Murine dendritic cells transfected with human GP100 elicit both antigen-specific CD8(+) and CD4(+) T-cell responses and are more effective than DNA vaccines at generating anti-tumor immunity

An Endogenous Retrovirus Vaccine Encoding an Envelope with a Mutated Immunosuppressive Domain in Combination with Anti-PD1 Treatment Eradicates Established Tumours in Mice

Endogenous retroviruses (ERVs) account for 8% of our genome, and, although they are usually silent in healthy tissues, they become reactivated and expressed in pathological conditions such as cancer. Several studies support a functional role of ERVs in tumour development and progression, specifically through their envelope (Env) protein, which contains a region described as an immunosuppressive domain (ISD). We have previously shown that targeting of the murine ERV (MelARV) Env using virus-like vaccine (VLV) technology, consisting of an adenoviral vector encoding virus-like particles (VLPs), induces protection against small tumours in mice. Here, we investigate the potency and efficacy of a novel MelARV VLV with a mutated ISD (ISDmut) that can modify the properties of the adenoviral vaccine-encoded Env protein. We show that the modification of the vaccine's ISD significantly enhanced T-cell immunogenicity in both prime and prime-boost vaccination regimens. The modified VLV in combination with an α-PD1 checkpoint inhibitor (CPI) exhibited excellent curative efficacy against large established colorectal CT26 tumours in mice. Furthermore, only ISDmut-vaccinated mice that survived CT26 challenge were additionally protected against rechallenge with a triple-negative breast cancer cell line (4T1), showing that our modified VLV provides cross-protection against different tumour types expressing ERV-derived antigens. We envision that translating these findings and technology into human ERVs (HERVs) could provide new treatment opportunities for cancer patients with unmet medical needs.

Strategies in DNA vaccine for melanoma cancer

According to reports of the international agency for cancer on research, although malignant melanoma shows less prevalence than nonmelanoma skin cancers, it is the major cause of skin cancer mortality. Given that, the production of effective vaccines to control melanoma is eminently required. In this regard, DNA-based vaccines have been extensively investigated for melanoma therapy. DNA vaccines are capable of inducing both cellular and humoral branches of immune responses. These vaccines possess some valuable advantages such as lack of severe side effects and high stability compared to conventional vaccination methods. The ongoing studies are focused on novel strategies in the development of DNA vaccines encoding artificial polyepitope immunogens based on the multiple melanoma antigens, the inclusion of molecular adjuvants to increase the level of immune responses, and the improvement of delivery approaches. In this review, we have outlined the recent advances in the field of melanoma DNA vaccines and described their implications in clinical trials as a strong strategy in the prevention and control of melanoma.

Heterologous Prime Boost Vaccination Induces Protective Melanoma-Specific CD8+ T Cell Responses

Cancer vaccination aims at inducing an adaptive immune response against tumor-derived antigens. In this study, we utilize recombinant human adenovirus serotype 5 (rAd5) and recombinant lymphocytic choriomeningitis virus (rLCMV)-based vectors expressing the melanocyte differentiation antigen gp100. In contrast to single or homologous vaccination, a heterologous prime boost vaccination starting with a rAd5-gp100 prime immunization followed by a rLCMV-gp100 boost injection induces a high magnitude of polyfunctional gp100-specific CD8⁺ T cells. Our data indicate that an optimal T cell induction is dependent on the order and interval of the vaccinations. A prophylactic prime boost vaccination with rAd5- and rLCMV-gp100 protects mice from a B16.F10 melanoma challenge. In the therapeutic setting, combination of the vaccination with low-dose cyclophosphamide showed a synergistic effect and significantly delayed tumor growth. Our findings suggest that heterologous viral vector prime boost immunizations can mediate tumor control in a mouse melanoma model.

Ex vivo-generated dendritic cell-based vaccines in melanoma: the role of nanoparticulate delivery systems

Melanoma is a poor immunogenic cancer and many treatment strategies have been used to enhance specific or nonspecific immunity against it. Dendritic cell (DC)-based cancer vaccine is the most effective therapies that have been used so far. Meanwhile, the efficacy of DC-based immunotherapy relies on critical factors relating to DCs such as the state of maturation and proper delivery of antigens. In this regard, the use of nanoparticulate delivery systems for effective delivery of antigen to ex vivo-generated DC-based vaccines that also poses adjuvanticity would be an ideal approach. In this review article, we attempt to summarize the role of different types of nanoparticulate antigen delivery systems used in the development of ex vivo-generated DC-based vaccines against melanoma and describe their adjuvanticity in mediation of DC maturation, cytoplasmic presentation of antigens to MHC class I molecules, which led to potent antigen-specific immune responses. As were represented, cationic liposomes were the most used approach, which suggest its potential applicability as delivery systems for further experiments in combination with either adjuvants or monoclonal antibodies.

Prophylactic Dendritic Cell-Based Vaccines Efficiently Inhibit Metastases in Murine Metastatic Melanoma

