Myeloid-derived Suppressor Cells In Mice Research Articles

Myeloid-Derived Suppressor Cells Ameliorate Cyclosporine A-Induced Hypertension in Mice.

The calcineurin inhibitor cyclosporine A (CsA) suppresses the immune system but promotes hypertension, vascular dysfunction, and renal damage. CsA decreases regulatory T cells and this contributes to the development of hypertension. However, CsA's effects on another important regulatory immune cell subset, myeloid-derived suppressor cells (MDSCs), is unknown. We hypothesized that augmenting MDSCs would ameliorate the CsA-induced hypertension and vascular and renal injury and dysfunction and that CsA reduces MDSCs in mice. Daily interleukin-33 treatment, which increased MDSC levels, completely prevented CsA-induced hypertension and vascular and renal toxicity. Adoptive transfer of MDSCs from control mice into CsA-treated mice after hypertension was established dose-dependently reduced blood pressure and vascular and glomerular injury. CsA treatment of aortas and kidneys isolated from control mice for 24 hours decreased relaxation responses and increased inflammation, respectively, and these effects were prevented by the presence of MDSCs. MDSCs also prevented the CsA-induced increase in fibronectin in microvascular and glomerular endothelial cells. Last, CsA dose-dependently reduced the number of MDSCs by inhibiting calcineurin and preventing cell proliferation, as other direct calcineurin signaling pathway inhibitors had the same dose-dependent effect. These data suggest that augmenting MDSCs can reduce the cardiovascular and renal toxicity and hypertension caused by CsA.

MiR-375 ameliorates sepsis by downregulating miR-21 level via inhibiting JAK2-STAT3 signaling.

Accumulating evidences have confirmed that miRNAs have important roles in sepsis. Myeloid-derived suppressor cells (MDSCs) enhance late sepsis development through immunosuppression in mice. Here, the functions and mechanisms of miR-375 in sepsis were revealed. We found that miR-375 level was downregulated but miR-21 level was upregulated in sepsis patients and that their levels were correlated negatively. Importantly, ectopic expression of miR-375 could decrease the number of sepsis Gr1+CD11b+ MDSCs in mice. Mechanistically, miR-375 could target Janus kinase 2 (JAK2) and further impaired signal transducer and activator of transcription 3 (STAT3) in sepsis Gr1+CD11b+ MDSC. Gain and loss of function of experiments showed that upregulation or downregulation of miR-375 level could decrease or increase miR-21 level. Moreover, pretreatment of JAK2 overexpressing vector could abolish the effects of miR-375 on miR-21 level and the amount of sepsis Gr1+CD11b+ MDSCs. Therefore, our results demonstrate that miR-375 could block JAK2-STAT3 pathway and thus modulate miR-21 level, which is involved in regulation of late sepsis.

