Chemokine nitration prevents intratumoral infiltration of antigen-specific T cells

Getting in front and behind the enemy lines to counter virus infection

How immune cells collaborate to clear a virus from the skin is not well understood. In this issue of Cell Host & Microbe,Hickman et al. (2013) show that spatially segregated Ly6G(+) monocytes and CD8(+) T cells act synergistically to clear vaccinia virus from a primary skin infection.

Deciphering and reversing tumor immune suppression

Generating an anti-tumor immune response is a multi-step process that is executed by effector T cells that can recognize and kill tumor targets. However, tumors employ multiple strategies to attenuate the effectiveness of T-cell-mediated attack. They achieve this by interfering with nearly every step required for effective immunity, from deregulation of antigen-presenting cells to establishment of a physical barrier at the vasculature that prevents homing of effector tumor-rejecting cells and the suppression of effector lymphocytes through the recruitment and activation of immunosuppressive cells such as myeloid-derived suppressor cells, tolerogenic monocytes, and T regulatory cells. Here, we review the ways in which tumors exert immune suppression and highlight the new therapies that seek to reverse this phenomenon and promote anti-tumor immunity. Understanding anti-tumor immunity, and how it becomes disabled by tumors, will ultimately lead to improved immune therapies and prolonged survival of patients.

Molecular pathways: inflammation-associated nitric-oxide production as a cancer-supporting redox mechanism and a potential therapeutic target

It is widely accepted that many cancers express features of inflammation, driven by both microenvironmental cells and factors, and the intrinsic production of inflammation-associated mediators from malignant cells themselves. Inflammation results in intracellular oxidative stress with the ultimate biochemical oxidants composed of reactive nitrogens and oxygens. Although the role of inflammation in carcinogensis is well accepted, we now present data showing that inflammatory processes are also active in the maintenance phase of many aggressive forms of cancer. The oxidative stress of inflammation is proposed to drive a continuous process of DNA adducts and crosslinks, as well as posttranslational modifications to lipids and proteins that we argue support growth and survival. In this perspective, we introduce data on the emerging science of inflammation-driven posttranslational modifications on proteins responsible for driving growth, angiogenesis, immunosuppression, and inhibition of apoptosis. Examples include data from human melanoma, breast, head and neck, lung, and colon cancers. Fortunately, numerous antioxidant agents are clinically available, and we further propose that the pharmacologic attenuation of these inflammatory processes, particularly the reactive nitrogen species, will restore the cancer cells to an apoptosis-permissive and growth-inhibitory state. Our mouse model data using an arginine antagonist that prevents enzymatic production of nitric oxide directly supports this view. We contend that selected antioxidants be considered as part of the cancer treatment approach, as they are likely to provide a novel and mechanistically justified addition for therapeutic benefit.

Modulation of CD8(+) T-cell activation events by monocytic and granulocytic myeloid-derived suppressor cells

Myeloid-derived suppressor cells are immature myeloid cells, consisting of a monocytic and a granulocytic fraction, that are known to suppress anti-tumor immune responses. Important targets of the immunosuppressive capacity of MDSC are CD8(+) T cells, which are crucial cytotoxic effector cells in immunotherapeutic settings. CD8(+) T-cell activation and differentiation comprises a well-orchestrated series of events, starting from early TCR-mediated signaling and leading to cytokine secretion, the expression of activation markers, proliferation and the differentiation into several subsets of effector and memory cells. In this review, we summarize the available data on how the production of reactive oxygen species, nitric oxide, the arginase-mediated depletion of l-arginine and Cystine depletion by MDSCs interfere with the signaling molecules necessary for normal CTL differentiation and activation.

The Activity of CCL18 is Principally Mediated through Interaction with Glycosaminoglycans

The CC chemokine ligand 18 (CCL18) was first identified as a chemoattractant for naïve T cells. It has been reported to recruit T and B lymphocytes, and we show here, natural killer (NK) cells, but with low efficacy. Investigation of its ability to elicit G-protein-coupled signaling showed that it does not involve extracellular signal-regulated kinase (ERK) phosphorylation, and it is not able to induce receptor internalization, as assessed on CCR3. CCL18 has recently been reported to possess activities unrelated to cellular recruitment, but it had no effect on T lymphocyte proliferation. We postulated that a more potent chemoattractant may be produced under inflammatory conditions but only minor truncations were observed, with the major form being the full-length protein. In view of the lack of potent immunomodulatory properties, we wondered if binding to CCL18 by the tick chemokine binding proteins Evasin-1 and -4 was an artifact of the methods used, but complex formation was confirmed by size exclusion chromatography, and abrogation of its binding to, and antagonism of, CCR3. Its receptor has remained elusive since its cloning in 1997, although it has been reported to induce migration of breast cancer cells by signaling through PITPNM3, but we show that this receptor is not expressed on lymphocytes. We have developed a radiolabeled equilibrium competition binding assay and demonstrated that it bound with high affinity to peripheral blood leukocytes (PBLs), but the binding was displaced similarly by both unlabelled CCL18 as well as heparin. Both heparin binding and binding to PBLs are considerably abrogated by mutation of the BBXB motif in the 40s loop suggesting an essential role of the CCL18-glycosaminoglycan interaction.