Recent data on the application of dendritic cells (DCs) as anti-tumor vaccines has shown their great potential in therapy and prophylaxis of cancer. Here we report on a comparison of two treatment schemes with DCs that display the models of prophylactic and therapeutic vaccination using three different experimental tumor models: namely, Krebs-2 adenocarcinoma (primary tumor), melanoma (B16, metastatic tumor without a primary node) and Lewis lung carcinoma (LLC, metastatic tumor with a primary node). Dendritic cells generated from bone marrow-derived DC precursors and loaded with lysate of tumor cells or transfected with the complexes of total tumor RNA with cationic liposomes were used for vaccination. Lipofectamine 2000 and liposomes consisting of helper lipid DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine) and cationic lipid 2D3 (1,26-Bis(1,2-de-O-tetradecyl-rac-glycerol)-7,11,16,20-tetraazahexacosan tetrahydrocloride) were used for RNA transfection. It was shown that DCs loaded with tumor lysate were ineffective in contrast to tumor-derived RNA. Therapeutic vaccination with DCs loaded by lipoplexes RNA/Lipofectamine 2000 was the most efficient for treatment of non-metastatic Krebs-2, where a 1.9-fold tumor growth retardation was observed. Single prophylactic vaccination with DCs loaded by lipoplexes RNA/2D3 was the most efficient to treat highly aggressive metastatic tumors LLC and B16, where 4.7- and 10-fold suppression of the number of lung metastases was observed, respectively. Antimetastatic effect of single prophylactic DC vaccination in metastatic melanoma model was accompanied by the reductions in the levels of Th2-specific cytokines however the change of the levels of Th1/Th2/Th17 master regulators was not found. Failure of double prophylactic vaccination is explained by Th17-response polarization associated with autoimmune and pro-inflammatory reactions. In the case of therapeutic DC vaccine the polarization of Th1-response was found nevertheless the antimetastatic effect was less effective in comparison with prophylactic DC vaccine.

Tumor-associated antigen/IL-21-transduced dendritic cell vaccines enhance immunity and inhibit immunosuppressive cells in metastatic melanoma

Dendritic cell (DC)-based vaccine approaches are being actively evaluated for developing immunotherapeutic agents against cancers. In this study, we investigated the use of engineered DCs expressing transgenic tumor-associated antigen hgp100 and the regulatory cytokine interleukin-21, namely DC-hgp100/mIL-21, as a therapeutic vaccine against melanoma. Tumor-bearing mice were injected intratumorally with transgenic DCs followed by three booster injections. Transgenic DC-hgp100/mIL-21 showed significant reduction in primary tumor growth and metastasis compared with DC-hgp100 alone and DC-mIL-21 alone. In vivo depletion of specific immune cell types (CD8(+) T, CD4(+) T and Natural killer (NK)-1.1(+) cells) effectively blocked the protective effect of this combinational vaccine. In adoptive transfer experiments, a survival rate of nearly 90% was observed at 60 days post-tumor inoculation for the combinational vaccine group. In contrast, all mice in the DC-hgp100 and DC-mIL-21-only groups died within 43-46 days after tumor challenge. Considerably increased levels of interferon (IFN)-γ, tumor necrosis factor (TNF)-α, granulocyte macrophage colony-stimulating factor (GM-CSF) and cytotoxic T lymphocytes (CTLs) were detected with the combination vaccine group compared with other individual treatment groups. In comparison with the DC-hgp100 or mIL-21 groups, the combinational DC-hgp100/mIL-21 vaccine also drastically suppressed the myeloid-derived suppressor cells (MDSCs) and T-regulatory (Treg) cell populations. Our findings suggest that a combinational DC- and gene-based hgp100 and mIL-21 vaccine therapy strategy warrants further evaluation as a clinically relevant cancer vaccine approach for human melanoma patients.

Dramatic efficacy improvement of a DC-based vaccine against AML by CD25 T cell depletion allowing the induction of a long-lasting T cell response

Dendritic cell (DC)-based vaccination is a promising approach to enhance anti-tumor immunity that could be considered for acute myeloid leukemia (AML) patients with high-risk of relapse. Our purpose was to study the efficiency and to optimize the immunogenicity of a DC-based vaccine in a preclinical AML murine model. In this report, C57BL6 mice were vaccinated with DC pulsed with peptides eluted (EP) from the syngeneic C1498 myelomonocytic leukemic cell line in a prophylactic setting. In this model, a natural antileukemic immunity mediated by NK cells was observed in the control unloaded DC-vaccinated group. On the other hand, we showed that the cytotoxic antileukemic immune response induced by vaccination with eluted peptides pulsed-DC (DC/EP), in vitro and in vivo, was mainly mediated by CD4(+) T cells. Treatment with anti-CD25 antibody to deplete CD4(+) CD25(+) regulatory T cells before DC-vaccination dramatically improved the antileukemic immune response induced by immunization, and allowed the development of long-lasting immune responses that were tumor protective after a re-challenge with leukemic cells. Our results suggest that this approach could be successful against weakly immunogenic tumors such as AML, and could be translated in human.

Xenogeneic immunization with human tyrosine hydroxylase DNA vaccines suppresses growth of established neuroblastoma

Neuroblastoma (NB) is a challenging malignancy of the sympathetic nervous tissue characterized by a very poor prognosis. One important marker for NB is the expression of tyrosine hydroxylase (TH), the first-step enzyme of catecholamine biosynthesis. We could show stable and high TH gene expression in 67 NB samples independent of the clinical stage. Based on this observation, we addressed the question of whether xenogeneic TH DNA vaccination is effective in inducing an anti-NB immune response. For this purpose, we generated three DNA vaccines based on pCMV-F3Ub and pBUD-CE4.1 plasmids encoding for human (h)THcDNA (A), hTH minigene (B), and hTHcDNA in combination with the proinflammatory cytokine interleukin 12 (C), and tested prophylactic and therapeutic efficacy to suppress primary tumor growth and spontaneous metastasis. Here we report that xenogeneic TH DNA vaccination was effective in eradicating established primary tumors and inhibiting metastasis. Interestingly, this effect could not be enhanced by adding the Th1 cytokine interleukin 12. However, increased IFN-gamma production and NB cytotoxicity of effector cells harvested from vaccinated mice suggested the participation of tumor-specific CTLs in the immune response. The depletion of CD8(+)T cells completely abrogated the hTH vaccine-mediated anti-NB immune response. Furthermore, rechallenging of surviving mice resulted in reduced primary tumor growth, indicating the induction of a memory immune response. In conclusion, xenogeneic immunization with TH-derived DNA vaccines is effective against NB, and may open a new venue for a novel and effective immunotherapeutic strategy against this challenging childhood tumor.

