Breathing is so effortless that we often take it for granted. However, our lungs are actually very delicate tissues with limited regeneration capacity. Upon lung failure, the only way to replace the tissue is by transplantation, yet this procedure is hampered by low survival rates and a lack of donors.
For decades, researchers have tried to create tissue-engineered lungs. Previous attempts have mostly involved reconstituting lung tissue on a microscopic scale by growing cells on a synthetic scaffold. But in a recent publication in Science, Laura Niklason, Professor of Anesthesiology and Biomedical Engineering, and her team reported progress in constructing a tissue-engineered lung using an alternative method, in which the acellular lung scaffold is derived from a native lung.
“We realized that what’s limiting us is that we don’t have this incredible, complex, three-dimensional scaffold of the native lung,” said Niklason, “So let’s see if we can borrow this scaffold from nature and repopulate it with cells.”
To create a scaffold from native lungs, the Niklason lab removed the lungs of an adult rat and decellularized it using detergent solutions. This procedure eliminated antigenic and cellular components, but mostly preserved the delicate extracellular scaffold that made up the hundreds of millions of microscopic air sacs and blood vessels in the original lung.
To repopulate the scaffold, neonatal rat lung epithelial cells were injected through the airway, and microvascular lung endothelial cells through the blood vessels. Meanwhile, the scaffold was maintained in a bioreactor that simulated fetal conditions by providing liquid ventilation of the lung through the trachea and perfusion with culture medium through the pulmonary artery.
Remarkably, the injected epithelial cells efficiently adhered to the scaffold and replicated without undergoing apoptosis. When neonatal rat lung epithelial cells are grown in standard tissue culture, most of the cells die; however, when the cells are grown on the scaffold, many cells survive and differentiate into six or seven important cells types. Niklason believes, “this shows that there’s a tremendous amount of information in the matrix—protein cues that help the cells survive, grow, and differentiate.”
As a final step, the researchers implanted the engineered lung in a rat and ventilated it with pure oxygen. The implanted lung was then monitored over the course of two to three hours to check if it inflated with air, perfused with blood, and exchanged gas as well as a native lung.
The engineered lung was found to function and exchange gas very effectively. But there were also several problems: the lung scaffold was slightly damaged, causing some bleeding into the airway. Additionally, the endothelial lining of the blood vessels was incomplete, resulting in clotting within a few hours of implantation.
The Niklason lab is now working on refining their construction of a tissue-engineered lung. “Our decellularization process has to become gentler still, and we have to be more careful about how we seed endothelium into the blood vessels,” stated Niklason.
As for extending this methodology to humans, perhaps the biggest challenge is finding a suitable source of human pulmonary epithelium. One possible route is to find a way to get induced pluripotent stem cells to differentiate into lung cells so that they can be used to seed a scaffold.
Engineers are still far from creating tissue-engineered lungs that can be implanted into patients, yet the recent developments made in the Niklason lab mark a significant step toward the engineering of lungs for implantation. Nikalson predicts that, “Realistically, maybe 20 to 25 years from now, our research will be able to benefit patients.”