Innate immune cellular therapeutics in transplantation

Successful organ transplantation provides an opportunity to extend the lives of patients with end-stage organ failure. Selectively suppressing the donor-specific alloimmune response, however, remains challenging without the continuous use of non-specific immunosuppressive medications, which have multiple adverse effects including elevated risks of infection, chronic kidney injury, cardiovascular disease, and cancer. Efforts to promote allograft tolerance have focused on manipulating the adaptive immune response, but long-term allograft survival rates remain disappointing. In recent years, the innate immune system has become an attractive therapeutic target for the prevention and treatment of transplant organ rejection. Indeed, contemporary studies demonstrate that innate immune cells participate in both the initial alloimmune response and chronic allograft rejection and undergo non-permanent functional reprogramming in a phenomenon termed "trained immunity." Several types of innate immune cells are currently under investigation as potential therapeutics in transplantation, including myeloid-derived suppressor cells, dendritic cells, regulatory macrophages, natural killer cells, and innate lymphoid cells. In this review, we discuss the features and functions of these cell types, with a focus on their role in the alloimmune response. We examine their potential application as therapeutics to prevent or treat allograft rejection, as well as challenges in their clinical translation and future directions for investigation.

Smad3 promotes adverse cardiovascular remodeling and dysfunction in doxorubicin-treated hearts

Many anticancer therapies cause serious cardiovascular complications that degrade quality of life and cause early mortality in treated patients. Specifically, doxorubicin is known as an effective anticancer agent that causes cardiomyopathy in treated patients. There has been growing interest in defining the role of endothelial cells in cardiac damage by doxorubicin. We have shown in the present study that endothelial nuclei accumulate more intravenously administered doxorubicin than other cardiac cell types. Doxorubicin enhanced cardiac production of the transforming growth factor-β (TGF-β) ligands and nuclear translocation of phospho-Smad3 in both cultured and in vivo cardiac endothelial cells. To examine the role of the TGF-β/mothers against decapentaplegic homolog 3 (Smad3) pathway in cardiac damage by doxorubicin, we used both Smad3 shRNA stable endothelial cell lines and Smad3-knockout mice. We demonstrated using endothelial transcriptome analysis that upregulation of the TGF-β and inflammatory cytokine/cytokine receptor pathways, as well as suppression of cell cycle and angiogenesis by doxorubicin, were alleviated in Smad3-deficient endothelial cells. The results of transcriptomic analysis were validated using qPCR, immunoblotting, and ex vivo aortic ring sprouting assays. Similarly, increased cardiac expression of cytokines and chemokines observed in treated wild-type mice was diminished in treated Smad3-knockout animals. We also detected increased end-diastolic diameter and depressed systolic function in doxorubicin-treated wild-type but not Smad3-knockout mice. This work provides evidence for the critical role of the canonical TGF-β/Smad3 pathway in cardiac damage by doxorubicin.NEW & NOTEWORTHY Microvascular endothelial cells in the heart accumulate more intravenously administered doxorubicin than nonendothelial cardiac cell types. The treatment enhanced the TGF-β/Smad3 pathway and elicited endothelial cell senescence and inflammatory responses followed by adverse cardiac remodeling and dysfunction in wild-type but not Smad3-deficient animals. Our study suggests that the TGF-β/Smad3 pathway contributes to the development of doxorubicin cardiomyopathy and the potential value of novel approaches to ameliorate cardiotoxicity by targeting the Smad3 transcription factor.

Skin and heart allograft rejection solely by long-lived alloreactive TRM cells in skin of severe combined immunodeficient mice

Whether induced tissue-resident memory T (T_RM) cells in nonlymphoid organs alone can mediate allograft rejection is unknown. By grafting alloskin or heart into severe combined immunodeficient or Rag2KO mice in which a piece of induced CD4⁺ and/or CD8⁺ T_RM cell-containing MHC-matched or syngeneic skin was transplanted in advance, we addressed this issue. The induced CD4⁺ T_RM cells in the skin alone acutely rejected alloskin or heart grafts. RNA-seq analysis showed that induced CD4⁺ T_RM cells in skin favorably differentiated into T_H17-like polarization during the secondary immune response. Inhibition of the key T_H17 signaling molecule RORγt attenuated T_RM cell-mediated graft rejection. Thus, we offer a unique mouse model to specifically study T_RM cell-mediated allograft rejection without the involvement of lymphocytes in lymphoid organs and tissues. Our study provides strong evidence supporting the hypothesis that long-lived alloreactive T_RM cells resident in other organs/tissues substantially contribute to organ allograft rejection.

miR-31, miR-155, and miR-221 expression profiles and their association with graft skin tolerance in a syngeneic vs. allogeneic murine skin transplantation model

Grafting is the gold standard for the treatment of severe skin burns. Frequently, allogeneic tissue is the only transient option for wound coverage, but their use risks damage to surrounding tissues. MicroRNAs have been associated with acute rejection of different tissues/organs. In this study, we analyzed the expression of miR-31, miR-155, and miR-221 and associate it with graft tolerance or rejection using a murine full-thickness skin transplantation model. Recipient animals for the syngeneic and allogeneic groups were BALB/c and C57BL/6 mice, respectively; donor tissues were obtained from BALB/c mice. After 7 days post-transplantation (DPT), the recipient skin and grafts in the syngeneic group maintained most of their structural characteristics and transforming growth factor (TGF)β1 and TGFβ3 expression. Allografts were rejected early (Banff grades II and IV at 3 and 7 DPT, respectively), showing damage to the skin architecture and alteration of TGFβ3 distribution. miRNAs skin expression changed in both mouse strains; miR-31 expression increased in the recipient skin of syngeneic grafts relative to that of allogeneic grafts at 3 and 7 DPT (p < 0.05 and p < 0.01, respectively); miR-221 expression increased in the same grafts at 7 DPT (p < 0.05). The only significant difference between donor tissues was observed for miR-155 expression at 7 DPT which was associated with necrotic tissue. Only miR-31 and miR-221 levels were increased in the blood of BALB/c mice that received syngeneic grafts after 7 DPT. Our data suggest that local and systemic miR-31 and miR-221 overexpression are associated with graft tolerance.

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.

Adoptive Transfer of Regulatory Immune Cells in Organ Transplantation

Chronic graft rejection remains a significant barrier to solid organ transplantation as a treatment for end-organ failure. Patients receiving organ transplants typically require systemic immunosuppression in the form of pharmacological immunosuppressants for the duration of their lives, leaving these patients vulnerable to opportunistic infections, malignancies, and other use-restricting side-effects. In recent years, a substantial amount of research has focused on the use of cell-based therapies for the induction of graft tolerance. Inducing or adoptively transferring regulatory cell types, including regulatory T cells, myeloid-derived suppressor cells, and IL-10 secreting B cells, has the potential to produce graft-specific tolerance in transplant recipients. Significant progress has been made in the optimization of these cell-based therapeutic strategies as our understanding of their underlying mechanisms increases and new immunoengineering technologies become more widely available. Still, many questions remain to be answered regarding optimal cell types to use, appropriate dosage and timing, and adjuvant therapies. In this review, we summarize what is known about the cellular mechanisms that underly the current cell-based therapies being developed for the prevention of allograft rejection, the different strategies being explored to optimize these therapies, and all of the completed and ongoing clinical trials involving these therapies.

