Methods to Measure MDSC Immune Suppressive Activity In Vitro and In Vivo

MDSC-targeted liposomal all-trans retinoic acid suppresses mMdscs and improves immunotherapy in HBV infection

BACKGROUND

Myeloid-derived suppressor cells (MDSCs) are evolving as a prominent determinant in cancer occurrence and development and are functionally found to suppress T cells in cancer. Not much research is done regarding its involvement in viral infections. This research was designed to investigate the role of MDSCs in hepatitis B virus (HBV) infection and how targeting these cells with our novel all-trans retinoic acid encapsulated liposomal formulation could improve immunotherapy in C57BL/6 mice.

METHODS

Ten micrograms (10 μg) of plasmid adeno-associated virus (pAAV/HBV 1.2, genotype A) was injected hydrodynamically via the tail vein of C57BL/6 mice. An all-trans retinoic acid encapsulated liposomal formulation (L-ATRA) with sustained release properties was used in combination with tenofovir disoproxil fumarate (TDF), a nucleotide analog reverse transcriptase inhibitor (nRTI) to treat the HBV infection. The L-ATRA formulation was given at a dose of 5 mg/kg intravenously (IV) twice a week. The TDF was given orally at 30 mg/kg daily.

RESULTS

Our results revealed that L-ATRA suppresses MDSCs in HBV infected mice and enhanced T-cell proliferation in vitro. In vivo studies showed higher and improved immunotherapeutic effect in mice that received L-ATRA and TDF concurrently in comparison with the groups that received monotherapy. Lower HBV DNA copies, lower concentrations of HBsAg and HBeAg, lower levels of ALT and AST and less liver damage were seen in the mice that received the combination therapy of L-ATRA + TDF.

CONCLUSIONS

In effect, targeting MDSCs with the combination of L-ATRA and TDF effectively reduced mMDSC and improved immunotherapy in the HBV infected mice. Targeting MDSCs could provide a breakthrough in the fight against hepatitis B virus infection.

Detection of myeloid-derived suppressor cells by flow cytometry

Recently discovered heterogeneous myeloid-derived suppressor cells (MDSCs) are some of the most discussed immunosuppressive cells in contemporary immunology, especially in the tumor microenvironment, and are defined primarily by their T cell immunosuppressive function. The importance of these cells extend to other chronic pathological conditions as well, including chronic infection, inflammation, and tissue remodeling. In many of these conditions, their accumulation/expansion correlates with disease progression, poor prognosis, and reduced survival, which highlights the potential of how these cells may be used in a clinical setting as both prognostic factor and therapeutic target. In healthy individuals, these cells are usually not present in the circulation. Therefore, monitoring this cell population is of potential clinical significance, and utility in basic research. However, these cells have a complex phenotype without one single marker of sufficient specificity for their identification. Flow cytometry is a powerful tool allowing multi-parameter analysis of heterogeneous cell populations, which makes it ideally suitable for the complex phenotypic analysis essential for identification and enumeration of circulating MDSCs. This approach has the potential to provide a novel clinically useful tool for assessment of prognosis and treatment outcomes. The protocol in this chapter describes a flow cytometric analysis to identify and quantify MDSCs from human or mouse whole blood leukocytes and peripheral blood mononuclear cells, as well as a single cell suspension from solid tissue, by using multicolor fluorescence-conjugated antibodies against their surface markers.

Immune checkpoint blockade therapy mitigates systemic inflammation and affects cellular FLIP-expressing monocytic myeloid-derived suppressor cells in non-progressor non-small cell lung cancer patients

Cancer cells favor the generation of myeloid cells with immunosuppressive and inflammatory features, including myeloid-derived suppressor cells (MDSCs), which support tumor progression. The anti-apoptotic molecule, cellular FLICE (FADD-like interleukin-1β-converting enzyme)-inhibitory protein (c-FLIP), which acts as an important modulator of caspase-8, is required for the development and function of monocytic (M)-MDSCs. Here, we assessed the effect of immune checkpoint inhibitor (ICI) therapy on systemic immunological landscape, including FLIP-expressing MDSCs, in non-small cell lung cancer (NSCLC) patients. Longitudinal changes in peripheral immunological parameters were correlated with patients' outcome. In detail, 34 NSCLC patients were enrolled and classified as progressors (P) or non-progressors (NP), according to the RECIST evaluation. We demonstrated a reduction in pro-inflammatory cytokines such as IL-8, IL-6, and IL-1β in only NP patients after ICI treatment. Moreover, using t-distributed stochastic neighbor embedding (t-SNE) and cluster analysis, we characterized in NP patients a significant increase in the amount of lymphocytes and a slight contraction of myeloid cells such as neutrophils and monocytes. Despite this moderate ICI-associated alteration in myeloid cells, we identified a distinctive reduction of c-FLIP expression in M-MDSCs from NP patients concurrently with the first clinical evaluation (T1), even though NP and P patients showed the same level of expression at baseline (T0). In agreement with the c-FLIP expression, monocytes isolated from both P and NP patients displayed similar immunosuppressive functions at T0; however, this pro-tumor activity was negatively influenced at T1 in the NP patient cohort exclusively. Hence, ICI therapy can mitigate systemic inflammation and impair MDSC-dependent immunosuppression.

