Tumor cell lysates as immunogenic sources for cancer vaccine design

Abstract

Autologous dendritic cells (DCs) loaded with tumor-associated antigens (TAAs) are a promising immunological tool for cancer therapy. These stimulate the antitumor response and immunological memory generation. Nevertheless, many patients remain refractory to DC approaches. Antigen (Ag) delivery to DCs is relevant to vaccine success, and antigen peptides, tumor-associated proteins, tumor cells, autologous tumor lysates, and tumor-derived mRNA have been tested as Ag sources. Recently, DCs loaded with allogeneic tumor cell lysates were used to induce a potent immunological response. This strategy provides a reproducible pool of almost all potential Ags suitable for patient use, independent of MHC haplotypes or autologous tumor tissue availability. However, optimizing autologous tumor cell lysate preparation is crucial to enhancing efficacy. This review considers the role of cancer cell-derived lysates as a relevant source of antigens and as an activating factor for ex vivo therapeutic DCs capable of responding to neoplastic cells. These promising therapies are associated with the prolonged survival of advanced cancer patients.

Keywords:

• dendritic cells
• cancer immunotherapy
• DAMPs
• Toll-like receptors
• immunogenic cell death

Abbreviations

TRIMEL	=	Allogeneic melanoma cell lysate
TRIPRO	=	Allogeneic prostate cell lysate
Ags	=	Antigens
CRT	=	Calreticulin
CDAMs	=	Cell death-associated molecules
AM	=	Cytokine-activated monocytes
CTLs	=	Cytotoxic T lymphocytes
DAMPs	=	Damage-associated molecular patterns
DTH	=	Delayed-type IV hypersensitivity
DCs	=	Dendritic cells
GM-CSF	=	Granulocyte and macrophage colony stimulating factor
HSPs	=	Heat shock proteins
HMGB1	=	High-mobility group box 1 protein
ICD	=	Immunogenic cell death
MHC	=	Major histocompatibility complex
MM	=	Malignant melanoma
MAAs	=	Melanoma-associated antigens
NKT	=	Natural killer T cell
PAMPs	=	Pathogen-associated molecular patterns
PRRs	=	Pattern recognition receptors
PBMCs	=	Peripheral blood mononuclear cells
PD1	=	Programmed cell death protein 1
PCCL	=	Prostate cancer cell lysate
PSA	=	Prostate specific antigen
RAGE	=	Receptor for advanced glycation endproducts
Tregs	=	Regulatory T lymphocytes
SNPs	=	Single nucleotide polymorphisms
TCRs	=	T cell receptors
TLRs	=	Toll-like receptors
TAPCells	=	Tumor antigen presenting cells
TNF	=	Tumor necrosis factor
TAAs	=	Tumor-associated antigens

Introduction

Standard treatments for disseminated tumors, through the use of surgical or radio/chemotherapy procedures, provide limited results that rarely change the disease outcome. During the last 2 decades, several immunotherapy approaches have been tested as alternative treatments against solid and hematological malignancies.¹ The use of recombinant cytokines such as IL-2 and IFN-α, T cell-mediated adoptive therapies, monoclonal antibodies, and dendritic cells (DCs)-based vaccines, among others, have provided improvements in the control of tumor growth and patient survival.¹ However, despite the relative effectiveness of these treatments, disadvantages are still prevalent in a large proportion of cases in regards to the percentage of refractory patients and side effects.

