Immunosuppressive CD14+HLA-DRlow/- monocytes in prostate cancer

Numerical defect of circulating dendritic cell subsets and defective dendritic cell generation from monocytes of patients with advanced melanoma

The behaviour of circulating dendritic cells (DCs) and DC generation from monocytes in melanoma patients during the progression of disease have not been described. We report a significant decrease in the absolute number of total DCs, which mainly affects plasmacytoid DCs in stage IV. Additionally, monocyte-DC generation is less efficient in advanced stages, resulting in DCs that exhibit increased phagocytic capacity, potentially indicating a less mature state. These findings elucidate aspects of basic tumour-mediated immunosuppression, which may have implications for immunotherapeutic approaches, suggesting that the selection of patients for immunotherapy should also be made on the basis of their immune status.

The impact of the myeloid response to radiation therapy

Radiation therapy is showing potential as a partner for immunotherapies in preclinical cancer models and early clinical studies. As has been discussed elsewhere, radiation provides debulking, antigen and adjuvant release, and inflammatory targeting of effector cells to the treatment site, thereby assisting multiple critical checkpoints in antitumor adaptive immunity. Adaptive immunity is terminated by inflammatory resolution, an active process which ensures that inflammatory damage is repaired and tissue function is restored. We discuss how radiation therapy similarly triggers inflammation followed by repair, the consequences to adaptive immune responses in the treatment site, and how the myeloid response to radiation may impact immunotherapies designed to improve control of residual cancer cells.

Clinical perspectives on targeting of myeloid derived suppressor cells in the treatment of cancer

Tumors escape immune recognition by several mechanisms, and induction of myeloid derived suppressor cells (MDSC) is thought to play a major role in tumor mediated immune evasion. MDSC arise from myeloid progenitor cells that do not differentiate into mature dendritic cells, granulocytes, or macrophages, and are characterized by the ability to suppress T cell and natural killer cell function. They are increased in patients with cancer including renal cell carcinoma (RCC), and their levels have been shown to correlate with prognosis and overall survival. Multiple methods of inhibiting MDSCs are currently under investigation. These can broadly be categorized into methods that (a) promote differentiation of MDSC into mature, non-suppressive cells (all trans retinoic acid, vitamin D), (b) decrease MDSC levels (sunitinib, gemcitabine, 5-FU, CDDO-Me), or (c) functionally inhibit MDSC (PDE-5 inhibitors, cyclooxygenase 2 inhibitors). Recently, several pre-clinical tumor models of combination therapy involving sunitinib plus vaccines and/or adoptive therapy have shown promise in MDSC inhibition and improved outcomes in the tumor bearing host. Current clinical trials are underway in RCC patients to assess not only the impact on clinical outcome, but how this combination can enhance anti-tumor immunity and reduce immune suppression. Decreasing immune suppression by MDSC in the cancer host may improve outcomes and prolong survival in this patient population.

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.

Circulating and tumor-infiltrating myeloid-derived suppressor cells in patients with colorectal carcinoma

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous family of myeloid cells that suppress T cell immunity in tumor-bearing hosts. In patients with colon cancer, MDSCs have recently been described as Lin^−/lowHLA-DR⁻CD11b⁺CD33⁺ cells correlating with cancer stage, metastasis and chemotherapy response. To learn in more detail the dynamic change and clinical relevance of circulating and tumor-infiltrating Lin^−/lowHLA-DR⁻CD11b⁺CD33⁺ MDSC in colorectal cancer, we harvested the blood from 64 patients with varying stage of colorectal cancer and tumor and matched paraneoplastic tissues from 5 patients with advanced colorectal cancer, subjected them to multicolor flow cytometric analysis of percentage, absolute number and phenotype of MDSC and finally characterized their immunosuppressive functions. Our results demonstrate that peripheral blood from colorectal cancer patients contains markedly increased percentage and absolute number of Lin^−/lowHLA-DR⁻CD11b⁺CD33⁺ MDSCs compared with healthy individuals, and this increase is closely correlated with clinical cancer stage and tumor metastasis but not primary tumor size and serum concentrations of cancer biomarker. A similar increase of MDSCs was also observed in the tumor tissues. Phenotyping MDSCs shows that they express high CD13 and CD39, low CD115, CD117, CD124 and PD-L1, and devoid of CD14, CD15 and CD66b, reminiscent of precursor myeloid cells. MDSCs from cancer patients but not healthy donors have the immunosuppressive activity and were able to inhibit in vitro autologous T-cell proliferation. Collectively, this study substantiates the presence of increased immunosuppressive circulating and tumor-resident Lin^−/lowHLA-DR⁻CD11b⁺CD33⁺ MDSCs in patients with colorectal cancers correlating with cancer stage and metastasis, and suggests that pharmacologic blockade of MDSCs should be considered in future clinical trials.

Phenotypic plasticity of MDSC in cancers

Myeloid-derived suppressor cells (MDSCs) were initially reported as suppressor of the adaptive immune responses against cancer and other diseases. However, emerging evidence suggest that MDSCs may also support anti-tumor immune responses under certain conditions or may inhibit tumor growth. In the present mini-review, we suggest that such opposing functions of MDSCs are due to phenotypic plasticity of the myeloid cells, allowing them to produce a diverse form of morphology, physiological state, and function in response to environmental conditions. Therefore, they can be manipulated by means of immune modulators to overcome their immune suppressive function.

Tumor-promoting immune-suppressive myeloid-derived suppressor cells in the multiple myeloma microenvironment in humans

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous, immature myeloid cell population with the ability to suppress immune responses. MDSCs have been characterized in infections, inflammatory diseases, and solid tumors; however, their presence and role in the tumor-promoting, immune-suppressive microenvironment in hematologic malignancies remains unclear. We assessed the presence, frequency, and functional characteristics of MDSCs in patients with newly diagnosed, relapsed, and relapsed/refractory multiple myeloma (MM) compared with healthy donors. Additionally, we evaluated the immunomodulatory effects of lenalidomide and bortezomib on MDSCs in MM. CD11b(+)CD14(-)HLA-DR(-/low)CD33(+)CD15(+) MDSCs were significantly increased in both the peripheral blood and the bone marrow of patients with active MM compared with healthy donors. Furthermore, MDSCs induced MM growth while suppressing T-cell-mediated immune responses. Conversely, MM cells induced the development of MDSCs from healthy donor peripheral blood mononuclear cells, confirming a bidirectional interaction between MDSCs and MM cells and immune effector cells. Our results further suggest that MDSCs may be associated with the activity of disease in MM. Importantly, our studies suggest that inhibition of the tumor-promoting and immune-suppressive functions of MDSCs in MM may represent a promising novel immune-based therapeutic strategy.

