Highlights on MolecularMechanisms of MDSC-Mediated ImmuneSuppression: Paving theWay for New WorkingHypotheses
Samantha Solito,1 Laura Pinton,1 Vera Damuzzo,1 and Susanna
Mandruzzato1,2
1Department of Surgery, Oncology and Gastroenterology, Oncology and ImmunologySection, University of Padova, Padova, Italy2IOV, IRCCS, Padova, Italy
MDSCshave been recognized in the last years as tolerogenic cells, potentially dangerous inthe context of neoplasia, since they are able to induce tolerance to a variety of anti-tumoreffectors, including CD4þ and CD8þ T cells. It is currently believed that the origin ofMDSCs is due to an arrest of the myeloid differentiation process caused by tumor-secretedfactors released in the tumor microenvironment that are able to exert an effect on myeloidprogenitors, rendering them unable to terminally differentiate into dendritic cells, granu-locytes and macrophages. As a consequence, these immature myeloid cells acquire sup-pressive activity through the activation of several mechanisms, controlled by differenttranscription factors. The lack of consensus about the phenotypical characterization ofhuman MDSCs is the result of the existence of different MDSC subsets, most likelydepending on the tumor in which they expand and on the tumor specific cytokine cocktaildriving their activation. This, in turn, might also influence the mechanisms of MDSC-mediated immune suppression. In this review article we address the role of tumor-derivedfactors (TDFs) in MDSC-recruitment and activation, discuss the complex heterogeneity ofMDSC phenotype and analyze the crosstalk between activated T cells and MDSCs.
Authors Solito and Pinton contributed equally to this work.Address correspondence to Susanna Mandruzzato, Department of Surgery, Oncologyand Gastroenterology, Oncology and Immunology Section, University of Padova, ViaGattamelata, 64 35128 Padova, Italy. E-mail: susanna. [email protected]
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
BACKGROUND
Tumor cells are able to orchestrate several tolerogenic responses in order toevade the control of the immune system and to induce proliferation, transforma-tion and invasion of malignant clones. One of these mechanisms involves theexpansion of Myeloid-Derived Suppressor Cells (MDSCs), an immaturemyeloidpopulation that originates frommyeloid precursors present in the bonemarrow,but unable to develop into terminally differentiated subsets, such as macro-phages, granulocytes and dendritic cells, therefore retaining an immature phe-notype and acquiring suppressive functions (Chioda et al., 2011; Gabrilovichand Nagaraj, 2009; Peranzoni et al., 2010).
Concerning the origin of MDSCs, the current hypothesis is that tumor cellsmay induce their expansion by means of different TDFs and the composition ofthis MDSC-inducing cocktail seems to be customized in each patient, dependingon tumor type and individual factors (Lechner et al., 2011); as a consequence,each tumor might preferentially induce a MDSC subset and thus differenttumors might expand phenotypically different MDSCs. In this regard, it hasbeen demonstrated that bothmurine and humanMDSCs consist of granulocyticand monocytic subsets, two distinct populations not only expressing differentsurface markers, but also suppressing immune responses through differentmolecular mechanisms (Filipazzi et al., 2012; Montero et al., 2012; Peranzoniet al., 2010; Youn andGabrilovich, 2010). Of note, it has been demonstrated thatMDSCs have the ability to suppress the immune response of T cells not onlyin vitro, but also in vivo (Dolcetti et al., 2010; Ilkovitch and Lopez, 2009; Li et al.,2009; Marigo et al., 2010; Vincent et al., 2010).
A major difference exists for murine and human MDSC, at least in terms ofphenotypic identification. In fact, murine MDSCs can be identified by the co-expression of the markers Gr-1 and CD11b; however, on the basis of the differ-ential expression of Gr-1, recently it was possible to identify three myeloidsubsets with different stages of maturation and the most immature populationwas endowed with the highest suppressive activity (Dolcetti et al., 2010). Theequivalent of the Gr-1 antigen is not known in humans and this, in turn, hashindered a precise identification of human MDSCs. For this reason, manyauthors used the combination of several myeloid markers to demonstrate anexpansion of immature myeloid cells, endowed with suppressive activity(Filipazzi et al., 2007; Mandruzzato et al., 2009; Rodriguez et al., 2009).
After the phase of recruitment at the tumor site, MDSCs are able to activatea strong inhibitory activity on T cells directed against tumor antigens; in thisregard, an intriguing interaction between activated T cells and MDSCs isemerging, resulting in an exchange not only of soluble factors, but also of surfacesignals through membrane receptors.
In this review, we address the role of TDFs in MDSC-recruitment andactivation, discuss the complex heterogeneity of MDSC phenotype, and showrecent advances in this field.
New Working Hypotheses on MDSC-Immune Suppression 723
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
Tumor-derived Factors: Key Regulators of MDSC RecruitmentThe aforementioned MDSC-inducing cocktail consists of myelopoietic fac-
tors (GM-CSF, G-CSF), angiogenic factors (VEGF) and cytokines (IL-6, IL-1β)(Bunt et al., 2007; Gabrilovich et al., 1998; Song et al., 2005). Interestingly, alsoinflammation appears to be connected to MDSC expansion, since inflammatorymediators as prostaglandins (PGE2), complement factors (C5a), and chemoat-tractants (S100A8, S100A9) have been described as molecules able to augmentMDSC expansion, recruitment and suppressive functions (Cheng et al., 2008;Markiewski et al., 2008; Sinha et al., 2007).
Colony StimulatingFactors (CSFs) are a family of cytokines involved in haema-topoiesis, myeloid compartment development and recruitment (reviewed in (Bronteet al., 2006)). It has been demonstrated that several human tumor lines releasespontaneouslyGM-CSF (Curranet al., 2011;Monti et al., 1993;Rokhlin et al., 1996).This cytokine is produced by themouse tumor cell lines CT26.WT and TS/A of colonadenocarcinoma and mammary tumor origin respectively (Bronte et al., 1999).When these GM-CSF-producing tumors were grown in vivo, they induced anincrease in a CD11bþGr-1þ cell population, which was able to suppress the CD8þTcell response. In order to establish the role of GM-CSF in the generation of thesuppressive population in tumor-bearing mice, the B16 melanoma cell line, whichnormally does not produce GM-CSF, was transduced with mouse GM-CSF cDNA.
