CCL2/CCR2 pathway mediates recruitment of myeloid suppressor cells to cancers

Home sweet home: the tumor microenvironment as a haven for regulatory T cells

CD4⁺Foxp3⁺ regulatory T cells (T_regs) have a fundamental role in maintaining immune balance by preventing autoreactivity and immune-mediated pathology. However this role of T_regs extends to suppression of anti-tumor immune responses and remains a major obstacle in the development of anti-cancer vaccines and immunotherapies. This feature of T_reg activity is exacerbated by the discovery that T_reg frequencies are not only elevated in the blood of cancer patients, but are also significantly enriched within tumors in comparison to other sites. These observations have sparked off the quest to understand the processes through which T_regs become elevated in cancer-bearing hosts and to identify the specific mechanisms leading to their accumulation within the tumor microenvironment. This manuscript reviews the evidence for specific mechanisms of intra-tumoral T_reg enrichment and will discuss how this information may be utilized for the purpose of manipulating the balance of tumor-infiltrating T cells in favor of anti-tumor effector cells.

Recruitment of γδ T lymphocytes to tumors: A new role for the pleiotropic chemokine CCL2

Despite the promise of targeting γδ T cells for cancer immunotherapy, the mechanisms underpinning the recruitment of this T-cell subsets to neoplastic lesions remain poorly understood. We have recently identified the pro-inflammatory chemokine CCL2 and its receptor CCR2 as key molecular determinants of γδ T-cell migration and tumor infiltration.

Premetastatic soil and prevention of breast cancer brain metastasis

BACKGROUND

As therapies for systemic cancer improve and patients survive longer, the risk for brain metastases increases. We evaluated whether immune mechanisms are involved in the development of brain metastasis.

METHODS

We conducted our studies using BALB/c mice bearing syngeneic 4T1 mammary adenocarcinoma cells in the mammary gland.

RESULTS

The brains of mice bearing 4T1 tumors at day 14 had no detectable metastatic tumor cells but presented with marked accumulation of bone marrow-derived CD11b(+)Gr1(+) myeloid cells, which express high levels of inflammatory chemokines S100A8 and S100A9. In vitro, S100A9 attracts 4T1 cells through Toll-like receptor 4 and CD11b(+)Gr1(+) myeloid cells through Toll-like receptor 4 and the receptor for advanced glycation end-products. Systemic treatment of 4T1-bearing mice with anti-Gr1 (RB6-8C5) monoclonal antibody reduces accumulation of CD11b(+)Gr1(+) myeloid cells in the day-14 premetastatic brain as well as subsequent brain metastasis of 4T1 cells detected on day 30. Furthermore, treatment of 4T1 tumor-bearing mice with the cyclooxygenase-2 inhibitor celecoxib or genetic disruption of cyclooxygenase-2 in 4T1 cells inhibits the inflammatory chemokines and infiltration of CD11b(+)Gr1(+) myeloid cells in the premetastatic brain and subsequent formation of brain metastasis.

CONCLUSIONS

Our results suggest that the primary tumor induces accumulation of CD11b(+)Gr1(+) myeloid cells in the brain to form "premetastatic soil" and inflammation mediators, such as S100A9, that attract additional myeloid cells as well as metastatic tumor cells. Celecoxib and anti-Gr1 treatment may be useful for blockade of these processes, thereby preventing brain metastasis in patients with breast cancer.

Chronic but not acute virus infection induces sustained expansion of myeloid suppressor cell numbers that inhibit viral-specific T cell immunity

Resolution of acute and chronic viral infections requires activation of innate cells to initiate and maintain adaptive immune responses. Here we report that infection with acute Armstrong (ARM) or chronic Clone 13 (C13) strains of lymphocytic choriomeningitis virus (LCMV) led to two distinct phases of innate immune response. During the first 72 hr of infection, dendritic cells upregulated activation markers and stimulated antiviral CD8(+) T cells, independent of viral strain. Seven days after infection, there was an increase in Ly6C(hi) monocytic and Gr-1(hi) neutrophilic cells in lymphoid organs and blood. This expansion in cell numbers was enhanced and sustained in C13 infection, whereas it occurred only transiently with ARM infection. These cells resembled myeloid-derived suppressor cells and potently suppressed T cell proliferation. The reduction of monocytic cells in Ccr2(-/-) mice or after Gr-1 antibody depletion enhanced antiviral T cell function. Thus, innate cells have an important immunomodulatory role throughout chronic infection.

Modulation of tumor immunity by soluble and membrane-bound molecules at the immunological synapse

To circumvent pathology caused by infectious microbes and tumor growth, the host immune system must constantly clear harmful microorganisms and potentially malignant transformed cells. This task is accomplished in part by T-cells, which can directly kill infected or tumorigenic cells. A crucial event determining the recognition and elimination of detrimental cells is antigen recognition by the T cell receptor (TCR) expressed on the surface of T cells. Upon binding of the TCR to cognate peptide-MHC complexes presented on the surface of antigen presenting cells (APCs), a specialized supramolecular structure known as the immunological synapse (IS) assembles at the T cell-APC interface. Such a structure involves massive redistribution of membrane proteins, including TCR/pMHC complexes, modulatory receptor pairs, and adhesion molecules. Furthermore, assembly of the immunological synapse leads to intracellular events that modulate and define the magnitude and characteristics of the T cell response. Here, we discuss recent literature on the regulation and assembly of IS and the mechanisms evolved by tumors to modulate its function to escape T cell cytotoxicity, as well as novel strategies targeting the IS for therapy.

Tumor necrosis factor-α blocks differentiation and enhances suppressive activity of immature myeloid cells during chronic inflammation

Elevated concentrations of tumor necrosis factor-α (TNF-α) are detected in pathologies characterized by chronic inflammation. Whether TNF-α plays a role in manipulating the host's immune system toward generating an immunosuppressive milieu, typical of ongoing chronic inflammation, is unclear. Here we showed that TNF-α exhibited a dual function during chronic inflammation: arresting differentiation of immature myeloid-derived suppressor cells (MDSCs) primarily via the S100A8 and S100A9 inflammatory proteins and their corresponding receptor (RAGE) and augmenting MDSC suppressive activity. These functions led to in vivo T and NK cell dysfunction accompanied by T cell antigen receptor ζ chain downregulation. Furthermore, administration of etanercept (TNF-α antagonist) during early chronic inflammatory stages reduced MDSCs' suppressive activity and enhanced their maturation into dendritic cells and macrophages, resulting in the restoration of in vivo immune functions and recovery of ζ chain expression. Thus, TNF has a fundamental role in promoting an immunosuppressive environment generated during chronic inflammation.

Mesenchymal contribution to recruitment, infiltration, and positioning of leukocytes in human melanoma tissues

To understand factors that regulate leukocyte entry and positioning within human melanoma tissues, we performed a multiparametric quantitative analysis of two separated regions: the intratumoral area and the peritumoral stroma. Using two mesenchymal markers, fibroblast activation protein (FAP) and CD90, we identified three subsets of mesenchymal cells (MCs): (i) intratumoral FAP(+)CD90(low/-) MC, (ii) peritumoral FAP(+)CD90(+) MC, and (iii) FAP(-)CD90(+) perivascular MC. We characterized CD90(+) MCs, which showed a stable CCL2-secretory phenotype when long-term expanded ex vivo, and heavily surrounded peritumoral Duffy antigen receptor for chemokine(+) (DARC) postcapillary venules, supporting a role for these vessels in peritumoral inflammatory leukocyte recruitment. Conversely, the intratumoral area was variably invaded by FAP(+)CD90(low/-) MCs that colocalized with a distinct extracellular matrix (ECM) network. A positive correlation was observed between intratumoral stromal cell/ECM networks and leukocyte infiltration among tumor cells (TCs), as well as in a stroma-dependent xenograft tumor model. Adoptively transferred T lymphocytes preferentially infiltrated tumors composed of TC+MC, compared with TCs only. Altogether, our results suggest that a variety of MCs contribute to regulate different steps of leukocyte tumor infiltration, that is, CD90(+) cells surrounding peritumoral vessels secrete CCL2 to recruit CCR2(+) leukocytes at the tumor periphery, whereas intratumoral FAP(+) cells organize a stromal scaffold that contact guide further invasion among densely packed tumor cells.

