Human fibrocytic myeloid‐derived suppressor cells express IDO and promote tolerance via Treg‐cell expansion

By restraining T‐cell activation and promoting Treg‐cell expansion, myeloid‐derived suppressor cells (MDSCs) and tolerogenic DCs can control self‐reactive and antigraft effector T cells in autoimmunity and transplantation. Their therapeutic use and characterization, however, is limited by their scarce availability in the peripheral blood of tumor‐free donors. In the present study, we describe and characterize a novel population of human myeloid suppressor cells, named fibrocytic MDSC, which are differentiated from umbilical cord blood precursors by 4‐day culture with FDA‐approved cytokines (recombinant human‐GM‐CSF and recombinant human‐G‐CSF). This MDSC subset, characterized by the expression of MDSC‐, DC‐, and fibrocyte‐associated markers, promotes Treg‐cell expansion and induces normoglycemia in a xenogeneic mouse model of Type 1 diabetes. In order to exert their protolerogenic function, fibrocytic MDSCs require direct contact with activated T cells, which leads to the production and secretion of IDO. This new myeloid subset may have an important role in the in vitro and in vivo production of Treg cells for the treatment of autoimmune diseases, and in either the prevention or control of allograft rejection.     

IFN‐γ regulates survival and function of tumor‐induced CD11b+Gr‐1high myeloid derived suppressor cells by modulating the anti‐apoptotic molecule Bcl2a1

Myeloid derived suppressor cells (MDSCs) play a critical role in suppression of immune responses in cancer and inflammation. Here, we describe how regulation of Bcl2a1 by cytokines controls the suppressor function of CD11b⁺Gr‐1^high granulocytic MDSCs. Coculture of CD11b⁺Gr‐1^high granulocytic MDSCs with antigen‐stimulated T cells and simultaneous blockade of IFN‐γ by the use of anti‐IFN‐γ blocking antibody, IFN‐γ^?/? effector T cells, IFN‐γR^?/? MDSCs or STAT1^?/? MDSCs led to upregulation of Bcl2a1 in CD11b⁺Gr‐1^high cells, improved survival, and enhanced their suppressor function. Molecular studies revealed that GM‐CSF released by antigen‐stimulated CD8⁺ T cells induced Bcl2a1 upregulation, which was repressed in the presence of IFN‐γ by a direct interaction of phosphorylated STAT‐1 with the Bcl2a1 promotor. Bcl2a1 overexpressing granulocytic MDSCs demonstrated prolonged survival and enhanced suppressor function in vitro. Our data suggest that IFN‐γ/ STAT1‐dependent regulation of Bcl2a1 regulates survival and thereby suppressor function of granulocytic MDSCs.     

Anti‐psoriatic effect of myeloid‐derived suppressor cells on imiquimod‐induced skin inflammation in mice

Myeloid‐derived suppressor cells (MDSCs) play an important role in controlling the immune response against cancer and in suppression of autoimmunity and allergic inflammation. However, the beneficial effects of MDSCs on the experimental mouse model of psoriasis have not been reported. Therefore, we investigated the anti‐psoriatic effect of MDSCs on IMQ‐induced skin inflammation in mice and explored the mechanisms involved. Our results showed that administration of MDSCs (1 × 10⁶ or 2 × 10⁶ cells) suppressed the development of IMQ‐induced skin inflammation in mice as exemplified by a significant reduction in clinical severity scores and was associated with a reduction of histopathological changes, including inflammatory infiltration, epidermal hyperplasia and hyperkeratosis. The immunosuppressive effect of MDSCs (1 × 10⁶ or 2 × 10⁶ cells) corresponded to the production of Th1 cytokines (TNF‐α, IFN‐γ) and Th17 cytokines (IL‐17A and IL‐23) in the serum and dorsal skin. Administration of MDSCs (1 × 10⁶ or 2 × 10⁶ cells) also inhibited splenomegaly. Moreover, an increased percentage of CD4⁺CD25⁺FoxP3⁺ regulatory T (Treg) cells and decreased percentage of Th1 and Th17 cells were found in mice treated with MDSCs. Taken together, these results imply that MDSCs have immunomodulatory and immunosuppressive effects on disease progression in a murine model of psoriasis and that MDSCs could be used in preventive or therapeutic strategies for the management of autoimmune inflammatory skin disorders, such as psoriasis.     

