Monocyte-Derived Suppressor Cells in Transplantation

Regulatory macrophages in solid organ xenotransplantation

Due to a critical organ shortage, pig organs are being explored for use in transplantation. Differences between species, particularly in cell surface glycans, can trigger elevated immune responses in xenotransplantation. To mitigate the risk of hyperacute rejection, genetically modified pigs have been developed that lack certain glycans and express human complement inhibitors. Nevertheless, organs from these pigs may still provoke stronger inflammatory and innate immune reactions than allotransplants. Dysregulation of coagulation and persistent inflammation remain obstacles in the transplantation of pig organs into primates. Regulatory macrophages (Mregs), known for their anti-inflammatory properties, could offer a potential solution. Mregs secrete interleukin 10 and transforming growth factor beta, thereby suppressing immune responses and promoting the development of regulatory T cells. These Mregs are typically induced via the stimulation of monocytes or macrophages with macrophage colony-stimulating factor and interferon gamma, and they conspicuously express the stable marker dehydrogenase/reductase 9. Consequently, understanding the precise mechanisms governing Mreg generation, stability, and immunomodulation could pave the way for the therapeutic use of Mregs generated in vitro. This approach has the potential to reduce the required dosages and durations of anti-inflammatory and immunosuppressive medications in preclinical and clinical settings.

Donor antigen-specific regulatory T cell administration to recipients of live donor kidneys: A ONE Study consortium pilot trial

Regulatory T cells (Tregs) can inhibit cellular immunity in diverse experimental models and have entered early phase clinical trials in autoimmunity and transplantation to assess safety and efficacy. As part of the ONE Study consortium, we conducted a phase I-II clinical trial in which purified donor antigen reactive (dar)-Tregs (CD4+CD25+CD127lo) were administered to 3 patients, 7 to 11 days after live donor renal transplant. Recipients received a modified immunosuppression regimen, without induction therapy, consisting of maintenance tacrolimus, mycophenolate mofetil, and steroids. Steroids were weaned off over 14 weeks. No rejection was seen on any protocol biopsy. Therefore, all patients discontinued mycophenolate mofetil 11 to 13 months posttransplant, per protocol. An early for-cause biopsy in 1 patient, 5 days after dar-Treg infusion, revealed absence of rejection and accumulation of Tregs in the kidney allograft. All patients had Treg-containing lymphoid aggregates evident on protocol biopsies performed 8 months posttransplant. The patients are now all >6 years posttransplant on tacrolimus monotherapy with excellent graft function. None experienced rejection episodes. No serious adverse events were attributable to Treg administration. These results support a favorable safety profile of dar-Tregs administered early after renal transplant, suggest early biopsy might be an instructive research endpoint and provide preliminary evidence of potential immunomodulatory activity.

Analysis of therapeutic potential of monocytic myeloid-derived suppressor cells in cardiac allotransplantation

BACKGROUND

Myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs) are attractive immune cells to induce immune tolerance. To explore a strategy for improving the efficacy of MDSC therapies, we examined the impact of adoptive transfer of several types of MDSCs on graft rejection in a murine heart transplantation model.

METHODS

We analyzed the effects of induced syngeneic and allogeneic bone marrow-derived MDSCs (BM-MDSCs) on graft survival and suppressive capacity. We also compared the ability of syngeneic monocytic MDSCs (Mo-MDSCs) and polymorphonuclear MDSCs (PMN-MDSCs) to inhibit graft rejection and investigated the suppression mechanisms.

RESULTS

Both syngeneic and allogeneic donor- or allogeneic third-party-derived BM-MDSCs prolonged graft survival, although syngeneic BM-MDSCs inhibited anti-donor immune responses most effectively in vitro. Syngeneic Mo-MDSCs, rather than PMN-MDSCs, were responsible for immune suppression through downregulating inducible nitric oxide synthase (iNOS) and expanded naturally occurring thymic originated Treg (nTreg) in vitro. Adoptive transfer of Mo-MDSCs, but not PMN-MDSCs, prolonged graft survival and increased Treg infiltration into the graft heart.

