Augusto C. Ochoa

Myeloid-derived suppressor cells (MDSCs) have emerged as major regulators of immune responses in cancer and other pathological conditions. In recent years, ample evidence supports key contributions of MDSC to tumour progression through both immune-mediated mechanisms and those not directly associated with immune suppression. MDSC are the subject of intensive research with >500 papers published in 2015 alone. However, the phenotypic, morphological and functional heterogeneity of these cells generates confusion in investigation and analysis of their roles in inflammatory responses. The purpose of this communication is to suggest characterization standards in the burgeoning field of MDSC research.

T cells infiltrating tumors have a decreased expression of signal transduction proteins, a diminished ability to proliferate, and a decreased production of cytokines. The mechanisms causing these changes have remained unclear. We demonstrated recently that peritoneal macrophages stimulated with interleukin 4 + interleukin 13 produce arginase I, which decreases the expression of the T-cell receptor CD3zeta chain and impairs T-cell responses. Using a 3LL murine lung carcinoma model we tested whether arginase I was produced in the tumor microenvironment and could decrease CD3zeta expression and impair T-cell function. The results show that a subpopulation of mature tumor-associated myeloid cells express high levels of arginase I, whereas tumor cells and infiltrating lymphocytes do not. Arginase I expression in the tumor was seen on day 7 after tumor injection. Tumor-associated myeloid cells also expressed high levels of cationic amino acid transporter 2B, which allowed them to rapidly incorporate L-Arginine (L-Arg) and deplete extracellular L-Arg in vitro. L-Arg depletion by tumor-associated myeloid cells blocked the re-expression of CD3zeta in stimulated T cells and inhibited antigen-specific proliferation of OT-1 and OT-2 cells. The injection of the arginase inhibitor N-hydroxy-nor-L-Arg blocked growth of s.c. 3LL lung carcinoma in mice. High levels of arginase I were also found in tumor samples of patients with non-small cell carcinoma. Therefore, arginase I production by mature myeloid cells in the tumor microenvironment may be a central mechanism for tumor evasion and may represent a target for new therapies.

L-arginine (L-Arg) plays a central role in several biologic systems including the regulation of T-cell function. L-Arg depletion by myeloid-derived suppressor cells producing arginase I is seen in patients with cancer inducing T-cell anergy. We studied how L-Arg starvation could regulate T-cell-cycle progression. Stimulated T cells cultured in the absence of L-Arg are arrested in the G0-G1phase of the cell cycle. This was associated with an inability of T cells to up-regulate cyclin D3 and cyclin-dependent kinase 4 (cdk4), but not cdk6, resulting in an impaired downstream signaling with a decreased phosphorylation of Rb protein and a low expression and binding of E2F1. Silencing of cyclin D3 reproduced the cell cycle arrest caused by L-Arg starvation. The regulation of cyclin D3 and cdk4 by L-Arg starvation occurs at transcriptional and posttranscriptional levels. Signaling through GCN2 kinase is triggered during amino acid starvation. Experiments demonstrated that T cells from GCN2 knock-out mice did not show a decreased proliferation and were able to up-regulate cyclin D3 when cultured in the absence of L-Arg. These results contribute to the understanding of a central mechanism by which cancer and other diseases characterized by high arginase I production may cause T-cell dysfunction.

Myeloid suppressor cells with high arginase activity are found in tumors and spleen of mice with colon and lung cancer. These cells, described as macrophages or immature dendritic cells, deplete arginine and impair T cell proliferation and cytokine production. Although arginase activity has been described in cancer patients, it is thought to originate from tumor cells metabolizing arginine to ornithine needed to sustain rapid cell proliferation. The goal of this study was to determine whether myeloid suppressor cells producing high arginase existed in renal cell carcinoma patients. Peripheral blood mononuclear cells from 123 patients with metastatic renal cell carcinoma, prior to treatment, were found to have a significantly increased arginase activity. These patients had a markedly decreased cytokine production and expressed low levels of T cell receptor CD3zeta chain. Cell separation studies showed that the increased arginase activity was limited to a specific subset of CD11b+, CD14-, CD15+ cells with a polymorphonuclear granulocyte morphology and markers, instead of macrophages or dendritic cells described in mouse models. Furthermore, these patients had low levels of arginine and high levels of ornithine in plasma. Depletion of the CD11b+, CD14- myeloid suppressor cells reestablished T cell proliferation and CD3zeta chain expression. These results showed, for the first time, the existence of suppressor myeloid cells producing arginase in human cancer patients. In addition, it supports the concept that blocking arginase may be an important step in the success of immunotherapy.

To the Editor:The recent study by Yang et al. (1) described antigen-specific immunosuppression by Gr-1+CD11b+ myeloid cells, which was mediated by the expression of CD80. This report continued a series of recent articles published in Cancer Research, which provided strong evidence in support of a critical role of these cells in tumor progression (1–6). Recent years have witnessed increasing interest in immunosuppressive cells of myeloid origin. During the last 18 months alone, more than 50 articles have appeared in peer-reviewed journals on this subject. Accumulation of these cells has been reported under pathologic conditions, including bacterial and parasitic infections, acute and chronic inflammation, and traumatic stress. However, most of the attention has been focused on the role of these cells in cancer. Immunosuppressive myeloid cells accumulate in large numbers in tumor-bearing mice, in practically all tested experimental models, as well as in patients with breast, lung, prostate, kidney, head and neck, and other types of cancer. These cells are produced in response to a variety of tumor-derived cytokines and are a heterogeneous mixture of myeloid cells at different stages of differentiation. The precise nature of the suppressor cell population (i.e., precursors of granulocytes, macrophages, dendritic cells, or early myeloid progenitors) depends on the tumor and tumor-derived factors of the hosts. Despite this heterogeneity, immunosuppressive myeloid cells share some common characteristics: lack or reduced expression of markers of mature myeloid cells, expression of both Gr-1 and CD11b molecules in mice, inability to differentiate into mature myeloid cells in the presence of tumor-derived factors, high levels of reactive oxygen species, and activation of arginase I and other molecules. Most importantly, these cells possess a high potential to suppress immune responses in vitro and in vivo. Immunosuppressive myeloid cells are now considered by many as a critical mechanism of tumor escape as well as an important immunosuppressive factor for other pathologic conditions.Because these cells play a key role in regulation of immunity, we feel it necessary to address one issue that causes confusion in this field. These cells lack a clear, unified name. In the literature, these cells have been called “immature myeloid cells” or “myeloid suppressor cells” (MSC). Although both of these names reflect the biology of cells, neither term is entirely accurate. The name “immature myeloid cells” implies that these cells are normal myeloid precursors. However, this may not be the case. Recent studies have shown clear differences in the biology of normal immature myeloid cells and the cells that accumulate in tumor-bearing hosts. In addition, this term does not reflect the most important feature of these cells: their ability to suppress immune responses. The name “MSC” implies that these cells include populations of mature myeloid cells, such as macrophages or dendritic cells, capable of displaying some immunosuppressive features under certain circumstances. However, this name is also not accurate, being too generic and potentially misleading because these cells are not mature myeloid cells. In addition, the abbreviation “MSC” is commonly used for the characterization of mesenchymal stem cells. We believe that the lack of an accurate name for these cells creates confusion and hampers attempts to develop a cohesive picture of the mechanisms of immunosuppression in cancer and other pathologic conditions.Therefore, we suggest that these cells be called “myeloid-derived suppressor cells”. We believe that this term more closely reflects the origin and function of these cells and hope that it will stimulate further scientific discussions and progress not only in immunology but also in cancer biology where undoubtedly the same or similar cell populations play a major functional role.