Myeloid-derived suppressor cells: Their role in the pathophysiology of hematologic malignancies and potential as therapeutic targets

Abstract

ABSTRACT Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells at various stages of differentiation/maturation that have a role in cancer induction and progression. They function as vasculogenic and immunosuppressive cells, utilizing multiple mechanisms to block both innate and adaptive anti-tumor immunity. Recently, their mechanism of action and clinical importance has been defined, and the cross talk between myeloid cells and cancer cells has been shown to contribute to tumor induction, progression, metastasis, and tolerance. In this review, we focus on the role of MDSCs in hematologic malignancies and the therapeutic approaches targeting MDSCs that are currently in clinical studies.

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Just Accepted by Leukemia & Lymphoma
Myeloid-derived suppressor cells: Their role in the
pathophysiology of hematologic malignancies and
potential as therapeutic targets
Ibrahim H. Younos, Fuminori Abe, and James E. Talmadge
Doi:10.3109/10428194.2014.987141
ABSTRACT
Myeloid-derived suppressor cells (MDSCs) are a heterogeneous
population of immature myeloid cells at various stages of
differentiation/maturation that have a role in cancer induction and
progression. They function as vasculogenic and immunosuppressive
cells, utilizing multiple mechanisms to block both innate and adaptive
anti-tumor immunity. Recently, their mechanism of action and clinical
importance has been defined, and the cross talk between myeloid
cells and cancer cells has been shown to contribute to tumor
induction, progression, metastasis, and tolerance. In this review, we
focus on the role of MDSCs in hematologic malignancies and the
therapeutic approaches targeting MDSCs that are currently in clinical
studies.
GLAL_A_987141_Coverpage.indd 1 11/15/2014 1:28:09 PM
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Myeloid-derived suppressor cells: Their role in the pathophysiology
of hematologic malignancies and potential as therapeutic targets
Ibrahim H. Younos1,2, Fuminori Abe3, and James E. Talmadge4
1Department of Clinical Pharmacology, Menoufia University, Gamal Abdel Nasser Street,
Shebin El-Kom, Al-Minufya, Egypt, 2Department of Pharmacology and Clinical Pharmacy,
College of Medicine and Health Sciences, Sultan Qaboos University, P.O. Box 35 Post Code
123, Alkhoudh, Muscat, Oman. 3SBI Pharmaceuticals Co., Ltd. Roppongi 1-6-1, Minato-ku,
Tokyo 106-6020, Japan, 4Department of Pathology & Microbiology, 686495 Nebraska
Medical Center, Omaha, Nebraska 68198-6495
Corresponding author: James E. Talmadge, Department of Pathology and
Microbiology, 986495 Nebraska Medical Center, Omaha, Nebraska 68495.
Tel: +1 (402) 559-5639. Fax: +1 (402) 559-4990. E-mail: jtalmadg@unmc.edu.
Short title: MDSCs in hematologic malignancies
ABSTRACT
Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population
of immature myeloid cells at various stages of differentiation/maturation that have a
role in cancer induction and progression. They function as vasculogenic and
immunosuppressive cells, utilizing multiple mechanisms to block both innate and
adaptive anti-tumor immunity. Recently, their mechanism of action and clinical
importance has been defined, and the cross talk between myeloid cells and cancer
cells has been shown to contribute to tumor induction, progression, metastasis, and
tolerance. In this review, we focus on the role of MDSCs in hematologic
malignancies and the therapeutic approaches targeting MDSCs that are currently in
clinical studies.
Keywords: Myeloid-derived suppressor cells, hematologic malignancies, multiple
myeloma, leukemia, lymphoma.
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INTRODUCTION
Preclinical and clinical studies have established that tumor progression is frequently associated
with the accumulation of immature myeloid cells in the peripheral blood (PB), spleen, and
tumor[1]. These immature myeloid cells are a hallmark of cancer[2] and a central mechanism of
immune evasion, supporting tumor progression[3]. Originally known as natural suppressor cells
or null cells[4], these myeloid cells are now commonly recognized as myeloid-derived
suppressor cells (MDSCs), representing a heterogeneous population of immature myeloid cells at
various stages of differentiation.
Expanded populations of MDSCs have been described in a broad range of disorders from viral
diseases[5] to solid tumors and hematological malignancies[6]. In addition to morphologic
identification, MDSCs are phenotypically subset into monocytic (mMDSC) and granulocytic
(gMDSC) cells based on GR1 expression levels, or Ly-6C and Ly-6G expression. Thus,
mMDSCs may be identified as CD11b+GR1lo/int or CD11b+Ly-6GlowLy-6Chi. These cells express
higher inducible nitric oxide synthase (iNOS) levels and have been reported to have increased T-
cell suppressive activity, in contrast to gMDSCs that are defined as CD11b+GR1bright or
CD11b+Ly-6G+Ly-6Clow. GMDSCs have high ARG1 levels and are found at significantly
increased numbers in most tumors, even in tumors where mMDSCs are substantially
increased[4]. MDSCs impede effective host control of neoplastic disease as intrinsic
immunosuppressive mechanisms associated with increasing tumor growth and have a critical role
in tumor-induced tolerance. Numerous tumor-secreted factors contribute to the proliferation,
trafficking, and accumulation of MDSCs and have been shown to parallel tumor burden[7,8].
MDSC-induced immune suppression is mediated primarily by the upregulation of nitric oxide
synthase 2 (NOS2)[9], reactive oxygen species (ROS)[10], and overexpression of arginase 1
(ARG1)[9]. As such, agents that inhibit expansion of MDSCs, and/or inhibit NOS2, ROS, or
ARG1 production have shown therapeutic activity in preclinical animal models and clinical
studies. In addition to their immunosuppressive activity, MDSCs can differentiate into vascular
cells[11] and osteoclasts (OCs)[12], which also contribute to tumor progression and
pathology[13]. However, most clinical studies concerning MDSCs have examined solid tumors,
while few have focused on their role in hematologic malignancies[6]. Thus, in this review, we
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discuss the role of MDSCs in the progression of hematologic tumors.
ROLE OF MDSCs IN NEOPLASTIC DISEASE
Although there are multiple mechanisms supporting tumor escape from host immunity[14], in
recent years, research has focused on MDSCs, in part, because of the direct correlation between
their presence in the PB and tumor progression and prognosis[15,16]. Studies have documented a
relationship between tumor burden and a reduced frequency and function of T-cells and the
potential to reduce this T-cell dysfunction by primary tumor resection[17-19]. Numerous papers
have examined the mechanisms by which MDSCs suppress T-cell function including a focus on
NOS2[20], ROS, and ARG1[21]as potential mechanisms (Table 1). The identification of
multiple subsets supports the plasticity of MDSCs and their potential to differentiate into not
only granulocytes, monocytes, and dendritic cells (DCs), but also OCs and endothelial cells.
There are now a number of phenotypic parameters under investigation including, but not limited
to, high levels of CD66b, low levels of CD62L, low CD16 expression[22], and vascular
endothelial growth-factor receptor 1 (VEGFR1) expression[23]. Myeloid cell populations are the
predominant population in the PB, both in terms of number and function. Granulocytes provide
the first line of defense against tumor challenge, and they also have a pro-tumorigenic role. The
basis for this “plasticity” observed in both mice and human MDSCs is based on their capability
to express different functional profiles in response to varying environmental signals. Similar to
mouse MDSCs, human MDSCs are hematopoietic progenitors that can differentiate into not only
granulocytes and monocytes, but also endothelial cells and OCs. While it is apparent that specific
growth factors (GFs) drive MDSCs to a differentiated phenotype, whether the difference in
MDSC profile between tumors is due to tumour heterogeneity, or based on tumour histiotype, is
unstudied. Further, tumor-secreted cytokines, chemokines, and GFs regulate the medullary
expansion of myeloid progenitors including MDSCs (Figure 1), their mobilization from the
marrow and extramedullary hematopoiesis (EMH), followed by de-margination and tumor
infiltration promoting tumor survival, growth, angiogenesis, and metastasis.
CLINICAL THERAPEUTIC STRATEGIES TARGETING MDSCs
Our understanding of the molecular mechanisms responsible for MDSC expansion,
accumulation, and regulation of T-cell function has allowed us to clinically limit their
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proliferation and function often with a suggestion of therapeutic activity. However, few
therapeutics act selectively on MDSCs. Current interventional strategies targeting MDSCs have
included approaches that induce their differentiation, reduce their number, inhibit their
proliferation, or inhibit their suppressive function (Figure 2). In this review, we have focused on
therapeutics that have been examined clinically and been reported to regulate MDSCs.
MDSC differentiation
One approach to inhibit MDSC immunosuppression is to reverse their arrested maturation or
induce their differentiation. Several studies have reported that all-trans retinoic acid (ATRA) can
differentiate MDSCs into mature monocytes and granulocytes. The initial studies using ATRA
focused on in vivo treatment of tumor-bearing (TB) mice[24] and in vitro studies of CD33+HLA
DR+ cells isolated from patients with metastatic renal cell carcinoma (RCC)[25]. These studies
showed that ATRA can reduce MDSC numbers and abrogate their function, resulting in restored
CD4+ and CD8+ T-cells numbers and functions, and delayed tumor progression[24,26]. In
clinical trials of patients with metastatic RCC given ATRA in combination with interleukin 2
(IL-2), ATRA improved DC function and T-cell responses associated with a decrease in MDSC
numbers[27]. In a similar study, small cell lung cancer (SCLC) patients were treated with
standard of care, a p53-transduced DC vaccine, or ATRA and the vaccine[28], with the finding
that no patients in the standard-of-care cohort developed detectable p53-specific responses. In
contrast, following immunization, in the vaccine-alone arm, 20% of patients developed a p53-
specific response, while 41.7% of patients given ATRA and the immunization developed
significant p53 responses and depressed numbers of MDSCs[28]. Taken together, these studies
support MDSC differentiation by ATRA resulting in an increased anti-tumor T-cell response.
Earlier studies focused on the differentiation of immunosuppressive CD34+ myeloid progenitors
(now known as MDSCs) into DCs by 1α,25-hydroxyvitamin D3, in mice bearing Lewis lung
carcinoma[29] tumors and patients with non-small cell lung cancer (NSCLC)[30]. In one clinical
study, NSCLC patients were either treated or not with 1α,25-hydroxyvitamin D3 for 3 weeks
before surgery[30]. Analysis of CD34+ cells and DCs by immunohistochemistry (IHC) in the
NSCLC tissues revealed reduced infiltration by CD34+ cells and immature DC-SIGN+ DCs and
increased numbers of intratumoral LAMP+ mature DCs. To assess the clinical significance of this
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observation, newly diagnosed NSCLC patients were examined, with patients either untreated or
treated with 1α,25-hydroxyvitamin D3 for 3 weeks before tumor excision[31]. IHC analysis of
the tumors revealed a significant increase in intratumoral CD4+ and CD8+ T-lymphocytes
expressing the activation marker, CD69, in treated patients. The untreated NSCLC patients had a
median time to recurrence of 181 days compared with 620 days in the treated group. Another
study examined the effects of 1α,25-hydroxyvitamin D3 on the cytokine profiles of NSCLC
patients revealed increased levels in the tumor tissue and plasma, of treated patients compared
with untreated patients[32]. In summary, the induction of MDSC differentiation appears to be
active clinically; however, several of these drugs are somewhat toxic.
