Researchers from Massachusetts General Hospital successfully regenerated functional heart muscles from decellularised human hearts. This accomplishment specifically involved bioengineering of functional myocardial tissue based on the combination of human cardiac matrix and human induced pluripotent stem cell–derived cardiomyocytes.
Current procedures involving heart transplantation remain problematic. The researchers noted that organ shortage in the United States and allograft rejection due to immune response are major limitations. Bioengineering a human heart through decellularisation before recellularisation by generation of cardiac muscle cells from induced pluripotent stem cells provides a viable workaround.
Decellularisation would strip the heart of components that would naturally generate an immune response. On the other hand, recellularisation using patient-obtained cells would not generate any immune response. The same recellularisation would repurpose extracted human hearts initially deemed as unsuitable for transplantation.
Procedure for decellularising an organ
Through their study, the researchers wanted to exploit these benefits to provide a theoretical alternative to allotransplantation—or transplantation to a recipient from genetically non-identical donor of the same species.
Harald Ott, lead researcher and MD at MGH Centre for Regenerative Medicine and the Department of Surgery, developed a procedure for organ decellularisation in 2008. The approach involves using a detergent solution to strip living cells from a donor organ before repopulating the remaining extracellular matrix scaffold with organ-appropriate types of cells.
The Ott-led MGH team of researchers subsequently used this procedure to decellularise organs of large animals and generate functional rat kidneys and lungs.
To translate the previous works to human scale, the researchers experimented with 73 human hearts determined unsuitable for transplantation and recovered under research consent. These organs came from brain-dead donors and those who had undergone cardiac death. Researchers used a process originally developed in earlier experiments involving rat hearts.
Detailed characterisation of the remaining cardiac scaffolds confirmed a high retention of matrix proteins and structure free of cardiac cells, as well as the preservation of coronary vascular and microvascular structures.
There was also freedom from human leukocyte antigens that could induce rejection. There was little difference between the reactions of organs from the two donor groups to the complex decellularisation process.
Procedure for recellularising an organ
Instead of using genetic manipulation to generate induced pluripotent stem cells from adult cells, the researchers used a newer procedure for reprogramming skills with messenger RNA factors. This is more efficient and less likely to incur regulatory hurdles.
They proceeded to inducing the pluripotent cells to differentiate into cardiac muscle cells or cardiomyocytes, documenting patterns of gene expression that reflected developmental milestones and generating cells in sufficient quantity for possible clinical application.
Cardiomyocytes were then reseeded into three-dimensional matrix tissue—first into thin matrix slices and then into 15 millimetre fibers, which developed into spontaneously contracting tissue after several days in culture.
The last step reflected the first regeneration of human heart muscle from pluripotent stem cells within a cell-free, human whole-heart matrix. The team delivered about 500 million iPSC-derived cardiomyocytes into the left ventricular wall of decellularised hearts. The organs were mounted for 14 days in an automated bioreactor system developed by the MGH team that both perfused the organ with nutrient solution and applied environmental stressors such as ventricular pressure to reproduce conditions within a living heart.
Analysis of the regenerated tissue found dense regions of iPSC-derived cells that had the appearance of immature cardiac muscle tissue and demonstrated functional contraction in response to electrical stimulation.
Key takeaways from the study
In further explaining the entire process, think of decellularisation as a process involving a donor organ and removing any traces that would make it recognisable or traceable back to the original owner. Of ourse, the organ needs cells to become fully functional. This is where recellularisation enters the picture. Reintroducing organ-specific cells grown from stem cells of the recipient would localise the organ. In other words, the organ becomes traceable back to the recipient instead of the owner.
The aforementioned procedures basically involve removing cells from a functioning organ or tissue and reintroducing organ-specific lab-grown cells. This two-fold process would make the bioengineered organ or tissue invisible from the immune system of the organ recipient. This is because the system would consider the organ as a natural biological part of the recipient. The aforementioned study demonstrated this possibility. However, it remains limited to bioengineering a tissue.
Bioengineering functional heart muscles nonetheless forms part of a bigger goal. Jacques Guyette, lead author of the study and PhD at MGH Centre for Regenerative Medicine, said that regenerating a whole heart is the long-term goal and this is several years away. Their current works centre on engineering a functional myocardial patch that could replace cardiac tissue damaged due a heart attack or heart failure.
Guyette also highlighted the fact that they need to improve methods for generating even more cardiac cells. Take note that recellularising a whole heart would take tends of billions of these cells. They also need to optimise bioreactor-based culture techniques to improve the maturation and function of engineered cardiac tissue, and electronically integrating regenerated tissue to function within the heart of the recipient.
Further details of the study Guyette and Ott are in the article “Bioengineering Human Myocardium on Native Extracellular Matrix” published in October 2015 in the journal Circulation Research. Other co-authors of the study are in the same article. Photo credit: Bernhard Jank, MD/Ott Lab/MGH/Adapted