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_H1 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.     

Myeloid-Derived Suppressor Cells Regulate Natural Killer Cell Response to Adenovirus-Mediated Gene Transfer

The attendant innate and adaptive immune responses to viral vectors have posed a significant hurdle for clinical application of viral vector-mediated gene therapy. Previous studies have shown that natural killer (NK) cells play a critical role in innate immune elimination of adenoviral vectors in the liver. However, it is not clear how the NK cell response to adenoviral vectors is regulated. In this study, we identified a role for granulocytic myeloid-derived suppressor cells (G-MDSCs) in this process. We show that in vivo administration of adenoviral vectors results in rapid accumulation of G-MDSCs early during adenoviral infection. In vivo depletion of both MDSC populations, but not monocytic MDSCs (M-MDSCs) alone, resulted in accelerated clearance of adenoviral vectors in the liver. This was accompanied by enhanced NK cell proliferation and activation, suggesting a role for MDSCs, probably G-MDSCs, in suppressing NK cell activation and function in vivo. We further demonstrate in vitro that G-MDSCs, but not M-MDSCs, are responsible for the suppression of NK cell activation. In addition, we show that adenoviral infection activated G-MDSCs to produce higher levels of reactive oxygen species (ROS) and that G-MDSC-mediated suppression of NK cells is mediated by ROS, specifically, H₂O₂. This study demonstrates for the first time that the NK cell response to adenoviral vectors is negatively regulated by G-MDSCs and suggests that G-MDSC-based strategies could potentially improve the outcome of viral vector-mediated gene therapy.     

Characterization of Liver Monocytic Myeloid-Derived Suppressor Cells and Their Role in a Murine Model of Non-Alcoholic Fatty Liver Disease

Myeloid-derived suppressor cells (MDSCs) are potent suppressors of T cell immunity in tumors and inflammatory diseases. They are identified by surface expression of CD11b⁺Gr1⁺ in mice, and CD11b⁺Gr1⁺ cells accumulate in the livers of obese mice. However, many myeloid cells share these CD11b⁺Gr1⁺ markers. Accordingly, the aim of this study was to identify the authentic phenotype of MDSCs and investigate their functions in non-alcoholic fatty liver disease (NAFLD). C57BL/6J mice were divided into 2 diet groups: a normal control group and high-fat group to induce NAFLD. We demonstrated that monocytic CD11b⁺Gr1^dim cells could be further divided into 2 populations based on side scatter (SSC) during flow cytometry. We found that SSC^lowCD11b⁺Gr1^dim cells accumulated in the livers of NAFLD mice over time, and that these cells were recruited by the chemokine CCL2 and its receptor CCR2 and might expand in the liver via macrophage colony-stimulating factor stimulation. Furthermore, SSC^lowCD11b⁺Gr1^dim cells had a strong suppressive ability on T cells; this effect was not observed for SSC^highCD11b⁺Gr1^dim cells, and was dependent on nitric oxide production by inducible nitric oxide synthase. Our findings demonstrate that SSC^lowCD11b⁺Gr1^dim cells represent authentic MDSCs in NAFLD livers, and might serve an important negative feedback function in liver inflammation.     

Polysaccharide from Lentinus edodes Inhibits the Immunosuppressive Function of Myeloid-Derived Suppressor Cells

Reversing the function of immune suppressor cells may improve the efficacy of cancer therapy. Here, we have isolated a novel polysaccharide MPSSS (577.2 Kd) from Lentinus edodes and examined its effects on differentiation and function of myeloid-derived suppressor cells (MDSCs). MPSSS is composed of glucose (75.0%), galactose (11.7%), mannose (7.8%), and xylose (0.4%). In vivo, it inhibits the growth of McgR32 tumor cells, which is correlated with a reduced percentage of MDSCs in peripheral blood. In vitro, it induces both morphological and biophysical changes in MDSCs. Importantly, MPSSS up-regulates MHC II and F4/80 expression on MDSCs, and reverses their inhibition effect on CD4⁺ T cells in a dose-dependent manner. The mechanism study shows that MPSSS may stimulate MDSCs through a MyD88 dependent NF-κB signaling pathway. Together, we demonstrated for the first time that MPSSS stimulates the differentiation of MDSCs and reverses its immunosuppressive functions, shedding new light on developing novel anti-cancer strategies by targeting MDSCs.     

