Myeloid-derived suppressor cells coming of age.

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells generated during a large array of pathologic conditions ranging from cancer to obesity. These cells represent a pathologic state of activation of monocytes and relatively immature neutrophils. MDSCs are characterized by a distinct set of genomic and biochemical features, and can, on the basis of recent findings, be distinguished by specific surface molecules. The salient feature of these cells is their ability to inhibit T cell function and thus contribute to the pathogenesis of various diseases. In this Review, we discuss the origin and nature of these cells; their distinctive features; and their biological roles in cancer, infectious diseases, autoimmunity, obesity and pregnancy.

Unique pattern of neutrophil migration and function during tumor progression.

Although neutrophils have been linked to the formation of the pre-metastatic niche, the mechanism of their migration to distant, uninvolved tissues has remained elusive. We report that bone marrow neutrophils from mice with early-stage cancer exhibited much more spontaneous migration than that of control neutrophils from tumor-free mice. These cells lacked immunosuppressive activity but had elevated rates of oxidative phosphorylation and glycolysis, and increased production of ATP, relative to that of control neutrophils. Their enhanced spontaneous migration was mediated by autocrine ATP signaling through purinergic receptors. In ectopic tumor models and late stages of cancer, bone marrow neutrophils demonstrated potent immunosuppressive activity. However, these cells had metabolic and migratory activity indistinguishable from that of control neutrophils. A similar pattern of migration was observed for neutrophils and polymorphonuclear myeloid-derived suppressor cells from patients with cancer. These results elucidate the dynamic changes that neutrophils undergo in cancer and demonstrate the mechanism of neutrophils' contribution to early tumor dissemination.

Ferroptosis of tumour neutrophils causes immune suppression in cancer.

Ferroptosis is a non-apoptotic form of regulated cell death that is triggered by the discoordination of regulatory redox mechanisms culminating in massive peroxidation of polyunsaturated phospholipids. Ferroptosis inducers have shown considerable effectiveness in killing tumour cells in vitro, yet there has been no obvious success in experimental animal models, with the notable exception of immunodeficient mice1,2. This suggests that the effect of ferroptosis on immune cells remains poorly understood. Pathologically activated neutrophils (PMNs), termed myeloid-derived suppressor cells (PMN-MDSCs), are major negative regulators of anti-tumour immunity3-5. Here we found that PMN-MDSCs in the tumour microenvironment spontaneously die by ferroptosis. Although decreasing the presence of PMN-MDSCs, ferroptosis induces the release of oxygenated lipids and limits the activity of human and mouse T cells. In immunocompetent mice, genetic and pharmacological inhibition of ferroptosis abrogates suppressive activity of PMN-MDSCs, reduces tumour progression and synergizes with immune checkpoint blockade to suppress the tumour growth. By contrast, induction of ferroptosis in immunocompetent mice promotes tumour growth. Thus, ferroptosis is a unique and targetable immunosuppressive mechanism of PMN-MDSCs in the tumour microenvironment that can be pharmacologically modulated to limit tumour progression.

Epigenetic silencing of retinoblastoma gene regulates pathologic differentiation of myeloid cells in cancer.

Two major populations of myeloid-derived suppressor cells (MDSCs), monocytic MDSCs (M-MDSCs) and polymorphonuclear MDSCs (PMN-MDSCs) regulate immune responses in cancer and other pathologic conditions. Under physiologic conditions, Ly6C(hi)Ly6G(-) inflammatory monocytes, which are the normal counterpart of M-MDSCs, differentiate into macrophages and dendritic cells. PMN-MDSCs are the predominant group of MDSCs that accumulates in cancer. Here we show that a large proportion of M-MDSCs in tumor-bearing mice acquired phenotypic, morphological and functional features of PMN-MDSCs. Acquisition of this phenotype, but not the functional attributes of PMN-MDSCs, was mediated by transcriptional silencing of the retinoblastoma gene through epigenetic modifications mediated by histone deacetylase 2 (HDAC-2). These data demonstrate a new regulatory mechanism of myeloid cells in cancer.

CD45 Phosphatase Inhibits STAT3 Transcription Factor Activity in Myeloid Cells and Promotes Tumor-Associated Macrophage Differentiation.

