key: cord-1028072-6tzawoc4
authors: Park, Soo-Jeung; Nam, Da-eun; Seong, Hae Chang; Hahn, Young S.
title: New Discovery of Myeloid-Derived Suppressor Cell’s Tale on Viral Infection and COVID-19
date: 2022-02-03
journal: Front Immunol
DOI: 10.3389/fimmu.2022.842535
sha: 45275eda509e4fbc372bc1b9d6db3bdb65819a1c
doc_id: 1028072
cord_uid: 6tzawoc4

Myeloid-derived suppressor cells (MDSCs) are generated under biological stress such as cancer, inflammatory tissue damage, and viral infection. In recent years, with occurrence of global infectious diseases, new discovery on MDSCs functions has been significantly expanded during viral infection and COVID-19. For a successful viral infection, pathogens viruses develop immune evasion strategies to avoid immune recognition. Numerous viruses induce the differentiation and expansion of MDSCs in order to suppress host immune responses including natural killer cells, antigen presenting cells, and T-cells. Moreover, MDSCs play an important role in regulation of immunopathogenesis by balancing viral infection and tissue damage. In this review article, we describe the overview of immunomodulation and genetic regulation of MDSCs during viral infection in the animal model and human studies. In addition, we include up-to-date review of role of MDSCs in SARS-CoV-2 infection and COVID-19. Finally, we discuss potential therapeutics targeting MDSCs.

Coronavirus Induced Disease 2019 (COVID-19) caused by Severe Acute Respiratory Syndrome Corona Virus (SARS-CoV)-2 started in the Hubei province, China in 2019 (1) . In the past two years, despite the efforts of many countries in response to the global pandemic situation, more than 260 million COVID-19 cases have been confirmed and over 5.2 million deaths have been reported to the World Health Organization (2) . Patients infected with SARS-CoV-2 show various pathologies ranging from asymptomatic to mild, moderate, severe, and fulminant symtoms. Among them, critical pathology is followed by complications such as respiratory failure, myocarditis, sepsis, and various organ failures (3, 4) .

It has been reported that the onset of COVID is not only directly affected by the virus, but also affected by immune responses such as cytokine storm syndrome, neutropenia, and lymphopenia (5, 6) . However, the main potential targets for therapeutic purposes in relation to these immune mechanisms still remain challenging. Reduced function and depletion of CD8 + T cells were observed in patients with severe symptoms of COVID-19 (7, 8) , CD4 + T cells have been reported to be important for patient recovery and protective immunity against SARS-CoV-2 (9, 10).

So far, immunological studies for COVID-19 individuals have mainly focused on innate and adaptive immune responses. Several studies report how the induction of MDSC and its role affect the progression of COVID-19 (11) (12) (13) (14) . Accordingly, studies confirm the role of the MDSC subset in severity of COVID-19 progression and provide potential therapeutic targets for COVID-19 (15) (16) (17) . Further research identifying new biomarkers could be critical in developing therapeutics for disease prevention and amelioration. In this review, we discuss the latest studies focusing on the immunoregulating properties of MDSC and with biomarkers that may influence the intervention in treatment of COVID-19.

Since the outbreak of COVID-19, numerous publications related to immune responses and immunopathogenesis have been reported. The pathophysiology of COVID-19 has been listed as a complex state according to age, sex, pregnancy, presence of underlying disease, etc. (18) (19) (20) . In the innate immune system, similar to Middle East respiratory syndrome (MERS)-CoV, SARS-CoV-2 modifies the signaling of TRAF3 and RNA sensor adapter molecules (MAVS) through proteins such as PLpro and inhibits type I interferon (IFN-1) production (21, 22) . Antagonism of IFN-1 production aids viral replication, promotes release of pyroptotic products, and induces additional inflammatory responses (23) . Pyroptosis is a form of programmed cell death within inflammatory cells (24) and is mediated by the production of IL-1b during SARS-CoV-2 infection (25) .

Severe COVID-19 patients showed impaired IFN-1 signaling compared to mild patients, and developed an inappropriate inflammatory state due to early delay in IFN-1 expression and activation of pro-inflammatory cytokines (IL-1, IL-6, IL-8, MCP-1, and CXCL-10) (26) (27) (28) . In addition, there were high viral titers and accumulation of monocyte-derived macrophages and neutrophils in the lungs. This condition leads to a systemic inflammatory response and cytokine storm syndrome via a massive release of cytokines. In addition, the risk of COVID-19 may increase or decrease differently depending on the number and activity of natural killer (NK) cells. Healthy children have been reported to have more NK cells than adults and the elderly (29) , which might explain why children are expected to have a better defense against SARS-CoV-2. According to a recent study, the number of NK cells in adults with severe COVID-19 was reported to be low, and the activated form of CD56 low NK cells increased to generate cytokines (30, 31) .

