**3. Current DC vaccination research in MM**

Usually, *ex vivo* DCs were generated from circulating blood precursors (i.e. monocytes) or bone marrow progenitor cells and educated them with myeloma-associated antigens prior to vaccination to patients with MM.

#### **3.1. Idiotype-pulsed DCs**

[10]. Cellular immunotherapy using DCs is emerging as a useful immunotherapeutic modality to treat MM [11]. While antigen-specific cytotoxic T lymphocytes (CTLs) and immune response can be induced by DC vaccination in MM patients, clinical responses so far have been largely unsatisfying to be observed only in a minority of treated patients with MM. Progress in understanding DC biology in cancer patients and the recruitment of suppressive cells of the adaptive and innate immune system in antitumor immunity of cellular immunotherapy is leading to new concept which aims at improved immune and clinical outcomes in MM. New concept is developing to generate novel therapeutic targets that could restore DC capacity to prime T cells and trigger effective anticancer responses in combination with other therapies to

DCs have a potent antigen-specific T cell stimulatory capacity and therefore should be considered to the one of the promising antitumor immunotherapeutic options. In tumorspecific immunity, secreted products or fragments from tumor cells enter into DCs through the endosome and are processed and presented on MHC class molecules of DCs [12]. Processed antigens presented on these molecules of DCs are recognized by CD4+ T helper cells, which not only enhance to the CD8+ T cell response but also facilitate to develop a humoral immune response for surface antigens expressed on the tumor cells. The antigens presented on MHC

Unfortunately, patients with MM have basically dysfunctional DCs that are functionally defective, evidenced by the decreased number of circulating precursors of DCs as well as the impaired T cell stimulatory capacities compared with normal controls [13, 14]. The defective functions of DCs in patients with MM are partially attributed to the production of IL-6 and other tumor-derived factors. DCs in MM patients are a target of tumor-associated suppressive factors, such as IL-10, transforming growth factor- beta (TGF-β), vascular endothelial growth factor (VEGF), and IL-6, resulting in their aberrant functions and impaired development of effector functions in tumor-specific lymphocytes [15]. There were only few patients with MM who responded clinically to vaccination with antigen-loaded autologous DCs. There may be several reasons for this failure from MM patients itself. MM is believed to induce immunopa‐ resis that interferes with DC function and hence affects the effective antitumor immune responses in these patients. They are able to escape immune surveillance by down-regulation of immune markers as well as through the production of immunosuppressive cytokines by the tumor cells or by activation of suppressor cells such as regulatory T cells and myeloid cells. Myeloma cells can produce immuno-inhibitory cytokines, such as IL-10, TGF-β, VEGF, and IL-6, which play major roles in the pathogenesis of MM [15]. In addition, the survival and proliferation of myeloma cells are partially facilitated by impaired endogenous immune surveillance against tumor antigens, including the abrogation of DC function, by constitutive activation of the signal transducer and activator of transcription 3 (STAT3) [13]. Impairment in both humoral and cellular immunity in MM is associated with impaired B cell responses;

CTLs, which have a direct cytotoxic effect on tumor cells.

T cells and impaired CTL responses; and dysfunction

offset tumor-induced suppression in MM.

180 Multiple Myeloma - A Quick Reflection on the Fast Progress

class I are recognized by CD8+

decreased T cell numbers including CD4+

**2. Dendritic cell in myeloma immunity**

Immunoglobulin idiotype (Id) is a tumor-specific antigen that is produced by each B cell tumor clone. Id protein has been used for immunotherapy in patients with MM [23, 24]. Id vaccination could induce immune responses by both antibodies and Id-specific T cells, including CD4+ and CD8+ T cells, through the presentation of Id protein on the surface of professional APCs [24]. Id-specific CTL lines that kill autologous primary myeloma cells *in vitro* have been generated [25, 26]. Autologous DCs that were generated from MM patients have been shown to efficiently endocytose different classes of Id proteins, and autologous Id-specific CTLs that were gener‐ ated by Id-pulsed DCs were able to recognize and kill autologous primary myeloma cells *in vitro* [25, 26]. Various studies of DC-based Id vaccination in MM have been reported [27-34]. Although Id-specific CTLs and immune responses could be induced in some patients, clinical responses have rarely been observed after vaccination possibly because Id protein is a weak antigen and immature DCs have been used in some studies [27].