CHBP induces stronger immunosuppressive CD127+ M-MDSC via erythropoietin receptor

Erythropoietin (EPO) is not only an erythropoiesis hormone but also an immune-regulatory cytokine. The receptors of EPO (EPOR)2 and tissue-protective receptor (TPR), mediate EPO's immune regulation. Our group firstly reported a non-erythropoietic peptide derivant of EPO, cyclic helix B peptide (CHBP), which could inhibit macrophages inflammation and dendritic cells (DCs) maturation. As a kind of innate immune regulatory cell, myeloid-derived suppressor cells (MDSCs) share a common myeloid progenitor with macrophages and DCs. In this study, we investigated the effects on MDSCs differentiation and immunosuppressive function via CHBP induction. CHBP promoted MDSCs differentiate toward M-MDSCs with enhanced immunosuppressive capability. Infusion of CHBP-induced M-MDSCs significantly prolonged murine skin allograft survival compared to its counterpart without CHBP stimulation. In addition, we found CHBP increased the proportion of CD11b+Ly6G-Ly6Chigh CD127+ M-MDSCs, which exerted a stronger immunosuppressive function compared to CD11b+Ly6G-Ly6Chigh CD127- M-MDSCs. In CHBP induced M-MDSCs, we found that EPOR downstream signal proteins Jak2 and STAT3 were upregulated, which had a strong relationship with MDSC function. In addition, CHBP upregulated GATA-binding protein 3 (GATA-3) protein translation level, which was an upstream signal of CD127 and regulator of STAT3. These effects of CHBP could be reversed if Epor was deficient. Our novel findings identified a new subset of M-MDSCs with better immunosuppressive capability, which was induced by the EPOR-mediated Jak2/GATA3/STAT3 pathway. These results are beneficial for CHBP clinical translation and MDSC cell therapy in the future.

Myeloid-derived suppressor cells in obstetrical and gynecological diseases

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of myeloid-origin cells which have immunosuppressive activities in several conditions, such as cancer and inflammation. Recent research has also associated MDSCs with numerous obstetrical and gynecological diseases. During pregnancy, MDSCs accumulate to ensure maternal-fetal immune tolerance, whereas they are decreased in patients who suffer from early miscarriage or pre-eclampsia. While the etiology of endometriosis is still unknown, abnormal accumulation of MDSCs in the peripheral blood and peritoneal fluid, alongside an increased level of reactive oxygen species (ROS), has been observed in these patients, which is central to the cellular immune regulations by MDSCs. Additionally, the regulation of MDSCs observed in tumours is also applicable to gynecologic neoplasms, including ovarian cancer and cervical cancer. More recently, emerging evidence has shown that there are high levels of MDSCs in premature ovarian failure (POF) and in vitro fertilization (IVF), but the underlying mechanisms are unknown. In this review, the generation and mechanisms of MDSCs are summarized. In particular, the modulation of these cells in immune-related obstetrical and gynecological diseases is discussed, including potential treatment options targeting MDSCs.

Apolipoprotein A-I Mimetic Peptide L-4F Suppresses Granulocytic-Myeloid-Derived Suppressor Cells in Mouse Pancreatic Cancer

L-4F is an apolipoprotein A-I (ApoA-I) mimetic peptide, it was engineered to imitate the anti-inflammatory and anti-oxidative activity of ApoA-I. In this paper, H7 cell was used to construct a mouse model of pancreatic cancer in situ, and the mice were treated with L-4F. Then, the development of pancreatic cancer and myeloid-derived suppressor cells (MDSCs) infiltration were investigated in vivo. After L-4F treatment, the differentiation, proliferation and apoptosis of MDSCs were detected in vitro. Moreover, we test its effects on the immunosuppressive function of MDSCs ex vivo. The results show that L-4F significantly reduced the tumorigenicity of H7 cells. L-4F suppressed granulocytic myeloid-derived suppressor cells (PMN-MDSCs) differentiation and inhibited the accumulation of PMN-MDSCs in the mouse spleen and tumor tissue. L-4F weakened the immunosuppressive function of MDSCs, resulting in decreased production of ROS and H₂O₂ by MDSCs, and increased T cell proliferation, interferon γ and tumor necrosis factor β secretion, and CD3⁺CD4⁺ T and CD3⁺CD8⁺ T cell infiltration into the mouse spleen and pancreatic cancer tissue. Furthermore, L-4F significantly down regulated the STAT3 signaling pathway in PMN-MDSCs. These results indicated that L-4F exerts an effective anti-tumor and immunomodulatory effect in pancreatic cancer by inhibiting PMN-MDSCs.

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.

Tolerogenic Role of Myeloid Suppressor Cells in Organ Transplantation

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of immature cells of myeloid origin with a specific immune inhibitory function that negatively regulates the adaptive immune response. Since MDSC participate in the promotion of tolerance in the context of organ transplantation, therapeutic strategies that regulate the induction and development of MDSC have been the center of scientist attention. Here we review literature regarding induction of MDSC with demonstrated suppressive function among different types of allografts and their mechanism of action. While manipulation of MDSC represents a potential therapeutic approach for the promotion of donor specific tolerance in solid organ transplantation, further characterization of their specific phenotype, which distinguishes MDSC from non-suppressive myeloid cells, and detailed evaluation of the inhibitory mechanism that determines their suppressive function, is necessary for the realistic application of MDSC as biomarkers in health and disease and their potential use as immune cell therapy in organ transplantation.

The Sweet Surrender: How Myeloid Cell Metabolic Plasticity Shapes the Tumor Microenvironment

Immune cells are one of the most versatile cell types, as they can tailor their metabolic activity according to their required function. In response to diverse environmental cues, immune cells undergo metabolic reprogramming to support their differentiation, proliferation and pro-inflammatory effector functions. To meet a dramatic surge in energetic demand, immune cells rewire their metabolism to utilize aerobic glycolysis. This preferential use of glycolysis even under aerobic conditions is well established in tumor cells, and is known as the "Warburg effect." Tumor cells avidly use glucose for aerobic glycolysis, thereby creating a nutrient-starved microenvironment, outcompeting T cells for glucose, and directly inhibiting T-cell anti-tumoral effector function. Given that both immune and tumor cells use similar modes of metabolism in the tumor stroma, it is imperative to identify a therapeutic window in which immune-cell and tumor-cell glycolysis can be specifically targeted. In this review, we focus on the Warburg metabolism as well as other metabolic pathways of myeloid cells, which comprise a notable niche in the tumor environment and promote the growth and metastasis of malignant tumors. We examine how differential immune-cell activation triggers metabolic fate, and detail how this forbidding microenvironment succeeds in shutting down the vigorous anti-tumoral response. Finally, we highlight emerging therapeutic concepts that aim to target immune-cell metabolism. Improving our understanding of immunometabolism and immune-cell commitment to specific metabolic fates will help identify alternative therapeutic approaches to battle this intractable disease.