In terms of the adaptive immune system, inefficient innate immune responses in SARS-CoV-2 infection lead to dampen adaptive immune responses and exacerbate inflammation (32) . Pro-inflammatory cytokines induce the expansion of CD4 + and CD8 + T cells, decrease regulatory T, and lead to activation of Th1-type and B cells. When patients with underlying severe disease are infected with SARS-CoV-2, the number of lymphocytes is decreased while the blood levels of CD4, CD8, and regulatory T cells were also significantly lower than those in patients with mild disease. At this time, monocytes, macrophages, and T cells are accumulated in the lungs, and T cells migrate from the blood to these tissues to regulate the depletion of blood lymphocytes (23) . In addition, a recent study showed that the lymphocyte counts of children infected with SARS-CoV-2 remained at a steady normal level compared to adults, and thus had less negative effect on immunomodulation (33) .

Extensive studies on analyzing immune responses in COVID-19 patients show that the number of T or B lymphocytes, DC, NK cells, and HLA-DR high cells are reduced in patients with severe symptoms (34) (35) (36) . Additionally, severe COVID-19associated hyperinflammatory syndromes have been reported that they originate from a host innate immune response (37) . Studies of transcriptome, proteomic, and epigenomics have revealed a wide range of functional impairments, including marked neutrophil hyperactivation symptoms in severe COVID-19 (38) (39) (40) (41) . Collectively, COVID-19 caused by SARS-CoV-2 is associated with a failure of innate and adaptive immune system regulation due to changes in other immune cells associated with a decrease in adaptive T cells. Although the link between the immune systems is still only partially explained, studies on MDSC, have been risen significantly and may provide explanation for dysregulated immune responses in COVID-19.

Myeloid-derived suppressor cells (MDSCs) are defined as a heterogeneous population of immature bone marrow cells that suppress T cell responses, and was first described in a mouse model of lung cancer in 1987. Together with myeloid progenitor cells, they have the ability to suppress the immune responses at the forefront of viral infection (42) . It has been reported that these cells have changed research fields related to cancer, inflammation, and immune response over the past 30 years, and even serve as a marker for distinguishing disease progression (43) (44) (45) . MDSCs are mainly classified into two distinct groupneutrophilic/granulocytic (PMN)-MDSCs and monocytic (M)-MDSCs (46, 47) ( Table 1) . Granulocytic MDSCs have multilobed nuclei similar to polymorphonuclear cells, and monocytic MDSCs have a single, round nucleus; therefore, they look similar to monocytes (48) . The morphological heterogeneity of these cells depends on the expression of Gr1, and the Gr1-specific antibody binds to both Ly6G and Ly6C, which are myeloid lineage differentiation antigens. Granulocyte and monocyte MDSCs of mice have phenotypes of CD11b + Ly6G + Ly6C low and CD11b + Ly6G -Ly6C high , respectively (49) . Human MDSCs are mainly identified as phenotypic markers Lin − HLA-DR − CD33 + or CD11b + CD14 − CD33 + (46) . In early studies on MDSC, the target of MDSC-mediated suppression was mainly T cells. After that, research on MDSC was gradually expanded, and it was additionally reported that it modulates innate immune cells such as NK cells, dendritic cells (DCs), and macrophages as well as adaptive immune cells such as B cells (50) (51) (52) .

Soluble factors related to MDSC function include reactive oxygen species (ROS), inducible nitric oxide synthase (iNOS), and arginase-1. Each of these key mediators independently attenuates the host immune response. Granulocytic MDSCs mainly use ROS generated by NADPH oxidase to cause immunosuppression. Monocytic MDSCs use iNOS to generate nitric oxide (NO) (48, 53, 54) . iNOS nitrosylates T-cell receptor (TCR) together with arginase-1 to generate reactive nitrogenoxide species that inhibit T-cell or induce apoptosis (55) . Interestingly, both granulocytic and monocytic MDSCs utilize the action of arginase-1 to deplete L-arginine, which is required for T cell proliferation and function (56, 57) . Effects of T-helper (Th)1 and Th2 cytokines, such as interleukin (IL)-2, IL-4, IL-13, and interferon (IFN)-g on aginase-1 led to the identification of crosstalk between MDSCs and T cells (58) (59) (60) (61) .

MDSCs from hepatocellular carcinoma patients have been shown to inhibit NK cell cytotoxicity and IFN-g release (62) . This was an arginase-1 independent, contact-dependent suppression effect that required the expression of NKp30, a receptor for NK cells. In the case of MDSCs expanded from tumor mice, membrane-bound transforming growth factor-b1 (TGF-b1) inhibited NK cell cytotoxicity, IFN-g production, and the expression of the activating receptor NKG2D (63) . In addition, MDSCs cause differentiation of immature DCs in cancer and limit the immune response by inhibiting the antigen uptake ability of DCs (64) (65) (66) .

