Study Finds That SARS-CoV-2 Infects Mouth Cells And Implicates Saliva As A Potential Route Of SARS-CoV-2 Transmission!

A new study by researchers from multi-research intuitions led by the National Institutes of Health and the University of North Carolina at Chapel Hill has found alarming evidence that the SARS-CoV-2 coronavirus also infects cells in the mouth.


 
Although there were obvious signs of oral infection including taste loss, dry mouth and mucosal lesions such as ulcerations, enanthema and macules, so far the involvement of the oral cavity in COVID-19 has been poorly understood.
 
In order to address this, the study team generated and analyzed two single-cell RNA sequencing datasets of the human minor salivary glands and gingiva (9 samples, 13,824 cells), identifying 50 cell clusters.
 
Utilizing integrated cell normalization and annotation, the team classified 34 unique cell subpopulations between glands and gingiva. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral entry factors such as ACE2 and TMPRSS members were broadly enriched in epithelial cells of the glands and oral mucosae.
 
By further using orthogonal RNA and protein expression assessments, the study team confirmed SARS-CoV-2 infection in the glands and mucosae. Saliva from SARS-CoV-2-infected individuals harbored epithelial cells exhibiting ACE2 and TMPRSS expression and sustained SARS-CoV-2 infection. Acellular and cellular salivary fractions from asymptomatic individuals were found to transmit SARS-CoV-2 ex vivo. Matched nasopharyngeal and saliva samples displayed distinct viral shedding dynamics, and salivary viral burden correlated with COVID-19 symptoms, including taste loss. Upon recovery, this asymptomatic cohort exhibited sustained salivary IgG antibodies against SARS-CoV-2.
 
Collectively, these data show that the oral cavity is an important site for SARS-CoV-2 infection and implicate saliva as a potential route of SARS-CoV-2 transmission.
 
The study findings were published in the peer reviewed journal: Nature Medicine. https://www.nature.com/articles/s41591-021-01296-8
 
Although it is  well known that the upper airways and lungs are primary sites of SARS-CoV-2 infection, there are clues the virus can infect cells in other parts of the body, such as the digestive system, blood vessels, kidneys, and, as this new study shows, the mouth.
 
Importantly the potential of the virus to infect multiple areas of the body might help explain the wide-ranging symptoms experienced by COVID-19 patients, including oral symptoms such as taste loss, dry mouth, and blistering.
 
Furthermore, the findings point to the possibility that the mouth plays a role in transmitting SARS-CoV-2 to the lungs or digestive system via saliva laden with virus from infected oral cells.
 
Significantly, a better understanding of the mouth's involvement could inform strategies to reduce viral transmission within and outside the body. The team was led by researchers at the National Institutes of Health and the University of North Carolina at Chapel Hill.
 
NIDCR Director Ms Rena D'Souza, D.D.S., M.S., Ph.D told Thailand Medical News,  & quot;Due to NIH's all-hands-on-deck response to the pandemic, researchers at the National Institute of Dental and Craniofacial Research were able to quickly pivot and apply their expertise in oral biology and medicine to answering key questions about COVID-19. The power of this approach is exemplified by the efforts of this scientific team, who identified a likely role for the mouth in SARS-CoV-2 infection and transmission, a finding that adds to knowledge critical for combatting this disease."
 
The research was led by Blake M. Warner, D.D.S., Ph.D., M.P.H., assistant clinical investigator and chief of NIDCR's Salivary Disorders Unit, and Kevin M. Byrd, D.D.S., Ph.D., at the time an assistant professor in the Adams School of Dentistry at the University of North Carolina at Chapel Hill. Byrd is now an Anthony R. Volpe Research Scholar at the American Dental Association Science and Research Institute. Ni Huang, Ph.D., of the Wellcome Sanger Institute in Cambridge, U.K., and Paola Perez, Ph.D., of NIDCR, were co-first authors.
 
Scientists already know that the saliva of individuals with COVID-19 can contain high levels of SARS-CoV-2, and studies suggest that saliva testing is nearly as reliable as deep nasal swabbing for diagnosing COVID-19.
 
However what researchers don't entirely know, however, is where SARS-CoV-2 in the saliva comes from. In people with COVID-19 who have respiratory symptoms, virus in saliva possibly comes in part from nasal drainage or sputum coughed up from the lungs.
 
Interestingly according to Warner, that may not explain how the virus gets into the saliva of people who lack those respiratory symptoms.
 
Blake M. Warner, DDS, PhD, MPH, Assistant Clinical Investigator and Chief of Salivary Disorders Unit, National Institute of Dental and Craniofacial Research added, “Based on data from our laboratories, we suspected at least some of the virus in saliva could be coming from infected tissues in the mouth itself."
 
In order to explore this possibility, the research team surveyed oral tissues from healthy people to identify mouth regions susceptible to SARS-CoV-2 infection.
 
Typically vulnerable cells contain RNA instructions for making "entry proteins" that the virus needs to get into cells. RNA for two key entry proteins-known as the ACE2 receptor and the TMPRSS2 enzyme was found in certain cells of the salivary glands and tissues lining the oral cavity.
 
Also in a small portion of salivary gland and gingival (gum) cells, RNA for both ACE2 and TMPRSS2 was expressed in the same cells. This indicated increased vulnerability because the virus is thought to need both entry proteins to gain access to cells.
 
Warner further added, "The expression levels of the entry factors are similar to those in regions known to be susceptible to SARS-CoV-2 infection, such as the tissue lining the nasal passages of the upper airway."
 
Upon confirming that parts of the mouth are susceptible to SARS-CoV-2, the researchers then looked for evidence of infection in oral tissue samples from people with COVID-19.
 
In samples collected at NIH from COVID-19 patients who had died, SARS-CoV-2 RNA was present in just over half of the salivary glands examined.
 
Alarmingly in salivary gland tissue from one of the people who had died, as well as from a living person with acute COVID-19, the scientists detected specific sequences of viral RNA that indicated cells were actively making new copies of the virus--further bolstering the evidence for infection.
 
Upon finding evidence of oral tissue infection, the study team wondered whether those tissues could be a source of the virus in saliva. This appeared to be the case. In people with mild or asymptomatic COVID-19, cells shed from the mouth into saliva were found to contain SARS-CoV-2 RNA, as well as RNA for the entry proteins.
 
In order to determine if the virus in saliva is infectious, the researchers exposed saliva from eight people with asymptomatic COVID-19 to healthy cells grown in a dish. Saliva from two of the volunteers led to the infection of the healthy cells, raising the possibility that even people without symptoms might transmit infectious SARS-CoV-2 to others through saliva.
 
Lastly, to explore the relationship between oral symptoms and virus in saliva, the team collected saliva from a separate group of 35 NIH volunteers with mild or asymptomatic COVID-19. Of the 27 people who experienced symptoms, those with the virus in their saliva were more likely to report loss of taste and smell, suggesting that oral infection might underlie oral symptoms of COVID-19.
 
In all, the researchers 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.
 
Byrd added, "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."
 
Further research will be needed to confirm the findings in a larger group of people and to determine the exact nature of the mouth's involvement in SARS-CoV-2 infection and transmission within and outside the body.
 
Warner further added, "By revealing a potentially underappreciated role for the oral cavity in SARS-CoV-2 infection, our study could open up new investigative avenues leading to a better understanding of the course of infection and disease. Such information could also inform interventions to combat the virus and alleviate oral symptoms of COVID-19.”
 
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