Interestingly, only transfected cells were able to induce the suppressive mye-loid subset. More recently, the same group demonstrated that silencing GM-CSFin a mammary carcinoma model determined a reduction in the accumulation ofMDSCs and in the systemic immunosuppression induced by this tumor (Dolcettiet al., 2010).Although the role ofGM-CSFasan inducer of tumor suppression orasan adjuvant in generating an immune response has been a controversial issue formany years (reviewed in (Parmiani et al., 2007)), recent data demonstrate thatGM-CSF is one of the major cytokines that induce and sustain a CD11bþ Gr-1þ
MDSC population in a murine model of breast carcinoma (Morales et al., 2010).GM-CSFalso appears to assumeakey role inMDSCexpansion in cancerpatients:in fact, melanoma patients who received GM-CSF as an adjuvant together with avaccine composed of tumor-derived heat-shock protein peptide complex gp96(HSPPC-96), expanded an MDSC population which suppressed PBMC prolifera-tion in a TGF-β dependent fashion (Filipazzi et al., 2007).
Furthermore, also G-CSF and IL-6 have been recognised as cytokines whichcould be involved in MDSC generation and are produced by either cancer cells ortumor-associated myeloid cells (Marigo et al., 2010). Consistently, Waight andcolleagues reported not only that high G-CSF levels are present in the sera of miceimplanted with AT-3 and 4T1 mammary cancer, but also that the treatment withanti-G-CSF mAb or down-regulation of G-CSF expression by RNA interference ledto a significant reductionof granulocyticCD11bþGr-1þMDSCs (Waight et al., 2011).
IL-6 is a pleiotropic cytokine with both pro-inflammatory and regulatoryfunctions that can be secreted both by transformed cells and tumor-infiltrating
724 S. Solito et al.
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
monocytes; interestingly, higher IL-6 concentration was found in peripheralblood of cancer patients in comparison to healthy controls (Heikkila et al.,2008; Mihara et al., 2012). In addition, Bunt and colleagues showed that IL-6promoted tumor progression by enhancing the accumulation of MDSCs actingas downstream mediator of the IL-1β signaling, an established TDF that trig-gers MDSC generation (Bunt et al., 2007; Song et al., 2005).
We decided to investigate if some of the cytokines present in the tumormicroenvironment can be used to generate MDSCs in vitro, starting frommyeloid precursors. We thus set up a protocol by treating human and murinebone marrow (BM) cells in vitro with GM-CSF, G-CSF and IL-6, alone or incombination, as substitutes of TDFs. Our results demonstrated that the combi-nation of GM-CSF with either G-CSF or IL-6 enriched an immature populationfrom human and murine BM cells, and that after 4 days of culture the resultingmyeloid cell population, named BM-MDSCs, was able to inhibit the prolifera-tion of both mitogen and alloantigen-activated T cells (Marigo et al., 2010).
Recently, Lechner and colleagues performed an extensive analysis of theexpression of 15 cytokines secreted by different human tumor cell lines, to set upcytokines mixture to induce in vitro MDSCs from PBMC of healthy donors(Lechner et al., 2010). Among the factors important for the generation of sup-pressive CD33þ cells, this group identified also GM-CSF; moreover, VEGF andTNFα were recognized as potent partners of GM-CSF to generate MDSCs.
Our studies assigned to GM-CSF a pivotal role in MDSC recruitment at thetumor site and therefore we believe that a deeper insight into GM-CSF signal-ling could disclose new targets for cancer therapy. Another interesting target isthe STAT-3 pathway, since differentMDSC-inducing TDFs, including GM-CSF,take part in STAT-3 signalling (Al-Shami et al., 1998; Stepkowski et al., 2008).Moreover, most of molecular mechanisms underlying aberrant MDSC matura-tion, recruitment and activity are efficiently regulated by transcription factors(reviewed in (Sonda et al., 2011)).
Among them, we decided to focus on CCAAT enhancer binding protein β(C/EBPβ), a key component of the emergency myelopoiesis triggered bystress and inflammation, since G-CSF, GM-CSF and IL-6 can signal throughthis transcriptional factor. We demonstrated that C/EBPβ is necessary for theimmunosuppressive program in both murine and human MDSCs and thattolerogenic activity of both tumor-induced and BM-derived MDSCs wasentirely dependent on this transcription factor (Marigo et al., 2010).
The Immature State of MDSCs: A Broad Spectrum of PhenotypicMarkersAs mentioned before, the treatment of human BM cells with the cytokines
G-CSF and GM-CSF allowed us to generate a population of immature myeloidcells able to suppress both the proliferation and the effector functions of T
New Working Hypotheses on MDSC-Immune Suppression 725
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
lymphocytes. Using the myeloid markers CD16 and CD11b, we analyzed theirmaturation stage and we observed that BM-MDSCs are represented by a het-erogeneous population characterized by different maturation stages rangingfrom the most immature CD11blow/-/CD16� cells, through the intermediatestage of maturation CD11bþ/CD16� and to more differentiated CD11bþ/CD16þ cells (Figure 1). We also observed that the treatment with G-CSF andGM-CSF induced an increase in the percentage of immature CD11bþ/CD16�
cells, in comparison to untreated BM cells (Marigo et al., 2010).When we set out to study whether BM-MDSCs contained subsets endowed
with different suppressive activity, we found that the only population responsi-ble for the immunosuppressive activity was the most immature subset consti-tuted byCD11blow/-/CD16� cells, mainly composed of basophilic cells, resemblingpromyelocytes. These cells had characteristics distinct from both mature mono-cytes and granulocytes because they lacked the expression of lineage markersand of the monocytic marker CD14, while expressing the myeloid marker CD33and the granulocytic antigenCD15. In addition, this cell population had a partialexpression of bothHLA-DR andCD66b and expressed IL4Rα, amarker known tobe associated with immunosuppressive activity (Solito et al., 2011b).
CD16, CD11b and CD66b: Myeloid Markers Identifying DifferentStages of MDSC DevelopmentA population of immature neutrophilic MDSCs was identified in the per-
ipheral blood of patients with urological cancers or head and neck cancer (HNC);this population could be divided into three cell subsets, according to the com-bined expression of CD11b and CD16. Although the whole population had a
Figure 1: Definition of BM-MDSC subsets. Flow cytometric evaluation ofCD11b andCD16markersin BM-MDSCs or sorted CD11blow/-/CD16�, CD11bþ/CD16� and CD11bþ/CD16þ cell populationsfrom BM-MDSCs. Three differentiation stages are distinguishable, based on the gradual increaseof CD11b and CD16 expression. The different fractions were sorted, transferred to slides bycytospin and stained with May-Grunwald-Giemsa (modified from (Solito et al., 2011b)).
