Marrow Stromal Cells for Cell-Based Therapy: The Role of Antiinflammatory Cytokines in Cellular Cardiomyoplasty

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1 Marrow Stromal Cells for Cell-Based Therapy: The Role of Antiinflammatory Cytokines in Cellular Cardiomyoplasty Guangyong Chen, MD, Madhur Nayan, BS, Minh Duong, BS, Juan-Francisco Asenjo, MD, Yin Ge, BS, Ray C-J Chiu, MD, PhD, and Dominique Shum-Tim, MD, MS Divisions of Cardiac Surgery and Surgical Research and Department of Anesthesia, McGill University Health Center, Montreal, Quebec, Canada Background. The mechanism by which marrow stromal cells (MSCs) improve cardiac function after myocardial infarction (MI) is still unclear. Because MI patients with lower circulating proinflammatory/antiinflammatory cytokine ratios have been reported to have a better prognosis and in vitro studies showed that MSCs express antiinflammatory cytokines, we hypothesized that changes in cytokine ratios in the infarct microenvironment after MSC therapy may play a role in improving early cardiac function after MI. Methods. Sixty-three rats that survived left coronary artery ligations were injected with culture media (group M) or MSCs (group C). Cardiac functional changes were assessed with echocardiography. Cytokine gene expressions of interleukin (IL)-1, IL-6, IL-8, (proinflammatory) and IL-10 (antiinflammatory) were quantified by real-time polymerase chain reaction. Extracellular matrix deposition, injury score, and the matrix metallopeptidase 2/tissue inhibitor of metallopeptidase 1 ratio were also analyzed. Results. The ratio of proinflammatory/antiinflammatory cytokine gene expression was decreased in group C at various times, particularly in the early postoperative period. In group C, the matrix metallopeptidase 2/tissue inhibitor of metallopeptidase 1 gene expression ratio was significantly lower than group M at the early phase (12 hours), which in group C was translated into significantly lower extracellular matrix deposition at 24 hours, 1, and 2 weeks. Functional recovery was also significantly better in cell therapy group C. Conclusions. Our data demonstrate that MSC therapy decreases the proinflammatory/antiinflammatory cytokine ratio in the microenvironment early after MI. This is associated with subsequent less scar formation and improved cardiac function. (Ann Thorac Surg 2010;90:190 8) 2010 by The Society of Thoracic Surgeons Accepted for publication Feb 26, Address correspondence to Dr Shum-Tim, Division of Cardiothoracic Surgery, McGill University Health Center, 1650 Cedar Ave, Ste C9-169, Montreal, QC H3G 1A4, Canada; dshumtim@yahoo.ca. Autologous marrow stromal cells (MSCs) transplanted into infarcted myocardium can differentiate into cells of a cardiomyocyte phenotype and improve left ventricular function [1 3]. To date, however, the precise mechanisms of such cardiac functional improvement remain controversial. The postulated mechanisms include stem cell transdifferentiation into cardiomyocytes or cell fusion between the transplanted stem cells and residual cardiomyocytes [4]. The functional improvement has far exceeded the extent of the neocardiomyocytes identified in the periinfarcted areas, which raises the further question of whether cell differentiation has any important role, at least during the early posttherapy phase. In addition, the improvement in cardiac function appeared to precede the time required for cell transdifferentiation. It has also been suggested that the MSCs could improve cardiac function through a paracrine or immunomodulatory effect, such as angiogenesis factor, secretion of various cytokines, and extracellular matrix (ECM) deposition [5 9]. A clinical study showed that a reduced ratio of proinflammatory to antiinflammatory cytokines inferred a better prognosis in patients with recent myocardial infarction (MI) [10]. Tögel and colleagues [11] suggested the role of antiinflammatory cytokines from implanted MSCs in early improvement of