Stem Cell banking

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Cell therapy for acute myocardial infarction: Requiescat in Pace

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Cell therapy for acute myocardial infarction: Requiescat in Pace
Abstract
Cell-based therapy is a promising option for treatment of ischemic diseases. Several cell types have experimentally been shown to increase the functional recovery of the heart after ischemia by physically forming new blood vessels, differentiating to cardiac myocytes and—additionally or alternatively—by providing proangiogenic and antiapoptotic factors promoting tissue repair in a paracrine manner. Clinical studies preferentially used adult bone marrow–derived cells for the treatment of patients with acute myocardial infarction. Most of the studies suggested that cell therapy reduced the infarct size and improved cardiac contractile function. However, cell therapy is in its early stages, and various questions remain. For example, the identification of those patients who benefit most from cell therapy, the optimal cell type and number for patient with acute and chronic diseases, the best time and way of cell delivery, and the mechanisms of action by which cells exhibit beneficial effects, need to be further evaluated. Although no major safety concerns were raised during the initial clinical trials, several potential side effects need to be carefully monitored. The present review article summarizes the results of the clinical studies and discusses the open issues.

Cell based therapy is a promising option for treatment of ischemic diseases. Several cell types have been shown to increase the functional recovery of the heart after ischemia. The present review article summarizes the results of the experimental and clinical studies and discusses open questions in cell-based therapies.

Ischemic diseases remain one of the major causes of morbidity and mortality in the industrialized world despite the development of several new therapeutic modalities, such as improved pharmacological therapies and improved interventional strategies (eg, catheter-based reopening of vessels). Peripheral ischemic vascular disease is still associated with a high morbidity and impairment of quality of life, whereas cardiac ischemia leading to postinfarction heart failure particularly in patients with large myocardial infarction is associated with a high mortality. Over the last decade, it has become apparent that the heart possesses regenerating capacities with respect to cardiomyocytes and new blood vessel formation (for review see1,2). Specifically, it has been shown that a sizeable fraction of cardiac myocytes “refreshes” the heart after injury insults like ischemia or pressure overload.3 Based on experimental data demonstrating that infusion or injection of stem/progenitor cells improves heart function after myocardial infarction and enhances blood flow in models of peripheral ischemia, clinical trials were initiated in 2001 to treat patients with peripheral or cardiac ischemia with circulating blood or bone marrow–derived cells. The present review article will summarize the experimental data and the clinical application of cell-based therapies focusing on patients with acute myocardial infarction, where most clinical data are available at present.

Types of Stem Cells
Numerous studies have experimentally addressed the potential of different types of stem cells to augment neovascularization and cardiac repair or regeneration. In general, two types of stem cells should be discussed separately: embryonic stem cells and adult stem cells. Whereas embryonic stem cells clearly have the capacity to differentiate into a variety of cell types and give rise to tissues and organs, most adult stem cells are more specified (more lineage committed) and the use of adult stem cells for organogenesis appears to be rather limited. This review will focus on the different types of adult stem cells and their therapeutic role in the salvage of ischemic tissue and in treatment of heart disease. Adult stem cells comprise at least 3 different groups: bone marrow–derived stem cells, the circulating pool of stem or progenitor cells, which, at least in part, are derived from the bone marrow, and tissue-resident stem cells.

Bone marrow–derived stem cells are the best characterized and have been used in the majority of clinical trials performed to date. Bone marrow contains a complex assortment of progenitor cells, including hematopoietic stem cells (HSCs); so-called “side population cells” (SP cells, defined by the expression of the Abcg2 transporter allowing to export a Hoechst dye),4 mesenchymal stem cells (MSCs) or stromal cells,5 and multipotential adult progenitor cells (MAPCs), a subset of MSCs.6 Several studies have shown the incorporation of these different bone marrow–derived cells into ischemic tissue, and it appears that these cells play a distinct role in the salvage of damaged tissue.

Another population of progenitor cells that has also been shown to have therapeutic potential is the pool of progenitor cells circulating within the blood. Circulating progenitor cells were initially discovered, when searching for proangiogenic cells for therapeutic vasculogenesis. Asahara and Isner isolated the so called “endothelial progenitor cells” defined by their function to form new blood vessels and enhance neovascularization after ischemia (for review see7,8). According to the assumption that these cells may represent adult hemangioblasts, these cells were characterized by the expression of at least 2 hematopoietic stem cell markers (CD133+or CD34+) and the endothelial marker VEGF-receptor 2 (also known as KDR or flk-1). The use of the marker combination CD34+CD133+KDR+ to identify clonally expandable circulating progenitor cells with a high capacity to acquire an endothelial phenotype has recently been challenged, and in vitro studies suggested that CD34+/CD45− cells have a higher capacity to acquire an endothelial phenotype, whereas CD34+CD133+KDR+ do not differentiate to endothelial cells.9 Although it is not entirely clear to what extent these data can be translated into the in vivo situation, where ischemic/necrotic tissue may provide an entirely different environment to dictate cell fate, it is evident from various studies that circulating progenitor cells, particularly when cultured in vitro, comprise several different cell populations. Whereas individual cells may indeed have clonal potential and stem cell characteristics, other cells may provide proangiogenic factors or promote vessel maturation or may act as pericytes together leading to neovascularization. For example, one subpopulation within these cultured or endogenously circulating cells consists of myeloid and myeloid precursor cells, which may preferentially act as proangiogenic cells,10,11 in addition (or alternatively) to their capacity to differentiate to endothelial cells.7,12 Alternatively, myeloid cells have been shown to fuse with skeletal muscle myotubes13,14 indicating that myeloid subpopulations may not only act to mediate neovascularization, but may also aid in muscle regeneration. Therefore, the so called “endothelial progenitor cells” most likely consist of several cell types that together may mediate salvage of ischemic tissue. As such the term “endothelial progenitor cells” refers to one functional aspect of a heterogeneous cell population, which is capable to induce neovascularization.

Other populations of stem cells that have been shown to have therapeutic potential in the setting of ischemia are derived from tissue and include mesoangioblasts, both mesenchymal and endothelial progenitor cells derived from adipose tissue, and tissue-resident cardiac stem cells. Mesoangioblasts are vessel-associated multipotent progenitors that express the key marker of angiopoietic progenitors, VEGF-receptor 2, but are distinct from hematopoietic endothelial progenitor cells. In vitro mesoangioblasts differentiate into many mesoderm cell types, such as smooth, cardiac and striated muscle, bone and endothelium, and have been shown in vivo to improve skeletal muscle function in a muscular dystrophy model as well as to improve heart function.15,16 Adipose tissue is a rich source of distinct subsets of stem/progenitor cells potentially useful for cardiac repair and neovascularization improvement.17,18 Both, mesenchymal stem cells and endothelial progenitor cells were isolated after enzymatic digestion of adipose tissue and showed beneficial effects in experimental studies.