Sipuleucel-T (Provenge) autologous vaccine approved for treatment of men with asymptomatic or minimally symptomatic castrate-resistant metastatic prostate cancer

Sipuleucel-T (Provenge) (Sip-T) is first -in class as a therapeutic autologous vaccine approved for the treatment of men with asymptomatic or minimally symptomatic castrate-resistant metastatic prostate cancer. This product is the culmination of decades of basic immunological and prostate cancer investigations and 13 y of clinical trial investigations. Sip-T represents a paradigm shift in cancer therapeutics and represents the first approved autologous therapeutic cancer vaccine, which has demonstrated a survival benefit. The potential benefit of this product is the excellent risk to benefit ratio, which will allow for the combination of this approach with other more toxic therapies. The favorable risk to benefit will also afford the opportunity for trials investigating this product earlier in the disease state and in combination with local therapies. The ability to target more localized or lower volume disease will maximize the therapeutic benefit over a longer period of time. The novelty of the platform of this approach could be used to treat any cancer with a tumor-specific cell surface target. The main product of Sip-T is the re-infusion of a patient's antigen presenting cells from leukapheresis after ex-vivo exposure to a chimeric protein of human GM-CSF and PAP. In metastatic CRPC patients three infusions of these activated cells over a month lead to statistically significant 4.1 mo increase in median survival and a 22.5% reduction in risk of death. The main side effect from this re-infusion of activated immune cells is a "flu-like" syndrome that includes chills, fatigue, fevers, back pain, nausea, joints aches and headaches in decreasing order of frequency. Immune monitoring during the clinical trials also demonstrated a specific cellular and antibody immune response, suggesting the proposed mechanism of adoptive immunotherapy to PAP was behind this survival benefit. This product also serves as a proof of principle for targeted immunotherapy for others cancers with defined cell surface markers. In summary, the approval of Sip-T based on a survival benefit and very tolerable safety profile will 1) enhance our ability to care for men with advanced prostate cancer, 2) allow for further investigations of this approach in combination with others therapies with different mechanisms of action and non-overlapping toxicities, and 3) allow further investigations earlier in the course of the disease.

Monocyte-derived cells of the brain and malignant gliomas: the double face of Janus

OBJECTIVE

Monocyte-derived cells of the brain (MDCB) are a diverse group of functional immune cells that are also highly abundant in gliomas. There is growing evidence that MDCB play essential roles in the pathogenesis of gliomas. The aim of this review was to collate and systematize contemporary knowledge about these cells as they relate to glioma progression and antiglioblastoma therapeutic modalities with a view toward improved effectiveness of therapy.

METHODS

We reviewed relevant studies to construct a summary of different MDCB subpopulations in steady state and in malignant gliomas and discuss their role in the development of malignant gliomas and potential future therapies.

RESULTS

Current studies suggest that MDCB subsets display different phenotypes and differentiation potentials depending on their milieu in the brain and exposure to tumoral influences. MDCB possess specific and unique functions, including those that are protumoral and those that are antitumoral.

CONCLUSIONS

Elucidating the role of mononuclear-derived cells associated with gliomas is crucial in designing novel immunotherapy strategies. Much progress is needed to characterize markers to identify cell subsets and their specific regulatory roles. Investigation of MDCB can be clinically relevant. Specific MDCB populations potentially can be used for glioma therapy as a target or as cell vehicles that might deliver cytotoxic substances or processes to the glioma microenvironment.

Myeloid cells obtained from the blood but not from the tumor can suppress T-cell proliferation in patients with melanoma

PURPOSE

Myeloid-derived suppressor cells (MDSC) have emerged as an immune-regulatory cell type that is expanded in tumor-bearing mice, but less is known about their immune-suppressive role in patients with cancer.

EXPERIMENTAL DESIGN

To study the importance of MDSC in patients with melanoma, we characterized the frequency, phenotype, and suppressive function of blood myeloid-derived cells and tumor-infiltrating myeloid cells in 26 freshly resected melanomas.

RESULTS

Blood and tumor-infiltrating myeloid cells (Lin(-) CD11b(+)) could be phenotypically and morphologically classified into monocytes/macrophages, neutrophils, eosinophils, and immature myeloid cells according to marker expression (CD14(+), CD14(-) CD15(hi), CD14(-) CD15(int), and CD14(-) CD15(-), respectively). In contrast to the expansion of MDSC reported in tumor-bearing mice, we found no differences in the frequency and phenotype of myeloid subsets in the blood of patients with melanoma compared with healthy donors. Myeloid cells represented 12% of the live cells in the melanoma cell suspensions, and were phenotypically diverse with high tumor-to-tumor variability. Interestingly, a positive association was found between the percentage of Tregs and granulocytic cells (Lin(-) CD11b(+) CD14(-)CD15(+)) infiltrating melanoma tumors. However, melanoma-infiltrating myeloid cells displayed impaired suppression of nonspecific T-cell proliferation compared with peripheral blood myeloid cells, in which monocytes and eosinophils were suppressive.

CONCLUSIONS

Our findings provide a first characterization of the nature and suppressive function of the melanoma myeloid infiltrate and indicate that the suppressive function of MDSC in patients with melanoma seems far less than that based on murine tumor models.

Generation of myeloid-derived suppressor cells using prostaglandin E2

Myeloid-derived suppressor cells (MDSCs) are natural immunosuppressive cells and endogenous inhibitors of the immune system. We describe a simple and clinically compatible method of generating large numbers of MDSCs using the cultures of peripheral blood-isolated monocytes supplemented with prostaglandin E₂ (PGE₂). We observed that PGE₂ induces endogenous cyclooxygenase (COX)2 expression in cultured monocytes, blocking their differentiation into CD1a⁺ dendritic cells (DCs) and inducing the expression of indoleamine 2,3-dioxygenase 1, IL-4Rα, nitric oxide synthase 2 and IL-10 - typical MDSC-associated suppressive factors. The establishment of a positive feedback loop between PGE₂ and COX2, the key regulator of PGE₂ synthesis, is both necessary and sufficient to promote the development of CD1a⁺ DCs to CD14⁺CD33⁺CD34⁺ monocytic MDSCs in granulocyte macrophage colony stimulating factor/IL-4-supplemented monocyte cultures, their stability, production of multiple immunosuppressive mediators and cytotoxic T lymphocyte-suppressive function. In addition to PGE₂, selective E-prostanoid receptor (EP)2- and EP4-agonists, but not EP3/1 agonists, also induce the MDSCs development, suggesting that other activators of the EP2/4- and EP2/4-driven signaling pathway (adenylate cyclase/cAMP/PKA/CREB) may be used to promote the development of suppressive cells. Our observations provide a simple method for generating large numbers of MDSCs for the immunotherapy of autoimmune diseases, chronic inflammatory disorders and transplant rejection.

