Targeting myeloid regulatory cells in cancer by chemotherapeutic agents

Sargassum henslowianum polysaccharides induce apoptosis of prostate cancer associated macrophages via the CD206-ERK-ROS pathway

Tumor-associated macrophages (TAMs) are a predominant immunosuppressive cell type within the tumor microenvironment. Whether polysaccharides from Sargassum henslowianum C. Ag. (SH) exert anti-tumor effects by modulating TAMs remains under investigation. In this study, we isolated and characterized a uniform sulfated polysaccharide, SHA1P-2, from SH, with a molecular weight of 68,000 g/mol. Monosaccharide composition analysis revealed that fucose and galactose were the predominant components, with minor amounts of xylose and mannose. RM1-Raw264.7 cells, simulating TAMs, were induced by culturing Raw264.7 cells in the supernatant of prostate cancer RM1 cells, and their effects on T-cell proliferation were assessed. Unlike Raw264.7 cells, RM1-Raw264.7 cells significantly suppressed concanavalin A-induced T-cell proliferation, a suppression alleviated by SHA1P-2 treatment. Interestingly, SHA1P-2 did not induce apoptosis in Raw264.7 cells but effectively triggered apoptosis in RM1-Raw264.7 cells. The apoptotic mechanism was mediated via the upregulation of CD206 in TAMs, as evidenced by reduced apoptosis and reactive oxygen species (ROS) levels when CD206 was knocked down or p-ERK was inhibited in TAMs. These findings demonstrate that SHA1P-2 promotes TAM apoptosis via the CD206-ERK-ROS axis, indicating that elevated ROS levels serve as a key trigger for TAM apoptosis, thereby presenting a novel strategy for TAM-based anti-tumor therapies.

Targeting myeloid-derived suppressor cells in the tumor microenvironment: potential therapeutic approaches for osteosarcoma

Osteosarcoma is a bone malignancy characterized by strong invasiveness and rapid disease progression. The tumor microenvironment of osteosarcoma contains various types of immune cells, including myeloid-derived suppressor cells, macrophages, T cells, and B cells. Imbalances of these immune cells can promote the proliferation and metastasis of osteosarcoma. Recent studies have indicated a substantial increase in the levels of myeloid-derived suppressor cells, an immune cell associated with immunosuppressive and pro-cancer effects, in the peripheral blood of patients with osteosarcoma. Moreover, the levels of the pro-inflammatory cytokine interleukin 18 are positively correlated with those of myeloid-derived suppressor cells in the peripheral blood of animal models of osteosarcoma. In this review, we explore the function of myeloid-derived suppressor cells in osteosarcoma based on the clinical diagnoses of patients with osteosarcoma and discuss future therapeutic approaches for targeting osteosarcoma. Targeting myeloid-derived suppressor cells represents a promising approach to improving the prognosis and survival rates of patients with osteosarcoma.

Role of Myeloid-derived suppressor cell (MDSC) in autoimmunity and its potential as a therapeutic target

Myeloid suppressor cells (MDSCs) are an important class of immune-regulating cells that can suppress T cell function. Most of our knowledge about the function of MDSC comes from studies of cancer models. Recent studies, however, have greatly contributed to the description of MDSC involvement in autoimmune diseases. They are known as a cell population that may negatively affect immune responses by regulating the function of CD4⁺ and CD8⁺ cells, which makes them an attractive target for autoimmune diseases therapy. However, many questions about MDSC activation, differentiation, and inhibitory functions remain unanswered. In this study, we have summarized the role of MDSCs in various autoimmune diseases, and the potential of targeting them for therapeutic benefits has been discussed.

Myeloid-derived suppressor cell depletion therapy targets IL-17A-expressing mammary carcinomas