In vivo ablation of plasmacytoid dendritic cells inhibits autoimmunity through expansion of myeloid-derived suppressor cells

Autoimmunity ensues upon breakdown of tolerance mechanism and priming of self-reactive T cells. Plasmacytoid dendritic cells (pDCs) constitute a unique cell subset that participates in the activation of autoreactive T cells but also has been shown to be critically involved in the induction of self-tolerance. However, their functional importance during the priming phase of an organ-specific autoimmune response remains unclear. In this study, we demonstrate that absence of pDCs during myelin antigenic challenge resulted in amelioration of experimental autoimmune encephalomyelitis and reduced disease severity. This was accompanied by significantly decreased frequency of myelin-specific T cells in the draining lymph nodes and inhibition of Th1 and Th17 immune responses. Unexpectedly, in vivo ablation of pDCs increased myelopoiesis in the bone marrow and specifically induced the generation of CD11b(hi)Gr1(+) myeloid-derived suppressor cells (MDSCs). Furthermore, we demonstrate that pDC depletion enhanced the mobilization of MDSCs in the spleen, and that sorted MDSCs could potently suppress CD4(+) T cell responses in vitro. Importantly, pDC-depleted mice showed increased levels of MCP-1 in the draining lymph nodes, and in vivo administration of MCP-1 increased the frequency and absolute numbers of MDSCs in the periphery of treated mice. Together, our results reveal that absence of pDCs during the priming of an autoimmune response leads to increased mobilization of MDSCs in the periphery in an MCP-1-dependent manner and subsequent amelioration of autoimmunity.

Immunosuppression in inflammatory melanoma: can it be resisted by adoptively transferred T cells?

Discovery of tumor antigen (TA) recognized by autologous T cells (TCs) in patients with melanoma has led to clinical protocols using either vaccination or adoptive transfer of TA-specific TCs. However, efficacy of these treatments has been hampered by inhibitory effects exerted on tumor-infiltrating TCs by tumor-intrinsic mediators or by recruitment of immunosuppressive cells. A mouse model of autochthonous melanoma recapitulates some aspects of inflammatory melanoma development in patients. These include a systemic Th2-/Th17-oriented chronic inflammation, recruitment of immunosuppressive myeloid cells and acquisition by tumor-infiltrating TCs of an 'exhausted' phenotype characterized by expression of multiple inhibitory receptors including programmed death-1, also expressed on patients' melanoma-infiltrating TCs. Rather than using extracellular blocking reagents to inhibitory surface molecules on TCs, we sought to dampen negative signaling exerted on them. Adoptively transferred TCs presenting increased cytokine receptor signaling due to expression of an active Stat5 transcription factor were efficient at inducing melanoma regression in the preclinical melanoma model. These transferred TCs thrived and retained expression of effector molecules in the melanoma microenvironment, defining a protocol endowing TCs with the ability to resist melanoma-induced immunosuppression.

Myeloid-derived suppressor cells function as novel osteoclast progenitors enhancing bone loss in breast cancer

Enhanced bone destruction is a hallmark of various carcinomas such as breast cancer, where osteolytic bone metastasis is associated with increased morbidity and mortality. Immune cells contribute to osteolysis in cancer growth, but the factors contributing to aggressive bone destruction are not well understood. In this study, we show the importance of myeloid-derived suppressor cells (MDSC) in this process at bone metastatic sites. Because MDSC originate from the same myeloid lineage as macrophages, which are osteoclast precursors, we hypothesized that MDSC may undergo osteoclast differentiation and contribute to enhanced bone destruction and tumor growth. Using an immunocompetent mouse model of breast cancer bone metastasis, we confirmed that MDSC isolated from the tumor-bone microenvironment differentiated into functional osteoclasts both in vitro and in vivo. Mechanistic investigations revealed that nitric oxide signaling was critical for differentiation of MDSC into osteoclasts. Remarkably, osteoclast differentiation did not occur in MDSC isolated from control or tumor-bearing mice that lacked bone metastasis, signifying the essential cross-talk between tumor cells and myeloid progenitors in the bone microenvironment as a requirement for osteoclast differentiation of MDSC. Overall, our results identify a wholly new facet to the multifunctionality of MDSC in driving tumor progression, in this case as a novel osteoclast progenitor that specifically drives bone metastasis during cancer progression.

Anti-Gr-1 antibody depletion fails to eliminate hepatic myeloid-derived suppressor cells in tumor-bearing mice

Recent studies show that the liver is a preferred organ for the accumulation of MDSC. In this study, we examined the effect of systemic RB6-8C5 treatment on hepatic MDSC in tumor-bearing mice. EL4 tumor-bearing mice were injected i.p. with RB6-8C5, and hepatic, splenic, and blood MDSCs were analyzed by flow cytometry. Unexpectedly, hepatic MDSC remained in the liver, although RB6-8C5 completely eliminated them from the spleen and peripheral blood 24 h after treatment. Secondary antibody staining confirmed the presence of RB6-8C5-bound MDSC in the liver of mice with s.c. tumors. Similar observations were made in two other (colon and melanoma) tumor models. Whereas RB6-8C5 injection induced cell death of hepatic MDSC, as shown by Annexin V/7-AAD staining, these cells were replaced immediately, leading to a constant, increased frequency of hepatic MDSC. Adoptively transferred MDSC migrated preferentially to the liver after RB6-8C5 treatment, suggesting that hepatic MDSCs are reconstituted rapidly after depletion. Finally, hepatic MDSC remained immunosuppressive despite RB6-8C5 injection. Our study demonstrates that RB6-8C5 is not suitable for depletion of hepatic MDSCs and analysis of their function.

Epithelial-mesenchymal-transition-like and TGFβ pathways associated with autochthonous inflammatory melanoma development in mice

We compared gene expression signatures of aggressive amelanotic (Amela) melanomas with those of slowly growing pigmented melanomas (Mela), identifying pathways potentially responsible for the aggressive Amela phenotype. Both tumors develop in mice upon conditional deletion in melanocytes of Ink4a/Arf tumor suppressor genes with concomitant expression of oncogene H-Ras(G12V) and a known tumor antigen. We previously showed that only the aggressive Amela tumors were highly infiltrated by leukocytes concomitant with local and systemic inflammation. We report that Amela tumors present a pattern of de-differentiation with reduced expression of genes involved in pigmentation. This correlates with reduced and enhanced expression, respectively, of microphthalmia-associated (Mitf) and Pou3f2/Brn-2 transcription factors. The reduced expression of Mitf-controlled melanocyte differentiation antigens also observed in some human cutaneous melanoma has important implications for immunotherapy protocols that generally target such antigens. Induced Amela tumors also express Epithelial-Mesenchymal-Transition (EMT)-like and TGFβ-pathway signatures. These are correlated with constitutive Smad3 signaling in Amela tumors and melanoma cell lines. Signatures of infiltrating leukocytes and some chemokines such as chemotactic cytokine ligand 2 (Ccl2) that contribute to leukocyte recruitment further characterize Amela tumors. Inhibition of the mitogen-activated protein kinase (MAPK) activation pathway in Amela tumor lines leads to reduced expression of EMT hallmark genes and inhibits both proinflammatory cytokine Ccl2 gene expression and Ccl2 production by the melanoma cells. These results indicate a link between EMT-like processes and alterations of immune functions, both being controlled by the MAPK pathway. They further suggest that targeting the MAPK pathway within tumor cells will impact tumor-intrinsic oncogenic properties as well as the nature of the tumor microenvironment.