During viral infection, similar increases in PMN-and M-MDSCs are initially observed in acute and chronic infection models, but rapidly return to baseline levels in acute cases. In chronic infection, it has been reported that it takes quite a long time to return to the baseline level. In the case of the mouse acute infection model, the inhibitory activity of M-MDSC did not appear at any time point. In the case of chronic infection model, it became more prominent from the 7th day to the 14th day after infection and decreased on the 30th day, but it was still detected. Recently, it has been reported that the ER stress response is essential for the inhibitory activity of M-MDSC in viral infection, and it is known that the acquisition of the most potent inhibitory activity is mediated by IFN-g signaling (67) .

Selective mediators have been shown to be responsible for the generation of MDSCs. Prostaglandin E2 (PGE2) exerts numerous biological actions, including anti-inflammatory and pro-inflammatory and is a key mediator for MDSC generation. Administration of PGE2 blocks DC differentiation and allows myeloid progenitor cells to acquire the characteristics of MDSC (68) . In addition, anti-inflammatory mediators such as NOS2, iNOS, indolamine-2,3-deoxygenase (IDO), and IL-10 are secreted between PGE2 and cyclooxygenase 2 (COX-2), and their function as MDSCs regulating immunosuppression has been reported (69, 70) . MDSCs suppress T cell effector function via co-expression of arginase1 and NOS1. When they are added to the co-culture of MDSC and activated T cells, T cell function is completely restored (71) . Since the expression level of PGE2 is elevated when tumors form, the inhibitory effect of COX-2 is considered to partially affect the reduction in MDSC production. Among the many factors that can induce MDSC production, IL-1b further stimulates the recruitment of MDSCs in the nontumor state (72) . S100A9 increases the immunosuppressive ability of MDSCs by increasing the expression of nuclear factor-kB (NF-kB) dependent arginase by binding to the receptor for advanced glycation end products (RAGE) (46, 73) .

To date, MDSCs have also been reported in numerous noncancer pathologies, including viral, parasitic, bacterial, and fungal infections (74, 75) . The functional role of accumulated MDSCs in most infectious diseases is to inhibit host defense and regulate inflammatory cytokines such as TNF-a, IL-1b, and IL-6 (76). Some studies have also mentioned a detrimental role of MDSC, but the study model, pathogen, disease stage, and T cell ratio show different results (77) (78) (79) . In relation to the study of MDSC viral infection, induction of MDSC expansion of viruses such as hepatitis B or C virus, Epstein-Barr virus, human papillomavirus, influenza, and SARS-CoV-2 has been reported, and these induce virus persistence; nonetheless, evidence of MDSC's tissue damage protective role has also been reported in other studies (80) (81) (82) . In these various inhibitory mechanisms, the activity of MDSCs effectively interferes with enhancing tumor and non-tumor immune responses ( Figure 1) .

The factors involved in the production and suppression of MDSC in various disease states include stem cell factor (SCF), HIF-1a, IL-6, macrophage colony stimulating factor (M-CSF), signal transducer and activator of transcription 3 (STAT3), myeloid-related protein S100A9, and IL-1b (51, 52, 70, 83, 84) . In these microenvironmental factors, MDSCs go through a journey to the site of immune response to exert an immunosuppressive effect. In particular, the efflux from the blood to the tumor depends on CXCR4 and also affects the chemotaxis of mature myeloid cells (85) . Another function of MDSC is epigenetic regulation. Histone deacetylase-2 (HDAC-2), which has been studied for a long time as a cancer treatment agent, converted monocyte MDSC to granulocyte MDSC, and the mechanism was suggested. In addition, HDAC-11 suppressed the expression of IL-10, an immunosuppressive cytokine, in chromosomal modification due to the action of tumor-derived factors (TDF) infiltrating MDSC (86) . Meanwhile, the DNA methylation inhibitor zebularine decreased the expression of IDO, a potent immunosuppressive mediator of MDSC (87) . This may support the epigenetic function of MDSCs regardless of acetylation or methylation.