Triple-negative breast cancer (TNBC) is an invasive subtype of breast cancer but paradoxically associated with increased tumor-infiltrating leukocytes. The molecular and cellular mechanisms underlying TNBC immunobiology are incompletely understood. Interleukin (IL)-17A is a pro-inflammatory cytokine that has both pro- and anti-tumor effects and found in 40-80% of TNBC samples. We report here that IL-17A mRNA and protein are detectable in some human TNBC cell lines and further upregulated by IL-23 and LPS stimulation. Furthermore, the impact of tumor-derived IL-17A in host immune response and tumor growth was examined using murine TNBC 4T1 mammary carcinoma cells transduced with an adenoviral vector expressing IL-17A (AdIL-17A) or control vector (Addl). Compared to Addl-transduction, AdIL-17A-transduction enhanced 4T1 tumor growth and lung metastasis in vivo, which was associated with a marked expansion of myeloid-derived suppressor cells (MDSCs). However, AdIL-17A-transduction also induced strong organ-specific and time-dependent immune activation indicated by dynamic changes of NK cells, B cells, CD4, and CD8 T cells in peripheral blood, lung, and tumor site, as well as the plasma levels of IFNγ. Such findings highlight that tumor-associated IL-17A induces concurrent immune activation and immune suppression. Administration of anti-Gr1 or anti-G-CSF antibody effectively depleted MDSCs in vivo, markedly reducing the growth of AdIL-17A-transduced 4T1 tumors, and eliminating lung metastasis. Collectively, our study demonstrates that MDSC depletion is an effective and practical approach for treating IL-17A-enriched mammary carcinomas.

The evolving landscape of immunotherapy in small-cell lung cancer: A focus on predictive biomarkers

Small cell lung cancer (SCLC) was defined as a "recalcitrant cancer" because of its dismal prognosis and lack of outcome improvements in the last 30 years. Immunotherapy with checkpoint inhibitors revolutionized treatment in many cancer types and results from the IMpower133 study, a double-blind placebo-controlled phase III trial, showed overall survival benefit for atezolizumab when added to standard platinum-etoposide chemotherapy in first-line SCLC setting for the first time since years. Trials with other checkpoint inhibitors, e.g. pembrolizumab, durvalumab, nivolumab and ipilimumab, are ongoing in various settings, but, to date, there are no defined factors to identify patients who are more likely to benefit from such treatments. This review summarizes results of immunotherapy trials in SCLC for first-line, maintenance and further-line therapies for single-agents and combinations with checkpoint inhibitors. Predictive factors from these trials are reviewed in order to identify their clinical value, with particular emphasis on PD-L1 expression on both tumor cells and in stroma, especially in pembrolizumab-treated patients, and tumor mutational burden, for patients treated with the ipilimumab and nivolumab combination.

Juglone eliminates MDSCs accumulation and enhances antitumor immunity

Myeloid-derived suppressor cells (MDSCs) contribute to immune activity suppression and promote the tumor progression. Elimination of MDSCs is a promising cancer therapeutic strategy, and some chemotherapeutic agents have been reported to hamper tumor progression by suppressing MDSCs. Juglone has been showed to exert a direct cytotoxic effect on tumor cells. However, the effect of juglone on MDSCs and anti-tumor immune statue has remained unexplored. In our study, we observed that juglone suppressed tumor growth and metastasis markedly, and the tumor growth suppression in immunocompetent mice was more drastic than that in immunodeficient mice. Juglone reduced the accumulation of MDSCs and increased IFN-γ production by CD8⁺ T cells. Consistently, juglone affected myeloid cells differentiation and maturation, impairing the immunosuppressive functions of MDSCs. Moreover, juglone down-regulated the level of IL-1β which was mediating accumulation of MDSCs. In addition, juglone inhibited 5FU-induced liver injury in a colorectal carcinoma-bearing mice model. Thus, our work suggests that the anti-tumor effect of juglone is mediated, at least in part, by eliminating accumulation of MDSCs.

Targeting myeloid regulators by paclitaxel-loaded enzymatically degradable nanocups

Tumor microenvironment is characterized by immunosuppressive mechanisms associated with the accumulation of immune regulatory cells - myeloid-derived suppressor cells (MDSC). Therapeutic depletion of MDSC has been associated with inhibition of tumor growth and therefore represents an attractive approach to cancer immunotherapy. MDSC in cancer are characterized by enhanced enzymatic capacity to generate reactive oxygen and nitrogen species (RONS) which have been shown to effectively degrade carbonaceous materials. We prepared enzymatically openable nitrogen-doped carbon nanotube cups (NCNC) corked with gold nanoparticles and loaded with paclitaxel as a therapeutic cargo. Loading and release of paclitaxel was confirmed through electron microscopy, Raman spectroscopy and LC-MS analysis. Under the assumption that RONS generated by MDSCs can be utilized as a dual targeting and oxidative degradation mechanism for NCNC, here we report that systemic administration of paclitaxel loaded NCNC delivers paclitaxel to circulating and lymphoid tissue MDSC resulting in the inhibition of growth of tumors (B16 melanoma cells inoculated into C57BL/6 mice) in vivo. Tumor growth inhibition was associated with decreased MDSC accumulation quantified by flow cytometry that correlated with bio-distribution of gold-corked NCNC resolved by ICP-MS detection of residual gold in mouse tissue. Thus, we developed a novel immunotherapeutic approach based on unique nanodelivery vehicles, which can be loaded with therapeutic agents that are released specifically in MDSC via NCNC selective enzymatic "opening" affecting change in the tumor microenvironment.

