Mesenchymal transition and dissemination of cancer cells is driven by myeloid-derived suppressor cells infiltrating the primary tumor

p16Ink4a and p21Cip1/Waf1 promote tumour growth by enhancing myeloid-derived suppressor cells chemotaxis

p16^Ink4a and p21^Cip1/Waf1 act as tumour suppressors through induction of cellular senescence. However, senescence-independent roles of these CDK inhibitors are not well understood. Here, we report an unexpected function of p16^Ink4 and p21^Cip1/Waf1, namely, tumour promotion through chemotaxis. In monocytic myeloid-derived suppressor cells (Mo-MDSCs), p16^Ink4 and p21^Cip1/Waf1 are highly expressed and stimulate CX3CR1 chemokine receptor expression by preventing CDK-mediated phosphorylation and inactivation of SMAD3. Thus, deletion of p16^Ink4 and p21^Cip1/Waf1 reduces CX3CR1 expression, thereby inhibiting Mo-MDSC accumulation in tumours expressing CX3CL1 and suppressing the tumour progression in mice. Notably, blockade of the CX3CL1/CX3CR1 axis suppresses tumour growth, whereas inactivation of CDKs elicits the opposite effect. These findings reveal an unexpected function of p16^Ink4a and p21^Waf1/Cip1 and indicate that regulation of Mo-MDSCs chemotaxis is a valuable potential strategy for control of tumour development.

Both p16^Ink4a and p21^Cip1/Waf1 are known oncosuppressors and have a role in senescence. Here, the authors show a pro-tumorigenic role for these two proteins: high expression in myeloid-derived suppressor cells stimulates their chemotactic function, favouring tumour progression.

Mesenchymal traits at the convergence of tumor-intrinsic and -extrinsic mechanisms of resistance to immune checkpoint blockers

Targeting of immune checkpoint blockers (ICBs), such as cytotoxic T-lymphocyte antigen-4 and programmed-death 1/programmed-death ligand 1, has dramatically changed the landscape of cancer treatment. Seeing patients who were refractory to conventional therapy recover after immunotherapy, with high rates of objective durable responses and increased overall survival, has raised great enthusiasm in cancer care and research. However, to date, only a restricted portion of patients benefit from these therapies, due to natural and acquired resistance relying on the ever-evolving cross-talk between tumor and stromal cells. Here, we review the convergence of tumor-intrinsic and -extrinsic cues, both affecting tumor plasticity and tumor stroma leading to an immunosuppressive tumor microenvironment, which may account for the heterogeneous responses and resistance to ICB therapies. A deeper knowledge of the mechanisms and fingerprints involved in natural and acquired resistance is likely to bring clinical benefit to the majority of patients, offering important clues for overcoming drug resistance and boosting the effectiveness of treatment. We discuss the need to define tumor subtypes based on the tumor, immune and stromal gene signature and propose that the better we understand tumor mesenchymal traits, the more we will be able to identify predictive biomarkers of response to ICB treatments.

Myeloid-Derived Suppressor Cells

Myeloid cells developed evolutionarily as a major mechanism to protect the host. They evolved as a critical barrier against infections and are important contributors to tissue remodeling. However, in cancer, myeloid cells are largely converted to serve a new master-tumor cells. This process is epitomized by myeloid-derived suppressor cells (MDSC). These cells are closely related to neutrophils and monocytes. MDSCs are not present in the steady state of healthy individuals and appear in cancer and in pathologic conditions associated with chronic inflammation or stress. These cells have emerged as an important contributor to tumor progression. Ample evidence supports a key role for MDSCs in immune suppression in cancer, as well as their prominent role in tumor angiogenesis, drug resistance, and promotion of tumor metastases. MDSCs have a fascinating biology and are implicated in limiting the effects of cancer immunotherapy. Therefore, targeting these cells may represent an attractive therapeutic opportunity. Cancer Immunol Res; 5(1); 3-8. ©2016 AACR.

Mesenchymal Transition of High-Grade Breast Carcinomas Depends on Extracellular Matrix Control of Myeloid Suppressor Cell Activity

The extracellular matrix (ECM) contributes to the biological and clinical heterogeneity of breast cancer, and different prognostic groups can be identified according to specific ECM signatures. In high-grade, but not low-grade, tumors, an ECM signature characterized by high SPARC expression (ECM3) identifies tumors with increased epithelial-to-mesenchymal transition (EMT), reduced treatment response, and poor prognosis. To better understand how this ECM3 signature is contributing to tumorigenesis, we expressed SPARC in isogenic cell lines and found that SPARC overexpression in tumor cells reduces their growth rate and induces EMT. SPARC expression also results in the formation of a highly immunosuppressive microenvironment, composed by infiltrating T regulatory cells, mast cells, and myeloid-derived suppressor cells (MDSCs). The ability of SPARC to induce EMT depended on the localization and suppressive function of myeloid cells, and inhibition of the suppressive function MDSCs by administration of aminobisphosphonates could revert EMT, rendering SPARC-overexpressing tumor cells sensitive to Doxil. We conclude that that SPARC is regulating the interplay between MDSCs and the ECM to drive the induction of EMT in tumor cells.

Neutralization of IL-8 decreases tumor PMN-MDSCs and reduces mesenchymalization of claudin-low triple-negative breast cancer

The complex signaling networks of the tumor microenvironment that facilitate tumor growth and progression toward metastatic disease are becoming a focus of potential therapeutic options. The chemokine IL-8 is overexpressed in multiple cancer types, including triple-negative breast cancer (TNBC), where it promotes the acquisition of mesenchymal features, stemness, resistance to therapies, and the recruitment of immune-suppressive cells to the tumor site. The present study explores the utility of a clinical-stage monoclonal antibody that neutralizes IL-8 (HuMax-IL8) as a potential therapeutic option for TNBC. HuMax-IL8 was shown to revert mesenchymalization in claudin-low TNBC models both in vitro and in vivo as well as to significantly decrease the recruitment of polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) at the tumor site, an effect substantiated when used in combination with docetaxel. In addition, HuMax-IL8 enhanced the susceptibility of claudin-low breast cancer cells to immune-mediated lysis with NK and antigen-specific T cells in vitro. These results demonstrate the multifaceted way in which neutralizing this single chemokine reverts mesenchymalization, decreases recruitment of MDSCs at the tumor site, assists in immune-mediated killing, and forms the rationale for using HuMax-IL8 in combination with chemotherapy or immune-based therapies for the treatment of TNBC.

