Myeloid-derived suppressor cells can be efficiently generated from human hematopoietic progenitors and peripheral blood monocytes

MDSC expansion during HIV infection: regulators, ART and immune reconstitution

Myeloid-derived suppressor cells (MDSCs) become expanded in different pathological conditions including human immunodeficiency virus (HIV) infection and this may worsen the disease status and accelerate disease progression. In HIV infection, MDSCs suppress anti-HIV immune responses and hamper immune reconstitution. Understanding the factors and mechanisms of MDSC expansion during HIV infection is central to understanding the pathophysiology of HIV infection. This may pave the way to developing new therapeutic targets or strategies. In this work we addressed (i) the mechanisms that regulate MDSC expansion, (ii) the impact of antiretroviral therapy (ART) on the frequency of MDSCs during HIV infection; (iii) the impact of MDSCs on immune reconstitution during successful ART; and (iv) the potential of MDSCs as a therapeutic target.

Radiation Therapy and Myeloid-Derived Suppressor Cells: Breaking Down Their Cancerous Partnership

Radiation therapy (RT) has been a primary treatment modality in cancer for decades. Increasing evidence suggests that RT can induce an immunosuppressive shift via upregulation of cells such as tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs). MDSCs inhibit antitumor immunity through potent immunosuppressive mechanisms and have the potential to be crucial tools for cancer prognosis and treatment. MDSCs interact with many different pathways, desensitizing tumor tissue and interacting with tumor cells to promote therapeutic resistance. Vascular damage induced by RT triggers an inflammatory signaling cascade and potentiates hypoxia in the tumor microenvironment (TME). RT can also drastically modify cytokine and chemokine signaling in the TME to promote the accumulation of MDSCs. RT activation of the cGAS-STING cytosolic DNA sensing pathway recruits MDSCs through a CCR2-mediated mechanism, inhibiting the production of type 1 interferons and hampering antitumor activity and immune surveillance in the TME. The upregulation of hypoxia-inducible factor-1 and vascular endothelial growth factor mobilizes MDSCs to the TME. After recruitment, MDSCs promote immunosuppression by releasing reactive oxygen species and upregulating nitric oxide production through inducible nitric oxide synthase expression to inhibit cytotoxic activity. Overexpression of arginase-1 on subsets of MDSCs degrades L-arginine and downregulates CD3ζ, inhibiting T-cell receptor reactivity. This review explains how radiation promotes tumor resistance through activation of immunosuppressive MDSCs in the TME and discusses current research targeting MDSCs, which could serve as a promising clinical treatment strategy in the future.

EP2 and EP4 blockade prevents tumor-induced suppressive features in human monocytic myeloid-derived suppressor cells

Tumors educate their environment to prime the occurrence of suppressive cell subsets, which enable tumor evasion and favors tumor progression. Among these, there are the myeloid-derived suppressor cells (MDSCs), their presence being associated with the poor clinical outcome of cancer patients. Tumor-derived prostaglandin E2 (PGE2) is known to mediate MDSC differentiation and the acquisition of pro-tumor features. In myeloid cells, PGE2 signaling is mediated via E-prostanoid receptor type 2 (EP2) and EP4. Although the suppressive role of PGE2 is well established in MDSCs, the role of EP2/4 on human MDSCs or whether EP2/4 modulation can prevent MDSCs suppressive features upon exposure to tumor-derived PGE2 is poorly defined. In this study, using an in vitro model of human monocytic-MDSCs (M-MDSCs) we demonstrate that EP2 and EP4 signaling contribute to the induction of a pro-tumor phenotype and function on M-MDSCs. PGE2 signaling via EP2 and EP4 boosted M-MDSC ability to suppress T and NK cell responses. Combined EP2/4 blockade on M-MDSCs during PGE2 exposure prevented the occurrence of these suppressive features. Additionally, EP2/4 blockade attenuated the suppressive phenotype of M-MDSCs in a 3D coculture with colorectal cancer patient-derived organoids. Together, these results identify the role of tumor-derived PGE2 signaling via EP2 and EP4 in this human M-MDSC model, supporting the therapeutic value of targeting PGE2-EP2/4 axis in M-MDSCs to alleviate immunosuppression and facilitate the development of anti-tumor immunity.

Sargramostim for Prophylactic Management of Gastrointestinal Immune-Related Adverse Events of Immune Checkpoint Inhibitor Therapy for Cancer

Immune checkpoint inhibitor (ICI) therapy improves outcomes in several cancers. Unfortunately, many patients experience grade 3-4 treatment-related adverse events, including gastrointestinal (GI) toxicities which are common. These GI immune-related adverse events (irAEs) induced by ICIs present significant clinical challenges, require prompt intervention, and result in treatment delays or discontinuations. The treatment for these potentially severe and even fatal GI irAEs which include enterocolitis, severe diarrhea, and hepatitis may interfere with the anti-cancer approach. Sargramostim (glycosylated, yeast-derived, recombinant human GM-CSF) is an agent that has been used in clinical practice for more than 30 years with a well-recognized safety profile and has been studied in many therapeutic areas. The mechanism of action of sargramostim may treat moderate-to-severe GI irAEs without impairing the anti-cancer therapy. Some early data also suggest a potential survival benefit. Through the differentiation/maturation of monocytes, macrophages, and neutrophils and induction of anti-inflammatory T cell responses, GM-CSF aids in GI homeostasis, mucosal healing, and mucosal immunity. GM-CSF knockout mice are susceptible to severe colitis which was prevented with murine GM-CSF administration. For some patients with GI mucosa and immune cell function impairment, e.g., Crohn's disease, sargramostim reduces disease severity. In a prospective, randomized study (ECOG 1608), advanced melanoma patients had a reduction in grade 3-5 GI irAEs and less frequent colonic perforation in the sargramostim plus ipilimumab arm compared to ipilimumab alone. Sargramostim continues to be studied with ICIs for the prophylactic management of irAEs while also potentially providing a survival benefit.