Lack of Significant Elevation of Myeloid-Derived Suppressor Cells in Peripheral Blood of Chronically Hepatitis C Virus-Infected Individuals

Myeloid-derived suppressor cells (MDSC) are immature myeloid cells with immunosuppressive function. Compared to the level in healthy controls (HC), no elevation of MDSC in chronic hepatitis C (cHEP-C) patients was found, and there was no difference in MDSC based on genotype or viral load (P > 0.25). Moreover, MDSC of cHEP-C patients inhibited CD8 T cell function as efficiently as MDSC of HC did. Since we detected neither quantitative nor qualitative differences in MDSC of cHEP-C patients relative to those of HC, we postulate that MDSC in peripheral blood are most likely not significant regarding immune dysfunction in cHEP-C.     

Effect of Maraviroc Intensification on HIV-1-Specific T Cell Immunity in Recently HIV-1-Infected Individuals

Background

The effect of maraviroc on the maintenance and the function of HIV-1-specific T cell responses remains unknown.

Methods

Subjects recently infected with HIV-1 were randomized to receive anti-retroviral treatment with or without maraviroc intensification for 48 weeks, and were monitored up to week 60. PBMC and in vitro-expanded T cells were tested for responses to the entire HIV proteome by ELISpot analyses. Intracellular cytokine staining assays were conducted to monitor the (poly)-functionality of HIV-1-specific T cells. Analyses were performed at baseline and week 24 after treatment start, and at week 60 (3 months after maraviroc discontinuation).

Results

Maraviroc intensification was associated with a slower decay of virus-specific T cell responses over time compared to the non-intensified regimen in both direct ex-vivo as well as in in-vitro expanded cells. The effector function profiles of virus-specific CD8⁺ T cells were indistinguishable between the two arms and did not change over time between the groups.

Conclusions

Maraviroc did not negatively impact any of the measured parameters, but was rather associated with a prolonged maintenance of HIV-1-specific T cell responses. Maraviroc, in addition to its original effect as viral entry inhibitor, may provide an additional benefit on the maintenance of virus-specific T cells which may be especially important for future viral eradication strategies.     

Gr1intCD11b+ Myeloid-Derived Suppressor Cells in Mycobacterium tuberculosis Infection

Background

Tuberculosis is one of the world’s leading killers, stealing 1.4 million lives and causing 8.7 million new and relapsed infections in 2011. The only vaccine against tuberculosis is BCG which demonstrates variable efficacy in adults worldwide. Human infection with Mycobacterium tuberculosis results in the influx of inflammatory cells to the lung in an attempt to wall off bacilli by forming a granuloma. Gr1^intCD11b⁺ cells are called myeloid-derived suppressor cells (MDSC) and play a major role in regulation of inflammation in many pathological conditions. Although MDSC have been described primarily in cancer their function in tuberculosis remains unknown. During M. tuberculosis infection it is crucial to understand the function of cells involved in the regulation of inflammation during granuloma formation. Understanding their relative impact on the bacilli and other cellular phenotypes is necessary for future vaccine and drug design.

Methodology/Principal Findings

We compared the bacterial burden, lung pathology and Gr1^intCD11b⁺ myeloid-derived suppressor cell immune responses in M. tuberculosis infected NOS2-/-, RAG-/-, C3HeB/FeJ and C57/BL6 mice. Gr-1⁺ cells could be found on the edges of necrotic lung lesions in NOS2-/-, RAG-/-, and C3HeB/FeJ, but were absent in wild-type mice. Both populations of Gr1⁺CD11b⁺ cells expressed high levels of arginase-1, and IL-17, additional markers of myeloid derived suppressor cells. We then sorted the Gr1^hi and Gr1^int populations from M. tuberculosis infected NOS-/- mice and placed the sorted both Gr1^int populations at different ratios with naïve or M. tuberculosis infected splenocytes and evaluated their ability to induce activation and proliferation of CD4+T cells. Our results showed that both Gr1^hi and Gr1^int cells were able to induce activation and proliferation of CD4+ T cells. However this response was reduced as the ratio of CD4⁺ T to Gr1⁺ cells increased. Our results illustrate a yet unrecognized interplay between Gr1⁺ cells and CD4⁺ T cells in tuberculosis.     