One microenvironment does not fit all: heterogeneity beyond cancer cells

Human cancers exhibit formidable molecular heterogeneity, to a large extent accounting for the incomplete and transitory efficacy of current anti-cancer therapies. However, neoplastic cells alone do not manifest the disease, but conscript a battery of non-tumor cells to enable and sustain hallmark capabilities of cancer. Escaping immunosurveillance is one of such capabilities. Tumors evolve immunosuppressive microenvironment to subvert anti-tumor immunity. In this review, we will focus on tumor-associated myeloid cells, which constitute an essential part of the immune microenvironment and reciprocally interact with cancer cells to establish malignancy toward metastasis. The diversity and plasticity of these cells constitute another layer of heterogeneity, beyond the heterogeneity of cancer cells themselves. We envision that immune microenvironment co-evolves with the genetic heterogeneity of tumor. Addressing the question of how genetically distinct tumors shape and are shaped by unique immune microenvironment will provide an attractive rationale to develop novel immunotherapeutic modalities. Here, we discuss the complex nature of tumor microenvironment, with an emphasis on the cellular and functional heterogeneity among tumor-associated myeloid cells as well as immune environment heterogeneity in the context of a full spectrum of human breast cancers.

Transforming Growth Factor-Beta1 and Myeloid-Derived Suppressor Cells Interplay in Cancer

Background:

Transforming growth factor-beta1 (TGF-β1) is a pleiotropic cytokine with a double role in cancer through its capacity to inhibit early stages of tumors while enhancing tumor progression at late stages of tumor progression. Moreover, TGF-β1 is a potent immunosuppressive cytokine within the tumor microenvironment that allows cancer cells to escape from immune surveillance, which largely contributes to the tumor progression.

Method:

It has been established that the cancer progression is commonly associated with increased number of Myeloid-derived suppressor cells (MDSC) that are a hallmark of cancer and a key mechanism of immune evasion.

Result:

MDSC represent a population of heterogeneous myeloid cells comprised of macrophages, granulocytes and dendritic cells at immature stages of development. MDSC promote tumor progression by regulating immune responses as well as tumor angiogenesis and cancer metastasis.

Conclusion:

In this review, we present an overview of the main key functions of both TGF-β1 and MDSC in cancer and in the immune system. Furthermore, the mutual contribution between TGF-β1 and MDSC in the regulation of immune system and cancer development will be analyzed.

EMT and Treatment Resistance in Pancreatic Cancer

Pancreatic cancer (PC) is the third leading cause of adult cancer mortality in the United States. The poor prognosis for patients with PC is mainly due to its aggressive course, the limited efficacy of active systemic treatments, and a metastatic behavior, demonstrated throughout the evolution of the disease. On average, 80% of patients with PC are diagnosed with metastatic disease, and the half of those who undergo surgery and adjuvant therapy develop liver metastasis within two years. Metastatic dissemination is an early event in PC and is mainly attributed to an evolutionary biological process called epithelial-to-mesenchymal transition (EMT). This innate mechanism could have a dual role during embryonic growth and organ differentiation, and in cancer progression, cancer stem cell intravasation, and metastasis settlement. Many of the molecular pathways decisive in EMT progression have been already unraveled, but little is known about the causes behind the induction of this mechanism. EMT is one of the most distinctive and critical features of PC, occurring even in the very first stages of tumor development. This is known as pancreatic intraepithelial neoplasia (PanIN) and leads to early dissemination, drug resistance, and unfavorable prognosis and survival. The intention of this review is to shed new light on the critical role assumed by EMT during PC progression, with a particular focus on its role in PC resistance.

Immunological Consequences of Epithelial-Mesenchymal Transition in Tumor Progression

Microenvironments that tumor cells encounter are different during the stages of cancer progression-primary tumor, metastasis, and at the metastatic site. This suggests potential differences in immune surveillance of primary tumor and metastasis. Epithelial-mesenchymal transition (EMT) is a key reversible process in which cancer cells transition into highly motile and invasive cells for dissemination. Only a tiny proportion successfully metastasize, supporting the notion of metastasis-specific immune surveillance. EMT involves extensive molecular reprogramming of cells conferring many clinically relevant features to cancer cells and affects tumor cell interactions within the tumor microenvironment. We review the impact of tumor immune infiltrates on tumor cell EMT and the consequences of EMT in shaping the immune microenvironment of tumors. The usefulness of EMT as a model to investigate metastasis-specific immune surveillance mechanisms are also explored. Finally, we discuss potential implications of EMT for tumor immunogenicity, as well as current immunotherapies and future strategies.

C-C Chemokine Receptor Type 2-Dependent Migration of Myeloid-Derived Suppressor Cells in Protection of Islet Transplants

BACKGROUND

Islet transplantation is a promising therapeutic approach to restore the physical response to blood glucose in type 1 diabetes. Current chronic use of immunosuppressive reagents for preventing islet allograft rejection is associated with severe complications. In addition, many of the immunosuppressive drugs are diabetogenic. The induction of transplant tolerance to eliminate the dependency on immunosuppression is ideal, but remains challenging.

METHODS

Addition of hepatic stellate cells allowed generation of myeloid-derived suppressor cells (MDSC) from precursors in mouse bone marrow. Migration of MDSC was examined in an islet allograft transplant model by tracking the systemic administered MDSC from CD45.1 congenic mice.

RESULTS

The generated MDSC were expressed C-C chemokine receptor type 2 (CCR2), which was enhanced by exposure to interferon-γ. A single systemic administration of MDSC markedly prolonged survival of islet allografts without requirement of immunosuppression. Tracking the administered MDSC showed that they promptly migrated to the islet graft sites, at which point they exerted potent immune suppressive activity by inhibiting CD8 T cells, enhancing regulatory T cell activity. MDSC generated from CCR2 mice failed to be mobilized and lost tolerogenic activity in vivo, but sustained suppressive activity in vitro.

CONCLUSIONS

MDSC migration was dependent on expression of CCR2, whereas CCR2 does not directly participate in immune suppression. Expression of CCR2 needs to be closely monitored for quality control purpose when MDSC are generated in vitro for immune therapy.

Dynamic interplay between tumour, stroma and immune system can drive or prevent tumour progression

In the tumour microenvironment, cancer cells directly interact with both the immune system and the stroma. It is firmly established that the immune system, historically believed to be a major part of the body's defence against tumour progression, can be reprogrammed by tumour cells to be ineffective, inactivated, or even acquire tumour promoting phenotypes. Likewise, stromal cells and extracellular matrix can also have pro-and anti-tumour properties. However, there is strong evidence that the stroma and immune system also directly interact, therefore creating a tripartite interaction that exists between cancer cells, immune cells and tumour stroma. This interaction contributes to the maintenance of a chronically inflamed tumour microenvironment with pro-tumorigenic immune phenotypes and facilitated metastatic dissemination. A comprehensive understanding of cancer in the context of dynamical interactions of the immune system and the tumour stroma is therefore required to truly understand the progression toward and past malignancy.

