The Crosstalk between Myeloid Derived Suppressor Cells and Immune Cells: To Establish Immune Tolerance in Transplantation

Differentiation of regulatory myeloid and T-cells from adult human hematopoietic stem cells after allogeneic stimulation

Introduction

Donor hematopoietic stem cell (DHSC) infusions are increasingly being studied in transplant patients for tolerance induction.

Methods

To analyze the fate of infused DHSCs in patients, we developed an in vitro culture system utilizing CD34+DHSCs stimulated with irradiated allogeneic cells in cytokine supplemented medium long-term.

Results

Flow cytometric analyses revealed loss of the CD34 marker and an increase in CD33+ myeloid and CD3+ T-cell proportion by 10.4% and 72.7%, respectively, after 21 days in culture. T-cells primarily expressed TcR-αβ and were of both CD4+ and CD8+ subsets. Approximately 80% of CD3+ T cells lacked expression of the co-stimulatory receptor CD28. The CD4+ compartment was predominated by CD4+CD25+CD127-FOXP3+ Tregs (>50% CD4+CD127- compartment) with <1% of all leukocytes exhibiting a CD4+CD127+ phenotype. Molecular analyses for T-cell receptor excision circles showed recent and increased numbers of TcR rearrangements in generated T cells over time suggesting de novo differentiation from DHSCs. CD33+ myeloid cells mostly expressed HLA-DR, but lacked expression of co-stimulatory receptors CD80 and CD83. When studied as modulators in primary mixed lymphocyte reactions where the cells used to stimulate the DHSC were used as responders, the DHSC-lines and their purified CD8+, CD4+, CD33+ and linage negative subsets inhibited the responses in a dose-dependent and non-specific fashion. The CD8+ cell-mediated inhibition was due to direct lysis of responder cells.

Discussion

Extrapolation of these results into the clinical situation would suggest that DHSC infusions into transplant recipients may generate multiple subsets of donor "chimeric" cells and promote recipient Treg development that could regulate the anti-donor immune response in the periphery. These studies have also indicated that T cell maturation can occur in vitro in response to allogeneic stimulation without the pre-requisite of a thymic-like environment or NOTCH signaling stimulatory cell line.

Could the Crosstalk Between Myeloid-Derived-Suppressor Cells and Regulatory T Cells Have a Role in Beta-Thalassemia?

Background

Secondary iron overload, alloimmunization, and increased risk of infection are common complications in patients with transfusion-dependent thalassemia (TDT). Regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) play an essential role in preventing excessive immune response. This research aimed to study the interaction between Tregs and MDSCs in TDT patients and to evaluate the association of these cell types with disease severity.

Methods

This case-control study included 26 patients with TDT and 23 healthy, age- and sex-matched controls. All patients were investigated for complete blood count (CBC), serum ferritin, and flow cytometric analysis of peripheral blood to detect Tregs, MDSCs, and MDSC subsets.

Results

A significant increase was observed in the frequencies of Tregs and MDSCs, particularly monocytic MDSCs (MO-MDSCs), in TDT patients compared with controls. The frequencies of these cells showed a direct association with ferritin level and total leukocyte count and an inverse association with hemoglobin level. Furthermore, a positive correlation was observed between Tregs and each of the total MDSCs and MO-MDSCs.

Conclusions

Levels of Tregs and MDSCs increased in TDT and may probably have a role in suppressing the active immune systems of TDT patients.

Discovery of Myeloid-Derived Suppressor Cell-Specific Metabolism by Metabolomic and Lipidomic Profiling

The endogenous factors that control the differentiation of myeloid-derived suppressor cells (MDSCs) are not yet fully understood. The purpose of this study was to find MDSC-specific biomolecules through comprehensive metabolomic and lipidomic profiling of MDSCs from tumor-bearing mice and to discover potential therapeutic targets for MDSCs. Partial least squares discriminant analysis was performed on the metabolomic and lipidomic profiles. The results showed that inputs for the serine, glycine, and one-carbon pathway and putrescine are increased in bone marrow (BM) MDSC compared to normal BM cells. Splenic MDSC showed an increased phosphatidylcholine to phosphatidylethanolamine ratio and less de novo lipogenesis products, despite increased glucose concentration. Furthermore, tryptophan was found to be at the lowest concentration in splenic MDSC. In particular, it was found that the concentration of glucose in splenic MDSC was significantly increased, while that of glucose 6-phosphate was not changed. Among the proteins involved in glucose metabolism, GLUT1 was overexpressed during MDSC differentiation but decreased through the normal maturation process. In conclusion, high glucose concentration was found to be an MDSC-specific feature, and it was attributed to GLUT1 overexpression. These results will help to develop new therapeutic targets for MDSCs.