Characterization and allergic role of IL-33-induced neutrophil polarization

Neutrophils are involved in the pathogenesis of allergy. However, the contribution of the different functionally polarized neutrophils in allergy needs to be clarified. We sought to define the characteristics of interleukin (IL)-33-induced neutrophils and the involvement of this subset of polarized neutrophils in allergic pathogenesis. Freshly isolated neutrophils were treated with different cytokines and the cytokine expression levels were detected by real-time PCR. The gene expression profile of IL-33-induced neutrophils was determined by microarray assay. Adoptive transfer assay was used to investigate the function of IL-33-induced neutrophils in an ovalbumin (OVA)-induced allergic asthma model. IL-33-treated neutrophils selectively produced IL-4, IL-5, IL-9 and IL-13 (referred as to N(IL-33) cells) and displayed a distinctive gene expression profile in sharp contrast to resting and lipopolysaccharide (LPS)-treated neutrophils. IL-33-induced neutrophils expressed high Levels of IL-1R2 on cell surface, whereas resting and LPS-treated neutrophils did not, indicating IL-1R2 might be used as a biomarker for N(IL-33) cells. Importantly, N(IL-33) neutrophils exist in the lungs of OVA-induced allergic asthma mice. Adoptive transfer of N(IL-33) neutrophils significantly promotes the severity of the lung pathogenesis in this model. IL-33 induces neutrophil polarization through c-Jun N-terminal kinase- and nuclear factor-κB-dependent pathways. A previously unappreciated neutrophil polarization driven by IL-33 with unique cell surface markers and cytokine/chemokine-producing gene profile was defined. The newly identified N(IL-33) subpopulation may have significant contribution to IL-33-related pathogenesis.

The Effect of Immunosuppressive Drugs on MDSCs in Transplantation

Myeloid-derived suppressor cells (MDSCs) are a group of innate immune cells that regulates both innate and adaptive immune responses. In recent years, MDSCs were shown to play an important negative regulatory role in transplant immunology even upstream of regulatory T cells. In certain cases, MDSCs are closely involved in transplantation immune tolerance induction and maintenance. It is known that some immunosuppressant drugs negatively regulate MDSCs but others have positive effects on MDSCs in different transplant cases. We herein summarized our recent insights into the regulatory roles of MDSCs in transplantation specially focusing on the effects of immunosuppressive drugs on MDSCs and their mechanisms of action. Studies on the effects of immunosuppressive drugs on MDSCs will significantly expand our understanding of immunosuppressive drugs on immune regulatory cells in transplantation and offer new insights into transplant tolerance. We hope to emphasize our concern for the negative effects of immunosuppressive agents on MDSCs, which may potentially attenuate the immune tolerance induction in transplanted recipients.

Evaluation of phenotypic and functional stability of RAW 264.7 cell line through serial passages

Established cell lines are widely used in research, however an appealing question is the comparability of the cells between various laboratories, their characteristics and stability in time. Problematic is also the cell line misidentification, genetic and phenotypic shift or Mycoplasma contamination which are often forgotten in research papers. The monocyte/macrophage-like cell line RAW 264.7 has been one of the most commonly used myeloid cell line for more than 40 years. Despite its phenotypic and functional stability is often discussed in literature or at various scientific discussion panels, their stability during the consecutive passages has not been confirmed in any solid study. So far, only a few functional features of these cells have been studied, for example their ability to differentiate into osteoclasts. Therefore, in the present paper we have investigated the phenotype and functional stability of the RAW 264.7 cell line from passage no. 5 till passage no. 50. We found out that the phenotype (expression of particular macrophage-characteristic genes and surface markers) and functional characteristics (phagocytosis and NO production) of RAW 264.7 cell line remains stable through passages: from passage no. 10 up to passage no. 30. Overall, our results indicated that the RAW 264.7 cell line should not be used after the passage no. 30 otherwise it may influence the data reliability.

mTORC2 controls Th9 polarization and allergic airway inflammation

BACKGROUND

T helper type 9 (Th9) cells, a subpopulation of CD4⁺ T cells, play a critical role in the pathogenesis of allergic airway inflammation. However, it remains unknown whether mTORC2 regulates Th9 differentiation or function during allergic inflammation.

METHODS

T-cell-specific Rictor-deficient mice, a mouse model of allergic airway inflammation induced by ovalbumin (OVA) sensitization and a mouse model of adoptive transfer of induced Th9 cells, were used to address the roles of mTORC2 in the pathogenesis of allergic airway inflammation. The in vitro Th9 induction, multiple colors flow cytometry, real-time PCR, and Western blots were used to investigate the molecular effects of mTORC2 in Th9 induction.

RESULTS

The differentiation of naïve CD4⁺ T cells into Th9 cells was significantly diminished in the absence of Rictor, the core component of mTORC2. Using a mouse model of allergic airway inflammation induced by OVA sensitization, T-cell-specific Rictor-deficient mice show much less severe allergic airway inflammation characterized by decreased pathological alterations and fibrosis of the lungs, which was accompanied with reduced Th9 differentiation and infiltration. Importantly, the isolated Rictor-deficient Th9 cells mediate less severe allergic pathogenesis upon adoptive transfer. Rictor deficiency impairs Th9 cell differentiation by reducing IRF4 expression rather than affecting Foxo1/Foxo3a transcriptional activity, which is likely due to decreased Akt and/or STAT6 activation.

CONCLUSIONS

These findings uncover a novel role of mTORC2 in Th9 cell differentiation and may have important implications for therapeutic intervention of allergic diseases.

Phosphatase wild-type p53-induced phosphatase 1 controls the development of TH9 cells and allergic airway inflammation

BACKGROUND

Allergic asthma is one of the most common diseases worldwide, resulting in a burden of diseases. No available therapeutic regimens can cure asthma thus far.

OBJECTIVE

We sought to identify new molecular targets for T_H9 cell-mediated allergic airway inflammation.

METHODS

Wild-type p53-induced phosphatase 1 (Wip1) gene knockout mice, Wip1 inhibitor-treated mice, and ovalbumin-induced allergic airway inflammation mouse models were used to characterize the roles of Wip1 in allergic airway inflammation. The induction of T_H cell subsets in vitro, real-time PCR, immunoblots, luciferase assays, and chromatin immunoprecipitation assays were used to determine the regulatory pathways of Wip1 in T_H9 differentiation.

RESULTS

Here we demonstrate that Wip1-deficient mice are less prone to allergic airway inflammation, as indicated by the decreased pathologic alterations in lungs. Short-term treatment with a Wip1-specific inhibitor significantly ameliorates allergic inflammation progression. Intriguingly, Wip1 selectively impaired T_H9 but not T_H1, T_H2, and T_H17 cell differentiation. Biochemical assays show that Wip1 deficiency increases c-Jun/c-Fos activity in a c-Jun N-terminal kinase-dependent manner and that c-Jun/c-Fos directly binds to Il9 promoter and inhibits Il9 transcription.

CONCLUSION

Wip1 controls T_H9 cell development through regulating c-Jun/c-Fos activity on the Il9 promoter and is important for the pathogenesis of allergic airway inflammation. These findings shed light on the previously unrecognized roles of Wip1 in T_H9 cell differentiation. The inhibitory effects of a Wip1 inhibitor on the pathogenesis of allergic airway inflammation can have important implications for clinical application of Wip1 inhibitors in allergy therapies.