726 S. Solito et al.
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
suppressive effect on T cell proliferation and IFN-γ production, the activity ofeach single myeloid population was not investigated, and therefore the tolero-genic activity associated to each subset is not known (Brandau et al., 2011). Thelack or reduced presence of CD16 antigen on MDSCs was confirmed also interminal cancer patients, in which a neutrophilic CD16lowCD15þ MDSCs popu-lation positively correlated with immunosuppressive activity and showed anegative correlation with patients survival (Choi et al., 2012).
MDSCs found in cancer patients are frequently described as CD11bþ cells,while in our model, BM-MDSCs show a low or absent expression of this marker.This apparent discrepancy probably depends on the site where they are enrolledand on the factors influencing the microenvironment in which they are found. AsCD11b is amember of the integrin’s family, required formonocytes andneutrophilsinteraction with the endothelium, we speculate that its expression could be up-regulated in MDSCs circulating in the blood to allowmigration to the tumor site.
A cell population of CD14�CD15þ MDSCs, showing the morphology of PMNand expressing CD11b, was described in the peripheral blood of renal cell carci-noma (RCC) patients. This cell population was responsible for reduced IFN-γproduction by T lymphocytes and only the depletion of CD11bþ cells revertedthe immunosuppressive effect, revealing that this marker is specific for MDSCs(Zea et al., 2005). Further studies of the same research group revealed thatMDCSs present in these patients were a subset of activated granulocytes withhigh CD11b and low CD16 expression (Rodriguez et al., 2009).
In the peripheral blood of NSCLC patients a suppressive myeloid cell popu-lation expressing CD11b, CD33 and CD15 was expanded; this phenotype isconsistent with MDSCs found by Zea and colleagues, while monocytic-MDSCsexpressing CD11b, CD14, CD33 and CD34 were identified in ascites of ovariancancer patients (Obermajer et al., 2011; Srivastava et al., 2008). Another anti-gen whose expression has often been reported inMDSCs is CD66b, a marker forsecondary granules of PMN that we also found partially expressed onCD11blow/-/CD16� BM-MDSCs. The specificity of CD66b as an MDSCs markerwas confirmed by functional assays, since the depletion of CD66bþ fraction fromRCC cancer patients’ PBMCs completely restored not only CD4þ and CD8þ Tcell proliferation, but also IFN-γ production; moreover its presence was detectedboth on MDSCs expanded in HNC cancer patients and on MDSCs derivedin vitro after culturing PBMCs from healthy donors with several tumor celllines (Brandau et al., 2011; Lechner et al., 2010; Rodriguez et al., 2009).
Absence of Lineage Markers as a Distinguishing Trait of MDSCsWe demonstrated that a specific subset isolated from BM-MDSCs, with
morphology and phenotype resembling promyelocytes, is responsible for thewhole immune suppression mediated by BM-MDSCs, and, importantly, thatthis myeloid subset is equivalent to MDSCs present in the blood of breast and
New Working Hypotheses on MDSC-Immune Suppression 727
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
colorectal cancer patients. One of the characteristics of these suppressive cells isthat they lack the markers of mature haematopoietic cells (CD3, CD14, CD16,CD19, CD20 and CD56) and are therefore lineage (Lin)neg (Solito et al., 2011b).Besides, these immature cells, expanded in the peripheral blood of patients withsolid tumors, are also negative for the HLA-DRmarker, but express themyeloidmarkers CD33þ and CD11bþ, and their presence is correlated with clinicalcancer stage, metastatic tumor burden and radiographic response to systemictherapy (Diaz-Montero et al., 2009).
Moreover, Lin�,HLA-DRlow/-, CD33þ, CD11bþMDSCs, circulating in thebloodof colorectal and advanced breast cancer patients, correlated with worse prognosisand radiographic progression (Solito et al., 2011b). A similar phenotype has beendescribed in RCCpatients, where immunosuppressive cells have been identified asLin�,HLA-DR�,CD33þ, butwithan intermediate cell surfaceexpressionofCD11b,thus suggesting again that CD11b expression might be modulated on MDSCs(Kusmartsev et al., 2008). Interestingly, the immunosuppressive cells obtainedafter cytokine treatment of PBMCs from healthy donor were Lin�, HLA-DRlow,CD33þ, a phenotype resembling that of BM-MDSCs (Lechner et al., 2011).
HLA-DR Expression is Low or Negative on MDSCsAs previously reported, the immature CD11b�/CD16� BM-MDSCs were
characterized by the presence of two populations with different expression ofHLA-DR. HLA class II is a marker expressed on a few specialized cell typesincluding dendritic cells andmonocytes, and its expression is used in associationwith other markers to define MDSCs (Filipazzi et al., 2007; Finke et al., 2011;Poschke et al., 2010; Yuan et al., 2011).
The immunosuppressive cells expanded in the peripheral blood of mela-noma patients after treatment with a vaccine containing GM-CSF as adjuvantwere defined as CD14þ, HLA-DRlow/- (Filipazzi et al., 2007). The same pheno-type was found also on MDSCs expanded in bladder cancer patients and asignificant association between the level of these cells and cancer pathologicalgrade and clinical stage was documented (Yuan et al., 2011). Another groupstudying melanoma patients identified suppressive myeloid cells asCD14þHLA-DRlow/-, i.e., a phenotype consistent with that reported byFilipazzi and colleagues, but, they demonstrated also that these cells expressedincreased levels of CD80, CD83 andDC-Sign, markers associated with dendriticcells and macrophages (Filipazzi et al., 2007; Poschke et al., 2010). As stated bythe authors, CD14þHLA-DRlow/- cells are monocyte-like and not only lack one ofthe main traits of MDSC, i.e. immaturity, but also express markers of moredifferentiated or mature cells. However, when myeloid cells are suppressive,many authors define them as MDSC, thus bypassing the requirement of one ofthe characteristics of these cells, i.e. lack or reduced expression of markers ofmature myeloid cells (Gabrilovich et al., 2007). Given the heterogeneity of this
728 S. Solito et al.
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
myeloid population, lack of defined phenotype, and cell culture conditionsrequired for the suppression assay, which in turn may alter the differentiationof myeloid cells, an assessment of MDSCs identity is not easy to reach.Cautiously we should keep in mind that tolerogenic myeloid populations otherthan MDSCs exist and it is not often easy to identify them unambiguously.