Although the adoptive transfer of tumor-specific CD8⁺ T cells with or without systemic immune suppression has been used in different clinical trials and has resulted in significant reductions of tumor size, some relevant adverse reactions were observed in treated patients.^2-4 More recently, the re-infusion of ex vivo, genetically manipulated, autologous CD8⁺ T cells that express high-affinity T cell receptors (TCRs) for melanoma-specific antigens has shown promising results in relation to tumor regression.⁵ In this context, the identification of new therapeutic targets in melanoma and immune cells has enabled the development of a wide range of monoclonal antibodies aimed at modulating the antitumor T cell-mediated immune response and at eradicating tumors.^6,7 In 2011, the United States Food and Drug Administration and the European Medicines Agency approved Ipilimumab, an anti-CTLA-4 monoclonal antibody, for the treatment of patients with advanced melanoma.⁸ Furthermore, an anti-PD1 monoclonal antibody which targets a co-inhibitory molecule expressed in activated T lymphocytes has shown promising results in phase I and II clinical trials.^9-11 Immunotherapy based on monoclonal antibodies (mAbs) is effective in up to 30% of treated patients. However, this type of treatment is frequently limited in duration. Additionally, some types of cancers are not responsive to mAb-based immunotherapy. Currently, little is known about the mechanisms underlying the differential clinical responses of patients to mAb-based immunotherapy.¹² An additional strategy for cancer immunotherapy is the use of DCs as cell-based vaccines. DC-based vaccination strategies are likely to be safe and, most importantly, capable of providing long-lasting protective immunity.^{13, 14} Nevertheless, although this strategy has been applied with success in clinical trials, an important percentage of treated patients remain refractory to these new approaches.

DCs are professional antigen presenting cells (APCs) that, upon encountering antigens (Ags), efficiently trigger adaptive immunity against pathogens and/or tumors,^13,15-19 thus establishing a link between the innate and adaptive arms of the immune system.²⁰ In humans, DCs that are found in blood, lymph nodes, and tonsils can be subdivided into plasmacytoid DCs and resident BDCA1⁺ DCs and BDCA3⁺ DCs.²¹ The latter of these share similarities with CD8α⁺ DCs that are found in mice and that excel at antigen presentation to CD8α⁺ T cells.²² In the skin, Langerhans cells, CD1a⁺ dermal DCs, and CD14⁺ dermal DCs can be found, with these having the ability to migrate to skin-draining lymph nodes.²¹ In addition, DCs residing in the periphery may also originate from peripheral blood monocytes when they are recruited to these tissues by pro-inflammatory signals.²³ In fact, human DCs can be differentiated in vitro from several cellular sources, including bone marrow, umbilical cord blood, and peripheral blood mononuclear cells (PBMCs), by using a variety of cytokines and activating factors such as the granulocyte and macrophage colony stimulating factor (GM-CSF) and IL-4.^24-26

With respect to the application of DC-based immunotherapy, our group has performed a series of clinical trials in advanced malignant melanoma (MM) and prostate cancer patients using ex vivo-generated DC-like tumor antigen presenting cells (referred to as TAPCells). TAPCells are generated from autologous cytokine-activated monocytes (AM) using allogeneic cell lysates (referred to as TRIMEL and TRIPRO) derived from 3 melanoma and prostate cancer cell lines, respectively.^14,27-30 In these studies, we reported a correlation between the positive immune response induced by DC-vaccination, as established by a patient tumor-specific delayed-type IV hypersensitivity (DTH) reaction, and improved long-term patient survival in late-stage MM, which is an excellent predictor for clinical response.^14,27,28 Additionally, in prostate cancer patients, TAPCell-based immunotherapy is a safe approach capable of inducing memory T lymphocytes, which might be associated with clinical responses, including decreased serum Prostate Specific Antigen (PSA) levels and increased PSA doubling time.³¹ Despite these positive outcomes, a large proportion of treated patients (∼40%) do not respond to the therapy and present the same survival rate as non-treated patients.^14,27,28 This lack of response could be explained, at least in part, by absence of sufficient immunogenic danger signals, either during DCs ex vivo generation or immunization and/or to deficiencies in antigen processing and presentation by injected DCs, which might also be related to a deficient delivery of danger signals to DCs.²⁸

Several factors impact the efficacy of vaccination protocols using ex vivo-generated DCs. One of the most relevant aspects includes the expression and biological properties of specific receptors activated during DCs stimulation, mainly pattern recognition receptors (PRRs), in addition to the subsequent process of DCs maturation and activation in response to PRR triggering. Given this context, cancer vaccine approaches need to include a strategy for efficiently activating ex vivo-produced DCs³² and, more importantly, for ensuring that this process results in a clinically effective and reproducible anticancer response in vivo. Taken together, these constraints support the imminent necessity to develop more tolerable, less expensive, and more effective therapeutic approaches that could particularly help patients with advanced metastatic disease.