Targeting myeloid cells in the tumor sustaining microenvironment

Myeloid cells are the most abundant cells in the tumor microenvironment (TME). The tumor recruits and modulates endogenous myeloid cells to tumor-associated macrophages (TAM), dendritic cells (DC), myeloid-derived suppressor cells (MDSC) and neutrophils (TAN), to sustain an immunosuppressive environment. Pathologically overexpressed mediators produced by cancer cells like granulocyte-macrophage colony-stimulating- and vascular endothelial growth factor induce myelopoiesis in the bone marrow. Excess of myeloid cells in the blood, periphery and tumor has been associated with tumor burden. In cancer, myeloid cells are kept at an immature state of differentiation to be diverted to an immunosuppressive phenotype. Here, we review human myeloid cells in the TME and the mechanisms for sustaining the hallmarks of cancer. Simultaneously, we provide an introduction into current and novel therapeutic approaches to redirect myeloid cells from a cancer promoting to a rather inflammatory, cancer inhibiting phenotype. In addition, the role of platelets for tumor promotion is discussed.

The innate and adaptive infiltrating immune systems as targets for breast cancer immunotherapy

A cancer cell-centric view has long dominated the field of cancer biology. Research efforts have focussed on aberrant cancer cell signalling pathways and on changes to cancer cell DNA. Mounting evidence demonstrates that many cancer-associated cell types within the tumour stroma co-evolve and support tumour growth and development, greatly modifying cancer cell behaviour, facilitating invasion and metastasis and controlling dormancy and sensitivity to drug therapy. Thus, these stromal cells represent potential targets for cancer therapy. Among these cell types, immune cells have emerged as a promising target for therapy. The adaptive and the innate immune system play an important role in normal mammary development and breast cancer. The number of infiltrating adaptive immune system cells with tumour-rejecting capacity, primarily, T lymphocytes, is lower in breast cancer compared with other cancer types, but infiltration occurs in a large proportion of cases. There is strong evidence demonstrating the importance of the immunosuppressive role of the innate immune system during breast cancer progression. A consideration of components of both the innate and the adaptive immune system is essential for the design and development of immunotherapies in breast cancer. In this review, we focus on the importance of immunosuppressive myeloid-derived suppressor cells (MDSCs) as potential targets for breast cancer therapy.

Myeloid-derived suppressor cells and myeloid regulatory cells in cancer and autoimmune disorders

Myeloid-derived suppressor cells (MDSCs) were originally described as a heterogeneous population of immature cells derived from myeloid progenitors with immune-suppressive functions in tumor-bearing hosts. In recent years, increasing number of studies have described various populations of myeloid cells with MDSC-like properties in murine models of cancer and autoimmune diseases. These studies have observed that the populations of MDSCs are increased during inflammation and autoimmune conditions. In addition, MDSCs can effectively suppress T cell responses and modulate the activity of natural killer cells and other myeloid cells. MDSCs have also been implicated in the induction of regulatory T cell production. Furthermore, these cells have the potential to suppress the autoimmune response, thereby limiting tissue injury. Myeloid regulatory cells (Mregs) are recently attracting increasing attention, since they function in proinflammatory and immune suppression in autoimmune diseases, as well as in various types of cancer. Currently, research focus is directed from MDSCs to Mregs in cancer and autoimmune diseases. The present study reviewed the suppressive roles of MDSCs in various autoimmune murine models, the immune modulation of MDSCs to T helper 17 lymphocytes, as well as the proinflammatory and immunosuppressive roles of Mregs in various types of cancer and autoimmune diseases.