Anti-cancer versus cancer-promoting effects of the interleukin-17-producing T helper cells

Research on T helper 17 (Th17) cells with regard to immunoediting has revealed elusive results. Whereas enhanced Th17 response and related molecules such as interleukin (IL)-17, IL-21, IL-22, IL-23 and STAT3 accompanied tumor induction and progression, finding that tumor growth/stage was negatively correlated with increased infiltration of Th17 cells in the tumor mass has prompted elucidation of various antitumor mechanisms elicited by Th17 and their related molecules. The pro-tumor efficacy of Th17 response included promotion of neutrophilia and induction of angiogenic (e.g. VEGF, MMP2 and MMP9) and anti-apoptotic factors (e.g. Bcl-XL), as well as expansion and activation of myeloid-derived suppressor cells, which facilitate generation of tumor-specific regulatory T cells. Other tumor immunogenic settings revealed anti-tumor pathways including induction of cytotoxic activity, expression of MHC antigens, the ability Th17 cells to reside within the tumor, and to convert into IFN-γ producers. Notably, Th17 cell related molecules exert indirect pro- or anti-tumor effects via inducing viral persistence or mediating protective mechanisms against bacterial and viral infection. Herein, the recent literature revealing such immunoediting events mediated by Th17 cells and their associated molecules as delivered by various experimental regimens and observed in cancer patient are revised, with a focus on some proposed anti-cancer therapies.

Impact of temozolomide on immune response during malignant glioma chemotherapy

Malignant glioma, or glioblastoma, is the most common and lethal form of brain tumor with a median survival time of 15 months. The established therapeutic regimen includes a tripartite therapy of surgical resection followed by radiation and temozolomide (TMZ) chemotherapy, concurrently with radiation and then as an adjuvant. TMZ, a DNA alkylating agent, is the most successful antiglioma drug and has added several months to the life expectancy of malignant glioma patients. However, TMZ is also responsible for inducing lymphopenia and myelosuppression in malignant glioma patients undergoing chemotherapy. Although TMZ-induced lymphopenia has been attributed to facilitate antitumor vaccination studies by inducing passive immune response, in general lymphopenic conditions have been associated with poor immune surveillance leading to opportunistic infections in glioma patients, as well as disrupting active antiglioma immune response by depleting both T and NK cells. Deletion of O6-methylguanine-DNA-methyltransferase (MGMT) activity, a DNA repair enzyme, by temozolomide has been determined to be the cause of lymphopenia. Drug-resistant mutation of the MGMT protein has been shown to render chemoprotection against TMZ. The immune modulating role of TMZ during glioma chemotherapy and possible mechanisms to establish a strong TMZ-resistant immune response have been discussed.

Nitric oxide-producing myeloid-derived suppressor cells inhibit vascular E-selectin expression in human squamous cell carcinomas

Squamous cell carcinomas (SCC) are sun-induced skin cancers that are particularly numerous and aggressive in immunosuppressed individuals. SCC evade immune detection at least in part by down-regulating E-selectin on tumor vessels, thereby restricting entry of skin homing T cells into tumors. We find that nitric oxide potently suppresses E-selectin expression on human endothelial cells and that SCC are infiltrated by nitric oxide-producing iNOS⁺ CD11b⁺ CD33⁺ CD11c⁻ HLA-DR⁻ myeloid-derived suppressor cells (MDSC). MDSC from SCC produced NO, TGFβ and arginase and inhibited endothelial E-selectin expression in vitro. MDSC from SCC expressed the chemokine receptor CCR2 and tumors expressed the CCR2 ligand HBD3, suggesting CCR2-HBD3 interactions may contribute to MDSC recruitment to SCC. Treatment of SCC in vitro with the iNOS inhibitor L-NNA induced E-selectin expression at levels comparable to imiquimod-treated SCC undergoing immunologic destruction. Our results suggest that local production of NO in SCC may impair vascular E-selectin expression. We show that MDSC are critical producers of NO in SCC and that NO inhibition restores vascular E-selectin expression, potentially enhancing T cell recruitment. iNOS inhibitors and other therapies that reduce NO production may therefore be effective in the treatment of SCC and their premalignant precursor lesions actinic keratoses.

The pros and cons of chemokines in tumor immunology

Innate and adaptive immune cells can intervene during tumor progression at different stages including initiation, angiogenesis, local spreading and distant metastasis formation. The net effect can be favorable or detrimental to tumor development, depending on the composition and activation status of the immune infiltrate. Chemokines can determine the distribution of immune cells in the tumor microenvironment and also affect stroma composition. Here we consider how a complex network of chemokines plays a key role in dictating the fate of a tumor. Although the field is in its infancy, we also highlight how targeting chemokines offers a tool to modulate the tumor environment with the aim of enhancing immune-mediated rejection of cancer.

Control of murine Ly6C(high) monocyte traffic and immunosuppressive activities by atypical chemokine receptor D6

The atypical chemokine receptor D6 is a decoy and scavenger receptor for most inflammatory CC chemokines and prevents the development of exacerbated inflammatory reactions. Here we report that mice lacking D6 expression in the nonhematopoietic compartment have a selective increase in the number of Ly6C(high) monocytes in the circulation and in secondary lymphoid tissues. Under inflammatory conditions, Ly6C(high) monocytes accumulate in increased number in secondary lymphoid organs of D6(-/-) mice in a CCR2-dependent manner. Ly6C(high) monocytes derived from D6(-/-) mice have enhanced immunosuppressive activity, inhibit the development of adaptive immune responses, and partially protect mice from the development of GVHD. Thus, control of CCR2 ligands by D6 regulates the traffic of Ly6C(high) monocytes and controls their immunosuppressive potential.

Coordinated regulation of myeloid cells by tumours

Myeloid cells are the most abundant nucleated haematopoietic cells in the human body and are a collection of distinct cell populations with many diverse functions. The three groups of terminally differentiated myeloid cells - macrophages, dendritic cells and granulocytes - are essential for the normal function of both the innate and adaptive immune systems. Mounting evidence indicates that the tumour microenvironment alters myeloid cells and can convert them into potent immunosuppressive cells. Here, we consider myeloid cells as an intricately connected, complex, single system and we focus on how tumours manipulate the myeloid system to evade the host immune response.

SCF and TLR4 ligand cooperate to augment the tumor-promoting potential of mast cells

Mast cells may have either antitumor or tumor-promoting potential. Nevertheless, mast cells in tumor microenvironment have been found to promote tumor growth. So far the mechanisms underlying the modulation of mast cell function in tumor microenvironment remains to be fully elucidated. Here, we report that tumor-promoting potential of mast cells could be augmented by molecules released from damaged tumor cells through cooperative stimulation of stem cell factor (SCF) and ligand for Toll-like receptor 4 (TLR4). Co-simulation with SCF and TLR4 ligand inhibited mast cell degranulation, but efficiently induced the production and secretion of VEGF, PDGF, and IL-10. Although TLR4 ligand alone may induce IL-12 expression in mast cells, co-stimulation with SCF and TLR4 ligand induced the expression of IL-10, but not IL-12, in mast cells. The phosphorylation of GSK3β was crucial for the effect of SCF and TLR4 ligand. In addition to inducing phosphorylation of GSK3β at Ser9 through PI3K pathway, SCF and TLR4 ligand cooperated to induce phosphorylation of GSK3β at Tyr216 by simultaneous activation of ERK and p38MAPK pathways. Both phospho-Ser9 and phospho-Tyr216 of GSK3β were required for IL-10 expression induced by SCF/TLR4 ligand, whereas suppressive effect of SCF/TLR4 ligand on mast cell degranulation was related to phospho-Tyr216. Importantly, the effect of SCF and TLR4 ligand on mast cells could be abrogated by inhibiting phosphorylation of GSK3β at Tyr216. These findings disclose the mechanisms underlying the modulation of mast cell function in tumor microenvironment, and suggest that inhibiting GSK3β in mast cells will be beneficial to the treatment of cancer.