Myeloid derived suppressor cells - a new therapeutic target in the treatment of cancer

Myeloid Derived Suppressor Cells (MDSC) are a heterogeneous population of immature myeloid cells that are increased in states of cancer, inflammation and infection. In malignant states, MDSC are induced by tumor secreted growth factors. MDSC play an important part in suppression of host immune responses through several mechanisms such as production of arginase 1, release of reactive oxygen species and nitric oxide and secretion of immune-suppressive cytokines. This leads to a permissive immune environment necessary for the growth of malignant cells. MDSC may also contribute to angiogenesis and tumor invasion. This review focuses on currently available strategies to inhibit MDSC in the treatment of cancer.

Myeloid-derived suppressor cells in breast cancer

Myeloid-derived suppressor cells (MDSCs) are a population of immature myeloid cells defined by their suppressive actions on immune cells such as T cells, dendritic cells, and natural killer cells. MDSCs typically are positive for the markers CD33 and CD11b but express low levels of HLADR in humans. In mice, MDSCs are typically positive for both CD11b and Gr1. These cells exert their suppressive activity on the immune system via the production of reactive oxygen species, arginase, and cytokines. These factors subsequently inhibit the activity of multiple protein targets such as the T cell receptor, STAT1, and indoleamine-pyrrole 2,3-dioxygenase. The numbers of MDSCs tend to increase with cancer burden while inhibiting MDSCs improves disease outcome in murine models. MDSCs also inhibit immune cancer therapeutics. In light of the poor prognosis of metastatic breast cancer in women and the correlation of increasing levels of MDSCs with increasing disease burden, the purposes of this review are to (1) discuss why MDSCs may be important in breast cancer, (2) describe model systems used to study MDSCs in vitro and in vivo, (3) discuss mechanisms involved in MDSC induction/function in breast cancer, and (4) present pre-clinical and clinical studies that explore modulation of the MDSC-immune system interaction in breast cancer. MDSCs inhibit the host immune response in breast cancer patients and diminishing MDSC actions may improve therapeutic outcomes.

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.

Immunosuppressive microenvironment in neuroblastoma

According to the cancer immunoediting model, the interplay between tumor cells and the host immune system is crucial for the control of tumor growth. NB is a pediatric tumor that presents with metastatic disease at diagnosis in about 50% of the cases, the majority of which have poor prognosis. In this Review article, immune escape pathways adopted by human neuroblastoma (NB) cells are reviewed. These include intrinsic defects of tumor cells such impaired expression of the HLA class I related antigen processing machinery and functional alterations of the tumor microenvironment (TM) induced by NB cell-derived immunosuppressive molecules as MICA and HLA-G. Finally, examples of therapeutic interventions targeting the TM are discussed to emphasize the concept that successful cancer treatment may be achieved using this strategy.

Beyond oxidative stress: an immunologist's guide to reactive oxygen species

Reactive oxygen species (ROS) react preferentially with certain atoms to modulate functions ranging from cell homeostasis to cell death. Molecular actions include both inhibition and activation of proteins, mutagenesis of DNA and activation of gene transcription. Cellular actions include promotion or suppression of inflammation, immunity and carcinogenesis. ROS help the host to compete against microorganisms and are also involved in intermicrobial competition. ROS chemistry and their pleiotropy make them difficult to localize, to quantify and to manipulate - challenges we must overcome to translate ROS biology into medical advances.

Immunity of human epithelial ovarian carcinoma: the paradigm of immune suppression in cancer

Epithelial ovarian cancer (EOC) is a significant cause of cancer-related mortality in women, and there has been no substantial decrease in the death rates due to EOC in the last three decades. Thus, basic knowledge regarding ovarian tumor cell biology is urgently needed to allow the development of innovative treatments for EOC. Traditionally, EOC has not been considered an immunogenic tumor, but there is evidence of an immune response to EOC in patients. Clinical data demonstrate that an antitumor immune response and immune evasion mechanisms are correlated with a better and lower survival, respectively, providing evidence for the immunoediting hypothesis in EOC. This review focuses on the immune response and immune suppression in EOC. The immunological roles of chemotherapy and surgery in EOC are also described. Finally, we detail pilot data supporting the efficiency of immunotherapy in the treatment of EOC and the emerging concept that immunomodulation aimed at counteracting the immunosuppressive microenvironment must be associated with immunotherapy strategies.

In vivo regulation of chemokine activity by post-translational modification

Cytokines and chemokines represent two important groups of proteins that control the immune system. Dysregulation of the network in which these immunomodulators function can result in uncontrolled inflammation leading to various diseases, including rheumatoid arthritis, characterized by chronic inflammation and bone erosion. Chemokine activity is regulated at multiple levels, such as post-translational modification (PTM) of chemokines and their receptors by specific enzymes including proteases and peptidylarginine deiminases. Many in vitro experiments underscore the importance of post-translational processing of human chemokines. PTMs may enhance or reduce chemokine activity or may alter the receptor specificity of chemokine ligands. However, identification of chemokine isoforms in physiological in vivo settings forms the ultimate proof that PTM of chemokines is relevant in regulating the biological activity of these molecules. This review summarizes current knowledge on the in vivo role for PTMs in the regulation of chemokine activity.

