Cellular approaches to tissue repair in cardiovascular disease: the more we know, the more there is to learn (2004)

CALL FOR PAPERSCellular Plasticity in the Cardiovascular SystemCellular approaches to tissue repair in cardiovascular disease: the more we know, the more there is to learnKeith L. March, and Brian H. JohnstoneKeith L. MarchIndiana Center for Vascular Biology and Medicine, Indiana University Medical Center, Indianapolis, Indiana 46202, and Brian H. JohnstoneIndiana Center for Vascular Biology and Medicine, Indiana University Medical Center, Indianapolis, Indiana 46202Published Online:01 Aug 2004https://doi.org/10.1152/ajpheart.00343.2004MoreSectionsPDF (102 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations the rapidly advancing field of cellular plasticity bears exciting implications for both basic understanding of cell biology and potentially important therapeutic approaches to a range of disease states that remain as areas of great patient need. Indeed, the use of stem cells, progenitor cells, or even mature cells for therapeutic intervention in cardiovascular disease holds great promise, as well as harboring significant controversy. This Call for Papers highlights the exciting advances that are being made in this broad area; and it illustrates the substantial breadth of cell types and approaches that have provided insights into the biology of cell-based repair processes and have promised implications for translation into the realm of patient therapy. The selections of the Call for Papers do indeed suggest several exciting potential therapeutic approaches, but they also help to underscore the many questions that remain to be answered about the underlying mechanisms that explain the beneficial effects that have been described after cell deliveries to vascular or cardiac targets. In this Prologue, a framework will first be provided by outlining key aspects currently under intense investigation in this area. The emphasis will be placed on some of the major questions that require consideration both with respect to the mechanism and future practical application. The manuscripts that comprise this Call will then be highlighted with regard to these issues.Therapy Recapitulates Physiology: A Working HypothesisA foundational hypothesis that may be used to direct and evaluate cell therapy protocols is that the organism employs endogenous reparative processes, involving cells that have also been proposed or studied as potentially important in therapeutic approaches. It logically follows that most effective therapies will be modeled on the recapitulation of endogenous physiology and will assist in pathophysiological states by directing toward the normal condition. The study of endogenous cellular plasticity and repair mechanisms, which will presumably play a key role in the homeostatic response to repair or reverse damage, is thus very germane to advancing the field of cell therapies. Many lines of evidence now suggest that inadequate cell plasticity may impair the potential for cardiovascular tissue repair and regeneration in disease states, contributing to the development of atherosclerosis as well as cardiac disease and diseases of other organs critically dependent on blood flow. However, the precise limitations of the endogenous reparative process are not yet clear. It is therefore important to know whether normal repair processes suffer deficits due to inadequate cell numbers or cell functions to consider thoughtfully whether more or indeed “better” cells must be provided.Two fundamental potential activities of stem cells and progenitor cells, which are most relevant to cardiovascular disease, are, respectively, 1) the use of cell therapies to expand the coronary or peripheral vasculature (via vasculogenesis, angiogenesis, or arteriogenesis/collateral vessel formation; hereafter collectively referred to as angiogenesis) and 2) the improvement in myocardial function by formation/modification of injured myocardial tissue (myocardial regeneration).Until recently, myocardial regeneration and angiogenesis have been treated as two distinct therapeutic goals. Recent studies, including the article below by Schuster et al. (Table 1, Article 6, Ref. 18a), however, suggest that they may be interrelated. For example, transdifferentiation may occur among relatively immature as well as mature cells (24); and endothelial cells can transdifferentiate into cardiomyocytes (6). Therefore, cell therapies directed to increases in the local numbers of vascular endothelial cells in the heart might result in enhanced formation of cardiomyocytes. Indeed, an interdependence between cell-induced myocardial regeneration and angiogenesis is suggested by the emerging concept that long-term engraftment and survival of delivered stem cells or progenitor cells will critically depend on establishing adequate blood flow through vascularizing the region surrounding the transplanted cells: a key point highlighted by the Schuster study.Table 1. Overview of articles