renal function after acute renal failure. On the basis of these observations, we sought to explore whether changes in the proinflammatory/ antiinflammatory cytokine gene expression ratio in the infarct microenvironment after MSC implantations were associated with early improved cardiac function. Material and Methods All procedures in this study were in compliance with the Guide for the Care and Use of Laboratory Animals (National Institutes of Health publication No ) and the Guide to the Care and Use of Experimental Animals of the Canadian Council on Animal Care by The Society of Thoracic Surgeons /$36.00 Published by Elsevier Inc doi: /j.athoracsur

2 Ann Thorac Surg CHEN ET AL 2010;90:190 8 MARROW STROMAL CELLS FOR CARDIOMYOPLASTY 191 Table 1. Cytokine Gene Expression of the Whole Heart Cytokine Group 12 Hours, Folds 24 Hours, Folds 1 Week, Folds 2 Weeks, Folds IL-1 M C a b IL-6 M C c IL-8 M C IL-10 M C d e a p 0.005; b p 0.005; c p 0.012; d p 0.006; e p IL interleukin. Isolation, Culture, and Labeling of Rat MSCs Rat MSC cultures were prepared according to Caplan s method, and then MSCs were transfected as previously described [12] with pmfg-lacz plasmid containing the -galactosidase gene for identification of the transplanted cells in the myocardium. The resulting MSCs expressing lacz were expanded until transplantation. Experimental Design Immunocompetent female syngenic Lewis rats (200 to 250 g, Charles River Canada, St. Constance, Que) were used. Proximal left coronary artery ligations were done in 72 rats and 9 rats died within 24 hours. The remaining 63 were randomly assigned to two groups, 32 to group C and 31 to group M. In group M, 150 L of medium was directly injected into three different sites around the periinfarct area of the myocardium 10 minutes after coronary ligation. In group C, isogenic MSCs ( ) suspended in medium were similarly injected after ligation. The injection of MSCs at the periinfarction area was essential because a previous study suggested that cell-tocell contact was necessary for the phenotypic expression of MSCs in a coculture in vitro model [13]. Rats were sacrificed at various predetermined time points. Cardiac Functional Assessment Transthoracic echocardiography was performed on all animals in groups M and C before operation (baseline), at 12 and 24 hours, and at 1 and 2 weeks after ligation and treatment. All animals were sedated in a quite room and in a right lateral decubitus position to ensure similar views in all echocardiographic assessments. A commercial echocardiographic system was obtained that used a 12-MHz probe with a small footprint (SonoSite, Seattle, WA). Scanning was performed as previously described [12]. The heart was scanned in longitudinal view in 2-dimensions and M-mode, where it is usually possible to see a 2-chamber view. Then the probe was turned 90 to get the best possible cross-sectional view of the left ventricle (LV) just above the papillary muscles. To minimize the intraobserver error in measurements, three consecutive measurements were averaged. The LV end-diastolic (LVEDD) and end-systolic (LVESD) diameters were measured according to the American Society of Echocardiology leading-edge method [12]. Fractional shortening (FS) was determined as [(LVEDD LVESD)/LVEDD]. The ejection fraction (EF) was estimated as [(LVEDV LVESV)/LVEDV]. To ensure reproducibility, all before and after intervention measurements were performed by one experienced observer (J.F.A.) who was blinded to the treatment groups. Harvesting of Hearts and Tissue Processing The recipient rats were sacrificed at 12 and 24 hours, and at 1 and 2 weeks after cell therapy and echocardiography. Their hearts were harvested and dissected from the Table 2. Proinflammatory/Antiinflammatory Cytokine Gene Expression Ratios Cytokine Group 12 Hours, Folds 24 Hours, Folds 1 Week, Folds 2 Weeks, Folds IL-1 /IL-10 M C a IL-6/IL-10 M C b IL-8/IL-10 M C c d e a p 0.001; b p 0.033; c p 0.027; d p ; e p IL interleukin.