Estradiol Augments Tumor-Induced Neutrophil Production to Promote Tumor Cell Actions in Lymphangioleiomyomatosis Models

Lymphangioleiomyomatosis (LAM) is a rare cystic lung disease caused by smooth muscle cell-like tumors containing tuberous sclerosis (TSC) gene mutations and found almost exclusively in females. Patient studies suggest LAM progression is estrogen dependent, an observation supported by in vivo mouse models. However, in vitro data using TSC-null cell lines demonstrate modest estradiol (E2) responses, suggesting E2 effects in vivo may involve pathways independent of direct tumor stimulation. We previously reported tumor-dependent neutrophil expansion and promotion of TSC2-null tumor growth in an E2-sensitive LAM mouse model. We therefore hypothesized that E2 stimulates tumor growth in part by promoting neutrophil production. Here we report that E2-enhanced lung colonization of TSC2-null cells is indeed dependent on neutrophils. We demonstrate that E2 induces granulopoiesis via estrogen receptor α in male and female bone marrow cultures. With our novel TSC2-null mouse myometrial cell line, we show that factors released from these cells drive E2-sensitive neutrophil production. Last, we analyzed single-cell RNA sequencing data from LAM patients and demonstrate the presence of tumor-activated neutrophils. Our data suggest a powerful positive feedback loop whereby E2 and tumor factors induce neutrophil expansion, which in turn intensifies tumor growth and production of neutrophil-stimulating factors, resulting in continued TSC2-null tumor growth.

In vitro-Generated MDSCs Reduce the Pregnancy Complications in an Abortion-Prone Murine Model

Recurrent spontaneous abortion (RSA) is one of the major pregnancy-related complications. The roles of different immune cells have been studied in pregnancy complications. The current study aimed to investigate myeloid-derived suppressor cells (MDSCs) in a murine abortion model and introduce a therapeutic approach by using in vitro-generated MDSCs in this model. CBA/J × DBA/2 (abortion prone) and CBA/J × Balb/C (normal pregnancy) mice were used. The frequency of granulocytic MDSCs, monocytic MDSCs, and Tregs was checked in the bone marrow and uteroplacental tissue of mice on three gestational days (gd9.5, gd13.5, and gd17.5) using the flow cytometry approach. MDSCs were generated in vitro from bone marrow-isolated cells using GM-CSF and IL-6 cytokines. Abortion-prone mice were injected intravenously with in vitro-generated MDSCs at gd0.5, and pregnancy outcomes were recorded in treated mice. The frequency of G-MDSCs and M-MDSCs in the bone marrow of abortion-prone mice was decreased at gd9.5 (p = 0.026 and p = 0.05, respectively). In uteroplacental tissue, the frequency of G-MDSCs was significantly lower at gd9.5 and gd13.5 (p = 0.001, p = 0.029, respectively), while M-MDSCs only showed decreased number at gd9.5 (p = 0.05) in abortion-prone mice. Injection of in vitro-generated MDSCs resulted in the increased fetus and placenta weights (p = 0.049 and p = 0.012, respectively) but showed no effect on the number of live fetuses and abortion rate. The reduced frequency of both G-MDSCs and M-MDSCs in the bone marrow and at the feto-maternal interface is associated with pregnancy complications. In vitro-generated MDSCs could be considered as a potential approach to reduce these complications.

Effect of Direct Acting Antivirals (DAAs) on Myeloid-Derived Suppressor Cells Population in Egyptian Chronic Hepatitis C Virus Patients: A Potential Immunomodulatory Role of DAAs

Chronic hepatitis C is a major health concern with high morbidity and mortality rates. The introduction of direct acting antivirals (DAAs) as a first-line treatment for hepatitis C virus (HCV) has significantly enhanced HCV eradication. However, DAA therapy is facing rising concerns regarding long-term safety, viral resistance, and reinfection. HCV is associated with different immune alteration mechanisms that can evade immunity and establish persistent infection. One of these suggested mechanisms is the accumulation of myeloid-derived suppressor cells (MDSCs), which is known to accumulate in chronic inflammatory conditions. Moreover, the role of DAA in restoring immunity after successful viral eradication is still unclear and needs further investigations. Thus, we aimed to investigate the role of MDSCs in chronic HCV Egyptian patients and its response to DAA in treated compared with untreated patients. Fifty untreated chronic hepatitis C (CHC) patients, 50 DAA-treated CHC patients, and 30 healthy individuals were recruited. We used flow cytometer analysis to measure MDSCs frequency and enzyme-linked immunosorbent assay analysis to evaluate the serum level of interferon (IFN)-γ. We found a significant elevation in MDSC% among the untreated group (34.5 ± 12.4%) compared with the DAA-treated group (18.3 ± 6.7%), while the control group had a mean of (3.8 ± 1.6%). IFN-γ concentration was higher in treated patients compared with untreated. We also found a significant negative correlation (rs -0.662) (p < 0.001) between MDSC% and IFN-γ concentration among treated HCV patients. Our results revealed important evidence of MDSCs accumulation in CHC patients and partial retrieval of the immune system regulatory function after DAA therapy.

