Biological substrate modification suppresses ventricular arrhythmias in a porcine model of chronic ischaemic cardiomyopathy.

AIMS: Cardiomyopathy patients are prone to ventricular arrhythmias (VA) and sudden cardiac death. Current therapies to prevent VA include radiofrequency ablation to destroy slowly conducting pathways of viable myocardium which support re-entry. Here, we tested the reverse concept, namely that boosting local tissue viability in zones of slow conduction might eliminate slow conduction and suppress VA in ischaemic cardiomyopathy. METHODS AND RESULTS: Exosomes are extracellular vesicles laden with bioactive cargo. Exosomes secreted by cardiosphere-derived cells (CDCEXO) reduce scar and improve heart function after intramyocardial delivery. In a VA-prone porcine model of ischaemic cardiomyopathy, we injected CDCEXO or vehicle into zones of delayed conduction defined by electroanatomic mapping. Up to 1-month post-injection, CDCEXO, but not the vehicle, decreased myocardial scar, suppressed slowly conducting electrical pathways, and inhibited VA induction by programmed electrical stimulation. In silico reconstruction of electrical activity based on magnetic resonance images accurately reproduced the suppression of VA inducibility by CDCEXO. Strong anti-fibrotic effects of CDCEXO, evident histologically and by proteomic analysis from pig hearts, were confirmed in a co-culture assay of cardiomyocytes and fibroblasts. CONCLUSION: Biological substrate modification by exosome injection may be worth developing as a non-destructive alternative to conventional ablation for the prevention of recurrent ventricular tachyarrhythmias.

Quantitative Hybrid Cardiac [18F]FDG-PET-MRI Images for Assessment of Cardiac Repair by Preconditioned Cardiosphere-Derived Cells.

Cardiosphere-derived cells (CDCs) are progenitor cells derived from heart tissue and have shown promising results in preclinical models. APOSEC, the secretome of irradiated peripheral blood mononuclear cells, has decreased infarct size in acute and chronic experimental myocardial infarction (MI). We enhanced the effect of CDCs with APOSEC preconditioning (apoCDC) and investigated the reparative effect in a translational pig model of reperfused MI. Supernatants of CDCs, assessed by proteomic analysis, revealed reduced production of extracellular matrix proteins after in vitro APOSEC preconditioning. In a porcine model of catheter-based reperfused anterior acute MI (AMI), CDCs with (apoCDC, n = 8) or without APOSEC preconditioning (CDC, n = 6) were infused intracoronary, 15 min after the start of reperfusion. Untreated AMI animals (n = 7) and sham procedures (n = 5) functioned as controls. 2-deoxy-2-(18 F)-fluoro-D-glucose-positron emission tomography-magnetic resonance imaging ([18F]FDG-PET-MRI), with late enhancement after 1 month, showed reduced scar volume and lower transmurality of the infarcted area in CDC and apoCDC compared to AMI controls. Segmental quantitative PET images displayed indicated more residual viability in apoCDC. The left-ventricle (LV) ejection fraction was improved nonsignificantly to 45.8% ± 8.6% for apoCDC and 43.5% ± 7.1% for CDCs compared to 38.5% ± 4.4% for untreated AMI. Quantitative hybrid [18F]FDG-PET-MRI demonstrated improved metabolic and functional recovery after CDC administration, whereas apoCDCs induced preservation of viability of the infarcted area.

Antegrade Conduction Rescues Right Ventricular Pacing-Induced Cardiomyopathy in Complete Heart Block.