MDSCs in cancer: Conceiving new prognostic and therapeutic targets

The incomplete clinical efficacy of anti-tumor immunotherapy can depend on the presence of an immunosuppressive environment in the host that supports tumor progression. Tumor-derived cytokines and growth factors induce an altered hematopoiesis that modifies the myeloid cell differentiation process, promoting proliferation and expansion of cells with immunosuppressive skills, namely myeloid derived suppressor cells (MDSCs). MDSCs promote tumor growth not only by shaping immune responses towards tumor tolerance, but also by supporting several processes necessary for the neoplastic progression such as tumor angiogenesis, cancer stemness, and metastasis dissemination. Thus, MDSC targeting represents a promising tool to eliminate host immune dysfunctions and increase the efficacy of immune-based cancer therapies.

Myeloid-derived suppressor cells in the tumor microenvironment: expect the unexpected

Our understanding of the role of myeloid-derived suppressor cells (MDSCs) in cancer is becoming increasingly complex. In addition to their eponymous role in suppressing immune responses, they directly support tumor growth, differentiation, and metastasis in a number of ways that are only now beginning to be appreciated. It is because of this increasingly complex role that these cells may become an important factor in the treatment of human cancer. In this Review, we discuss the most pertinent and controversial issues of MDSC biology and their role in promoting cancer progression and highlight how these cells may be used in the clinic, both as prognostic factors and as therapeutic targets.

Frequencies of circulating MDSC correlate with clinical outcome of melanoma patients treated with ipilimumab

Metastatic melanoma has a poor prognosis with high resistance to chemotherapy and radiation. Recently, the anti-CTLA-4 antibody ipilimumab has demonstrated clinical efficacy, being the first agent to significantly prolong the overall survival of inoperable stage III/IV melanoma patients. A major aim of patient immune monitoring is the identification of biomarkers that predict clinical outcome. We studied circulating myeloid-derived suppressor cells (MDSC) in ipilimumab-treated patients to detect alterations in the myeloid cell compartment and possible correlations with clinical outcome. Lin(-) CD14(+) HLA-DR(-) monocytic MDSC were enriched in peripheral blood of melanoma patients compared to healthy donors (HD). Tumor resection did not significantly alter MDSC frequencies. During ipilimumab treatment, MDSC frequencies did not change significantly compared to baseline levels. We observed high inter-patient differences. MDSC frequencies in ipilimumab-treated patients were independent of baseline serum lactate dehydrogenase levels but tended to increase in patients with severe metastatic disease (M1c) compared to patients with metastases in skin or lymph nodes only (M1a), who had frequencies comparable to HD. Interestingly, clinical responders to ipilimumab therapy showed significantly less lin(-) CD14(+) HLA-DR(-) cells as compared to non-responders. The data suggest that the frequency of monocytic MDSC may be used as predictive marker of response, as low frequencies identify patients more likely benefitting from ipilimumab treatment. Prospective clinical trials assessing MDSC frequencies as potential biomarkers are warranted to validate these observations.

Immunological Mechanisms of Low and Ultra-Low Dose Cancer Chemotherapy

Traditionally, anticancer chemotherapy has been generally considered to be strongly immunosuppressive. However, increasing evidence suggests that certain chemotherapeutic agents rely on the induction of antitumor immune responses, in both experimental animal models and patients with cancer. Many of these chemotherapeutic agents exert immunogenic effects via the induction and release of immunostimulatory "danger" signals from dying cancerous cells when used in low doses. New data suggests that several common chemotherapeutic agents may also display direct stimulating effects on immune cells even when applied in ultra-low concentrations (chemoimmunomodulation). Importantly, it is becoming clear that both immune effector cells and immune regulatory cells can be targeted by various chemotherapeutic agents to produce favorable antitumor immune responses. Therefore, utilizing cancer drugs to enhance host antitumor immunity should be considered a feasible therapeutic approach; and recent characterization of the immunomodulatory mechanisms of anticancer chemotherapy using both new and traditional cytotoxic agents suggests that combinations of these approaches with "classical" immunomodulatory agents could lead to a viable new therapeutic paradigm for the treatment of cancer.