Mannan-Binding Lectin Deficiency Limits Inflammation-induced Myeloid-Derived Suppressor Cells Expansion via Modulating Tumor Necrosis Factor Alpha-triggered Apoptosis

Background: Mannan-binding lectin (MBL), a soluble pattern recognition molecule in the innate immune system, is reported to be associated with the function of immune cells. Myeloid-derived suppressor cells (MDSCs) are mainly characterized by immunosuppressive activities involving several inflammatory diseases such as cancer, infection, and arthritis. Some of the factors inducing their apoptosis are known, however, mechanisms have not been identified. The underlying impact of MBL on the MDSCs especially under inflammatory conditions remains unknown. This study was designed to investigate whether MBL affects MDSCs survival during inflammation conditions.Methods: WT and MBL-deficient (MBL^-/-) mice were induced on day 0 of the experiment by subcutaneous injection of complete Freund''s adjuvant and then injected with incomplete Freund''s adjuvant into the knee joint space under general anesthesia on day 14 to induce inflammatory arthritis. The proportions of MDSCs in the spleen and blood and the serum level of the inflammatory cytokines were measured. In vitro study, MDSCs were isolated from the bone marrow of WT and MBL^-/- mice and cultured in the presence of interleukin-6 (IL-6) and granulocyte-macrophage colony-stimulating factor (GM-CSF) for 5 days with or without tumor necrosis factor-alpha (TNF-α).Results: After adjuvant treatment, MBL^-/- mice had a significantly lower frequency of MDSCs as well as elevated serum inflammatory cytokines levels compared to WT mice. MBL deficiency markedly inhibited the MDSCs frequency from mice bone marrow induced by IL-6 and GM-CSF in the presence of TNF-α in vitro. Mechanistic studies established that MBL inhibited MDSCs apoptosis via down-regulation of TNF-α/tumor necrosis factor-alpha receptor 1 (TNFR1) signaling pathway and subsequent caspase 3-dependent manner.Conclusion: Mannan-binding lectin deficiency inhibits myeloid-derived suppressor cells expansion via modulating TNF-α triggered apoptosis.     

New Tools to Expand Regulatory T Cells from HIV-1-infected Individuals

CD4+ Regulatory T cells (Tregs) are potent immune modulators and serve an important function in human immune homeostasis. Depletion of Tregs has led to measurable increases in antigen-specific T cell responses in vaccine settings for cancer and infectious pathogens. However, their role in HIV-1 immuno-pathogenesis remains controversial, as they could either serve to suppress deleterious HIV-1-associated immune activation and thus slow HIV-1 disease progression or alternatively suppress HIV-1-specific immunity and thereby promote virus spread. Understanding and modulating Treg function in the context of HIV-1 could lead to potential new strategies for immunotherapy or HIV vaccines. However, important open questions remain on their role in the context of HIV-1 infection, which needs to be carefully studied.Representing roughly 5% of human CD4+ T cells in the peripheral blood, studying the Treg population has proven to be difficult, especially in HIV-1 infected individuals where HIV-1-associated CD4 T cell and with that Treg depletion occurs. The characterization of regulatory T cells in individuals with advanced HIV-1 disease or tissue samples, for which only very small biological samples can be obtained, is therefore extremely challenging. We propose a technical solution to overcome these limitations using isolation and expansion of Tregs from HIV-1-positive individuals.Here we describe an easy and robust method to successfully expand Tregs isolated from HIV-1-infected individuals in vitro. Flow-sorted CD3⁺CD4⁺CD25⁺CD127^low Tregs were stimulated with anti-CD3/anti-CD28 coated beads and cultured in the presence of IL-2. The expanded Tregs expressed high levels of FOXP3, CTLA4 and HELIOS compared to conventional T cells and were shown to be highly suppressive. Easier access to large numbers of Tregs will allow researchers to address important questions concerning their role in HIV-1 immunopathogenesis. We believe answering these questions may provide useful insight for the development of an effective HIV-1 vaccine.     