Imaging the microanatomy of astrocyte-T-cell interactions in immune-mediated inflammation

The role of astrocytes in the immune-mediated inflammatory response in the brain is more prominent than previously thought. Astrocytes become reactive in response to neuro-inflammatory stimuli through multiple pathways, contributing significantly to the machinery that modifies the parenchymal environment. In particular, astrocytic signaling induces the establishment of critical relationships with infiltrating blood cells, such as lymphocytes, which is a fundamental process for an effective immune response. The interaction between astrocytes and T-cells involves complex modifications to both cell types, which undergo micro-anatomical changes and the redistribution of their binding and secretory domains. These modifications are critical for different immunological responses, such as for the effectiveness of the T-cell response, for the specific infiltration of these cells and their homing in the brain parenchyma, and for their correct apposition with antigen-presenting cells (APCs) to form immunological synapses (ISs). In this article, we review the current knowledge of the interactions between T-cells and astrocytes in the context of immune-mediated inflammation in the brain, based on the micro-anatomical imaging of these appositions by high-resolution confocal microscopy and three-dimensional rendering. The study of these dynamic interactions using detailed technical approaches contributes to understanding the function of astrocytes in inflammatory responses and paves the way for new therapeutic strategies.

Protumor activities of the immune response: insights in the mechanisms of immunological shift, oncotraining, and oncopromotion

Experimental and clinical studies indicate that cells of the innate and adaptive immune system have both anti- and pro-tumor activities. This dual role of the immune system has led to a conceptual shift in the role of the immune system's regulation of cancer, in which immune-tumor cell interactions are understood as a dynamic process that comprises at least five phases: immunosurveillance, immunoselection, immunoescape, oncotraining, and oncopromotion. The tumor microenvironment shifts immune cells to perform functions more in tune with the tumor needs (oncotraining); these functions are related to chronic inflammation and tissue remodeling activities. Among them are increased proliferation and survival, increased angiogenesis and vessel permeability, protease secretion, acquisition of migratory mesenchymal characteristics, and self-renewal properties that altogether promote tumor growth and metastasis (oncopromotion). Important populations in all these pro-tumor processes are M2 macrophages, N2 neutrophils, regulatory T cells, and myeloid derived suppressor cells; the main effectors molecules are CSF-1, IL-6, metalloproteases, VEGF, PGE-2, TGF- β , and IL-10. Cancer prognosis correlates with densities and concentrations of protumoral populations and molecules, providing ideal targets for the intelligent design of directed preventive or anticancer therapies.

The immunosuppressive tumour network: myeloid-derived suppressor cells, regulatory T cells and natural killer T cells

Myeloid-derived suppressor cells (MDSC) and regulatory T (Treg) cells are major components of the immune suppressive tumour microenvironment (TME). Both cell types expand systematically in preclinical tumour models and promote T-cell dysfunction that in turn favours tumour progression. Clinical reports show a positive correlation between elevated levels of both suppressors and tumour burden. Recent studies further revealed that MDSCs can modulate the de novo development and induction of Treg cells. The overlapping target cell population of Treg cells and MDSCs is indicative for the importance and flexibility of immune suppression under pathological conditions. It also suggests the existence of common pathways that can be used for clinical interventions aiming to manipulate the TME. Elimination or reprogramming of the immune suppressive TME is one of the major current challenges in immunotherapy of cancer. Interestingly, recent findings suggest that natural killer T (NKT) cells can acquire the ability to convert immunosuppressive MDSCs into immunity-promoting antigen-presenting cells. Here we will review the cross-talk between MDSCs and other immune cells, focusing on Treg cells and NKT cells. We will consider its impact on basic and applied cancer research and discuss how targeting MDSCs may pave the way for future immunocombination therapies.

The kinship of neutrophils and granulocytic myeloid-derived suppressor cells in cancer: cousins, siblings or twins?

Neutrophils in the tumor host may promote tumor progression by enhancing angiogenesis, invasion and metastasis. Granulocytic myeloid-derived suppressor cells (MDSC) share many features with neutrophils. Classically, MDSC are viewed as and defined as immunosuppressive cells. In this article we summarize and critically review evidence for a role of MDSC in promoting angiogenesis, invasion and metastasis of solid tumors. We also attempt to provide a critical evaluation of the relationship between neutrophils and G-MDSC in the tumor host with a particular focus on human cancer.

Protein tyrosine nitration: biochemical mechanisms and structural basis of functional effects

