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Discovery by Wellcome Sanger Institute scientists may explain why Covid-19 leads to taste loss for some patients

Scientists have shown that the Covid-19 virus can infect specific cells in the salivary gland in the mouth in a study that could aid efforts to reduce transmission within and outside the body.

The research, involving the Wellcome Sanger Institute and collaborators, also revealed that live cells from the mouth were found in saliva and the virus was able to reproduce within these infected cells.

The study could help explain some of the oral symptoms experienced by Covid-19 patients, including taste loss, dry mouth and blistering.

Prof Kevin M Byrd, joint lead author on the study and a co-ordinator of the HCA Oral & Craniofacial Biological Network, who carried out the work at the University of North Carolina, said: “The study’s findings suggest that the mouth, via infected oral cells, plays a bigger role in SARS-CoV-2 infection than previously thought.

“When infected saliva is swallowed or tiny particles of it are inhaled, we think it can potentially transmit SARS-CoV-2 further into our throats, our lungs, or even our guts.”

Earlier studies have shown that cells in the nose and lung contain high levels of RNA for key proteins that allow the SARS-CoV-2 virus to enter cells. But the role of the mouth in Covid-19 transmission has been poorly understood. While it was known that saliva of people with Covid-19 can contain the virus, it was unclear if mouth cells were involved.

Researchers used single cell RNA sequencing technology and bioinformatics method to examine healthy volunteers for cells that expressed two key entry proteins – ACE2 and the TMPRSS2 protease – which SARS-CoV-2 uses to infect human cells. They found salivary gland ductal cells and some gingival, or gum, cells expressed both proteins and were therefore vulnerable to infection.

The virus’ RNA was also found in salivary gland cells from Covid-19 patients and there was evidence that the virus was replicating in some of these cells.

And when saliva from eight people with mild or asymptomatic cases was added to monkey cells grown in dishes, some of these cells became infected. This raises the possibility that even people without symptoms might transmit infectious SARS-CoV-2 to others through saliva.

It is hoped the work could open up new investigative avenues leading to a better understanding of the course of infection and disease.

Dr Sarah Teichmann in front of the computer cluster at Sanger. Picture: Pari Naderi
Dr Sarah Teichmann in front of the computer cluster at Sanger. Picture: Pari Naderi

The work was carried out as part of the global Human Cell Atlas consortium, which aims to create reference maps of all human cells.

Dr Sarah Teichmann, a senior author from the Wellcome Sanger Institute and co-chair of the Human Cell Atlas organising committee, said: “Human Cell Atlas data is being used to understand Covid-19 and identify which of our cells are critical for initial infection and transmission.

“This first integrated adult Human Oral Cell Atlas is openly available, to help understand SARS-Cov-2 transmission and inform preventative measures to reduce the spread of this coronavirus. The global Human Cell Atlas community will continue to investigate cells and targets likely to be involved in Covid-19.”

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