Role of myeloid-derived suppressor cells during Trypanosoma cruzi infection

Chagas disease (CD), caused by the protozoan parasite Trypanosoma cruzi, is the third largest parasitic disease burden globally. Currently, more than 6 million people are infected, mainly in Latin America, but international migration has turned CD into an emerging health problem in many nonendemic countries. Despite intense research, a vaccine is still not available. A complex parasite life cycle, together with numerous immune system manipulation strategies, may account for the lack of a prophylactic or therapeutic vaccine. There is substantial experimental evidence supporting that T. cruzi acute infection generates a strong immunosuppression state that involves numerous immune populations with regulatory/suppressive capacity. Myeloid-derived suppressor cells (MDSCs), Foxp3+ regulatory T cells (Tregs), regulatory dendritic cells and B regulatory cells are some of the regulatory populations that have been involved in the acute immune response elicited by the parasite. The fact that, during acute infection, MDSCs increase notably in several organs, such as spleen, liver and heart, together with the observation that depletion of those cells can decrease mouse survival to 0%, strongly suggests that MDSCs play a major role during acute T. cruzi infection. Accumulating evidence gained in different settings supports the capacity of MDSCs to interact with cells from both the effector and the regulatory arms of the immune system, shaping the outcome of the response in a very wide range of scenarios that include pathological and physiological processes. In this sense, the aim of the present review is to describe the main knowledge about MDSCs acquired so far, including several crosstalk with other immune populations, which could be useful to gain insight into their role during T. cruzi infection.

Monocytic Myeloid-Derived Suppressor Cells from Tumor Tissue Are a Differentiated Cell with Limited Fate Plasticity

Owing to ease of access and high yield, most murine myeloid-derived suppressor cell (MDSC) knowledge comes from the study of spleen-derived MDSCs rather than those isolated from the tumor. Although several studies have identified subtle differences in suppressive function between these MDSCs, a recent report demonstrated that the whole peripheral myeloid compartment poorly reflects myeloid populations found at the tumor. We confirm and extend these observations by presenting data that indicate extensive differences exist between peripheral and tumor MDSCs, suggesting that it may be inappropriate to use spleen MDSCs as surrogates for studying tumor MDSCs. Using cytospins, we observed that tumor MDSCs have undergone a morphologic shift from immature myeloid cell forms commonly seen in bone marrow (BM) and spleen MDSCs and acquired mature myeloid cell characteristics. Spleen and BM monocyte-like MDSCs (M-MDSCs) readily responded to differentiation signals for multiple myeloid cell types whereas tumor M-MDSCs had remarkably reduced cellular plasticity. At the time of isolation, M-MDSCs from BM or spleen have little to no T cell suppressive activity whereas those from the tumor possess immediate and efficient T cell suppressive function. Finally, microarray analysis revealed that the transcriptomes of tumor and spleen M-MDSCs possessed >4500 differentially expressed transcripts. We conclude that tumor M-MDSCs are more differentiated and mature, and that they are morphologically, genetically, and functionally distinct from spleen and BM M-MDSCs. These observations have important implications for the design of anti-MDSC therapies and suggest that preclinical studies using nontumor MDSCs could lead to results not applicable to tumor MDSCs.

Aryl hydrocarbon receptor activation drives polymorphonuclear myeloid-derived suppressor cell response and efficiently attenuates experimental Sjögren's syndrome

Myeloid-derived suppressor cells (MDSCs) comprise heterogeneous myeloid cell populations with immunosuppressive capacity that contribute to immune regulation and tolerance induction. We previously reported impaired MDSC function in patients with primary Sjögren's syndrome (pSS) and mice with experimental SS (ESS). However, the molecular mechanisms underlying MDSC dysfunction remain largely unclear. In this study, we first found that aryl hydrocarbon receptor (AhR) was highly expressed by human and murine polymorphonuclear MDSCs (PMN-MDSCs). Indole-3-propionic acid (IPA), a natural AhR ligand produced from dietary tryptophan, significantly promoted PMN-MDSC differentiation and suppressive function on CD4+ T cells. In contrast, feeding a tryptophan-free diet resulted in a decreased PMN-MDSC response, a phenotype that could be reversed by IPA supplementation. The functional importance of PMN-MDSCs was demonstrated in ESS mice by using a cell-depletion approach. Notably, AhR expression was reduced in PMN-MDSCs during ESS development, while AhR antagonism resulted in exacerbated ESS pathology and dysregulated T effector cells, which could be phenocopied by a tryptophan-free diet. Interferon regulatory factor 4 (IRF4), a repressive transcription factor, was upregulated in PMN-MDSCs during ESS progression. Chromatin immunoprecipitation analysis revealed that IRF4 could bind to the promoter region of AhR, while IRF4 deficiency markedly enhanced AhR-mediated PMN-MDSC responses. Furthermore, dietary supplementation with IPA markedly ameliorated salivary glandular pathology in ESS mice with restored MDSC immunosuppressive function. Together, our results identify a novel function of AhR in modulating the PMN-MDSC response and demonstrate the therapeutic potential of targeting AhR for the treatment of pSS.