#### **3.2. Myeloma-associated antigens-based DC immunotherapy**

In general, the production of DC vaccines using whole tumor antigens has become a promising tool for immunotherapy against MM. There are several types of myeloma-associated antigen for loading onto DCs: loading with myeloma lysates [35, 36], loading with dying myeloma cells [37-39], transfection with myeloma-derived RNA [40], pulsing with myeloma-derived heat shock protein (HSP) gp96 [41, 42], and hybridization with myeloma cells [43, 44]. These techniques have the advantage of allowing the presentation of multiple epitopes to MHC on DCs, therefore inducing polyclonal T cell responses from many potentially unknown tumorassociated antigens (TAAs) and reducing the probability of immune escape by a single TAA.

Various myeloma-associated antigens that may induce immune responses from DC-based vaccines have been identified in MM patients. MUC1-specific CTLs that were induced *in vitro* using peptide-pulsed DCs or plasma cell RNA-loaded DCs efficiently killed not only target cells pulsed with the antigenic peptide but also MM cells [40, 45]. DCs transfected with PTD-NY-ESO-1 protein can induce CD8+ cellular antitumor immunity superior to that achieved with NY-ESO-1 protein alone [46]. Sp17-specific HLA class I-restricted CTLs were successfully generated by DCs that had been loaded with recombinant Sp17 protein and were able to kill autologous tumor cells that expressed Sp17 [47]. The overexpression of hTERT on MM compared to the expression levels in normal cells indicated that this telomerase also could be used as a myeloma-associated antigen. hTERT was capable of triggering antitumor-CTL responses and killing hTERT+ tumor cells [48]. Recently, a report demonstrated that activated T lymphocytes were able to successfully kill myeloma cells after stimulation by DCs loaded with hTERT- and MUC1-derived nonapeptides [49]. DKK1, a novel protein that is not expressed in most normal tissues but is expressed in almost all myeloma cells, may be an important antigenic target for anti-myeloma immunotherapy. DKK1-specific CTLs that were generated by DCs pulsed with DKK1 peptides were specifically lysed by autologous primary myeloma cells and DKK1-positive cell lines [50].

inducing potent α-type 1-polarizing DCs (αDC1s) by exposing immature DCs to α-type 1 polarizing cytokine cocktail containing IL-1β, TNF-α, IFN-α, IFN-γ, and polyinosinic:polycy‐ tidylic acid [poly(I:C)] to generate strong functional CTLs on average 20-fold higher than sDCs [58-62]. Recently, we successfully generated αDC1s from MM patients with high expression of costimulatory molecules, significant production of IL-12p70, and potent generation of myeloma-specific CTLs [37, 38]. Such a novel strategy would provide improved potency of *ex*

Cellular Immunotherapy Using Dendritic Cells in Multiple Myeloma: New Concept to Enhance Efficacy

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183

The other strategy to induce new potent DCs from patients with MM was the use of helper cells to promote type 1-polarization of DCs. Indeed, it has been demonstrated that NK cells play a major immunoregulatory role in the development of protective T cell-mediated immunity against intracellular pathogens and cancers [63]. Such helper activity of NK cells is at least partially mediated by the functional modulation of DCs. This phenomenon depended on the production of IFN-γ and TNF-α from the activated NK cells [63] and was associated with enhanced cross-presentation of tumor antigen and the induction of Th1 and CTL responses [39, 64, 65]. Recent data from our laboratory and other groups has demon‐ strated that NK-DC interactions promote the subsequent induction of tumor-specific responses in CD4+ and CD8+ T cells, allowing NK cells to act as helper cells in the develop‐ ment of type 1-polarized DCs in responses against cancer [39, 64, 65]. Resting NK cells that are activated in the presence of toll-like receptor (TLR) agonists, IL-2, and IFN-α can induce potent DCs with enhanced IL-12p70 production *in vitro*, generating strong antigen-specific