Exposure to inflammatory cytokines selectively limits GM‐CSF production by induced T regulatory cells

Interest in manipulating the immunosuppressive powers of Foxp3‐expressing T regulatory cells as an immunotherapy has been tempered by their reported ability to produce proinflammatory cytokines when manipulated in vitro, or in vivo. Understanding processes that can limit this potentially deleterious effect of Treg cells in a therapeutic setting is therefore important. Here, we have studied this using induced (i) Treg cells in which de novo Foxp3 expression is driven by TCR‐stimulation in vitro in the presence of TGF‐β. We show that iTreg cells can produce significant amounts of three proinflammatory cytokines (IFN‐γ, GM‐CSF and TNF‐α) upon secondary TCR stimulation. GM‐CSF is a critical T‐cell derived cytokine for the induction of EAE in mice. Despite their apparent capacity to produce GM‐CSF, myelin autoantigen‐responsive iTreg cells were unable to provoke EAE. Instead, they maintained strong suppressive function in vivo, preventing EAE induction by their CD4⁺Foxp3^? counterparts. We identified that although iTreg cells maintained the ability to produce IFN‐γ and TNF‐α in vivo, their ability to produce GM‐CSF was selectively degraded upon antigen stimulation under inflammatory conditions. Furthermore, we show that IL‐6 and IL‐27 individually, or IL‐2 and TGF‐β in combination, can mediate the selective loss of GM‐CSF production by iTreg cells.     

CD73 expression on extracellular vesicles derived from CD4+CD25+Foxp3+ T cells contributes to their regulatory function

CD4⁺CD25⁺Foxp3⁺ Treg cells maintain immunological tolerance. In this study, the possibility that Treg cells control immune responses via the production of secreted membrane vesicles, such as exosomes, was investigated. Exosomes are released by many cell types, including T cells, and have regulatory functions. Indeed, TCR activation of both freshly isolated Treg cells and an antigen‐specific Treg‐cell line resulted in the production of exosomes as defined morphologically by EM and by the presence of tetraspanin molecules LAMP‐1/CD63 and CD81. Expression of the ecto‐5‐nucleotide enzyme CD73 by Treg cells has been shown to contribute to their suppressive function by converting extracellular adenosine‐5‐monophosphate to adenosine, which, following interaction with adenosine receptors expressed on target cells, leads to immune modulation. CD73 was evident on Treg cell derived exosomes, accordingly when these exosomes were incubated in the presence of adenosine‐5‐monophosphate production of adenosine was observed. Most importantly, CD73 present on Treg cell derived exosomes was essential for their suppressive function hitherto exosomes derived from a CD73‐negative CD4⁺ T‐cell line did not have such capabilities. Overall our findings demonstrate that CD73‐expressing exosomes produced by Treg cells following activation contribute to their suppressive activity through the production of adenosine.     

Polysaccharide Agaricus blazei Murill stimulates myeloid derived suppressor cell differentiation from M2 to M1 type,which mediates inhibition of tumour immune‐evasion via the Toll‐like receptor 2 pathway

