Myeloid-derived Suppressor Cells In Cancer Patients Research Articles

<i>In vitro</i> production of myeloid-derived suppressor cells from peripheral blood monocytes

Myeloid-derived suppressor cells (MDSCs) are of interest as key regulators of the immune response for the development and improvement of cellular technologies in biomedicine. Enhancing the suppressive activity of these cells is important for developing therapies for autoimmune diseases and miscarriages, and their suppression may be useful in the treatment of cancer, since MDSCs are known to suppress antitumor immunity. However, there is a problem that prevents the active study of MDSCs, i.e., the difficulty in obtaining sufficient numbers of this cell population. Isolation of MDSCs in cancer patients poses an ethical challenge. Moreover, these MDSC may differ in subpopulation composition and suppressive activity due to individual factors. Researchers who generate human MDSC from bone marrow cells may also face similar problems. Therefore, finding a reliable and affordable source of these cells to facilitate the study of their functions is extremely important. Attempts to obtain human MDSCs in vitro have been ongoing for a long time. GM- CSF, IL-6, IL- 1β, IL-4, PGE2, LPS, M-CSF, IFNγ are described as factors that induce the ex vivo MDSC differentiation. However, despite multiple factors used, not all protocols are clearly reproducible, leading to generation of a sufficient number of cells in the target population. Previously, we had also developed a scheme for MDSC differentiation from CD11b+ cells derived from human peripheral blood, which made it possible to obtain a tangible but still insufficient percentage of cells to study functional activity.
 To increase the number of MDSCs in cultures, we developed a protocol aimed for differentiation of these cells from peripheral blood monocytes (CD14+ cells) previously transformed into PCMO (programmed cells of monocytic origin). The monocytes isolated by immunomagnetic separation were cultured in a de-differentiating medium (complete culture medium supplemented with M-CSF, IL-3 and β-mercaptoethanol) for one week. Later on, the medium was replaced by the addition of GM-CSF, being cultured for three days, followed by addition of LPS and IL-1β in order to induce suppressive activity. We have found that culturing CD14+ cells on a two-week schedule with prior creation of dedifferentiation conditions resulted in a slightly decreased percentage of viable cells in culture. However, there was a trend towards an increased ratio of MDSCs in culture (from an average of 34 to 40%) and an increase in their suppressive activity (arginase and IDO expression). The percentage of Arg+ cells increased by average of 10%, and IDO+ cells, by 16%. Moreover, the percentage of mature M-MDSCs was significantly (several-fold) higher when compared with differentiation protocol using CD11b+ cells. Hence, this method of MDSCs production enables us to increase the number of cells belonging to the conditionally “mature” monocyte subpopulation of MDSCs, as well as the percentage of functional suppressor cells in the population. The described scheme may be used to improve the quality of studies aimed at modulating MDSC functions in order to develop new therapeutic approaches.

The adaptor protein TRAF3 is an immune checkpoint that inhibits myeloid-derived suppressor cell expansion.

Myeloid-derived suppressor cells (MDSCs) are aberrantly expanded in cancer patients and under other pathological conditions. These cells orchestrate the immunosuppressive and inflammatory network to facilitate cancer metastasis and mediate patient resistance to therapies, and thus are recognized as a prime therapeutic target of human cancers. Here we report the identification of the adaptor protein TRAF3 as a novel immune checkpoint that critically restrains MDSC expansion. We found that myeloid cell-specific Traf3-deficient (M-Traf3 -/-) mice exhibited MDSC hyperexpansion during chronic inflammation. Interestingly, MDSC hyperexpansion in M-Traf3 -/- mice led to accelerated growth and metastasis of transplanted tumors associated with an altered phenotype of T cells and NK cells. Using mixed bone marrow chimeras, we demonstrated that TRAF3 inhibited MDSC expansion via both cell-intrinsic and cell-extrinsic mechanisms. Furthermore, we elucidated a GM-CSF-STAT3-TRAF3-PTP1B signaling axis in MDSCs and a novel TLR4-TRAF3-CCL22-CCR4-G-CSF axis acting in inflammatory macrophages and monocytes that coordinately control MDSC expansion during chronic inflammation. Taken together, our findings provide novel insights into the complex regulatory mechanisms of MDSC expansion and open up unique perspectives for the design of new therapeutic strategies that aim to target MDSCs in cancer patients.

