of October 7, 2015.This information is current as

PathogenesisTumor-Infiltrating Dendritic Cells in Cancer

Karyampudi and Keith L. KnutsonJo Marie Tran Janco, Purushottam Lamichhane, Lavakumar

http://www.jimmunol.org/content/194/7/2985doi: 10.4049/jimmunol.1403134

2015; 194:2985-2991; ;J Immunol 

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Tumor-Infiltrating Dendritic Cells in Cancer PathogenesisJo Marie Tran Janco,* Purushottam Lamichhane,†,‡ Lavakumar Karyampudi,‡ andKeith L. Knutson†,‡

Dendritic cells (DCs) play a pivotal role in the tumor mi-croenvironment, which is known to affect disease progres-sion inmany humanmalignancies. Infiltration by mature,active DCs into the tumors confers an increase in im-mune activation and recruitment of disease-fightingimmune effector cells and pathways. DCs are the pref-erential target of infiltrating T cells. However, tumorcells have means of suppressing DC function or of al-tering the tumor microenvironment in such a way thatimmune-suppressive DCs are recruited. Advances inunderstanding these changes have led to promisingdevelopments in cancer-therapeutic strategies target-ing tumor-infiltrating DCs to subdue their immuno-suppressive functions and enhance their immune-stimulatory capacity. The Journal of Immunology,2015, 194: 2985–2991.

In recent years, it has become increasingly appreciated thatthe immune system’s role in modulating malignancy isfar more complex than anticipated. A number of studies

found correlations between the presence of infiltrating im-mune cells in the tumor microenvironment (TME) andprognosis of many cancers, such as ovarian, renal cell, colo-rectal, and breast cancers (1). The immune component of theTME consists of predominantly CD4+ and CD8+ T cells,dendritic cells (DCs), macrophages, and regulatory T cells(Tregs) (2). In general, T cell infiltration portends a betteroutcome (1, 3–5). One key example, published by Zhang et al(1), found that tumor-infiltrating T cells were observed in55% of tumors obtained from advanced ovarian cancerpatients. The 5-y overall survival rate for patients whosetumors contained tumor-infiltrating T cells was 38% incomparison with a 4.5% rate of survival for those whosetumors did not. In contrast, many other studies over the pastdecade demonstrated that other subsets of adaptive immunecells are typically, but not always, associated with worse prog-nosis, seeming to promote tumorigenesis (6–9). For example,Tregs (CD4+/CD25+FOXP3+) in ovarian cancer confera significantly higher risk for death, even when controlledfor stage and degree of surgical reduction of disease (8). In

addition to immune-suppressive T cells, tumors by them-selves are adept at preventing the destructive capabilities ofinfiltrating antitumor immune effector cells. Tumors pro-mote apoptosis and paralyze antitumor effector cells throughthe release of immune-suppressive factors like NO, IL-10,IL-6, arginase-I, vascular endothelial growth factor (VEGF),IDO, and TGF-b (10–14).Also, suppressive cells of the innate arm of the immune

system, such as inflammation-induced myeloid-derived sup-pressor cells and tumor-associated macrophages, are known tobe correlated with poor outcome and rapid disease progression(15–22). Although the negative roles of these innate immune-suppressive cells in the TME are widely demonstrated, therole of others, such as DCs, has been subject to debate becauseof conflicting observations (23–26). DCs have an integral rolein influencing the immune response and are the subset of cellsin the TME to which antitumor T cells are attracted; how-ever, they may alter their role from being immunostimulatoryto immunosuppressive at different stages of cancer progres-sion (27). The focus of this article is on tumor-infiltratingDCs (TIDCs). We discuss their interaction with the pro-gression or suppression of malignancy and highlight the newdirections for the therapeutic manipulation of such immu-nosuppressive DCs to tip the balance in favor of antitumorimmunity.

