Role of myeloid-derived suppressor cells in mouse pre-sensitized cardiac transplant model

Metabolic reprogramming of myeloid-derived suppressor cells in the context of organ transplantation

Myeloid-derived suppressor cells (MDSCs) are naturally occurring leukocytes that develop from immature myeloid cells under inflammatory conditions that were discovered initially in the context of tumor immunity. Because of their robust immune inhibitory activities, there has been growing interest in MDSC-based cellular therapies for transplant tolerance induction. Indeed, various pre-clinical studies have introduced in vivo expansion or adoptive transfer of MDSC as a promising therapeutic strategy leading to a profound extension of allograft survival due to suppression of alloreactive T cells. However, several limitations of cellular therapies using MDSCs remain to be addressed, including their heterogeneous nature and limited expansion capacity. Metabolic reprogramming plays a crucial role for differentiation, proliferation and effector function of immune cells. Notably, recent reports have focused on a distinct metabolic phenotype underlying the differentiation of MDSCs in an inflammatory microenvironment representing a regulatory target. A better understanding of the metabolic reprogramming of MDSCs may thus provide novel insights for MDSC-based treatment approaches in transplantation. In this review, we will summarize recent, interdisciplinary findings on MDSCs metabolic reprogramming, dissect the underlying molecular mechanisms and discuss the relevance for potential treatment approaches in solid-organ transplantation.

Aging Affects the Role of Myeloid-Derived Suppressor Cells in Alloimmunity

Myeloid-derived suppressor cells (MDSC) are defined as a group of myeloid cells with potent immunoregulatory functions that have been shown to be involved in a variety of immune-related diseases including infections, autoimmune disorders, and cancer. In organ transplantation, MDSC promote tolerance by modifying adaptive immune responses. With aging, however, substantial changes occur that affect immune functions and impact alloimmunity. Since the vast majority of transplant patients are elderly, age-specific modifications of MDSC are of relevance. Furthermore, understanding age-associated changes in MDSC may lead to improved therapeutic strategies. Here, we provide a comprehensive update on the effects of aging on MDSC and discuss potential consequences on alloimmunity.

Rapamycin delays allograft rejection in obese graft recipients through induction of myeloid-derived suppressor cells

Obesity has become a relevant problem in transplantation medicine with steadily increasing numbers of obese graft recipients. However, the effect of immunomodulatory drugs on transplant-related outcomes among obese patients are unknown. Therefore, we evaluated the impact of rapamycin on allograft rejection and alloimmune response in a murine model of diet-induced obesity and fully-mismatched skin transplantation. Rapamycin significantly delayed allograft rejection in obese recipient mice compared to treated lean mice (14.5 days vs. 10.7 days, p = 0.005). Treatment with rapamycin increased frequencies of monocytic myeloid-derived suppressor cells (M-MDSCs), augmented the immunosuppressive activity of M-MDSCs on T cells through indoleamine 2,3-dioxygenase pathway and shifted CD4⁺T cells towards regulatory T cells in obese graft recipients. In summary, our results demonstrate that rapamycin delays allograft rejection in obese graft recipients by enhancing suppressive immune cell function and shifting immune cell subsets towards anti-inflammatory conditions.

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.

Impact of Regulatory T Cells on Innate Immune Cells in a Pre-Sensitized Heart Transplant Model

Background

Although our previous studies revealed the role of Tregs (regulatory T cells) and MDSCs (myeloid-derived suppressor cells) in a pre-sensitized cardiac transplant model, interplay between Tregs and NK cells, neutrophils, and macrophages remain undefined.

Material/Methods

Mice heart transplantation with skin pre-sensitization was performed, in which prolonged-cold ischemia time (PCI) was used for donor treatment. Syngeneic heterotopic heart transplant recipients with PCI were treated with PC61 (monoclonal anti-CD25 antibodies), adoptive cell transfer with Tregs, and rapamycin.