Recruitment of monocytic myeloid-derived suppressor cells (MDSCs) and differentiation of tumor-associated macrophages (TAMs) are the major factors contributing to tumor progression and metastasis. We demonstrated that differentiation of TAMs in tumor site from monocytic precursors was controlled by downregulation of the activity of the transcription factor STAT3. Decreased STAT3 activity was caused by hypoxia and affected all myeloid cells but was not observed in tumor cells. Upregulation of CD45 tyrosine phosphatase activity in MDSCs exposed to hypoxia in tumor site was responsible for downregulation of STAT3. This effect was mediated by the disruption of CD45 protein dimerization regulated by sialic acid. Thus, STAT3 has a unique function in the tumor environment in controlling the differentiation of MDSC into TAM, and its regulatory pathway could be a potential target for therapy.

Fatty acid transport protein 2 reprograms neutrophils in cancer.

Polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) are pathologically activated neutrophils that are crucial for the regulation of immune responses in cancer. These cells contribute to the failure of cancer therapies and are associated with poor clinical outcomes. Despite recent advances in the understanding of PMN-MDSC biology, the mechanisms responsible for the pathological activation of neutrophils are not well defined, and this limits the selective targeting of these cells. Here we report that mouse and human PMN-MDSCs exclusively upregulate fatty acid transport protein 2 (FATP2). Overexpression of FATP2 in PMN-MDSCs was controlled by granulocyte-macrophage colony-stimulating factor, through the activation of the STAT5 transcription factor. Deletion of FATP2 abrogated the suppressive activity of PMN-MDSCs. The main mechanism of FATP2-mediated suppressive activity involved the uptake of arachidonic acid and the synthesis of prostaglandin E2. The selective pharmacological inhibition of FATP2 abrogated the activity of PMN-MDSCs and substantially delayed tumour progression. In combination with checkpoint inhibitors, FATP2 inhibition blocked tumour progression in mice. Thus, FATP2 mediates the acquisition of immunosuppressive activity by PMN-MDSCs and represents a target to inhibit the functions of PMN-MDSCs selectively and to improve the efficiency of cancer therapy.

Can the suppressive activity of myeloid-derived suppressor cells be "chop"ped?

Myeloid-derived suppressor cells (MDSCs) are one of the major negative regulators of immune responses. In this issue of Immunity, Thevenot et al. (2014) showed that in tumors, the suppressive activity of MDSCs is regulated by transcription factor Chop.

Redox lipid reprogramming commands susceptibility of macrophages and microglia to ferroptotic death.

Ferroptotic death is the penalty for losing control over three processes-iron metabolism, lipid peroxidation and thiol regulation-that are common in the pro-inflammatory environment where professional phagocytes fulfill their functions and yet survive. We hypothesized that redox reprogramming of 15-lipoxygenase (15-LOX) during the generation of pro-ferroptotic signal 15-hydroperoxy-eicosa-tetra-enoyl-phosphatidylethanolamine (15-HpETE-PE) modulates ferroptotic endurance. Here, we have discovered that inducible nitric oxide synthase (iNOS)/NO•-enrichment of activated M1 (but not alternatively activated M2) macrophages/microglia modulates susceptibility to ferroptosis. Genetic or pharmacologic depletion/inactivation of iNOS confers sensitivity on M1 cells, whereas NO• donors empower resistance of M2 cells to ferroptosis. In vivo, M1 phagocytes, in comparison to M2 phagocytes, exert higher resistance to pharmacologically induced ferroptosis. This resistance is diminished in iNOS-deficient cells in the pro-inflammatory conditions of brain trauma or the tumour microenvironment. The nitroxygenation of eicosatetraenoyl (ETE)-PE intermediates and oxidatively truncated species by NO• donors and/or suppression of NO• production by iNOS inhibitors represent a novel redox mechanism of regulation of ferroptosis in pro-inflammatory conditions.

Applying pressure on macrophages.

In this issue of Immunity, Cortez-Retamozo et al. (2013) demonstrate that the increased production of angiotensin II in tumor-bearing mice induces the expansion of macrophage progenitors and the supply of macrophages.

Neutrophils and PMN-MDSC: Their biological role and interaction with stromal cells.

