Keywords

Introduction

Vaccination is an effective method for preventing the spread of infectious diseases. Edward Jenner is credited with developing the first vaccine, which he used against smallpox in 1796 [1]. In the twenty-first century, the use of vaccines has become indispensable to the eradication of infectious diseases [2]. Vaccines can either be preventative or therapeutic. Preventive vaccines block the spread of a disease within populations by inducing the generation of specific antibodies and the formation of long-lived memory B cells. In addition, preventive vaccines can also induce cellular immunity [3]. Therapeutic vaccines, on the other hand, stimulate the immune system to help eliminate the cause of the disease in the body. Therapeutic vaccines can be used to treat infectious diseases or cancer [4].

The word “cancer ” refers to a group of diseases involving abnormal growth of cells. These cells have the potential to spread to other parts of the body. Moreover, cancer cells tend to form tumors, which are complex tissues comprised of multiple types of heterogeneous neoplastic cells [5]. Cancer immunotherapy is a targeted therapy that utilizes proteins associated with the tumor for therapy. Due to the heterogeneous nature of tumors, multiple combinations of targeted therapies are useful to achieve maximum clinical success.

A recent search on www.clinicaltrials.gov for the term “cancer vaccines ” yielded 1837 clinical studies as of October 31, 2016. One hundred eighty-three of these were phase III clinical trials, and 833 were phase II clinical trials. This highlights the importance of cancer vaccine studies. The activity of therapeutic vaccines depends on antigen-specific CD8+ T cells, which generate cytotoxic T lymphocytes (CTLs) to reject cancer or infected cells. The CTLs generated by a vaccine need to have (1) higher affinity T cell receptor and higher T cell avidity against peptide-MHC molecules expressed on tumor cells [6], (2) higher expression of the secretory molecules perforin and granzyme [6], (3) expression of certain chemokines (e.g., CXCR3) which allow them to migrate to the site of the tumor [7], (4) persistence at the tumor site (e.g., integrins CD103 [8] and CD49a [9]), and (5) high expression of co-stimulatory molecules (e.g., CD137 [10]) and low expression of inhibitory molecules (e.g., cytotoxic T lymphocyte antigen 4 (CTLA-4) [11] or PD-1 [12]). The generation of such CD8+ T cells requires antigen presentation by appropriate antigen-presenting cells (APCs) [13, 14] and the generation of CD4+ T cells that produce cytokines such as IL-21, which promote CD8+ T cell proliferation and differentiation [15]. Therapeutic vaccines should also induce the formation of long-lived memory CD8+ T cells, which act to prevent relapse of the disease [16].

Dendritic Cell Subpopulations

Dendritic cells (DCs) were identified by Steinman and colleagues more than 40 years ago on the basis of their cytolytic features and the absence of surface immunoglobulin (Ig) molecules, thy-1, and brain antigens [17, 18]. DCs are professional APCs, whose principle function is to present antigens to resting naïve T lymphocytes in order to induce primary immune responses. DCs have high cell surface expression of major histocompatibility complex (MHC) class I molecules, MHC class II molecules, and CD86 [19].

DCs are an essential component of vaccines, due to their ability to capture an antigen, process it, and present it to T cells [20]. In peripheral tissues, immature DCs (iDCs) can efficiently capture antigen and present it to T cells, resulting in immune tolerance due to the lack of co-stimulatory molecules [21]. Targeting DCs for vaccination requires a detailed understanding of the function of the various DC subsets in immunity. DC subtypes differ in location, migratory pathways, immunological function, and dependence on infection or other types of inflammatory stimuli. Depending on their localization in lymphoid tissues, mouse and human DCs are divided into two major subsets: plasmacytoid DCs and conventional/myeloid DCs.

Plasmacytoid Dendritic Cells

Plasmacytoid DCs (pDCs) are a rare subset of DCs that are morphologically and functionally unique from conventional DCs. At steady state, pDCs express low levels of MHC class II and co-stimulatory molecules, as well as low levels of integrin CD11c [22]. During viral infections, or upon recognition of foreign nucleic acids, pDCs produce large amounts of type I interferon (IFN) in response to toll-like receptor (TLR) 7 and TLR9 ligation [23]. pDCs have poor antigen presentation capacity and have been associated with immune tolerance in mice [24]. However, whether a similar tolerogenic function exists in human pDCs remains unknown.