In RCC patients MDSCs have been identified both as monocytic expressingCD14 and HLA-DRlow/- and as neutrophilic expressing CD15 and CD33 butlacking HLA-DR, thus highlighting again the different nature of immunosup-pressive cells and the coexistence of different MDSCs subtypes in the samepatient (Finke et al., 2011).
IL4Rα: A Marker Expressed by MDSCsThe α chain of IL4 receptor (IL4Rα or CD124) is one of the chains of the
receptor for IL-4 and IL-13, cytokines involved in MDSC activation, and it is alsoamarker of tumor-induced circulatingMDSCs in colon andmammary carcinoma,lymphoma and fibrosarcomamouse models (Gallina et al., 2006). Experiments ofIL4Rα genetic ablation showed that the expression of thismarker is necessary formouse MDSC immunosuppressive activity (Gallina et al., 2006).
These data were further confirmed by a study on a colon carcinoma mousemodel, in which IL4Rα expression on MDSCs was significantly decreased aftertreatment with sildenafil, a phosphodiesterase-5 inhibitor (Serafini et al., 2006).Moreover, the same group extended their studies to a murine lymphoma modeland found that IL4Rα expression onMDSCs significantly correlated with tumorprogression and could be inhibited by sildenafil, although this treatment did notreduce significantly tumor outgrowth (Serafini et al., 2008).
Our group demonstrated that MDSCs are present both in mononuclear andpolymorphonuclear fractions of circulating blood leukocytes of patients withcolon cancer and melanoma, and that IL4Rα is up-regulated in both myeloidpopulations although its presence positively correlated only with the immuno-suppressive activity of mononuclear cells (Mandruzzato et al., 2009). The pre-sence of IL4Rα was further confirmed by a study on melanoma patients, inwhichMDSCswere defined as CD14þHLA-DRlow /- cells, as previously discussed(Poschke et al., 2010). Although it is known that IL4Rα engagement triggers theexpression of TGFβ, Arg1, and iNOS, key molecules potentially involved inMDSC activity, the functional relevance of IL4Rα is still an open area of inves-tigation and needs to be further explored (Gabrilovich and Nagaraj, 2009).
The Interplay Between Activated T Cells and MDSCsFrom the previous considerations it appears evident that it is difficult at the
moment to define unambiguously human MDSCs with a defined set of markersto evaluate their presence in cancer patients. In fact, MDSC phenotypemight be
New Working Hypotheses on MDSC-Immune Suppression 729
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
deeply influenced by the cytokines and growth factors present; as a conse-quence, not only the characteristics of MDSC’s phenotype might be tumor-dependent, but also the mechanisms of MDSC-immune suppression mightreflect the microenvironment present in different tumors.
It is well known thatMDSCs are able to suppress T-cell responses through avariety of mechanisms, such as arginase and iNOS activation, or induction ofother tolerogenic cell populations, such as the Tregs. However, many aspects ofMDSC’s biology still need to be clarified and in this respect we believe that thesuppression exerted byMDSCsmight be seen as the result of a complex networkof interactions between activated T cells and myeloid cells recruited to thetumor microenvironment; in this context, it should be important to dissect notonly the mechanisms by which MDSCs directly exert their inhibitory functionon T cells, but also the reciprocal signals orchestrated during the interactionsbetween T cells and MDSCs.
We recently observed that BM-MDSCs are able to proliferate, a requirementthat appears important also in tumor-bearing mouse, since 5-Fluorouracil, ananti-proliferative drug, is able to deplete both granulocytic and monocytic sup-pressive subsets of MDSCs, by abrogating its suppressive function (Solito et al.,2011b; Ugel et al., 2009; Vincent et al., 2010). BM-MDSC proliferative rateincreases over time in the presence of activated T cells, suggesting that theactivation of T cells is a key factor driving MDSC-immune suppression; more-over, MDSCs differentiate toward more mature myeloid cells only in the pre-sence of resting T cells, whereas the contact with activated T lymphocytesinduces a block in the MDSC-maturation process, leading to a decrease inCD11b, CD16, and CD66b expression and an increase in the expression ofHLA-DR (Solito et al., 2011b). Once again the lack of mature markers appearsto be critical to define the inhibitory activity of MDSCs.
An intriguing aspect of MDSCs concerns the antigen specificity of immunesuppression exerted by these cells; several studies on different mouse models ortumor patients revealed that in some cases MDSC-immune suppressionappears as an antigen-dependent mechanism inducing CD8þ and CD4þ T celltolerance, while in other cases the inhibitory function might not require thepresence of antigen specific T cells (reviewed in (Solito et al., 2011a)).
An emerging hypothesis on the relationship between activated T cells andMDSCs stemmed from recent work in which it was demonstrated that granulo-cytic MDSCs express the death receptor Fas. Interestingly, the interactionbetween activated T cells, expressing FasL, and Fasþ MDSCs induce apoptosisof the suppressive cells thus contributing to the homeostatic regulation ofMDSCs (Sinha et al., 2011).
Another interesting observation focusing on the interaction betweenMDSCs and T lymphocytes was recently documented. In this work it wasdemonstrated that antigen-specific CD4þ T cells could switch granulocyticMDSC to non-specific suppressor cells in vitro and in vivo. Of note, this effect was
730 S. Solito et al.
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
mediated by direct cell to cell contact and depended on the expression and cross-linking of MHC class II on MDSC surface (Nagaraj et al., 2012). The same grouppreviously identified a new mechanism of immune-suppression exerted byMDSCs, represented by the alteration of TCR signaling; specifically it was shownthat reactive nitrogen species (RNS) produced by MDSCs, are able to nitratetyrosine residues in the TCR and CD8 receptors, thus resulting in a decreasedrecognition of peptide-MHC complexes by the TCR (Nagaraj et al., 2007).
More recently, the importance of RNS in the tumor context was highlightedby another study in which it was demonstrated that CCL2, an inflammatorychemokine involved in the recruitment of both CTLs and myeloid cells totumors, can be modified in the tumor microenvironment by RNS. Such altera-tion is a stable posttranslational modification, which changes CCL2 functionalproperties resulting in an impaired capacity of T cells to bind the modifiedchemokine. As a result, nitrated CCL2 loses its ability to recruit tumor-specificCTL, while retaining its ability to attract myeloid cells to the tumor.