ZEB1-regulated inflammatory phenotype in breast cancer cells

Zinc finger E‐box binding protein 1 (ZEB1) and ZEB2 induce epithelial‐mesenchymal transition (EMT) and enhance cancer progression. However, the global view of transcriptional regulation by ZEB1 and ZEB2 is yet to be elucidated. Here, we identified a ZEB1‐regulated inflammatory phenotype in breast cancer cells using chromatin immunoprecipitation sequencing and RNA sequencing, followed by gene set enrichment analysis (GSEA) of ZEB1‐bound genes. Knockdown of ZEB1 and/or ZEB2 resulted in the downregulation of genes encoding inflammatory cytokines related to poor prognosis in patients with cancer, including IL6 and IL8, therefore suggesting that ZEB1 and ZEB2 have similar functions in terms of the regulation of production of inflammatory cytokines. Antibody array and ELISA experiments confirmed that ZEB1 controlled the production of the IL‐6 and IL‐8 proteins. The secretory proteins regulated by ZEB1 enhanced breast cancer cell proliferation and tumor growth. ZEB1 expression in breast cancer cells also affected the growth of fibroblasts in cell culture, and the accumulation of myeloid‐derived suppressor cells in tumors in vivo. These findings provide insight into the role of ZEB1 in the progression of cancer, mediated by inflammatory cytokines, along with the initiation of EMT.

Tumor-Induced Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) represent a heterogeneous, immune-suppressive leukocyte population that develops systemically and infiltrates tumors. MDSCs can restrain the immune response through different mechanisms including essential metabolite consumption, reactive oxygen and nitrogen species production, as well as display of inhibitory surface molecules that alter T-cell trafficking and viability. Moreover, MDSCs play a role in tumor progression, acting directly on tumor cells and promoting cancer stemness, angiogenesis, stroma deposition, epithelial-to-mesenchymal transition, and metastasis formation. Many biological and pharmaceutical drugs affect MDSC expansion and functions in preclinical tumor models and patients, often reversing host immune dysfunctions and allowing a more effective tumor immunotherapy.

EMT and inflammation: inseparable actors of cancer progression

Tumors can be depicted as wounds that never heal, and are infiltrated by a large array of inflammatory and immune cells. Tumor-associated chronic inflammation is a hallmark of cancer that fosters progression to a metastatic stage, as has been extensively reviewed lately. Indeed, inflammatory cells persisting in the tumor establish a cross-talk with tumor cells that may result in a phenotype switch into tumor-supporting cells. This has been particularly well described for macrophages and is referred to as tumor-associated 'M2' polarization. Epithelial-to-mesenchymal transition (EMT), the embryonic program that loosens cell-cell adherence complexes and endows cells with enhanced migratory and invasive properties, can be co-opted by cancer cells during metastatic progression. Cancer cells that have undergone EMT are more aggressive, displaying increased invasiveness, stem-like features, and resistance to apoptosis. EMT programs can also stimulate the production of proinflammatory factors by cancer cells. Conversely, inflammation is a potent inducer of EMT in tumors. Therefore, the two phenomena may sustain each other, in an alliance for metastasis. This is the focus of this review, where the interconnections between EMT programs and cellular and molecular actors of inflammation are described. We also recapitulate data linking the EMT/inflammation axis to metastasis.

Chemotherapeutic agent-mediated elimination of myeloid-derived suppressor cells

Immunotherapy has shown great promise in the fight against cancer, as evidenced by the clinical efficacy of chimeric antigen receptor T cells in hematologic malignancies and checkpoint blockade in certain solid tumors. However, a considerable number of patients fail to respond to these therapies. Induction of myeloid-derived suppressor cells (MDSCs) by growing tumors has been shown to be one important factor limiting the efficacy of cancer immunotherapy. Recently, several chemotherapeutic agents used in conventional cancer chemotherapy have been found to reduce MDSC numbers in tumor tissues as well as in the peripheral lymphoid organs, and combining these agents with immunotherapy improved survival of tumor-bearing hosts. In this review, we will highlight the effects of chemotherapeutic agents on MDSC accumulation, and examine the various factors likely to influence these effects.

EMT and MET: necessary or permissive for metastasis?

Epithelial‐to‐mesenchymal transition (EMT) and its reverse mesenchymal‐to‐epithelial transition (MET) have been suggested to play crucial roles in metastatic dissemination of carcinomas. These phenotypic transitions between states are not binary. Instead, carcinoma cells often exhibit a spectrum of epithelial/mesenchymal phenotype(s). While epithelial/mesenchymal plasticity has been observed preclinically and clinically, whether any of these phenotypic transitions are indispensable for metastatic outgrowth remains an unanswered question. Here, we focus on epithelial/mesenchymal plasticity in metastatic dissemination and propose alternative mechanisms for successful dissemination and metastases beyond the traditional EMT/MET view. We highlight multiple hypotheses that can help reconcile conflicting observations, and outline the next set of key questions that can offer valuable insights into mechanisms of metastasis in multiple tumor models.

Deciphering the loop of epithelial-mesenchymal transition, inflammatory cytokines and cancer immunoediting

Tumorigenesis and tumor progression relies on the dialectics between tumor cells, the extracellular matrix and its remodelling enzymes, neighbouring cells and soluble cues. The host immune response is crucial in eliminating or promoting tumor growth and the reciprocal coevolution of tumor and immune cells, during disease progression and in response to therapy, shapes tumor fate by activating innate and adaptive mechanisms. The phenotypic plasticity is a common feature of epithelial and immune cells and epithelial-mesenchymal transition (EMT) is a dynamic process, governed by microenvironmental stimuli, critical in tumor cell shaping, increased tumor cell heterogeneity and stemness. In this review we will outline how the dysregulation of microenvironmental signaling is crucial in determining tumor plasticity and EMT, arguing how therapy resistance hinges on these dynamics.

Involvement of the TGFβ1/Smad2/MMP3 signaling pathway in SB431542-induced inhibition of cell invasion in multiple myeloma RPMI 8226 cells

Multiple myeloma (MM) is a malignancy characterized by plasma cell hyperplasia. The majority of patients with MM suffer from mortality due to tumor recurrence and metastasis, which has become an emerging clinical problem. Transforming growth factor β1 (TGFβ1) has been implicated in tumor metastasis; however, its role in RPMI 8226 cells remains to be elucidated. In the present study, RPMI 8226 cells were treated with various concentrations of SB431542, a TGFβ1 inhibitor, for 12, 24 and 48 h. RPMI 8226 cells were transfected with lentiviral-TGFβ1 vectors to overexpress TGFβ1. Cell proliferation rate was subsequently determined by cell-counting kit-8 assay and cell invasion was assessed by Transwell assay. Expression of TGFβ1, SMAD family member 2 (Smad2) and matrix metallopeptidase 3 (MMP3) were analyzed by western blotting. The results demonstrated that cell proliferation and invasion of RPMI 8226 cells was significantly inhibited by SB431542 (P<0.05). SB431542 was able to significantly downregulate the expression of TGFβ1, phosphorylated (p)-Smad2 and MMP3; however, the overexpression of TGFβ1 significantly upregulated the expression of TGFβ1, p-Smad2 and MMP3. In conclusion, SB431542 reduced cell invasion in RPMI 8226 cells, and this effect may be mediated via the TGFβ1/Smad2/MMP3 signaling pathway.