3 192 CHEN ET AL Ann Thorac Surg MARROW STROMAL CELLS FOR CARDIOMYOPLASTY 2010;90:190 8 Fig 1. Infarct scar area and the total area of left ventricular myocardium are shown at 12 hours and extending to 2 weeks. The red represents extracellular matrix deposition in scar tissue, and the grey area represents myocardium (original magnification 4). Compared with group M, group C had a significantly less area of extracellular matrix deposition. surrounding tissue. The LV was cut transversely in half, evenly along the largest circumference of the infarcted area. One half was fixed and stained for -galactosidase activity using X-gal staining, as described previously [12]. The remaining half of the heart specimen was finely minced and put into a tube with the RNALater solution (Qiagen, Valencia, CA) and stored at 4 C for RNA extraction. Quantitative Real-Time Polymerase Chain Reaction Total RNA was isolated with an RNeasy kit (Qiagen) according to the manufacturer s instructions. Total RNA (5 g) was subjected to first-strand complimentary DNA synthesis with QuantiTect Reverse Transcription Kit (Qiagen). Quantitative real-time polymerase chain reaction (RT- PCR) was performed to compare the changes in expression of different inflammatory cytokines using the following primers: -Actin: F 5=-CACGCCATCCTGCGTCTGGA-3=; R5=-GCACCGTGTTGGCGTAGAG-3= IL-1 : F5=-TCAGGCAGATGGTGTCTGTC-3=; R5=-GGTCTATATCCTCCAGCTGC-3= IL-6: F 5=-AACGCCTGGAAGAAGATGCC-3=; R5=-CTCAGGCTGAACTGCAGGAA-3= IL-8: F 5=-TTTCTGCAGCTCTCTGTGAGG-3=; R5=-CTGCTGTTGTTGTTGCTTCTC-3= IL-10: F 5=-GCGACTTGTTGCTGACCGG-3=; R5=-GAACCTTGGAGCAGATTTTG-3= Fig 2. Histologic examination of the hearts shows marrow stromal cell retention in the left ventricle but a decrease in number over time. X-gal staining (original magnification 200) confirmed successful engraftment of marrow stromal cells (blue cells) at various times points (arrows).

4 Ann Thorac Surg CHEN ET AL 2010;90:190 8 MARROW STROMAL CELLS FOR CARDIOMYOPLASTY 193 Table 3. Matrix Metallopeptidase 2/Tissue Inhibitor of Matrix Metallopeptidase 1 Gene Expression Ratio Group 12 Hours, Folds 24 Hours, Folds 1 Week, Folds 2 Weeks, Folds M C a a p The My-iQ system (Bio-Rad, Hercules, CA) was used to monitor RT-PCR amplification using SYBR Green I. Reaction conditions were as follows: hot start for 120 seconds at 95 C, melting at 95 C for 15 seconds, annealing at 56 C for 12 seconds, and amplification at 72 C for 15 seconds. Samples were run in duplicate, and the average crossing point value was used for calculations by the method of 2- CT. The relative quantitation value of a target gene, normalized to -actin as the internal control gene, was expressed as a number, which indicated the relative expression compared with that gene. The possibility of amplifying contaminating DNA and nonspecific amplification were avoided with precautions [11]. Conventional PCR and Gel Bands Semiquantification Using the same samples as in quantitative RT-PCR assay, the complimentary DNA was also used in regular PCR to compare changes in the genes expressing matrix metallopeptidase 2 (MMP-2) and tissue inhibitor of metallopeptidase 1 (TIMP-1). We mixed the primers of -actin and TIMP-1 together in one tube for each running, and MMP-2 was done separately with the following thermal protocol: 50 C for 30 minutes, 95 C for 15 minutes, 30 cycles at 94 C for 30 seconds, 55 C for 30 seconds, 72 C for 1 minute, and the final extension step at 72 C for 10 minutes. Primers used for amplification were: MMP-2: F 5=-AGGACAAGTGGTCCGCGTAAAG-3=; R5=- CACTTCCGGTCATCATCGTAGT-3= TIMP-1: F 5=-ACTAAGATGCTCAAAGGATTCG-3=; R5=-ATCGCTCTGGTAGCCCTTCT-3= -Actin: F 5=-CACGCCATCCTGCGTCTGGA-3=; R5=-AGCACCGTGTTGGCGTAGAG-3= Images of the PCR ethidium bromide-stained agarose gels were acquired with an image system (Cohu Inc, Poway, CA), and quantification of the bands was performed by ImageJ-1.41 software (National Institutes of Health, Bethesda, MD). Band intensity was expressed as relative absorbance units normalized to -actin. The ratio between the samples was determined and compared. Collagen Content and Tissue Injury Measurement Five consecutive 5- m sections were prepared from the midportion of the infracted area in all samples. Sections were stained with the collagen-specific dye Sirius red 3BA in a saturated picric acid solution to allow clear discrimination between cardiomyocytes and ECM. The slides were scanned at 2400 dpi with 4 magnification, and the pictures were red-green-blue split to the green color layer for measurement. In the digital images, scar area and the total area of myocardium were traced manually by a blinded observer and measured automatically by the computer by the ImageJ-1.41 software. Infarct size, expressed as a percentage, was calculated