In vivo administration of AT38, a small molecule blocking the generation ofperoxynitrites, to tumor-bearing mice controls RNS generation and induces amassive T cell infiltration within the tumor microenvironment and enhancesprotocols of adoptive immunotherapy with tumor-specific CTLs (Molon et al.,2011). This result supports the concept that restoring the altered microenviron-ment may be an important step in the success of immunotherapy.
The down-regulation of CD3ζ chain is another alteration frequently asso-ciated with an impairment of TCR signal both in tumor and chronic inflamma-tion models (Ezernitchi et al., 2006; Otsuji et al., 1996; Schule et al., 2002). Wedemonstrated that BM-MDSCs are able to decrease the expression of CD3ζchain of activated T cells (Solito et al., 2011b). The down modulation of thisprotein was also observed in T lymphocytes in the peripheral blood of RCCpatients, who had increased arginase activity present in a specific subset ofgranulocytic MDSCs and the depletion of these suppressive cells restored CD3ζchain expression (Zea et al., 2005).
The CD3ζ chainwas significantly reduced also in TILs and lymphatic organsin ret transgenicmice that spontaneously developmelanoma and distantmetas-tases; moreover, the decrease in expression in the ζ chain correlated signifi-cantly with tumor progression (Meyer et al., 2011). Analogously, the reductionin CD3ζ chain expression on T cells of patients with uveal melanoma wascorrelated significantly with the percentage of granulocytic CD11bþ cells inthe PBMCs (McKenna et al., 2009). We observed that the reduction of CD3ζchain expression on T cells induced by BM-MDSCs was also accompanied by adecrease in the surface expression of CD3ε chain thus unveiling an increasedlevel of T-cell dysfunction. The reduction of CD3ε chain expression was alsoinduced by tolerogenic dendritic cells and it was evidenced before the onset ofT-cell apoptosis (Kuang et al., 2008; Solito et al., 2011b); however, in this casethe expression of CD3ζ chain was not affected.
New Working Hypotheses on MDSC-Immune Suppression 731
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
In lung adenocarcinoma patients, significant decreases of expression CD3ε,but not CD3ζ, was detected in T cells obtained from pleural effusion as comparedto peripheral blood of healthy donors.
Taken together these results indicate that CD3ε chain down-regulationcould also be used as a marker of T cell dysfunction induced by tolerogeniccells. All together, several advances have been made in recent years in thefield of MDSCs-mediated immune suppression and new working hypothesisare emerging (Table 1). Although some of the data demonstrate conflictingevidences from different laboratories, only a better knowledge of the biology ofthese cells and of the environment properties in which they work will help tosolve the complexity of MDSCs. This, in turn, will open new perspectives todevelop innovative approaches for cancer immunotherapy.
ACKNOWLEDGMENTS
Thisworkwas supported by a grant from the ItalianMinistry of Health. S.S. andV.D. are recipients of an AIRC fellowship.
Declaration of interest: The authors report no conflicts of interest. Theauthors alone are responsible for the content and writing of the paper.
Table 1: Mechanisms of MDSC Suppression.
ESTABLISHED MECHANISMS OF MDSC SUPPRESSION
Depletion of L-arginine in the tumormilieu byiNOS and Arg-1
Reviewed in (Bronte and Zanovello,2005); (Rodriguez andOchoa, 2008)
Production of peroxynitrite leading tonitration and nitrosilation of TCR and CD8molecules
(Nagaraj et al., 2007)
Down-regulation of CD3ζ and/or CD3"molecules associates with reduced TCRsignaling
(Kuang et al., 2008; McKenna et al.,2009; Solito et al., 2011b; Zea et al.,2005)
Induction of T regulatory subsets (Huang et al., 2006; Serafini et al.,2008)
Depletion of cysteine leading to impairmentin the metabolism of amino-acids
(Srivastava et al., 2010)
NEW TARGETS OF INVESTIGATION ON MDSCReverse signalling of MDSC-T cell interaction (Nagaraj et al., 2012)Influence of MDSC on T lymphocytestrafficking
(Hanson et al., 2009; Molon et al.,2011)
Antigen specificity of immune suppressionby MDSCs
Reviewed in (Solito et al., 2011a)
Transcription factors involved in MDSCrecruitment, activation and immunesuppression
Reviewed in (Sonda et al., 2011)
Fas/FasL dependent apoptosis of MDSCs (Sinha et al., 2011)
732 S. Solito et al.
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
REFERENCES
Al-Shami, A., Mahanna,W., andNaccache, P.H. (1998). Granulocyte-macrophage colony-stimulating factor-activated signaling pathways in human neutrophils. Selectiveactivation of Jak2, Stat3, and Stat5b. J. Biol. Chem. 273:1058–1063.
Brandau, S., Trellakis, S., Bruderek, K., Schmaltz, D., Steller, G., Elian, M., Suttmann,H., Schenck, M., Welling, J., Zabel, P., and Lang, S. (2011). Myeloid-derived suppres-sor cells in the peripheral blood of cancer patients contain a subset of immatureneutrophils with impaired migratory properties. J. Leukoc. Biol. 89:311–317.
Bronte, V., Chappell, D.B., Apolloni, E., Cabrelle, A., Wang, M., Hwu, P., Restifo, N.P.(1999). Unopposed production of granulocyte-macrophage colony-stimulating factorby tumors inhibits CD8þ T cell responses by dysregulating antigen-presenting cellmaturation. J. Immunol. 162:5728–5737.
Bronte, V., Cingarlini, S., Marigo, I., De Santo, C., Gallina, G., Dolcetti, L., Ugel, S.,Peranzoni, E., Mandruzzato, S., and Zanovello, P. (2006). Leukocyte infiltration incancer creates an unfavorable environment for antitumor immune responses: anovel target for therapeutic intervention. Immunol. Invest. 35:327–357.
Bronte, V., and Zanovello, P. (2005). Regulation of immune responses by L-argininemetabolism. Nat. Rev. Immunol. 5:641–654.
Bunt, S.K., Yang, L., Sinha, P., Clements, V.K., Leips, J., and Ostrand-Rosenberg, S.(2007). Reduced inflammation in the tumor microenvironment delays the accumula-tion of myeloid-derived suppressor cells and limits tumor progression. Cancer Res.67:10019–10026.