BACKGROUND: Right ventricular (RV) pacing-induced cardiomyopathy (PICM) occurs in ∼30% of patients with RV leads. This study evaluated the long-term effects of restoring antegrade conduction with a biological pacemaker in a porcine model of RV PICM. OBJECTIVES: The goal of this study was to determine if antegrade biological pacing can attenuate RV PICM. METHODS: In pigs with complete atrioventricular (AV) block, transcription factor T-box 18 (TBX18) was injected into the His bundle region in either of 2 experimental protocols: protocol A sought to prevent PICM, and protocol B sought to reverse PICM. In protocol A, we injected adenoviral vectors expressing TBX18 (or the reporter construct green fluorescent protein) after AV node ablation, and observed the animals for 8 weeks. In protocol B, PICM was established by using AV node ablation and 4 weeks of electronic RV pacing, at which point TBX18 was injected into the His bundle region. RESULTS: In protocol A, TBX18 biological pacing led to superior chronotropic support (62.4 ± 3 beats/min vs. 50.4 ± 0.4 beats/min; p = 0.01), lower backup pacemaker utilization (45 ± 2.6% vs. 94.6 ± 1.4%; p = 0.001), and greater ejection fraction (58.5 ± 1.3% vs. 46.7 ± 2%; p = 0.001). In protocol B, full-blown RV PICM was evident 4 weeks after complete AV block in both groups; subsequent intervention led to higher mean heart rate (56 ± 2 beats/min vs. 50.1 ± 0.4 beats/min; p = 0.05), less backup pacemaker utilization (53 ± 8.2% vs. 95 ± 1.6%; p = 0.003), and a greater ejection fraction (61.7 ± 1.3% vs. 49 ± 1.6%; p = 0.0003) in TBX18-injected animals versus control animals. CONCLUSIONS: In a preclinical model, pacemaker-induced cardiomyopathy can be prevented, and reversed, by restoring antegrade conduction with TBX18 biological pacing.

Diffusion Tensor Cardiac Magnetic Resonance Reveals Exosomes From Cardiosphere-Derived Cells Preserve Myocardial Fiber Architecture After Myocardial Infarction.

The object of the study was to reveal the fiber microstructural response with diffusion tensor cardiac magnetic resonance after intramyocardial exosomes secreted by cardiosphere-derived cells (CDCEXO) in chronic porcine myocardial infarction. Porcine with myocardial infarction underwent intramyocardial delivery of human CDCEXO and placebo in a randomized placebo-controlled study. Four weeks after injection, viability improved in the CDCEXO group, whereas myocardial fiber architecture and cardiac function were preserved. In the placebo group, fiber architecture and cardiac function declined. Myocardial regeneration by CDCEXO is not tumor-like; instead, details of tissue architecture are faithfully preserved, which may foster physiological excitation and contraction.

Optimized CEST cardiovascular magnetic resonance for assessment of metabolic activity in the heart.

BACKGROUND: Previous studies have linked cardiac dysfunction to loss of metabolites in the creatine kinase system. Chemical exchange saturation transfer (CEST) is a promising metabolic cardiovascular magnetic resonance (CMR) imaging technique and has been applied in the heart for creatine mapping. However, current limitations include: (a) long scan time, (b) residual cardiac and respiratory motion, and (c) B0 field variations induced by respiratory motion. An improved CEST CMR technique was developed to address these problems. METHODS: Animals with chronic myocardial infarction (N = 15) were scanned using the proposed CEST CMR technique and a late gadolinium enhancement (LGE)  sequence as reference. The major improvements of the CEST CMR technique are: (a) Images were acquired by single-shot FLASH, significantly increasing the scan efficiency. (b) All images were registered to reduce the residual motion. (c) The acquired Z-spectrum was analyzed using 3-pool-model Lorentzian-line fitting to generate CEST signal, reducing the impact of B0 field shifting due to respiratory motion. Feasibility of the technique was tested in a porcine model with chronic myocardial infarction. CEST signal was measured in the scar, border zone and remote myocardium. Initial studies were performed in one patient. RESULTS: In all animals, healthy remote myocardial CEST signal was elevated (0.16 ± 0.02) compared to infarct CEST signal (0.09 ± 0.02, P < 0.001) and the border zone (0.12 ± 0.02, P < 0.001). For both animal and patient studies, the hypointense regions in the CEST contrast maps closely match the bright areas in the LGE images. CONCLUSIONS: The proposed CEST CMR technique was developed to address long scan times, respiratory and cardiac motion, and B0 field variations. Lower CEST signal in bright region of the LGE image is consistent with the fact that myocardial infarction has reduced metabolic activity.

Exosomal MicroRNA Transfer Into Macrophages Mediates Cellular Postconditioning.