by dividing the sum of infarct areas by the total sum of LV areas, including those without infarct scar, and multiplying by 100. The following morphologic criteria were applied in a blinded fashion for tissue injury assessment: score 0, no damage; score 1 (mild), interstitial edema and focal necrosis; score 2 (moderate), diffuse myocardial cell swelling and necrosis; score 3 (severe), necrosis with presence of contraction bands and neutrophil infiltrate; score 4 (highly severe), widespread necrosis with presence of contraction bands, neutrophil infiltrate, and hemorrhage. Statistical Analysis All data are expressed as mean standard deviation. Data were analyzed using SPSS 16.0 software (SPSS Inc, Chicago, IL). The t test and two-way repeated analysis of variance (ANOVA) were used to make comparisons and test the differences for continuous variables. A value of p 0.05 was considered statistically significant. Table 4. Scar Area of the Left Ventricle Group 12 Hours, % 24 Hours, % 1 Week, % 2 Weeks, % M C a b c a p ; b p 0.030; c p

5 194 CHEN ET AL Ann Thorac Surg MARROW STROMAL CELLS FOR CARDIOMYOPLASTY 2010;90:190 8 Table 5. Tissue Injury Group 12 Hours 24 Hours 1 Week 2 Weeks M C a b c a p ; b p 0.014; c p Results Inflammatory Cytokines Gene Expression Changes in the LV The fresh heart tissue slices for the total RNA extraction included infarcted area, periinfarcted area, and normal heart tissue from the LV; therefore, the gene expression results reflected the global gene expression changes of the entire LV. Group C showed more of an increase than group M in gene expressions for all the measured proinflammatory and antiinflammatory cytokines IL-1, IL-6, and IL-10 from 12 hours to 2 weeks and for IL-8 from 12 to 24 hours. The increase in cytokines was significant, especially during the first 12 to 24 hours after MI and gradually tapered over time. In particular, the increase in antiinflammatory IL-10 was much more exaggerated during this early phase (Table 1). Repeated measures ANOVA revealed that differences were significant over time within groups in the expression of IL-1, IL-8, and IL-10. The ratios of proinflammatory/antiinflammatory cytokine gene expression reflected the relationship of proinflammatory vs antiinflammatory cytokines changes in Table 1 and were significantly lower in group C, especially during the first 12 to 24 hours after cell therapy. The significance gradually tapered off over time, except for the IL-8/IL-10 ratio at 2 weeks postoperatively (Table 2). All of the ratios were also significantly different over time within groups, as was demonstrated by repeated measures ANOVA. The proinflammatory/antiinflammatory cytokine ratio suggested that MSC therapy modified the cytokine profiles in the infarct microenvironment in favor of an antiinflammatory effect early after cell implantation. MMP-2 and TIMP-1 Gene Expressions in LV The PCR analysis of MMP-2 and TIMP-1 gene expression indicated that MSC injections caused a reduction in the expression of MMP-2 at 12 hours, 24 hours, 1 week, and 2 weeks (0.50-, 0.57-, 0.95, 0.80-fold, respectively), and an increase of TIMP-1 at corresponding times (1.24-, 1.09-, 1.02-, 2.05-fold, respectively). The ratio of MMP-2/ TIMP-1 was significantly reduced in group C especially during early phase at 12 hours, and a similar trend was observed at 24 hours, 1 week, and 2 weeks (Table 3). A significant difference over time within groups was found with repeated measures ANOVA. ECM Deposition Measurement Histopathologic analysis revealed a significant reduction of scar ECM deposition in the cell therapy group at 24 hours, 1 week, and at 2 weeks. Repeated measures ANOVA demonstrated that scar ECM deposition was very significantly reduced over time in group C (Fig 1 and Table 4). Histopathologic Evaluation Histologic examination of serial sections of every heart in group C confirmed the successful engraftment of laczlabeled MSCs (blue cells) at 12 hours, 24 hours, and 1 week after cell implantation, but labeled MSCs were scarce at 2 weeks (Fig 2). Tissue injury and inflammatory cell infiltration early after MI and cell therapy at 12 and 24 hours were actually higher in group C than in group M (Table 5). However, subacute myocardial injury and inflammatory cell infiltration at 1 and 2 weeks showed a significant decrease in group C compared with group M. The difference over time within groups was significant, as was demonstrated by repeated measures ANOVA. Cardiac Functional Assessment The baseline EF and FS were not significantly different between groups. Compared with group M, group C showed improvement in EF as early as 12 hours after treatment (group M vs group C , p 0.069), and reached significance at 24 hours (group M vs group C , p 0.028), 