Myeloid-derived suppressor cells and vaccination against pathogens

It is widely accepted that the immune system includes molecular and cellular components that play a role in regulating and suppressing the effector immune response in almost any process in which the immune system is involved. Myeloid-derived suppressor cells (MDSCs) are described as a heterogeneous population of myeloid origin, immature state, with a strong capacity to suppress T cells and other immune populations. Although the initial characterization of these cells was strongly associated with pathological conditions such as cancer and then with chronic and acute infections, extensive evidence supports that MDSCs are also involved in physiological/non-pathological settings, including pregnancy, neonatal period, aging, and vaccination. Vaccination is one of the greatest public health achievements and has reduced mortality and morbidity caused by many pathogens. The primary goal of prophylactic vaccination is to induce protection against a potential pathogen by mimicking, at least in a part, the events that take place during its natural interaction with the host. This strategy allows the immune system to prepare humoral and cellular effector components to cope with the real infection. This approach has been successful in developing vaccines against many pathogens. However, when the infectious agents can evade and subvert the host immune system, inducing cells with regulatory/suppressive capacity, the development of vaccines may not be straightforward. Notably, there is a long list of complex pathogens that can expand MDSCs, for which a vaccine is still not available. Moreover, vaccination against numerous bacteria, viruses, parasites, and fungi has also been shown to cause MDSC expansion. Increases are not due to a particular adjuvant or immunization route; indeed, numerous adjuvants and immunization routes have been reported to cause an accumulation of this immunosuppressive population. Most of the reports describe that, according to their suppressive nature, MDSCs may limit vaccine efficacy. Taking into account the accumulated evidence supporting the involvement of MDSCs in vaccination, this review aims to compile the studies that highlight the role of MDSCs during the assessment of vaccines against pathogens.

New Perspectives on Myeloid-Derived Suppressor Cells and Their Emerging Role in Haematology

Myeloid-derived suppressor cells (MDSCs) are immature cells of myeloid origin that have gained researchers’ attention, as they constitute promising biomarkers and targets for novel therapeutic strategies (i.e., blockage of development, differentiation, depletion, and deactivation) in several conditions, including neoplastic, autoimmune, infective, and inflammatory diseases, as well as pregnancy, obesity, and graft rejection. They are characterised in humans by the typical immunophenotype of CD11b⁺CD33⁺HLA-DR^–/low and immune-modulating properties leading to decreased T-cell proliferation, induction of T-regulatory cells (T-regs), hindering of natural killer (NK) cell functionality, and macrophage M2-polarisation. The research in the field is challenging, as there are still difficulties in defining cell-surface markers and gating strategies that uniquely identify the different populations of MDSCs, and the currently available functional assays are highly demanding. There is evidence that MDSCs display altered frequency and/or functionality and could be targeted in immune-mediated and malignant haematologic diseases, although there is a large variability of techniques and results between different laboratories. This review presents the current literature concerning MDSCs in a clinical point of view in an attempt to trigger future investigation by serving as a guide to the clinical haematologist in order to apply them in the context of precision medicine as well as the researcher in the field of experimental haematology.

Immunosuppressive tumor microenvironment in Uterine Serous Carcinoma via CCL7 signal with myeloid-derived suppressor cells

Serous carcinoma of the uterus (USC) is a pathological subtype of high-grade endometrial cancers, with no effective treatment for advanced cases. Since such refractory tumors frequently harbor anti-tumor immune tolerance, many immunotherapies have been investigated for various malignant tumors using immuno-competent animal models mimicking their local immunities. In this study, we established an orthotopic mouse model of high-grade endometrial cancer and evaluated the local tumor immunity to explore the efficacy of immunotherapies against USC. A multivariate analysis of 62 human USC cases revealed that the tumor infiltrating cell status, few CD8+ cells and abundant myeloid-derived suppressor cells (MDSCs), was an independent prognostic factor (p<0.005). A murine endometrial cancer cell (mECC) was obtained from C57BL/6 mice via endometrium-specific deletion of Pten and Tp53, and another high-grade cell (HPmECC) was established by further overexpressing Myc in mECCs. HPmECCs exhibited higher capacities of migration and anchorage-independent proliferation than mECCs (p<0.01, p<0.0001), and when both types of cells were inoculated into the uterus of C57BL/6 mice, the prognosis of mice bearing HPmECC-derived tumors was significantly poorer (p<0.001). Histopathological analysis of HPmECC orthotopic tumors showed serous carcinoma-like features with prominent tumor infiltration of MDSCs (p<0.05), and anti Gr-1 antibody treatment significantly prolonged the prognosis of HPmECC-derived tumor-bearing mice (p<0.05). High CCL7 expression was observed in human USC and HPmECC, and MDSCs migration was promoted in a CCL7 concentration-dependent manner. These results indicate that anti-tumor immunity is suppressed in USC due to increased number of tumor-infiltrating MDSCs via CCL signal.

Glucosamine Interferes With Myelopoiesis and Enhances the Immunosuppressive Activity of Myeloid-Derived Suppressor Cells

Glucosamine (GlcN) is the most widely consumed dietary supplement and exhibits anti-inflammatory effects. However, the influence of GlcN on immune cell generation and function is largely unclear. In this study, GlcN was delivered into mice to examine its biological function in hematopoiesis. We found that GlcN promoted the production of immature myeloid cells, known as myeloid-derived suppressor cells (MDSCs), both in vivo and in vitro. Additionally, GlcN upregulated the expression of glucose transporter 1 in hematopoietic stem and progenitor cells (HSPCs), influenced HSPC functions, and downregulated key genes involved in myelopoiesis. Furthermore, GlcN increased the expression of arginase 1 and inducible nitric oxide synthase to produce high levels of reactive oxygen species, which was regulated by the STAT3 and ERK1/2 pathways, to increase the immunosuppressive ability of MDSCs. We revealed a novel role for GlcN in myelopoiesis and MDSC activity involving a potential link between GlcN and immune system, as well as the new therapeutic benefit.

