Thomas Condamine

HIF-1α regulates function and differentiation of myeloid-derived suppressor cells in the tumor microenvironment.

The hypoxic environment of tumors dictates the phenotype of local myeloid-derived suppressor cells (MDSCs) via HIF-1a expression; hypoxia converts splenic MDSCs from specific into nonspecific suppressors.

Molecular mechanisms regulating myeloid-derived suppressor cell differentiation and function

Myeloid-derived suppressor cells (MDSCs) are one of the main cell populations responsible for regulating immune responses. MDSCs accumulate during tumor progression, autoimmunity, chronic infection and other pathological conditions, and can potently suppress T cell function. Recent studies have demonstrated the ability of MDSCs to modulate the activity of NK and myeloid cells and have implicated MDSCs in the induction of regulatory T cells. Here, we discuss recent findings that describe the molecular mechanisms that regulate the expansion and function of MDSCs, as well as recent attempts to use MDSCs in cell therapy for different pathologic conditions.

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, Ly6ChiLy6G− 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.

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.

ER stress regulates myeloid-derived suppressor cell fate through TRAIL-R–mediated apoptosis

Myeloid-derived suppressor cells (MDSCs) dampen the immune response thorough inhibition of T cell activation and proliferation and often are expanded in pathological conditions. Here, we studied the fate of MDSCs in cancer. Unexpectedly, MDSCs had lower viability and a shorter half-life in tumor-bearing mice compared with neutrophils and monocytes. The reduction of MDSC viability was due to increased apoptosis, which was mediated by increased expression of TNF-related apoptosis-induced ligand receptors (TRAIL-Rs) in these cells. Targeting TRAIL-Rs in naive mice did not affect myeloid cell populations, but it dramatically reduced the presence of MDSCs and improved immune responses in tumor-bearing mice. Treatment of myeloid cells with proinflammatory cytokines did not affect TRAIL-R expression; however, induction of ER stress in myeloid cells recapitulated changes in TRAIL-R expression observed in tumor-bearing hosts. The ER stress response was detected in MDSCs isolated from cancer patients and tumor-bearing mice, but not in control neutrophils or monocytes, and blockade of ER stress abrogated tumor-associated changes in TRAIL-Rs. Together, these data indicate that MDSC pathophysiology is linked to ER stress, which shortens the lifespan of these cells in the periphery and promotes expansion in BM. Furthermore, TRAIL-Rs can be considered as potential targets for selectively inhibiting MDSCs.

Myeloid derived suppressor cells regulate growth of multiple myeloma by inhibiting T cells in bone marrow

Myeloid-derived suppressor cells (MDSC) are one of the major factors limiting the immune response in cancer. However, their role in bone marrow (BM), the site of primary localization of multiple myeloma (MM), is poorly understood. In this study, we found a significant accumulation of CD11b+CD14−CD33+ immunosuppressive MDSC in BM of patients with newly diagnosed MM. To assess the possible role of MDSC in MM, we used immunocompetent mouse models. Immunosuppressive MDSC accumulated in BM of mice as early as 1 wk after tumor inoculation. S100A9 knockout (KO) mice, which are deficient in their ability to accumulate MDSC in tumor-bearing hosts, demonstrated reduced MDSC accumulation in BM after injection of MM cells compared with wild-type mice. Growth of the immunogenic MM cells was significantly reduced in S100A9KO mice. This effect was associated with the accumulation of Ag-specific CD8+ T cells in BM and spleens of S100A9KO mice, but not wild-type mice, and was abrogated by the administration of anti-CD8 Ab or adoptive transfer of MDSC. Thus, the accumulation of MDSC at early stages of MM plays a critical role in MM progression and suggests that MDSC can be considered a possible therapeutic target in this disease.

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.

Lectin-type oxidized LDL receptor-1 distinguishes population of human polymorphonuclear myeloid-derived suppressor cells in cancer patients

PMN-MDSC in cancer patients can be distinguished from neutrophils by a genomic signature and by expression of the LOX-1 receptor. Stressing myeloid-derived suppressor cells in cancer Immunotherapies for cancer have shown promising results in part because they overcome the suppressive effects of the tumor microenvironment on immune cells. Condamine et al. now report that polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) can be distinguished from neutrophils in the same cancer patient by the expression of the lipid metabolism–related molecule lectin-type oxidized LDL receptor-1 (LOX-1). LOX-1–expressing neutrophils were nearly undetectable in healthy individuals but were found prominently in tumor tissues. Moreover, exposing neutrophils from healthy individuals to endoplasmic reticulum stress resulted in up-regulation of LOX-1 and increased suppressive function. These data support the specific targeting of LOX-1–expressing PMN-MDSC for cancer immunotherapy. Polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) are important regulators of immune responses in cancer and have been directly implicated in the promotion of tumor progression. However, the heterogeneity of these cells and the lack of distinct markers hamper the progress in understanding the biology and clinical importance of these cells. Using partial enrichment of PMN-MDSC with gradient centrifugation, we determined that low-density PMN-MDSC and high-density neutrophils from the same cancer patients had a distinct gene profile. The most prominent changes were observed in the expression of genes associated with endoplasmic reticulum (ER) stress. Unexpectedly, low-density lipoprotein (LDL) was one of the most increased regulators, and its receptor oxidized LDL receptor 1 (OLR1) was one of the most overexpressed genes in PMN-MDSC. Lectin-type oxidized LDL receptor-1 (LOX-1) encoded by OLR1 was practically undetectable in neutrophils in peripheral blood of healthy donors, whereas 5 to 15% of total neutrophils in cancer patients and 15 to 50% of neutrophils in tumor tissues were LOX-1+. In contrast to their LOX-1− counterparts, LOX-1+ neutrophils had gene signature, potent immunosuppressive activity, up-regulation of ER stress, and other biochemical characteristics of PMN-MDSCs. Moreover, induction of ER stress in neutrophils from healthy donors up-regulated LOX-1 expression and converted these cells to suppressive PMN-MDSCs. Thus, we identified a specific marker of human PMN-MDSC associated with ER stress and lipid metabolism, which provides new insights into the biology and potential therapeutic targeting of these cells.

Transcriptional regulation of myeloid-derived suppressor cells

Myeloid‐derived suppressor cells are a heterogeneous group of pathologically activated immature cells that play a major role in the negative regulation of the immune response in cancer, autoimmunity, many chronic infections, and inflammatory conditions, as well as in the regulation of tumor angiogenesis, tumor cell invasion, and metastases. Accumulation of myeloid‐derived suppressor cells is governed by a network of transcriptional regulators that could be combined into 2 partially overlapping groups: factors promoting myelopoiesis and preventing differentiation of mature myeloid cells and factors promoting pathologic activation of myeloid‐derived suppressor cells. In this review, we discuss the specific nature of these factors and their impact on myeloid‐derived suppressor cell development.

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.