Myeloid cells in tumor inflammation

Bone marrow derived myeloid cells progressively accumulate in tumors, where they establish an inflammatory microenvironment that is favorable for tumor growth and spread. These cells are comprised primarily of monocytic and granulocytic myeloid derived suppressor cells (MDSCs) or tumor-associated macrophages (TAMs), which are generally associated with a poor clinical outcome. MDSCs and TAMs promote tumor progression by stimulating immunosuppression, neovascularization, metastasis and resistance to anti-cancer therapy. Strategies to target the tumor-promoting functions of myeloid cells could provide substantial therapeutic benefit to cancer patients.

Comparative analysis of monocytic and granulocytic myeloid-derived suppressor cell subsets in patients with gastrointestinal malignancies

Myeloid-derived suppressor cells (MDSC) are a heterogenous population of cells comprising myeloid progenitor cells and immature myeloid cells, which have the ability to suppress the effector immune response. In humans, MDSC have not been well characterized owing to the lack of specific markers, although it is possible to broadly classify the MDSC phenotypes described in the literature as being predominantly granulocytic (expressing markers such as CD15, CD66, CD33) or monocytic (expressing CD14). In this study, we set out to perform a direct comparative analysis across both granulocytic and monocytic MDSC subsets in terms of their frequency, absolute number, and function in the peripheral blood of patients with advanced GI cancer. We also set out to determine the optimal method of sample processing given that this is an additional source of heterogeneity. Our findings demonstrate consistent changes across sample processing methods for monocytic MDSC, suggesting that reliance upon cryopreserved PBMC is acceptable. Although we did not see an increase in the population of granulocytic MDSC, these cells were found to be more suppressive than their monocytic counterparts.

Establishment of single-cell screening system for the rapid identification of transcriptional modulators involved in direct cell reprogramming

Combinatorial interactions of transcription modulators are critical to regulate cell-specific expression and to drive direct cell reprogramming (e.g. trans-differentiation). However, the identification of key transcription modulators from myriad of candidate genes is laborious and time consuming. To rapidly identify key regulatory factors involved in direct cell reprogramming, we established a multiplex single-cell screening system using a fibroblast-to-monocyte transition model. The system implements a single-cell 'shotgun-transduction' strategy followed by nested-single-cell-polymerase chain reaction (Nesc-PCR) gene expression analysis. To demonstrate this, we simultaneously transduced 18 monocyte-enriched transcription modulators in fibroblasts followed by selection of single cells expressing monocyte-specific CD14 and HLA-DR cell-surface markers from a heterogeneous population. Highly multiplex Nesc-PCR expression analysis revealed a variety of gene combinations with a significant enrichment of SPI1 (86/86) and a novel transcriptional modulator, HCLS1 (76/86), in the CD14(+)/HLA-DR(+) single cells. We could further demonstrate the synergistic role of HCLS1 in regulating monocyte-specific gene expressions and phagocytosis in dermal fibroblasts in the presence of SPI1. This study establishes a platform for a multiplex single-cell screening of combinatorial transcription modulators to drive any direct cell reprogramming.

Neutrophils and granulocytic myeloid-derived suppressor cells: immunophenotyping, cell biology and clinical relevance in human oncology

Accumulating evidence indicates that myeloid cells are critically involved in the pathophysiology of human cancers. In contrast to the well-characterized tumor-associated macrophages, the significance of granulocytes in cancer has only recently begun to emerge. A number of studies found increased numbers of neutrophil granulocytes and granulocytic myeloid-derived suppressor cells (GrMDSCs) both in the peripheral blood and in the tumor tissues of patients with different types of cancer. Most importantly, granulocytes have been linked to poor clinical outcome in cancer patients which suggests that these cells might have important tumor-promoting effects. In this review, we will address in detail the following major topics: (1) neutrophils and GrMDSCs in the peripheral blood of cancer patients-phenotype and functional changes; (2) neutrophils and GrMDSCs in the tumor tissue-potential mechanisms of tumor progression and (3) relevance of neutrophils and GrMDSCs for the clinical outcome of cancer patients. Furthermore, we will discuss the advantages and disadvantages of the current strategies used for identification and monitoring of human MDSCs. We propose a six-color immunophenotyping protocol that discriminates between monocytic MDSCs (MoMDSCs), two subsets of GrMDSCs and two subsets of immature myeloid cells in human cancer patients, thus, allowing for an improved characterization and understanding of these multifaceted cells.

Silencing of Irf7 pathways in breast cancer cells promotes bone metastasis through immune escape

Breast cancer metastasis is a key determinant of long-term patient survival. By comparing the transcriptomes of primary and metastatic tumor cells in a mouse model of spontaneous bone metastasis, we found that a substantial number of genes suppressed in bone metastases are targets of the interferon regulatory factor Irf7. Restoration of Irf7 in tumor cells or administration of interferon led to reduced bone metastases and prolonged survival time. In mice deficient in the interferon (IFN) receptor or in natural killer (NK) and CD8(+) T cell responses, metastasis was accelerated, indicating that Irf7-driven suppression of metastasis was reliant on IFN signaling to host immune cells. We confirmed the clinical relevance of these findings in over 800 patients in which high expression of Irf7-regulated genes in primary tumors was associated with prolonged bone metastasis-free survival. This gene signature may identify patients that could benefit from IFN-based therapies. Thus, we have identified an innate immune pathway intrinsic to breast cancer cells, the suppression of which restricts immunosurveillance to enable metastasis.

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.

Myeloid-derived suppressor cells evolve during sepsis and can enhance or attenuate the systemic inflammatory response

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous Gr1(+) CD11b(+) population of immature cells containing granulocytic and monocytic progenitors, which expand under nearly all inflammatory conditions and are potent repressors of T-cell responses. Studies of MDSCs during inflammatory responses, including sepsis, suggest they can protect or injure. Here, we investigated MDSCs during early and late sepsis. To do this, we used our published murine model of cecal ligation and puncture (CLP)-induced polymicrobial sepsis, which transitions from an early proinflammatory phase to a late anti-inflammatory and immunosuppressive phase. We confirmed that Gr1(+) CD11b(+) MDSCs gradually increase after CLP, reaching ∼88% of the bone marrow myeloid series in late sepsis. Adoptive transfer of early (day 3) MDSCs from septic mice into naive mice after CLP increased proinflammatory cytokine production, decreased peritoneal bacterial growth, and increased early mortality. Conversely, transfer of late (day 12) MDSCs from septic mice had the opposite effects. Early and late MDSCs studied ex vivo also differed in their inflammatory phenotypes. Early MDSCs expressed nitric oxide and proinflammatory cytokines, whereas late MDSCs expressed arginase activity and anti-inflammatory interleukin 10 (IL-10) and transforming growth factor β (TGF-β). Late MDSCs had more immature CD31(+) myeloid progenitors and, when treated ex vivo with granulocyte-macrophage colony-stimulating factor (GM-CSF), generated fewer macrophages and dendritic cells than early MDSCs. We conclude that as the sepsis inflammatory process progresses, the heterogeneous MDSCs shift to a more immature state and from being proinflammatory to anti-inflammatory.

