COVID-19: myeloid-derived suppressor cells impact impact disease severity


Individual variations in how the immune system responds to SARS-CoV-2 appear to impact the severity of disease. Researchers at Karolinska Institutet in Sweden have now been able to show that patients with severe COVID-19 have significantly elevated levels of a certain type of immune cells in their blood, called myeloid-derived suppressor cells.

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

Most individuals with COVID-19 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 of disease varies so much between patients.

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

Dampens T cell activity

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 study consisted of 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.

Unbalanced immune system

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.

There is also a strong clinical connection, as you could potentially use the results to find new biomarkers for severe illness,” says Anna Smed Sörensen, associate professor at the Department of Medicine, Solna, Karolinska Institutet, and the study’s last author.

An inherent limitation of the study is the number of patients and amount of sample material that could be collected, why each sample was used as efficiently as possible.

“The next step in our research is to further study the connection between different parts of the immune system, such as M-MDSC, T cells, and antibodies,” says Anna Smed Sörensen.

The clinical presentations of Covid-19 range from asymptomatic, mild, moderate to severe pneumonia and fulminant disease1.

The immunological mechanisms underlying the heterogeneous clinical expression of SARS-CoV-2 infection and those underlying factors influencing the clinical outcome remain to be defined. Tissue damage has been associated with excessive and uncontrolled immune activation and pro-inflammatory cytokines2,3.

A massive infiltration of mononuclear cells has been detected in the lungs, with parallel low levels of hyperactive T cells in the peripheral blood4. Moreover, depletion of lymphocytes and increase of neutrophils in the peripheral blood are changes typically associated with an unfavorable disease course.

In particular, it has been reported that the Lymphocyte/Neutrophil ratio is an independent risk factor of mortality for Covid-19 patients5–7. Notably, lymphocytes from patients with severe Covid-19 often present an exhausted phenotype8,9, and the macrophage activation syndrome has been described to occur in patients with respiratory failure10.

Together, these data indicate that SARS-CoV-2 infection associated with excessive and dysregulated immune activation, and massive migration of cells to the infected tissue, to control viral replication, could contribute to tissue damage.

During evolution, immune system has developed regulatory mechanisms able to control excessive inflammation/activation, such as the induction of inhibitory receptor expression, production of specialized anti-inflammatory cytokines, and expansion of regulatory cells. However, their role during SARS-CoV-2 infection is poorly understood.

We recently reported the expansion of myeloid-derived suppressor cells (MDSC) during SARS-CoV-2 infection, correlated with inflammatory milieu11,12. Whether MDSCs play a beneficial anti-inflammatory and/or a detrimental immune suppressive role during COVID-19 has not yet been elucidated.

MDSC, defined in humans as CD11b +CD14−CD33+CD15+ and HLADR-low (polymorphonuclear MDSCs), or CD11b+CD14+CD33+ and HLA-DR-low (monocytic MDSCs), are known to have the remarkable ability to reduce overt inflammation and suppress T-cell responses through several mechanisms, including inducible nitric oxide synthase (iNOS), arginase-1(Arg-1), nicotinamide adenine dinucleotide phosphate oxidase (NOX2), and transforming growth factor beta (TGF-β)13–16.

In the present work, we provide evidence that early PMN-MDSC expansion inhibits SARS-CoV-2 specific T-cell responses, and might predict fatal outcome.

High PMN-MDSC frequency is associated with SARS-CoV-2 fatal infection

T-cell response is central in the adaptive immune-mediated elimination of pathogens. Since we showed the detrimental impact of PMN-MDSC on T-cell response to SARS-CoV-2, we wondered whether PMN-MDSC frequency could be predictive of disease outcome. We retrospectively grouped patients into different groups, namely, those who recovered (survivors, n = 59) and those who succumbed to the disease (non-survivors, n = 19), and analyzed the relative PMN-MDSC frequency at the time of admission.

We found a significantly higher frequency of PMN-MDSC in the non-survivor compared with the survivor group (Fig. ​(Fig.4A),4A), suggesting that PMN-MDSC percentage could be predictive of disease outcome. To validate this hypothesis, we performed a ROC analysis and found that PMN-MDSC frequency at the admission could distinguish between survivors and non-survivors (Fig. ​(Fig.4B;4B; area under the curve 0.82, p < 0.0001), identifying a cut off value of 54.91% (77.8% and 78% of sensibility and specificity, respectively). This data were further confirmed by estimating the HR of death for MDSC in a Cox regression analysis adjusting for age and gender: the increase of 1% in PMN-MDSC frequency was independently associated with an augmentation of 3% of risk of fatal outcome (Table ​(Table11).

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Fig. 4
High PMN-MDSC frequency associate with COVID-19 fatal outcome.
A PMN-MDSC frequency at the time of admission grouped in survivors (n = 59) and non-survivors (n = 19). Results are shown as Box and Whiskers. Mann–Whitney test was applied, and a p value < 0.05 was considered significant. B Receiver operating characteristic (ROC) curve for the PMN-MDSC frequency as marker of COVID-19 fatal outcome.

Table 1 – Cox regression analysis.