In Vitro 3D Staphylococcus aureus Abscess Communities Induce Bone Marrow Cells to Expand into Myeloid-Derived Suppressor Cells

Staphylococcus aureus is the main causative pathogen of subcutaneous, bone, and implant-related infections, forming structures known as staphylococcal abscess communities (SACs) within tissues that also contain immunosuppressive myeloid-derived suppressor cells (MDSCs). Although both SACs and MDSCs are present in chronic S. aureus infections, it remains unknown whether SACs directly trigger MDSC expansion. To investigate this, a previously developed 3D in vitro SAC model was co-cultured with murine and human bone marrow cells. Subsequently, it was shown that SAC-exposed human CD11b^low/− myeloid cells or SAC-exposed murine CD11b⁺ Gr-1⁺ cells were immunosuppressive mainly by reducing absolute CD4⁺ and CD8α⁺ T cell numbers, as shown in T cell proliferation assays and with flow cytometry. Monocytic MDSCs from mice with an S. aureus bone infection also strongly reduced CD4⁺ and CD8α⁺ T cell numbers. Using protein biomarker analysis and an immunoassay, we detected in SAC–bone marrow co-cultures high levels of GM-CSF, IL-6, VEGF, IL-1β, TNFα, IL-10, and TGF-β. Furthermore, SAC-exposed neutrophils expressed Arg-1 and SAC-exposed monocytes expressed Arg-1 and iNOS, as shown via immunofluorescent stains. Overall, this study showed that SACs cause MDSC expansion from bone marrow cells and identified possible mediators to target as an additional strategy for treating chronic S. aureus infections.

Artepillin C Reduces Allergic Airway Inflammation by Induction of Monocytic Myeloid-Derived Suppressor Cells

Propolis is a natural product produced by bees that is primarily used in complementary and alternative medicine and has anti-inflammatory, antibacterial, antiviral, and antitumoral biological properties. Some studies have reported the beneficial effects of propolis in models of allergic asthma. In a previous study, our group showed that green propolis treatment reduced airway inflammation and mucus secretion in an ovalbumin (OVA)-induced asthma model and resulted in increased regulatory T cells (Treg) and polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) frequencies in the lungs, two leukocyte populations that have immunosuppressive functions. In this study, we evaluated the anti-inflammatory effects of artepillin C (ArtC), the major compound of green propolis, in the context of allergic airway inflammation. Our results show that ArtC induces in vitro differentiation of Treg cells and monocytic MDSC (M-MDSC). Furthermore, in an OVA-induced asthma model, ArtC treatment reduced pulmonary inflammation, eosinophil influx to the airways, mucus and IL-5 secretion along with increased frequency of M-MDSC, but not Treg cells, in the lungs. Using an adoptive transfer model, we confirmed that the effect of ArtC in the reduction in airway inflammation was dependent on M-MDSC. Altogether, our data show that ArtC exhibits an anti-inflammatory effect and might be an adjuvant therapy for allergic asthma.

A Complex Metabolic Network Confers Immunosuppressive Functions to Myeloid-Derived Suppressor Cells (MDSCs) within the Tumour Microenvironment

Myeloid-derived suppressor cells (MDSCs) constitute a plastic and heterogeneous cell population among immune cells within the tumour microenvironment (TME) that support cancer progression and resistance to therapy. During tumour progression, cancer cells modify their metabolism to sustain an increased energy demand to cope with uncontrolled cell proliferation and differentiation. This metabolic reprogramming of cancer establishes competition for nutrients between tumour cells and leukocytes and most importantly, among tumour-infiltrating immune cells. Thus, MDSCs that have emerged as one of the most decisive immune regulators of TME exhibit an increase in glycolysis and fatty acid metabolism and also an upregulation of enzymes that catabolise essential metabolites. This complex metabolic network is not only crucial for MDSC survival and accumulation in the TME but also for enhancing immunosuppressive functions toward immune effectors. In this review, we discuss recent progress in the field of MDSC-associated metabolic pathways that could facilitate therapeutic targeting of these cells during cancer progression.

Plasma from patients with bacterial sepsis or severe COVID-19 induces suppressive myeloid cell production from hematopoietic progenitors in vitro

Bacterial sepsis and severe COVID-19 share similar clinical manifestations and are both associated with dysregulation of the myeloid cell compartment. We previously reported an expanded CD14+ monocyte state, MS1, in patients with bacterial sepsis and validated expansion of this cell subpopulation in publicly available transcriptomics data. Here, using published datasets, we show that the gene expression program associated with MS1 correlated with sepsis severity and was up-regulated in monocytes from patients with severe COVID-19. To examine the ontogeny and function of MS1 cells, we developed a cellular model for inducing CD14+ MS1 monocytes from healthy bone marrow hematopoietic stem and progenitor cells (HSPCs). We found that plasma from patients with bacterial sepsis or COVID-19 induced myelopoiesis in HSPCs in vitro and expression of the MS1 gene program in monocytes and neutrophils that differentiated from these HSPCs. Furthermore, we found that plasma concentrations of IL-6, and to a lesser extent IL-10, correlated with increased myeloid cell output from HSPCs in vitro and enhanced expression of the MS1 gene program. We validated the requirement for these two cytokines to induce the MS1 gene program through CRISPR-Cas9 editing of their receptors in HSPCs. Using this cellular model system, we demonstrated that induced MS1 cells were broadly immunosuppressive and showed decreased responsiveness to stimulation with a synthetic RNA analog. Our in vitro study suggests a potential role for systemic cytokines in inducing myelopoiesis during severe bacterial or SARS-CoV-2 infection.