Characterization and biological significance of IL-23-induced neutrophil polarization

Neutrophils are heterogeneous with distinct subsets, and can switch phenotypes to exert regulatory functions on immunity. We herein demonstrate that IL-23-treated neutrophils selectively produce IL-17A, IL-17F and IL-22, and display a distinct gene expression profile in contrast to resting and lipopolysaccharide-treated neutrophils. IL-17⁺ neutrophils are present in the colons of mice with dextran sulfate sodium-induced colitis. Adoptive transfer of IL-23-treated neutrophils significantly promotes pathogenesis in this model. IL-23 induces neutrophil polarization through STAT3-dependent RORγt and BATF pathways. Thus, IL-23-induced neutrophil polarization expresses a unique cytokine-producing profile, which may contribute to IL-23-mediated inflammatory diseases.

Specific and Complex Reprogramming of Cellular Metabolism in Myeloid Cells during Innate Immune Responses

Renewed interest in immune cell metabolism has led to the emergence of a research field aimed at studying the importance of metabolic processes for an effective immune response. In addition to the adaptive immune system, cells of the myeloid lineage have been shown to undergo robust metabolic changes upon activation. Whereas the specific metabolic requirements of myeloid cells after lipopolysaccharide/TLR4 stimulation have been extensively studied, recent evidence suggested that this model does not represent a metabolic blueprint for activated myeloid cells. Instead, different microbial stimuli, pathogens, or tissue microenvironments lead to specific and complex metabolic rewiring of myeloid cells. Here we present an overview of the metabolic heterogeneity in activated myeloid cells during health and disease. Directions for future research are suggested to ultimately provide new therapeutic opportunities. The uniqueness of metabolic signatures accompanying different conditions will require tailor-made interventions to ultimately modulate aberrant myeloid cell activation during disease.

The Diverse Biological Functions of Neutrophils, Beyond the Defense Against Infections

Polymorphonuclear neutrophils are among the first defense against infection and closely involved in the initiation of inflammatory response. It is well recognized that this function of neutrophils was mainly mediated by phagocytosis, intracellular degradation, releasing of granules, and formation of neutrophil extracellular traps after sensing dangerous stress. However, accumulating data showed that neutrophils had a variety of important biological functions in both innate and adaptive immunities, far beyond cytotoxicity against pathogens. Neutrophils can differentially switch phenotypes and display distinct subpopulations under different microenvironments. Neutrophils can produce a large variety of cytokines and chemokines upon stimulation. Furthermore, neutrophils directly interact with dendritic cells (DCs), macrophages, natural killer cells, T cells, and B cells so as to either potentiate or down-modulate both innate and adaptive immunity. In the present review, we summarize the recent progress on the functional plasticity and the regulatory ability on immunity of neutrophils in physiological and pathological situations.

Glucocorticoid receptor promotes the function of myeloid-derived suppressor cells by suppressing HIF1α-dependent glycolysis

Immunomodulatory signaling imposes tight regulations on metabolic programs within immune cells and consequentially determines immune response outcomes. Although the glucocorticoid receptor (GR) has been recently implicated in regulating the function of myeloid-derived suppressor cells (MDSCs), whether the dysregulation of GR in MDSCs is involved in immune-mediated hepatic diseases and how GR regulates the function of MDSCs in such a context remains unknown. Here, we revealed the dysregulation of GR expression in MDSCs during innate immunological hepatic injury (IMH) and found that GR regulates the function of MDSCs through modulating HIF1α-dependent glycolysis. Pharmacological modulation of GR by its agonist (dexamethasone, Dex) protects IMH mice against inflammatory injury. Mechanistically, GR signaling suppresses HIF1α and HIF1α-dependent glycolysis in MDSCs and thus promotes the immune suppressive activity of MDSCs. Our studies reveal a role of GR-HIF1α in regulating the metabolism and function of MDSCs and further implicate MDSC GR signaling as a potential therapeutic target in hepatic diseases that are driven by innate immune cell-mediated systemic inflammation.

Phosphatase Wip1 Masters IL-17-producing Neutrophil-mediated Colitis in Mice

Wild-type p53-induced phosphatase 1 (Wip1) is currently believed to be a promising drug target for cancer therapy. Our recent studies showed that deletion of Wip1 remarkably promoted neutrophil inflammatory response. Whether Wip1 is involved in the regulation of inflammatory bowel disease is unknown. In the present study, we found that Wip1 knockout (KO) mice were more susceptible to colitis induced by dextran sulphate sodium (DSS) than wild-type mice as substantiated by the lower mouse survival ratio, rapid bodyweight loss, increased disease activity index, shorter colon length, and more severe pathology of colons in Wip1KO mice. Using full bone marrow chimera mouse models, we demonstrated that Wip1 intrinsically controls inflammatory response of immune cells. Deletion of IL-17 (Wip1/IL-17 double KO mice) significantly rescued the pathology in Wip1KO mice. Neutrophils of DSS-treated wild-type and Wip1KO mice expressed significantly higher IL-17. After adoptive transfer of sorted Wip1KO or double KO neutrophils into IL-17KO mice, mice receiving double KO neutrophils were more resistant to DSS-induced colitis than mice receiving Wip1KO neutrophils. These data collectively indicate that Wip1 modulates host sensitivity to colitis by intrinsically regulating immune cells. The enhanced IL-17 expression in neutrophils contributed to the increased sensitivity and severity of colitis in Wip1KO mice. Thus, Wip1 may be used as a drug target to treat colitis.

Deletion of Smad3 improves cardiac allograft rejection in mice

T cells play a critical role in acute allograft rejection. TGF-β/Smad3 signaling is a key pathway in regulating T cell development. We report here that Smad3 is a key transcriptional factor of TGF-β signaling that differentially regulates T cell immune responses in a mouse model of cardiac allograft rejection in which donor hearts from BALB/c mice were transplanted into Smad3 knockout (KO) and wild type (WT) mice. Results showed that the cardiac allograft survival was prolonged in Smad3 KO recipients. This allograft protection was associated with a significant inhibition of proinflammatory cytokines (IL-1β, TNF-α, and MCP-1) and infiltration of neutrophils, CD3⁺ T cells, and F4/80⁺ macrophages. Importantly, deletion of Smad3 markedly suppressed T-bet and IFN-γ while enhancing GATA3 and IL-4 expression, resulting in a shift from the Th1 to Th2 immune responses. Furthermore, mice lacking Smad3 were also protected from the Th17-mediated cardiac injury, although the regulatory T cell (Treg) response was also suppressed. In conclusion, Smad3 is an immune regulator in T cell-mediated cardiac allograft rejection. Loss of Smad3 results in a shift from Th1 to Th2 but suppressing Th17 immune responses. Thus, modulation of TGF-β/Smad3 signaling may be a novel therapy for acute allograft rejection.

The effect of immunosuppressive drug cyclosporine A on myeloid-derived suppressor cells in transplanted mice

OBJECTIVE

Myeloid-derived suppressor cells (MDSCs) play important roles in preventing graft rejection. Immunosuppressive drug cyclosporine A (CsA) is widely used in clinics to treat patients with allografts and autoimmune diseases. However, the effect of CsA on CD11b(+)Gr1(+) MDSCs has not been studied.

