Researchers have discovered that goblet and ciliated cells in the nose have high levels of the entry proteins that the COVID-19 coronavirus uses to get into our cells. The identification of these cells could help explain the high transmission rate of the virus.
Published in the journal Nature Medicine, the finding shows that cells in the eye and some other organs also contain the viral-entry proteins.
The study also predicts how a key entry protein is regulated with other immune system genes and reveals potential targets for the development of treatments to reduce transmission.
While it is known that the virus that causes COVID-19 disease, known as SARS-CoV-2, uses a similar mechanism to infect our cells as a related coronavirus that caused the 2003 SARS epidemic, the exact cell types involved in the nose had not previously been pinpointed.
To discover which cells could be involved in COVID-19 transmission, researchers analysed multiple Human Cell Atlas (HCA) consortium datasets of single-cell RNA sequencing, from more than 20 different tissues of non-infected people. These included cells from the lung, nasal cavity, eye, gut, heart, kidney and liver.
The researchers looked for which individual cells expressed both of two key entry proteins that are used by the COVID-19 virus to infect our cells. "We found that the receptor protein - ACE2 - and the TMPRSS2 protease that can activate SARS-CoV-2 entry are expressed in cells in different organs, including the cells on the inner lining of the nose," said study first author Dr Waradon Sungnak from Wellcome Sanger Institute in the UK.
"We then revealed that mucus-producing goblet cells and ciliated cells in the nose had the highest levels of both these COVID-19 virus proteins, of all cells in the airways. This makes these cells the most likely initial infection route for the virus," Sungnak added.
"Our findings are consistent with the rapid infection rates of the virus seen so far. The location of these cells on the surface of the inside of the nose make them highly accessible to the virus, and also may assist with transmission to other people," said study researcher Martijn Nawijn from the University Medical Center Groningen in the Netherlands.
According to the researchers, the two key entry proteins ACE2 and TMPRSS2 were also found in cells in the cornea of the eye and in the lining of the intestine. This suggests another possible route of infection via the eye and tear ducts, and also revealed a potential for faecal-oral transmission.
"By pinpointing the exact characteristics of every single cell type, the Human Cell Atlas is helping scientists to diagnose, monitor and treat diseases including COVID-19 in a completely new way," the researchers noted.