Gr‐1⁺ CD11b⁺ myeloid‐derived suppressor cells (MDSCs) accumulate in tumor‐bearing animals and play a critical negative role during tumor immunotherapy. Strategies for inhibition of MDSCs are expected to improve cancer immunotherapy. Polysaccharide Agaricus blazei Murill (pAbM) has been found to have anti‐cancer activity, but the underlying mechanism of this is poorly understood. Here, pAbM directly activated the purified MDSCs through inducing the expression of interleukin‐6 (IL‐6), IL‐12, tumour necrosis factor and inducible nitric oxide synthase (iNOS), CD86, MHC II, and pSTAT1 of it, and only affected natural killer and T cells in the presence of Gr‐1⁺ CD11b⁺ monocytic MDSCs. On further analysis, we demonstrated that pAbM could selectively block the Toll‐like receptor 2 (TLR2) signal of Gr‐1⁺ CD11b⁺ MDSCs and increased their M1‐type macrophage characteristics, such as producing IL‐12, lowering expression of Arginase 1 and increasing expression of iNOS. Extensive study showed that Gr‐1⁺ CD11b⁺ MDSCs by pAbM treatment had less ability to convert the CD4⁺ CD25^? cells into CD4⁺ CD25⁺ phenotype. Moreover, result from selective depletion of specific cell populations in xenograft mice model suggested that the anti‐tumour effect of pAbM was dependent on Gr‐1⁺ CD11b⁺ monocytes, nether CD8⁺ T cells nor CD4⁺ T cells. In addition to, pAbM did not inhibit tumour growth in TLR2^–/– mice. All together, these results suggested that pAbM, a natural product commonly used for cancer treatment, was a specific TLR2 agonist and had potent anti‐tumour effects through the opposite of the suppressive function of Gr‐1⁺ CD11b⁺ MDSCs.     

Tumor‐induced myeloid‐derived suppressor cell subsets exert either inhibitory or stimulatory effects on distinct CD8+ T‐cell activation events

Tumor growth coincides with an accumulation of myeloid‐derived suppressor cells (MDSCs), which exert immune suppression and which consist of two main subpopulations, known as monocytic (MO) CD11b⁺CD115⁺Ly6G^?Ly6C^high MDSCs and granulocytic CD11b⁺CD115^?Ly6G⁺Ly6C^int polymorphonuclear (PMN)‐MDSCs. However, whether these distinct MDSC subsets hamper all aspects of early CD8⁺ T‐cell activation — including cytokine production, surface marker expression, survival, and cytotoxicity — is currently unclear. Here, employing an in vitro coculture system, we demonstrate that splenic MDSC subsets suppress antigen‐driven CD8⁺ T‐cell proliferation, but differ in their dependency on IFN‐γ, STAT‐1, IRF‐1, and NO to do so. Moreover, MO‐MDSC and PMN‐MDSCs diminish IL‐2 levels, but only MO‐MDSCs affect IL‐2Rα (CD25) expression and STAT‐5 signaling. Unexpectedly, however, both MDSC populations stimulate IFN‐γ production by CD8⁺ T cells on a per cell basis, illustrating that some T‐cell activation characteristics are actually stimulated by MDSCs. Conversely, MO‐MDSCs counteract the activation‐induced change in CD44, CD62L, CD162, and granzyme B expression, while promoting CD69 and Fas upregulation. Together, these effects result in an altered CD8⁺ T‐cell adhesiveness to the extracellular matrix and selectins, sensitivity to FasL‐mediated apoptosis, and cytotoxicity. Hence, MDSCs intricately influence different CD8⁺ T‐cell activation events in vitro, whereby some parameters are suppressed while others are stimulated.     

Tumor‐induced CD11b+ Gr‐1+ myeloid‐derived suppressor cells exacerbate immune‐mediated hepatitis in mice in a CD40‐dependent manner