CONCLUSION

Recipient-derived Mo-MDSCs are most effective in prolonging graft survival via inhibiting T cell response and nTreg infiltration.

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.

The Immunological Basis of Liver Allograft Rejection

Liver allograft rejection remains a significant cause of morbidity and graft failure in liver transplant recipients. Rejection is caused by the recognition of non-self donor alloantigens by recipient T-cells. Antigen recognition results in proliferation and activation of T-cells in lymphoid tissue before migration to the allograft. Activated T-cells have a variety of effector mechanisms including direct T-cell mediated damage to bile ducts, endothelium and hepatocytes and indirect effects through cytokine production and recruitment of tissue-destructive inflammatory cells. These effects explain the histological appearances of typical acute T-cell mediated rejection. In addition, donor specific antibodies, most typically against HLA antigens, may give rise to antibody-mediated rejection causing damage to the allograft primarily through endothelial injury. However, as an immune-privileged site there are several mechanisms in the liver capable of overcoming rejection and promoting tolerance to the graft, particularly in the context of recruitment of regulatory T-cells and promotors of an immunosuppressive environment. Indeed, around 20% of transplant recipients can be successfully weaned from immunosuppression. Hence, the host immunological response to the liver allograft is best regarded as a balance between rejection-promoting and tolerance-promoting factors. Understanding this balance provides insight into potential mechanisms for novel anti-rejection therapies.

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.

Myeloid-Derived Suppressor Cells in Kidney Transplant Recipients and the Effect of Maintenance Immunotherapy

Myeloid-derived suppressor cells (MDSC) represent a heterogeneous group of myeloid regulatory cells that were originally described in cancer. Several studies in animal models point to MDSC as important players in the induction of allograft tolerance due to their immune modulatory function. Most of the published studies have been performed in animal models, and the data addressing MDSCs in human organ transplantation are scarce. We evaluated the phenotype and function of different MDSCs subsets in 38 kidney transplant recipients (KTRs) at different time points. Our data indicate that monocytic MDSCs (Mo-MDSC) increase in KTR at 6 and 12 months posttransplantation. On the contrary, the percentages of polymorphonuclear MDSC (PMN-MDSC) and early-stage MDSC (e-MDSC) are not significantly increased. We evaluated the immunosuppressive activity of Mo-MDSC in KTR and confirmed their ability to increase regulatory T cells (Treg) in vitro. Interestingly, when we compared the ability of Mo-MDSC to suppress T cell proliferation, we observed that tacrolimus, but not rapamycin-treated KTR, was able to inhibit CD4⁺ T cell proliferation in vitro. This indicates that, although mTOR inhibitors are widely regarded as supportive of regulatory responses, rapamycin may impair Mo-MDSC function, and suggests that the choice of immunosuppressive therapy may determine the tolerogenic pathway and participating immune cells that promote organ transplant acceptance in KTR.

SIRPα/CD47 axis controls the maintenance of transplant tolerance sustained by myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of immature hematopoietic precursors known to suppress immune responses. Interaction of SIRP alpha (SIRPα), expressed by myeloid cells, with the ubiquitous receptor CD47 is an important immune checkpoint of the innate response regulating macrophages and dendritic cells functions. We previously described that MDSC expressing SIRPα accumulated after transplantation and maintained kidney allograft tolerance. However, the role of the SIRPα/CD47 axis on MDSC function remained unknown. Here, we found that blocking SIRPα or CD47 with monoclonal antibodies (mAbs) induced differentiation of MDSC into myeloid cells overexpressing MHC class II, CD86 costimulatory molecule and increased secretion of macrophage-recruiting chemokines (eg, MCP-1). Using a model of long-term kidney allograft tolerance sustained by MDSC, we observed that administration of blocking anti-SIRPα or CD47 mAbs induced graft dysfunction and rejection. Loss of tolerance came along with significant decrease of MDSC and increase in MCP-1 concentration in the periphery. Graft histological and transcriptomic analyses revealed an inflammatory (M1) macrophagic signature at rejection associated with overexpression of MCP-1 mRNA and protein in the graft. These findings indicate that the SIRPα-CD47 axis regulates the immature phenotype and chemokine secretion of MDSC and contributes to the induction and the active maintenance of peripheral acquired immune tolerance.