Clinical evaluation of systemic and local immune responses in cancer: time for integration

The immune system has a dual role in cancer development and progression. On the one hand, it can eradicate emerging malignant cells, but on the other hand, it can actively promote growth of malignant cells, their invasive capacities and their ability to metastasize. Immune cells with predominantly anti-tumor functionality include cells of the innate immune system, such as natural killer cells, and cells of adaptive immunity, such as conventional dendritic cells and cytotoxic T lymphocytes. Immune cells with predominantly pro-tumor functionality include a broad spectrum of cells of the innate and adaptive immune system, such as type 2 neutrophils and macrophages, plasmacytoid DC, myeloid-derived suppressor cells and regulatory T lymphocytes. The presence of immune cells with tumor-suppressive and tumor-promoting activity in the cancer microenvironment and in peripheral blood is usually associated with good clinical outcomes and poor clinical outcomes, respectively. Significant advances in experimental and clinical oncoimmunology achieved in the last decade open an opportunity for the use of modern morphologic, flow cytometric and functional tests in clinical practice. In this review, we describe an integrated approach to clinical evaluation of the immune status of cancer patients for diagnostic purposes, prognostic/predictive purposes (evaluation of patient prognosis and response to treatment) and for therapeutic purposes.

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.

Update on the challenges and recent advances in cancer immunotherapy

This overview provides an analysis of some of the immunotherapies currently in use and under investigation, with a special focus on the tumor microenvironment, which we believe is a major factor responsible for the general failure of immunotherapy to date. It is our expectation that combining immunotherapy with methods of altering the tumor microenvironment and targeting regulatory T cells and myeloid cells will yield favorable results.

The role of TLR4 in the paclitaxel effects on neuronal growth in vitro

Paclitaxel (Pac) is an antitumor agent that is widely used for treatment of solid cancers. While being effective as a chemotherapeutic agent, Pac in high doses is neurotoxic, specifically targeting sensory innervations. In view of these toxic effects associated with conventional chemotherapy, decreasing the dose of Pac has been recently suggested as an alternative approach, which might limit neurotoxicity and immunosuppression. However, it remains unclear if low doses of Pac retain its neurotoxic properties or might exhibit unusual effects on neuronal cells. The goal of this study was to analyze the concentration-dependent effect of Pac on isolated and cultured DRG neuronal cells from wild-type and TLR4 knockout mice. Three different morphological parameters were analyzed: the number of neurons which developed neurites, the number of neurites per cell and the total length of neurites per cell. Our data demonstrate that low concentrations of Pac (0.1 nM and 0.5 nM) do not influence the neuronal growth in cultures in both wild type and TLR4 knockout mice. Higher concentrations of Pac (1–100 nM) had a significant effect on DRG neurons from wild type mice, affecting the number of neurons which developed neurites, number of neurites per cell, and the length of neurites. In DRG from TLR4 knockout mice high concentrations of Pac showed a similar effect on the number of neurons which developed neurites and the length of neurites. At the same time, the number of neurites per cell, indicating the process of growth cone initiation, was not affected by high concentrations of Pac. Thus, our data showed that Pac in high concentrations has a significant damaging effect on axonal growth and that this effect is partially mediated through TLR4 pathways. Low doses of Pac are devoid of neuronal toxicity and thus can be safely used in a chemomodulation mode.

Immunomodulatory effects of low dose chemotherapy and perspectives of its combination with immunotherapy

Given that cancer is one of the main causes of death worldwide, many efforts have been directed toward discovering new treatments and approaches to cure or control this group of diseases. Chemotherapy is the main treatment for cancer; however, a conventional schedule based on maximum tolerated dose (MTD) shows several side effects and frequently allows the development of drug resistance. On the other side, low dose chemotherapy involves antiangiogenic and immunomodulatory processes that help host to fight against tumor cells, with lower grade of side effects. In this review, we present evidence that metronomic chemotherapy, based on the frequent administration of low or intermediate doses of chemotherapeutics, can be better than or as efficient as MTD. Finally, we present some data indicating that noncytotoxic concentrations of antineoplastic agents are able to both up-regulate the immune system and increase the susceptibility of tumor cells to cytotoxic T lymphocytes. Taken together, data from the literature provides us with sufficient evidence that low concentrations of selected chemotherapeutic agents, rather than conventional high doses, should be evaluated in combination with immunotherapy.