Pak2 Regulates MDSC Development and Function

Myeloid derived suppressor cells (MDSCs) are a heterogeneous cell population at various stages of differentiation that can increase under various pathologic conditions such as cancer, infection or inflammation, displaying suppressive function. It is well recognized that MDSCs contribute to tumor evasion by suppressing cell-mediated immunity. Based on the differential expression of Ly6C and/or Ly6G in mice, MDSCs are characterized as granulocytic (CD11b+Ly6G+Ly6Clow) or monocytic MDSCs (CD11b+Ly6Glow/−Ly6Chi). These subsets induce T-cell hyporesponsiveness and can have various functions and distribution depending on their environment. Although much research has focused on the tumorigenic effects of MDSCs, studies on the regulation of their development during hematopoiesis remain limited.p21-activated kinases (Paks) are serine/threonine kinases that regulate diverse cellular activities including cytoskeletal remodeling, cell motility, proliferation, apoptosis and mitosis. Despite active research on pharmacological inhibition of group I Paks in treating solid tumors, few studies have examined the role of Paks in modulating normal hematopoiesis. Knowledge of the role of Pak2 in regulating long-term hematopoiesis and lineage commitment remains limited. Utilizing a conditional Pak2-KO murine model, we have previously demonstrated that Pak2 disruption in hematopoietic stem/progenitor cells (HSPCs) induces myeloid lineage skewing and CD11b+Gr1+ cell expansion in mice. Compared to mice reconstituted with wild type (WT) bone marrow (BM), mice transplanted with Pak2-KO BM displayed a significantly higher percentage of granulocyte-monocyte progenitors (GMPs) in the BM and higher numbers of CD11b+Gr1+ cells in the spleen.In this study, we demonstrated that CD11b+Gr1high cells isolated from the spleens of mice with Pak2-KO BM displayed significantly greater suppressive function on T cell proliferation in vitro, consistent with MDSC phenotype. There was a near 2-fold increase in the numbers of both granulocytic and monocytic splenic MDSCs in mice reconstituted with Pak2-KO BM. At HSPC level, Pak2-KO BM yielded greater than 3-fold more colonies in response to GM-CSF but not G-CSF or M-CSF when compared to WT cells, indicating selective hypersensitivity to GM-CSF. In parallel experiments, Pak2-KO and WT BM C-kit+ cells that were enriched for hematopoietic progenitor cells (HPCs) were cultured in liquid culture in the presence of GM-CSF. Pak2-KO BM C-kit+ cells yielded greater than 2-fold higher numbers of CD11b+Gr1+ MDSCs that displayed potent suppression on CD8+ T cell proliferation. These data demonstrate that Pak2 disruption increases HPC sensitivity to GM-CSF signaling and drives lineage commitment toward granulocyte-monocyte lineage thus promoting MDSC development. In addition, we have also found that Pak2 deficient MDSCs are more proliferative and more resistant to apoptosis when compared to WT CD11b+Gr1+cells, thus contributing to expansion of this population in vivo. Loss of Pak2 decreases MDSC sensitivity to apoptosis through differential regulation of multiple pro- and anti-apoptotic gene expression. Furthermore, Pak2 disruption down regulates the expression of IRF8, a well-described myeloid transcription factor. Together, our data indicate that loss of Pak2 promotes HPC myeloid lineage commitment and CD11b+Gr1+ MDSC proliferation while suppressing apoptotic cell death in these cells. Further studies are ongoing to determine the interaction between Pak2 and IRF8. DisclosuresNo relevant conflicts of interest to declare.

Stat3 and C/EBPβ synergize to induce miR-21 and miR-181b expression during sepsis.

Myeloid-derived suppressor cells (MDSCs) increase late sepsis immunosuppression and mortality in mice. We reported that microRNA (miR) 21 and miR-181b expression in Gr1+CD11b+ myeloid progenitors increase septic MDSCs in mice by arresting macrophage and dendritic cell differentiation. Here, we report how sepsis regulates miR-21 and miR-181b transcription. In vivo and in vitro binding studies have shown that C/EBPα transcription factor, which promotes normal myeloid cell differentiation, binds both miRNA promoters in Gr1+CD11b+ cells from sham mice. In contrast, in sepsis Gr1+CD11b+ MDSCs miR-21 and miR-181b promoters bind both transcription factors Stat3 and C/EBPβ, which co-imunoprecipitate as a single complex. Mechanistically, transcription factor Rb phosphorylation supports Stat3 and C/EBPβ accumulation at both miRNA promoters, and C/EBPβ or Stat3 depletion by siRNA in sepsis Gr1+CD11b+ MDSCs inhibits miR-21 and miR-181b expression. To further support this molecular path for MDSC accumulation, we found that Stat3 and C/EBP binding at miR-21 or miR-181b promoter was induced by IL-6, using a luciferase reporter gene transfection into naive Gr1+CD11b+ cells. Identifying how sepsis MDSCs are generated may inform new treatments to reverse sepsis immunosuppression.

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+ Ly6Chigh 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.

Myeloid-derived suppressor cells contribute to A2B adenosine receptor-induced VEGF production and angiogenesis in a mouse melanoma model.

