Researchers have characterized the specific ways in which SARS-CoV-2, the coronavirus that causes COVID-19, infects the nasal cavity, bronchia, and lungs.

The findings, published in the journal Cell, suggest that the virus primarily establishes in the nasal cavity, but when aspirated into the lungs, it may cause more serious, or potentially fatal pneumonia.

Researchers infected cells from different parts of the human airway with the virus isolated from different patients. They found that the virus was able to highly infect the cells lining the nasal passages. It was less likely to infect the cells lining the throat and bronchia, with the lowest infectivity in lung cells. The cell surface receptor, ACE2, which is the receptor that the virus uses to enter cells, was more abundant on nasal-lining cells rather than lower airway cells. This could be explain, in part, why upper airway nasal-lining cells were more susceptible to infection. Other experiments focused on the proteins TMPRSS2 and furin that can also help the virus enter into cells.

Other airway-lining cells called epithelial cells, and to a limited extent the pneumocyte lung cell, can help transfer inhaled oxygen into the bloodstream. But the virus infects almost no other airway cells, according to the study.

The virus also did not infect airway-lining cells called club cells. And similar types of airway epithelial cells from different human donors, especially lower-airway epithelial cells, varied significantly in their susceptibility to infection. Such findings suggest that there are undiscovered factors in airway cells that influence the course of infection. The study was partly funded by NHLBI.