Myeloid-derived suppressor cells impair the quality of dendritic cell vaccines

Myeloid-derived suppressor cells (MDSC) are important regulators of the immune system and key players in tumor-induced suppression of T-cell responses. CD14+HLA-DR-/low MDSC have been detected in a great number of malignancies, including melanoma. MDSC are known to be impaired in their ability to differentiate along the myeloid lineage, e.g., into dendritic cells (DC). This is a concern for utilization of monocyte-derived DC for vaccination of patients with melanoma or other cancers exhibiting accumulation of CD14+ MDSC. When producing DC according to standard operating procedures of two currently ongoing clinical trials, we found that MDSC co-purified with monocytes isolated by elutriation. MDSC frequencies did not affect yield or viability of the produced DC, but induced a dose-dependent decrease in DC maturation, ability to take up antigen, migrate and induce T-cell IFNγ production. Changes in DC characteristics were most notable when 'pathological' frequencies of >50% CD14+HLA-DR- cells were present in the starting culture. The impaired DC quality could not be explained by altered cytokine production or increased oxidative stress in the cultures. Tracking of HLA-DR- cells throughout the culture period revealed that the observed changes were partially due to the impaired maturation and functionality of the originally HLA-DR- population, but also to their negative effects on HLA-DR+ cells. In conclusion, MDSC could be induced to differentiate into DC but, due to the impairment of overall DC vaccine quality when >50% HLA-DR- cells were present in the starting culture, their removal could be advisable.

Immunosuppressive activity of CD14+ HLA-DR- cells in squamous cell carcinoma of the head and neck

Myeloid-derived suppressor cells (MDSC) represent a heterogeneous population and have the potential to suppress immune responses via diverse mechanisms. In recent studies, a new subset of MDSC was identified by the markers CD14(+) and HLA-DR(-) in the peripheral blood from cancer patients. In this study, we investigated the proportions and characteristics of CD14(+) HLA-DR(-) cells in patients with squamous cell carcinoma of the head and neck (SCCHN). As expected, the percentage of CD14(+) HLA-DR(-) cells was significantly elevated in patients relative to healthy donors and the sorted CD14(+) HLA-DR(-) cells were able to suppress effectively both the proliferation and IFN-γ production of anti-CD3/anti-CD28 stimulated T cells, suggesting that CD14(+) HLA-DR(-) cells in patients with SCCHN contribute to the immune suppressive status. Furthermore, CD14(+) HLA-DR(-) cells revealed a higher level of CD86 and PD-L1 expression and transforming growth factor (TGF)-β production than CD14(+) HLA-DR(+) cells. Addition of anti-CD86 mAb, anti-PD-L1 mAb and anti-TGF-β mAb partially restored T-cell proliferation and IFN-γ production, respectively, indicating that the suppressive effects of CD14(+) HLA-DR(-) cells appear to be mediated by various molecules, including coinhibitory molecules and cytokines. Our data suggest that CD14(+) HLA-DR(-) cells act as potent immunosuppressive cells and particularly contribute to tumor escape from the host immune system in patients with SCCHN.

Wnt5a induces a tolerogenic phenotype of macrophages in sepsis and breast cancer patients

A well-orchestrated inflammatory reaction involves the induction of effector functions and, at a later stage, an active downregulation of this potentially harmful process. In this study we show that under proinflammatory conditions the noncanonical Wnt protein, Wnt5a, induces immunosuppressive macrophages. The suppressive phenotype induced by Wnt5a is associated with induction of IL-10 and inhibition of the classical TLR4-NF-κB signaling. Interestingly, this phenotype closely resembles that observed in reprogrammed monocytes in sepsis patients. The Wnt5a-induced feedback inhibition is active both during in vitro LPS stimulation of macrophages and in patients with sepsis caused by LPS-containing, gram-negative bacteria. Furthermore, using breast cancer patient tissue microarrays, we find a strong correlation between the expression of Wnt5a in malignant epithelial cells and the frequency of CD163(+) anti-inflammatory tumor-associated macrophages. In conclusion, our data point out Wnt5a as a potential target for an efficient therapeutic modality in severe human diseases as diverse as sepsis and malignancy.

Myeloid-derived suppressor cell measurements in fresh and cryopreserved blood samples

Myeloid-derived suppressor cells (MDSC) present in the human peripheral blood, represent a heterogeneous population of cells with monocytic and granulocytic features. To provide guidelines for reliable assessments of the frequency and function of MDSC, we compared fresh vs. cryopreserved peripheral blood mononuclear cell (PBMC) samples obtained from normal controls and patients with cancer. PBMC were obtained from 4 healthy donors and 21 patients with cancer. They were stained with labeled antibodies, and the frequency of DR⁻/LIN⁻/CD11b+, DR⁻/LIN⁻/CD15+, DR⁻/LIN⁻/CD33+ and DR(-/low)/CD14+ cells was determined by flow cytometry before and after cryopreservation. CFSE-based suppressor assays were used to test inhibitory functions of MDSC. Arginase I expression and reactive oxygen species (ROS) upregulation in MDSC subsets were evaluated by flow cytometry. The DR(-/low)/CD14+ and DR⁻/LIN⁻/CD11b+ subsets of MDSC were found to be more resistant to the cryopreservation/thawing procedure compared to the DR⁻/LIN⁻/CD15+ and DR⁻/LIN⁻/CD33+ subsets. The frequency of the latter two MDSC subsets was significantly reduced after cryopreservation. All but DR⁻/LIN⁻/CD15+ cells inhibited proliferation of autologous CSFE-labeled CD4+ cells but lost suppressor activity after cryopreservation. Only DR⁻/LIN-/CD15+ cells were positive for Arginase I, but lost its expression after cryopreservation. Only fresh DR⁻/LIN⁻/CD11b+ and DR⁻/LIN⁻/CD15+ cells produced ROS after in vitro stimulation. Studies of human MDSC should be performed in fresh blood samples. If samples have to be cryopreserved, monitoring of CD11b+ and CD14+ MDSC subsets provides the most reliable results. Arginase I expression or stimulated ROS production assessed by flow cytometry are useful markers for MDSC subsets only in fresh samples.

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.