Myeloid-derived suppressor cells and tolerogenic dendritic cells are distinctively induced by PI3K and Wnt signaling pathways

Imbalanced immune responses are a prominent hallmark of cancer and autoimmunity. Myeloid cells can be overly suppressive, inhibiting protective immune responses or inactive not controlling autoreactive immune cells. Understanding the mechanisms that induce suppressive myeloid cells, such as myeloid-derived suppressor cells (MDSCs) and tolerogenic dendritic cells (TolDCs), can facilitate the development of immune-restoring therapeutic approaches. MDSCs are a major barrier for effective cancer immunotherapy by suppressing antitumor immune responses in cancer patients. TolDCs are administered to patients to promote immune tolerance with the intent to control autoimmune disease. Here, we investigated the development and suppressive/tolerogenic activity of human MDSCs and TolDCs to gain insight into signaling pathways that drive immunosuppression in these different myeloid subsets. Moreover, monocyte-derived MDSCs (M-MDSCs) generated in vitro were compared to M-MDSCs isolated from head-and-neck squamous cell carcinoma patients. PI3K-AKT signaling was identified as being crucial for the induction of human M-MDSCs. PI3K inhibition prevented the downregulation of HLA-DR and the upregulation of reactive oxygen species and MerTK. In addition, we show that the suppressive activity of dexamethasone-induced TolDCs is induced by β-catenin-dependent Wnt signaling. The identification of PI3K-AKT and Wnt signal transduction pathways as respective inducers of the immunomodulatory capacity of M-MDSCs and TolDCs provides opportunities to overcome suppressive myeloid cells in cancer patients and optimize therapeutic application of TolDCs. Lastly, the observed similarities between generated- and patient-derived M-MDSCs support the use of in vitro-generated M-MDSCs as powerful model to investigate the functionality of human MDSCs.

M-MDSC in vitro generation from mouse bone marrow with IL-3 reveals high expression and functional activity of arginase 1

Myeloid-derived suppressor cells (MDSC) represent major regulators of immune responses, which can control T cells via their inducible nitric oxide synthase (iNOS)- and arginase 1 (Arg1)-mediated effector functions. While GM-CSF is well documented to promote MDSC development, little is known about this potential of IL-3, an established growth factor for mast cells. Here, we show that IL-3, similar to GM-CSF, generates monocytic MDSC (M-MDSC) from murine bone marrow (BM) cells after 3 days of in vitro culture. At this time point, predominantly CD11b⁺ CD49a⁺ monocytic and CD11b⁺ CD49a^- FcεR I^- neutrophilic cells were detectable, while CD11b^low/neg FcεR I⁺ mast cells accumulated only after extended culture periods. Both growth factors were equivalent in generating M-MDSC with respect to phenotype, cell yield and typical surface markers. However, IL-3 generated M-MDSC produced less TNF, IL-1β and IL-10 after activation with LPS + IFN-γ but showed higher Arg1 expression compared to GM-CSF generated M-MDSC. Arg1 was further induced together with iNOS after MDSC activation. Accordingly, an increased Arg1-dependent suppressor activity by the IL-3 generated M-MDSC was observed using respective iNOS and Arg1 inhibitors. Together, these data indicate that M-MDSC can be generated in vitro by IL-3, similar to GM-CSF, but with increased Arg1 expression and Arg1-mediated suppression capacity. This protocol now allows further in vitro studies on the role of IL-3 for MDSC biology.

Increase in Frequency of Myeloid-Derived Suppressor Cells in the Bone Marrow of Myeloproliferative Neoplasm: Potential Implications in Myelofibrosis

The Philadelphia-negative myeloproliferative neoplasms (MPNs), defined as clonal disorders of the hematopoietic stem cells, are characterized by the proliferation of mature myeloid cells in the bone marrow and a chronic inflammatory status impacting the initiation, progression, and symptomatology of the malignancies. There are three main entities defined as essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF), and genetically classified by JAK2^V617F, CALR, or MPL mutations. In MPNs, due to the overproduction of inflammatory cytokines by the neoplastic cells and non-transformed immune cells, chronic inflammation may provoke the generation and expansion of myeloid-derived suppressors cells (MDSCs) that highly influence the adaptive immune response. Although peripheral blood MDSC levels are elevated, their frequency in the bone marrow of MPNs patients is not well elucidated yet. Our results indicated increased levels of total (T)-MDSCs (CD33⁺HLA-DR^-/low) and polymorphonuclear (PMN)-MDSCs (CD33⁺/HLA-DR^low/CD15⁺/CD14^-) in the bone marrow and peripheral blood of all three types of MPNs malignancies. However, these bone marrow MDSCs-increased frequencies did not correlate with the clinical parameters, such as hepatomegaly, leukocytes, hemoglobin, or platelet levels, or with JAK2 and CALR mutations. Besides, bone marrow MDSCs, from ET, PV, and PMF patients, exhibited immunosuppressive function, determined as T-cell proliferation inhibition. Notably, the highest T-MDSCs and PMN-MDSC levels were found in PMF samples, and the increased MDSCs frequency strongly correlated with the degree of myelofibrosis. Thus, these data together indicate that the immunosuppressive MDSCs population is increased in the bone marrow of MPNs patients and may be implicated in generating a fibrotic microenvironment.