Vascular endothelial growth factor (VEGF) is an angiogenic factor critically involved in tumor progression. Adenosine A2B receptor plays a pivotal role in promoting tumor growth. The aim of this study was to investigate the role of myeloid-derived suppressor cells (MDSCs) in the pro-angiogenic effects of A2B and to determine whether A2B blockade could enhance the effectiveness of anti-VEGF treatment. Mice treated with Bay60-6583, a selective A2B receptor agonist, showed enhanced tumor VEGF-A expression and vessel density. This effect was associated with accelerated tumor growth, which could be reversed with anti-VEGF treatment. Bay60-6583 increased the accumulation of tumor CD11b+Gr1+ cells. Depletion of MDSCs in mice significantly reduced A2B-induced VEGF production. However, A2B receptor stimulation did not directly regulate VEGF expression in isolated tumor myeloid cells. Mechanistically, Bay60-6583-treated melanoma tissues showed increased STAT3 activation. Inhibition of STAT3 significantly decreased the pro-tumor activity of Bay60-6583 and reduced tumor VEGF expression. Pharmacological blockade of A2B receptor with PSB1115 significantly reduced tumor growth by inhibiting tumor angiogenesis and increasing T cells numbers within the tumor microenvironment. These effects are, at least in part, dependent on impaired tumor accumulation of Gr1+ cells upon A2B receptor blockade. PSB1115 increased the effectiveness of anti-VEGF treatment.

Differential induction of Ly6G and Ly6C positive myeloid derived suppressor cells in chronic kidney and liver inflammation and fibrosis.

CD11b+Gr1+ myeloid derived suppressor cells (MDSC) are known to be very potent suppressors of T cell immunity and can be further stratified into granulocytic MDSC and monocytic MDSC in mice based on expression of Ly6G or Ly6C, respectively. Here, using these markers and functional assays, we aimed to identify whether MDSC are induced during chronic inflammation leading to fibrosis in both kidney and liver and whether additional markers could more specifically identify these MDSC subsets. In an adenine-induced model of kidney inflammation/fibrosis suppressive Ly6Gpos MDSC were induced. The suppressive function within the Ly6G+ MDSC population was exclusively present in IFNγRβ expressing cells. In contrast, in chronic inflammation in the liver induced by bile duct ligation, suppressive capacity was exclusively present in the Ly6Cpos MDSC subset. Gene expression analyses confirmed the differential origins and regulation of those MDSC subsets. Additionally, depletion of MDSC in either kidney or liver fibrosis enhanced fibrosis markers, indicating a protective role for MDSC in organ fibrosis. Thus, our data demonstrate that during liver inflammation and kidney fibrosis MDSC with similar function arise bearing a distinct marker profile and arising from different cell populations.

The protective role of myeloid-derived suppressor cells in concanavalin A-induced hepatic injury

The mechanism underlying T cell-mediated fulminant hepatitis is not fully understood. In this study, we investigated whether myeloid derived suppressor cells (MDSCs) could prevent the concanavalin A (ConA)-induced hepatitis through suppressing T cell proliferation. We observed an increase in the frequencies of MDSCs in mouse spleen and liver at early stage of ConA treatment, implicating that the MDSCs might be involved in the initial resistance of mice against ConA-mediated inflammation. Subpopulation analysis showed that the MDSCs in liver of ConA-induced mice were mainly granulocytic MDSCs. Adoptive transfer of the bone marrow-derived MDSCs into ConA-treated mice showed that the MDSCs migrated into the liver and spleen where they suppressed T cell proliferation through ROS pathway. In addition, the frequencies of MDSCs in mice were also significantly increased by the treatment with immune suppressor glucocorticoids. Transfer of MDSCs into the regulatory T cell (Treg)-depleted mice showed that the protective effect of MDSCs on ConA-induced hepatitis is Treg-independent. In conclusion, our results demonstrate that MDSCs possess a direct protective role in T cell-mediated hepatitis, and increasing the frequency of MDSCs by either adoptive transfer or glucocorticoid treatment represents a potential cell-based therapeutic strategy for the acute inflammatory disease.

Myeloid-derived suppressor cell function is diminished in aspirin-triggered allergic airway hyperresponsiveness in mice