Neutrophils and polymorphonucler myeloid-derived suppressor cells (PMN-MDSC) share origin and many morphological and phenotypic features. However, they have different biological role. Neutrophils are one of the major mechanisms of protection against invading pathogens, whereas PMN-MDSC have immune suppressive activity and restrict immune responses in cancer, chronic infectious disease, trauma, sepsis, and many other pathological conditions. Although in healthy adult individuals, PMN-MDSC are not or barely detectable, in patients with cancer and many other diseases they accumulate at various degree and co-exist with neutrophils. Recent advances allow for better distinction of these cells and better understanding of their biological role. Accumulating evidence indicates PMN-MDSC as pathologically activated neutrophils, with important role in regulation of immune responses. In this review, we provide an overview on the definition and characterization of PMN-MDSC and neutrophils, their pathological significance in a variety of diseases, and their interaction with other stromal components.

Transcriptional factor ATF3 protects against colitis by regulating follicular helper T cells in Peyer's patches.

Disruption of mucosal immunity plays a critical role in the pathogenesis of inflammatory bowel disease, yet its mechanism remains not fully elucidated. Here, we found that activating transcription factor 3 (ATF3) protects against colitis by regulating follicular helper T (TFH) cells in the gut. The expression of ATF3 in CD4+ T cells was negatively correlated with the severity of ulcerative colitis in clinical patients. Mice with ATF3 deficiency in CD4+ T cells (CD4creAtf3fl/fl ) were much more susceptible to dextran sulfate sodium-induced colitis. The frequencies of TFH cells, not other T cell subsets, were dramatically decreased in Peyer's patches from CD4creAtf3fl/fl mice compared with Atf3fl/fl littermate controls. The defective TFH cells significantly diminished germinal center formation and IgA production in the gut. Importantly, adoptive transfer of TFH or IgA+ B cells caused significant remission of colitis in CD4creAtf3fl/fl mice, indicating the TFH-IgA axis mediated the effect of ATF3 on gut homeostasis. Mechanistically, B cell lymphoma 6 was identified as a direct transcriptional target of ATF3 in CD4+ T cells. In summary, we demonstrated ATF3 as a regulator of TFH cells in the gut, which may represent a potential immunotherapeutic target in colitis.

Myc-mediated transcriptional regulation of the mitochondrial chaperone TRAP1 controls primary and metastatic tumor growth.

The role of mitochondria in cancer continues to be debated, and whether exploitation of mitochondrial functions is a general hallmark of malignancy or a tumor- or context-specific response is still unknown. Using a variety of cancer cell lines and several technical approaches, including siRNA-mediated gene silencing, ChIP assays, global metabolomics and focused metabolite analyses, bioenergetics, and cell viability assays, we show that two oncogenic Myc proteins, c-Myc and N-Myc, transcriptionally control the expression of the mitochondrial chaperone TNFR-associated protein-1 (TRAP1) in cancer. In turn, this Myc-mediated regulation preserved the folding and function of mitochondrial oxidative phosphorylation (OXPHOS) complex II and IV subunits, dampened reactive oxygen species production, and enabled oxidative bioenergetics in tumor cells. Of note, we found that genetic or pharmacological targeting of this pathway shuts off tumor cell motility and invasion, kills Myc-expressing cells in a TRAP1-dependent manner, and suppresses primary and metastatic tumor growth in vivo We conclude that exploitation of mitochondrial functions is a general trait of tumorigenesis and that this reliance of cancer cells on mitochondrial OXPHOS pathways could offer an actionable therapeutic target in the clinic.

Selective targeting of different populations of myeloid-derived suppressor cells by histone deacetylase inhibitors.

Myeloid-derived suppressor cells (MDSCs) are widely implicated in negative regulation of immune responses in cancer. Inhibition of class I histone deacetylases (HDAC) with entinostat has anti-MDSC activity. However, as single agent, it did not delay tumor growth in EL4 and LLC tumor models. Here, we found that entinostat reduced immune suppressive activity of only one type of MDSC-polymorphonuclear, PMN-MDSC, whereas it had no effect on monocytic M-MDSC or macrophages. M-MDSC had high amount of class II HDAC-HDAC6, which was further increased after the treatment of mice with entinostat. Inhibition of HDAC6 with ricolinostat reduced suppressive activity of M-MDSC, but did not affect PMN-MDSC or delayed tumor growth. However, combination of entinostat and ricolinostat abrogated suppressive activity of both populations of MDSC and substantially delayed tumor progression. Thus, inactivation of MDSC required targeting of both major subsets of these cells via inhibitors of class I and class II HDAC.