Adipose Tissue-Derived CCL5 Enhances Local Pro-Inflammatory Monocytic MDSCs Accumulation and Inflammation via CCR5 Receptor in High-Fat Diet-Fed Mice

The C-C chemokine motif ligand 5 (CCL5) and its receptors have recently been thought to be substantially involved in the development of obesity-associated adipose tissue inflammation and insulin resistance. However, the respective contributions of tissue-derived and myeloid-derived CCL5 to the etiology of obesity-induced adipose tissue inflammation and insulin resistance, and the involvement of monocytic myeloid-derived suppressor cells (MDSCs), remain unclear. This study used CCL5-knockout mice combined with bone marrow transplantation (BMT) and mice with local injections of shCCL5/shCCR5 or CCL5/CCR5 lentivirus into bilateral epididymal white adipose tissue (eWAT). CCL5 gene deletion significantly ameliorated HFD-induced inflammatory reactions in eWAT and protected against the development of obesity and insulin resistance. In addition, tissue (non-hematopoietic) deletion of CCL5 using the BMT method not only ameliorated adipose tissue inflammation by suppressing pro-inflammatory M-MDSC (CD11b⁺Ly6G^-Ly6C^hi) accumulation and skewing local M1 macrophage polarization, but also recruited reparative M-MDSCs (CD11b⁺Ly6G^-Ly6C^low) and M2 macrophages to the eWAT of HFD-induced obese mice, as shown by flow cytometry. Furthermore, modulation of tissue-derived CCL5/CCR5 expression by local injection of shCCL5/shCCR5 or CCL5/CCR5 lentivirus substantially impacted the distribution of pro-inflammatory and reparative M-MDSCs as well as macrophage polarization in bilateral eWAT. These findings suggest that an obesity-induced increase in adipose tissue CCL5-mediated signaling is crucial in the recruitment of tissue M-MDSCs and their trans-differentiation to tissue pro-inflammatory macrophages, resulting in adipose tissue inflammation and insulin resistance.

TGF-β Enhances Immunosuppression of Myeloid-Derived Suppressor Cells to Induce Transplant Immune Tolerance Through Affecting Arg-1 Expression

Myeloid-derived suppressor cells (MDSCs) are a class of heterogeneous myeloid cells, which play an important role in immunosuppression. We intended to find an effective method that can produce MDSCs with significantly better efficiency and promote immune tolerance for transplant rejection through cell therapy. It has been reported that granulocyte and macrophage colony-stimulating factor (GM-CSF) could induce MDSCs in vitro to cause immunosuppression. In the present study, transforming growth factor β (TGF-β) was added to the induction system, and flow cytometry analysis was used to detect the phenotypes of induced MDSCs. Their potential immunosuppressive function and mechanisms were determined by co-culturing MDSCs with stimulated T cells in vitro and transferring MDSCs to the skin grafted C57BL/6J mouse models in vivo. It was found that the addition of TGF-β could effectively cause bone marrow cells to differentiate into a group of cells with stronger immunosuppressive functions, thereby inhibiting the proliferation of stimulated T cells. The population of CD11b+Gr-1+ MDSCs also increased significantly as compared with GM-CSF alone treatment. While detecting for immunosuppressive effectors, we found that expression of arginase 1 (Arg-1) was significantly upregulated in these MDSCs, and inhibitor of Arg-1 significantly suppressed their immunosuppressive capabilities. Moreover, an adoptive transfer of these cells significantly prolonged survival of allo-skin and improved immune tolerance in vivo. These findings indicated that TGF-β + GM-CSF could serve as an effective and feasible method to induce powerful immunosuppressive MDSCs in vitro. Thus, TGF-β + GM-CSF–induced MDSCs may have a promising role in prevention of the graft rejection.

Myeloid-Derived Suppressor Cells (MDSC) in the Umbilical Cord Blood: Biological Significance and Possible Therapeutic Applications

Myeloid-derived suppressor cells (MDSCs) represent a heterogeneous population of myeloid cells that suppress immune responses in cancer, infection, and trauma. They mainly act by inhibiting T-cells, natural-killer cells, and dendritic cells, and also by inducing T-regulatory cells, and modulating macrophages. Although they are mostly associated with adverse prognosis of the underlying disease entity, they may display positive effects in specific situations, such as in allogeneic hematopoietic stem cell transplantation (HSCT), where they suppress graft-versus-host disease (GVHD). They also contribute to the feto-maternal tolerance, and in the fetus growth process, whereas several pregnancy complications have been associated with their defects. Human umbilical cord blood (UCB) is a source rich in MDSCs and their myeloid progenitor cells. Recently, a number of studies have investigated the generation, isolation, and expansion of UCB-MDSCs for potential clinical application associated with their immunosuppressive properties, such as GVHD, and autoimmune and inflammatory diseases. Given that a significant proportion of UCB units in cord blood banks are not suitable for clinical use in HSCT, they might be used as a significant source of MDSCs for research and clinical purposes. The current review summarizes the roles of MDSCs in the UCB, as well as their promising applications.