Mitigating the prevalence and function of myeloid-derived suppressor cells by redirecting myeloid differentiation using a novel immune modulator

BackgroundImmune suppression is common in neoplasia and a major driver is tumor-induced myeloid dysfunction. Yet, overcoming such myeloid cell defects remains an untapped strategy to reverse suppression and improve host...

Targeting myeloid-derived suppressor cells using all-trans retinoic acid in melanoma patients treated with Ipilimumab

BackgroundImmune checkpoint inhibitors have improved overall survival rates for many cancers, yet the majority of patients do not respond to treatment and succumb to disease progression. One tumor-related mechanism limiting the efficacy of immunotherapies in melanoma is the recruitment and expansion of myeloid-derived suppressor cells (MDSCs). Therefore, targeting MDSCs in combination with immunotherapies is an attractive strategy to improve response rates and effectiveness. MethodsWe tested this strategy by designing a randomized phase II clinical trial treating advanced melanoma patients with either Ipilimumab monotherapy or Ipilimumab plus all-trans retinoic acid (ATRA). Clinicaltrails.gov identifier (NCT02403778). The frequency of circulating MDSCs and the activation of CD8(+) T cells was measured by flow cytometry. Expression of immunosuppressive genes was measured with quantitative real time-PCR. T cell suppressive functions were measured by mixed lymphocyte reaction. ResultsHere we show that in vitro treatment with ATRA decreases immunosuppressive function of MDSCs in mixed lymphocyte reactions. Additionally, ATRA reduces the expression of immunosuppressive genes including PD-L1, IL-10, and indoleamine 2,3‑dioxygenase by MDSCs. Furthermore, the addition of ATRA to standard of care Ipilimumab therapy appears safe, as ATRA did not increase the frequency of grade 3 or 4 adverse events. Finally, ATRA significantly decreased the frequency of circulating MDSCs compared to Ipilimumab treatment alone in advanced-stage melanoma patients. ConclusionsThese results illustrate the importance of MDSCs in immunotherapy resistance and provide evidence that targeting MDSCs in cancer patients may augment immunotherapeutic approaches.

Myeloid-Derived Suppressor Cells Hinder the Anti-Cancer Activity of Immune Checkpoint Inhibitors

Immune checkpoint inhibitors (ICI) used for cancer immunotherapy were shown to boost the existing anti-tumor immune response by preventing the inhibition of T cells by tumor cells. Antibodies targeting two negative immune checkpoint pathways, namely cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), and programmed cell death-ligand 1 (PD-L1), have been approved first for patients with melanoma, squamous non-small cell lung cancer (NSCLC), and renal cell carcinoma. Clinical trials are ongoing to verify the efficiency of these antibodies for other cancer types and to evaluate strategies to block other checkpoint molecules. However, a number of patients do not respond to this treatment possibly due to profound immunosuppression, which is mediated partly by myeloid-derived suppressor cells (MDSC). This heterogeneous population of immature myeloid cells can strongly inhibit anti-tumor activities of T and NK cells and stimulate regulatory T cells (Treg), leading to tumor progression. Moreover, MDSC can contribute to patient resistance to immune checkpoint inhibition. Accumulating evidence demonstrates that the frequency and immunosuppressive function of MDSC in cancer patients can be used as a predictive marker for therapy response. This review focuses on the role of MDSC in immune checkpoint inhibition and provides an analysis of combination strategies for MDSC targeting together with ICI to improve their therapeutic efficiency in cancer patients.