Immunosuppressive CD11b⁺Gr‐1⁺ myeloid‐derived suppressor cells (MDSCs) accumulate in the livers of tumor‐bearing (TB) mice. We studied hepatic MDSCs in two murine models of immune‐mediated hepatitis. Unexpectedly, treatment of TB mice with Concanavalin A (Con A) or α‐galactosylceramide resulted in increased alanine aminotransferase (ALT) and aspartate aminotransferase (AST) serum levels in comparison to tumor‐free mice. Adoptive transfer of hepatic MDSCs into naïve mice exacerbated Con A induced liver damage. Hepatic CD11b⁺Gr‐1⁺ cells revealed a polarized proinflammatory gene signature after Con A treatment. An IFN‐γ‐dependent upregulation of CD40 on hepatic CD11b⁺Gr‐1⁺ cells along with an upregulation of CD80, CD86, and CD1d after Con A treatment was observed. Con A treatment resulted in a loss of suppressor function by tumor‐induced CD11b⁺Gr‐1⁺ MDSCs as well as enhanced reactive oxygen species (ROS)‐mediated hepatotoxicity. CD40 knockdown in hepatic MDSCs led to increased arginase activity upon Con A treatment and lower ALT/AST serum levels. Finally, blockade of arginase activity in Cd40^?/? tumor‐induced myeloid cells resulted in exacerbation of hepatitis and increased ROS production in vivo. Our findings indicate that in a setting of acute hepatitis, tumor‐induced hepatic MDSCs act as proinflammatory immune effector cells capable of killing hepatocytes in a CD40‐dependent manner.     

Monocytic myeloid‐derived suppressor cells regulate T‐cell responses against vaccinia virus

Vaccinia virus (VV) can potently activate NK‐ and T‐cell responses, leading to efficient viral control and generation of long‐lasting protective immunity. However, immune responses against viral infections are often tightly controlled to avoid collateral damage and systemic inflammation. We have previously shown that granulocytic myeloid‐derived suppressor cells (g‐MDSCs) can suppress the NK‐cell response to VV infection. It remains unknown what regulates T‐cell responses to VV infection in vivo. In this study, we first showed that monocytic MDSCs (m‐MDSCs), but not g‐MDSCs, from VV‐infected mice could directly suppress CD4⁺ and CD8⁺ T‐cell activation in vitro. We then demonstrated that defective recruitment of m‐MDSCs to the site of VV infection in CCR2^?/? mice enhanced VV‐specific CD8⁺ T‐cell response and that adoptive transfer of m‐MDSCs into VV‐infected mice suppressed VV‐specific CD8⁺ T‐cell activation, leading to a delay in viral clearance. Mechanistically, we further showed that T‐cell suppression by m‐MDSCs is mediated by indication of iNOS and production of NO upon VV infection, and that IFN‐γ is required for activation of m‐MDSCs. Collectively, our results highlight a critical role for m‐MDSCs in regulating T‐cell responses against VV infection and may suggest potential strategies using m‐MDSCs to modulate T‐cell responses during viral infections.     

MDSC subtypes and CD39 expression on CD8+ T cells predict the efficacy of anti-PD-1 immunotherapy in patients with advanced NSCLC

The major suppressive immune cells in tumor sites are myeloid derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs), and Treg cells, and the major roles of these suppressive immune cells include hindering T-cell activities and supporting tumor progression and survival. In this study, we analyzed the pattern of circulating MDSC subtypes in patients with non-small cell lung cancer (NSCLC) whether those suppressive immune cells hinder T-cell activities leading to poor clinical outcomes. First, we verified PMN-MDSCs, monocytic-MDSCs (M-MDSCs), and Treg cells increased according to the stages of NSCLC, and MDSCs effectively suppressed T-cell activities and induced T-cell exhaustion. The analysis of NSCLC patients treated with anti-PD-1 immunotherapy demonstrated that low PMN-MDSCs, M-MDSCs, and CD39⁺CD8⁺ T cells as an individual and all together were associated with longer progression free survival and overall survival, suggesting PMN-MDSCs, M-MDSCs, and CD39⁺CD8⁺ T cells frequencies in peripheral blood might be useful as potential predictive and prognostic biomarkers.     