We also found that the selected combinations of TLR agonists synergistically triggered a Th1-polarizing capacity through production of high amounts of IL-12p70 [66]. However, the major limitation of this combination was the decreased ability of these cells to migrate into lymph nodes compared to that of conventional sDCs. When DCs are activated by individu‐ al TLR agonists, such as lipopolysaccharide (LPS) or poly(I:C), or by a combination of 2 TLR agonists, all cells mature and produce high levels of bioactive IL-12p70 in early phase of maturation and after subsequent stimulation with T cell-related DC activating signal CD40L. In addition, the phenotyes of these matured DCs were markedly enhanced when a combina‐ tion of type I and type II IFN was added. These combinations of stimuli also regulated the expression of CD38 and CD74, markers related to the full activation of DCs [67, 68]. We demonstrated that, at the optimal concentration used to stimulate DCs, the combination of 2 TLR agonists with type I and II IFNs can be used to generate fully mature DCs that have high migratory capacity and can maintain IL-12p70-producing capacity. The regulation of CD38 and CD74 in DCs could in turn enhance the migratory activity of DCs in the presence

Ursolic acid (URC) is isolated from *Uncaria rhynchophylla* and phytochemically classified as a triterpene. Triterpene compounds have been identified as a unique class of natural products possessing diverse biological activities. Recently, we reported that URC activates human DCs in a fashion that favors Th1 polarization via the activation of TLR2- and/or TLR4-dependent

nation URC and IFN-γ enhanced the activation of DCs via promotion of IFN-γ-induced Th1

naïve T cells [70]. In addition, combi‐

*vivo*-generated DCs for cancer immunotherapy.

CTLs against myeloma cells [39].

of a combination of 2 TLR agonists and IFNs [69].

IL-12p70 and induces the production of IFN-γ by CD4<sup>+</sup>

## **4. New concepts to enhance the efficacy of cellular immunotherapy in MM**

#### **4.1. How to enhance the efficacy of DC vaccinations**

Because of unsatisfied clinical response of DC vaccination trials in MM, a number of groups have looked at whether the DC vaccination may be more effective if better cytokine combina‐ tions are used to enhance DC function, effective tumor antigens are investigated to use, suppressive signal transcriptions are blocked to overcome defective DC function, the interac‐ tion with immunosuppressor cells is interrupted to avoid the effect of these suppressor cells, or DC vaccines need to be combined with other therapies.

#### **4.2. The next generation of DCs**

To improve DC vaccination, the investigators exploit to the microbial activation signals leading to generate potent DCs with high secretion of cytokines such as IL-12p70, which generate strong tumor-specific Th1 response and helper function for the generation of memory T cells, high production of polarizing signals, which help the generation of high avidity in CTLs that may be resistant to tumor microenvironment, and strong costimulation mediated via several costimulatory molecular pathways [51, 52]. This induces to eliminate Tregs and block tumor microenvironment results in the full activity of elicited CTLs and tumor rejection.

The initial phase of DC-based vaccines involving immature or partially-mature "first-genera‐ tion" DCs has been reported [53, 54]. However, such DCs express suboptimal levels of costimulatory molecules and constitute weaker immunogens than subsequently implemented mature DCs, the "second-generation" of clinically applied standard DCs (sDCs), which induced by cytokine cocktails containing IL-1β/TNF-α/IL-6/prostaglandin E<sup>2</sup> (PGE2) [55]. However, to date, sDC vaccines still have some drawbacks, including the mediation of Th2 polarization by increased secretion of the immunosuppressive cytokine IL-10 from DCs and high activity in activating Tregs [56, 57]. Therefore, several investigators, including our group, have tried to develop new generation of potent DC that possess all required features for inducing effective tumor-specific immune responses. We demonstrated the feasibility of inducing potent α-type 1-polarizing DCs (αDC1s) by exposing immature DCs to α-type 1 polarizing cytokine cocktail containing IL-1β, TNF-α, IFN-α, IFN-γ, and polyinosinic:polycy‐ tidylic acid [poly(I:C)] to generate strong functional CTLs on average 20-fold higher than sDCs [58-62]. Recently, we successfully generated αDC1s from MM patients with high expression of costimulatory molecules, significant production of IL-12p70, and potent generation of myeloma-specific CTLs [37, 38]. Such a novel strategy would provide improved potency of *ex vivo*-generated DCs for cancer immunotherapy.

achieved with NY-ESO-1 protein alone [46]. Sp17-specific HLA class I-restricted CTLs were successfully generated by DCs that had been loaded with recombinant Sp17 protein and were able to kill autologous tumor cells that expressed Sp17 [47]. The overexpression of hTERT on MM compared to the expression levels in normal cells indicated that this telomerase also could be used as a myeloma-associated antigen. hTERT was capable of triggering antitumor-CTL