Conventional/Myeloid Dendritic Cells

Conventional DCs (cDCs) have long dendrite extensions and high MHC class II expression. Although recent evidence suggests that mouse and human cDCs have functional homology, cDCs express species-specific surface markers [25]. Lymphoid tissue-resident cDCs are found in the thymus, spleen, lymph nodes, and Peyer’s patches in mice. They express either CD8 alpha or CD11b. Mouse CD8 alpha+ cDCs recognize and cross-present extracellular antigens to CD8+ T cells, which produce IL-12 and IFN-γ upon activation, and prime Th1 and cytotoxic T cell responses. On the other hand, the CD11b+ cDC subset preferentially activates CD4+ T cells and induces Th2 or Th17 differentiation. In addition to the lymphoid tissue-resident cDCs, three subsets of mouse migratory/nonlymphoid tissue cDCs have also been identified. These include CD103−CD11b+ cDCs, CD103+CD11b− cDCs, and CD103+CD11b+ intestinal cDCs. Migratory/nonlymphoid tissue CD11b+ cDCs are functionally similar to lymphoid tissue-resident CD11b+ cDCs, while CD103+ cDCs share similarities with lymphoid tissue-resident CD8 alpha+ cDCs, including their ability to cross-present antigens to CD8+ T cells.

Skin Dendritic Cells

The skin also contains some DC subsets that can also be found in the lymphoid organs. Mouse and human epidermis contains Langerhans cells (LCs) , which are characterized by the expression of langerin and E-cadherin, and are strong stimulators of CD4+ T cells [26,27,28]. Ex vivo experimental models have shown that both mouse and human LCs are strong cross-presenting cells [29]. However, there is conflicting evidence regarding their in vivo cross-presentation capacity [30].

Inflammatory DCs

Inflamed tissues and draining lymphoid organs contain another subset of DCs called inflammatory DCs. Inflammatory DCs are generated from monocytes and express macrophage-specific markers such as F4/80, CD64, and the high-affinity IgE receptor, FcɛRI [31]. Inflammatory DCs can be generated in culture from human monocyte-derived DCs (moDCs) using granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 [32]. Mouse inflammatory DCs function in both innate and adaptive immunity and activate CD4+ T cells for polarizing into Th1-type and Th2-type immunity [33]. During bacterial infection, inflammatory DCs produce iNOS and TNF-α, and their activation is dependent on TLR4 and TLR9 [34]. The first evidence for the role of inflammatory DCs in inducing Th2-type immunity came from the study by Kool et al. [35], where they demonstrated a rapid recruitment of CD11b+F4/80IntLy6Chigh “inflammatory monocytes” to the peritoneal cavity within 6 h after intraperitoneal injection of OVA-albumin. In mice, several infectious models have suggested that inflammatory DCs are critical for Th1-type immunity. Upon immunization or viral infection, these inflammatory DCs produce IL-12 and stimulate T cell-mediated IFN-γ production [33].