Gene delivery efficiency in bone marrow-derived dendritic cells: comparison of four methods and optimization for lentivirus transduction

Many gene delivery methods have been used to transduce or transfect bone marrow-derived dendritic cells (BMDCs) for genetic engineered DC vaccine research. The present study, for the first time, evaluated the efficiencies of four methods (lipofection, DNA electroporation, recombinant adeno-associated virus type 2 (rAAV2) transduction, and recombinant lentivirus (rLV) transduction) using EGFP as a report gene in the same BMDC culture system. Our data demonstrate that rLV transduction is the most effective method; both lipofection and DNA electroporation transfect BMDCs at lower efficiencies; rAAV2 can hardly transduce BMDCs. Furthermore, our results, for the first time, demonstrate that rLV transduction efficiency on BMDCs can be improved significantly by co-centrifugation and repeated transduction.

Factors involved in the maturation of murine dendritic cells transduced with adenoviral vector variants

Adenoviral vector (Ad)-mediated gene transfer is an attractive method for manipulating the immunostimulatory properties of dendritic cells (DCs) for cancer immunotherapy. DCs treated with Ad have phenotype alterations (maturation) that facilitate T cell sensitization. We investigated the mechanisms of DC maturation with Ad transduction. Expression levels of a maturation marker (CD40) on DCs treated with conventional Ad, fiber-modified Ads (AdRGD, AdF35, AdF35DeltaRGD), or a different serotype Ad (Ad35) were correlated with their transduction efficacy. The alphav-integrin directional Ad, AdRGD, exhibited the most potent ability to enhance both foreign gene expression and CD40 expression, and induced secretion of interleukin-12, tumor necrosis factor-alpha, and interferon-alpha in DCs. The presence of a foreign gene expression cassette in AdRGD was not necessary for DC maturation. Maturation of DCs treated with AdRGD was suppressed by destruction of the Ad genome, inhibition of endocytosis, or endosome acidification, whereas proteasome inhibition increased CD40 expression levels on DCs. Moreover, inhibition of alphav-integrin signal transduction and blockade of cytokine secretion affected the maturation of DCs treated with AdRGD only slightly or not at all, respectively. Thus, our data provide evidence that Ad-induced DC maturation is due to Ad invasion of the DCs, followed by nuclear transport of the Ad genome, and not to the expression of foreign genes.

Exploiting dendritic cells for active immunotherapy of cancer and chronic infections

Dendritic cells (DCs) are important antigen-presenting cells (APCs) that can prime naive T cells and control adaptive immune responses with respect to magnitude, memory and self-tolerance. Understanding the biology of these cells is central to the development of new generation immunotherapies for cancer and chronic infections. This review presents a brief overview of DC biology and of the preparation and use of DC-based vaccines.

Murine dendritic cells modified with CXCL10 gene and tumour cell lysate mediate potent antitumour immune responses in mice

The aim of our present study was to estimate the effect of a therapeutic vaccine against tumour based on dendritic cells (DC) vaccine modified with tumour cell lysate and chemokine CXCL10 gene. In this study, mouse bone marrow DC were pulsed with tumour cell (RM-1) lysate and then transfected with a plasmid vector expressing CXCL10 cDNA by DOTAP liposome. The protective and therapeutic effects of the DC vaccine in RM-1 tumour model were assessed (divided into CXCL10/Lysate-DC, CXCL10/DC, pcDNA/Lysate-DC, Lysate-DC, pcDNA-DC, DC and PBS). The DC transfected with CXCL10 gene were capable of synthesizing and secreting CXCL10 chemokine. The highest CTL activity against RM-1 cells was induced in mice immunized with DC vaccine that was modified with RM-1 lysate and CXCL10 gene (CXCL10/Lysate-DC) when compared with its counterpart in mice. The CXCL10/Lysate-DC immunized mice also exhibited resistance to tumour challenge most effectively. In the RM-1 tumour model, immunization of CXCL10/Lysate-DC inhibited the tumour growth most significantly when compared with other groups and the survival time of the mice treated with CXCL10/Lysate-DC was greatly extended. These findings provide a potential strategy to improve the efficacy of DC-based tumour vaccine.

Hydrodynamic Limb Vein Delivery of a Xenogeneic DNA Cancer Vaccine Effectively Induces Antitumor Immunity

Tumor-associated antigens (TAA) are typically poorly immunogenic "self" antigens. An effective strategy to break tolerance and induce antitumor immunity is by genetic vaccination, employing the orthologous TAA-sequence from a different species. We recently developed a clinically relevant approach for intravascular hydrodynamic limb vein (HLV) delivery of nucleic acids to skeletal muscle. Using the human gp100 xenogeneic TAA in the murine B16 melanoma model, we show that genetic vaccination of mice by HLV plasmid DNA delivery was highly effective at breaking tolerance against the homologous murine gp100 (mgp100) TAA and induced prophylactic antitumor protection. HLV vaccination resulted in an anti-hgp100 humoral and cellular response, with 4-5% of CD8(+) T cells being gp100(25-33)-epitope-specific. Vaccinated animals demonstrated in vivo cytolytic activity against human and mgp100(25-33) peptide-pulsed targets. Antitumor immunity could be adoptively transferred by splenocytes from human gp100-vaccinated animals. Furthermore, a durable antitumor memory response was established as approximately 3% of CD8(+) T cells were gp100(25-33) antigen-specific in mice 6 months after vaccination. Following a single HLV human gp100 DNA boost, this level increased to approximately 17% and protected animals from subsequent B16 tumor rechallenge. Our results warrant further consideration of HLV as a clinically relevant method for cancer gene therapy.

