A joint research team of Korea Advanced Institute for Science and Technology (KAIST) and Chungnam National University Hospital (CNUH) has identified the abnormality of natural killer (NK) cells in Covid-19 patients for the first time.

The symptom disappears in mild patients after a week but continues for a long time in severe patients. The researchers have revealed the functional abnormality of NK cells, which play a key role in innate immune response in severe Covid-19 patients. 

When the human body gets infected by a virus, an innate immune response happens as primary resistance. The NK cells are in charge of this antiviral innate immune response. Most of these NK cells are made up of cytotoxic molecules that kill cells infected by the virus. A decrease in the number or function of NK cell cytotoxicity in Covid-19 patients has been reported. 

However, specific changes in NK cells and a decrease in functional status had not been identified.

The joint team carried out a longitudinal study on Korean Covid-19 patients from early diagnosis to recovery in this research. As a result, the researchers identified changes in NK cells in the progression of Covid-19 infection for the first time. Also, they clarified the difference between the characteristics and functions of NK cells in severe and mild patients. Furthermore, advanced immunology research techniques and gene expression analysis were used to analyze and identify the changes in the NK cells of Covid-19 patients.

The researchers discovered typical abnormality characteristics found only in the NK cells of Covid-19 patients, different from healthy people or influenza patients. They also disclosed that these abnormal NK cells have reduced cytotoxicity compared to that of general NK cells.

These abnormal NK cells increase rapidly in the early stages of Covid-19 patients regardless of severity, weakening the innate immune response of Covid-19 patients. Also, through the longitudinal study of NK cells in Covid-19 progression, they found that the rapid increase of abnormal NK cells continues longer in severe Covid-19 patients related to the damage in the innate immune response.

It is the first discovery worldwide of an increase in the number of abnormal NK cells in Covid-19 patients, drawing attention from the immunology field for reporting the mechanism of damage in the innate immune response of Covid-19 patients.

The research was published in the Journal of Allergy and Clinical Immunology titled “Abnormality in the NK cell population is prolonged in severe Covid-19 patients.”

“We found an increase of abnormal NK cells in Covid-19 patients,” said Dr. Leem Ga-lam of KAIST, the lead author, “In other respiratory infections, this change in NK cells was not observed. It is understood that this is a typical characteristic of Covid-19 infection. This will provide a basis for giving preemptive treatment for severe Covid-19 patients.

Professor Kim Yeon-sook of CNUH said, “This longitudinal study on the progression of Covid-19 patients, from the early stages to recovery observing the characteristics and change NK cell, was the first in the world. It is significant by revealing the damage on the innate immune response of Covid-19 patients.”