Studying SARS-CoV-2 Infection With Lung Organoids | Expert Q&A | Corning

Abraham: Strictly speaking, there are two different pathways used to form lung organoids: either through pluripotent stem cells or through adult epithelial stem cells present in the human bronchial tract or lung alveoli. The latter takes less time since scientists can simply mix the lung-derived cells in a validated extracellular matrix and then add media with specific growth factors to differentiate and grow from there. This process takes about 14 to 28 days.

With pluripotent stem cells, you'll be spending more time, but you have a distinct cost advantage since gaining access to lung specimens is not trivial. For this method, researchers start with expanded populations of undifferentiated pluripotent stem cells that are subject to directed differentiation using specific growth factors. From there, the material forms small clusters called spheroids after 10 days. When you mix these spheroids with an extracellular matrix and overlay them with more growth factors, they'll then generate lung organoids, forming airway-like structures and markers typical of lung epithelium. Supported by air-liquid interface technology, this process takes anywhere between 50 to 85 days.

Zhang: That really depends on your lab's capabilities and resources. Many academic labs tend to study organoids via pluripotent stem cells since students commonly work on those experiments and time is not as much of an issue. Even if it's a longer pathway with more labor, it could be less expensive than buying the cells or having access to a hospital with adult specimens. On the other hand, using adult epithelial cells can be cost-effective if labor costs would exceed the cost of purchasing the cells.

Zhang: We've seen lung organoids make significant contributions in the fight against the pandemic, particularly with drug screening. Lung organoids infected with SARS-CoV-2 can serve as a disease model to study infection and provide a valuable resource for identifying COVID-19 therapeutics. For example, one of the key requirements for SARS-CoV-2 infection is the presence of ACE2 receptors—and research has identified several drugs that can inhibit that mechanism of viral entry.

Abraham: ACE2 is required for the spike proteins on SARS-CoV-2 to enter the cell, so the fact that a receptor is present is the first sign that there's a possibility for infection—the same is true with the gastrointestinal tract. Then, once the ACE2 receptor is attached, viral entry begins as the viral and organoid membranes combine. From there, the virus reproduces and infection advances.

In terms of cellular damage, the cytokine storm is especially relevant. Researchers have used alveolar organoids to study this mechanism in particular. Research has shown that those alveolar organoids produce proinflammatory cytokines when infected with SARS-CoV-2, and the increase in these cytokines is believed to be the cause for the most severe symptoms of COVID-19, including organ failure. The lung tissue initiates the cytokine storm, which results in cell death and tissue damage even before the immune cells arrive in the lungs.

Then again, we're still learning about this virus. Other researchers suggest that the true cause of cellular death is still a mystery. It could be because of damage caused by the virus or self-induced destruction, or that immune cells reach the site and eat the cells from there. There's still so much we do not fully know.

Zhang: There's always potential to study and learn more about this novel coronavirus, even a year or more after COVID-19 was first seen. Studying viral variants, such as those that have made news in Europe, the U.S., and elsewhere, is one area where we could potentially expand knowledge through the use of organoids. For example, if there's a mutation in the protein that binds to the ACE2, then it would affect COVID-19 infectivity. Still, it's just too new to know for sure.

The exciting thing about using organoids for this type of work is the potential for learning a lot in a relatively small amount of time, particularly through the use of high-throughput screening. I think you'll undoubtedly see more interest and uptake in organoid technology in the future, and I'm looking forward to seeing where it takes science as a whole.

Learn more about how lung organoids are defining the future of infectious disease modeling.