Regulation of MDSCs by cytotoxic drugs
Chemotherapeutic drugs, including gemcitabine[33,34] and 5-fluorouracil (5FU)[35], can reduce
MDSC numbers demonstrating a decrease in MDSC frequency in the PB of TB mice.
Gemcitabine, a pyrimidine nucleoside analogue, has also been combined with a cancer vaccine
strategy in murine tumor models with encouraging outcomes[36,37]. In clinical studies that
evaluated the combination of peptide vaccination with gemcitabine, boosting of cellular and
humoral responses were observed[38]. A similar result was observed in pancreatic cancer
patients treated concurrently with gemcitabine and capecitabine and with an administered
GV1001 vaccine and adjuvant, granulocyte macrophage colony-stimulating factor (GM-CSF),
documenting a decreased MDSC frequency[39]. Together, these and other studies suggest that
gemcitabine can induce host anti-tumor immunity resulting in improved therapeutic activity[34].
5FU has been shown in mice to significantly decrease MDSC numbers[35]. In contrast to 5FU
and gemcitabine, some chemotherapeutic drugs, such as cyclophosphamide, can increase MDSC
frequency and reduce cancer immunity. An increase in MDSCs was also observed in breast
cancer patients treated with doxorubicin and cyclophosphamide as compared to pre-treatment
levels[8]. In another study, the levels of circulating MDSCs in a trial of 41 women diagnosed
with stage II-IIIa HER2/neu-negative breast cancer[40] who achieved a pathologic complete
response (pCR) were lower as compared to patients who did not achieve a pCR. Thus, the
potential duel activity by cytotoxic drugs is attractive as they may reduce both tumor burden and
MDSC frequency.
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Inhibitors of VEGF activity
Sunitinib is a multi-kinase inhibitor that inhibits platelet-derived growth factor, fms-like tyrosine
kinase 3 (FLt3)-ligand, VEGFR 1-3, colony-stimulating factor-1 receptor (CSF1R), and stem-
cell factor (SCF) activities. Most of its tumor inhibitory and immune stimulatory effects have
been attributed to its inhibition of signal transducers and activators of transcription 3 (STAT3)
signaling and VEGF inhibitory activity[41]. Treatment with sunitinib has been shown to result in
a 50% reduction in PB MDSCs in RCC patients. The decline was associated with improved Th1
lymphocyte function and decreased numbers of T-regulatory cells (Tregs)[35,42]. Multiple
rodent studies have examined the effect on MDSC formation by the inhibition of SCF[42,43].
These studies include the knock-down of SCF with siRNA[43] and inhibition of SCF signaling
by anti-c-kit antibodies[43] or by tyrosine kinase inhibitors (TKIs), such as sunitinib[42],
pazopanib, or sorafenib[44]. Animal models have also suggested that sunitinib inhibition of
MDSCs may be an effective vaccine adjuvant[44,45]. In contrast, in a phase-III trial combining
the TroVax® (MVA-5T4) vaccine with either sunitinib, IL-2, or interferon alpha (IFN-α) in
RCC patients, no effect on survival relative to sunitinib, IL-2 or IFN-α alone was reported[46].
However, this lack of activity may be due to sequencing of sunitinib and the vaccine[47].
Consistent with sunitinib inhibition of MDSCs, the anti-VEGF antibody, bevacizumab, has also
been shown to have a therapeutic effect via tumor blood vessel inhibition and to reduce the
number of MDSCs in TB mice[48] and patients[49]. However, cancer patients treated with
VEGF-Trap, a VEGF-binding protein, did not have a significant reduction in MDSC levels[50].
Nonetheless, increased DC frequencies and improved T-cell responses were observed in those
patients that did have lowered MDSC levels[50]. In other studies, treatment of RCC patients with
bevacizumab did not reduce circulating MDSC levels, but instead increased MDSC and plasma
ARG1 levels when given in combination with IL-2[23], which was likely due to the IL-2
administration. Consistent with this suggestion, a significant decrease in the total number of
CD11b+VEGFR1β+ MDSCs was observed in metastatic RCC patients treated only with
bevacizumab[49]. Studies, with vemurafenib, a small molecule inhibitor of the BrafV600E
mutation[51], which causes constitutive activation of the mitogen-activated protein kinase
(MAPK) pathway[52], have been shown to significantly decrease the frequencies of mMDSCs
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and gMDSCs in the PB[51]. In one clinical study using vemurafenib, patients responded to
treatment with a reduction in the frequency of mMDSCs, whereas the gMDSC responses varied.
Vemurafenib treatment is thought to modulate the tumor microenvironment, thereby, impacting
MDSC proliferation[51]. The demonstration that drugs which inhibit angiogenesis may also
reduce MDSCs have obscured their mechanism of action and suggested that they may not only
restrict angiogenesis, but also vasculogenesis.
Inhibitors of colony-stimulating factors and their receptors
Growth factors have an important role in expanding and recruiting MDSCs to tumor sites and in
inducing angiogenesis[53]. Treatment of TB mice with a small molecule inhibitor of the M-CSF
receptor, GW2580, inhibited mMDSC recruitment into tumors and reduced the expression of
pro-angiogenic and immunosuppressive genes within the tumor[54]. PLX3397, another inhibitor
of CSF1 activity, has also been shown to increase the efficacy of adoptive cellular therapy for
murine tumors[55]. These observations have progressed into the clinic, and monoclonal
antibodies that block CSF1R (e.g., IMC-CS4 and RG7155), as well as small molecule inhibitors
of CSF1R (e.g. PLX-3397), are undergoing phase-I clinical trials[56]. Indeed, the initial clinical
study with RG7155 resulted in reduced tumor infiltration by macrophages and objective
responses in patients with diffuse-type, giant-cell tumors[56]. As part of a dose-escalation
toxicity assessment, a variety of patients with advanced solid tumors were treated, and
investigators found that 5 out of 7 patients who received high doses of the antibody had an
increased CD4 and CD8 T-cell infiltration and a decrease in tumor associated macrophages
(TAMs). The authors suggested that antibody administration not only reduces TAMs, but may
also increase T-cell tumor infiltration potentially improving therapeutic outcomes.
Amiloride, a drug used to treat high blood pressure, can also reduce exosome release[57].
Exosomes can “encapsulate” hematopoietic GFs and contribute to MDSC expansion in cancer
patients [57,58]. MDSCs isolated from the PB of patients with colorectal metastatic carcinoma
who were treated with amiloride for 3 weeks had reduced suppressive activity ex vivo, and
exosomes from the autologous serum had a decreased ability to induce STAT3 phosphorylation
of MDSCs and impaired their suppressive activity[57]. The potential to inhibit MDSC expansion
is attractive and supports the utility of this class of drugs in combination with drugs that inhibit
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MDSC immunosuppression, as well as when combined with immunization. Further, given the
multiple mechanisms of immune suppression by MDSCs combination therapy, focus on multiple
mechanisms of immune suppression is also an attractive approach.
Drugs interfering with MDSC immunosuppression
One strategy to reduce MDSC suppression of T-cells is to target their function. In this respect,
STAT3 inhibition is an optimal target since it is a key regulator of MDSC biology and is
continuously activated in malignant, but not normal cells[59]. Several STAT3 inhibitors are
currently under investigation[60] clinically including a Janus kinase 2 (JAK2)/STAT3 inhibitor,
which blocks STAT3 phosphorylation by inhibiting JAK2[61]. In addition to these inhibitors,
numerous other mediators of MDSC immunosuppression have been studied clinically as
discussed below.
Nitro-bisphosphonates (N-bisphosphonates) as an inhibitor of matrix metalloproteinases
(MMPs)
N-bisphosphonates inhibit bone-resorbing OCs[62] by decreased prenylation of proteins, such as
MMP 9, which can influence MDSC generation/function by cleaving c-kit, thereby supporting
MDSC proliferation and mobilization from the BM[63]. MMP 9 has also been found to make
VEGF available for receptor binding on MDSCs[64]. Co-administration of zoledronic acid (ZA)
with a plasmid DNA vaccine encoding rat p185/Her-2 demonstrated improved T-cell responses
measured as delayed tumor growth and increased induction of anti-p185/Her-2 antibodies as
compared to controls[64]. Other studies that examined the effects of the ZA on mice injected
with the pancreatic cancer cell line, Panc02, observed reduced intra-tumoral MDSC
accumulation that was associated with delayed tumor growth rate, prolonged median survival,
and increased recruitment of intratumoral T-cells[65]. The relevance of this observation to the
clinic was extended by studies in pancreatic cancer patients who had decreased MDSC numbers
in the PB, marrow, and pancreas following ZA administration[66].
Phosphodiesterase-5 (PDE5) inhibitors
PDE5 inhibitors are clinically indicated for the treatment of erectile dysfunction, as well as
pulmonary hypertension through their effect on NO release[67]. The PDE5 inhibitor, sildenafil,
down-regulates ARG1 and NOS2 expression, thereby reducing the suppressive effect of
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CD11b+Gr-1+ MDSCs both in mice and in vitro by blood cells obtained from patients with
multiple myeloma (MM)[67]. Treatment of MM patients with PDE5 inhibitors, including
sildenafil and tadalafil, has been shown to significantly reduce MDSCs and M protein in these
patients[68]. These observations have been extended to include not only a reduction in PB
MDSCs, but also to MDSCs in the marrow, as well as to decrease ROS levels in MDSCs.
Further, treatment with tadalafil has been shown to improve T-cell function in patients with end-
stage MM[69].
Inhibition of MDSC NO metabolism with nitroaspirin (NO-aspirin)
NO-aspirin, a NO-releasing aspirin composed of aspirin covalently linked to a NO donor group,
interferes with NO-metabolism. Oral administration of nitroaspirin to TB mice was reported to
increase the number and function of tumor-antigen-specific T lymphocytes by interfering with
inhibitory myeloid cell enzymatic activities[70]. Decreased T-cell responsiveness in the presence
of MDSCs is partially due to MDSC production of NO leading to increased nitration of the T-
cell receptor, chemokine (C-C motif) ligand (CCL) 2, or STAT1[71-73]. NO-aspirins also
suppress ROS production and inhibit NOS2 feedback[74]. A potent NO-aspirin, AT-38, inhibits
NOS2 via various mechanisms and leads to the reversal of MDSC-induced inhibition of T-cell
responses in vitro by reducing CCL2 chemokine nitration[73]. NCX-4016 is presently under
investigation in a phase-I clinical trial for the prevention of colorectal cancer in patients at high
risk of developing this malignancy (NCT00331786).