In proteins, the nitration of tyrosine residues to 3-nitro-tyrosine represents an oxidative post-translational modification that disrupts nitric oxide ((•)NO) signaling and skews metabolism towards pro-oxidant processes. Indeed, excess levels of reactive oxygen species in the presence of (•)NO or (•)NO-derived metabolites lead to the formation of nitrating species such as peroxynitrite. Thus, protein 3-nitrotyrosine has been established as a biomarker of cell, tissue, and systemic "nitroxidative stress". Moreover, tyrosine nitration modifies key properties of the amino acid: phenol group pK(a), redox potential, hydrophobicity, and volume. Thus, the incorporation of a nitro group (-NO(2)) into protein tyrosines can lead to profound structural and functional changes, some of which contribute to altered cell and tissue homeostasis. In this Account, I describe our current efforts to define (1) biologically-relevant mechanisms of protein tyrosine nitration and (2) how this modification can cause changes in protein structure and function at the molecular level. First, I underscore the relevance of protein tyrosine nitration via free-radical-mediated reactions (in both peroxynitrite-dependent and -independent pathways) involving a tyrosyl radical intermediate (Tyr(•)). This feature of the nitration process is critical because Tyr(•) can follow various fates, including the formation of 3-nitrotyrosine. Fast kinetic techniques, electron paramagnetic resonance (EPR) studies, bioanalytical methods, and kinetic simulations have all assisted in characterizing and fingerprinting the reactions of tyrosine with peroxynitrite and one-electron oxidants and its further evolution to 3-nitrotyrosine. Recent findings show that nitration of tyrosines in proteins associated with biomembranes is linked to the lipid peroxidation process via a connecting reaction that involves the one-electron oxidation of tyrosine by lipid peroxyl radicals (LOO(•)). Second, immunochemical and proteomic-based studies indicate that protein tyrosine nitration is a selective process in vitro and in vivo, preferentially directed to a subset of proteins, and within those proteins, typically one or two tyrosine residues are site-specifically modified. The nature and site(s) of formation of the proximal oxidizing or nitrating species, the physicochemical characteristics of the local microenvironment, and the structural features of the protein account for part of this selectivity. How this relatively subtle chemical modification in one tyrosine residue can sometimes cause dramatic changes in protein activity has remained elusive. Herein, I analyze recent structural biology data of two pure and homogenously nitrated mitochondrial proteins (i.e., cytochrome c and manganese superoxide dismutase, MnSOD) to illustrate regioselectivity and structural effects of tyrosine nitration and subsequent impact in protein loss- or even gain-of-function.

Highlights on molecular mechanisms of MDSC-mediated immune suppression: paving the way for new working hypotheses

MDSCs have been recognized in the last years as tolerogenic cells, potentially dangerous in the context of neoplasia, since they are able to induce tolerance to a variety of anti-tumor effectors, including CD4(+) and CD8(+) T cells. It is currently believed that the origin of MDSCs is due to an arrest of the myeloid differentiation process caused by tumor-secreted factors released in the tumor microenvironment that are able to exert an effect on myeloid progenitors, rendering them unable to terminally differentiate into dendritic cells, granulocytes and macrophages. As a consequence, these immature myeloid cells acquire suppressive activity through the activation of several mechanisms, controlled by different transcription factors. The lack of consensus about the phenotypical characterization of human MDSCs is the result of the existence of different MDSC subsets, most likely depending on the tumor in which they expand and on the tumor specific cytokine cocktail driving their activation. This, in turn, might also influence the mechanisms of MDSC-mediated immune suppression. In this review article we address the role of tumor-derived factors (TDFs) in MDSC-recruitment and activation, discuss the complex heterogeneity of MDSC phenotype and analyze the crosstalk between activated T cells and MDSCs.

Anatomically restricted synergistic antiviral activities of innate and adaptive immune cells in the skin

Despite extensive ex vivo investigation, the spatiotemporal organization of immune cells interacting with virus-infected cells in tissues remains uncertain. To address this, we used intravital multiphoton microscopy to visualize immune cell interactions with virus-infected cells following epicutaneous vaccinia virus (VV) infection of mice. VV infects keratinocytes in epidermal foci and numerous migratory dermal inflammatory monocytes that outlie the foci. We observed Ly6G(+) innate immune cells infiltrating and controlling foci, while CD8(+) T cells remained on the periphery killing infected monocytes. Most antigen-specific CD8(+) T cells in the skin did not interact with virus-infected cells. Blocking the generation of reactive nitrogen species relocated CD8(+) T cells into foci, modestly reducing viral titers. Depletion of Ly6G(+) and CD8(+) cells dramatically increased viral titers, consistent with their synergistic but spatially segregated viral clearance activities. These findings highlight previously unappreciated differences in the anatomic specialization of antiviral immune cell subsets.

Vascular E-selectin expression correlates with CD8 lymphocyte infiltration and improved outcome in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is an aggressive, polyomavirus-linked skin cancer. Although CD8 lymphocyte infiltration into the tumor is strongly correlated with improved survival, these cells are absent or sparse in most MCCs. We investigated whether specific mechanisms of T-cell migration may be commonly disrupted in MCC tumors with poor CD8 lymphocyte infiltration. Intratumoral vascular E-selectin, critical for T-cell entry into skin, was downregulated in the majority (52%) of MCCs (n=56), and its loss was associated with poor intratumoral CD8 lymphocyte infiltration (P<0.05; n=45). Importantly, survival was improved in MCC patients whose tumors had higher vascular E-selectin expression (P<0.05). Local nitric oxide (NO) production is one mechanism of E-selectin downregulation and it can be tracked by quantifying nitrotyrosine, a stable biomarker of NO-induced reactive nitrogen species (RNS). Indeed, increasing levels of nitrotyrosine within MCC tumors were associated with low E-selectin expression (P<0.05; n=45) and decreased CD8 lymphocyte infiltration (P<0.05, n=45). These data suggest that one mechanism of immune evasion in MCC may be restriction of T-cell entry into the tumor. Existing therapeutic agents that modulate E-selectin expression and/or RNS generation may restore T-cell entry and could potentially synergize with other immune-stimulating therapies.