Transforming Growth Factor-β1 in Cancer Immunology: Opportunities for Immunotherapy

Transforming growth factor-beta1 (TGF-β) regulates a plethora of cell-intrinsic processes that modulate tumor progression in a context-dependent manner. Thus, although TGF-β acts as a tumor suppressor in the early stages of tumorigenesis, in late stages, this factor promotes tumor progression and metastasis. In addition, TGF-β also impinges on the tumor microenvironment by modulating the immune system. In this aspect, TGF-β exhibits a potent immunosuppressive effect, which allows both cancer cells to escape from immune surveillance and confers resistance to immunotherapy. While TGF-β inhibits the activation and antitumoral functions of T-cell lymphocytes, dendritic cells, and natural killer cells, it promotes the generation of T-regulatory cells and myeloid-derived suppressor cells, which hinder antitumoral T-cell activities. Moreover, TGF-β promotes tumor-associated macrophages and neutrophils polarization from M1 into M2 and N1 to N2, respectively. Altogether, these effects contribute to the generation of an immunosuppressive tumor microenvironment and support tumor promotion. This review aims to analyze the relevant evidence on the complex role of TGF-β in cancer immunology, the current outcomes of combined immunotherapies, and the anti-TGF-β therapies that may improve the success of current and new oncotherapies.

Induction of M‑MDSCs with IL6/GM‑CSF from adherence monocytes and inhibition by WP1066

Peripheral blood monocytes acquire the phenotype of myeloid-derived suppressor cells (MDSCs) by induction of cytokine or co-culture with cancer cells and are widely used to model MDSCs for in vitro studies. However, the simplest method of plastic adhesive sorting is poorly described as the purity of monocyte resulting from this method is the lowest compared with flow cytometry cell-sorting and magnetic beads sorting. Therefore, the present study aimed at investigating the effect of the plastic adhesive monocyte isolation techniques on the resulting MDSCs phenotype. Monocytes were allowed to adhere for 1 h and cultured with IL6 and granulocyte-macrophage colony-stimulating factors (GM-CSF) for 7 days. Plastic adhesion sorting resulted in early low monocyte yield and purity, but high purity of MDSCs was obtained by refreshing the induction medium. The resulting MDSCs were the major subpopulation of CD33⁺CD11b⁺CD14⁺CD15^-human leukocyte antigen (HLA)^-/low cells and provided the potent capacity to suppress T cell proliferation and cytokine IFN-γ production. Moreover, the induced MDSCs were inhibited by STAT3 inhibitor WP1066, resulting in downregulation of phosphorylated-STAT3 and PD-L1 expression and upregulation of apoptosis respectively. In conclusion, the present study described the generation of monocytic MDSCs from adherence monocytes and the inhibition of STAT3 inhibitor WP1066 on the induced MDSCs. The present study contributed to the development of a new clinical drug, WP1066 targeting MDSC.

Pseudocowpox virus, a novel vector to enhance the therapeutic efficacy of antitumor vaccination

Objective

Antitumor viral vaccines, and more particularly poxviral vaccines, represent an active field for clinical development and translational research. To improve the efficacy and treatment outcome, new viral vectors are sought, with emphasis on their abilities to stimulate innate immunity, to display tumor antigens and to induce a specific T‐cell response.

Methods

We screened for a new poxviral backbone with improved innate and adaptive immune stimulation using IFN‐α secretion levels in infected PBMC cultures as selection criteria. Assessment of virus effectiveness was made in vitro and in vivo.

Results

The bovine pseudocowpox virus (PCPV) stood out among several poxviruses for its ability to induce significant secretion of IFN‐α. PCPV produced efficient activation of human monocytes and dendritic cells, degranulation of NK cells and reversed MDSC‐induced T‐cell suppression, without being offensive to activated T cells. A PCPV‐based vaccine, encoding the HPV16 E7 protein (PCPV‐E7), stimulated strong antigen‐specific T‐cell responses in TC1 tumor‐bearing mice. Complete regression of tumors was obtained in a CD8⁺ T‐cell‐dependent manner after intratumoral injection of PCPV‐E7, followed by intravenous injection of the cancer vaccine MVA‐E7. PCPV also proved active when injected repeatedly intratumorally in MC38 tumor‐bearing mice, generating tumor‐specific T‐cell responses without encoding a specific MC38 antigen. From a translational perspective, we demonstrated that PCPV‐E7 effectively stimulated IFN‐γ production by T cells from tumor‐draining lymph nodes of HPV⁺‐infected cancer patients.

Conclusion

We propose PCPV as a viral vector suitable for vaccination in the field of personalised cancer vaccines, in particular for heterologous prime‐boost regimens.

Anatomical Distribution of Myeloid-Derived Suppressor Cells During HIV Infection

In recent years, expansion of myeloid-derived suppressor cells (MDSCs) has been reported to play a detrimental role in the pathogenesis of human immunodeficiency virus (HIV) infection. Much effort has been focused to comprehend the mechanisms and factors that regulate the expansion of such unwanted immune cell populations. Of particular interest has been the mechanisms by which MDSCs could contribute to the pathogenesis of HIV infection. So far, the studies have been restricted to MDSCs in the circulatory system of HIV patients, but not in other tissue compartments. In fact, lymphatic tissues/organs are the primary sites where HIV replication and immune depletion/dysfunction occur during the course of HIV infection. Therefore, investigating the anatomical distribution of MDSCs in such compartments is essential to understand the role that they play in the pathogenesis of HIV infection. Hence, we aim to shed light on the available literature about the anatomical distribution of MDSCs during HIV infection and compare it with the distribution of MDSCs in other pathological conditions, mainly cancer.

Selectively targeting myeloid-derived suppressor cells through TRAIL receptor 2 to enhance the efficacy of CAR T cell therapy for treatment of breast cancer

BACKGROUND

Successful targeting of solid tumors such as breast cancer (BC) using chimeric antigen receptor (CAR) T cells has proven challenging, largely attributed to the immunosuppressive tumor microenvironment (TME). Myeloid-derived suppressor cells (MDSCs) inhibit CAR T cell function and persistence within the breast TME. To overcome this challenge, we have developed CAR T cells targeting tumor-associated mucin 1 (MUC1) with a novel chimeric costimulatory receptor that targets tumor necrosis factor-related apoptosis-inducing ligand receptor 2 (TR2) expressed on MDSCs.