The discovery of tissue-resident stem cells in the heart, the “cardiac stem” cells, offers the potential for in vivo induction of proliferation and differentiation of these cells, which are primed to acquire a cardiac phenotype and, therefore, might be optimally suited for cardiac repair. Several different populations have been identified and characterized including c-Kit+ cells,19 Sca-1+ cells,20 side population cells (SP),21 and cells expressing the protein Islet-1.22 Whereas c-Kit+ cells, Sca-1+ cells, and cardiac SP cells have been isolated from adult hearts, cells expressing Islet-1 so far only have been detected in neonatal hearts. Whether c-Kit+, Sca-1+, and cardiac SP cells comprise 3 different cell populations is not entirely clear. Another type of cardiac stem cell has been identified by growing self-adherent clusters (termed “cardiospheres”) from subcultures of murine or human biopsy specimens.23,24 Others have generated cardiac SP-cell derived cardiospheres by adapting a method used for creating neurospheres suggesting that cardiac neural crest cells may contribute to cardiac SP cells.25 Cardiosphere-derived cardiac stem cells as well as c-Kit+ cardiac stem cells are capable of long-term self-renewal and can differentiate into the major specialized cell types of the heart: myocytes and vascular cells expressing both endothelial or smooth muscle cell markers. The exact origin of these c-Kit+, Sca-1+, SP, Islet-1+, or cardiosphere-derived cardiac stem cells and the mechanisms maintaining the cardiac stem cell pool are unclear. Two recent studies suggest that c-Kit+ and cardiac SP cells may arise from the bone marrow,26,27 however these studies cannot entirely exclude that specific subpopulations of cardiac stem cells originate from the heart and these cardiac stem cells may represent remnants from embryonic development in selected niches within the heart.

In summary, although several different types of adult stems have been identified and used for improving cardiac function after ischemia, it remains unclear which of these cells have the greatest therapeutic potential. In an attempt to discern which adult stem cell population produces the greatest functional efficacy, one study compared mesoangioblasts and bone marrow–derived progenitor cells with fibroblasts and endothelial cells. Both cell types showed a similar capacity to improve heart function, whereas endothelial cells and fibroblasts were not effective.16 A single nonhematopoetic MSC subpopulation, unpurified MSC, bone marrow mononuclear cells, and peripheral blood mononuclear cells were compared in another study. This study suggested that single clonally purified MSC are most efficient for cardiac repair. Interestingly, unpurified MSC had similar beneficial effects on adverse remodeling of infarcted hearts compared with freshly isolated bone marrow mononuclear cells.28 However, because the rats were treated with cyclosporin A, one cannot exclude that the immunosuppression itself modulated the effects of the different stem cell populations. Clearly, a ranking of cells used for cell therapy will not only be based on the assessment of the functional capacities of the cells, but also on the safety and feasibility of the treatment in the clinical setting.

Clinical Application
Clinically Used Cell Types
Currently, a variety of autologous adult progenitor cells are undergoing preclinical evaluation. Bone marrow is, at present, the most frequent source used clinically for cardiac repair.29 The rapid transition from bench to bedside was facilitated by the more than 30 years of clinical experience and the excellent safety profile of infused bone marrow–derived mononuclear cells (BMCs) used for bone marrow reconstitution. After bone marrow aspiration the mononuclear cell fraction is obtained by density gradient centrifugation in most of the studies (only the BOOST trial used a sedimentation protocol). The mononuclear fraction includes a heterogeneous mixture of cells with varying percentages of hematopoietic stem cells, endothelial progenitor cells, mesenchymal stem cells, and side population cells. So far, isolated bone marrow–derived cells are injected into the heart without further ex vivo expansion. In a few studies, specific subpopulations such as a fraction of hematopoietic and endothelial progenitor cells expressing the marker protein CD133+ are purified. Lastly, peripheral blood–derived progenitor cells are clinically used both for cardiac repair and peripheral ischemia. Circulating blood–derived cells have been isolated from mononuclear blood cells and selected ex vivo by culturing in “endothelium-specific” medium for 3 days,30,31 or a specific subfraction, the hematopoietic progenitor cells CD34+, is enriched from whole blood after G colony–stimulating factor (CSF) mediated mobilization from the bone marrow into the blood.32

Clinical Results
The results of clinical trials published to date, aiming at progenitor cell-based myocardial repair in patients with acute myocardial infarction, are summarized in the Table. Overall, the published studies demonstrate that the intracoronary infusion of autologous BMC is safe and feasible in patients with acute myocardial infarction. The initial pilot studies by Strauer,33 the TOPCARE-AMI,30 the BOOST-trial,34 and the study performed by Fernandez-Aviles35 reported nearly identical results—an improvement in global LV ejection fraction by an absolute 6 to 9 percentage point, reduced end-systolic LV volumes, and improved perfusion in the infarcted area 4 to 6 months after cell transplantation. A randomized controlled trial by Janssens36 did not reveal a significant effect on global ejection fraction, but showed an improvement in regional ejection fraction and a reduction of the infarct size in the BMC group. Only one larger study, the ASTAMI trial, did not show any benefit on left ventricular functional parameters.37 The reason for the failure of the ASTAMI trial to show a benefit of cell therapy may have been because of the different cell isolation and storage protocol, which significantly affected the functional capacity of the cells.38 Because, however, no preclinical functional testing of the cells used for the ASTAMI trial was reported, it is essentially impossible to judge the negative outcome of this trial.
curncman
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Cell therapy for acute myocardial infarction:

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Review ArticleStem Cell Therapy in Acute Myocardial Infarction:A Pot of Gold or Pandora’s BoxV. K . S hah1andK.K.Shalia21Interventional Cardiologist, Sir H.N. Hospital and Research Centre, Raja Rammohan Roy Road, Mumbai 400 004, India2Sir H.N. Medical Research Society, Sir H.N. Hospital and Research Centre, Raja Rammohan Roy Road, Mumbai 400 004, IndiaCorrespondence should be addressed to V. K. Shah, vkshah45@hotmail.comReceived 1 September 2010; Revised 18 December 2010; Accepted 29 December 2010Academic Editor: Shijun HuCopyright © 2011 V. K. Shah and K. K. Shalia. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.Stem cell therapy for conditions characterized by myocyte loss in myocardial infarction and heart failure is intuitively appealing.Stem cells from various sources, including heart itself in preclinical and animal studies, have shown the potential to improve thefunction of ventricular muscle after ischaemic injury. The clinical experience from worldwide studies have indicated the safetyprofile but with modest benefits. The predominant mechanisms of transplanted cells for improving cardiac function have pointedtowards paracrine effects rather than transdifferentiation into cardiomyocytes. Thus, further investigations should be encouragedtowards bench side and bedside to resolve various issues for ensuring the correct type and dosing of cells, time, and method ofdelivery and identify correct mechanism of functional improvement. An interdisciplinary effort at the scientific, clinical, and thegovernment front w ill bring successful realization of this therapy for healing the heart and may convert what seems now a Pandora’sBox into a Pot of Gold.1. Clinical NeedMyocardial infarction (MI) remains a major cause of mor-bidity and mortality. Rapid reperfusion of the occludedcoronary arteries is of great importance in salvaging ischemicmyocardium and limiting the size of infarct. This reducesearly complications and improves survival rates. Unfortu-nately, myocardial necrosis starts rapidly before reperfusioncanbeachievedinmostofthepatients,leavinganinfarctzone that contains nonfunctional myocytes that are remod-eled into the scar tissues surrounded by region of ischemia.Contemporary reperfusion strategies using percutaneousinterventions aided with pharmacotherapy and mechanicaldeviceshaveshowntoresolvetheischemiawithonlymodestimprovements in global Left Ventricular Function (LVF) asevidenced by 2% to 4% increase in LV Ejection Fraction(LVEF) at six months after an acute MI [1,2]. This loss ofviable myocardium initiates a process of adverse ventricularremodeling and a downward spiral leading to congestiveheart failure. This is followed by repeated hospitalizationand increased economic burden on the society with 50% ofthe patients dying within five years of the diagnosis. Scartissue is incapable of performing the vital function of cardiacmuscle and suffers from decreased cardiac output. Revival ofthe cardiac tissue in infarct zone can enhance the functionalactivity of the heart. Thus, heart muscle salvage after heartattack is the single important determinant factor for theevent-free long-term survival.Considered as terminally differentiated organ, regener-ating the myocardium was never thought of as an optionfor heart muscle salvage. Stem cell-based therapy becamea realistic option to replace damaged heart muscles dueto series of experimental findings of myocyte turnover inmammalian heart (Ta ble 1 ). Evidence such as fraction ofcardiomyocytes may be able to reenter the cell cycle andthat limited regeneration can occur through recruitmentof resident and circulating stem cells were presented [3–11]. But it was also realized that these endogenous repairmechanisms are overwhelmed by the substantial damageto the myocardium from the injury that it faces duringMI. However, the existence of these endogenous repairmechanisms as well as the concept of adult stem cell plasticity
2Stem Cells InternationalTab le 1: Evidence of myocardial regeneration.Study FindingsKajstura et al. 1998[3]14 ×106 myocytes in mitosis by confocalmicroscopy.Beltrami et al. 2001[4]4% myocytes in mitosis by labeling withnuclei antigen Ki-67.Hierlihy et al. 2002[5]Endogenous resident cardiac lineagenegative [L-] C-Kit + stem cellsdifferentiated into all three mainmyocardial cell types; myocardial,endothelial, and smooth muscle celltypes.Laflamme et al.2002 [6]Quaini et al. 2002[7]Sex-mismatched cardiactransplantations, homing of recipient’sprogenitor cells in the myocardium wasdemonstrated. In the procedure Ychromosome in situ hybridization wasused to track the male cells in the femaleallografts coupled with immunostainingto define the identity that these cells hadacquired.Jackson et al. 2001[8] and Bittira etal. 2003 [9]Marrow-derived progenitor cells circulateand home to injured tissues similarly toleukocytes, where they contribute to theformation of new tissues.suggested that cardiac repair may be achieved therapeuticallyin these clinical settings and gave a way for preclinical trials.Subsequent promising reports of these same trials promptedrapid initiation of human clinical trials. In the present paper,we discuss the different types of stem cells and their journeyin healing the heart, certain unresolved issues and discuss keypoints for the design of future stem cell therapy trials.2. Stem CellsStem cells are primitive, undifferentiated, undefined pluripo-tent multilineage cells that retain the ability to renew them-selves through mitotic cell division and can divide and createacellmoredifferentiated than itself. Every single cell in thebody originates from this type of cell. They are obtained notonly from embryo and fetus but also from various parts ofthe adult body. Adult stem cells are defined as undifferen-tiated progenitor cells from an individual after embryonicdevelopment. Multiple tissues have been shown to containorgan-specific progenitor cells. However, adult stem cellshave less potential to differentiate without assistance. Stemcells are usually classified according to the following cri-teria: origin, type of organ or tissue from which the cellsare derived, surface markers, and final differentiation fate(Table 2 ).2.1. Embryonic Stem Cells (ESCs). ESCs are totipotent stemcells derived from the inner mass of the blastocyst stage late inthe first week of fertilization. They differentiate into multicel-lular embryoid bodies containing differentiating cells fromallthreegermlayers,ectoderm,mesoderm,andendoderm,and are able to give rise to most somatic cell lineages [12–14]. Since the mid-eighties, it has been shown that duringin vitro differentiation into cystic embryoid bodies, ESCsdifferentiate into beating cardiac myocytes [12]andareelectromechanically coupled to the host cardiac cells [15,16]. ESC-derived cardiac myocytes most closely resembleembryonic cardiac myocytes and express the complete reper-toire of cardiac-restricted transcription factors includingGATA4, Nkx2.5, MEF2C, and Irx4 [17]. In several rodentmodels it has been shown that, when transplanted intoinfarcted myocardium, ESCs-derived cardiomyocytes engraftand improve cardiac function [18–21]. However, there arecertain limitations for their use. The first is the likelihoodof teratoma formation at the implantation site. This canbe resolved by their differentiation prior to implantationand thus yielding a pure cardiac myocyte population [22].Thesecondissuepertainstoimmunity.Oncethoughtto be uniquely immunoprivileged, increasing evidence hasdemonstrated that ESCs express specific human leukocyteantigen (HLA) subclasses [23]. This raises the worry ofgraft rejection and might necessitate immunosuppression.However, Steroid use without concomitant stem cell implan-tation has been known for some time to be harmful toischemic myocardium [24]. There is currently ongoing re-search to help limit the immunogenicity of the cells forallogeneic transplantation. Finally, the origin of ESCs hasraised considerable ethical concerns and led to heateddebates among scientists and the wider public. The recentdiscovery that it is possible to generate ESC-like cells, calledinducible pluripotent stem (iPS) cells, by reprogrammingadult somatic cells with genes regulating ESC pluripotencymay resolve the ethical and immunogenic issues associatedwith the use of ESCs [25–27].2.2. Induced Pluripotent Stem (iPS) Cells. Induced pluripo-tent stem (iPS) cells are the stem cells artificially derived fromadult somatic cells which have been induced to express a geneprofile characteristic of ESCs (Oct3/4, Sox2, KLF4, cMyc)in response to genetic engineering [25,28–30]. iPScells arethought to be therapeutically equivalent to ESCs, in manyrespects, such as the expression of certain embryonic markers(SSEA-1) and proteins, chromatin methylation patterns,doubling time, embryoid body formation, teratoma forma-tion, viable chimera formation, potency, and differentiability.These cells are genetically identical to thedonor cells [30,31].In contrast to ESCs, the use of iPS does not generate ethicalcontroversies. Expansion of iPS in stem cell media can yieldasufficient number of cells that can subsequently be used forstudies on cardiac differentiation. But the full extent of theirpotential and possible toxicity is still being assessed [28,32].2.3. Cardiac Stem Cells (CSCs). Several groups of investiga-tors reported that the postnatal heart includes niches of CSCsand/or cardiac progenitors with the capacity to replicateand differentiate into cardiac myocytes [18,19,33–39].These cell populations included side population (SP) cells(Hoechst 33342 and Rhodamine 123 dye negative) [5,40],cell expressing the stem cell factor c-Kit (CD117) [41], cell