Myeloid-Derived Suppressor Cells in Solid Tumors

Myeloid-derived suppressor cells (MDSCs) are one of the main suppressive cell population of the immune system. They play a pivotal role in the establishment of the tumor microenvironment (TME). In the context of cancers or other pathological conditions, MDSCs can differentiate, expand, and migrate in large quantities during circulation, inhibiting the cytotoxic functions of T cells and NK cells. This process is regulated by ROS, iNOS/NO, arginase-1, and multiple soluble cytokines. The definition of MDSCs and their phenotypes in humans are not as well represented as in other organisms such as mice, owing to the absence of the cognate molecule. However, a comprehensive understanding of the differences between different species and subsets will be beneficial for clarifying the immunosuppressive properties and potential clinical values of these cells during tumor progression. Recently, experimental evidence and clinical investigations have demonstrated that MDSCs have a close relationship with poor prognosis and drug resistance, which is considered to be a leading marker for practical applications and therapeutic methods. In this review, we summarize the remarkable position of MDSCs in solid tumors, explain their classifications in different models, and introduce new treatment approaches to target MDSCs to better understand the advancement of new approaches to cancer treatment.

Early Myeloid Derived Suppressor Cells (eMDSCs) Are Associated With High Donor Myeloid Chimerism Following Haploidentical HSCT for Sickle Cell Disease

Haploidentical hematopoietic stem cell transplantation (haplo-HSCT) is a widely available curative option for patients with sickle cell disease (SCD). Our original non-myeloablative haplo-HSCT trial employing post-transplant (PT) cyclophosphamide had a low incidence of GVHD but had high rejection rates. Here, we aimed to evaluate immune reconstitution following haplo-HSCT and identify cytokines and cells associated with graft rejection/engraftment. 50 cytokines and 10 immune cell subsets were screened using multiplex-ELISA and flow cytometry, respectively, at baseline and PT-Days 30, 60, 100, and 180. We observed the most significant differences in cytokine levels between the engrafted and rejected groups at PT-Day 60, corresponding with clinical findings of secondary graft rejection. Of the 44 cytokines evaluated, plasma concentrations of 19 cytokines were different between the two groups at PT-Day 60. Factor analysis suggested two independent factors. The first factor (IL-17A, IL-10, IL-7, G-CSF, IL-2, MIP-1a, VEGF, and TGFb1 contributed significantly) was strongly associated with engraftment with OR = 2.7 (95%CI of 1.4 to 5.4), whereas the second factor (GROa and IL-18 contributed significantly) was not significantly associated with engraftment. Sufficient donor myeloid chimerism (DMC) is critical for the success of HSCT; here, we evaluated immune cells among high (H) DMC (DMC≥20%) and low (L) DMC (DMC<20%) groups along with engrafted and rejected groups. We found that early myeloid-derived suppressor cell (eMDSC) frequencies were elevated in engrafted patients and patients with HDMC at PT-Day 30 (P< 0.04 & P< 0.003, respectively). 9 of 20 patients were evaluated for the source of eMDSCs. The HDMC group had high mixed chimeric eMDSCs as compared to the LDMC group (P< 0.00001). We found a positive correlation between the frequencies of eMDSCs and Tregs at PT-Day 100 (r=0.72, P <0.0007); eMDSCs at BSL and Tregs at PT-Day 100 (r=0.63, P <0.004). Of 10 immune regulatory cells and 50 cytokines, we observed mixed chimeric eMDSCs and IL-17A, IL-10, IL-7, G-CSF, IL-2, MIP-1a, VEGF, TGFb1 as potential hits which could serve as prognostic markers in predicting allograft outcome towards engraftment following haploidentical HSCT employing post-transplant cyclophosphamide. The current findings need to be replicated and further explored in a larger cohort.

Myeloid-derived suppressor cells: Important communicators in systemic lupus erythematosus pathogenesis and its potential therapeutic significance

Systemic lupus erythematosus (SLE) is a recognized chronic condition associated with immune system disorders that affect women nine times more commonly than men. SLE is characterized by over-secretion and release of autoantibodies in response to different cellular compartments and self-tolerance breaks to its own antigens. The detailed immunological dysregulation as an associated event that elicits the onset of clinical manifestations of SLE has not been clarified yet. Though, research using several animal models in the last two decades has indicated the role of the immune system in the pathogenesis of this disease. Myeloid-derived suppressor cells (MDSCs) as heterogeneous myeloid cells, are responsible for severe pathological conditions, including infection, autoimmunity, and cancer, by exerting considerable immunosuppressive effects on T-cells responses. It has been reported that these cells are involved in the regulation process of the immune response in several autoimmune diseases, particularly SLE. The function of MDSC is deleterious in infection and cancer diseases, though their role is more complicated in autoimmune diseases. In this review, we summarized the role and function of MDSCs in the pathogenesis and progression of SLE and its possible therapeutic approach.

MDSCs interactions with other immune cells and their role in maternal-fetal tolerance

MDSCs (myeloid-derived suppressor cells) are a population of immature and heterogeneous bone marrow cells with immunosuppressive functions, and they are mainly divided into two subgroups: granulocytic MDSCs (G-MDSCs) and monocytic MDSCs (M-MDSCs). Immunosuppression is the main and most important function of MDSCs, and they mainly exert an inhibitory effect through endoplasmic reticulum stress and some enzymes related to inhibitors, as well as some cytokines and other factors. In addition, MDSCs also interact with other immune cells, especially NK cells, DCs and Tregs, to participate in immune regulation. A large number of MDSCs are found during normal pregnancy. Combined with their immunosuppressive effects, these results suggest that MDSCs are likely to be closely related to maternal-fetal immune tolerance. This review mainly shows the interaction of MDSCs with other immune cells and the important role of MDSCs in maternal-fetal tolerance. The current research shows that MDSCs are mainly mediated by STAT3, HLA-G, CXCR2, Arg-1 and HIF1-α in immune regulation during pregnancy. Interpreting maternal-fetal tolerance from the perspective of MDSCs provides a special perspective for research on immune regulation and maternal-fetal tolerance of MDSCs to obtain a more comprehensive understanding of immune regulation and immune tolerance.