Inhibition of ROS activity
Up-regulation of ROS is a mechanism for MDSC suppressive activity. CDDO-Me, a synthetic
triterpenoid, at nanomolar concentrations can up-regulate antioxidant genes and reduce
intracellular ROS[75] through nuclear factor E2-related factor 2 (Nrf2) transcription factor
activation[76]. Thus, it was hypothesized that triterpenoids might also regulate MDSC
function[77]. Subsequent murine studies have shown that it effectively reduces the immune-
suppressive activity in TB hosts and restores T-cell responses and activity[78,79]. A clinical
study that evaluated the activity of CDDO-Me[80] on MDSCs and immunity, examined serum
samples from patients with pancreatic adenocarcinoma treated in a phase-I trial. Patients were
treated weekly with gemcitabine on days 1, 8, and 15 and with CDDO-Me orally daily for 21
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days. Treatment with CDDO-Me or gemcitabine alone, had no effect on MDSC frequency;
however, after 2 weeks of treatment with CDDO-Me and gemcitabine, a significant increase in
T-cell responses to tetanus toxoid and phytohemagglutinin (PHA) was observed[80].
COX-2 inhibition
Chronic inflammation correlates with tumor induction and progression supporting a causative
relationship between inflammation and cancer. Thus, patients and experimental animals
receiving nonsteroidal anti-inflammatory drugs are often protected against the initiation and
progression of primary and metastatic disease[81] due to the inhibition of tumor secretion of
prostaglandin E2 (PGE-2)[82]. The cyclooxygenase 2 (COX-2) inhibitor, celecoxib, has been
shown to significantly reduce the number of MDSCs during 1,2-dimethylhydrazine diHCl (1,2-
DMH) induction of intestinal tumors in Swiss mice in association with chemopreventive
activity[83]. The 1,2-DMH tumor induction was associated with a four-fold increase in MDSCs
supporting potential chemopreventive activity for COX-2 inhibition and the regulation of MDSC
expansion[83]. Further, oral celecoxib can also decrease both MDSC numbers and function via a
reduction in ROS levels, and has a synergistic effect when used in combination with DC-based
immunotherapy[84].
ROLE OF MDSCS IN LEUKEMIA PATHOGENESIS, PROGRESSION, AND
THERAPY
Leukemias are malignant disorders of hematologic cells that result in the overproduction of
undifferentiated and immature leukocytes that function abnormally within the BM, the
circulation, and at extramedullary sites[85]. Multipotent, hematopoietic stem/progenitor cells
(HSCs/HPCs), which are the dominant hematopoietic population in the BM, are the source of
leukemic cells[86]. The leukemia microenvironment differs from that of solid tumors; although,
there is an overlap in immune evasion mechanisms supporting tumor progression[87]. The
anatomical origin and immaturity shared by both myeloid leukemic tumor cells and MDSCs
support their roles in immune evasion where their increased numbers may limit the response to
immunotherapy[8,88]. Although chronic lymphocytic leukemia pathogenesis does not imply
increased expansion of BM immature myeloid cells, untreated patients with CLL have
demonstrated significantly increased levels of immunosuppressive mMDSCs characterized as
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CD14+HLA-DRlow[89].
Murine acute myeloid leukemia (AML) models have documented MDSC expansion; however, in
one study, their immune suppressive functions were observed to not result in T-cell dysfunction
as it was not reversed by MDSC depletion using anti-Ly-6G antibody[90]. Consistent with the
membrane phenotypes of AML cells and MDSCs, enzyme phenotypes are also similar, including
increased arginase and NOS2 levels. Thus, as determined in a recent clinical study of AML
patients, the immunosuppression may not be limited to MDSC expansion, but may also be
associated with the leukemic cells themselves[91]. This suggested mechanism is supported by
clinical studies, investigating gMDSC levels in chronic myeloid leukemia patients. In these
studies no significant differences as compared to normal individuals were observed; although,
high-risk patients had significantly higher MDSC numbers and ARG1 mRNA levels as
compared to low-risk patients and control donors[92].
AML is a common sequela to myelodysplastic syndrome (MDS) where leukemic cancer stem
cells escape the immune response, and MDSC levels in the PB of high-risk MDS patients are
significantly higher than in low-risk patients and control subjects dictating the MDS prognosis.
This observation is associated with disease progression as following 3 months of supportive
treatment, the PB MDSC frequency was increased in MDS patients with progressive disease as
compared to patients in remission[93].
Impaired hematopoiesis in MDS patients is also mediated, in part, by a significant expansion of
BM MDSCs[94]. However, these BM-derived MDSCs express neither common leukemic gene
mutations nor chromosome deletions, which are evident only in the non-MDSC populations. In
ex vivo studies, BM-derived MDSCs have been shown to impair T-cell proliferation and reduce
IFN-γ production, which is reversed by MDSC depletion. Further, MDSCs isolated from MDS
patients produce high VEGF, IL-10, Tumor growth factor-beta (TGF-β), NO, and arginase levels
as compared to MDSCs from healthy individuals. In the BM microenvironment of MDS patients,
MDSCs contributed to the apoptotic phenotype of the adjacent HPCs that affect their
developmental pathway[94]. Phenotypic differentiation of marrow progenitor cells and
infiltrating MDSCs is difficult and, classically, is based on immunosuppressive activity.
However, additional biomarkers can be used including cellular ARG1 and NOS2 levels.
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Consistent with the increased arginine and NOS2 levels in patients with AML[91], early,
primitive HPC populations have low, or nonexistent, CD123 (IL-3R) expression[95], while
committed myeloid progenitors have high expression levels[96,97]. Although CD123 is
expressed by human plasmacytoid DCs, rather than myeloid DCs[98,99], it is also found on
myeloid DCs with MDSC characteristics including arginase and NOS2 over-expression
following monocyte co-culture with IL-4 or IL-13 and IL-3[100]. Some leukemias, including
blastic pDC neoplasms, AML, acute lymphoblastic leukemia, and hairy cell leukemia[101],
exhibit rapid progression associated with CD123 overexpression. In addition, only a small
percentage of HSCs express CD123. Thus, the majority of HSCs are not affected by receptor
blocking or signaling pathway inhibition[102], supporting this as a potentially useful molecular
target for the inhibition of MDSCs.
The antigen-specific T-cell suppression by MDSCs in leukemic patients is partially mediated by
ROS upregulation[10,103] that, in turn, prevents MDSC differentiation[104]. The role of ROS in
the development of a leukemic phenotype is important as leukemia cells are affected by
intracellular and extracellular ROS-related redox disturbances that may support immune evasion
and disease progression[105]. Some tumor-derived GFs that expand MDSCs also contribute to
leukemic cell growth[106] and protect against apoptosis[107], including upregulation of ROS. In
AML, malignant cells have significantly higher ROS levels than normal cells, thereby providing
a mechanism for immune evasion[108,109].
Immature HPCs, including MDSCs, express FLt3, which has been reported to expand and
activate MDSCs[110,111] and increase their suppressive potential as shown in a murine allograft
rejection model[112]. FLt3 mutations also have a role in leukemogenesis, especially for AML
(30–35%), which is associated with a poor prognosis, potentially providing a target to improve
therapeutic outcomes[113,114]. FLt3 inhibitors also curtail the progression of leukemias with
mutated FLt3 as shown in murine models[114]; although, FLt3L administration has also
exhibited antitumor effects in a syngeneic FDCP1 leukemia murine model[115] and has been
shown to support leukemia blast proliferation and resistance to apoptosis and survival[116,117],
either alone or in synergy with other hematopoietic GFs[118,119].
In vitro studies have revealed that ATRA induces MDSC and myeloid leukemia cell
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differentiation[26,120,121], which can result in therapeutic activity in patients[122] and in
murine tumor models[24] where it decreases MDSC numbers and boosts immunity[123].
Retinoic acid receptor alpha (RARα) gene mutations are commonly seen in acute promyelocytic
leukemia (APL) resulting in hematopoietic differentiation and arrest at the myeloid
promyelocytic stage[124,125]. Therefore, the RARα ligand, ATRA, has proven effective as a
therapeutic remedy for APL as it stimulates the differentiation of immature myeloid progenitors
into DCs and improves their antigen-specific, immune function[126-128]. However, resistance is
common and necessitates its administration in combination with standard chemotherapy[24].
MDSCS IN PATIENTS WITH MM
MM is a hematologic malignancy characterized by the accumulation of malignant plasma cells
within the BM[129]. Studies into the mechanism of MM bone disease have demonstrated the
regulation of MDSCs[6,130] and their functional heterogeneity. MDSCs isolated from the PB of
patients with MM[67] reportedly have an inhibitory effect on T-cell proliferation in vitro. In this
study, lymphocytes from MM patients were reported to have an impaired proliferative response.
Further, the addition of an ARG1-specific inhibitor, a NOS2 inhibitor, or a PDE5 inhibitor
reversed this inhibition documenting that the immunosuppressive function was due to MDSCs.
Studies into MDSC levels[6,131] in MM and monoclonal gammopathy of undetermined
significance (MGUS) patients, a pre-stage of MM, were found to be significantly increased in
MM patients. Interestingly, MGUS patients also had an increase in MDSCs, although at lower
levels than in MM patients, suggesting that MDSC numbers in MM patients were dependent on
disease progression. A different MDSC phenotypic population in the BM and PB of patients with
MM was observed together with a significant accumulation of MDSCs in the PB as compared to
healthy donors[132]. These MDSCs were primarily granulocytic, but, mMDSCs were also
observed, although, their frequency was similar to healthy donors. Interestingly, in contrast to
normal donors, MDSCs from MM patients could suppress T-cell responses. Furthermore, a
syngeneic murine model of MM was developed in which investigators observed rapid
accumulation of MDSCs in BM, lymph nodes, and spleens of MM-bearing mice[132] in a
S100A9 dependent manner. One aspect of MDSC pathobiology is the differential mechanisms of
action and role in neoplasia by mMDSCs and gMDSCs. This is difficult to clarify as frequently
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both are expanded by inflammatory conditions. They have differing biomarker expression;
however, their discrete role(s) in neoplasia requires additional clarification.
In a study of the presence, frequency, and functional characteristics of MDSCs in patients with
newly diagnosed MM as compared to healthy donors[133], MDSCs were reported to be
increased in number and frequency in relapse/refractory MM as compared to healthy donors.
MDSCs also had increased immune-suppressive activity, including an inhibition of both T-cell
and natural killer T (NKT) cell proliferation using cells isolated from both the PB and BM. This
supported a role for MDSCs in promoting tumor growth and in suppressing antitumor immunity
suggesting their use as a therapeutic target in patients with MM. Favaloro, et al., also reported an
increase in gMDSCs in the PB and BM of MM patients[134], which was significantly higher
than in normal donors. A higher proportion of gMDSCs was also observed in the BM as
compared to matched PB samples. These studies revealed a direct correlation between MDSC
and Treg numbers. Mechanistic studies using flow-sorted MDSCs from MM patients revealed
that MDSCs induced Treg generation and that G-CSF mobilization significantly increased
MDSC frequency.