MDSCs are innate immune cells that can be increased in activity by infection-causing factors as previously described for various viral, parasitic, and bacterial infections, and also regulate the adaptive immune system. Several studies related to COVID-19 have reported that the high frequency of MDSC is associated with symptoms of severe disease and appears in the form of myeloid cell compartments that are difficult to control. Expansion of MDSC that occur in blood of severe COVID-19 patients had a close effect on lymphopenia and enhanced arginase activity (88) . In particular, the ratio of MDSC to effector CD8 + T cells was increased in patients with severe pneumonia accompanied by acute respiratory distress syndrome (ARDS). MDSC frequencies in total circulating mononuclear cells ranging from mild to severe cases were recorded at a maximum of 25% and 90%, respectively (89) . In addition, MDSC is involved in not only the inhibitory effect on T cell proliferation and activation, but also functional impairment of NK cells, B cell inhibition, Treg expansion induction, and downregulation of cytokine production by macrophages (89, 90) . Markers suggesting granulocytes, such as eosinophils, neutrophils, and basophils, were highly expressed in COVID-19 patients, predicting the activity of granulocytic MDSCs. Decrease in the expression of granulocytes identified by the integrin CD11b, increase in the number of neutrophils identified in CD15 + CD16 + , and down regulation of Th2-related CRTH2 in eosinophils and basophils were established as the signature of COVID-19. In addition, the appearance of PD-L1 checkpoint expression in eosinophils and basophils was found to be related to severity (91) .

In the immune cell metabolism program of COVID-19 patients, voltage-dependent anion channel 1 (VDAC1) was expressed in the T cell population, which is associated with mitochondrial dysfunction and apoptosis. This may provide a means to predict disease severity, follow-up, and design metabolic therapy regimens (92) .

PMN-MDSC was expanded during COVID-19, especially in patients requiring intensive care. A positive correlation was found between PMN-MDSC and the concentration level of inflammatory cytokines (IL-1b, IL-6, IL-8, and TNF-a) in the blood (93, 94) . Inflammatory cytokines play a central role in inducing the expansion of MDSCs (46) . The expression level of lectin-type oxidized LDL receptor 1 (LOX-1) was suggested as a marker to distinguish a subset of MDSCs with strong immunosuppressive ability in patients suffering from ARDS (16) . In addition, a significant increase in hexokinase II + PMN-MDSC was confirmed in severe COVID-19 patients with moderate or severe disease. In mild COVID-19 patients, IFNstimulating inflammatory HLA-DR hi CD11c hi monocytes increased, and IFN-1 deficiency was confirmed in severe patients. On the other hand, the high frequency of monocytes in HLA-DR low and neutrophils in CD10 low CD101CXCR4 +/suggest emergent myelopoiesis as immunosuppressive markers in the blood and lungs of severe patients (95) (96) (97) (98) (99) (100) (101) . Reductions in MDSCs during the COVID-19 recovery phase were associated with increases in inflammatory cytokines in the patient's blood, including decreases in TGF-b (88) . A multivariate regression analysis showed an association between the PMN-MDSC rate and fatal disease state, and the frequency of PMN-MDSC was higher in the non-survivors group than that in the recovered group (93) . A recent study reported a significant correlation between C-reactive protein, ferritin, and lactate dehydrogenase levels and MDSC in patients with COVID-19. This indicates that immature PMN-MDSCs are associated with disease severity (102) . Expansion of PMN-MDSCs and immature neutrophils in severe COVID-19 conditions indicates Th1 cell suppression and an increase in the frequency of Th17 cells due to strong polar migration to Th17 cells (103) .

M-MDSCs that have been mainly identified in PBMCs from acute COVID-19 patients, are associated with disease severity, and also suppress T cell responses (17) . They are characterized by expressing VDAC and carnitine palmitoyltransferase I. M-MDSC isolated from COVID-19 patient inhibited T cell proliferation and IFN-g production through an Arg-1dependent mechanism, and increased Arg-1 and IL-6 levels (104) . In an in vitro study that tested T cell proliferation, arginine supplementation helped restore T cell proliferation in patients with COVID-19, which had been reduced (88) . In addition, the monocyte distribution width (MDW), which has recently emerged as a promising early biomarker of sepsis, has been considered as a key mediator of hyperinflammatory disorders in severe COVID-19 conditions. High MDW values have been reported to be associated with prognostic lethal outcome in COVID-19 patients (105) . The presence of neutrophils and macrophages was confirmed in the bronchoalveolar lavage fluid (BALF) from patients with severe COVID-19, while large amounts of cytokines and chemokines were secreted. Among the gene signatures identified in single-cell RNA sequencing (scRNAseq) data, gene sinatures such as H3F3B, IFITM1/2, SAT1, and S100A8 are associated with neutrophils, and CCL2/3/8, KLF6, and SPP1 are associated with macrophages (106, 107) . This was the similar result as the high level of S100A8/9 found in the plasma of severe patients (98) . High levels of proinflammatory cytokines and chemokines such as CXCL8, IL-6, and IL-10 were associated with upregulation of the monocyte compartment (108, 109) . Another additional serum chemokines and cytokines (IL-6, IFN-l3, IP-10, CXCL9, CXCR1/2/4 and CCL17), virus-sensing TLRs, HIF1a, and several genes involved in various metabolic regulation were identified in COVID-19 (110, 111) . Furthermore, it was reported that soluble triggering receptor expressed on myeloid cells and an IL-6-based algorithm could serve as a very sensitive marker for early discrimination among patients with adverse reactions among COVID-19 patients (112) . Although more research is needed, one of the markers that can predict the correct mortality rate among COVID-19 ICU patients is mid-regional pro-adrenomedullin (MR-proADM): it presented as high levels in non-survivors (113) . In addition, several recent studies have shown new markers such as neutrophil-to-lymphocyte ratio, neutrophil-to-platelet ratio, uric acid level, total antioxidant capacity, eosinophil/PMN ratio, high-density lipoprotein, and apoprotein A1 (114) (115) (116) (117) (118) (119) .