XBP1s repression regulates Kupffer cell polarization leading to immune suppressive effects protecting liver allograft in rats

BACKGROUND

Polarized kupffer cells (KCs) influence the immune response after liver transplantation. We report an undiscovered immune regulatory role of X-box binding protein 1 (XBP1) on immune function of kupffer cells (KCs).

METHODS

Acute rejection model using rats.

RESULTS

We found that suppression of XBP1s in lipopolysaccharide (LPS) -activated KCs could increase the expression of arginase-1 (Arg-1) and CD204 but also decrease the expression levels of MHC-II and CD40 and shift the phenotype markers of KCs toward M2 via the janus kinase (JAK) 3- Signal Transducer And Activator Of Transcription (STAT) 6 pathway, presenting an immunosuppressive function by enhancing anti-inflammatory cytokine secretion and accelerating apoptosis of activated T cells. XBP1s over-expression in KCs shift the phenotype markers on KCs towards M1 via the JAK1-STAT1 pathway and have shown a strong pro-inflammatory property. Down-regulation of XBP1s in KCs changed the phenotype and cytokine secretion profile towards M2 and markedly protected the function and structure of allograft liver, prolonging the recipient's survival compared with control and normal saline groups in rats.

CONCLUSIONS

Our findings reveal a novel regulatory mechanism of XBP1 in an induced immuno-suppressive state to protect rat's liver allograft via JAK-STAT mediated KCs polarization.

Myeloid-Derived Suppressor Cells Alleviate Renal Fibrosis Progression via Regulation of CCL5-CCR5 Axis

BACKGROUND

Renal fibrosis is inevitable in all progressive chronic kidney diseases (CKDs) and represents a serious public health problem. Immune factors contribute to the progression of renal fibrosis. Thus, it is very possible that immunosuppression cells, such as myeloid-derived suppressor cells (MDSCs), could bring benefits to renal fibrosis. Herein, this study investigated the antifibrotic and reno-protective effect of MDSCs and the possible mechanisms.

METHODS

Murine and cell models of unilateral ureter obstruction (UUO) renal fibrosis were used. Bone marrow-induced MDSCs and granulocyte-macrophage colony-stimulating factor (GM-CSF) were pretreated before surgery. Kidney weight, pathological injury, extracellular matrix deposition, and epithelial-mesenchymal transition progression were examined. Transforming growth factor (TGF)-β1)/Smad/Snail signaling pathway involvement was investigated through Western blotting and quantitative PCR (qPCR). Accumulation of MDSC, CD4+ T cell, regulatory T (Treg), and T helper 1 (T_H1) cell accumulation, and CCL5 and CCR5 expression level in MDSCs and non-MDSCs were evaluated using flow cytometry.

RESULTS

In vitro- and in vivo-induced MDSCs significantly ameliorated UUO-induced tubulointerstitial fibrosis, inhibited the TGF-β1/Smad/Snail signaling pathway, and enhanced MDSC and Treg infiltration in the kidney while downregulating the T_H1 cells. Both in vitro and in vivo experiments confirmed CCL5 elevation in the two MDSC-treated groups.

CONCLUSION

In vitro- and in vivo-induced MDSCs alleviated renal fibrosis similarly through promoting the CCL5-CCR5 axis interaction and TGF-β1/Smad/Snail signaling pathway inhibition. Our results indicate an alternative treatment for renal fibrosis.

Myeloid-derived suppressor cells expand after transplantation and their augmentation increases graft survival

Myeloid-derived suppressor cells (MDSCs) expand in an inflammatory microenvironment such as cancer and autoimmunity. To study if transplantation induces MDSCs and these cells regulate allograft survival, C57BL/6 donor hearts were transplanted into BALB/c recipients and endogenous MDSCs were characterized. The effects of adoptive transfer of transplant (tx), tumor (tm), and granulocyte-colony stimulating factor (g-csf)-expanded MDSCs or depletion of MDSC were assessed. MDSCs expanded after transplantation (1.7-4.6-fold) in the absence of immunosuppression, homed to allografts, and suppressed proliferation of CD4 T cells in vitro. Tx-MDSCs differed phenotypically from tm-MDSCs and g-csf-MDSCs. Among various surface markers, Rae-1 expression was notably low and TGF-β receptor II was high in tx-MDSCs when compared to tm-MDSCs and g-csf-MDSCs. Adoptive transfer of these three MDSCs led to differential graft survival: control (6 days), tx-MDSCs (7.5 days), tm-MDSCs (9.5 days), and g-csf-MDSCs (19.5 days). In combination with anti-CD154 mAb, MDSCs synergistically extended graft survival from 40 days (anti-CD154 alone) to 86 days with tm-MDSCs and 132 days with g-csf-MDSCs. Early MDSC depletion (day 0 or 20), however, abrogated graft survival, but late depletion (day 25) did not. In conclusion, MDSCs expanded following transplantation, migrated to cardiac allografts, prolonged graft survival, and were synergistic with anti-CD154 mAb.