Cheng, P., Corzo, C.A., Luetteke, N., Yu, B., Nagaraj, S., Bui, M.M., Ortiz, M., Nacken,W., Sorg, C., Vogl, T., Roth, J., and Gabrilovich, D.I. (2008). Inhibition of dendriticcell differentiation and accumulation of myeloid-derived suppressor cells in cancer isregulated by S100A9 protein. J. Exp. Med. 205:2235–2249.
Chioda, M., Peranzoni, E., Desantis, G., Papalini, F., Falisi, E., Solito, S., Mandruzzato,S., and Bronte, V. (2011). Myeloid cell diversification and complexity: an old conceptwith new turns in oncology. Cancer Metastasis Rev. 30:27–43.
Choi, J., Suh, B., Ahn, Y.O., Kim, T.M., Lee, J.O., Lee, S.H., and Heo, D.S. (2012). CD15(þ)/CD16 (low) human granulocytes from terminal cancer patients: granulocytic myeloid-derived suppressor cells that have suppressive function. Tumour Biol. 33:121–129.
Curran, C.S., Evans, M.D., and Bertics, P.J. (2011). GM-CSF production by glioblastomacells has a functional role in eosinophil survival, activation, and growth factorproduction for enhanced tumor cell proliferation. J. Immunol. 187:1254–1263.
Diaz-Montero, C.M., Salem, M.L., Nishimura, M.I., Garrett-Mayer, E., Cole, D.J., andMontero, A.J. (2009). Increased circulating myeloid-derived suppressor cells corre-late with clinical cancer stage, metastatic tumor burden, and doxorubicin-cyclophosphamide chemotherapy. Cancer Immunol. Immunother. 58:49–59.
Dolcetti, L., Peranzoni, E., Ugel, S., Marigo, I., Fernandez Gomez, A., Mesa, C., Geilich,M., Winkels, G., Traggiai, E., Casati, A., Grassi, F., and Bronte, V. (2010). Hierarchyof immunosuppressive strength among myeloid-derived suppressor cell subsets isdetermined by GM-CSF. Eur. J. Immunol. 40:22–35.
Ezernitchi, A.V., Vaknin, I., Cohen-Daniel, L., Levy, O., Manaster, E., Halabi, A.,Pikarsky, E, Shapira, L., and Baniyash, M. (2006). TCR zeta down-regulationunder chronic inflammation is mediated by myeloid suppressor cells differentiallydistributed between various lymphatic organs. J. Immunol. 177:4763–4772.
New Working Hypotheses on MDSC-Immune Suppression 733
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
Filipazzi, P., Huber, V., and Rivoltini, L. (2012). Phenotype, function and clinical implica-tions of myeloid-derived suppressor cells in cancer patients. Cancer Immunol.Immunother. 61:255–263.
Filipazzi, P., Valenti, R., Huber, V., Pilla, L., Canese, P., Iero,M., Castelli, C., Mariani, L.,Parmiani, G., and Rivoltini, L. (2007). Identification of a new subset of myeloidsuppressor cells in peripheral blood of melanoma patients with modulation by agranulocyte-macrophage colony-stimulation factor-based antitumor vaccine. J. Clin.Oncol. 25:2546–2553.
Finke, J., Ko, J., Rini, B., Rayman, P., Ireland, J., and Cohen, P. (2011). MDSC as amechanism of tumor escape from sunitinib mediated anti-angiogenic therapy. Int.Immunopharmacol. 11:856–861.
Gabrilovich, D., Ishida, T., Oyama, T., Ran, S., Kravtsov, V., Nadaf, S., and Carbone, D.P.(1998). Vascular endothelial growth factor inhibits the development of dendritic cellsand dramatically affects the differentiation of multiple hematopoietic lineagesin vivo. Blood 92:4150–4166.
Gabrilovich, D.I., Bronte, V., Chen, S.H., Colombo, M.P., Ochoa, A., Ostrand-Rosenberg,S., and Schreiber, H. (2007). The terminology issue for myeloid-derived suppressorcells. Cancer Res 67:425; author reply 426.
Gabrilovich, D.I., and Nagaraj, S. (2009). Myeloid-derived suppressor cells as regulatorsof the immune system. Nat. Rev. Immunol. 9:162–174.
Gallina, G., Dolcetti, L., Serafini, P., De Santo, C., Marigo, I., Colombo, M.P., Basso, G.,Brombacher, F., Borrello, I., Zanovello, P., Bicciato, S., andBronte, V. (2006). Tumorsinduce a subset of inflammatory monocytes with immunosuppressive activity onCD8þ T cells. J. Clin. Invest. 116:2777–2790.
Hanson, E.M., Clements, V.K., Sinha, P., Ilkovitch, D., andOstrand-Rosenberg, S. (2009).Myeloid-derived suppressor cells down-regulate L-selectin expression on CD4þ andCD8þ T cells. J. Immunol. 183:937–944.
Heikkila, K., Ebrahim, S., and Lawlor, D.A. (2008). Systematic review of the associationbetween circulating interleukin-6 (IL-6) and cancer. Eur. J. Cancer 44:937–945.
Huang, B., Pan, P.Y., Li, Q., Sato, A.I., Levy, D.E, Bromberg, J., Divino, C.M., and Chen,S.H. (2006). Gr-1þCD115þ immature myeloid suppressor cells mediate the devel-opment of tumor-induced T regulatory cells and T-cell anergy in tumor-bearing host.Cancer Res. 66:1123–1131.
Ilkovitch, D., and Lopez, D.M. (2009). The liver is a site for tumor-induced myeloid-derived suppressor cell accumulation and immunosuppression. Cancer Res.69:5514–5521.
Kuang, D.M., Zhao, Q., Xu, J., Yun, J.P., Wu, C., and Zheng, L. (2008). Tumor-educatedtolerogenic dendritic cells induce CD3epsilon down-regulation and apoptosis of Tcells through oxygen-dependent pathways. J. Immunol. 181:3089–3098.
Kusmartsev, S., Su, Z., Heiser, A., Dannull, J., Eruslanov, E., Kubler, H., Yancey, D.,Dahm, P., and Vieweg, J. (2008). Reversal of myeloid cell-mediated immunosuppres-sion in patients withmetastatic renal cell carcinoma.Clin. Cancer Res. 14:8270–8278.