Abstract BS1-2: Myeloid-derived suppressor cells: many facets of regulation of tumor progression

Abstract Tumor associated macrophages (TAM), myeloid derived suppressor cells (MDSC), represented by PMN-MDSC and M-MDSC, and dendritic cells (DC) are major components of tumor microenvironment playing critical role in tumor progression. There are now compelling evidence linking MDSC with negative clinical outcomes in many types of cancer. Therefore, these cells are attractive therapeutic targets. Recently, several therapeutic strategies have been developed that target MDSC in mouse animal models and in first clinical trials. Recent findings of selective targeting of MDSC in cancer patients using agonistic TRAIL receptor DR5 antibody will be discussed. In 16 patients with advanced solid tumors this antibody demonstrated selective depletion of MDSC without affecting populations of neutrophils, monocytes, or lymphocytes. Another therapeutic strategy involves histone deacetilase inhibitor – entinostat. Recent study in tumor-bearing mice revealed that entinostat slightly increased expansion and accumulation of granulocytic cells in tumors. However, in contrast to untreated mice, these cells lacked suppressive activity. This effect was associated with potent antitumor activity of combination of entinostat with check-point inhibitors. By studying the effect of CSF1R inhibitor in tumor-bearing mice we identified the complex network regulating different groups of myeloid cells in tumor microenvironment. This network involves tumor cells and cancer associated fibroblast and protect tumors from loss of individual component of myeloid cell compartment. We determined that this network is operational not only in mice but also in cancer patients. Targeting of this network dramatically enhance the effect of cancer therapy. Citation Format: Gabrilovich DI. Myeloid-derived suppressor cells: many facets of regulation of tumor progression [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr BS1-2.

Abstract B78: The influence of breast cancer on the metabolic status of myeloid cells

Abstract Breast cancer is the most common cancer and the second leading cause of cancer death in women. Except for surgery, radiotherapy, chemotherapy and targeting therapy, breast cancer vaccine is now actively developed. However, the outcome of most therapeutic cancer vaccines was not satisfying probably due to the induction of immunosuppression in cancer patients. Myeloid-derived suppressor cells (MDSCs) are one of the major immunoregulatory cell populations, which play important roles not only in T-cell suppression but also in angiogenesis, lymphangiogenesis and metastasis during tumor progression. MDSCs are heterogeneous populations of immature myeloid cells, which expand and developed in responses to growth factors, cytokines and chemokines derived from cancer cells and then are released from bone marrow to peripheral tissues including tumor site. It is well known as tumor cells could influence the activation and proliferation of myeloid cells through release of cytokines during tumor progression to result in the accumulation of large number of MDSCs in cancer patients. However little is known about the relevance of metabolic status and accumulation of MDSCs in cancer patients during tumor progression. We therefore try to elucidate the metabolic changes during differentiation of myeloid cells into MDSCs. To intensively study the transcriptional program of MDSCs in comparison to their healthy counterparts, we perform DNA microarray analysis using mRNAs from monocytic MDSCs, granulocytic MDSCs from primary tumor sites of 4T1 tumor-bearing mice, monocytes and neutrophils from bone marrow of healthy mice. MDSCs up-regulated mRNA levels of genes related to angiogenesis and lymphangiogenesis e.g. VEGFa, angiopoietin 1, PROK2 (BV8), MMP9, MMP12, MMP13 and MMP14, genes of pro-inflammatory cytokines e.g. IL-1α, IL-6 and TNFα, genes of immunosuppressive molecules e.g. arginase 1, iNOS, PD-L1, PD-L2, CD83 and CD155. We are currently examining the metabolomics of these MDSCs and their healthy counterparts and expect to prove the association of certain metabolic changes with the differentiation of MDSCs. Citation Format: Li-Rung Huang, Shiou-Ling Jian, Yi-Jyun Jhou. The influence of breast cancer on the metabolic status of myeloid cells. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr B78.

Abstract 273: TLR8 agonist VTX-2337 (motolimod) decreases monocytic myeloid-derived suppressor cells by inducing differentiation to mature antigen-presenting cells