DC-based vaccine loaded with acid-eluted peptides in acute myeloid leukemia: the importance of choosing the best elution method

Tumor-associated peptides isolated by acid elution are frequently used for therapeutic immunization against various tumors both in mice and in humans. In acute myeloid leukemia (AML), the frequent accessibility of a large tumor burden allows for extraction of peptides from leukemia cells by using either citrate-phosphate (CP) or trifluoroacetic acid (TFA) buffer. To develop an optimal immunotherapeutic protocol for AML patients, we evaluated both in mice and in humans, the immunogenicity of peptides eluted from leukemia cells with the two acids (TFA or CP). Although ex vivo studies in mice showed that both prophylactic immunizations with mature dendritic cells (DC) loaded with TFA-peptides (DC/TFA), or CP-peptides (DC/CP), were able to stimulate specific antileukemia immune responses, only vaccination with DC/TFA was able to prevent leukemia outgrowth. Moreover, in humans, only DC/TFA generated significant antileukemia CD4(+) and cytotoxic CD8(+) T cell responses in vitro. In summary, these data demonstrate that the choice of the acid elution procedure to isolate immunogenic peptides strongly influences the efficacy of the antileukemia immune responses. These finding raise essential considerations for the development of immunotherapeutic protocols for cancer patients. In our model, our results argue for the use of the TFA elution method to extract immunogenic AML-associated peptides.

From pathogen to medicine: HIV-1-derived lentiviral vectors as vehicles for dendritic cell based cancer immunotherapy

Over the years, the unique capacity of dendritic cells (DC) for efficient activation of naive T cells has led to their extensive use in cancer immunotherapy protocols. In order to be able to fulfil their role as antigen-presenting cells, the antigen of interest needs to be efficiently introduced and subsequently correctly processed and presented by the DC. For this purpose, a variety of both viral and non-viral antigen-delivery systems have been evaluated. Amongst those, HIV-1-derived lentiviral vectors have been used successfully to transduce DC. This review considers the use of HIV-1-derived lentiviral vectors to transduce human and murine DC for cancer immunotherapy. Lentivirally transduced DC have been shown to present antigenic peptides, prime transgene-specific T cells in vitro and elicit a protective cytotoxic T-lymphocyte (CTL) response in animal models. Different parameters determining the efficacy of transduction are considered. The influence of lentiviral transduction on the DC phenotype and function is described and the induction of immune responses by lentivirally transduced DC in vitro and in vivo is discussed in detail. In addition, direct in vivo administration of lentiviral vectors aiming at the induction of antigen-specific immunity is reviewed. This strategy might overcome the need for ex vivo generation and antigen loading of DC. Finally, future perspectives towards the use of lentiviral vectors in cancer immunotherapy are presented.

Efficient Generation of Canine Bone Marrow-Derived Dendritic Cells

Because of their unsurpassed potency in presenting antigens to naive T cells, dendritic cells are considered to be an important candidate in the development of immunotherapeutic strategies. Despite the high potential of dendritic cell-based immunotherapy, as a so-called dendritic cell vaccination, few clinical approaches using dendritic cell vaccination have been performed in the dog because of very limited information regarding the generation of canine dendritic cells and their functional properties. We therefore established a protocol for the efficient generation of dendritic cells from canine bone marrow cells using recombinant feline granulocyte-macrophage colony-stimulating factor and canine interleukin-4. Dendritic cells were generated efficiently: a yield of 1-9 x 10(6) cells per approximately 0.5 ml of canine bone marrow aspiration was achieved. These dendritic cells showed features shared with mouse and human dendritic cells: dendrite morphology, expression of surface markers MHC class II and CD11c, and up-regulation of molecules related to antigen presentation (MHC class II, B7-1, and B7-2) by activation with lipopolysaccharide. Moreover, the dendritic cells demonstrated phagocytic activity, processing activity of pinocytosed proteins, and activation of allogeneic T cells far more potent than that by macrophages. Our findings suggest that the bone marrow-derived dendritic cells are functional for the capturing and processing of antigens and the initiation of T cell responses.

Clinical Trials With Tumor Antigen Genetically Modified Dendritic Cells

Tumor antigen genetically modified dendritic cells (DC) have been extensively tested as cancer vaccine approaches in preclinical models. This testing has provided evidence of their ability to generate coordinated antitumor CD8+ cytotoxic T lymphocyte (CTL) and CD4+ T-helper cell responses. Their antitumor activity compared favorably to multiple other vaccination strategies in mice. This approach has been brought to patients within nine pilot clinical trials reported to date. These clinical trials have tested both RNA and DNA as means to introduce the foreign genetic material into the DC. Administration to human subjects has proven to be both feasible and safe. There is clear evidence of the ability to activate both CD8+ CTL and CD4+ T-helper cells, which has been the major scientific endpoint in most of these trials. However, antitumor activity has been marginal thus far. In conclusion, tumor antigen genetically modified DC are a feasible strategy to activate tumor-specific T cells in humans.

Immunization with dendritic cells retrovirally transduced with mycobacterial antigen 85A gene elicits the specific cellular immunity including cytotoxic T-lymphocyte activity specific to an epitope on antigen 85A

In the present study, we evaluated antigen 85A (Ag85A) gene-transduced dendritic cells (DCs) vaccine against Mycobacterium tuberculosis. Murine bone marrow-derived DCs were retrovirally transduced with mycobacterial Ag85A gene and injected to BALB/c mice intravenously. The DC vaccine was capable of inducing purified protein derivative (PPD)- and the antigen-specific spleen cell proliferation and IFN-gamma production from both CD4+ and CD8+ T cells in spleens of the immune mice. In addition, the DC vaccination induced cytotoxic T-lymphocytes (CTL) and IFN-gamma-producing cells specific for a 9-mer CTL epitope on Ag85A molecule. This eliciting cellular immunity led to protection against wasting disease due to M. tuberculosis infection and induction of moderate bacterial clearance.