Lechner, M.G., Liebertz, D.J., and Epstein, A.L. (2010). Characterization of cytokine-induced myeloid-derived suppressor cells from normal human peripheral bloodmononuclear cells. J. Immunol. 185:2273–2284.
Lechner, M.G., Megiel, C., Russell, S.M., Bingham, B., Arger, N., Woo, T., and Epstein,A.L. (2011). Functional characterization of human Cd33þ and Cd11bþ myeloid-derived suppressor cell subsets induced from peripheral blood mononuclear cellsco-cultured with a diverse set of human tumor cell lines. J. Transl. Med. 9:90.
734 S. Solito et al.
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
Li, H., Han, Y., Guo, Q., Zhang,M., and Cao, X. (2009). Cancer-expandedmyeloid-derivedsuppressor cells induce anergy of NK cells through membrane-bound TGF-beta 1.J. Immunol. 182:240–249.
Mandruzzato, S., Solito, S., Falisi, E., Francescato, S., Chiarion-Sileni, V., Mocellin, S.,Zanon, A., Rossi, C.R., Nitti, D., Bronte, V., and Zanovello, P. (2009). IL4Ralphaþmyeloid-derived suppressor cell expansion in cancer patients. J. Immunol.182:6562–6568.
Marigo, I., Bosio, E., Solito, S., Mesa, C., Fernandez, A., Dolcetti, L., Ugel, S., Sonda, N.,Bicciato, S., Falisi, E., Calabrese, F., Basso, G., Zanovello, P., Cozzi, E.,Mandruzzato, S. and Bronte, V. (2010). Tumor-induced tolerance and immune sup-pression depend on the C/EBPbeta transcription factor. Immunity 32:790–802.
Markiewski, M.M., DeAngelis, R.A., Benencia, F., Ricklin-Lichtsteiner, S.K., Koutoulaki,A., Gerard, C., Coukos, G., and Lambris, J.D. (2008/0. Modulation of the antitumorimmune response by complement. Nat Immunol 9:1225–1235.
McKenna, K.C., Beatty, K.M., Bilonick, R.A., Schoenfield, L., Lathrop, K.L., and Singh,A.D. (2009). Activated CD11bþCD15þ granulocytes increase in the blood of patientswith uveal melanoma. Invest. Ophthalmol. Vis. Sci. 50:4295–4303.
Meyer, C., Sevko, A., Ramacher, M., Bazhin, A.V., Falk, C.S.; Osen, W., Borrello, I., Kato,M., Schadendorf, D., Baniyash, M., and Umansky, V. (2011). Chronic inflammationpromotes myeloid-derived suppressor cell activation blocking antitumor immunityin transgenic mouse melanoma model. Proc Natl Acad Sci USA 108:17111–17116.
Mihara, M., Hashizume, M., Yoshida, H., Suzuki, M., and Shiina, M. (2012). IL-6/IL-6receptor system and its role in physiological and pathological conditions. Clin. Sci.(Lond.) 122:143–159.
Molon, B., Ugel, S., Del Pozzo, F., Soldani, C., Zilio, S., Avella, D., De Palma, A., Mauri, P.,Monegal, A., Rescigno,M., Savino, B., Colombo, P., Jonjic, N., Pecanic, S., Lazzarato, L.,Fruttero, R., Gasco, A., Bronte, V., and Viola, A. (2011). Chemokine nitrationprevents intratumoral infiltration of antigen-specific T cells. J. Exp. Med.208:1949–1962.
Montero, A.J., Diaz-Montero, C.M., Kyriakopoulos, C.E., Bronte, V., andMandruzzato, S.(2012). Myeloid-derived suppressor cells in cancer patients: A clinical perspective.J. Immunother. 35:107–115.
Monti, F., Szymczuk, S., Motta, M.R., Benini, C., Fattori, P.P., Pini, E., Pasquini, E., Zoli,W., Amadori, D., and Ravaioli, A. (1993). GM-CSF production in human adenocarci-noma cell lines. J. Biol. Regul. Homeost. Agents 7:85–91.
Morales, J.K., Kmieciak, M., Knutson, K.L., Bear, H.D., and Manjili. M.H. (2010).GM-CSF is one of the main breast tumor-derived soluble factors involved in thedifferentiation of CD11b-Gr1- bone marrow progenitor cells into myeloid-derivedsuppressor cells. Breast Cancer Res. Treat. 123:39–49.
Nagaraj, S., Gupta, K.,Pisarev, V., Kinarsky, L., Sherman, S., Kang, L., Herber, D.L.,Schneck, J., Gabrilovich, D.I. (2007). Altered recognition of antigen is amechanism ofCD8þ T cell tolerance in cancer. Nat. Med. 13:828–835.
Nagaraj, S., Nelson, A., Youn, J.I., Cheng, P., Quiceno, D., and Gabrilovich, D.I. (2012).Antigen-specific CD4þ T cells regulate function of myeloid-derived suppressor cellsin cancer via retrograde MHC class II signaling. Cancer Res
Obermajer, N., Muthuswamy, R., Odunsi, K., Edwards, R.P., and Kalinski, P. (2011).PGE2-Induced CXCL12 Production and CXCR4 expression controls the accumu-lation of human MDSCs in ovarian cancer environment. Cancer Res. 71:7463–7470.
New Working Hypotheses on MDSC-Immune Suppression 735
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
Otsuji, M., Kimura, Y., Aoe, T., Okamoto, Y., and Saito, T. (1996). Oxidative stress bytumor-derived macrophages suppresses the expression of CD3 zeta chain of T-cellreceptor complex and antigen-specific T-cell responses. Proc. Natl. Acad. Sci. USA93:13119–13124.
Parmiani, G., Castelli, C., Pilla, L., Santinami, M., Colombo, M.P., and Rivoltini, L.(2007). Opposite immune functions of GM-CSF administered as vaccine adjuvantin cancer patients. Ann. Oncol. 18:226–232.
Peranzoni, E., Zilio, S., Marigo, I., Dolcetti, L., Zanovello, P., Mandruzzato, S., andBronte, V. (2010). Myeloid-derived suppressor cell heterogeneity and subset defini-tion. Curr. Opin. Immunol. 22:238–244.
Poschke, I., D. Mougiakakos, J. Hansson, G.V. Masucci, and R. Kiessling. 2010.Immature immunosuppressive CD14þHLA-DR-/low cells in melanoma patientsare Stat3hi and overexpress CD80, CD83, and DC-sign. Cancer Res 70:4335–4345.