Myeloid-derived suppressor cells participate in preventing graft rejection

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells and have a tremendous potential to suppress immune responses. MDSCs accumulate during tumor progression, autoimmunity, chronic infection, transplantation, and other pathological conditions and can potently suppress T-cell function. Here, we discuss recent findings that describe the molecular mechanisms of MDSCs suppressing T-cell immune responses as well as recent observations that MDSCs may have roles in transplant tolerance.

Identification of a novel HLA-G+ regulatory population in blood: expansion after allogeneic transplantation and de novo HLA-G expression at graft-versus-host disease sites

BACKGROUND

The human leukocyte antigen-G (HLA-G) has been considered to be an important tolerogeneic molecule playing an essential role in maternal-fetal tolerance, which constitutes the perfect example of successful physiological immunotolerance of semi-allografts. In this context, we investigated the putative role of this molecule in the allogeneic hematopoietic cell transplantation setting.

DESIGN AND METHODS

The percentage of HLA-G(+) cells in peripheral blood of healthy donors and allo-transplanted patients was evaluated by flow cytometry. Their immunoregulatory and tolerogeneic properties were investigated in in vitro immunostimulatory and immunosuppression assays. Immunohistochemical analysis for HLA-G expression was performed in skin biopsies from allo-transplanted patients and correlated with the occurrence of graft-versus-host disease.

RESULTS

We identified a CD14(+)HLA-G(pos) population with an HLA-DR(low) phenotype and decreased in vitro immunostimulatory capacity circulating in peripheral blood of healthy individuals. Naturally occurring CD14(+)HLA-G(pos) cells suppressed T-cell responses and exerted an immunotolerogenic action on T cells by rendering them hyporesponsive and immunosuppressive in vitro. After allogeneic hematopoietic cell transplantation, HLA-G(pos) cells increase in blood. Interestingly, besides an increase in CD14(+)HLA-G(pos) cells, there was also a pronounced expansion of CD3(+)HLA-G(pos) cells. Of note, CD3(+)HLA-G(pos) and CD14(+)HLA-G(pos) cells from transplanted patients were suppressive in in vitro lymphoproliferation assays. Furthermore, we found an upregulation of HLA-G expression in skin specimens from transplanted patients that correlated with graft-versus-host disease. Inflammatory cells infiltrating the dermis of transplanted patients were also HLA-G(pos).

CONCLUSIONS

We report the presence of naturally occurring HLA-G(pos) monocytic cells with in vitro suppressive properties. HLA-G expressing regulatory blood cells were found in increased numbers after allogeneic transplantation. Epithelial cells in skin affected by graft-versus-host disease revealed elevated HLA-G expression.

Expansion of a subset of CD14highCD16negCCR2low/neg monocytes functionally similar to myeloid-derived suppressor cells during SIV and HIV infection

Monocytes have been categorized in three main subpopulations based on CD14 and CD16 surface expression. Classical monocytes express the CD14(++)CD16(-)CCR2(+) phenotype and migrate to inflammatory sites by quickly responding to CCL2 signaling. Here, we identified and characterized the expansion of a novel monocyte subset during HIV and SIV infection, which were undistinguishable from classical monocytes, based on CD14 and CD16 expression, but expressed significantly lower surface CCR2. Transcriptome analysis of sorted cells demonstrated that the CCR2(low/neg) cells are a distinct subpopulation and express lower levels of inflammatory cytokines and activation markers than their CCR2(high) counterparts. They exhibited impaired phagocytosis and greatly diminished chemotaxis in response to CCL2 and CCL7. In addition, these monocytes are refractory to SIV infection and suppress CD8(+) T cell proliferation in vitro. These cells express higher levels of STAT3 and NOS2, suggesting a phenotype similar to monocytic myeloid-derived cells, which suppress expansion of CD8(+) T cells in vivo. They may reflect an antiproliferative response against the extreme immune activation observed during HIV and SIV infections. In addition, they may suppress antiviral responses and thus, have a role in AIDS pathogenesis. Antiretroviral therapy in infected macaque and human subjects caused this population to decline, suggesting that this atypical phenotype is linked to viral replication.

Broad-spectrum immunosuppression by classless monocytes in non-Hodgkin's lymphoma

The mechanisms of systemic immunosuppression in B-cell non-Hodgkin's lymphoma (NHL) are poorly characterized. Lin and colleagues collected blood from 40 NHL patients prior to therapy. Monocytes from NHL patients suppressed T-cell proliferation, were unresponsive to Toll-like receptor stimulation by CpG and resistant to maturing into CD83(+) dendritic cells. This suppression was mediated in part through arginine metabolism, as exogenous arginine supplementation reversed this, and NHL patients had elevated arginase I in their plasma. These cells had decreased HLA-DR and TNF-α receptor II (CD120b) expression compared with controls. Patients with increased ratios of CD14(+)HLA-DR(low/-) monocytes had more advanced disease and suppressed immune functions, indicating that CD14(+)HLA-DR(low/-) monocytes are a pivotal and profoundly effective contributor to systemic immunosuppression in NHL.

Increased circulating immunosuppressive CD14(+)HLA-DR(-/low) cells correlate with clinical cancer stage and pathological grade in patients with bladder carcinoma

This study investigated CD14(+)HLA-DR(-/low) cells in peripheral blood mononuclear cells (PBMCs) from 64 patients with bladder carcinoma (BC) and 14 healthy controls. Cell phenotypes were determined and CD14(+)HLA-DR(-/low) cells, CD14(+)HLA-DR(+) cells and PBMCs depleted of CD14(+)HLA-DR(-/low) cells were isolated. Proliferation of stimulated PBMCs and interferon-γ (IFN-γ) production after addition of CD14(+)HLA-DR(-/low) and CD14(+)HLA-DR(+) cells at different ratios were measured. IFN-γ production was also measured after addition of L-arginine and/or antitransforming growth factor-β (TGF-β) neutralizing monoclonal antibody, and in PBMCs depleted of CD14(+)HLA-DR(-/low) cells. The proportion of CD14(+)HLA-DR(-/low) cells in BC patients was significantly higher than in controls. CD14(+)HLA-DR(-/low) cells significantly decreased T-cell proliferation and IFN-γ production in a dose-dependent manner. This suppressive activity was partially reversed by L-arginine or anti-TGF-β. Enhanced IFN-γ secretion was also seen in PBMCs depleted of CD14(+)HLA-DR(-/low) cells. The level of CD14(+)HLA-DR(-/low) cells was associated with gender, tumour size, number of tumours, cancer pathological grade and clinical stage. CD14(+)HLA-DR(-/low) cells may represent a subpopulation of myeloid-derived suppressor cells in BC patients.