presentedArticle (Ref. No.)TitleAuthorsCell Source/PurificationDelivery MethodImmune StatusEnd PointsProposed Role of Cells1 (24a)Tissue-engineered microvessels on three-dimensional biodegradable scaffolds using human endothelial progenitor cells.Wu, Aikawa-Rabkin, Gulserian, Perry, Masuda, Sutherland, Schoen, Mayer, BischoffHuman smooth muscle cells and CD34+/CD133+ cells from Umbilical CordNANAStructural assemblyIncorporation2 (9a)c-kit-Immunopositive vascular progenitor cells populate human coronary in-stent restenosis but not primary atherosclerotic lesionsHibbert, Chen, O'BrienNANANALocal tissue and circulating cell pool numbersNA3 (8a)Modulation of the vascular response to injury by autologous blood-derived outgrowth endothelial cellsGulati, Jevremovic, Witt, Kleppe, Vile, Lerman, SimariRabbit outgrowth endothelial cells and culture-modified mononuclear cellsLocal catheter-based application (20 mins)AutologousCell incorporation, vasoreactivity, neointimal reductionIncorporation into vessel lumen and paracrine stimulation4 (9c)Use of blood outgrowth endothelial cells as virus-producing vectors for gene delivery to tumorsJevremovic, Gulati, Hennig, Diaz, Cole, Kleppe, Cosset, Simari, VileHuman outgrowth endothelial cells and culture-modified mononuclear cellsTail vein injection (systemic)Athymic nude miceTransfectability, homing, local vector disseminationParacrine stimulation via gene transfer5 (8b)Autologous vascular smooth muscle cell-based myocardial gene therapy to induce coronary collateral growthHattan, Warltier, Gu, Kolz, Chilian, WeihrauchCanine jugular vein smooth muscle cellsIntracoronaryAutologousCollateral flow, gene-product secretionParacrine stimulation via gene transfer6 (18a)Myocardial neovascularization by bone marrow angioblasts results in cardiomyocyte regenerationSchuster, Kocher, Seki, Way, Xiang, Homma, ItescuHuman G-CSF mobilized, CD34+ enriched PBMCTail vein injection (systemic)Athymic nude ratsNeovascularization, CMC proliferation, myocardial functionIncorporation (vascular), vascular nutritive function, and paracrine stimulation7 (9b)Stable benefit of embryonic stem cell therapy in myocardial infarctionHodgson, Behfar, Zingman, Kane, Perez-Terzic, Alekseev, Puceat, TerzicMurine embryonic stem cells (CGR8 line)Direct intramyocardial injectionImmunocompetent ratsCell incorporation, myocardial function, and depolarizationIncorporation into myocardium8 (15a)Bone marrow stromal cells improve cardiac performance in healed infracted rat heartsOlivares, Ribeiro, deCastro, Ribeiro, Mattos, Goldenberg, Mill, Dohmann, de dos SantosRat adherent bone marrow stromal cellsDirect intramyocardial injectionIsogenic Wistar Rat donor/recipientsCell incorporation, myocardial function, and depolarizationIncorporation into myocardium9 (13a)Autologous stem cell transplantation for myocardial repairLiu, Hu, Wang, Gong, Mansoor, Hou, Zeng, Zhang, Jerosch-Herold, Guo, Bache, ZhangPorcine adherent bone marrow stromal cellsEpicardial placement of pre-seeded fibrin matrixAutologousNeovascularization, cell incorporation, myocardial function, in vitro myocyte differentiationIncorporation into neovasculature, vascular nutritive function, and paracrine stimulationNA, not applicable.Major Issues to be Investigated in the Field of Cell Plasticity and TherapyFigure 1 provides a conceptual overview of key questions to be more completely answered regarding potential features of cell repair. The following features are some of the most prominent.Fig. 1.Conceptual overview of key questions regarding the potential features of cell repair.Download figureDownload PowerPoint1) What are the major identities of the cells that undertake endogenous repair? In Fig. 1, several resident populations are depicted, but the actual number of relevant populations is unknown. Are these cells common among many tissues? Is there functional redundancy among multiple cell repair populations? Conversely, is an interactive synergy among multiple (>1) cell populations critical; i.e., must more than one population of repair cells be present and active to induce optimal tissue repair?2) Where do the reparative cells grossly reside in the organism? Is this reservoir compartment the bone marrow or other extrahematopoietic tissue? Another possibility is that key repair cells reside within the repair target tissue, as recent studies have suggested in the case of the heart (10, 15, 20). In cases such as the latter, will repair cells from other sources function as effectively (from a therapeutic perspective) in certain circumstances? What is the microscopic niche/location of these cells within their tissues? How do they gain access to their local niche in the target tissue?3) With the onset of disease (or the presence of risk factors), to what extent do the relevant cells decrease in number? To what extent are they functionally impaired? Which of these issues is amenable to complementation by the input of additional cells? Can autologous cells actually complement the deficits, or