Dendritic cells

Described in the early nineteenth century by Paul Langerhansand termed “dendritic cells” in 1973 by Ralph M. Steinmanand Zanvil A. Cohn, DCs are key decision makers, deter-mining whether the adaptive arm of the immune systemshould or should not be activated. Crucial as professionalAPCs, they present Ags and provide a multitude of othernecessary signals (costimulatory molecules and cytokines) forT cell activation and differentiation, thereby shaping theimmune response. DCs also interact with other immune cells,including NK cells and B cells (28, 29). Many subsets of DCswith unique and specific functions, morphology, and locali-zation have been described (30). These include Langerhanscells, monocyte-derived DCs (CD14+ DCs), myeloid DCs,and plasmacytoid DCs (pDCs). Furthermore, each of these

*Division of Gynecologic Surgery, Mayo Clinic, Rochester, MN 55906; †Department ofImmunology, Mayo Clinic, Rochester, MN 55906; and ‡Cancer Vaccines and ImmuneTherapies Program, Vaccine and Gene Therapy Institute, Port St. Lucie, FL 34987

Received for publication December 16, 2014. Accepted for publication February 3,2015.

This work was supported by the Minnesota Ovarian Cancer Alliance, the Fred C. andKatherine B. Andersen Foundation, the Marsha Rivkin Foundation, and the MayoClinic Ovarian Cancer Specialized Program of Research Excellence Award (P50-CA136393, National Institutes of Health/National Cancer Institute).

Address correspondence and reprint requests to Dr. Keith L. Knutson at the currentaddress: Mayo Clinic, 4500 San Pablo Road, Jacksonville, FL 32224. E-mail address:knutson.keith@mayo.edu

Abbreviations used in this article: CDP, common DC progenitor; DC, dendritic cell;pDC, plasmacytoid DC; PD-L1, programmed death 1 ligand; TIDC, tumor-infiltratingDC; TIM-3, T cell Ig and mucin domain 3; TME, tumor microenvironment; Treg,regulatory T cell; VEGF, vascular endothelial growth factor.

Copyright� 2015 by TheAmerican Association of Immunologists, Inc. 0022-1767/15/$25.00

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has different maturation states that add to the complexity.Identification of DCs, because of their heterogeneity, can bechallenging and variable between studies, with some usingsingle surface cell markers and others using multiple surfacecell markers. At least three subsets of splenic murine DCs havebeen described: CD11chighCD8a+CD11b2DEC205+ lym-phoid DCs, CD11chighCD8a2CD11b+DEC205+ myeloidDCs, and CD11cintermediateCD8a+/2CD11b2B220+Gr-1+

pDCs (31). Generally, myeloid DCs are thought to exhibitstimulatory effects, whereas lymphoid DCs or pDCs are in-volved with regulation or tolerogenesis. However, there arereports demonstrating immunosuppressive activities onthe part of myeloid DCs and Ag presentation by pDCs (32).DC lineage is not as well characterized in humans; theyare thought to arise from a myeloid progenitor, shared withmacrophages, called a macrophage–DC progenitor, which, inturn, gives rise to common DC progenitors (CDPs). CDPsthen give rise to pDCs and pre-DCs that are progenitors oftwo myeloid subsets. Human pDCs express surface markersCD123, CD303 or BDCA-2, and CD304 or BCDA-4.Human myeloid DCs are defined largely by CD1c/BDCA-1+

or CD141/BDCA-3+ expression. There are many conservedmolecular pathways when comparing murine and humanDCs, suggesting preservation of function (33).DCs tend to congregate in blood and lymphoid tissues,

although they are found widely in the body, as Langerhans cellsin the skin and as intestinal DCs in the gastrointestinal tract.The characteristics of human TIDCs have perhaps best beenstudied in cutaneous melanoma. Noted though, is the diffi-culty of clearly characterizing human TIDCs because ofsignificant patient/population heterogeneity, relatively lowfrequency of TIDCs, and differences in the use of markers toassess TIDC populations. In melanoma, TIDC frequencytends to be higher in the peritumoral area, and those cellsexhibit a more mature phenotype, whereas more immatureTIDCs are found within the tumors. However, the relationshipbetween TIDC location and clinical outcome is not wellunderstood, because it depends on TIDC type and commu-nication with other cells (34). In general, mature DCs havebeen considered immune stimulatory, whereas immature oneshave been considered suppressive and tolerogenic. However,based on their attributes, such as activation status, maturity,and polarization in TME, DCs’ functional plasticity is con-sidered complicated (35–39). This makes it difficult to makeany generalized inference about the roles of DCs in the TME.