The immune modulatory effects of umbilical cord-derived mesenchymal stromal cells in severe COVID-19 pneumonia

Coronavirus disease 2019 (COVID-19) may result in a life-threatening condition due to a hyperactive immune reaction to severe acute respiratory syndrome-coronavirus-2 infection, for which no effective treatment is available. Based on the potent immunomodulatory properties of mesenchymal stromal cells (MSCs), a growing number of trials are ongoing. This prompted us to carry out a thorough immunological study in a patient treated with umbilical cord-derived MSCs and admitted to the Intensive Care Unit for COVID-19-related pneumonia. The exploratory analyses were assessed on both peripheral blood and bronchoalveolar fluid lavage samples at baseline and after cellular infusion by means of single-cell RNA sequencing, flow cytometry, ELISA, and functional assays. Remarkably, a normalization of circulating T lymphocytes count paralleled by a reduction of inflammatory myeloid cells, and a decrease in serum levels of pro-inflammatory cytokines, mostly of interleukin-6 and tumor necrosis factor-α, were observed. In addition, a drop of plasma levels of those chemokines essential for neutrophil recruitment became evident that paralleled the decrease of lung-infiltrating inflammatory neutrophils. Finally, circulating monocytes and low-density gradient neutrophils acquired immunosuppressive function. This scenario was accompanied by an amelioration of respiratory, renal, inflammatory, and pro-thrombotic indexes. Our results provide the first immunological data possibly related to the use of umbilical cord-derived MSCs in severe COVID-19 context.

Synchronous effects of targeted mitochondrial complex I inhibitors on tumor and immune cells abrogate melanoma progression

Metabolic heterogeneity within the tumor microenvironment promotes cancer cell growth and immune suppression. We determined the impact of mitochondria-targeted complex I inhibitors (Mito-CI) in melanoma. Mito-CI decreased mitochondria complex I oxygen consumption, Akt-FOXO signaling, blocked cell cycle progression, melanoma cell proliferation and tumor progression in an immune competent model system. Immune depletion revealed roles for T cells in the antitumor effects of Mito-CI. While Mito-CI preferentially accumulated within and halted tumor cell proliferation, it also elevated infiltration of activated effector T cells and decreased myeloid-derived suppressor cells (MDSC) as well as tumor-associated macrophages (TAM) in melanoma tumors in vivo. Anti-proliferative doses of Mito-CI inhibited differentiation, viability, and the suppressive function of bone marrow-derived MDSC and increased proliferation-independent activation of T cells. These data indicate that targeted inhibition of complex I has synchronous effects that cumulatively inhibits melanoma growth and promotes immune remodeling.

Deciphering the state of immune silence in fatal COVID-19 patients

Since the beginning of the SARS-CoV-2 pandemic, COVID-19 appeared as a unique disease with unconventional tissue and systemic immune features. Here we show a COVID-19 immune signature associated with severity by integrating single-cell RNA-seq analysis from blood samples and broncho-alveolar lavage fluids with clinical, immunological and functional ex vivo data. This signature is characterized by lung accumulation of naïve lymphoid cells associated with a systemic expansion and activation of myeloid cells. Myeloid-driven immune suppression is a hallmark of COVID-19 evolution, highlighting arginase-1 expression with immune regulatory features of monocytes. Monocyte-dependent and neutrophil-dependent immune suppression loss is associated with fatal clinical outcome in severe patients. Additionally, our analysis shows a lung CXCR6⁺ effector memory T cell subset is associated with better prognosis in patients with severe COVID-19. In summary, COVID-19-induced myeloid dysregulation and lymphoid impairment establish a condition of 'immune silence' in patients with critical COVID-19.

Deciphering the state of immune silence in fatal COVID-19 patients

Since the beginning of the SARS-CoV-2 pandemic, COVID-19 appeared as a unique disease with unconventional tissue and systemic immune features. Here we show a COVID-19 immune signature associated with severity by integrating single-cell RNA-seq analysis from blood samples and broncho-alveolar lavage fluids with clinical, immunological and functional ex vivo data. This signature is characterized by lung accumulation of naïve lymphoid cells associated with a systemic expansion and activation of myeloid cells. Myeloid-driven immune suppression is a hallmark of COVID-19 evolution, highlighting arginase-1 expression with immune regulatory features of monocytes. Monocyte-dependent and neutrophil-dependent immune suppression loss is associated with fatal clinical outcome in severe patients. Additionally, our analysis shows a lung CXCR6+ effector memory T cell subset is associated with better prognosis in patients with severe COVID-19. In summary, COVID-19-induced myeloid dysregulation and lymphoid impairment establish a condition of 'immune silence' in patients with critical COVID-19.