Correlation between MDSC and Immune Tolerance in Transplantation: Cytokines, Pathways and Cell-cell Interaction

MDSCs play an important role in the induction of immune tolerance. Cytokines and chemokines (GM-CSF, IL-6) contributed to the expansion, accumulation of MDSCs, and MDSCs function through iNOS, arginase and PD-L1. MDSCs are recruited and regulated through JAK/STAT, mTOR and Raf/MEK/ERK signaling pathways. MDSCs' immunosuppressive functions were realized through Tregs-mediated pathways and their direct suppression of immune cells. All of the above contribute to the MDSC-related immune tolerance in transplantation. MDSCs have huge potential in prolonging graft survival and reducing rejection through different ways and many other factors worthy to be further investigated are also introduced.

"High Treg" Inflammations Promote (Most) Non-Hematologic Cancers While "Low Treg" Inflammations Promote Lymphoid Cancers

In an earlier publication, a binary classification of chronic diseases has been proposed. Chronic diseases were classified as “high Treg” or “low Treg” diseases depending on whether the pro-inflammatory or the anti-inflammatory arms of the immune response are deficient. The present work uses this model to analyze the interplay between cancer and the immune system, based on published literature. The work leans upon the etiology of alcohol and tobacco-related malignancies. The main conclusions are: triggers of specific “high Treg” immune reaction promote most non-hematologic cancers, whereas triggers of “low Treg” immune reaction promote lymphomas. The opposite is also true: triggers of specific “high Treg” immune reaction suppress lymphoma, whereas triggers of “low Treg” immune reaction suppress non-hematologic cancers. Both lymphoma and autoimmune diseases are “low Treg” conditions. For this reason, both are promoted by the same panel of “low Treg” bacteria and parasites and are inhibited by “high Treg” triggers. For example, alcohol consumption, a “high Treg” trigger, protects against lymphoma and autoimmune hypothyroidism. In addition, the same immune-modulatory drugs are effective in the treatment of both lymphoma and autoimmune diseases. Like other cancers, lymphoma transforms from a “low Treg” type at early stage of the disease into a “high Treg” type at advanced stages. However, lymphoma is distinguished from most other cancers by the length of time it dwells at an indolent “low Treg” state (many years) before lymphoma cells sensitivity to transforming growth factor-beta is impaired. This impairment stimulates the switch from “low Treg” into “high Treg” response and results in immune escape. The application of this analysis to the pharmacological activity of checkpoint inhibitors forecasts that checkpoint inhibitors would not be effective in low-grade, indolent lymphomas. As of now, checkpoint inhibitors are approved for the treatment of advanced lymphoma only.

Myeloid-derived suppressor cells in transplantation tolerance induction

Myeloid-derived suppressor cells (MDSCs) are a group of heterogeneous cells derived from bone marrow. These cells are developed from immature myeloid cells and have strong negative immunomodulatory effects. In the context of pathology (such as tumor, autoimmune disease, trauma, and burns), MDSCs accumulate around tumor and inflammatory tissues, where their main role is to inhibit the function of effector T cells and promote the recruitment of regulatory T cells. MDSCs can be used in organ transplantation to regulate the immune responses that participate in rejection of the transplanted organ. This effect is achieved by increasing the production of MDSCs in vivo or transfusion of MDSCs induced in vitro to establish immune tolerance and prolong the survival of the graft. In this review, we discuss the efficacy of MDSCs in a variety of transplantation studies as well as the induction of immune tolerance to prevent transplant rejection through the use of common clinical immunosuppressants combined with MDSCs.

Can we prevent or treat graft-versus-host disease with cellular-therapy?

Acute and chronic graft-versus-host disease (GvHD) are the most important causes of treatment-related morbidity and mortality after allogeneic hematopoietic cell transplants for various diseases. Corticosteroids are an effective therapy in only about one-half of affected individuals and new therapy options are needed. We discuss novel strategies to treat GvHD using cellular-therapy including adoptive transfer of regulatory T-cells (T_regs), mesenchymal stromal cells (MSCs), cells derived from placental tissues, invariant natural killer T-cells (iNKTs), and myeloid-derived suppressor cells (MDSCs).These strategies may be more selective than drugs in modulating GvHD pathophysiology, and may be safer and more effective than conventional pharmacologic therapies. Additionally, these therapies have not been observed to substantially compromise the graft-versus-tumor effect associated with allotransplants. Many of these strategies are effective in animal models but substantial data in humans are lacking.

