Disruption in lung cell repair may underlie acute respiratory distress syndrome in COVID-19 and other respiratory diseases

A study suggests potential avenues of research to develop new therapies to improve survival and prevent chronic lung disease in COVID-19 and non-COVID-19 acute respiratory distress syndrome.

11:28 AM

Author | Eileen Leahy

lungs drawn in blue with black background and red cells inside
Getty Images

Investigators studying lung cells have discovered that the normal repair process that occurs after lung disease or injury appears to be incomplete but still ongoing in patients who died of COVID-19 and non-COVID acute respiratory distress syndrome. In patients who survive but develop scarring in the lungs, it appears that the repair process is permanently arrested, leading to chronic fibrotic lung disease. These findings may lead to novel therapies to promote healthy regeneration to increase survival and prevent fibrosis, they report in The American Journal of Pathology, published by Elsevier.

"We wanted to understand whether incomplete or impaired repair of the damaged lung might contribute to the high mortality rates from COVID-19 and non-COVID ARDS," explained lead investigator Rachel L. Zemans, MD, Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine and Program in Cellular and Molecular Biology, School of Medicine, University of Michigan, Ann Arbor, MI, USA. "We also wanted to understand why some survivors of ARDS from COVID-19 have normal lungs, but others have scarring that leads to lifelong symptoms."

The alveolar epithelium in the lungs is made up of two types of cells. Type 1 alveolar epithelial cells (AEC1) are flat and broad and cover most of the alveolar surface. They play a critical role in barrier integrity and facilitate efficient oxygen absorption. Type 2 alveolar endothelial cells (AEC2) are small cuboidal cells that cover the rest of the surface. They produce a pulmonary surfactant to inflate the lungs and remove the fluid. AEC1 and AEC2 are damaged in ARDS due to COVID-19 or other causes. It is known that during lung injury in mice, AEC2 proliferate, exit the cell cycle, and enter a transitional state before changing into AEC1 to repair the alveolar epithelium. In humans with idiopathic pulmonary fibrosis (IPF), AEC2 never leave the transitional state, and change into AEC1, leading to the development of scar tissue known as fibrosis. The state of epithelial injury and regeneration in COVID-19 and non-COVID-19 ARDS without fibrosis had not been well characterized.

The investigators recovered lung tissue from the autopsies of patients who died of COVID-19 or non-COVID-19 ARDS within two weeks of hospitalization. They were compared with patients with IPF. The tissue was examined for evidence of AEC2 proliferation, transitional cells, AEC1 differentiation, indications of the loss of the ability to divide (senescence), and fibrosis. Investigators also compared the gene expression profiles of transitional cells in two mouse models of physiological regeneration without fibrosis, early human COVID-19 and non-COVID-19 ARDS, and human IPF.

The early ARDS lungs had extensive epithelial damage and a regenerative response in which ACE2 proliferated and entered the transitional state. The transitional cells occasionally assumed a flat AEC1 morphology but rarely expressed AEC1 markers. In contrast to patients with IPF, these lungs had not yet developed fibrosis.

"It appears that the stem cells that repair the lungs are able to begin the repair process and assume a 'transitional' or intermediate state," said Dr. Zemans. "However, they have not completely regenerated the cells that were damaged during the injury, leading to acute respiratory failure."

The investigators propose that in COVID-19 survivors who recover normal lungs, the transitional cells ultimately regenerate the damaged cells. However, in survivors who develop scarred lungs, AEC1 are never regenerated; cells are stuck in the transitional state which can lead to scarring and lifelong respiratory impairment.

"The COVID-19 pandemic has focused attention on severe lung injury, and this new work uses COVID-19 cases to generate a new perspective on lung injury and repair in ARDS. The results could provide clues about new therapeutic approaches to promote normal repair and/or prevent fibrosis," observed by Thomas R. Martin, MD, Division of Pulmonary, Critical Care and Sleep Medicine, Department of Medicine, University of Washington School of Medicine, Seattle, WA, USA, in an accompanying commentary. "The results also could be useful in evaluating whether currently available therapies for chronic fibrosis might be helpful early after the onset of acute lung injury."

The study's first author Christopher Ting, MD, Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Michigan, added, "As a pulmonary and critical care fellow training during COVID-19, I have seen the pandemic highlight a disease process that we have made frustratingly slow progress on. I hope that pursuing the questions our work poses will continue to shed light on how the lung may heal or not heal after injury."

Paper cited: "Fatal COVID-19 and Non-COVID-19 Acute Respiratory Distress Syndrome Is Associated with Incomplete Alveolar Type 1 Epithelial Cell Differentiation from the Transitional State without Fibrosis," The American Journal of Pathology. DOI: 10.1016/j.ajpath.2021.11.014

This study was supported by grants from the National Heart Lung and Blood Institute (grants R01HL131608, R01HL147920, R01HL1127203, and T32HL007749).

This article was originally published by Elsevier.

More Articles About: Lab Report All Research Topics Lungs and Breathing
Health Lab word mark overlaying blue cells
Health Lab

Explore a variety of health care news & stories by visiting the Health Lab home page for more articles.

Media Contact Public Relations

Department of Communication at Michigan Medicine

[email protected]


Stay Informed

Want top health & research news weekly? Sign up for Health Lab’s newsletters today!

Featured News & Stories Doctors trying to unclot a clogged artery
Health Lab
Subset of COVID-19 Patients Have Increased Bleeding Risk
A new potential biomarker raises concerns over the current standard for treating COVID-19 induced blood clots with high dose blood thinners.
graphic of pink orange skin tissue cell
Health Lab
National Research Effort Discovers Relationship Between Inflammation, Metabolism and Scleroderma Scarring
Study finds NAD+ break down leads to multi organ scarring, providing now a previously undiscovered pathogenic role of the enzyme CD38 in disease scarring.
purple-blue cells floating around on dark background
Health Lab
Exploring a New Weapon Against COVID-19 Cytokine Storm
Researchers have created an experimental device that, instead of inhibiting inflammatory proteins in COVID-19 patients, changes the phenotype of circulating white blood cells, helping wean two patients off ECMO.
Health Lab
Bacteria in Healthy Lungs Linked to Immunity
While gut bacteria have been credited with boosting the body’s immune system, bacteria in the lungs may play a more direct role in protecting their own backyard.
Health Lab
Bacteria in Your Lungs? New Microbiome Study Shows How They Get There
Read about a new lung microbiome study reviewing new research on bacteria in lungs and how it got there, and what happens to the and our bodies as a result.
Health Lab
Discovery of Gut Bacteria in Critically Ill Lungs May Change ICU Care
University of Michigan researchers discover gut bacteria in critically ill lungs. Learn how this may influence illnesses such as sepsis and acute lung failure.