TIDCs

TIDCs have an immature and/or paralyzed phenotype. TIDCshave been found in the TME in many cancer types, such asbreast, colorectal, lung, renal, head and neck, bladder, gastric,and ovarian (40). Not surprisingly, because of the complexityof phenotype, as well as the methods of identification, DCinfiltration into tumors was reported to have an associationwith both good and poor prognosis in different tumor types.For example, in one study, patients with colorectal carcinomaand high numbers of TIDCs exhibited shorter disease-freeand overall survival than did those with low numbers (41).Conversely, TIDC presence was correlated with regression ofmelanoma (32). Prognostic impact may be more related toa shift in TIDC phenotype or the relative proportions ofsubtypes within the tumor infiltrate, rather than simply the

degree of TIDC infiltration into the tumor (42). TIDCs tendto exhibit a phenotype of low costimulatory moleculeexpression (26, 37), blunted Ag cross-presentation (26), andhigh expression of regulatory molecules and receptors (37, 43)and are usually associated with immunosuppression. Animalmodeling studies showed that the type, phenotype, and amountof TIDCs are dynamic over time and may influence diseaseprogression significantly at different stages of tumor growth.For example, Krempski et al. (37) showed in an ID8 mousemodel of ovarian cancer that, as the cancer progresses from lightto heavy tumor burden, the numbers of TIDCs increase. Nearlyall of the DCs progressively upregulated immune-suppressivemolecules; this change is associated with a concomitant lossof T cell infiltration (37). Scarlett et al. (38) made similarobservations in a spontaneous murine model of ovariancancer. In that model, tumor progression was associatedwith increasingly dense immune infiltrates, which includedDCs, macrophages, myeloid-derived suppressor cells, andT cells. A turning point was observed, at which aggressivegrowth correlated with a switch from immune-stimulatoryDCs to immune-suppressive DCs. Depletion of DCs in thatmodel early in the course of disease led to increased tumorexpansion and aggression, whereas depletion later led toabrogation of tumor growth, suggesting that tumors eventuallydevelop strategies to prevent DCs from sensing endogenousdanger signals and responding with immune stimulation.Although it is not possible to determine whether this isrelevant to human cancer, there are some indications in theliterature. For example, Kocian et al. (44) found, in a study ofcolorectal cancer, that relapsed patients were more likely to havehad fewer TIDCs in their primary tumors. Importantly, therewas a distinct difference in the proportion of mature andimmature (i.e., tolerogenic) TIDCs noted, with patientsexperiencing disease recurrence having higher densities ofimmature TIDCs and lower densities of mature TIDCs intheir primary lesions. A study of non–small cell lung cancerTIDCs demonstrated that there are three DC subsets: thosewith high, intermediate, and no CD11c expression. In particular,the intermediate CD11c–expressing group appeared to have lowlevels of costimulatory molecules and high levels of the immune-inhibitory molecule programmed death 1 ligand (PD-L1) (25).TIDCs as a group exhibited poor response to TLR stimulationin terms of Ag-presentation capability. In breast cancer, pDCswithin the primary tumor correlated with worse prognosis.When isolated from breast tumor biopsies from patients withhigh mitotic index and triple-negative breast tumors (aggressivebreast tumors), pDCs produced low amounts of IFN-a in com-parison with pDCs isolated from patients’ blood. These cells alsosustained FOXP3+ Treg expansion, potentially contributing toimmune tolerance and worsened clinical outcomes (45).Although it is not yet clear what specifically induces these shiftsin DC subset distribution and/or phenotypic shift, release ofcytokines or other factors into the TME are likely involved.