Dendritic Cell Functions

Activating Signals from DCs

Dendritic cells express co-stimulatory molecules on their surface, which are important for the induction of immune responses. T cell activation occurs through the recognition of antigenic MHC peptides on DCs by T cell receptors. However, robust T cell responses also require interactions between co-stimulatory ligands on T cells and their receptors expressed on DCs. Without this co-stimulatory molecule interaction, antigen-specific T cells become anergic. DC vaccines can be useful by upregulating co-stimulatory signals in therapeutic immunity [36, 37]. Two major families of co-stimulatory molecules are expressed on dendritic cells: the B7/CD28 family and the tumor necrosis factor (TNF) family receptors . The initiation of the cell-mediated immune response is mediated by B7/CD28 family members, whereas the later phase of T cell activation is dependent on TNF-receptor family members. In the B7/CD28 family, the expression of CD80 (B7.1) and CD86 (B7.2) [37] on DCs is the most important co-stimulatory pathway in T cell activation. Others include ICOS-ligand (ICOSL; B7-H2, [38]), programmed death-1 ligand (PD-L1 or B7-H1), PD-L2 or B7-DC, as well as B7-H3 [39] and B7-H4, also known as B7x and B7S1 [40]. The TNF superfamily co-stimulatory molecules in DCs include CD40, OX40L, CD27, 4-1BB (CD137), TNF receptor superfamily member 4 (TNFRSF4), TNF ligand superfamily member 14 (TNFSF14), and glucocorticoid-induced tumor necrosis factor receptor (GITR) [41,42,43]. T cells express the activating receptor CD28 and the inhibitory receptor CTLA-4 (CD152) on their surface. The co-stimulatory molecules CD80 (B7.1) and CD86 (B7.2) on DCs bind with either the activating receptor CD28 or the inhibitory receptor CTLA-4 on T cells (Fig. 6.1). Interaction of B7.1 or B7.2 with CD28 stimulates the activation of T cells. In contrast, engagement of B7.1 or B7.2 with CTLA-4 inhibits T cell-mediated immune responses. Downregulation of immune responses leads to immune tolerance and autoimmunity. Therefore, the blocking of inhibitory signals through CTLA-4, in addition to the upregulation of B7.1 and B7.2 by immunostimulants, is essential for enhancing antitumor responses [44].

Fig. 6.1
figure 1

(a) Signals generated between T cells and DCs. Both T cells and DCs express complex networks of transmembrane receptors and ligands, which interact with each other to enhance T cell activation. Interaction of T cell receptor (TCR) with MHC complex on DCs generates signal-1. Co-stimulatory signal (signal-2) is required for full activation of T cells, which is mediated by the interaction of CD28 (expressed on T cells) with CD80 and CD86 on DCs. For functional activation of T cells, signal-3 is important. This signal is mediated by soluble factors such as IL-12, IL-15, IL-16, or TNF-α. Generation of signal-3 also depends on the activation of PRRs by PAMPs or TFs, which activates DCs to produce TH1- or TH2-cell polarizing factors. ICOS, expressed on T cells upon activation, interacts with ICOS-ligand (ICOS-L: B7-H2) on DCs to regulate T cell growth, proliferation, survival, and polarization. Ligation of CD40 (expressed on T cells) with CD40L on DCs induces expression of co-stimulatory, adhesion, and MHC molecules and promotes the secretion of T cell stimulatory cytokines like IL-12. T cells also express inhibitory receptors such as CTLA-4 and PD-1, which suppress T cell activation by interacting with ligands (PD-L1, PD-L2) expressed on DCs. (b) Interaction of T cells with tumor. The inhibitory receptors (PD-1, TIM-3) expressed on T cells can bind with their ligands (PD-L1, Galectin-9) expressed on tumor cells. This will inactivate the T cells, providing an immunosuppressive environment for the tumor

Full size image

DCs are pulsed with multiple ligands , including TLR agonists, CD40 ligand, and TNFRSF4 ligand, which induce the expression of co-stimulatory molecules on their surface. TLRs are expressed on most immune cells, and in some cases also on tumor cells. For example, GL261 cells express TLR2, TLR3, and TLR4 and increase MHC class I expression [45]. Activation of DCs with TLR signals induces the upregulation of co-stimulatory molecules [46]. Activated DCs secrete immunomodulatory cytokines (i.e., IL-12) and increase antigen processing and presentation to T cells and B cells. TLR agonist injection at the tumor site (intratumoral injection) produced a survival benefit in multiple tumor models in rodents [47].

DCs also talk to invariant natural killer T cells (iNKTs) through the interaction of CD40 and CD40 ligand. The synthetic iNKT agonist α-galactosylceramide (α-GalCer) promotes T cell responses to DC vaccines. DCs can acquire and present α-GalCer to CD1d molecules (an MHC class I-like molecule, highly expressed in NKT cells) and induce the expression of the co-stimulatory molecule CD40 upon interaction with iNKTs [48].