1 week (group M vs group C , p ), and 2 weeks (group M vs group C , p ). Repeated measures ANOVA revealed that EF differences were significant over time within groups (p 0.003) and between groups (p ). FS showed a similar trend with improvement in group C at 12 hours (group M vs group C , p 0.051) and was significantly increased at 24 hours (group M vs group C , p 0.038), 1 week (group M vs group C , p 0.001), and 2 weeks (group M vs group C , p 0.001), respectively. Repeated measures ANOVA also revealed that FS differences were significant over time within groups (p 0.006) and between groups (p ). Taken together, these data suggest that the beneficial effects of stem cell therapy actually occurred quite early after myocardial injury. Comment Recent studies have reported that transplanting MSCs improves cardiac function and that MSCs could survive

6 Ann Thorac Surg CHEN ET AL 2010;90:190 8 MARROW STROMAL CELLS FOR CARDIOMYOPLASTY 195 from several days to beyond 6 months [1 3]. There have been controversies about whether stem cells actually differentiate into functioning myocytes and whether the number of engrafted cells could account for the improved function [14 18]. In an acute renal failure model, Tögel and colleagues [11] observed significant renal functional recovery within 3 days after MSC therapy, a period in which none of the administered MSCs had differentiated into kidney tubular or endothelial cell phenotypes. In the current study, we also demonstrated that isogeneic MSCs provided a significant functional benefit because group C showed a significant improvement in cardiac function as early as 24 hours after cell therapy and up to 2 weeks. Although few studies have evaluated the early cardiac functional changes after MSC treatment, Francis and colleagues [14] showed improved cardiac function as early as 1 hour after transplanting MSCs, suggesting that mechanisms other than cell differentiation or cell fusion might be responsible at different stages of cell-based therapy. Tang and colleagues [19] also raised doubts about stem cell differentiation as a sole contributor to myocardial regeneration and subsequent functional recovery because very few engrafted MSCs were shown to express specific cardiac markers, such as connexin-43 and cardiac troponin I. The small number of engrafted cells in proportion to the functional improvement suggested that increased myocardial mass as a sole mechanism for improved cardiac function appeared less plausible. Some reports have suggested other mechanisms, among which the secretion of various mediators through a paracrine effect from the transplanted MSCs has gained increasing attention [6, 20]. Persistent proinflammatory cytokine elevation has been associated with a worse clinical outcome in patients suffering recent MI. Induction and release of the proinflammatory cytokines IL-1, tumor necrosis factor- (TNF- ), IL-6, and IL-8 are consistently found in experimental models and clinical cases of MI [21, 22]. There is also evidence showing a dramatic reduction in infarct size with the use of specific antiinflammatory strategies [23 25]. Our current study suggests that transplanted MSCs up-regulated both proinflammatory and antiinflammatory gene expressions in the infracted myocardium early after implantation. This cytokine profile was contrary to our original hypothesis that MSCs might inhibit the expression of proinflammatory cytokines while increasing the antiinflammatory cytokine activity. Our findings, however, are consistent with a clinical study by Kilic and colleagues [10], who showed that serum concentrations of IL-1 and IL-6 were significantly higher in patients with new coronary events. On the other hand, Tögel and colleagues [11] showed that at 24 hours after cell therapy, the expression of proinflammatory/antiinflammatory cytokines reversed significantly in favor of antiinflammatory IL-10, basic fibroblastic growth factor, transforming growth factor-, and B-cell lymphoma 2 up-regulation in MSC treated kidneys. This was associated with measurable recovery in renal function too early to be related to neoglomerular cell formation. Interestingly, such renoprotection was not obtained with syngenic fibroblasts. Histologic results of our study also demonstrated that acute myocardial injury caused fluxes of inflammatory cells infiltration, cytokine mediators, and tissue injury in groups M and C within the first 24 hours. However, the antiinflammatory cytokines surge exceeded that of the proinflammatory counterparts, leading to more favorable ECM remodeling and functional improvement in longer follow-up. These findings suggested that the transplanted MSCs did not change or inhibit the inflammatory reaction histopathologically after acute MI. Reactive