Abstract There is increasing evidence that myeloid-derived suppressor cells (MDSCs) contribute to the progression of cancer by inhibiting tumor-directed immune responses and producing mediators that promote the growth and survival of tumor cells. While present at low numbers in the peripheral blood of healthy individuals, levels of circulating MDSC increase in cancer patients as tumors produce factors that drive both the expansion and recruitment of these immune cells. MDSCs are a heterogeneous population and include both granulocytic MDSC (G-MDSC) and monocytic MDSC (M-MDSC) sub-populations, each of which can inhibit T cell function through arginase activity and the production of nitric oxide. Novel approaches to decrease MDSC expansion, or inhibit their immunosuppressive functions, hold great promise for augmenting anti-tumor immunity. In this study, we examined the effect of VTX-2337, a selective TLR8 agonist, on MDSC function. We initially measured the expression of TLR8 in RNA extracted from FACS-sorted MDSC isolated from peripheral blood mononuclear cells (PBMCs) of three healthy donors. TLR8 levels found in the M-MDSC subpopulation were comparable to those found in monocytes and myeloid dendritic cells (mDC), and are known to express high levels of TLR8 protein. To assess the M-MDSC response to TLR8 activation, PBMCs from healthy donors, were treated overnight in medium alone, low dose (167 nM) or high dose (500 nM) VTX-2337. Treatment with VTX-2337 resulted in a significant loss of the M-MDSC (HLA-DR-CD14+) population. Interestingly, we also observed that older (>45 years) donors demonstrated a greater loss of the M-MDSC sub-population than younger (<45 years) donors. In contrast, there was no significant difference in the recovery of the G-MDSC subtype (Lin-HLA-DR-CD33+) following VTX-2337 treatment. Additionally, treatment with either the TLR7 agonist imiquimod, or TLR9 agonist CpG ODN2006, did not result in a loss of the M-MDSC population. We hypothesized that the selective decrease in the M-MDSC subpopulation following TLR8 activation may be attributed to their differentiation into mature antigen-presenting cells. To test this hypothesis, we cultured CFSE-labelled MACS-separated HLA-DR-CD14+ M-MDSC and autologous PBMC with VTX-2337 or medium only (control). TLR8 activation of labelled M-MDSC cells resulted in HLA-DR expression and promoted their differentiation into HLA-DR+CD14+ monocytes. Whether VTX-2337 activation can also decrease the immunosuppressive and/or tumor-promoting activities of MDSC is currently under investigation. Our finding, where TLR8-activated M-MDSC can differentiate into mature antigen-presenting cells is highly novel, and suggests the potential for using VTX-2337 to modulate MDSCs in cancer patients and enable a more effective, immune response to tumors. Citation Format: Zina J. Rutnam, Yushe Dang, Gregory Diestch, Hailing Lu, Yi Yang, Robert Hershberg, Mary L. Disis. TLR8 agonist VTX-2337 (motolimod) decreases monocytic myeloid-derived suppressor cells by inducing differentiation to mature antigen-presenting cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 273. doi:10.1158/1538-7445.AM2015-273

Myeloid-Derived Suppressor Cells in Cancer: Therapeutic, Predictive, and Prognostic Implications

Immune evasion is a hallmark of cancer. While there are multiple different mechanisms that cancer cells employ, myeloid-derived suppressor cells (MDSCs) are one of the key drivers of tumor-mediated immune evasion. MDSCs begin as myeloid cells recruited to the tumor microenvironment, where they are transformed into potent immunosuppressive cells. However, our understanding of the clinical relevance of MDSCs in cancer patients has significantly lagged behind the preclinical literature in part due to the absence of a cognate molecule present in mice, as well as to the considerable heterogeneity of MDSCs. However, if one evaluates the clinical literature through the filter of clinically robust endpoints, such as overall survival, three important phenotypes emerge: promyelocytic, monocytic, and granulocytic. Based on these studies, MDSCs have clear prognostic importance in multiple solid tumors, and emerging data support the utility of circulating MDSCs as a predictive marker for cancer immunotherapy, and even as an early leading marker for predicting clinical response to systemic chemotherapy in patients with advanced solid tumors. More recent preclinical data in immunosuppressed murine models suggest that MDSCs play an important role in tumor progression and the metastatic process that is independent of their immunosuppressive properties. Consequently, targeting MDSCs either in combination with cancer immunotherapy or independently as part of an approach to inhibit the metastatic process appears to be a very clinically promising strategy. We review different approaches to target MDSCs that could potentially be tested in future clinical trials in cancer patients.