Stem Cells International 3Tab le 2: Major cell types with potentials for cardiac cell therapy.Type Markers Advantages DisadvantagesEmbryonic stem cells (ESCs)Blastocysts (inner cellmass)— Totipotent and highly expandableImmunosuppression required, ethicaldebate, lack of availability, and tumourpotentialIPS (induced pluripotentcell)Fibroblast (byreprogramming adultsomatic cells with genesregulating ESCpluripotency)—Pluripotent indistinguishable fromESCs at the epigenetic andfunctional levels. Embryonic stemcell like autologous adult cells forcell therapyTumourigenesisAdult/Fetal cardiomyocytes Isl+,Lin−c-kit+Sca-1+cardiosphere cells, SP cellsMultipotentCardiomyocyte phenotypeElectro-physiologically compatibleImmunosuppression required, ethicaldebate, short survival, and limitedsupplySkeletal myoblasts satellitecells CD56+Autologous transplantation, lack ofimmunogenicity and high yield andfatigue resistant, slow twitch fibersElectrophysiologically uncompatible,lack of gap junction, arrhythmogenicHematopoietic stem cellsBone marrow/peripheralbloodCD34+,CD45+, CD133+Multipotent, lack ofimmunogenicity and autologoustransplantation, different lineage ofcellsQuantum of cell population notadequateMesenchymal Stem CellsBone marrowStromal/muscle, skin, andadipose tissueAdhesion molecules(ALCAM/CD44)Antigens(SH2/SH3/SH4/STRO-1)Allogenic/autologoustransplantation, lack ofimmunogenicity (lack MHCII andB7 expression), pluripotent andcryopreservable for future useRequires expansionEndothelial progenitor cellsBone marrow/peripheralbloodCD133+Autologous transplantation,monopotent, lack ofimmunogenicityNeed for expansion because of limitedsupplyexpressing the stem cell antigen 1 (Sca-1+)[42] cardiosphere-derived cells [43] and expressing the protein Islet-1 detectedin the neonatal hearts. These cells are approximately 1/10ththe size of adult cardiac myocytes. When isolated by repeatedpanning or FACS sorting, 7–10% of these cells expressedthe early cardiac-restricted markers GATA4, Nkx2.5, andMEF2 [39,41,44]. Expression of these markers does notdefinitively mark a cell as cardiac in origin but does supportthis conclusion. Since they are cardiac in origin, perhaps suchcells might provide a mechanically and electrophysiologicallycompatible source of cells for transplantation. These cells canbe harvested from cardiac biopsies. They were demonstratedto give rise to cardiomyocytes, endothelial cells (ECs), andsmooth muscle cells (SMCs) in preclinical and some animalexperiments with improved LV functions. SP cells [45–47],c-kit cells [48–50], and cardiosphere cells [43,51–54]weredemonstrated to give rise to cardiomyocytes, ECs, and SMCsin preclinical and some animal experiments with improvedLVF, whil e S ca-1+CD31−cells were shown to differentiateinto cardiomyocytes and ECs in culture as well as in miceafter MI and improved cardiac function by promoting newblood vessel formation [55]. CSCs isolated and cloned fromthe heart ventricles of rat subjects have been shown to beeffective in the treatment of myocardial ischemia, thereforemaking the heart a viable source of stem cells for myocardialrepair [48].Cardiac stem cells (as well as stem cells from othertissues) appear to reside in specialized niches, which supportthe growth and maintenance of the stem cell pool [56,57].Putative niches have been localized throughout the myo-cardium, concentrated in deep tissue at the atria and apex[41,58]. Recent evidence has also shown that there is amarked increase in the number and migration of such cells tothe injury areas following an ischemic insult [42]. Althoughthe different cardiac stem cell pools are small relative tothe mature resident cardiomyocytes, they are believed tobe the source of new cells in normal organ homeostasisas well as in stressed myocardium [59]. At present, it isunclear if the various cardiac stem cells are distinct typesor whether they represent different stages of a single celllineage. Furthermore, it appears that the cardiac stem cellpool diminishes with ageing, possibly contributing to thelack of efficacy of regeneration in elderly individuals [59].2.4. Skeletal Myoblasts. Often called “Satellite Cells” whichare found beneath the basal membrane of muscle fibres liedormant till stimulated to proliferate by muscle injury ordisease [60]. These cells were the first to enter the clinicalarena after completion of a decade of experimental testingresulting in at least 40 studies. Myoblasts can be isolated fromskeletal muscle biopsies and expanded in vitro. These studies
4Stem Cells Internationalconsistently showed differentiation of implanted myoblastsinto multinucleated myotubes (not cardiomyocytes) with thelack of connexin activity and absence of electromechanicalcoupling with the host cardiomyocytes. Despite these appar-ent short comings, a definite improvement in regional andglobal LVF was demonstrated. These data along with theclinically appealing characteristics of skeletal myoblasts (ahigh in vitro scalability of the initial biopsy, an advance stageof differentiation virtually eliminating tumorigenicity, anda high resistance to ischemia) paved the way for the initialhuman trials which started in June 2000 [61,62]. Nonethe-less, it would appear that enthusiasm for this approach iswaning. However, considerations for modified or preselectedproducts have been formulated, and a “second generation” ofskeletal myoblasts modified by cell enhancement techniqueshave been hypothesized [63,64].2.5. Bone Marrow Stem Cells (BMSCs). The bone marrow ex-emplifies a typical adult stem cell source containing differentcell populations that have the potential to migrate and trans-differentiate into cells of diverse phenotypes. Unfractionatedbone marrow cells contain different stem and progenitorscell populations including Haemopoietic Stem Cells (HSCs),Endothelial Progenitor Cells (EPCs), and Mesenchymal StemCells (MSCs). Apart from these there are Multipotent AdultProgenitors Cells (MAPCs) also derived from bone marrowstromal cells. They have the ability to differentiate in vitroin cells of three germ layers and differentiate into cardiac,endothelial, and smooth muscle cell phenotypes.2.6. Hematopoietic Stem Cells (HSCs). HSCs can be isolatedfrom bone marrow cells through selective sorting for aparticular set of surface antigen (Lineage negative [Lin−]c-kit+,Sca-1+,CD34lo,andCD38hi)[65,66] and represent theprototypic adult stem cell population. They were shown todifferentiate into cardiomyocytes in culture, making them ofparticular interest in the treatment of cardiac disease becausethey represent a well-characterized and ample source ofprogenitor cells [67–70]. In vivo demonstration of the samewas given by Orlic et al. [71] by direct injection of Lin−c-kit+cells into the infarct region. Number of landmark studiesfollowed then which showed significant improvement incardiac function when these bone marrow-derived cells wereimplanted directly or mobilized from endogenousreservoirs.Some actually demonstrated regeneration of contractingcardiomyocytes and improved ventricular function [72–74],while others found beneficial effect independent of tissueregeneration [75–77]. Nevertheless, the improvements seenin ventricular function prompted a number of clinical trialsusing autologous BMSCs to treat heart failure patients orpatients who had suffered an MI.2.7. Mesenchymal Stem Cells (MSCs). MSCs represent a rarepopulation of cells with absence of HSC markers CD34and CD133. They are about 0.01% of the mononuclear cellfraction of the bone marrow and are also present in adiposetissue. They are less immunogenic due to lack of MHC-II andB-7 costimulatory molecule expression thereby preventing T-cell responses. They can differentiate into osteoblasts, chon-drocytes, and adipocytes [78,79]. Differentiation of MSCs tocardiomyocytes-likecells was observed under specific cultureconditions wherein MSCs were induced to transdifferentiateinto cardiomyocyte by 5-azacytidine, a DNA methylationagent [80]. Animal studies have also shown that MSCs havepotential for site-specific differentiation into heart musclecells, vascular-like structures, as well as gap junction protein[80–90]. These results suggest that MSCs act by regeneratingfunctionally effective, integrated cardiomyocytes and possi-bly new blood vessels. MSCs also have been injected intoinfarcted myocardium via a catheter-based approach in pigs,resulting in regeneration of myocardium, reduced infarctsize, and improved regional and global cardiac contractilefunction. Importantly, the latter study used allogenic MSCs,which did not produce evidence of rejection [89]. BecauseMSCs clones can be expanded in vitro and reportedly havea low immunogenicity, they might be used in an allogenicsetting in the future as cost-effective “off-the-shelf” allogeniccell product [91].MSCs were derived from adipose tissue; adipose tissue-derived stem cells (ASCs) were first identified by Zuk et al.