MM-expanded MDSCs in osteolysis
In MM patients, tumor cells in the BM microenvironment continually interact with
hematopoietic and stromal cells[135]. It has been suggested that MDSCs affect MM marrow
metastases and, together, contribute to osteolysis, which is a common complication in breast and
prostate cancer patients, as well as MM patients, resulting in a poor prognosis. This occurs in
part due to OCs, which are bone-degrading cells whose activity and viability are increased by
MM cells. OC differentiation and survival regulate the receptor activator of nuclear factor-κβ
(RANK) and its ligand, RANKL, requiring the presence of both macrophage colony-stimulating
factor (M-CSF) and RANKL. Tumors often enhance the number and activity of osteoclasts;
however, there is increasing evidence that MDSCs may exert a dual effect on
immunosuppression and bone pathology thereby serving as OC progenitors in patients with
MM[136].
MDSCs isolated from TB mice with bone metastases have been reported to differentiate into
OCs in vitro and in vivo[136]. In another study, MDSCs isolated from bones with tumor
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metastases demonstrated increased OC differentiation[137], and other investigators have
reported that cancer-induced MDSCs resulted in bone destruction by directly differentiating into
OC progenitors[138]. A murine myeloma tumor model has been used to study the response of
MDSCs in mice to tumor growth[137]. Additionally, in studies comparing the capacities of naïve
and tumor-associated MDSCs to differentiate into functional OCs, it was found that
bisphosphonate, a potent bone resorption inhibitor, reduced MDSC differentiation into OCs.
Further, the CD11b-/loLy6Chi BM population was shown to differentiate into OCs in vivo when
adoptively transferred into OC-deficient transgenic mice[139].
MDSCs inhibition provides a novel approach to controlling MM
Standard of care for patients with MM usually consists of chemotherapy and stem-cell
transplantation[140]. In addition, lenalidomide is approved for patients with MM and is usually
administered in combination with low-dose glucocorticoids. Ghosh, et al., hypothesized that
inhibiting MDSCs could augment the antitumor activity of this immunomodulating drug[141].
However, further investigation is needed to examine the possibility of targeting MDSCs and
Tregs as part of this therapeutic strategy[142].
Differing miRNA expression has been observed between MM and its pre-cancerous stage,
MGUS[143]. For example, miRNA-21 and miRNA-155 are up-regulated in MM patients.
Further, miRNA-21 transcription is controlled by IL-6 through a STAT3 mechanism including
IL-6-dependent growth of MM cell lines. Anti-myeloma-related drugs, such as denosumab, a
RANKL neutralizing antibody; LY2127599 a BAFF (MM GF produced by DCs and BM stromal
cells) neutralizing antibody; MLN3897, a CCR1-inhibitor; BHQ880, DKK-1, a Wnt inhibitor;
and, Sotatercept, an activin-A antagonist, are all currently under development for the treatment
of MM[129]. Further, all of these drugs directly or indirectly involve MDSCs. It is hoped that by
inhibiting MDSCs and Tregs, we may control MM, bone disease, and tumor growth, in addition
to MDSC mediated immunosuppression and osteolysis.
MDSCS AS A PROGNOSTIC TOOL AND THERAPEUTIC TARGET IN LYMPHOMA
PATIENTS
Studies investigating lymphomas, including adult T-cell/lymphoma (ATL)[144,145], diffuse
large B-cell lymphoma (DLBCL)[146,147], follicular non-Hodgkin lymphoma (NHL)[148,149]
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and Hodgkin lymphoma (HL)[150] have revealed an increase in blood monocytes and MDSCs as
prognostic parameters[6]. In one study, 16 out of 79 adult ATL patients had an increased blood
monocyte count that correlated with a poor prognosis[144]. M-CSF was also found to be
increased in the serum of 3 progressing patients prior to treatment and 2 patients after treatment,
although, in vitro cultured ATL cells did not secrete this GF, indicating that its source may not be
tumor cells[144]. Moreover, in a study of 168 ATL patients, overall survival (OS) correlated
with blood monocytic number supporting its use as a prognostic parameter[151].
In a retrospective study of 94 T-cell lymphoma patients with an “aggressive-typically nodal
presentation” as classified by the World Health Organization[152], a correlation between the
absolute monocyte count and prognosis was identified[145]. Consistent with these observations,
one study examined monocyte engraftment in T-cell lymphoproliferative disorders, specifically
T-cell non-Hodgkin lymphoma (NHL), using a NOD/SCID murine model with 10 cutaneous and
12 peripheral T-cell NHL patients. In this study, CCL5 was shown to recruit non-malignant
mononuclear cells into the tumor microenvironment resulting in enhanced growth and survival of
malignant T-cells, which in turn prevented mature DC development via upregulation of IL-10
leading to impaired immune responses[153].
Retrospectively, using univariate and multivariate analyses, two studies of 99[146] and 366[147]
treated diffuse large B-cell lymphoma (DLBCL) patients were examined to assess the
relationship between absolute monocyte count and absolute lymphocyte count, which provided a
prognostic marker for OS and progression-free survival (PFS) independent of tumor histiotype.
In another study of 476 HL patients, followed for a median of 5.6 years, the absolute
lymphocyte/monocyte ratio before treatment was found to be an independent factor for
predicting clinical outcomes in terms of OS, lymphoma-specific survival, PFS, and time to
progression[150].
In NHL patients, tumor-derived factors were shown to enhance hematopoiesis and tumor-
associated myeloid cell accumulation, suppress T-cell immunity, stimulate tumor angiogenesis,
and decrease patient survival[154-156]. In one study, non-lymphoma immune cells from 200
follicular lymphoma (FL) patients were examined before therapy revealing an unfavorable
genetic signature similar to circulating mononuclear cells[157]. In another study of 99 FL
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patients, microarray and IHC analysis revealed an accumulation of tumor-associated
mononuclear cells indicating their importance as a poor prognostic parameter[154]. In addition,
IHC studies of 194 FL patients have shown that low CD68 expression by intratumoral
macrophages correlates with improved prognosis[155]. Following analysis of 355 FL patients, an
increased absolute monocytic count was associated with poor survival, but was not predictive of
therapeutic response[148]. Moreover, macrophage recruitment correlated with progression in 42
NHL cases[158]. Taken together, these findings support the analysis of absolute monocytic count
as a potential independent prognostic factor.
In a study of 40 NHL patients, a significant increase in circulating, immunosuppressive MDSCs
was reported to directly correlate with disease progression and, potentially, to act as a prognostic
marker[149]. In other studies, hematopoietic myeloid progenitor cell levels were found to
directly correlate with lymphoma severity where they, together with circulating and lymph
nodes-resident-endothelial progenitors, were considered significant markers for evaluating
angiogenesis and recruitment in lymphomagenesis[158,159]. In another study of B-cell NHL,
nodal biopsies were investigated in 71 patients using histopathology, IHC, and electron
microscopy to measure CD68+ hemangiogenic macrophage counts. These were found to be
significantly higher in B-NHL as compared to 30 benign lymphadenopathies samples, especially
high-grade tumors, which paralleled microvessel density[160]. Gene expression and phenotypic
analysis of PB and lymph node samples from 70 lymphoma patients revealed a significant
increase in endothelial progenitors, especially in younger patients, and was found to decrease in
the PB of responder patients as compared to patients who showed poor response to treatment.
Further, an increased frequency of endothelial progenitors in lymph nodes was found to correlate
with increased angiogenesis[161].
In preclinical studies, decreased CTL activity in a subcutaneous T-cell lymphoma murine model
(BW-Sp3) was reported to emanate from activated splenic CD11b+ myeloid cells resulting in
disease progression. These cells had reversible T-cell suppression mediated by high arginase and
NO expression in a cell-to-cell, contact-dependent manner. However, induction of anti-tumor
APC activity was shown in other cohorts of mice to be associated with disease regression[9]. In a
murine B-cell lymphoma model, IL4Rα up-regulation on tumor-induced MDSCs was found to
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suppress antigen-driven CD8+ T-cell proliferation, but not CD4+ T-cell and enhanced Treg
expansion. Sildenafil blocked IL4Rα expression and led to Treg suppression in vivo. This was
reportedly associated with improved T-cell proliferation after in vitro use of arginase and NO
inhibitors rather than TGFβ neutralizing antibody. Thus, MDSC suppression was associated with
arginase and NO activity, but independent of TGFβ[162].
SUMMARY
In conclusion, most of the pharmacological manipulations of MDSCs described herein were
performed using murine models, in vitro cultured cells, or cells from patients with solid tumors,
and few reports have examined MDSCs in patients with MM, leukemia, or lymphoma.
Nevertheless, a few studies have examined the influence of drugs on MDSC populations in
cancer patients and hold promise in terms of targeted therapy. However, abrogating MDSC-
associated immunosuppression is not likely to be adequate as a single therapeutic strategy, but
must be combined with other modalities or used as an adjuvant with other immune
manipulations.
Conflicts of Interest: The authors declare no conflicts of interest.
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References
1. Talmadge JE. Pathways Mediating the Expansion and Immunosuppressive Activity of
Myeloid-Derived Suppressor Cells and Their Relevance to Cancer Therapy. Clin
Cancer Res 2007;13:5243-5248.
2. Sonnenfeld A. Leukamische reaktiones bei carcinoma. Zeitschrift f Klin Med 1929;111.
3. Drake CG, Jaffee E, Pardoll DM. Mechanisms of immune evasion by tumors. Adv
Immunol 2006;90:51-81.
4. Talmadge JE, Gabrilovich DI. History of myeloid-derived suppressor cells. Nat Rev
Cancer 2013;13:739-752.
5. Goh C, Narayanan S, Hahn YS. Myeloid-derived suppressor cells: the dark knight or
the joker in viral infections? Immunol Rev 2013;255:210-221.
6. Tadmor T, Attias D, Polliack A. Myeloid-derived suppressor cells--their role in
haemato-oncological malignancies and other cancers and possible implications for
therapy. Br J Haematol 2011;153:557-567.
7. Donkor MK, Lahue E, Hoke TA, et al. . Mammary tumor heterogeneity in the
expansion of myeloid-derived suppressor cells. Int Immunopharmacol 2009;9:937-948.
8. Diaz-Montero CM, Salem ML, Nishimura MI, Garrett-Mayer E, Cole DJ, Montero AJ.
Increased circulating myeloid-derived suppressor cells correlate with clinical cancer
stage, metastatic tumor burden, and doxorubicin-cyclophosphamide chemotherapy.
Cancer Immunol Immunother 2009;58:49-59.