Interaction Between Invariant NKT Cells and Myeloid-derived Suppressor Cells in Cancer Patients

Despite advances in therapeutic strategies, the ability of cancer cells to evade destruction remains a significant obstacle to the development of effective anticancer treatment. In recent years a subset of immune cells, myeloid-derived suppressor cells (MDSCs), has been shown to play a key role in evasion of the patient's immune system by tumor cells. A number of different tumor types are associated with increased numbers of circulating MDSCs in cancer patients, suppressing the immune response and permitting continued tumor cell proliferation. Invariant NKT (iNKT) cells have recently been defined as a unique subset of immune cells that are able to act as a link between the innate and adaptive arms of the immune system. iNKT cells have the ability to carry out immune surveillance of tumor cells and control proliferation of malignant cells. Recently, we presented evidence that iNKT cells are able to interact with and decrease the numbers of circulating MDSCs in melanoma patients. This review discusses the evidence for MDSCs in tumor progression and the implication that iNKT cells could be developed as a potent therapeutic strategy.

Regulating the suppressors: apoptosis and inflammation govern the survival of tumor-induced myeloid-derived suppressor cells (MDSC)

Immune suppressive myeloid-derived suppressor cells (MDSC) are present in most cancer patients where they inhibit innate anti-tumor immunity and are a significant obstacle to cancer immunotherapy. Inflammation is a known inducer of Gr1(+)CD11b(+) MDSC; however, the factors/conditions that regulate MDSC survival and half-life have not been identified. We have used mass spectrometry (MS) and proteomic analysis to identify proteins and pathways that regulate MDSC survival. This analysis revealed high expression of caspase family proteins and the Fas-FasL, p38 MAPK, and TGFβ pathways, suggesting that Fas-FasL apoptosis regulates MDSC survival. Flow cytometry, confocal microscopy, and western blot analyses confirmed the MS findings and demonstrated that Fas(+) MDSC are susceptible to Fas-mediated killing in vitro. In vivo studies with FasL-deficient and Fas-deficient mice demonstrated that Fas-FasL interactions are essential for MDSC apoptosis and for rejection of established metastatic disease and survival and that FasL(+) T cells are the effector population mediating MDSC apoptosis. MS findings validated by biological experiments demonstrated that inflammation increases MDSC levels by protecting MDSC from Fas-mediated apoptosis, possibly by activating p38 MAPK. These results demonstrate that MDSC half-life in vivo is regulated by FasL(+) T cells and that inflammation increases MDSC levels by conferring resistance to Fas-mediated apoptosis and identifies T cells as the relevant effector cells causing MDSC apoptosis in vivo. This newly recognized mechanism for regulating MDSC levels identifies potential new targets for decreasing MDSC in cancer patients.

Abstract A67: Increased circulating HLA-DRneg, Lin1 lo/neg, CD33+ CD11b+ myeloidderived suppressor cells correlate with increased arginase levels in patients with prostate cancer

Abstract Due to our increased understanding of tumor immunology a number of immunotherapeutic approaches have been successfully introduced into the clinic and further promising therapeutic strategies are being investigated in on-going clinical trials. However, evaluation of anti-tumor immunity in such trials 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. One of the main cell types that have emerged as potent suppressors of T cell-mediated immunity are myeloid derived suppressor cells (MDSCs). MDSCs are a heterogeneous population of immature cells of myeloid origin with a wide repertoire of effector mechanisms. While there are published data on circulating MDSC in cancer patients with diverse primary tumors, there is little published data investigating MDSCs specifically in prostate cancer. Following on from our recent study looking at HLADR- Lin1low/− CD33+ CD11b+ MDSCs in pancreatic, esophageal and gastric cancer patients we sought to determine whether this population of MDSCs also played a role in prostate cancer. Peripheral blood was collected from 75 prostate cancer patients and 50 healthy controls. PBMC were harvested with subsequent flow cytometric analysis of MDSC. MDSCs were highly statistically significantly elevated in prostate cancer patients compared with healthy controls (p<0.0001). In this cohort there was no evidence of increasing MDSC % with stage of disease. MDSCs exert their immunosuppressive activity through multiple mechanisms, including arginine depletion. MDSC expressing arginase I deplete L-arginine from the microenvironment and profoundly inhibit T-cell function. Analysis of arginase I levels by ELISA on the plasma from the same prostate cancer patient cohort showed that Arginase I levels were also highly statistically significantly elevated in prostate cancer patients compared with controls (p<0.0001). There was a clear association between increasing Arginase I level and increasing MDSC percentage (p=0.0195). In contrast to our finding in upper gastrointestinal cancers where significant elevations of the Th2 cytokine interleukin-13 (IL-13) were found and correlated with MDSC levels, we were unable to detect IL-13 in the plasma of prostate cancer patients. This probably reflects the diverse combinations of cytokines produced by different tumors and the large number of potential MDSC-inducing and MDSC activating factors. These data not only add to our understanding of the immunobiology of prostate cancer but provide the rationale for combining immunotherapy studies with MDSC inhibition strategies. Citation Format: Nicola Annels, Rachel Gabitass, Gary Middleton, Hardev Pandha. Increased circulating HLA-DRneg, Lin1 lo/neg, CD33+ CD11b+ myeloidderived suppressor cells correlate with increased arginase levels in patients with prostate cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr A67.