Expanding Diversity and Common Goal of Regulatory T and B Cells. II: In Allergy, Malignancy, and Transplantation

Regulation of immune response was found to play an important role in the course of many diseases such as autoimmune diseases, allergy, malignancy, organ transplantation. The studies on immune regulation focus on the role of regulatory cells (Tregs, Bregs, regulatory myeloid cells) in these disorders. The number and function of Tregs may serve as a marker of disease activity. As in allergy, the depletion of Tregs is observed and the results of allergen-specific immunotherapy could be measured by an increase in the population of IL-10⁺ regulatory cells. On the basis of the knowledge of anti-cancer immune response regulation, new directions in therapy of tumors are introduced. As the proportion of regulatory cells is increased in the course of neoplasm, the therapeutic action is directed at their inhibition. The depletion of Tregs may be also achieved by an anti-check-point blockade, anti-CD25 agents, and inhibition of regulatory cell recruitment to the tumor site by affecting chemokine pathways. However, the possible favorable role of Tregs in cancer development is considered and the plasticity of immune regulation should be taken into account. The new promising direction of the treatment based on regulatory cells is the prevention of transplant rejection. A different way of production and implementation of classic Tregs as well as other cell types such as double-negative cells, Bregs, CD4⁺ Tr1 cells are tested in ongoing trials. On the basis of the results of current studies, we could show in this review the significance of therapies based on regulatory cells in different disorders.

EMT, CSCs, and drug resistance: the mechanistic link and clinical implications

The success of anticancer therapy is usually limited by the development of drug resistance. Such acquired resistance is driven, in part, by intratumoural heterogeneity - that is, the phenotypic diversity of cancer cells co-inhabiting a single tumour mass. The introduction of the cancer stem cell (CSC) concept, which posits the presence of minor subpopulations of CSCs that are uniquely capable of seeding new tumours, has provided a framework for understanding one dimension of intratumoural heterogeneity. This concept, taken together with the identification of the epithelial-to-mesenchymal transition (EMT) programme as a critical regulator of the CSC phenotype, offers an opportunity to investigate the nature of intratumoural heterogeneity and a possible mechanistic basis for anticancer drug resistance. In fact, accumulating evidence indicates that conventional therapies often fail to eradicate carcinoma cells that have entered the CSC state via activation of the EMT programme, thereby permitting CSC-mediated clinical relapse. In this Review, we summarize our current understanding of the link between the EMT programme and the CSC state, and also discuss how this knowledge can contribute to improvements in clinical practice.

Selective blockade of B7-H3 enhances antitumour immune activity by reducing immature myeloid cells in head and neck squamous cell carcinoma

Immature myeloid cells including myeloid-derived suppressor cells (MDSCs) and tumour-associated macrophages (TAMs) promote tumour growth and metastasis by facilitating tumour transformation and angiogenesis, as well as by suppressing antitumour effector immune responses. Therefore, strategies designed to reduce MDSCs and TAMs accumulation and their activities are potentially valuable therapeutic goals. In this study, we show that negative immune checkpoint molecule B7-H3 is significantly overexpressed in human head and neck squamous cell carcinoma (HNSCC) specimen as compared with normal oral mucosa. Using immunocompetent transgenic HNSCC models, we observed that targeting inhibition of B7-H3 reduced tumour size. Flow cytometry analysis revealed that targeting inhibition of B7-H3 increases antitumour immune response by decreasing immunosuppressive cells and promoting cytotoxic T cell activation in both tumour microenvironment and macroenvironment. Our study provides direct in vivo evidence for a rationale for B7-H3 blockade as a future therapeutic strategy to treat patients with HNSCC.

The Sabotaging Role of Myeloid Cells in Anti-Angiogenic Therapy: Coordination of Angiogenesis and Immune Suppression by Hypoxia

Tumor angiogenesis has become a promising target for anti-tumor therapy. Unfortunately, the somewhat inevitable occurrence of resistance has limited the efficacy of anti-angiogenic therapy. In addition to their well-established role in immune suppression, bone marrow-derived myeloid cells actively contribute to tumor angiogenesis. More importantly, myeloid cells constitute one of the major mechanisms of resistance to angiogenesis inhibition. As the most pervasive feature in tumor microenvironment, hypoxia is able to initiate both pro-angiogenic and immunosuppressive capacities of myeloid cells. Tumor adapts to hypoxic stress primarily through signaling mediated by hypoxic inducible factors (HIFs) and consequently utilizes hypoxia to its own advantage. In this regard, hypoxia orchestrates both angiogenesis and immune evasion to support tumor growth. In this article, we will review available information on the sabotaging role of myeloid cells in anti-angiogenic therapy. We will also discuss how hypoxia coordinates the dual-role cellular and molecular participants in microenvironment to maximize the efficiency of angiogenesis and immunosuppression to promote tumor progression. J. Cell. Physiol. 232: 2312-2322, 2017. © 2016 Wiley Periodicals, Inc.

In Brief: Myeloid-derived suppressor cells in cancer

The role of myeloid‐derived suppressor cells (MDSCs) in cancer development has become clear over recent years, and MDSC targeting is an emerging opportunity for enhancing the effectiveness of current anticancer therapies. As MDSCs are not only able to limit anti‐tumour T‐cell responses, but also to promote tumour angiogenesis and invasion, their monitoring has prognostic and predictive value. Herein, we review the key features of MDSCs in cancer promotion. © 2017 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Determinants of Organotropic Metastasis

The spread of cancer from a primary tumor to distant organ sites is the most devastating aspect of malignancy. Dissemination to specific organs depends upon blood flow patterns and characteristics of the distant organ environment, such as the vascular architecture, stromal cell content, and the biochemical milieu of growth factors, signaling molecules, and metabolic substrates, which can be permissive or antagonistic to metastatic colonization. Metastatic tumor cells possess intrinsic cellular properties selected for adaptation to specific organ environments, where they co-opt growth and survival signals, undergo metabolic reprogramming, and subvert resident stromal cell activities to promote extravasation, immune evasion, angiogenesis, and overt metastatic growth. Recent work and new experimental models of metastatic organotropism are uncovering crucial details of how malignant cells metastasize to specific tissues, revealing key mediators that prepare metastatic niches in specific organs and identifying new targets that offer attractive options for therapeutic intervention.