SUBJECTS

The subjects of the study include BALB/c skin-grafted C57BL/6 mice and the in vitro MDSCs induction system.

TREATMENT

Skin-grafted mice were treated with CsA (30 mg/kg, i.p.) or control buffer daily. 0.01 μg/ml CsA was added during MDSC induction.

METHODS

Flow cytometry was used to check cell phenotypes and proliferation. Real-time PCR was used for gene expressions. Inducible nitric oxide synthase iNOS-knockout mice were used for the role of iNOS in the immunosuppression of MDSCs.

RESULTS

CsA in MDSC-induction system significantly increased the number of CD11b(+)Gr1(+)MDSCs without detectable effects on the expressions of CD31, CD115 and CD274. However, GM-CSF + CsA-induced MDSCs express higher iNOS than control MDSCs. Blocking iNOS activity by inhibitor or gene deletion significantly reversed the inhibitory effects of GM-CSF + CsA-induced MDSCs on T cell proliferation. Importantly, CsA treatment significantly increased the number and the immunosuppressive ability of CD11b(+)Gr1(+)MDSCs in allogeneic skin-grafted mice.

CONCLUSIONS

CsA promotes MDSC induction and immunosuppressive function, which might be of clinical importance in treating graft rejection and autoimmune diseases.

TNFα-induced M-MDSCs promote transplant immune tolerance via nitric oxide

Efficient induction of functional competent myeloid-derived suppressor cells (MDSCs) will be critical for the clinical application of MDSCs to treat autoimmune diseases and to induce transplantation immune tolerance. In the present study, we tried to establish the MDSC induction system with M-CSF and tumor necrosis factor α (TNFα) and investigated the immunosuppressive function of M-CSF + TNFα-induced MDSCs in transplant mouse models. Monocytic MDSCs (M-MDSCs) were induced by culture of the non-adherent mouse bone marrow cells with M-CSF or M-CSF + TNFα, respectively, for 7 days. Phenotype analysis revealed that the majority of M-CSF- and M-CSF + TNFα-induced MDSCs express F4/80. The addition of TNFα in the induction period increased Gr-1, Ly6C, CD80, and CD274 expressions on these cells. M-CSF + TNFα-induced M-MDSCs showed poor TNFα, IL-12, and IL-6 expressions after lipopolysaccharide (LPS) stimulation and decreased arginase 1 (Arg-1) and Fizz expressions after IL-4 stimulation compared with M-CSF-induced M-MDSCs. M-CSF + TNFα-induced M-MDSCs showed enhanced ability to suppress T cell proliferation and cytokine production than M-CSF-induced M-MDSCs. M-CSF + TNFα-induced M-MDSCs express high levels of inducing nitric oxide synthase (iNOS) and blocking iNOS activity by a chemical inhibitor or gene deficiency significantly reversed the inhibitory effects of M-CSF + TNFα-induced M-MDSCs on T cells. Adoptive transfer of M-CSF + TNFα-induced M-MDSCs promoted immune tolerance in a male-to-female skin-grafted mice, but M-CSF + TNFα-induced iNOS-deficient M-MDSCs failed to do so. Thus, M-CSF + TNFα-induced M-MDSCs have powerful immunosuppressive activity, which is mediated by an iNOS-dependent pathway. M-CSF + TNFα-induced M-MDSCs can promote immune tolerance to donor antigens in a transplant mouse model.

KEY MESSAGE

The combination of M-CSF and TNFα efficiently induces functional M-MDSCs in vitro. M-CSF + TNFα-induced M-MDSCs promote immune tolerance in a transplant mouse model. The immunosuppressive ability of M-CSF + TNFα-induced M-MDSCs is dependent on iNOS.

In Vivo Mobilization and Functional Characterization of Nonhuman Primate Monocytic Myeloid-Derived Suppressor Cells

Increasing evidence from small animal models shows that myeloid-derived suppressor cells (MDSCs) can play a crucial role in inhibiting allograft rejection and promoting transplant tolerance. We identified CD3(-)CD20(-)HLA-DR(-)CD14(+)CD33(+)CD11b(+) cells in peripheral blood of healthy rhesus macaques. These putative monocytic MDSCs constituted 2.1% ± 1.7% of lin(-)HLA-DR(-) peripheral blood mononuclear cells. Administration of granulocyte-macrophage colony-stimulating factor (CSF) and granulocyte CSF increased their incidence to 5.3% ± 3.4%. The total number of MDSCs that could be flow sorted from a single whole rhesus leukapheresis product was 38 ± 13 × 10(6) (n = 10 monkeys). Freshly isolated or cryopreserved MDSCs from mobilized monkeys incorporated in cultures of anti-CD3- and anti-CD28-stimulated autologous T cells markedly suppressed CD4(+) and CD8(+) T cell proliferation and cytokine secretion (interferon γ, IL-17A). Moreover, these MDSCs enhanced CD4(+)CD25(hi)Foxp3(+) regulatory T cell (Treg) expansion while inhibiting proliferation of activated memory T cells and increasing Treg relative to effector and terminally differentiated memory T cells. Inhibition of arginase-1, but not inducible nitric oxide synthase activity, partially reversed the inhibitory effect of the MDSCs on CD8(+) T cell proliferation. Consequently, functional MDSCs can be isolated from nonhuman primates for prospective use as therapeutic cellular vaccines in transplantation.

mTOR masters monocytic myeloid-derived suppressor cells in mice with allografts or tumors

CD11b(+) Gr1(+) myeloid-derived suppressor cells (MDSCs) play critical roles in controlling the processes of tumors, infections, autoimmunity and graft rejection. Immunosuppressive drug rapamycin (RPM), targeting on the key cellular metabolism molecule mTOR, is currently used in clinics to treat patients with allo-grafts, autoimmune diseases and tumors. However, the effect of RPM on MDSCs has not been studied. RPM significantly decreases the cell number and the immunosuppressive ability on T cells of CD11b(+) Ly6C(high) monocytic MDSCs (M-MDSCs) in both allo-grafts-transplanted and tumor-bearing mice respectively. Mice with a myeloid-specific deletion of mTOR have poor M-MDSCs after grafting with allo-skin tissue or a tumor. Grafting of allo-skin or tumors significantly activates glycolysis pathways in myeloid precursor cells in bone marrow, which is inhibited by RPM or mTOR deletion. 2-deoxyglucose (2-DG), an inhibitor of the glycolytic pathway, inhibits M-MDSC differentiation from precursors, while enhancing glycolysis by metformin significantly rescues the RPM-caused deficiency of M-MDSCs. Therefore, we offer evidence supporting that mTOR is an intrinsic factor essential for the differentiation and immunosuppressive function of M-MDSCs and that these metabolism-relevant medicines may impact MDSCs-mediated immunosuppression or immune tolerance induction, which is of considerable clinical importance in treating graft rejection, autoimmune diseases and cancers.