9. Liu Y, van Ginderachter JA, Brys L, de BP, Raes G, Geldhof AB. Nitric oxide-
independent CTL suppression during tumor progression: association with arginase-
producing (M2) myeloid cells. J Immunol. 2003;170:5064-5074.
10. Corzo CA, Cotter MJ, Cheng P, et al. . Mechanism regulating reactive oxygen species
in tumor-induced myeloid-derived suppressor cells. J Immunol 2009;182:5693-5701.
11. Schmid MC, Varner JA. Myeloid cells in the tumor microenvironment: modulation of
tumor angiogenesis and tumor inflammation. Journal of Oncology 2010;2010:201026.
12. Sawant A, Deshane J, Jules J, et al. . Myeloid-derived suppressor cells function as novel
osteoclast progenitors enhancing bone loss in breast cancer. Cancer Res 2013;73:672-
682.
13. Abe F, Dafferner AJ, Donkor M, et al. . Myeloid-derived suppressor cells in mammary
tumor progression in FVB Neu transgenic mice. Cancer Immunol Immunother
2010;59:47-62.
14. Whiteside TL. Tricks tumors use to escape from immune control. Oral Oncol
2009;45:e119-e123.
15. Montero AJ, Diaz-Montero CM, Kyriakopoulos CE, Bronte V, Mandruzzato S.
Myeloid-derived suppressor cells in cancer patients: a clinical perspective. J
Immunother 2012;35:107-115.
16. Poschke I, Kiessling R. On the armament and appearances of human myeloid-derived
suppressor cells. Clin Immunol 2012;144:250-268.
17. Rashid OM, Nagahashi M, Ramachandran S, et al. . Resection of the primary tumor
improves survival in metastatic breast cancer by reducing overall tumor burden. Surgery
2013.
18. Salvadori S, Martinelli G, Zier K. Resection of solid tumors reverses T cell defects and
restores protective immunity. J Immunol 2000;164:2214-2220.
19. Predina JD, Kapoor V, Judy BF, et al. . Cytoreduction surgery reduces systemic
myeloid suppressor cell populations and restores intratumoral immunotherapy
effectiveness. J Hematol Oncol 2012;5:34.
20. Mazzoni A, Bronte V, Visintin A, et al. . Myeloid suppressor lines inhibit T cell
responses by an NO-dependent mechanism. J Immunol 2002;168:689-695.
21. Rodriguez PC, Quiceno DG, Zabaleta J, et al. . Arginase I production in the tumor
microenvironment by mature myeloid cells inhibits T-cell receptor expression and
antigen-specific T-cell responses. Cancer Res 2004;64:5839-5849.
22. Dumitru CA, Moses K, Trellakis S, Lang S, Brandau S. Neutrophils and granulocytic
myeloid-derived suppressor cells: immunophenotyping, cell biology and clinical
relevance in human oncology. Cancer Immunol Immunother 2012;61:1155-1167.
Leuk Lymphoma Downloaded from informahealthcare.com by Sultan Qaboos University on 12/07/14
For personal use only.

JUST ACCEPTED
20
23. Rodriguez PC, Ernstoff MS, Hernandez C, et al. . Arginase I-producing myeloid-
derived suppressor cells in renal cell carcinoma are a subpopulation of activated
granulocytes. Cancer Res 2009;69:1553-1560.
24. Kusmartsev S, Cheng F, Yu B, et al. . All-trans-retinoic acid eliminates immature
myeloid cells from tumor-bearing mice and improves the effect of vaccination. Cancer
Res 2003;63:4441-4449.
25. Kusmartsev S, Su Z, Heiser A, et al. . Reversal of myeloid cell-mediated
immunosuppression in patients with metastatic renal cell carcinoma. Clin Cancer Res
2008;14:8270-8278.
26. Nefedova Y, Fishman M, Sherman S, Wang X, Beg AA, Gabrilovich DI. Mechanism of
All-Trans Retinoic Acid Effect on Tumor-Associated Myeloid-Derived Suppressor
Cells. Cancer Res 2007;67:11021-11028.
27. Lee JM, Seo JH, Kim YJ, Kim YS, Ko HJ, Kang CY. The restoration of myeloid-
derived suppressor cells as functional antigen-presenting cells by NKT cell help and all-
trans-retinoic acid treatment. Int J Cancer 2011.
28. Iclozan C, Antonia S, Chiappori A, Chen DT, Gabrilovich D. Therapeutic regulation of
myeloid-derived suppressor cells and immune response to cancer vaccine in patients
with extensive stage small cell lung cancer. Cancer Immunol Immunother 2013;62:909-
918.
29. Young MR, Wright MA, Vellody K, Lathers DM. Skewed differentiation of bone
marrow CD34+ cells of tumor bearers from dendritic toward monocytic cells, and the
redirection of differentiation toward dendritic cells by 1alpha,25-dihydroxyvitamin D3.
Int.J.Immunopharmacol. 1999;21:675-688.
30. Kulbersh JS, Day TA, Gillespie MB, Young MR. 1alpha,25-Dihydroxyvitamin D(3) to
skew intratumoral levels of immune inhibitory CD34(+) progenitor cells into dendritic
cells. Otolaryngol.Head Neck Surg. 2009;140:235-240.
31. Walsh JE, Clark A-M, Day TA, Gillespie MB, Young MRI. Use of α,25-
Dihydroxyvitamin D3 treatment to stimulate immune infiltration into head and neck
squamous cell carcinoma. Hum Immunol 2010;71:659-665.
32. Walker DD, Reeves TD, de Costa AM, Schuyler C, Young MR. Immunological
modulation by 1alpha,25-dihydroxyvitamin D3 in patients with squamous cell
carcinoma of the head and neck. Cytokine 2012;58:448-454.
33. Le HK, Graham L, Cha E, Morales JK, Manjili MH, Bear HD. Gemcitabine directly
inhibits myeloid derived suppressor cells in BALB/c mice bearing 4T1 mammary
carcinoma and augments expansion of T cells from tumor-bearing mice. Int
Immunopharmacol 2009;9:900-909.
34. Tomihara K, Fuse H, Heshiki W, et al. . Gemcitabine chemotherapy induces phenotypic
alterations of tumor cells that facilitate antitumor T cell responses in a mouse model of
oral cancer. Oral Oncol 2014;50:457-467.
35. Vincent J, Mignot G, Chalmin F, et al. . 5-Fluorouracil selectively kills tumor-
associated myeloid-derived suppressor cells resulting in enhanced T cell-dependent
antitumor immunity. Cancer Res 2010;70:3052-3061.
36. Mikyskova R, Indrova M, Simova J, Bieblova J, Bubenik J, Reinis M. Genetically
modified tumour vaccines producing IL-12 augment chemotherapy of HPV16-
associated tumours with gemcitabine. Oncol Rep 2011;25:1683-1689.
37. Esaki S, Goshima F, Kimura H, Murakami S, Nishiyama Y. Enhanced antitumoral
activity of oncolytic herpes simplex virus with gemcitabine using colorectal tumor
models. Int J Cancer 2013;132:1592-1601.
38. Yanagimoto H, Shiomi H, Satoi S, et al. . A phase II study of personalized peptide
vaccination combined with gemcitabine for non-resectable pancreatic cancer patients.
Oncol Rep 2010;24:795-801.
39. Annels NE, Shaw VE, Gabitass RF, et al. . The effects of gemcitabine and capecitabine
combination chemotherapy and of low-dose adjuvant GM-CSF on the levels of
myeloid-derived suppressor cells in patients with advanced pancreatic cancer. Cancer
Immunol Immunother 2014;63:175-183.
40. Montero AJ, Diaz-Montero CM, Deutsch YE, et al. . Phase 2 study of neoadjuvant
treatment with NOV-002 in combination with doxorubicin and cyclophosphamide
Leuk Lymphoma Downloaded from informahealthcare.com by Sultan Qaboos University on 12/07/14
For personal use only.

JUST ACCEPTED
21
followed by docetaxel in patients with HER-2 negative clinical stage II-IIIc breast
cancer. Breast Cancer Res Treat 2012;132:215-223.
41. Xin H, Zhang C, Herrmann A, Du Y, Figlin R, Yu H. Sunitinib inhibition of Stat3
induces renal cell carcinoma tumor cell apoptosis and reduces immunosuppressive cells.
Cancer Res 2009;69:2506-2513.
42. Ko JS, Zea AH, Rini BI, et al. . Sunitinib mediates reversal of myeloid-derived
suppressor cell accumulation in renal cell carcinoma patients. Clin Cancer Res
2009;15:2148-2157.
43. Pan PY, Wang GX, Yin B, et al. . Reversion of immune tolerance in advanced
malignancy: modulation of myeloid-derived suppressor cell development by blockade
of stem-cell factor function. Blood 2008;111:219-228.
44. Ozao-Choy J, Ma G, Kao J, et al. . The novel role of tyrosine kinase inhibitor in the
reversal of immune suppression and modulation of tumor microenvironment for
immune-based cancer therapies. Cancer Res 2009;69:2514-2522.
45. Bose A, Taylor J, Alber S, et al. . Sunitinib facilitates the activation and recruitment of
therapeutic anti-tumor immunity in concert with specific vaccination. Int J Cancer
2011;129:2158 - 2170.
46. Amato RJ, Hawkins RE, Kaufman HL, et al. . Vaccination of metastatic renal cancer
patients with MVA-5T4: a randomized, double-blind, placebo-controlled phase III
study. Clin Cancer Res 2010;16:5539-5547.
47. Farsaci B, Higgins JP, Hodge JW. Consequence of dose scheduling of sunitinib on host
immune response elements and vaccine combination therapy. Int J Cancer
2012;130:1948-1959.
48. Gabrilovich D, Ishida T, Oyama T, et al. . Vascular endothelial growth factor inhibits
the development of dendritic cells and dramatically affects the differentiation of
multiple hematopoietic lineages in vivo. Blood 1998;92:4150-4166.
49. Kusmartsev S, Eruslanov E, Kubler H, et al. . Oxidative stress regulates expression of
VEGFR1 in myeloid cells: link to tumor-induced immune suppression in renal cell
carcinoma. J.Immunol. 2008;181:346-353.
50. Fricke I, Mirza N, Dupont J, et al. . Vascular endothelial growth factor-trap overcomes
defects in dendritic cell differentiation but does not improve antigen-specific immune
responses. Clin Cancer Res 2007;13:4840-4848.
51. Schilling B, Sucker A, Griewank K, et al. . Vemurafenib reverses immunosuppression
by myeloid derived suppressor cells. Int J Cancer 2013;133:1653-1663.
52. Sumimoto H, Imabayashi F, Iwata T, Kawakami Y. The BRAF-MAPK signaling
pathway is essential for cancer-immune evasion in human melanoma cells. J Exp Med
2006;203:1651-1656.