BACKGROUND: Cardiosphere-derived cells (CDCs) confer cardioprotection in acute myocardial infarction by distinctive macrophage (Mϕ) polarization. Here we demonstrate that CDC-secreted exosomes (CDCexo) recapitulate the cardioprotective effects of CDC therapy known as cellular postconditioning. METHODS: Rats and pigs underwent myocardial infarction induced by ischemia/reperfusion before intracoronary infusion of CDCexo, inert fibroblast exosomes (Fbexo; control), or vehicle. Two days later, infarct size was quantified. Macrophages were isolated from cardiac tissue or bone marrow for downstream analyses. RNA sequencing was used to determine exosome content and alterations in gene expression profiles in Mϕ. RESULTS: Administration of CDCexo but not Fbexo after reperfusion reduces infarct size in rat and pig models of myocardial infarction. Furthermore, CDCexo reduce the number of CD68+ Mϕ within infarcted tissue and modify the polarization state of Mϕ so as to mimic that induced by CDCs. CDCexo are enriched in several miRNAs (including miR-146a, miR-181b, and miR-126) relative to Fbexo. Reverse pathway analysis of whole-transcriptome data from CDCexo-primed Mϕ implicated miR-181b as a significant (P=1.3x10-21) candidate mediator of CDC-induced Mϕ polarization, and PKCδ (protein kinase C δ) as a downstream target. Otherwise inert Fbexo loaded selectively with miR-181b alter Mϕ phenotype and confer cardioprotective efficacy in a rat model of myocardial infarction. Adoptive transfer of PKCδ-suppressed Mϕ recapitulates cardioprotection. CONCLUSIONS: Our data support the hypothesis that exosomal transfer of miR-181b from CDCs into Mϕ reduces PKCδ transcript levels and underlies the cardioprotective effects of CDCs administered after reperfusion.

Exosomes secreted by cardiosphere-derived cells reduce scarring, attenuate adverse remodelling, and improve function in acute and chronic porcine myocardial infarction.

Aims: Naturally secreted nanovesicles known as exosomes are required for the regenerative effects of cardiosphere-derived cells (CDCs), and exosomes mimic the benefits of CDCs in rodents. Nevertheless, exosomes have not been studied in a translationally realistic large-animal model. We sought to optimize delivery and assess the efficacy of CDC-secreted exosomes in pig models of acute (AMI) and convalescent myocardial infarction (CMI). Methods and Results: In AMI, pigs received human CDC exosomes (or vehicle) by intracoronary (IC) or open-chest intramyocardial (IM) delivery 30 min after reperfusion. No-reflow area and infarct size (IS) were assessed histologically at 48 h. Intracoronary exosomes were ineffective, but IM exosomes decreased IS from 80 ± 5% to 61 ± 12% (P= 0.001) and preserved left ventricular ejection fraction (LVEF). In a randomized placebo-controlled study of CMI, pigs 4 weeks post-myocardial infarction (MI) underwent percutaneous IM delivery of vehicle (n = 6) or CDC exosomes (n = 6). Magnetic resonance imaging (MRI) performed before and 1 month after treatment revealed that exosomes (but not vehicle) preserved LV volumes and LVEF (−0.1 ± 2.2% vs. −5.4 ± 3.6%, P= 0.01) while decreasing scar size. Histologically, exosomes decreased LV collagen content and cardiomyocyte hypertrophy while increasing vessel density. Conclusion: Cardiosphere-derived cell exosomes delivered IM decrease scarring, halt adverse remodelling and improve LVEF in porcine AMI and CMI. While conceptually attractive as cell-free therapeutic agents for myocardial infarction, exosomes have the disadvantage that IM delivery is necessary.

Durable Benefits of Cellular Postconditioning: Long-Term Effects of Allogeneic Cardiosphere-Derived Cells Infused After Reperfusion in Pigs with Acute Myocardial Infarction.

BACKGROUND: Infusion of allogeneic cardiosphere-derived cells (allo-CDCs) postreperfusion elicits cardioprotective cellular postconditioning in pigs with acute myocardial infarction. However, the long-term effects of allo-CDCs have not been assessed. We performed a placebo-controlled pivotal study for long-term evaluation, as well as shorter-term mechanistic studies. METHODS AND RESULTS: Minipigs underwent 1.5-hour mid-left anterior descending balloon occlusion followed by reperfusion and were randomized to receive intracoronary allo-CDCs or vehicle 30 minutes postreperfusion. Left ventriculography (LVG) demonstrated preserved ejection fraction (EF) and attenuation of LV remodeling in CDC-treated pigs. Pigs underwent cardiac magnetic resonance imaging (MRI) and LVG 1 hour and 8 weeks after therapy to evaluate efficacy. MRI showed improvement of EF and attenuation of LV remodeling immediately after allo-CDC infusion. In addition, allo-CDCs improved regional function and decreased hypertrophy 2 months post-treatment. Histological analysis revealed increased myocardial salvage index, enhanced vascularity, sustained reductions in infarct size/area at risk and scar transmurality, and attenuation of collagen deposition in the infarct zone of allo-CDC-treated pigs at 2 months. Allo-CDCs did not evoke lymphohistiocytic infiltration or systemic humoral memory response. Short-term experiments designed to probe mechanism revealed antiapoptotic effects of allo-CDCs on cardiomyocytes and increases in cytoprotective macrophages, but no increase in overall inflammatory cell infiltration 2 hours after cell therapy. CONCLUSIONS: Allo-CDC infusion postreperfusion is safe, improves cardiac function, and attenuates scar size and remodeling. The favorable effects persist for at least 2 months after therapy. Thus, cellular postconditioning confers not only acute cardioprotection, but also lasting structural and functional benefits.