Cell-based cancer gene therapy: breaking tolerance or inducing autoimmunity?

This review examines the mechanisms involved in anti-tumor immunity and how peptides present in many tumor types (tumor-associated antigens) are recognized by T cells from tumor-bearing cancer patients. Tumor-associated antigens are derived from proteins that are also expressed in normal cells. It is predicted that immune responses to such peptides will be compromised by self-tolerance or that stimulation of effective immune responses will be accompanied by autoimmunity. We also consider that the immunity induced against two autoantigens, which are highly conserved in vertebrates, involve qualitatively different mechanisms, such as the production of antibodies and cell-mediated immune responses. However, both pathways lead to tumor immunity and identical phenotypic manifestations of autoimmunity. Appropriate selection of the optimal tumor antigen is critical for the induction of an anti-tumor immune response. Thus, we stress that the methods for antigen presentation using dendritic cells play a critical role in the development of tumor vaccines, to break immune tolerance and induce a strong immune response against them. The viability and feasibility of expansion of canine dendritic cells from bone marrow and peripheral blood ex vivo for the treatment of spontaneous cancers in dogs is also discussed.

Natural killer cells play a critical role in the immune response following immunization with melanoma-antigen-engineered dendritic cells

Tumor antigen gene-modified dendritic cells (DC) generates robust antigen-specific protective antitumor responses. Though the role of CD4 positive and CD8 positive cells in the immunological response to gene-modified DC has been well-characterized, the role of NK cells in this response has been somewhat less clear. Owing to the significant contribution of innate immunity in other model systems, we postulated that NK cells would hold a critical position in the generation of an immune response following immunization with tumor antigen-engineered DC. Immunization with MART-1 melanoma antigen-engineered DC in C57BL/6 mice resulted in the generation of antigen-specific cytotoxic T lymphocytes and in vivo protective responses to the murine B16 melanoma. These responses were dependent on the presence of functional NK cells, although NK cells alone were not sufficient in generating protective responses. Adoptive transfer of NK cells into an NK-deficient but T-cell-competent environment restored the protective response to gene-modified DC immunization. In conclusion, protective immunity after tumor antigen gene-modified DC immunization requires collaboration between CD4+ and CD8+ T cells and NK cells.

Exploiting dendritic cells for cancer immunotherapy: genetic modification of dendritic cells

Dendritic cells (DCs) are pivotal regulators of immune reactivity and immune tolerance. The observation that DCs can recruit naive T cells has invigorated cancer immunology and led to the proposal of DCs as the basis for vaccines designed for the treatment of cancer. Designing effective strategies to load DCs with antigens is a challenging field of research. The successful realization of gene transfer to DCs will be highly dependent on the employed vector system. Here, we review various viral and non-viral gene transfer systems, and discuss their distinct characteristics and possible advantages and disadvantages in respect to their use in DC-based immunotherapy.

Dendritic cells transduced with gp100 gene by RGD fiber-mutant adenovirus vectors are highly efficacious in generating anti-B16BL6 melanoma immunity in mice

Dendritic cells (DCs) are the most potent professional antigen-presenting cells for the initiation of antigen-specific immune responses, and antigen-loaded DCs have been regarded as promising vaccines in cancer immunotherapy. We previously demonstrated that RGD fiber-mutant adenovirus vector (AdRGD) could attain highly efficient gene transduction into human and murine DCs. The aim of the present study is to demonstrate the predominance of ex vivo genetic DC manipulation using AdRGD in improving the efficacy of DC-based immunotherapy targeting gp100, a melanoma-associated antigen (MAA). Vaccination with murine bone marrow-derived DCs transduced with AdRGD encoding gp100 (AdRGD-gp100/mBM-DCs) dramatically improved resistance to B16BL6 melanoma challenge and pulmonary metastasis as compared with immunization with conventional Ad-gp100-transduced mBM-DCs. The improvement in antimelanoma effects upon immunization with AdRGD-gp100/mBM-DCs correlated with enhanced cytotoxic activities of natural killer (NK) cells and B16BL6-specific cytotoxic T lymphocytes (CTLs). Furthermore, in vivo depletion analysis demonstrated that CD8(+) CTLs and NK cells were the predominant effector cells responsible for the anti-B16BL6 immunity induced by vaccination with AdRGD-gp100/mBM-DCs, and that helper function of CD4(+) T cells was necessary for sufficiently eliciting effector activity. These findings clearly revealed that highly efficient MAA gene transduction to DCs by AdRGD could greatly improve the efficacy of DC-based immunotherapy against melanoma.

Effective induction of therapeutic antitumor immunity by dendritic cells coexpressing interleukin-18 and tumor antigen

Dendritic cell (DC) based cancer vaccine can induce potent antitumor immunity in murine models; however, objective clinical responses have been observed only in a minority of cancer patients. To improve the antitumor effect of DC vaccine, Th1-biasing cytokine interleukin (IL) 18 and melanoma-associated antigen gp100 were cotransfected into bone marrow-derived DC (IL-18/gp100-DC), which were used as vaccine to induce the protective and therapeutic immunity in a B16 melanoma model. Immunization with IL-18/gp100-DC resulted in tumor resistance in 87.5% of the mice challenged with B16 cells; however, 12.5% and 25% of mice immunized with gp100 gene-modified DC (gp100-DC) or IL-18 gene-modified DC (IL-18-DC) were tumor free, respectively. Most importantly, IL-18/gp100-DC immunization led to the generation of potent therapeutic immunity that significantly inhibited the tumor growth and improved the survival period of mice bearing established melanoma. Immune cell depletion experiments identified that CD4(+) T cells also played an important role in the priming phase of antitumor immunity and CD8(+) T lymphocytes were the primary effectors. gp100-specific CTL response were induced most markedly in the tumor-bearing mice immunized with IL-18/gp100-DC. Administration with such vaccine also significantly increased the production of Th1 cytokine (IL-2 and interferon-gamma) and induced infiltration of inflammatory cells inside and around the tumors. In addition, natural killer cell activity was also augmented. These results indicate that immunization with DC vaccine coexpressing Th1 cytokine IL-18 and tumor antigen gene may be an effective strategy for a successful therapeutic vaccination.