Phenotype, development, and biological function of myeloid-derived suppressor cells

CD11b⁺Gr-1⁺ myeloid-derived suppressor cells (MDSCs) are an important population of innate regulatory cells mainly comprising monocytic MDSCs (M-MDSCs) with a phenotype of CD11b⁺Ly6G^-Ly6C^high and granulocytic MDSCs (G-MDSCs) with a phenotype of CD11b⁺Ly6G⁺Ly6C^low in mice. They play crucial roles in the pathogenesis of cancers, chronic infections, autoimmune diseases, and transplantation. Various extracellular factors such as lipopolysaccharide (LPS), macrophage colony-stimulating factor (M-CSF), granulocyte macrophage colony-stimulating factor (GM-CSF), stem cell factor (SCF), interleukin (IL)-6, interferon gamma (IFNγ), IL-1β, vascular endothelial growth factor (VEGF), Hsp72, IL-13, C5a, and prostaglandin E2 (PGE2) can induce MDSC differentiation, whereas IL-4 and all-trans-retinoic acid can inhibit this process. For the intracellular signals, signal transducer and activator of transcription (STAT) family members, C/EBPβ and cyclooxigenase-2 (COX-2) promote MDSC function, whereas interferon regulatory factor-8 (IRF-8) and Smad3 downregulate MDSC activity. The immunosuppressive function of MDSCs is mediated through various effector molecules, primarily cellular metabolism-related molecules such as nitric oxide (NO), arginase, reactive oxygen species (ROS), transforming growth factor β (TGFβ), IL-10, indoleamine 2,3-dioxygenase (IDO), heme oxygenase-1 (HO-1), carbon monoxide (CO), and PGE2. In this article, we will summarize the molecules involved in the induction and function of MDSCs as well as the regulatory pathways of MDSCs.

TSC1 controls IL-1β expression in macrophages via mTORC1-dependent C/EBPβ pathway

The tuberous sclerosis complex 1 (TSC1) is a tumor suppressor that inhibits the mammalian target of rapamycin (mTOR), which serves as a key regulator of inflammatory responses after bacterial stimulation in monocytes, macrophages, and primary dendritic cells. Previous studies have shown that TSC1 knockout (KO) macrophages produced increased inflammatory responses including tumor necrosis factor-α (TNF-α) and IL-12 to pro-inflammatory stimuli, but whether and how TSC1 regulates pro-IL-1β expression remains unclear. Here using a mouse model in which myeloid lineage-specific deletion of TSC1 leads to constitutive mTORC1 activation, we found that TSC1 deficiency resulted in impaired expression of pro-IL-1β in macrophages following lipopolysaccharide stimulation. Such decreased pro-IL-1β expression in TSC1 KO macrophages was rescued by reducing mTORC1 activity with rapamycin or deletion of mTOR. Rictor deficiency has no detectable effect on pro-IL-1β synthesis, suggesting that TSC1 positively controls pro-IL-1β expression through mTORC1 pathway. Moreover, mechanism studies suggest that mTORC1-mediated downregulation of the CCAAT enhancer-binding protein (C/EBPβ) critically contributes to the defective pro-IL-1β expression. Overall, these findings highlight a critical role of TSC1 in regulating innate immunity by control of the mTOR1-C/EBPβ pathway.

Insights into Myeloid-Derived Suppressor Cells in Inflammatory Diseases

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells involved in immune regulation. This population subdivides into granulocytic MDSCs and monocytic MDSCs, which regulate immune responses via the production of various molecules including reactive oxygen species, nitric oxide, arginase-1, interleukin-10, and transforming growth factor-β. Most studies of MDSCs focused on their role in tumors. MDSCs protect tumor cells from immune responses, and thus the frequency of MDSCs associates with poor prognosis. Many recent studies reported an important role for MDSCs in inflammatory diseases via the regulation of immune cells. In addition, the utilization of MDSCs by infectious pathogens suggests an immune evasion mechanism. Thus, MDSCs are important immune regulators in inflammatory diseases, as well as in tumors. This review focuses on the role of MDSCs in the regulation of inflammation in non-tumor settings.

Monocyte-Derived Suppressor Cells in Transplantation

Myeloid-derived suppressor cells (MDSC) are cells of myeloid origin with enhanced suppressive function. They are negative regulators of the immune responses and comprise a heterogeneous mixture of immunosuppressive cells of monocytic (M-MDSC) and granulocytic (G-MDSC) origin. A more recent nomenclature proposes the term "suppressive monocyte derived cells" (suppressive MCs) to define CSF1/CSF2-dependent mouse suppressor cells that develop from common monocyte progenitors (cMoPs) after birth. Here, we review the literature about monocytic-derived cells with demonstrated suppressor function in vitro and in vivo within the context of solid organ transplantation.

Microgravity activates p38 MAPK-C/EBPβ pathway to regulate the expression of arginase and inflammatory cytokines in macrophages

OBJECTIVE AND DESIGN

Molecular mechanisms of microgravity-caused immunosuppression are not fully elucidated. In the present study, we investigated the effects of simulated microgravity on macrophage functions and tried to identify the related intracellular signal pathways.

MATERIAL OR SUBJECTS

Primary mouse macrophages were used in the present study. The gene expression and function of IL-4-treated mouse macrophages were detected after simulated microgravity or 1 g control.

METHODS

Freshly isolated primary mouse macrophages were cultured in a standard simulated microgravity situation using a rotary cell culture system (RCCS-1) and 1 g control conditions. Real-time PCR, western blots and flow cytometry were used to investigate the related intracellular signals and molecule expression.

RESULTS

The arginase mRNA and protein levels in freshly isolated primary mouse macrophages under simulated microgravity using RCCS-1 were significantly higher than those under normal gravity. Meanwhile, simulated microgravity induced over-expression of C/EBPβ, a transcription factor of arginase promoter, and activation of p38 MAPK, which could increase C/EBPβ expression. Furthermore, up-regulation of Interleukin-6 (IL-6) and down-regulation of IL-12 p40 (IL-12B) in LPS-stimulated macrophages were also detected after simulated microgravity, which is regulated by C/EBPβ.

CONCLUSIONS

Simulated microgravity activates a p38 MAPK-C/EBPβ pathway in macrophages to up-regulate arginase and IL-6 expression and down-regulate IL-12B expression. Both increased arginase expression and decreased IL-12B expression in macrophages during inflammation could result in immunosuppression under microgravity.

Influenza viral neuraminidase primes bacterial coinfection through TGF-β-mediated expression of host cell receptors

Influenza infection predisposes the host to secondary bacterial pneumonia, which is a major cause of mortality during influenza epidemics. The molecular mechanisms underlying the bacterial coinfection remain elusive. Neuraminidase (NA) of influenza A virus (IAV) enhances bacterial adherence and also activates TGF-β. Because TGF-β can up-regulate host adhesion molecules such as fibronectin and integrins for bacterial binding, we hypothesized that activated TGF-β during IAV infection contributes to secondary bacterial infection by up-regulating these host adhesion molecules. Flow cytometric analyses of a human lung epithelial cell line indicated that the expression of fibronectin and α5 integrin was up-regulated after IAV infection or treatment with recombinant NA and was reversed through the inhibition of TGF-β signaling. IAV-promoted adherence of group A Streptococcus (GAS) and other coinfective pathogens that require fibronectin for binding was prevented significantly by the inhibition of TGF-β. However, IAV did not promote the adherence of Lactococcus lactis unless this bacterium expressed the fibronectin-binding protein of GAS. Mouse experiments showed that IAV infection enhanced GAS colonization in the lungs of wild-type animals but not in the lungs of mice deficient in TGF-β signaling. Taken together, these results reveal a previously unrecognized mechanism: IAV NA enhances the expression of cellular adhesins through the activation of TGF-β, leading to increased bacterial loading in the lungs. Our results suggest that TGF-β and cellular adhesins may be potential pharmaceutical targets for the prevention of coinfection.