Curcumin induces the tolerogenic dendritic cell that promotes differentiation of intestine‐protective regulatory T cells

The gut is home to a large number of Treg, with both CD4⁺ CD25⁺ Treg and bacterial antigen‐specific Tr1 cells present in normal mouse intestinal lamina propria. It has been shown recently that intestinal mucosal DC are able to induce Foxp3⁺ Treg through production of TGF‐β plus retinoic acid (RA). However, the factors instructing DC toward this mucosal phenotype are currently unknown. Curcumin has been shown to possess a number of biologic activities including the inhibition of NF‐κB signaling. We asked whether curcumin could modulate DC to be tolerogenic whose function could mimic mucosal DC. We report here that curcumin modulated BM‐derived DC to express ALDH1a and IL‐10. These curcumin‐treated DC induced differentiation of naïve CD4⁺ T cells into Treg resembling Treg in the intestine, including both CD4⁺CD25⁺ Foxp3⁺ Treg and IL‐10‐producing Tr1 cells. Such Treg induction required IL‐10, TGF‐β and retinoic acid produced by curcumin‐modulated DC. Cell contact as well as IL‐10 and TGF‐β production were involved in the function of such induced Treg. More importantly, these Treg inhibited antigen‐specific T‐cell activation in vitro and inhibited colitis due to antigen‐specific pathogenic T cells in vivo.     

Human antigen‐specific CD4+CD25+CD134+CD39+ T cells are enriched for regulatory T cells and comprise a substantial proportion of recall responses

Human Ag‐specific CD4⁺ T cells can be detected by their dual expression of CD134 (OX40) and CD25 after a 44 hours stimulation with cognate Ag. We show that surface expression of CD39 on Ag‐specific cells consistently identifies a substantial population of CD4⁺CD25⁺CD134⁺CD39⁺ T cells that have a Treg‐cell‐like phenotype and mostly originate from bulk memory CD4⁺CD45RO⁺CD127^lowCD25^highCD39⁺ Treg cells. Viable, Ag‐specific CD25⁺CD134⁺CD39⁺ T cells could be expanded in vitro as cell lines and clones, and retained high Forkhead Box Protein 3, CTLA‐4 and CD39 expression, suppressive activity and Ag specificity. We also utilised this combination of cell surface markers to measure HIV‐Gag responses in HIV⁺ patients before and after anti‐retroviral therapy (ART). Interestingly, we found that the percentage of CD39^? cells within baseline CD4⁺ T‐cell responses to HIV‐Gag was negatively correlated with HIV viral load pre‐ART and positively correlated with CD4⁺ T‐cell recovery over 96 weeks of ART. Collectively, our data show that Ag‐specific CD4⁺CD25⁺CD134⁺CD39⁺ T cells are highly enriched for Treg cells, form a large component of recall responses and maintain a Treg‐cell‐like phenotype upon in vitro expansion. Identification and isolation of these cells enables the role of Treg cells in memory responses to be further defined and provides a development pathway for novel therapeutics.     

β‐Glucan enhances antitumor immune responses by regulating differentiation and function of monocytic myeloid‐derived suppressor cells

Myeloid‐derived suppressor cells (MDSCs) accumulate in tumor‐bearing hosts and play a major role in tumor‐induced immunosuppression, which hampers effective immuno‐therapeutic approaches. β‐Glucans have been reported to function as potent immuno‐modulators to stimulate innate and adaptive immune responses, which contributes to their antitumor property. Here, we investigated the effect of particulate β‐glucans on MDSCs and found that β‐glucan treatment could promote the differentiation of M‐MDSCs (monocytic MDSCs) into a more mature CD11c⁺ F4/80⁺ Ly6C^low population via dectin‐1 pathway in vitro, which is NF‐κB dependent, and the suppressive function of M‐MDSCs was significantly decreased. Treatment of orally administered yeast‐derived particulate β‐glucan drastically downregulated MDSCs but increased the infiltrated DCs and macrophages in tumor‐bearing mice, thus eliciting CTL and Th1 responses, inhibiting the suppressive activity of regulatory T cells, thereby leading to the delayed tumor progression. We show here for the first time that β‐glucans induce the differentiation of MDSCs and inhibit the regulatory function of MDSCs, therefore revealing a novel mechanism for β‐glucans in immunotherapy and suggesting their potential clinical benefit.     