The present report discusses the role of stressed, cancer cell-derived lysates as a strong immunological stimulus for therapeutically used ex vivo-generated DCs, in addition to exploring the potential contribution of these in developing more efficient DC-based immunotherapies against cancer.

Allogeneic tumor cell lysates as a source of antigens for loading of DCs

An effective antigen presentation from DCs for initiating specific anticancer cellular immunity requires optimal activation and migration to secondary draining lymphoid organs¹³ where they can engage Ag-specific, naïve CD4⁺ and CD8⁺ T cells. This process results in T-cell activation, proliferation, and mobilization to peripheral tissues where these cells carryout effector functions.^13,27,33-35

Remarkably, DCs are the main cell type able to present exogenous peptides loaded onto major histocompatibility complex (MHC) class I molecules to naïve CD8⁺ T cells in a process denoted cross-presentation.^36,37 The cross-presentation process has proved essential for the generation of cytotoxic T lymphocytes (CTLs) against viruses, transplanted cells, and tumor cells.^38,39 During an antitumor immune response, DC-mediated cross-presentation subsequent to the uptake and processing of material derived from apoptotic, necrotic, or even live cancer cells constitutes a relevant natural mode of TAA-presentation, thereby contributing to an efficient triggering of immunity against tumors.⁴⁰

Optimal delivery of tumor Ags is also a crucial aspect in DC-based immunotherapy success. Several methods for Ags preparation/delivery to DCs have been developed to improve capturing and presentation of Ags by DCs.⁴¹ These methods include the use of RNA and DNA derived from tumor cells; TAA-derived synthetic peptides; the generation of recombinant proteins; tumor-derived apoptotic bodies; and transfection of DCs with vectors codifying for TAAs, among others. Synthetic peptide-based vaccines require knowledge of the patient's haplotypes and specific epitopes suitable for binding the MHC. Most clinical trials using synthetic peptide-induced immunological responses mediated by CD4⁺ and CD8⁺ T-cells have failed to produce objective clinical responses or improvements in patient survival.^42-45 One possible explanation may be due to the probable induction of tolerance through high affinity peptides, the limited persistence of peptide-MHC complexes on DCs, tumor escape by clones lacking antigen expression, or the absence of immunological danger signals associated with Ags.²⁸ The combined results of these studies suggest that peptide-loaded DC immunization could be a highly specific strategy, but further efforts are required to produce significant therapeutic effects.^42-44,46 Related to this, longer peptides (28–35 amino acids long) have been recently tested in cervical cancer patients and have been shown safe and immunogenic.^47-49 These peptides are unable to directly bind MHC molecules, and must be internalized and processed by DCs for presentation.⁵⁰ To date, one of the most successful immunotherapeutic approaches has been sipuleucel-T (Provenge®, Dendreon), which is the first FDA-approved cell-based therapy for the treatment of hormone-refractory prostate cancer patients.⁵¹ Sipuleucel-T consists of autologous PBMCs, including DCs, B cells, monocytes, and natural killer (NK) cells, that have been activated ex vivo with a recombinant fusion protein containing prostatic acid phosphatase, a prostate cancer associated antigen, fused to GM-CSF. Sipuleucel-T has shown overall, prolonged survival with moderate side effects among men with metastatic castration-resistant prostate cancer.⁵¹

Transfection of DCs with tumor-derived cDNA or mRNA appears to be an interesting approach for TAAs delivery. Furthermore, the mRNA coding for co-stimulatory molecules could be co-transfected to ensure the induction of a mature phenotype on DCs. Nevertheless, this technique allows the delivery of a limited amount of antigens due to a damaged integrity and viability of DCs.⁵²

Another tested method consists in loading DCs with attenuated pathogenic particles (derived from bacteria or viruses) containing genes encoding for TAAs with the purpose of inducing their expression coupled with pathogen associated molecular patterns (PAMPs).⁵² Despite being an interesting concept, it is important to evaluate the immune response developed against DNA or proteins from the vector, which could limit the clinical efficacy of this approach.