The calcineurin-NFAT axis controls allograft immunity in myeloid-derived suppressor cells through reprogramming T cell differentiation

While cyclosporine (CsA) inhibits calcineurin and is highly effective in prolonging rejection for transplantation patients, the immunological mechanisms remain unknown. Herein, the role of calcineurin signaling was investigated in a mouse allogeneic skin transplantation model. The calcineurin inhibitor CsA significantly ameliorated allograft rejection. In CsA-treated allograft recipient mice, CD11b(+) Gr1(+) myeloid-derived suppressor cells (MDSCs) were functional suppressive immune modulators that resulted in fewer gamma interferon (IFN-γ)-producing CD8(+) T cells and CD4(+) T cells (T(H)1 T helper cells) and more interleukin 4 (IL-4)-producing CD4(+) T cells (T(H2)) and prolonged allogeneic skin graft survival. Importantly, the expression of NFATc1 is significantly diminished in the CsA-induced MDSCs. Blocking NFAT (nuclear factor of activated T cells) with VIVIT phenocopied the CsA effects in MDSCs and increased the suppressive activities and recruitment of CD11b(+) Gr1(+) MDSCs in allograft recipient mice. Mechanistically, CsA treatment enhanced the expression of indoleamine 2,3-dioxygenase (IDO) and the suppressive activities of MDSCs in allograft recipients. Inhibition of IDO nearly completely recovered the increased MDSC suppressive activities and the effects on T cell differentiation. The results of this study indicate that MDSCs are an essential component in controlling allograft survival following CsA or VIVIT treatment, validating the calcineurin-NFAT-IDO signaling axis as a potential therapeutic target in transplantation.

Cellular metabolism and macrophage functional polarization

Macrophages are a functionally heterogeneous cell population that is mainly shaped by a variety of microenvironmental stimuli. Interferon γ (IFN-γ), interleukin-1β (IL-1β), and lipopolysaccharide (LPS) induce a classical activation of macrophages (M1), whereas IL-4 and IL-13 induce an alternative activation program in macrophages (M2). Reprogramming of intracellular metabolisms is required for the proper polarization and functions of activated macrophages. Similar to the Warburg effect observed in tumor cells, M1 macrophages increase glucose consumption and lactate release and decreased oxygen consumption rate. In comparison, M2 macrophages mainly employ oxidative glucose metabolism pathways. In addition, fatty acids, vitamins, and iron metabolisms are also related to macrophage polarization. However, detailed metabolic pathways involved in macrophages have remained elusive. Understanding the bidirectional interactions between cellular metabolism and macrophage functions in physiological and pathological situations and the regulatory pathways involved may offer novel therapies for macrophage-associated diseases.

TSC1 controls macrophage polarization to prevent inflammatory disease

Macrophages acquire distinct phenotypes during tissue stress and inflammatory responses, but the mechanisms that regulate the macrophage polarization are poorly defined. Here we show that tuberous sclerosis complex 1 (TSC1) is a critical regulator of M1 and M2 phenotypes of macrophages. Mice with myeloid-specific deletion of TSC1 exhibit enhanced M1 response and spontaneously develop M1-related inflammatory disorders. However, TSC1-deficient mice are highly resistant to M2-polarized allergic asthma. Inhibition of the mammalian target of rapamycin (mTOR) fails to reverse the hypersensitive M1 response of TSC1-deficient macrophages, but efficiently rescues the defective M2 polarization. Deletion of mTOR also fails to reverse the enhanced inflammatory response of TSC1-deficient macrophages. Molecular studies indicate that TSC1 inhibits M1 polarization by suppressing the Ras GTPase-Raf1-MEK-ERK pathway in mTOR-independent manner, whereas TSC1 promotes M2 properties by mTOR-dependent CCAAT/enhancer-binding protein-β pathways. Overall, these findings define a key role for TSC1 in orchestrating macrophage polarization via mTOR-dependent and independent pathways.

Preemptive donor apoptotic cell infusions induce IFN-γ-producing myeloid-derived suppressor cells for cardiac allograft protection

We have previously shown that preemptive infusion of apoptotic donor splenocytes treated with the chemical cross-linker ethylcarbodiimide (ECDI-SPs) induces long-term allograft survival in full MHC-mismatched models of allogeneic islet and cardiac transplantation. The role of myeloid-derived suppressor cells (MDSCs) in the graft protection provided by ECDI-SPs is unclear. In this study, we demonstrate that infusions of ECDI-SPs increase two populations of CD11b(+) cells in the spleen that phenotypically resemble monocytic-like (CD11b(+)Ly6C(high)) and granulocytic-like (CD11b(+)Gr1(high)) MDSCs. Both populations suppress T cell proliferation in vitro and traffic to the cardiac allografts in vivo to mediate their protection via inhibition of local CD8 T cell accumulation and potentially also via induction and homing of regulatory T cells. Importantly, repeated treatments with ECDI-SPs induce the CD11b(+)Gr1(high) cells to produce a high level of IFN-γ and to exhibit an enhanced responsiveness to IFN-γ by expressing higher levels of downstream effector molecules ido and nos2. Consequently, neutralization of IFN-γ completely abolishes the suppressive capacity of this population. We conclude that donor ECDI-SPs induce the expansion of two populations of MDSCs important for allograft protection mediated in part by intrinsic IFN-γ-dependent mechanisms. This form of preemptive donor apoptotic cell infusions has significant potential for the therapeutic manipulation of MDSCs for transplant tolerance induction.