must cells be provided from a nondiseased source (e.g., typically crossing immunologic barriers)? Alternatively, will the reparative function of autologous cells be enhanced by the incorporation of exogenous genetic material before their provision, so that the cells become gene vectors providing therapeutic levels of proteins to target tissues?4) How do target sites stimulate the release of the cells from reservoir sites to make them potentially available for repair? Is this mechanism altered by disease status or the presence of risk factors?5) What “recruits” or calls in the cells, once liberated, to home to target sites? What ligands function to bind them there?6) For how long can/do the putative repair cells survive in the target tissue, once physiologically recruited? How long do they survive if placed artificially? Will redosing be necessary to achieve sustained benefit?7) How do the cells function in the target tissue? Do they predominantly function to support the tissue by 1) providing paracrine signals with informational or survival content [recruiter/manager (17, 18)/“software” (9) roles]; 2) enhancing the local vascular network (supplier role); or 3) adding to the parenchymal mass of the tissue [“building block”/“hardware” (9) role]?8) If the cells incorporate into the parenchyma, do they do so by 1) a differentiation event from a relatively primitive precursor cell type; 2) a transdifferentiation event from another lineage; or 3) by fusion with the preexisting parenchymal cells?9) How do the cells contribute to functional improvement in the target tissue (e.g., myocardial contractile or electrical function)? Which set of the above mechanisms plays a role in the functional behavior of these cells?10) Is it in fact the case that the best approaches to therapy will employ elements that recapitulate normal physiological processes that are impaired due to disease?11) How is delivery of putative repair cell populations best conducted to achieve maximal tissue investment with the cells in a fashion that contributed to functional improvement?12) To what extent is it necessary to understand the answers to these questions before embarking on large clinical trials, when preclinical studies show intriguing evidence of functional improvements in blood flow or indexes of myocardial function?Can What You Don't Know Help You?The final point, the topic of question 12, is a key issue of debate. One distinct position holds that the positive preclinical data, coupled with the severity of disease found in cardiovascular patient populations that are not readily addressed by current therapy, justify the early clinical trials that have already begun. A clearly opposing viewpoint suggests that knowledge of the identity and mechanisms of action of endogenous repair cell populations must be much more fully developed before clinical trials are reasonable. However, a consideration of the history of the cell therapy, which is most firmly grounded in clinical practice, that of bone marrow transplantation, reveals that approaches to severe diseases have indeed met with significant successes even before the entire definition of hematopoietic stem cell identity. Accordingly, a moderate approach may be suggested, involving cautious attempts to pursue clinical trials directed to initially evaluate the safety of cell-based approaches, then to pursue efficacy after once establishing safe parameters of cell isolation and delivery.Indeed, numerous clinical studies are presently underway with a variety of cell types or cell mixtures in patient populations that have experienced recent infarction or have manifestations of ischemic cardiomyopathy; despite the situation that the identity of the key therapeutic cell types and their mechanisms of function have yet to be fully characterized. A multitude of both controlled preclinical and largely uncontrolled clinical studies have suggested remarkable biological effects and potential improvements. Several of the manuscripts in this Call collection provide examples of impressive functional effects of cell delivery.As this collection demonstrates, a very wide range of cell populations are being tested, raising the distinct question of why so many cell types appear to be able to confer benefit. It is conceivable that such observations have a basis in a set of shared cellular mechanisms that exist more broadly than originally expected: the ability of many cell types to modulate their environment by the secretion of paracrine factors is one such mechanism. On the other hand, particular outcomes may require cellular properties, which are much less commonly held, such as the self-renewability and inducible differentiation, which are key hallmarks of stem or progenitor cells.Construction Managers or Building Blocks?Many recent studies indeed support the concept that cell-based repair mechanisms rely on the plasticity of their protein expression, leading them to conduct paracrine factor secretion facilitating angiogenesis or mediating cell