Widespread Myocardial Delivery of Heart-Derived Stem Cells by Nonocclusive Triple-Vessel Intracoronary Infusion in Porcine Ischemic Cardiomyopathy: Superior Attenuation of Adverse Remodeling Documented by Magnetic Resonance Imaging and Histology.

Single-vessel, intracoronary infusion of stem cells under stop-flow conditions has proven safe but achieves only limited myocardial coverage. Continuous flow intracoronary delivery to one or more coronary vessels may achieve broader coverage for treating cardiomyopathy, but has not been investigated. Using nonocclusive coronary guiding catheters, we infused allogeneic cardiosphere-derived cells (CDCs) either in a single vessel or sequentially in all three coronary arteries in porcine ischemic cardiomyopathy and used magnetic resonance imaging (MRI) to assess structural and physiological outcomes. Vehicle-infused animals served as controls. Single-vessel stop-flow and continuous-flow intracoronary infusion revealed equivalent effects on scar size and function. Sequential infusion into each of the three major coronary vessels under stop-flow or continuous-flow conditions revealed equal efficacy, but less elevation of necrotic biomarkers with continuous-flow delivery. In addition, multi-vessel delivery resulted in enhanced global and regional tissue function compared to a triple-vessel placebo-treated group. The functional benefits after global cell infusion were accompanied histologically by minimal inflammatory cellular infiltration, attenuated regional fibrosis and enhanced vessel density in the heart. Sequential multi-vessel non-occlusive delivery of CDCs is safe and provides enhanced preservation of left ventricular function and structure. The current findings provide preclinical validation of the delivery method currently undergoing clinical testing in the Dilated cardiomYopathy iNtervention With Allogeneic MyocardIally-regenerative Cells (DYNAMIC) trial of CDCs in heart failure patients.

Intracoronary delivery of self-assembling heart-derived microtissues (cardiospheres) for prevention of adverse remodeling in a pig model of convalescent myocardial infarction.

BACKGROUND: Preclinical studies in rodents and pigs indicate that the self-assembling microtissues known as cardiospheres may be more effective than dispersed cardiosphere-derived cells. However, the more desirable intracoronary route has been assumed to be unsafe for cardiosphere delivery: Cardiospheres are large (30-150 µm), raising concerns about likely microembolization. We questioned these negative assumptions by evaluating the safety and efficacy of optimized intracoronary delivery of cardiospheres in a porcine model of convalescent myocardial infarction. METHODS AND RESULTS: First, we standardized the size of cardiospheres by modifying culture conditions. Then, dosage was determined by infusing escalating doses of cardiospheres in the left anterior descending artery of naive pigs, looking for acute adverse effects. Finally, in a randomized efficacy study, 14 minipigs received allogeneic cardiospheres (1.3 × 10(6)) or vehicle 1 month after myocardial infarction. Animals underwent magnetic resonance imaging before infusion and 1 month later to assess left ventricular ejection fraction, scar mass, and viable mass. In the dosing study, we did not observe any evidence of microembolization after cardiosphere infusion. In the post-myocardial infarction study, cardiospheres preserved LV function, reduced scar mass and increased viable mass, whereas placebo did not. Moreover, cardiosphere decreased collagen content, and increased vessel densities and myocardial perfusion. Importantly, intracoronary cardiospheres decreased left ventricular end-diastolic pressure and increased cardiac output. CONCLUSIONS: Intracoronary delivery of cardiospheres is safe. Intracoronary cardiospheres are also remarkably effective in decreasing scar, halting adverse remodeling, increasing myocardial perfusion, and improving hemodynamic status after myocardial infarction in pigs. Thus, cardiospheres may be viable therapeutic candidates for intracoronary infusion in selected myocardial disorders.

Cellular postconditioning: allogeneic cardiosphere-derived cells reduce infarct size and attenuate microvascular obstruction when administered after reperfusion in pigs with acute myocardial infarction.

