Some respiratory cells are especially vulnerable to coronavirus because they are especially well endowed with two transmembrane proteins, ACE2 and TMPRSS2. ACE2, a receptor protein, engages with the coronavirus’ spike protein, and TMPRSS2, a protease, cleaves the spike protein, allowing the viral and cellular membranes to fuse. ACE2 and TMPRSS2, a new single-cell analysis has determined, are expressed more abundantly in certain progenitor cells. These cells normally develop into respiratory tract cells lined with hair-like projections called cilia that sweep mucus and bacteria out of the lungs.

To distinguish cells of this type from other, vastly more abundant cells representing other cell types, scientists working under the auspices of the German Center for Lung Research examined samples from non-virus infected patients. Working cell by cell, the scientists used RNA sequencing to measure ACE2 and TMPRSS2 expression levels.

“We analyzed a total of nearly 60,000 cells to determine whether they activated the gene for the receptor and potential cofactors, thus in principle allowing them to be infected by the coronavirus,” reported Soeren Lukassen, a scientist affiliated with the Berlin Institute of Health (BIH) and Charité—Universitätsmedizin Berlin. “We only found the gene transcripts for ACE2 and for the cofactor TMPRSS2 in very few cells, and only in very small numbers.”

Lukassen is one of the lead authors of an article that appeared in the EMBO Journal. The article, entitled “SARS‐CoV‐2 receptor ACE2 and TMPRSS2 are primarily expressed in bronchial transient secretory cells,” may inform investigators interested in countering coronavirus infection and pathogenesis.

“Here, we investigate ACE2 and TMPRSS2 expression levels and their distribution across cell types in lung tissue (twelve donors, 39,778 cells) and in cells derived from subsegmental bronchial branches (four donors, 17,521 cells) by single-nuclei and single-cell RNA sequencing, respectively,” the article’s authors wrote. “While TMPRSS2 is expressed in both tissues, in the subsegmental bronchial branches ACE2 is predominantly expressed in a transient secretory cell type. Interestingly, these transiently differentiating cells show an enrichment for pathways related to RHO GTPase function and viral processes suggesting increased vulnerability for SARS‐CoV‐2 infection.”

Study participants representing the University of Heidelberg initially intended to study why lung cancer sometimes occurs in people who have never smoked. They began by analyzing samples of twelve lung cancer patients. These samples were obtained from the Heidelberg Lung Biobank and came from both the cancerous part of the lungs and the surrounding healthy lung tissue. They also studied cells from the airways of healthy patients, which had been collected in a minimally invasive manner during a bronchoscopy examination performed to rule out lung cancer.

The rapidly spreading coronavirus prompted the researchers to take another look at these existing but so far unpublished data. “I was convinced that the data we gathered from these non-coronavirus infected patients would provide important information for understanding the viral infection,” said Roland Eils, PhD, a professor of bioinformatics at the University of Heidelberg and a founding director of the BIH Digital Health Center.

The scientists who participated in the current study knew from previous studies that for coronavirus infection to occur, the virus’ spike protein must engage an ACE2 receptor on the cell surface, and that in addition, the virus needs one or more cofactors. Otherwise, it cannot penetrate cells.

“[But we also] wanted to find out which specific cells the coronavirus attacks,” emphasized Christian Conrad, PhD, another co-author of the current study and, like Eils, a researcher at the BIH Digital Health Center.

After conducting single-cell sequencing, the scientists discovered that certain progenitor cells in the bronchi are mainly responsible for producing the coronavirus receptors. “Armed with the knowledge of which cells are attacked, we can now develop targeted therapies,” asserted the study’s corresponding author, Michael Kreuter, MD, from the Thorax Clinic at Heidelberg University Hospital.

An interesting additional finding of the study was that the ACE2 receptor density on the cells increased with age and was generally higher in men than in women. “This was only a trend, but it could explain why SARS-CoV-2 has infected more men than women,” Eils noted. “Our sample sizes are still much too small to make conclusive statements, so we need to repeat the study in larger patient cohorts.”

“These results show us that the virus acts in a highly selective manner, and that it is dependent on certain human cells in order to spread and replicate,” Eils continued. “The better we understand the interaction between the virus and its host, the better we will be able to develop effective counterstrategies.”

He and the other researchers will next study COVID-19 patients to ascertain whether the virus has actually infected these cells. “We want to understand why the infection takes a benign course in some patients, while causing severe disease in others,” Eils stressed. “So, we will also look closely at the immune cells in the infected tissue.”