Additionally, autologous tumor cell lysates, whole tumor cells, and tumor-derived mRNA have also been tested as antigen providers for DCs.^53-56 When fused or loaded with autologous tumor cells or tumor lysates, these cells induce a stronger and more extensive immunological response against tumors.^53-59 Still, these therapies are limited to a reduced proportion of patients that have tumor masses at surgically accessible sites, therefore ensuring the possibility of obtaining the amount of biological material required.

One of the simplest and most promising sources of tumor Ags is the preparation of allogeneic cancer cell lysates.^{14,28-30,60-62} An advantage of this strategy is that it provides a standardized, applicable source of tumor-specific Ags, which is also useful in high-risk, tumor-free patients. Furthermore, allogeneic cancer cell lysates constitute a valuable alternative for obtaining immunogenic DCs (Table 1).^28,60,63,64

Table 1. Comparison of different strategies for antigen-delivery to DCs

Loading methods	Advantages	Disadvantages
Tumor cell lysates	Allows the loading of a wide variety of antigens. Capable to induce both CD4^+ and CD8^+ T-cell response. Gives different DAMPs to DCs to ensure maturation. (6)	Can provide immunoregulatory cytokines to dendritic cells that could induce a tolerogenic transformation. (6)
Purified Tumor-associated antigens	Activates antigen-specific T cell response Capable of inducing both CD4^+ and CD8^+ T-cell response. (1–4)	Requires the knowledge of the patient's HLA haplotype if short peptides are used. (3) Gives a small amount of different antigens. Does not induce maturation on dendritic cells.
Tumor-derived mRNA	Allows the transfection of TAAs and co-stimulatory molecules. Ensure presentation in MHC class I without requiring cross-presentation. (5)	Could not induce a potent CD4^+ immune response. Gives a small amount of different antigens.
Tumor-associated antigen transfected vectors	Gives specific TAAs in a pro-inflammatory way. Could be used to load DCs both in vivo or in vitro. (3)	Requires the use of vectors Could reduce effectiveness by inducing anti-vector immune responses. (3)

Tumor cell lysates are excellent sources for delivering of a wide variety of Ags associated with MHC class I/II molecules, inducing a more integral immune response. Importantly, the method for inducing cell death or chemical protein modifications during whole tumor lysate preparation could impact the immunogenicity and efficacy of the therapy. Studies in murine models using DCs pulsed with tumor lysates have shown significant results in the induction of potent immune responses, as evidenced by the generation of specific CTLs against tumor Ags and a significant reduction of tumors in these animals.^65-67 Moreover, several studies using DCs loaded with mainly autologous, but also allogeneic, tumor lysates have been performed with positive results in mice and humans.^68-71 Likewise, positive results using allogeneic lysates from a variety of human tumor cell lines have been obtained in several clinical trials for the treatment of different types of cancer.^{14,28, 72,73} However, a potential problem lays in the fact that some cancer cells are able to secrete immunoregulatory cytokines such as IL-10 and TGF-β during the cell culture process, thus inducing a more tolerogenic phenotype on DCs. Related to this, the majority of protocols designed to produce tumor cell lysates for DC loading include several washing steps, which result in a minimal amount of these cytokines present in preparations.²⁷ Taken together, these studies show that DCs pulsed with tumor lysates or apoptotic bodies are safe and capable of inducing immune responses in a broad vaccinated population, thus representing a promising approach for future studies. In this same line, TRIMEL, a tumor lysate generated from a mixture of 3 established metastatic melanoma cell lines, contains the majority of the currently described MAAs (Table 2).²⁸ In fact, we have previously shown that TAPCells induced IFN-γ release by an HLAA2⁺-restricted/MART-1–specific CD8⁺ T-cell clone (epitope MART-1_27-35), therefore highlighting the ability of TAPCells to cross-present exogenous MAAs in the context of MHC class I to melanoma-specific CD8⁺ T lymphocytes.²⁸ Tumor-associated danger signals derived from the lysate may be responsible for an efficient Ag cross-presentation process mediated by TAPCells.²⁸ Related to this, it is important to distinguish between the capacity of cancer cell lysates to provide a wide spectrum of TAAs from their capacity to provide danger signals that trigger the optimal maturation and activation of APCs.