In summary, monocytes and segmented neutrophils from peripheral blood migrate to immature myeloid cell candidates due to the elevation of cytokines and pro-inflammatory mediators during COVID-19, demonstrating the generation of emergency myeloid cells. Bone marrow cells identified in severe COVID-19 conditions are a subset of the primary immune cells that have initiated their activity, and studies on their ratio, inflammation, and identification of chemotactic genes will lead to potential diagnostics and therapeutics. Many studies related to COVID-19 have addressed the roles of MDSCs and their subsets, suggesting their selection as biomarkers for immune dysregulation in COVID-19 (120) . This is clearly clinically meaningful given its correlation with disease. Candidates for newly added biomarkers related to COVID-19 are shown in Table 2 .

Based on the expansion of MDSC in COVID-19, several molecular mechanisms for MDSC differentiation have been elucidated, making it possible to target and develop therapeutic agents. As cancer therapy, MDSC removal is beneficial to boost anti-tumor immunity such that chemotherapy reduces MDSCmediated inhibition of T cells (125, 126) . Rather this therapy was able to concert MDSCs into pro-inflammatory cells and disrupt tumor growth (127) . Given the immunophenotype and suppression mechanism of the existing tumor microenvironment (TME) are diverse, it is challenging to target various human MDSC types (128) . By using information gained from cancer studies, MDSCs-target therapeutic strategies can be applied to COVID-19 (70, 129) . Table 3 summarizes the core of MDSCs targeting strategies for disease control and inhibition of MDSCs activity identified so far and the candidates applicable to COVID-19. Various types of drugs have been reported as 1) drugs that differentiate MDSCs into mature myeloid cells, 2) drugs that interfere with MDSC maturation from cell precursors, 3) drugs that reduce MDSC accumulation in peripheral organs, and 4) drugs that affect MDSC inhibitory function (120). First, as a potential MDSC target, COX-2 inhibitors are useful because of significant role of the PGE-COX-2 axis in MDSC generation. COX-2 inhibitors inhibit the migration of MDSCs to tumor sites and reduce the incidence of several cancers by regulating transcriptomes (137) (138) (139) . Prostaglandin D2 (PGD2) plays a role as a key mediator for lymphopenia in a recent COVID-19 study, and induces upregulation of M-MDSC through the DP2 receptor in group 2 innate lymphoid cells. Targeting PGD2/DP2 signaling using a related receptor antagonist (ramatroban) has been consedered as a therapy to address immune dysfunction and lymphopenia in COVID-19 (153) .

Second, a phosphodiesterase-5 inhibitor (tadalafil) approved by the Federal Drug Administration (FDA) can inhibit MDSC by inducing downregulation of Arg1 and iNOS activity in several preclinical models (140) (141) (142) . In a study using an animal model, tadalafil decreased the levels of glutamic oxaloacetic transaminase, an enzyme that promotes carbohydrate and protein metabolism in aflatoxin-induced liver cancer cells (154) . Similarly, all-trans retinoic acid (ATRA), which has been previously used as a treatment for acute promyelocytic leukemia, induces MDSD differentiation, allowing NKT cells to mature into a state where they can be helped. ATRA also induces the expression of glutathione synthase, which leads to the production of glutathione, a ROS neutralizing agent (130) . 1a,25-dihydroxyvitamin D3 (calcitriol) is able to inhibit IL-6-stimulated MDSC proliferation in a mouse esophageal squamous cell carcinoma model (131) . This treatment can be extended to treatment of other diseases. Strong pathogen molecular patterns, such as CpG oligonucleotides and paclitaxel, have also been shown to differentiate MDSCs into mature myeloid cells (132, 133) .