Immunological monitoring of the tumor immunoenvironment for clinical trials

Monitoring of immunotherapeutic clinical trials has undergone a considerable change in the last decade resulting in a general agreement that immune monitoring should guide the development of cancer vaccines. The emphasis on immune cell functions and quantitation of antigen-specific T cells have been playing a major role in the attempts to establish meaningful correlations between therapy-induced alterations in immune responses and clinical endpoints. However, one significant unresolved issue in modern immunotherapy is that when a tumor-specific cellular immune response is observed following the course of immunotherapy, it does not always lead to clinically proven cancer regression. This disappointing lack of a correlation between the tumor-specific cytotoxic immune responses and the clinical efficacy of immunotherapy may be explained, among other reasons, by the notion that the analysis of any single immunological parameter is not sufficient to provide clinically feasible information about the complex interactions between different cell subsets in the peripheral blood and immune, tumor, and stromal cells in the tumor milieu. By contrast, a systemic approach is required for improving the quality of a serial monitoring to ensure that it adequately and reliably measures potential changes induced in patients by administered vaccines or immunomodulators. Comprehensive evaluation of the balance between the immunostimulatory and immunosuppressive compartments of the immune system could be critical for a better understanding of how a given immunotherapy works or does not work in a particular clinical trial. New approaches to characterize tumor-infiltrating leukocytes, their phenotypic, biochemical, and genetic characteristics within the tumor microenvironment need to be developed and validated and should complement current monitoring techniques. These immune-monitoring assays for the local tumor immunoenvironment should be developed, validated, and standardized for reliability and consistency in order to establish the overall performance standards.

Phenotype, function and clinical implications of myeloid-derived suppressor cells in cancer patients

The involvement of a smouldering microenvironment is currently considered a cancer hallmark and a required step for tumour cells to disable specific immunity while promoting angiogenesis and stroma remodelling. Nevertheless, the molecular pathways driving such aberrant interactions in human cancer and their actual implication in disease progression are still poorly defined. Here, we will report about the remarkable efforts devoted by our group as well as many other scientists to dissect this process focusing on tumour-mediated activation of myeloid dysfunctional pathways occurring in cancer patients. Indeed, myeloid-derived suppressor cells (MDSC), playing a crucial role as cellular regulators of immune responses, have been extensively shown to restrain tumour immunity through a vast array of molecular mechanisms and to promote tumour progression in different murine models. Although in mice the phenotypic features of these cells were defined initially rather generally by Gr1(+) and CD11b(+) co-expression, more recent studies have unravelled the actual complexity of this population and the existence of different cell subsets. This complexity is even more remarked in the human setting, where heterogeneous populations of myeloid cells with variable phenotype and immunosuppressive features have been described in patients affected by different types of tumours. The lack of homogeneous properties of human MDSC has made these cells a controversial and still unacknowledged player in cancer-related immune suppression and disease progression. Nevertheless, with the efforts of the scientific community, MDSC will soon reveal their key role thereby becoming novel targets for innovative therapeutic strategies.

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.

Antigen-specific CD4(+) T cells regulate function of myeloid-derived suppressor cells in cancer via retrograde MHC class II signaling

Myeloid-derived suppressor cells (MDSC) play a major role in cancer-related immune suppression, yet the nature of this suppression remains controversial. In this study, we evaluated the ability of MDSCs to elicit CD4(+) T-cell tolerance in different mouse tumor models. In contrast to CD8(+) T-cell tolerance, which could be induced by MDSCs in all the tumor models tested, CD4(+) T-cell tolerance could be elicited in only one of the models (MC38) in which a substantial level of MHC class II was expressed on MDSCs compared with control myeloid cells. Mechanistic investigations revealed that MDSCs deficient in MHC class II could induce tolerance to CD8(+) T cells but not to CD4(+) T cells. Unexpectedly, antigen-specific CD4(+) T cells (but not CD8(+) T cells) could dramatically enhance the immune suppressive activity of MDSCs by converting them into powerful nonspecific suppressor cells. This striking effect was mediated by direct cell-cell contact through cross-linking of MHC class II on MDSCs. We also implicated an Ets-1 transcription factor-regulated increase in expression of Cox-2 and prostaglandin E2 in MDSCs in mediating this effect. Together, our findings suggest that activated CD4(+) T cells that are antigen specific may enhance the immune suppressive activity of MDSCs, a mechanism that might serve normally as a negative feedback loop to control immune responses that becomes dysregulated in cancer.

Role of myeloid-derived suppressor cells in tumor immunotherapy

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells that infiltrate human and experimental tumors and strongly inhibit anticancer immune response directly or by inducing regulatory T-lymphocyte activity. Consequently, MDSCs are important actors of cancer-induced immune tolerance and a major obstacle to efficiency of cancer immunotherapy. Several means of preventing MDSCs accumulation or inhibiting their immunosuppressive effect were recently discovered in cancer-bearing hosts, contributing to restoring antitumor immunity and consequently to control of tumor growth. In experimental tumor models, targeting MDSCs can enhance the effects of active or passive immunotherapy. While similar effects have not yet been noted in cancer-bearing patients, recent preclinical findings demonstrating that the selective toxicity of conventional chemotherapies such as gemcitabine and 5-fluorouracil on MDSCs might contribute to their anticancer effect provide impetus to pursue investigations to unravel novel therapeutics that target MDSCs in humans.

PGE(2)-driven induction and maintenance of cancer-associated myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSCs) are critical mediators of tumor-associated immune suppression, with their numbers and activity strongly increased in most human cancers and animal models. MDSCs suppress anti-tumor immunity through multiple mechanisms, including the manipulation of arginine and tryptophan metabolism by such factors as arginase (Arg), inducible nitric oxide synthase (iNOS/NOS2), and indoleamine-2,3-dioxygenase (IDO). Prostaglandin E(2) (PGE(2)), a mediator of chronic inflammation and tumor progression, has emerged as a key molecule in MDSC biology. PGE(2) promotes MDSC development and their induction by additional factors, directly suppresses T cell immune responses and participates in the induction of other MDSC-associated suppressive factors, including Arg, iNOS and IDO. It further promotes MDSC recruitment to tumor environments through the local induction of CXCL12/SDF-1 and the induction and stabilization of the CXCL12 receptor, CXCR4, on tumor-associated MDSCs. The establishment of a positive feedback loop between PGE(2) and cyclooxygenase 2 (COX-2), the key regulator of PGE(2) synthesis, stabilizes the MDSC phenotype and is required for their suppressive function. The central role of PGE(2) in MDSC biology provides for a feasible target for counteracting MDSC-mediated immune suppression in cancer.