Table 2. MAA expression of the 3 melanoma cell lines that compose the TRIMEL lysate.

	Cell line
Antigens	Mel1	Mel2	Mel3
MART-1/MelanA^#	+	+	+
Gp100*	+	+	+
MC1R^#	+	+	+
MCSP^#	+	+	+
S-100*	+	+	+
NY-ESO-1^¶	+	+	+
Her2/Neu^#	+	−	+
MAGE1^§	+	−	−
MAGE3^§	−	+	+

(*) Immunohistochemistry, (#) flow cytometry, (§) RT-PCR, and (¶) Western blot.

Tumor cell lysates as immune activators in DC-based vaccines: Interplay of damage-associated molecular patterns (DAMPs), PRRs, and tumor immunogenic cell death (ICD)

Recent studies suggest that DCs, through their PRRs, interpret signals from peripheral physiological or pathological microenvironments. Depending on the nature, amount, and combinations of these stimuli, DCs acquire different functional capabilities.^26,74

Several endogenous factors are translocated to the cell membrane or are released into the extracellular milieu by dying, stressed, or injured cells. These signals can alert the immune system and initiate repair and remodeling mechanisms in damaged tissues.⁷⁵ These so-called damage-associated molecular patterns (DAMPs) can function as either as adjuvants or danger signals for immune cells or can also play an important role in homeostatic mechanisms. A remarkable characteristic of DAMPs is that the majority of these molecules have completely distinct, non-immunological related functions under normal physiological conditions.⁷⁶

DAMPs are normally absent or found in very low concentrations in the extracellular matrix of any given tissue, but in conditions of tissue damage, these molecules are either exposed or secreted and can interact with almost all types of immune cells, such as DCs, through PRRs on their surface. This interaction is primarily mediated by Toll-like receptors (TLRs), which compose a family of membrane-spanning proteins that recognize structurally conserved self- and pathogen-related molecules.⁷⁷ To date, 10 different TLRs have been described in humans that recognize different DAMPs⁷⁸ and PAMPs.⁷⁹ These receptors mediate the interaction between immune system cells and pathogens and play a central role in the innate immune response.⁸⁰ The signals mediated by different TLRs also have a crucial impact on the induction and regulation of effective adaptive immune responses against pathogens and tumors.^{29, 81-83} In addition to TLRs, several PRRs have been found involved in sensing DAMPs. These include receptors belonging to the family of C-type lectins, such as CLEC9a and Mincle,⁸⁴ and cytosolic PRRs, such as DAI, AIM, RIG-I, MDA-5, and NLRP3.⁸⁵

In fact, the events associated with DCs maturation and migration are partly the result of PRR activation expressed on the cell surface,⁸⁶ the synthesis of which is regulated by the environment where these cells reside.^87,88 Due to this aspect, it is important to recognize that adequate PRRs stimulation during the ex vivo-generation of DCs can minimize the possibility of obtaining a tolerogenic phenotype, ensuring instead an immunogenic DC phenotype with effective priming CD4⁺ and CD8⁺ T lymphocytes that could trigger an in vivo cellular response against neoplastic cells.

Recent evidence suggests that the way in which tumor cells die could be a key factor in triggering an appropriate anti-tumor immune response.^89,90 It is therefore relevant to identify danger signals induced during the cell death process that could be involved in both antigenicity and adjuvanticity. Recent studies have suggested that radiation and certain chemotherapeutic agents are able to induce a variety of stress signals, such as Heat Shock Proteins (HSPs), the translocation of calreticulin (CRT, a well described “eat-me” signal), and the chromatin-associated protein high-mobility group box 1 (HMGB1) that can act as an adjuvants in Ag delivery.^65,91-95