Oncostatin M suppresses oestrogen receptor-α expression and is associated with poor outcome in human breast cancer

The most important clinical biomarker for breast cancer management is oestrogen receptor alpha (ERα). Tumours that express ER are candidates for endocrine therapy and are biologically less aggressive, while ER-negative tumours are largely treated with conventional chemotherapy and have a poor prognosis. Despite its significance, the mechanisms regulating ER expression are poorly understood. We hypothesised that the inflammatory cytokine oncostatin M (OSM) can downregulate ER expression in breast cancer. Recombinant OSM potently suppressed ER protein and mRNA expression in vitro in a dose- and time-dependent manner in two human ER+ breast cancer cell lines, MCF7 and T47D. This was dependent on the expression of OSM receptor beta (OSMRβ) and could be blocked by inhibition of the MEKK1/2 mitogen-activated protein kinases. ER loss was also necessary for maximal OSM-induced signal transduction and migratory activity. In vivo, high expression of OSM and OSMR mRNA (determined by RT-PCR) was associated with reduced ER (P<0.01) and progesterone receptor (P<0.05) protein levels in a cohort of 70 invasive breast cancers. High OSM and OSMR mRNA expression was also associated with low expression of ESR1 (ER, P<0.0001) and ER-regulated genes in a previously published breast cancer gene expression dataset (n=321 cases). In the latter cohort, high OSMR expression was associated with shorter recurrence-free and overall survival in univariate (P<0.0001) and multivariate (P=0.022) analyses. OSM signalling may be a novel factor causing suppression of ER and disease progression in breast cancer.

Application of paclitaxel in low non-cytotoxic doses supports vaccination with melanoma antigens in normal mice

Chemotherapeutic agents such as paclitaxel applied in ultra-low, non-cytotoxic doses were previously shown to stimulate dendritic cell activity and anti-tumor immune responses upon vaccination in mouse transplantable tumor models. However, the mechanisms of these alterations-termed chemoimmunomodulation or chemomodulation-are still not clear. This study investigated the effect of paclitaxel applied in ultra-low, non-cytotoxic doses on the efficiency of immunization of healthy C57BL/6 mice with the peptide derived from tyrosinase related protein (TRP)-2 as a model melanoma antigen. Using an IFNγ ELISPOT assay, it was found that administration of 1 mg paclitaxel/kg in combination with the peptide vaccination strongly increased the frequencies of TRP-2 specific spleen T-cells as compared to levels due to the vaccination alone. This was associated with a significant decrease in the levels of regulatory T-cells (T(reg)) and immature myeloid cells (known as a counterpart of myeloid derived suppressor cells [MDSC] in healthy mice). Such impairments of potential immunosuppressive cells were found to correlate with a strong increase in the amount of effector CD8+ and CD4+ T-cells in the bone marrow and spleen. Furthermore, in paclitaxel-treated mice, a significant augmentation of natural killer (NK) cell numbers in the bone marrow and their ability to produce IFNγ were observed. In addition, the level of NK-T-cells in the lymph nodes was also increased. It is suggested that paclitaxel applied in ultra-low, non-cytotoxic doses may potentially enhance the efficacy of anti-tumor vaccinations by neutralizing immunosuppressive T(reg) and MDSC populations in tumor-bearing hosts.

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.

Paclitaxel promotes differentiation of myeloid-derived suppressor cells into dendritic cells in vitro in a TLR4-independent manner

Myeloid cells play a key role in the outcome of anti-tumor immunity and response to anti-cancer therapy, since in the tumor microenvironment they may exert both stimulatory and inhibitory pressures on the proliferative, angiogenic, metastatic, and immunomodulating potential of tumor cells. Therefore, understanding the mechanisms of myeloid regulatory cell differentiation is critical for developing strategies for the therapeutic reversal of myeloid derived suppressor cell (MDSC) accumulation in the tumor-bearing hosts. Here, using an in vitro model system, several potential mechanisms of the direct effect of paclitaxel on MDSC were tested, which might be responsible for the anti-tumor potential of low-dose paclitaxel therapy in mice. It was hypothesized that a decreased level of MDSC in vivo after paclitaxel administration might be due to (i) the blockage of MDSC generation, (ii) an induction of MDSC apoptosis, or (iii) the stimulation of MDSC differentiation. The results revealed that paclitaxel in ultra-low concentrations neither increased MDSC apoptosis nor blocked MDSC generation, but stimulated MDSC differentiation towards dendritic cells. This effect of paclitaxel was TLR4-independent since it was not diminished in cell cultures originated from TLR4-/- mice. These results support a new concept that certain chemotherapeutic agents in ultra-low non-cytotoxic doses may suppress tumor progression by targeting several cell populations in the tumor microenvironment, including MDSC.