Positive feedback between PGE2 and COX2 redirects the differentiation of human dendritic cells toward stable myeloid-derived suppressor cells

Dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) show opposing roles in the immune system. In the present study, we report that the establishment of a positive feedback loop between prostaglandin E(2) (PGE(2)) and cyclooxygenase 2 (COX2), the key regulator of PGE(2) synthesis, represents the determining factor in redirecting the development of CD1a(+) DCs to CD14(+)CD33(+)CD34(+) monocytic MDSCs. Exogenous PGE(2) and such diverse COX2 activators as lipopolysaccharide, IL-1β, and IFNγ all induce monocyte expression of COX2, blocking their differentiation into CD1a(+) DCs and inducing endogenous PGE(2), IDO1, IL-4Rα, NOS2, and IL-10, typical MDSC-associated suppressive factors. The addition of PGE(2) to GM-CSF/IL-4-supplemented monocyte cultures is sufficient to induce the MDSC phenotype and cytotoxic T lymphocyte (CTL)-suppressive function. In accordance with the key role of PGE(2) in the physiologic induction of human MDSCs, the frequencies of CD11b(+)CD33(+) MDSCs in ovarian cancer are closely correlated with local PGE(2) production, whereas the cancer-promoted induction of MDSCs is strictly COX2 dependent. The disruption of COX2-PGE(2) feedback using COX2 inhibitors or EP2 and EP4 antagonists suppresses the production of MDSC-associated suppressive factors and the CTL-inhibitory function of fully developed MDSCs from cancer patients. The central role of COX2-PGE(2) feedback in the induction and persistence of MDSCs highlights the potential for its manipulation to enhance or suppress immune responses in cancer, autoimmunity, or transplantation.

Positive feedback between PGE2 and COX2 redirects the differentiation of human dendritic cells toward stable myeloid-derived suppressor cells

Dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) show opposing roles in the immune system. In the present study, we report that the establishment of a positive feedback loop between prostaglandin E(2) (PGE(2)) and cyclooxygenase 2 (COX2), the key regulator of PGE(2) synthesis, represents the determining factor in redirecting the development of CD1a(+) DCs to CD14(+)CD33(+)CD34(+) monocytic MDSCs. Exogenous PGE(2) and such diverse COX2 activators as lipopolysaccharide, IL-1β, and IFNγ all induce monocyte expression of COX2, blocking their differentiation into CD1a(+) DCs and inducing endogenous PGE(2), IDO1, IL-4Rα, NOS2, and IL-10, typical MDSC-associated suppressive factors. The addition of PGE(2) to GM-CSF/IL-4-supplemented monocyte cultures is sufficient to induce the MDSC phenotype and cytotoxic T lymphocyte (CTL)-suppressive function. In accordance with the key role of PGE(2) in the physiologic induction of human MDSCs, the frequencies of CD11b(+)CD33(+) MDSCs in ovarian cancer are closely correlated with local PGE(2) production, whereas the cancer-promoted induction of MDSCs is strictly COX2 dependent. The disruption of COX2-PGE(2) feedback using COX2 inhibitors or EP2 and EP4 antagonists suppresses the production of MDSC-associated suppressive factors and the CTL-inhibitory function of fully developed MDSCs from cancer patients. The central role of COX2-PGE(2) feedback in the induction and persistence of MDSCs highlights the potential for its manipulation to enhance or suppress immune responses in cancer, autoimmunity, or transplantation.

Myeloid-derived suppressor cell accumulation and function in patients with newly diagnosed glioblastoma

To assess the accumulation of myeloid-derived suppressor cells (MDSCs) in the peripheral blood of patients with glioma and to define their heterogeneity and their immunosuppressive function. Peripheral blood mononuclear cells (PBMCs) from healthy control subjects and from patients with newly diagnosed glioma were stimulated with anti-CD3/anti-CD28 and T cells assessed for intracellular expression of interferon (IFN)-γ. Antibody staining of PBMCs from glioma patients and healthy donors (CD33, HLADR, CD15, and CD14) followed by 4-color flow cytometry analysis-defined MDSC levels in the peripheral blood. To assess the role of MDSCs in suppressing T cell IFNγ production, PBMCs were depleted of MDSCs using anti-CD33 and anti-CD15 antibody-coated beads prior to T cell stimulation. Enzyme-linked immunosorbent assays were used to assess plasma arginase activity and the level of granulocyte colony-stimulating factor (G-CSF). Patients with glioblastoma have increased MDSC counts (CD33+HLADR-) in their blood that are composed of neutrophilic (CD15(+); >60%), lineage-negative (CD15(-)CD14(-); 31%), and monocytic (CD14(+); 6%) subsets. After stimulation, T cells from patients with glioblastoma had suppressed IFN-γ production when compared with healthy, age-matched donor T cells. Removal of MDSCs from the PBMCs with anti-CD33/CD15-coated beads significantly restored T cell function. Significant increases in arginase activity and G-CSF levels were observed in plasma specimens obtained from patients with glioblastoma. The accumulation of MDSCs in peripheral blood in patients with glioma likely promotes T cell immune suppression that is observed in this patient population. Increased plasma levels of arginase and G-CSF may relate to MDSC suppressor function and MDSC expansion, respectively, in patients with glioma.

Origin and Functions of Tumor-Associated Myeloid Cells (TAMCs)

The construction of an inflammatory microenvironment provides the fuel for cancer development and progression. Hence, solid tumors promote the expansion and the recruitment of leukocyte populations, among which tumor-associated myeloid cells (TAMCs) represent a paradigm for cancer-promoting inflammation. TAMCs group heterogeneous phagocytic populations stemming from a common myeloid progenitor (CMP), that orchestrate various aspects of cancer, including: diversion and skewing of adaptive responses; immunosuppression; cell growth; angiogenesis; matrix deposition and remodelling; construction of a metastatic niche and actual metastasis. Several evidence indicate that TAMCs show plasticity and/or functional heterogeneity, suggesting that tumour-derived factors promote their functional "reprogramming" towards protumoral activities. While recent studies have attempted to address the role of microenvironment signals, the interplay between cancer cells, innate and adaptive immunity is now emerging as a crucial step of the TAMCs reprogramming. Here we discuss the evidence for the differentiation of TAMCs during the course of tumor progression and the molecular mechanisms that regulate such event.

The immune microenvironment of myeloma

The bone marrow (BM) is the site of disease in myeloma and possesses unique immune characteristics involved in the pathobiology of the disease. Interactions of plasma cells with stromal cells, osteoclasts, osteoblasts, myeloid and lymphoid cells make up the unique bone marrow milieu that mediates myeloma disease progression. Independently or through a complex network of interactions these cells impart immune changes leading to immune evasion and disease progression. The critical role of these factors in disease progression has led to the intense development of therapeutic strategies aimed at either disrupting the immune mechanisms mediating disease progression or augmenting those with anti-tumor benefits. This review discusses the major contributors of immunity in the bone marrow microenvironment, their interactions, and mechanisms whereby immune modulation can be translated into therapies with anti-myeloma efficacy.