T lymphocytes were able to successfully kill myeloma cells after stimulation by DCs loaded with hTERT- and MUC1-derived nonapeptides [49]. DKK1, a novel protein that is not expressed in most normal tissues but is expressed in almost all myeloma cells, may be an important antigenic target for anti-myeloma immunotherapy. DKK1-specific CTLs that were generated by DCs pulsed with DKK1 peptides were specifically lysed by autologous primary

**4. New concepts to enhance the efficacy of cellular immunotherapy in MM**

Because of unsatisfied clinical response of DC vaccination trials in MM, a number of groups have looked at whether the DC vaccination may be more effective if better cytokine combina‐ tions are used to enhance DC function, effective tumor antigens are investigated to use, suppressive signal transcriptions are blocked to overcome defective DC function, the interac‐ tion with immunosuppressor cells is interrupted to avoid the effect of these suppressor cells,

To improve DC vaccination, the investigators exploit to the microbial activation signals leading to generate potent DCs with high secretion of cytokines such as IL-12p70, which generate strong tumor-specific Th1 response and helper function for the generation of memory T cells, high production of polarizing signals, which help the generation of high avidity in CTLs that may be resistant to tumor microenvironment, and strong costimulation mediated via several costimulatory molecular pathways [51, 52]. This induces to eliminate Tregs and block tumor

The initial phase of DC-based vaccines involving immature or partially-mature "first-genera‐ tion" DCs has been reported [53, 54]. However, such DCs express suboptimal levels of costimulatory molecules and constitute weaker immunogens than subsequently implemented mature DCs, the "second-generation" of clinically applied standard DCs (sDCs), which induced by cytokine cocktails containing IL-1β/TNF-α/IL-6/prostaglandin E<sup>2</sup> (PGE2) [55]. However, to date, sDC vaccines still have some drawbacks, including the mediation of Th2 polarization by increased secretion of the immunosuppressive cytokine IL-10 from DCs and high activity in activating Tregs [56, 57]. Therefore, several investigators, including our group, have tried to develop new generation of potent DC that possess all required features for inducing effective tumor-specific immune responses. We demonstrated the feasibility of

microenvironment results in the full activity of elicited CTLs and tumor rejection.

tumor cells [48]. Recently, a report demonstrated that activated

responses and killing hTERT+

182 Multiple Myeloma - A Quick Reflection on the Fast Progress

**4.2. The next generation of DCs**

myeloma cells and DKK1-positive cell lines [50].

**4.1. How to enhance the efficacy of DC vaccinations**

or DC vaccines need to be combined with other therapies.

The other strategy to induce new potent DCs from patients with MM was the use of helper cells to promote type 1-polarization of DCs. Indeed, it has been demonstrated that NK cells play a major immunoregulatory role in the development of protective T cell-mediated immunity against intracellular pathogens and cancers [63]. Such helper activity of NK cells is at least partially mediated by the functional modulation of DCs. This phenomenon depended on the production of IFN-γ and TNF-α from the activated NK cells [63] and was associated with enhanced cross-presentation of tumor antigen and the induction of Th1 and CTL responses [39, 64, 65]. Recent data from our laboratory and other groups has demon‐ strated that NK-DC interactions promote the subsequent induction of tumor-specific responses in CD4+ and CD8+ T cells, allowing NK cells to act as helper cells in the develop‐ ment of type 1-polarized DCs in responses against cancer [39, 64, 65]. Resting NK cells that are activated in the presence of toll-like receptor (TLR) agonists, IL-2, and IFN-α can induce potent DCs with enhanced IL-12p70 production *in vitro*, generating strong antigen-specific CTLs against myeloma cells [39].

We also found that the selected combinations of TLR agonists synergistically triggered a Th1-polarizing capacity through production of high amounts of IL-12p70 [66]. However, the major limitation of this combination was the decreased ability of these cells to migrate into lymph nodes compared to that of conventional sDCs. When DCs are activated by individu‐ al TLR agonists, such as lipopolysaccharide (LPS) or poly(I:C), or by a combination of 2 TLR agonists, all cells mature and produce high levels of bioactive IL-12p70 in early phase of maturation and after subsequent stimulation with T cell-related DC activating signal CD40L. In addition, the phenotyes of these matured DCs were markedly enhanced when a combina‐ tion of type I and type II IFN was added. These combinations of stimuli also regulated the expression of CD38 and CD74, markers related to the full activation of DCs [67, 68]. We demonstrated that, at the optimal concentration used to stimulate DCs, the combination of 2 TLR agonists with type I and II IFNs can be used to generate fully mature DCs that have high migratory capacity and can maintain IL-12p70-producing capacity. The regulation of CD38 and CD74 in DCs could in turn enhance the migratory activity of DCs in the presence of a combination of 2 TLR agonists and IFNs [69].