Probing the Bi-directional Interaction Between Microglia and Gliomas in a Tumor Microenvironment on a Microdevice

It has been proven that microglia are involved in both early and late stages of glioma progression and contribute substantially to the tumor mass of gliomas. Because no appropriate in vitro or in vivo investigative approach is available, the dynamic interaction between microglia and gliomas during tumor formation remains unclear. In this study, three types of microfluidic assay were developed to examine the outcomes of the dynamic interaction between microglia and gliomas. Co-migration assay and two-dimensional cell co-culture assay have been used to show that microglial BV-2 cells migrate toward C6 glioma cells and inhibit tumor growth during the early stage of tumorigenesis. However, in three-dimensional cell spheres (three-dimensional cell co-culture assay) that contain a large amount of glioma cells, mimicking the late stage of glioma growth, the phagocytosis of microglia was suppressed, which suggests that glioma cells could reeducate classically activated microglia into a tumor-promoting state at some point during tumor progression. Notably, we found that microglia could contribute to tumor invasion and acquisition of the epithelial-mesenchymal transition phenotype in the glioma microenvironment during the early stage and the late stage of tumor progression. In conclusion, we have developed a potential quantitative method for in vitro study of glioma immunity and provided evidence for the duality of glioma-associated microglia.

The TRAIL-Induced Cancer Secretome Promotes a Tumor-Supportive Immune Microenvironment via CCR2

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is known for specifically killing cancer cells, whereas in resistant cancers, TRAIL/TRAIL-R can promote metastasis via Rac1 and PI3K. It remains unknown, however, whether and to what extent TRAIL/TRAIL-R signaling in cancer cells can affect the immune microenvironment. Here we show that TRAIL-triggered cytokine secretion from TRAIL-resistant cancer cells is FADD dependent and identify the TRAIL-induced secretome to drive monocyte polarization to myeloid-derived suppressor cells (MDSCs) and M2-like macrophages. TRAIL-R suppression in tumor cells impaired CCL2 production and diminished both lung MDSC presence and tumor growth. In accordance, the receptor of CCL2, CCR2, is required to facilitate increased MDSC presence and tumor growth. Finally, TRAIL and CCL2 are co-regulated with MDSC/M2 markers in lung adenocarcinoma patients. Collectively, endogenous TRAIL/TRAIL-R-mediated CCL2 secretion promotes accumulation of tumor-supportive immune cells in the cancer microenvironment, thereby revealing a tumor-supportive immune-modulatory role of the TRAIL/TRAIL-R system in cancer biology.

Resolution of Cancer-Promoting Inflammation: A New Approach for Anticancer Therapy

Inflammation is a protective response that eliminates harmful stimuli and restores tissue homeostasis, whereas the failure to resolve inflammation leads to the development of malignancies. Immune cells in the tumor inflammatory microenvironment endow cancer cells with their specific hallmarks, including mutations, metabolic reprograming, angiogenesis, invasion, and metastasis. Targeting the inflammatory microenvironment with anti-inflammatory drugs (e.g., aspirin) or by enhancing antitumor immunity (e.g., chimeric antigen receptor T cell therapy) has been extensively investigated and has achieved promising results in many cancers. Recently, a novel approach promoting antitumor immunity via a dual anti-inflammatory and pro-resolving strategy was proposed based on the discovery of potent, endogenous, specialized pro-resolving mediators, including lipoxins, resolvins, protectins, and maresins. In this review, we describe the updated principal cellular and molecular mechanisms of inflammation resolution and cancer immunity and discuss the pro-resolution strategy in cancer treatment and prevention.

Transmembrane tumor necrosis factor-α promotes the recruitment of MDSCs to tumor tissue by upregulating CXCR4 expression via TNFR2

Myeloid-derived suppressor cells (MDSCs) accumulated in tumor sites promote immune evasion. We found that TNFR deficiency-induced rejection of transplanted tumor was accompanied with markedly decreased accumulation of MDSCs. However, the mechanism(s) behind this phenomenon is not completely understood. Here, we demonstrated that TNFR deficiency did not affect the amount of MDSCs in bone marrow (BM), but decreased accumulation of Gr-1⁺CD11b⁺ MDSCs in the spleen and tumor tissues. The chemotaxis of Tnfr^-/- MDSCs was prominently decreased in response to both tumor cell culture supernatants and tumor tissue homogenates from Tnfr^-/- and wild-type mice, indicating an effect of TNFR signaling on chemokine receptor expression in MDSCs. We used real-time PCR to detect gene expression for several chemokine receptors in MDSCs from BM and found that CXCR4 was the most affected molecule at the transcriptional level in Tnfr^-/- MDSCs. Neutralizing CXCR4 in wild-type MDSCs by a specific antibody blocked their chemotactic migration. Interestingly, it was tmTNF-α, but not sTNF-α, that induced CXCR4 expression in MDSCs. This effect of tmTNF-α was totally blocked in TNFR2^-/- but not in TNFR1^-/- MDSCs, and partially inhibited by PDTC or SB203580, an inhibitor of NF-κB or p38 MAPK pathway, respectively. Adoptive transfer of wild-type MDSCs restored MDSCs accumulation in tumors of Tnfr^-/- mice, but this could be partially blocked by treatment with a CXCR4 inhibitor AMD3100. Our data suggest that tmTNF-α upregulates CXCR4 expression that promotes chemotaxis of MDSCs to tumor, and give a new insight into a novel mechanism by which tmTNF-α facilitates tumor immune evasion.

IL4-induced gene 1 promotes tumor growth by shaping the immune microenvironment in melanoma

Amino acid catabolizing enzymes emerged as a crucial mechanism used by tumors to dampen immune responses. The L-phenylalanine oxidase IL-4 induced gene 1 (IL4I1) is expressed by tumor-associated myeloid cells of most solid tumors, including melanoma. We previously provided the only evidence that IL4I1 accelerates tumor growth by limiting the CD8⁺ T cell mediated immune response, in a mouse model of melanoma cell transplantation. Here, we explored the role of IL4I1 in Ret mice, a spontaneous model of melanoma. We found that IL4I1 was expressed by CD11b⁺ myeloid cells and that its activity correlated with disease aggressiveness. IL4I1 did not enhance tumor cell proliferation or angiogenesis, but orchestrated the remodeling of the immune compartment within the primary tumor. Indeed, the inactivation of IL4I1 limited the recruitment of polymorphonuclear myeloid-derived suppressor cells and enhanced the infiltration by Th1 and cytotoxic T cells, thus delaying tumor development and metastatic dissemination. Accordingly, human primary melanomas that were poorly infiltrated by IL4I1⁺ cells exhibited a higher density of CD8⁺ T cells. Collectively, our findings strengthen the rationale for therapeutic targeting of IL4I1 as one of the key immune regulators.