Activation of immunosuppressive network in the aging process

Chronic low-grade inflammation has a key role in the aging process, a state called inflammaging. It is known that the chronic inflammatory condition generates counteracting immunosuppressive state in many diseases. Inflammaging is also associated with an immune deficiency; generally termed as immunosenescence, although it is not known whether it represents the senescence of immune cells or the active remodeling of immune system. Evidence has accumulated since the 1970's indicating that immunosenescence might be caused by an increased activity of immunosuppressive cells rather than cellular senescence. Immune cells display remarkable plasticity; many of these cells can express both proinflammatory and immunosuppressive phenotypes in a context-dependent manner. The immunosuppressive network involves the regulatory subtypes of T (Treg) and B (Breg) cells as well as regulatory phenotypes of macrophages (Mreg), dendritic (DCreg), natural killer (NKreg), and type II natural killer T (NKT) cells. The immunosuppressive network also includes monocytic (M-MDSC) and polymorphonuclear (PMN-MDSC) myeloid-derived suppressor cells which are immature myeloid cells induced by inflammatory mediators. This co-operative network is stimulated in chronic inflammatory conditions preventing excessive inflammatory responses but at the same time they exert harmful effects on the immune system and tissue homeostasis. Recent studies have revealed that the aging process is associated with the activation of immunosuppressive network, especially the functions of MDSCs, Tregs, and Mregs are increased. I will briefly review the properties of the regulatory phenotypes of immune cells and examine in detail the evidences for an activation of immunosuppressive network with aging.

Myeloid-derived suppressor cells are bound and inhibited by anti-thymocyte globulin

Myeloid-derived suppressor cells (MDSCs) inhibit T cell responses and are relevant to cancer, autoimmunity and transplant biology. Anti-thymocyte globulin (ATG) is a commonly used T cell depletion agent, yet the effect of ATG on MDSCs has not been investigated. MDSCs were generated in Lewis Lung Carcinoma 1 tumor-bearing mice. MDSC development and function were assessed in vivo and in vitro with and without ATG administration. T cell suppression assays, RT-PCR, flow cytometry and arginase activity assays were used to assess MDSC phenotype and function. MDSCs increased dramatically in tumor-bearing mice and the majority of splenic MDSCs were of the polymorphonuclear subset. MDSCs potently suppressed T cell proliferation. ATG-treated mice developed 50% fewer MDSCs and these MDSCs were significantly less suppressive of T cell proliferation. In vitro, ATG directly bound 99.6% of MDSCs. CCR7, L-selectin and LFA-1 were expressed by both T cells and MDSCs, and binding of LFA-1 was inhibited by ATG pre-treatment. Arg-1 and PD-L1 transcript expression were reduced 30–40% and arginase activity decreased in ATG-pretreated MDSCs. MDSCs were bound and functionally inhibited by ATG. T cells and MDSCs expressed common Ags which were also targets of ATG. ATG may be helpful in tumor models seeking to suppress MDSCs. Alternatively, ATG may inadvertently inhibit important T cell regulatory events in autoimmunity and transplantation.

Arginine deficiency is involved in thrombocytopenia and immunosuppression in severe fever with thrombocytopenia syndrome

Severe fever with thrombocytopenia syndrome (SFTS) caused by a recently identified bunyavirus, SFTSV, is an emerging infectious disease with extensive geographical distribution and high mortality. Progressive viral replication and severe thrombocytopenia are key features of SFTSV infection and fatal outcome, whereas the underlying mechanisms are unknown. We revealed arginine deficiency in SFTS cases by performing metabolomics analysis on two independent patient cohorts, suggesting that arginine metabolism by nitric oxide synthase and arginase is a key pathway in SFTSV infection and consequential death. Arginine deficiency was associated with decreased intraplatelet nitric oxide (Plt-NO) concentration, platelet activation, and thrombocytopenia. An expansion of arginase-expressing granulocytic myeloid-derived suppressor cells was observed, which was related to T cell CD3-ζ chain down-regulation and virus clearance disturbance, implicating a role of arginase activity and arginine depletion in the impaired anti-SFTSV T cell function. Moreover, a comprehensive measurement of arginine bioavailability, global arginine bioavailability ratio, was shown to be a good prognostic marker for fatal prediction in early infection. A randomized controlled trial demonstrated that arginine administration was correlated with enhanced Plt-NO concentration, suppressed platelet activation, and elevated CD3-ζ chain expression and eventually associated with an accelerated virus clearance and thrombocytopenia recovery. Together, our findings revealed the arginine catabolism pathway-associated regulation of platelet homeostasis and T cell dysregulation after SFTSV infection, which not only provided a functional mechanism underlying SFTS pathogenesis but also offered an alternative therapy choice for SFTS.

Apoptotic cell-based therapies for promoting transplantation tolerance

PURPOSE OF REVIEW

This article is aimed to provide readers with an updated review on the applicability, efficacy, and challenges of employing donor apoptotic cell-based therapies to promote transplantation tolerance in various experimental and clinical settings.

