It is estimated that approximately 5 million Americans currently live with heart failure (HF) and an additional 500,000 patients are newly diagnosed each year.1 End-stage HF has a worse prognosis than most types of cancer,2 and heart transplantation remains the gold standard of all therapies for advanced HF.3 However, the number of donor organs significantly fails to meet demand. Therefore, alternative treatment options for advanced HF are necessary. In principle, the nature of the HF determines the choice of therapeutic strategy. Cell therapy and tissue engineering are important new treatment options that promise to open new therapeutic routes for the treatment of HF.
Different therapeutic approaches are available for the treatment of end-stage HF. They can be divided into two categories: treatments that directly target the cause of end-stage HF, such as coronary artery bypass graft, mitral valve repair and ventricular restoration; and treatments that support heart function by regenerating cardiac tissue. As a last approach, and as the only curative treatment option, heart transplantation might be considered for some patients.3
Recent developments in the field of cell therapy and tissue engineering have made new methods of treatment available, and these have been shown, in initial clinical trials, to generate good results in combination with established surgical treatments. Extensive research has been conducted investigating the effects, on cardiac remodelling, of the transplantion of stem cells at different stages of development and from different germ layers. Cell-mediated effects of cells also have profound impacts on tissue reconstruction. The aim of cell transplantation is to repopulate diseased myocardium with cells that could restore myocardial perfusion and contractility. Results of experimental studies have shown that bone marrow-derived stem cells (BMCs) can differentiate into myogenic and angiogenic cell lines.
The term ‘adult stem cell’ refers to undifferentiated cells located in adult, differentiated tissue. These cells are capable of self-renewal and can generate all the specialised cell types found in the relevant tissue. Adult stem cells have been described from most mammalian tissues, including the haematopoietic and neural systems. They contribute to tissue homeostasis and replace damaged cells. Stem cell function in adult tissue is regulated in a complex local environment in response to the needs of the organism.
This specific microenvironment, called the ‘stem cell niche’, integrates signals from neighbouring cells, matrix components and soluble mediators to balance the stem cell response4 to hypoxia or inflammation, for example.5 Adult stem cells have been shown to respond to tissue injury, including tissue-specific homing and retention signals.6 Tissue injury might also be a determinant for stem cell differentiation as well as self-renewal.
Neural stem cells, for example, were shown to be pluripotent in experimental transplantation settings. These cells were found to produce a variety of haematopoietic cells after transplantation into irradiated hosts.7 In further studies, neural and haematopoietic stem cells were injected into mouse blastocysts and contributed to ectodermal, endodermal and mesodermal tissue.8,9 However, as shown by other studies, neurosphere-derived cells10,11 as well as BMCs12 can spontaneously fuse with embryonic stem cells in vitro. The resulting cells show comparable differentiation potential to embryonic stem (ES) cells.13 In addition, murine haematopoietic stem cells (HSCs) have been reported to transdifferentiate into liver,14 muscle15 and brain cells16in vivo.
New experimental data have challenged the model of stem cell transdifferentiation and direct cell replacement. HSCs and mesenchymal BMCs are the two groups of stem cells that have been used for end-stage HF therapy. We have shown that the clinvical application of CD 133+ human BMCs as endothelial progenitor cells can improve cardiac function. The CD 133+ cells can be enriched successfully by the application of CD 133+ antibody (Miltenyi Biotec, Bergisch Gladbach, Germany) under good manufacturing practice (GMP) conditions.
Unfortunately, over the past few years, no GMP-grade antibody for the isolation of mesenchymal somatic stem cells has been available for clinical application. The first multicentre surgical trial in the US (the IMPACT trial) showed an improvement in cardiac function after the transepicardial injection of mesenchymal stem cells in patients with end-stage HF.15 The mesenchymal stem cells were prepared by in vitro expansion, similar to many other previous studies.
Available data regarding mesenchymal stem cell isolation describe cell selection by the use of in vitro expanded adherent cell populations. Several crucial points need to be considered during the in vitro expansion process, including the exposition of the cells during expansion to the animal serum, the decrease of the cell purity, and the differentiation of the cells. In addition, the cell culture environment, possible contamination and cost problems favour a shift toward a straightforward selection method. Our aim was to carry out cell enrichment without in vitro manipulation of the cells and to overcome the adverse effects of cell expansion.
CD271 is a recently discovered marker for multipotent mesenchymal stem cells. Also known as nerve growth factor receptor, it has an extracellular domain containing four 40-amino acids. Quirici et al. detailed the homogenous multipotent capacity of CD271+ mesenchymal stem cells.16 In addition, several groups have shown that CD271 can be used as the most specific marker for the enrichment of the mesenchymal stem cells without in vitro expansion, and have published evidence for the high expansion potential of the CD271+ enriched cell population.17,18
We have successfully isolated human CD271+ cells by processing human bone marrow cells from sternal bone marrow aspirates in patients undergoing coronary artery bypass graft surgery. We used the CD271 GMP grade antibody and the CliniMACS® device as appropriate tools for the enrichment of mesenchymal BMCs. The protocol first published by our group enables isolation of the positive cell fraction from the bone marrow aspirate without the need for Ficoll preparation (see Figure 1).19 In addition, we plan to use an acute and chronic porcine ischaemia model for a transepicardial CD271+ cell injection. The results are likely to show the effect of stem cell injection on the regeneration of failing myocardium.
Published data have provided clear evidence of the improvement of cardiac function after the unloading of the heart by using mechanical cardiac assist devices. We intend to enhance this by injecting stem cells after the unloading of the failing heart by using mechanical cardiac assist devices in our established animal model (see Figure 2). Future results regarding the application of enriched CD271+ cells as a novel mesenchymal cell line are likely to highlight its future role in cardiac cell therapy.