How scientists are uncovering the mysteries of ARDS

COVID-19 virus finds a weak spot in the lungs

COVID-19 is “a classic example of an attack on the alveolar epithelium,” Dr Martin said.

By chance, the virus evolved to recognize receptors in the epithelium, allowing it to enter and propagate. “To make matters worse, defense mechanisms in the body attack those dying epithelial cells because the virus is visible on the surface cells. So lymphocytes from the immune system and macrophages attack the outer walls and cause further damage.”

Other scientists agree about this general picture of ARDS. “Studies of human lung tissue support the notion that failure of alveolar repair and regeneration mechanisms underlie the chronic lung dysfunction that can result from ARD,” wrote researchers from Cedars-Sinai Medical Center, Los Angeles, California, and Icahn School of Medicine at Mount Sinai, New York, NY, in a 2022 report.

According to Dr Martin, researchers and clinicians have discovered a pair of strategies to help vanquish COVID-19: Control viral entry through antiviral medication and dampen the body’s inflammatory response via steroids.

Still, “although we’ve learned lessons from COVID-19, we’re not good at all at promoting repair,” Dr Martin said. While new drugs have dramatically improved treatment for lung diseases such as cystic fibrosis, he said, “we don’t have good examples of new therapies that promote repair in ARDS.”
 

Looking for a way to turn the tide of fluid buildup

Dr Zemans and colleagues have uncovered a crucial obstacle to repair: the failure of stem cells to fully differentiate and become functional alveolar epithelial cells.

Researchers only began to understand a few years ago that the stem cells go through a transitional stage from type 2 to type 1, which make up 98% of cells in the epithelial surface, Dr Zemans said. In patients with ARDS who don’t get better quickly, “it looks like the cells get hung up in this intermediate state. They can’t finish that regeneration.”

As a 2022 study by Dr Zemans and colleagues put it, this process can lead to “ongoing barrier permeability, noncardiogenic pulmonary edema, and ventilator dependence, and mortality.” In fact, she said, “when we look at the lungs of people who died of ARDS, their cells were all in that intermediate stage.”

The discovery of the intermediate state only came about because of new technology called single-cell RNA sequencing, she said. “Now, these transitional cells are being found in other organs.”

Why do the epithelial cells get only part way through the regeneration process? It’s not entirely clear, Dr Zemans said, but researcher are intrigued by the idea that “cross-talk” between cells is playing a role.

“When the cells are in that stage, they also activate neighboring cells, including inflammatory cells, like macrophages, and fibroblasts,” she said. “And once those cells become activated, they become pathologic. What we think is that those cells then can talk back to the epithelial cells and prevent the epithelial cells from finishing that differentiation. It’s really hard to snap out of that positive feedback loop.”

This interaction probably evolved “for a good reason,” she said, “but it also became pathologic.” If the cells stay in the intermediate stage too long, she said, fibrosis develops. “They have scar tissue that never goes away. It takes a lot of work to expand the lungs when they’re so stiff when they should be stretchy like a rubber band. Scar tissue also gets in the way of the oxygen absorption, so some people have low oxygen levels.”