Circulating and Tumor-Infiltrating Myeloid-Derived Suppressor Cells in Patients with Colorectal Carcinoma

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous family of myeloid cells that suppress T cell immunity in tumor-bearing hosts. In patients with colon cancer, MDSCs have recently been described as Lin^−/lowHLA-DR⁻CD11b⁺CD33⁺ cells correlating with cancer stage, metastasis and chemotherapy response. To learn in more detail the dynamic change and clinical relevance of circulating and tumor-infiltrating Lin^−/lowHLA-DR⁻CD11b⁺CD33⁺ MDSC in colorectal cancer, we harvested the blood from 64 patients with varying stage of colorectal cancer and tumor and matched paraneoplastic tissues from 5 patients with advanced colorectal cancer, subjected them to multicolor flow cytometric analysis of percentage, absolute number and phenotype of MDSC and finally characterized their immunosuppressive functions. Our results demonstrate that peripheral blood from colorectal cancer patients contains markedly increased percentage and absolute number of Lin^−/lowHLA-DR⁻CD11b⁺CD33⁺ MDSCs compared with healthy individuals, and this increase is closely correlated with clinical cancer stage and tumor metastasis but not primary tumor size and serum concentrations of cancer biomarker. A similar increase of MDSCs was also observed in the tumor tissues. Phenotyping MDSCs shows that they express high CD13 and CD39, low CD115, CD117, CD124 and PD-L1, and devoid of CD14, CD15 and CD66b, reminiscent of precursor myeloid cells. MDSCs from cancer patients but not healthy donors have the immunosuppressive activity and were able to inhibit in vitro autologous T-cell proliferation. Collectively, this study substantiates the presence of increased immunosuppressive circulating and tumor-resident Lin^−/lowHLA-DR⁻CD11b⁺CD33⁺ MDSCs in patients with colorectal cancers correlating with cancer stage and metastasis, and suggests that pharmacologic blockade of MDSCs should be considered in future clinical trials.     

Analysis of Suppressor and Non-Suppressor FOXP3+ T Cells in HIV-1-Infected Patients

Recently, it was shown that peripheral blood FOXP3+CD4+ T cells are composed of three phenotypic and functionally distinct subpopulations. Two of them having in vitro suppressive effects were characterized as resting Treg cells (rTregs) and activated Treg cells (aTregs). A third subset, identified as FOXP3+ non-Tregs, does not display any suppressor activity and produce high levels of Th1 and Th17 cytokines upon stimulation. In the present study we focus on the characteristics of these three subsets of FOXP3+CD4+ T cells in untreated HIV-1-infected patients. We found that the absolute counts of rTregs, aTregs and FOXP3+ non-Tregs were reduced in HIV-1 patients compared with healthy donors. The relative frequency of rTregs and aTregs was similar in HIV-1 patients and healthy donors, while the frequency of FOXP3+ non-Tregs was significantly higher in HIV-1 patients, reaching a maximum in those patients with the lower values of CD4 counts. Contrasting with the observations made in FOXP3- CD4+ T cells, we did not find a negative correlation between the number of rTregs, aTregs or FOXP3+ non-Tregs and virus load. Studies performed with either whole PBMCs or sorted aTregs and FOXP3+ non-Tregs cells showed that these two populations of FOXP3+ T cells were highly permissive to HIV-1 infection. Upon infection, FOXP3+ non-Tregs markedly down-regulates its capacity to produce Th1 and Th17 cytokines, however, they retain the ability to produce substantial amounts of Th2 cytokines. This suggests that FOXP3+ non-Tregs might contribute to the polarization of CD4+ T cells into a Th2 profile, predictive of a poor outcome of HIV-1-infected patients.     

