Mucus Protects Lungs by Forming Layers, Preventing Worsening of Lung Diseases, Studies Reveal

Two new studies highlight the important role of mucus in cleaning and protecting the lungs, and how its accumulation and impaired clearance can contribute to the worsening of lung diseases such as chronic obstructive pulmonary disease (COPD).

In the studies, researchers show how mucus in the lungs forms a protective layer to trap and prevent bacteria from getting in contact with the lung epithelium.

They also emphasize that this mucus eventually needs to be cleared from the body — likely the reason why, after a cold, people develop a cough to expel mucus. Failure to get rid of this mucus means that it becomes stagnant allowing bacteria trapped in the mucus to grow, which can promote the worsening of lung diseases such as COPD.

“This is new knowledge, but there are no instant cures to be found yet. But to make progress, it’s important to understand how the mucus clearance works,” Gunnar C. Hansson, MD, PhD, professor of medical biochemistry at Sahlgrenska Academy, University of Gothenburg, and the studies’ lead author, said in a press release.

In the normal human lung, mucus is produced largely by glands that lead to the formation of mucus bundles. These mucus bundles, with the help of cilia — hair-like structures that stem from the surface of certain airway cells — sweep the airways to remove debris and bacteria.

In one of the studies, titled “Attached stratified mucus separates bacteria from the epithelial cells in COPD lungs,” published in the Journal of Clinical Investigation (JCI) Insight, researchers compared the composition of bronchoalveolar lavages (a procedure by which cells and other components from the airways are collected for analysis) from COPD patients and mucus collected from a mouse model of COPD.

Results showed that the composition of both types of samples were similar. Moreover, both had increased levels of several proteins known to be present in the protective mucus layer in the large intestine.

Researchers also found that, in both COPD patients and the mouse model, the mucus accumulated in layers, closely resembling the mucus structure in the intestine. This layered mucus in the intestines serves as a barrier to protect the intestinal epithelium from bacteria.

In the lungs, researchers found, the principle is maintained. When they exposed the COPD mouse model to the bacteria Pseudomonas aeruginosa, they saw that, without a mucus layer, the bacteria were in direct contact with the epithelial cells.

“It is then reasonable to believe that the tracheobronchial airway mucus layer may act as an innate defense mechanism aimed at trapping and keeping the bacteria away from the host epithelial cell surface,” the researchers wrote.

While in a normal lung epithelium, the mucus is eventually released by coughing, in patients with COPD or diseases such as cystic fibrosis, the mucus remains attached to the lungs. As a result, while initially offering a protective layer, this mucus then accumulates bacteria that may cause damage to the lungs.

In a second study, published in the European Respiratory Journal, researchers showed that inhalation of the spray Atrovent (ipratropium bromide), a common medication used in COPD and cystic fibrosis, is a way to keep mucus bundles moving, preventing it from accumulating.

The study is titled “The mucus bundles responsible for airway cleaning are retained in cystic fibrosis and by cholinergic stimulation.”

“Our observations explain some of the beneficial effects of this inhaled drug in the treatment of COPD,” said Anna Ermund, PhD, the study’s first author.

According to the team, these findings on the function of mucus may contribute to developing new treatments for both acute and chronic lung diseases.