Preliminary assessment of the feasibility of autologous myeloid-derived suppressor cell infusion in non-human primate kidney transplantation

Myeloid-derived suppressor cells (MDSC) are a heterogenous population of immunosuppressive myeloid cells now considered important immune regulatory cells in diverse clinical conditions, including cancer, chronic inflammatory disorders and transplantation. In rodents, MDSC administration can inhibit graft-versus-host disease lethality and enhance organ or pancreatic islet allograft survival. There is also evidence, however, that under systemic inflammatory conditions, adoptively-transferred MDSC can rapidly lose their suppressive function. To our knowledge, there are no reports of autologous MDSC administration to either human or clinically-relevant non-human primate (NHP) transplant recipients. Monocytic (m) MDSC have been shown to be more potent suppressors of T cell responses than other subsets of MDSC. Following their characterization in rhesus macaques, we have conducted a preliminary analysis of the feasibility and preliminary efficacy of purified mMDSC infusion into MHC-mismatched rhesus kidney allograft recipients. The graft recipients were treated with rapamycin and the high affinity variant of the T cell co-stimulation blocking agent cytotoxic T lymphocyte antigen 4 Ig (Belatacept) that targets the B7-CD28 pathway. Graft survival and histology were not affected by infusions of autologous, leukapheresis product-derived mMDSC on days 7 and 14 post-transplant (cumulative totals of 3.19 and 1.98 × 10⁶ cells/kg in n = 2 recipients) compared with control monkeys that did not receive MDSC (n = 2). Sequential analyses of effector T cell populations revealed no differences between the groups. While these initial findings do not provide evidence of efficacy under the conditions adopted, further studies in NHP, designed to ascertain the appropriate mMDSC source and dose, timing and anti-inflammatory/immunosuppressive agent support are likely to prove instructive regarding the therapeutic potential of MDSC in organ transplantation.

Role of Complement Activation in Allograft Inflammation

PURPOSE

Novel paradigms have broadened our understanding of mechanisms through which complement mediates allograft inflammation/injury. Herein we review advances in the field and highlight therapeutic implications.

RECENT FINDINGS

Pre-clinical and translational human trials have elucidated complement-dependent mechanisms of post-transplant ischemia-reperfusion (I/R) injury. Immune cell-derived, and intracellular, complement activation are newly linked to proinflammatory T cell immunity relevant to allograft rejection. Complement-induced immune regulation, including C5a ligation of C5a receptor 2 on T cells, C5a/C5a receptor 1 interactions on regulatory myeloid cells, and C1q binding to CD8+ T cells can inhibit proinflammatory T cells and/or prolong murine allograft survival. Pilot trials of complement inhibition to treat/prevent human I/R- or antibody-initiated allograft injury show promise.

SUMMARY

The complement system participates in allograft injury through multiple context- dependent mechanisms involving various components and receptors. These new insights along with development and implementation of individualized complement inhibitory strategies have potential to improve transplant outcomes.