Pten Null Prostate Epithelium Promotes Localized Myeloid-Derived Suppressor Cell Expansion and Immune Suppression during Tumor Initiation and Progression

Chronic inflammation is known to be associated with prostate cancer development, but how epithelium-associated cancer-initiating events cross talk to inflammatory cells during prostate cancer initiation and progression is largely unknown. Using the Pten null murine prostate cancer model, we show an expansion of Gr-1⁺ CD11b⁺ myeloid-derived suppressor cells (MDSCs) occurring intraprostatically immediately following epithelium-specific Pten deletion without expansion in hematopoietic tissues. This MDSC expansion is accompanied by sustained immune suppression. Prostatic Gr-1⁺ CD11b⁺ cells, but not those isolated from the spleen of the same tumor-bearing mice, suppress T cell proliferation and express high levels of Arginase 1 and iNOS. Mechanistically, the loss of PTEN in the epithelium leads to a significant upregulation of genes within the inflammatory response and cytokine-cytokine receptor interaction pathways, including Csf1 and Il1b, two genes known to induce MDSC expansion and immunosuppressive activities. Treatment of Pten null mice with the selective CSF-1 receptor inhibitor GW2580 decreases MDSC infiltration and relieves the associated immunosuppressive phenotype. Our study indicates that epithelium-associated tumor-initiating events trigger the secretion of inflammatory cytokines and promote localized MDSC expansion and immune suppression, thereby promoting tumor progression.     

Age-Related Expansion of Tim-3 Expressing T Cells in Vertically HIV-1 Infected Children

As perinatally HIV-1-infected children grow into adolescents and young adults, they are increasingly burdened with the long-term consequences of chronic HIV-1 infection, with long-term morbidity due to inadequate immunity. In progressive HIV-1 infection in horizontally infected adults, inflammation, T cell activation, and perturbed T cell differentiation lead to an “immune exhaustion”, with decline in T cell effector functions. T effector cells develop an increased expression of CD57 and loss of CD28, with an increase in co-inhibitory receptors such as PD-1 and Tim-3. Very little is known about HIV-1 induced T cell dysfunction in vertical infection. In two perinatally antiretroviral drug treated HIV-1-infected groups with median ages of 11.2 yr and 18.5 yr, matched for viral load, we found no difference in the proportion of senescent CD28⁻CD57⁺CD8⁺ T cells between the groups. However, the frequency of Tim-3⁺CD8⁺ and Tim-3⁺CD4⁺ exhausted T cells, but not PD-1⁺ T cells, was significantly increased in the adolescents with longer duration of infection compared to the children with shorter duration of HIV-1 infection. PD-1⁺CD8⁺ T cells were directly associated with T cell immune activation in children. The frequency of Tim-3⁺CD8⁺ T cells positively correlated with HIV-1 plasma viral load in the adolescents but not in the children. These data suggest that Tim-3 upregulation was driven by both HIV-1 viral replication and increased age, whereas PD-1 expression is associated with immune activation. These findings also suggest that the Tim-3 immune exhaustion phenotype rather than PD-1 or senescent cells plays an important role in age-related T cell dysfunction in perinatal HIV-1 infection. Targeting Tim-3 may serve as a novel therapeutic approach to improve immune control of virus replication and mitigate age related T cell exhaustion.     

Osteoclasts in Multiple Myeloma Are Derived from Gr-1+CD11b+Myeloid-Derived Suppressor Cells

Osteoclasts play a key role in the development of cancer-associated osteolytic lesions. The number and activity of osteoclasts are often enhanced by tumors. However, the origin of osteoclasts is unknown. Myeloid-derived suppressor cells (MDSCs) are one of the pre-metastatic niche components that are induced to expand by tumor cells. Here we show that the MDSCs can differentiate into mature and functional osteoclasts in vitro and in vivo. Inoculation of 5TGM1-GFP myeloma cells into C57BL6/KaLwRij mice led to a significant expansion of MDSCs in blood, spleen, and bone marrow over time. When grown in osteoclastogenic media in vitro, MDSCs from tumor-challenged mice displayed 14 times greater potential to differentiate into mature and functional osteoclasts than those from non-tumor controls. Importantly, MDSCs from tumor-challenged LacZ transgenic mice differentiated into LacZ+osteoclasts in vivo. Furthermore, a significant increase in tumor burden and bone loss accompanied by increased number of osteoclasts was observed in mice co-inoculated with tumor-challenged MDSCs and 5TGM1 cells compared to the control animals received 5TGM1 cells alone. Finally, treatment of MDSCs from myeloma-challenged mice with Zoledronic acid (ZA), a potent inhibitor of bone resorption, inhibited the number of osteoclasts formed in MDSC cultures and the expansion of MDSCs and bone lesions in mice. Collectively, these data provide in vitro and in vivo evidence that tumor-induced MDSCs exacerbate cancer-associated bone destruction by directly serving as osteoclast precursors.     

