Non-human primate studies for cardiomyocyte transplantation-ready for translation?

Non-human primates (NHP) are valuable models for late translational pre-clinical studies, often seen as a last step before clinical application. The unique similarity between NHPs and humans is often the subject of ethical concerns. However, it is precisely this analogy in anatomy, physiology, and the immune system that narrows the translational gap to other animal models in the cardiovascular field. Cell and gene therapy approaches are two dominant strategies investigated in the research field of cardiac regeneration. Focusing on the cell therapy approach, several xeno- and allogeneic cell transplantation studies with a translational motivation have been realized in macaque species. This is based on the pressing need for novel therapeutic options for heart failure patients. Stem cell-based remuscularization of the injured heart can be achieved via direct injection of cardiomyocytes (CMs) or patch application. Both CM delivery approaches are in the late preclinical stage, and the first clinical trials have started. However, are we already ready for the clinical area? The present review concentrates on CM transplantation studies conducted in NHPs, discusses the main sources and discoveries, and provides a perspective about human translation.

Immature human engineered heart tissues engraft in a guinea pig chronic injury model.

Engineered heart tissue (EHT) transplantation represents an innovative, regenerative approach for heart failure patients. Late preclinical trials are underway, and a first clinical trial started recently. Preceding studies revealed functional recovery after implantation of in vitro-matured EHT in the subacute stage, whereas transplantation in a chronic injury setting was less efficient. When transplanting matured EHTs, we noticed that cardiomyocytes undergo a dedifferentiation step before eventually forming structured grafts. Therefore, we wanted to evaluate whether immature EHT (EHTIm) patches can be used for transplantation. Chronic myocardial injury was induced in a guinea pig model. EHTIm (15×106 cells) were transplanted within hours after casting. Cryo-injury led to large transmural scars amounting to 26% of the left ventricle. Grafts remuscularized 9% of the scar area on average. Echocardiographic analysis showed some evidence of improvement of left-ventricular function after EHTIm transplantation. In a small translational proof-of-concept study, human scale EHTIm patches (4.5×108 cells) were epicardially implanted on healthy pig hearts (n=2). In summary, we provide evidence that transplantation of EHTIm patches, i.e. without precultivation, is feasible, with similar engraftment results to those obtained using matured EHT.

Contractile Force of Transplanted Cardiomyocytes Actively Supports Heart Function After Injury.

BACKGROUND: Transplantation of pluripotent stem cell-derived cardiomyocytes represents a promising therapeutic strategy for cardiac regeneration, and the first clinical studies in patients with heart failure have commenced. Yet, little is known about the mechanism of action underlying graft-induced benefits. Here, we explored whether transplanted cardiomyocytes actively contribute to heart function. METHODS: We injected cardiomyocytes with an optogenetic off-on switch in a guinea pig cardiac injury model. RESULTS: Light-induced inhibition of engrafted cardiomyocyte contractility resulted in a rapid decrease of left ventricular function in ≈50% (7/13) animals that was fully reversible with the offset of photostimulation. CONCLUSIONS: Our optogenetic approach demonstrates that transplanted cardiomyocytes can actively participate in heart function, supporting the hypothesis that the delivery of new force-generating myocardium can serve as a regenerative therapeutic strategy.

Human engineered heart tissue transplantation in a guinea pig chronic injury model.

Myocardial injury leads to an irreversible loss of cardiomyocytes (CM). The implantation of human engineered heart tissue (EHT) has become a promising regenerative approach. Previous studies exhibited beneficial, dose-dependent effects of human induced pluripotent stem cell (hiPSC)-derived EHT patch transplantation in a guinea pig model in the subacute phase of myocardial injury. Yet, advanced heart failure often results from a chronic remodeling process. Therefore, from a clinical standpoint it is worthwhile to explore the ability to repair the chronically injured heart. In this study human EHT patches were generated from hiPSC-derived CMs (15 × 106 cells) and implanted epicardially four weeks after injury in a guinea pig cryo-injury model. Cardiac function was evaluated by echocardiography after a follow-up period of four weeks. Hearts revealed large transmural myocardial injuries amounting to 27% of the left ventricle. EHT recipient hearts demonstrated compact muscle islands of human origin in the scar region, as indicated by a positive staining for human Ku80 and dystrophin, remuscularizing 5% of the scar area. Echocardiographic analysis demonstrated no significant functional difference between animals that received EHT patches and animals in the cell-free control group (fractional area change 36% vs. 34%). Thus, EHT patches engrafted in the chronically injured heart but in contrast to the subacute model, grafts were smaller and EHT patch transplantation did not improve left ventricular function, highlighting the difficulties for a regenerative approach.