METHODS

The function of the TR2.41BB costimulatory receptor was assessed by exposing non-transduced (NT) and TR2.41BB transduced T cells to recombinant TR2, after which nuclear translocation of NFκB was measured by ELISA and western blot. The cytolytic activity of CAR.MUC1/TR2.41BB T cells was measured in a 5-hour cytotoxicity assay using MUC1+ tumor cells as targets in the presence or absence of MDSCs. In vivo antitumor activity was assessed using MDSC-enriched tumor-bearing mice treated with CAR T cells with or without TR2.41BB.

RESULTS

Nuclear translocation of NFκB in response to recombinant TR2 was detected only in TR2.41BB T cells. The presence of MDSCs diminished the cytotoxic potential of CAR.MUC1 T cells against MUC1+ BC cell lines by 25%. However, TR2.41BB expression on CAR.MUC1 T cells induced MDSC apoptosis, thereby restoring the cytotoxic activity of CAR.MUC1 T cells against MUC1+ BC lines. The presence of MDSCs resulted in an approximately twofold increase in tumor growth due to enhanced angiogenesis and fibroblast accumulation compared with mice with tumor alone. Treatment of these MDSC-enriched tumors with CAR.MUC1.TR2.41BB T cells led to superior tumor cell killing and significant reduction in tumor growth (24.54±8.55 mm³) compared with CAR.MUC1 (469.79±81.46 mm³) or TR2.41BB (434.86±64.25 mm³) T cells alone. CAR.MUC1.TR2.41BB T cells also demonstrated improved T cell proliferation and persistence at the tumor site, thereby preventing metastases. We observed similar results using CAR.HER2.TR2.41BB T cells in a HER2+ BC model.

CONCLUSIONS

Our findings demonstrate that CAR T cells that coexpress the TR2.4-1BB receptor exhibit superior antitumor potential against breast tumors containing immunosuppressive and tumor promoting MDSCs, resulting in TME remodeling and improved T cell proliferation at the tumor site.

Polymorphonuclear myeloid-derived suppressor cells link inflammation and damage response after trauma

Elimination of the posttraumatic inflammatory response and recovery of homeostasis are crucial for the positive prognosis of trauma patients. Myeloid-derived suppressor cells (MDSCs) are known to play a regulatory role in the posttraumatic immune response in mice, but their induction source and involved potential mechanism are poorly understood. Here, we report that polymorphonuclear MDSCs (PMN-MDSCs) are activated after trauma and are closely associated with the progression of the posttraumatic inflammatory response. In humans, lectin-type oxidized LDL receptor 1 (LOX1) was used to specifically characterize LOX1⁺ PMN-MDSCs. Trauma patients showed high intracellular reactive oxygen species (ROS) production, as well as activation of LOX1⁺ PMN-MDSCs. These MDSCs contribute to the anti-inflammatory immune response by regulating the Treg/Th17 and Th2/Th1 balances after trauma, increasing the levels of anti-inflammatory factors, and decreasing the levels of proinflammatory factors. The number of LOX1⁺ PMN-MDSCs was positively correlated with the positive clinical prognosis of trauma patients with infection. Activation of LOX1⁺ PMN-MDSCs is mediated by NF-κB signal, and TGF-β1 may be as an important inducer for LOX1⁺ PMN-MDSCs in the posttraumatic cytokine environment. In a pseudofracture trauma mouse model, we also observed the activation of PMN-MDSCs, accompanying high levels of intracellular ROS production, NF-κB phosphorylation, and changes in the inflammatory environment, in particularly by regulating the Treg/Th17 and Th2/Th1 balance. And more significantly, posttraumatic inflammation was alleviated in mice after transferring trauma-derived PMN-MDSCs, but aggravated after injecting with Gr1 agonistic antibody. These findings provide evidence for the specific role of PMN-MDSCs in the regulation of posttraumatic inflammation.

Cancer-associated fibroblasts induce monocytic myeloid-derived suppressor cell generation via IL-6/exosomal miR-21-activated STAT3 signaling to promote cisplatin resistance in esophageal squamous cell carcinoma

Drug resistance remains the major obstacle limiting the effectiveness of chemotherapy for esophageal squamous cell carcinoma (ESCC)[1]. However, how stromal cells cooperate with immune cells to contribute to drug resistance is not yet fully understood. In this study, we observed that monocytic myeloid-derived suppressor cells (M-MDSCs) were correlated with cisplatin resistance in patients with ESCC. Furthermore, CAFs promoted differentiation of monocytes into M-MDSCs phenotypically and functionally in vitro. Mechanically, both interleukin (IL)-6 and exosome-packed microRNA-21 (miR-21) secreted by CAFs synergistically promoted the generation of M-MDSCs via activating the signal transducing activator of transcription 3 (STAT3) by IL-6 in an autocrine manner. Combined blocking of IL-6 receptor and inhibition of miR-21 significantly reversed CAF-mediated M-MDSC generation. Notably, the effects of CAFs on M-MDSC induction were abolished by inhibiting STAT3 signaling. Functionally, CAF-induced M-MDSCs promoted drug resistance of tumor cells upon cisplatin treatment. Clinically, ESCC patients with high infiltration of CAFs and CD11b⁺ myeloid cells had unfavorable predicted overall survival both in our cohort and in TCGA data. Taken together, our study reveals a paracrine and autocrine of IL-6 caused by CAFs co-activate STAT3 signaling, promoting the generation of M-MDSCs, and highlights the important role of CAFs in cooperation with M-MDSCs in promoting drug resistance, thus providing potential opportunities for reversing drug resistance through inhibition of STAT3 signaling.