Local induction of paralysis of TIDCs. Tumor cells and the TMErelease factors that inhibit or reverse DC maturation andnormal function. For example, Michielsen et al. (46) showedthat conditioned media obtained from culturing humancolorectal tumor explant tissue were high in VEGF and thechemokines CCL2, CXCL1, and CXCL5. Pretreatment ofDCs in vitro with this media resulted in inhibition ofmaturation. A dynamic network, consisting of cell surface

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molecules and soluble factors, regulates TIDC-mediatedtumor-immune interactions in the TME (Fig. 1). Our groupobserved that one way in which tumors paralyze DCs isthrough the induction of PD-1 expression (37). PD-1 and itsligands PD-L1 and PD-L2 constitute an important immune-regulatory pathway that suppresses or impairs effective T, B,and myeloid cell responses in the initiation (priming) andeffector phases of the immune response. Our data show thatPD-1 expression on murine DCs is minimal in the earlystages of tumor growth; however, as the disease progresses,nearly all TIDCs eventually have high levels of PD-1 (37, 47).Blockade of PD-1 on these TIDCs in vitro enhanced theproduction of immune-stimulatory cytokines, increased NF-kB activation in DCs, improved costimulatory moleculeexpression, and improved the ability of these DCs toactivate T cells (37, 47). Thus, the paralyzed phenotype ofTIDCs is likely reversible, at least pharmacologically. Anothermechanism by which TIDCs are locked into an immune-suppressive or nonimmunogenic phenotype is through theupregulation of T cell Ig and mucin domain 3 (TIM-3) atthe cell surface. TIM-3 has been largely studied as a suppressivemarker of Th1 type T cells, and blockade of TIM-3 exacerbatesvarious T cell–mediated autoimmune diseases (48). Morerecently, its expression and role in modulating innate immuneactivation have become clearer (49). Chiba et al. (50) found thatboth murine and human tumors induce upregulation of TIM-3on DCs through multiple factors found in the TME, such asIL-10, TGF-b1, VEGF-A, IDO, and arginase. More importantly,they found that TIM-3 interacted with HMGB1 and prevented it

from sensing tumor-derived nucleic acid danger signals in theTME. In addition to directly suppressing danger signal sensing,TIM-3 ligation with cross-linking Ab results in activation ofBruton’s tyrosine kinase and c-SRC, leading to the release ofimmune-suppression factors by DCs that suppressed NF-kBsignaling (51).Emerging evidence suggests that TIDCs have an impaired

Ag-presentation capacity. For example, using the B.16 mousemelanoma tumor model, Stoitzner et al. (52) showed that,despite uptake of Ag, TIDCs (CD11c-sorted cells) cannotcross-present the Ag to OVA-specific CD8+ OT-I and CD4+

OT-II T cells. We used PD-1+ TIDCs from the ID8 mousemodel of ovarian cancer and observed that blockade of PD-1 onthese PD-1+ TIDCs that were pulsed with OVA enhanced theirability to activate OT-1 T cells (K. Karyampudi, P. Lamichhane,and K. Knutson, unpublished observations). This suggests thatPD-1 expressed on TIDCs might act as a switch that regulatesthe Ag cross-presentation capacity of TIDCs. In contrast,Engeldardt et al. (27) showed that murine TIDCs (CD1232)ingest tumor-derived protein and present processed Ags to in-filtrating tumor-specific T cells, even engaging in long-livedinteractions with TIDCs; however, these interactions are non-productive. In addition to regulating Ag-presentation capacity,tumors and/or TME are known to play a role in modifying DCactivity by inducing the upregulation of microRNAs in thesecells. Min et al. (53) showed that multiple microRNAs, such asmiR-16-1, miR-22, miR-155, and miR-503, are upregulated inbone marrow–derived DCs by mouse ovarian, melanoma, cer-vical, lung, and breast cancer cell lines. In the presence of tumor

↑Apoptosis

↑Proliferation↑InvasionTumor cell

Tumor cell

T cell

T cell Treg

Treg

Retinoic acid

IDO

↓CD80↓CD86↓MHC class I

TNF-αGZMPerforin

IL-10TGFβArginase

IL-10TGFβ

↑STAT3↑FOXO3↓NFκB

PD-1

PD-L1

HMGB1

TIM-3

Galectin-9

Key:

FIGURE 1. TIDCs are central to tumor-immune interactions in the TME. DCs are recruited into the TME and induced to upregulate PD-1 and TIM-3.