53. Xu J, Escamilla J, Mok S, et al. . CSF1R signaling blockade stanches tumor-infiltrating
myeloid cells and improves the efficacy of radiotherapy in prostate cancer. Cancer Res
2013;73:2782-2794.
54. Priceman SJ, Sung JL, Shaposhnik Z, et al. . Targeting distinct tumor-infiltrating
myeloid cells by inhibiting CSF-1 receptor: combating tumor evasion of antiangiogenic
therapy. Blood 2010;115:1461-1471.
55. Mok S, Koya RC, Tsui C, et al. . Inhibition of CSF-1 receptor improves the antitumor
efficacy of adoptive cell transfer immunotherapy. Cancer Res 2014;74:153-161.
56. Ries Carola H, Cannarile Michael A, Hoves S, et al. . Targeting Tumor-Associated
Macrophages with Anti-CSF-1R Antibody Reveals a Strategy for Cancer Therapy.
Cancer Cell.
57. Chalmin F, Ladoire S, Mignot G, et al. . Membrane-associated Hsp72 from tumor-
derived exosomes mediates STAT3-dependent immunosuppressive function of mouse
and human myeloid-derived suppressor cells. J Clin Invest 2010;120:457-471.
58. Filipazzi P, Valenti R, Huber V, et al. . Identification of a New Subset of Myeloid
Suppressor Cells in Peripheral Blood of Melanoma Patients With Modulation by a
Granulocyte-Macrophage Colony-Stimulation Factor-Based Antitumor Vaccine. J Clin
Oncol 2007;25:2546-2553.
59. Kujawski M, Kortylewski M, Lee H, Herrmann A, Kay H, Yu H. Stat3 mediates
myeloid cell-dependent tumor angiogenesis in mice. J Clin Invest 2008;118:3367-3377.
Leuk Lymphoma Downloaded from informahealthcare.com by Sultan Qaboos University on 12/07/14
For personal use only.

JUST ACCEPTED
22
60. Poschke I, Mougiakakos D, Hansson J, Masucci GV, Kiessling R. Immature
immunosuppressive CD14+HLA-DR-/low cells in melanoma patients are Stat3hi and
overexpress CD80, CD83, and DC-sign. Cancer Res 2010;70:4335-4345.
61. Nefedova Y, Cheng P, Gilkes D, et al. . Activation of dendritic cells via inhibition of
Jak2/STAT3 signaling. J Immunol. 2005;175:4338-4346.
62. Rodan GA, Fleisch HA. Bisphosphonates: mechanisms of action. J Clin Invest
1996;97:2692-2696.
63. Heissig B, Hattori K, Dias S, et al. . Recruitment of stem and progenitor cells from the
bone marrow niche requires MMP-9 mediated release of kit-ligand. Cell 2002;109:625-
637.
64. Melani C, Sangaletti S, Barazzetta FM, Werb Z, Colombo MP. Amino-Biphosphonate
Mediated MMP-9 Inhibition Breaks the Tumor-Bone Marrow Axis Responsible for
Myeloid-Derived Suppressor Cell Expansion and Macrophage Infiltration in Tumor
Stroma. Cancer Res 2007;67:11438-11446.
65. Porembka MR, Mitchem JB, Belt BA, et al. . Pancreatic adenocarcinoma induces bone
marrow mobilization of myeloid-derived suppressor cells which promote primary tumor
growth. Cancer Immunol Immunother 2012.
66. Porembka M, Mitchem J, Belt B, et al. . Pancreatic adenocarcinoma induces bone
marrow mobilization of myeloid-derived suppressor cells which promote primary tumor
growth. Cancer Immunol Immunother 2012;61:1373 - 1385.
67. Serafini P, Meckel K, Kelso M, et al. . Phosphodiesterase-5 inhibition augments
endogenous antitumor immunity by reducing myeloid-derived suppressor cell function.
J Exp Med 2006;203:2691-2702.
68. Noonan KA, Ghosh N, Rudraraju L, Bui M, Borrello I. Targeting Immune Suppression
with PDE5 Inhibition in End-Stage Multiple Myeloma. Cancer Immunol Res 2014.
69. Administration Of An Oral PDE5 Inhibitor, Tadalafil In Conjunction With a
Lenalidomide Containing Regimen In Patients With Multiple Myeloma. Rudraraju L,
Ye X, Noonan K, Huff CA, Borrello IM, editors2013. 1959-1959 p.
70. De Santo C. Nitroaspirin corrects immune dysfunction in tumor-bearing hosts and
promotes tumor eradication by cancer vaccination. Proc Natl Acad Sci 2005;102:4185-
4190.
71. Mundy-Bosse BL, Lesinski GB, Jaime-Ramirez AC, et al. . Myeloid-derived suppressor
cell inhibition of the IFN response in tumor-bearing mice. Cancer Res 2011;71:5101-
5110.
72. Nagaraj S, Gupta K, Pisarev V, et al. . Altered recognition of antigen is a mechanism of
CD8+ T cell tolerance in cancer. Nat Med 2007;13:828-835.
73. Molon B, Ugel S, Del Pozzo F, et al. . Chemokine nitration prevents intratumoral
infiltration of antigen-specific T cells. J Exp Med 2011;208:1949-1962.
74. Fiorucci S, Santucci L, Cirino G, et al. . IL-1 beta converting enzyme is a target for
nitric oxide-releasing aspirin: new insights in the antiinflammatory mechanism of nitric
oxide-releasing nonsteroidal antiinflammatory drugs. J Immunol 2000;165:5245-5254.
75. Thimmulappa R, Fuchs R, Malhotra D, et al. . Preclinical evaluation of targeting the
Nrf2 pathway by triterpenoids (CDDO-Im and CDDO-Me) for protection from LPS-
induced inflammatory response and reactive oxygen species in human peripheral blood
mononuclear cells and neutrophils. Antioxid Redox Signal 2007;9:1963 - 1970.
76. Li B, Abdalrahman A, Lai Y, et al. . Dihydro-CDDO-trifluoroethyl amide suppresses
inflammatory responses in macrophages via activation of Nrf2. Biochem Biophys Res
Commun 2014;444:555-561.
77. Ling X, Konopleva M, Zeng Z, et al. . The Novel Triterpenoid C-28 Methyl Ester of 2-
Cyano-3, 12-Dioxoolen-1, 9-Dien-28-Oic Acid Inhibits Metastatic Murine Breast
Tumor Growth through Inactivation of STAT3 Signaling. Cancer Res 2007;67:4210-
4218.
78. Wang YY, Zhe H, Zhao R. Preclinical evidences toward the use of triterpenoid CDDO-
Me for solid cancer prevention and treatment. Mol Cancer 2014;13:30.
79. Place AE, Suh N, Williams CR, et al. . The novel synthetic triterpenoid, CDDO-
imidazolide, inhibits inflammatory response and tumor growth in vivo. Clin Cancer Res
2003;9:2798-2806.
Leuk Lymphoma Downloaded from informahealthcare.com by Sultan Qaboos University on 12/07/14
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23
80. Nagaraj S, Youn J, Weber H, et al. . Anti-inflammatory triterpenoid blocks immune
suppressive function of MDSCs and improves immune response in cancer. Clin Cancer
Res 2010;16:1812 - 1823.
81. Ma X, Kundu N, Rifat S, Walser T, Fulton AM. Prostaglandin E receptor EP4
antagonism inhibits breast cancer metastasis. Cancer Res 2006;66:2923-2927.
82. Chan G, Boyle JO, Yang EK, et al. . Cyclooxygenase-2 expression is up-regulated in
squamous cell carcinoma of the head and neck. Cancer Res 1999;59:991-994.
83. Talmadge J, Hood K, Zobel L, Shafer L, Coles M, Toth B. Chemoprevention by
cyclooxygenase-2 inhibition reduces immature myeloid suppressor cell expansion. Int
Immunopharmacol 2007;7:140 - 151.
84. Veltman J, Lambers M, van Nimwegen M, et al. . COX-2 inhibition improves
immunotherapy and is associated with decreased numbers of myeloid-derived
suppressor cells in mesothelioma. Celecoxib influences MDSC function. BMC Cancer
2010;10:464.
85. Sokal JE, Baccarani M, Russo D, Tura S. Staging and prognosis in chronic
myelogenous leukemia. Semin Hematol 1988;25:49-61.
86. Gabrilove JL, Jakubowski A. Hematopoietic growth factors: biology and clinical
application. J Natl Cancer Inst Monogr 1990:73-77.
87. Chen X, Liu S, Wang L, Zhang W, Ji Y, Ma X. Clinical significance of B7-H1 (PD-L1)
expression in human acute leukemia. Cancer Biol Ther 2008;7:622-627.
88. Gupta V, Tallman MS, Weisdorf DJ. Allogeneic hematopoietic cell transplantation for
adults with acute myeloid leukemia: myths, controversies, and unknowns. Blood
2011;117:2307-2318.
89. Jitschin R, Braun M, Buttner M, et al. . CLL-cells induce IDOhi CD14+HLA-DRlo
myeloid-derived suppressor cells that inhibit T-cell responses and promote TRegs.
Blood 2014;124:750-760.
90. Zhang L, Chen X, Liu X, et al. . CD40 ligation reverses T cell tolerance in acute
myeloid leukemia. J Clin Invest 2013;123:1999-2010.
91. Mussai F, De Santo C, Abu-Dayyeh I, et al. . Acute myeloid leukemia creates an
arginase-dependent immunosuppressive microenvironment. Blood 2013;122:749-758.
92. Christiansson L, Soderlund S, Svensson E, et al. . Increased level of myeloid-derived
suppressor cells, programmed death receptor ligand 1/programmed death receptor 1,
and soluble CD25 in Sokal high risk chronic myeloid leukemia. PLoS One
2013;8:e55818.
93. Jiang HJ, Fu R, Wang HQ, et al. . Increased circulating of myeloid-derived suppressor
cells in myelodysplastic syndrome. Chin Med J (Engl) 2013;126:2582-2584.
94. Chen X, Eksioglu EA, Zhou J, et al. . Induction of myelodysplasia by myeloid-derived
suppressor cells. J Clin Invest 2013;123:4595-4611.
95. Sato N, Caux C, Kitamura T, et al. . Expression and factor-dependent modulation of the
interleukin-3 receptor subunits on human hematopoietic cells. Blood 1993;82:752-761.
96. Huang S, Chen Z, Yu JF, et al. . Correlation between IL-3 receptor expression and
growth potential of human CD34+ hematopoietic cells from different tissues. Stem
Cells 1999;17:265-272.
97. Testa U, Fossati C, Samoggia P, et al. . Expression of growth factor receptors in
unilineage differentiation culture of purified hematopoietic progenitors. Blood
1996;88:3391-3406.
98. Robinson SP, Patterson S, English N, Davies D, Knight SC, Reid CD. Human
peripheral blood contains two distinct lineages of dendritic cells. Eur J Immunol
1999;29:2769-2778.