Myeloid-Derived Suppressor Cells

Myeloid cells developed evolutionarily as a major mechanism to protect the host. They evolved as a critical barrier against infections and are important contributors to tissue remodeling. However, in cancer, myeloid cells are largely converted to serve a new master-tumor cells. This process is epitomized by myeloid-derived suppressor cells (MDSC). These cells are closely related to neutrophils and monocytes. MDSCs are not present in the steady state of healthy individuals and appear in cancer and in pathologic conditions associated with chronic inflammation or stress. These cells have emerged as an important contributor to tumor progression. Ample evidence supports a key role for MDSCs in immune suppression in cancer, as well as their prominent role in tumor angiogenesis, drug resistance, and promotion of tumor metastases. MDSCs have a fascinating biology and are implicated in limiting the effects of cancer immunotherapy. Therefore, targeting these cells may represent an attractive therapeutic opportunity. Cancer Immunol Res; 5(1); 3-8. ©2016 AACR.

Myeloid-Derived Suppressor Cells

Myeloid cells developed evolutionarily as a major mechanism to protect the host. They evolved as a critical barrier against infections and are important contributors to tissue remodeling. However, in cancer, myeloid cells are largely converted to serve a new master-tumor cells. This process is epitomized by myeloid-derived suppressor cells (MDSC). These cells are closely related to neutrophils and monocytes. MDSCs are not present in the steady state of healthy individuals and appear in cancer and in pathologic conditions associated with chronic inflammation or stress. These cells have emerged as an important contributor to tumor progression. Ample evidence supports a key role for MDSCs in immune suppression in cancer, as well as their prominent role in tumor angiogenesis, drug resistance, and promotion of tumor metastases. MDSCs have a fascinating biology and are implicated in limiting the effects of cancer immunotherapy. Therefore, targeting these cells may represent an attractive therapeutic opportunity. Cancer Immunol Res; 5(1); 3-8. ©2016 AACR.

Chemokine Receptor Signaling and the Hallmarks of Cancer

The chemokines are a family of chemotactic cytokines that mediate their activity by acting on seven-transmembrane-spanning G protein-coupled receptors. Both the ability of the chemokines and their receptors to form homo- and heterodimers and the promiscuity of the chemokine-chemokine receptor interaction endow this protein family with enormous signaling plasticity and complexity that are not fully understood at present. Chemokines were initially identified as essential regulators of homeostatic and inflammatory trafficking of innate and adaptive leucocytes from lymphoid organs to tissues. Chemokines also mediate the host response to cancer. Nevertheless, chemokine function in this response is not limited to regulating leucocyte infiltration into the tumor microenvironment. It is now known that chemokines and their receptors influence most-if not all-hallmark processes of cancer; they act on both neoplastic and untransformed cells in the tumor microenvironment, including fibroblasts, endothelial cells (blood and lymphatic), bone marrow-derived stem cells, and, obviously, infiltrating leucocytes. This review begins with an overview of chemokine and chemokine receptor structure, to better define how chemokines affect the proliferation, survival, stemness, and metastatic potential of neoplastic cells. We also examine the main mechanisms by which chemokines regulate tumor angiogenesis and immune cell infiltration, emphasizing the pro- and antitumorigenic activity of this protein superfamily in these interrelated processes.

Pro-Tumoral Inflammatory Myeloid Cells as Emerging Therapeutic Targets

Since the observation of Virchow, it has long been known that the tumor microenvironment constitutes the soil for the infiltration of inflammatory cells and for the release of inflammatory mediators. Under certain circumstances, inflammation remains unresolved and promotes cancer development. Here, we review some of these indisputable experimental and clinical evidences of cancer related smouldering inflammation. The most common myeloid infiltrate in solid tumors is composed of myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs). These cells promote tumor growth by several mechanisms, including their inherent immunosuppressive activity, promotion of neoangiogenesis, mediation of epithelial-mesenchymal transition and alteration of cellular metabolism. The pro-tumoral functions of TAMs and MDSCs are further enhanced by their cross-talk offering a myriad of potential anti-cancer therapeutic targets. We highlight these main pro-tumoral mechanisms of myeloid cells and give a general overview of their phenotypical and functional diversity, offering examples of possible therapeutic targets. Pharmacological targeting of inflammatory cells and molecular mediators may result in therapies improving patient condition and prognosis. Here, we review experimental and clinical findings on cancer-related inflammation with a major focus on creating an inventory of current small molecule-based therapeutic interventions targeting cancer-related inflammatory cells: TAMs and MDSCs.

Lymphadenectomy promotes tumor growth and cancer cell dissemination in the spontaneous RET mouse model of human uveal melanoma

Resection of infiltrated tumor-draining lymph nodes (TDLNs) is a standard practice for the treatment of several cancers including breast cancer and melanoma. However, many randomized prospective trials have failed to show convincing clinical benefits associated with LN removal and the role of TDLNs in cancer dissemination is poorly understood. Here, we found in a well-characterized spontaneous mouse model of uveal melanoma that the growth of the primary tumor was accompanied by increased lymphangiogenesis and cancer cell colonization in the LNs draining the eyes. But, unexpectedly, early resection of the TDLNs increased the growth of the primary tumor and associated blood vessels as well as promoted cancer cell survival and dissemination. These effects were accompanied by increased tumor cell proliferation and expression of phosphorylated AKT. Topical application of a broad anti-inflammatory agent, Tobradex, or an oral treatment with cyclooxygenase-2 specific inhibitor, Celecoxib, reversed tumor progression observed after complete lymphadenectomy. Our study confirms the importance of tumor homeostasis in cancer progression by showing the enhancing effects of TDLN removal on tumor growth and cancer cell dissemination, and suggests that TDLN resection may only be beneficial if used in combination with anti-inflammatory drugs such as Tobradex and Celecoxib.

Tackling Cancer Stem Cells via Inhibition of EMT Transcription Factors

Cancer stem cell (CSC) has become recognized for its role in both tumorigenesis and poor patient prognosis in recent years. Traditional therapeutics are unable to effectively eliminate this group of cells from the bulk population of cancer cells, allowing CSCs to persist posttreatment and thus propagate into secondary tumors. The therapeutic potential of eliminating CSCs, to decrease tumor relapse, has created a demand for identifying mechanisms that directly target and eliminate cancer stem cells. Molecular profiling has shown that cancer cells and tumors that exhibit the CSC phenotype also express genes associated with the epithelial-to-mesenchymal transition (EMT) feature. Ample evidence has demonstrated that upregulation of master transcription factors (TFs) accounting for the EMT process such as Snail/Slug and Twist can reprogram cancer cells from differentiated to stem-like status. Despite being appealing therapeutic targets for tackling CSCs, pharmacological approaches that directly target EMT-TFs remain impossible. In this review, we will summarize recent advances in the regulation of Snail/Slug and Twist at transcriptional, translational, and posttranslational levels and discuss the clinical implication and application for EMT blockade as a promising strategy for CSC targeting.