RECENT FINDINGS

Recently, donor apoptotic cell-based therapies have been employed in various models of cell (including pancreatic islets and bone marrow hematopoietic stem cells) and solid organ (heart and kidney) transplantation to promote donor-specific tolerance. Published data, thus far, have revealed a high potential of this approach in inducing robust transplantation tolerance. Recent clinical trials have also underscored the safety and potential efficacy of this approach in alleviating graft-versus-host disease (GVHD) in bone marrow transplantation (BMT). Host factors including prior allo-sensitization and opportunistic infections pose major obstacles in establishing transplantation tolerance employing this strategy. However, emerging data provide strategies for overcoming such obstacles in these clinically relevant settings.

SUMMARY

Donor apoptotic cell therapy is an emerging strategy in promoting transplantation tolerance, with recent data emphasizing its efficacy and applicability for transplantation tolerance in the clinic.

Emerging strategies in cancer therapy combining chemotherapy with immunotherapy

Cancer immunotherapy holds great potential to battle cancer by exerting a durable immunity effect. However, this process might be limited by various constraints existing in the tumor microenvironment (TME), such as the lack of available neoantigen, insufficient T cells from the naive repertoire, or immunosuppressive networks in which immunogenic tissue is protected from immune attacks. Certain chemotherapeutic drugs could elicit immune-potentiating effects by either inducing immunogenicity or relieving tumor-induced immunosuppression. Some also leave tumors directly susceptible to cytotoxic T cell attacks. Mounting evidence accumulated from preclinical and clinical studies suggests that these two treatment modalities might be mutually reinforcing as an effective "chemo-immunotherapy" strategy. Herein, we reviewed the latest advances in cancer immunotherapy and related mechanisms involved in chemotherapeutic-mediated immune activation. The emerging combination strategies and synergistic effects in response to chemo-immunotherapy are highlighted. We also discuss the challenges and critical considerations in its future development.

The Yin and Yang of Myeloid Derived Suppressor Cells

In recent years, most of our knowledge about myeloid derived suppressor cells (MDSCs) has come from cancer studies, which depicts Yin side of MDSCs. In cancer, inherent immunosuppressive action of MDSCs favors tumor progression by inhibiting antitumor immune response. However, recently Yang side of MDSCs has also been worked out and suggests the role in maintenance of homeostasis during non-cancer situations like pregnancy, obesity, diabetes, and autoimmune disorders. Continued work in this area has armored the biological importance of these cells as master regulators of immune system and prompted scientists all over the world to look from a different perspective. Therefore, explicating Yin and Yang arms of MDSCs is obligatory to use it as a double edged sword in a much smarter way. This review is an attempt toward presenting a synergistic coalition of all the facts and controversies that exist in understanding MDSCs, bring them on the same platform and approach their "Yin and Yang" nature in a more comprehensive and coherent manner.

Myeloid-Derived Suppressor Cells and Their Potential Application in Transplantation

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immunosuppressive cells of the myeloid lineage upregulated by mediators of inflammation, such as IL-2, granulocyte colony-stimulating factor, and S100A8/A9. These cells have been studied extensively by tumor biologists. Because of their robust immunosuppressive potential, MDSCs have stirred recent interest among transplant immunologists as well. MDSCs inhibit T-cell responses through, among other mechanisms, the activity of arginase-1 and inducible nitric oxide synthase, and the expansion of T regulatory cells. In the context of transplantation, MDSCs have been studied in several animal models, and to a lesser degree in humans. Here, we will review the immunosuppressive qualities of this important cell type and discuss the relevant studies of MDSCs in transplantation. It may be possible to exploit the immunosuppressive capacity of MDSCs for the benefit of transplant patients.

Dendritic Cells and Programmed Death-1 Blockade: A Joint Venture to Combat Cancer

Two decades of clinical cancer research with dendritic cell (DC)-based vaccination have proved that this type of personalized medicine is safe and has the capacity to improve survival, but monotherapy is unlikely to cure the cancer. Designed to empower the patient’s antitumor immunity, huge research efforts are set to improve the efficacy of next-generation DC vaccines and to find synergistic combinations with existing cancer therapies. Immune checkpoint approaches, aiming to breach immune suppression and evasion to reinforce antitumor immunity, have been a revelation in the immunotherapy field. Early success of therapeutic antibodies blocking the programmed death-1 (PD-1) pathway has sparked the development of novel inhibitors and combination therapies. Hence, merging immunoregulatory tumor-specific DC strategies with PD-1-targeted approaches is a promising path to explore. In this review, we focus on the role of PD-1-signaling in DC-mediated antitumor immunity. In the quest of exploiting the full potential of DC therapy, different strategies to leverage DC immunopotency by impeding PD-1-mediated immune regulation are discussed, including the most advanced research on targeted therapeutic antibodies, lessons learned from chemotherapy-induced immune activation, and more recent developments with soluble molecules and gene-silencing techniques. An overview of DC/PD-1 immunotherapy combinations that are currently under preclinical and clinical investigation substantiates the clinical potential of such combination strategies.