Pancreatic adenocarcinoma induces bone marrow mobilization of myeloid-derived suppressor cells which promote primary tumor growth

PURPOSE

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of immunosuppressive cells that are upregulated in cancer. Little is known about the prevalence and importance of MDSC in pancreas adenocarcinoma (PA).

EXPERIMENTAL DESIGN

Peripheral blood, bone marrow, and tumor samples were collected from pancreatic cancer patients, analyzed for MDSC (CD15(+)CD11b(+)) by flow cytometry and compared to cancer-free controls. The suppressive capacity of MDSC (CD11b(+)Gr-1(+)) and the effectiveness of MDSC depletion were assessed in C57BL/6 mice inoculated with Pan02, a murine PA, and treated with placebo or zoledronic acid, a potent aminobisphosphonate previously shown to target MDSC. The tumor microenvironment was analyzed for MDSC (Gr1(+)CD11b(+)), effector T cells, and tumor cytokine levels.

RESULTS

Patients with PA demonstrated increased frequency of MDSC in the bone marrow and peripheral circulation which correlated with disease stage. Normal pancreas tissue showed no MDSC infiltrate, while human tumors avidly recruited MDSC. Murine tumors similarly recruited MDSC that suppressed CD8(+) T cells in vitro and accelerated tumor growth in vivo. Treatment with zoledronic acid impaired intratumoral MDSC accumulation resulting in delayed tumor growth rate, prolonged median survival, and increased recruitment of T cells to the tumor. This was associated with a more robust type 1 response with increased levels of IFN-γ and decreased levels of IL-10.

CONCLUSIONS

MDSC are important mediators of tumor-induced immunosuppression in pancreatic cancer. Inhibiting MDSC accumulation with zoledronic acid improves the host anti-tumor response in animal studies suggesting that efforts to block MDSC may represent a novel treatment strategy for pancreatic cancer.

The Tumor-Promoting Flow of Cells Into, Within and Out of the Tumor Site: Regulation by the Inflammatory Axis of TNFα and Chemokines

Tumors are dynamic organs, in which active processes of cell motility affect disease course by regulating the composition of cells at the tumor site. While sub-populations of tumor-promoting leukocytes are recruited inward and endothelial cell migration stands in the basis of vascular branching throughout the tumor, cancer cells make their way out of the primary site towards specific metastatic sites. This review describes the independent and cross-regulatory roles of inflammatory chemokines and of the inflammatory cytokine tumor necrosis factor α (TNFα) in determining cell motility processes that eventually have profound effects on tumor growth and metastasis. First, the effects of inflammatory chemokines such as CCL2 (MCP-1), CCL5 (RANTES) and CXCL8 (IL-8) are described, regulating the inward flow of leukocyte sub-populations with pro-tumoral activities, such as tumor-associated macrophages (TAM), myeloid-derived suppressor cells (MDSC), tumor-associated neutrophils (TAN), Th17 cells and Tregs. Then, the ability of inflammatory chemokines to induce endothelial cell migration, sprouting and tube formation is discussed, with its implications on tumor angiogenesis. This part is followed by an in depth description of the manners by which TNFα potentiates the above activities of the inflammatory chemokines, alongside with its ability to directly induce migratory processes in the tumor cells thus promoting metastasis. Note worthy is the ability of TNFα to induce in the tumor cells the important process of epithelial-to-mesenchymal transition (EMT). Emphasis is given to the ability of TNFα to establish an inflammatory network with the chemokines, and in parallel to form a cell re-modeling network together with transforming growth factor β (TGFβ). The review concludes by discussing the implications of such networks on disease course, and on the future design of therapeutic measures in cancer.

Murine pancreatic adenocarcinoma dampens SHIP-1 expression and alters MDSC homeostasis and function

Background

Pancreatic cancer is one of the most aggressive cancers, with tumor-induced myeloid-derived suppressor cells (MDSC) contributing to its pathogenesis and ineffective therapies. In response to cytokine/chemokine receptor activation, src homology 2 domain-containing inositol 5′-phosphatase-1 (SHIP-1) influences phosphatidylinositol-3-kinase (PI3K) signaling events, which regulate immunohomeostasis. We hypothesize that factors from murine pancreatic cancer cells cause the down-regulation of SHIP-1 expression, which may potentially contribute to MDSC expansion, and the suppression of CD8⁺ T cell immune responses. Therefore, we sought to determine the role of SHIP-1 in solid tumor progression, such as murine pancreatic cancer.

Methodology and Principal Findings

Immunocompetent C57BL/6 mice were inoculated with either murine Panc02 cells (tumor-bearing [TB] mice) or Phosphate Buffer Saline (PBS) (control mice). Cytometric Bead Array (CBA) analysis of supernatants of cultured Panc02 detected pro-inflammatory cytokines such as IL-6, IL-10 and MCP-1. TB mice showed a significant increase in serum levels of pro-inflammatory factors IL-6 and MCP-1 measured by CBA. qRT-PCR and Western blot analyses revealed the in vivo down-regulation of SHIP-1 expression in splenocytes from TB mice. Western blot analyses also detected reduced SHIP-1 activity, increased AKT-1 and BAD hyper-phosphorylation and up-regulation of BCL-2 expression in splenocytes from TB mice. In vitro, qRT-PCR and Western blot analyses detected reduced SHIP-1 mRNA and protein expression in control splenocytes co-cultured with Panc02 cells. Flow cytometry results showed significant expansion of MDSC in peripheral blood and splenocytes from TB mice. AutoMACS sorted TB MDSC exhibited hyper-phosphorylation of AKT-1 and over-expression of BCL-2 detected by western blot analysis. TB MDSC significantly suppressed antigen-specific CD8⁺ T cell immune responses in vitro.

Conclusion/Significance

SHIP-1 may regulate immune development that impacts MDSC expansion and function, contributing to pancreatic tumor progression. Thus, SHIP-1 can be a potential therapeutic target to help restore immunohomeostasis and improve therapeutic responses in patients with pancreatic cancer.

Chemokines in tumor development and progression

Chemokines were originally identified as mediators of the inflammatory process and regulators of leukocyte trafficking. Subsequent studies revealed their essential roles in leukocyte physiology and pathology. Moreover, chemokines have profound effects on other types of cells associated with the inflammatory response, such as endothelial cells and fibroblasts. Thus, chemokines are crucial for cancer-related inflammation, which can promote tumor development and progression. Increasing evidence points to the vital effects of several chemokines on the proliferative and invasive properties of tumor cells. The wide range of activities of chemokines in tumorigenesis highlights their roles in tumor development and progression.

