Modeling the lung in the lab

Elizabeth Corteselli used an NC TraCS pilot grant to create a new model for studying lung diseases and potential treatments.

For most of us, breathing is something we take for granted. Multiple times per minute, all day and every day, we inhale, gathering the oxygen we rely on, and exhale, releasing carbon dioxide and other gases back into the air.

But for many people with lung diseases, breathing can be a constant struggle. One particularly challenging respiratory illness is idiopathic pulmonary fibrosis (IPF), which causes a person's lungs to progressively become stiff and scar over. This can lead to coughing, shortness of breath, and fatigue—and since IPF is a chronic illness, these symptoms can last for the rest of a person's life.

To make matters worse, only a few medications are currently approved to treat IPF in the United States, and these can sometimes cause significant side effects. Pirfenidone, for example, is commonly prescribed to slow the progression of lung fibrosis but can also cause gastrointestinal issues like vomiting and nausea. Sometimes, these side effects are severe enough that people stop taking the medication entirely, says Elizabeth Corteselli, a pediatric allergy and immunology researcher at the University of North Carolina School of Medicine.

Part of the problem is how pirfenidone is delivered. Taken as a pill, the drug diffuses through the bloodstream to reach the lungs. That means it also travels to other parts of the body, often causing unpleasant side effects. Theoretically, if the medication could be delivered through an inhaler, it could hit the lungs directly with a lower overall dosage, potentially reducing those side effects.

Researchers are already exploring this possibility. But Corteselli points out that to truly understand how inhaled pirfenidone might work, researchers need a way to study how this medication interacts with the lung in vitro—and most of today's lab research tools just aren't advanced enough for researchers to investigate how an inhaled drug affects the many types of lung cell inside a living, breathing person.

So, with support from a pilot award from the North Carolina Translational and Clinical Sciences (NC TraCS) Institute, Corteselli recently set out to create a new lab model of the lung. With this grant, she, her research technician Lexi Field, and their colleagues built a multi-layered cross-section of an airway in a dish—giving researchers an up-close and sophisticated way to observe how our lungs interact with everything we might breathe in, from pollution to dust to medications.

Corteselli earned her PhD at UNC, studying environmental science and public health. Before returning to Carolina as a faculty member, she completed a postdoc at the Vermont Lung Center, where she first started looking at pulmonary fibrosis.

In patients with pulmonary fibrosis, lung cells called fibroblasts become abnormally "activated," which can lead to scarring of the lungs over time. Some people develop pulmonary fibrosis because of a known risk factor, like asbestos exposure or autoimmune conditions. When the cause isn't known, it is called idiopathic pulmonary fibrosis, or IPF.

"This is a really devastating disease," Corteselli says.

The medications approved to treat IPF work by slowing down the stiffening and scarring process, but adherence can be a major challenge because of the drug's intense side effects. But many scientists are hopeful that aerosolized medications could make treatment easier on patients.

In theory, researchers could test this by spritzing those medications over a clump of fibroblasts and watching what happens. But in our lungs, the fibroblasts that are activated in patients with pulmonary fibrosis aren't directly exposed to the outside air or inhaled medications. Instead, they're tucked away behind a layer of cells called the respiratory epithelium, which lines the inner walls of our airways and forms the boundary between the air we breathe and the interstitial space of our lung. For an inhaled medication to reach the fibroblasts, it must first pass through this layer.

"You couldn't just treat fibroblasts with the inhaled drug," Corteselli says. "That's not how they're going to see it."

To study how inhaled medications might help treat IPF, scientists need a way to apply the drug to epithelial cells and observe whether it filters through to the fibroblasts. But most common research methods can't replicate this interaction.

A clump of cells grown in a standard Petri dish, for example, won't have all the complexity of a human lung's cell architecture. Animal models also fall short. Mice are often used to study pulmonary fibrosis, but their disease is induced by a chemical treatment that creates scarring that can often heal on its own, Corteselli says. The scarring also tends to occur in the upper parts of a mouse's lungs, while in people, IPF usually begins deep in the lower regions. Many researchers study the lung's epithelial cells by growing them in a thin layer with a liquid medium underneath and air above. This has become the "gold standard" for research on the health effects of air pollution, Corteselli says. But that model doesn't include any fibroblasts, and it's difficult to study pulmonary fibrosis without fibroblasts.

Other, more advanced systems come closer but still aren't quite right. Three-dimensional organoids, for example, create miniature versions of lung tissue but don't fully mimic the structure of the air-lung interface. Precision-cut lung slices, thin slices of tissue removed from lungs, preserve some of the lung's cell architecture, are typically kept entirely in air or submerged in liquid. These conditions, again, don't reflect how these cells interact with air in a living body.

With her NC TraCS pilot award, Corteselli combined many of the best features of these existing technologies to build a new model for studying the lung. In her design, a layer of epithelial cells is grown in a dish, exposed on top to the air. Below that is a layer of fibroblasts submerged in a liquid medium, with no exposure to the air. Using this setup, researchers can spray a medication over the epithelial cells—just as it would be inhaled—and watch what happens.

After developing the model, Corteselli and her research team doused it with a pro-fibrotic cocktail, a mixture of chemicals that makes the cells behave like those from a person with IPF. They then tested pirfenidone in two ways: by spraying a nebulized version of the drug over the epithelial layer and by adding the drug into the liquid medium where fibroblasts grow, mimicking how the drugs might reach the lungs through the bloodstream.

They found that the aerosolized version of pirfenidone could pass through the epithelial layer and reach the fibroblasts, reversing some of the pro-fibrotic gene expression induced by the cocktail. The fibroblasts received higher concentrations of the drug when it was added to the liquid medium, but that may be because the aerosolized version was applied twice, while the liquid exposure was continuous.

As researchers continue to assess whether inhaled pirfenidone can help patients with IPF, insights from Corteselli's model will be crucial. She also hopes researchers will use this system to study other respiratory health issues and environmental exposures, such as chronic obstructive pulmonary disease (COPD).

"In order for pre-clinical models and in vitro models to accurately predict what's going to happen when we give someone a drug," Corteselli says, "we need those models to be the best they can be—to truly represent both the disease and the organ, and to include all the right components."