Programmed Cell Death-1 Deficiency Exacerbates T Cell Activation and Atherogenesis despite Expansion of Regulatory T Cells in Atherosclerosis-Prone Mice

T cell activation represents a double-edged sword in atherogenesis, as it promotes both pro-inflammatory T cell activation and atheroprotective Foxp3⁺ regulatory T cell (Treg) responses. Here, we investigated the role of the co-inhibitory receptor programmed cell death-1 (PD-1) in T cell activation and CD4⁺ T cell polarization towards pro-atherogenic or atheroprotective responses in mice. Mice deficient for both low density lipoprotein receptor and PD-1 (Ldlr^−/−Pd1^−/−) displayed striking increases in systemic CD4⁺ and CD8⁺ T cell activation after 9 weeks of high fat diet feeding, associated with an expansion of both pro-atherogenic IFNγ-secreting T helper 1 cells and atheroprotective Foxp3⁺ Tregs. Importantly, PD-1 deficiency did not affect Treg suppressive function in vitro. Notably, PD-1 deficiency exacerbated atherosclerotic lesion growth and entailed a massive infiltration of T cells in atherosclerotic lesions. In addition, aggravated hypercholesterolemia was observed in Ldlr^−/−Pd1^−/− mice. In conclusion, we here demonstrate that although disruption of PD-1 signaling enhances both pro- and anti-atherogenic T cell responses in Ldlr^−/− mice, pro-inflammatory T cell activation prevails and enhances dyslipidemia, vascular inflammation and atherosclerosis.     

Immune Activation Is Associated with CD8 T Cell Interleukin-21 Production in HIV-1-Infected Individuals

Interleukin-21 (IL-21) can be produced by CD8 T cells from HIV-1-infected individuals and those with autoimmune disease, but the mechanism remains poorly understood. Here we demonstrate that IL-21-producing CD8 T cells are not associated with CD4 depletion and are absent in patients with idiopathic CD4 lymphocytopenia. Instead, IL-21 production by CD8 T cells was associated with high levels of activation, suggesting that these cells emerge as a consequence of excessive chronic immune activation rather than CD4 lymphopenia.     

Mast Cells Mobilize Myeloid-Derived Suppressor Cells and Treg Cells in Tumor Microenvironment via IL-17 Pathway in Murine Hepatocarcinoma Model

Tumor immunosuppression is commonly braided with chronic inflammation during tumor development. However, the relationship between immunosuppression and inflammation in tumor microenvironment is still unclear. We have demonstrated that mast cells are accumulated and exacerbate the inflammation and immunosuppression in tumor microenvironment via SCF/c-kit signaling pathway. Here, we further elucidate the underlying mechanism, which involves both myeloid-derived suppressor cells (MDSCs) and regulatory T (Treg) cells. Our data showed that mast cells mobilized the infiltration of MDSCs to tumor and induced the production of IL-17 by MDSCs; MDSCs-derived IL-17 indirectly attracted Treg cells, enhanced their suppressor function, and induced the IL-9 production by Treg cells; in turn, IL-9 strengthened the survival and protumor effect of mast cells in tumor microenvironment. Our findings disclose a closed loop among mast cells, MDSCs and Treg cells in tumor microenvironment, which provides a new insight into the paralleled developments of inflammation and immunosuppression in tumor microenvironment. Based on these findings, we propose that targeting tumor inflammation might be a potential strategy to reverse the immunosuppression of tumor microenvironment, thus facilitating cancer immunotherapy.