Third, potential drug can be designed toward aiming at the migration/recruitment of myeloid cells among treatment strategies in order to recover COVID-19-related hyperinflammation and the resulting immunomodulatory disorders (155) . CXCR2 and CCR2/ 5 inhibitors are known to decrease the migration of MDSCs from the bone marrow to the circulation (143, 144) . The importance of inhibiting MDSC proliferation and migration to TME with strategies such as anti-CXCR2 monoclonal antibody has also been reported (145) . Leronlimab (CCR5 blocking antibody) decreased plasma IL-6 levels, restored the CD4/CD8 ratio, and induced a decrease in SARS-CoV-2 plasma viremia (156) . The previously mentioned alamin S100A8 is strongly induced in COVID-19 patients and SARS-CoV-2 infected models. Paquinimod, known as an inhibitor of S100A8/9, induced a decrease in viral load in mice infected with SARS-CoV-2, resulting in relief from pneumonia (151) . Although the number of neutrophils increases in the onset of COVID-19 and is accompanied by uncontrollable pathological damage, paquinimod decreased the number of neutrophils. The treatment of this drug can be suggested as a method for therapeutic purposes while simultaneously detecting abnormal changes in S100A8/9 and neutrophils in the COVID-19 state (151) . Lastly, drug can be designed to regulate metabolism of MDSCs. The inhibition of arginase-1 or supplementation of arginine in severe COVID-19 patients could be a treatment to restore depleted arginine and impaired T cell function (157) . Moreover, reprogramming of MDSCs targeting epigenetics based on immune metabolism is also expected to solve the pulmonary inflammatory state of COVID-19 (158) . This diverse list of drugs capable of manipulating MDSC populations could play an important role in MDSC-related treatment modalities in chronic inflammation, cancer, as well as in COVID-19.

MDSCs plays a pivotal role in regulating the innate immunity and adaptive immunity. Dysregulated immune response and MDSCs expansion have been reported in COVID-19 and other viral infections. The removal of MDSC leads to an increase in the immune response against the viral infection. Thereby, active research has been conducted to identify various MDSC phenotypic markers and discover therapeutic agents targeting MDSCs. Despite of the detrimental role of MDSCs in human inflammatory diseases, MDSCs are resistant to allograft transplantation and autoimmune diseases, limits the inflammatory damaging, and shows a tendency to return to a non-inflammatory state by homeostasis. There are still few research reports that MDSC directly inhibits hyperinflammation and helps clinical recovery, but additional studies are needed because the possibility cannot be ruled out. In this regard, it is necessary to keep the tolerogenic properties of MDSC in mind to develop MDSC-targeted therapeutics (81) .

In patients infected with COVID-19, both PMN-MDSC and M-MDSC accumulation and their expansion have been confirmed. Thereby, various markers for identifying MDSCs are important in the development of diagnostic systems. It can also open up several possibilities for treatment by targeting immunosuppressive function of MDSCs. The correlation between BALF and serum MDSC frequency and clinical biomarkers will facilitate the consideration and selection of future therapeutics. In many of these studies, the severity of COVID-19 was clinically evaluated using metabolites, cytokines, chemokines, and several proteins related to various mechanisms such as inflammation and apoptosis. Candidates considered as therapeutic agents for COVID-19 were typically presented as specific cytokine inhibitors or immunomodulatory agents. Although official approval of future treatments will be necessary, reports on various therapeutic approaches and treatment prognosis for approved drugs will still be required.

S-JP contributed to manuscript research and writing. D-eN contributed to manuscript research and writing. HS contributed to manuscript writing and review. YH contribute to manuscript supervision, writing, and review. All authors contributed to the article and approved the submitted version.

This work was supported by NIH Grant (DK122737 to YH).

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Indoleamine 2,3-Dioxygenase and Dendritic Cell Tolerogenicity

Myeloid-Derived Suppressor Cells in the Era of Increasing Myeloid Cell Diversity

Metabolism of L-Arginine by Myeloid-Derived Suppressor Cells in Cancer: Mechanisms of T Cell Suppression and Therapeutic Perspectives

IL-1b Regulates a Novel Myeloid-Derived Suppressor Cell Subset That Impairs NK Cell Development and Function

Increased Myeloid-Derived Suppressor Cells in Gastric Cancer Correlate With Cancer Stage and Plasma S100A8/A9 Proinflammatory Proteins

Myeloid-Derived Suppressor Cells in Infection: A General Overview

Myeloid-Derived Suppressor Cells and the Pathogenesis of Human Immunodeficiency Virus Infection

Tumor NLRP3-Derived IL-1b Drives the IL-6/STAT3

Myeloid-Derived Suppressor Cell Subtypes Differentially Influence T-Cell Function, T-Helper Subset Differentiation, and Clinical Course in CLL

HIV-1 Nef Induces Hck/Lyn-Dependent Expansion of Myeloid-Derived Suppressor Cells Associated With Elevated Interleukin-17/G-CSF Levels

MDSC in Mice and Men: Mechanisms of Immunosuppression in Cancer

Increased Blood Monocytic Myeloid Derived Suppressor Cells But Low Regulatory T Lymphocytes in Patients With Mild COVID-19

Myeloid-Derived Suppressor Cells: The Dark Knight or the Joker in Viral Infections?