Evaluation of the therapeutic potential of bone marrow-derived myeloid suppressor cell (MDSC) adoptive transfer in mouse models of autoimmunity and allograft rejection

Therapeutic use of immunoregulatory cells represents a promising approach for the treatment of uncontrolled immunity. During the last decade, myeloid-derived suppressor cells (MDSC) have emerged as novel key regulatory players in the context of tumor growth, inflammation, transplantation or autoimmunity. Recently, MDSC have been successfully generated in vitro from naive mouse bone marrow cells or healthy human PBMCs using minimal cytokine combinations. In this study, we aimed to evaluate the potential of adoptive transfer of such cells to control auto- and allo-immunity in the mouse. Culture of bone marrow cells with GM-CSF and IL-6 consistently yielded a majority of CD11b⁺Gr1^hi/lo cells exhibiting strong inhibition of CD8⁺ T cell proliferation in vitro. However, adoptive transfer of these cells failed to alter antigen-specific CD8⁺ T cell proliferation and cytotoxicity in vivo. Furthermore, MDSC could not prevent the development of autoimmunity in a stringent model of type 1 diabetes. Rather, loading the cells prior to injection with a pancreatic neo-antigen peptide accelerated the development of the disease. Contrastingly, in a model of skin transplantation, repeated injection of MDSC or single injection of LPS-activated MDSC resulted in a significant prolongation of allograft survival. The beneficial effect of MDSC infusions on skin graft survival was paradoxically not explained by a decrease of donor-specific T cell response but associated with a systemic over-activation of T cells and antigen presenting cells, prominently in the spleen. Taken together, our results indicate that in vitro generated MDSC bear therapeutic potential but will require additional in vitro factors or adjunct immunosuppressive treatments to achieve safe and more robust immunomodulation upon adoptive transfer.

mTOR limits the recruitment of CD11b+Gr1+Ly6Chigh myeloid-derived suppressor cells in protecting against murine immunological hepatic injury

The mTOR pathway integrates diverse environmental inputs, including immune signals and metabolic cues, to direct the innate and adaptive immune responses. MDSCs are a heterogeneous cell population that plays a crucial regulatory effect in immune-related diseases. However, whether mTOR signaling affects the functions of MDSCs remains largely unknown. Here, we show that mTOR signaling is a pivotal negative determinant of MDSC recruitment in IMH disease. In the context of IMH, inhibition of mTOR with rapamycin in CD11b⁺Gr1⁺ MDSCs mediates protection against IMH and serves as a functional, suppressive immune modulator that results in increased CD11b⁺Gr1⁺Ly6C(high) MDSC recruitment to inflammatory sites. In agreement with this, mTOR down-regulation promotes CD11b⁺Gr1⁺Ly6C(high) MDSC migration in vitro and in vivo. Mechanistically, mTOR activity down-regulation in MDSCs induced iNOS expression and NO production. Pharmacologic inhibition of iNOS completely eliminated MDSC recruitment. This study identifies MDSCs as an essential component for protection against IMH following rapamycin treatment. Rapamycin treatment or mTOR inhibition promotes CD11b⁺Gr1⁺Ly6C(high) MDSC recruitment and is critically required for protection against hepatic injury. This study further validates the targeting of mTOR signaling as a potential therapeutic approach to IMH-related diseases.

Innate immune cells in transplantation

PURPOSE OF REVIEW

To examine the recent literature on the role of innate cells in immunity to transplanted tissue. It specifically addresses the impact of monocytes/macrophages, neutrophils, natural killer cells, and platelets.

RECENT FINDINGS

Current research indicates that innate immunity plays a dual role in response to transplanted tissue with the ability to either facilitate rejection or promote tolerance. Intriguingly, some of these cells are even capable of reacting to allogeneic cells, a feature usually only attributed to cells of the adaptive immune system.

SUMMARY

This review highlights the new therapeutic targets in the innate immune system that may be useful in the treatment of transplant recipients. It also emphasizes the need to use caution in exploring these new therapeutics.

Targeting S1P1 receptor protects against murine immunological hepatic injury through myeloid-derived suppressor cells

Although FTY720 may alter migration and homing of lymphocytes via sphingosine-1-phosphate (S1P) receptors, our recent studies indicated that FTY720 directly controls the differentiation of Th1 cells to regulatory T cells (Tregs) by targeting S1P1. However, the pharmacological function of FTY720 in immunological hepatic injury remains unknown. In this study, the role and regulatory signaling pathway of S1P receptor were investigated using a pharmacological approach in immune-mediated hepatic injury (IMH). In the context of IMH, FTY720 significantly ameliorated mortality and hepatic pathology. In FTY720-treated mice, recruited CD11b(+)Gr1(+) myeloid-derived suppressor cells (MDSCs) mediate protection against IMH and are functional suppressive immune modulators that result in fewer IFN-γ-producing Th1 cells and more Foxp3(+) Tregs. In agreement, FTY720-treated MDSCs promote the reciprocal differentiation between Th1 cells and Tregs in vitro and in vivo. Mechanistically, FTY720 treatment induced inducible NO synthase expression and NO production in MDSCs. Pharmacologic inhibition of inducible NO synthase completely eliminates MDSC suppressive function and eradicates their inducible effects on T cell differentiation. Finally, the mTOR inhibitor, rapamycin, photocopies the effects of FTY720 on MDSCs, implicating mTOR as a downstream effector of S1P1 signaling. This study identifies MDSCs as an essential component that provides protection against IMH following FTY720 or rapamycin treatment, validating the S1P1-mTOR signaling axis as a potential therapeutic target in hepatic injury.

Phosphatase Wip1 negatively regulates neutrophil migration and inflammation

Neutrophils are critically involved in host defense and tissue damage. Intrinsic signal mechanisms controlling neutrophil activities are poorly defined. We found that the expression of wild-type p53-induced phosphatase 1 (Wip1) in mouse and human neutrophils was downregulated quickly after neutrophil activation through JNK-microRNA-16 pathway. Importantly, the Wip1 expression level was negatively correlated with inflammatory cytokine productions of neutrophils in sepsis patients. Wip1-deficient mice displayed increased bactericidal activities to Staphylococcus aureus and were hypersensitive to LPS-induced acute lung damage with increased neutrophil infiltration and inflammation. Mechanism studies showed that the enhanced inflammatory activity of neutrophils caused by Wip1 deficiency was mediated by p38 MAPK-STAT1 and NF-κB pathways. The increased migration ability of Wip1KO neutrophils was mediated by the decreased CXCR2 internalization and desensitization, which was directly regulated by p38 MAPK activity. Thus, our findings identify a previously unrecognized function of Wip1 as an intrinsic negative regulator for neutrophil proinflammatory cytokine production and migration through multiple signal pathways.

Kinase AKT1 negatively controls neutrophil recruitment and function in mice

Neutrophils are critically involved in host defense and inflammatory injury. However, intrinsic signaling mechanisms controlling neutrophil recruitment and activities are poorly defined. In this article, we showed that protein kinase AKT1 (also known as PKBα) is the dominant isoform expressed in neutrophils and is downregulated upon bacterial infection and neutrophil activation. AKT1 deficiency resulted in severe disease progression accompanied by recruitment of neutrophils and enhanced bactericidal activity in the acute inflammatory lung injury (ALI) and the Staphylococcus aureus infection mouse models. Moreover, the depletion of neutrophils efficiently reversed the aggravated inflammatory response, but adoptive transfer of AKT1(-/-) neutrophils could potentiate the inflammatory immunity, indicating an intrinsic effect of the neutrophil in modulating inflammation in AKT1(-/-) mice. In the ALI model, the infiltration of neutrophils into the inflammatory site was associated with enhanced migration capacity, whereas inflammatory stimuli could promote neutrophil apoptosis. In accordance with these findings, neutralization of CXCR2 attenuated neutrophil infiltration and delayed the occurrence of inflammation. Finally, the enhanced bactericidal activity and inflammatory immunity of AKT-deficient neutrophils were mediated by a STAT1-dependent, but not a mammalian target of rapamycin-dependent, pathway. Thus, our findings indicated that the AKT1-STAT1 signaling axis negatively regulates neutrophil recruitment and activation in ALI and S. aureus infection in mice.