A Th1 cytokine-enriched microenvironment enhances tumor killing by activated T cells armed with bispecific antibodies and inhibits the development of myeloid-derived suppressor cells

In this study, we investigated whether activated T cells (ATC) armed with bispecific antibodies (aATC) can inhibits tumor growth and MDSC development in a Th1 cytokine-enriched (IL-2 and IFN-γ) microenvironment. Cytotoxicity mediated by aATC was significantly higher (P < 0.001) against breast cancer cell lines in the presence of Th1 cytokines as compared with control co-cultures. In the presence of aATC, CD33+ /CD11b+ /CD14- /HLA-DR- MDSC population was reduced significantly under both control (P < 0.03) and Th1-enriched (P < 0.036) culture conditions. Cytokine analysis in the culture supernatants showed high levels of MDSC suppressive chemokines CXCL9 and CXCL10 in Th1-enriched culture supernatants with highly significant increase (P < 0.001) in the presence of aATC. Interestingly, MDSC recovered from co-cultures without aATC showed potent ability to suppress activated T-cell-mediated cytotoxicity (P < 0.001), IFN-γ production (P < 0.01) and T-cell proliferation (P < 0.05) compared to those recovered from aATC-containing co-cultures. These data suggest that aATC can mediate enhanced killing of tumor cells and may suppress MDSC and T(reg) differentiation, and presence of Th() cytokines potentiates aATC-induced suppression of MDSC, suggesting that Th1-enriching immunotherapy may be beneficial in cancer treatment.

Chemokine nitration prevents intratumoral infiltration of antigen-specific T cells

Blocking CCL2 nitration in tumors promoted CD8⁺ influx and reduced tumor growth and prolonged survival in mice when combined with adoptive cell therapy.

Tumor-promoted constraints negatively affect cytotoxic T lymphocyte (CTL) trafficking to the tumor core and, as a result, inhibit tumor killing. The production of reactive nitrogen species (RNS) within the tumor microenvironment has been reported in mouse and human cancers. We describe a novel RNS-dependent posttranslational modification of chemokines that has a profound impact on leukocyte recruitment to mouse and human tumors. Intratumoral RNS production induces CCL2 chemokine nitration and hinders T cell infiltration, resulting in the trapping of tumor-specific T cells in the stroma that surrounds cancer cells. Preconditioning of the tumor microenvironment with novel drugs that inhibit CCL2 modification facilitates CTL invasion of the tumor, suggesting that these drugs may be effective in cancer immunotherapy. Our results unveil an unexpected mechanism of tumor evasion and introduce new avenues for cancer immunotherapy.

Mast cell: insight into remodeling a tumor microenvironment

Mast cells are of paramount importance to allergies, pathogen immune responses during infections, and angiogenesis, as well as innate and adaptive immune regulations. Beyond all these roles, mast cells are now more and more being recognized as modulators of tumor microenvironment. Notwithstanding mounting evidences of mast cell accumulation in tumors, their exact role in tumor microenvironment is still incompletely understood. In this review, we discuss the significant role of mast cells in the remodeling of tumor microenvironment by either releasing various factors after activation or interacting with other cells within tumor and, as a result, the possible role of mast cell in cancer invasion and metastasis. We also discuss recent findings that mast cells actively release microparticles, which account for the transfer of membrane-type receptor signal and regulatory molecules such as microRNAs to tumor cells and immune cells. These findings on mast cells provide further insights into the complexity of tumor microenvironment remodeling.

Myeloid-derived suppressor cells: general characteristics and relevance to clinical management of pancreatic cancer

Recent studies describe a heterogeneous population of cells of the myeloid lineage, termed myeloid derived suppressor cells (MDSC), which are observed with increased prevalence in the peripheral blood and tumor microenvironment of cancer patients, including pancreatic cancer. Accumulation of MDSC in the peripheral circulation has been related to extent of disease, and correlates with stage. MDSC have primarily been implicated in promoting tumor growth by suppressing antitumor immunity. There is also compelling evidence MDSC are also involved in angiogenesis and metastatic spread. Two main subsets of MDSC have been identified in cancer patients: a monocytic subset, characterized by expression of CD14, and a granulocytic subset characterized by expression of CD15. Both subsets of MDSC actively suppress host immunity through a variety of mechanisms including production of reactive oxygen species and arginase. Just as in humans, accumulation of monocytic and granulocytic MDSC has been noted in the bone marrow, spleen, peripheral circulation, and tumors of tumor bearing mice. Successful targeting of MDSC in mice is associated with improved immune responses, delayed tumor growth, improved survival, and increased efficacy of vaccine therapy. By further elucidating mechanisms of MDSC recruitment and maintenance in the tumor environment, strategies could be developed to reverse immune tolerance to tumor. We discuss here what is currently known about MDSC as well as some potential strategies targeting MDSC in the context of our work on pancreatic cancer and recent literature. Due to the number of new reports on MDSC, the most pertinent ones have been selected.

Sunitinib facilitates the activation and recruitment of therapeutic anti-tumor immunity in concert with specific vaccination

The multikinase inhibitor sunitinib malate (SUT) has been reported to reduce levels of myeloid suppressor cells and Treg cells in cancer patients, hypothetically diminishing intrinsic impediments for active immunization against tumor-associated antigens in such individuals. The goal of this study was to identify longitudinal immune molecular and cellular changes associated with tumor regression and disease-free status after the treatment of established day 7 s.c. MO5 (B16.OVA) melanomas with SUT alone (1 mg/day via oral gavage for 7 days), vaccination using ovalbumin (OVA) peptide-pulsed dendritic cell [vaccine (VAC)] alone, or the combination of SUT and VAC (SUT/VAC). We observed superior anti-tumor efficacy for SUT/VAC combination approaches, particularly when SUT was applied at the time of the initial vaccination or the VAC boost. Treatment effectiveness was associated with the acute loss of (and/or failure to recruit) cells bearing myeloid-derived suppressor cells or Treg phenotypes within the tumor microenvironment (TME) and the corollary, prolonged enhancement of Type-1 anti-OVA CD8(+) T cell responses in the tumor-draining lymph node and the TME. Enhanced Type-1 T cell infiltration of tumors was associated with treatment-induced expression of vascular cell adhesion molecule-1 (VCAM-1) and CXCR3 ligand chemokines in vascular/peri-vascular cells within the TME, with SUT/VAC therapy benefits conditionally negated upon adminsitration of CXCR3 or VCAM-1 blocking antibodies. These data support the ability of a short 7 day course of SUT to (re)condition the TME to become more receptive to the recruitment and prolonged therapeutic action of (VAC-induced) anti-tumor Tc1 cells.

Expression of the inflammatory chemokines CCL2, CCL5 and CXCL2 and the receptors CCR1-3 and CXCR2 in T lymphocytes from mammary tumor-bearing mice

Chemokines and their receptors have been studied in several solid tumor models as mediators of inflammation. In turn, inflammation has been implicated in the promotion and progression of tumors, and as such, chemokines have been proposed as novel molecular targets for chemotherapy. While the expression of these molecules has been described in tumor cells, endothelial cells, macrophages and neutrophils, less attention has been paid to the expression profile of these molecules by T lymphocytes in the periphery or infiltrating the tumor. Using the D1-DMBA-3 murine mammary adenocarcinoma model, we aimed to better characterize the differential expression of chemokines and/or their receptors in the host and in the tumor microenvironment, and specifically, in the T cells of tumor-bearing mice compared to normal control animals. We found that T lymphocytes from tumor-bearing mice express the pro-inflammatory chemokines, CCL2, CCL5 and CXCL2, as well as the chemokine receptors, CCR1, CCR2, CCR3 and CXCR2.

