et al. conducted a study in 33 patients with DCM using an intracoronary infusion of BMDSC. After 3 months, cardiac function improved significantly and at 12 months N-terminal prohormone brain natriuretic peptide (NT-proBNP) serum levels were also decreased [125]. Improvements in quality of life, clinical symptoms, exercise capacity and cardiac function were reported in a similar study conducted by Martino et al. [126].

In the first randomized, open label, controlled trial performed by Seth et al., patients with DCM (n = 24) were administered autologous bone marrow mononuclear cells, and compared to 20 control patients. Cell administration was by intracoronary infusion, during balloon occlusion of the coronary sinus effluent. At 6 months, the treatment group demonstrated significant improvements in the LVEF, NYHA scale, and cardiac volumes compared to controls [127]. A later report from the same group continued to show improved cardiac function and clinical symptoms with treatment (n = 41) compared to control (n = 40) [128].

Vrtovec has reported the long-term follow-up of 110 patients = DCM who were randomized to receive open-label intracoronary infusion of CD34+ cells obtained by GM-CSF mobilization and apheresis, or control. LVEF increased up to 3 years, after which improvement declined, but at 5 years, the patients receiving cells still had higher LVEF and 6-min walk distance, and lower N-terminal B-type natriuretic peptide (a blood biomarker of heart failure.) [129].

Taken in context, these results do suggest that the treatment of non-ischemic cardiomyopathies is feasible and can be considered; randomized blinded placebo controlled trials will be required before firm conclusions can be drawn about efficacy.

As surveyed herein, a variety of autologous cell products have been subjected to early stage testing with a variety of strategies. While autologous cells would be expected to avoid the potential of host rejection, compared with the possibility of donor-derived allogeneic products, autologous cells do have important practical and clinical limitations. Depending on the cell type being prepared, tissue acquisition and processing, and culture expansion impose logistical obstacles to treatment when it might be needed. Furthermore, it has been suggested that stem cells from older, sicker patients are marked by smaller number or less potency [130]. Finally, the requirement to prepare (and perform pre-release quality testing for) a cell product for each individual patient is an expensive undertaking.