[92] as a source of adult MSCs. After lineage-specific stimu-lation, ASCs show multiple lineage differentiation potential.They can differentiate into adipogenic, chondrogenic, myo-genic, cardiomyogenic, osteogenic, endothelial, and neu-rogenic lineages [93,94].Adiposetissueisanabundantexpandable and easily accessible source of MSCs also eval-uated for their therapeutic potential in regenerating heartin animal model after MI [95]. In culture ASCs express cellsurface markers similar to those expressed by bone marrowMSCs including CD117 (stem cell factor R), CD29 (betaintegrin), CD105 (multilineage differentiation markers),CD54 (intercellular adhesion molecule-1 (ICAM-1), andCD44 [78,96].2.8. Endothelial Progenitor Cells (EPCs). Cells with phe-notypic and functional characteristics similar to the fetalangioblast also are present in adult human bone marrow[11]. Endothelial progenitor cells (EPCs) represent a subsetof HSCs that are able to acquire an endothelial phenotype. Invitro [97–100] EPCs express the HSC markers CD34 and theendothelial marker Flk-1 (vascular endothelial growth factorreceptor-2 (VEGFR-2)) [99]. EPC can be isolated directlyfrom the bone marrow or from the peripheral circulationand expanded in vitro. Preclinical trials indicated that EPCscontribute to 1–25% of vessel formation after ischemicinjury for several diseases [101]. They promote neovascu-larization by secreting proangiogenic growth factors andstimulate reendothelialization thereby contribute to vascularhomeostasisandperhapsmyogenesis[102]. In the animalexperiments injection of EPCs into infarcted myocardiumimproved LVF and inhibited fibrosis [11,103,104]. Althoughthere was no change in the noninfarcted regions of theheart, there was a significant reduction in collagen depositionand apoptosis of cardiomyocytes and an improvement incardiac function on echocardiography [11]. It appeared
Stem Cells International 5that neovascularization induced by these cells led to theprevention of apoptosis and LV remodeling and led to somedegree of cardiomyocyte regeneration [105].The cell surface antigen CD133+is expressed on earlyHSCs and EPCs and less than 1% of nucleated BMSCs, and,because these cells cannot be expanded ex vivo, only limitednumbers of CD133+cells can be obtained for therapeuticpurposes [106].2.9. Fetal and Umbilical Cord Blood Cells (UCBCs). Becauseof their prenatal origin, fetal and UCBCs may possess greaterplasticity than adult cells. Human umbilical cord bloodcontains a number of progenitor cell populations, includingHSCs and MSCs, in addition to a population of unrestrictedsomatic stem cells, which have been shown to have prolifera-tive potential [37,107]. However, animal studies have shownconflicting results with regard to improvements in LVF.Ma et al. [108] injected human mononuclear UCBCs, asmall fraction (≈1%) of which were CD34+, intravenously1 day after MI in NOD/scid mice. The cells homed to theinfarcted hearts, reduced infarct size, and enhanced neovas-cularization with capillary endothelial cells of both humanand mouse origin. Interestingly, they found no evidence ofmyocytes of human origin, arguing against cardiomyogenicdifferentiation. In a rat model of MI [109] UCBCs CD34+improved cardiac function when injected into the peri-infarct rim immediately after MI compared with controlanimals that received injection of medium. Apart from these,K¨ogler and colleagues [110] have described a population ofcells from human UCBCs called unrestricted somatic stemcells. These cells which are fibroblast like in appearance andadhere to culture dishes are negative for c-kit, CD34, andCD45 and are capable of differentiating, both in vitro andinvivo,intoavarietyoftissues,includingcardiomyocytes.These stem cells [111], when delivered by direct injection atthoracotomy in immunosuppressed pigs after MI, improvedperfusion and wall motion, reduced infarct scar size, andenhanced global cardiac function.3. Human Clinical TrialsAs already mentioned in the beginning, modern reperfusionstrategies and advances in pharmacological managementthat resolve the ischemia but not the infarct zone haveresulted in an increasing proportion of AMI survivors atheightened risk of developing LV remodeling and heartfailure. None of our current therapies address the underlyingcause of the remodeling process, that is, the damage ofcardiomyocytes and the vasculature in the infarcted area.BMSCs gained attention as early as in the year 1968, withthe first report of their clinical use for restoring the bloodand the immune system in children with congenital immu-nodeficiencies [112]. However, host HSCs, used for bloodborne malignancies replace the donor HSCs and they donot have to differentiate into another cell type. Therefore,the revolutionary paper of Orlic et al. [71]withaveryprovocative finding which suggested that directly injectingHSCs resulted in extensive myocardial regeneration andTab le 3: Animal experiments demonstrating myocardial genera-tion with BMSCs.Study FindingsTomita et al. 1999[90]Transplantation of autologous bonemarrow cells to stimulate angiogenesis inthe recipient ischemic myocardium.Functional improvement was observedonly in recipients of the mesenchymal stemcells that had been treated with5-azacytidine.Orlic et al. 2001[71]Haematopoietic stem cells injected weredemonstrated to occupy the infarctedregion and resulted in extensivemyocardial regeneration.Jackson et al. 2001[8]The engrafted SP cells (CD34(−)/low,c-Kit(+), Sca-1(+)) or their progenymigrated into ischemic cardiac muscle andblood vessels, differentiated tocardiomyocytes and endothelial cells, andcontributed to the formation of functionaltissue.Kocher et al. 2001[11]Systemic infusions of human bonemarrow-derived endothelial cell precursorswere able to intercept the remodelingprocess of the left ventricle. The observedneovascularization prevented apoptosis ofhypertrophied myocytes reducing collagendeposition and subsequent scar formation.Posttransplantation ventricle functionimproved as well.Orlic et al. 2001[72]That mobilization of animal’s own bonemarrow with G-CSF before and aftermyocardial infarction in mice resulted ingrowth of new cardiomyocytes in theinfarct zone, improved ventricularfunction, and substantially decreasedmortality by 68%.subsequent various similar reports of animal experiments(Table 3 ) gave a hope of using stem cells as tool in the handsof mankind for regenerating myocardium [8,11,71,72,90].Although their findings were subsequently challenged byBalsam et al. [75], Murry and colleagues [76], and Chien[113], the journey of stem cells as therapy in regeneratingthe human myocardium had already begun with yet anotherpath breaking clinical study by Strauer et al. (2002) whoreported not only improved LVF in human trial but alsosafety and efficacy of infusing bone marrow mononuclearfraction (BMMNCs) through intracoronary route althoughin a very small study population [114].Since then, there have been many published studies[115]withdifferent types of cells including composite ofBMMNCs, EPCs, MSCs, adipose cells, and cord blood cells.The evidence that precursors of both cardiomyocytes andendothelial cells exist within the mononuclear cell fraction ofadultbonemarrowformsthebasisfortheuseofBMMNCsin most of the clinical trials to date. After bone marrowaspiration from large bones, most commonly the iliac crestthen mixed with heparin; the mononuclear cell fraction is
6Stem Cells Internationalobtained by density gradient centrifugation or sedimentationprotocol. The mononuclear fraction is injected into the heartwithout further ex vivo expansion. In addition, there hasbeen great variability in the number of cells transplanted (1–400 million), and the route of administration has includedintracoronary (by using the stop-flow balloon catheterapproach), intravenous, epicardial, and intramural methods[116]. The results of clinical trials publishedto date aiming atprogenitor cell-based myocardial repair in patients with AMIare summarized in Ta ble 4 (a).Oneimportantpointthathastobekeptinmindregarding the human trials is that the clinical studies differedsignificantly from the animal studies: (1) in the animals theinfarct-related artery was never reperfused, but cells weredirectly injected into myocardium in the AMI condition,(2) the majority of these trials utilized relatively unpurifiedpopulations of BMMNCs which represent less than 0.1%of stem cells, and none of these trials utilized the Lin−c-kit+cells described in animal experiments, and (3) mostimportantly, the infarction was created in animal by coronaryligation and was not thrombus related.The initial pilot studies by Strauer et al. [114], theTOPCARE-AMI [117,118], the BOOST-trial [119], and thestudy performed by Fernandez-Aviles [120]aswellours[121] reported nearly identical results—an improvement inglobal LVEF by an absolute 6 to 9% and reduced LV end-systolic volume (LVESV) at 6 months after cell transplanta-tion. Overall, the published studies demonstrated that theintracoronary infusion of autologous BMSCs is safe andfeasible in patients with AMI and on top of the benefits asso-ciated with established interventional and medical strategiesto promote functional recovery after AMI. Further improve-ment of LVEF was mostly due to the improved regionalwall motion in the infarct