Myeloid-derived Suppressor Cells in Cancer Patients

Myeloid-derived suppressor cells (MDSCs) represent a heterogenous collection of immature myeloid cells endowed with suppressive function on the immune response. Their presence has been extensively investigated in preclinical models, especially in the context of cancer. One of the major obstacles in their accurate identification has been the definition of an unambiguous phenotype, shared between mice and humans, and clearly correlating with their suppressive function. In this paper, we review the literature concerning the phenotype in mouse and in humans, showing that at least 2 subsets of MDSCs are present under different situations. We also address the role of MDSCs in tumor progression, evaluate the prognostic significance of MDSC in cancer patients, and their possible role as marker of clinical outcome and response to therapy. Finally, we examine the strategies designed to modulate MDSCs in cancer patients, which might represent an innovative approach to enhance the effectiveness of immune-based therapies.

Abstract 453: Myeloid regulatory cells in cancer patients

Abstract Adaptive immunotherapy can augment host immunity, although the outcomes to date in cancer patients have been limited, due in part, to increased numbers of myeloid derived suppressor cells (MDSCs) in the peripheral blood. MDSCs are key cellular regulators that can limit T-cell and dendritic cell (DC) number and function. They are believed to be myeloid precursors that are increased as part of the hematopoietic response to myelosuppression, stress, chemotherapy, and tumor growth. Recently, we found that mononuclear cell apheresis of some cancer patients (melanoma, leukemia and lymphoma), but not normal donors were enriched for hypodense myeloid cells as confirmed by elutriation and Ficoll-Hypaque gradients. These cells are also found in the peripheral blood, although their frequency is enriched by mononuclear cell apheresis. Despite their low density, cytologically they appear as granulocytes containing < 1% bands or metamyelocytes. Phenotypic analysis by flow cytometry revealed that >98% of the granulocytes (FSxSS), were MDSCs, defined as linage−(CD3, CD19 and CD56)DR−CD14dullCD13dull CD11b+CD33+. Consistent with an immature granulocyte phenotype > 50% of the granulocytes were CD16dull/− as compared to <3% of the granulocytes in the PB or apheresis products of normal donors. The frequency of these cells in an apheresis product or the peripheral blood from cancer patients can be as high as 60% of total nucleated cells and following mononuclear cell elutriation >95% of nucleated cells. It is emphasized that this is not observed in all patients and in our limited experience occurs in 20% or less of patients with advanced disease. Enrichment of hypodense MDSCs by Miltenyi magnetic bead selection of CD11b+ cells following subtractive depletion of DR+ cells provided a 60-85% pure population of MDSCs that can suppress an allogeneic T-cell response and by quantitative TaqMan analysis have significantly increased levels of NOS2, IL10, LTB4R1, MPO, MMP9 and VEGFα. We conclude that the majority of MDSCs in cancer patients are immature granulocytes; rather than myeloid precursors, as evidenced by their CD16dull phenotype, a lack of CD34 expression and segmented granulocyte morphology. Further, the presence of hypodense MDSCs has a significant impact on monocyte enrichment (apheresis, elutriation and Ficoll Hypaque separation) for maturation of monocytes into dendritic cells and use as a vaccine. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 453. doi:10.1158/1538-7445.AM2011-453