IDO (indoleamine 2,3-dioxygenase) enzyme inhibitors have entered clinical trials for cancer treatment based on preclinical studies indicating that they can defeat immune escape and broadly enhance other therapeutic modalities. However, clear genetic evidence of IDO’s impact on tumorigenesis in physiologic models of primary or metastatic disease is lacking. Investigating the impact of Ido1 gene disruption in mouse models of oncogenic KRAS-induced lung carcinoma and breast carcinoma-derived pulmonary metastasis, we have found that IDO-deficiency resulted in reduced lung tumor burden and improved survival in both models. Micro-CT imaging further revealed that the density of the underlying pulmonary blood vessels was significantly reduced in Ido1-nullizygous mice. During lung tumor and metastasis outgrowth, IL6 induction was greatly attenuated in conjunction with the loss of IDO. Biologically, this resulted in a consequential impairment of pro-tumorigenic MDSCs (myeloid-derived suppressor cells), as restoration of IL6 recovered both MDSC suppressor function and metastasis susceptibility in Ido1-nullizygous mice. Together, our findings define IDO as a prototypical integrative modifier that bridges inflammation, vascularization and immune escape to license primary and metastatic tumor outgrowth.

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that suppress innate and adaptive immunity. MDSCs are present in many disease settings; however, in cancer, they are a major obstacle for both natural antitumor immunity and immunotherapy. Tumor and host cells in the tumor microenvironment (TME) produce a myriad of pro-inflammatory mediators that activate MDSCs and drive their accumulation and suppressive activity. MDSCs utilize a variety of mechanisms to suppress T cell activation, induce other immune-suppressive cell populations, regulate inflammation in the TME, and promote the switching of the immune system to one that tolerates and enhances tumor growth. Because MDSCs are present in most cancer patients and are potent immune-suppressive cells, MDSCs have been the focus of intense research in recent years. This review describes the history and identification of MDSCs, the role of inflammation and intracellular signaling events governing MDSC accumulation and suppressive activity, immune-suppressive mechanisms utilized by MDSCs, and recent therapeutics that target MDSCs to enhance antitumor immunity.

Chronic inflammation often precedes malignant transformation and later drives tumor progression. Likewise, subversion of the immune system plays a role in tumor progression, with tumoral immune escape now well recognized as a crucial hallmark of cancer. Myeloid-derived suppressor cells (MDSC) are elevated in most individuals with cancer, where their accumulation and suppressive activity are driven by inflammation. Thus, MDSC may define an element of the pathogenic inflammatory processes that driven immune escape. The secreted alarmin HMGB1 is a pro-inflammatory partner, inducer and chaperone for many pro-inflammatory molecules that MDSC development. Therefore, in this study we examined HMGB1 as a potential regulator of MDSC. In murine tumor systems, HMGB1 was ubiquitous in the tumor microenvironment, activating the NF-κB signal transduction pathway in MDSC and regulating their quantity and quality. We found that HMGB1 foments the development of MDSC from bone marrow progenitor cells, contributing to their ability to suppress antigen-driven activation of CD4+ and CD8+ T cells. Further, HMGB1 increased MDSC-mediated production of IL-10, enhanced crosstalk between MDSC and macrophages and facilitated the ability of MDSC to down-regulate expression of the naive T cell homing receptor L-selectin. Overall, our results revealed a pivotal role for HMGB1 in the development and cancerous contributions of MDSC in cancer patients.

