The analysis of samples of three types of salivary gland obtained in a minimally invasive autopsy procedure performed on patients who died from complications of the COVID-19 in the hospital complex of the Faculty of Medicine of the University of São Paulo (FM-USP), showed that tissues specialized in the production and secretion of saliva function as reservoirs for the novo coronavírus SARS-CoV-2.
In the study, already published in the Journal of Pathology, researchers show that SARS-CoV-2 infects and replicates in salivary glands, which helps explain the abundance of coronaviruses in saliva and which led scientists to develop saliva-based diagnostic tests.
“This is the first study on the ability of a respiratory virus to infect and replicate in salivary glands. Until now, it was thought that only viruses that cause highly prevalent diseases, such as herpes, used salivary glands as reservoirs. The discovery may help explain why SARS-CoV-2 is so infectious,” Bruno Fernandes Matuck, from the USP Dentistry School and first author of the article, told the São Paulo State Research Foundation Agency.
A previous study by the same team of investigators had already demonstrated the presence of SARS-CoV-2 RNA in periodontal tissue from patients who died of COVID-19.
As SARS-CoV-2 is highly infectious compared to other respiratory viruses, the researchers hypothesized that the coronavirus could replicate in salivary gland cells and therefore be present in saliva without coming into contact with secretions. nasal and pulmonary. Previous research has detected ACE2 receptors in salivary gland ducts. The “spike” protein in SARS-CoV-2 binds to ACE2 to invade and infect cells. More recently, other research groups have carried out studies in animals showing that other receptors, in addition to ACE2, such as transmembrane serine protease 2 and furin, both present in the salivary glands, are targets of SARS-CoV-2.
To test this hypothesis in humans, ultrasound-guided autopsies were performed on 24 patients who died from COVID-19, with a mean age of 53 years, to extract tissue samples from the parotid, submandibular, and minor salivary glands.
Tissue samples were subjected to molecular analysis (RT-PCR), which detected the presence of virus in more than two thirds. Immunohistochemistry – a form of immunostaining in which antibodies bind to the antigen in the tissue sample, a dye is activated and the antigen can then be seen under a microscope – also demonstrated the presence of the virus in the tissue. Finally, examination under an electron microscope detected not only the presence of the virus, but also its replication in cells and the type of organelle it uses to replicate itself.
“We observed several viruses clump together in the cells of the salivary glands, which showed that they were there replicating. They weren’t passively in these cells,” said Bruno Matuck.
Mouth as a direct gateway for coronavirus
The researchers now plan to see if the mouth can be a direct entry point for SARS-CoV-2, as ACE2 and transmembrane serine protease 2 are found in various parts of the cavity, as well as in gingival tissue and oral mucosa. . In addition, the mouth has a larger contact area than the nasal cavity, which is widely considered the main entry point for the virus.
Luiz Fernando Ferraz da Silva, principal investigator on the study, said the team will partner “with researchers at the University of North Carolina, in the United States, to map the distribution of receptors in the mouth and quantify viral replication in oral tissues” , given that “the mouth may be a viable means for the virus to enter the body directly”.
The researchers also want to know if older people have more ACE2 receptors in their mouths than younger people, given the decrease in salivary secretion with age. However, researchers found a high viral load even in older patients, who have less salivary tissue. “These patients had almost no salivary tissue, just adipose tissue. Even so, the viral load was relatively high,” explained researcher Bruno Matuck.