Randomized-controlled phase II trial of salvage chemotherapy after immunization with a TP53-transfected dendritic cell-based vaccine (Ad.p53-DC) in patients with recurrent small cell lung cancer.

Small cell lung cancer TP53 mutations lead to expression of tumor antigens that elicits specific cytotoxic T-cell immune responses. In this phase II study, dendritic cells transfected with wild-type TP53 (vaccine) were administered to patients with extensive-stage small cell lung cancer after chemotherapy. Patients were randomized 1:1:1 to arm A (observation), arm B (vaccine alone), or arm C (vaccine plus all-trans-retinoic acid). Vaccine was administered every 2 weeks (3 times), and all patients were to receive paclitaxel at progression. Our primary endpoint was overall response rate (ORR) to paclitaxel. The study was not designed to detect overall response rate differences between arms. Of 69 patients enrolled (performance status 0/1, median age 62 years), 55 were treated in stage 1 (18 in arm A, 20 in arm B, and 17 in arm C) and 14 in stage 2 (arm C only), per 2-stage Simon Minimax design. The vaccine was safe, with mostly grade 1/2 toxicities, although 1 arm-B patient experienced grade 3 fatigue and 8 arm-C patients experienced grade 3 toxicities. Positive immune responses were obtained in 20% of arm B (95% confidence interval [CI], 5.3-48.6) and 43.3% of arm C (95% CI 23.9-65.1). The ORRs to the second-line chemotherapy (including paclitaxel) were 15.4% (95% CI 2.7-46.3), 16.7% (95% CI 2.9-49.1), and 23.8% (95% CI 9.1-47.5) for arms A, B, and C, with no survival differences between arms. Although our vaccine failed to improve ORRs to the second-line chemotherapy, its safety profile and therapeutic immune potential remain. Combinations with the other immunotherapeutic agents are reasonable options.

The Nature of Myeloid-Derived Suppressor Cells in the Tumor Microenvironment.

Myeloid-derived suppressor cells (MDSC) are one of the major components of the tumor microenvironment. The main feature of these cells is their potent immune suppressive activity. MDSC are generated in the bone marrow and, in tumor-bearing hosts, migrate to peripheral lymphoid organs and the tumor to contribute to the formation of the tumor microenvironment. Recent findings have revealed differences in the function and fate of MDSC in the tumor and peripheral lymphoid organs. We review these findings here and, in this context, we discuss the current understanding as to the nature of these differences, the underlying mechanisms, and their potential impact on the regulation of tumor progression.

Notch and wingless signaling cooperate in regulation of dendritic cell differentiation.

Dendritic cell (DC) differentiation is regulated by stroma via a network of soluble and cell-bound factors. Notch is one of the major elements of this network. Its role in DC differentiation, however, is controversial. Here, we demonstrate that activation of Notch signaling in hematopoietic progenitor cells (HPCs) promoted differentiation of conventional DCs via activation of the canonical Wingless (Wnt) pathway. Inhibition of the Wnt pathway abrogated the effect of Notch on DC differentiation. The fact that activation of the Wnt pathway in Notch-1-deficient embryonic stem cells restored DC differentiation indicates that Wnt signaling is downstream of the Notch pathway in regulating DC differentiation. Notch signaling activated the Wnt pathway in HPCs via expression of multiple members of the Frizzled family of Wnt receptors, which was directly regulated by the CSL (RPB-Jkappa) transcription factor. Thus, these data suggest a model of DC differentiation via cooperation between Wnt and Notch pathways.

Regulation of tumor metastasis by myeloid-derived suppressor cells.

Accumulation of pathologically activated immature myeloid cells with potent immune-suppressive activity is one of the major immunological hallmarks of cancer. In recent years, it became clear that in addition to their immune-suppressive activity, myeloid-derived suppressor cells (MDSCs) influence tumor progression in a variety of ways. They are directly implicated in the promotion of tumor metastases by participating in the formation of premetastatic niches, promoting angiogenesis and tumor cell invasion. In this review, we discuss recent data describing various roles of MDSCs in the formation of tumor metastases.