border zone. However, thereare contradictory reports as well. Janssens and colleagues[122] did not find any improvement in their primary endpoint after intracoronary transfer of BMMNCs, However,they demonstrated a significant reduction in scar size andan improvement in regional function, but there was noimprovement in LVEF (P=.36). Their patient populationdiffered from the BOOST trial in that they were reperfusedearlier and may therefore have gained only a small benefitfrom cell therapy because they derived maximal benefit fromearlier reperfusion. The beneficial effects observed in mostof the pilot phase I/II studies were confirmed in the sofar largest double-blind, randomized, multicenter REPAIR-AMI trial [123] which demonstrated not only improvedLVF but also showed a reduction in the combined clinicalendpoint of death, MI, or revascularization in the BMSCs-treated patients compared with placebo after 1 year followup.Patients with a lower baseline EF (≤48.9%) showed asignificant 3-fold higher recovery in global LVEF as well as onclinical end points indicating that patients with more severeMI profit most from BMSCs therapy. Only one larger study,the ASTAMI trial [124], did not show any benefit on LVFparameters. The reason for the failure is considered to be dueto their different cell isolation and storage protocol, whichsignificantly affected the functional capacity of the cells. Sofar, no trial has demonstrated a significant effect of BMSCstransfer on LV end-diastolic volumes (LVEDV), suggestingthat unselected BMSCs may have a limited impact on LVremodeling after AMI. Again, larger studies are requiredto settle this issue. Followup data from the BOOST trial[125]aswellasours[126] show that the improvement ofLVEF is maintained after 18 and 24 months, respectively,indicating that BMSCs transfer prevents progression ofdiastolic dysfunction after AMI.The therapeutic effects of MSC transplantation after AMIhave been investigated in two clinical trials. Chen et al.[127] infused autologous MSCs by intracoronary route anddemonstrated no arrhythmias or other side effects. After sixmonths of MSC transfer, regional wall motion and globalLVEF were improved, and LVEDV was decreased comparedwith a randomized control group that had received anintrac-oronary infusion of saline [127]. Unfortunately, it was notreported whether intracoronary MSC delivery promotedischemic damage to the myocardium, a complication thathad occurred after intracoronary MSC infusions in dogs[128]. Another study by Hare et al. [129] also demonstratedthat intravenous allogenic MSCs were safe in patients afterAMI with increased LVEF and reverse remodeling. Currently,several studies have been undertaken for allogenic MSCs inclinical trials for myocardial regeneration in the United Statesunder the sponsorship of Osiris Therapeutics. Such an off-the-shelf strategy for cell therapy would potentially makethe procedural logistics easier. Taken together, these studiessuggested that BMSCs or their selected cell populations aresafe and may improve cardiac function by a substantialand clinically meaningful degree following MI. An extensivemeta-analysis by Abdel-Latif et al. [130] on eighteen eligiblestudies (N=999 patients) involving adult BMSCs such asBMNNC, MSCs, and EPCs measuring the same outcomesdemonstrated that, as compared to controls, bone marrowtransplantation improved LVEF (pooled difference of 3.66%;95% confidence interval (CI), 1.93% to 5.4%, P<.001),reduced infarct scar size (−5.49%; 95% CI: −9.1% to−1.8%; P=.003), and reduced LVESV (−4.8% mL; 95%CI: −8.2 to −1.41 mL; P=.006). The available evidencesuggests that BMC transplantation is associated with modestimprovements in physiologic and anatomic parameters inpatients with both acute MI and chronic IHD, above andbeyond conventional therapy. This further suggests carryingout multicentric randomized large trials targeted to addressthe impact of intracoronary cell therapy on importantoutcomes and long-term event-free survival as compared tothe conventional therapy.Studies like those by Werner et al. [131]havealsoprovid-ed evidence of increased survival following AMI in patientswith greater number of circulating EPCs. This and positiveresults of preclinical trials led to human trials to assess safetyand feasibility of EPCs [132–134]. The results of these trialsshowed trend towards improvement of LVF in both acute andchronic ischemia, without adverse effects [133,135–137].The ability of injured myocardium to recruit extra-cardiac stem cells following injury is critical in myocardialrepair and regeneration. Little is known with regard to theregulatory mechanisms that control the homing and holdingof stem cells to injured tissues. The precise time course,
Stem Cells International 7Tab le 4(a) Summary of major cell-based clinical trialsStudy Method ofdeliveryPatientstreated/control Placebo/controlCell typecell/number ordoseTime of celldelivery (daysafter MI)ResultsStrauer et al., 2002[114](Germany) IC 20/10 Case controlled BM-MNC9–28 ×1067Improvedcontractility andreduced infarct size at6monthsTOPCARE-AMI,Assmus et al. 2002[117], Sch¨achinger2004 [118](Germany)IC 30/29ControlNonrandomizedopen-labeledBM-MNC2.4 ×108CPC 1.3 ×1073to7Improved LVFE andreduced infract size at4–12 monthsBOOST,Wollert et al. 2004[119],IC 30/30 Control BM-MNC24 ×1096Improved EF at 6months, increasedregional contractility,Meyer et al. 2006 [125](Germany)no difference at 18monthsREPAIR-AMI,Sch¨achinger et al.2006 [123](Germany)IC 102/102 Placebo BM-MNC2.4 ×1084Improved EF andreduced infarct size at4monthsFernandez et al. 2004[120]IC 20/13 Control BM-MNC11–90 ×10610–15Significant functionalimprovement andreduced infarct sizeJanssens et al., 2005[122](Belgium) IC 33/34 Placebo BM-MNC3.0 ×108cells 1Decrease scar size butno improvement inLVEF at 4 mo nthsASTAMI, lunde et al.2006 [124](Norway) IC 50/50ControlRandomized + placebocontrolledBM-MNC8.7 ×1075to8 No difference at 6monthsShah et al. 2007[121,126](India) IC 20/10ControlOpen-labelnonrandomizedBM-MNC13.4 ×1076to8Improved LVfunction at 6 monthsand sustained at 24monthsChen et al. 2004 [127](China) IC 34/35 PlaceboControlledMCSs48–68 ×1010 18Inc LVEF, Inc regionalcontractility, increaseviability of infarctzone/wall after 3 and6monthsHare et al. 2009 [129](USA) IV 39/21 Double-blind placebocontrolledMSCs 0.5, 1.6,5 million cells/kg1, 2, 3, 6 monthsfollowupImproved LVEF andreverse modelingIC: intracoronary, IV: intravenous, BM-MNC: unfractionated bone marrow mononuclear cells, CPC: Circulating progenitor cells, MSCs: mesenchymal stemcells.(b) Summary of major cell-based clinical trialsStudy Method ofdeliveryPatientstreated/controlled Placebo/controlCell type, cellnumber, ordoseTime of celldelivery (daysafter MI)ResultsInce et al. 2005 [152]First Line-AMI(Germany)Mobilizationof G-CSF 15/15 Randomized +controlledBM-MNCCD34+ 1–6After 4 and 12 monthfollowup improvedLVEF and systolic wallthicknessRipa et al. 2006 [153],STEMMI (Denmark)Mobilizationof G-CSF 39/39Randomized +placebocontrolledBM-MNCCD34+ 1–6After 6 monthfollowup systolic wallthickness ↑viability ofinfarct zone/wall
8Stem Cells International(b) Continued.Study Method ofdeliveryPatientstreated/controlled Placebo/controlCell type, cellnumber, ordoseTime of celldelivery (daysafter MI)ResultsZohlnh¨ofer et al. 2006[154], REVIVAL(Germany)Mobilizationof G-CSF 56/58Randomized +placebocontrolledBM-MNCCD34+ 1–6 After 4 and 6 monthfollowup No effectsEngelman et al. 2006[155],G-CSF-STEMI(Germany)Mobilizationof G-CSF 22/22Randomized +placebocontrolledBM-MNCCD34+ 1–5After 4 and 6 monthfollowupNo effectskinetics, and factors stimulating bone marrow mobilizationremain the subject of intense investigation. Several crucialfactors have been shown to promote the mobilization ofBMSCs into peripheral circulation, including granulocytecolony-stimulating factor (G-CSF), granulocyte/macrophagecolony-stimulating factor (GM-CSF), stem cell factor (SCF),vascular endothelial growth factor (VEGF), hepatocytegrowth factor (HGF), and erythropoietin (EPO) [138]. Myo-cardial ischemia is known to induce several “mobilizing cy-tokines”, including, but not limited to, G-CSF [139–141],SCF [139–141], VEGF [141–145], stromal derived factor(SDF-1) [139,141,145,146], and EPO [147,148]. Thesecytokines may be responsible for the observed homingof BMSCs following MI. Mobilization of BMSCs throughcytokine stimulants increases their concentration in theperipheral circulation substantially. In addition to well-recognized HSCs mobilizing agents such as G-CSF andSCF, VEGF, and EPO and statins have been shown topromote EPC recruitment [148–151]. Several clinical trials(Table 4 (b)) were carried out with mobilization of BMSCswith G-CSF [152–155]. Abdel-Latif et al. [156] also carriedout meta-analysis of clinical trials wherein BMSCs weremobilized with G-CSF. The analysis revealed that G-CSFtherapy in unselected patients with AMI appeared safe butdid not provide benefit. Subgroup analyses suggest that G-CSF therapy may be salutary in AMI patients with severeLV dysfunction and when started early. Larger randomizedstudies may be conducted to evaluate the potential benefitsof early G-CSF therapy in AMI patients with LV dysfunction.3.1. Safety and Long-Term Benefit of Cell Therapy. Stem cellpotency is a double-aged sword, and therefore, although