Three distinct tolerogenic CD14+ myeloid cell types to actively manage autoimmune disease: Opportunities and challenges

Current treatment for patients with autoimmune disorders including rheumatoid arthritis, multiple sclerosis and type 1 diabetes, often consists of long-term drug regimens that broadly dampen immune responses. These non-specific treatments are frequently associated with severe side effects creating an urgent need for safer and more effective therapy to promote peripheral tolerance in autoimmune diseases. Cell-based immunotherapy may offer an encouraging alternative, where tolerogenic CD14⁺ myeloid cells are infused to inhibit autoreactive effector cells. In this review, we compared in depth three promising tolerogenic CD14⁺ candidates for the treatment of autoimmune disease: 1) tolerogenic dendritic cells, 2) monocytic myeloid-derived suppressor cells and 3) CD14⁺ type 2 conventional dendritic cells. TolDC-based therapy has entered clinical testing whereas evidence from the latter two cell types m-MDSCs and CD14⁺ cDC2s is predominantly coming from cancer immunology research. These three cell types have distinct cellular properties and immunosuppressive mechanisms offering unique opportunities to be explored. However, these cells differ in stage of development towards immunotherapy each facing additional hurdles. Therefore, we speculate on the potential benefits and risks of these cell types as novel cell-based immunotherapies to control autoimmune disease in patients.

Transforming growth factor-beta1 and myeloid-derived suppressor cells: A cancerous partnership

Transforming growth factor-beta1 (TGF-β1) plays a crucial role in tumor progression. It can inhibit early cancer stages but promotes tumor growth and development at the late stages of tumorigenesis. TGF-β1 has a potent immunosuppressive function within the tumor microenvironment that largely contributes to tumor cells' immune escape and reduction in cancer immunotherapy responses. Likewise, myeloid-derived suppressor cells (MDSCs) have been postulated as leading tumor promoters and a hallmark of cancer immune evasion mechanisms. This review attempts to analyze the prominent roles of both TGF-β1 and MDSCs and their interplay in cancer immunity. Furthermore, therapies against either TGF-β1 or MDSCs, and their potential synergistic combination with immunotherapies are discussed. Simultaneous TGF-β1 and MDSCs inhibition suggest a potential improvement in immunotherapy or subverted tumor immune resistance.

Comments on the ambiguity of selected surface markers, signaling pathways and omics profiles hampering the identification of myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSC) are important immune-regulatory cells but their identification remains difficult. Here, we provide a critical view on selected surface markers, transcriptional and translational pathways commonly used to identify MDSC by specific, their developmental origin and new possibilities by transcriptional or proteomic profiling. Discrimination of MDSC from their non-suppressive counterparts is a prerequisite for the development of successful therapies. Understanding the switch mechanisms that direct granulocytic and monocytic development into a pro-inflammatory or anti-inflammatory direction will be crucial for therapeutic strategies. Manipulation of these myeloid checkpoints are exploited by tumors and pathogens, such as M. tuberculosis (Mtb), HIV or SARS-CoV-2, that induce MDSC for immune evasion. Thus, specific markers for MDSC identification may reveal also novel molecular candidates for therapeutic intervention at the level of MDSC.

Myeloid-derived suppressor cells as cellular immunotherapy in transplantation and autoimmune diseases

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells, which have been characterized for their immunosuppressive capacity through multiple mechanisms. These cells have been extensively studied in the field of tumor immunity. Emerging evidence has highlighted its essential role in maintaining immune tolerance in transplantation and autoimmunity. Because of their robust immune inhibitory activities, there has been growing interest in MDSC-based cellular therapy. Various pre-clinical studies have demonstrated that the adoptive transfer of MDCS represented a promising therapeutic strategy for immune-related disorders. In this review, we summarize relevant studies of MDSC-based cell therapy in transplantation and autoimmune diseases and discuss the challenges and future directions for clinical application of MDSC-based cell therapy.

Reactive myelopoiesis and the onset of myeloid-mediated immune suppression: Implications for adoptive cell therapy

Adoptive T cell therapy (ACT) in combination with lymphodepleting chemotherapy is an effective strategy to induce the eradication of cancer, providing long-term regressions in patients. However, only a minority of patients that receive ACT with tumor-specific T cells exhibit durable benefit. Thus, there is an urgent need to characterize mechanisms of resistance and define strategies to alleviate immunosuppression in the context of ACT in cancer. This article reviews the importance of lymphodepleting regimens in promoting the optimal engraftment and expansion of T cells in hosts after adoptive transfer. In addition, we discuss the role of concomitant immunosuppression and the accumulation of myeloid derived suppressor cells (MDSCs) during immune recovery after lymphodepleting regimens and mobilization regimens.

Recent advances in myeloid-derived suppressor cell biology

In recent years, studying the role of myeloid-derived suppressor cells (MDSCs) in many pathological inflammatory conditions has become a very active research area. Although the role of MDSCs in cancer is relatively well established, their role in non-cancerous pathological conditions remains in its infancy resulting in much confusion. Our objectives in this review are to address some recent advances in MDSC research in order to minimize such confusion and to provide an insight into their function in the context of other diseases. The following topics will be specifically focused upon: (1) definition and characterization of MDSCs; (2) whether all MDSC populations consist of immature cells; (3) technical issues in MDSC isolation, estimation and characterization; (4) the origin of MDSCs and their anatomical distribution in health and disease; (5) mediators of MDSC expansion and accumulation; (6) factors that determine the expansion of one MDSC population over the other; (7) the Yin and Yang roles of MDSCs. Moreover, the functions of MDSCs will be addressed throughout the text.