Immunotherapy of human papilloma virus induced disease

Immunotherapy is the generic name for treatment modalities aiming to reinforce the immune system against diseases in which the immune system plays a role. The design of an optimal immunotherapeutic treatment against chronic viruses and associated diseases requires a detailed understanding of the interactions between the target virus and its host, in order to define the specific strategies that may have the best chance to deliver success at each stage of disease. Recently, a first series of successes was reported for the immunotherapy of Human Papilloma Virus (HPV)-induced premalignant diseases but there is definitely room for improvement. Here I discuss a number of topics that in my opinion require more study as the answers to these questions allows us to better understand the underlying mechanisms of disease and as such to tailor treatment.

Tumor microenvironment and myeloid-derived suppressor cells

Tumor progression has been demonstrated to be supported by chronic inflammatory conditions developed in the tumor microenvironment and characterized by the long-term secretion of various inflammatory soluble factors (including cytokines, chemokines, growth factors, reactive oxygen and nitrogen species, prostaglandins etc.) and strong leukocyte infiltration. Among leukocytes infiltrating tumors, myeloid-derived suppressor cells (MDSCs) represent one of the most important players mediating immunosuppression. These cells may not only strongly inhibit an anti-tumor immune reactions mediated by T cells but also directly stimulate tumorigenesis, tumor growth and metastasis by enhancing neoangiogenesis and creating a suitable environment for the metastatic formation. This review provides an overview of interactions between MDSCs and tumor cells leading to MDSC generation, activation and migration to the tumor site, where they can strongly enhance tumor progression. Better understanding of the MDSC-tumor interplay is critical for the development of new strategies of tumor immunotherapy.

Instruction of myeloid cells by the tumor microenvironment: Open questions on the dynamics and plasticity of different tumor-associated myeloid cell populations

The versatility and plasticity of myeloid cell polarization/differentiation has turned out to be crucial in health and disease, and has become the subject of intense investigation during the last years. On one hand, myeloid cells provide a critical contribution to tissue homeostasis and repair. On the other hand, myeloid cells not only play an important role as first line defense against pathogens but also they are involved in a broad array of inflammation-related diseases such as cancer. Recent studies show that macrophages can exist in different activation states within the same tumor, underlining their plasticity and heterogeneity. In this review, we will discuss recent evidence on how the tumor microenvironment, as it evolves, shapes the recruitment, function, polarization and differentiation of the myeloid cell compartment, leading to the selection of myeloid cells with immunosuppressive and angiogenic functions that facilitate tumor progression and dissemination.

Immune tolerance to tumor antigens occurs in a specialized environment of the spleen

Peripheral tolerance to tumor antigens (Ags) is a major hurdle for antitumor immunity. Draining lymph nodes are considered the privileged sites for Ag presentation to T cells and for the onset of peripheral tolerance. Here, we show that the spleen is fundamentally important for tumor-induced tolerance. Splenectomy restores lymphocyte function and induces tumor regression when coupled with immunotherapy. Splenic CD11b(+)Gr-1(int)Ly6C(hi) cells, mostly comprising proliferating CCR2(+)-inflammatory monocytes with features of myeloid progenitors, expand in the marginal zone of the spleen. Here, they alter the normal tissue cytoarchitecture and closely associate with memory CD8(+) T cells, cross-presenting tumor Ags and causing their tolerization. Because of its high proliferative potential, this myeloid cell subset is also susceptible to low-dose chemotherapy, which can be exploited as an adjuvant to passive immunotherapy. CCL2 serum levels in cancer patients are directly related to the accumulation of immature myeloid cells and are predictive for overall survival in patients who develop a multipeptide response to cancer vaccines.

Adoptive T-cell therapy using autologous tumor-infiltrating lymphocytes for metastatic melanoma: current status and future outlook

Immunotherapy using autologous T cells has emerged to be a powerful treatment option for patients with metastatic melanoma. These include the adoptive transfer of autologous tumor-infiltrating lymphocytes (TILs), T cells transduced with high-affinity T cell receptors against major tumor antigens, and T cells transduced with chimeric antigen receptors composed of hybrid immunoglobulin light chains with endodomains of T-cell signaling molecules. Among these and other options for T-cell therapy, TILs together with high-dose interleukin 2 have had the longest clinical history with multiple clinical trials in centers across the world consistently demonstrating durable clinical response rates near 50% or more. A distinct advantage of TIL therapy making it still the T-cell therapy of choice is the broad nature of the T-cell recognition against both defined and undefined tumors antigens against all possible major histocompatibility complex, rather than the single specificity and limited major histocompatibility complex coverage of the newer T cell receptors and chimeric antigen receptor transduction technologies. In the past decade, significant inroads have been made in defining the phenotypes of T cells in TIL-mediating tumor regression. CD8+ T cells are emerging to be critical, although the exact subset of CD8+ T cells exhibiting the highest clinical activity in terms of memory and effector markers is still controversial. We present a model in which both effector-memory and more differentiated effector T cells ultimately may need to cooperate to mediate long-term tumor control in responding patients. Although TIL therapy has shown great potential to treat metastatic melanoma, a number of issues have emerged that need to be addressed to bring it more into the mainstream of melanoma care. First, we have a reached the point where a pivotal phase II or phase III trial is needed in an attempt to gain regulatory approval of TILs as standard of care. Second, improvements in how we expand TILs for therapy are needed that minimize the time the T cells are in culture and improve the memory and effector characteristics of the T cells for longer persistence and enhanced anti-tumor activity in vivo. Third, there is a critical need to identify surrogate and predictive biomarkers to better select suitable patients for TIL therapy to improve response rate and duration. Overall, the outlook for TIL therapy for melanoma is very bright. We predict that TILs will indeed emerge to become an approved treatment in the upcoming years through pivotal clinical trials. Moreover, new approaches combining TILs with targeted signaling pathway drugs, such as mutant B-RAF inhibitors, and synergistic immunomodulatory interventions enhancing T-cell costimulation and preventing negative regulation should further increase therapeutic efficacy and durable complete response rates.