BACKGROUND: Intracoronary delivery of cardiosphere-derived cells (CDCs) has been demonstrated to be safe and effective in porcine and human chronic myocardial infarction. However, intracoronary delivery of CDCs after reperfusion in acute myocardial infarction has never been assessed in a clinically-relevant large animal model. We tested CDCs as adjunctive therapy to reperfusion in a porcine model of myocardial infarction. METHODS AND RESULTS: First, escalating doses (5, 7.5, and 10 million cells) of allogeneic CDCs were administered intracoronary 30 minutes after reperfusion. Forty-eight hours later, left ventriculography was performed and animals euthanized to measure area at risk, infarct size (IS), and microvascular obstruction. Second, identical end points were measured in a pivotal study of minipigs (n=14) that received 8.5 to 9 million allogeneic CDCs, placebo solution, or sham. Multiple indicators of cardioprotection were observed with 7.5 and 10 million allogeneic CDCs, but not 5 million CDCs, relative to control. In the pivotal study, IS, microvascular obstruction, cardiomyocyte apoptosis, and adverse left ventricular remodeling were all smaller in the CDC group than in sham or placebo groups. In addition, serum troponin I level at 24 hours was lower after CDC infusion than that in the placebo or sham groups, consistent with the histologically-demonstrated reduction in IS. CONCLUSIONS: Intracoronary delivery of allogeneic CDCs is safe, feasible, and effective in cardioprotection, reducing IS, preventing microvascular obstruction, and attenuating adverse acute remodeling. This novel cardioprotective effect, which we call cellular postconditioning, differs from previous strategies to reduce IS in that it works even when initiated with significant delay after reflow.

Allogeneic cardiospheres delivered via percutaneous transendocardial injection increase viable myocardium, decrease scar size, and attenuate cardiac dilatation in porcine ischemic cardiomyopathy.

BACKGROUND: Epicardial injection of heart-derived cell products is safe and effective post-myocardial infarction (MI), but clinically-translatable transendocardial injection has never been evaluated. We sought to assess the feasibility, safety and efficacy of percutaneous transendocardial injection of heart-derived cells in porcine chronic ischemic cardiomyopathy. METHODS AND RESULTS: We studied a total of 89 minipigs; 63 completed the specified protocols. After NOGA-guided transendocardial injection, we quantified engraftment of escalating doses of allogeneic cardiospheres or cardiosphere-derived cells in minipigs (n = 22) post-MI. Next, a dose-ranging, blinded, randomized, placebo-controlled ("dose optimization") study of transendocardial injection of the better-engrafting product was performed in infarcted minipigs (n = 16). Finally, the superior product and dose (150 million cardiospheres) were tested in a blinded, randomized, placebo-controlled ("pivotal") study (n = 22). Contrast-enhanced cardiac MRI revealed that all cardiosphere doses preserved systolic function and attenuated remodeling. The maximum feasible dose (150 million cells) was most effective in reducing scar size, increasing viable myocardium and improving ejection fraction. In the pivotal study, eight weeks post-injection, histopathology demonstrated no excess inflammation, and no myocyte hypertrophy, in treated minipigs versus controls. No alloreactive donor-specific antibodies developed over time. MRI showed reduced scar size, increased viable mass, and attenuation of cardiac dilatation with no effect on ejection fraction in the treated group compared to placebo. CONCLUSIONS: Dose-optimized injection of allogeneic cardiospheres is safe, decreases scar size, increases viable myocardium, and attenuates cardiac dilatation in porcine chronic ischemic cardiomyopathy. The decreases in scar size, mirrored by increases in viable myocardium, are consistent with therapeutic regeneration.

In vivo contrast free chronic myocardial infarction characterization using diffusion-weighted cardiovascular magnetic resonance.

