New study shows cilia in human airways create vertical fluid flow to protect lungs from infection

Scientists from the University of Cambridge, in collaboration with the University of Strathclyde, have discovered that cilia—tiny hair-like structures lining the human airways—not only sweep mucus laterally but also generate vertical fluid movement that may help prevent infections in the lungs.

The study, published in Proceedings of the National Academy of Sciences (PNAS), was led by Dr Erika Causa and colleagues in Professor Pietro Cicuta’s group at Cambridge, with mathematical modelling by Dr Debasish Das from the University of Strathclyde. The research shows that ciliary motion creates both horizontal and vertical flows, forming what the authors describe as a “dynamic barrier” that may prevent bacteria, viruses and harmful particles from reaching airway lining cells.

Dr Das, from the Department of Mathematics and Statistics at the University of Strathclyde, said, “Our findings show that cilia don’t just move mucus along the airways; they also push fluid upward, away from the lung lining. Coordinated ciliary beating not only maintains respiratory health by clearing mucus but also provides a dynamic barrier against pathogen entry. This new understanding could lead to better treatments for lung conditions in which cilia don’t work properly.”

The research builds on the established understanding that cilia beat in a coordinated manner to move mucus and trapped particles toward the mouth. However, using real-time imaging of fluid flow within cultured human airway epithelia, the team at Cambridge observed that this coordinated beating also produces upward vertical flow, a feature that had not been previously documented.

To track this vertical movement, the researchers employed a method to ‘uncage’ fluorescent markers within human airway samples. This allowed them to visualise both the faster horizontal and the slower vertical flows resulting from ciliary movement.

To complement the experimental observations, Dr Das developed computational simulations using slender-body theory. These simulations revealed that vertical flow is dependent on synchronised ciliary beating. The flow was only present when cilia beat in coordination, such as in metachronal waves, and was absent when their motion was random, highlighting the importance of coordinated activity in the airway’s defence mechanism.

The findings may offer new insights into conditions such as cystic fibrosis and primary ciliary dyskinesia, in which ciliary dysfunction leads to impaired mucus clearance and increased infection risk. Previous studies have also shown that immobile cilia (ciliostasis) are associated with higher viral replication, including in pathogens like H3N2 influenza.

SARS-CoV-2, the virus responsible for COVID-19, has been observed to infect ciliated airway cells, often impairing their function. The resulting ciliary dysfunction may weaken airway defences and allow viral particles to reach deeper into the lung.