Induction of protective immunity to Listeria monocytogenes with dendritic cells retrovirally transduced with a cytotoxic T lymphocyte epitope minigene

In the present study, we developed a cytotoxic T lymphocyte (CTL) epitope minigene-transduced dendritic cell (DC)-based vaccine against Listeria monocytogenes. Murine bone marrow-derived DCs were retrovirally transduced with a minigene for listeriolysin O (LLO) 91-99, a dominant CTL epitope of L. monocytogenes, and were injected into BALB/c mice intravenously. We found that the DC vaccine was capable of generating peptide-specific CD8+ T cells exhibiting LLO 91-99-specific cytotoxic activity and gamma interferon production, leading to induction of protective immunity to the bacterium. Furthermore, we demonstrated that the retrovirally transduced DC vaccine was more effective than a CTL epitope peptide-pulsed DC vaccine and a minigene DNA vaccine for eliciting antilisterial immunity. These results provide an alternative strategy in which retrovirally transduced DCs are used to design vaccines against intracellular pathogens.

Genetically modified dendritic cells--a new, promising cancer treatment strategy?

Dendritic cells (DCs), the most potent antigen-presenting cells (APCs), were discovered almost 30 years ago. Due to the priming of antigen-specific immune responses mediated by CD4+ and CD8+ lymphocytes, DCs are crucial for the induction of adaptive immunity against cancer. Therefore, vaccination of cancer patients with DCs presenting tumour-associated antigens (TAAs) have been believed to be a promising anticancer strategy. Multiple clinical trials have been carried out in order to evaluate the safety and efficacy of cancer vaccines based on antigen-pulsed DCs. However, pulsing of DCs with particular peptides has several disadvantages: i) short-time duration of antigen-major histocompatability complex (MHC) complexes, ii) a requirement for matching defined peptides with MHC complexes and iii) exclusive presentation of single antigen epitopes. Application of gene transfer technologies in the field of DC-based vaccines made possible the development of novel, anticancer immunisation strategies. In several animal models, DCs modified with genes encoding TAA or immunostimulatory proteins have been shown to be effective in the induction of antitumour immune responses. Based on these encouraging results, a first clinical trial of prostate cancer patients vaccinated with gene modified DCs has recently been initiated. In this article, methods used for genetic modification of DCs and anticancer vaccination strategies based on genetically modified DCs are reviewed.

Generation of potent and specific cellular immune responses via in vivo stimulation of dendritic cells by pNGVL3-hFLex plasmid DNA and immunogenic peptides

Dendritic cells (DC) are the most potent professional antigen-presenting cells with exquisite capacity to interact with T cells to initiate strong primary cellular immune responses. The antigen-presenting capability of DC makes them attractive vehicles for the delivery of therapeutic cancer vaccines. Recently, we have demonstrated that the introduction of a recombinant gene encoding the human Flt3L gene into mice could result in the expansion of the DC population in vivo. In this report, we have introduced the human Flt-3L gene via naked DNA-based immunization in combination with the muc-1 tumor peptide to immunize mice. We demonstrated that the population of DC expanded following stimulation with the human Flt-3L gene in vivo is functional and they are able to elicit potent muc-1 peptide-specific cellular responses. The strategy described here allows the efficient generation of antigen-specific CTL immunity in vivo and has the potential to be applied in developing efficient protocols for antigen-specific immunotherapy of human malignancies.

Mature CD83(+) dendritic cells infected with recombinant gp100 vaccinia virus stimulate potent antimelanoma T cells

BACKGROUND

Mature dendritic cells (DCs) are potent antigen-presenting cells that activate naive T lymphocytes and initiate cellular immune responses. The ability of CD83(+) mature DCs infected with vaccinia virus encoding the gp100 melanoma transgene (rV-gp100) to stimulate an antimelanoma CD8(+) T-cell response was investigated.

METHODS

Monocyte-derived immature or CD83(+) mature DCs were infected with rV-gp100. The activation state of the DCs and the expression of gp100 protein were evaluated by flow cytometry. The reactivity of antimelanoma CD8(+) T cells was confirmed by measuring specific interferon gamma secretion by using enzyme-linked immunosorbent assay in a mixed-tumor lymphocyte culture.

RESULTS

Both immature and CD83(+) mature DCs expressed gp100 protein when the DCs were infected with rV-gp100. Calcium-signaling agents were required to induce maturation of both infected and noninfected immature DCs. Only rV-gp100-infected CD83(+) DCs induced CD8(+) T cells, after a single stimulation that recognized both peptide-pulsed target cells to multiple gp100 epitopes and a melanoma cell line that endogenously expressed gp100 antigen.

CONCLUSIONS

CD83(+) DCs transduced with rV-gp100 are capable of generating a strong CD8(+) T-cell response against melanoma tumor cells. Expression of melanoma antigens by mature DCs offers the potential advantage of presenting multiple endogenously processed T-cell epitopes and using multiple HLA restriction elements for antimelanoma vaccine therapy.