Myeloid-derived suppressor cells (MDSCs) have recently been implicated in the pathogenesis of asthma, but their regulation in patients with aspirin-intolerant asthma (AIA) remains unclear. We sought to characterize MDSC accumulation and pathogenic functions in allergic airway inflammation mediated by COX-1 deficiency or aspirin treatment in mice. Allergic airway inflammation was induced in mice by means of ovalbumin challenge. The distribution and function of MDSCs in mice were analyzed by using flow cytometry and pharmacologic/gene manipulation approaches. CD11b(+)Gr1(high)Ly6G(+)Ly6C(int) MDSCs (polymorphonuclear MDSCs [PMN-MDSCs]) recruited to the lungs are negatively correlated with airway inflammation in allergen-challenged mice. Aspirin-treated and COX-1 knockout (KO) mice showed significantly lower accumulation of PMN-MDSCs in the inflamed lung and immune organs accompanied by increased TH2 airway responses. The TH2-suppressive function of PMN-MDSCs was notably impaired by COX-1 deletion or inhibition, predominantly through downregulation of arginase-1. COX-1-derived prostaglandin E2 promoted PMN-MDSC generation in bone marrow through E prostanoid 2 and 4 receptors (EP2 and EP4), whereas the impaired arginase-1 expression in PMN-MDSCs in COX-1 KO mice was mediated by dysregulation of the prostaglandin E2/EP4/cyclic AMP/protein kinase A pathway. EP4 agonist administration alleviated allergy-induced airway hyperresponsiveness in COX-1 KO mice. Moreover, the immunosuppressive function of PMN-MDSCs from patients with AIA was dramatically decreased compared with that from patients with aspirin-tolerant asthma. The immunosuppressive activity of PMN-MDSCs was diminished in both allergen-challenged COX-1 KO mice and patients with AIA, probably through an EP4-mediated signaling pathway, indicating that activation of PMN-MDSCs might be a promising therapeutic strategy for asthma, particularly AIA.

α-Galactosylceramide but Not Phenyl-Glycolipids Induced NKT Cell Anergy and IL-33–Mediated Myeloid-Derived Suppressor Cell Accumulation via Upregulation of egr2/3

Strategies for cancer immunotherapy include activating immune system for therapeutic benefit or blockade of immune checkpoints. To harness innate immunity to fight cancer, α-galactosylceramide (α-GalCer) has been used to activate NKT cells. Unfortunately, administration of α-GalCer causes long-term NKT cell anergy, but the molecular mechanism is unclear. In this study, we showed that α-GalCer-triggered egr2/3, which induced programmed death 1 and cbl-b in NKT cells, leading to NKT cell anergy. We also uncovered the induction of the immunosuppressive myeloid-derived suppressor cells (MDSCs) in the spleen by α-GalCer that might attenuate its antitumor efficacy. The accumulation of MDSC was accompanied by 20-fold rise in their arg-1 mRNAs and enhanced expression of programmed death 1/programmed death ligand 1. Furthermore, α-GalCer-induced egr-2/3 in hepatic NKT cells upregulated their TRAIL in addition to Fas ligand (FasL) and induced alarm signaling molecule IL-33 in Kupffer cells, presumably because of liver damage triggered by TRAIL/FasL. We further demonstrated that IL-33-stimulated macrophages produce G-CSF, which in turn, boosted MDSCs. Thus, α-GalCer-induced FasL/TRAIL and IL-33 provided a novel mechanism underlying α-GalCer-induced hepatotoxicity and MDSC accumulation. In contrast, analogs of α-GalCer containing phenyl group in the lipid tail could neither induce NKT anergy nor enhance MDSCs accumulation. Furthermore, tumor-infiltrating MDSCs in mice injected repeatedly with α-GalCer were 2-fold higher than those treated with phenyl-glycolipids. These results not only revealed the induction of MDSC via IL-33 as a new mechanism for α-GalCer-elicited immunosuppression but also provided one of the mechanisms underlying the superior antitumor potency of phenyl-glycolipids. Our findings have important implications for the development of NKT-stimulatory glycolipids as vaccine adjuvants and anticancer therapeutics.

Housing Mice At Sub-Thermoneutral Temperatures Influences Severity Of Gvhd In Mouse Models