99. Cella M, Jarrossay D, Facchetti F, et al. . Plasmacytoid monocytes migrate to inflamed
lymph nodes and produce large amounts of type I interferon. Nat Med 1999;5:919-923.
100. Testa U, Riccioni R, Diverio D, Rossini A, Lo Coco F, Peschle C. Interleukin-3
receptor in acute leukemia. Leukemia 2004;18:219-226.
101. Testa U, Pelosi E, Frankel A. CD 123 is a membrane biomarker and a therapeutic target
in hematologic malignancies. Biomark Res 2014;2:4.
Leuk Lymphoma Downloaded from informahealthcare.com by Sultan Qaboos University on 12/07/14
For personal use only.

JUST ACCEPTED
24
102. Jin L, Lee EM, Ramshaw HS, et al. . Monoclonal antibody-mediated targeting of
CD123, IL-3 receptor alpha chain, eliminates human acute myeloid leukemic stem cells.
Cell Stem Cell 2009;5:31-42.
103. Kusmartsev S, Nefedova Y, Yoder D, Gabrilovich DI. Antigen-specific inhibition of
CD8+ T cell response by immature myeloid cells in cancer is mediated by reactive
oxygen species. J.Immunol. 2004;172:989-999.
104. Kusmartsev S. Inhibition of myeloid cell differentiation in cancer: the role of reactive
oxygen species. J Leukoc Biol 2003;74:186-196.
105. Sallmyr A, Fan J, Datta K, et al. . Internal tandem duplication of FLT3 (FLT3/ITD)
induces increased ROS production, DNA damage, and misrepair: implications for poor
prognosis in AML. Blood 2008;111:3173-3182.
106. Zhu QS, Xia L, Mills GB, Lowell CA, Touw IP, Corey SJ. G-CSF induced reactive
oxygen species involves Lyn-PI3-kinase-Akt and contributes to myeloid cell growth.
Blood 2006;107:1847-1856.
107. Maraldi T, Prata C, Fiorentini D, Zambonin L, Landi L, Hakim G. Induction of
apoptosis in a human leukemic cell line via reactive oxygen species modulation by
antioxidants. Free Radic Biol Med 2009;46:244-252.
108. Zhou FL, Zhang WG, Wei YC, et al. . Involvement of oxidative stress in the relapse of
acute myeloid leukemia. J Biol Chem 2010;285:15010-15015.
109. Zhou F, Shen Q, Claret FX. Novel roles of reactive oxygen species in the pathogenesis
of acute myeloid leukemia. J Leukoc Biol 2013;94:423-429.
110. Sonoda Y, Kimura T, Sakabe H, et al. . Human FLT3 ligand acts on myeloid as well as
multipotential progenitors derived from purified CD34+ blood progenitors expressing
different levels of c-kit protein. Eur.J.Haematol. 1997;58:257-264.
111. Solheim J, Reber A, Ashour A, et al. . Spleen but not tumor infiltration by dendritic and
T cells is increased by intravenous adenovirus-Flt3 ligand injection. Cancer Gene Ther
2007;14:364-371.
112. Rosborough BR, Mathews LR, Matta BM, et al. . Cutting Edge: Flt3 Ligand Mediates
STAT3-Independent Expansion but STAT3-Dependent Activation of Myeloid-Derived
Suppressor Cells. J Immunol 2014;192:3470-3473.
113. Drexler HG, Quentmeier H. FLT3: receptor and ligand. Growth Factors 2004;22:71-73.
114. Gilliland DG, Griffin JD. Role of FLT3 in leukemia. Curr Opin Hematol. 2002;9:274-
281.
115. Wang A, Braun SE, Sonpavde G, Cornetta K. Antileukemic activity of Flt3 ligand in
murine leukemia. Cancer Res 2000;60:1895-1900.
116. Drexler HG. Expression of FLT3 receptor and response to FLT3 ligand by leukemic
cells. Leukemia 1996;10:588-599.
117. Dehmel U, Zaborski M, Meierhoff G, et al. . Effects of FLT3 ligand on human leukemia
cells. I. Proliferative response of myeloid leukemia cells. Leukemia 1996;10:261-270.
118. Piacibello W, Fubini L, Sanavio F, et al. . Effects of human FLT3 ligand on myeloid
leukemia cell growth: heterogeneity in response and synergy with other hematopoietic
growth factors. Blood 1995;86:4105-4114.
119. Eder M, Hemmati P, Kalina U, et al. . Effects of Flt3 ligand and interleukin-7 on in
vitro growth of acute lymphoblastic leukemia cells. Exp Hematol 1996;24:371-377.
120. Almand B, Clark JI, Nikitina E, et al. . Increased production of immature myeloid cells
in cancer patients: a mechanism of immunosuppression in cancer. J Immunol
2001;166:678-689.
121. Gabrilovich DI, Velders MP, Sotomayor EM, Kast WM. Mechanism of immune
dysfunction in cancer mediated by immature Gr-1+ myeloid cells. J Immunol
2001;166:5398-5406.
122. Mirza N. All-trans-Retinoic Acid Improves Differentiation of Myeloid Cells and
Immune Response in Cancer Patients. Cancer Res 2006;66:9299-9307.
123. Gervais A, Leveque J, Bouet-Toussaint F, et al. . Dendritic cells are defective in breast
cancer patients: a potential role for polyamine in this immunodeficiency. Breast Cancer
Res 2005;7:R326-R335.
124. van Bockstaele DR, Lenjou M, Snoeck HW, Lardon F, Stryckmans P, Peetermans ME.
Direct effects of 13-cis and all-trans retinoic acid on normal bone marrow (BM)
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progenitors: comparative study on BM mononuclear cells and on isolated CD34+ BM
cells. Ann Hematol 1993;66:61-66.
125. Van Bockstaele DR, Lenjou M, Snoeck HW, Lardon F, Peetermans ME. Effect of
retinoic acid (RA) on myeloid progenitors in normal bone marrow (BM). Leukemia
1994;8:214-215.
126. Castaigne S, Chomienne C, Daniel MT, et al. . All-trans retinoic acid as a differentiation
therapy for acute promyelocytic leukemia. I. Clinical results. Blood 1990;76:1704-1709.
127. Warrell RP, Jr., Frankel SR, Miller WH, Jr., et al. . Differentiation therapy of acute
promyelocytic leukemia with tretinoin (all-trans-retinoic acid). N.Engl.J.Med.
1991;324:1385-1393.
128. Breitman TR, Collins SJ, Keene BR. Terminal differentiation of human promyelocytic
leukemic cells in primary culture in response to retinoic acid. Blood 1981;57:1000-
1004.
129. Oranger A, Carbone C, Izzo M, Grano M. Cellular mechanisms of multiple myeloma
bone disease. Clin Dev Immunol 2013;2013:289458.
130. Escors D. Tumour immunogenicity, antigen presentation and immunological barriers in
cancer immunotherapy. New J Sci 2014;2014.
131. Parrinello N, LaCava P, Tibullo D, et al. . Myeloid-derived suppressor cells in patients
with multiple myeloma and monoclnal gammopathy of undetermined significance.
Haematologica 2010;95[suppl.2]:abs. 0930.
132. Ramachandran IR, Martner A, Pisklakova A, et al. . Myeloid-derived suppressor cells
regulate growth of multiple myeloma by inhibiting T cells in bone marrow. J Immunol
2013;190:3815-3823.
133. Gorgun GT, Whitehill G, Anderson JL, et al. . Tumor-promoting immune-suppressive
myeloid-derived suppressor cells in the multiple myeloma microenvironment in
humans. Blood 2013;121:2975-2987.
134. Favaloro J, Liyadipitiya T, Brown R, et al. . Myeloid derived suppressor cells are
numerically, functionally and phenotypically different in patients with multiple
myeloma. Leuk Lymphoma 2014.
135. Noll JE, Williams SA, Tong CM, et al. . Myeloma plasma cells alter the bone marrow
microenvironment by stimulating the proliferation of mesenchymal stromal cells.
Haematologica 2014;99:163-171.
136. Sawant A, Ponnazhagan S. Myeloid-derived suppressor cells as osteoclast progenitors:
a novel target for controlling osteolytic bone metastasis. Cancer Res 2013;73:4606-
4610.
137. Danilin S, Merkel AR, Johnson JR, Johnson RW, Edwards JR, Sterling JA. Myeloid-
derived suppressor cells expand during breast cancer progression and promote tumor-
induced bone destruction. Oncoimmunology 2012;1:1484-1494.
138. Zhuang J, Zhang J, Lwin ST, et al. . Osteoclasts in multiple myeloma are derived from
Gr-1+CD11b+myeloid-derived suppressor cells. PLoS One 2012;7:e48871.
139. Charles JF, Hsu LY, Niemi EC, Weiss A, Aliprantis AO, Nakamura MC. Inflammatory
arthritis increases mouse osteoclast precursors with myeloid suppressor function. J Clin
Invest 2012;122:4592-4605.
140. Sahebi F, Shen Y, Thomas SH, et al. . Late relapses following reduced intensity
allogeneic transplantation in patients with multiple myeloma: a long-term follow-up
study. Br J Haematol 2013;160:199-206.
141. Ghosh N, Rudraraju L, Ye X, Noonan K, Huff C, Borrello I. Administration Of An Oral
PDE5 Inhibitor, Tadalafil In Conjunction With a Lenalidomide Containing Regimen In
Patients With Multiple Myeloma. 55th ASH Annual Meeting and Exposition. New
Orleans, LA; 2013.
142. Semeraro M, Vacchelli E, Eggermont A, et al. . Trial Watch: Lenalidomide-based
immunochemotherapy. Oncoimmunology 2013;2:e26494.
143. Li L, Zhang J, Diao W, et al. . MicroRNA-155 and MicroRNA-21 promote the
expansion of functional myeloid-derived suppressor cells. J Immunol 2014;192:1034-
1043.
Leuk Lymphoma Downloaded from informahealthcare.com by Sultan Qaboos University on 12/07/14
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144. Tokioka T, Shimamoto Y, Motoyoshi K, Yamaguchi M. Clinical significance of
monocytosis and human monocytic colony-stimulating factor in patients with adult T-
cell leukaemia/lymphoma. Haematologia (Budap) 1994;26:1-9.
145. Bari A, Tadmor T, Sacchi S, et al. . Monocytosis has adverse prognostic significance
and impacts survival in patients with T-cell lymphomas. Leuk Res 2013;37:619-623.
146. Porrata LF, Ristow K, Habermann TM, et al. . Absolute monocyte/lymphocyte count
prognostic score is independent of immunohistochemically determined cell of origin in
predicting survival in diffuse large B-cell lymphoma. Leuk Lymphoma 2012;53:2159-
2165.
147. Wilcox RA, Ristow K, Habermann TM, et al. . The absolute monocyte and lymphocyte
prognostic score predicts survival and identifies high-risk patients in diffuse large-B-
cell lymphoma. Leukemia 2011;25:1502-1509.