Lymphotactin cotransfection enhances the therapeutic efficacy of dendritic cells genetically modified with melanoma antigen gp100

Lymphotactin (Lptn) is a C chemokine that attracts T cells and NK cells. Dendritic cells (DC) are highly efficient, specialized antigen-presenting cells and antigen-pulsed DC has been regarded as promising vaccines in cancer immunotherapy. The aim of our present study is to improve the therapeutic efficacy of DC-based tumor vaccine by increasing the preferential chemotaxis of DC to T cells. In this study, Lptn and/or melanoma-associated antigen gp100 were transfected into mouse bone marrow-derived DC, which were used as vaccines in B16 melanoma model. Immunization of C57BL/6 mice with DC adenovirally cotransfected with Lptn and gp100 (Lptn/gp100-DC) could enhance the cytotoxicities of CTL and NK cells, increase the production of IL-2 and interferon-gamma significantly, as compared with immunization with gp100-DC, Lptn-DC, LacZ-DC, DC or PBS counterparts. The Lptn/gp100-DC immunized mice exhibited resistance to tumor challenge most effectively. It was found that the tumor mass of mice vaccinated by Lptn/gp100-DC showed obvious necrosis and inflammatory cell infiltration. In vivo depletion analysis demonstrated that CD8(+) T cells are the predominant T cell subset responsible for the antitumor effect of Lptn/gp100-DC and CD4(+) T cells were necessary in the induction phase of tumor rejection, while NK cells were less important although they participated in the antitumor response either in the induction phase or in the effector phase. In the murine model with the pre-established subcutaneous B16 melanoma, immunization with Lptn/gp100-DC inhibited the tumor growth most significantly when compared with other counterparts. These findings provide a potential strategy to improve the efficacy of DC-based tumor vaccines.

Effect of liposomal antigens on the priming and activation of the immune system by dendritic cells

Dendritic cells (DCs) are recognized as the sole professional antigen-presenting cells capable of priming naive T cells of the helper and cytotoxic phenotypes. This property is presently exploited with success in vaccinal strategies against pathogens or tumor cells that otherwise escape immune recognition, but the repeated infusions of ex vivo expanded and sensitized DCs are usually required to achieve protection. In this paper, we demonstrate that liposomal antigens can efficiently relay and propagate the action of DCs, inducing a strong long-term response against their associated antigen. Their effect is mainly achieved by improving the ex vivo loading of DCs and by efficiently channeling the activation stimulus into the induction of effector function. This is demonstrated by the sustained immunoglobulin production as well as by the sustained lymphoproliferation and the increased cytokine secretion that can be achieved upon restimulation of DC-primed immune cells with limited amount of liposomal antigenic material. Being well-tolerated and easily prepared, liposomal antigens could therefore be expected to significantly contribute to the efficiency and to a more general utilization of the highly promising but rather cumbersome DC-based immunotherapies.

Cancer vaccines

Attempts to generate an anticancer immune response in vivo in patients with cancer have taken several forms. Although to date there have been relatively few published studies describing the effects of the approach in hematologic malignancy, that circumstance is expected to change rapidly during the next few years. In solid tumors, it is not known which, if any, of the approaches being explored will be able to produce responses of sufficient effectiveness and duration to be of general clinical value. Despite the documented increase in survival of patients developing an immune response to tumor immunization, no randomized clinical trial has been entirely convincing. As knowledge of the molecular basis of the immune response and of the immune defenses used by cancer cells improves, it is reasonable to expect to see increasing benefits from tumor vaccines, which are likely to complement, long before they replace, conventional therapies.

Gene delivery by attenuated Salmonella typhimurium: comparing the efficacy of helper versus cytotoxic T cell priming in tumor vaccination

Using the murine B16F1 melanoma, we compared a CTL- versus helper T cell (TH)-directed vaccination approach. Mice were either orally vaccinated with attenuated Salmonella typhimurium (SL) or subcutaneously with dendritic cells (DCs) loaded with gp100 peptides predicted to bind to H2-Kb/H2-Db molecules. SL were transformed with the murine gp100 cDNA (SL-gp100) or with a fusion construct of gp100 and a fragment of invariant chain cDNA (SL-gp100/Ii). Transcription of these genes in vivo has been readily observed in monocytes and DC. Retardation of B16F1 growth was more efficiently achieved by vaccination with SL-gp100 than with DC. Vaccination with SL-gp100/Ii aiming at preferential presentation by MHC II molecules provided some further improvement due to a stronger expansion of TH and CTL. The importance of help was further sustained by a prolongation of the survival time when mice concomitantly received IL2. Notably, prophylactic, compared to therapeutic, vaccination had no additional impact on survival time/rate. This was due to a striking decrease in frequencies of gp100-specific TH, CTL, and cytokine-expressing cells during tumor growth. Thus, the efficacy of vaccination was limited by tumor-induced immunosuppression. Our data demonstrate the oral route of vaccination via Salmonella as a most convenient transfer regimen and confirm the superiority of protocols aiming at preferential activation of TH.

Prophylactic tumor vaccination: comparison of effector mechanisms initiated by protein versus DNA vaccination

Clinical success in tumor vaccination frequently does not reach expectation. Since vaccination protocols are quite variable, we used the murine renal cell carcinoma line RENCA transfected with the lacZ gene (RENCA-beta-gal) to compare the efficacy of two different vaccination strategies or their combination and to elaborate on the underlying mechanisms. BALB/c mice were vaccinated either with naked lacZ DNA or with attenuated Salmonella typhimurium transformed with lacZ DNA or with dendritic cells (DC) loaded with the beta-galactosidase protein or mice were vaccinated with both DNA and protein. Although all regimens led to a prolongation of survival time, oral vaccination with transfected S. typhimurium followed by i.v. transfer of protein-loaded DC provided the optimal schedule. In this setting, >50% of mice remained tumor free after challenge with 10 times the lethal tumor dose of RENCA-beta-gal. As explored in transfer experiments, the superior efficacy of combining DNA and protein vaccination is due to the facts that 1) optimal protection depends on both activated CD4(+) and CD8(+) cells and 2) CD8(+) CTL are most strongly activated by vaccination with transformed Salmonella, whereas vaccination with protein-loaded DC is superior for the activation of Th. The latter induced sustained activation of CTL and recruitment of nonadaptive defense mechanisms. The data demonstrate the strength of DNA vaccination, particularly by the oral route, and provide evidence that a combined treatment with protein-loaded DC can significantly increase the therapeutic efficacy.