Inhibitory Signals from DCs

Certain co-stimulatory molecules on DCs, including PD-L1, PD-L2, and B7-H4, inhibit T cell-mediated immune activation [40, 49]. Antibody blocking of PD-L1 and PD-L2 on DCs improves the proliferation and cytokine production of CD4+ T cells. DCs also express suppressive molecules, including zinc finger protein (ZFP) A20, DC-derived immunoglobulin receptor 2 (DIgR2), Notch ligands, and suppressor of cytokine signaling 1 (SOCS1) [50]. Targeting of these molecules could potentially increase the efficacy of DC-based therapeutic vaccines.

Antigen Loading into DCs

The therapeutic efficacy of DC vaccination depends on the effectiveness of the uptake and loading of tumor-specific antigens into MHC complexes, as well as the expansion of DC subsets that can prime naïve T cells for targeted immune activation. Antigens can be comprised of RNA, DNA, proteins, peptides, tumor lysate, fusion proteins, or apoptotic cells [51]. Research on antigen presentation for DC vaccination has shown that whole tumor antigens induced greater clinical responses than single peptide antigens, in consideration of the heterogeneous properties of tumors [52, 53]. In contrast, DCs loaded with apoptotic bodies of GBMs can increase the risk for induction of tolerogenic DCs [54].

DC-Mediated Immune Responses Against Tumors

The goal of DC-mediated cancer vaccines is to induce the formation of tumor antigen-specific cytotoxic CD8+ T cells, which can recognize and kill target tumor cells [55]. Recent evidence has suggested that DCs can capture tumor antigens, process them, and cross-present them to T cells in tumor-draining lymph nodes. This generates tumor-specific cytotoxic CD8+ T cells, which contribute greatly to tumor rejection [56, 57]. Data from clinical trials have indicated that DC vaccination also induces natural killer (NK) cell immunity, which includes enhancing NK cell number and functional activation [58]. DCs have been used for tumor eradication in a mouse melanoma model, and the effect was completely abrogated after depletion of NK cells, indicating the strong positive role of NK cells in DC-mediated vaccination against tumors [59, 60].

DCs are present in most tumors and play an important role in the tumor microenvironment in controlling tumor progression [61]. DCs infiltrate into the tumor site and serve to recruit and activate disease-fighting immune effector cells. Recent advances in the understanding of the tumor microenvironment have led to targeting tumor-infiltrating DCs for cancer therapeutics. Altering immunosuppressive DCs to become immune-stimulatory DCs is one of the strategies employed for effective cancer immunotherapy [61, 62]. DCs can take up tumor-specific antigens by interacting with live tumor cells or by capturing dying tumor cells. Dying tumor cells not only release tumor antigen for uptake by DCs but also express many signaling molecules on their surface, which could be either stimulatory or inhibitory for DCs. For example, tumor cells can express phagocytic markers like CX3CL1 , which activates DCs for phagocytic activity, or CD47, which interacts with signal regulatory protein-a on phagocytes to provide inhibitory signals that prevent phagocytosis. Using a CD47-blocking antibody in combination with rituximab (a CD20 antibody that depletes B cells) results in increased phagocytosis in tumor models in mice [63].

Tumor cells have mechanisms to suppress DC function, or to recruit immune-suppressive DCs to the tumor site [55]. The tumor can switch the differentiation of monocytes to macrophages and prevent the priming of tumor-specific cytotoxic T cells by DCs. In addition, tumor glycoprotein antigens can be endocytosed into early endosomal compartments, which prevents efficient processing and presentation to T cells [64, 65]. Tumor cells also secrete the cytokine IL-10 [66], which can inhibit DC maturation, leading to anergy [67]. In addition, the maturation of mDCs can be altered by tumor-derived factors, leading to the development of DC cell types that indirectly promote tumor growth. pDCs can also play an important role in tumor progression by inducing naïve CD4+ T cells to differentiate into IL-10-producing T cells with immunosuppressive functions [68]. In breast carcinoma, infiltrating pDCs produce little IFN-α upon ligation with TLRs, due to the inhibitory signals from ligation of immunoglobulin-like transcript 7 (ILT7) on pDCs with bone marrow stromal antigen 2 (BST2). DCs require IFN-α for cross-presenting tumor antigens to T cells. Thus, the inhibition of IFN-α signaling affects the generation of tumor antigen-specific cytotoxic T cells [57, 69].

