Researchers at the Karolinska Institutet in Sweden have shown through their study that Covid-19 patients with severe symptoms have significantly elevated levels of a certain type of immune cell in their blood called myeloid-derived suppressor cells.

The study, published in the Journal of Clinical Investigation , intends to bring an increased understanding of how early immune responses impact disease severity.

The researchers noted that most Covid-19 positive individuals develop mild to moderate symptoms and recover without needing hospital treatment. In severe cases, however, Covid-19 can lead to respiratory failure or even death. It is not yet known why the severity varies to extremes in patients.

Researchers at the Karolinska Institutet, Karolinska University Hospital, Stemirna Therapeutics in Shanghai, and Stanford University in the US have now studied one type of immune cell, monocytic myeloid-derived suppressor cells, or M-MDSC, and their potential role in Covid-19.

The researchers wrote: T cells are part of the immune system and play an important part in the body's protection against viral infections such as Covid-19. M-MDSCs have been shown to increase in other inflammatory conditions, and their suppressive effect on T cell activity has been established.

The role of M-MDSC in respiratory infections, however, is largely unknown. Since low levels of T cells are a hallmark of Covid-19, it is of interest to understand the role of M-MDSCs in this disease, the authors of the study added.

For the research, the team involved 147 patients with mild to fatal Covid-19, who were sampled repeatedly from blood and the respiratory tract. These were then compared with patients with influenza and healthy individuals.

The results show that patients with severe Covid-19 have significantly elevated levels of M-MDSCs in blood compared with milder cases and healthy individuals. Covid-19 patients had fewer T cells in blood than healthy subjects, and they showed signs of impaired function.

The analysis also showed that the levels of M-MDSCs early in the course of disease seemed to reflect subsequent disease severity.

"Our results help increase the understanding of what causes severe Covid-19 and is an important piece of the puzzle in understanding the connection between the early, innate immune system, which includes M-MDSC, and the later, adaptive immune system, which includes T cells,” says Anna Smed Sorensen, associate professor at the Department of Medicine, Solna, Karolinska Institutet, and the study's last author.

She added: “There is also a strong clinical connection, as you could potentially use the results to find new biomarkers for severe illness."

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