Hypoxia augments MHC class I antigen presentation via facilitation of ERO1-α-mediated oxidative folding in murine tumor cells

To establish an effective cancer immunotherapy, it is crucial that cancer cells present a cancer-specific antigen in a hypoxic area, a hallmark of the tumor microenvironment. Here, we show the impact of hypoxia on MHC class I antigen presentation in vitro and in vivo in murine tumors. Activation of antigen-specific CTLs by tumor cells that had been pre-incubated under a condition of hypoxia was enhanced compared with that by tumor cells pre-incubated under a condition of normoxia. Cell surface expression of MHC class I-peptide complex on the tumor cells was increased under a condition of hypoxia, thereby leading to higher susceptibility to specific CTLs. We show that the hypoxia-inducible ER-resident oxidase ERO1-α plays an important role in the hypoxia-induced augmentation of MHC class I-peptide complex expression. ERO1-α facilitated oxidative folding of MHC class I heavy chains, thereby resulting in the augmentation of cell surface expression of MHC class I-peptide complex under hypoxic conditions. These results suggest that since the expression of MHC class I-peptide complex is augmented in a hypoxic tumor microenvironment, strategies for inhibiting the function of regulatory T cells and myeloid-derived suppressor cells and/or immunotherapy with immune checkpoint inhibitors are promising for improving cancer immunotherapy.

Neutrophils and Granulocytic MDSC: The Janus God of Cancer Immunotherapy

Neutrophils are the most abundant circulating blood cell type in humans, and are the first white blood cells recruited at the inflammation site where they orchestrate the initial immune response. Although their presence at the tumor site was recognized in the 1970s, until recently these cells have been neglected and considered to play just a neutral role in tumor progression. Indeed, in recent years neutrophils have been recognized to play a dual role in tumor development by either assisting the growth, angiogenesis, invasion, and metastasis or by exerting tumoricidal action directly via the secretion of antitumoral compounds, or indirectly via the orchestration of antitumor immunity. Understanding the biology of these cells and influencing their polarization in the tumor micro- and macro-environment may be the key for the development of new therapeutic strategies, which may finally hold the promise of an effective immunotherapy for cancer.

The interplay of epigenetic therapy and immunity in locally recurrent or metastatic estrogen receptor-positive breast cancer: Correlative analysis of ENCORE 301, a randomized, placebo-controlled phase II trial of exemestane with or without entinostat

Entinostat, a class I-selective histone deacetylase inhibitor, has shown promising activity in ENCORE 301, a randomized, placebo-controlled, phase II trial of exemestane with or without entinostat in women with locally recurrent or metastatic estrogen receptor-positive breast cancer progressing on a nonsteroidal aromatase inhibitor. ENCORE 301 showed an 8.3-mo improvement in median overall survival among patients who received entinostat. We investigated the impact of entinostat on immune subsets with CD40, HLA-DR, and immune checkpoint receptor expression analyses in 34 patient blood samples from ENCORE 301. We found that entinostat significantly decreased granulocytic and monocytic MDSCs at cycle 1 day 15. MDSC CD40 was significantly downregulated by entinostat. A significant increase in HLA-DR expression on CD14⁺ monocytes by entinostat was observed. Entinostat did not impact T-cell subsets or T-cell immune checkpoint receptor expression. Our findings suggest that a significant interplay between this epigenetic regimen and host immune homeostatic mechanisms may impact therapeutic outcome.

TLR3 agonist and Sorafenib combinatorial therapy promotes immune activation and controls hepatocellular carcinoma progression

Hepatocellular carcinoma (HCC) is associated with high mortality and the current therapy for advanced HCC, Sorafenib, offers limited survival benefits. Here we assessed whether combining the TLR3 agonist: lysine-stabilized polyinosinic-polycytidylic-acid (poly-ICLC) with Sorafenib could enhance tumor control in HCC. Combinatorial therapy with poly-ICLC and Sorafenib increased apoptosis and reduced proliferation of HCC cell lines in vitro, in association with impaired phosphorylation of AKT, MEK and ERK. In vivo, the combinatorial treatment enhanced control of tumor growth in two mouse models: one transplanted with Hepa 1-6 cells, and the other with liver tumors induced using the Sleeping beauty transposon. Tumor cell apoptosis and host immune responses in the tumor microenvironment were enhanced. Particularly, the activation of local NK cells, T cells, macrophages and dendritic cells was enhanced. Decreased expression of the inhibitory signaling molecules PD-1 and PD-L1 was observed in tumor-infiltrating CD8⁺ T cells and tumor cells, respectively. Tumor infiltration by monocytic-myeloid derived suppressor cells (Mo-MDSC) was also reduced indicating the reversion of the immunosuppressive tumor microenvironment. Our data demonstrated that the combinatorial therapy with poly-ICLC and Sorafenib enhances tumor control and local immune response hence providing a rationale for future clinical studies.

Nitric oxide synthase 2 is involved in the pro-tumorigenic potential of γδ17 T cells in melanoma

γδ T lymphocytes may exert either protective or tumor-promoting functions in cancer, mostly based on their polarization toward interferon (IFN)-γ or interleukin (IL)-17 productions, respectively. Here, we demonstrate that γδ T cells accelerate the spontaneous metastatic melanoma development in a model of transgenic mice for the human RET oncogene (Ret mice). We identify unanticipated roles of inducible nitric oxide synthase (NOS2) in favoring the recruitment of pro-tumor γδ T cells within the primary tumor. γδ T cells isolated from Ret mice deficient for NOS2 produced more IFNγ and less IL-17 than their counterparts from Ret mice. By supporting IL-17 production by γδ T cells, NOS2 leads to the recruitment of polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) and metastasis formation. NOS2 also reduces the cytotoxicity of γδ T cells toward melanoma cells. Finally, we detected NOS2 expressing γδ T cells in the primary tumor and tumor-draining lymph nodes in Ret mice, but also in human melanoma. Overall our results support that this NOS2 autocrine expression is responsible for the polarization of γδ T cells toward a pro-tumor profile.