neutrophil cells and damaged endothelial cells of blood vessels classically produce the proinflammatory cytokines, and therefore, it is not expected that MSCs implantation after MI would reverse this phenomenon immediately. The tissue trauma associated with MSC implantation per se may perhaps further aggravate the inflammatory response. On the other hand, these findings could also reflect the insufficient number of retained MSCs to completely suppress the acute inflammation, because many injected cells might have been lost early after injection due to mechanical leakage or washout, as suggested by Teng and colleagues [26]. By 12 to 24 hours after MSC implantation, however, the retained stem cells may respond and subsequently reversed the proinflammatory/antiinflammatory ratio of the microenvironment. In the current study, IL-10 gene expression was remarkably up-regulated relative to the proinflammatory cytokines early after MSC therapy and showed evidence of strong cytoprotective effects on the cardiomyocytes. This was associated with decreased MMP-2 and increased TIMP-1 gene expression during the repair process after MSC therapy. MMP-mediated matrix degradation is crucial in infarct healing and the pattern of left ventricular remodeling, leading to various LV functional changes in patients with heart failure. Berry and colleagues [27] showed that MSC injections limited cell apoptosis and fibrosis and thus improved LV function and reduced dilatation. Recent work by Guo and colleagues [28] examined the cytokines and functional parameters 4 weeks after cell therapy and revealed a decrease in deposition of collagen types I and III and decreased gene and protein expression of MMP-1 and TIMP-1. These results correlated with attenuated ventricular dilatation and thickening. Interestingly, the authors also reported decreased expression of proinflammatory cytokines such as TNF-, IL-1, and IL-6, suggesting a possible role for inflammatory mediators in the remodeling process [28, 29]. IL-1 and TNF- have been shown to increase MMP-2 and MMP-9 matrix degradation and activity [30], whereas IL-10 and transforming growth factor- enhanced TIMP synthesis, regulating the matrix degradation and producing a favorable myocardial remodeling for better functional recovery. Therefore, changes in the cytokine milieu might play a key role in regulating ECM after myocardial injury. In our study, the analysis of antiinflammatory IL-10 gene expression, ECM formation, and functional recovery cor-

7 196 CHEN ET AL Ann Thorac Surg MARROW STROMAL CELLS FOR CARDIOMYOPLASTY 2010;90:190 8 related with result reported by others and suggests a potential mechanism of stem cell therapy in an MI model. The temporal and spatial discrepancies of various cytokine expressions during different scenarios of tissue organ injuries have been suggested to be more important in determining the outcome rather than the individual mediator per se. One study showed IL-6 expression was enhanced early during the first 6 hours after reperfusion of the previously ischemic myocardium [31]. Others have suggested that after 24 hours of reperfusion, the IL-6 mrna expression was down-regulated in the same ischemic segments in which IL-10 mrna up-regulation was found [11]. On the other hand, in a non-reperfused infarct model, IL-6 over-expression lasted as long as 24 hours [32], but in such a permanent coronary occlusion model, both IL-10 and IL-6 mrna over-expression persisted simultaneously [33]. These findings suggested that although a surge in proinflammatory cytokines invariably occurred after injury, the expression of antiinflammatory mediators reflected a counter-regulatory response that is important for survival and favorable organ function recovery. Recent studies have supported the notion that a lack of an appropriate antiinflammatory counter-balance in response to a certain proinflammatory reaction, rather than the individual proinflammatory or antiinflammatory mechanism, might be the culprit of the increased coronary risk [34, 35]. In our study, proinflammatory and antiinflammatory cytokine gene expressions were both up-regulated after MSC therapy. However, the LV functional recovery suggested that the two opposing cytokine forces had an unbalanced effect, such that the antiinflammatory effects predominated and resulted in more favorable histopathologic remodeling and subsequent improved cardiac functional improvement. This possible paracrine mechanism of MSC therapy may at least partly explain the early functional improvement after MI after stem cell therapy, before transdifferentiation or other more protracted mechanisms take place at a later period. The current study has some limitations that merit further investigations. First, it is