Murine CMV induces type 1 IFN that impairs differentiation of MDSCs critical for transplantation tolerance

Clinical tolerance without immunosuppression has now been achieved for organ transplantation, and its scope will likely continue to expand. In this context, a previously understudied and now increasingly relevant area is how microbial infections might affect the efficacy of tolerance. A highly prevalent and clinically relevant posttransplant pathogen is cytomegalovirus (CMV). Its impact on transplantation tolerance and graft outcomes is not well defined. Employing a mouse model of CMV (MCMV) infection and allogeneic pancreatic islet transplantation in which donor-specific tolerance was induced by infusing donor splenocytes rendered apoptotic by treatment with ethylenecarbodiimide, we investigated the effect of CMV infection on transplantation tolerance induction. We found that acute MCMV infection abrogated tolerance induction and that this abrogation correlated with an alteration in the differentiation and function of myeloid-derived suppressor cells (MDSCs). These effects on MDSCs were mediated in part through MCMV induced type 1 interferon (IFN) production. During MCMV infection, the highly immunosuppressive Gr1^HI-granulocytic MDSCs were markedly reduced in numbers, and the accumulating Ly6C^HI-monocytic cells lost their MDSC-like function but instead acquired an immunostimulatory phenotype to cross-present alloantigens and prime alloreactive CD8 T cells. Consequently, the islet allograft exhibited an altered effector to regulatory T-cell ratio that correlated with the ultimate graft demise. Blocking type 1 IFN signaling during MCMV infection rescued MDSC populations and partially restored transplantation tolerance. Our mechanistic studies now provide a solid foundation for seeking effective therapies for promoting transplantation tolerance in settings of CMV infection.

Macrophages: versatile players in renal inflammation and fibrosis

Macrophages have important roles in immune surveillance and in the maintenance of kidney homeostasis; their response to renal injury varies enormously depending on the nature and duration of the insult. Macrophages can adopt a variety of phenotypes: at one extreme, M1 pro-inflammatory cells contribute to infection clearance but can also promote renal injury; at the other extreme, M2 anti-inflammatory cells have a reparative phenotype and can contribute to the resolution phase of the response to injury. In addition, bone marrow monocytes can differentiate into myeloid-derived suppressor cells that can regulate T cell immunity in the kidney. However, macrophages can also promote renal fibrosis, a major driver of progression to end-stage renal disease, and the CD206⁺ subset of M2 macrophages is strongly associated with renal fibrosis in both human and experimental diseases. Myofibroblasts are important contributors to renal fibrosis and recent studies provide evidence that macrophages recruited from the bone marrow can transition directly into myofibroblasts within the injured kidney. This process is termed macrophage-to-myofibroblast transition (MMT) and is driven by transforming growth factor-β1 (TGFβ1)-Smad3 signalling via a Src-centric regulatory network. MMT may serve as a key checkpoint for the progression of chronic inflammation into pathogenic fibrosis.

Role of myeloid regulatory cells (MRCs) in maintaining tissue homeostasis and promoting tolerance in autoimmunity, inflammatory disease and transplantation

Myeloid cells play a pivotal role in regulating innate and adaptive immune responses. In inflammation, autoimmunity, and after transplantation, myeloid cells have contrasting roles: on the one hand they initiate the immune response, promoting activation and expansion of effector T-cells, and on the other, they counter-regulate inflammation, maintain tissue homeostasis, and promote tolerance. The latter activities are mediated by several myeloid cells including polymorphonuclear neutrophils, macrophages, myeloid-derived suppressor cells, and dendritic cells. Since these cells have been associated with immune suppression and tolerance, they will be further referred to as myeloid regulatory cells (MRCs). In recent years, MRCs have emerged as a therapeutic target or have been regarded as a potential cellular therapeutic product for tolerance induction. However, several open questions must be addressed to enable the therapeutic application of MRCs including: how do they function at the site of inflammation, how to best target these cells to modulate their activities, and how to isolate or to generate pure populations for adoptive cell therapies. In this review, we will give an overview of the current knowledge on MRCs in inflammation, autoimmunity, and transplantation. We will discuss current strategies to target MRCs and to exploit their tolerogenic potential as a cell-based therapy.