COX-2 blockade suppresses gliomagenesis by inhibiting myeloid-derived suppressor cells

Epidemiologic studies have highlighted associations between the regular use of nonsteroidal anti-inflammatory drugs (NSAID) and reduced glioma risks in humans. Most NSAIDs function as COX-2 inhibitors that prevent production of prostaglandin E₂ (PGE₂). Because PGE₂ induces expansion of myeloid-derived suppressor cells (MDSC), we hypothesized that COX-2 blockade would suppress gliomagenesis by inhibiting MDSC development and accumulation in the tumor microenvironment (TME). In mouse models of glioma, treatment with the COX-2 inhibitors acetylsalicylic acid (ASA) or celecoxib inhibited systemic PGE₂ production and delayed glioma development. ASA treatment also reduced the MDSC-attracting chemokine CCL2 (C-C motif ligand 2) in the TME along with numbers of CD11b(+)Ly6G(hi)Ly6C(lo) granulocytic MDSCs in both the bone marrow and the TME. In support of this evidence that COX-2 blockade blocked systemic development of MDSCs and their CCL2-mediated accumulation in the TME, there were defects in these processes in glioma-bearing Cox2-deficient and Ccl2-deficient mice. Conversely, these mice or ASA-treated wild-type mice displayed enhanced expression of CXCL10 (C-X-C motif chemokine 10) and infiltration of cytotoxic T lymphocytes (CTL) in the TME, consistent with a relief of MDSC-mediated immunosuppression. Antibody-mediated depletion of MDSCs delayed glioma growth in association with an increase in CXCL10 and CTLs in the TME, underscoring a critical role for MDSCs in glioma development. Finally, Cxcl10-deficient mice exhibited reduced CTL infiltration of tumors, establishing that CXCL10 limited this pathway of immunosuppression. Taken together, our findings show that the COX-2 pathway promotes gliomagenesis by directly supporting systemic development of MDSCs and their accumulation in the TME, where they limit CTL infiltration.

Mast cell 5-lipoxygenase activity promotes intestinal polyposis in APCDelta468 mice

Arachidonic acid metabolism has been implicated in colon carcinogenesis, but the role of hematopoietic 5-lipoxygenase (5LO) that may impact tumor immunity in development of colon cancer has not been explored. Here we show that tissue-specific deletion of the 5LO gene in hematopoietic cells profoundly attenuates polyp development in the APC(Δ468) murine model of colon polyposis. In vitro analyses indicated that mast cells in particular utilized 5LO to limit proliferation of intestinal epithelial cells and to mobilize myeloid-derived suppressor cells (MDSCs). Mice lacking hemapoietic expression of 5LO exhibited reduced recruitment of MDSCs to the spleen, mesenteric lymph nodes, and primary tumor site. 5LO deficiency also reduced the activity in MDSCs of arginase-1, which is thought to be critical for MDSC function. Together, our results establish a pro-tumorigenic role of hematopoietic 5LO in the immune microenvironment and suggest 5LO inhibition as an avenue for future investigation in treatment of colorectal polyposis and cancer.

Systemic delivery of neutralizing antibody targeting CCL2 for glioma therapy

Tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) inhibit anti-tumor immune responses and facilitate tumor growth. Precursors for these immune cell populations migrate to the tumor site in response to tumor secretion of chemokines, such as monocyte chemoattractant protein-1 (MCP-1/CCL2), which was originally purified and identified from human gliomas. In syngeneic mouse GL261 glioma and human U87 glioma xenograft models, we evaluated the efficacy of systemic CCL2 blockade by monoclonal antibodies (mAb) targeting mouse and/or human CCL2. Intraperitoneal (i.p.) administration of anti-mouse CCL2 mAb as monotherapy (2 mg/kg/dose, twice a week) significantly, albeit modestly, prolonged the survival of C57BL/6 mice bearing intracranial GL261 glioma (P = 0.0033), which was concomitant with a decrease in TAMs and MDSCs in the tumor microenvironment. Similarly, survival was modestly prolonged in severe combined immunodeficiency mice bearing intracranial human U87 glioma xenografts treated with both anti-human CCL2 mAb and anti-mouse CCL2 antibodies (2 mg/kg/dose for each, twice a week) compared to mice treated with control IgG (P = 0.0159). Furthermore, i.p. administration of anti-mouse CCL2 antibody in combination with temozolomide (TMZ) significantly prolonged the survival of C57BL/6 mice bearing GL261 glioma with 8 of 10 treated mice surviving longer than 70 days, while only 3 of 10 mice treated with TMZ and isotype IgG survived longer than 70 days (P = 0.0359). These observations provide support for development of mAb-based CCL2 blockade strategies in combination with the current standard TMZ-based chemotherapy for treatment of malignant gliomas.

Depletion of regulatory T cells facilitates growth of established tumors: a mechanism involving the regulation of myeloid-derived suppressor cells by lipoxin A4

Regulatory T cells (Tregs) are thought to facilitate tumor development by suppressing protective antitumor immune responses. However, recent clinical and laboratory studies show that Tregs are a favorable element against cancer. In this study, we provide evidence that Tregs have both promoting and inhibiting effects on tumors, depending on the stage of tumor development. By using 0.5 mg cyclophosphamide, we constructed a murine liver cancer model in which Tregs were continuously and selectively depleted. Under such conditions, we found that tumor growth was inhibited at early stages but accelerated later on. Analysis of the tumor microenvironment disclosed that long-term Treg depletion by 0.5 mg cyclophosphamide treatment induced Gr-1(+)CD11b(+) myeloid-derived suppressor cells (MDSCs). Ablation of MDSCs by anti-Gr-1 Ab blocked Treg depletion-induced promotion of tumor growth. Furthermore, lipoxygenases 5 and 12, two enzymes participating in the biosynthesis of the lipid anti-inflammatory mediator lipoxin A(4), were upregulated or downregulated by Treg depletion or adoptive transfer. Correspondingly, the levels of lipoxin A(4) were increased or decreased. Lipoxin A(4) thus regulated the induction of MDSCs in response to Treg depletion. These findings suggest that Tregs may play different roles at different stages of tumor growth: promoting early and inhibiting late tumor growth. Our study also suggests that the interplay among Tregs, MDSCs, and lipoxin A(4) tunes the regulation of tumor-associated inflammation.

Type 1 T helper cells induce the accumulation of myeloid-derived suppressor cells in the inflamed Tgfb1 knockout mouse liver

Immune-mediated liver injury in hepatitis is due to activated T cells producing interferon-γ (IFN-γ). It is important to identify negative feedback immune mechanisms that can regulate T cell activity. In this study, we demonstrate that liver inflammation mediated by type 1 T helper (Th1) cells can induce the accumulation of myeloid-derived suppressor cells (MDSCs), pleiomorphic cells capable of modulating T cell-mediated immunity, that heretofore have been studied almost exclusively in the context of tumor-associated inflammation. Mice deficient in the gene encoding transforming growth factor-β1 (Tgfb1(-/-) mice) acutely develop liver necroinflammation caused by IFN-γ-producing clusters of differentiation 4-positive (CD4(+)) T cells. Liver Th1 cell accumulation was accompanied by myeloid cells expressing CD11b and Gr1, phenotypic hallmarks of MDSCs. Isolated Tgfb1(-/-) liver CD11b(+)Gr1(+) cells were functional MDSCs, readily suppressing T cell proliferation in vitro. Pharmacologic inhibitors of inducible nitric oxide (NO) synthase completely eliminated suppressor function. Suppressor function and the production of NO were dependent on cell-cell contact between MDSCs and T cells, and upon IFN-γ, and were specifically associated with the "monocytic" CD11b(+)Ly6G(-) Ly6C(hi) subset of liver Tgfb1(-/-) CD11b(+) cells. The rapid accumulation of CD11b(+)Gr1(+) cells in Tgfb1(-/-) liver was abrogated when mice were either depleted of CD4(+) T cells or rendered unable to produce IFN-γ, showing that Th1 activity induces MDSC accumulation.

CONCLUSION

Th1 liver inflammation mobilizes an MDSC response that, through the production of NO, can inhibit T cell proliferation. We propose that MDSCs serve an important negative feedback function in liver immune homeostasis, and that insufficient or inappropriate activity of this cell population may contribute to inflammatory liver pathology.