A novel methodology of the myeloid-derived suppressor cells (MDSCs) generation with splenic stroma feeder cells

Myeloid-derived suppressor cells (MDSCs) are a significant obstacle for immunotherapy of cancer. It is of great clinical relevance to study the mechanism of MDSCs accumulation in mouse spleens and establish a stable method to obtain high-purity MDSCs in vitro for further research. Here, we established a new method for amplifying a large number of highly pure MDSCs in vitro. To mimic the microenvironment of MDSCs development in vivo, mouse splenic stroma feeder cells and serum-free medium containing granulocyte-macrophage colony stimulating factor (GM-CSF) were used to induce myeloid precursors in mouse bone marrow cells, which differentiate into MDSCs. Development and immunological functions of the cells were monitored both in vivo and in vitro. A total of 4 × 10⁸ MDSCs could be obtained from the bone marrow from one mouse, the ratio of CD11b⁺Gr-1⁺ MDSCs could reach 93.8% ± 3.3% after nine days of culture in vitro. Cultured MDSCs maintained a similar immunophenotype with MDSCs found in tumor-bearing mice. Colony forming assay in vitro and in vivo demonstrated that these were myeloid precursor cells. These cells generated high levels of reactive oxygen species and arginase 1 to prevent proliferation of CD8⁺ T cells in vitro. These also increased regulatory T (Treg) cells in blood while promoting the growth of lymphoma in vivo. In addition, cultured MDSCs effectively inhibited acute graft-versus-host disease (aGVHD). Our findings suggest that mouse splenic stroma plays an important role in the generation of MDSCs and represent a preliminary mechanism for the accumulation of MDSCs in spleens, and thereby lay the foundation for basic research and the clinical application of MDSCs.

Myeloid-Derived Suppressor Cells in the Context of Allogeneic Hematopoietic Stem Cell Transplantation

Myeloid-derived suppressor cells (MDSCs) are innate immune cells that acquire the capacity to suppress adaptive immune responses. In the context of allogeneic hematopoietic stem cell transplantation (allo-HSCT), MDSCs (in the donor graft and in the recipient, after allo-HSCT) might mediate immune suppression through multiple mechanisms. However, it remains unclear how MDSCs can be distinguished from their normal myeloid counterparts in the hematopoietic stem cell donor graft and during immune reconstitution after allo-HSCT in the recipient. Our ability to understand their exact role in allo-HSCT is limited by the absence of a specific gene signature or surface markers for identifying MDSCs among myeloid cells and by their plasticity in different microenvironments. According to various studies, MDSCs might induce transplant tolerance and control graft vs. host disease (GVHD), but their impact on the graft vs. tumor effect (GVT) is not fully understood. In fact, we know that MDSCs commonly expand in patients with cancer, and they are thought to promote hematological malignancy progression. However, little is known about whether depleting them might be an effective strategy for enhancing GVT effects. Here, we review data published over the past 40 years on allo-HSCT to delineate the different MDSC subsets, and their abilities to induce transplant tolerance and preserve the GVT effect. This review will provide a basis for determining whether one MDSC subset might be proposed as the most appropriate candidate for cellular therapies, due to its ability to modulate GVHD.

Cysteine cathepsins L and X differentially modulate interactions between myeloid-derived suppressor cells and tumor cells

Increased proteolytic activity of cysteine cathepsins has long been known to facilitate malignant progression, and it has also been associated with tumor-promoting roles of myeloid-derived suppressor cells (MDSCs). Consequently, cysteine cathepsins have gained much attention as potential targets for cancer therapies. However, cross-talk between tumor cells and MDSCs needs to be taken into account when studying the efficacy of cathepsin inhibitors as anti-cancer agents. Here, we demonstrate the potential of the MDA-MB-231 breast cancer cell line to generate functional MDSCs from CD14⁺ cells of healthy human donors. During this transition to MDSCs, the overall levels of cysteine cathepsins increased, with the largest responses for cathepsins L and X. We used small-molecule inhibitors of cathepsins L and X (i.e., CLIK-148, Z9, respectively) to investigate their functional impact on tumor cells and immune cells in this co-culture system. Interactions with peripheral blood mononuclear cells reduced MDA-MB-231 cell invasion, while inhibition of cathepsin X activity by Z9 restored invasion. Inhibition of cathepsin L activity using CLIK-148 resulted in significantly increased CD8⁺ cytotoxicity. Of note, inhibition of cathepsins L and X in separate immune or tumor cells did not promote these functional changes. Together, our findings underlie the importance of tumor cell-immune cell interactions in the evaluation of the anti-cancer potential of cysteine cathepsin inhibitors.

Mycobacterium tuberculosis and myeloid-derived suppressor cells: Insights into caveolin rich lipid rafts

Mycobacterium tuberculosis (M.tb) is likely the most successful human pathogen, capable of evading protective host immune responses and driving metabolic changes to support its own survival and growth. Ineffective innate and adaptive immune responses inhibit effective clearance of the bacteria from the human host, resulting in the progression to active TB disease. Many regulatory mechanisms exist to prevent immunopathology, however, chronic infections result in the overproduction of regulatory myeloid cells, like myeloid-derived suppressor cells (MDSC), which actively suppress protective host T lymphocyte responses among other immunosuppressive mechanisms. The mechanisms of M.tb internalization by MDSC and the involvement of host-derived lipid acquisition, have not been fully elucidated. Targeted research aimed at investigating MDSC impact on phagocytic control of M.tb, would be advantageous to our collective anti-TB arsenal. In this review we propose a mechanism by which M.tb may be internalized by MDSC and survive via the manipulation of host-derived lipid sources.