148. Wilcox RA, Ristow K, Habermann TM, et al. . The absolute monocyte count is
associated with overall survival in patients newly diagnosed with follicular lymphoma.
Leuk Lymphoma 2012;53:575-580.
149. Lin Y, Gustafson MP, Bulur PA, Gastineau DA, Witzig TE, Dietz AB.
Immunosuppressive CD14+HLA-DR(low)/- monocytes in B-cell non-Hodgkin
lymphoma. Blood 2011;117:872-881.
150. Porrata LF, Ristow K, Habermann TM, et al. . Peripheral blood lymphocyte/monocyte
ratio at diagnosis and survival in nodular lymphocyte-predominant Hodgkin lymphoma.
Br J Haematol 2012;157:321-330.
151. Takasaki Y, Iwanaga M, Tsukasaki K, et al. . Impact of visceral involvements and blood
cell count abnormalities on survival in adult T-cell leukemia/lymphoma (ATLL). Leuk
Res 2007;31:751-757.
152. Armitage JO. The aggressive peripheral T-cell lymphomas: 2012 update on diagnosis,
risk stratification, and management. Am J Hematol 2012;87:511-519.
153. Wilcox RA, Wada DA, Ziesmer SC, et al. . Monocytes promote tumor cell survival in
T-cell lymphoproliferative disorders and are impaired in their ability to differentiate into
mature dendritic cells. Blood 2009;114:2936-2944.
154. Farinha P, Masoudi H, Skinnider BF, et al. . Analysis of multiple biomarkers shows that
lymphoma-associated macrophage (LAM) content is an independent predictor of
survival in follicular lymphoma (FL). Blood 2005;106:2169-2174.
155. Canioni D, Salles G, Mounier N, et al. . High numbers of tumor-associated
macrophages have an adverse prognostic value that can be circumvented by rituximab
in patients with follicular lymphoma enrolled onto the GELA-GOELAMS FL-2000
trial. J Clin Oncol 2008;26:440-446.
156. Andjelic B, Mihaljevic B, Todorovic M, Bila J, Jakovic L, Jovanovic MP. The number
of lymphoma-associated macrophages in tumor tissue is an independent prognostic
factor in patients with follicular lymphoma. Appl Immunohistochem Mol Morphol
2012;20:41-46.
157. Dave SS, Wright G, Tan B, et al. . Prediction of survival in follicular lymphoma based
on molecular features of tumor-infiltrating immune cells. N Engl J Med 2004;351:2159-
2169.
158. Ruan J, Hyjek E, Kermani P, et al. . Magnitude of stromal hemangiogenesis correlates
with histologic subtype of non-Hodgkin's lymphoma. Clin Cancer Res 2006;12:5622-
5631.
159. Monestiroli S, Mancuso P, Burlini A, et al. . Kinetics and viability of circulating
endothelial cells as surrogate angiogenesis marker in an animal model of human
lymphoma. Cancer Res 2001;61:4341-4344.
160. Vacca A, Ribatti D, Ruco L, et al. . Angiogenesis extent and macrophage density
increase simultaneously with pathological progression in B-cell non-Hodgkin's
lymphomas. Br J Cancer 1999;79:965-970.
161. Igreja C, Courinha M, Cachaco AS, et al. . Characterization and clinical relevance of
circulating and biopsy-derived endothelial progenitor cells in lymphoma patients.
Haematologica 2007;92:469-477.
Leuk Lymphoma Downloaded from informahealthcare.com by Sultan Qaboos University on 12/07/14
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162. Serafini P, Mgebroff S, Noonan K, Borrello I. Myeloid-Derived Suppressor Cells
Promote Cross-Tolerance in B-Cell Lymphoma by Expanding Regulatory T Cells.
Cancer Res 2008;68:5439-5449.
163. Lathers D, Clark J, Achille N, Young M. Phase 1B study to improve immune responses
in head and neck cancer patients using escalating doses of 25-hydroxyvitamin D3.
Cancer Immunol Immunother 2004;53:422 - 430.
164. Gabitass RF, Annels NE, Crawshaw J, Pandha HS, Middleton GW. Use of gemcitabine-
(Gem) and fluropyrimidine (FP)-based chemotherapy to reduce myeloid-derived
suppressor cells (MDSCs) in pancreatic (PC) and esophagogastric cancer (EGC). 2011
ASCO Annual Meeting. Volume 29: J Clin Oncol; 2011. p suppl; abstr 2588.
165. Finke JH, Rini B, Ireland J, et al. . Sunitinib Reverses Type-1 Immune Suppression and
Decreases T-Regulatory Cells in Renal Cell Carcinoma Patients. Clinical Cancer
Research 2008;14:6674-6682.
166. Rodriguez P, Ernstoff M, Hernandez C, et al. . Arginase I-producing myeloid-derived
suppressor cells in renal cell carcinoma are a subpopulation of activated granulocytes.
Cancer Res 2009;69:1553 - 1560.
167. Obermajer N, Muthuswamy R, Odunsi K, Edwards RP, Kalinski P. PGE(2)-induced
CXCL12 production and CXCR4 expression controls the accumulation of human
MDSCs in ovarian cancer environment. Cancer Res 2011;71:7463-7470.
168. Wesolowski R, Markowitz J, Carson W. Myeloid derived suppressor cells - a new
therapeutic target in the treatment of cancer. Journal for ImmunoTherapy of Cancer
2013;1:10.
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FIGURE LEGENDS
Figure 1: Cross-talk between tumors and myelopoiesis. Tumors secrete growth factors
(GFs) and cytokines that induce the expansion and mobilization of myeloid progenitors
including myeloid-derived suppressor cells (MDSCs) from the marrow that traffic to
extramedullary sites including the spleen, liver and tumor where they can also
proliferate, demarginate, and circulate to other organs, as well as, infiltrate tumors.
Dependent upon the infiltrating cellular subset and extent of maturation, these cells are
critical components and regulators of angiogenesis, vasculogenesis,
immunosuppression, and tumor growth.
De-margination
of MDSCs into
the circulation
Margination of MDSCs and
extramedullary
hematopoiesis
Circulating
pool of MDSCs
Medullary expansion of
mature and precursor
myeloid cells including
MDSCs
Inflammation
Mobilization
MDSC arrest and
tumor infiltration
Tumor secretion of
GFs and chemokines
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Figure 2: Biologics regulating myeloid-derived suppressor cells (MDSCs). The primary
mechanisms and mediators regulating MDSC expansion, recruitment, differentiation, and
function. ARG-1, arginase 1; BV8, Bombina variagata peptide 8; CCR2, C-C chemokine
receptor type 2; CCR5, C-C chemokine receptor type 5; CXCL1, C-X-C motif ligand 1; CXCR4,
C-X-C chemokine receptor type 4; FLT3, Fms-like tyrosine kinase 3; G-CSF, granulocyte-
colony stimulating factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; IFN-γ,
interferon-gamma; IL-6, interleukin 6; IL-10, interleukin 10 ; KLF4, Kruppel-like factor 4; LPS,
lipopolysaccharide; LTB4, leukotriene B4; M-CSF, macrophage-colony stimulating factor;
NOS-2, nitric oxide synthase 2; PGE-2, prostaglandin E2; ROS, reactive oxygen species; SCF,
stem cell factor; S100A9, calcium-binding protein; STAT3, signal transducer and activator of
transcription 3; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor.
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Table Legend:
Table 1: Therapeutics that are being tested clinically for myeloid-derived suppressor cell
(MDSC) regulation.
Mechanism Therapeutic Agent Target Type of
Cancer Tested Species Effect on
Myeloid-derived
Suppressor Cells
(MDSCs)
Differentiation All trans-retinoic acid
(ATRA)[122]
Differentiation Metastatic renal
cell carcinoma
(RCC) and small-
cell lung cancer
(SCLC)
Human MDSC differentiation
Differentiation Vitamin D3[163] Differentiation Non-small-cell
lung cancer
(NSCLC),
squamous cell
carcinoma of head
and neck
Human MDSC differentiation
Cytotoxic drugs Gemcitabine[38] Cytotoxic Pancreatic cancer Human Inhibition of
proliferation
Cytotoxic drugs 5-fluorouracil (5FU)[164] Cytotoxic Pancreatic and
esophagogastric
cancer
Human Inhibition of MDSC
expansion
Vascular endothelial
growth factor (VEGF)
inhibition
VEGF-tartrate-resistant
acid phosphatase,
Tartrate-resistant acid
phosphatase (Trap)[50]
Growth factor (GF)
inhibition
Solid tumors Human No activity on MDSCs
VEGF inhibition Tyrosine-kinase inhibitor
(TKI) (Sunitinib)[165]
GF/signal inhibition RCC Human Inhibition of
proliferation
VEGF inhibition Antibody to VEGF
(Bevacizumab
(Avastin))[166]
GF inhibition Metastatic RCC Human Weak inhibition of
proliferation
VEGF inhibition Vemurafenib[51] BRAF inhibitor Melanoma Human Decreased MDSC
frequency
Colony-stimulating
factor (CSF) and
receptor inhibition
TKI PLX3397[53] GF inhibition (FLT3) Prostate cancer Murine &
human
Inhibition of tumor
associated macrophage
(TAM) Recruitment
CSF and receptor
inhibition
Monoclonal antibodies to
macrophage colony-
stimulating factor (M-
CSF) (RG7155)[56]
GF inhibition (M-
CSF)
Diffuse-type, giant-
cell tumors
Murine &
human
Reduce tumor
infiltration
CSF and receptor
inhibition
Amiloride[57] Reduces exosome
release of GFs
Metastatic
colorectal
carcinoma
Murine &
human
Inhibition of MDSC
suppressive activity
Interference with
immunosuppression
Celebrex[167] Enzyme inhibition
(cyclooxygenase 2)
Ovarian cancer Human Inhibition of
proliferation/activation
and chemotaxsis
Interference with
immunosuppression
Amino-
bisphosphonate[66]
Enzyme inhibition
(farnesyl-diphosphate
synthase)
Pancreatic
adenocarcinoma
Human Inhibition of
proliferation
Interference with
immunosuppression
Bardoxolone Methyl
(CDDO-Me)[80]
Enzyme Inhibition
reactive oxygen
species (ROS)
Pancreatic
adenocarcinoma
Murine &
human
Inhibition of MDSC T-
cell suppression
Interference with
immunosuppression
Nitrous-oxide (NO)-
aspirin[168]
Enzyme inhibition
(inducible nitric
oxide synthase and
arginase-1 (ARG1)
Prostate cancer Human Inhibition of MDSC
suppression of T-cell
function
Interference with
immunosuppression
Phosphodiesterase type 5
(PDE5) Inhibitors[67]
Enzyme inhibition Multiple myeloma
(MM) and NSCLC
Murine &
human
Inhibition of MDSC
proliferation and T-cell
suppression
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