Involvement of CD11b+ GR-1 low cells in autoimmune disorder in MRL-Fas lpr mouse

OBJECTIVE

Myeloid-derived suppressor cells (MDSCs) have been identified as immunosuppressive cells in tumor-related inflammation. However, the pathogenesis of MDSCs for autoimmune disease has not been investigated as yet. The aim of this study was to address whether MDSCs contribute to autoimmune organ injury in lupus-prone mice.

METHODS

MDSCs were analyzed by flow cytometric staining of CD11b(+) GR-1(+) in MRL-Fas ( lpr ) mice. CD4(+) T-cell proliferation assay was performed by coculture with CD11b(+) GR-1(+) splenocytes. The percentage of immunosuppressive cells was examined during disease progression. Expression of chemokine receptor on immunosuppressive cells was analyzed, and chemotaxis assay was performed.

RESULTS

CD11b(+) GR-1(low) cells had a suppressive effect on CD4(+) T-cell proliferation, which was restored by an arginase-1 inhibitor. CD11b(+) GR-1(low) cells increased in percentage during disease progression in kidney and blood. The number of migrated CD11b(+) GR-1(low) cells increased in the presence of monocyte chemoattractant protein-1/CCL2.

CONCLUSION

We assessed the involvement of CD11b(+) GR-1(low) cells in autoimmune disorder in MRL-Fas(lpr) mice. These cells regulate immunological responses via CCL2/CCR2 signaling. The regulation of immunosuppressive monocytes may provide novel therapeutic strategy for organ damage in autoimmune diseases.

Role of type 1 IFNs in antiglioma immunosurveillance--using mouse studies to guide examination of novel prognostic markers in humans

PURPOSE

We hypothesized that the type 1 IFNs would play a pivotal role in antiglioma immunosurveillance through promotion of type 1 adaptive immunity and suppression of immunoregulatory cells.

EXPERIMENTAL DESIGN

We induced de novo gliomas in Ifnar1(-/-) (deficient for type 1 IFN receptors) or wild-type mice by intracerebroventricuar transfection of NRas and a short hairpin RNA against P53 using the Sleeping Beauty transposon system. We analyzed the survival of 587 glioma patients for single nucleotide polymorphisms (SNP) in type 1 IFN-related genes.

RESULTS

Ifnar1(-/-) mice exhibited accelerated tumor growth and death. Analyses of brain tumor-infiltrating lymphocytes in Ifnar1(-/-) mice revealed an increase of cells positive for CD11b(+)Ly6G(+) and CD4(+)FoxP3(+), which represent myeloid-derived suppressor cells and regulatory T cells, respectively, but a decrease of CD8(+) cytotoxic T lymphocytes (CTLs) compared with wild-type mice. Ifnar1(-/-) mouse-derived glioma tissues exhibited a decrease in mRNA for the CTL-attracting chemokine Cxcl10, but an increase of Ccl2 and Ccl22, both of which are known to attract immunoregulatory cell populations. Dendritic cells generated from the bone marrow of Ifnar1(-/-) mice failed to function as effective antigen-presenting cells. Moreover, depletion of Ly6G(+) cells prolonged the survival of mice with developing gliomas. Human epidemiologic studies revealed that SNPs in IFNAR1 and IFNA8 are associated with significantly altered overall survival of patients with WHO grade 2 to 3 gliomas.

CONCLUSIONS

The novel Sleeping Beauty-induced murine glioma model led us to discover a pivotal role for the type 1 IFN pathway in antiglioma immunosurveillance and relevant human SNPs that may represent novel prognostic markers.

Cancer-associated myeloproliferation: old association, new therapeutic target

The association between malignancy and development of a paraneoplastic leukocytosis, the so-called leukemoid reaction, has long been appreciated. Although a leukemoid reaction has conventionally been defined as a peripheral blood leukocytosis composed of both mature and immature granulocytes that exceeds 50,000/microL, a less profound leukocytosis may be appreciated in many patients harboring a malignant disease. More recent insights have shed new light on this long-recognized association, because research performed in both murine models and cancer patients has uncovered multiple mechanisms by which tumors both drive myelopoiesis, sometimes leading to a clinically apparent leukocytosis, and inhibit the differentiation of myeloid cells, resulting in a qualitative change in myelopoiesis. This qualitative change leads to the accumulation of immature myeloid cells, which due to their immune suppressive effects have been collectively called myeloid-derived suppressor cells. More recently, myeloid cells have been shown to promote tumor angiogenesis. Cancer-associated myeloproliferation is not merely a paraneoplastic phenomenon of questionable importance but leads to the suppression of host immunity and promotion of tumor angiogenesis, both of which play an integral part in tumorigenesis and metastasis. Therefore, cancer-associated myeloproliferation represents a novel therapeutic target in cancer that, decades after its recognition, is only now being translated into clinical practice.

Monocyte chemoattractant protein-1 blockade inhibits lung cancer tumor growth by altering macrophage phenotype and activating CD8+ cells

The role of chemokines in the pathogenesis of lung cancer has been increasingly appreciated. Monocyte chemoattractant protein-1 (MCP-1, also known as CCL2) is secreted from tumor cells and associated tumor stromal cells. The blockade of CCL2, as mediated by neutralizing antibodies, was shown to reduce tumorigenesis in several solid tumors, but the role of CCL2 in lung cancer remains controversial, with evidence of both protumorigenic and antitumorigenic effects. We evaluated the effects and mechanisms of CCL2 blockade in several animal models of non-small-cell lung cancer (NSCLC). Anti-murine-CCL2 monoclonal antibodies were administered in syngeneic flank and orthotopic models of NSCLC. CCL2 blockade significantly slowed the growth of primary tumors in all models studied, and inhibited lung metastases in a model of spontaneous lung metastases of NSCLC. In contrast to expectations, no significant effect of treatment was evident in the number of tumor-associated macrophages recruited into the tumor after CCL2 blockade. However, a change occurred in the polarization of tumor-associated macrophages to a more antitumor phenotype after CCL2 blockade. This was associated with the activation of cytotoxic CD8(+) T lymphocytes (CTLs). The antitumor effects of CCL2 blockade were completely lost in CB-17 severe combined immunodeficient mice or after CD8 T-cell depletion. Our data from NSCLC models show that CCL2 blockade can inhibit the tumor growth of primary and metastatic disease. The mechanisms of CCL2 blockade include an alteration of the tumor macrophage phenotype and the activation of CTLs. Our work supports further evaluation of CCL2 blockade in thoracic malignancies.

Systemic immune suppression in glioblastoma: the interplay between CD14+HLA-DRlo/neg monocytes, tumor factors, and dexamethasone

Patients with glioblastoma (GBM) exhibit profound systemic immune defects that affect the success of conventional and immune-based treatments. A better understanding of the contribution of the tumor and/or therapy on systemic immune suppression is necessary for improved therapies, to monitor negative effects of novel treatments, to improve patient outcomes, and to increase understanding of this complex system. To characterize the immune profile of GBM patients, we phenotyped peripheral blood and compared these to normal donors. In doing so, we identified changes in systemic immunity associated with both the tumor and dexamethasone treated tumor bearing patients. In particular, dexamethasone exacerbated tumor associated lymphopenia primarily in the T cell compartment. We have also identified unique tumor and dexamethasone dependent altered monocyte phenotypes. The major population of altered monocytes (CD14(+)HLA-DR(lo/neg)) had a phenotype distinct from classical myeloid suppressor cells. These cells inhibited T cell proliferation, were unable to fully differentiate into mature dendritic cells, were associated with dexamethasone-mediated changes in CCL2 levels, and could be re-created in vitro using tumor supernatants. We provide evidence that tumors express high levels of CCL2, can contain high numbers of CD14(+) cells, that tumor supernatants can transform CD14(+)HLA-DR(+) cells into CD14(+)HLA-DR(lo/neg) immune suppressors, and that dexamethasone reduces CCL2 in vitro and is correlated with reduction of CCL2 in vivo. Consequently, we have developed a model for tumor mediated systemic immune suppression via recruitment and transformation of CD14(+) cells.