Results

We unveiled that both rapamycin treatment and adoptive transfer of Tregs could lead to a remarkable decrease of frequency of splenic Gr1+ cells (P=0.058 and P=0.016, respectively). Although administration of PC61 did not affect frequency of splenic Gr1+ cells, it dramatically increased frequency of splenic F4/80+ macrophages (P=0.052). Intriguingly, use of both exogenous PC61 and rapamycin induced a dramatic augmentation of frequency of Gr-1+ neutrophils in the grafts (PC61: P=0.00029; rapamycin: P=0.0096). Noticeably, all different regimens including PC61, rapamycin, and adoptive transfer of Tregs, consistently resulted in a remarked augmentation of frequency of F4/80+ macrophages within grafts (PC61, P=0.0013; rapamycin, P=0.015; Tregs transfer, P=0.013). Although rapamycin and adoptive transfer of Tregs did not affect frequency of NK1.1+ cells, administration of PC61 dramatically increased frequency of NK1.1+ cells within grafts (P=0.033).

Conclusions

Tregs depletion or Tregs induced by rapamycin or exogenous cell transfer could affect frequencies of both splenic and intragraft neutrophils, macrophages, and NK cells, but not splenic NK cells. Our data might shed light on understanding sensitized transplant biology.

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.

The roles of sepsis-induced myeloid derived suppressor cells in mice corneal, skin and combined transplantation

PURPOSE

To explore the effects of adoptive transferring sepsis induced myeloid-derived suppressor cells (iMDSCs) in mice corneal, skin, and combined corneal-skin survival.

METHODS

Allogeneic full-thickness corneal transplantation, fully mismatched skin transplantation, and corneal-skin combined transplantation (donor C57BL/6 to recipient Balb/c mice) were performed. Sepsis-induced infectious-MDSCs (iMDSCs), were purified from bone marrow of cecal ligated and punctured (CLP) Balb/c mice. Recipient-derived iMDSCs were adoptively transferred into different recipient groups by retro-orbital injection after surgeries. Corneal and skin grafts were examined and photographed routinely for a period of 45days. Histopathology was performed to evaluate corneal-graft inflammation. Bone marrow and/or corneal grafts in each group were harvested from executed recipients on postoperative days 15, 25, 35. Corneal cells and bone marrow cells were stained with CD11b-PE and Gr1-FITC, analyzed by FACS.

RESULTS

iMDSCs were able to significantly prolong allograft survival in both corneal and corneal-skin combined transplant groups. A substantial expansion of MDSCs was observed in recipients' bone marrow, particularly in combined groups at an early stage postoperatively, and accordingly the concentration of MDSCs in corneal grafts increased significantly in adoptive transferred groups.

CONCLUSIONS

Sepsis-induced MDSCs may suggest a novel cellular therapeutic approach for preventing various types of allograft rejection.

The Role and Potential Therapeutic Application of Myeloid-Derived Suppressor Cells in Allo- and Autoimmunity

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells that consists of myeloid progenitor cells and immature myeloid cells. They have been identified as a cell population that may affect the activation of CD4(+) and CD8(+) T-cells to regulate the immune response negatively, which makes them attractive targets for the treatment of transplantation and autoimmune diseases. Several studies have suggested the potential suppressive effect of MDSCs on allo- and autoimmune responses. Conversely, MDSCs have also been found at various stages of differentiation, accumulating during pathological situations, not only during tumor development but also in a variety of inflammatory immune responses, bone marrow transplantation, and some autoimmune diseases. These findings appear to be contradictory. In this review, we summarize the roles of MDSCs in different transplantation and autoimmune diseases models as well as the potential to target these cells for therapeutic benefit.

Tolerance induced by IL-6 deficient donor heart is significantly involved in myeloid-derived suppressor cells (MDSCs)

OBJECTIVES

Transplant tolerance induced by IL-6 deficient donor is supported by regulatory T cells (Tregs). However, it is unknown whether innate immunoregulatory cells such as myeloid-derived suppressor cells (MDSCs) are involved in the process.

MATERIALS AND METHODS

In this study, we demonstrate the role of MDSCs by transplanting IL-6 deficient heart grafts into wild-type recipients in a murine allogeneic transplant model.

RESULTS

Our data further revealed that utilization of IL-6 deficient heart grafts could cause a significant prolongation of allograft survival (Mantel-Cox Test, p = 0.001; Gehan-Breslow-Wilcoxon Test, p = 0.0016) and a remarkable increase of the frequency of CD11b + Gr1(-low) in the recipients' spleens (p = 0.0028).

CONCLUSIONS

MDSCs rather than Th17 cells are closely involved in induced tolerance by IL-6 deficient donor heart. This unveiled mechanism of targeting IL-6 or its signaling pathway may provide a novel insight into preventing allograft rejection for non-sensitized transplant recipients.