On the armament and appearances of human myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSC) have frequently been observed in patients with cancer. This heterogeneous population of myeloid cells can exert potent suppression of lymphocyte function and thereby poses a significant hurdle to natural or therapeutically induced anti-tumor immunity. On the other hand, the natural function of MDSC is not yet well understood and their role in infection, inflammation and autoimmune disease is still puzzling. Understanding MDSC biology will provide the tools necessary for therapeutic targeting of this population, but also permit exploitation of their strong tolerogenic function in the treatment of inflammatory conditions and the prevention of graft rejection.

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.

Molecular pathways: tumor-infiltrating myeloid cells and reactive oxygen species in regulation of tumor microenvironment

Tumor-associated myeloid cells are the major type of inflammatory cells involved in the regulation of antitumor immune responses. One key characteristic of these cells is the generation of reactive oxygen (ROS) and reactive nitrogen species (RNS) in the tumor microenvironment. Recent studies have shown the important role of ROS and RNS, especially peroxynitrite, in immune suppression in cancer. ROS and RNS are involved in induction of antigen-specific T-cell tolerance, inhibition of T-cell migration to the tumor site, and tumor cell evasion of recognition by cytotoxic T cells. In preclinical settings, a number of potential therapeutic agents showed activity in blocking ROS/RNS in cancer and in improving the efficacy of cancer immune therapy. A better understanding of ROS/RNS-associated pathways in myeloid cells will help to identify more specific and direct targets to facilitate the development of more effective immune therapy for cancer.

Tumor microenvironment and lymphocyte infiltration

There is ample evidence that the presence of tumor-infiltrating T lymphocytes is associated with a favorable prognostic in patients. These observations suggest that a limiting step to immune resistance and immunotherapy could be the capacity of tumor-specific T cells to reach tumor bed. In this article, we review some factors that may influence this infiltration, and in particular the nature of the vasculature, the expression of chemokines or tumor antigens and the presence of dendritic cells and CD4+ T lymphocytes.

Modulation of microenvironment acidity reverses anergy in human and murine tumor-infiltrating T lymphocytes

Stimulating the effector functions of tumor-infiltrating T lymphocytes (TIL) in primary and metastatic tumors could improve active and adoptive T-cell therapies for cancer. Abnormal glycolysis, high lactic acid production, proton accumulation, and a reversed intra-extracellular pH gradient are thought to help render tumor microenvironments hostile to roving immune cells. However, there is little knowledge about how acidic microenvironments affect T-cell immunity. Here, we report that lowering the environmental pH to values that characterize tumor masses (pH 6-6.5) was sufficient to establish an anergic state in human and mouse tumor-specific CD8(+) T lymphocytes. This state was characterized by impairment of cytolytic activity and cytokine secretion, reduced expression of IL-2Rα (CD25) and T-cell receptors (TCR), and diminished activation of STAT5 and extracellular signal-regulated kinase (ERK) after TCR activation. In contrast, buffering pH at physiologic values completely restored all these metrics of T-cell function. Systemic treatment of B16-OVA-bearing mice with proton pump inhibitors (PPI) significantly increased the therapeutic efficacy of both active and adoptive immunotherapy. Our findings show that acidification of the tumor microenvironment acts as mechanism of immune escape. Furthermore, they illustrate the potential of PPIs to safely correct T-cell dysfunction and improve the efficacy of T-cell-based cancer treatments.

Tumor-expressed inducible nitric oxide synthase controls induction of functional myeloid-derived suppressor cells through modulation of vascular endothelial growth factor release