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

We sought to identify tumor-secreted factors that altered the frequency of MDSCs and correlated with clinical outcomes in advanced melanoma patients. We focused our study on several of the many factors involved in the expansion and mobilization of MDSCs. These were identified by measuring circulating concentrations of 13 cytokines and growth factors in stage IV melanoma patients (n = 55) and healthy controls (n = 22). Based on these results, we hypothesized that IL-6 and IL-8 produced by melanoma tumor cells participate in the expansion and recruitment of MDSCs and together would be predictive of overall survival in melanoma patients. We then compared the expression of IL-6 and IL-8 in melanoma tumors to the corresponding plasma concentrations and the frequency of circulating MDSCs. These measures were correlated with clinical outcomes. Patients with high plasma concentrations of either IL-6 (40%) or IL-8 (63%), or both (35%) had worse median overall survival compared to patients with low concentrations. Patients with low peripheral concentrations and low tumoral expression of IL-6 and IL-8 showed decreased frequencies of circulating MDSCs, and patients with low frequencies of MDSCs had better overall survival. We have previously shown that IL-6 is capable of expanding MDSCs, and here we show that MDSCs are chemoattracted to IL-8. Multivariate analysis demonstrated an increased risk of death for subjects with both high IL-6 and IL-8 (HR 3.059) and high MDSCs (HR 4.265). Together these results indicate an important role for IL-6 and IL-8 in melanoma patients in which IL-6 potentially expands peripheral MDSCs and IL-8 recruits these highly immunosuppressive cells to the tumor microenvironment. This study provides further support for identifying potential therapeutics targeting IL-6, IL-8, and MDSCs to improve melanoma treatments.

Insights Into Mechanisms of Tumor and Immune System Interaction: Association With Wound Healing

A large number of studies have presented a great deal of information about tumor and immune system interaction. Nevertheless, the problem of tumor evasion from the immune reaction is still difficult to resolve. Understanding the ways in which immunosuppressive tumor microenvironment develops and maintains its potential is of utmost importance to ensure the best use of the suppressed immune functions. The study presents a review covering the data on tumor-associated antigens, mechanisms of tumor evasion from the immune reactions, and search for common immunosuppressive processes of tumor growth and normal wound healing. The study discusses the important role of monocytes/macrophages in the regulation of immune system reactions. We suggest that the simultaneous actions of growth factors and pro-inflammatory cytokines may result in the suppression of the immune system. The study describes intracellular signaling molecules that take part in the regulation of the myeloid cell functions. If the hypothesis is proved correct, the indicated interaction of cytokines could be regarded as a prospective target for antitumor therapy.

AMPK Alpha-1 Intrinsically Regulates the Function and Differentiation of Tumor Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSC) represent a primary mechanism of immune evasion in tumors and have emerged as a major obstacle for cancer immunotherapy. The immunoinhibitory activity of MDSC is tightly regulated by the tumor microenvironment and occurs through mechanistic mediators that remain unclear. Here, we elucidated the intrinsic interaction between the expression of AMP-activated protein kinase alpha (AMPKα) and the immunoregulatory activity of MDSC in tumors. AMPKα signaling was increased in tumor-MDSC from tumor-bearing mice and patients with ovarian cancer. Transcription of the Ampkα1-coding gene, Prkaa1, in tumor-MDSC was induced by cancer cell-derived granulocyte-monocyte colony-stimulating factor (GM-CSF) and occurred in a Stat5-dependent manner. Conditional deletion of Prkaa1 in myeloid cells, or therapeutic inhibition of Ampkα in tumor-bearing mice, delayed tumor growth, inhibited the immunosuppressive potential of MDSC, triggered antitumor CD8⁺ T-cell immunity, and boosted the efficacy of T-cell immunotherapy. Complementarily, therapeutic stimulation of AMPKα signaling intrinsically promoted MDSC immunoregulatory activity. In addition, Prkaa1 deletion antagonized the differentiation of monocytic-MDSC (M-MDSC) to macrophages and re-routed M-MDSC, but not granulocytic-MDSC (PMN-MDSC), into cells that elicited direct antitumor cytotoxic effects through nitric oxide synthase 2-mediated actions. Thus, our results demonstrate the primary role of AMPKα1 in the immunosuppressive effects induced by tumor-MDSC and support the therapeutic use of AMPK inhibitors to overcome MDSC-induced T-cell dysfunction in cancer. SIGNIFICANCE: AMPKα1 regulates the immunosuppressive activity and differentiation of tumor-MDSC, suggesting AMPK inhibition as a potential therapeutic strategy to restore protective myelopoiesis in cancer.

The presence and suppressive activity of myeloid-derived suppressor cells are potentiated after interferon-β treatment in a murine model of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune demyelinating disease of the human central nervous system (CNS), mainly affecting young adults. Among the immunomodulatory disease modifying treatments approved up to date to treat MS, IFN-β remains to be one of the most widely prescribed for the Relapsing-Remitting (RR) variant of the disease, although its mechanism of action is still partially understood. RR-MS variant is characterized by phases with increasing neurological symptoms (relapses) followed by periods of total or partial recovery (remissions), which implies the existence of immunomodulatory agents to promote the relapsing-to-remitting transition. Among these agents, it has been described the immunosuppressive role of a heterogeneous population of immature myeloid cells, namely the myeloid-derived suppressor cells (MDSCs) during the clinical course of the experimental autoimmune encephalomyelitis (EAE), the most used MS model to study RRMS. However, it is still unknown how the current MS disease modifying treatments, e.g. IFN- β, affects to MDSCs number or activity. Our present results show that a single injection of IFN-β at the onset of the clinical course reduces the severity of the EAE, enhancing the presence of MDSCs within the smaller demyelinated areas. Moreover, the single dose of IFN-β promotes MDSC immunosuppressive activity both in vivo and in vitro, augmenting T cell apoptosis. Finally, we show that IFN-ß preserves MDSC immaturity, preventing their differentiation to mature and less suppressive myeloid cell subsets. Taking together, all these data add new insights into the mechanism of IFN-β treatment in EAE and point to MDSCs as a putative endogenous mediator of its beneficial role in this animal model of MS.