Interactions of PD-1 with PD-L1 in the TME blocks responsiveness to danger signals and prevents T cell activation through reduced Ag presentation and

costimulation. Danger signals are also reduced as a result of high TIM-3 expression, which bindsHMGB1. T cells are preferentially drawn to TIDCs as they enter the

TME. In addition to the lack of appropriate activating signals, T cell responses are blocked by engagement of PD-1 by PD-L1 on the DC surface. Tregs are also

induced by the TIDCs to establish a tolerogenic environment.

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cells, DCs undergo apoptosis at higher rates compared withDCs that were cocultured with normal fibroblasts. Fur-thermore, it was observed that these microRNAs specificallytargeted PI3K/AKT, MAPK, and apoptotic signaling path-ways in DCs.

TIDCs block adaptive immunity. Immature and paralyzedTIDCs suppress both innate and adaptive immune effectorsusing a wide variety of mechanisms (Fig. 1). The ligand forPD-1, PD-L1, is constitutively expressed on activated DCs,and its expression is further upregulated by the TME (54).Studies by Curiel et al. (54) and Krempski et al. (37) showedthat blockade of PD-L1 specifically on murine TIDCsresulted in a better capability of TIDCs to stimulate T cellactivation. Krempski et al. (37) also showed that the TIDCscoexpress PD-1 and PD-L1, suggesting the possibility thatsuppression of T cell activation may be mediated by eitherof the molecule’s interactions with PD-L1 or PD-1 on theT cells. Using PD-L1–knockout T cells, Krempski et al. wenton to show that TIDC-bound PD-L1 mediated direct immediateT cell suppression, as assessed in MLR, whereas DC-bound PD-1paralyzed TIDCs, rendered them unresponsive to danger signals,and downregulated costimulatory molecules and cytokines. Thus,the suppressive nature of TIDCs mediated by PD-L1 appearsto be constitutively present but is overcome by increased levelsof costimulatory molecules and cytokines. This balance betweeninhibitory and activating potential is reminiscent of the killerphenotype of NK cells, whose activity is a balance of severalinhibitory and activating receptors (55).Consistent with the idea that the immune-activating po-

tential of TIDCs is a balance between multiple inhibitory andactivating molecules, DCs possess mechanisms other than justPD-L1 to block T cell activation. Liu et al. (56) reported thatmurine lung tumor cells release large amounts of PGE2 andTGF-b, which results in the conversion of immune-activatingDCs into immune-suppressive DCs (CD11clowCD11bhigh

Ialow). These DCs suppress T cell responses that are mediated,in part, through the upregulation of arginase I, whichdegrades arginine, an amino acid that T cells are unable toproduce themselves and that is required for CD4 T cellproliferation. Higher arginase expression also may lead tohigher levels of reactive oxygen intermediates, which blockAg-specific CD8 T cell responses (57). IDO expression byTIDCs also plays an important role in mediating the sup-pression of adaptive-immune responses (58). Tumor-derivedPGE2 induces IDO expression in TIDCs, and these IDO+

TIDCs suppress CD8 T cell responses to Ags presented bythese TIDCs themselves, as well as those presented by third-party, nonsuppressive DCs. TIDCs also may suppress adaptive-immune responses indirectly. Indirect mechanisms includethe induction of Tregs (by human pDCs) (59). Cytokines,such as TGF-b, IL-10, and IL-2, induce DCs to stimulateTreg formation in vitro (60). Recently, it was described thatthese cytokines may act together with cosignaling/surfacemolecules, such as inducible T cell costimulator ligand,PD-L1, CD80, and CD86, to induce DCs to stimulate Tregformation (59, 61–64).