Immunogenic cell death (ICD) of cancer cells is a novel concept that has emerged during the last decade, and it underlines the fundamental role of the immune system in cancer biology in regards to the identification of DAMPs released by tumor cells during ICD.⁷⁵ In an attempt to differentiate the specific origin of DAMPs between distinct mechanisms, specific danger signals exposed or released during ICD have been referred as to cell death-associated molecules (CDAMs).⁸⁵ Several studies have reported that cancer cell lines treated ex vivo with chemotherapeutic drugs, photodynamic therapy, or gamma-irradiation and implanted subcutaneously into syngeneic immunocompetent mice work as a cancer vaccine, even in the absence of any adjuvant.^92,96-98 Additionally, a proportion of these mice are protected against subsequent challenges with untreated live cancer cell lines. Moreover, tumor cell death caused by radiotherapy also promotes cross-presentation.^83,99,100 In this context, specific DAMPs such as surface exposed CRT, secreted ATP, and passively released HMGB1 and subsequent interactions with phagocytosis receptors, purinergic receptors, and PRRs, respectively, are required for ICD, and this ultimately leads to the activation of potent anticancer immunity (Table 3).^{85,96,101-105}

Table 3. Danger signals (DAMPs) associated with cancer cell death and their immune effects.

DAMPs	Best known immune effects	References
HMGB1	Mediates inflammatory response; chemoattractant for immune cells; DCs maturation; enhances Ags cross-presentation	33, 95, 105, 107, 117
CRT	“Eat me” signal that contributes to DCs surface expression of MHC-II molecules	33, 114, 117
HSP proteins family (HSP70, HSP90)	Peptide carrier function; adjuvants in processing and presenting Ags by DCs	92, 115–117
ATP	“Find me” signal; IL-1β maturation; inflammasome activation	88, 89, 125

Additionally, radio- and chemotherapy ICD are 2 well-described and important factors in patient response to treatment. In other words, the release of danger signals during cancer cell death, as induced by these anticancer therapies, is relevant to increase patient survival.⁸³ Interestingly, ICD of cancer cells appears to not only play a role in the in vivo response to gold standard anticancer therapies, but it is also a fundamental mechanism that can be exploited during DC-based immunotherapy.

Tumor ICD depends, at least in part, on the specific DAMPs released by dying/stressed cells and the danger signaling response triggered by these on immune cells.⁷⁵ By far, most of the studied ICD inducers are chemotherapeutical agents. Enhanced immunogenicity of tumor cell death depends, at least in part, on the death-initiating stimulus. In this context, some but not all cancer cell death inducers cause exposure of danger signals on the cell surface (such as ecto-CRT and ecto-HSP70) or release into the extracellular space (such as HMGB1 or ATP).¹⁰⁶ In fact, Gamma and UVC-irradiation are able to trigger CRT exposure, an “eat me” signal for APCs, on apoptotic cancer cells.^92,107 Recently, the use of hypochlorous acid before lysate purification was examined as a way to improve the immunogenicity of ovarian cancer lysates through oxidation.^108,73

Interestingly, heat shock is another strong stimulator of ICD. In an in vitro study, it was shown that human DCs loaded with melanoma cells that were heat-treated at 42°C prior to cell lysis were more efficient at cross-priming naïve human CD8⁺ T cells than DCs loaded with unheated, killed melanoma cells.¹⁰⁹ These heat-treated melanoma cells expressed enhanced amounts of HSP70, and the enhanced cross-priming could also be reproduced by overexpressing HSP70. In the lysate generation protocol used by our group, the ICD of cancer cells was induced in vitro by heat shock at 42°C for one hour followed by an additional 37°C for 2 hours.¹⁴ Importantly, heat-shock conditioning of cancer cells increased their CRT plasma membrane translocation and induced the release of HMGB1 protein.²⁸ CRT and HMGB1 mobilization were associated with enhanced maturation of DCs and efficient Ag cross-presentation capacity, respectively.²⁸

Additionally, HMGB1 co-localizes with TLR4 in monocytes, therefore the blockage of TLR4 inhibits the expression of maturation-associated markers, pro-inflammatory cytokines, and the CCR7 chemokine receptor induced by tumor cell lysates.²⁹ Interestingly, TLR4 gene-specific single nucleotide polymorphisms (SNPs) are thought to be related with a diminished immune response in TAPCell-vaccinated MM patients and in breast cancer patients treated with radiotherapy and chemotherapy.^29,83 The ability of DCs to migrate in vivo to draining lymph nodes, a relevant pre-requisite for its clinical efficacy, is also increased through tumor cell lysate stimulation.¹¹⁰ In summary, accumulated evidence supports the notion that heat conditioning is able to induce ICD in cancer cells and, in turn, generate cancer cell-derived lysates that establish proper conditions for ex vivo-generated antitumor-DCs.