Searching for Effective Antiviral Small Molecules Against Influenza A Virus: A Patent Review

Molecular Mechanisms Regulating Myeloid-Derived Suppressor Cell Differentiation and Function

Flagellin Induces Myeloid-Derived Suppressor Cells: Implications for Pseudomonas Aeruginosa Infection in Cystic Fibrosis Lung Disease

PGE 2 -Induced CXCL12 Production and CXCR4 Expression Controls the Accumulation of Human MDSCs in Ovarian Cancer Environment

Epigenetic Silencing of Retinoblastoma Gene Regulates Pathologic Differentiation of Myeloid Cells in Cancer

Low Dose Zebularine Treatment Enhances Immunogenicity of Tumor Cells

SARS-CoV-2-Induced ARDS Associates With MDSC Expansion, Park et al. Immunoregulatory Role of MDSCs Frontiers in Immunology | www

Lymphocyte Dysfunction, and Arginine Shortage

Expansion of Myeloid-Derived Suppressor Cells in Patients With Severe Coronavirus Disease (COVID-19)

Expansion of Myeloid Derived Suppressor Cells Contributes to Platelet Activation by L-Arginine Deprivation During SARS-CoV-2 Infection

A Granulocytic Signature Identifies COVID-19 and Its Severity

Metabolic Programs Define Dysfunctional Immune Responses in Severe COVID-19 Patients

Early Expansion of Myeloid-Derived Suppressor Cells Inhibits SARS

Cell Response and may Predict Fatal COVID-19 Outcome

Myeloid Cell Dynamics Correlating With Clinical Outcomes of Severe COVID-19 in Japan

Severe COVID-19 Is Marked by a Dysregulated Myeloid Cell Compartment

Multi-Omic Profiling Reveals Widespread Dysregulation of Innate Immunity and Hematopoiesis in COVID-19

Plasma From Patients With Bacterial Sepsis or Severe COVID-19 Induces Suppressive Myeloid Cell Production From Hematopoietic Progenitors In Vitro

Elevated Calprotectin and Abnormal Myeloid Cell Subsets Discriminate Severe From Mild COVID-19

Quantitative and Qualitative Alterations of Circulating Myeloid Cells and Plasmacytoid DC in SARS-CoV-2 Infection

Monocyte HLA-DR Measurement by Flow Cytometry in COVID -19 Patients: An Interim Review

Systems Biological Assessment of Immunity to Mild Versus Severe COVID-19 Infection in Humans

Correlation of Myeloid-Derived Suppressor Cells With C-Reactive Protein

Neutrophils Mediate Th17 Promotion in COVID-19 Patients

Functional Monocytic Myeloid-Derived Suppressor Cells Increase in Blood But Not Airways and Predict COVID-19 Severity

Monocyte Distribution Width (MDW) as Novel Inflammatory Marker With Prognostic Significance in COVID-19 Patients

Single-Cell Transcriptome Analysis Highlights a Role for Neutrophils and Inflammatory Macrophages in the Pathogenesis of Severe COVID-19

The Differential Immune Responses to COVID-19 in Peripheral and Lung Revealed by Single-Cell RNA Sequencing

Plasma From Patients With Bacterial Sepsis or Severe COVID-19 Induces Suppressive Myeloid Cell Production From Hematopoietic Progenitors In Vitro

Re-Analysis of Single Cell Transcriptome Reveals That the NR3C1-CXCL8-Neutrophil Axis Determines the Severity of COVID-19

Serum CCL17 Level Becomes a Predictive Marker to Distinguish Between Mild/Moderate and Severe/Critical Disease in Patients With

Increased Expression of Hypoxia-Induced Factor 1a mRNA and Its Related Genes in Myeloid Blood Cells From Critically Ill

COVID-19 Risk Stratification Algorithms Based on sTREM-1 and IL-6 in Emergency Department

Effectiveness of Mid-Regional Pro-Adrenomedullin (MR-proADM) as Prognostic Marker in COVID-19 Critically Ill Patients: An Observational Prospective Study

Hemogram as Marker of in-Hospital Mortality in COVID-19

Uric Acid as a Prognostic Factor and Critical Marker of COVID-19

Total Antioxidant Capacity as a Marker of Severity of COVID-19 Infection: Possible Prognostic and Therapeutic Clinical Application

Eosinopenia <100/ml as a Marker of Active COVID-19: An Observational Prospective Study

High-Density Lipoprotein Cholesterol: A Marker of COVID-19 Infection Severity?