The paralyzed, immune-suppressive phenotype of TIDCs is regulatedby hyperactivation of tumor-induced transcription factors. TIDCsalso were reported to overexpress several important regulatorygenes that regulate their immune-suppressive and/or paralyzedphenotype. The most prominently studied is the transcription

factor STAT3, which was found to be hyperactivated in DCsfollowing exposure to tumor-derived factors (65, 66). STAT3hyperactivation induces S100A9 protein, which prevents fullmaturation of DCs, thereby blocking their responsiveness tolocal danger signals (67). We also found that IL-10, which isoverexpressed in the TME of various tumors, can activateSTAT3 and induce enhanced expression of PD-1 and PD-L1on DCs (K. Karyampudi, P. Lamichhane, and K. Knutson,unpublished observations), rendering them ineffective. Humanand murine prostate cancer TIDCs (pDCs and non-pDCs)also were reported to overexpress another cell cycle–associatedtranscription factor, FOXO3. Its overexpression upregulatesIDO, arginase, and TGF-b while diminishing the expressionof costimulatory molecules and the release of inflammatorycytokines (43).

Therapeutic manipulation of TIDCs

Inroads have been made into the issue of how to modifyTIDCs effectively. Their central role in the maintenance ofinnate- and adaptive-immune responses, as well as their abilityto present Ag to effector cells, marks them as attractive targetsfor cancer treatment (68, 69). Over the past two decades, DCshave been used in vaccine models in preclinical and clinicalsettings. However, an intriguing new area of potential thera-peutic impact is the manipulation of TIDCs. This has notbeen translated into the clinical setting, but several promisingpathways and targets have been investigated recently.Targeting the pathways that paralyze TIDCs appears to be

a promising approach; certainly, one of the most attractivepathways is to specifically target the PD-1/PD-L1 inhibitoryaxis, particularly considering the rapid advances in the clinicaldevelopment of biologics that block PD-1 and PD-L1 inter-actions (70). This may be particularly true with respect toadvanced cancers in which PD-1 becomes highly expressed onTIDCs (37). Blocking the ligation of PD-1 on DCs, as wasdiscussed above, restores danger responsiveness with increasedT cell–activation capabilities (37). Furthermore, recent datafrom our group showed that PD-1 blockade, specifically onmurine TIDCs, also leads to an increase in IL-7R expressionby T cells, resulting in increased persistence in the TME (47).Further studies to validate the anti–PD-1–mediated rescue ofimmune-stimulatory capacities of TIDCs will open up pathsfor combination therapies in the clinical setting that are aimedat enhancement of antitumor immunity via the TIDC com-partment of the TME. Anti–PD-1 Ab is under investigationand showed some efficacy and safety in patients with differenttumor types (71). It was approved recently by the U.S. Foodand Drug Administration for use in patients with unresectableor metastatic melanoma if disease progresses following ipili-mumab or in case of BRAF V600 mutation following BRAF-inhibitor treatment. Similarly, therapeutic biologics targetingthe immune-suppressive molecule TIM-3 are under devel-opment and showing great promise (72). Given the multiplecell types (e.g., TIDCs, T cells, macrophages) in the tumorthat express both PD-1 and TIM-3, it will be necessary tocarefully craft translational studies that enable a better un-derstanding of the impact of these therapeutic modalities onTIDCs. Alternatively, another potential modality is to blocktumor-produced factors that induce TIDC paralysis, such as IL-10, one of the key cytokines known to upregulate both TIM-3and PD-1 on TIDCs (50) (K. Karyampudi, P. Lamichhane, and

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K. Knutson, unpublished observations). Vicari et al. (73) showedthat TIDCs (most of which were of the immature phenotype)that were refractive to stimulation by a combination of LPS,anti-CD40, and IFN-g could be rescued by treatment witha combination of CpG and anti–IL-10R Ab both in vitro and invivo, leading to better de novo IL-12 production and antitumorimmune responses.Other studies aimed at inducing the maturation of TIDCs

