Myeloid-derived suppressor cells (MDSC) producing arginase I are increased in the peripheral blood of patients with renal cell carcinoma (RCC). MDSC inhibit T-cell function by reducing the availability of L-arginine and are therefore considered an important tumor escape mechanism. We aimed to determine the origin of arginase I-producing MDSC in RCC patients and to identify the mechanisms used to deplete extracellular L-arginine. The results show that human MDSC are a subpopulation of activated polymorphonuclear (PMN) cells expressing high levels of CD66b, CD11b, and VEGFR1 and low levels of CD62L and CD16. In contrast to murine MDSC, human MDSC do not deplete L-arginine by increasing its uptake but instead release arginase I into the circulation. Activation of normal PMN induces phenotypic and functional changes similar to MDSC and also promotes the release of arginase I from intracellular granules. Interestingly, although activation of normal PMN usually ends with apoptosis, MDSC showed no increase in apoptosis compared with autologous PMN or PMN obtained from normal controls. High levels of VEGF have been shown to increase suppressor immature myeloid dendritic cells in cancer patients. Treatment of RCC patients with anti-VEGF antibody bevacizumab, however, did not reduce the accumulation of MDSC in peripheral blood. In contrast, the addition of interleukin-2 to the treatment increased the number of MDSC in peripheral blood and the plasma levels of arginase I. These results may provide new insights on the mechanisms of tumor-induced anergy/tolerance and may help explain why some immunotherapies fail to induce an antitumor response. [Cancer Res 2009;69(4):1553-60]

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.

Myeloid suppressor cells (MSCs) producing high levels of arginase I block T cell function by depleting l-arginine in cancer, chronic infections, and trauma patients. In cancer, MSCs infiltrating tumors and in circulation are an important mechanism for tumor evasion and impair the therapeutic potential of cancer immunotherapies. However, the mechanisms that induce arginase I in MSCs in cancer are unknown. Using the 3LL mouse lung carcinoma, we aimed to characterize these mechanisms. Arginase I expression was independent of T cell–produced cytokines. Instead, tumor-derived soluble factors resistant to proteases induced and maintained arginase I expression in MSCs. 3LL tumor cells constitutively express cyclooxygenase (COX)-1 and COX-2 and produce high levels of PGE2. Genetic and pharmacological inhibition of COX-2, but not COX-1, blocked arginase I induction in vitro and in vivo. Signaling through the PGE2 receptor E-prostanoid 4 expressed in MSCs induced arginase I. Furthermore, blocking arginase I expression using COX-2 inhibitors elicited a lymphocyte-mediated antitumor response. These results demonstrate a new pathway of prostaglandin-induced immune dysfunction and provide a novel mechanism that can help explain the cancer prevention effects of COX-2 inhibitors. Furthermore, an addition of arginase I represents a clinical approach to enhance the therapeutic potential of cancer immunotherapies.

Myeloid-derived suppressor cells (MDSC) promote tumor growth by inhibiting T-cell immunity and promoting malignant cell proliferation and migration. The therapeutic potential of blocking MDSCs in tumors has been limited by their heterogeneity, plasticity, and resistance to various chemotherapy agents. Recent studies have highlighted the role of energy metabolic pathways in the differentiation and function of immune cells; however, the metabolic characteristics regulating MDSCs remain unclear. We aimed to determine the energy metabolic pathway(s) used by MDSCs, establish its impact on their immunosuppressive function, and test whether its inhibition blocks MDSCs and enhances antitumor therapies. Using several murine tumor models, we found that tumor-infiltrating MDSCs (T-MDSC) increased fatty acid uptake and activated fatty acid oxidation (FAO). This was accompanied by an increased mitochondrial mass, upregulation of key FAO enzymes, and increased oxygen consumption rate. Pharmacologic inhibition of FAO blocked immune inhibitory pathways and functions in T-MDSCs and decreased their production of inhibitory cytokines. FAO inhibition alone significantly delayed tumor growth in a T cell-dependent manner and enhanced the antitumor effect of adoptive T-cell therapy. Furthermore, FAO inhibition combined with low-dose chemotherapy completely inhibited T-MDSCs immunosuppressive effects and induced a significant antitumor effect. Interestingly, a similar increase in fatty acid uptake and expression of FAO-related enzymes was found in human MDSCs in peripheral blood and tumors. These results support the possibility of testing FAO inhibition as a novel approach to block MDSCs and enhance various cancer therapies.

Tumor-induced tolerance is a well-established phenomenon in cancer patients that can severely impair the therapeutic efficacy of immunotherapy. One mechanism leading to T-cell tolerance is the generation of myeloid-derived suppressor cells (MDSC) by soluble factors produced by the tumor. MDSC express CD11b + as a common marker but may vary in their stage of maturation, depending on the tumor factors being produced. Arginase production by MDSC depletes arginine from the tumor microenvironment and impairs T-cell signal transduction and function. We studied whether an increase in MDSC could explain the molecular alterations and dysfunction found in T cells of patients with renal cell carcinoma (RCC). Arginase activity in the peripheral blood mononuclear cells of 117 RCC patients was increased between 6-to 8-fold compared with normal controls. The increased arginase activity was limited to the CD11b + CD14 À myeloid cells and resulted in significantly decreased serum levels of arginine and increased ornithine in patients. Depletion of MDSC restored IFN-g production and T-cell proliferation. Preliminary data suggest that prostaglandin E 2 produced by the tumor induces arginase I expression in MDSC. Therefore, blocking MDSC activity may enhance the therapeutic efficacy of immunotherapy in RCC. Immunotherapy with cytokines, such as interleukin-2 (IL-2),has become a standard of care for patients with renal cell carcinoma (RCC). However, only 20% to 30% of patients have a partial or complete response (1), which is significantly lower than the therapeutic efficacy suggested by animal models. One possible explanation is that tumor-induced tolerance diminishes the potential therapeutic effect of T cells, the principal effector cells in most forms of immunotherapy. Over the last decade, several mechanisms by which tumors escape the immune response have been described. These can be divided into three major groups: alterations in antigen expression in tumor cells to make them less detectable, the active suppression of dendritic and T-cell function by inhibitory molecules or factors produced by tumors, and the induction of cells that can suppress the immune response, including myeloid-derived suppressor cells (MDSC) and regulatory T cells. We have focused our work on characterizing MDSC and the mechanisms by which they cause T-cell dysfunction in cancer.