survival. This has been described for a population of cells cultured from peripheral blood mononuclear cells, which have been variably termed circulating cultured angiogenic cells (CACs) [(17) and Call Ref. 3], culture-modified mononuclear cells (CMMCs) [(8) and Call Refs. 4 and 5], and endothelial progenitor cells (1, 3, 4, 7, 22, 23); for adipose stromal cells (18); and most recently for bone marrow-derived stromal cells (MSCs) (13). As alluded to above, this perspective in which such cells contribute their main effects by the synthesis of a complex mixture of bioactive molecules has led to their description as providing an instructive “manager” function (18) or “software” function (9).Although several studies clearly support the ability of particular progenitor or cardiac-specified cells of embryonic origin to incorporate into adult myocardium (19) with resultant functional improvement after infarction (Table 1, Article 7, Ref. 9b), other studies that support an unexpected cardiomyocytic fate of bone marrow (16) have been recently challenged by findings that call into question the ability of such cells to transdifferentate into cardiomyocytes. Some studies (2, 21) have suggested that significant mechanisms responsible for the appearance of cardiomyocyte fates involve cell fusion, particularly after marrow transplantation, although the functional outcomes of such fusion have not been documented in vivo, and the role of such fusion events in typical physiological repair of cardiovascular tissues is not yet known. More recent studies (5, 14) have not been able to confirm a cardiomyocytic fate of several marker-purified populations of hematopoietic-lineage cells after local intramyocardial injection. However, one of these studies did demonstrate a very low-frequency potential transdifferentiation event after bone marrow transplantation (14), highlighting the possibility that various methods of cell delivery to the putative niche for repair may lead to disparate biological outcomes.In a parallel study directed toward understanding cell incorporation into nascent vasculature, it has been most recently suggested that bone marrow-derived cells do not incorporate into nascent neovasculature with high frequency in an ischemic hindlimb model (28) but rather accumulate around growing collateral arteries, expressing various angiogenic factors and chemokines.It is important to recognize that these studies, which call into question particular mechanisms, such as transdifferentiation, do not imply the absence of potential for functional improvements in ischemic or other injury; they merely raise the awareness of the need for better assessment of mechanisms by which these improvements might be expected to occur.To Stem or Not to Stem?The redirection of the focus on cell repair processes from transdifferentiation and tissue incorporation toward paracrine effects naturally leads to a key question that also emerges from a consideration of the studies in the general literature as well as those within this Call. To succeed in a therapy, is it necessary to isolate cell populations with high-frequency stem or progenitor properties, or might less-specialized populations achieve important therapeutic goals by mechanisms including factor secretion and possibly enhancement of blood flow? Although the answer to this and many of the questions posed earlier have yet to emerge, the very possibility that more easily accessible, abundant populations may help in tissue repair is quite interesting from the perspective that ultimate clinical utilization is likely to depend not only on the functional effects and progressively defined mechanisms, but also critically on a reasonable modicum of practicality for widespread application of any therapy, which is put forward as a broadly useful approach.Implications of the Studies Featured Within This CallTable 1 lists the manuscripts of this Call by title and authors, and then provides key information about the manuscripts with respect to the cell source studied, the method of delivery where appropriate, the immune status of the cellular transplantations performed, the major end points of each study, and finally, the primary mechanisms by which the cells studied in each case mediated the observed end point effects. This table makes apparent the plurality of cell types, delivery approaches, and cell mechanisms suggested by these studies: these data are summarized at the bottom. It is shown that four studies studied cell populations isolated only by attachment or growth-selection phenotype, whereas four studies evaluated cells that were purified by their surface markers, or were relatively well-characterized discrete populations. Two studies evaluated systemic cell delivery, whereas the balance used some form of local delivery to place the cells where desired. Ultimately, six studies showed in vitro or in vivo incorporation of the studied cells into either vascular or cardiac structures, whereas five studies demonstrated an important