Rodriguez, P.C., Ernstoff, M.S., Hernandez, C., Atkins, M., Zabaleta, J., Sierra, R., andOchoa, A.C. 2009. Arginase I-producing myeloid-derived suppressor cells in renal cellcarcinoma are a subpopulation of activated granulocytes. Cancer Res. 69:1553–1560.
Rodriguez, P.C., and Ochoa, A.C. (2008). Arginine regulation by myeloid derived sup-pressor cells and tolerance in cancer: mechanisms and therapeutic perspectives.Immunol. Rev. 222:180–191.
Rokhlin, O.W., Griebling, T.L., Karassina, N.V., Raines, M.A., and Cohen, M.B. (1996).Human prostate carcinoma cell lines secrete GM-CSF and express GM-CSF-receptoron their cell surface. Anticancer Res. 16:557–563.
Schule, J., Bergkvist, L., Hakansson, L., Gustafsson, B., and Hakansson, A. (2002).Down-regulation of the CD3-zeta chain in sentinel node biopsies from breast cancerpatients. Breast Cancer Res. Treat. 74:33–40.
Serafini, P., Meckel, K., Kelso,M., Noonan, K., Califano, J., Koch,W., Dolcetti, L., Bronte,V., and Borrello, I. (2006). Phosphodiesterase-5 inhibition augments endogenousantitumor immunity by reducing myeloid-derived suppressor cell function. J. Exp.Med. 203:2691–2702.
Serafini, P., Mgebroff, S., Noonan, K., and Borrello, I. (2008). Myeloid-derived suppressorcells promote cross-tolerance in B-cell lymphoma by expanding regulatory T cells.Cancer Res. 68:5439–5449.
Sinha, P., Chornoguz, O., Clements, V.K., Artemenko, K.A., Zubarev, R.A., and Ostrand-Rosenberg, S. (2011). Myeloid-derived suppressor cells express the death receptorFas and apoptose in response to T cell-expressed FasL. Blood 117:5381–5390.
Sinha, P., Clements, V.K., Fulton, A.M., and Ostrand-Rosenberg, S. (2007).Prostaglandin E2 promotes tumor progression by inducingmyeloid-derived suppres-sor cells. Cancer Res. 67:4507–4513.
Solito, S., Bronte, V., and Mandruzzato, S. (2011a). Antigen specificity of immune sup-pression by myeloid-derived suppressor cells. J. Leukoc. Biol. 90:31–36.
Solito, S., Falisi, E., Diaz-Montero, C.M., Doni, A., Pinton, L., Rosato, A., Francescato, S.,Basso, G., Zanovello, P., Onicescu, G., Garrett-Mayer, E., Montero, A.J., Bronte, V.,and Mandruzzato, S. (2011b). A human promyelocytic-like population is responsiblefor the immune suppression mediated by myeloid-derived suppressor cells. Blood118:2254–2265.
Sonda, N., Chioda, M., Zilio, S., Simonato, F., and Bronte, V. (2011). Transcription factorsin myeloid-derived suppressor cell recruitment and function. Curr. Opin. Immunol.23:279–285.
736 S. Solito et al.
Imm
unol
Inv
est D
ownl
oade
d fr
om in
form
ahea
lthca
re.c
om b
y U
nive
rsity
of
Suss
ex L
ibra
ry o
n 03
/11/
13Fo
r pe
rson
al u
se o
nly.
Song, X., Krelin, Y., Dvorkin, T., Bjorkdahl, O., Segal, S., Dinarello, C.A., Voronov, E., andApte, R.N. (2005). CD11bþ/Gr-1þ immature myeloid cells mediate suppression of Tcells in mice bearing tumors of IL-1beta-secreting cells. J. Immunol. 175:8200–8208.
Srivastava, M.K., Bosch, J.J., Thompson, J.A., Ksander, B.R., Edelman, M.J., andOstrand-Rosenberg, S. (2008). Lung cancer patients’ CD4(þ) T cells are activatedin vitro by MHC II cell-based vaccines despite the presence of myeloid-derivedsuppressor cells. Cancer Immunol. Immunother. 57:1493–1504.
Srivastava, M.K., Sinha, P.. Clements, V.K., Rodriguez, P., and Ostrand-Rosenberg, S.(2010).Myeloid-derived suppressor cells inhibit T-cell activation by depleting cystineand cysteine. Cancer Res 70:68–77.
Stepkowski, S.M., Chen, W., Ross, J.A., Nagy, Z.S., and Kirken, R.A. (2008). STAT3: animportant regulator of multiple cytokine functions. Transplantation 85:1372–1377.
Vincent, J., Mignot, G., Chalmin, F., Ladoire, S., Bruchard, M., Chevriaux, A., Martin, F.,Apetoh, L., Rebe, C., and Ghiringhelli, F. (2010). 5-Fluorouracil selectively killstumor-associated myeloid-derived suppressor cells resulting in enhanced T cell-dependent antitumor immunity. Cancer Res. 70:3052–3061.
Waight, J.D., Hu, Q., Miller, A., Liu, S., and Abrams, S.I. (2011). Tumor-derived G-CSFfacilitates neoplastic growth through a granulocytic myeloid-derived suppressor cell-dependent mechanism. PLoS One 6:e27690.
Youn, J.I., and Gabrilovich, D.L. (2010). The biology of myeloid-derived suppressor cells:the blessing and the curse of morphological and functional heterogeneity. Eur.J. Immunol. 40:2969–2975.
Yuan, X.K., X.K. Zhao, Y.C. Xia, X. Zhu, and P. Xiao. 2011. Increased circulating immu-nosuppressive CD14(þ)HLA-DR(-/low) cells correlate with clinical cancer stage andpathological grade in patients with bladder carcinoma. J. Int. Med. Res. 39:1381–1391.
Zea, A.H., Rodriguez, P.C., Atkins, M.B., Hernandez, C., Signoretti, S., Zabaleta, J.,McDermott, D., Quiceno, D., Youmans, A., O’Neill, A., Mier, J., and Ochoa, A.C.(2005). Arginase-producing myeloid suppressor cells in renal cell carcinomapatients: a mechanism of tumor evasion. Cancer Res 65:3044–3048.
New Working Hypotheses on MDSC-Immune Suppression 737