Dendritic Cell Immunotherapy

The main goal of cancer immunotherapy is to stimulate tumor antigen-specific T cells to inhibit the malignant activity of cancer cells. Vaccines should target the cancer cells but also leave healthy cells unaffected [70]. The discovery that vaccination with DCs loaded with tumor antigens stimulates strong and broad immune responses in tumor-bearing animals has rejuvenated the field of cancer vaccine research and has led to many clinical trials. These trials have involved multiple different methods of generating DCs, introducing antigens to DCs in vitro or in vivo, as well as introducing DCs into the body.

Generation of DCs and Antigen Loading

As DCs constitute only about 1% of peripheral blood mononuclear cells (PBMCs) , therapeutic DCs need to be generated from precursors. In the presence of IL-4 and GM-CSF, considerable numbers of DCs can be generated from monocytes or CD34+ progenitors [71,72,73,74]. However, it is unclear whether ex vivo-differentiated DCs are the optimal source for DC-based immunotherapy. Long-term culture of DCs ex vivo, with the cytokines and mediators required for their activation and differentiation, might negatively affect the function of DCs when injected into patients. The newly injected DCs might express different receptors or lose the ability to produce some pro-inflammatory cytokines [75].

DCs loaded with tumor antigens linked to keyhole limpet hemocyanin (KLH) , an immunogenic protein, produce an enhanced antitumor immune response [76]. Tumor-bearing mice immunized with DCs loaded with anti-idiotype protein produced tumor-specific immune responses [77, 78]. Helper antigens can also be coupled with tumor antigens for presentation by DCs, which can produce more robust immune responses against both the tumor target antigen and the helper antigen [79]. Antigens from whole cell lysate can also be loaded into DCs for vaccination. DCs pulsed with myeloma lysates induced myeloma-associated immunity [80]. Whole-cell RNA, DNA, or apoptotic bodies are also used for antigen loading into DCs [81, 82]. Whole cell lysate or cancer cells fused with DCs are used to present a wide variety of tumor antigens, which can ultimately enhance co-stimulatory functions of DCs. MC38 carcinoma cells, which express MHC classes I and II, co-stimulating molecules, and intercellular cell adhesion molecule-1 (ICAM-1), when fused with DCs for vaccination, strongly induced M38 tumor-specific immunity in vivo. Multiple myeloma cells freshly isolated from patients and fused with autologous DCs induced myeloma-specific cytotoxicity [83,84,85,86]. Mycobacterium tuberculosis heat shock protein X (HspX) has been used as an immunoadjuvant in DC-based tumor immunotherapy, which has significant potential in immunotherapeutics. The activated DCs induced pro-inflammatory cytokine production, and in vivo injection into mice significantly attenuated the metastatic capacity of B16-BL6 melanoma cancer cells toward the lung [87].