Apolipoprotein A1 Level in Plasma of Patients With Diabetes and Diabetic Patients With COVID-19 as a Possible Marker of Disease

Myeloid-Derived Suppressor Cells as a Potential Biomarker and Therapeutic Target in COVID-19

A Dynamic COVID-19 Immune Signature Includes Associations With Poor Prognosis

Dynamic Relationship Between D-Dimer and COVID-19 Severity

Prognostic Value of Interleukin-6, C-Reactive Protein, and Procalcitonin in Patients With COVID-19

Targeting Neutrophils to Treat Acute Respiratory Distress Syndrome in Coronavirus Disease

Metronomic Chemotherapy With Low-Dose Cyclophosphamide Plus Gemcitabine can Induce Anti-Tumor T Cell Immunity In Vivo

Rosiglitazone and Gemcitabine in Combination Reduces Immune Suppression and Modulates T Cell Populations in Pancreatic Cancer

Intratumoral Injection of Attenuated Salmonella Vaccine can Induce Tumor Microenvironmental Shift From Immune Suppressive to Immunogenic

Derived Suppressor Cells: Not Only in Tumor Immunity

Mechanism of All-Trans Retinoic Acid Effect on Tumor-Associated Myeloid-Derived Suppressor Cells

25-Dihydroxyvitamin D3 Inhibits Esophageal Squamous Cell Carcinoma Progression by Reducing IL6 Signaling

Intratumoral Injection of CpG Oligonucleotides Induces the Differentiation and Reduces the Immunosuppressive Activity of Myeloid-Derived Suppressor Cells

Paclitaxel Promotes Differentiation of Myeloid-Derived Suppressor Cells Into Dendritic Cells In Vitro in a TLR4-Independent Manner

Generation of Antigen-Presenting Cells From Tumor-Infiltrated CD11b Myeloid Cells With DNA Demethylating Agent 5-Aza-2′-Deoxycytidine

An Intragenic Long Noncoding RNA Interacts Epigenetically With the RUNX1 Promoter and Enhancer Chromatin DNA in Hematopoietic Malignancies: RUNX1 Intragenic Noncoding RNA

Targeting Myeloid-Derived Suppressor Cells in Ovarian Cancer

Case-Control Study of Use of Nonsteroidal Antiinflammatory Drugs and Glioblastoma Multiforme

A Randomized Trial of Aspirin to Prevent Colorectal Adenomas in Patients With Previous Colorectal Cancer

COX-2 Blockade Suppresses Gliomagenesis by Inhibiting Myeloid-Derived Suppressor Cells

Phosphodiesterase-5 Inhibition Augments Endogenous Antitumor Immunity by Reducing Myeloid-Derived Suppressor Cell Function

Phosphodiesterase-5 Inhibition Suppresses Colonic Inflammation-Induced Tumorigenesis via Blocking the Recruitment of MDSC

Targeting the Crosstalk Between Cytokine-Induced Killer Cells and Myeloid-Derived Suppressor Cells in Hepatocellular Carcinoma

CXCR2: From Bench to Bedside

Epigenetic Therapy Inhibits Metastases by Disrupting Premetastatic Niches

Disruption of CXCR2-Mediated MDSC Tumor Trafficking Enhances Anti-PD1 Efficacy

Coordinated Regulation of Myeloid Cells by Tumours

Clinical Perspectives on Targeting of Myeloid Derived Suppressor Cells in the Treatment of Cancer

Myeloid-Derived Suppressor Cells in Gastrointestinal Cancers: A Systemic Review

Depletion and Maturation of Myeloid-Derived Suppressor Cells in Murine Cancer Models

IL-6 and IL-8 Are Linked With Myeloid-Derived Suppressor Cell Accumulation and Correlate With Poor Clinical Outcomes in Melanoma Patients

Induction of Alarmin S100A8/A9 Mediates Activation of Aberrant Neutrophils in the Pathogenesis of COVID-19

Myeloid-Derived Suppressor Cells in Cancer: Therapeutic, Predictive, and Prognostic Implications

Prostaglandin D2 as a Mediator of Lymphopenia and a Therapeutic Target in COVID-19 Disease

Tadalafil Inhibits Elevated Glutamic Oxaloacetic Transaminase During Alcohol Aflatoxin Induced Hepatocellular Carcinoma in Rats

Longitudinal Immune Profiling Reveals Key Myeloid Signatures Associated With COVID-19

Disruption of the CCL5/RANTES-CCR5 Pathway Restores Immune Homeostasis and Reduces Plasma Viral Load in Critical COVID-19

Transcriptome and Functions of Granulocytic Myeloid-Derived Suppressor Cells Determine Their Association With Disease Severity of COVID-19

COVID-19 and Myeloid Cells: Complex Interplay Correlates With Lung Severity

Conflict of Interest: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest

This review article is dedicated to the memory of Hugo Rosen, MD who has done pioneering studies on chronic liver diseases and liver immunology. We also thank the former members of the Hahn lab (Drs. Robert Tacke, Celeste Goh) for initiating first MDSCs studies in viral infection and elucidating the immunosuppressive function of MDSCs on T cells and NK cell responses.