CD84 is a regulator of the immunosuppressive microenvironment in multiple myeloma

Multiple myeloma (MM) is characterized by an accumulation of malignant plasma cells (PCs) within the BM. The BM microenvironment supports survival of the malignant cells and is composed of cellular fractions that foster myeloma development and progression by suppression of the immune response. Despite major progress in understanding the biology and pathophysiology of MM, this disease is still incurable and requires aggressive treatment with significant side effects. CD84 is a self-binding immunoreceptor belonging to the signaling lymphocyte activation molecule (SLAM) family. Previously, we showed that CD84 bridges between chronic lymphocytic leukemia cells and their microenvironment, and it regulates T cell function. In the current study, we investigated the role of CD84 in MM. Our results show that MM cells express low levels of CD84. However, these cells secrete the cytokine macrophage migration inhibitory factor (MIF), which induces CD84 expression on cells in their microenvironment. Its activation leads to an elevation of expression of genes regulating differentiation to monocytic/granulocytic–myeloid-derived suppressor cells (M-MDSCs and G-MDSCs, respectively) and upregulation of PD-L1 expression on MDSCs, which together suppress T cell function. Downregulation of CD84 or its blocking reduce MDSC accumulation, resulting in elevated T cell activity and reduced tumor load. Our data suggest that CD84 might serve as a novel therapeutic target in MM.

Single-Cell Transcriptome Analysis Workflow for Splenic Myeloid-Derived Suppressor Cells from Murine Breast Cancer Models

Single-cell transcriptomics is a powerful tool to study previously unrealized cellular heterogeneity at the resolution of individual cells. Most of the previous knowledge in cell biology is based on data generated by bulk analysis methods, which provide averaged readouts that usually mask cellular heterogeneity. This approach is challenging when the biological effect of interest is limited to a subpopulation within a cell type. This may particularly apply immune cell populations as these cells are highly mobile and swiftly respond to changes in cytokines or chemokines. For example, in cancer certain subset of myeloid immune cells may acquire immunosuppressive features to suppress antitumor immune responses, and thus described as myeloid-derived suppressor cells (MDSCs). Advances in single-cell RNA sequencing (scRNAseq) allowed scientists to overcome this limitation and enable in-depth interrogation of these subsets of immune cells including MDSCs. Here, we provide a detailed protocol for using scRNAseq to explore MDSCs in the context of splenic myeloid cells from breast tumor-bearing mice in comparison to wildtype controls to define the unique molecular features of immunosuppressive myeloid cells.

Phenotypical Characterization and Isolation of Tumor-Derived Mouse Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous cell population composed of mature and immature cells of myeloid origin that play a major role in tumor progression by inhibiting the antitumor immune responses mediated by T cells. In this chapter, we describe protocols for isolation, phenotypical and functional evaluation of MDSCs isolated from mouse tumors, with the aim at unifying and standardizing protocols set up by different laboratories.

Induction of a regulatory myeloid program in bacterial sepsis and severe COVID-19

A recent estimate suggests that one in five deaths globally are associated with sepsis ¹ . To date, no targeted treatment is available for this syndrome, likely due to substantial patient heterogeneity ^2,3 and our lack of insight into sepsis immunopathology ⁴ . These issues are highlighted by the current COVID-19 pandemic, wherein many clinical manifestations of severe SARS-CoV-2 infection parallel bacterial sepsis ^5-8 . We previously reported an expanded CD14+ monocyte state, MS1, in patients with bacterial sepsis or non-infectious critical illness, and validated its expansion in sepsis across thousands of patients using public transcriptomic data ⁹ . Despite its marked expansion in the circulation of bacterial sepsis patients, its relevance to viral sepsis and association with disease outcomes have not been examined. In addition, the ontogeny and function of this monocyte state remain poorly characterized. Using public transcriptomic data, we show that the expression of the MS1 program is associated with sepsis mortality and is up-regulated in monocytes from patients with severe COVID-19. We found that blood plasma from bacterial sepsis or COVID-19 patients with severe disease induces emergency myelopoiesis and expression of the MS1 program, which are dependent on the cytokines IL-6 and IL-10. Finally, we demonstrate that MS1 cells are broadly immunosuppressive, similar to monocytic myeloid-derived suppressor cells (MDSCs), and have decreased responsiveness to stimulation. Our findings highlight the utility of regulatory myeloid cells in sepsis prognosis, and the role of systemic cytokines in inducing emergency myelopoiesis during severe bacterial and SARS-CoV-2 infections.

Physical activity delays accumulation of immunosuppressive myeloid-derived suppressor cells

Background

Myeloid-derived suppressor cells (MDSCs) are potent suppressors of immune function and may play a key role in the development and progression of metastatic cancers. Aerobic exercise has been shown to have anticancer effects, yet the mechanisms behind this protection are largely unknown. Therefore, we examined the effects of physical activity on MDSC accumulation and function.

Methods

Female BALB/c mice were assigned to one of two primary groups: sedentary tumor (SED+TUM) or wheel run tumor (WR+TUM). After 6 weeks of voluntary wheel running, all animals were randomly subdivided into 4 different timepoint groups; 16, 20, 24, and 28 days post-tumor injection. All mice were inoculated with 4T1 mammary carcinoma cells in the mammary fat pad and WR groups continued to run for the specified time post-injection. Spleen, blood, and tumor samples were analyzed using flow cytometry to assess proportions of MDSCs.