have been conducted. Saito et al. (74) showed that, whentumor-bearing mice are treated with plasmids containingcDNA for Flt3L or intratumoral injection with adenoviralvector carrying the IL-18 gene, the TIDCs expressed higherCD86 and exhibited better potential to activate T cells. Po-tent antitumor activity also developed in distant uninjectedtumors from the treated mice. In addition to this, given theprominent role of microRNAs in maintaining the functionof TIDCs, delivery of modulators of microRNAs may be oftherapeutic benefit, although low bioavailability and cellularuptake are the challenges associated with the delivery of thesetypes of agents. Taking advantage of the endocytic activity ofDCs to deliver nanoparticles carrying oligonucleotide complexes,the activity of endogenous immune-regulating microRNAs ortranscription factors can be augmented or suppressed. Cubillos-Ruiz et al. (75) showed that such an approach can be used todeliver miR-155, which resulted in the subsequent skewing ofTIDCs toward being immune stimulatory rather than immunesuppressive and eventually slowed the progression of ovariancancer in a mouse model. However, a caveat is that somemicroRNAs have multiple roles, making them difficult targets.For example, miR-155 also was shown to be associated withenhanced apoptosis of TIDCs; therefore, therapeutic targetingmay not have the desired effects (75). Luo et al. (76) found thatincorporating STAT3 small interfering RNA into a nanovaccineand immunizing mice i.p. resulted in replacement of suppressiveTIDCs in the TME with more immune-stimulatory DCs, in-dicating that the TIDC compartment in the TME is beingrenewed continuously.Lastly, other interesting avenues for modification of TIDC

behavior and phenotype are being explored. In one study ofa murine model of lung cancer, a population of TIDCs wasidentified that increased in the tumor as the cancer progressed.Polarization of the TIDCs was found to be influenced by smallrho GTPase signaling, which was reported previously as beingintegral to DC endocytosis, Ag processing/presentation, andmotility. Rho GTPase inhibitor toxin B blocked the effect ofpaclitaxel in preventing a tumor-induced shift toward regu-latory DCs, offering a possible mechanism for tumor im-munologic escape and a future point of intervention (77).Lactic acid production by tumor cells may be another meansby which cancer cells are able to induce immune-suppressiveTIDCs. For example, in spheroid cultures of tumor cell lines,melanoma and prostate carcinoma cells were noted to pro-duce high levels of lactic acid. The addition of lactic acid to invitro–cultured DCs produced a phenotype similar to mela-noma and prostate TIDCs (i.e., decreased IL-12 productionand immature phenotype), and blockade of lactic acid revertedthe phenotype to normal (78).Targeting of these immune therapies to the TME in clinical

practice will be a challenge, given the complexity of immuneregulation; there are several approaches under investigationby which these pathways may be used. The development of

a systemic Ab affecting a tolerogenic pathway, such as an anti–PD-1 Ab or biologic targeting of TIM-3, is one such method,and several agents are currently in development or underclinical investigation. Manipulation of immune tolerance andincreased activation of DCs in combination with develop-ment of a DC-based vaccine for cancer Ag presentation wouldbe another approach, as well as intratumoral delivery of agentsor TIDC-modulating agents given in conjunction withchemotherapy to boost efficacy.

ConclusionsOver the past few years, the concept of the TME in the settingof malignancy has become well developed; although manyimmune cells play integral roles in the TME, it is increasinglyclear that DCs are central mediators. The complexities of theirrole have yet to be fully elucidated, but studies have yieldedpromising insights into their function within the TME anddirections for DC-targeted cancer therapy. Evident now is thefact that TIDCs are impaired in a variety of ways, which leadthese DCs to confer immune suppression, rather than immunestimulation, at the local TME. Preclinical studies demonstratepromising areas for therapeutic intervention, such as the use ofchemotherapy drugs at lower, noncytotoxic doses for immunemodification, blocking the pathways that induce tumor-induced shifts to a suppressive or regulatory DC phenotype,and targeted activation of TIDCs. Understanding the mech-anisms and players involved in inducing such impaired andimmune-suppressive DCs will open up avenues to exploretherapies directed toward rescuing the immune-stimulatorypotential of TIDCs.

DisclosuresThe authors have no financial conflicts of interest.

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