New research published in American Journal of Respiratory and Critical Care Medicine revealed that autopsied lungs of COVID-19 victims had high amounts of airway secretions in the small airways, specifically MUC5B mucins. This “widespread” airway occlusion due to mucus had not been previously reported in COVID-19 patients, the researchers say.

After analyzing autopsied lungs of patients who died of COVID-19, researchers found “high degrees of production of the secreted airway mucin MUC5B and moderate amounts of MUC5AC. Strikingly, they also found occlusion of ∼50% of the small airways by mucus, as well as widespread aberrant expression of MUC5B within microcysts in damaged alveolar parenchyma,” according to the study.

Scientists called the discovery of widespread small airway occlusion by mucus “somewhat surprising,” based on existing belief that COVID-related cough “has generally been reported to be nonproductive” and prior analyses failing to report the presence of airway mucus occlusion.

They added that the predominance of MUC5B rather than MUC5AC mucins was also significant. In most respiratory viral infections, such as rhinoviruses and influenza viruses, they say, MUC5AC is the more prevalent of the two. Researchers attribute this to evidence that “IL-1β increases expression of MUC5B moderately more than MUC5AC, whereas IL-13 increases expression of MUC5AC far more than MUC5B.”

A third finding of interest is the presence of numerous mucin-expressing microcysts in the lung parenchyma. Researchers liken this microcysts to “‘pods’ observed as a sequela of respiratory viral infections in mice and to the ‘honeycomb cysts’ of advanced idiopathic pulmonary fibrosis.”

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