theinitial experimental studies confirmed that the infusion ofBMSCs do not cause major side effects, several potentialissues were raised such as electrical stability, increased res-tenosis, or progression of atherosclerotic disease. However,none of the clinical studies with BMSCs so far have reportedan increased incidence of arrhythmias (as have been seen insome of the myoblast trials), bleeding complications, addi-tional ischemic injury, or promoted inflammatory reactionas no further increase in CRP, and troponin was observedincluding in our study.Restenosis, which was considered as potential side effectby progenitor cell-mediated plaque angiogenesis or plaqueinflammation, was only increased using CD133+cells [157,158]. This is surprising, because the isolation of selectedprogenitor cells excluding contaminating proinflammatorycells would have been assumed to reduce rather than increasethe risk of restenosis and atherosclerotic disease progression.Because CD133+cells were isolated by using a mouse an-tibody, one may speculate that the remaining antibodymight have elicited a local proinflammatory reaction despitethe failure to detect systemic antimouse antibodies in thepatients. All other studies did not observe an augmentedrisk for restenosis [159]; if anything, there was a decreasednecessity for revascularization procedures in the REPAIR-AMI trial [123].Intramyocardial calcification which was reported tooccur in murine models of MI after direct injection of un-purified BMSCs or MSCs [160,161]wasnotreportedinthe various clinical trials as reported by MRI imaging. Thismay be explained by the enrichment of mononuclear cells bydensity gradient centrifugation used in the majority of theclinical studies.It had been discussed that the proangiogenic capacityparticularly of EPCs might relate to an increased tumorvascularization. However, during followup of the availablestudies, no increased incidence of cancer was seen in BMSCs-treated patient. Most of the clinical trials did exclude patientswith known tumors. It is unclear whether a single applicationof EPCs is sufficient to promote tumor growth. However,because of the low incidence of such events, this needs to becarefully monitored in the future.An important issue is whether the improvement seenduring the initial 6 months after cell therapy is maintainedfor a prolonged time. Careful evaluation of the 18 monthsfollowup data of the BOOST trial indicates that the EF ofthe cell therapy group is maintained from 6 to 18 monthsfollowup [125]; however, the difference between the celltherapy and the control group was no longer statisticallysignificant. The small number of patients (30 per group)may preclude detecting a statistical difference between the 2groups. The long-term 5-year followup MRI-derived data ofthe TOPCARE-AMI trial showed that the EF was maintainedand even further augmented in the treated patients, inparallel with a sustained reduction in NT-proBNP serumlevels suggesting a sustained beneficial effect on long-termLV remodeling (S. Dimmeler and A. M. Zeiher, unpublisheddata). In our study also the improvement seen at 6 monthsin LVF was sustained at 24 months. However, longer-termfollowup in larger-scale randomized trials will finally address
Stem Cells International 9this important question. Overall, the clinical data available atpresent indicate that cell therapy with bone marrow-derivedcells is feasible and safe at least for the duration of followuppresently available (up to 5 years for the initial studies).4. Mechanism of Myocardial RepairOne could see that, although the early phase of researchin cardiac repair aimed at histologic outcomes, the humantrial of last five years demonstrated improvement of heart’sfunction as their clinical end point and have erroneouslyreasoned that, because ventricular function was improved,the heart was regenerated. This shift towards physiologymade mechanism less evident. There is still controversy as towhether actual differentiation occurs versus large cell fusionwith resident myocytes. This is because on one hand themyocyte deficit in infarction-induced heart failure is on theorder of one billion cardiomyocytes and on the other handthe documentation of LVF improvement within 72 hours isfar earlier than would be expected for cell regeneration of anymeaningful extent [162].The fact that after transplantation of hundreds ofmillions of cells, less than 2% of the cells are actually stillpresent in the tissue within 2 weeks of implant; the prevailingconcept of stem cell efficacy has shifted toward the cytokine-paracrine hypothesis [163]. It has also been proposed thatthrough paracrine mechanisms stem cells release angiogenicligands, protect cardiomyocytes from apoptotic cell death,induce proliferation of endogenous cardiomyocytes, andmay recruit resident CSC (Figure 1). Indeed, various studiesshowed that progenitor cells secrete survival factors suchas endothelial growth factor, stromal-derived factor (SDF-1), angiopoietin 1, hepatocyte growth factor, insulin-likegrowth factor 1, and periostin [77,164–169], thymosinb4 which promotes wound healing or the Wnt antagonistsecreted frizzled-related protein 2 (SFRP-2) which protectscardiomyocytes from hypoxia-induced apoptosis [170–172]and thus stimulate tissue recovery after ischemic injury andminimize the infarct size [165,167,173–176]. Regardless ofthe mechanisms, there appears to be general agreement thatstem cell therapy has the potential to improve perfusion andcontractile performance of the injured heart.5. Issues to Be Addressed in the Future StudiesThe ultimate aim of the cellular transplantation remainsto be the regeneration of lost heart muscle along withthe reversal of the remodeling process. It is possible thatthe apparently variable results among different trials aresecondary to differences in the protocols. Despite growingclinical experience, use of these heterogeneous parametersalong with various clinical end points among human trialshave left us with fundamental questions regarding the use ofthe ideal cell type; the number of cells needed to be deliveredfor maximal efficacy; optimal isolation, purification, andstorage techniques; ideal route of delivery; ideal time ofadministration after injury to improve the efficacy of thistherapy as well as for this therapy to be included under treat-ment guidelines. The trials discussed above were not poweredto address the effects on these hard clinical end points butcan give us some direction towards standardization of thetherapy on above parameters.5.1. Which Cell Populations Should Be Delivered?While the ideal cell type for stem cell therapies remainsto be determined to date, bone marrow-derived stem cells,isolated from whole bone marrow aspirate, remain the mostcommonly used cell type for human studies. Unfraction-ated bone marrow cells gained advantage over above cellsdue to many reasons. It has the feasibility of procuring,no requirement of in vitro expansion and above all theavailability of mixed population of cells with characteristicfor differentiating into various populations of cells. And ofcourse it has no ethical issues. However, importantly MSCshave also emerged as most promising cell population withtheir inherent property of transdifferentiating into cardiom-yocytes and to be tolerated by the immune system giving usthe most convenient “off-the-shelf” reagent.5.2. What Number of Cells Should Be Given?Murry et al. [162] have pointed out that number of cellsadministered reported studies range by as much as 6700-fold.Myocardium contains approximately 20 million cardiomy-ocytes per gm of tissue [177]. The average left ventricle ap-proximately weighs 200g and therefore contains approxi-mately 4 billion cardiomyocytes. To cause a heart failure,an infarct needs to kill approximately 25% of the ventricle(for comparison, infarcting 40% of the ventricle resultsin acute cardiogenic shock) [178]. Therefore, the myocytedeficit in infarction-induced heart failure is in the order ofone billion cardiomyocytes. True cardiac regeneration wouldtherefore require restoring approximately one billion cardi-omyocytes and ensuring their synchronous contraction viaelectromechanical junctions with host myocardium.5.3. When Dose Cells Should Be Transplanted?In the first 48 hours of AMI attack, debridement andformation of a fibrin-based provisional matrix predominatesbefore a healing phase ensues [179]. At the initial 3-4 daysafter MI cell adhesion, molecule concentration which hasnot yet declined may promote the transplanted cells intoinflammatory process than in the formation of functionalmyocardium [180]. It is only by 7th day after MI that VEGFconcentration peaks and cell adhesion molecule concentra-tion declines [181,182]. By 2 weeks after scar formation,the benefits achieved due to cell transplantation are reduced.Therefore, the ideal time point of transplantation remains 7–14 days [105]. This was very much evident in the REPAIRAMI trial [123] wherein patients being treated up to 4 daysafter the MI showed no benefit, whereas later treatment(day 4 to 8) provided an enhanced improvement of EFduring follow-up. This suggests that microenvironment after
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New insights into molecular pathways in colorectal cancer: Adiponectin, interleukin-6 and opioid signaling

