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

Kidney transplantation is the most mature and common type of solid organ transplantation worldwide. The first report of MDSCs in an experimental kidney transplantation animal model was in 2008. A rat model of anti-CD28-induced kidney allograft tolerance showed an accumulation of plastic-adherent CD11b⁺ myeloid cells expressing CD80/86 that were defined as MDSCs. This study indicated that MDSCs had nonspecific immunosuppressive activity in vivo and in vitro involving the action of inducible nitric oxide synthase (iNOS), which was upregulated after contact with activated effector T cells but not with regulatory T cells (Tregs) [28]. Dilek et al. analyzed gene expression in blood-derived MDSCs from rat recipients of kidney allografts using DNA microarray [43]. They found that CCL5 (Rantes), a chemokine for Tregs, was strongly downregulated after treatment with a tolerizing regimen. The results indicated the contribution of MDSCs to the establishment of a graft-to-periphery CCL5 gradient in tolerant kidney allograft recipients, which controlled the recruitment of Tregs to the graft where they likely contributed to maintaining tolerance [43].

In the clinic, Hock et al. reported renal transplant recipients had elevated frequencies of circulating CD11b⁺CD33⁺HLA-DR⁻ MDSCs (both M-MDSCs and G-MDSCs) [44]. They further traced the change in MDSCs for 1 year post-transplantation. These observational studies showed that MDSC numbers increased rapidly and peaked following commencement of immunosuppression [45]. Luan et al. also found an increase in CD11b⁺CD33⁺HLA-DR⁻ MDSCs in renal transplant recipients [46]. It is well known that Tregs play a key role in immune tolerance induction and maintenance [47]. The authors found that MDSCs isolated from kidney transplant recipients were highly efficient in suppressing the proliferation of CD4⁺ T cells in mixed leukocyte reactions. In addition, CD11b⁺CD33⁺HLA-DR⁻ MDSCs were capable of expanding Tregs in vitro, and their accumulation over time after transplantation was linearly correlated with an increase in Tregs in vivo. This was the first study to link the presence of MDSCs with the emergence of Tregs in vivo in transplant recipients, and to define the subpopulation of MDSCs derived from transplant recipients responsible for generation of Tregs [46]. The results of Meng et al. also supplement the previous findings of MDSCs in kidney transplantation [48]. In this study, 50 renal transplant recipients with acute T cell-mediated rejection were enrolled. The patients were divided into two groups according to the ratio of MDSCs in the peripheral blood mononuclear cell population tested by flow cytometry. As expected, the 1- and 5-year graft survival rates in the high-MDSC group were 93 and 79%, respectively, but were only 68 and 36%, respectively, in the low-MDSC group. As expected, interferon (IFN)-γ- and tumor necrosis factor (TNF)-α-related renal tissue injury was significantly alleviated in the high-MDSC group compared with the low-MDSC group. Furthermore, IL-17, a strong pro-inflammatory cytokine, was inhibited by Tregs expanded by MDSCs [48]. These results indicated that the number of MDSCs in the renal allograft recipients was associated with long-term graft survival and that MDSCs may regulate the imbalance between Tregs and Th17 cells.

In a prolonged murine cold ischemia time-mediated donor cardiac injury model [49], Gong et al. detected three MDSC subsets, CD11b⁺Gr-1^low, CD11b⁺Gr-1^int and CD11b⁺Gr-1^high. It should be noted that the definition of MDSC subsets was based on Gr-1, without Ly6C. The findings revealed that CD11b⁺Gr-1^low MDSCs had strong suppressive activity. MDSC subsets from the tolerant mice exhibited higher suppressive capacities compared with subsets from naive (untreated) mice. Depletion of Tregs increased peripheral and intragraft CD11b⁺Gr-1^low MDSC frequency. Intriguingly, boosting Tregs caused a remarkable increase in CD11b⁺Gr-1^low MDSC frequency in the graft, peripheral blood and spleen. This study indicated for the first time the possible interaction between MDSCs and Tregs in cardiac transplantation [49]. Furthermore, Nakamura et al. demonstrated that MDSCs induced by PD-L1 were capable of recruiting Foxp3⁺ Tregs [50].