Ursolic acid (URC) is isolated from *Uncaria rhynchophylla* and phytochemically classified as a triterpene. Triterpene compounds have been identified as a unique class of natural products possessing diverse biological activities. Recently, we reported that URC activates human DCs in a fashion that favors Th1 polarization via the activation of TLR2- and/or TLR4-dependent IL-12p70 and induces the production of IFN-γ by CD4<sup>+</sup> naïve T cells [70]. In addition, combi‐ nation URC and IFN-γ enhanced the activation of DCs via promotion of IFN-γ-induced Th1

cell polarization that was dependent on the activation of IL-12p70 and independent of TLR4 [71, 72]. The potential of natural products to enhance DC maturation and activation has important implications for the use of DCs as a cancer vaccine.

**4.4. Blocking immunosuppressive activity during the loading of tumor antigens for DC**

Cellular Immunotherapy Using Dendritic Cells in Multiple Myeloma: New Concept to Enhance Efficacy

http://dx.doi.org/10.5772/54100

185

The suppressive effects of tumor cells during DC generation have been explained previously by the ability of the tumor microenvironment to suppress DC differentiation [73]. This process can influence STAT3 and ERK phosphorylation, resulting in hyperactivation of STAT3 and ERK, which may be responsible for defective generation of DCs [74]. The immune-mediated antitumor effects of DCs are enhanced by inhibition of the JAK2/STAT3 pathway [75], inhibition of p38 or activation of the MEK/ERK or MAP kinase pathways, and neutralization of IL-6 [76]. Recently, we found that when MM-derived DCs were generated by loading tumor lysates from autologous myeloma cells, these DCs showed lower phenotypic maturation, less T cell stimulatory capacity, less CTL activity, and highly abnormal IL-6 and IL-12 secretion compared to the secretion by unloaded DCs. Moreover, the levels of VEGF, phospho-STAT3, and phospho-ERK1/2 in these DCs were significantly higher than in unloaded DCs. After neutralization of VEGF activity, DC functions, signal transduction, and cytokine production were returned to normal level. Therefore, inhibitory factors and abnormal signaling pathways during maturation with tumor antigens in DCs may be responsible for the defective activity of DCs in MM, and these abnormalities may be overcome by neutralizing the signaling that

**5. Combination therapy: New concept to enhance efficacy of DC vaccines**

Many factors contribute to the limited clinical efficacy of DC vaccines. The tumor microenvir‐ onment contains different kinds of inhibitory cells, such as Tregs and MDSC, and inhibitory molecules, such as IL-10, IL-6, TGF-β, and VEGF, all of which prevent the activation of effector T cells in response to DC responses [16-21, 23, 78, 79]. Although DC vaccines showed effective antitumor effect in experimental *ex vivo* systems, they didn't effectively induce strong immune responses that were enough to kill tumors *in vivo*. Therefore, strategies to improve the efficacy of DC vaccines are to overcome the immune tolerance/suppression induced by these cells, which are involved in the use of a combination of DC vaccine with either stimulatory cytokines or the targeting elimination of inhibitory cells and molecules in tumor microenvironment.

Cytokines, such as GM-CSF or IL-2, known to enhance cell-mediated immune responses may be administered as adjuvants with the vaccines aiming to create an environment where specific immune responses are readily induced [80, 81]. To enhance the efficacy of DC vaccination, Idpulsed DCs were combined with GM-CSF [80, 82-84], with immunogenic carrier molecules such as KLH [27, 28, 31-33, 82, 85], or cytokine IL-2 [80, 83] to improve the effectiveness of these DC vaccines in patients with MM. Recently, a phase I study was performed in patients with MM using autologous DCs/tumor cells fusion in combination with GM-CSF administration at the day of DC vaccination [86]. The expansion of circulating CD4+ and CD8+ T cells reactive with autologous myeloma cells were detected in 11 of 15 evaluable patients. A majority of

would lead to a suppressed immune response [77].