MDSC, for each additional unit1.
Age, for each additional year1.020.991.060.179
Gender, Female vs Male0.780.272.240.644


We identified the PMN-MDSC frequency as an early marker of COVID-19 fatal outcome involved in the inhibition of SARS-CoV-2 specific T-cell response. The increased proportion of circulating MDSCs, mainly described in cancer, is attracting great interest in infectious diseases18–22 Although MDSC typically arise in in peripheral tissues to dampen inflammation and inflammation-induced tissue damage, a detrimental role of this cell population in cancer and infectious diseases has been widely reported21,23,24.

We recently found an expansion of PMN-MDSC in the peripheral blood of COVID-19 patients compared to HD, in particular in patients experiencing severe disease11,12. We observed an expansion of PMN-MDSC in non-ICU and ICU requiring patients, with the latter group presenting with a higher PMN-MDSC frequency.

Following the recommendation for MDSC nomenclature and characterization25, the cells phenotypically here identified as PMN-MDSC were confirmed to act as suppressive cells, since they were able to effectively inhibit T-cell proliferation.

It has been proposed that inflammation promotes MDSC expansion and up-regulation of their suppressive capability17. In COVID-19 patients, a hyper-inflammation has been described2,26 that possibly induced PMN-MDSC differentiation and expansion. Indeed, we observed a correlation between PMN-MDSC frequency and the plasma level of IL-1β, IL-6, IL-8, and TNF-α.

Interestingly, patients who will ultimately succumbed to SARS-CoV-2 infection showed a higher frequency of PMN-MDSC at the time of admission as compared to patients who ultimately survived, suggesting that PMN-MDSC frequency can be used as a prognostic marker of disease outcome.

The non-survivors PMN-MDSC frequency matched with a high IL-8 level at the admission time, which decreased at later time points, in parallel to the reducing trend of PMN-MDSC. Among the pro-inflammatory mediators, IL-8 has been linked to the recruitment of MDSCs27,28, suggesting that IL-8 could have a major role in PMN-MDSC maintenance.

Of note, in non-survivors a significant increase of IL-6 was found after 2 weeks from admission, suggesting that, the decrease of PMN-MDSC frequency could account for IL-6 rise. High IL-6 levels has been recently associated with fatal COVID-19 infection29, suggesting that immune suppression, possibly mediated by expanded MDSC, could be highly beneficial in reducing inflammation.

PMN-MDSC from COVID-19 patients expressed typical mRNA associated with MDSC suppressive functions such as ArgI, TGF-β, and iNOS, providing evidence that these cells exhibited a highly suppressive potential. Moreover, PMN-MDSC frequency correlated with the plasma level of TGF-β, indicating that PMN-MDSC contribute to TGF-β release, which, in turn, may act as a potent enhancer of the MDSC inhibition function30.

L-Arginine, nitric oxide, and TGFβ pathways have been reported to play a major role in the regulation of innate and adaptive immune response31–33. In particular, these molecules influence human T-cell proliferation, differentiation, and survival34. The importance of the specific T cells has been demonstrated for virus clearance, for limiting tissue damage, and dampening overactive innate immune responses35–37.

Two recently published studies demonstrated a SARS-CoV-2 specific T-cell response in convalescent/recovered patients38,39. However, timing, composition, magnitude, persistence, and protective role of T-cell response during COVID-19 are still to be fully elucidated.

Our data showed a low SARS-CoV-2 specific T-cell response during COVID-19, defined by IFN-γ production, which was increased by depleting MDSC, suggesting that the low T cell responses are due to in vivo suppressive activity, not associated with the absence of antigen-specific T cells. We also found that TGF-β and iNOS produced by PMN-MDSC mediated their suppressive activity. PMN-MDSC from COVID-19 patients also inhibit PHA-induced IFN-γ production, indicating a powerful suppressive activity on T cells.

The suppressive potential of expanded MDSC can inhibit also NK-cell function16. The observation of reduction in the granzime A content of NK cells in patients with COVID-1940 could therefore also be associated with the MDSC-induced suppression of NK activity. We have also reported a negative correlation between PMN-MDSC frequency and perforin expressing NK cells11, consolidating that PMN-MDSC are indeed involved in inhibiting the cytotoxic potential of NK cells.

The ROC analysis confirmed the detrimental role of PMN-MDSC during COVID-19, as their high frequency at early stage of the disease correlated with fatal disease outcome. A logistic regression analysis showed that patients with an early increased PMN-MDSC frequency had a high risk of mortality after adjustment for other cofounders. Altogether, our data indicate that a high PMN-MDSC frequency, even if reduces inflammation, can inhibit both adaptive and innate anti-viral immune response, thus preventing virus elimination and ultimately patients’ recovery.

In conclusion, despite the low number of patients analyzed in this report, this explorative study indicates new biologically and clinically relevant factors in the pathogenesis of SARS-CoV2 infection, namely the MDSC and TGF-β-mediated suppression of virus specific T cell function. Our data also highlights the rational for a possible use of therapeutic approaches focused on reducing MDSC number/function, thereby increasing anti-virus directed T-cell responses as a viable therapeutic approach for patients with cancer41,42.

reference link:

More information: Functional monocytic myeloid-derived suppressor cells increase in blood but not airways and predict COVID-19 severity. DOI: 10.1172/JCI44734.


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