Myeloid-derived suppressor cells (MDSCs IntroductionA major barrier to effective cancer immunotherapy is immune suppression, and the accumulation of myeloid-derived suppressor cells (MDSCs) has recently been recognized as a major mechanism to promote immune suppression (1, 2). MDSCs comprise a mixture of myeloid cells reflecting various stages of differentiation, and in mouse models, these cells are typically distinguished from other inhibitory myeloid populations based on their unique coexpression of macrophage (CD11b) and granulocyte (Gr-1) markers (1).Tumor-induced MDSCs are further dichotomized into monocytic and granulocytic subsets based on the differential expression of the Ly6G and Ly6C epitopes (3, 4). Intriguingly, granulocytic MDSCs outnumber monocytic MDSCs in numerous mouse tumor models (3, 5), although the basis for this subset dichotomy remains unclear. The phenotypes in humans are more complex and vary with tumor type. However, there is general agreement that a common lineage-negative MDSC subset observed among a range of human cancers bears the core phenotype CD33 + HLA-DR -(6-11). Interestingly, this subset resembles promyelocytes, a granulocytic population reflecting an early stage of differentiation (6, 7).Although many studies have been dedicated to the phenotypic characterization of MDSCs and unraveling mechanisms by which these cells mediate tumor progression, a large gap remains in our understanding of the mechanisms that initiate their development. It is known, however, that MDSC subsets emerge in response to tumor-derived factors (TDFs) and the signaling pathways these molecules engage. As a number of TDFs engage the STAT3 or STAT5 signaling pathway, STAT3 or STAT5 activation has been associated with various stages in MDSC biology (1,(12)(13)(14)(15)(16)(17)(18)(19).

We show here that fundamental aspects of antitumor immunity in mice are significantly influenced by ambient housing temperature. Standard housing temperature for laboratory mice in research facilities is mandated to be between 20-26°C; however, these subthermoneutral temperatures cause mild chronic cold stress, activating thermogenesis to maintain normal body temperature. When stress is alleviated by housing at thermoneutral ambient temperature (30-31°C), we observe a striking reduction in tumor formation, growth rate and metastasis. This improved control of tumor growth is dependent upon the adaptive immune system. We observe significantly increased numbers of antigen-specific CD8 + T lymphocytes and CD8 + T cells with an activated phenotype in the tumor microenvironment at thermoneutrality. At the same time there is a significant reduction in numbers of immunosuppressive MDSCs and regulatory T lymphocytes. Notably, in temperature preference studies, tumor-bearing mice select a higher ambient temperature than non-tumor-bearing mice, suggesting that tumor-bearing mice experience a greater degree of coldstress. Overall, our data raise the hypothesis that suppression of antitumor immunity is an outcome of cold stress-induced thermogenesis. Therefore, the common approach of studying immunity against tumors in mice housed only at standard room temperature may be limiting our understanding of the full potential of the antitumor immune response. murine tumor models | metabolism

Myeloid-derived suppressor cells (MDSC) are induced under diverse pathologic conditions, including neoplasia, and suppress innate and adaptive immunity. While the mechanisms by which MDSC mediate immunosuppression are well-characterized, details on how they develop remain less understood. This is complicated further by the fact that MDSC comprise multiple myeloid cell types, namely monocytes and granulocytes, reflecting diverse stages of differentiation and the proportion of these subpopulations vary among different neoplastic models. Thus, it is thought that the type and quantities of inflammatory mediators generated during neoplasia dictate the composition of the resultant MDSC response. Although much interest has been devoted to monocytic MDSC biology, a fundamental gap remains in our understanding of the derivation of granulocytic MDSC. In settings of heightened granulocytic MDSC responses, we hypothesized that inappropriate production of G-CSF is a key initiator of granulocytic MDSC accumulation. We observed abundant amounts of G-CSF in vivo, which correlated with robust granulocytic MDSC responses in multiple tumor models. Using G-CSF loss- and gain-of-function approaches, we demonstrated for the first time that: 1) abrogating G-CSF production significantly diminished granulocytic MDSC accumulation and tumor growth; 2) ectopically over-expressing G-CSF in G-CSF-negative tumors significantly augmented granulocytic MDSC accumulation and tumor growth; and 3) treatment of naïve healthy mice with recombinant G-CSF protein elicited granulocytic-like MDSC remarkably similar to those induced under tumor-bearing conditions. Collectively, we demonstrated that tumor-derived G-CSF enhances tumor growth through granulocytic MDSC-dependent mechanisms. These findings provide us with novel insights into MDSC subset development and potentially new biomarkers or targets for cancer therapy.

SUMMARY The co-engagement of fragment crystallizable (Fc) gamma receptors (FcγRs) with the Fc region of recombinant immunoglobulin monoclonal antibodies (mAbs) and its contribution to therapeutic activity has been extensively studied. For example, Fc-FcγR interactions have been shown to be important for mAb-directed effector cell activities, as well as mAb-dependent forward signaling into target cells via receptor clustering. Here we identify a function of mAbs targeting T cell-expressed antigens that involves FcγR co-engagement on antigen-presenting cells (APCs). In the case of mAbs targeting CTLA-4 and TIGIT, the interaction with FcγR on APCs enhanced antigen-specific T cell responses and tumoricidal activity. This mechanism extended to an anti-CD45RB mAb, which led to FcγR-dependent regulatory T cell expansion in mice.

Myeloid-derived suppressor cells (MDSC) induced during neoplasia display potent pro-tumorigenic activities. Tumor-derived factors influence MDSC development, yielding monocytic and granulocytic subsets. In contrast to monocytic MDSC, little is known about how granulocytic MDSC develop. We demonstrated that tumor-derived G-CSF drives granulocytic MDSC formation, thus providing new insights into myeloid-tumor biology.