MDSCs: Key Criminals of Tumor Pre-metastatic Niche Formation

The emergence of disseminated metastases remains the primary cause of mortality in cancer patients. Formation of the pre-metastatic niche (PMN), which precedes the establishment of tumor lesions, is critical for metastases. Bone marrow-derived myeloid cells (BMDCs) are indispensable for PMN formation. Myeloid-derived suppressor cells (MDSCs) are a population of immature myeloid cells that accumulate in patients with cancer and appear in the early PMN. The mechanisms by which MDSCs establish the pre-metastatic microenvironment in distant organs are largely unknown, although MDSCs play an essential role in metastasis. Here, we summarize the key factors associated with the recruitment and activation of MDSCs in the PMN and review the mechanisms by which MDSCs regulate PMN formation and evolution. Finally, we predict the potential value of MDSCs in PMN detection and therapy.

The role of myeloid-derived suppressor cells (MDSC) in the inflammaging process

A chronic low-grade inflammation is one of the hallmarks of the aging process. This gradually augmenting inflammatory state has been termed inflammaging. Inflammaging is associated with increased myelopoiesis in the bone marrow. This myelopoiesis-biased process increases the generation not only of mature myeloid cells, e.g. monocytes, macrophages, and neutrophils, but also immature myeloid progenitors and myeloid-derived suppressor cells (MDSCs). It is known that the aging process is associated with a significant increase in the presence of MDSCs in the bone marrow, blood, spleen, and peripheral lymph nodes. Consequently, MDSCs will become recruited into inflamed tissues where they suppress acute inflammatory responses and trigger the resolution of inflammation. However, if the perpetrator cannot be eliminated, the long-term presence of MDSCs suppresses the host's immune defence and increases the susceptibility to infections and tumorigenesis. Chronic immunosuppression also impairs the clearance of waste products and dead cells, impairs energy metabolism, and disturbs tissue proteostasis. This immunosuppressive state is reminiscent of the immunosenescence observed in inflammaging. It seems that proinflammatory changes in tissues with aging stimulate the myelopoietic production of MDSCs which subsequently induces immunosenescence and maintains the chronic inflammaging process. We will briefly describe the functions of MDSCs and then examine in detail how inflammaging enhances the generation MDSCs and how MDSCs are involved in the control of immunosenescence occurring in inflammaging.

Plasticity and biological diversity of myeloid derived suppressor cells

Myeloid derived suppressor cells (MDSCs) are immature myeloid cells characterized by diverse phenotypes and functions. They impair effector functions of immune cells and promote tumor growth, angiogenesis, and tissue damage. In pathologies characterized by chronic inflammation, MDSCs are arrested in their immature state and migrate from the bone marrow to the periphery and to the site of inflammation, where they mediate immunosuppression. When reaching new environments, which exhibit a different array of cytokines, chemokines, and pro-inflammatory mediators, MDSCs sense and adapt to the altered micro-environment by virtue of acquiring different suppressive features/functions that involve changing their cell fate, surface receptors, metabolism and intracellular as well as secreted molecules. This review summarizes some of the latest publications highlighting various layers of MDSC plasticity in relation to different pathologies. We discuss treatments capitalizing on MDSC plasticity aimed at combating MDSCs or manipulating their suppressive activity for improved therapy.

Myeloid-derived suppressor cells coming of age

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells generated during a large array of pathologic conditions ranging from cancer to obesity. These cells represent a pathologic state of activation of monocytes and relatively immature neutrophils. MDSCs are characterized by a distinct set of genomic and biochemical features, and can, on the basis of recent findings, be distinguished by specific surface molecules. The salient feature of these cells is their ability to inhibit T cell function and thus contribute to the pathogenesis of various diseases. In this Review, we discuss the origin and nature of these cells; their distinctive features; and their biological roles in cancer, infectious diseases, autoimmunity, obesity and pregnancy.

Conditioned media from the renal cell carcinoma cell line 786.O drives human blood monocytes to a monocytic myeloid-derived suppressor cell phenotype

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells critical in mediating immune suppression in cancer patients. To develop an in vitro assay system that functionally mimics the tumor microenvironment, we cultured human monocytes with conditioned media from several cancer cell lines. Conditioned media from five tumor cell lines induced survival and differentiation of monocytes into cells characteristically similar to macrophages and MDSCs. Notably, media from the 786.O renal cell carcinoma line induced monocytes to acquire a monocytic MDSC phenotype characterized by decreased HLA-DR expression, increased nitric oxide production, enhanced proliferation, and ability to suppress autologous CD3⁺ T cell proliferation. We further demonstrated that these in vitro MDSCs are phenotypically and functionally similar to patient-derived MDSCs. Inhibitors of STAT3, CK2, and GM-CSF resulted in partial reversal of the MDSC phenotype. MDSCs generated in vitro from 786.O tumor conditioned media represent a platform to identify potential therapeutics that inhibit MDSC activities.

Transforming Growth Factor-Beta1 and Myeloid-Derived Suppressor Cells Interplay in Cancer

Background:

Transforming growth factor-beta1 (TGF-β1) is a pleiotropic cytokine with a double role in cancer through its capacity to inhibit early stages of tumors while enhancing tumor progression at late stages of tumor progression. Moreover, TGF-β1 is a potent immunosuppressive cytokine within the tumor microenvironment that allows cancer cells to escape from immune surveillance, which largely contributes to the tumor progression.

Method:

It has been established that the cancer progression is commonly associated with increased number of Myeloid-derived suppressor cells (MDSC) that are a hallmark of cancer and a key mechanism of immune evasion.

Result:

MDSC represent a population of heterogeneous myeloid cells comprised of macrophages, granulocytes and dendritic cells at immature stages of development. MDSC promote tumor progression by regulating immune responses as well as tumor angiogenesis and cancer metastasis.

Conclusion:

In this review, we present an overview of the main key functions of both TGF-β1 and MDSC in cancer and in the immune system. Furthermore, the mutual contribution between TGF-β1 and MDSC in the regulation of immune system and cancer development will be analyzed.