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Changes to immune cells in patients with COVID-19

A recent research paper published in the official BSI journal Immunology looked at how levels of various immune cells change in the blood of patients with different severities of COVID-19 and the crucial role of inflammation during SARS-CoV-2 infection.

Researchers from the Careggi University Hospital in Florence, Italy, studied blood samples of 40 patients infected with SARS-CoV-2 to measure the levels of different immune cells and biological molecules.

Myeloid cells are a group of specialised immune cells that circulate in the blood and are associated with the innate immune system. There are a variety of myeloid cells, each with a unique role during the initial immune response to an infection. The researchers focused on the myeloid cells called dendritic cells, monocytes, and neutrophils.

Monocytes have inflammatory and microbe killing functions and neutrophils are the initial response cell in the immune system. One type of dendritic cell is called a plasmacytoid dendritic cell (pDC), which promotes immune responses to viruses. In the presence of a virus, they release significant levels of signalling molecules, leading to a variety of immune mechanisms.

Proteins found on the surface of cells can be used as markers to understand functioning in an immune response. For example, some markers assist in the movement of the cells to infection sites or activate the cell.

What tests were carried out?

Forty patients diagnosed with COVID-19 agreed to participate in the research. They were categorised into three groups – patients admitted to an emergency department (ED), to an intensive care unit (ICU) and not admitted to an intensive care unit (non-ICU). These groups were linked to the severity of the patient’s symptoms. Eight healthy patients also took part in the research.

Blood was taken from each patient. The samples were then analysed in a series of laboratory experiments to identify the type of cells and surface proteins present.

What were the results of this study?

The researchers found blood levels of dendritic cells decreased significantly in patients with COVID-19. Levels of monocytes and neutrophils were unchanged in infected patients.

In the subsequent analysis of surface proteins, certain makers were found elevated on monocytes and neutrophils in patients with COVID-19 compared with healthy patients.

The severity of COVID-19 symptoms was associated with:

  • Levels of different cell types in blood – ICU patients had significantly lower blood pDC levels than ED patients.
  • Expression of cell surface proteins – certain markers were expressed more in immune cells of ICU patients compared to non‐ICU patients and significantly more when compared to healthy patients.
  • Levels of monocyte subsets in blood
  • Levels of inflammation markers – blood serum levels of signalling molecules, including C‐reactive protein, increased with symptom severity.

What can we conclude from this study?

The research shows that SARS-CoV-2 infection affects myeloid immune cells and markers of inflammation. The monocyte and neutrophil cell surface markers that were expressed more in patients with COVID-19 assist in the activation of each immune cell. This suggests that inflammation has a crucial role in the impaired immune response seen in patients with COVID-19. It will be important to further understand how the alterations in levels of myeloid cells and their activation impacts severity of disease.


Peruzzi, B., Bencini, S., Capone, M., Mazzoni, A., Maggi, L., Salvati, L., Vanni, A., Orazzini, C., Nozzoli, C., Morettini, A., Poggesi, L., Pieralli, F., Peris, A., Bartoloni, A., Vannucchi, A.M., Liotta, F., Caporale, R., Cosmi, L. and Annunziato, F. (2020) Quantitative and qualitative alterations of circulating myeloid cells and plasmacytoid DC in SARS‐CoV‐2 infection. Immunology. doi: 10.1111/imm.13254

First published 01 September 2020

Summary author Matt Perryman, BSI Editorial Co-ordinator