Clinical outcome of patients with various advanced cancer types vaccinated with an optimized cryptic human telomerase reverse transcriptase (TERT) peptide: results of an expanded phase II study

BACKGROUND

TERT (telomerase reverse transcriptase) plays a critical role in tumor cell growth and survival. In an expanded phase II study, we evaluated the immunological and clinical responses to the TERT-targeting Vx-001 vaccine in patients with advanced solid tumors.

METHODS

HLA-A*0201-positive patients received two subcutaneous injections of the optimized TERT(572Y) peptide followed by four injections of the native TERT(572) peptide, every 3 weeks. Peptide-specific immune responses were evaluated by enzyme-linked immunosorbent spot at baseline, and after the second and the sixth vaccinations.

RESULTS

Fifty-five patients were enrolled and 34 (62%) completed the six vaccinations. A TERT-specific T-cell immune response was observed in 55% and 70% of patients after the second and the sixth vaccinations, respectively. The disease control rate (DCR) was 36% [95% confidence interval (CI) 24% to 49%], including one complete and one partial response. Immunologically responding patients had a better clinical outcome than nonresponders [DCR: 44% versus 14% (P = 0.047); progression-free survival (PFS): 5.2 versus 2.2 months (P = 0.0001) and overall survival: 20 versus 10 months (P = 0.041)]. Multivariate analysis revealed that the immunological response was an independent variable associated with increased PFS (hazard ratio = 3.35; 95% CI 1.7-6.7).

CONCLUSION

Vx-001 vaccine was well tolerated and induced a TERT-specific immunological response, which was significantly correlated with improved clinical outcome.

Camouflage and sabotage: tumor escape from the immune system

The field of tumor immunology has made great progress in understanding tumor immune interactions. As a consequence a number of immuno-therapeutic approaches have been successfully introduced into the clinic and a large number of promising therapeutic strategies are investigated in ongoing clinical trials. Evaluation of anti-tumor immunity in such trials as well as in animal models has shown that tumor escape from immune recognition and tumor-mediated suppression of anti-tumor immunity can pose a significant obstacle to successful cancer therapy. Here, we review mechanisms of tumor immune escape and immune-subversion with a focus on the research interests in our laboratory: loss of MHC class I on tumor cells, increased oxidative stress, recruitment of myeloid-derived suppressor cells, and regulatory T cells.

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

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

Myeloid derived suppressor cells in human diseases

Myeloid derived suppressor cells (MDSC) have been described as a heterogeneous cell population with potent immune suppressor function in mice. Limited data are available on MDSC in human diseases. Interpretation of these data is complicated by the fact that different markers have been used to analyze human MDSC subtypes in various clinical settings. Human MDSC are CD11b+, CD33+, HLA-DR(neg/low) and can be divided into granulocytic CD14⁻ and monocytic CD14+ subtypes. Interleukin 4Rα, VEGFR, CD15 and CD66b have been suggested to be more specific markers for human MDSC, however these markers can only be found on some MDSC subsets. Until today the best marker for human MDSC remains their suppressor function, which can be either direct or indirect through the induction of regulatory T cells. Immune suppressor activity has been associated with high arginase 1 and iNOS activity as well as ROS production by MDSC. Not only in murine models, but even more importantly in patients with cancer, different drugs have been shown to either reverse the immune suppressor function of MDSC or directly target these cells. Systemic treatment with all-trans-retinoic acid has been shown to mature human MDSC and reverse their immune suppressor function. Alternatively, MDSC can be targeted by treatment with the multi-targeted receptor tyrosine kinase inhibitor sunitinib. This review will provide a comprehensive summary of the recent literature on human MDSC.

Renal transplant recipients have elevated frequencies of circulating myeloid-derived suppressor cells

BACKGROUND

Cancer, particularly cutaneous squamous cell carcinoma (SCC), is a major cause of mortality in renal transplant recipients (RTRs). Myeloid-derived suppressor cells (MDSC) play a central role in suppressing cancer immunosurveillance but their potential mobilisation in RTRs and levels relative to those of other immunoregulatory dendritic cell (DC) populations have not been analysed.

METHODS

The circulating frequencies of MDSC and DC were analysed by multicolour flow cytometry in immunocompetent patients without (n = 13) or with (ICI-SCC(Pos), n = 14) current SCC, normal donors (NDs, n = 34), chronic kidney disease patients (CKD patients, n = 22) and RTRs (n = 31).

RESULTS

Compared to NDs, RTRs had significantly elevated levels of both CD14(Neg) and CD14(Pos) MDSC subsets (P < 0.001), while CKD patients and ICI-SCC(Pos) had significantly elevated levels of only the CD14(Neg)-MDSC subset. DC frequencies were significantly decreased in RTRs and CKD patients but were at normal levels in ICI-SCC(Pos). The MDSC/DC ratio was significantly elevated (P < 0.05) in RTRs (median = 5.7), CKD patients (median = 3.2) and ICI-SCC(Pos) (median = 3.5) relative to NDs (median = 0.7). The use of immunosuppressive drugs in CKD patients and past/current occurrence of SCC in RTRs was associated with significantly increased CD14(Neg)-MDSC frequencies. MDSC enriched from RTRs, when co-cultured with activated NDs T cells significantly suppressed extracellular IL-10 levels and can, when activated with formyl-methionyl-leucyl-phenylalanine, inhibit T-cell proliferation.

CONCLUSIONS

RTRs, CKD patients and ICI-SCC(Pos) have increased MDSC frequencies and MDSC/DC ratios. These changes may impact on cancer immunosurveillance. Therefore, MDSC represent both a potential therapeutic target and prognostic marker in these patients, with respect to the development of SCC and other malignancies.

Myeloid-derived suppressor cells--their role in haemato-oncological malignancies and other cancers and possible implications for therapy

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells at different stages of maturation that play a role in cancer tolerance and function as an immune-suppressive cell subpopulation. They utilize different mechanisms to block both innate and adaptive arms of anti-tumour immunity, mostly through inhibition of T cell activation and expansion. Further advances in our understanding of this cell population in both murine models and humans has enabled more accurate characterization of their phenotype and the recognition of two major classes of MDSCs: granulocytic and monocytic. Recently, the mechanism of action and clinical importance of MDSCs has been more clearly defined and their interactions with cancer cells have been shown to be among the factors influencing tumour development and induction of tolerance. Most of the earlier studies were performed using murine models, but recent clinical investigations have shown their potential role in human cancers. Here, we review the origin of MDSCs, their mechanisms of action, the factors influencing their production and related signalling pathways. We focus on their role in human solid tumours and haemato-oncological malignancies, and relate to possible novel therapeutic approaches targeting MDSCs which could be considered together with other anticancer strategies in the not too distant future.