Plasmodium yoelii infection of BALB/c mice results in expansion rather than induction of CD4+ Foxp3+ regulatory T cells

Recently, we demonstrated elevated numbers of CD4⁺ Foxp3⁺ regulatory T (Treg) cells in Plasmodium yoelii‐infected mice contributing to the regulation of anti‐malarial immune response. However, it remains unclear whether this increase in Treg cells is due to thymus‐derived Treg cell expansion or induction of Treg cells in the periphery. Here, we show that the frequency of Foxp3⁺ Treg cells expressing neuropilin‐1 (Nrp‐1) decreased at early time‐points during P. yoelii infection, whereas percentages of Helios⁺ Foxp3⁺ Treg cells remained unchanged. Both Foxp3⁺ Nrp‐1⁺ and Foxp3⁺ Nrp‐1^? Treg cells from P. yoelii‐infected mice exhibited a similar T‐cell receptor Vβ chain usage and methylation pattern in the Treg‐specific demethylation region within the foxp3 locus. Strikingly, we did not observe induction of Foxp3 expression in Foxp3^? T cells adoptively transferred to P. yoelii‐infected mice. Hence, our results suggest that P. yoelii infection triggered expansion of naturally occurring Treg cells rather than de novo induction of Foxp3⁺ Treg cells.     

Clinical Significance of Myeloid-Derived Suppressor Cells in Human Renal Transplantation with Acute T Cell-Mediated Rejection

Myeloid-derived suppressor cells (MDSCs) are negative regulators of the immune response and are in part responsible for the inhibition of the T cell-mediated immune response. A recent paper indicated that MDSCs were involved in prolonged allograft survival in animal models of transplantation, but the significance of MDSCs in human renal transplantation is still unknown. In our study, 50 patients with biopsy-proven acute T cell-mediated rejection (ATCMR) were included. The ratio of MDSCs in peripheral blood mononuclear cell (PBMC) was evaluated with FACS, and the patients were divided into the MDSCs high group (MDSCs, >10 %) or the MDSCs low group (MDSCs, <10 %). We compared the allograft function, severity of tissue injury, and long-time survival between the two groups. In the MDSCs high group, allograft function was significantly increased compared with the MDSCs low group. Furthermore, we found that isolated MDSCs from transplant recipients are capable of expanding regulatory T cell (Treg), meanwhile, inhibiting production of IL-17 in vitro. We also found that the ratio between Foxp3⁺ and IL-17-producing CD4⁺ T cells positively correlated with MDSCs frequency in PBMC. In conclusion, we demonstrated a potential role for MDSCs in prolonging allograft survival after ATCMR, and this was associated with higher CD4⁺Foxp3⁺/CD4⁺IL-17⁺ ratio in PBMC.     

CD8α+ dendritic cells prime TCR‐peptide‐reactive regulatory CD4+FOXP3− T cells

CD4⁺ T cells with immune regulatory function can be either FOXP3⁺ or FOXP3^?. We have previously shown that priming of naturally occurring TCR‐peptide‐reactive CD4⁺FOXP3^? Treg specifically controls Vβ8.2⁺CD4⁺ T cells mediating EAE. However, the mechanism by which these Treg are primed to recognize their cognate antigenic determinant, which is derived from the TCRVβ8.2‐chain, is not known. In this study we show that APC derived from splenocytes of naïve mice are able to stimulate cloned CD4⁺ Treg in the absence of exogenous antigen, and their stimulation capacity is augmented during EAE. Among the APC populations, DC were the most efficient in stimulating the Treg. Stimulation of CD4⁺ Treg was dependent upon processing and presentation of TCR peptides from ingested Vβ8.2TCR⁺CD4⁺ T cells. Additionally, DC pulsed with TCR peptide or apoptotic Vβ8.2⁺ T cells were able to prime Treg in vivo and mediate protection from disease in a CD8‐dependent fashion. These data highlight a novel mechanism for the priming of CD4⁺ Treg by CD8α⁺ DC and suggest a pathway that can be exploited to prime antigen‐specific regulation of T‐cell‐mediated inflammatory disease.