role for paracrine stimulation by the cells. None of the studies directly evaluated for the possibility of cell fusion as a partial explanation for the findings of incorporation. Two studies explicitly modified cells by gene and one of these on cultured vascular smooth muscle cells, not even to study cells with a The of the studies studied various types of these cells, including endothelial outgrowth endothelial cells which which do not at factor peripheral CD34+ a embryonic stem cell and bone the range of cells studied among these Call for each cell delivery demonstrated important end points (Table 1, 1 and Refs. first six studies (Table 1, Refs. various potential of progenitor cells in vascular and whereas the four studies (Table 1, Refs. myocardial The study by Schuster et al. (Table 1, Article 6, Ref. plays a role in that it that systemic delivery of mononuclear cells leads to cardiac effects presumably as a of enhanced study of et al. (Table 1, Article 1, Ref. is an in vitro of a that reveals the of an population in vascular in a dependent on smooth whereas the study of et al. (Table 1, Article Ref. between numbers of cells and circulating cells in with local atherosclerotic those with and nondiseased cell populations are clearly quite yet the that both significant in these two states of vascular disease that these local are with cellular the local vascular disease these or as a result of them however, be from this The study of et al. (Table 1, Article 3, Ref. of the with regard to their ability to incorporate into a vessel after This study highlights the ability of to help repair which in to the yet potentially effects of circulating smooth muscle as two studies both aspects of using cells as gene et al. (Table 1, Article 4, Ref. directly the endothelial progenitor and cells with regard to their and were found to be more and to home more effectively into a neovasculature, thus possibly toward a for the This also provides for a between the and cell the use of the by to both of these cell types a of on the of an (e.g., primary on surface proliferation, or other The study of et al. (Table 1, Article Ref. is interesting in of a enhancement of blood flow to myocardium in a model of by the local of autologous vascular smooth muscle cells, which were with a expressing This study has features of particular to clinical including the of the model to the easily of the of such cells, and the of autologous cells as the for the It also the potential of cells, which function by factor as by the of of cell an approach may also be of great with cells from other autologous sources not such as adipose stromal cells study of Schuster et al. (Table 1, Article 6, Ref. several interesting it employs systemic delivery of a CD34+ of G-CSF cells after rat myocardial and suggested significant of such cells to the with of neovascularization and improvement of cardiomyocyte survival as well as an of cells incorporation and by It also showed functional and improvement at Although these remarkable findings will likely using approaches, will be present in of the recent with a G-CSF et al. (Table 1, Article 7, Ref. and et al. Ref. also rat myocardial which showed significant improvements in myocardial function, as well as evidence for incorporation of cells from the cell and rat cells, The used were in that the study cell delivery after and showed sustained functional improvement for to delivery, whereas the study cell delivery 4 after and evaluated functional outcomes after only The study, that of et al. (Table 1, Article Ref. a tissue approach of the described earlier 1, Ref. but the placement of autologous in a model of myocardial infarction with This stem cell approach both neovascularization and a myocardial functional improvement with the of the into the presumably a significant ability of the in this the use of local or systemic cellular approaches for the of cardiovascular disease a very important therapeutic concept for to be answered about the optimal cell types and which will be useful and to to widespread clinical Indeed, mature cells have also been used for cell therapy and have at shown therapeutic efficacy that is to that found in studies using stem cells and progenitor cells. However, it appear that cell-based therapies may have significant potential to the substantial that remain in with impaired cardiac function due to or other myocardial As with other which require a mixture of biological and many such as and will whether the can be made from the in the Call for Papers to of of this article were in by the of The article must therefore be in with to this and of the tissue of transplanted human endothelial progenitor cells by and of cells with cardiomyocytes and and of and in and of blood-derived human adult endothelial progenitor cells into functionally active cardiomyocytes. and stem cells mature fates in and induce endothelial cells to transdifferentiate into cardiac implications for myocardium and of progenitor cells after myocardial of and myocardial