Despite currently significant research in this field, there is no consensus about whether the immunomodulatory effects of DAMPs on APCs can be categorized based on their timing. In fact, DAMP-mediated effects in the early stage, such as chemotaxis, phagocytosis, and pro-inflammatory cytokine production, have not yet been defined in relation to late stage effects, such as DCs migration to draining lymph nodes, the proper expression of co-stimulatory molecules, and TAAs cross-presentation to naïve CD8⁺ T lymphocytes.

Furthermore, there is still debate in defining which ICD inducers are more suitable for ex vivo and/or in vivo immune system stimulation, or whether specific combinations of these are more efficient in triggering an adaptive anticancer immune response.

Concluding remarks

The biology of DCs and their interaction with other immune cells is not completely understood. Several aspects of DCs generation and vaccination require optimization, including adequate stimulation using specific signals through specific receptors for DCs activation and maturation, correct antigen loading of DCs, and an adequate delivery of DCs to ensure appropriate migration to T lymphocyte areas in draining lymphoid tissues. The generation of cancer cell-derived lysates, after ICD induction, seems to combine TAAs and proper danger signal to ensure a committed phenotype of ex vivo-generated DCs. This process results in an efficient polarization of T lymphocytes in an anticancer-Th1/Th17 immune response, resulting in objective clinical benefits for cancer patients (Fig. 1). Immunotherapeutic approaches have been applied in a variety of tumors with different clinical and immunological results. Implementing combination therapies that target distinct arms of antitumor immunity, including immunization and checkpoint antibody therapy, might be synergistic and may result in improved clinical benefits that could accomplish stronger, more sustained responses and long-lasting tumor destruction, therefore leading to better survival and quality of life for patients.

Figure 1. Summary of the effects of stressed cancer cell-derived lysates on the phenotypic and functional characteristics of ex vivo-generated DCs. The ex vivo stimulation of therapeutic TAPCells with 2 different cancer cell-derived lysates (TRIMEL and TRIPRO) induced rapid (24–48 hours) and committed maturation to Th1/Th17 polarizing DCs. This is achieved through the engagement of different cancer cell-expressed DAMPs (e.g. CRT and HMGB1), and by induction through a previous heat conditioning of cancer cells, with different innate immune-receptors (e.g., TLR2, TLR4, and RAGE) expressed on the surface of AM (an immature DC phenotype). These functional characteristics include the secretion of pro-inflammatory cytokines (TNF-α, IL-6, and IL-1β), the migration capacity to draining lymph nodes, Th1 and Th17 polarizing cytokines (IL-12, IFN-α, and IL-17) and the presentation and cross-presentation of TAAs to naïve CD4⁺ and CD8⁺ T lymphocytes, respectively, which can explain, at least in part, the in vivo responses observed in vaccinated MM and prostate cancer patients.^{14,27–29,31,110}

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

The authors thank Mr. Eugenio Rivas and Mr. Cristián Pereda for artwork assistance, and Mr. Dagoberto Donoso for technical assistance.

Funding

This work was funded by grants from the Millennium Science Initiative of the Ministry of Economy, Development and Tourism (P09/016-F, F.S.-O.); the Chilean National Fund for Scientific and Technological Development (FONDECYT 1130320 F.S-O., 11130607 F.E.G., and 1130324 M.L.); Funding for the Promotion of Scientific and Technological Development (FONDEF D11I1036 F.S.-O. and ML); and the National Commission of Scientific and Technological Research (CONICYT, Advanced Human Capital Program, F.E.G., CONICYT Programa de Atracción de Capital Humano Avanzado, PAI 82130031, F.O.).