Mast cells mobilize myeloid-derived suppressor cells and Treg cells in tumor microenvironment via IL-17 pathway in murine hepatocarcinoma model

Tumor immunosuppression is commonly braided with chronic inflammation during tumor development. However, the relationship between immunosuppression and inflammation in tumor microenvironment is still unclear. We have demonstrated that mast cells are accumulated and exacerbate the inflammation and immunosuppression in tumor microenvironment via SCF/c-kit signaling pathway. Here, we further elucidate the underlying mechanism, which involves both myeloid-derived suppressor cells (MDSCs) and regulatory T (Treg) cells. Our data showed that mast cells mobilized the infiltration of MDSCs to tumor and induced the production of IL-17 by MDSCs; MDSCs-derived IL-17 indirectly attracted Treg cells, enhanced their suppressor function, and induced the IL-9 production by Treg cells; in turn, IL-9 strengthened the survival and protumor effect of mast cells in tumor microenvironment. Our findings disclose a closed loop among mast cells, MDSCs and Treg cells in tumor microenvironment, which provides a new insight into the paralleled developments of inflammation and immunosuppression in tumor microenvironment. Based on these findings, we propose that targeting tumor inflammation might be a potential strategy to reverse the immunosuppression of tumor microenvironment, thus facilitating cancer immunotherapy.

CC chemokine ligand 2 (CCL2) promotes prostate cancer tumorigenesis and metastasis

CCL2 is a chemokine known to recruit monocytes and macrophages to sites of inflammation. A growing body of research suggests CCL2 is progressively overexpressed in tumor beds and may play a role in the clinical progression of solid tumors. Cancer cells derived from several solid tumor types demonstrate functional receptors for CCL2, suggesting this chemokine may achieve tumorigenicity through direct effects on malignant cells; however, a variety of normal host cells that co-exist with cancer in the tumor microenvironment also respond to CCL2. These cells include macrophages, osteoclasts, endothelial cells, T-lymphocytes, and myeloid-derived immune suppressor cells (MDSCs). CCL2 mediated interactions between normal and malignant cells in the tumor microenvironment and plays a multi-faceted role in tumor progression.

Mononuclear phagocytes in head and neck squamous cell carcinoma

The head and neck squamous cell carcinoma microenvironments contain many immune cells and their secretory products. Many of these cells belong to the mononuclear phagocyte system. The aim of this review is to study the interactions between mononuclear phagocytes and head and neck squamous cell carcinoma tissue. The role of inflammation in tumours and the cytokine interleukin-6 will be highlighted. Future therapy strategies in the treatment of head and neck cancer might be directed towards mononuclear phagocytes and their cytokine production.

Phenotypic and functional delineation of murine CX(3)CR1 monocyte-derived cells in ovarian cancer

Ovarian tumor progression is marked by the peritoneal accumulation of leukocytes. Among these leukocytes, an immunosuppressive CD11b(+)CD11c(+) population has been identified in both human and ovarian tumors. The use of transplantable models of murine ovarian tumors has demonstrated that this population promotes ovarian tumor growth, whereas elimination of this population has been shown to inhibit ovarian tumor progression. Despite the demonstrated importance of these cells to ovarian tumor progression, the mechanisms by which these cells are recruited to the peritoneal tumor are largely unknown. Therefore, this study analyzes the mechanisms these cells use to migrate to the peritoneum with the goal of therapeutically blocking their recruitment and subsequent immunosuppressive activity. Recent studies have identified that CX(3)CR1, Gr-1, and CCR2 delineate phenotypic and functional murine monocyte subsets. Here, we report that CX(3)CR1(lo)Gr-1(hi) cells dominate the population of peritoneal CD11b(+) leukocytes early in murine tumor development; however, the CX(3)CR1(hi) population of cells present in the peritoneum dramatically increases in both total numbers and percentage during tumor progression. Functional analyses reveal that both of these CX(3)CR1 subsets are immunosuppressive to naive CD8(+) and CD4(+) T-cell responses. Importantly, we demonstrate that CCR2 is a critical functional facilitator of leukocyte recruitment to the ovarian tumor microenvironment, and its genetic deletion results in a reduced tumor burden compared with wild-type mice. These results demonstrate that subsets of immunosuppressive leukocytes are recruited to the ovarian tumor environment through the CCR2 pathway, which offers a viable therapeutic target to inhibit their migration to the tumor site.

Successful immunotherapy with IL-2/anti-CD40 induces the chemokine-mediated mitigation of an immunosuppressive tumor microenvironment

Treatment of mice bearing orthotopic, metastatic tumors with anti-CD40 antibody resulted in only partial, transient anti-tumor effects whereas combined treatment with IL-2/anti-CD40, induced tumor regression. The mechanisms for these divergent anti-tumor responses were examined by profiling tumor-infiltrating leukocyte subsets and chemokine expression within the tumor microenvironment after immunotherapy. IL-2/anti-CD40, but not anti-CD40 alone, induced significant infiltration of established tumors by NK and CD8(+) T cells. To further define the role of chemokines in leukocyte recruitment into tumors, we evaluated anti-tumor responses in mice lacking the chemokine receptor, CCR2. The anti-tumor effects and leukocyte recruitment mediated by anti-CD40 alone, were completely abolished in CCR2(-/-) mice. In contrast, IL-2/anti-CD40-mediated leukocyte recruitment and reductions in primary tumors and metastases were maintained in CCR2(-/-) mice. Treatment of mice with IL-2/anti-CD40, but not anti-CD40 alone, also caused an IFN-gamma-dependent increase in the expression of multiple Th1 chemokines within the tumor microenvironment. Interestingly, although IL-2/anti-CD40 treatment increased Tregs in the spleen, it also caused a coincident IFN-gamma-dependent reduction in CD4(+)/FoxP3(+) Tregs, myeloid-derived suppressor cells and Th2 chemokine expression specifically within the tumor microenvironment that was not observed after treatment with anti-CD40 alone. Similar effects were observed using IL-15 in combination with anti-CD40. Taken together, our data demonstrate that IL-2/anti-CD40, but not anti-CD40 alone, can preferentially reduce the overall immunosuppressive milieu within the tumor microenvironment. These results suggest that the use of anti-CD40 in combination with IL-2 or IL-15 may hold substantially more promise for clinical cancer treatment than anti-CD40 alone.

Monocytes promote tumor cell survival in T-cell lymphoproliferative disorders and are impaired in their ability to differentiate into mature dendritic cells

A variety of nonmalignant cells present in the tumor microenvironment promotes tumorigenesis by stimulating tumor cell growth and metastasis or suppressing host immunity. The role of such stromal cells in T-cell lymphoproliferative disorders is incompletely understood. Monocyte-derived cells (MDCs), including professional antigen-presenting cells such as dendritic cells (DCs), play a central role in T-cell biology. Here, we provide evidence that monocytes promote the survival of malignant T cells and demonstrate that MDCs are abundant within the tumor microenvironment of T cell-derived lymphomas. Malignant T cells were observed to remain viable during in vitro culture with autologous monocytes, but cell death was significantly increased after monocyte depletion. Furthermore, monocytes prevent the induction of cell death in T-cell lymphoma lines in response to either serum starvation or doxorubicin, and promote the engraftment of these cells in nonobese diabetic/severe combined immunodeficient mice. Monocytes are actively recruited to the tumor microenvironment by CCL5 (RANTES), where their differentiation into mature DCs is impaired by tumor-derived interleukin-10. Collectively, the data presented demonstrate a previously undescribed role for monocytes in T-cell lymphoproliferative disorders.