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Article by Daniel Rivas, Margaretha Morsink & Roberta Lock
Engineering of Lung Alveolar Epithelium
Source Publication:
Rational engineering of lung alveolar epithelium, Nature npj Regenerative Medicine, 2023
Leibi et al., Laura Niklason Lab.
End-stage lung diseases remain one of the leading causes of death in the United States (150,000 annually), where the only effective treatment is lung transplantation. However, this comes with significant challenges; there aren't enough people donating organs, and when someone does get a new set of lungs, their body might reject them, which means their immune system fights against the new lungs instead of accepting them. Engineered whole lungs based on decellularized lung extracellular matrix (ECM) scaffolds offer a potential solution to fill the need for more lungs and the need to avoid rejection. Lung scaffolds prove to provide the optimal microenvironment necessary for cells to grow and recapitulate normal lung features. This study focused on the lung alveoli, which are tiny air sacs were gas exchange occurs, and combined decellularized lung scaffolds with a combination of lung-specific cell types in a bioreactor to engineer an alveolar epithelium.
What did these researchers do?
This study engineered an alveolar epithelium based on decellularized whole rat lung scaffolds seeded with endothelial cells (EC), alveolar fibroblasts (FB), and alveolar epithelial type 2 (AEC2) and type 1 (AEC1) cells cultured in bioreactors. The ECs and FBs are supporting cell types necessary to keep the AECs –the most important cells in the alveoli- healthy. AEC2 cells are primary cells that secrete surfactant, and they can divide and differentiate towards AEC1s when repair is needed, and AEC1s are the main cells in the alveolar barrier, responsible for gas exchange with blood capillaries.
First, the researchers showed they could produce AEC2s repopulation on the scaffold with native-like morphology (cell shape), localization, and organization. Then, differentiation from AEC2s to AEC1s was achieved by removing certain growth factors and adding mechanical stimulation simulating breathing-like motions. The cellular identity (type of cell – the lung has over 40 types!), morphology, and organization of the engineered alveolar epithelium were assessed via multiple imaging and gene and protein expression assessments.
Why is this important?
End-stage lung diseases cause more than 150,000 deaths annually, while lung transplants remain the only effective therapeutic answer. However, transplants do not always perform optimally, and their availability is heavily compromised by organ shortages even in first-world countries like the United States. Engineering whole lungs based on decellularized lungs could offer a therapeutic alternative to overcome organ shortage limitations. Moreover, if the scaffolds are repopulated with the patient’s own cells, it could overcome organ rejection and patients could potentially live without immunosuppressant therapy, which is extremely harsh on the body.
How did the researchers do this?
The researchers first isolated rat AEC2s and seeded them into decellularized whole rat lungs to generate the alveolar epithelium. For proper cell growth and maintenance of AEC2s, they seeded isolated rat ECs and FBs into the decellularized lung scaffolds as well. Co-culturing all three cell types is required to promote the formation of AEC2-rich alveolar-like units with high phenotypic, surfactant secretion, and mechanical compliance similarity to native lungs.
The next step to achieve functional alveolar-like units is ACE2 differentiation to AEC1s to determine whether both cell types can be supported in the scaffold. This was investigated in small-scale engineered lung tissue (ELT) platforms supporting the culture of decellularized lung scaffold slices with mechanical strain, mimicking the breath. The combination of mechanical strain with the removal of the growth factors keeping the AEC2 alive resulted in the differentiation of AEC2 to AEC1. These results suggest that to drive differentiation of AEC2s into AEC1s to form a gas exchange alveolar barrier, both withdrawal of the AEC2s pro-proliferative factors (to prevent further proliferation of AEC2s) and the application of breathing-like mechanical strain (specific mechanical stimuli is known to affect cell fate and processes) are necessary.
Histology of triculture lung and decellularized lung
What comes next?
As the researchers were able to establish a protocol demonstrating an alveolar epithelium in decellularized rat lung scaffolds similar to native lungs, the next step would be to move to larger animal models that are more physiologically relevant to humans such as pigs. By this proof of concept study in rats, the researchers can focus on replicating their experiments in decellularized pig lungs. This will develop an alveolar epithelium in a much larger scale. Ultimately, these experiments can lead to decellularization of human donor lungs that can act as scaffolds for the recipient’s own lung cells. This will serve as a solution to the current donor organ shortage as well as the organ rejection.