Myeloid-derived suppressor cells in transplantation: the dawn of cell therapy

Myeloid-derived suppressor cells (MDSCs) are a series of innate cells that play a significant role in inhibiting T cell-related responses. This heterogeneous population of immature cells is involved in tumor immunity. Recently, the function and importance of MDSCs in transplantation have garnered the attention of scientists and have become an important focus of transplantation immunology research because MDSCs play a key role in establishing immune tolerance in transplantation. In this review, we summarize recent studies of MDSCs in different types of transplantation. We also focus on the influence of immunosuppressive drugs on MDSCs as well as future obstacles and research directions in this field.

Targeting Cannabinoid Signaling in the Immune System: "High"-ly Exciting Questions, Possibilities, and Challenges

It is well known that certain active ingredients of the plants of Cannabis genus, i.e., the "phytocannabinoids" [pCBs; e.g., (-)-trans-Δ⁹-tetrahydrocannabinol (THC), (-)-cannabidiol, etc.] can influence a wide array of biological processes, and the human body is able to produce endogenous analogs of these substances ["endocannabinoids" (eCB), e.g., arachidonoylethanolamine (anandamide, AEA), 2-arachidonoylglycerol (2-AG), etc.]. These ligands, together with multiple receptors (e.g., CB₁ and CB₂ cannabinoid receptors, etc.), and a complex enzyme and transporter apparatus involved in the synthesis and degradation of the ligands constitute the endocannabinoid system (ECS), a recently emerging regulator of several physiological processes. The ECS is widely expressed in the human body, including several members of the innate and adaptive immune system, where eCBs, as well as several pCBs were shown to deeply influence immune functions thereby regulating inflammation, autoimmunity, antitumor, as well as antipathogen immune responses, etc. Based on this knowledge, many in vitro and in vivo studies aimed at exploiting the putative therapeutic potential of cannabinoid signaling in inflammation-accompanied diseases (e.g., multiple sclerosis) or in organ transplantation, and to dissect the complex immunological effects of medical and "recreational" marijuana consumption. Thus, the objective of the current article is (i) to summarize the most recent findings of the field; (ii) to highlight the putative therapeutic potential of targeting cannabinoid signaling; (iii) to identify open questions and key challenges; and (iv) to suggest promising future directions for cannabinoid-based drug development.

FAP positive fibroblasts induce immune checkpoint blockade resistance in colorectal cancer via promoting immunosuppression

Immune checkpoint blockades that significantly prolonged survival of melanoma patients have been less effective on colorectal cancer (CRC) patients. Growing evidence suggested that fibroblast activation protein-alpha (FAP) on cancer associate fibroblasts (CAFs) has critical roles in regulating antitumor immune response by inducing tumor-promoting inflammation. In this study, we explored the roles of FAP in regulating the tumor immunity and immune checkpoint blockades resistance in CRC experimental systems. We found that CAFs with high FAP expression could induce immune checkpoint blockade resistance in CRC mouse model. Mechanistically, CAFs with high FAP expression promoted immunosuppression in the CRC tumor immune microenvironment by up-regulating CCL2 secretion, recruiting myeloid cells, and decreasing T-cell activity. In human CRC samples, FAP expression was proportional to myeloid cells number, but inversely related to T-cell number. High FAP expression also predicted poor survival of CRC patients. Taken together, our study suggested that high FAP expression in CAFs is one reason leading to immune checkpoint blockades resistance in CRC patients and FAP is an optional target for reversing immune checkpoint blockades resistance.

Targeting the Monocyte-Macrophage Lineage in Solid Organ Transplantation

There is an unmet clinical need for immunotherapeutic strategies that specifically target the active immune cells participating in the process of rejection after solid organ transplantation. The monocyte–macrophage cell lineage is increasingly recognized as a major player in acute and chronic allograft immunopathology. The dominant presence of cells of this lineage in rejecting allograft tissue is associated with worse graft function and survival. Monocytes and macrophages contribute to alloimmunity via diverse pathways: antigen processing and presentation, costimulation, pro-inflammatory cytokine production, and tissue repair. Cross talk with other recipient immune competent cells and donor endothelial cells leads to amplification of inflammation and a cytolytic response in the graft. Surprisingly, little is known about therapeutic manipulation of the function of cells of the monocyte–macrophage lineage in transplantation by immunosuppressive agents. Although not primarily designed to target monocyte–macrophage lineage cells, multiple categories of currently prescribed immunosuppressive drugs, such as mycophenolate mofetil, mammalian target of rapamycin inhibitors, and calcineurin inhibitors, do have limited inhibitory effects. These effects include diminishing the degree of cytokine production, thereby blocking costimulation and inhibiting the migration of monocytes to the site of rejection. Outside the field of transplantation, some clinical studies have shown that the monoclonal antibodies canakinumab, tocilizumab, and infliximab are effective in inhibiting monocyte functions. Indirect effects have also been shown for simvastatin, a lipid lowering drug, and bromodomain and extra-terminal motif inhibitors that reduce the cytokine production by monocytes–macrophages in patients with diabetes mellitus and rheumatoid arthritis. To date, detailed knowledge concerning the origin, the developmental requirements, and functions of diverse specialized monocyte–macrophage subsets justifies research for therapeutic manipulation. Here, we will discuss the effects of currently prescribed immunosuppressive drugs on monocyte/macrophage features and the future challenges.