In addition to focusing on the mechanism of MDSC induction, some studies have also examined the possibility of increasing the induction of MDSCs, which may indicate a possible direction for immune therapy in transplantation. Mammalian target of rapamycin (mTOR) inhibitors are the main immunosuppressive drugs for organ transplant recipients. Nevertheless, the mechanisms by which mTOR inhibitors induce immunosuppression are not fully understood. Interestingly, in a murine cardiac transplantation model, rapamycin treatment led to the recruitment of MDSCs and increased their expression of iNOS [51]. Adoptive transcoronary arterial transfer of MDSCs from rapamycin-treated recipients prolonged allograft survival. Co-administration of rapamycin and the mitogen-activated protein kinase (MEK) inhibitor trametinib reversed rapamycin-mediated MDSC recruitment. Thus, the author concluded that the mTOR and Raf/MEK/extracellular signal regulated kinase (ERK) signaling pathways appeared to play an important role in MDSC expansion [51]. IL-33, which is thought to be able to facilitate Th2 responses, significantly increased CD11b⁺Gr1^int M-MDSCs in a chronic cardiac rejection model, reduced antibody-mediated rejection and ultimately prolonged allograft survival [52, 53]. These findings support IL-33-based MDSC therapy in cardiac transplantation. The induction of other molecules, such as IL-6 [54, 55] and IFN-γ [56], has also been tightly associated with MDSCs in cardiac transplant protection.

Skin transplantation is a convenient animal model for the study of rejection and tolerance. In 2008, a study conducted by Zhang et al. demonstrated the expansion of MDSCs by immunoglobulin-like transcript 2 receptor and its ligands [57]. Moreover, the immunosuppressive function of MDSCs was enhanced by this specific kind of receptor, which led to a better survival rate of alloskin grafts after transplantation. Other studies have illustrated that MDSCs induced by IL-2C or neupogen [58], IL-33 [59], Δ9-tetrahydrocannabinol (via the activation of cannabinoid receptor 1) [60] and TNF-α (via an iNOS-dependent mechanism) [61] were able to suppress T cell proliferation, promote Tregs and induce immune tolerance. Considering the close relationship between MDSCs and Tregs, Adeegbe et al. combined induced-MDSCs, which were shown to be superior to naïve MDSCs, with induced-Tregs to promote transplantation tolerance. The results showed that co-administration of these two regulatory cells had a much better effect on graft survival than administration of either one alone [58], which highlights the combination of MDSCs and Tregs as a potential cell therapy for tolerance induction. In another study, the adoptive transfer of MDSCs prolonged skin graft survival but failed to alter antigen-specific CD8⁺ T cell proliferation and cytotoxicity. The authors attributed this result to the over-activation of donor-specific T cells in the spleen [62]. Furthermore, in order to assess the effect of MDSCs in immunosuppressive treatments, Carretero-Iglesia et al. directly compared the function of three regulatory myeloid cell types in a skin transplant model, including autologous tolerogenic dendritic cells, suppressor macrophages (suppMφs) and MDSCs [63]. They found that these three types of cells perform their roles in T cell inhibition in three different ways. autologous tolerogenic dendritic cells mainly regulate the activation, proliferation and reactivation process of T cells, while suppMφs are thought to be responsible for inducing and expanding Treg cells. However, MDSCs appear to exert their immunosuppressive function through Treg expansion as well as induction of T cell death.

Even though the acceptance rates of penetrating keratoplasty are relatively high [72, 73], either inflammation or neovascularization can induce corneal graft rejection and failure [74]. When retroorbitally injected into the recipients immediately after keratoplasty surgery, both iMDSCs and tMDSCs had a suppressive effect on CD4⁺ T cell proliferation, and they improved the histological condition and decreased neovascularization of corneal grafts. However, a supplementary injection of iMDSCs did not cause graft improvements at later stages [75]. This result was partly consistent with a study in which CD11b⁺ cells were induced by lipopolysaccharide (LPS) [76].

Type 1 diabetes accompanied by diabetic complications usually occurs when patients lose insulin-producing pancreatic β cells. The routine therapy for this disease is the administration of exogenous insulin. However, insulin may not fulfill the function of normal β cells. Pancreas transplantation has been considered to address this situation, but deleterious side effects are unavoidable [77‐79]. In recent years, islet transplantation in optimal situations has appeared as an alternative to pancreas transplantation [80, 81]. As a type of cell transplant, islet grafts have promising prospects, and the induction of tolerance to remove the dependency on immunosuppressive drugs is challenging and necessary. Considering the significance of MDSCs in transplantation, Arakawa et al. co-transplanted MDSCs (generated from bone marrow cells cultured with hepatic stellate cells and GM-CSF with islet grafts in diabetic recipient mice [82]. The results showed that the co-transplantation of these two types of cells significantly promoted allograft survival. As expected, MDSCs exerted their inhibitory function on T cells in an iNOS-dependent manner. MDSCs from inos-deficient mice failed to protect islet allografts [82], which indicated the key role of iNOS in MDSC-induced tolerance in islet transplantation. The conclusion agreed with a previous study in which MDSCs expressing a high level of arginase-1 enhanced antigen-specific Tregs in the B7H1 pathway [83].