Selective Deletion of Heparan Sulfotransferase Enzyme, Ndst1, in Donor Endothelial and Myeloid Precursor Cells Significantly Decreases Acute Allograft Rejection

Early damage to transplanted organs initiates excess inflammation that can cause ongoing injury, a leading cause for late graft loss. The endothelial glycocalyx modulates immune reactions and chemokine-mediated haptotaxis, potentially driving graft loss. In prior work, conditional deficiency of the glycocalyx-modifying enzyme N-deacetylase-N-sulfotransferase-1 (Ndst1^f/f TekCre⁺) reduced aortic allograft inflammation. Here we investigated modification of heparan sulfate (HS) and chemokine interactions in whole-organ renal allografts. Conditional donor allograft Ndst1 deficiency (Ndst1^−/−; C57Bl/6 background) was compared to systemic treatment with M-T7, a broad-spectrum chemokine-glycosaminoglycan (GAG) inhibitor. Early rejection was significantly reduced in Ndst1^−/− kidneys engrafted into wildtype BALB/c mice (Ndst1^+/+) and comparable to M-T7 treatment in C57Bl/6 allografts (P < 0.0081). M-T7 lost activity in Ndst1^−/− allografts^, while M-T7 point mutants with modified GAG-chemokine binding displayed a range of anti-rejection activity. CD3+ T cells (P < 0.0001), HS (P < 0.005) and CXC chemokine staining (P < 0.012), gene expression in NFκB and JAK/STAT pathways, and HS and CS disaccharide content were significantly altered with reduced rejection. Transplant of donor allografts with conditional Ndst1 deficiency exhibit significantly reduced acute rejection, comparable to systemic chemokine-GAG inhibition. Modified disaccharides in engrafted organs correlate with reduced rejection. Altered disaccharides in engrafted organs provide markers for rejection with potential to guide new therapeutic approaches in allograft rejection.

Neutrophil derived CSF1 induces macrophage polarization and promotes transplantation tolerance

The colony-stimulating factor 1 (CSF1) regulates the differentiation and function of tissue macrophages and determines the outcome of the immune response. The molecular mechanisms behind CSF1-mediated macrophage development remain to be elucidated. Here we demonstrate that neutrophil-derived CSF1 controls macrophage polarization and proliferation, which is necessary for the induction of tolerance. Inhibiting neutrophil production of CSF1 or preventing macrophage proliferation, using targeted nanoparticles loaded with the cell cycle inhibitor simvastatin, abrogates the induction of tolerance. These results provide new mechanistic insights into the developmental requirements of tolerogenic macrophages and identify CSF1 producing neutrophils as critical regulators of the immunological response.

Memory T cells are significantly increased in rejected liver allografts of rhesus monkeys

The rhesus monkey (RM) is an excellent preclinical model in kidney, heart, and islet transplantation that has provided the basis for new immunosuppressive protocols for clinical studies. However, there remain relatively few liver transplantation (LT) models in nonhuman primates. In this study, we analyzed the immune cell populations of peripheral blood mononuclear cells (PBMCs) and secondary lymphoid organs along with livers of normal RMs and compared them with those of rejected LT recipients following withdrawal of immunosuppression. We undertook 5 allogeneic ABO compatible orthotopic LTs in monkeys using 5 normal donor monkey livers. We collected tissues including lymph nodes, spleens, blood, and recipient livers, and we performed flow cytometric analysis using isolated immune cells. We found that CD4 or CD8 naïve T cells were normally seen at low levels, and memory T cells were seen at high levels in the liver rather than lymphoid organs or PBMC. However, regulatory cells such as CD4+ forkhead box P3+ T cells and CD8+ CD28- cells remained in high numbers in the liver, but not in the lymph nodes or PBMC. The comparison of CD4/8 T subpopulations in normal and rejected livers and the various tissues showed that naïve cells were dramatically decreased in the spleen, lymph node, and PBMCs of rejected LT monkeys, but rather, the memory CD4/8 T cells were increased in all tissues and PBMC. The normal liver has large numbers of CD4 regulatory T cells, CD8+ CD28-, and myeloid-derived suppressor cells, which are known immunosuppressive cells occurring at much higher levels than those seen in lymph node or peripheral blood. Memory T cells are dramatically increased in rejected liver allografts of RMs compared with those seen in normal RM tissues. Liver Transplantation 24 256-268 2018 AASLD.