Induction of ErbB-2/neu-specific protective and therapeutic antitumor immunity using genetically modified dendritic cells: enhanced efficacy by cotransduction of gene encoding IL-12

Overexpression of ErbB-2/neu occurs in 20-30% of patients with breast cancer and indicates a poor prognosis. The presence of a detectable immune response to ErbB-2/neu in some patients suggests that this oncogene may be a useful target for vaccine therapy. We evaluated whether genetic immunization using dendritic cells (DC) transduced ex vivo with an adenovirus expressing the ErbB-2/neu gene (AdNeuTK) could induce protective and therapeutic immunity against a breast tumor cell line overexpressing ErbB-2/neu. Subcutaneous (s.c.) immunization with the DC vaccine elicited protective immunity in an average of 60% of animals. CTL analysis demonstrated specific cytotoxic activity against breast tumor cells, as well as syngeneic fibroblasts transduced with AdNeuTK. In vivo depletion studies demonstrated both CD4+ and CD8+ T cells were required. In a therapeutic setting, immunization with the DC vaccines could cure mice with pre-established tumors and efficacy was further enhanced by cotransducing DCs with a vector expressing murine IL-12 (AdmIL-12). These studies support DC vaccines as a therapeutic strategy for human breast cancer, while emphasizing the importance of optimizing an immune response by combining tumor antigen presentation with immunostimulatory cytokines.

HLA-A2.1/K(b) transgenic murine dendritic cells transduced with an adenovirus encoding human gp100 process the same A2.1-restricted peptide epitopes as human antigen-presenting cells and elicit A2.1-restricted peptide-specific CTL

HLA-A2.1/K(b) transgenic mice (A2.1/K(b) mice) were used to investigate the processing of human gp100 melanoma antigen by murine antigen presenting cells (APC). Bone marrow-derived dendritic cells (DC) from A2.1/K(b) mice were transduced with adenovirus encoding human gp100 (Ad2/hugp100v2). The Ad2/hugp100v2-transduced DC express human gp100, as documented by immunoperoxidase staining. Flow cytometric analysis demonstrates that Ad vector transduction does not downregulate expression of several markers, including MHC class I. We show that Ad2/hugp100v2-transduced DC are recognized by peptide-specific, A2.1-restricted CTL, suggesting correct processing and presentation of the hugp100 antigen by murine DC. To assess dominance among the various A2.1-restricted epitopes encoded by hugp100, A2.1/K(b) transgenic mice were immunized with Ad2/hugp100v2-transduced DC. Resulting effector cytotoxic T lymphocytes (CTL) were assayed for peptide specificity using a panel of six synthetic peptides known to encode A2.1-restricted epitopes of human gp100 (denoted G154, G177, G209, G280, G457, G476). CTL obtained from Ad2/hugp100v2-transduced DC immunized A2.1/K(b) mouse lysed target cells presenting five of the six epitopes, supporting the observation that murine cells correctly process the hugp100 antigen. The immunogenicity of individual gp100 epitopes correlates with their binding affinity to A2.1. CTL generated from A2.1/K(b) mice immunized with Ad2/hugp100v2-transduced DC also specifically recognize A2.1(+)/gp100(+) human melanoma cells. These data suggest that murine APC process and present the same set of HLA-restricted peptides, similar to human APC. HLA transgenic mice serve as a useful model system to study class I-restricted epitopes of human tumor-associated antigens.

Dendritic cells infected with a vaccinia vector carrying the human gp100 gene simultaneously present multiple specificities and elicit high-affinity T cells reactive to multiple epitopes and restricted by HLA-A2 and -A3

To investigate the ability of human dendritic cells (DC) to process and present multiple epitopes from the gp100 melanoma tumor-associated Ags (TAA), DC from melanoma patients expressing HLA-A2 and HLA-A3 were pulsed with gp100-derived peptides G9154, G9209, or G9280 or were infected with a vaccinia vector (Vac-Pmel/gp100) containing the gene for gp100 and used to elicit CTL from autologous PBL. CTL were also generated after stimulation of PBL with autologous tumor. CTL induced with autologous tumor stimulation demonstrated HLA-A2-restricted, gp100-specific lysis of autologous and allogeneic tumors and no lysis of HLA-A3-expressing, gp100+ target cells. CTL generated by G9154, G9209, or G9280 peptide-pulsed, DC-lysed, HLA-A2-matched EBV transformed B cells pulsed with the corresponding peptide. CTL generated by Vac-Pmel/gp100-infected DC (DC/Pmel) lysed HLA-A2- or HLA-A3-matched B cell lines pulsed with the HLA-A2-restricted G9154, G9209, or G9280 or with the HLA-A3-restricted G917 peptide derived from gp100. Furthermore, these DC/Pmel-induced CTL demonstrated potent cytotoxicity against allogeneic HLA-A2- or HLA-A3-matched gp100+ melanoma cells and autologous tumor. We conclude that DC-expressing TAA present multiple gp100 epitopes in the context of multiple HLA class I-restricting alleles and elicit CTL that recognize multiple gp100-derived peptides in the context of multiple HLA class I alleles. The data suggest that for tumor immunotherapy, genetically modified DC that express an entire TAA may present the full array of possible CTL epitopes in the context of all possible HLA alleles and may be superior to DC pulsed with limited numbers of defined peptides.