While a very high proportion of humans develop serious and fatal GVHD following bone marrow transplantation (BMT), mice typically do not develop lethal graft versus host disease (GVHD) when receiving bone marrow alone, even when the donor and recipient mice are MHC mismatched. Thus, the major pre-clinical model in which novel treatments for GVHD are being tested has a fundamental resistance to GVHD. Researchers therefore add donor-derived T cells from spleen or lymph node in order to generate a similar incidence and severity of GVHD as seen in humans, thus using a model system that has fundamental differences from the clinical situation.Recent work from our lab has revealed that the standard cool (sub-thermoneutral) temperature in which all laboratory mice must be housed (creating a chronic condition of metabolic cold stress) results in impaired CD8+ immune function, while bolstering the action of immunosuppressive cells. Current work suggests that the excessive stimulation of cold-stress induced sympathetic nerve activity, and subsequent production of norepinephrine is the cause of this immunosuppressive environment. Because GVHD depends upon the activation and function of donor CD8+ T cells in the host environment, we hypothesized that mouse models do not develop expected amounts of GVHD following a bone marrow only transplant because the host environment, which is metabolically cold-stressed, blocks the function of effector T cells.To test our hypothesis we first housed BALB/c mice at standard (22°C) and thermoneutral temperatures (30°C) for two weeks. To generate a MHC disparate model, we lethally irradiated the mice and transplanted C57BL/6 bone marrow without additional splenocytes. We used both weight loss and survival following BMT as a readout for GVHD severity. As expected, under standard conditions of housing at 22° mice were able to recover from the transplant and return to their initial weight within the same 17 day post-transplant timeframe (percent initial weight at 22°: 101% ± 1%; N=10). In support of our hypothesis we saw weight loss in the mice maintained at 30° as early as 17 days post-transplant (percent initial weight at 30°: 93% ± 2%; N=10). Interestingly, weight loss continued in the mice maintained at thermoneutrality (percent initial weight loss 48 days post-transplant at 30°: 0.84 ± 0.03, N=10). Beginning 49 days post-transplant some mice at thermoneutrality began to succumb to GVHD. Thus, housing mice at thermoneutrality decreased survival suggesting a greater incidence/more severe GVHD (Figure 1; p=0.01, N=10). As our previous data in solid tumor models suggests that numbers of immunosuppressive myeloid derived suppressor cells (MDSCs) are increased when mice are metabolically stressed at 22°, we next examined differences in this cell population in the spleens of transplanted mice. We found an obvious trend towards decreased splenic MDSCs in mice housed at 30° compared to 22°. [Display omitted] To retest this hypothesis in a clinically relevant minor antigen mismatch model we housed male C57BL/6 mice at ST or TT for two weeks. We then lethally irradiated the mice and transplanted female 129/SvJ bone marrow. Again, we found a greater weight loss (p=0.02; N=10) in the mice receiving bone marrow only maintained at 30° compared to 22° as early as 28 days post-transplant (percent initial weight at 22°: 103% ± 2%, N=10; percent initial weight at 30°: 96% ± 2%, N=10). We also found increased numbers of MDSCs in the spleens of mice housed at 22° compared to 30° (p=0.09, N=10).The increasing number of patients receiving BMT for various malignancies clearly highlights the critical importance of improving mouse models for understanding GVHD as this problem remains a major barrier to achieving greater survival for patients. These data demonstrate the existence of a fundamental relationship between cool standard housing temperature used in all research institutes and a resistance to GVHD, which is likely linked to a reduced immune response and/or increased immunosuppression. Achieving a better understanding of this potential confounding environmental factor could help to improve the use of mouse models for identifying the best therapies to attenuate or eliminate GVHD in humans, while at the same time, promoting effective immune reconstitution and graft versus tumor effect. Disclosures:No relevant conflicts of interest to declare.

Chronic psychological stress induces the accumulation of myeloid-derived suppressor cells in mice.

Chronic psychological stress has been shown to adversely impact immune system functions and compromise host defenses against various infections. However, the underlying mechanisms remain elusive. Recent studies have demonstrated that myeloid-derived suppressor cells (MDSCs) play an important role in regulating immunity. It is of interest to explore whether or not chronic psychological stress plays immunosuppressive functions partially by inducing MDSCs accumulation. In this work, we report that chronic psychological stress led to the accumulation of CD11b+Gr1+ cells in the bone marrow of BALB/c mice. Repeated β-agonist infusion showed no such effect. However, β-adrenergic blockade, but not glucocorticoids blockade, partially reversed the accumulation of CD11b+Gr1+ cells under the condition of chronic psychological stress, suggesting catecholamines collaborate with other factors to induce the accumulation. Further exploration indicates that cyclooxygenase 2 (COX-2)-prostaglandin E2 (PGE2) loop might act downstream to induce the accumulation. A majority of the accumulated CD11b+Gr1+ cells were Ly6G+Ly6Clow immature neutrophils, which inhibited cytokine release of macrophages as well as T cell responsiveness. Moreover, the accumulated CD11b+Gr1+ cells under the condition of chronic psychological stress expressed multiple inhibitory molecules. Taken together, our data demonstrate for the first time that chronic psychological stress induces MDSCs accumulation in mice, which can contribute to immunosuppression.