Inducible NO synthase (iNOS) is a hallmark of chronic inflammation that is also overexpressed in melanoma and other cancers. Whereas iNOS is a known effector of myeloid-derived suppressor cell (MDSC)-mediated immunosuppression, its pivotal position at the interface of inflammation and cancer also makes it an attractive candidate regulator of MDSC recruitment. We hypothesized that tumor-expressed iNOS controls MDSC accumulation and acquisition of suppressive activity in melanoma. CD11b(+)GR1(+) MDSC derived from mouse bone marrow cells cultured in the presence of MT-RET-1 mouse melanoma cells or conditioned supernatants expressed STAT3 and reactive oxygen species (ROS) and efficiently suppressed T cell proliferation. Inhibition of tumor-expressed iNOS with the small molecule inhibitor L-NIL blocked accumulation of STAT3/ROS-expressing MDSC, and abolished their suppressive function. Experiments with vascular endothelial growth factor (VEGF)-depleting Ab and recombinant VEGF identified a key role for VEGF in the iNOS-dependent induction of MDSC. These findings were further validated in mice bearing transplantable MT-RET-1 melanoma, in which L-NIL normalized elevated serum VEGF levels; downregulated activated STAT3 and ROS production in MDSC; and reversed tumor-mediated immunosuppression. These beneficial effects were not observed in iNOS knockout mice, suggesting L-NIL acts primarily on tumor- rather than host-expressed iNOS to regulate MDSC function. A significant decrease in tumor growth and a trend toward increased tumor-infiltrating CD8(+) T cells were also observed in MT-RET transgenic mice bearing spontaneous tumors. These data suggest a critical role for tumor-expressed iNOS in the recruitment and induction of functional MDSC by modulation of tumor VEGF secretion and upregulation of STAT3 and ROS in MDSC.

Prostate cancer, tumor immunity and a renewed sense of optimism in immunotherapy

The recent FDA approval of the first therapeutic vaccine against prostate cancer has revitalized the public interest in the fields of cancer immunology and immunotherapy. Yet, clinical results are modest. A reason for this limited success may reside in the capacity of the tumor to convert inflammation in a tumor-promoting condition and eventually escape immune surveillance. Here we present the main known interactions between the prostate tumor and the immune system, showing how the malignancy can dodge the immune system by also exerting several immunosuppressive mechanisms. We also discuss experimental and clinical strategies proposed to counteract cancer immune evasion and emphasize the importance of implementing appropriate murine models like the transgenic adenocarcinoma of the mouse prostate model for investigating the biology of prostate cancer and novel immunotherapy approaches against it.

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.

Accessories to the crime: functions of cells recruited to the tumor microenvironment

Mutationally corrupted cancer (stem) cells are the driving force of tumor development and progression. Yet, these transformed cells cannot do it alone. Assemblages of ostensibly normal tissue and bone marrow-derived (stromal) cells are recruited to constitute tumorigenic microenvironments. Most of the hallmarks of cancer are enabled and sustained to varying degrees through contributions from repertoires of stromal cell types and distinctive subcell types. Their contributory functions to hallmark capabilities are increasingly well understood, as are the reciprocal communications with neoplastic cancer cells that mediate their recruitment, activation, programming, and persistence. This enhanced understanding presents interesting new targets for anticancer therapy.

Identification of myeloid derived suppressor cells in dogs with naturally occurring cancer

Dogs with naturally occurring cancer represent an important large animal model for drug development and testing novel immunotherapies. However, poorly defined immunophenotypes of canine leukocytes have limited the study of tumor immunology in dogs. The accumulation of myeloid derived suppressor cells (MDSCs) is known to be a key mechanism of immune suppression in tumor-bearing mice and in human patients. We sought to identify MDSCs in the blood of dogs with cancer. Peripheral blood mononuclear cells (PBMCs) from dogs with advanced or early stage cancer and from age-matched healthy controls were analyzed by flow cytometry and microscopy. Suppressive function was tested in T cell proliferation and cytokine elaboration assays. Semi-quantitative RT-PCR was used to identify potential mechanisms responsible for immunosuppression. PBMCs from dogs with advanced or metastatic cancer exhibited a significantly higher percentage of CD11b⁺CD14⁻MHCII⁻ cells compared to dogs diagnosed with early stage non-metastatic tumors and healthy dogs. These CD11b⁺ CD14⁻MHCII⁻ cells constitute a subpopulation of activated granulocytes that co-purify with PBMCs, display polymorphonuclear granulocyte morphology, and demonstrate a potent ability to suppress proliferation and IFN-γ production in T cells from normal and tumor-bearing donors. Furthermore, these cells expressed hallmark suppressive factors of human MDSC including ARG1, iNOS2, TGF-β and IL-10. In summary our data demonstrate that MDSCs accumulate in the blood of dogs with advanced cancer and can be measured using this three-marker immunophenotype, thereby enabling prospective studies that can monitor MDSC burden.

Differential macrophage programming in the tumor microenvironment

Of the multiple unique stromal cell types common to solid tumors, tumor-associated macrophages (TAMs) are significant for fostering tumor progression. The protumor properties of TAMs derive from regulation of angiogenic programming, production of soluble mediators that support proliferation, survival and invasion of malignant cells, and direct and indirect suppression of cytotoxic T cell activity. These varied activities are dependent on the polarization state of TAMs that is regulated in part by local concentrations of cytokines and chemokines, as well as varied interactions of TAMs with normal and degraded components of the extracellular matrix. Targeting molecular pathways regulating TAM polarization holds great promise for anticancer therapy.