**5.1. DC vaccine and cytokine combination**

**vaccines**

#### **4.3. New sources of myeloma-associated antigens for DC vaccines**

Another important consideration to improve the efficacy of DC vaccination in patients with MM is an effective tumor antigen, instead of using idiotype proteins with a weak antigenicity. The use of whole tumor cells, instead of single antigen, may help to enhance antitumor effects to target multiple tumor variants. It is necessary to use purified, optimized myeloma cells, if possible, as a source of tumor antigens for loading onto DCs to generate potent myelomaspecific CTLs [35]. However, it is not only impractical to obtain sufficient amounts of purified autologous myeloma cells for tumor antigens in the clinical setting from patients with MM and it is also unsuitable for those with a lower tumor burden status. As an alternative source of tumor-relevant antigens, allogeneic tumor cells or established cancer cell lines have been used to overcome the limitation in various tumors [37, 38]. DCs loaded with tumor antigens derived from allogeneic myeloma cells could generate myeloma-specific CTLs against autologous myeloma cells in patients with MM [37, 39]. The success of using an allogeneic myeloma cell line as tumor antigens led to the possibility that allogeneic myeloma cells could also be used as a viable source of tumor antigens in the context of appropriate major MHC alleles to autologous CTLs. In addition, autologous DCs loaded with dying myeloma cells of allogeneic matched monoclonal immunoglobulin subtype showed to generate potent myelo‐ ma-specific CTLs against autologous myeloma cells in MM patients [38] These findings suggested that allogeneic myeloma cell lines and allogeneic matched monoclonal immuno‐ globulin subtype of myeloma were effective tumor antigens capable of inducing functional CTLs against patients' own myeloma cells.

Improved understanding of which specific anticancer agents lead to immunogenic cell death and whether these process can enhance antitumor immunity may facilitate the mechanism how chemotherapy and immunotherapy combination can induce immune responses against cancer. Recently, we have worked to develop strategies that recover dysfunction of DCs caused from loading tumor antigens through treatment of myeloma cells with a combination of the selective JAK/STAT3 inhibitor, JSI-124, and a kind of proteasome inhibitor, bortezomib. We observed that production of inhibitory cytokines, such as IL-10, IL-23, and especially IL-6, which induces DC dysfunction in MM patients, was down-regulated in DCs loaded with dying myeloma tumor cells that induced by these agents. Furthermore, phospho-STAT3 was also down-regulated in the DCs. These DCs displayed a superior ability to induce myeloma-specific responses of CTLs. More recently, we are investigating whether chaetocin could be used to induce dying tumor cells for loading onto DCs to enhance myeloma-specific antitumor responses. We show that anti-myeloma drug-induced dying tumor cells can be used as the source of myeloma antigens to loading onto DCs that could elicit potent anti-myeloma activity of CTLs due to the expression of HSP and cancer testis antigens as a mechanism of immuno‐ genic death of human MM cells.

#### **4.4. Blocking immunosuppressive activity during the loading of tumor antigens for DC vaccines**

The suppressive effects of tumor cells during DC generation have been explained previously by the ability of the tumor microenvironment to suppress DC differentiation [73]. This process can influence STAT3 and ERK phosphorylation, resulting in hyperactivation of STAT3 and ERK, which may be responsible for defective generation of DCs [74]. The immune-mediated antitumor effects of DCs are enhanced by inhibition of the JAK2/STAT3 pathway [75], inhibition of p38 or activation of the MEK/ERK or MAP kinase pathways, and neutralization of IL-6 [76]. Recently, we found that when MM-derived DCs were generated by loading tumor lysates from autologous myeloma cells, these DCs showed lower phenotypic maturation, less T cell stimulatory capacity, less CTL activity, and highly abnormal IL-6 and IL-12 secretion compared to the secretion by unloaded DCs. Moreover, the levels of VEGF, phospho-STAT3, and phospho-ERK1/2 in these DCs were significantly higher than in unloaded DCs. After neutralization of VEGF activity, DC functions, signal transduction, and cytokine production were returned to normal level. Therefore, inhibitory factors and abnormal signaling pathways during maturation with tumor antigens in DCs may be responsible for the defective activity of DCs in MM, and these abnormalities may be overcome by neutralizing the signaling that would lead to a suppressed immune response [77].