GVHD-associated, inflammasome-mediated loss of function in adoptively transferred myeloid-derived suppressor cells.

Myeloid-derived suppressor cells (MDSCs) are a naturally occurring immune regulatory population associated with inhibition of ongoing inflammatory responses. In vitro generation of MDSCs from bone marrow has been shown to enhance survival in an acute model of lethal graft-versus-host disease (GVHD). However, donor MDSC infusion only partially ameliorates GVHD lethality. In order to improve the potential therapeutic benefit and ultimately survival outcomes, we set out to investigate the fate of MDSCs after transfer in the setting of acute GVHD (aGVHD). MDSCs transferred to lethally irradiated recipients of allogeneic donor hematopoietic grafts are exposed to an intense inflammatory environment associated with aGVHD, which we now show directly undermines their suppressive capacity. Under a conditioning regimen and GVHD inflammatory settings, MDSCs rapidly lose suppressor function and their potential to inhibit GVHD lethality, which is associated with their induced conversion toward a mature inflammasome-activated state. We find even brief in vitro exposure to inflammasome-activating mediators negates the suppressive potential of cultured murine and human-derived MDSCs. Consistent with a role for the inflammasome, donor MDSCs deficient in the adaptor ASC (apoptosis-associated speck-like protein containing a CARD), which assembles inflammasome complexes, conferred improved survival of mice developing GVHD compared with wild-type donor MDSCs. These data suggest the use of MDSCs as a therapeutic approach for preventing GVHD and other systemic inflammatory conditions will be more effective when combined with approaches limiting in vivo MDSC inflammasome activation, empowering MDSCs to maintain their suppressive potential.

COX-1-derived thromboxane A2 plays an essential role in early B-cell development via regulation of JAK/STAT5 signaling in mouse.

Cyclooxygenases (COXs) and their prostanoid products play important roles in a diverse range of physiological processes, including in the immune system. Here, we provide evidence that COX-1 is an essential regulator in early stages of B-cell development. COX-1-deficient mice displayed systematic reduction in total B cells, which was attributed to the arrest of early B-cell development from pro-B to pre-B stage. We further demonstrated that this defect was mediated through downregulation of the Janus kinase/signal transducer and activator of transcription 5 (JAK/STAT5) signaling and its target genes, including Pax5, in COX-1(-/-) mice. Mechanistic studies revealed that COX-1-derived thromboxane A2 (TxA2) could regulate JAK3/STAT5 signaling through the cyclic adenosine monophosphate-protein kinase A pathway, via binding with its receptor thromboxane A2 receptor (TP). Administration of the TP agonist could rescue the defective B-cell development and JAK/STAT5 signaling activity in COX-1-deficient mice. Moreover, administration of low-dose aspirin caused a significant reduction in total B cells in peripheral blood of healthy human volunteers, coincidentally with reduced TxA2 production and downregulation of JAK/STAT5 signaling. Taken together, our results demonstrate that COX-1-derived TxA2 plays a critical role in the stage transition of early B-cell development through regulation of JAK/STAT5 signaling and indicate a potential immune-suppressive effect of low-dose aspirin in humans.

Oxidized lipids block antigen cross-presentation by dendritic cells in cancer.

Cross-presentation is one of the main features of dendritic cells (DCs), which is critically important for the development of spontaneous and therapy-inducible antitumor immune responses. Patients, at early stages of cancer, have normal presence of DCs. However, the difficulties in the development of antitumor responses in patients with low tumor burden raised the question of the mechanisms of DC dysfunction. In this study, we found that, in differentiated DCs, tumor-derived factors blocked the cross-presentation of exogenous Ags without inhibiting the Ag presentation of endogenous protein or peptides. This effect was caused by intracellular accumulation of different types of oxidized neutral lipids: triglycerides, cholesterol esters, and fatty acids. In contrast, the accumulation of nonoxidized lipids did not affect cross-presentation. Oxidized lipids blocked cross-presentation by reducing the expression of peptide-MHC class I complexes on the cell surface. Thus, this study suggests the novel role of oxidized lipids in the regulation of cross-presentation.