Results

Cells expressing MDSC biomarkers were detected in the spleen, blood, and tumor beginning at d16. However, since there was no evidence of immunosuppressive function until d28, we refer to them as immature myeloid cells (IMCs). Compared to SED+TUM, levels of IMCs in the spleen were significantly lower (p < 0.05) in WR+TUM at day 16 (33.0 ± 5.2%; 23.1 ± 10.2% of total cells, respectively) and day 20 (33.9 ± 8.1%; 24.3 ± 5.1% of total cells, respectively). Additionally, there were fewer circulating IMCs in WR+TUM at day 16 and MDSC levels were significantly lower (p < 0.05) in the tumor at day 28 in WR+TUM. Additionally, a non-significant 62% and 26% reduction in metastatic lung nodules was observed at days 24 and 28, respectively. At day 28, MDSCs harvested from SED+TUM significantly suppressed CD3⁺CD4⁺ T cell proliferation (3.2 ± 1.3 proliferation index) while proliferation in WR+TUM MDSC co-cultures (5.1 ± 1.7 proliferation index) was not different from controls.

Conclusions

These findings suggest that physical activity may delay the accumulation of immunosuppressive MDSCs providing a broader window of opportunity for interventions with immunotherapies.

The Engagement Between MDSCs and Metastases: Partners in Crime

Tumor metastases represent the major cause of cancer-related mortality, confirming the urgent need to identify key molecular pathways and cell-associated networks during the early phases of the metastatic process to develop new strategies to either prevent or control distal cancer spread. Several data revealed the ability of cancer cells to establish a favorable microenvironment, before their arrival in distant organs, by manipulating the cell composition and function of the new host tissue where cancer cells can survive and outgrow. This predetermined environment is termed “pre-metastatic niche” (pMN). pMN development requires that tumor-derived soluble factors, like cytokines, growth-factors and extracellular vesicles, genetically and epigenetically re-program not only resident cells (i.e., fibroblasts) but also non-resident cells such as bone marrow-derived cells. Indeed, by promoting an “emergency” myelopoiesis, cancer cells switch the steady state production of blood cells toward the generation of pro-tumor circulating myeloid cells defined as myeloid-derived suppressor cells (MDSCs) able to sustain tumor growth and dissemination. MDSCs are a heterogeneous subset of myeloid cells with immunosuppressive properties that sustain metastatic process. In this review, we discuss current understandings of how MDSCs shape and promote metastatic dissemination acting in each fundamental steps of cancer progression from primary tumor to metastatic disease.

Immunoregulation and Clinical Implications of ANGPT2/TIE2+ M-MDSC Signature in Non-Small Cell Lung Cancer

Myeloid-derived suppressor cells (MDSC) promote immunosuppression and are a target in the field of immuno-oncology. Accumulation of MDSCs is associated with poor prognosis and resistance to immunotherapy for several cancers. Here, we describe an accumulation of a subset of circulating monocytic MDSCs (M-MDSC) overexpressing TIE2, the receptor for angiopoietin-2 (ANGPT2), in patients with non-small cell lung cancer (NSCLC). Greater numbers of circulating TIE2⁺ M-MDSCs were detected in patients with NSCLC compared with healthy subjects, and this accumulation correlated with ANGPT2 concentration in blood. The presence of an ANGPT2-rich environment was associated with impairment of preexisting T-cell responses against tumor-associated antigens (TAA) in patients with NSCLC. We demonstrated that ANGPT2 sensitizes TIE2⁺ M-MDSCs such that these cells suppress TAA-specific T cells. In patients with NSCLC, upregulation of the ANGPT2/TIE2⁺ M-MDSC signature in blood was associated with a poor prognosis. Our results identify the ANGPT2/TIE2⁺ M-MDSC axis as a participant in tumor immune evasion that should be taken into account in future cancer immunotherapy.

Immunosuppression by monocytic myeloid-derived suppressor cells in patients with pancreatic ductal carcinoma is orchestrated by STAT3

Background

Pancreatic ductal adenocarcinoma (PDAC) is a highly devastating disease with an overall 5-year survival rate of less than 8%. New evidence indicates that PDAC cells release pro-inflammatory metabolites that induce a marked alteration of normal hematopoiesis, favoring the expansion and accumulation of myeloid-derived suppressor cells (MDSCs). We report here that PDAC patients show increased levels of both circulating and tumor-infiltrating MDSC-like cells.

Methods

The frequency of MDSC subsets in the peripheral blood was determined by flow cytometry in three independent cohorts of PDAC patients (total analyzed patients, n = 117). Frequency of circulating MDSCs was correlated with overall survival of PDAC patients. We also analyzed the frequency of tumor-infiltrating MDSC and the immune landscape in fresh biopsies. Purified myeloid cell subsets were tested in vitro for their T-cell suppressive capacity.

Results

Correlation with clinical data revealed that MDSC frequency was significantly associated with a shorter patients’ overall survival and metastatic disease. However, the immunosuppressive activity of purified MDSCs was detectable only in some patients and mainly limited to the monocytic subset. A transcriptome analysis of the immunosuppressive M-MDSCs highlighted a distinct gene signature in which STAT3 was crucial for monocyte re-programming. Suppressive M-MDSCs can be characterized as circulating STAT3/arginase1-expressing CD14⁺ cells.

Conclusion

MDSC analysis aids in defining the immune landscape of PDAC patients for a more appropriate diagnosis, stratification and treatment.