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.

Functional Characterization of Regulatory Macrophages That Inhibit Graft-reactive Immunity

Macrophage accumulation in transplanted organs has long been recognized as a feature of allograft rejection¹. Immunogenic monocytes infiltrate the allograft early after transplantation, mount a graft reactive response against the transplanted organ, and initiate organ rejection². Recent data suggest that suppressive macrophages facilitate successful long-term transplantation³ and are required for the induction of transplantation tolerance⁴. This suggests a multidimensional concept of macrophage ontogeny, activation, and function, which demands a new roadmap for the isolation and analysis of macrophage function⁵. Due to the plasticity of macrophages, it is necessary to provide a methodology to isolate and characterize macrophages, depending on the tissue environment, and to define their functions according to different scenarios. Here, we describe a protocol for immune characterization of graft-infiltrating macrophages and the methods we used to functionally evaluate their capacity to inhibit CD8⁺ T proliferation and to promote CD4⁺Foxp3⁺ Treg expansion in vitro.

Functional analysis of CD14+HLA-DR-/low myeloid-derived suppressor cells in patients with lung squamous cell carcinoma

Immunomodulatory therapy is a potential effective treatment for advanced cancer that may provide an alternative to chemotherapy, which patients can experience adverse side effects to. Myeloid-derived suppressor cells (MDSCs) have been demonstrated to cause T-cell tolerance in rodents and humans; however, little is known about the role of MDSCs in squamous cell carcinoma. In the present study, the role of MDSCs in lung squamous cell carcinoma was investigated. Peripheral blood from 78 patients with lung squamous cell carcinoma and 30 healthy controls was examined for the presence and function of human MDSCs, denoted as monocyte differentiation antigen CD14-positive HLA class II histocompatibility antigen DR-negative/low (CD14⁺ HLA-DR^-/low) cells by flow cytometry. The sorted T-cell surface glyoprotein CD3 (CD3)⁺ cells and CD14⁺HLA-DR^-/low cells were subsequently co-cultured with a tumor cell line (NCI-H226). T-cell apoptosis was detected using annexin-V-fluorescein isothicyanate and 7-aminoactinomycin D. Interferon-γ (IFN-γ) levels were detected using an ELISA. The frequency of MDSCs in the peripheral blood mononuclear cells (PBMCs) from patients with lung squamous cell carcinoma was significantly higher compared with that of the healthy controls (P<0.05), whereas the frequency of T-cell surface glyoprotein CD4 (CD4)⁺ T cells and CD8⁺ T cells in PBMCs was significantly decreased (P<0.05). In an MDSC/CD8⁺ co-culture system, the proportion of CD8⁺ T-cell apoptosis significantly increased with the increase in ratio of MDSCs (P<0.05), while the proportion of tumor cell apoptosis significantly decreased (P<0.05). The concentration of IFN-γ significantly decreased with the increase in MDSCs (P<0.05). Therefore, MDSCs participate in the immune escape of lung squamous cell carcinoma, and may provide a possible therapeutic strategy for the treatment of this disease.