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New insights into molecular pathways in colorectal cancer: Adiponectin, interleukin-6 and opioid signaling

Colorectal cancer (CRC) is one of the most common cause of death among neoplasms around the world. The environmental factors, like diet and obesity, are crucial in CRC pathogenesis by creating cancer-favorable microenvironment and hormonal changes. Adiponectin, the adipose tissue-specific hormone, is generally considered to negatively correlate with CRC development. The interleukin 6 (IL-6) is one of the most important pro-inflammatory cytokine connected with CRC, which is strongly inflammation-associated cancer. The opioids are variable group substantially correlated with cancers - the endogenous opioids affect immune system and cell cycle including proliferation and cell death whereas exogenous opioids are leading clinically used analgesics in terminal cancer patients. In this review we discuss the involvement of adiponectin, IL-6 and opioids in CRC pathogenesis, their link with obesity, possible cross-talk and potential novel therapeutic approach in CRC treatment.
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Your Healthy Self Podcast: Benefits of Cell Therapy

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Your Healthy Self Podcast: Benefits of Cell Therapy

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Re: Stem Cell banking

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Webinar: "Human Cell Manufacturing for Clinical Applications"

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Clinical outcomes of cryopreserved graft recipients following allo-HCT

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Clinical outcomes of cryopreserved graft recipients following allo-HCT


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Why is Dr. Foss Excited for Stem Cells? Let's Find Out!

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Why is Dr. Foss Excited for Stem Cells? Let's Find Out!

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Stem Cells and Synthetic Biology featuring Dr. Krishanu Saha | The Stem Cell Podcast

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Stem Cells and Synthetic Biology featuring Dr. Krishanu Saha | The Stem Cell Podcast

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Toxicities after CAR-T

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Toxicities after CAR-T

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Development of Immune Competent Small Animal Models for HBV | Helene Strick-Marchand

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Development of Immune Competent Small Animal Models for HBV | Helene Strick-Marchand
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