Downregulation of Hematopoietic MUC1 during Experimental Colitis Increases Tumor-Promoting Myeloid-Derived Suppressor Cells

MUC1 is a tumor-associated antigen that is aberrantly expressed in cancer and inflammatory bowel disease (IBD). Even though immune cells express low MUC1 levels, their modulations of MUC1 are important in tumor progression. Consistent with previous clinical data that show increased myeloid-derived suppressor cells (MDSCs) in IBD, we now show that downregulation of MUC1 on hematopoietic cells increases MDSCs in IBD, similar to our data in tumor-bearing mice. We hypothesize that MDSC expansion in IBD is critical for tumor progression. To mechanistically confirm the linkage between Muc1 downregulation and MDSC expansion, we generated chimeric mice that did not express Muc1 in the hematopoietic compartment (KO→WT). These mice were used in two models of colitis and colitis-associated cancer (CAC) and their responses were compared with wild-type (WT) chimeras (WT→WT). KO→WT mice show increased levels of MDSCs during colitis and increased protumorigenic signaling in the colon during CAC, resulting in larger colon tumors. RNA and protein analysis show increased upregulation of metalloproteinases, collagenases, defensins, complements, growth factors, cytokines, and chemokines in KO→WT mice as compared with WT→WT mice. Antibody-mediated depletion of MDSCs in mice during colitis reduced colon tumor formation during CAC. Development of CAC is a serious complication of colitis and our data highlight MDSCs as a targetable link between inflammation and cancer. In addition, the lack of MUC1 expression on MDSCs can be a novel marker for MDSCs, given that MDSCs are still not well characterized in human cancers.

Comparative cancer immunology: Inducing myeloid derived suppressor cells in vitro (P6092)

Abstract Comparative oncology has broad applications in the development and optimization of novel therapeutics for cancer in both humans and animals. In this study, we developed in vitro tools to study canine myeloid derived suppressor cells (MDSCs). Previously, we have shown that tumor derived soluble factors (TDSFs) modified the function of a dendritic cell line. In this study, we investigated TDSF’s ability to induce MDSCs. TDSFs were generated from canine tumor cell lines and canine bone marrow precursors were differentiated in the presence of GM-CSF with or without TDSFs. We found that TDSFs dramatically increased the expression of CD11b+CADO48A+ (a canine MDSC marker akin to CD11b+Gr1+ MDSCs in mice) in cells between days 3-7 compared to controls with the highest percentage (40%) occurring on day 5 of differentiation. Day 5 TDSF-exposed cells suppressed the proliferation of responder immune cells. Similar results were found in companion studies using immunocompetent mice and murine in vitro TDSF-generated MDSCs. Taken together, these findings support that TDSFs can generate canine MDSCs in vitro that are similar to mouse MDSCs and that the in vitro generation of MDSCs mimics the immunosuppression seen with ex vivo derived MDSCs in humans and mice. These data have important implications both in understanding canine cancer immunology and for translational and therapeutic applications utilizing the dog as a model for human cancer.

MDSC

MDSC

Myeloid-derived suppressor cells: general characteristics and relevance to clinical management of pancreatic cancer.

Recent studies describe a heterogeneous population of cells of the myeloid lineage, termed myeloid derived suppressor cells (MDSC), which are observed with increased prevalence in the peripheral blood and tumor microenvironment of cancer patients, including pancreatic cancer. Accumulation of MDSC in the peripheral circulation has been related to extent of disease, and correlates with stage. MDSC have primarily been implicated in promoting tumor growth by suppressing antitumor immunity. There is also compelling evidence MDSC are also involved in angiogenesis and metastatic spread. Two main subsets of MDSC have been identified in cancer patients: a monocytic subset, characterized by expression of CD14, and a granulocytic subset characterized by expression of CD15. Both subsets of MDSC actively suppress host immunity through a variety of mechanisms including production of reactive oxygen species and arginase. Just as in humans, accumulation of monocytic and granulocytic MDSC has been noted in the bone marrow, spleen, peripheral circulation, and tumors of tumor bearing mice. Successful targeting of MDSC in mice is associated with improved immune responses, delayed tumor growth, improved survival, and increased efficacy of vaccine therapy. By further elucidating mechanisms of MDSC recruitment and maintenance in the tumor environment, strategies could be developed to reverse immune tolerance to tumor. We discuss here what is currently known about MDSC as well as some potential strategies targeting MDSC in the context of our work on pancreatic cancer and recent literature. Due to the number of new reports on MDSC, the most pertinent ones have been selected.