BACKGROUND: Despite the established role of late gadolinium enhancement (LGE) cardiovascular magnetic resonance (CMR) in characterizing chronic myocardial infarction (MI), a significant portion of chronic MI patients are contraindicative for the use of contrast agents. One promising alternative contrast free technique is diffusion weighted CMR (dwCMR), which has been shown ex vivo to be sensitive to myocardial fibrosis. We used a recently developed in vivo dwCMR in chronic MI pigs to compare apparent diffusion coefficient (ADC) maps with LGE imaging for infarct characterization. METHODS: In eleven mini pigs, chronic MI was induced by complete occlusion of the left anterior descending artery for 150 minutes. LGE, cine, and dwCMR imaging was performed 8 weeks post MI. ADC maps were derived from three orthogonal diffusion directions (b = 400 s/mm2) and one non-diffusion weighted image. Two semi-automatic infarct classification methods, threshold and full width half max (FWHM), were performed in both LGE and ADC maps. Regional wall motion (RWM) analysis was performed and compared to ADC maps to determine if any observed ADC change was significantly influenced by bulk motion. RESULTS: ADC of chronic MI territories was significantly increased (threshold: 2.4 ± 0.3 µm2/ms, FWHM: 2.4 ± 0.2 µm2/ms) compared to remote myocardium (1.4 ± 0.3 µm2/ms). RWM was significantly reduced (threshold: 1.0 ± 0.4 mm, FWHM: 0.9 ± 0.4 mm) in infarcted regions delineated by ADC compared to remote myocardium (8.3 ± 0.1 mm). ADC-derived infarct volume and location had excellent agreement with LGE. Both LGE and ADC were in complete agreement when identifying transmural infarcts. Additionally, ADC was able to detect LGE-delineated infarcted segments with high sensitivity, specificity, PPV, and NPV. (threshold: 0.88, 0.93, 0.87, and 0.94, FWHM: 0.98, 0.97, 0.93, and 0.99, respectively). CONCLUSIONS: In vivo diffusion weighted CMR has potential as a contrast free alternative for LGE in characterizing chronic MI.

Biological pacemaker created by minimally invasive somatic reprogramming in pigs with complete heart block.

Somatic reprogramming by reexpression of the embryonic transcription factor T-box 18 (TBX18) converts cardiomyocytes into pacemaker cells. We hypothesized that this could be a viable therapeutic avenue for pacemaker-dependent patients afflicted with device-related complications, and therefore tested whether adenoviral TBX18 gene transfer could create biological pacemaker activity in vivo in a large-animal model of complete heart block. Biological pacemaker activity, originating from the intramyocardial injection site, was evident in TBX18-transduced animals starting at day 2 and persisted for the duration of the study (14 days) with minimal backup electronic pacemaker use. Relative to controls transduced with a reporter gene, TBX18-transduced animals exhibited enhanced autonomic responses and physiologically superior chronotropic support of physical activity. Induced sinoatrial node cells could be identified by their distinctive morphology at the site of injection in TBX18-transduced animals, but not in controls. No local or systemic safety concerns arose. Thus, minimally invasive TBX18 gene transfer creates physiologically relevant pacemaker activity in complete heart block, providing evidence for therapeutic somatic reprogramming in a clinically relevant disease model.

Validation of contrast-enhanced magnetic resonance imaging to monitor regenerative efficacy after cell therapy in a porcine model of convalescent myocardial infarction.

BACKGROUND: Magnetic resonance imaging (MRI) in the CArdiosphere-Derived aUtologous stem CElls to reverse ventricUlar dySfunction (CADUCEUS) trial revealed that cardiosphere-derived cells (CDCs) decrease scar size and increase viable myocardium after myocardial infarction (MI), but MRI has not been validated as an index of regeneration after cell therapy. We tested the validity of contrast-enhanced MRI in quantifying scarred and viable myocardium after cell therapy in a porcine model of convalescent MI. METHODS AND RESULTS: Yucatan minipigs underwent induction of MI and 2-3 weeks later were randomized to receive intracoronary infusion of 12.5×10(6) mismatched allogeneic CDCs or vehicle. Allogeneic CDCs induced mild local mononuclear infiltration but no systemic immunogenicity. MRI revealed that allogeneic CDCs attenuated remodeling, improved global and regional function, decreased scar size, and increased viable myocardium compared with placebo 2 months post-treatment. Extensive histological analysis validated quantitatively the MRI measurements of scar size, scar mass, and viable mass. CDCs neither altered gadolinium contrast myocardial kinetics nor induced changes in vascular density or architecture in viable and scarred myocardium. Histology demonstrated that CDCs lead to cardiomyocyte hyperplasia in the border zone, consistent with the observed stimulation of endogenous regenerative mechanisms (cardiomyocyte cycling, upregulation of endogenous progenitors, angiogenesis). CONCLUSIONS: Contrast-enhanced MRI accurately measures scarred and viable myocardium after cell therapy in a porcine model of convalescent MI. MRI represents a useful tool for assessing dynamic changes in the infarct and monitoring regenerative efficacy.