Exosomes from human induced pluripotent stem cell- (hiPSC)-derived cardiac cells (CCs) have, for the first time, been conclusively shown to improve recovery from myocardial infarction (MI) in pigs, in a collaborative study begun 7 years ago by U.S. researchers.

Using membrane-bound exosomal microvesicles to copy the regenerative effects of cardiac cell transplants could potentially avoid the safety and efficacy issues that have prevented clinical use of whole-cell heart therapies, the authors reported in the September 16, 2020, edition of Science Translational Medicine.

Animal studies have identified hiPSCs as a promising source of cardiomyocytes (CMs), smooth muscle cells (SMCs), and endothelial cells (ECs) for regenerative myocardial therapies.

However, their clinical use has been limited by poor cell retention and engraftment rates, together with concerns regarding potential tumorigenesis and, in particular, ventricular arrhythmia.

"The major problem with whole-cell therapy concerns the extremely low successful engraftment rate," said study leader Jianyi Zhang, professor and chair of the Department of Biomedical Engineering, School of Medicine and School of Engineering, University of Alabama at Birmingham.

Moreover, "studies in nonhuman primate (NHP) models have shown that when given at an extremely high dose, a significant graft size is accompanied by arrhythmia," Zhang told BioWorld Science.

Therefore, strategies using the cardioprotective and regenerative properties of hiPSCs and hiPSC-derived cells and cell products, while avoiding cell transplantation difficulties, could lead to the development of new cardiovascular therapeutics.

Because the engraftment rate of transplanted cells is low, benefits are usually attributed to the cells' paracrine activity, so paracrine factors produced within transplanted cells are often loaded into exosomes for transport to their targets.

Exosomes

Exosomes and other microvesicles secreted from hiPSC-CMs contain biologically active proteins, RNAs and microRNAs (miRNAs).

Previous research has shown that hiPSC-CM exosomes applied as a patch to infarcted rat hearts was associated with improved cardiac function, infarct size, hypertrophy, CM apoptosis, and arrhythmia.

Exosomes are also important in angiogenesis, but it is unknown whether the protective and regenerative properties of exosomes from different types of hiPSC-derived cardiac cells are distinct, necessitating studies in larger animals.

These questions were addressed in the new STM study using exosomes from different hiPSC-derived cell types and by evaluating myocardial recovery in female pigs treated with cardiac cells or with the exosomes secreted by cardiac cells after experimentally induced MI.

Pigs were preferred to NHP primarily for economic reasons, said Zhang. "NHPs are too expensive, costing 10-times more than pigs, when all factors are considered, whereas for a comparable level of cardiovascular physiological information obtained, pigs are much more cost-effective."

The pig models received sham surgery; MI without treatment (MI group); or MI and treatment with hIPSC cells, fragments from those cells, or exosomes derived from several cell types.

All cells and exosomes were administered via direct injection into the injured myocardium.

In vitro, exosomes were shown to promote endothelial cell tube formation and microvessel sprouting from mouse aortic arches and to protect hiPSC-CMs by reducing apoptosis, maintaining intracellular calcium homeostasis, and increasing ATP.

"These in vitro findings show that the benefits of the exosomes are very clear, but that said, exosomes are dynamic, meaning that the same cell types can release different functional exosomes under stressed conditions," said Zhang.

"Therefore, quality control is important, necessitating the establishment of a bank of different cell-type exosomes for mechanistic in vitro studies," he said.

In vivo, left ventricular ejection fraction, wall stress, cardiac hypertrophy, scar size, cell apoptosis, and angiogenesis assessments in the infarcted region were better in the treated groups than in the untreated group 4 weeks after infarction. Moreover, frequencies of arrhythmic events in animals from the groups treated with either cells or exosomes were comparable to those in untreated animals.

"These in vivo findings are very encouraging, showing levels of improvement that are consistent four weeks after the transplantation. However the real question is whether these functional and structural benefits can be sustained at 6 months or beyond," said Zhang.

Collectively, these data suggest that acellular exosomes "could enable exploitation of the cardioprotective and reparative properties of hiPSC-derived cells, while avoiding complexities associated with cell storage, transportation, and immune rejection," he concluded.

In future, "if we can improve exosomal quality control and demonstrate that the benefits of exosomal delivery can be sustained in the long term, we can attempt repeated peripheral intravenous exosomal delivery, which would be much more convenient in the clinical setting." Moreover, "our studies were not designed to directly compare the potency of [hiPSC-derived cells and] exosomes for myocardial protection/regeneration," noted Zhang.

"So although our results suggest the two treatments are comparable, additional studies are required to determine which approach may be most beneficial for patients with acute MI, heart failure, or other cardiovascular diseases." (Gao, L. et al. Sci Transl Med 2020, 12: eaay1318).

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