Activated NKT cells facilitated functional switch of myeloid-derived suppressor cells at inflammation sites in fulminant hepatitis mice

Myeloid-derived suppressor cells (MDSCs) confer immunosuppressive properties, but their roles in fulminant hepatitis have not been well defined. In this study, we systematically examined the distribution of MDSCs in bone marrow (BM), liver and spleen, and their functional and differentiation status in an acute fulminant hepatitis mouse model induced by lipopolysaccharide and D-galactosamine (LPS-GalN). Moreover, the interaction between NKT cells and MDSCs was determined. Our study revealed that BM contained the largest pool of MDSCs during pathogenesis of fulminant hepatitis compared with liver and spleen. MDSCs in liver/spleen expressed higher levels of chemokine receptors such as CCR2, CX3CR1 and CXCR2. At inflamed tissues such as liver or spleen, activated NKT cells induced differentiation of MDSCs through cell-cell interaction, which markedly dampened the immunosuppressive effects and promoted MDSCs to produce pro-inflammatory cytokines and activate inflammatory cells. Our findings thus demonstrated an unexpected pro-inflammatory state for MDSCs, which was mediated by the activated NKT cells that precipitated the differentiation and functional evolution of these MDSCs at sites of inflammation.

Macrophage depletion reduces postsurgical tumor recurrence and metastatic growth in a spontaneous murine model of melanoma

Surgical resection of tumors is often followed by regrowth at the primary site and metastases may emerge rapidly following removal of the primary tumor. Macrophages are important drivers of tumor growth, and here we investigated their involvement in postoperative relapse as well as explore macrophage depletion as an adjuvant to surgical resection. RETAAD mice develop spontaneous metastatic melanoma that begins in the eye. Removal of the eyes as early as 1 week of age did not prevent the development of metastases; rather, surgery led to increased proliferation of tumor cells locally and in distant metastases. Surgery-induced increase in tumor cell proliferation correlated with increased macrophage density within the tumor. Moreover, macrophages stimulate tumor sphere formation from tumor cells of post-surgical but not control mice. Macrophage depletion with a diet containing the CSF-1R specific kinase inhibitor Ki20227 following surgery significantly reduced postoperative tumor recurrence and abrogated enhanced metastatic outgrowth. Our results confirm that tumor cells disseminate early, and show that macrophages contribute both to post-surgical tumor relapse and growth of metastases, likely through stimulating a population of tumor-initiating cells. Thus macrophage depletion warrants exploration as an adjuvant to surgical resection.

Mesenchymal Stem Cells and Myeloid Derived Suppressor Cells: Common Traits in Immune Regulation

To protect host against immune-mediated damage, immune responses are tightly regulated. The regulation of immune responses is mediated by various populations of mature immune cells, such as T regulatory cells and B regulatory cells, but also by immature cells of different origins. In this review, we discuss regulatory properties and mechanisms whereby two distinct populations of immature cells, mesenchymal stem cells, and myeloid derived suppressor cells mediate immune regulation, focusing on their similarities, discrepancies, and potential clinical applications.

Immune Regulation of the Metastatic Process: Implications for Therapy

Metastatic disease is the major cause of fatalities in cancer patients, but few therapies are designed to target the metastatic process. Cancer cells must perform a number of steps to successfully establish metastatic foci, including local invasion, intravasation, survival, extravasation, and growth in ectopic tissue. Due to the nonrandom distribution of metastasis, it has long been recognized that the tissue microenvironment must be an important determinant of colonization. More recently it has been established in animal models that immune cells regulate the metastatic process, including a dominant role for monocytes and macrophages, and emerging roles for neutrophils and various lymphocyte populations. While most research has focused on the early dissemination process, patients usually present clinically with disseminated, if not macroscopic, disease. Identifying pathways by which immune cells regulate growth and therapeutic resistance within metastatic sites is therefore key to the development of pharmacological agents that will significantly extend patient survival.

Pharmacological Modulation of Proton Channel Hv1 in Cancer Therapy: Future Perspectives

The pharmacological modulation of the immunosuppressive tumor microenvironment has emerged as a relevant component for cancer therapy. Several approaches aiming to deplete innate and adaptive suppressive populations, to circumvent the impairment in antigen presentation, and to ultimately increase the frequency of activated tumor-specific T cells are currently being explored. In this review, we address the potentiality of targeting the voltage-gated proton channel, Hv1, as a novel strategy to modulate the tumor microenvironment. The function of Hv1 in immune cells such as macrophages, neutrophils, dendritic cells, and T cells has been associated with the maintenance of NADPH oxidase activity and the generation of reactive oxygen species, which are required for the host defense against pathogens. We discuss evidence suggesting that the Hv1 proton channel could also be important for the function of these cells within the tumor microenvironment. Furthermore, as summarized here, tumor cells express Hv1 as a primary mechanism to extrude the increased amount of protons generated metabolically, thus maintaining physiologic values for the intracellular pH. Therefore, because this channel might be relevant for both tumor cells and immune cells supporting tumor growth, the pharmacological inhibition of Hv1 could be an innovative approach for cancer therapy. With that focus, we analyzed the available compounds that inhibit Hv1, highlighted the need to develop better drugs suitable for patients, and commented on the future perspectives of targeting Hv1 in the context of cancer therapy.

NK Cells, Tumor Cell Transition, and Tumor Progression in Solid Malignancies: New Hints for NK-Based Immunotherapy?

Several evidences suggest that NK cells can patrol the body and eliminate tumors in their initial phases but may hardly control established solid tumors. Multiple factors, including the transition of tumor cells towards a proinvasive/prometastatic phenotype, the immunosuppressive effect of the tumor microenvironment, and the tumor structure complexity, may account for limited NK cell efficacy. Several putative mechanisms of NK cell suppression have been defined in these last years; conversely, the cross talk between NK cells and tumor cells undergoing different transitional phases remains poorly explored. Nevertheless, recent in vitro studies and immunohistochemical analyses on tumor biopsies suggest that NK cells could not only kill tumor cells but also influence their evolution. Indeed, NK cells may induce tumor cells to change the expression of HLA-I, PD-L1, or NKG2D-L and modulate their susceptibility to the immune response. Moreover, NK cells may be preferentially located in the borders of tumor masses, where, indeed, tumor cells can undergo Epithelial-to-Mesenchymal Transition (EMT) acquiring prometastatic phenotype. Finally, the recently highlighted role of HMGB1 both in EMT and in amplifying the recruitment of NK cells provides further hints on a possible effect of NK cells on tumor progression and fosters new studies on this issue.

Neutrophils: Critical components in experimental animal models of cancer

Neutrophils have a crucial role in tumor development and metastatic progression. The contribution of neutrophils in tumor development is multifaceted and contradictory. On the one hand, neutrophils prompt tumor inception, promote tumor development by mediating the initial angiogenic switch and facilitate colonization of circulating tumor cells, and on the other hand, have cytotoxic and anti-metastatic capabilities. Our understanding of the role of neutrophils in tumor development has greatly depended on different experimental animal models of cancer. In this review we cover important findings that have been made about neutrophils in experimental animal models of cancer, point to their advantages and limitations, and discuss novel techniques that can be used to expand our knowledge of how neutrophils influence tumor progression.