MDSC and macrophages are present in most solid tumors and are important drivers of immune suppression and inflammation. It is established that cross-talk between MDSC and macrophages impacts anti-tumor immunity; however, interactions between tumor cells and MDSC or macrophages are less well studied. To examine potential interactions between these cells, we studied the impact of MDSC, macrophages, and four murine tumor cell lines on each other, both in vitro and in vivo. We focused on IL-6, IL-10, IL-12, TNF-α, and NO, as these molecules are produced by macrophages, MDSC, and many tumor cells; are present in most solid tumors; and regulate inflammation. In vitro studies demonstrated that MDSC-produced IL-10 decreased macrophage IL-6 and TNF-α and increased NO. IL-6 indirectly regulated MDSC IL-10. Tumor cells increased MDSC IL-6 and vice versa. Tumor cells also increased macrophage IL-6 and NO and decreased macrophage TNF-α. Tumor cell-driven macrophage IL-6 was reduced by MDSC, and tumor cells and MDSC enhanced macrophage NO. In vivo analysis of solid tumors identified IL-6 and IL-10 as the dominant cytokines and demonstrated that these molecules were produced predominantly by stromal cells. These results suggest that inflammation within solid tumors is regulated by the ratio of tumor cells to MDSC and macrophages and that interactions of these cells have the potential to alter significantly the inflammatory milieu within the tumor microenvironment.

Phosphoinositides (PIs) are signaling molecules that regulate cellular events including vesicle targeting and interactions between membrane and cytoskeleton. Phosphatidylinositol (PtdIns)(4,5)P 2 is one of the best characterized PIs; studies in which PtdIns(4,5)P 2 localization or concentration is altered lead to defects in the actin cytoskeleton and exocytosis. PtdIns(4,5)P 2 and its derivative Ins(1,4,5)P 3 accumulate in salt, cold, and osmotically stressed plants. PtdIns(4,5)P 2 signaling is terminated through the action of inositol polyphosphate phosphatases and PI phosphatases including supressor of actin mutation (SAC) domain phosphatases. In some cases, these phosphatases also act on Ins(1,4,5)P 3 . We have characterized the Arabidopsis (Arabidopsis thaliana) sac9 mutants. The SAC9 protein is different from other SAC domain proteins in several ways including the presence of a WW protein interaction domain within the SAC domain. The rice (Oryza sativa) and Arabidopsis SAC9 protein sequences are similar, but no apparent homologs are found in nonplant genomes. High-performance liquid chromatography studies show that unstressed sac9 mutants accumulate elevated levels of PtdIns(4,5)P 2 and Ins(1,4,5)P 3 as compared to wildtype plants. The sac9 mutants have characteristics of a constitutive stress response, including dwarfism, closed stomata, and anthocyanin accumulation, and they overexpress stress-induced genes and overaccumulate reactive-oxygen species. These results suggest that the SAC9 phosphatase is involved in modulating phosphoinsitide signals during the stress response.Phosphoinositides (PIs) are a family of eight molecules in which the hydroxyl groups on the inositol moiety can be phosphorylated in a variety of combinations (Stevenson et al., 2000;Meijer and Munnik, 2003;van Leeuwen et al., 2004). PIs undergo cycles of phosphorylation and dephosphorylation through organelle-specific PI kinases and phosphatases, leading to distinct subcellular distributions of PI species (De Matteis and Godi, 2004). PIs control the timing and location of many cellular events including vesicle targeting, interactions between the membrane and the cytoskeleton, membrane budding and fusing, nuclear and cytoplasmic signal transduction, and activity of membrane channels (Hilgemann and Ball, 1996;Martin, 1998;Czech, 2000;Odorizzi et al., 2000;Stevenson et al., 2000; Simonsen et al., 2001;Hardie, 2003;Meijer and Munnik, 2003;Oliver et al., 2004;van Leeuwen et al., 2004). Specific PI-binding sites have been found on a variety of effector proteins including protein kinases, actin-binding proteins, GTPases, and membrane trafficking proteins, and it is thought that binding to PIs can target effector proteins to specific membrane locations (Martin, 1998;Hu et al., 1999;Yao et al., 1999;Dowler et al., 2000;Tall et al., 2000;Ellson et al., 2002;Itoh and Takenawa, 2002).Unraveling the specific functions of the PI species and the enzymes that modify them is challenging for several reasons. Enzyme specificities do not always correlate ...