Myeloid-derived suppressor cells: The green light for myeloma immune escape

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous, immature myeloid cell population with the ability to suppress innate and adaptive immune responses that promote tumor growth. MDSCs are increased in patients with multiple myeloma (MM) and have bidirectional interaction with tumors within the MM microenvironment. MM-MDSCs promote MM tumor growth and induce immune suppression; conversely, MM cells induce MDSC development and survival. Although the role of MDSCs in infections, inflammatory diseases and solid tumors has been extensively characterized, their tumor-promoting and immune-suppressive role in MM and the MM microenvironment is only beginning to emerge. The presence and activation of MDSCs in MM patients has been well documented; however, the direct actions and functional consequences of MDSCs on cancer cells is poorly defined. Immunosuppressive MDSCs play an important role in tumor progression primarily because of their capability to promote immune-escape, angiogenesis, drug resistance and metastasis. However, their role in the bone marrow (BM), the primary MM site, is poorly understood. MM remains an incurable malignancy, and it is likely that the BM microenvironment protects MM against chemotherapy agents and the host immune system. A growing body of evidence suggests that host immune cells with a suppressive phenotype contribute to a myeloma immunosuppressive network. Among the known suppressor cells, MDSCs and T regulatory cells (Tregs) have been found to be significantly increased in myeloma patients and their levels correlate with disease stage and clinical outcome. Furthermore, it has been shown that MDSC can mediate suppression of myeloma-specific T-cell responses through the induction of T-cell anergy and Treg development in the MM microenvironment. Here, we review clinical correlations and the preclinical proof-of-principle data on the role of MDSCs in myeloma immunotolerance and highlight the mechanistically relevant MDSC-targeted compounds and their potential utility in a new approach for anti-myeloma therapy.

The Mononuclear Phagocyte System in Organ Transplantation

The mononuclear phagocyte system (MPS) comprises monocytes, macrophages and dendritic cells (DCs). Over the past few decades, classification of the cells of the MPS has generated considerable controversy. Recent studies into the origin, developmental requirements and function of MPS cells are beginning to solve this problem in an objective manner. Using high-resolution genetic analyses and fate-mapping studies, three main mononuclear phagocyte lineages have been defined, namely, macrophage populations established during embryogenesis, monocyte-derived cells that develop during adult life and DCs. These subsets and their diverse subsets have specialized functions that are largely conserved between species, justifying the introduction of a new, universal scheme of nomenclature and providing the framework for therapeutic manipulation of immune responses in the clinic. In this review, we have commented on the implications of this novel MPS classification in solid organ transplantation.

Immune Tolerance for Autoimmune Disease and Cell Transplantation

The undesired destruction of healthy cells, either endogenous or transplanted, by the immune system results in the loss of tissue function or limits strategies to restore tissue function. Current therapies typically involve nonspecific immunosuppression that may prevent the appropriate response to an antigen, thereby decreasing humoral immunity and increasing the risks of patient susceptibility to opportunistic infections, viral reactivation, and neoplasia. The induction of antigen-specific immunological tolerance to block undesired immune responses to self- or allogeneic antigens, while maintaining the integrity of the remaining immune system, has the potential to transform the current treatment of autoimmune disease and serve as a key enabling technology for therapies based on cell transplantation.

Prospective Clinical Testing of Regulatory Dendritic Cells in Organ Transplantation

Dendritic cells (DC) are rare, professional antigen-presenting cells with ability to induce or regulate alloimmune responses. Regulatory DC (DCreg) with potential to down-modulate acute and chronic inflammatory conditions that occur in organ transplantation can be generated in vitro under a variety of conditions. Here, we provide a rationale for evaluation of DCreg therapy in clinical organ transplantation with the goal of promoting sustained, donor-specific hyporesponsiveness, while lowering the incidence and severity of rejection and reducing patients’ dependence on anti-rejection drugs. Generation of donor- or recipient-derived DCreg that suppress T cell responses and prolong transplant survival in rodents or non-human primates has been well-described. Recently, good manufacturing practice (GMP)-grade DCreg have been produced at our Institution for prospective use in human organ transplantation. We briefly review experience of regulatory immune therapy in organ transplantation and describe our experience generating and characterizing human monocyte-derived DCreg. We propose a phase I/II safety study in which the influence of donor-derived DCreg combined with conventional immunosuppression on subclinical and clinical rejection and host alloimmune responses will be examined in detail.