possible that MSCs act through other collective multifactorial mechanisms by many complex mediators. The role of inflammatory cytokines may only be a fraction of the overall mechanisms of stem cell therapy in this MI model. It is, however, interesting that a brief and possibly small amount of retained MSCs might exert profound longer protective effects on the injured myocardium, as many others have demonstrated. Second, we did not determine the fate, the subtype, and the actual number of MSCs required to produce the beneficial effects. This is, however, a question that is actively pursued by many others in the field, and no agreement has yet been reached at present. Third, the current rodent model is far from clinical relevance, and a study in larger animals should be considered. On the other hand, the use of small rodents is a well accepted and more preferable model in the study of molecular mechanisms of stem cell therapy. Better understanding of the mechanisms of stem cell therapy will undoubtedly lead to future larger-animal studies and provide information for evidence-based research trials in the clinical setting. The present study proposes a new mechanism implicating antiinflammatory cytokines in the early benefit of MSC implantation after MI. Fourth, this study did not address the relative contribution of various cytokines that surge from damaged tissue or injected MSCs, and further investigation using a knock-out rodent model will help to answer this question. However, we confirmed that the presence of MSCs modified these mediators and resulted in better functional recovery. Also, whether injection of other cell sources would produce the observed benefit is a pertinent question that we did not address. Tögel and colleagues [11], however, showed that the administration of fibroblasts did not reproduce the renoprotective effects in a renal injury model. Finally, the cell therapy was injected 10 minutes after coronary ligation, which may not necessarily simulate clinical settings. However, the current model maximizes the effect of inflammatory cytokines after acute MI. Further study in a large-animal model will allow the introduction of MI in a noninvasive fashion, followed by cell therapy. This will overcome the current limitation with our rodent model in which creation of MI and reintervention for MSC injection at a later time will produce prohibitively high surgical mortality. In summary, our study shows that isogeneic MSCs injected into periinfarcted regions of ischemic myocardium after coronary ligation reduces the proinflammatory/ antiinflammatory cytokine gene expression ratio that is correlated with improved cardiac function. The ratio of cytokine gene expressions favoring antiinflammation with less scar formation suggests that a paracrine mechanism may at least partly explain the early functional improvement. These findings open up a new and potentially important mode of action for MSCs, which also provides further impetus to study the role of IL-10 in the pathophysiology of MI. We thank Dr James Hanley, Department of Epidemiology and Statistics, McGill University, for his assistance with statistics. This research was supported in part by the Natural Sciences and Engineering Research Council (NSERC) and Fonds de la Recherche en Sante du Quebec (FRSQ). References 1. Amado LC, Saliaris AP, Schuleri KH, et al. 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Soluble CD40 ligand: interleukin-10 ratio predicts in-hospital adverse events in patients with ST-segment elevation myocardial infarction. Thromb Res 2007;121: INVITED COMMENTARY Despite significant medical advancements, myocardial ischemic disease is still a leading cause of death in the world. The regenerative capacity of the heart is now generally believed to exist; however, its capacity to regenerate an adequate amount of myocardial tissue during ischemic insults is insufficient. Subsequently, cardiomyocytes are lost as a result of myocardial ischemia, triggering cardiac dysfunction and eventual cardiac failure. One of the most promising novel therapies for cardiac failure is cellular cardiomyoplasty, which relies on transferring cells into the compromised myocardium to improve function. Although initial results have shown promise, the effect of cellular cardiomyoplasty has been limited due to the conflicting results seen in experimental animal models and clinical trials. Many questions remain unanswered in this approach to treating heart failure. First, the mechanism whereby cellular cardiomyoplasty improves myocardial performance is still unknown. Second, the optimal cell type skeletal myoblasts vs bone marrow-derived cells 2010 by The Society of Thoracic Surgeons /$36.00 Published by Elsevier Inc doi: /j.athoracsur