In Vivo DC Targeting for Immunotherapy

A recent area for DC cancer immunotherapy is to target DCs in vivo, using antibodies of activating receptors in addition to tumor antigens [88,89,90]. DCs process and present the antigen to T cells in order to stimulate an antitumor response. The optimal induction of this immune response also requires the presence of adjuvants, which stimulate DC activation. If an antigen-antibody complex is presented to DCs without adjuvants, the DCs can induce tolerance rather than immunity [91]. Nonactivated (immature) DCs can present self-antigens to T cells, which leads to immune tolerance, either through T cell depletion or through the differentiation of regulatory or suppressor T cells [92,93,94,95]. Naturally occurring regulatory T cells (CD4+) are important for the maintenance of immune tolerance. CD4+ regulatory T cells express CD25 (IL-2Ra), CTLA-4, and the transcription factor FoxP3, and exert their immunosuppressive effect either in a cell contact-dependent manner or by producing the cytokines IL-10 and TGFβ [96, 97]. Several studies have shown that using adjuvants (TLR agonists) combined with antigen-antibody conjugates stimulates antigen-specific immunity and prevents tolerance [98]. Other studies have shown that targeting the molecules expressed on DCs for delivering the antigen elicits antitumor responses. These include endocytic receptor DEC-205 [99,100,101], CD11c [102, 103], C-type lectin domain family 9 member A (Clec9A), MHC class II [104, 105], lectin-like oxidized low-density lipoprotein (LDL) receptor 1 (LOX1), mannose receptor (MR) [106, 107], CD36 (also known as fatty acid translocase (FAT)) [108], and bone marrow stromal cell antigen 2 (Bst2 or CD317), a molecule expressed on pDC [109]. DCs upregulate surface expression of CXCL16 following in vivo injection of the glycolipid antigen α-GalCer, which interacts with iNKT cells for IFN-γ production and tumor control against the metastatic B16 melanoma model in the liver and lung [110]. T cells activated by autologous tumor antigen-pulsed DCs were used for immunotherapy in human patients with endometrial cancer, where tumor-specific cytotoxic CD8+ T cells were generated [111]. Also in humans, DC-mediated idiotype vaccination was used against multiple myeloma [112] and against prostate cancer [113]. Antigen-loaded upconversion nanoparticles (UCNPs) have been used to label and stimulate DCs in vivo to induce antigen-specific immune responses by producing IFN-γ and generating CTLs [114]. Polymers of lactic acid and glycolic acid (PLGA) have also been used to supply DCs with cytokines, TLR ligands, and tumor lysates in vivo, which stimulated tumor-specific T cell responses against melanoma and glioma in tumor-bearing animals [115, 116]. Here in Table 6.1, we have summarized the results of clinical trials that used DCs as immunotherapy in multiple different tumor targets, either alone or in combination with other agents.

Table 6.1 Published clinical trial results of dendritic cell-based cancer immunotherapy
Full size table

DC Vaccines Combined with Other Therapies

Animal studies using DC vaccines combined with IL-2 for the treatment of sarcoma and squamous cell carcinoma (SCC) showed significantly suppressed tumor growth [134, 135]. Intratumoral injection of DCs combined with systemic chemotherapy (cyclophosphamide) led to complete tumor regression in a murine CT26 colon adenocarcinoma model [136]. Recent studies have shown that DC-based immunotherapy can also be combined with suppressive signals to positively regulate anticancer immunity. T cells express PD-1, which can bind with its ligand PD-L1 (expressed on the tumor cells or cancer cells) and suppress the immune system. Targeting this inhibitory signal, either by using siRNA of PD-L1 or using anti-PD-1 and anti-PD-L1 agents , has demonstrated huge clinical success in a wide range of malignancies [137]. Ipilimumab , an anti-CTLA-4 antibody, was also used in combination with DCs for the treatment of metastatic melanoma. Adriamycin plus DC combination therapy was also used for the B16 melanoma model [136]. CpG oligonucleotides (TLR ligand) combined with mature antigen-pulsed DCs were used in a syngeneic murine colon carcinoma model [138]. Autologous DCs loaded with tumor lysate or glioma-associated antigen in combination with radiotherapy were used to treat patients with malignant glioma [139]. Patients with treatment-refractory cancers were injected intratumorally with immature DCs that had been combined with KLH and a cytokine-based adjuvant. This was followed by an IV infusion of activated T cells. This treatment course was administered before and after radiation, which killed tumor cells so that DCs could use them as a source of antigen. Half of the patients in the trial showed a complete clinical response [140]. Table 6.2 summarizes the list of current phase III clinical trials using DC-based immunotherapy combined with chemotherapy or other agents. A recent cell type-based immunotherapy called sipuleucel-T consists of APCs (including DCs) activated by a recombinant fusion protein (PA2024) consisting of prostatic acid phosphatase fused to GM-SCF. Sipuleucel-T was used to treat patients with metastatic castration-resistant prostate cancer in a phase III clinical trial. Data from the study indicated short-term survival benefits and immune activation [141,142,143]. These studies suggest that combination therapy of DCs with other conventional therapies could be very effective against different types of cancer.

