In a study published in the recently published journal Cell, Dr. Peter Jackson, professor of pathology, microbiology, and immunology at Stanford University in the United States, has pinpointed the way COVID-19 enters and leaves nasal cells. Researchers said that the upper respiratory tract is not only the source of lung infection but also the source of transmission to others. Facts have proved that inhibiting the virus in and out of respiratory cells is effective in reducing the transmission of highly infectious COVID-19.

The epithelial tissue of the nasal cavity and respiratory tract is mainly composed of three cell types: basal cells, goblet cells, and poly ciliary cells, which account for about 80% of the total number of nasal epithelial cells. Multiciliated cells form a protective barrier to prevent the virus from entering the respiratory tract. The researchers magnified two structures found on poly ciliated epithelial cells: cilia and microvilli.

Researchers used a complex tissue culture method to produce respiratory epithelial organs to imitate the normal respiratory tract. Although lacking blood vessels and immune cells, these organs completely cover the structure of nasal mucosa epithelium in other aspects, including intact mucus layer and well-developed ciliated cells.

The researchers put the cultured organ like COVID-19 in the same culture dish. The electron microscope showed that the virus was initially only attached to the cilia. After incubation with COVID-19 for 6 hours, many virus particles from the tip down spread on both sides of the cilia. Even after 24 hours, the virus only replicates in a few cells. Mass replication takes 48 hours.

The study found that reducing the level of a protein crucial to cilia formation in nasal epithelial cells can significantly slow down the infection of COVID-19, which proves that human ciliated nasal epithelial cells are the main entry site of COVID-19 in nasal epithelial tissue.

The researchers precisely located the enzymes in the cells. These enzymes were activated in large quantities after infection with COVID-19, resulting in the microvilli becoming a huge, branching tree structure to which virus particles were attached. These viruses can be pushed into the mucus-mucin layer, where they can "float" along the mucus and infect other cells farther away. Inhibiting these enzymes stops the mutation and greatly reduces the spread of the virus to other cells.

These findings have identified new targets for the development of nasal drugs, which can prevent the movement of cilia or the enlargement of microvilli to prevent even unknown respiratory virus infections.