CpG Blocks Immunosuppression by Myeloid-Derived Suppressor Cells in Tumor-Bearing Mice

The Toll-like receptor (TLR) 9 ligand CpG has been used successfully for the immunotherapy of cancer. Chronic CpG application in tumor-free hosts leads, however, to the expansion of myeloid-derived suppressor cells (MDSC), which can cause T-cell suppression and may thus hamper the development of an effective immune response. Here, we investigated the effect of TLR9 activation on the function of MDSC in tumor-bearing mice. We investigated the effect of CpG treatment on the number, phenotype, and function of MDSC in mice bearing subcutaneous C26 tumors and in CEA424-TAg mice bearing autochthonous gastric tumors. CpG treatment blocks the suppressive activity of MDSC on T-cell proliferation in both tumor models. Inhibition of MDSC function by CpG was particularly pronounced for a highly suppressive Ly6G(hi) polymorphonuclear subset of MDSC. We further show that TLR9 activation by CpG promotes maturation and differentiation of MDSC and strongly decreases the proportion of Ly6G(hi) MDSC in both tumor-bearing and tumor-free mice. We demonstrate that IFN-α produced by plasmacytoid dendritic cells upon CpG stimulation is a key effector for the induction of MDSC maturation in vitro and show that treatment of mice with recombinant IFN-α is sufficient to block MDSC suppressivity. We show here for the first time that TLR9 activation inhibits the regulatory function of MDSC in tumor-bearing mice and define a role for the antitumoral cytokine IFN-α in this process.

Invariant NKT cells reduce the immunosuppressive activity of influenza A virus–induced myeloid-derived suppressor cells in mice and humans

Infection with influenza A virus (IAV) presents a substantial threat to public health worldwide, with young, elderly, and immunodeficient individuals being particularly susceptible. Inflammatory responses play an important role in the fatal outcome of IAV infection, but the mechanism remains unclear. We demonstrate here that the absence of invariant NKT (iNKT) cells in mice during IAV infection resulted in the expansion of myeloid-derived suppressor cells (MDSCs), which suppressed IAV-specific immune responses through the expression of both arginase and NOS, resulting in high IAV titer and increased mortality. Adoptive transfer of iNKT cells abolished the suppressive activity of MDSCs, restored IAV-specific immune responses, reduced IAV titer, and increased survival rate. The crosstalk between iNKT and MDSCs was CD1d- and CD40-dependent. Furthermore, IAV infection and exposure to TLR agonists relieved the suppressive activity of MDSCs. Finally, we extended these results to humans by demonstrating the presence of myeloid cells with suppressive activity in the PBLs of individuals infected with IAV and showed that their suppressive activity is substantially reduced by iNKT cell activation. These findings identify what we believe to be a novel immunomodulatory role of iNKT cells, which we suggest could be harnessed to abolish the immunosuppressive activity of MDSCs during IAV infection.

Overexpression of Interleukin-1β Induces Gastric Inflammation and Cancer and Mobilizes Myeloid-Derived Suppressor Cells in Mice

Polymorphisms of interleukin-1beta (IL-1beta) are associated with an increased risk of solid malignancies. Here, we show that stomach-specific expression of human IL-1beta in transgenic mice leads to spontaneous gastric inflammation and cancer that correlate with early recruitment of myeloid-derived suppressor cells (MDSCs) to the stomach. IL-1beta activates MDSCs in vitro and in vivo through an IL-1RI/NF-kappaB pathway. IL-1beta transgenic mice deficient in T and B lymphocytes develop gastric dysplasia accompanied by a marked increase in MDSCs in the stomach. Antagonism of IL-1 receptor signaling inhibits the development of gastric preneoplasia and suppresses MDSC mobilization. These results demonstrate that pathologic elevation of a single proinflammatory cytokine may be sufficient to induce neoplasia and provide a direct link between IL-1beta, MDSCs, and carcinogenesis.