Table 6.2 Current phase III clinical trials of dendritic cell-based cancer immunotherapy
Full size table

DC Vaccine Clinical Trials

Many of the DC vaccine discoveries from animal models have been translated to human clinical trials, with varying levels of success (Fig. 6.2). The first clinical study was performed in 1996 and involved the administration of immature DCs pulsed with tumor-specific idiotype protein to B-cell lymphoma patients [117]. Two years later, DCs were being pulsed with tumor lysate or MHC I peptide [118]. By 1999, mature DCs were being explored [119, 144, 145], which included stimulating these DCs with IL-2 [135]. Subsequent studies revealed that immature DCs were tolerogenic [146, 147]. In the early 2000s, researchers continued to test different antigens, including MHC I and II peptides [148, 149] and tumor-derived RNA [122,123,124]. Researchers also generated DC/tumor cell hybrid vaccines [121], as well as DC vaccines using xenoantigen [150]. The route of DC administration was also found to be important for clinical outcomes [151]. DC exosome-based vaccines were also tested [152]. The combination of a DC vaccine with cyclophosphamide was later used in human renal cell carcinoma (RCC) patients [153]. Allogenic DCs were fused with tumor cells to produce a hybrid vaccine against metastatic RCC [125]. DCs were also activated with type I IFN [154] or IFN-γ [155] to enhance the immune response. In the second half of the 2000s, researchers began loading DCs with allogenic tumor cell lysates [127, 128]. DCs have also been combined with poly (I:C) [129] and vaccine-primed T cells [131]. Langerhans cell-based vaccines were also found to be effective at inducing immune responses to certain types of cancer [130]. DC vaccines have been used in conjunction with the depletion of regulatory T cells [126] or myeloid-derived suppressor cells (MDSCs) [156]. A DC/tumor fusion vaccine has also been tested in multiple myeloma patients following autologous stem cell transplant [132]. A recent study also revealed that preconditioning the vaccination site with tetanus/diphtheria toxoid (Td) improves DC migration and survival in human glioblastoma patients [133].

Fig. 6.2
figure 2

Timeline of the development of the dendritic cell-based cancer vaccines . DCs were first discovered in 1973. However, they were not successfully generated ex vivo until 1994. This allowed researchers to begin exploring the potential of DCs for therapeutic cancer vaccines. The first clinical study was performed in 1996 and involved the administration of immature DCs pulsed with tumor-specific idiotype protein to B-cell lymphoma patients. Two years later, DCs were pulsed with tumor lysate or MHC I peptide. By 1999, mature DCs were being explored, which included stimulating these DCs with IL-2. Subsequent studies revealed that immature DCs were tolerogenic, so researchers fully shifted to mature DCs. In the early 2000s, researchers continued to test different antigen-loading conditions and different antigens, including MHC I and II peptides and tumor-derived RNA. In the second half of the decade, researchers continued to refine antigen sourcing and loading while combining DC-based vaccines with additional treatments. These included depletion of regulatory immune cells, as well as combination with other therapies such as chemotherapy and IFN-γ. This focus on combination therapies and removing inhibitory cells has generally been maintained over the last few years. Recent studies have involved combining DC vaccines with stimulated T cells and using DC/tumor cell hybrid vaccines after stem cell transplant for hematological cancers

Full size image

Conclusions and Future Directions

DCs have been used for cancer immunotherapy for the past 20 years with diverse clinical outcomes. In animal model studies, DCs have been effective at generating tumor antigen-specific cytotoxic T cells, which ultimately kill tumor cells and produce long-term survival. So far, data from human clinical trials has suggested that DC-based vaccines are safe, yet the clinical responses have been generally less impressive than in animal models. Even with these setbacks, the FDA approved the first therapeutic DC-based vaccine in 2010 (sipuleucel-T, Provenge®) for use in patients with asymptomatic or minimally symptomatic metastatic hormone-refractory prostate cancer [141, 157].

Researchers continue to make new discoveries that improve the efficacy of DC-based cancer vaccines. DC immunotherapy has inherent limitations due to the immunosuppressive environment in the tumor site. However, combination therapy with antibodies or other immunomodulating agents might play an important role for generating antigen-specific antitumor immunity. Hopefully, further optimization of these therapy regimens will result in improved clinical outcomes in patients.