Melanoma-induced immunosuppression and its neutralization

Malignant melanoma is characterized by a rapid progression, metastasis to distant organs, and resistance to chemo- and radiotherapy. Well-defined immunogenic capacities of melanoma cells should allow a successful application of different immunotherapeutic strategies. However, the overall results of immunotherapeutic clinical studies are not satisfactory. These paradoxical observations are supposed to be due to the profound immunosuppression mediated by different mechanisms dealing with alterations in tumor and surrounding stroma cells. Melanoma microenvironment has been characterized by a remarkable accumulation of highly immunosuppressive regulatory leucocytes, in particular, myeloid-derived suppressor cells (MDSCs). Their migration, retention and high activity in the tumor lesions have been demonstrated to be induced by chronic inflammatory conditions developing in the tumor microenvironment and characterized by the long-term secretion of various inflammatory mediators (cytokines, chemokines, growth factors, reactive oxygen and nitrogen species, prostaglandins etc.) leading to further cancer progression. Here, we discuss the role of chronic inflammation in the recruitment and activation of MDSCs in melanoma lesions as well as therapeutic approaches of MDSC targeting to overcome tumor immunosuppressive microenvironment induced by chronic inflammation and enhance the efficiency of melanoma immunotherapies.

Cross-talk between myeloid-derived suppressor cells (MDSC), macrophages, and dendritic cells enhances tumor-induced immune suppression

The tumor microenvironment is a complex milieu of tumor and host cells. Host cells can include tumor-reactive T cells capable of killing tumor cells. However, more frequently the tumor and host components interact to generate a highly immune suppressive environment that frustrates T cell cytotoxicity and promotes tumor progression through a variety of immune and non-immune mechanisms. Myeloid-derived suppressor cells (MDSC) are a major host component contributing to the immune suppressive environment. In addition to their inherent immune suppressive function, MDSC amplify the immune suppressive activity of macrophages and dendritic cells via cross-talk. This article will review the cell-cell interactions used by MDSC to inhibit anti-tumor immunity and promote progression, and the role of inflammation in promoting cross-talk between MDSC and other cells in the tumor microenvironment.

Matrix architecture defines the preferential localization and migration of T cells into the stroma of human lung tumors

Appropriate localization and migration of T cells is a prerequisite for antitumor immune surveillance. Studies using fixed tumor samples from human patients have shown that T cells accumulate more efficiently in the stroma than in tumor islets, but the mechanisms by which this occurs are unknown. By combining immunostaining and real-time imaging in viable slices of human lung tumors, we revealed that the density and the orientation of the stromal extracellular matrix likely play key roles in controlling the migration of T cells. Active T cell motility, dependent on chemokines but not on β1 or β2 integrins, was observed in loose fibronectin and collagen regions, whereas T cells migrated poorly in dense matrix areas. Aligned fibers in perivascular regions and around tumor epithelial cell regions dictated the migratory trajectory of T cells and restricted them from entering tumor islets. Consistently, matrix reduction with collagenase increased the ability of T cells to contact cancer cells. Thus, the stromal extracellular matrix influences antitumor immunity by controlling the positioning and migration of T cells. Understanding the mechanisms by which this collagen network is generated has the potential to aid in the development of new therapeutics.

Overcoming immunosuppression in the melanoma microenvironment induced by chronic inflammation

Malignant melanoma is known by its rapid progression and poor response to currently applied treatments. Despite the well-documented melanoma immunogenicity, the results of immunotherapeutic clinical trials are not satisfactory. This poor antitumor reactivity is due to the development of chronic inflammation in the tumor microenvironment characterized by infiltrating leukocytes and soluble mediators, which lead to an immunosuppression associated with cancer progression. Using the ret transgenic mouse melanoma model that closely resembles human melanoma, we demonstrated increased levels of chronic inflammatory factors in skin tumors and metastatic lymph nodes, which correlated with tumor progression. Furthermore, Gr1(+)CD11b(+) myeloid-derived suppressor cells (MDSC), known to block tumor-reactive T cells, were enriched in melanoma lesions and showed an enhanced immunosuppressive capacity. This MDSC accumulation was associated with a strong TCR ζ-chain downregulation in T cells suggesting that the tumor inflammatory microenvironment supports MDSC recruitment and immunosuppressive activity. Indeed, upon administration of phosphodiesterase-5 inhibitor sildenafil or paclitaxel in non-cytotoxic doses, we observed reduced levels of chronic inflammatory mediators in association with decreased MDSC amounts and immunosuppressive function. This led to a partial restoration of ζ-chain expression in T cells and to a significantly increased survival of tumor-bearing mice. CD8 T-cell depletion resulted in an abrogation of beneficial outcome of both drugs, suggesting the involvement of MDSC and CD8 T cells in the observed therapeutic effects. Our data imply that inhibition of chronic inflammation in the tumor microenvironment should be applied in conjunction with melanoma immunotherapies to increase their efficacy.

Regulation of suppressive function of myeloid-derived suppressor cells by CD4+ T cells

Myeloid derived suppressor cells play a critical role in T cell suppression in cancer. Here, we discuss the mechanisms of how MDSC suppress CD4(+) or CD8(+) T cells in an antigen dependent or non-dependent manner.

Hiding the road signs that lead to tumor immunity

Schaer et al. discuss mechanisms of immune evasions by tumors, including the recent finding that CCL2 nitrosylation prevents T cell infiltration into tumors.

Tumors exploit many strategies to evade T cell–mediated destruction. For example, tumors can prevent T cell infiltration by modifying gene expression in the endothelial cells and pericytes that form their vasculature. New work showing that the T cell–attracting chemokine CCL2 can be posttranslationally modified in the tumor microenvironment adds another mechanism to the already formidable arsenal of immunoevasion tactics used by solid tumors.